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

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Abstract
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. 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 μm, n = 3). The initial short latency component of DR‐VRP was resistant to AP5. 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 μm, n = 3 and 14.3 ± 3.9 μm, n = 6 respectively) for this reversal were not significantly different. Tizanidine, clonidine and diazepam had no effect on the AP5‐insensitive component of the DR‐VRP. 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 μm). Flumazenil (1 μm) failed to reverse the depressant effect of tizanidine. The depressant effect of diazepam was reversed by flumazenil (1 μm) but not by idazoxan (1 μm). Naloxone 1 μm did not reverse the effects of either tizanidine or diazepam.6 In the presence of tetrodotoxin (0.1 μm) which abolished synaptic activity, clonidine, tizanidine or diazepam (10, 100 and 10 μm 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 μm, EC25 15.3 ± 3.0 nm) and tizanidine (1 μm, EC25 227 ± 83 nm). These depressant effects were reversed by idazoxan (1 μm). 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 μm. The latter depressant action was reversed by the GABAB receptor antagonist, CGP35348 (1 mm). Diazepam (1 μm) had no significant effect on ganglionic transmission.8 It is concluded that the activation of benzodiazepine or α2‐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 identify central myorelaxant actions that are mediated through functional benzodiazepine or α2‐noradrenaline receptors.