RESUMO
Prenatal stress is believed to increase the risk of developing neuropsychiatric disorders, including major depression. Adverse genetic and environmental impacts during early development, such as glucocorticoid hyper-exposure, can lead to changes in the foetal brain, linked to mental illnesses developed in later life. Dysfunction in the GABAergic inhibitory system is associated with depressive disorders. However, the pathophysiology of GABAergic signalling is poorly understood in mood disorders. Here, we investigated GABAergic neurotransmission in the low birth weight (LBW) rat model of depression. Pregnant rats, exposed to dexamethasone, a synthetic glucocorticoid, during the last week of gestation, yielded LBW offspring showing anxiety- and depressive-like behaviour in adulthood. Patch-clamp recordings from dentate gyrus granule cells in brain slices were used to examine phasic and tonic GABAA receptor-mediated currents. The transcriptional levels of selected genes associated with synaptic vesicle proteins and GABAergic neurotransmission were investigated. The frequency of spontaneous inhibitory postsynaptic currents (sIPSC) was similar in control and LBW rats. Using a paired-pulse protocol to stimulate GABAergic fibres impinging onto granule cells, we found indications of decreased probability of GABA release in LBW rats. However, tonic GABAergic currents and miniature IPSCs, reflecting quantal vesicle release, appeared normal. Additionally, we found elevated expression levels of two presynaptic proteins, Snap-25 and Scamp2, components of the vesicle release machinery. The results suggest that altered GABA release may be an essential feature in the depressive-like phenotype of LBW rats.
Assuntos
Depressão , Ácido gama-Aminobutírico , Gravidez , Feminino , Ratos , Animais , Ácido gama-Aminobutírico/metabolismo , Peso ao Nascer , Glucocorticoides/metabolismo , Hipocampo/metabolismo , Receptores de GABA-A/metabolismoRESUMO
Amyotrophic lateral sclerosis (ALS) is a progressive degenerative disease of the central nervous system. Symptomatic and presymptomatic ALS patients demonstrate cortical hyperexcitability, which raises the possibility that alterations in inhibitory gamma-aminobutyric acid (GABA)ergic system could underlie this dysfunction. Here, we studied the GABAergic system in cortex using patch-clamp recordings in the wobbler mouse, a model of ALS. In layer 5 pyramidal neurons of motor cortex, the frequency of GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents was reduced by 72% in wobbler mice. Also, miniature inhibitory postsynaptic currents recorded under blockade of action potentials were decreased by 64%. Tonic inhibition mediated by extrasynaptic GABA(A) receptors was reduced by 87%. In agreement, we found a decreased density of parvalbumin- and somatostatin-positive inhibitory interneurons and reduced vesicular GABA transporter immunoreactivity in the neuropil. Finally, we observed an increased input resistance and excitability of wobbler excitatory neurons, which could be explained by lack of GABA(A) receptor-mediated influences. In conclusion, we demonstrate decreases in GABAergic inhibition, which might explain the cortical hyperexcitability in wobbler mice.
Assuntos
Esclerose Lateral Amiotrófica/fisiopatologia , Potenciais Pós-Sinápticos Inibidores/fisiologia , Córtex Motor/fisiopatologia , Inibição Neural/fisiologia , Ácido gama-Aminobutírico/metabolismo , Esclerose Lateral Amiotrófica/metabolismo , Animais , Modelos Animais de Doenças , Imuno-Histoquímica , Camundongos , Potenciais Pós-Sinápticos em Miniatura/fisiologia , Córtex Motor/metabolismo , Técnicas de Cultura de Órgãos , Técnicas de Patch-Clamp , Transmissão Sináptica/fisiologiaRESUMO
In major depression, one line of research indicates that a dysfunctional GABAergic inhibitory system is linked to the appearance of depressive symptoms. However, as the mechanistic details of such GABAergic deficit are largely unknown, we undertook a functional investigation of the GABAergic system in the rat chronic mild stress model of depression. Adult rats were exposed to an eight-week long stress protocol leading to anhedonic-like behavior. In hippocampal brain slices, phasic, and tonic GABA(A) receptor-mediated currents in dentate gyrus granule cells were examined using patch-clamp recordings. In granule cells, the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) was reduced to 41% in anhedonic-like rats, which was associated with a reduced probability of evoked GABA release. Using immunohistochemical analysis, there was no change in the number of parvalbumin-positive interneurons in the dentate gyrus. Notably, we observed a 60% increase in THIP-activated tonic GABA(A) mediated current in anhedonic-like rats, suggesting an upregulation of extrasynaptic GABA(A) receptors. Finally, five weeks treatment with the antidepressant escitalopram partially reversed the sIPSCs frequency. In summary, we have revealed a hippocampal dysfunction in the GABAergic system in the chronic mild stress model of depression in rats, caused by a reduction in action potential-dependent GABA release. Since the function of the GABAergic system was improved by antidepressant treatment, in parallel with behavioral read outs, it suggests a role of the GABAergic system in the pathophysiology of depression.
