Mating-induced FOS-like immunoreactivity in the rat forebrain: a sex comparison and a dimorphic effect of pelvic nerve transection

Previous research has shown that mating induces the expression of the immediate-early gene, c-fos, as detected by the increased presence of nuclear FOS-like immunoreactivity (FOS-IR), in specific forebrain regions of both male and female rats. In the male both olfactory-vomeronasal (O/V) and genital/somatosensory (G/S) inputs appear to contribute to the neural FOS response to mating whereas in the female G/S input carried by the pelvic nerves appears to mediate the forebrain FOS response. To date, however, no direct sex comparison of the mating-induced forebrain FOS response has been made in rats maintained under the same steroidal conditions nor has the contribution of afferent sensory input from the pelvic nerves been assessed in males. We first compared the level of FOS-IR in brain regions of mated and unpaired gonadectomized male and female rats given 5 micrograms/kg estradiol benzoate (EB) for 7 days and 500 micrograms progesterone (P) 4 h prior to testing. One h after experiencing 1 ejaculation, both sexes showed increased FOS-IR in the medial preoptic a (MPOA), bed nucleus of the stria terminalis (BNST), medial amygdala, the ventro-lateral portion of the ventromedial nucleus of the hypothalamus (VMN), and the midbrain central tegmental field (CTF). This increase was significantly greater in the MPOA and medial amygdala of mated females than of males. Bilateral transection of the pelvic nerves significantly attenuated the increase in FOS-IR after mating in the CTF of male rats and in the MPOA, BNST, VMN, medial amygdala and CTF of females. Thus, following mating there is no sex difference in the brain regions which express c-fos, but there is a dimorphism in the contribution of afferent information conveyed by the pelvic nerves to the mating-induced FOS response. The neural FOS response of the female to mating is heavily dependent upon the G/S afferent inputs carried by the pelvic nerves whereas the male's neural c-fos response may depend on O/V input plus G/S input conveyed via other efferent pathways such as the pudendal nerves.

Sex and regional differences in intracellular localization of estrogen receptor immunoreactivity in adult ferret forebrain

Estrogen receptors were visualized in adult ferret brains using the H222 estrogen receptor antibody and immunocytochemical techniques. H222 immunoreactive (H222ir) cell nuclei were present in many forebrain regions in gonadectomized ferrets of both sexes. In many instances, H222ir cells also had immunoreaction product in their processes. All cells with H222ir processes also contained H222ir nuclei. More H222ir processes were observed in females in the medial and lateral preoptic area/anterior hypothalamus, and at the level of the descending fornix and caudal anterior commissure. Quantitative image analysis confirmed that females had significantly more (approximately 50%) extranuclear H222 immunoreaction product than males in cells in the magnocellular or preoptic subnuclei of the bed nucleus of the stria terminalis. Cells in the principal subnucleus of the bed nucleus of the stria terminalis and ventrolateral septum were notable for the relative paucity of H222ir processes. Sex differences in the intracellular extranuclear distribution of estrogen receptor protein in particular brain regions might contribute to the differential regulation of estrogen-dependent functions in the two sexes.

Equivalent levels of mating-induced neural c-fos immunoreactivity in castrated male rats given androgen, estrogen, or no steroid replacement

The nuclear protein product (FOS) of the immediate-early gene, c-fos, was visualized immunocytochemically in the brains of male rats after they either achieved 8 intromissions with an estrous female or were left alone in a test arena. Mating induced equivalent increments in the number of FOS immunoreactive (IR) neurons present in the medial preoptic area (mPOA), the bed nucleus of the stria terminalis (BNST), and the medial amygdala in groups of males that were gonadally intact or had been castrated and treated for 7 days with either testosterone propionate, dihydrotestosterone propionate, estradiol benzoate (EB), or oil vehicle. Equivalent, low numbers of FOS-IR neurons were seen in these brain regions in additional groups of castrated males that received either EB or oil vehicle but were not paired with a female before being killed. Circulating sex steroids apparently contribute little to the mating-induced stimulation of c-fos gene expression, even in brain regions known to contain high levels of androgen and estrogen receptor.

