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1.
Nature ; 618(7965): 513-518, 2023 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-37015289

RESUMO

The replacement of benzene rings with sp3-hybridized bioisosteres in drug candidates generally improves pharmacokinetic properties while retaining biological activity1-5. Rigid, strained frameworks such as bicyclo[1.1.1]pentane and cubane are particularly well suited as the ring strain imparts high bond strength and thus metabolic stability on their C-H bonds. Cubane is the ideal bioisostere as it provides the closest geometric match to benzene6,7. At present, however, all cubanes in drug design, like almost all benzene bioisosteres, act solely as substitutes for mono- or para-substituted benzene rings1-7. This is owing to the difficulty of accessing 1,3- and 1,2-disubstituted cubane precursors. The adoption of cubane in drug design has been further hindered by the poor compatibility of cross-coupling reactions with the cubane scaffold, owing to a competing metal-catalysed valence isomerization8-11. Here we report expedient routes to 1,3- and 1,2-disubstituted cubane building blocks using a convenient cyclobutadiene precursor and a photolytic C-H carboxylation reaction, respectively. Moreover, we leverage the slow oxidative addition and rapid reductive elimination of copper to develop C-N, C-C(sp3), C-C(sp2) and C-CF3 cross-coupling protocols12,13. Our research enables facile elaboration of all cubane isomers into drug candidates, thus enabling ideal bioisosteric replacement of ortho-, meta- and para-substituted benzenes.

2.
J Physiol ; 602(9): 2061-2087, 2024 May.
Artigo em Inglês | MEDLINE | ID: mdl-38554126

RESUMO

Motoneuron properties and their firing patterns undergo significant changes throughout development and in response to neuromodulators such as serotonin. Here, we examined the age-related development of self-sustained firing and general excitability of tibialis anterior motoneurons in a young development (7-17 years), young adult (18-28 years) and adult (32-53 years) group, as well as in a separate group of participants taking selective serotonin reuptake inhibitors (SSRIs, aged 11-28 years). Self-sustained firing, as measured by ΔF, was larger in the young development (∼5.8 Hz, n = 20) compared to the young adult (∼4.9 Hz, n = 13) and adult (∼4.8 Hz, n = 8) groups, consistent with a developmental decrease in self-sustained firing mediated by persistent inward currents (PIC). ΔF was also larger in participants taking SSRIs (∼6.5 Hz, n = 9) compared to their age-matched controls (∼5.3 Hz, n = 26), consistent with increased levels of spinal serotonin facilitating the motoneuron PIC. Participants in the young development and SSRI groups also had higher firing rates and a steeper acceleration in initial firing rates (secondary ranges), consistent with the PIC producing a steeper acceleration in membrane depolarization at the onset of motoneuron firing. In summary, both the young development and SSRI groups exhibited increased intrinsic motoneuron excitability compared to the adults, which, in the young development group, was also associated with a larger unsteadiness in the dorsiflexion torque profiles. We propose several intrinsic and extrinsic factors that affect both motoneuron PICs and cell discharge which vary during development, with a time course similar to the changes in motoneuron firing behaviour observed in the present study. KEY POINTS: Neurons in the spinal cord that activate muscles in the limbs (motoneurons) undergo increases in excitability shortly after birth to help animals stand and walk. We examined whether the excitability of human ankle flexor motoneurons also continues to change from child to adulthood by recording the activity of the muscle fibres they innervate. Motoneurons in children and adolescents aged 7-17 years (young development group) had higher signatures of excitability that included faster firing rates and more self-sustained activity compared to adults aged ≥18 years. Participants aged 11-28 years of age taking serotonin reuptake inhibitors had the highest measures of motoneuron excitability compared to their age-matched controls. The young development group also had more unstable contractions, which might partly be related to the high excitability of the motoneurons.


Assuntos
Neurônios Motores , Humanos , Neurônios Motores/fisiologia , Neurônios Motores/efeitos dos fármacos , Adulto , Adolescente , Feminino , Masculino , Criança , Adulto Jovem , Pessoa de Meia-Idade , Potenciais de Ação/fisiologia , Músculo Esquelético/fisiologia , Músculo Esquelético/crescimento & desenvolvimento , Músculo Esquelético/inervação , Inibidores Seletivos de Recaptação de Serotonina/farmacologia
3.
J Physiol ; 601(10): 1897-1924, 2023 05.
Artigo em Inglês | MEDLINE | ID: mdl-36916205

RESUMO

Sensory and corticospinal tract (CST) pathways activate spinal GABAergic interneurons that have axoaxonic connections onto proprioceptive (Ia) afferents that cause long-lasting depolarizations (termed primary afferent depolarization, PAD). In rodents, sensory-evoked PAD is produced by GABAA receptors at nodes of Ranvier in Ia afferents, rather than at presynaptic terminals, and facilitates spike propagation to motoneurons by preventing branch-point failures, rather than causing presynaptic inhibition. We examined in 40 human participants whether putative activation of Ia-PAD by sensory or CST pathways can also facilitate Ia afferent activation of motoneurons via the H-reflex. H-reflexes in several leg muscles were facilitated by prior conditioning from low-threshold proprioceptive, cutaneous or CST pathways, with a similar long-lasting time course (∼200 ms) to phasic PAD measured in rodent Ia afferents. Long trains of cutaneous or proprioceptive afferent conditioning produced longer-lasting facilitation of the H-reflex for up to 2 min, consistent with tonic PAD in rodent Ia afferents mediated by nodal α5-GABAA receptors for similar stimulation trains. Facilitation of H-reflexes by this conditioning was likely not mediated by direct facilitation of the motoneurons because isolated stimulation of sensory or CST pathways did not alone facilitate the tonic firing rate of motor units. Furthermore, cutaneous conditioning increased the firing probability of single motor units (motoneurons) during the H-reflex without increasing their firing rate at this time, indicating that the underlying excitatory postsynaptic potential was more probable, but not larger. These results are consistent with sensory and CST pathways activating nodal GABAA receptors that reduce intermittent failure of action potentials propagating into Ia afferent branches. KEY POINTS: Controlled execution of posture and movement requires continually adjusted feedback from peripheral sensory pathways, especially those that carry proprioceptive information about body position, movement and effort. It was previously thought that the flow of proprioceptive feedback from Ia afferents was only reduced by GABAergic neurons in the spinal cord that sent axoaxonic projections to the terminal endings of sensory axons (termed GABAaxo neurons). Based on new findings in rodents, we provide complementary evidence in humans to suggest that sensory and corticospinal pathways known to activate GABAaxo neurons that project to dorsal parts of the Ia afferent also increase the flow of proprioceptive feedback to motoneurons in the spinal cord. These findings support a new role for spinal GABAaxo neurons in facilitating afferent feedback to the spinal cord during voluntary or reflexive movements.


