Intracerebroventricular Oxytocin Self-Administration in Female Rats.

Oxytocin (OT) is a neuromodulator that facilitates pair-bonding, maternal care and social approach. OT is considered to promote these social behaviours by enhancing the salience and reinforcing effects of relevant social stimuli. There is the additional possibility that OT per se may be rewarding. To test this, we investigated whether female rats would voluntarily self-administer OT. Female Long-Evans rats were ovariectomised and then received an oestrogen implant and an i.c.v. cannula. Rats were tested in an operant chamber with active and inactive levers. They were initially tested for 4 h/day on a fixed-ratio 5 schedule for self-administration of artificial cerebral spinal fluid (aCSF) for 5 days, followed by aCSF, or OT, at 1 or 10 ng/µl for another 5 days. Rats self-administering aCSF made 36.2 ± 6.2 active lever responses/4 h versus 14.9 ± 3.4 inactive responses. Responses for 1 ng/µl OT were similar. However, rats self-administering 10 ng/µl OT made significantly more active lever responses (67.8 ± 12.0 per 4 h), and received 121.4 ± 21.0 ng OT/4 h. To determine whether reduced anxiety contributes to the reinforcing effects of OT, rats received an infusion of aCSF or OT at 0.3 or 3.0 µg immediately before testing on the elevated plus maze. There was no effect of OT on anxiety as reflected by percentage time spent on the open arms, as well as no effect of OT on locomotion as measured either by the number of closed arm entries or the number of total arm entries. These results suggest that OT may be rewarding, and that this is not a result of the anxiolytic effects of OT.

Oxytocin induces a conditioned social preference in female mice.

Friendships and other rewarding affilliative bonds are associated with the actions of the nonapeptide hormone oxytocin (OT) in humans and many social mammals. We investigated whether OT itself is rewarding, and if that reward is dependent upon the presence of conspecifics. We evaluated the reinforcing effects of OT infusion in female mice on social (conditioned social preference; CSP) and nonsocial tests (conditioned place preference; CPP). Ovariectomised females received oestradiol implants and i.c.v. cannulas. During a pre-test, they were introduced to a three-chamber apparatus for 10 min. Social and place apparatus were identical, except that each end-chamber contained a novel stimulus female for CSP, whereas they were distinguished by visual and tactile cues for CPP. For CSP, test females received OT (0, 100, 200 or 100 ng) and were paired for 30 min with one stimulus female. On alternating days, they received saline vehicle and were paired with the opposite female, for a total of four pairings each. The final conditioned preference test was identical to the pre-test. OT induced CSP. Test mice that received 100 ng of OT increased their preference score from -67.4 ± 22.1 s in pre-test to +55.7 ± 35.1 s during the conditioned preference test (P < 0.05). It was observed that 200 ng OT induced an increase in preference score from -162.7 ± 47.3 to +74.3 ± 23.7 s (P < 0.001). There was no effect of 0 or 1000 ng of OT on CSP. An additional group of mice was tested for CPP at 200 ng of OT. Testing and pairings were identical to CSP. OT induced a small but significant CPP. Mice increased their preference score from -222.4 ± 38.0 to -126.0 ± 58.7 s (P < 0.05). OT had no effect on anxiety or odour recognition, as assessed by elevated plus maze and olfactory habituation/dishabituation tests, respectively. In conclusion, OT, similar to other motivating stimuli (drugs, food), not only is rewarding when tested under solitary conditions, but also is reinforcing in a social setting.

Anabolic-androgenic steroid effects on nociception and morphine antinociception in male rats.

