Erla R. Smith

The relationship between circulating testosterone levels and male sexual behavior in rats

Abstract The relationships between plasma testosterone (T) and various parameters of male sexual behavior were examined in intact and castrated T-treated male rats. Repeated blood sampling and behavioral testing revealed no correlation between any measure of sexual behavior and plasma T in normal untreated sexually active males. T-Filled Silastic capsules, implanted subcutaneously at the time of castration, were found to produce plasma T levels proportional to capsule size. Plasma T titers less than 10% of normal (0.2 ng/ml) maintained ejaculatory behavior near normal levels for the 58 days of the experiment. Measures of sexual behavior which showed androgen dependence were intromission latency, postejaculatory interval, and intromission frequency. The plasma T concentration required to maintain these parameters within the intact range was 0.7 ng/ml, which is less than one-third of the mean intact level (2.6 ng/ml). No significant improvement in the sex behavior measures was seen with plasma T levels between 0.7 and 3.1 ng/ml. It was concluded that the absence of relationships between circulating T and sexual behavior in the normal rat population is due to the androgen requirement for this behavior being less than the amount normally present. Findings on T levels and T treatment in noncopulator males are also presented.

Evidence for the modulation of sexual behavior by α-adrenoceptors in male rats

Clonidine, a commonly used antihypertensive agent believed to act by stimulation of central α- adrenoceptors, produced a dose-related suppression of ejaculatory behavior in sexually vigorous male rats

The effects of estrogen and progesterone on female rat proceptive behavior

Abstract The relative importance of estrogen (EB) and progesterone (P) in stimulating proceptivity in ovariectomized female rats was studied. Proceptive behavior was measured quantitatively, providing a clear measure of response to experimental manipulation. When rats were tested biweekly after daily treatment with 0.4 μg/100 g body wt EB for 4 days, they showed maximal lordosis but low levels of proceptive behavior by the second test. Additional EB (3.0 μg/100 g body wt daily) failed to stimulate additional proceptivity. A graded increase in proceptive behavior resulted from administration of increasing doses of P (50, 100, 500 μg and 1.0 mg) to animals receiving EB priming as described above. The level of “soliciting” was significantly higher than EB-only-treated rats when 500 μg or 1.0 mg P was given. Ovariectomized, adrenalectomized rats, primed with 2.5 μg/100 g body wt EB daily for 7 days and tested on Day 8, were significantly less proceptive than ovariectomized, sham-adrenalectomized rats with the same hormone treatment. Four hours after injection of 1.0 mg P, there was no difference in proceptive or receptive behavior between sham- and adrenalectomized rats. It was concluded that if an EB dose is sufficient to induce maximal receptivity, additional estrogen does not stimulate proceptivity; unlike previous studies, the present data are not consistent with a global effect of ovarian steroids on both components of female behavior. Progesterone is more effective than estrogen in stimulating proceptive behavior, although proceptivity is not absolutely dependent on progesterone for expression. Proceptivity in EB-only-treated rats appears to be facilitated by adrenal P.

Relative thresholds of behavioral and somatic responses to estrogen

Abstract The effects of 17 β-estradiol benzoate on sexual receptivity, vaginal and pituitary cytology, and uterine growth were studied in spayed rats in order to compare the relative sensitivities of the behavioral and somatic responses to this steroid. The most sensitive measure was the vaginal smear; full cornification followed administration of 0.2 μg of the estrogen per day. The threshold for attainment of normal levels of sexual receptivity was between 0.8 and 1.6 μg/day. Similar dosage was required to reverse completely the changes in adenohypophyses and uteri resulting from ovariectomy. These results demonstrate that apparently normal sexual behavior may be restored in spayed rats by daily administration of “physiological” doses of estrogen in the absence of progesterone treatment.

Hormonal responses accompanying fear and agitation in the squirrel monkey

The adrenocortical and gonadal responses of 14 male monkeys were evaluated during four experimental conditions in order to evaluate the influence of social interactions on endocrine responsiveness. Plasma hormone levels were determined during the establishment of social relations, after 60-min exposures to a novel environment, after 60-min exposures to a snake, and 60 min after ACTH administration. Both adrenal and gonadal secretion changed significantly during the first day after social relations were established, although only dominant males showed increases in testosterone, whereas cortisol levels rose in all subjects. Increases in cortisol, but not testosterone, were also observed following exposure to novelty or a snake. The presence of a social partner reduced signs of behavioral disturbance during these test conditions, although the adrenal responses were equivalent or greater than when tested alone. This finding qualifies earlier research which indicated that social support was beneficial for reducing stress when squirrel monkeys were tested in larger groups in their home environment.

