The effect of centrally acting myorelaxants on NMDA receptor‐mediated synaptic transmission in the immature rat spinal cord in vitro

Abstract

1 The effect of the myorelaxant drugs baclofen, diazepam and tizanidine have been compared on in vitro preparations of baby rat spinal cord and adult rat superior cervical ganglion. 2 Dorsal root-elicited long duration (time to half decay 9.71 ± 0.29 s.e.mean, n = 31) ipsilateral ventral root reflexes (DR-VRP), measured as integrated area, of immature rat spinal cord preparations were abolished by RS-2-amino-5-phosphonopentanoate (AP5) (EC50 8.13 ± 0.92 ILM, n = 3). The initial short latency component of DR-VRP was resistant to AP5. 3 Baclofen abolished both components of the DR-VRP. Respective EC50 values for the AP5-insensitive and AP5-sensitive components were 237 ± 68 nM (n ± 7) and 57 ± 10 nM (n = 7). These effects of baclofen were reversed by the GABAB antagonist, CGP35348. The apparent Kd values (16.7 ± 6.4 pLM, n = 3 and 14.3 ± 3.9 JtM, n = 6 respectively) for this reversal were not significantly different. 4 Tizanidine, clonidine and diazepam had no effect on the AP5-insensitive component of the DR-VRP. 5 The AP5-sensitive long duration component of the DR-VRP was depressed to respective maximal levels of 23.2 ± 1.4% (n = 7), 18.8 ± 3.8% (n = 4) and 47.6 ± 1.6% (n = 5) of control (100%) levels by tizanidine (EC50 135 ± 33 nM), clonidine (EC50 26.0 ± 2.2 nM) and diazepam (EC25 114 ± 12 nM, n = 4).The depressant effects of tizanidine and clonidine were reversed by idazoxan (1 1AM). Flumazenil (I tiM) failed to reverse the depressant effect of tizanidine. The depressant effect of diazepam was reversed by flumazenil (1 jLM) but not by idazoxan (1 JAM). Naloxone 1 jAM did not reverse the effects of either tizanidine or diazepam. 6 In the presence of tetrodotoxin (0.1 SAM) which abolished synaptic activity, clonidine, tizanidine or diazepam (10, 100 and 101JM respectively) produced no significant antagonism of NMDA-induced depolarizations recorded from ventral roots. 7 Control (100%) synaptic responses of rat superior cervical ganglion preparations were depressed respectively to near maximal levels of 60.0 ± 5.2% (n = 4) and 60.7 ± 5.6% (n = 5) by clonidine (0.5 JAM, EC25 15.3 ± 3.0 nM) and tizanidine (1 JAM, EC25 227 ± 83 nM). These depressant effects were reversed by idazoxan (1 AM). Baclofen (EC25 28.7 ± 10.0, n = 3) depressed the postganglionic response to a maximum level of 71.8 + 2.4% (n = 4) control at a concentration of 100 JAM. The latter depressant action was reversed by the GABAB receptor antagonist, CGP35348 (1 mM). Diazepam (1 JAM) had no significant effect on ganglionic transmission. 8 It is concluded that the activation of benzodiazepine or M2-noradrenaline receptors can modulate NMDA receptor-mediated excitatory synaptic pathways whereas synaptic excitation from primary afferent terminals, mediated by non-NMDA receptors, appears to lack the propensity for this type of modulation. The results show also that the isolated spinal preparation can be used to (Davies & Quinlan, 1985). Thus in the present investigation the action of the myorelaxant, tizanidine, has been compared with the prototypic...

The effects of central myorelaxants on synaptically‐evoked primary afferent depolarization in the immature rat spinal cord in vitro

“…The results suggest that the previously reported depression of output from motoneurones by tizanidine or diazepam (Siarey et al, 1992b) is independent of their actions on the DR-DRP.…”

“…The NMDA receptor antagonist AP5 produced a preferential depression of the long duration com- (Siarey et al, 1992b). In the present study these two drugs were tested on the DR-DRP at concentrations higher than were required for the above actions.…”

“…In a previous study (Siarey et al, 1992b) it was found that the myorelaxant drugs diazepam and tizanidine produced similar levels of depression of N-methyl-D-aspartate (NMDA) receptor-mediated synaptic excitation of motoneurones, as reflected in recordings from ventral roots of in vitro spinal cord preparations. These depressant actions were mediated respectively via flumazenil-and idazoxan-sensitive receptors.…”

Tolperisone-Type Drugs Inhibit Spinal Reflexes via Blockade of Voltage-Gated Sodium and Calcium Channels

“…1A) consisted of a biphasic population spike (monosynaptic reflex, MSR; time to peak, 7.0 Ϯ 0.4 ms) superimposed on the early phase of a tonic, long-lasting potential shift, reaching a maximum at 10 to 15 ms following the stimulation of the dorsal root. This latter potential basically represents population excitatory postsynaptic potential (EPSP) of motoneurons (Siarey et al, 1992). Nevertheless, small waves attributable to asynchronous firing of motoneurons were often discernibly superimposed on this tonic potential.…”

Comparison of the Novel Subtype-Selective GABAA Receptor-Positive Allosteric Modulator NS11394 [3′-[5-(1-Hydroxy-1-methyl-ethyl)-benzoimidazol-1-yl]-biphenyl-2-carbonitrile] with Diazepam, Zolpidem, Bretazenil, and Gaboxadol in Rat Models of Inflammatory and Neuropathic Pain

“…In the light of a spinal action of diazepam (Siarey et al, 1992(Siarey et al, , 1994, we performed electrophysiological experiments on the isolated spinal cord to learn about possible mechanisms of action of NS11934 in the spinal circuits that mediate withdrawal reflexes. To this end, we first checked the effects of NS11394 on native spinal cord GABA A receptors localized to the central terminals of primary afferents.…”