The purpose of this study was to investigate the effects of acute and chronic administration of anabolic-androgenic steroids (AAS) on nociception and morphine antinociception in acute pain models, as well as on chronic inflammatory nociception. In Experiment 1, adult, gonadally intact male rats were injected s.c. for 28 days with either 5 mg/kg testosterone (T), dihydrotestosterone (DHT), stanozolol (STAN), or safflower oil vehicle (N=12-25/group). On day 28, rats in each group were tested on acute thermal and mechanical nociceptive assays, before and after morphine treatment. In Experiment 2, rats in each group (N=8-10/group) were injected with mineral oil or complete Freund's adjuvant (CFA) into one hindpaw after 28 days of AAS treatment, and then tested for thermal hyperalgesia, mechanical allodynia, inflammation and locomotor suppression intermittently for 28 days. Experiment 3 replicated nociceptive measurements in Experiments 1 and 2, but with a single AAS or vehicle injection occurring 3h prior to testing (N=10-12/group). While chronic AAS administration tended to decrease body weight gain and alter reproductive organ weights in the expected manner, it did not significantly alter acute nociception nor attenuate the development of various chronic pain indices after CFA administration. Morphine antinociceptive potency was significantly decreased by chronic DHT on the hot plate test only. Acute AAS administration also did not significantly alter acute or chronic nociception, or morphine antinociceptive potency. Comparisons between acute and chronic AAS administration suggest that steroid tolerance did not occur in rats treated with AAS chronically. Taken together, these data do not support the hypothesis that AAS exposure alters nociception or morphine antinociception in gonadally intact males.

The bed nucleus of the stria terminalis in the Syrian hamster: subnuclei and connections of the posterior division.

The bed nucleus of the stria terminalis is a key part of a ring of cells extending between the centromedial amygdala and bed nucleus of the stria terminalis referred to as the extended amygdala. The present study describes the architecture of the bed nucleus of the stria terminalis and the connections of subnuclei in posterior bed nucleus of the stria terminalis. The hamster bed nucleus of the stria terminalis is readily allotted to anterior and posterior divisions separated by the fibers of the body of the anterior commissure. The anterior division has four subnuclei: anteromedial, anterointermediate, anterolateral, and anteroventral. Within the posterior division, there are three distinct regions: posteromedial, posterointermediate, and posterolateral. In hamsters, the posterior bed nucleus of the stria terminalis contributes to male sexual behavior, particularly chemoinvestigation. Moreover, the posterior bed nucleus of the stria terminalis is part of a neural circuit essential for mating, including the medial amygdaloid nucleus and medial preoptic area. The connections of bed nucleus of the stria terminalis, posteromedial part, bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posterolateral part were visualized by co-injection of anterograde (Phaseolus vulgaris leucoagglutinin) and retrograde (cholera toxin B) tract tracers. The bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posteromedial part have dense bidirectional connections with medial amygdaloid nucleus and cortical amygdala via the stria terminalis and ventral amygdalofugal pathway. These subnuclei also maintain bidirectional connections with steroid-concentrating areas including lateral septum, medial preoptic area, hypothalamus, and periaqueductal gray. The bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posteromedial part receive projections from the subiculum and send projections to deep mesencephalic nuclei. By contrast, the bed nucleus of the stria terminalis, posterolateral part is connected with the central amygdala, lateral hypothalamus, subthalamic nucleus, nucleus accumbens, substantia innominata, substantia nigra and thalamus. Thus, the bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posteromedial part have similar connections with areas involved in social behaviors. The bed nucleus of the stria terminalis, posterolateral part maintains connections with areas involved in motivational circuits. This supports the concept of distinct circuits within the extended amygdala which differentially link the centromedial amygdala and bed nucleus of the stria terminalis.

Androgen dependence in hamsters: overdose, tolerance, and potential opioidergic mechanisms.

