The pontine parabrachial region mediates some of the descending inhibitory effects of stimulating the anterior pretectal nucleus

Abstract

Electrical stimulation of the anterior pretectal nucleus (APtN) elicits antinociception by inhibiting the responses of spinal multireceptive neurones to noxious stimuli. This descending inhibition is mediated, in part, by activating cells in the ventrolateral medulla. Neuronal tract tracing has previously shown that the APtN also projects directly to the pontine parabrachial region (PPR). The PPR, investigated by Katayama et al. ( Brain Res., 296 (1984) 263–283), corresponds to the cholinergic cell group Ch5 of Mesulam et al. ( Neuroscience, 10 (1983) 1185–1201). In this study, the pathway from APtN to PPR was investigated using urethane anaesthetised rats. Electrical stimulation (single square wave 0.2 ms pulses, 1–10 V, 5 Hz) of the APtN potently excites 40% of the cells recorded in the PPR. In the reverse experiment, stimulation of the PPR at the same parameters excited 36% of the cells recorded in the APtN. The contribution of this pathway to the spinal inhibitory effects of APtN stimulation was then examined. Unanaesthetised animals received electrical stimulation to the APtN (35 μ A r.m.s., 15 s) and the increase in tail-flick latencies was measured. Bilateral electrolytic lesions of the PPR caused a 67% reduction of the antinociceptive effect of APtN stimulation. In urethane anaesthetised rats, microinjection of tetracaine into the PPR blocked the inhibition of multireceptive dorsal horn neurones caused by APtN stimulation (20 s train of 50 μA square wave 0.1 ms pulses, 100 Hz). In conclusion, these experiments strongly sugget that the PPR may be an important part of a descending antinociceptive pathway originating in the APtN.