Electrophysiological studies on the effects of opiates on dorsal horn nociceptive neurones at 51 atmospheres absolute

Abstract

The purpose of this study was to investigate the effects, if any, of high pressure on opiate analgesia. The interaction of high pressure with intrathecal morphine (2.5-15 micrograms) and pethidine (10-145 micrograms) was studied on the responses of 51 dorsal horn neurones in the intact rat under urethane anaesthesia to electrical stimulation applied to their receptive fields. Two types of response to the addition of the opiates were found. Cells were either rapidly and maximally inhibited by the lowest dose of morphine (2.5 micrograms) or pethidine (10 micrograms) or slowly inhibited in a dose-dependent manner with an ED50 of 13.6 nmol for morphine and 401 nmol for pethidine. Pressure did not significantly affect the time-response curves for these two types of response but did change the relative numbers of each type recorded. The number of cells totally inhibited by the lowest of drug concentrations was increased for morphine at pressure but decreased for pethidine.

Electrophysiological studies on the postnatal development of the spinal antinociceptive effects of the delta opioid receptor agonist DPDPE in the rat

“…Injection (1.75 g kg −1 ). Halothane and urethane anaesthesia have been compared previously in the adult rat and no differences in neuronal activity or neuronal responses to opioids were observed ( Smallman et al ., 1989 ). Complete areflexia and surgical anaesthesia was sufficiently maintained for the duration of the experiment without any obvious depression of respiratory or other physiological functions.…”

A method for recording single unit activity in lumbar spinal cord in rats anesthetized with nitrous oxide in a hyperbaric chamber

“…A motor-controlled device could be constructed that would permit the investigator to indirectly touch the hindpaw and thereby assess the receptive field and neuronal responses to non-noxious and noxious mechanical stimulation. For reasons of simplicity, we decided to use an electrical search stimulus, as has been done previously in hyperbaric chambers (Smallman, Dickenson et al, 1989). Thus, we maximized the chance of finding a suitable neuron by placing electrodes spanning the entire paw of each hindlimb.…”

“…Performing simple studies (such as determination of anesthetic requirements, i.e., minimum alveolar concentration, MAC) in a hyperbaric chamber is relatively straightforward (Gonsowski and Eger, 1994). But more complex neurophysiological experiments in whole animals (such as single unit recording in spinal cord) are technically demanding even at normobaric conditions, and have rarely been attempted under hyperbaric conditions (Smallman, Dickenson et al, 1989).…”