Assuntos
Giro Denteado/metabolismo , Agonistas de Receptores de GABA-A/farmacologia , Potenciais Pós-Sinápticos Inibidores/fisiologia , Isoxazóis/farmacologia , Receptores de GABA-A , Ácido gama-Aminobutírico/metabolismo , Potenciais de Ação/efeitos dos fármacos , Animais , Antidepressivos de Segunda Geração/farmacologia , Citalopram/farmacologia , Depressão/tratamento farmacológico , Modelos Animais de Doenças , Hipocampo/efeitos dos fármacos , Hipocampo/metabolismo , Hipocampo/fisiopatologia , Imuno-Histoquímica , Interneurônios/metabolismo , Masculino , Parvalbuminas/metabolismo , Técnicas de Patch-Clamp , Ratos , Ratos Wistar , Receptores de GABA-A/efeitos dos fármacos , Receptores de GABA-A/metabolismo , Inibidores Seletivos de Recaptação de Serotonina/farmacologiaRESUMO
Mature BDNF and its precursor proBDNF may both be secreted to exert opposite effects on synaptic plasticity in the hippocampus. However, it is unknown how proBDNF and mature BDNF affect the excitability of GABAergic interneurons and thereby regulate GABAergic inhibition. We made recordings of GABAergic spontaneous IPSCs (sIPSCs) in mouse dentate gyrus granule cells and found that chronic or acute BDNF reductions led to large increases in the sIPSC frequencies, which were TTX (tetrodotoxin) sensitive and therefore action-potential driven. Conversely, addition of mature BDNF, but not proBDNF, within minutes led to a decrease in the sIPSC frequency to 44%. Direct recordings from fast-spiking GABAergic interneurons revealed that mature BDNF reduced their excitability and depressed their action potential firing, whereas proBDNF had no effect. Using the TrkB inhibitor K-252a, or mice deficient for the common neurotrophin receptor p75(NTR), the regulation of GABAergic activity was shown specifically to be mediated by BDNF binding to the neurotrophin receptor TrkB. In agreement, immunohistochemistry demonstrated that TrkB, but not p75(NTR), was expressed in parvalbumin-positive interneurons. Our results suggest that mature BDNF decreases the excitability of GABAergic interneurons via activation of TrkB, while proBDNF does not impact on GABAergic activity. Thus, by affecting the firing of GABAergic interneurons, mature BDNF may play an important role in regulating network oscillations in the hippocampus.