Assuntos
Neurônios Motores , Medula Espinal , Humanos , Neurônios Motores/fisiologia , Medula Espinal/fisiologia , Tratos Piramidais/fisiologia , Transmissão Sináptica/fisiologia , Músculo Esquelético/fisiologia , Vias Aferentes , Ácido gama-Aminobutírico , Neurônios Aferentes/fisiologia
4.
J Physiol ; 601(10): 1925-1956, 2023 05.
Artigo em Inglês | MEDLINE | ID: mdl-36928599

RESUMO

Suppression of the extensor H-reflex by flexor afferent conditioning is thought to be produced by a long-lasting inhibition of extensor Ia afferent terminals via GABAA receptor-activated primary afferent depolarization (PAD). Given the recent finding that PAD does not produce presynaptic inhibition of Ia afferent terminals, we examined in 28 participants if H-reflex suppression is instead mediated by post-activation depression of the extensor Ia afferents triggered by PAD-evoked spikes and/or by a long-lasting inhibition of the extensor motoneurons. A brief conditioning vibration of the flexor tendon suppressed both the extensor soleus H-reflex and the tonic discharge of soleus motor units out to 150 ms following the vibration, suggesting that part of the H-reflex suppression during this period was mediated by postsynaptic inhibition of the extensor motoneurons. When activating the flexor afferents electrically to produce conditioning, the soleus H-reflex was also suppressed but only when a short-latency reflex was evoked in the soleus muscle by the conditioning input itself. In mice, a similar short-latency reflex was evoked when optogenetic or afferent activation of GABAergic (GAD2+ ) neurons produced a large enough PAD to evoke orthodromic spikes in the test Ia afferents, causing post-activation depression of subsequent monosynaptic EPSPs. The long duration of this post-activation depression and related H-reflex suppression (seconds) was similar to rate-dependent depression that is also due to post-activation depression. We conclude that extensor H-reflex inhibition by brief flexor afferent conditioning is produced by both post-activation depression of extensor Ia afferents and long-lasting inhibition of extensor motoneurons, rather than from PAD inhibiting Ia afferent terminals. KEY POINTS: Suppression of extensor H-reflexes by flexor afferent conditioning was thought to be mediated by GABAA receptor-mediated primary afferent depolarization (PAD) shunting action potentials in the Ia afferent terminal. In line with recent findings that PAD has a facilitatory role in Ia afferent conduction, we show here that when large enough, PAD can evoke orthodromic spikes that travel to the Ia afferent terminal to evoke EPSPs in the motoneuron. These PAD-evoked spikes also produce post-activation depression of Ia afferent terminals and may mediate the short- and long-lasting suppression of extensor H-reflexes in response to flexor afferent conditioning. Our findings highlight that we must re-examine how changes in the activation of GABAergic interneurons and PAD following nervous system injury or disease affects the regulation of Ia afferent transmission to spinal neurons and ultimately motor dysfunction in these disorders.


Assuntos
Reflexo H , Receptores de GABA-A , Animais , Camundongos , Reflexo H/fisiologia , Neurônios Aferentes/fisiologia , Neurônios Motores/fisiologia , Músculo Esquelético , Estimulação Elétrica
5.
J Neurophysiol ; 130(4): 799-823, 2023 10 01.
Artigo em Inglês | MEDLINE | ID: mdl-37609680

RESUMO

When a muscle is stretched, sensory feedback not only causes reflexes but also leads to a depolarization of sensory afferents throughout the spinal cord (primary afferent depolarization, PAD), readying the whole limb for further disturbances. This sensory-evoked PAD is thought to be mediated by a trisynaptic circuit, where sensory input activates first-order excitatory neurons that activate GABAergic neurons that in turn activate GABAA receptors on afferents to cause PAD, though the identity of these first-order neurons is unclear. Here, we show that these first-order neurons include propriospinal V3 neurons, as they receive extensive sensory input and in turn innervate GABAergic neurons that cause PAD, because optogenetic activation or inhibition of V3 neurons in mice mimics or inhibits sensory-evoked PAD, respectively. Furthermore, persistent inward sodium currents intrinsic to V3 neurons prolong their activity, explaining the prolonged duration of PAD. Also, local optogenetic activation of V3 neurons at one segment causes PAD in other segments, due to the long propriospinal tracts of these neurons, helping to explain the radiating nature of PAD. This in turn facilitates monosynaptic reflex transmission to motoneurons across the spinal cord. In addition, V3 neurons directly innervate proprioceptive afferents (including Ia), causing a glutamate receptor-mediated PAD (glutamate PAD). Finally, increasing the spinal cord excitability with either GABAA receptor blockers or chronic spinal cord injury causes an increase in the glutamate PAD. Overall, we show the V3 neuron has a prominent role in modulating sensory transmission, in addition to its previously described role in locomotion.NEW & NOTEWORTHY Locomotor-related propriospinal neurons depolarize sensory axons throughout the spinal cord by either direct glutamatergic axoaxonic contacts or indirect innervation of GABAergic neurons that themselves form axoaxonic contacts on sensory axons. This depolarization (PAD) increases sensory transmission to motoneurons throughout the spinal cord, readying the sensorimotor system for external disturbances. The glutamate-mediated PAD is particularly adaptable, increasing with either an acute block of GABA receptors or chronic spinal cord injury, suggesting a role in motor recovery.


