Fictive Scratching Patterns in Brain Cortex-Ablated, Midcollicular Decerebrate, and Spinal Cats.

Background: The spinal cord's central pattern generators (CPGs) have been explained by the symmetrical half-center hypothesis, the bursts generator, computational models, and more recently by connectome circuits. Asymmetrical models, at odds with the half-center paradigm, are composed of extensor and flexor CPG modules. Other models include not only flexor and extensor motoneurons but also motoneuron pools controlling biarticular muscles. It is unknown whether a preferred model can explain some particularities that fictive scratching (FS) in the cat presents. The first aim of this study was to investigate FS patterns considering the aiming and the rhythmic periods, and second, to examine the effects of serotonin (5HT) on and segmental inputs to FS. Methods: The experiments were carried out first in brain cortex-ablated cats (BCAC), then spinalized (SC), and for the midcollicular (MCC) preparation. Subjects were immobilized and the peripheral nerves were used to elicit the Monosynaptic reflex (MR), to modify the scratching patterns and for electroneurogram recordings. Results: In BCAC, FS was produced by pinna stimulation and, in some cases, by serotonin. The scratching aiming phase (AP) initiates with the activation of either flexor or extensor motoneurons. Serotonin application during the AP produced simultaneous extensor and flexor bursts. Furthermore, WAY 100635 (5HT1A antagonist) produced a brief burst in the tibialis anterior (TA) nerve, followed by a reduction in its electroneurogram (ENG), while the soleus ENG remained silent. In SC, rhythmic phase (RP) activity was recorded in the soleus motoneurons. Serotonin or WAY produced FS bouts. The electrical stimulation of Ia afferent fibers produced heteronymous MRes waxing and waning during the scratch cycle. In MCC, FS began with flexor activity. Electrical stimulation of either deep peroneus (DP) or superficial peroneus (SP) nerves increased the duration of the TA electroneurogram. Medial gastrocnemius (MG) stretching or MG nerve electrical stimulation produced a reduction in the TA electroneurogram and an initial MG extensor burst. MRes waxed and waned during the scratch cycle. Conclusion: Descending pathways and segmental afferent fibers, as well as 5-HT and WAY, can change the FS pattern. To our understanding, the half-center hypothesis is the most suitable for explaining the AP in MCC.

Examining Monosynaptic Connections in Drosophila Using Tetrodotoxin Resistant Sodium Channels.

Here, a new technique termed Tetrotoxin (TTX) Engineered Resistance for Probing Synapses (TERPS) is applied to test for monosynaptic connections between target neurons. The method relies on co-expression of a transgenic activator with the tetrodotoxin-resistant sodium channel, NaChBac, in a specific presynaptic neuron. Connections with putative post-synaptic partners are determined by whole-cell recordings in the presence of TTX, which blocks electrical activity in neurons that do not express NaChBac. This approach can be modified to work with any activator or calcium imaging as a reporter of connections. TERPS adds to the growing set of tools available for determining connectivity within networks. However, TERPS is unique in that it also reliably reports bulk or volume transmission and spillover transmission.

MODULATION OF THE MONOSYNAPTIC REFLEX POTENTIALSIN THE DECEREBRATED RATS UNDER THE INFLUENCE OF HYDROXYTRYPTOPHAN.

We studied the serotonin effect on monosynaptic reflex potentials (MSR) of spinal motorneurons in the decerebrated rats in control and after intraperitoneal administration of serotonin precursor ­ 5-Hydroxytryptophan (5-HTP). MSR of motorneurons in the lumbar spinal cord were registered using electrical stimulation of dorsal root of the 5th lumbar section. During stimulation physiological saline or 5-hydroxytryptophan was injected intraperitoneally. In comparison with average amplitude of the control MSR there were registered significant increase in amplitudes of the MSR (169% and +172%, P <0,001) in animals with injection 5-HTP. These data suggest that serotonin release after 5-HTP administration leads to activation of motorneurons in the lumbar spinal cord. The mechanism of this activation may be related to the weakening of the inhibitory control of interneurons in the transmission pathways of the excitatory influences from muscle afferent to motorneurons and to the postural (antigravity) reflex reactions which necessary for the initiation of locomotion.