Assuntos
Fator Neurotrófico Derivado do Encéfalo/metabolismo , Giro Denteado/metabolismo , Interneurônios/metabolismo , Potenciais de Ação , Animais , Giro Denteado/citologia , Masculino , Camundongos , Camundongos Mutantes , Inibição Neural/fisiologia , Plasticidade Neuronal/fisiologia , Técnicas de Patch-Clamp , Receptor trkB/metabolismo , Regulação para CimaRESUMO
The striatum integrates motor behavior using a well-defined microcircuit whose individual components are independently affected in several neurological diseases. The glial cell line-derived neurotrophic factor (GDNF), synthesized by striatal interneurons, and Sonic hedgehog (Shh), produced by the dopaminergic neurons of the substantia nigra (DA SNpc), are both involved in the nigrostriatal maintenance but the reciprocal neurotrophic relationships among these neurons are only partially understood. To define the postnatal neurotrophic connections among fast-spiking GABAergic interneurons (FS), cholinergic interneurons (ACh), and DA SNpc, we used a genetically induced mouse model of postnatal DA SNpc neurodegeneration and separately eliminated Smoothened (Smo), the obligatory transducer of Shh signaling, in striatal interneurons. We show that FS postnatal survival relies on DA SNpc and is independent of Shh signaling. On the contrary, Shh signaling but not dopaminergic striatal innervation is required to maintain ACh in the postnatal striatum. ACh are required for DA SNpc survival in a GDNF-independent manner. These data demonstrate the existence of three parallel but interdependent neurotrophic relationships between SN and striatal interneurons, partially defined by Shh and GDNF. The definition of these new neurotrophic interactions opens the search for new molecules involved in the striatal modulatory circuit maintenance with potential therapeutic value.
Assuntos
Corpo Estriado/fisiologia , Neurônios Dopaminérgicos/fisiologia , Interneurônios/fisiologia , Rede Nervosa/fisiologia , Substância Negra/fisiologia , Acetilcolina/metabolismo , Potenciais de Ação , Animais , Animais Recém-Nascidos , Sobrevivência Celular , Fator Neurotrófico Derivado de Linhagem de Célula Glial/metabolismo , Proteínas Hedgehog/metabolismo , Camundongos Endogâmicos C57BL , Camundongos Transgênicos , Degeneração Neural/patologia , Transdução de SinaisRESUMO
The release of chemical mediators is an essential element of cell-to-cell communication. Signaling molecules such as neurotransmitters and hormones are stored in membrane-bound organelles called secretory vesicles. Some of these organelles can store molecules at high concentrations, overcoming the osmotic shock that could burst the organelle. These organelles contain a proteinaceous matrix that traps the molecules and avoids high intravesicular osmotic pressure. The functional nanostructure and internal organization of the matrix is not well understood. A report by Lovric et al. in this issue of ACS Nano provides insight into the storage of a small molecule-dopamine-within the intraluminal compartments of a secretory vesicle. Lovric et al. used a powerful combination of high spatial resolution mass spectrometry and transmission electron microscopy in conjunction with amperometric measurements of exocytotic release to delineate the temporal and spatial fate of intravesicular dopamine and its interaction with the matrix.
Assuntos
Dopamina , Vesículas Secretórias , Exocitose , Nanoestruturas , Espectrometria de Massa de Íon SecundárioRESUMO
Two main questions are important for understanding and treating affective disorders: why are certain individuals susceptible or resilient to stress, and what are the features of treatment response and resistance? To address these questions, we used a chronic mild stress (CMS) rat model of depression. When exposed to stress, a fraction of rats develops anhedonic-like behavior, a core symptom of major depression, while another subgroup of rats is resilient to CMS. Furthermore, the anhedonic-like state is reversed in about half the animals in response to chronic escitalopram treatment (responders), while the remaining animals are resistant (non-responder animals). Electrophysiology in hippocampal brain slices was used to identify a synaptic hallmark characterizing these groups of animals. Presynaptic properties were investigated at GABAergic synapses onto single dentate gyrus granule cells. Stress-susceptible rats displayed a reduced probability of GABA release judged by an altered paired-pulse ratio of evoked inhibitory postsynaptic currents (IPSCs) (1.48 ± 0.25) compared with control (0.81 ± 0.05) and stress-resilient rats (0.78 ± 0.03). Spontaneous IPSCs (sIPSCs) occurred less frequently in stress-susceptible rats compared with control and resilient rats. Finally, a subset of stress-susceptible rats responding to selective serotonin reuptake inhibitor (SSRI) treatment showed a normalization of the paired-pulse ratio (0.73 ± 0.06) whereas non-responder rats showed no normalization (1.2 ± 0.2). No changes in the number of parvalbumin-positive interneurons were observed. Thus, we provide evidence for a distinct GABAergic synaptopathy which associates closely with stress-susceptibility and treatment-resistance in an animal model of depression.