Estrogen receptor immunoreactive neurons in the fetal ferret forebrain

The development of estrogen receptors was studied in the preoptic area/anterior hypothalamus (POA/AH) of fetal male and female ferrets. In males this region includes a nucleus (MN-POA/AH), delineated by Nissl stains, which is not discernible in females. The results reveal the distribution of estrogen receptor containing cells during the period when estrogen is known to induce the differentiation of the male ferret's MN-POA/AH. Brains were taken from ferret kits on days 30, 34, 37 and 40 of a 41-42 day gestation, and were processed utilizing the H222 monoclonal antibody to reveal estrogen receptors. At E30 there were numerous H222 immunoreactive (ir) cells in central regions of the POA/AH. From E30 to E40 there was a striking increase in the number of H222ir cells in the POA/AH. A broad sweep of H222ir cells extended from the ventral POA dorsally and laterally into the caudal POA and AH of both males and females. H222ir cells were not restricted to the region of the MN-POA/AH at any fetal age. H222 immunoreaction product at E30 was restricted to nuclear compartments. By E40, H222ir processes extended from some cells with H222ir nuclei in the medial and lateral POA/AH in both males and females. At the older fetal ages immunopositive cell numbers increased in lateral positions. At E34 and E37 (but not E30) selective ventricular zones, and regions between the hypothalamus and amygdala contained H222ir cells, suggesting the presence of estrogen receptors in cells during migration. Although the amygdala contained a few H222ir cells as early as E34, the cortex lacked H222ir cells even as late as E40. The appearance of H222ir cells in positions suggestive of migration is consistent with the hypothesis that estrogen receptors play some role in determining cell positions in certain regions of the developing nervous system.

A sex comparison of serotonin immunoreactivity and content in the ferret preoptic area/anterior hypothalamus

Previous studies with rats raised the possibility that sexually dimorphic features of the medial preoptic area/anterior hypothalamus (POA/AH) may result, in part, from a sex difference in the serotonergic innervation of this region. We asked whether a similar phenomenon may occur in a carnivore, the ferret. A sexually dimorphic male nucleus of the dorsal POA/AH (Mn-POA/AH) has previously been characterized in Nissl-stained sections of the male ferret forebrain; this nucleus is absent in females. A nondimorphic ventral nucleus of the POA/AH is found in both sexes. In the present study numerous serotonin (5-HT) immunoreactive (ir) fibers were observed in the dorsal POA/AH of gonadectomized adult ferrets of both sexes. By contrast, in both sexes the ventral nucleus of the POA/AH had many fewer 5-HTir fibers. A similar difference in the distribution of immunoreactivity between dorsal and ventral POA/AH was observed for tyrosine hydroxylase (TH) localized in cell bodies and in nerve fibers and for H222ir estrogen receptors localized in cell nuclei. Likewise, in both sexes the content of 5-HT and dopamine (DA), measured by high pressure liquid chromatography with electrochemical detection, were significantly higher in the dorsal than the ventral POA/AH, thereby corroborating observed regional differences in 5-HTir and THir fibers, respectively. The present findings provide no support for the notion that sexually dimorphic cytoarchitectonic features of the dorsal POA/AH in ferrets are associated with a sex difference in the serotonergic innervation of this region.

Sexual dimorphism in the effects of mating on the in vitro release of LHRH from the ferret mediobasal hypothalamus

A sexually dimorphic pattern in the secretion of luteinizing hormone (LH) has previously been shown to occur in response to mating in an induced ovulating species, the ferret, with mating augmenting the secretion of LH in females but not in males. The aim of this study was to determine whether this dimorphic pattern in the postcoital secretion of LH reflects a dimorphic effect of mating on the neural release of luteinizing hormone releasing hormone (LHRH). The effect of mating on the in vitro release of LHRH from mediobasal hypothalami (MBH) collected from breeding male and female ferrets was studied. Luteinizing hormone releasing hormone release and content were significantly reduced in tissues from estrous females sacrificed 0.25 h after mating compared to unpaired estrous females and estrous females sacrificed 1 or 2.6 h after the mating stimulus. By contrast, the release of LHRH from MBH fragments and LHRH tissue content were equivalent in breeding males that were sacrificed 0.25 h after mating and in breeding males that were left unpaired. These data suggest that the postcoital surge of LH in the female ferret is preceded by a release of LHRH that initially depletes neuronal terminals within the MBH, whereas LHRH release, like pituitary LH secretion, is minimally affected by mating in males.