Effects of centrally acting analgesics on spinal segmental reflexes and wind-up.

BACKGROUND: The spinal cord is a prime site of action for analgesia. Here we characterize the effects of established analgesics on segmental spinal reflexes. The aim of the study was to look for the pattern of action or signature of analgesic effects on these reflexes. METHODS: We used a spinal cord in vitro preparation of neonate mice to record ventral root responses to dorsal root stimulation. Pregabalin, clonidine, morphine and duloxetine and an experimental sigma-1 receptor antagonist (S1RA) were applied to the preparation in a cumulative concentration protocol. Drug effects on the wind-up produced by repetitive stimulation of C-fibres and on responses to single A- and C-fibre intensity stimuli were analysed. RESULTS: All compounds produced a concentration-dependent inhibition of total spikes elicited by repetitive stimulation. Concentrations producing ∼50% reduction in this parameter were (in µM) clonidine (0.01), morphine (0.1), pregabalin (1), duloxetine (10) and S1RA (30). At these concentrations clonidine, pregabalin and S1RA had significant effects on the wind-up index and little depressant effects on responses to single stimuli. Morphine and duloxetine did not depress wind-up index and showed large effects on responses to single stimuli. None of the compounds had strong effects on the amplitude of the non-nociceptive monosynaptic reflex. CONCLUSIONS: morphine and duloxetine had general depressant effects on spinal reflexes, whereas the effects of clonidine, pregabalin and S1RA appeared to be restricted to signals originated by strong repetitive activation of C-fibres. Results are discussed in the context of reported behavioural effects of the compounds studied.

Daily passive cycling attenuates the hyperexcitability and restores the responsiveness of the extensor monosynaptic reflex to quipazine in the chronic spinally transected rat.

Activity-based interventions such as locomotor training or passive cycling have a positive influence on the spinal circuitry and recovery following a spinal cord injury (SCI). The use of quipazine in combination with exercise training has demonstrated a greater functional recovery than has exercise training alone. However, the influence of exercise or training on the responsiveness of the spinal cord to quipazine has not been examined following a chronic spinal transection. The purpose of this study was to characterize the flexor and extensor monosynaptic reflex (MSR) response pre- and post-quipazine in chronic complete spinally transected rats that either underwent daily passive cycling for 3 months or did not receive passive cycling. Following a chronic spinal transection, the extensor MSR demonstrated a hyperreflexive response (fivefold increase) to afferent stimuli, and did not respond to quipazine injection. With daily passive cycling, the extensor MSR hyperexcitability was attenuated, and the MSR amplitude increased 72% following quipazine injection (p<0.004), which was comparable to the extensor MSR response (94%) in the control group. For both chronic spinal transection groups, the flexor MSR amplitudes were not altered following quipazine injection, whereas in the control group the flexor MSR amplitude increased 86% in response to quipazine (p<0.004). These results demonstrate that passive cycling attenuates the hyperreflexive response of the extensor MSR following a chronic SCI, and restores the MSR response to quipazine.

Removal of supraspinal input reveals a difference in the flexor and extensor monosynaptic reflex response to quipazine independent of motoneuron excitation.

The purpose of this study was to determine if quipazine, a serotonergic agonist, differentially modulates flexor and extensor motor output. This was achieved by examining the monosynaptic reflex (MSR) of the tibial (extensor) and peroneal (flexor) nerves, by determining the basic and rhythmic properties of extensor and flexor motoneurons, and by recording extracellular Ia field potentials of the tibial and peroneal nerves in the in vivo adult decerebrate rat in both spinal intact and acute spinalized preparations. In the spinal intact preparation, the tibial and peroneal MSR amplitude significantly increased compared with baseline in response to quipazine, with no difference between nerves (P < 0.05). In the spinalized preparation, the MSR was significantly increased in both the tibial and peroneal nerves with the latter increasing more than the former (5.7 vs. 3.6 times; P < 0.05). Intracellular motoneuron experiments demonstrated that rheobase decreased, while input resistance, afterhyperpolarization amplitude, and the firing rate at a given current injection increased in motoneurons following quipazine administration with no differences between extensor and flexor motoneurons. Both the tibial and peroneal nerve extracellular Ia field potentials increased with the peroneal demonstrating a significantly greater increase (7 vs. 38%; P < 0.05) following quipazine. It is concluded that in the spinal intact preparation quipazine does not have a differential effect on flexor or extensor motor output. However, in the acute spinalized preparation, quipazine preferentially affects the flexor MSR compared with the extensor MSR, likely due to the removal of a descending tonic inhibition on flexor Ia afferents.