Anabolic steroids are drugs of abuse. However, the potential for steroid reward and addiction remains largely unexplored. This study used i.c.v. testosterone self-administration and controlled infusions of testosterone or vehicle in hamsters to explore central mechanisms of androgen overdose. Forty-two hamsters used nose-pokes to self-administer 1 microg/microl testosterone i.c.v. 4 h/day in an operant chamber. During 1-56 days of androgen self-administration, 10 (24%) hamsters died. Deaths correlated with peak daily intake of testosterone. Of the hamsters that self-administered a peak intake of <20 microg/day, there was 100% survival (10/10). Survival decreased to 86% (19/22) when daily testosterone intake peaked at 20-60 microg/day. Only 30% (three of 10) survived when daily testosterone intake exceeded 60 microg/day. Deaths are not due to volume or vehicle because i.c.v. infusions of 80 mul vehicle had no effect. Testosterone overdose resembles opiate intoxication. When male hamsters received infusions of 40 microg testosterone, locomotion (25.1+/-18.8 grid-crossings/10 min), respiration (72.7+/-5.4 breaths/min) and body temperature (33.5+/-0.4 degrees C) were significantly reduced, compared with males receiving vehicle infusions (186.1+/-8.1 crossings/10 min, 117.6+/-1.0 breaths/min, 35.9+/-0.1 degrees C, P<0.05). However, males developed tolerance to continued daily testosterone infusion. After 15 days, locomotion (170.2+/-6.3 crossings), respiration (118.4+/-1.3 breaths/min), and body temperature (35.3+/-0.3 degrees C) in testosterone-infused males were equivalent to that in vehicle controls (P>0.05). The depressive effects of testosterone infusion are blocked by the opioid antagonist, naltrexone. With naltrexone pre-treatment (10 mg/kg s.c.), locomotion (183.7+/-1.8 crossings/10 min), respiration (116.9+/-0.3 breaths/min), and body temperature (36.1+/-0.4 degrees C) during testosterone infusion were equivalent to vehicle controls. Likewise, naltrexone prevents the reinforcing effects of i.c.v. testosterone self-administration. These results indicate that testosterone at high doses causes central autonomic depression, which may be a factor in deaths during self-administration. As well, the depressive effects of large quantities of testosterone may be mediated, at least in part, by an opioidergic mechanism.

Sexual differentiation of the neuroendocrine control of gonadotrophin secretion: concepts derived from sheep models.

In our laboratory the sheep is used as an experimental model to study the early programming of the neuroendocrine mechanisms timing the pubertal increase in GnRH secretion. This interest has arisen because puberty in male lambs occurs much earlier than that in female lambs. Such sex differences in the timing of puberty are present in most species, as well as in the patterns of reproduction in the adult. Although this finding could merely reflect differences in the function of the ovary and testes, many of these differences arise from early sexual differentiation of central mechanisms controlling GnRH secretion. Two models are used for our studies. One model (Model I) has been developed to understand how the male reproductive neuroendocrine system becomes differentiated from that of the female system. The other (Model II) is used to study abnormal female sexual differentiation and the possible aetiologies of reproductive diseases. The discussion focuses on how these two models can be used to study the organizational action of steroids on the mechanisms timing puberty and the secretion patterns of reproductive hormones in the adult. Broadly, our findings indicate that an extended period of steroid action on the developing brain programmes sex differences in GnRH secretion that are manifest later in life: in the expression of pulsatile GnRH release after birth or earlier; in its amplification during puberty; in its differential regulation during young adulthood. Inappropriate programming of the control of GnRH secretion can lead to impaired fertility.

Steroidal control of male hamster sexual behavior in Me and MPOA: effects of androgen dose and tamoxifen.

Steroids stimulate male sexual behavior through interconnected limbic nuclei, including the medial amygdala (Me) and medial preoptic area (MPOA). Although Me and MPOA each can transduce hormonal cues to induce sexual activity in castrated male hamsters, simultaneous stimulation of Me and MPOA fails to amplify mating. The present study extends our investigations of redundancy in the hormonal control of mating by testing the behavioral effects of (1) increasing steroid dose in a single brain region or (2) locally blocking steroid action with an estrogen antagonist. In Experiment 1, sexually experienced castrates received a single testosterone implant in Me, bilateral testosterone implants, or a single implant of a highly potent androgen, 7a-methyl-19-nortestosterone (MENT). These treatments stimulated mating behavior: 2 weeks after surgery, mounting was observed in > or =50% of the males in each group. In Experiment 2, castrated males received intracerebral implants of the estrogen antagonist tamoxifen in Me or MPOA, combined with systemic testosterone replacement. Tamoxifen in MPOA had minimal effects on the recovery of mating behavior. With tamoxifen in Me, mounts and intromissions were significantly reduced 18 days after surgery. However, the percent of males in each group that expressed mounts, intromissions or ejaculations was not different. Thus, in Experiment 1, increasing the amount of steroid does not amplify mating. Likewise, local blockade of hormone action in Experiment 2 does not prevent behavior. These findings support the concept that steroids are largely permissive for male sex behavior. Steroid stimulation of either Me or MPOA is sufficient for sexual activity. Conversely, neither Me nor MPOA has an absolute requirement for hormones to facilitate expression of mating.

Oral testosterone self-administration in male hamsters.