Increased expression of c-fos in the medial preoptic area after mating in male rats: role of afferent inputs from the medial amygdala and midbrain central tegmental field

Immunocytochemical methods were used to localize the protein product of the immediate-early gene, c-fos, in male rats after exposure to, or direct physical interaction with, oestrous females. Increasing amounts of physical contact with a female, with resultant olfactory-vomeronasal and/or genital-somatosensory inputs, caused corresponding increments in c-fos expression in the medial preoptic area, the caudal part of the bed nucleus of the stria terminalis, the medial amygdala, and the midbrain central tegmental field. Males bearing unilateral electrothermal lesions of the olfactory peduncle showed a significant reduction in c-fos expression in the ipsilateral medial amygdala, but not in other structures, provided their coital interaction with oestrous females was restricted to mount-thrust and occasional intromissive patterns due to repeated application of lidocaine anaesthetic to the penis. No such lateralization of c-fos expression occurred in other males with unilateral olfactory lesions which were allowed to intromit and ejaculate with a female. These results suggest that olfactory inputs, possibly of vomeronasal origin, contribute to the activation of c-fos in the medial amygdala. However, lesion-induced deficits in this type of afferent input to the nervous system appear to be readily compensated for by the genital somatosensory input derived from repeated intromissions. Unilateral excitotoxic lesions of the medial preoptic area, made by infusing quinolinic acid, failed to reduce c-fos expression in the ipsilateral or contralateral medial amygdala or central tegmental field following ejaculation. By contrast, combined, unilateral excitotoxic lesions of the medial amygdala and the central tegmental field significantly reduced c-fos expression in the ipsilateral bed nucleus of the stria terminalis and medial preoptic area after mating; no such asymmetry in c-fos expression occurred when lesions were restricted to either the medial amygdala or central tegmental field. This suggests that afferent inputs from the central tegmental field (probably of genital-somatosensory origin) and from the medial amygdala (probably of olfactory-vomeronasal origin) interact to promote cellular activity, and the resultant induction of c-fos, in the ipsilateral bed nucleus of the stria terminalis and medial preoptic area. The monitoring of neuronal c-fos expression provides an effective means of studying the role of sensory factors in governing the activity of integrated neural structures which control the expression of a complex social behaviour.

Effects of sex and androgen treatment on dendritic dimensions of neurons in the sexually dimorphic preoptic/anterior hypothalamic area of male and female ferrets

A sexually dimorphic group of cells at the dorsal border of the preoptic/anterior hypothalamic area (POA/AH) of ferrets has been previously identified in Nissl-stained tissue. In this study, Golgi-stained tissue was examined in order 1) to determine whether sex differences exist in dendritic dimensions of neurons from this region, and 2) to assess the effects of adult androgen treatment on dendritic morphology in ferrets of both sexes. Brains from adult ferrets given daily injections of testosterone propionate (5 mg/kg body weight) or oil vehicle for 5 weeks after gonadectomy were impregnated by Golgi-Cox procedures. After sectioning at 120 microns, 78 multipolar neurons were selected from the sexually dimorphic POA/AH of 12 ferrets and reconstructed in three dimensions with the aid of a computer-assisted neuron tracing system. Large sex differences were observed in somal area and most aspects of dendritic morphology, including total length, number of branches, and total dendritic surface area. Androgen also appeared to accentuate dendritic arborization in both sexes, but this effect was weaker than the sex effect, more apparent in males than females, and restricted to fewer variables. The most statistically significant effects of adult androgen treatment in males were found for total dendritic surface area and percentage of fourth order dendrites, and in females, average dendritic thickness. These data show that strong sex differences exist in dendritic structure of neurons in the POA/AH, and suggest that alterations in levels of gonadal steroids in adulthood may promote synaptic remodeling in a region of the brain involved in the control of sexually dimorphic behaviors.