Assuntos
Neurônios Motores , Medula Espinal , Animais , Camundongos , Axônios , Neurônios GABAérgicos , Ácido Glutâmico
6.
J Neurophysiol ; 123(5): 1657-1670, 2020 05 01.
Artigo em Inglês | MEDLINE | ID: mdl-32208883

RESUMO

The loss of descending serotonin (5-HT) to the spinal cord contributes to muscle spasms in chronic spinal cord injury (SCI). Hyperexcitable motoneurons receive long-lasting excitatory postsynaptic potentials (EPSPs), which activate their persistent inward currents to drive muscle spasms. Deep dorsal horn (DDH) neurons with bursting behavior could be involved in triggering the EPSPs due to loss of inhibition in the chronically 5-HT-deprived spinal cord. Previously, in an acutely transected preparation, we found that bursting DDH neurons were affected by administration of the 5-HT1B/1D receptor agonist zolmitriptan, which suppressed their bursts, and by N-methyl-d-aspartate (NMDA), which enhanced their bursting behavior. Nonbursting DDH neurons were not influenced by these agents. In the present study, we investigate the firing characteristics of bursting DDH neurons following chronic spinal transection at T10 level in adult mice and examine the effects of replacing lost endogenous 5-HT with zolmitriptan. Terminal experiments using our in vitro preparation of the sacral cord were carried out ~10 wk postransection. Compared with the acute spinal stage of our previous study, DDH neurons in the chronic stage became more responsive to dorsal root stimulation, with burst duration doubling with chronic injury. The suppressive effects of zolmitriptan were stronger overall, but the facilitative effects of NMDA were weaker. In addition, the onset of DDH neuron activity preceded ventral root output and the firing rates of DDH interneurons correlated with the integrated long-lasting ventral root output. These results support a contribution of the bursting DDH neurons to muscle spasms following SCI and inhibition by 5-HT.NEW & NOTEWORTHY We investigate the firing characteristics of bursting deep dorsal horn (DDH) neurons following chronic spinal transection. DDH neurons in the chronic stage are different from those in the acute stage as noted by their increase in excitability overall and their differing responses serotonin (5-HT) and N-methyl-d-aspartate (NMDA) receptor agonists. Also, there is a strong relationship between DDH neuron activity and ventral root output. These results support a contribution of the bursting DDH neurons to muscle spasms following chronic spinal cord injury (SCI).


Assuntos
Potenciais de Ação/fisiologia , Potenciais Pós-Sinápticos Excitadores/fisiologia , Interneurônios/fisiologia , Neurônios Motores/fisiologia , Células do Corno Posterior/fisiologia , Agonistas do Receptor 5-HT1 de Serotonina/farmacologia , Serotonina/metabolismo , Espasmo , Traumatismos da Medula Espinal , Raízes Nervosas Espinhais , Potenciais de Ação/efeitos dos fármacos , Animais , Doença Crônica , Modelos Animais de Doenças , Potenciais Pós-Sinápticos Excitadores/efeitos dos fármacos , Interneurônios/efeitos dos fármacos , Interneurônios/metabolismo , Camundongos , Camundongos Endogâmicos C57BL , Neurônios Motores/efeitos dos fármacos , Neurônios Motores/metabolismo , N-Metilaspartato/farmacologia , Oxazolidinonas/farmacologia , Células do Corno Posterior/efeitos dos fármacos , Células do Corno Posterior/metabolismo , Espasmo/metabolismo , Espasmo/fisiopatologia , Traumatismos da Medula Espinal/metabolismo , Traumatismos da Medula Espinal/fisiopatologia , Raízes Nervosas Espinhais/efeitos dos fármacos , Raízes Nervosas Espinhais/metabolismo , Raízes Nervosas Espinhais/fisiopatologia , Triptaminas/farmacologia
7.
J Neurophysiol ; 124(1): 49-62, 2020 07 01.
Artigo em Inglês | MEDLINE | ID: mdl-32459560

RESUMO

The aim of the study was to examine whether the sustained increases in the excitability of afferent fibers traversing the dorsal columns evoked by their polarization depend on the branching points of these fibers. To this end, the effects of epidural polarization were compared in four spinal regions in deeply anesthetized rats; two with the densest collateralization of muscle afferent fibers (above motor nuclei and Clarke's column) and two where the collateralization is more sparse (rostral and caudal to motor nuclei, respectively. The degree of collateralization in different segments was reconstructed in retrogradely labeled afferent fibers in the rat. Nerve volleys evoked in peripheral nerves by electrical stimulation of the dorsal columns within these regions were used as a measure of the excitability of the stimulated fibers. Potent increases in the excitability were evoked by polarization above motor nuclei and Clarke's column, both during constant direct current (DC) polarization (1 µA for 1 min) and for at least 30 min following DC polarization. Smaller excitability increases occurred during the polarization within other regions and were thereafter either absent or rapidly declined after its termination. The postpolarization increases in excitability were counteracted by the GABAA receptor antagonist bicuculline and the α5GABAA extrasynaptic receptor antagonist L655708 and enhanced by the GABAA receptor agonist muscimol and by ionophoretically applied GABA. As extrasynaptic α5GABAA receptors have been found close to Na channels within branching points, these results are consistent with the involvement of branching points in the induction of the sustained postpolarization increases in fiber excitability.NEW & NOTEWORTHY Polarization of sensory fibers traversing dorsal columns of the spinal cord may considerably increase the excitability of these fibers. We show that this involves the effects of current at branching points of afferent fibers and depends on extrasynaptic effects of GABA. These results contribute to our understanding of the mechanism underlying plasticity of activation of nerve fibers and may be used to increase the effectiveness of epidural stimulation in humans and recovery of spinal functions.