The addiction potential of anabolic steroids remains largely unexplored. Here, we demonstrate voluntary oral testosterone intake in hamsters. Using a 2-bottle choice test, males preferred an aqueous solution of 200 microg/ml testosterone over vehicle. However, the taste of testosterone is not highly preferred. Addition of testosterone at 400 microg/ml increased fluid consumption from the nonpreferred bottle in a 2-bottle choice test, but cholesterol at the same concentration reduced drinking, suggesting that testosterone reward is not common to all sterols. With food-induced drinking, testosterone maintained fluid intake when food was withdrawn. These data demonstrate that oral self-administration of testosterone is reinforcing in hamsters, suggesting the potential for dependence in human users.

Central inhibition of gonadotropin-releasing hormone secretion in the growth-restricted hypogonadotropic female sheep.

Growth retardation induced by dietary restriction results in hypogonadotropism, and thus, puberty is delayed. The present studies determined 1) whether reduced LH secretion in the growth-retarded condition is due to a reduction in the frequency and/or in the amplitude of GnRH secretion, and 2) whether the mechanism regulating LH secretion is being actively inhibited via central mechanisms. To determine whether GnRH pulse frequency and/or amplitude are reduced during growth restriction, blood samples were simultaneously collected from pituitary portal blood for GnRH and from jugular blood for LH determinations over a 4-h period in ovariectomized lambs (52 wk of age) that were either growth restricted (28 kg; n = 8) or growing normally (60 kg; n = 7). As expected, the growth-restricted females were hypogonadotropic and exhibited a long LH interpulse interval compared with the normally growing females. However, although the GnRH interpulse interval was longer in the growth-restricted lambs compared with that in the normally growing lambs, the pattern of GnRH secretion did not directly correspond with that of LH secretion in the growth-restricted group. In addition, high amplitude GnRH pulses that coincided with LH pulses and small, low amplitude GnRH pulses without a concomitant LH pulse occurred. The second study tested the hypothesis that diet-induced hypogonadotropism is the result of actively inhibited central mechanisms by investigating the effects of the nonspecific central nervous system inhibitor, sodium pentobarbital, on pulsatile LH secretion in the growth-restricted lamb. Serial blood samples were collected from 11 ovariectomized lambs that were maintained at weaning weight (approximately 20 kg) by reduced diet. After a 4-h pretreatment period, six of the lambs were anesthetized with sodium pentobarbital for 4 h; the other five lambs were untreated and served as controls. Pentobarbital anesthesia reduced the LH interpulse interval (increased the frequency) and increased mean LH levels. These findings suggest that during growth restriction hypogonadotropism arises from a central inhibition of GnRH neurons and is manifest as a decrease in both frequency and amplitude of GnRH pulses.

A new method for simultaneous demonstration of anterograde and retrograde connections in the brain: co-injections of biotinylated dextran amine and the beta subunit of cholera toxin.

In studying reciprocally connected brain networks, it is advantageous to use techniques that allow simultaneous visualization of both efferent and afferent connections from a single injection site. We report on a new technique to achieve this using pressure injections of a mixture of biotinylated dextran amine (BDA) and the beta subunit of cholera toxin (Ctb). Adult male hamsters (n = 12) received 20-30-nl injections of either a 1:1 mixture of BDA (Sigma, 10%) and Ctb (List Biological, 0.5%), or each tracer by itself, into the medial amygdala. Adult female sheep (n = 4) received 200-300 nl of the combined tracer into the A15 region of the hypothalamus. After 1 (hamster) or 2 weeks' (sheep) survival, animals were perfused with 4% paraformaldehyde. Sections were double-labeled, first for BDA histochemistry using nickel-enhanced DAB, then for Ctb using a PAP technique and unenhanced DAB. In all animals, combined injections resulted in clear and consistent patterns of both anterograde and retrograde labeling. Ctb immunoreactivity was distinct and easily distinguished from BDA labeling. There was no evidence for loss of sensitivity of either tracer due to the combined delivery; no differences were seen between combined or single tracer injections in numbers of retrogradely-labeled cells or in the distribution of anterogradely-labeled fibers. In summary, the combined delivery of BDA and Ctb is an easy and reliable technique for simultaneous afferent and efferent tract tracing in both small and large animals; it could potentially be combined with immunocytochemistry to determine the neurochemical content of labeled cells or fibers.

