Transition from precopulatory to copulatory behaviour in male rats with lesions in medial preoptic area: dependence on precopulatory pattern of female

In the present study, both the precopulatory behaviour and the copulatory readiness of male rats following the bilateral medial preoptic area lesions was compared with their intact states. In behavioural testing, the intensity of female precopulatory behaviour was used as an experimental variable. The results showed that the natural threshold of copulatory readiness of males was increased in the lesioned state, the animals were more dependent on the soliciting patterns of the female. However, all the males exhibited conspicuous precopulatory behaviour towards the stimulus females used. Apparently, further brain structures participate in the regulation of sexual behaviour of males, above all, in activation or maintenance of precopulatory activity.     

Independent control of sexual and scent marking behaviors of male gerbils by cells in or near the medial preoptic area

This research studied the role of the medial preoptic area and adjacent cell populations in androgen control of scent marking and sexual behavior in male gerbils (Meriones unguiculatus). Experiment 1 replicated previous research showing that implants of testosterone propionate in or near the medial preoptic area reinstate marking behavior in castrates. Implant sites near the diagonal band of Broca or in the posterior part of the medial preoptic area, near the anterior hypothalamus, are more effective than other sites. Experiment 2 showed that medial preoptic area lesions permanently impair sexual behavior despite testosterone stimulation. Experiments 2–4 showed that lesions in or near the medial preoptic area can also disrupt scent marking; however, this behavior gradually recovered in many lesioned males, especially if they received testosterone. The data suggest that both scent marking and sexual behavior are controlled by androgens acting on cells in or near the medial preoptic area, but the cell populations involved in these two behaviors are probably not the same.     

Dissociated functional pathways for appetitive and consummatory reproductive behaviors in male Syrian hamsters

In many species, including Syrian hamsters, the generation of male reproductive behavior depends critically on the perception of female odor cues from conspecifics in the environment. The behavioral response to these odors is mediated by a network of steroid-sensitive ventral forebrain nuclei including the medial amygdala (MA), posterior bed nucleus of the stria terminalis (BNST) and medial preoptic area (MPOA). Previous studies have demonstrated that each of these three nuclei is required for appropriate sexual behavior and that MA preferentially sends female odor information directly to BNST and MPOA. It is unknown, however, how the functional connections between MA and BNST and/or MPOA are organized to generate different aspects of reproductive behavior. Therefore, the following experiments used the asymmetrical pathway lesion technique to test the role of the functional connections between MA and BNST and/or MPOA in odor preference and copulatory behaviors. Lesions that functionally disconnected MA from MPOA eliminated copulatory behavior but did not affect odor preference. In contrast, lesions that functionally disconnected MA from BNST eliminated preference for volatile female odors but did not affect preference for directly contacted odors or copulatory behavior. These results therefore demonstrate a double dissociation in the functional connections required for attraction to volatile sexual odors and copulation and, more broadly, suggest that appetitive and consummatory reproductive behaviors are mediated by distinct neural pathways.     

Afferent and efferent connections of the sexually dimorphic medial preoptic nucleus of the male quail revealed by in vitro transport of DiI

The medial preoptic nucleus of the Japanese quail is a testosterone-sensitive structure that is involved in the control of male copulatory behavior. The full understanding of the role played by this nucleus in the control of reproduction requires the identification of its afferent and efferent connections. In order to identify neural circuits involved in the control of the medial preoptic nucleus, we used the lipophilic fluorescent tracer DiI implanted in aldheyde-fixed tissue. Different strategies of brain dissection and different implantation sites were used to establish and confirm afferent and efferent connections of the nucleus. Anterograde projections reached the tuberal hypothalamus, the area ventralis of Tsai, and the substantia grisea centralis. Dense networks of fluorescent fibers were also seen in several hypothalamic nuclei, such as the anterior medialis hypothalami, the paraventricularis magnocellularis, and the ventromedialis hypothalami. A major projection in the dorsal direction was also observed from the medial preoptic nucleus toward the nucleus septalis lateralis and medialis. Afferents to the nucleus were seen from all these regions. Implantation of DiI into the substantia grisea centralis also revealed massive bidirectional connections with a large number of more caudal mesencephalic and pontine structures. The substantia grisea centralis therefore appears to be an important center connecting anterior levels of the brain to brain-stem nuclei that may be involved in the control of male copulatory behavior.     