Assuntos
Fenômenos Eletrofisiológicos/fisiologia , Fibras Nervosas Mielinizadas/fisiologia , Plasticidade Neuronal/fisiologia , Neurônios Aferentes/fisiologia , Nervos Periféricos/fisiologia , Medula Espinal/fisiologia , Ácido gama-Aminobutírico/fisiologia , Anestesia , Animais , Estimulação Elétrica , Fenômenos Eletrofisiológicos/efeitos dos fármacos , Espaço Epidural , Feminino , Agonistas de Receptores de GABA-A/farmacologia , Antagonistas de Receptores de GABA-A/farmacologia , Masculino , Plasticidade Neuronal/efeitos dos fármacos , Neurônios Aferentes/efeitos dos fármacos , Nervos Periféricos/efeitos dos fármacos , Ratos , Ratos Sprague-Dawley , Medula Espinal/efeitos dos fármacos , Ácido gama-Aminobutírico/farmacologia
8.
J Neurophysiol ; 124(1): 63-85, 2020 07 01.
Artigo em Inglês | MEDLINE | ID: mdl-32459555

RESUMO

Persistent inward calcium and sodium currents (IP) activated during motoneuron recruitment help synaptic inputs maintain self-sustained firing until derecruitment. Here, we estimate the contribution of the IP to self-sustained firing in human motoneurons of varying recruitment threshold by measuring the difference in synaptic input needed to maintain minimal firing once the IP is fully activated compared with the larger synaptic input required to initiate firing before full IP activation. Synaptic input to ≈20 dorsiflexor motoneurons simultaneously recorded during ramp contractions was estimated from firing profiles of motor units decomposed from high-density surface electromyography (EMG). To avoid errors introduced when using high-threshold units firing in their nonlinear range, we developed methods where the lowest threshold units firing linearly with force were used to construct a composite (control) unit firing rate profile to estimate synaptic input to higher threshold (test) units. The difference in the composite firing rate (synaptic input) at the time of test unit recruitment and derecruitment (ΔF = Frecruit - Fderecruit) was used to measure IP amplitude that sustained firing. Test units with recruitment thresholds 1-30% of maximum had similar ΔF values, which likely included both slow and fast motor units activated by small and large motoneurons, respectively. This suggests that the portion of the IP that sustains firing is similar across a wide range of motoneuron sizes.NEW & NOTEWORTHY A new method of estimating synaptic drive to multiple, simultaneously recorded motor units provides evidence that the portion of the depolarizing drive from persistent inward currents that contributes to self-sustained firing is similar across motoneurons of different sizes.


Assuntos
Neurônios Motores/fisiologia , Músculo Esquelético/fisiologia , Recrutamento Neurofisiológico/fisiologia , Potenciais Sinápticos/fisiologia , Adulto , Eletromiografia , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Adulto Jovem
9.
J Neurophysiol ; 121(5): 1591-1608, 2019 05 01.
Artigo em Inglês | MEDLINE | ID: mdl-30625007

RESUMO

The monosynaptic stretch reflex (MSR) plays an important role in feedback control of movement and posture but can also lead to unstable oscillations associated with tremor and clonus, especially when increased with spinal cord injury (SCI). To control the MSR and clonus after SCI, we examined how serotonin regulates the MSR in the sacrocaudal spinal cord of rats with and without a chronic spinal transection. In chronic spinal rats, numerous 5-HT receptor agonists, including zolmitriptan, methylergonovine, and 5-HT, inhibited the MSR with a potency highly correlated to their binding affinity to 5-HT1D receptors and not other 5-HT receptors. Selective 5-HT1D receptor antagonists blocked this agonist-induced inhibition, although antagonists alone had no action, indicating a lack of endogenous or constitutive receptor activity. In normal uninjured rats, the MSR was likewise inhibited by 5-HT, but at much higher doses, indicating a supersensitivity after SCI. This supersensitivity resulted from the loss of the serotonin transporter SERT with spinal transection, because normal and injured rats were equally sensitive to 5-HT after SERT was blocked or to agonists not transported by SERT (zolmitriptan). Immunolabeling revealed that the 5-HT1D receptor was confined to superficial lamina of the dorsal horn, colocalized with CGRP-positive C-fibers, and eliminated by dorsal rhizotomy. 5-HT1D receptor labeling was not found on large proprioceptive afferents or α-motoneurons of the MSR. Thus serotonergic inhibition of the MSR acts indirectly by modulating C-fiber activity, opening up new possibilities for modulating reflex function and clonus via pain-related pathways. NEW & NOTEWORTHY Brain stem-derived serotonin potently inhibits afferent transmission in the monosynaptic stretch reflex. We show that serotonin produces this inhibition exclusively via 5-HT1D receptors, and yet these receptors are paradoxically mostly confined to C-fibers. This suggests that serotonin acts by gating of C-fiber activity, which in turn modulates afferent transmission to motoneurons. We also show that the classic supersensitivity to 5-HT after spinal cord injury results from a loss of SERT, and not 5-HT1D receptor plasticity.


