Neuronal activation at cardioreactive sites in the anterior hypothalamus is often accompanied by profound inhibition of spinal nociception (Lumb & Lovick, 1993). Nociceptive information from the periphery is conveyed to the spinal cord by C- and Aδ-nociceptors and these fibres mediate different qualities of the nociceptive message. The present study was designed to test for any selective effects of hypothalamic control on responses evoked by these different classes of nociceptor.

Experiments were carried out on nine male Wistar rats (270-325 g) anaesthetised with alphaxalone/alphadolone (Saffan, 14-24 mg kg^-1 h^-1 I.V.) and instrumented to record arterial blood pressure and rectal temperature. Micropipettes were placed stereotaxically at sites in the hypothalamus at which pressure-injection of DL-homocysteic acid (DLH; 0.05 M, 50 nl) evoked pressor or depressor responses. Slow (2.5 °C s^-1) or fast (7.5 °C s^-1) ramps of contact heat (30-55 °C) were applied to the dorsal surface of a hindpaw to preferentially activate C-or Aδ-nociceptors, respectively (Yeomans & Proudfit, 1996) and withdrawal EMG thresholds were recorded from the biceps femoris before and after injection of DLH. At the end of the experiment animals were killed humanely with an overdose of pentobarbitone. Twelve hypothalamic sites were tested for their effects on responses to slow (n = 11) and fast (n = 10) heating ramps. Following DLH, thresholds to withdrawal were increased in response to eight of the slow ramps: from 50.7 ± 0.4 to 53.0 ± 0.7 °C (mean ± S.E.M., n = 4) or to above cut-off (n = 4). In the other three, thresholds remained within 1 °C of control values. In contrast, mean thresholds for withdrawal to fast ramps decreased following DLH injection to 49.8 ± 1.1 °C compared with control values (53.6 ± 0.6 °C, n = 5). At the other five sites where fast ramps were tested, thresholds were unchanged (n = 3) or increased (n = 2) post-DLH.

These data provide evidence that the hypothalamus has differential effects on the responses to nociceptive input arising from activity in C-fibres compared with input arising from Aδ-fibres. This may have important functional implications as it suggests that descending control can suppress the slowly conducted, poorly localised aspects of the pain signal, and simultaneously enhance the rapidly conducted, well localised signal which, from a behavioural point of view, conveys the protective aspects of the nociceptive input.This work was supported by the Wellcome Trust. D.A.A.S. is a BBSRC scholar.

Lumb, B.M. & Lovick, T.A. (1993). J. Neurophysiol. 70, 1570-1577.

Yeomans, D.C. & Proudfit, H.K. (1996). Pain 68, 141-150.