Ontogeny of the sexually dimorphic male nucleus in the preoptic/anterior hypothalamus of ferrets and its manipulation by gonadal steroids.

A sexually dimorphic nucleus exists in the dorsal region of the ferret preoptic/anterior hypothalamic area (POA/AH), and is called the male nucleus of the POA/AH (MN-POA/AH) because it is found only in males. Development of the MN-POA/AH was studied in male ferrets, and for comparison a sexually nondimorphic ventral POA/AH nucleus was studied in both sexes. The MN-POA/AH was conspicuous in males as early as embryonic day 37 (E37) of a 41-day gestation, and its volume increased until postnatal day 56 (P56). No nucleus was present in the dorsal POA/AH of females at any age. The densities and average somal areas of cells in the dorsal POA/AH were similar in males and females at E33, before the MN-POA/AH could be visualized. However, at E37 and E41 dorsal cells were greater in density and/or somal area in males than in females, accounting for the appearance of a nucleus in males at these ages. To insure that the dorsal POA/AH nucleus seen in males at E37 and E41 was the presumptive MN-POA/AH present in adult males, pregnant ferrets were given progesterone and either implanted subcutaneously (s.c.) with testosterone (T) or ovariectomized and implanted s.c. with the aromatase inhibitor, 1,4,6-androstatriene-3,17-dione (ATD), on day 30 of gestation. As predicted from previous studies in which subjects were sacrificed in adulthood, formation of a dorsal POA/AH nucleus was promoted in female ferrets by T, and blocked in males by maternal ovariectomy and ATD treatment for animals sacrificed at E41. Much evidence suggests that behavioral sexual differentiation is accomplished in the male ferret between age E28 and P20. The MN-POA/AH is present and potentially functional in males during a considerable portion of this perinatal period.

Role of androgens in the regulation of sexual behavior in the female musk shrew.

These experiments were conducted to determine whether androgens are involved in the regulation of female sexual behavior in the musk shrew (Suncus murinus). In the musk shrew, sexual behavior can be reinstated after ovariectomy by the administration of supraphysiological doses of either estradiol (E2) or testosterone. However, physiological doses of E2 are not effective in this regard. To examine the role of androgens, ovariectomized (OVX) females received testosterone-filled hormone implants. These implants reinstated sexual behavior in a dose-dependent manner. To determine whether the aromatization of androgen is essential for restoration of sexual behavior, the nonaromatizable androgen, dihydrotestosterone (DHT), was administered to OVX females either alone or with an implant containing E2; sexual behavior was not restored. In the third experiment, gonadally intact females, treated with the aromatization inhibitor 1,4,6-androstatriene-3,17-dione (ATD) exhibited no sexual behavior and had significantly lower levels of aromatase activity in the medial basal hypothalamus/preoptic area than control females. In the last experiment, OVX females implanted with testosterone and given concurrent ATD treatment demonstrated significantly less sexual behavior as compared with controls. These results suggest that either estrogens produced via androgen aromatization in the brain and/or an intermediate product in the aromatization process are involved in the regulation of sexual behavior in the female musk shrew.

Effect of sex, intrauterine position and androgen manipulation on the development of brain aromatase activity in fetal ferrets.

Abstract Experiments were conducted to explore the possible relationship between testicular androgen secretion and the development of brain aromatase activity in fetal ferrets. Aromatase activity in the preoptic+mediobasal hypothalamus and temporal lobe was similar in fetuses of both sexes between embryonic Days 26 and 36 even though whole body androgen content was invariably higher in males than females. Whole body androgen content was significantly higher in females located caudally (downstream) from two or more as opposed to zero or one males in the same uterine horn; nevertheless their brain aromatase activity was similar. Finally, maternal treatment with either the androgen receptor antagonist Flutamide or 5alpha-dihydrotestosterone propionate beginning on gestational Day 24 did not affect brain aromatase activity in fetal offspring of either sex, delivered on embryonic Day 34. Previous studies suggest that the biosynthesis of estrogen in the fetal ferret brain is normally greater in males than females. The present results suggest that this sex difference results primarily from increased androgenic substrate being available to non-saturated aromatizing enzymes and not from an androgen-dependent activation of aromatase.

Annual reproductive cycle in the male red-sided garter snake, Thamnophis sirtalis parietalis: field and laboratory studies.