Assuntos
Fibras Nervosas Amielínicas/metabolismo , Receptor 5-HT1D de Serotonina/metabolismo , Reflexo de Estiramento , Traumatismos da Medula Espinal/metabolismo , Animais , Feminino , Fibras Nervosas Amielínicas/efeitos dos fármacos , Fibras Nervosas Amielínicas/fisiologia , Ratos , Agonistas do Receptor 5-HT1 de Serotonina/farmacologia , Antagonistas do Receptor 5-HT1 de Serotonina/farmacologia , Proteínas da Membrana Plasmática de Transporte de Serotonina/metabolismo , Traumatismos da Medula Espinal/fisiopatologia
10.
J Neurophysiol ; 121(4): 1352-1367, 2019 04 01.
Artigo em Inglês | MEDLINE | ID: mdl-30625014

RESUMO

Spinal cord injury leads to a devastating loss of motor function and yet is accompanied by a paradoxical emergence of muscle spasms, which often involve complex muscle activation patterns across multiple joints, reciprocal muscle timing, and rhythmic clonus. We investigated the hypothesis that spasms are a manifestation of partially recovered function in spinal central pattern-generating (CPG) circuits that normally coordinate complex postural and locomotor functions. We focused on the commissural propriospinal V3 neurons that coordinate interlimb movements during locomotion and examined mice with a chronic spinal transection. When the V3 neurons were optogenetically activated with a light pulse, a complex coordinated pattern of motoneuron activity was evoked with reciprocal, crossed, and intersegmental activity. In these same mice, brief sensory stimulation evoked spasms with a complex pattern of activity very similar to that evoked by light, and the timing of these spasms was readily reset by activation of V3 neurons. Given that V3 neurons receive abundant sensory input, these results suggest that sensory activation of V3 neurons is alone sufficient to generate spasms. Indeed, when we silenced V3 neurons optogenetically, sensory evoked spasms were inhibited. Also, inhibiting general CPG activity by blocking N-methyl-d-aspartate (NMDA) receptors inhibited V3 evoked activity and associated spasms, whereas NMDA application did the opposite. Furthermore, overwhelming the V3 neurons with repeated optogenetic stimulation inhibited subsequent sensory evoked spasms, both in vivo and in vitro. Taken together, these results demonstrate that spasms are generated in part by sensory activation of V3 neurons and associated CPG circuits. NEW & NOTEWORTHY We investigated whether locomotor-related excitatory interneurons (V3) play a role in coordinating muscle spasm activity after spinal cord injury (SCI). Unexpectedly, we found that these neurons not only coordinate reciprocal motor activity but are critical for initiating spasms, as well. More generally, these results suggest that V3 neurons are important in initiating and coordinating motor output after SCI and thus provide a promising target for restoring residual motor function.


Assuntos
Interneurônios/fisiologia , Espasticidade Muscular/fisiopatologia , Músculo Esquelético/fisiopatologia , Traumatismos da Medula Espinal/fisiopatologia , Animais , Geradores de Padrão Central/fisiopatologia , Extremidades/inervação , Extremidades/fisiologia , Feminino , Masculino , Camundongos , Neurônios Motores/fisiologia , Contração Muscular , Músculo Esquelético/inervação , Nervos Espinhais/fisiopatologia
11.
Brain ; 141(7): 1946-1962, 2018 07 01.
Artigo em Inglês | MEDLINE | ID: mdl-29860396

RESUMO

Rehabilitative training is one of the most successful therapies to promote motor recovery after spinal cord injury, especially when applied early after injury. Polytrauma and management of other medical complications in the acute post-injury setting often preclude or complicate early rehabilitation. Therefore, interventions that reopen a window of opportunity for effective motor training after chronic injury would have significant therapeutic value. Here, we tested whether this could be achieved in rats with chronic (8 weeks) dorsolateral quadrant sections of the cervical spinal cord (C4) by inducing mild neuroinflammation. We found that systemic injection of a low dose of lipopolysaccharide improved the efficacy of rehabilitative training on forelimb function, as assessed using a single pellet reaching and grasping task. This enhanced recovery was found to be dependent on the training intensity, where a high-intensity paradigm induced the biggest improvements. Importantly, in contrast to training alone, the combination of systemic lipopolysaccharide and high-intensity training restored original function (reparative plasticity) rather than enhancing new motor strategies (compensatory plasticity). Accordingly, electrophysiological and tract-tracing studies demonstrated a recovery in the cortical drive to the affected forelimb muscles and a restructuration of the corticospinal innervation of the cervical spinal cord. Thus, we propose that techniques that can elicit mild neuroinflammation may be used to enhance the efficacy of rehabilitative training after chronic spinal cord injury.


Assuntos
Mielite/reabilitação , Traumatismos da Medula Espinal/reabilitação , Traumatismos da Medula Espinal/terapia , Animais , Medula Cervical/lesões , Feminino , Membro Anterior/inervação , Inflamação , Lipopolissacarídeos/uso terapêutico , Mielite/terapia , Regeneração Nervosa/fisiologia , Plasticidade Neuronal/fisiologia , Tratos Piramidais/fisiopatologia , Ratos , Ratos Endogâmicos Lew , Recuperação de Função Fisiológica/fisiologia , Medula Espinal/fisiopatologia , Traumatismos da Medula Espinal/fisiopatologia
12.
J Neurophysiol ; 120(6): 2953-2974, 2018 12 01.
Artigo em Inglês | MEDLINE | ID: mdl-30256739