Chondroitinase ABC promotes plasticity of spinal reflexes following peripheral nerve injury.

Peripheral nerve transection, even with optimal repair, can result in an extensive disruption of central connectivity, which can lead to long-lasting impairments in motor and sensory function. We hypothesised that removal of spinal cord chondroitin sulphate proteoglycans (CSPGs) would promote plasticity in the spinal cord, allowing compensation for inaccurate peripheral reinnervation. In adult rats, the median and radial nerves were cut and repaired, either correctly (median to median and radial to radial), or incorrectly (median to radial and vice versa). This produced two levels of inaccuracy of peripheral reinnervation. Whole nerve recordings from a third brachial plexus nerve, the ulnar, were made during median or radial nerve stimulation. Low and high threshold reflexes were characterised in uninjured animals and a clear difference in the pattern of ulnar response to flexor (median) or extensor (radial) stimulation was established. This included the phenomenon of wind-up, where repetitive median nerve stimulation at supramaximal C-fibre threshold leads to a progressive increase in the number of spikes recorded. To achieve digestion of CSPGs a lentiviral vector expressing ChABC was delivered to the spinal cord via intraspinal injection. Following ChABC treatment, we found several indicators of reorganisation of central connections. Firstly, we found that the amplitude of a low threshold, polysynaptic reflex could be increased after nerve injury, only following treatment with ChABC. Secondly, wind-up of motor responses in the ulnar nerve to supramaximal stimulation of afferents in the median nerve, which collapses after nerve injury (to ~25% of uninjured value), could be restored by ChABC after correct repair (to ~90% of uninjured value). Thirdly, wind-up in ulnar motor axons to stimulation of radial nerve afferents, which is minimal in the uninjured state, becomes significantly stronger after nerve injury and ChABC treatment (a 10 fold increase). We propose that application of a plasticity-promoting treatment to the spinal cord allows the amplification of adaptive changes in response to inaccurate wiring in the periphery.

The role of muscle spindles in the development of the monosynaptic stretch reflex.

Muscle sensory axons induce the development of specialized intrafusal muscle fibers in muscle spindles during development, but the role that the intrafusal fibers may play in the development of the central projections of these Ia sensory axons is unclear. In the present study, we assessed the influence of intrafusal fibers in muscle spindles on the formation of monosynaptic connections between Ia (muscle spindle) sensory axons and motoneurons (MNs) using two transgenic strains of mice. Deletion of the ErbB2 receptor from developing myotubes disrupts the formation of intrafusal muscle fibers and causes a nearly complete absence of functional synaptic connections between Ia axons and MNs. Monosynaptic connectivity can be fully restored by postnatal administration of neurotrophin-3 (NT-3), and the synaptic connections in NT-3-treated mice are as specific as in wild-type mice. Deletion of the Egr3 transcription factor also impairs the development of intrafusal muscle fibers and disrupts synaptic connectivity between Ia axons and MNs. Postnatal injections of NT-3 restore the normal strengths and specificity of Ia-motoneuronal connections in these mice as well. Severe deficits in intrafusal fiber development, therefore, do not disrupt the establishment of normal, selective patterns of connections between Ia axons and MNs, although these connections require the presence of NT-3, normally supplied by intrafusal fibers, to be functional.

Involvement of AMPA receptors for Mesobuthus tamulus Pocock venom-induced depression of monosynaptic reflex in neonatal rat spinal cord in vitro.

