Acute ethanol exposure decreases the analgesic potency of morphine in mice.

Chronic (7 days), forced ethanol drinking can decrease the analgesic potency of opioid agonists in mice. In the present study, the effect of short-term ethanol treatment was examined using forced ethanol access and ethanol injection protocols. Mice were given forced access to 1, 3 or 7% (v/v) ethanol for 24 hr and then tested for s.c. morphine analgesia using the tailflick assay. Controls had access to water. Another group of mice was injected i.p. with 2.5 g/kg ethanol or water 4 times over a 21 hr period and tested 3 hr after the final injection for morphine analgesia. Other mice were injected once i.p. with 1, 2 or 3 g/kg ethanol or water and tested 24 hr later using the tailflick. In the forced access study, ethanol dose-dependently decreased morphine's analgesic potency with the highest dose (7%) producing a 1.6-fold shift in the ED50. This decrease in morphine potency was similar to that found in a related study using 7% ethanol for 7 days (1.8-fold shift). Repeated ethanol injections significantly reduced the analgesic potency of morphine (1.9-fold shift), whereas, a single injection of 1, 2 or 3 g/kg ethanol did not alter the potency of morphine. Control studies indicated that neither 24 hr water nor food deprivation affected morphine potency. Overall, these data show that sustained exposure to ethanol over a 24 hr period will dose-dependently decrease morphine's analgesic potency.

The effect of cumulative dosing on the analgesic potency of morphine in mice.

Opioid analgesic potency can be evaluated using cumulative dosing, in which subjects are repeatedly administered a drug and tested after each dose until a criterion effect is reached. Although many laboratories use cumulative dosing, the effects of varying the starting dose and the magnitude of the increment dose on morphine analgesia (tail flick) in mice have not been evaluated. In experiment 1. mice were injected with the same starting dose [0.5 mg/kg subcutaneously (SC)] and 30 min later were tested for analgesia. Mice that were not analgesic were administered an increment dose (0.5, 1.0, 2.0, 2.5, or 3.0 mg/kg) and retested. The process was continued until all mice were analgesic. There was a significant effect of increment dose on morphine potency, with the relative potency increasing as the increment dose was increased. In experiment 2, different starting doses (0.5, 1.0, 2.0, or 3.0 mg/kg) were used with a constant increment dose of 1.0 mg/kg. There was a significant effect of starting dose on the potency of morphine, with the relative potency increasing as the starting dose increased. To determine if increment and starting dose affect tolerance estimates, mice were implanted SC with a 25- or 75-mg morphine or placebo pellet for 7 days and then tested using cumulative dose-response. Changes in the increment dose significantly affected the degree of tolerance for mice implanted with a 25-mg morphine pellet but not for mice implanted with a 75-mg morphine pellet. Changes in the starting dose did not significantly alter estimates of tolerance. Overall, these data indicate that the starting dose and increment dose can impact on morphine's potency determined by cumulative dosing protocols. Furthermore, estimates of tolerance can be affected by dosing parameters in the cumulative dosing protocol. These results suggest that cumulative dosing procedures should be standardized across experiments.

The effect of ethanol drinking on opioid analgesia and receptors in mice.

Recent studies suggest substantial interactions between opioids and ethanol (EtOH). Both in vivo and in vitro experiments indicate that EtOH can regulate opioid systems and that opioids can modify EtOH consumption. In the present studies, we examined if EtOH consumption altered opioid receptors and the potency of opioid analgesics. Mice were given unlimited access to 6-7% EtOH alone for 7 days or were allowed to drink increasing concentrations (3-6%) of EtOH over 13-14 days. Controls had access to water. The EtOH groups drank significantly less volume than controls, although there were no significant differences in body weight or baseline nociception. The analgesic (tail flick) potency of SC morphine was decreased by approximately 1.6-2.0-fold in EtOH-treated mice. A single acute dose of EtOH (1 g/kg) that produced blood alcohol levels in excess of that for 7 day exposure to EtOH, did not change morphine's analgesic ED50, suggesting that chronic exposure to EtOH was necessary for the reduction in potency. The change in morphine potency was not due to pharmacokinetic differences because EtOH consumption did not modify the concentration of morphine in brain and spinal cord. The analgesic potency of a delta-opioid receptor agonist (ICV DSLET) was also decreased by approximately 2-fold. Saturation binding studies indicated no changes in the density or affinity of brain and spinal cord delta-opioid ([3H]DPDPE, [3H]DSLET, [3H]DeltorphinII) and mu-opioid ([3H]DAMGO) receptors. Similarly, there was no significant effect of EtOH on delta-opioid receptor mRNA in either brain or spinal cord preparations. Taken together, these data suggest that EtOH consumption decreases the analgesic potency of opioids in mice through a mechanism that is unrelated to pharmacokinetics or opioid receptor changes in brain and cord.