Sex difference in the effect of mating on c-fos expression in luteinizing hormone-releasing hormone neurons of the ferret forebrain

The pulsatile secretion of LH was previously found to rise in female ferrets after receipt of an intromission, whereas in males that achieved an intromission, both LH and testosterone secretion were either reduced or unchanged. We sought to determine whether this sexually dimorphic pattern of LH secretion reflects a sex difference in the effect of mating on the activity of forebrain neurons that secrete LHRH. Immunocytochemical methods were used to localize the nuclear protein product of the immediate early gene, c-fos, as an index of increased neuronal activity after mating. Nuclear FOS immunoreactivity (FOS-IR) was monitored in LHRH-IR neurons as well as other non-LHRH forebrain neurons. In confirmation of previous reports, LHRH-IR perikarya in ferrets of both sexes were located medially along the base of the brain at rostral, medial, and caudal levels of the preoptic-hypothalamic continuum. In each of these regions a significantly higher percentage of LHRH-IR neurons was colabeled with nuclear FOS-IR in mated than in unpaired females. By contrast, an equivalent low percentage of LHRH-IR neurons was colabeled with FOS-IR in mated and unpaired male ferrets. Significantly more FOS-IR neurons (not colabeled with LHRH) were detected in the bed nucleus of the stria terminalis, the medial preoptic area, the dorsal-medial hypothalamus, and the medial amygdala (MA) of mated vs. unpaired females. By contrast, mating significantly augmented FOS-IR only in the MA of male ferrets. The results suggest that the sexually dimorphic pattern of LH secretion that occurs in ferrets after mating reflects a selective activation of LHRH neurons in the female forebrain. This sex-specific increase in the responsiveness of LHRH neurons to mating may depend on input from a limbic circuit which includes the medial amygdala, bed nucleus of the stria terminalis, and medial preoptic area.

Reciprocal relationships between pulsatile androgen secretion and the expression of mating behavior in adult male ferrets

The pulsatile secretion of androgen was similar over a 12-hr period in breeding male ferrets implanted with jugular catheters which either achieved an intromission with an estrous female or received no socio-sexual contact. This negative result contrasts with the previous demonstration (Carroll, Erskine, and Baum, 1987, Endocrinology 121, 1349-1359) of a significant, delayed rise in mean plasma androgen concentrations in breeding male ferrets 5-12 hr after mating. Males used in that previous study had lower initial mean plasma levels of androgen and smaller testis diameters than the present males. We therefore asked whether differences in circulating androgen levels, characteristic of males in different phases of the seasonal breeding cycle, might affect the expression of mating behavior. Castrated males given 0, 0.2, 2.0, or 5.0 mg/kg of testosterone propionate (TP) showed dose-related increases in the expression of different components of sexual behavior, including neck gripping, mounting, and intromitting. Surprisingly, intromissive performance was significantly better in intact breeding males than in castrates given even the highest dosage of TP. These results suggest that ferrets' mating performance may vary with seasonal variations in androgen availability, and that the ability of males to exhibit a postcoital increase in the testicular secretion of androgen may be limited to the beginning or end of the breeding season, when circulating levels of androgen are relatively low. Mating-induced increments in androgen secretion at these times may enhance subsequent reproductive success by facilitating males' intromissive capacity, which is required for the induction of ovulation and optimal sperm transport in female partners.