Assuntos
Depressão/fisiopatologia , Plasticidade Neuronal , Sinapses/fisiologia , Animais , Masculino , Ratos , Ratos Wistar , Ácido gama-Aminobutírico/fisiologiaRESUMO
Autism spectrum disorders (ASDs) comprise a group of clinical phenotypes characterized by repetitive behavior and social and communication deficits. Autism is generally viewed as a neurodevelopmental disorder where insults during embryonic or early postnatal periods result in aberrant wiring and function of neuronal circuits. Neurexins are synaptic proteins associated with autism. Here, we generated transgenic ßNrx1ΔC mice in which neurexin function is selectively impaired during late postnatal stages. Whole-cell recordings in cortical neurons show an impairment of glutamatergic synaptic transmission in the ßNrx1ΔC mice. Importantly, mutant mice exhibit autism-related symptoms, such as increased self-grooming, deficits in social interactions, and altered interaction for nonsocial olfactory cues. The autistic-like phenotype of ßNrx1ΔC mice can be reversed after removing the mutant protein in aged animals. The defects resulting from disruption of neurexin function after the completion of embryonic and early postnatal development suggest that functional impairment of mature circuits can trigger autism-related phenotypes.
Assuntos
Transtorno Autístico/genética , Moléculas de Adesão de Célula Nervosa/metabolismo , Neurônios/metabolismo , Animais , Transtorno Autístico/metabolismo , Transtorno Autístico/fisiopatologia , Proteínas de Ligação ao Cálcio , Células Cultivadas , Córtex Cerebral/citologia , Córtex Cerebral/crescimento & desenvolvimento , Córtex Cerebral/metabolismo , Córtex Cerebral/fisiopatologia , Asseio Animal , Potenciais Pós-Sinápticos Inibidores , Camundongos , Mutação , Moléculas de Adesão de Célula Nervosa/genética , Neurônios/fisiologia , Fenótipo , Comportamento SocialRESUMO
In addition to their classical roles in neuronal growth, survival and differentiation, neurotrophins are also rapid regulators of excitability, synaptic transmission and activity-dependent synaptic plasticity. We have recently shown that mature BDNF (Brain Derived Neurotrophic Factor), but not proBDNF, modulates the excitability of interneurons in dentate gyrus within minutes. Here, we used brain slice patch-clamp recordings to study the mechanisms through which BDNF modulates the firing of interneurons in rat dentate gyrus by binding to TrkB receptors. Bath application of BDNF (15 ng/ml) under current-clamp decreased the firing frequency (by 80%) and input resistance, blocking the delayed firing observed at near-threshold voltage ranges, with no changes in resting membrane potential or action potential waveform. Using TEA (tetraethylammonium), or XE991(a Kv7/KCNQ channel antagonist), the effect of BDNF was abolished, whereas application of retigabine (a Kv7/KCNQ channel opener) mimicked the effect of BDNF, suggesting that the M-current could be implicated in the modulation of the firing. In voltage-clamp experiments, BDNF increased the M-like current amplitude with no change in holding current. This effect was again blocked by XE991 and mimicked by retigabine, the latter accompanied with a change in holding current. In agreement with the electrophysiology, parvalbumin-positive interneurons co-expressed TrkB receptors and Kv7.2/KCNQ2 channels. In conclusion, BDNF depresses the excitability of interneurons by activating an M-like current and possibly blocking Kv1 channels, thereby controlling interneuron resting membrane potential and excitability.