RESUMO

Activation of GABAA receptors on sensory axons produces a primary afferent depolarization (PAD) that modulates sensory transmission in the spinal cord. While axoaxonic synaptic contacts of GABAergic interneurons onto afferent terminals have been extensively studied, less is known about the function of extrasynaptic GABA receptors on afferents. Thus, we examined extrasynaptic α5GABAA receptors on low-threshold proprioceptive (group Ia) and cutaneous afferents. Afferents were impaled with intracellular electrodes and filled with neurobiotin in the sacrocaudal spinal cord of rats. Confocal microscopy was used to reconstruct the afferents and locate immunolabelled α5GABAA receptors. In all afferents α5GABAA receptors were found throughout the extensive central axon arbors. They were most densely located at branch points near sodium channel nodes, including in the dorsal horn. Unexpectedly, proprioceptive afferent terminals on motoneurons had a relative lack of α5GABAA receptors. When recording intracellularly from these afferents, blocking α5GABAA receptors (with L655708, gabazine, or bicuculline) hyperpolarized the afferents, as did blocking neuronal activity with tetrodotoxin, indicating a tonic GABA tone and tonic PAD. This tonic PAD was increased by repeatedly stimulating the dorsal root at low rates and remained elevated for many seconds after the stimulation. It is puzzling that tonic PAD arises from α5GABAA receptors located far from the afferent terminal where they can have relatively little effect on terminal presynaptic inhibition. However, consistent with the nodal location of α5GABAA receptors, we find tonic PAD helps produce sodium spikes that propagate antidromically out the dorsal roots, and we suggest that it may well be involved in assisting spike transmission in general. NEW & NOTEWORTHY GABAergic neurons are well known to form synaptic contacts on proprioceptive afferent terminals innervating motoneurons and to cause presynaptic inhibition. However, the particular GABA receptors involved are unknown. Here, we examined the distribution of extrasynaptic α5GABAA receptors on proprioceptive Ia afferents. Unexpectedly, these receptors were found preferentially near nodal sodium channels throughout the afferent and were largely absent from afferent terminals. These receptors produced a tonic afferent depolarization that modulated sodium spikes, consistent with their location.


Assuntos
Potenciais da Membrana , Neurônios Aferentes/metabolismo , Propriocepção , Receptores de GABA-A/metabolismo , Canais de Sódio/metabolismo , Medula Espinal/metabolismo , Animais , Feminino , Antagonistas de Receptores de GABA-A/farmacologia , Inibição Neural , Neurônios Aferentes/efeitos dos fármacos , Neurônios Aferentes/fisiologia , Ratos , Ratos Sprague-Dawley , Medula Espinal/citologia , Medula Espinal/fisiologia , Sinapses/metabolismo , Sinapses/fisiologia
13.
J Neurophysiol ; 111(1): 145-63, 2014 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-24068759

RESUMO

Spinal cord transection leads to elimination of brain stem-derived monoamine fibers that normally synthesize most of the monoamines in the spinal cord, including serotonin (5-hydroxytryptamine, 5-HT) synthesized from tryptophan by enzymes tryptophan hydroxylase (TPH, synthesizing 5-hydroxytryptophan, 5-HTP) and aromatic l-amino acid decarboxylase (AADC, synthesizing 5-HT from 5-HTP). Here we examine whether spinal cord caudal to transection remains able to manufacture and metabolize 5-HT. Immunolabeling for AADC reveals that, while most AADC is confined to brain stem-derived monoamine fibers in spinal cords from normal rats, caudal to transection AADC is primarily found in blood vessel endothelial cells and pericytes as well as a novel group of neurons (NeuN positive and GFAP negative), all of which strongly upregulate AADC with injury. However, immunolabeling for 5-HT reveals that there is no detectable endogenous 5-HT synthesis in any structure in the spinal cord caudal to a chronic transection, including in AADC-containing vessels and neurons, consistent with a lack of TPH. In contrast, when we applied exogenous 5-HTP (in vitro or in vivo), AADC-containing vessels and neurons synthesized 5-HT, which contributed to increased motoneuron activity and muscle spasms (long-lasting reflexes, LLRs), by acting on 5-HT2 receptors (SB206553 sensitive) located on motoneurons (TTX resistant). Blocking monoamine oxidase (MAO) markedly increased the sensitivity of the motoneurons (LLR) to 5-HTP, more than it increased the sensitivity of motoneurons to 5-HT, suggesting that 5-HT synthesized from AADC is largely metabolized in AADC-containing neurons and vessels. In summary, after spinal cord injury AADC is upregulated in vessels, pericytes, and neurons but does not endogenously produce 5-HT, whereas when exogenous 5-HTP is provided AADC does produce functional amounts of 5-HT, some of which is able to escape metabolism by MAO, diffuse out of these AADC-containing cells, and ultimately act on 5-HT receptors on motoneurons.


Assuntos
Descarboxilases de Aminoácido-L-Aromático/metabolismo , Serotonina/biossíntese , Traumatismos da Medula Espinal/metabolismo , Medula Espinal/metabolismo , Animais , Descarboxilases de Aminoácido-L-Aromático/genética , Tronco Encefálico/metabolismo , Feminino , Neurônios Motores/metabolismo , Especificidade de Órgãos , Pericitos/metabolismo , Ratos , Serotonina/metabolismo , Serotonina/farmacologia , Medula Espinal/citologia , Medula Espinal/efeitos dos fármacos
14.
bioRxiv ; 2024 Mar 17.
Artigo em Inglês | MEDLINE | ID: mdl-38558998

RESUMO

While considerable progress has been made in understanding the neuronal circuits that underlie the patterning of locomotor behaviours such as walking, less is known about the circuits that amplify motoneuron output to enable adaptable increases in muscle force across different locomotor intensities. Here, we demonstrate that an excitatory propriospinal neuron population (V3 neurons, Sim1 + ) forms a large part of the total excitatory interneuron input to motoneurons (∼20%) across all hindlimb muscles. Additionally, V3 neurons make extensive connections among themselves and with other excitatory premotor neurons (such as V2a neurons). These circuits allow local activation of V3 neurons at just one segment (via optogenetics) to rapidly depolarize and amplify locomotor-related motoneuron output at all lumbar segments in both the in vitro spinal cord and the awake adult mouse. Interestingly, despite similar innervation from V3 neurons to flexor and extensor motoneuron pools, functionally, V3 neurons exhibit a pronounced bias towards activating extensor muscles. Furthermore, the V3 neurons appear essential to extensor activity during locomotion because genetically silencing them leads to slower and weaker mice with a poor ability to increase force with locomotor intensity, without much change in the timing of locomotion. Overall, V3 neurons increase the excitability of motoneurons and premotor neurons, thereby serving as global command neurons that amplify the locomotion intensity.