Glutamate is a putative neurotransmitter at Ia-alpha motoneuron synapse in the spinal cord and mediate the action via N-methyl-D-aspartate (NMDA) and a-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors. Since NMDA receptors are not involved in M. tamulus Pocock (MBT) venom-induced depression of spinal monosynaptic reflex (MSR), the present study was undertaken to evaluate the role of AMPA receptors in mediating the depression of MSR by MBT venom. The experiments were performed on isolated hemisected spinal cord from 4-6 day old rats. Stimulation of a dorsal root with supramaximal voltage evoked MSR and polysynaptic reflex (PSR) potentials in the corresponding segmental ventral root. Superfusion of MBT venom (0.3 microg/ml) depressed the spinal reflexes in a time-dependent manner. The maximum depression of MSR(approximately 66%) was seen at 10 min and it was 25 min for PSR (approximately 75%). The time to produce 50% depression of MSR and PSR was 6.7+/- 1.5 and 10.8 +/- 2.6 min, respectively. Pretreatment of the cords with 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX, 0.1 microM), an AMPA receptor antagonist, blocked the venom-induced depression of MSR but not PSR. The results indicate that venom-induced depression of MSR is mediated via AMPA receptors.

Involvement of NO-guanylyl cyclase pathway for the depression of spinal monosynaptic reflex by Mesobuthus tamulus venom in neonatal rat in vitro.

AIMS: The present study was undertaken to evaluate the role of nitric oxide (NO) in Mesobuthus tamulus (MBT) venom-induced depression of spinal reflexes. MAIN METHODS: Experiments were performed on isolated hemisected spinal cords from 4 to 6day old rats. Stimulation of a dorsal root with supramaximal strength evoked monosynaptic (MSR) and polysynaptic reflex (PSR) potentials in the corresponding segmental ventral root. KEY FINDINGS: Superfusion of MBT venom (0.3µg/ml) depressed the spinal reflexes in a time-dependent manner and the maximum depression was seen at 10min (MSR by 63%; PSR by 79%). The time to produce 50% depression (T-50) of MSR and PSR was 7.7±1.3 and 5.7±0.5min, respectively. Pretreatment with bicuculline (1µM; GABA(A) receptor antagonist) or strychnine (1µM; glycine(A) receptor antagonist) did not block the venom-induced depression of spinal reflexes. However, Nω-nitro-L-arginine methyl ester (L-NAME, 100 or 300µM; NO synthase inhibitor) or hemoglobin (Hb, 100µM; NO scavenger) antagonized the venom-induced depression of MSR. Further, soluble guanylyl cylase inhibitors (1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one, ODQ; 1µM or methylene blue, 100µM) also antagonized the venom-induced depression of MSR but not PSR. Nitrite concentration (indicator of NO activity) of the cords exposed to venom (0.3µg/ml) was not different from the control group. SIGNIFICANCE: The results indicate that venom-induced depression of MSR is mediated via NO-guanylyl cyclase pathway without involving GABAergic or glycinergic system.

Involvement of 5-hydroxytryptaminergic transmission for the Mesobuthus tamulus venom-induced depression of spinal reflexes in neonatal rat in vitro.

Mesobuthus tamulus (MBT) venom is shown to depress the spinal reflexes through a mechanism unrelated to the NMDA receptors. 5-Hydroxytryptamine (5-HT) is another excitatory transmitter in the spinal cord therefore, the present study was undertaken to examine the involvement of 5-HT in the venom-induced depression of reflexes. The experiments were performed on isolated hemisected spinal cords from 4 to 6-day-old rats. Stimulation of a dorsal root with supramaximal strength evoked monosynaptic reflex (MSR) and polysynaptic reflex (PSR) potentials in the corresponding segmental ventral root. MBT venom (0.3 microg/ml) depressed the spinal reflexes in a time-dependent manner and the maximal depression was seen at 10 min. The time to produce 50% depression (T-50) of MSR and PSR was 8.1+/-1.41 and 6.8+/-0.5 min, respectively. Pretreatment with pindolol (1 microM; 5-HT(1A/1B) receptor antagonist) blocked the reflexes up to 15 min. On the other hand, ketanserin (10 microM; 5-HT(2A/2C) receptor antagonist) or ondansetron (0.1 microM; 5-HT(3) receptor antagonist) blocked the venom-induced depression of MSR and PSR during entire exposure time (30 min). The 5-HT concentration of the cords exposed to venom (1.6+/-0.04 microg/g tissue) was significantly greater than the control group (0.98+/-0.08 microg/g tissue). The results indicate that venom-induced depression of spinal reflexes is mediated via 5-HTergic transmission involving 5-HT(1A/1B), 5-HT(2A/2C) and 5-HT(3) receptors.