Sex difference in whole-body androgen content in rats on fetal days 18 and 19 without evidence that androgen passes from males to females

The whole-body content of androgen (testosterone + 5 alpha-dihydrotestosterone) was significantly higher on Fetal Days 18 and 19 in male than in female rats; androgen content was equivalent in the two sexes at other fetal ages, including Days 16, 17, 20, and 21, and prior to parturition on Fetal Day 22. These results partially corroborate previous data of Weisz and Ward (Endocrinology 1980; 106:306-316), who measured testosterone in pooled plasma from rat fetuses and suggest that androgens contribute to masculine brain sexual differentiation only briefly during fetal life. No significant differences in whole-body androgen content were observed among groups of females situated in utero between 0, 1, or 2 males on each side (contiguous male model) or among groups of females with 0, 1, or 2 or more males located caudally (on the cervical side) in the same uterine horn, regardless of whether combined data from Fetal Days 17-22 or only Fetal Days 18 and 19 were considered. These results provide no evidence that androgens from males reach female fetuses in the same uterine horn.

Estradiol binding in the sexually dimorphic nucleus of the preoptic/anterior hypothalamic area of adult male ferrets and in the equivalent region of females

A large number of estradiol-concentrating cells were visualized by autoradiography in a subpopulation of large neurons located in and around the sexually dimorphic male nucleus of the preoptic/anterior hypothalamic area (MN-POA/AH) of castrated male ferrets and in a comparable dorsal portion of the POA/AH of ovariectomized females. Considerably fewer estradiol-labelled cells were seen in the non-dimorphic ventral POA/AH nucleus of both sexes. Estrogen binding in cells in or around the MN-POA/AH may contribute to the formation of this sexually dimorphic nucleus in fetal males and may mediate specific estrogen-dependent behavioral functions in adulthood.

Neonatal testosterone masculinizes sexual behavior without affecting the morphology of the dorsal preoptic/anterior hypothalamic area of female ferrets

We examined whether testosterone (T) administered to female ferrets neonatally--a treatment known to enhance masculine coital capacity--induces formation of the sexually dimorphic male nucleus in the dorsal preoptic/anterior hypothalamic area (MN-POA/AH), and/or sensitizes dorsal POA/AH neurons to the stimulatory effect of later androgen treatment on somal dimensions. In males, the MN-POA/AH was present in all subjects, and exposure to androgen following castration at postnatal day 56 (P56) increased both MN-POA/AH volume as well as mean somal areas of MN-POA/AH neurons relative to oil-treated controls. Females given androgen from P5 to P20 and for one month beginning after ovariectomy on P56 failed to develop the MN-POA/AH, but displayed high levels of masculine sexual behavior. Somal areas of dorsal POA/AH neurons in females that received either T or a control neonatally did not increase following androgen treatment at P56. Thus, the correlation that exists between somal enlargement of dorsal POA/AH neurons and masculine sexual behavior in androgen-treated males is not found in behaviorally masculinized females. Masculine coital ability does not appear related to aspects of dorsal POA/AH morphology, supporting data from a previous study in which lesions of the MN-POA/AH caused negligible deficits in masculine sexual behavior of adult male ferrets.

Differential regulation of brain aromatase by androgen in adult and fetal ferrets

The contribution of androgens to the regulation of aromatase activity (AA), measured by quantifying the in vitro formation of [3H]estrone from 19-[3H]hydroxyandrostenedione precursor, was studied in equivalent microdissected brain regions of adult and fetal ferrets. In adulthood, AA was similar in the bed nucleus of the stria terminalis, medial (M) and lateral (L) preoptic area (POA), medial (MA) and lateral amygdala (LA), ventromedial hypothalamus (VMH), and parietal cortex of gonadectomized males and females given no concurrent steroid treatment. Daily sc injections of the androgen dihydrotestosterone propionate significantly stimulated AA in MPOA, MA, and VMH of adult males and in MA of females; a similar trend was seen in MPOA and VMH of females. By contrast, no evidence of androgenic regulation of AA was obtained in these three brain regions microdissected from fetuses killed on embryonic day 35 (E35; 41-day gestation). Transplacental administration of the antiandrogen flutamide beginning on day E24 failed to affect AA in MPOA, LPOA, MA, LA, or parietal cortex, although this treatment significantly reduced AA in bed nucleus of the stria terminalis of fetal males. The results suggest that the responsiveness of aromatizing enzymes to androgenic induction is similar in several subcortical brain regions of adult ferrets of both sexes. In breeding males such an action of androgen may augment the neural production of estrogen, which has previously been implicated in the control of sexual behavior and the feedback regulation of LH secretion. By contrast, androgen apparently contributes minimally to the regulation of AA in brain regions of fetal ferrets, particularly in the MPOA, in which a sexually dimorphic nucleus differentiates in males around E37 in response to estrogen produced locally.