Prenatal testosterone masculinizes synaptic input to gonadotropin-releasing hormone neurons in sheep.

In sheep, the control of tonic and surge GnRH secretion is sexually differentiated by testosterone in utero. However, GnRH neurons are not sexually dimorphic with respect to number, distribution, or gross morphology. Therefore, this study tested the hypothesis that prenatal steroids influence synaptic input to GnRH neurons. We compared the number of synapses on GnRH neurons from male, female, and androgenized female lambs (n = 5 each). Androgenized females were exposed to testosterone during mid-gestation. Yearling lambs were perfused, and GnRH neurons were visualized using the LR-1 antibody. Five to seven GnRH neurons from the rostral preoptic area in each animal were viewed at the ultrastructural level. Afferent synapses and glial ensheathment on each neuron were counted in a single section through the plane of the nucleus. GnRH neurons from females received approximately twice as many contacts (3.6 +/- 0.7 synapses/100 microm plasma membrane) as those from male lambs (1.6 +/- 0.3; p < 0.05), similar to previous reports in rats. In addition, the number of synapses on GnRH neurons from androgenized female lambs (1.5 +/- 0.5) was similar to that from male lambs, suggesting that prenatal steroids give rise to sex differences in synaptic input to GnRH neurons.

Prenatal dihydrotestosterone differentially masculinizes tonic and surge modes of luteinizing hormone secretion in sheep.

The control of LH secretion in sheep is sexually differentiated. Males begin to reduce their sensitivity to inhibitory steroid feedback, leading to a pubertal increase in tonic LH secretion by 10 weeks of age, but females remain hypersensitive until 30 weeks. Moreover, only females can respond to the positive feedback action of estradiol to produce a preovulatory LH surge. Prenatal exposure of the female lamb to testosterone masculinizes tonic LH and abolishes the LH surge postnatally. However, the type of steroid involved is not known because testosterone can be converted to estradiol or dihydrotestosterone (DHT). This study tested the hypothesis that DHT, which cannot be converted to an estrogen, masculinizes tonic LH without defeminizing the LH surge. Pregnant ewes were treated with DHT (800, 400, or 200 mg/week) during the critical period for sexual differentiation of gonadotropin secretion (days 30-90; 145 days is term). To evaluate the time of the decrease in responsiveness to steroid inhibition, a constant steroid feedback signal was produced. At 4 weeks of age, androgenized females (800 mg, n = 5; 400 mg, n = 4; 200 mg, n = 5) and control males (n = 7) and females (n = 9) were gonadectomized and implanted with a SILASTIC brand estradiol capsule. Tonic LH secretion in males began to increase at 6.7 +/- 0.5 weeks (mean +/- SEM). In DHT-treated females, the LH increase began at the same time (800 mg DHT, 10.7 +/- 3.9 weeks; 400 mg DHT, 9.9 +/- 5.9 weeks; 200 mg DHT, 7.1 +/- 4.9 weeks). This was several months earlier than in control females (29.1 +/- 0.8 weeks; P < 0.05). After puberty, estradiol induced LH surges in 8 of 9 control females and 11 of 12 DHT-treated females, but not in any control males. These results lead to the hypothesis that in the sheep, distinct requirements exist for differentiation of 2 types of reproductive hormone control systems, and that conversion of testosterone to an estrogen is not essential for both. Aromatization is necessary to prevent the surge control of GnRH from operating in the male, but nonaromatizable androgens differentiate the tonic control to permit high GnRH secretion earlier in life.

Androgen receptor immunoreactivity in the male and female Syrian hamster brain.

