Failure of naloxone to block the reduction in burying behaviour after ejaculation in male rats

Opioid antagonists and the sexual satiation phenomenon

This study evaluates the effects of the IP injection of naloxone (0.3, 3 and 30 mg/kg) and naltrexone (0.2, 2 and 20 mg/kg) on the sexual satiation phenomenon. It was found that both antagonists exert a dose-based biphasic effect on the proportion of sexually exhausted rats displaying copulation. The intermediate doses of both opioid antagonists were more effective than the low and high doses in increasing the percentage of animals engaged in copulation. The analysis of the specific sexual behaviour parameters revealed that naloxone produces a slight inhibitory effect at the lowest dose, evidenced as an increase in the intromission number. The higher doses of this compound facilitated copulation reflected as a shortening of the ejaculation latency and the interintromission interval (III) and an increase in the copulatory rate. Naltrexone treatment had only facilitatory effects at the lower doses by reducing the III. The higher doses of naloxone (3 and 30 mg/kg) and the intermediate dose of naltrexone (2 mg/kg) decreased the spontaneous ambulatory behaviour of sexually satiated rats without impairing sexual behaviour execution. Data suggest a participation of the endogenous opioid systems in the sexual inhibition resulting from sexual exhaustion.

Ejaculation induced changes in escape latency in the hot plate test: pharmacological analysis of anxiolytic versus analgesic effect

The possible changes in nociception at various stages of male sexual behaviour were explored in the hot plate test. Although other authors have reported an antinociceptive effect of mating, we failed to find this effect on the hot plate test after several sexual behaviour events. To further explore the possible antinociceptive action of copulation we administered a suboptimal analgesic dose of morphine (0.3 mg/kg i.p.). No change in nociception were observed in animals treated with a subthreshold dose of morphine and tested in the nociception test after ejaculation were observed. Since previous reports have refered that ejaculation produces hypoalgesia when measured in the hot plate test, we attempted to replicate these findings. We found that one ejaculation produces an increase in the latency to escape in male rats previously habituated to the hot plate test. These results suggest a hypoalgesic effect. However, such changes could also be interpreted as alterations in the animals' emotionality. Thus, a group of habituated animals was tested on the switched off plate. An important increase in the escape latency behaviour after ejaculation was observed, while no differences between control animals, without sexual behaviour display, tested on switched on and switched off plate were observed. The participation of the benzodiazepine and opiod systems in the ejaculation effect on the switched off plate was explored. A similar increase in escape latency to that induced by ejaculation was caused by diazepam (1.0 and 2.0 mg/kg). The increase in escape latency induced by ejaculation was prevented by the benzodiazepine antagonist, flumazenil (20.0 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.