Role of androgen in sexual reflexes of the male rat.

Abstract Studies were conducted on genital reflexes (erections, cups and flips), which occur spontaneously in supine unanesthetized male rats. The androgen dependence of this behavior was first demonstrated by a significant decline in all reflexes after castration. Second, the establishment of stable blood androgen levels by testosterone administration via Silastic capsules resulted in partial or complete maintenance of the reflexes after castration, depending on the dose. The circulating titer of androgen required for reflex performance which resembled that of intact rats was considerably less than the normal mean blood level, as previously found for copulatory behavior. Apart from very low androgen situations, there seems to be no necessary relationship of copulatory behavior performance to reflex responses and the variance in the latter is apparently unrelated to plasma testosterone level in normal intact adults. Nevertheless, the androgen dependence of genital reflexes is an essential component of the normal androgenic regulation of copulatory behavior.

Neuroendocrine Mechanisms Mediating the Suppression of Circulating Testosterone Levels Associated with Chronic Stress in Male Rats

Neuroendocrine mechanisms involved in the reduction of testosterone (T) levels induced by the chronic surgical stress of s.c. gauze implantation were investigated in male rats. The reduction resulted from a decline in T secretion and not an increase in clearance of the hormone from the circulation. In most cases, the decrement was accompanied by a reduction in LH levels, indicating that decreased LH stimulation is the primary factor mediating the drop in T levels. The suppressive effect of surgical stress on LH and T levels was apparently not mediated by the pituitary-adrenal system.

Sexual behavior in developing male rats.

Abstract During normal development, the onset of reproductive behavior in male rats was not preceded by any change in plasma testosterone (T) levels. Implantation of Silastic capsules containing T in 14-day-old male rats advanced the onset of all parameters of sexual behavior by 20 days. Implantation of Silastic capsules containing estradiol in 14-day-old male rats stimulated precocious mounting and intromitting, but not ejaculation. Implantation of dihydrotestosterone-filled Silastic capsules in 14-day-old male rats completely inhibited the development of sexual behavior. All hormones suppressed plasma LH levels. These findings in immature male rats are similar to previous findings in adult males. Immature male rats were behaviorally less responsive to T than adult males, and it was suggested that, during development, male rats become progressively more sensitive to the behavior-stimulating effects of circulating T. No effects of copulatory experience on plasma concentration of T or on the weights of testes, penes, or accessory sexual glands were detected.

Hormonal regulation of penile erection in castrated male rats

Abstract The effects of various androgen agonists and antagonists on the restoration of penile erection in long-term castrated male rats were investigated. Erections were evaluated in spinally intact animals held in a supine position with the penile sheath retracted. Mating behavior was measured by placing the male in an observation arena with a receptive female. Both testosterone and dihydrotestosterone (DHT) were very effective in restoring penile erections. Testosterone was also effective in restoring mating behavior, but DHT was totally ineffective. Estradiol was incapable of restoring erections but was effective in stimulating mounting behavior. The non-steroidal antiandrogen flutamide inhibited the testosterone-induced restoration of penile erections, when administered alone, it had a slight excitatory effect. The latency for restoration of erections by testosterone was quite rapid, with a substantial increase within 24 hr. It is suggested that: (1) the androgen-induced restoration of penile erection derives more from neural changes than growth of somatic tissues; and (2) the neural elements regulating erection demonstrate different response characteristics to various hormones than those regulating mounting.

Hormonal Response to Stress in the Squirrel Monkey (Saimiri sciureus)

The pituitary-adrenal and gonadal responses following stress were evaluated in the squirrel monkey. Plasma levels of cortisol (CS), ACTH and testosterone (T) were determined during a 4-h period following the combined stress of capture and ether anesthesia. The results indicated that the squirrel monkey manifests higher basal levels of steroids than typically found in other mammals. The endocrine response following stress was biphasic, involving an initial elevation and subsequent decline in hormone levels. Males manifested significantly higher plasma levels of CS and T and lower plasma levels of ACTH than did females.