To investigate potential mechanisms for sex differences in the physiologic response to androgens, the present study compared the hormonal regulation of intracellular androgen receptor partitioning and the distribution of androgen receptor immunoreactivity in select brain regions from male and female hamsters. Androgen receptors were visualized on coronal brain sections. Two weeks after castration, androgen receptor immunoreactivity filled the neuronal nuclei and cytoplasm in males and females. In gonad-intact males and females, androgen receptor immunoreactivity was limited to the cell nucleus. Whereas exogenous dihydrotestosterone prevented cytoplasmic immunoreactivity, estrogen at physiologic levels did not. These results suggest that nuclear androgen receptor immunoreactivity in gonad-intact females is maintained by endogenous androgens, and that androgens have the potential to influence neuronal activity in either sex. However, sex differences in the number and staining intensity of androgen-responsive neurons were apparent in select brain regions. In the ventral premammillary nucleus, ventromedial nucleus of the hypothalamus, and medial amygdaloid nucleus, androgen receptor staining was similar in gonadectomized males and females. In the lateral septum, posteromedial bed nucleus of the stria terminalis (BNSTpm), and medial preoptic nucleus, the number of androgen receptor-immunoreactive neurons was significantly lower in females (p < .05). Moreover, the integrated optical density/cell in BNSTpm was significantly less in females (1.28+/-0.3 units) than in males (2.21+/-0.2 units; p < .05). These sex differences in the number and staining intensity of androgen-responsive neurons may contribute to sex differences in the behavioral and neuroendocrine responses to androgens.

Testosterone stimulation of the medial preoptic area and medial amygdala in the control of male hamster sexual behavior: redundancy without amplification.

Receptors for gonadal steroids are present in an interconnected network of limbic nuclei. The existence of this network structure has important implications for how steroids control reproductive physiology and behavior. In 1986, Cottingham and Pfaff proposed that properties of a steroid-responsive neural network could include redundancy, amplification, stability and selective filtering. The present study tested the concept of steroid amplification, using male hamster sexual behavior as a model. In the male hamster, the medial amygdaloid nucleus (Me) and medial preoptic area (MPOA) are essential for mating behavior, and both nuclei transduce steroid cues to facilitate copulation. To determine if steroid action at multiple interconnected nuclei amplifies mating, the present study compared sexual behavior in castrated male hamsters bearing unilateral intracranial implants of testosterone in Me or MPOA with that of males with dual testosterone implants in Me and MPOA. Implants that stimulated androgen receptor-containing neurons in Me or MPOA stimulated copulatory behavior above castrate levels. However, behavior of males with dual implants was not significantly different from that of males with single implants. This suggests that steroid action at either MPOA or Me is sufficient to facilitate mating, but dual stimulation of these reciprocally-connected nuclei does not amplify sexual behavior.

Bidirectional connections of the medial amygdaloid nucleus in the Syrian hamster brain: simultaneous anterograde and retrograde tract tracing.

In the male Syrian hamster, mating is dependent on chemosensory and hormonal stimuli, and interruption of either input prevents copulation. The medial amygdaloid nucleus (Me) is a key nodal point in the neural circuitry controlling male sexual behavior because it relays both odor and steroid cues. Me is comprised of two major subdivisions, anterior (MeA) and posterior (MeP), which have distinct, although overlapping efferent projections. The present study investigated the afferents and efferents of MeA and MeP by using combined anterograde and retrograde tract tracing. Phaseolus vulgaris-leucoagglutinin and cholera toxin B were injected by iontophoresis through a single glass micropipette and detected by immunohistochemistry. MeA has widespread connections with olfactory structures, whereas MeP is heavily interconnected with steroid-responsive brain regions. The efferent projections of MeA and MeP were similar to those reported previously for the rat and hamster. In particular, MeP projects to the posteromedial subdivision of the bed nucleus of the stria terminalis (BNST) and to the medial preoptic nucleus, whereas MeA projects to adjacent subnuclei in BNST and the preoptic area. MeA and MeP also have distinct patterns of afferent input. Furthermore, the combination of anterograde and retrograde tract tracers shows that MeA and MeP are each bidirectionally connected with each other and with limbic nuclei. These results demonstrate that subnuclei of Me are interconnected with limbic structures in hamster brain. These connections may contribute to chemosensory and hormonal integration to control male sexual behavior.

Sexual differentiation of reproductive neuroendocrine function in sheep.