Vaginocervical stimulation of ferrets induces release of luteinizing hormone-releasing hormone

Abstract Vaginocervical stimulation of ovariectomized estradiol-primed ferrets (which are reflex ovulators) with a glass rod in the presence of a neck-gripping male induced an increase in plasma luteinizing hormone (LH) from undetectable levels (</=0.50 ng/ml) before stimulation, to 2.4 +/- 0.43 ng/ml 75 min after stimulation (stimulated females). Forty-eight h after stimulation plasma LH returned to baseline levels (post-stimulated females). A significant decrease in the number of perikarya containing LH-releasing hormone (LHRH), detected by immunocytochemistry, was associated with the increase in plasma LH following stimulation. Approximately one half of the number of immunoreactive LHRH neurons (243+/-27) were detected in the forebrain of stimulated females, compared to those detected in the forebrain of post-stimulated animals (436 +/- 88) using antiserum AR 744. Equivalent results were obtained with a different antiserum (RM 1076) capable of detecting the extended decapeptide, or precursor, as well as partially or fully processed decapeptide. We conclude that controlled Vaginocervical stimulation of female ferrets evokes the release of LHRH as well as LH, depleting approximately 50% of the LHRH perikarya of detectable LHRH. Additionally, electron microscopy of LHRH perikarya of stimulated females revealed more Golgi complexes/cell compared to baseline females. We propose that Vaginocervical stimulation also augments the processing of extended precursor forms of LHRH to generate the decapeptide.

Effect of naloxone on the pulsatile secretion of luteinizing hormone in gonadectomized male and female ferrets before and after oestradiol replacement

Abstract Intravenous infusions of the opioid receptor antagonist, naloxone, caused a significant rise in luteinizing hormone (LH) pulse frequency in male and female ferrets which had been gonadectomized 10 days earlier while in breeding condition; mean LH concentrations and LH pulse amplitudes were not affected. By contrast, naloxone failed to stimulate LH pulse frequency, or other LH parameters in gonadectomized ferrets of either sex which received daily injections of a low dose of oestradiol. Our results for the ferret, in which the female is an induced ovulator, resemble those previously obtained in another induced ovulator, the rabbit. They contrast, however, with the results of numerous studies using spontaneously ovulating species in which sex steroids, if anything, facilitate the ability of naloxone to stimulate LH secretion.

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.

Effects of lesions of a sexually dimorphic nucleus in the preoptic/anterior hypothalamic area on the expression of androgen- and estrogen-dependent sexual behaviors in male ferrets

The male nucleus of the preoptic/anterior hypothalamic area (MN-POA/AH) is a sexually dimorphic structure present in male, but not in female ferrets. Ovariectomized female ferrets given increasing dosages of estradiol benzoate (EB) normally run faster towards a stud male in an L-maze (i.e. become more proceptive). In two separate experiments, only gonadectomized males with bilateral damage to the MN-POA/AH following large or small electrolytic lesions approached stud males more quickly in response to EB. By contrast, males which received sham lesions, unilateral large POA/AH lesions, or bilateral lesions which missed the MN-POA/AH on at least one side failed to show EB-induced reductions in approach latencies in pre- or post-operative tests. Males with large POA/AH lesions also displayed significant post-operative decrements in masculine sexual behaviors during treatment with a high dose of testosterone propionate (TP). Less severe, but statistically significant deficits in masculine coital performance were also observed in males with small lesions which damaged the MN-POA/AH bilaterally; however, the ability of these males to achieve intromissions appeared normal. Together, these results suggest that the MN-POA/AH of the male ferret exerts an inhibitory influence on estrogen-dependent proceptive responsiveness, but play only a minor role in the control of masculine coital behavior.