Pre- and postsynaptic modulation of monosynaptic reflex by GABAA receptors on turtle spinal cord.

There is growing evidence that activation of high affinity extrasynaptic GABA(A) receptors in the brain, cerebellum and spinal cord substantia gelatinosa results in a tonic inhibition controlling postsynaptic excitability. The aim of the present study was to determine if GABA(A) receptors mediating tonic inhibition participate in the modulation of monosynaptic reflex (MSR) in the vertebrate spinal cord. Using an in vitro turtle lumbar spinal cord preparation, we show that conditioning stimulation of a dorsal root depressed the test monosynaptic reflex (MSR) at long condition-test intervals. This long duration inhibition is similar to the one seen in mammalian spinal cord and it is dependent on GABA(A) as it was completely blocked by 20 microm picrotoxin (PTX) or bicuculline (BIC) or 1 microm gabazine, simultaneously depressing the dorsal root potential (DRP) without MSR facilitation. Interestingly 100 microm picrotoxin or BIC potentiated the MSR, depressed the DRP, and produced a long lasting motoneurone after-discharge. Furosemide, a selective antagonist of extrasynaptic GABA(A) receptors, affects receptor subtypes with alpha(4/6) subunits, and in a similar way to higher concentrations of PTX or BIC, also potentiated the MSR but did not affect the DRP, suggesting the presence of alpha(4/6) GABA(A) receptors at motoneurones. Our results suggest that (1) the turtle spinal cord has a GABA(A) mediated long duration inhibition similar to presynaptic inhibition observed in mammals, (2) GABA(A) receptors located at the motoneurones and primary afferents might produce tonic inhibition of monosynaptic reflex, and (3) GABA(A) receptors modulate motoneurone excitability reducing the probability of spurious and inappropriate activation.

Involvement of nitric oxide in 3-nitropropionic acid-induced depression of spinal reflexes in neonatal rat spinal cord in vitro.

The objective of the present investigation is to study the involvement of nitric oxide (NO) in 3-nitropropionic acid (3-NPA)-induced depression of spinal reflexes. Experiments were conducted on preparations of hemisected spinal cord isolated from 4 to 8 day old rats. Stimulation of a dorsal root evoked reflex potentials (monosynaptic, MSR; polysynaptic, PSR) in the corresponding segmental ventral root. Superfusion of 3-NPA (3.4 mM) depressed the spinal reflexes in a time-dependent manner and the reflexes were abolished after 35 min. The time required to produce 50% depression of the reflexes (T-50) was 17.8+/-5.3 min for MSR and 17.5+/-2.1 min for PSR. L-NAME (Nomega-nitro-L-arginine methyl ester; 100 microM), a nitric oxide synthase inhibitor, antagonized the 3-NPA (3.4 mM)-induced depression of reflexes and increased the T-50 values (34 and 30 min for MSR and PSR, respectively) significantly (P<0.05). In addition, hemoglobin (Hb, 100 microM), a NO scavenger, blocked the 3-NPA-induced depression of reflexes significantly (P<0.05). T-50 values in Hb pretreated cords were 57 and 45 min for MSR and PSR, respectively which were greater than the cords pretreated with L-NAME. The nitrite (NO(2)(-)) content of the 3-NPA exposed cords was 84 microM/g of tissue which was significantly greater than the control (13 microM/g; P<0.05). Pretreatment of cords with L-NAME or Hb antagonized the 3-NPA-induced increase in NO(2)(-). The results indicate that NO produced by 3-NPA is involved in the 3-NPA-induced depression of spinal reflexes.

5-HT-induced depression of the spinal monosynaptic reflex potential utilizes different types of 5-HT receptors depending on Mg2+ availability.