In many species, the timing of puberty is different in males and females. This does not simply reflect differences in the time course of activation of the testes and ovaries. Rather, sex differences in pubertal onset reside within brain mechanisms controlling GnRH secretion, as exemplified by studies conducted in sheep. Exposure of sheep fetuses to testicular steroids alters the timing of puberty, principally by reducing photoperiod responsiveness. This is manifest as an early increase in LH secretion in males or in females exposed experimentally to testosterone before birth. Steroids also act on non-photoperiodic mechanisms to abolish the preovulatory gonadotrophin surge. In view of these multiple organizational actions of steroids to control postnatal gonadotrophin secretion, it is becoming clear that there are many critical periods of brain development for organizing the GnRH neurosecretory system, and that these may be sensitive to different testosterone metabolites. Although GnRH neurones are not sexually dimorphic with respect to number, distribution or gross morphology, fundamental questions remain as to how steroids exert their effects at the cell through actions on GnRH afferents. Teleologically, these early sex-specific changes in mechanisms timing puberty maximize the chance that reproductive activity will ultimately be successful in each sex.

Integration of chemosensory and hormonal input in the male Syrian hamster brain.

Mating in the male Syrian hamster requires the interaction of chemosensory and hormonal stimuli. Chemosensory cues from the vomeronasal organ and olfactory mucosa are transmitted through limbic nuclei that contain receptors for gonadal steroid hormones, including the medial amygdaloid nucleus (Me) and medial preoptic area (MPOA). This pathway is essential for mating, as lesions that interrupt transmission of chemosensory cues to MPOA will abolish copulation. Likewise, gonadal steroids facilitate sexual behavior through Me and MPOA, as demonstrated using intracranial implants in the brains of castrate males. In addition, odor and hormonal signals must be integrated in the brain for copulation to occur. Mating is prevented when olfactory bulbectomy is performed ipsilateral to an intracranial testosterone implant, thereby preventing the interaction of odors and hormones. According to our current model, hormones may act as a gating signal to strengthen synaptic contacts along the chemosensory pathway, thereby permitting or enhancing transmission of chemosensory cues.

Thinking about networks in the control of male hamster sexual behavior.

Motivated social behaviors such as mating are controlled by a complex network of limbic nuclei. Concepts of network organization derived from computational neuroscience may aid our understanding of the links between the neuroanatomical circuitry and what is represented by the anatomy. Research in my laboratory uses mating behavior in the male Syrian hamster as a model to elucidate how chemosensory and steroid cues are integrated in the brain. An interaction of odors and hormones is required for mating in this species. These two essential stimuli are transmitted through separate parallel pathways in the limbic system. The functional organization of the hamster mating behavior circuit is characterized by distributed representation, divergent and convergent neural pathways, and recurrent feedback. Odors and hormones have different modes of action on this neural network. While chemosensory cues stimulate the input units of the network, steroids facilitate behavior through the hidden units. In this manner, steroids appear to create a permissive environment for subsequent activation by odor cues.

Integration of chemosensory and hormonal cues is essential for sexual behaviour in the male Syrian hamster: role of the medial amygdaloid nucleus.

Mating behaviour in the male hamster requires chemosensory and hormonal cues, and copulation is abolished if either signal is interrupted. In addition, the integration of chemosensory stimuli with steroid signals is essential for mating. In castrated male hamsters, implantation ofa testosterone-filled cannula in the preoptic area stimulates mating behaviour. However, removal of the ipsilateral olfactory bulb prevents steroid facilitation of sexual activity. The present studies determined if the integration of chemosensory and hormonal cues necessary for mating behaviour is distributed within steroid-sensitive nuclei in the brain, or is restricted to the preoptic area. Specifically, the hypothesis was tested that the medial amygdala is capable of odour and hormone integration. Castrated male hamsters received an intracerebral implant of testosterone in the medial amygdala combined with removal of a single olfactory bulb, ipsilateral or contralateral to the implant. Mating behaviour did not increase after implant surgery and bulbectomy in either ipsilateral or contralateral bulbectomized males. In a second study, males were bulbectomized three weeks after implant surgery, to demonstrate the ability of testosterone in the medial amygdala to stimulate male sexual behaviour, and the loss of behaviour following bulbectomy. The results confirm that integration of odour and steroid cues is essential for mating in the male hamster. Moreover, the medial amygdaloid nucleus contributes to chemosensory and hormonal integration. However, compared with steroid stimulation in the preoptic area, the behavioural effects of testosterone in the medial amygdaloid nucleus are more sensitive to manipulations of the olfactory system, suggesting that the amygdala requires bilateral chemosensory input.