Frank Beach's research on the sexual differentiation of behavior and his struggle with the "organizational" hypothesis

Highlights are described of Frank Beach's research contributions in the area of steroidal control of behavioral sexual differentiation in rats and beagle dogs. Beach's initial struggle with and his subsequent partial acceptance of the organizational hypothesis of steroidal action in the developing male nervous system as an explanation of these steroidal effects on behavior, advanced by W. C. Young and co-workers, is also chronicled.

Endocrine correlates of mating-induced reductions in estrous behaviour in an induced ovulator, the ferret

Experiments were conducted to assess the time course of behavioral and endocrine changes which occur in female ferrets as they switch from estrus to the pseudopregnant state. Significant reductions in females' acceptance of neck gripping by a stimulus male (receptivity) and in their latency to approach a stimulus male in an L-maze (proceptivity) were first observed 3 days after receipt of an intromission; no such changes occurred in other females which were only neck gripped by stimulus males during the initial test session. Corpora lutea were later found only in the ovaries of females which received intromissions, confirming that ovulation had occurred in these animals. Plasma concentrations of prostaglandin E1, prostaglandin F2 alpha, and the 13,14-dihydro 15-keto metabolite of prostaglandin F2 alpha (PGFM) were unchanged in female ferrets for 4-5 days after receipt of an intromission. By contrast, plasma concentrations of progesterone were significantly elevated beginning 5 days after, whereas plasma estradiol was significantly reduced beginning 4 days after receipt of an intromission. Daily sc administration of the progesterone receptor antagonist. RU 38486, significantly retarded the lengthening in females' approach latencies to a stimulus male, suggesting that postcoital elevations in circulating progesterone normally contribute to the expected decline in proceptive responsiveness. By contrast, postcoital reductions in acceptance quotients occurred at equivalent rates in females treated with RU 38486 versus vehicle, leading us to infer that postcoital reductions in estrogenic stimulation may cause this decline in ferrets' receptive responsiveness.

Prenatal and neonatal testosterone exposure interact to affect differentiation of sexual behavior and partner preference in female ferrets

Implanting testosterone (T) subcutaneously over Postnatal Days 5-20 masculinized sexual behavior, reduced proceptive responsiveness, and shifted sexual preference more readily in male than in female ferrets gonadectomized on Day 5. This enhanced sensitivity of males to neonatal T was best duplicated in females exposed transplacentally to T over Embryonic Days (E) 27-39 (41-day gestation) and injected at birth with T (2.5 micrograms sc in oil: 10% ethanol). Extended exposure of male ferrets to high levels of T, beginning shortly after the onset of testicular steroidogenesis (E25) and continuing for several hours after birth (E41) normally sensitizes their brains to the subsequent organizational effects on coital performance and sexual motivation of the relatively low levels of T that circulate in male ferrets during the first 3 postnatal weeks.

Naloxone disrupts the expression but not the acquisition by male rats of a conditioned place preference response for an oestrous female

Two experiments were conducted to investigate the possible role of endogenous opioid peptides in the regulation of masculine sexual reward. In experiment 1 sexually experienced male rats, which had recently been castrated or left gonadally intact, were allowed to mate with an oestrous female in an initially "non-preferred" chamber of a test apparatus. On alternate days these males were placed alone in the initially "preferred" chamber of the same apparatus. After eight such conditioning sessions both intact and castrated males had acquired a conditioned place preference (CPP) for the initially "non-preferred" chamber whereas control males, which were never given access to an oestrous female, showed no evidence of a significant shift in their preference for either chamber. Administration of the opioid receptor antagonist, naloxone (1 or 5 mg/kg, SC) prior to each conditioning session had no significant influence on the acquisition of a CPP for an oestrous female. By contrast, in experiment 2 naloxone treatment significantly attenuated the expression of a previously established CPP for an oestrous female in both gonadally intact and castrated males. The results suggest that opioid components of neural reward circuits are normally activated in the male rat by conditioned incentive stimuli, but not by the primary rewarding stimuli associated with access to and mating with an oestrous female.

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.