Receptor subtypes involved in the 5-hydroxytryptamine (5-HT)-induced depression of synaptic transmission in neonatal rat spinal cords in vitro were evaluated in the absence or presence of Mg(2+) in the medium. Stimulation of a dorsal root evoked monosynaptic reflex potential (MSP) and polysynaptic reflex potential (PSP) in the segmental ventral root in Mg(2+)-free medium where the voltage-dependent blockade of NMDA receptors is absent. The 5-HT (0.3-50 microM) in the Mg(2+)-free medium depressed the MSP and PSP in a concentration-dependent manner. At 30 microM of 5-HT, the depression was 57% and 95% for MSP and PSP, respectively, and no further depression was seen at 50 microM. The 5-HT-induced depression of the reflexes in the Mg(2+)-free medium was blocked by ondansetron (5-HT(3) receptor antagonist), but not by spiperone (5-HT(2A/2C) antagonist). In the Mg(2+)-free medium, phenylbiguanide (5-HT(3) agonist) also depressed the MSP and PSP in a concentration-dependent manner and was blocked by ondansetron. Addition of Mg(2+) (1.3 mM) to the medium abolished the PSP and decreased the MSP by 30%. In the presence of Mg(2+), 5-HT (1-50 microM) also depressed the MSP in a concentration-dependent manner. At 10 microM of 5-HT, there was approximately 20% depression and at 50 microM the depression was 100%. The 5-HT-induced depression of MSP in the Mg(2+)-containing medium was antagonized by spiperone (p < 0.05, two-way ANOVA), but not by ondansetron. The results indicate that the 5-HT-induced depression of MSP involves 5-HT(3) receptors in the Mg(2+)-free medium and 5-HT(2A/2C) in the presence of Mg(2+) when NMDA receptors are in the closed state.

Neuropeptide modulation of a lumbar spinal reflex: potential implications for female sexual function.

INTRODUCTION: Neuropeptides are known to modulate female receptivity. However, even though receptivity is a spinal reflex, the role of neuropeptides within the spinal cord remains to be elucidated. AIM: The aims were to (i) investigate neuropeptides in the lumbosacral region; and (ii) determine how neuropeptides modulate glutamate release from stretch Ia fibers, touch sensation Abeta fibers and Adelta/C pain fibers. MAIN OUTCOME MEASURES: Neuropeptide modulation of the lumbosacral dorsal-root ventral-root reflex in vitro. METHODS: Spinal cords were removed from Sprague-Dawley rats in compliance with UK Home Office guidelines. Hemisected cords were superfused with aCSF and the dorsal root (L4-S1) was stimulated to evoke glutamate release. A biphasic reflex response was evoked from the opposite ventral root consisting of a monosynaptic (Ia fibers) and polysynaptic (Abeta, Adelta/C fibers) component. RESULTS: The micro opioid receptor (MOR) agonist DAMGO inhibited the monosynaptic (EC(50) 0.02 +/- 0.02 nM) and polysynaptic area (EC(50) 125 +/- 167 nM) but not polysynaptic amplitude. Oxytocin and corticotrophin releasing factor (CRF) inhibited the monosynaptic amplitude (EC(50), 1.4 +/- 1.0 nM and EC(50) 4.3 +/- 3.5 nM, respectively), polysynaptic amplitude (EC(50) 18.2 +/- 28.0 nM and EC(50), 9.5 +/- 13.3 nM, respectively), and area (EC(50) 11.6 +/- 13.0 nM and EC(50), 2.8 +/- 3.3 nM, respectively); effects that were abolished by oxytocin and CRF(1) antagonists, L-368899 and 8w. Melanocortin agonists solely inhibited the monosynaptic component, which were blocked by the MC(3/4) receptor antagonist SHU9119. CONCLUSION: These data suggest endogenous neuropeptides are released within the lumbosacral spinal cord. Melanocortin agonists, oxytocin, CRF, and DAMGO via MC(4), oxytocin, CRF(1), and MOR inhibit glutamate release but with differing effects on afferent fiber subtypes. Melanocortins, oxytocin, CRF, and DAMGO have the ability to modulate orgasm whereas oxytocin, CRF and DAMGO can increase pain threshold. Oxytocin and CRF may dampen touch sensation.

