To determine the relationship between potency and lipophilicity for intrathecal opioids, morphine, normorphine, pethidine and methadone were studied in an electrophysiological model in the rat. Dose-response curves were constructed for the opioid effects on C fibre evoked activity of dorsal horn nociceptive neurones following intrathecal application of each opioid, and the ED50 values were correlated with lipid solubility. A significant inverse correlation was found (P = 0.002; r = 0.998) so that the most lipid soluble drugs were the least potent. The possible mechanism of this relationship, the implications for spinal opioid use and the effect of different administration routes on the relationship between lipid solubility and potency are considered.
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.
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