Assuntos
Potenciais de Ação , Fator Neurotrófico Derivado do Encéfalo/fisiologia , Giro Denteado/fisiologia , Interneurônios/fisiologia , Animais , Giro Denteado/citologia , Giro Denteado/efeitos dos fármacos , Feminino , Proteínas de Ligação ao GTP/fisiologia , Técnicas In Vitro , Interneurônios/efeitos dos fármacos , Interneurônios/metabolismo , Masculino , Parvalbuminas/metabolismo , Técnicas de Patch-Clamp , Bloqueadores dos Canais de Potássio/farmacologia , Ratos , Ratos Wistar , Fosfolipases Tipo C/fisiologiaRESUMO
δ-subunit containing extrasynaptic GABA(A) receptors are potential targets for modifying neuronal activity in a range of brain disorders. With the aim of gaining more insight in synaptic and extrasynaptic inhibition, we used a new positive modulator, AA29504, of δ-subunit containing GABA(A) receptors in mouse neurons in vitro and in vivo. Whole-cell patch-clamp recordings were carried out in the dentate gyrus in mouse brain slices. In granule cells, AA29504 (1 µM) caused a 4.2-fold potentiation of a tonic current induced by THIP (1 µM), while interneurons showed a potentiation of 2.6-fold. Moreover, AA29504 (1 µM) increased the amplitude and prolonged the decay of miniature inhibitory postsynaptic currents (mIPSCs) in granule cells, and this effect was abolished by Zn²âº (15 µM). AA29504 (1 µM) also induced a small tonic current (12.7 ± 3.2 pA) per se, and when evaluated in a nominally GABA-free environment using Ca²âº imaging in cultured neurons, AA29504 showed GABA(A) receptor agonism in the absence of agonist. Finally, AA29504 exerted dose-dependent stress-reducing and anxiolytic effects in mice in vivo. We propose that AA29504 potentiates δ-containing GABA(A) receptors to enhance tonic inhibition, and possibly recruits perisynaptic δ-containing receptors to participate in synaptic phasic inhibition in dentate gyrus.
Assuntos
GABAérgicos/farmacologia , Agonistas GABAérgicos/farmacologia , Neurônios/efeitos dos fármacos , Neurônios/fisiologia , Receptores de GABA-A/fisiologia , Animais , Ansiedade/tratamento farmacológico , Ansiedade/psicologia , Encéfalo/metabolismo , Cálcio/metabolismo , Interpretação Estatística de Dados , Giro Denteado/citologia , Giro Denteado/efeitos dos fármacos , Fenômenos Eletrofisiológicos , Febre/etiologia , GABAérgicos/metabolismo , Isoxazóis/farmacologia , Masculino , Camundongos , Camundongos Endogâmicos BALB C , Camundongos Endogâmicos C57BL , Técnicas de Patch-Clamp , Células Piramidais/efeitos dos fármacos , Ensaio Radioligante , Receptores de GABA-A/efeitos dos fármacos , Estresse Psicológico/fisiopatologia , Transmissão Sináptica/efeitos dos fármacosRESUMO
Cysteine string protein-α (CSP-α) is a synaptic vesicle protein that prevents activity-dependent neurodegeneration by poorly understood mechanisms. We have studied the synaptic vesicle cycle at the motor nerve terminals of CSP-α knock-out mice expressing the synaptopHluorin transgene. Mutant nerve terminals fail to sustain prolonged release and the number of vesicles available to be released decreases. Strikingly, the SNARE protein SNAP-25 is dramatically reduced. In addition, endocytosis during the stimulus fails to maintain the size of the recycling synaptic vesicle pool during prolonged stimulation. Upon depolarization, the styryl dye FM 2-10 becomes trapped and poorly releasable. Consistently with the functional results, electron microscopy analysis revealed characteristic features of impaired synaptic vesicle recycling. The unexpected defect in vesicle recycling in CSP-α knock-out mice provides insights into understanding molecular mechanisms of degeneration in motor nerve terminals.
