Acetazolamide and amiloride inhibit pentobarbital-induced facilitation of nocifensive reflexes

Neuronal excitation may result from stimulation of gamma-aminobutyric acid A (GABA(A)) receptors that prolong the channel opening, depolarizing the postsynaptic membrane. Drugs such as acetazolamide or amiloride can block GABA depolarization. Barbiturates facilitate nociceptive reflexes and also prolong the GABA(A) channel open-time. To evaluate the possible mechanism, the authors studied the impact of acetazolamide and amiloride on pentobarbital-induced nocifensive reflex facilitation. Because nitric oxide (NO) is a mediator of reflex facilitation, the authors evaluated the effects of NO synthase inhibition.

Nocifensive reflex thresholds were quantified with the hind paw withdrawal latency from radiant heat (HPW latency) in the rat. Nocifensive reflexes were facilitated with intraperitoneal injection of pentobarbital (30 mg/kg). The authors tested the roles of GABA-mediated depolarization and NO in reflex facilitation by pretreatment with acetazolamide and amiloride and inhibition of NO synthase with L-NAME and 7-NI, respectively. Sedative effects of pentobarbital were evaluated with the righting reflex, the response to vibrissal stimulation, and plasma drug concentrations.

Pentobarbital decreased the hind paw withdrawal latency from 11.2+/-1 to 8.3+/-1 s (P < 0.001). Pretreatment with each of the four test drugs limited the reduction in reflex facilitation after pentobarbital to 1.3 s or less, similar to the reduction seen after saline injection, without altering sedation. L-NAME increased plasma pentobarbital concentrations by 10% without changing the concentration associated with return of responsiveness.

Pentobarbital-induced nocifensive reflex facilitation was inhibited by all four tested drugs without evidence of increased sedation. The results are consistent with a role for GABA(A) receptor-mediated depolarization in barbiturate-induced hyper-reflexia.