Enhancement by morphine of the central descending inhibitory influence on spinal sensory transmission

Abstract

The effect of morphine on central inhibitory influenced acting upon the spinal sensory transmission was studied in cats. Splanchnic afferent impulses recorded from the ventrolateral funiculus were inhibited by small doses (2–4 mg/kg) of morphine in intact cats, with or without pentobarbital anesthesia, but were only slightly depressed in spinal cats unless the dose was increased to 10 mg/kg. This inhibitory effect of 4 mg/kg of morphine in intact cats was reversed by an acute transection of the spinal cord at the level of C-1. Furthermore, similar experiments showed that the above-mentioned depressive effect of morphine was not diminished by transections of the brain stem at levels higher than posterior 11 mm (stereotaxic coordinate) but was markedly reduced by transection at levels posterior 14–16 mm. These results suggest that the suppressive effect of small doses of morphine is mainly mediated through its facilitatory effect on the lower brain stem which has descending inhibitory influences on the spinal sensory transmission.