Assuntos
Exocitose/fisiologia , Proteínas de Choque Térmico HSP40/metabolismo , Proteínas de Membrana/metabolismo , Neurônios Motores/metabolismo , Transmissão Sináptica/fisiologia , Vesículas Sinápticas/metabolismo , Animais , Proteínas de Fluorescência Verde/metabolismo , Camundongos , Camundongos Knockout , Camundongos Transgênicos , Neurônios Motores/ultraestrutura , Músculo Esquelético/metabolismo , Músculo Esquelético/ultraestrutura , Junção Neuromuscular/metabolismo , Junção Neuromuscular/ultraestrutura , Proteínas Recombinantes de Fusão/metabolismo , Vesículas Sinápticas/ultraestrutura , Proteína 25 Associada a Sinaptossoma/metabolismoRESUMO
The size principle dictates the orderly recruitment of motoneurons (Mns). This principle assumes that Mns of different sizes have a similar voltage threshold, cell size being the crucial property in determining neuronal recruitment. Thus, smaller neurons have higher membrane resistance and require a lower depolarizing current to reach spike threshold. However, the cell size contribution to recruitment in Mns during postnatal development remains unknown. To investigate this subject, rat oculomotor nucleus Mns were intracellularly labeled and their electrophysiological properties recorded in a brain slice preparation. Mns were divided into 2 age groups: neonatal (1-7 postnatal days, n = 14) and adult (20-30 postnatal days, n = 10). The increase in size of Mns led to a decrease in input resistance with a strong linear relationship in both age groups. A well-fitted inverse correlation was also found between input resistance and rheobase in both age groups. However, input resistance versus rheobase did not correlate when data from neonatal and adult Mns were combined in a single group. This lack of correlation is due to the fact that decrease in input resistance of developing Mns did not lead to an increase in rheobase. Indeed, a diminution in rheobase was found, and it was accompanied by an unexpected decrease in voltage threshold. Additionally, the decrease in rheobase co-varied with decrease in voltage threshold in developing Mns. These data support that the size principle governs the recruitment order in neonatal Mns and is maintained in adult Mns of the oculomotor nucleus; but during postnatal development the crucial property in determining recruitment order in these Mns was not the modifications of cell size-input resistance but of voltage threshold.
Assuntos
Fenômenos Eletrofisiológicos , Neurônios Motores/fisiologia , Nervo Oculomotor/fisiologia , Potenciais de Ação/fisiologia , Animais , Animais Recém-Nascidos , Tamanho Celular , Feminino , Masculino , Ratos , Ratos WistarRESUMO
Human and murine succinic semialdehyde dehydrogenase (SSADH; gamma-hydroxybutyric (GHB) aciduria) deficiency represents an epileptic disorder associated with hyperGABA- and hyperGHB-ergic states. Despite significant neurotransmitters alterations, well-defined single-cell electrophysiological studies, aimed to provide insight into regional neuropathology, have been lacking. In this study, we characterized the effect of residual SSADH enzyme function/increased GABA levels on single-cell hippocampal electrophysiology in SSADH+/+ (wild-type; WT), SSADH+/- (heterozygous; HET), and SSADH-/- (knock-out; KO) mice. Tonic extrasynaptic GABAA receptor (GABAAR)-mediated currents were elevated in HET and KO mice, whereas phasic synaptic GABAAR currents were unaltered in dentate gyrus granule cells. Similarly, tonic GABAAR-mediated currents were increased in dentate gyrus interneurons of KO animals, while phasic GABAergic neurotransmission was unaffected in the same cells. Our results indicate global disruption of cortical networks in SSADH KO mice, affecting both excitatory and inhibitory neurons. Our findings provide new clues concerning seizure evolution in the murine model (absence-->tonic-clonic-->status epilepticus), and extend pathophysiological insight into human SSADH deficiency.
Assuntos