3-Nitropropionic acid-induced depression of spinal reflexes involves mechanisms different from ischemia-induced depression.

Effect of 3-nitropropionic acid (3-NPA) and ischemia (glucose- and O(2)-free solution) on synaptic transmission in hemisected spinal cord from 4 to 8 day old rats was examined in vitro. Stimulation of a dorsal root (L3-5 segments) evoked monosynaptic (MSR) and polysynaptic reflex (PSR) potentials in the segmental ventral root. Superfusion of 3-NPA (0.17-3.4 mM) depressed the reflexes in a concentration- and time-dependent manner. At 3.4 mM of 3-NPA, the reflexes were abolished by 35 min. Time required to produce 50% depression (T-50) was around 170, 80, 40 and 17 min for MSR and 110, 70, 25 and 16 min for PSR at 0.17, 0.51, 1.7 and 3.4mM of 3-NPA, respectively. Ischemia also produced a time-dependent depression of reflexes and abolished them by 35 min and the T-50 values were around 18 min. Presence of creatine phosphate (10mM) in the superfusing medium did not alter the time course of 3-NPA-induced depression of reflexes but prolonged the ischemia-induced depression. dl-2-amino-5-phosphonovaleric acid (NMDA receptor antagonist; 10 microM) failed to block the 3-NPA (3.4 mM)-induced depression of reflexes, but blocked the ischemia-induced depression. The results indicate that 3-NPA-induced depression of spinal reflexes does not involve NMDA receptors and is different from ischemia-induced depression.

In vivo evidence for serotonin 5-HT2C receptor-mediated long-lasting excitability of lumbar spinal reflex and its functional interaction with 5-HT1A receptor in the mammalian spinal cord.

The purpose of the present study was to investigate the 5-HT(2C) receptor-mediated effects on the spinal monosynaptic mass reflex activities and also its functional interactions with 5-HT(1A) receptors in anesthetized, acutely spinalized mammalian adult spinal cord in vivo. Intravenous administration of (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) (0.1 mg/kg), an agonist of 5-HT(2A/2C) receptors, significantly increased the excitability of spinal motoneurons as reflected by an increase in the spinal monosynaptic mass reflex amplitude to 150-200% of the control. 5-HT(2A/2C) receptor-induced motoneuron excitability was slow, persistent and long-lasting for more than 2h that was significantly inhibited by 5-HT(2C) receptor specific antagonist SB 242084 administered 10 min prior to DOI. Simultaneous administration of DOI (0.1 mg/kg, i.v.) along with (+/-)-8-hydroxy dipropylaminotetraline hydrobromide (8-OH-DPAT) (0.1 mg/kg, i.v.) completely inhibited DOI-induced spinal monosynaptic mass reflex facilitation. In another separate study, administration of 8-OH-DPAT (0.1 mg/kg, i.v.) at the maximum response of DOI also inhibited the motoneuron's excitability; however, the inhibition lasted only for a period of 40-60 min after administration of 8-OH-DPAT, after which the spinal monosynaptic mass reflex amplitude reached its maximum level. These findings suggest that the 5-HT(2C) receptor is primarily involved in the mediation of the long-lasting excitability of spinal motoneurons and possibly interacts with its functional counterpart, 5-HT(1A) receptors in the mammalian adult spinal cord.

Differential inhibitory control of semicircular canal nerve afferent-evoked inputs in second-order vestibular neurons by glycinergic and GABAergic circuits.