The reproductive cycle of male red-sided garter snakes (Thamnophis sirtalis parietalis) was studied in a laboratory-maintained population and in nature. Both populations exhibited a dissociated reproductive pattern in which spermatogenesis was initiated after the breeding season and sperm were stored in the vas deferens over winter. Although animals in both the laboratory and the field exhibited a dissociated reproductive pattern, temporal differences were evident in the two populations. Animals maintained in the laboratory initiated and completed spermiation much sooner than animals collected in the field. In the captive population, the renal sexual segment (RSS) could be identified but did not exhibit a seasonal fluctuation, indicating perhaps that androgen production was continuous, but at a reduced level. In nature, the RSS was hypertrophied at emergence, regressed following the breeding season, and again hypertrophied as circulating androgens increased in the fall, prior to hibernation. In both laboratory-maintained and field animals the circulating concentrations of testosterone and dihydrotestosterone at emergence were initially similar to the levels observed prior to hibernation but by 2 weeks postemergence had decreased to a relatively low level. Both the high androgen levels prior to hibernation and the rapid decrease during the spring courtship season may result from temperature influences on clearance rates. Plasma levels of corticosterone varied during the spring, first increasing upon emergence and then declining approximately 2 weeks following emergence.

Temperature activation of courtship behavior in the male red-sided garter snake (Thamnophis sirtalis parietalis): role of the anterior hypothalamus-preoptic area.

Adult male red-sided garter snakes (Thamnophis sirtalis parietalis) received radio-frequency lesions prior to 17 weeks of low-temperature hibernation. Animals found to have bilateral lesions of the anterior hypothalamus-preoptic area (AH-POA) failed to exhibit courtship behavior on emergence from hibernation. Those individuals in which the major portion of the destruction was centered in the anterior POA also exhibited deficits in thermoregulatory behavior. Animals that received unilateral lesions of the AH-POA initiated courtship behavior after controls and had an abbreviated period of courtship; these animals exhibited normal responses to thermal stimuli. Male snakes with lesions outside the AH-POA courted normally and demonstrated no differences in thermoregulatory behavior compared with the surgical controls. These results indicate that in male red-sided garter snakes, an intact AH-POA is critical for the integration of thermal stimuli that activate seasonal courtship behavior.

Pinealectomy blocks vernal courtship behavior in red-sided garter snakes.

Male red-sided garter snakes (Thamnophis sirtalis parietalis) court only on emergence from winter dormancy. The salient proximate factor responsible for the initiation of courtship behavior appears to be warm ambient temperatures subsequent to sustained exposure to cold temperatures. In the present study, autumn-captured garter snakes were pinealectomized or sham-pinealectomized 1-2 weeks prior to hibernation. After 17 weeks of maintenance in several ambient lighting and temperature conditions, all males were tested for the presence of courtship behavior. No pinealectomized animals exhibited any indications of reproductive behavior, whereas 53% of sham-pinealectomized males actively courted females. Photoperiod did not influence the prevalence of mating behavior. These results suggest that the pineal gland mediates non-photoperiodic seasonal information in the garter snake. This is the first demonstration, in any vertebrate species, that the pineal gland can directly influence reproductive behavior.

Facilitation of courtship behavior in the male red-sided garter snake (Thamnophis sirtalis parietalis) following lesions of the septum or nucleus sphericus.

Courtship behavior in male red-sided garter snakes is limited to the period immediately following spring emergence. Lesions of the septum or nucleus sphericus prior to hibernation facilitate courtship behavior upon emergence the following spring. These lesions also had a significant effect on the reproductive physiology of the male snake. Snakes with lesions in the nucleus sphericus had a significantly higher concentration of circulating androgens compared to the control group. While animals with lesions in the septum exhibited elevated levels of circulating androgens, they were not statistically elevated when compared to the control group. The quantity of sexual granules in the epithelium of the renal sexual segment, a secondary sexual characteristic of male squamate reptiles, was significantly greater in lesioned animals. These results are the first to demonstrate a central inhibitory control of sexual behavior in a snake.

Effects of mammalian gonadotropins (oFSH and oLH) on testicular development in the immature water snake, Nerodia sipedon.

Ovine FSH (oFSH) or ovine LH (oLH) was injected into immature male Nerodia sipedon and was assessed as to its effects on testicular development and steroid synthesis as indicated by the subsequent development of the renal sexual segment, a known secondary sexual character. FSH-injected animals had significantly larger testis mass, seminiferous tubule diameter, and epithelial height when compared to LH-injected and control groups. While FSH induced testicular development, LH induced lipid accumulation and hypertrophy of the mitochondria in the Leydig cell, indicative of androgen production. While both FSH and LH induced development of the renal sexual segment, LH stimulated the production of sexual granules prior to FSH, implying that androgens enter the circulation faster under LH treatment.