Dosagem de Genes/genética , Hipocampo/patologia , Potenciais da Membrana/genética , Neurônios/fisiologia , Convulsões , Succinato-Semialdeído Desidrogenase/deficiência , Animais , Animais Recém-Nascidos , Biofísica , Modelos Animais de Doenças , Relação Dose-Resposta a Droga , Estimulação Elétrica/métodos , Feminino , Antagonistas GABAérgicos/farmacologia , Humanos , Técnicas In Vitro , Modelos Lineares , Lisina/análogos & derivados , Masculino , Potenciais da Membrana/efeitos dos fármacos , Camundongos , Camundongos Knockout , Neurônios/classificação , Neurônios/efeitos dos fármacos , Técnicas de Patch-Clamp/métodos , Piridazinas/farmacologia , Convulsões/genética , Convulsões/patologia , Convulsões/fisiopatologia , Ácido gama-Aminobutírico/metabolismoRESUMO
This work investigates the somatodendritic shaping of rat oculomotor nucleus motoneurons (Mns) during postnatal development. The Mns were functionally identified in slice preparation, intracellularly injected with neurobiotin, and three-dimensionally reconstructed. Most of the Mns (approximately 85%) were multipolar and the rest (approximately 15%) bipolar. Forty multipolar Mns were studied and grouped as follows: 1-5, 6-10, 11-15, and 21-30 postnatal days. Two phases were distinguished during postnatal development (P1-P10 and P11-P30). During the first phase, there was a progressive increase in the dendritic complexity; e.g., the number of terminals per neuron increased from 26.3 (P1-P5) to 47.7 (P6-P10) and membrane somatodendritic area from 11,289.9 microm(2) (P1-P5) to 19,235.8 microm(2) (P6-P10). In addition, a few cases of tracer coupling were observed. During the second phase, dendritic elongation took place; e.g., the maximum dendritic length increased from 486.7 microm (P6-P10) to 729.5 microm in adult Mns, with a simplification of dendritic complexity to values near those for the newborn, and a slow, progressive increase in membrane area from 19,235.8 microm(2) (P6-P10) to 24,700.3 microm(2) (P21-P30), while the somatic area remained constant. In conclusion, the electrophysiological changes reported in these Mns with maturation (Carrascal et al. [2006] Neuroscience 140:1223-1237) cannot be fully explained by morphometric variations; the dendritic elongation and increase in dendritic area are features shared with other pools of Mns, whereas changes in dendritic complexity depend on each population; the first phase paralleled the establishment of vestibular circuitry and the second paralleled eyelid opening.
Assuntos
Tronco Encefálico/citologia , Tronco Encefálico/crescimento & desenvolvimento , Dendritos/fisiologia , Neurônios Motores/citologia , Plasticidade Neuronal/fisiologia , Nervo Oculomotor/crescimento & desenvolvimento , Potenciais de Ação/fisiologia , Análise de Variância , Animais , Animais Recém-Nascidos , Biotina/análogos & derivados , Biotina/metabolismo , Feminino , Imageamento Tridimensional/métodos , Técnicas In Vitro , Masculino , Modelos Anatômicos , Ratos , Ratos WistarRESUMO
Above recruitment threshold, ocular motoneurons (Mns) show a firing rate linearly related with eye position. Current hypothesis suggests that synaptic inputs are determinant for establishing the recruitment threshold and firing rate gain in these Mns. We investigated this proposal by studying the cholinergic modulation in oculomotor nucleus Mns by intracellular recordings in rat brain slice preparation. All recorded Mns were silent at their resting membrane potential. Bath application of carbachol (10 microm) produced a depolarization and a sustained firing that was not silenced on returning membrane potential to the precarbachol value via DC injection. In response to similar membrane depolarization or equal-current steps, carbachol-exposed Mns produced a higher firing rate and a shorter spike afterhyperpolarization phase with lower amplitude. The relationship between injected current and firing rate (I-F) was linear in control and carbachol-exposed Mns. The slope of these relationships (I-F gain) decreased with carbachol exposure. Bath application of agonist and antagonist of nicotinic and muscarinic acetylcholine receptors in addition to immunohistochemical studies support the notion that muscarinic receptors are primarily involved in the preceding responses. We conclude that muscarinic inputs play an important role in determining the recruitment threshold and firing rate gain observed in oculomotor Mns in vivo.
