Differential inhibitory mechanisms in VPL versus intralaminar nociceptive neurons of the cat: II. Effects of systemic morphine and CCK.

In urethane-chloralose anesthetized cats, the effects of intravenous morphine on responses of thalamic nociceptive units were studied. In both nociceptive specific (NS) and wide dynamic range (WDR) units recorded from the nucleus ventralis posterolateralis (VPL), intravenous morphine suppressed unit responses to the greater splanchnic nerve (SPL) stimulation, but had little effect on responses to stimulation of spinothalamic tract fibers in the ventrolateral funiculus. In nociceptive units recorded from nuclei centralis lateralis (CL) and parafascicularis (Pf) of the intralaminar nuclei, intravenous morphine suppressed responses to stimulation of the mesencephalic reticular formation as well as to SPL stimulation. Intravenous cholecystokinin (CCK) antagonized the suppressive action of morphine on responses of VPL units, but did not antagonize the suppressive action of morphine on responses of intralaminar units. The results suggest that intravenous morphine inhibits synaptic transmission of nociceptive impulses in the intralaminar nuclei as well as in the spinal cord, but not in the VPL, and that CCK antagonizes the antinociceptive action of morphine in the spinal cord, but not in the intralaminar nuclei.