Differential effects of beta-endorphin infused into the hypothalamic preoptic area at various phases of the male rat's sexual behaviour.
Beta-Endorphin, infused into the pre-optic/anterior hypothalamus (40 pmoles bilaterally) of the male rat before he was placed in an arena containing an oestrous female, inhibited mounting, intromitting and ejaculation, but investigative behaviour continued at control levels. If the infusion was delayed until the male had made an intromission, then beta-endorphin no longer had any effect on sexual interaction, the male mounting and ejaculating as if he had received a control infusion of artificial cerebrospinal fluid. However, if the male was returned to a different female after the infusion had been completed, then the suppressive effects of beta-endorphin returned. Males infused during the refractory period following an ejaculation (and returned to the same female) showed unimpaired return of sexual activity. Imposing a delay of up to 2 h after an intromission and an infusion showed that the effect of beta-endorphin was still antagonized when the male was again paired with the same female; however, by 6 h, its inhibitory effects were beginning to return. Allowing the male to mount (but not intromit) a female whose vagina had been taped partially counteracted the behavioural effect of beta-endorphin. If the female was separated from the male by a small wire cage which allowed limited interaction with her, subsequently infusing the males with beta-endorphin suppressed their mounting behaviour. These results show that both investigative and mounting behaviour can occur after infusions of beta-endorphin into the pre-optic/anterior hypothalamus, but that the transition between them is prevented if infusions are made before a critical point in the behavioural sequence. This is the onset of following the female and mounting her. Analysis of the behavioural sequence after either artificial cerebrospinal fluid or beta-endorphin infusions confirmed this; beta-endorphin interrupted the sequence at the first transition between investigative and mounting behaviour. These results suggest that beta-endorphin acts on a neural mechanism in the medial preoptic area/anterior hypothalamus which allows matching of incentive stimulus to specific behavioural response, and this may be a general property of this part of the brain.
Stavy
M
M
Department of Anatomy, University of Cambridge, U.K.
Herbert
J
J
D000818:Animals; D004723:Endorphins / Q000494:pharmacology*; D008297:Male; D011301:Preoptic Area / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D011930:Reaction Time / Q000502:physiology; D012726:Sexual Behavior, Animal / Q000187:drug effects*
D004723:Endorphins