15.
J Neurophysiol ; 109(6): 1485-93, 2013 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-23221401

RESUMO

Activation of receptors by serotonin (5-HT1) and norepinephrine (α2) on primary afferent terminals and excitatory interneurons reduces transmission in spinal sensory pathways. Loss or reduction of descending sources of serotonin and norepinephrine after spinal cord injury (SCI) and the subsequent reduction of 5-HT1/α2 receptor activity contributes, in part, to the emergence of excessive motoneuron activation from sensory afferent pathways and the uncontrolled triggering of persistent inward currents that depolarize motoneurons during muscle spasms. We tested in a double-blind, placebo-controlled study whether facilitating 5-HT1B/D receptors with the agonist zolmitriptan reduces the sensory activation of motoneurons during an H-reflex in both noninjured control and spinal cord-injured participants. In both groups zolmitriptan, but not placebo, reduced the size of the maximum soleus H-reflex with a peak decrease to 59% (noninjured) and 62% (SCI) of predrug values. In SCI participants we also examined the effects of zolmitriptan on the cutaneomuscular reflex evoked in tibialis anterior from stimulation to the medial arch of the foot. Zolmitriptan, but not placebo, reduced the long-latency, polysynaptic component of the cutaneomuscular reflex (first 200 ms of reflex) by ∼50%. This ultimately reduced the triggering of the long-lasting component of the reflex (500 ms poststimulation to end of reflex) known to be mediated by persistent inward currents in the motoneuron. These results demonstrate that facilitation of 5-HT1B/D receptors reduces sensory transmission in both monosynaptic and polysynaptic reflex pathways to ultimately reduce long-lasting reflexes (spasms) after SCI.


Assuntos
Reflexo H/efeitos dos fármacos , Oxazolidinonas/farmacologia , Agonistas do Receptor 5-HT1 de Serotonina/farmacologia , Traumatismos da Medula Espinal/fisiopatologia , Transmissão Sináptica/efeitos dos fármacos , Triptaminas/farmacologia , Potenciais de Ação/efeitos dos fármacos , Adulto , Método Duplo-Cego , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Neurônios Motores/metabolismo , Neurônios Motores/fisiologia , Espasticidade Muscular/metabolismo , Espasticidade Muscular/fisiopatologia , Músculo Esquelético/inervação , Músculo Esquelético/fisiopatologia , Receptor 5-HT1B de Serotonina/metabolismo , Receptor 5-HT1D de Serotonina/metabolismo , Receptores Adrenérgicos alfa 2/metabolismo , Traumatismos da Medula Espinal/metabolismo
16.
J Neurophysiol ; 109(2): 375-88, 2013 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-23100134

RESUMO

Neurons of the dorsal spinocerebellar tracts (DSCT) have been described to be rhythmically active during walking on a treadmill in decerebrate cats, but this activity ceased following deafferentation of the hindlimb. This observation supported the hypothesis that DSCT neurons primarily relay the activity of hindlimb afferents during locomotion, but lack input from the spinal central pattern generator. The ventral spinocerebellar tract (VSCT) neurons, on the other hand, were found to be active during actual locomotion (on a treadmill) even after deafferentation, as well as during fictive locomotion (without phasic afferent feedback). In this study, we compared the activity of DSCT and VSCT neurons during fictive rhythmic motor behaviors. We used decerebrate cat preparations in which fictive motor tasks can be evoked while the animal is paralyzed and there is no rhythmic sensory input from hindlimb nerves. Spinocerebellar tract cells with cell bodies located in the lumbar segments were identified by electrophysiological techniques and examined by extra- and intracellular microelectrode recordings. During fictive locomotion, 57/81 DSCT and 30/30 VSCT neurons showed phasic, cycle-related activity. During fictive scratch, 19/29 DSCT neurons showed activity related to the scratch cycle. We provide evidence for the first time that locomotor and scratch drive potentials are present not only in VSCT, but also in the majority of DSCT neurons. These results demonstrate that both spinocerebellar tracts receive input from the central pattern generator circuitry, often sufficient to elicit firing in the absence of sensory input.


Assuntos
Potenciais de Ação , Locomoção/fisiologia , Neurônios Aferentes/fisiologia , Tratos Espinocerebelares/fisiologia , Animais , Gatos , Estado de Descerebração , Membro Posterior/inervação
17.
J Neurophysiol ; 109(6): 1473-84, 2013 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-23221402

RESUMO

In animals, the recovery of motoneuron excitability in the months following a complete spinal cord injury is mediated, in part, by increases in constitutive serotonin (5-HT2) and norepinephrine (α1) receptor activity, which facilitates the reactivation of calcium-mediated persistent inward currents (CaPICs) without the ligands serotonin and norepinephrine below the injury. In this study we sought evidence for a similar role of constitutive monoamine receptor activity in the development of spasticity in human spinal cord injury. In chronically injured participants with partially preserved sensory and motor function, the serotonin reuptake inhibitor citalopram facilitated long-lasting reflex responses (spasms) previously shown to be mediated by CaPICs, suggesting that in incomplete spinal cord injury, functional descending sources of monoamines are present to activate monoamine receptors below the lesion. However, in participants with motor or motor/sensory complete injuries, the inverse agonist cyproheptadine, which blocks both ligand and constitutive 5-HT2/α1 receptor activity, decreased long-lasting reflexes, whereas the neutral antagonist chlorpromazine, which only blocks ligand activation of these receptors, had no effect. When tested in noninjured control participants having functional descending sources of monoamines, chlorpromazine was effective in reducing CaPIC-mediated motor unit activity. On the basis of these combined results, it appears that in severe spinal cord injury, facilitation of persistent inward currents and muscle spasms is mainly mediated by the activation of constitutive 5-HT2 and α1 receptor activity. Drugs that more selectively block these constitutively active monoamine receptors may provide better oral control of spasticity, especially in motor complete spinal cord injury where reducing motoneuron excitability is the primary goal.


