Abstract
In rats anaesthetised with alphaxalone/alphadolone (Saffan), bilateral microinjections of the GABA antagonist, bicuculline, into a restricted region of nucleus paragigantocellularis lateralis (PGL), ventromedial to the caudal pole of the facial nucleus, produced an increase in the latency of the tail flick response to noxious heat. The analgesia was always accompanied by a rise in mean arterial blood pressure but the time course of the cardiovascular and antinociceptive changes was different. Guanethidine (7 mg/kg i.v.) blocked the presser response but had no effect on the magnitude or time course of the analgesia. In contrast, microinjection of physostigmine into PGL produced a presser response but no change in the latency of the tail flick response. It is concluded that there are functionally distinct pools of neurones within PGL which respectively produce antinociception and changes in vasomotor activity. Ongoing activity in both types of neurone is regulated by a tonic inhibitory GABAergic influence. In addition, the cardiovascular neurones receive a tonic excitatory cholinergic input.
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