mise à jour du
29 août 2010
Pharmacol Biochem Behav

Effects of Copulation on Apomorphine-Induced
Erection in Rats
Benjamin D. Sachs, Keiko Akasofu, Sara S. McEldowney
Department of Psychology, University of Connecticut


By testing the effects of antecedent copulation on subsequent apomorphine-induced penile erection we sought to test an implicit assumption in the research on drug-induced "spontaneous" erection-namely, that this research provides information relevant to the regulation of erection in copula. In experiment 1, male rats were observed after being injected SC with 0, 15, 30, 60, or 120 tg/kg apomorphine (APO); 60 ag/kg yielded the maximum probability of erection and yawning.
In experiment 2, males were injected with 60 &g/kg APO after no exposure to females, after three intromissions, or after copulation to sexual satiety. There was no significant effect of three intromissions, but sexually sated males displayed no erections, the first evidence that copulation affects drug-induced erections. In experiment 3, males had one ejaculation, three intromissions, or no exposure to females immediately before injection with APO (60 sg/kg, SC) or ascorbic acid vehicle. APO induced both erection and yawning, but neither behavior was reliably affected by copulation in APO-treated males. Among vehicle-treated males, those having three intromissions or one ejaculation before the test had shorter erection latencies and more erections than males not exposed to females.
Thus, a relatively small amount of copulation resulted in a level of erectile response similar to that of APO-treated males. Optimal doses of APO may be no more effective in promoting erection in male rats than are the natural neurochemical sequelae to copulation.

FOR over 20 years, a major tool in the study of the neurochemical basis of erection has been the use of pharmacological agents that promote "spontaneous" penile erection in isolated male rats. In much of the research on drug-induced erection there appears to be an implicit assumption that this approach provides information relevant to the regulation of erection in copula. Apomorphine (APO), a D1/D2 dopamine agonist, is among the most frequently used of these drugs because of its efficacy with systemic injection, and its use has been proposed as a reference model for the study of the physiological regulation of erection.
However, the relation of drug-induced erections to erections occurring in other contexts remains untested, and therefore uncertain, because the physiological regulation of erection is known to vary with the context. For example, hypogonadal men have an expected decrement in spontaneous erections, including nocturnal penile tumescence, and in encounters with sexual partners, but these men have normal erections while viewing erotic videos. Castrated male rats treated with estrogen have erections adequate for copulation but fail to display reflexive erections, whereas dIhydrotestosterone maintains reflexive erections but not copulation.
Evidence that antecedent copulation affected subsequent reflexive erection helped to demonstrate the sexual relevance of reflexive erection tests. In contrast, the failure of ejaculation induced by brain stimulation to influence subsequent copulation was taken to mean that this mode of ejaculation involved mechanisms that were not relevant to copulatory behavior. 1f drug-induced erections have relevance to sexual activity, then antecedent copulation should affect them. Therefore, we inquired whether copulation would influence subsequent APO-induced erections.
With regard to yawning, injection of APO increased its incidence, as expected from previous studies, but neither APO- nor-vehicle-injected males showed an effect of copulation on yawning. We infer that yawning and erection are readily dissociable and that any changes in brain dopamine resulting from copulating to ejaculation are not sufficient to promote reliable changes in yawning latency or frequency.
Experiment 2 had demonstrated no influence of three intromissions on APO-induced erection, but there was a strong inhibitory effect of sexual satiety. In experiment 3, three intromissions were again without reliable effect on APO-induced erection, as was one ejaculation. Since a few intromissions or one ejaculation have repeatedly been shown to enhance reflexive erections, it appears that APO-induced erections are less susceptible than reflexive erections to the influence of antecedent copulation. Nonetheless, in experiment 3 there was a nonsignificant progressive reduction in the erection latencies of APO-injected males after three intromissions or one ejaculation. This reduction may hint at the possibility of an effect that might be revealed under other experimental conditions (e.g., in males given a suboptimal dose of APO). Pending such evidence, however, an interaction between subsatiety amounts of copulation and APO-induced erections remains to be demonstrated.
Unexpectedly, three intromissions or one ejaculation reliably increased the probability of spontaneous erections in vehicle-injected rats in the absence of a female. In fact, these treatments were no less effective than 60 ig APO in promoting erection, and the erectile response pattern appeared to be identical to APO-induced erection. Brain levels of dopamine and other neurotransmitters change dramatically during and after copulation or even with noncontact exposure to estrous females. Such changes may have contributed to the display of postcopulatory erections. In any case, it appears that optimal doses of APO may be no more effective than the natural neurochemical sequelae to copulation in promoting erection in male rats.
Psychogenic erection is commonly regarded as erection that results from fantasy, spontaneous brain activity, or non-contact exposure to sexual stimuli, and reflexive erections are those resulting from somesthetic stimulation, especially of the perigenital area. Erections in copula probably include elements of both sources of stimulation. The postcopulatory occurrence of erection in male rats for several minutes after removal of the female has elements of reflexive erection, in that genital stimulation precedes the erections, and of psychogenic stimulation, in that erections appear to occur spontaneously for several minutes after genital stimulation. Appropriate categorization of the response is less important thanand is likely to follow from-analysis of the bases for the response. Research in progress addresses the stimulus basis and physiological mediation of psychogenic erection in rats.