Diazepam attenuates hyperexcitability and mechanical hypersensitivity of dorsal horn convergent neurones during reperfusion of the rat's tail following ischaemia

Abstract

We have investigated the involvement of the GABA A-benzodiazepine receptor complex in nociceptive activity of convergent neurones in the spinal cord during ischaemia and reperfusion of their receptive fields on the rat's tail. In enflurane anaesthetized rats, extracellular recordings were made from convergent neurones located throughout the dorsal horn before, during and after 30 min of ischaemia. Following intrathecal saline pretreatment, there was a significant increase in spontaneous firing rate during ischaemia (219 ± 21%, P < 0.02, n = 10) which persisted durig reperfusion. After 10 min of reperfusion, the neurones exhibited a greater response than before ischaemia to both innocuous brush (54 ± 11%, P < 0.05, n= 10) and noxious pinch (72 ± 14%, P < 0.02, n = 10) and the enhanced sensitivity persisted over the 60-minreperfusion period. During reperfusion, receptive field size increased in most neurones tested. Intrathecal diazepam (100 and 500 μg) abolished the hyperexcitability and the hypersensitivity to both innocuous and noxious mechanical stimulation during reperfusion. The highest dose of diazepam (500 μg) also attenuated the increase in spontaneous firing rate during ischaemia. Diazepam, at the doses tested, had no effect on receptive field enlargements during reperfusion. The effect of 100 μg of diazepam was partially reversed by flumazenil (1 mg/kg i.p.) but not by naloxone (1 mg/kg i.p.). In the absence of ischaemia, diazepam had no effect on spontaneous firing rate nor on the responses to innocuous or noxious mechanical stimulation. Our results support an antinociceptive role for benzodiazepines in the dorsal horn elements responsible for reperfusion hyperalgesia. On the other hand, we found no evidence that intrathecal diazepam affected spinal nociceptive processes in the absence of the conditioning stimulus of transient ischaemia.