κ-Opioid receptor-mediated analgesia:: hotplate temperature and sex differences

The present experiment was designed to investigate the dose-effect and time-effect functions of the kappa - opioid receptor agonist [5 alpha ,7 Phi ,8 beta)-(-)-N-Methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro(4,5)dec-8-yl]benzeneacetamide] (U69,593) in intact and gonadectomized male and female rats as a function of hotplate temperature (45 degreesC or 51 degreesC). At 45 degreesC baseline Lick latencies were longer in female rats than in male rats. Lick latencies increased dose-dependently in all subjects except intact female rats. U69,593 increased lick latencies as a function of time since its administration in all subjects. At 51 degreesC baseline lick latencies did not differ between groups and they increased dose-dependently in all subjects. The effectiveness of U69,593 decreased as a function of time since its administration, but not in castrated male rats. These observations suggest that gonadal hormones could play a role in modulating the behavioral effects of U69,593 when subjects are tested at different hotplate temperatures. (C) 2000 Elsevier Science B.V. All rights reserved.