Cholinergic control of male mating behavior in hamsters: effects of systemic agonist or antagonist treatment

Sexual behavior in male rats is thought to depend in part on central cholinergic activity. In particular, previous studies of responses to systemically administered cholinergic drugs suggest that male rat behavior can be facilitated by the muscarinic agonist oxotremorine but is disrupted by the muscarinic antagonist scopolamine. However, it is not clear how broadly these effects generalize across species. To address this issue, we observed the impact on sexual behavior in male hamsters of systemic treatment with oxotremorine or scopolamine. In each case, the peripheral muscarinic antagonist methylscopolamine was used as an auxiliary or control treatment to better isolate central cholinergic effects. Both oxotremorine and scopolamine disrupted male behavior in hamsters. For example, both increased the likelihood of failure to achieve intromission or ejaculation. Further, even on completed tests oxotremorine treatment led to changes including increases in mount latency and postejaculatory interval while scopolamine treatment caused changes including increases in ejaculation latency and intromission frequency. The many changes caused by these treatments suggest that acetylcholine helps to control many elements of male behavior, probably by acting at multiple brain sites. The generally similar responses to a cholinergic agonist and antagonist suggest the dependence of efficient mating behavior on optimal levels of central cholinergic activity.     

Sexual experience in female rodents: cellular mechanisms and functional consequences

The neurobiology of female sexual behavior has largely focused on mechanisms of hormone action on nerve cells and how these effects translate into the display of copulatory motor patterns. Of equal importance, though less studied, are some of the consequences of engaging in sexual behavior, including the rewarding properties of sexual interactions and how sexual experience alters copulatory efficiency. This review summarizes the effects of sexual experience on reward processes and copulation in female Syrian hamsters. Neural correlates of these sexual interactions include long-term cellular changes in dopamine transmission and postsynaptic signaling pathways related to neuronal plasticity (e.g., dendritic spine formation). Taken together, these studies suggest that sexual experience enhances the reinforcing properties of sexual behavior, which has the coincident outcome of increasing copulatory efficiency in a way that can increase reproductive success.     

Reflexive testosterone release: a model system for studying the nongenomic effects of testosterone upon male behavior

Male mammals of many species exhibit reflexive testosterone release in mating situations. In house mice (Mus musculus), the dramatic robustness of such release, occurring primarily in response to a novel female, suggests some function. The resulting testosterone elevations typically peak during copulatory behavior and may serve to activate transitory motivational and physiological responses that facilitate reproduction. However, such a function requires that testosterone be working through either nongenomic, or very quick genomic, mechanisms. The first part of the review describes reflexive sex hormone release in house mice. The second part summarizes research implicating testosterone’s fast actions in affecting anxiety, reward, learning, analgesia, and penile reflexes in rodents, all of which could optimize male mating success. The review concludes with a speculative model of how spontaneous and reflexive hormone release might interact to regulate reproductive behavior and why mice appear to be an ideal species for examining testosterone’s quick effects.     

Effects of anti-estrogens on testosterone stimulated male sexual behavior and peripheral target tissues in the castrate male rat.

In the first experiment castrated male rats were injected daily with either vehicle or 800 mug testosterone together with either 6 hr pretreatment or concurrent treatment with the anti-estrogens CI-628 (4 mg) or MER-25 (20 mg). The only treatment found to significantly reduce male copulatory behavior was concurrent treatment with CI-628. Anti-estrogen treatment was also found to slightly reduce body weights, increase seminal vesicle weights in response to testosterone and to have no significant effects on androgen stimulated increases in penis weight and length. In the second experiment castrated male rats were injected daily with either vehicle or 500 mug testosterone together with 2.5 mg injections of CI-628 given 6 hr before and concurrent with the androgen injections. The anti-estrogen treatment was found to markedly inhibit the desplay of male sexual behavior, to reduce body weights, and to stimulate seminal vesicle weights. Penile weights and lengths were again not effected by the anti-estrogen therapy. These results were interpreted as supporting the theory that testosterone stimulated male sexual behavior in the rat following its aromatization to estradiol in the brain.     

