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.
DISCUSSION
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.