Labyrinthine nerve-evoked monosynaptic excitatory postsynaptic potentials (EPSPs) in second-order vestibular neurons (2 degrees VN) sum with disynaptic inhibitory postsynaptic potentials (IPSPs) that originate from the thickest afferent fibers of the same nerve branch and are mediated by neurons in the ipsilateral vestibular nucleus. Pharmacological properties of the inhibition and the interaction with the afferent excitation were studied by recording monosynaptic responses of phasic and tonic 2 degrees VN in an isolated frog brain after electrical stimulation of individual semicircular canal nerves. Specific transmitter antagonists revealed glycine and GABA(A) receptor-mediated IPSPs with a disynaptic onset only in phasic but not in tonic 2 degrees VN. Compared with GABAergic IPSPs, glycinergic responses in phasic 2 degrees VN have larger amplitudes and a longer duration and reduce early and late components of the afferent nerve-evoked subthreshold activation and spike discharge. The difference in profile of the disynaptic glycinergic and GABAergic inhibition is compatible with the larger number of glycinergic as opposed to GABAergic terminal-like structures on 2 degrees VN. The increase in monosynaptic excitation after a block of the disynaptic inhibition in phasic 2 degrees VN is in part mediated by a N-methyl-d-aspartate receptor-activated component. Although inhibitory inputs were superimposed on monosynaptic EPSPs in tonic 2 degrees VN as well, the much longer latency of these IPSPs excludes a control by short-latency inhibitory feed-forward side-loops as observed in phasic 2 degrees VN. The differential synaptic organization of the inhibitory control of labyrinthine afferent signals in phasic and tonic 2 degrees VN is consistent with the different intrinsic signal processing modes of the two neuronal types and suggests a co-adaptation of intrinsic membrane properties and emerging network properties.

Ethanol reduces motoneuronal excitability and increases presynaptic inhibition of Ia afferents in the human spinal cord.

INTRODUCTION: Already low blood concentrations of ethanol acutely impair motor control and coordination. In vitro experiments have given evidence that spinal effects of ethanol contribute to this by reducing spinal excitability and enhancing presynaptic inhibition of Ia fibers. In this study, we investigated the influence of 0.7 g per kilogram of bodyweight ethanol on motoneuronal excitability and presynaptic inhibition in humans. METHODS: The study was performed in 10 volunteers. Spinal excitability was measured by the maximal H-reflex of the soleus muscle normalized to the maximal muscular response (Hmax/Mmax). Presynaptic inhibition was measured by changes in heteronymous Ia-facilitation of the soleus H-reflex, which is achieved by stimulation of the femoral nerve. A decrease in facilitation can be ascribed to an increase in presynaptic inhibition. Changes of these parameters under the influence of 0.7 g per kilogram of bodyweight ethanol were assessed in comparison to control measurements before ethanol application. RESULTS: Both parameters, Hmax/Mmax and Heteronymous facilitation, were significantly reduced under the influence of ethanol (Wilcoxon signed-rank test with Bonferroni correction for each, p<0.01). DISCUSSION: The increase in presynaptic inhibition by ethanol is probably caused by an increase in GABAA receptor-mediated Cl-conductance, which has been shown in spinal cord cultures. The role of presynaptic inhibition in movement is assumed to be there to control the afferent input of muscle spindles and tendon organs as a mechanism of specific input-selection. This study demonstrated that ethanol reduces spinal excitability and increases GABAergic presynaptic inhibition on Ia afferent fibers in humans.

Differential effects of spinal 5-HT1A receptor activation and 5-HT2A/2C receptor desensitization by chronic haloperidol.

The effects of 7- and 21-day haloperidol treatment on the spinal serotonergic system were examined in vivo in acutely spinalized adult rats. Intravenous administration of a selective 5-HT(2A/2C) receptor agonist, (+/-)-2,5-Dimethoxy-4-iodoamphetamine hydrochloride (0.1 mg/kg) significantly increased the excitability of spinal motoneurones as reflected by increased monosynaptic mass reflex amplitude. This was significantly reduced in rats treated with haloperidol (1 mg/kg/day, i.p.) for 7 and 21 days. Administration of a 5-HT(1A/7) receptor agonist, (+/-)-8-Hydroxy dipropylaminotetraline hydrobromide (0.1 mg/kg, i.v.) significantly inhibited the monosynaptic mass reflex. This inhibition was greatly prolonged in haloperidol treated animals. These results demonstrate that the effects of haloperidol on the activation and desensitization of 5-HT(1A) and 5-HT(2A/2C) receptors respectively, may be mediated via intracellular mechanisms shared by these receptors with dopamine D(2) receptors in the mammalian spinal cord. The above serotonergic mechanisms may be partly responsible for haloperidol-induced extrapyramidal motor dysfunction.