Relation of Intromissions to the Female''s Postejaculatory Refractory Period in Rats

The objectives of this study were to investigate the temporal aspects of female sexual behavior during single and multiple ejaculatory tests. Females were tested in a two-compartment chamber where they could escape from the male following sexual contacts. In Experiment 1, correlation analysis showed that the number of intromissions received by the female over 3 ejaculatory series was positively correlated with the female's postejaculatory refractory period (PER). In Experiment 2, females receiving 2–4 intromissions before ejaculation had a PER that did not differ from those receiving 5–15 (average 10) intromissions preceding ejaculation. However, if the male ejaculated on the first or second intromission, the female's PER was significantly shorter than the other groups and did not differ from her return latency after an intromission without ejaculation. Females receiving 24–31 intromissions preceding ejaculation exhibited the longest PER. Analysis revealed that the number of intromissions received by females before ejaculation was positively correlated with the female's PER. We concluded that the male's ejaculatory reflex, seminal emission, and postejaculatory behavior alone without at least 2 preceding intromissions were not sufficient to induce a female's PER comparable to that seen after an ejaculation during normal copulation. In addition, the number of intromissions received by the female preceding ejaculation was positively correlated with the female's PER if the range of intromission frequency was large enough.     

Coition-induced inhibition of sexual receptivity in female lizards (Anolis carolinensis)

Sexually receptive female lizards (Anolis carolinensis) were tested for sexual receptivity at varying intervals after exposure to selected behavioral components of normal copulation. These components were: (1) complete copulation with a male; (2) exposure to an intact male that was allowed to court and take a neck grip, but not mount or intromit or; (3) exposure to a hemipenectomized male who could court, take a neck grip, and assume the copulation posture, but not intromit or ejaculate. Results indicated that normal mating inhibits subsequent sexual receptivity within 1 min but does not affect receptivity during subsequent follicular cylces. Intromission and possibly ejaculation by the male are critical for this inhibition of female sexual receptivity. In addition, intromission and ejaculation are important in the male's termination of copulation.     

Behavioral and EEG patterns of the female toad Bufo arenarum during artificial clasping

A study was made of the possible existence of a receptive area in the female toad, involved in the sexual synchronization during copulation. Motor behavior depends on whether axillary or flank pressure is applied. Motor passivity and periodic large amplitude chest movements were observed immediately after clasping was directed at the axillary region. Also enhancement of spindle alpha-like activity in the EEG was noted four to six hours afterwards. Conversely, when clasped by its flank regions the female increased its thoraco-abdominal cavity and forward-kicking with its hindlimbs tended to release the clasp. Hence, a different functional significance is suggested between the two regions in the female toad and for the same area in the two sexes. The possible relationships between the observed motor and EEG changes and hormonal release during the sexual embrace deserve further studies. Based on the above mentioned results, a sexually receptive area is proposed for the female toad, complementary of that classically described for the male.     

Effects of Δ1(2)-tetrahydrocannabinol on copulation in the male rat

The mating behavior of 15 male rats was measured after treatment with ¹⁽²⁾-tetrahydrocannabinol (THC) and propylene glycol (vehicle). Administration of either 2 mg per kg or 3 mg per kg body weight of THC was followed by significant increases in latency to the first mount, latency to ejaculation, and latency to the first mount following ejaculation. No significant changes were found in the number of intromissions or mounts. The deterioration in sexual performance is interpreted as reflecting a decreased motivation to copulate under the influence of the drug.     

Insensible weight loss as an indicator of metabolic rate

Insensible weight loss has been used in large animals as an index of metabolic rate. By using the microprocessor controlled electronic balance to minimize the effects of movement, the feasibility of utilizing insensible weight loss as an estimate of metabolic rate in the rat is considered in terms of convenience and accuracy. Using various metabolic stimulants, a high correlation between insensible weight loss and metabolic rate was obtained. Moreover, a calculation is described that corrects for respiratory water loss and results in even higher correlations. These findings support the use of insensible weight loss as an indicator of metabolic rate in controlled situations for small animals.     

Olfactory and vomeronasal system participation in male hamsters' attraction to female vaginal secretions.

The effects of olfactory (OLF) vomeronasal (VN), or combined deafferentations of male hamsters on their attraction to female hamster vaginal secretions (FHVS) were determined using 2 different attraction tests. In the first, FHVS was placed on one wall of a plastic test chamber, while in the second test, FHVS was rubbed onto 1 of 2 anesthetized castrate hamsters. OLF deafferentation abolished the males' attraction to FHVS in the first test but had no effect in the second. The persistence of FHVS attraction in the anesthetized castrate test depended on the VN system in that its subsequent deafferentation greatly attenuated the attraction. When the VN system was deafferented alone, FHVS attraction was significantly reduced only in males exhibiting severe mating behavior deficits. These results are interpreted to support the hypothesis that the OLF and VN systems may be preferentially responsive to volatile and non-volatile odorants, respectively. Our findings emphasize that both the OLF and VN systems participate in subserving males' attraction to FHVS and their mating with receptive females. Thus each chemosensory system may influence both sexual arousal and copulatory mechanisms.