Assuntos
Espasticidade Muscular/fisiopatologia , Receptor 5-HT2A de Serotonina/metabolismo , Receptores Adrenérgicos alfa 1/metabolismo , Traumatismos da Medula Espinal/fisiopatologia , Potenciais de Ação/efeitos dos fármacos , Adulto , Idoso , Monoaminas Biogênicas/metabolismo , Cálcio/metabolismo , Estudos de Casos e Controles , Clorpromazina/farmacologia , Citalopram/farmacologia , Antagonistas de Dopamina/farmacologia , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Neurônios Motores/metabolismo , Neurônios Motores/fisiologia , Espasticidade Muscular/metabolismo , Músculo Esquelético/inervação , Músculo Esquelético/fisiopatologia , Recrutamento Neurofisiológico/efeitos dos fármacos , Reflexo/efeitos dos fármacos , Inibidores Seletivos de Recaptação de Serotonina/farmacologia , Traumatismos da Medula Espinal/diagnóstico , Traumatismos da Medula Espinal/metabolismo
18.
bioRxiv ; 2023 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-38234767

RESUMO

Sensory input flow is central to voluntary movements. For almost a century, GABA was believed to modulate this flow by inhibiting sensory axons in the spinal cord to sculpt neural inputs into skilled motor output. Instead, here we show that GABA can also facilitate sensory transmission in monkeys and consequently increase spinal and cortical neural responses to sensory inputs challenging our understanding of generation and perception of movement.

19.
J Neurophysiol ; 108(11): 2991-8, 2012 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-22972966

RESUMO

N-methyl-d-aspartate (NMDA) receptors are of critical importance for locomotion in the developing neonatal spinal cord in rats and mice. However, due to profound changes in the expression of NMDA receptors in development between the neonatal stages and adulthood, it is unclear whether NMDA receptors are still an important component of locomotion in the adult rodent spinal cord. To shed light on this issue, we have taken advantage of recently developed preparations allowing the intracellular recording of adult motoneurons that control the tail in the sacrocaudal spinal cord of adult mice and rats. We show that in the adult sacrocaudal spinal cord, NMDA induces rhythmic activity recorded on the ventral roots, often coordinated from left to right, as in swimming motions with the tail (fictive locomotion). The adult motoneurons themselves are intrinsically sensitive to NMDA application. That is, when motoneurons are synaptically isolated with TTX, NMDA still causes spontaneous bursts of rhythmic activity, depending on the membrane potential. We show that these bursts in motoneurons depend on an NMDA-mediated persistent inward current and are terminated by the progressive activation of a persistent outward current. These results indicate that motoneurons, along with the central pattern generator, can actively participate in the production of swimminglike locomotor activity in adult rodents.


Assuntos
Potenciais de Ação/efeitos dos fármacos , Agonistas de Aminoácidos Excitatórios/farmacologia , Neurônios Motores/fisiologia , N-Metilaspartato/farmacologia , Animais , Locomoção/fisiologia , Camundongos , Bloqueadores dos Canais de Potássio/farmacologia , Ratos , Medula Espinal/fisiologia , Raízes Nervosas Espinhais/fisiologia , Cauda/inervação , Tetrodotoxina/farmacologia
20.
Behav Brain Res ; 422: 113731, 2022 03 26.
Artigo em Inglês | MEDLINE | ID: mdl-34979221

RESUMO

Animal models of cervical spinal cord injury (SCI) have frequently utilized partial transection injuries to evaluate plasticity promoting treatments such as rehabilitation training of skilled reaching and grasping tasks. Though highly useful for studying the effects of cutting specific spinal tracts that are important for skilled forelimb motor function, cervical partial-transection SCI-models underappreciate the extensive spread of most human SCIs, thus offering poor predictability for the clinical setting. Conversely, moderate cervical contusion SCI models targeting the spinal tracts important for skilled reaching and grasping can better replicate the increased size of most human SCIs and are often considered more clinically relevant. However, it is unknown whether animals with moderate cervical contusion SCIs that damage key spinal motor tracts can train in skilled reaching and grasping tasks. In this study, we quantify the impact of injury size and distribution on recovery in a skilled motor task called the single pellet reaching, grasping and retrieval (SPRGR) task in rats with cervical unilateral contusion injuries (UCs), and compare to rats with a partial transection SCIs (i.e., dorsolateral quadrant transection; DLQ). We found that UCs damage key tracts important for performing skilled motor tasks, similar to DLQs, but UCs also produce more extensive grey matter damage and more ventral white matter damage than DLQs. We also compared forelimb functionality at 1, 3, and 5 weeks of rehabilitative motor training between trained and untrained rats and found a more severe drop in SPRGR performance than in DLQ SCIs. Nevertheless, despite more severe injuries and initially low SPRGR performance, rehabilitative training for contusion animals resulted in significant improvements in SPRGR performance and proportionally more recovery than DLQ rats. Our findings show that rehabilitative motor training can facilitate considerable amounts of motor recovery despite extensive spinal cord damage, especially grey matter damage, thus supporting the use of contusion or compression SCI models and showing that ventral grey and white matter damage are not necessarily detrimental to recovery after training.


Assuntos
Medula Cervical/lesões , Terapia por Exercício , Membro Anterior/fisiopatologia , Destreza Motora/fisiologia , Reabilitação Neurológica , Condicionamento Físico Animal/fisiologia , Traumatismos da Medula Espinal/fisiopatologia , Traumatismos da Medula Espinal/reabilitação , Animais , Comportamento Animal/fisiologia , Contusões/fisiopatologia , Contusões/reabilitação , Modelos Animais de Doenças , Ratos
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