Assuntos
Neurônios Motores/fisiologia , Nervo Oculomotor/fisiologia , Sistema Nervoso Parassimpático/fisiologia , Receptores Muscarínicos/fisiologia , Recrutamento Neurofisiológico/fisiologia , Anestésicos Locais/farmacologia , Animais , Western Blotting , Interpretação Estatística de Dados , Estimulação Elétrica , Eletrofisiologia , Imuno-Histoquímica , Potenciais da Membrana/efeitos dos fármacos , Agonistas Muscarínicos/farmacologia , Antagonistas Muscarínicos/farmacologia , Músculos Oculomotores/inervação , Músculos Oculomotores/fisiologia , Sistema Nervoso Parassimpático/efeitos dos fármacos , Ratos , Ratos Wistar , Receptores Muscarínicos/efeitos dos fármacos , Recrutamento Neurofisiológico/efeitos dos fármacos , Sinapses/fisiologia , Tetrodotoxina/farmacologiaRESUMO
Alert-chronic studies show that ocular motoneurons (Mns) exhibit a phasic and tonic firing correlated with eye saccade-velocity and position (fixation), respectively. Differences in the phasic and tonic firing among Mns depend on synaptic inputs and/or the intrinsic membrane properties. We have used in vitro slice preparation to investigate the contribution of membrane properties to firing properties of Wistar rat oculomotor nucleus Mns. We recorded different discharge patterns and focused on Mns with sustained discharge (type I) because they were the most abundant, and their firing pattern resembles that reported in alert preparations. Various differences divided these Mns into types I(A) and I(B); the afterhyperpolarization (AHP) phase of the spike was monophasic in I(A) and biphasic in I(B); I(A) Mns showed tonic or phasic-tonic firing depending on the current intensity, while I(B) Mns showed phasic-tonic discharge; the phasic firing was higher in I(B) than in I(A) Mns; I(A) Mns fired in a narrower range than did I(B) Mns; and I(A) Mns showed lower maximum frequency than did I(B) Mns. In conclusion, I(A) and I(B) Mns show different phasic firing properties and dynamic range, supported by intrinsic membrane properties. We suggest that I(A) and I(B) Mns innervate fast-twitch muscle fibres with different contraction speeds, and could contribute to generating a fine phasic signal for a graded muscle contraction. Finally, we have demonstrated an inverse relationship between Mn thresholds and tonic firing gain, concluding that intrinsic membrane properties could not support the covariation between tonic firing gain and recruitment thresholds reported in alert studies.
Assuntos
Potenciais de Ação/fisiologia , Movimentos Oculares/fisiologia , Mesencéfalo/fisiologia , Neurônios Motores/fisiologia , Músculos Oculomotores/inervação , Nervo Oculomotor/fisiologia , Animais , Biotina/análogos & derivados , Forma Celular/fisiologia , Corantes , Dendritos/fisiologia , Dendritos/ultraestrutura , Feminino , Masculino , Mesencéfalo/citologia , Neurônios Motores/citologia , Contração Muscular/fisiologia , Fibras Musculares de Contração Rápida/fisiologia , Fibras Musculares de Contração Lenta/fisiologia , Músculos Oculomotores/fisiologia , Nervo Oculomotor/citologia , Ratos , Ratos Wistar , Movimentos Sacádicos/fisiologiaRESUMO
The postnatal maturation of rat brainstem (oculomotor and hypoglossal nuclei) and spinal motoneurons, based on data collected from in vitro studies, is reviewed here. Membrane input resistance diminishes with age, but to a greater extent for hypoglossal than for oculomotor motoneurons. The time constant of the membrane diminishes with age in a similar fashion for both oculomotor and hypoglossal motoneurons. The current required to reach threshold (rheobase) decreases in oculomotor motoneurons, in contrast with the increase observed in hypoglossal motoneurons. The depolarization voltage required to generate an action potential also diminishes in oculomotor motoneurons, whereas it remains constant in hypoglossal motoneurons. A membrane potential rectification (sag) appears in response to negative current steps, hyperpolarizing brainstem motoneurons more than 20 mV relative to the rest. This membrane response is more frequent in adult motoneurons. The durations of the action potential and its medium afterhyperpolarization (mAHP) decrease with postnatal development in all motoneurons studied, although the shortening of mAHP is more evident in oculomotor motoneurons. A rise in firing rate for all motoneurons with age is universal; this trend is also more pronounced in oculomotor motoneurons. Developing motoneurons exhibit a postinhibitory rebound depolarization that is capable of triggering an action potential or a short burst of spikes. This phenomenon is voltage-dependent and requires less of a membrane hyperpolarization to elicit an action potential in adult than in neonatal cells. In all developing brainstem and spinal motoneurons, the adult somal size is reached within the newborn period, although their dendrites continue to elongate. In summary, input resistance, time constant, and durations of action potential and mAHP decrease, while the frequency of sag and postinhibitory rebound, as well as the motoneuron firing rate and dendritic length, increase with postnatal age. These trends are universal to all the motoneuronal populations studied; however, the extent of these changes differs for each motoneuronal pool. A further distinction is evident in the inconsistent age-dependent change in rheobase and depolarization voltage for the two brainstem motoneuron nuclei.