Effects of the nonsteroidal inhibitor R76713 on testosterone-induced sexual behavior in the Japanese quail (Coturnix coturnix japonica)

A new triazole derivative, R76713 (6-[4-chlorophenyl)(1H-1,2,4-triazol-1-yl)methyl]-1-methyl-1H- benzotriazole), was recently shown to inhibit aromatase selectively without affecting other steroid-metabolizing enzymes and without interacting with estrogen, progestin, or androgen receptors. This compound was tested for its capacity to intefere with the induction of copulatory behavior by testosterone (T) in castrated Japanese quail (Coturnix coturnix japonica). In a first experiment, R76713 inhibited (range 0.01 to 1 mg/kg) the activation of sexual behavior by T silastic implants and hypothalamic aromatase activity in castrated male quail in a dose-dependent manner. The 5 alpha- and 5 beta- reductases of T were not systematically affected. Stereotaxic implantation of R76713 in the medial preoptic area similarly blocked the behavior activated by systemic treatment with T, demonstrating that central aromatization of androgen is implicated in the activation of behavior. These inhibiting effects of R76713 on behavior were observed when implants were placed in the medial part of the nucleus preopticus medialis, confirming the implication of this brain area in the control of male copulatory behavior. Finally, the behavioral inhibition produced by R76713 could be reversed by simultaneous treatment with a dose of estradiol, which was not behaviorally effective by itself. This suggests that the behavioral deficit induced by the inhibitor was specifically due to the suppression of estrogen production. This also shows that the activation of copulatory behavior probably results from the interaction of androgens and estrogens at the brain level, as the two treatments separately providing these hormonal stimuli (T with the aromatase inhibitor on one hand and a low dose of estradiol on the other hand) had almost no behavioral effects but they synergized to activate copulation when given concurrently. These data confirm the critical role of preoptic aromatase in the activation of reproductive behavior and demonstrate that R76713 is a useful tool for the in vivo study of estrogen-dependent processes.     

Testosterone and sexual experience alter levels of plasma membrane binding sites for progesterone in the male rat brain.

Physiological levels of progesterone act in conjunction with androgens to facilitate copulatory behavior in male rats, mice, and lizards. Radiolabeled progesterone conjugated to bovine serum albumin measured specific binding sites in membrane fractions from male rats that were gonadectomized and testosterone treated, or remained gonadally intact, to determine the role of gonadal steroids on mPR binding. To determine whether behavioral experience could alter binding levels, males either remained sexually na?ve or became sexually experienced. In sexually na?ve males, the highest levels of specific binding occurred in the dorsal portions of the medial preoptic area, with only moderate levels of binding in ventral portions of the medial preoptic area and the dorsal and ventral medial hypothalamus. However, conjugated progesterone binding in these brain regions did not change as a function of testosterone or behavioral manipulations. In contrast, the amygdala responded to behavioral experience with significantly (4-fold) increased binding in gonadectomized, T-treated males with sexual experience. These data indicate that the neuronal plasticity for membrane-associated progesterone binding is regionally specific, being regulated by sexual experience following the reinstatement of testosterone levels, thus suggesting a functional role for plasma membrane activity of progesterone in male rat reproduction.     

Sex differences and effects of neonatal aromatase inhibition on masculine and feminine copulatory potentials in prairie voles

Copulatory behaviors in most rodents are highly sexually dimorphic, even when circulating hormones are equated between the sexes. Prairie voles (Microtus ochrogaster) are monomorphic in their display of some social behaviors, including partner preferences and parenting, but differences between the sexes in their masculine and feminine copulatory behavior potentials have not been studied in detail. Furthermore, the role of neonatal aromatization of testosterone to estradiol on the development of prairie vole sexual behavior potentials or their brain is unknown. To address these issues, prairie vole pups were injected daily for the first week after birth with 0.5 mg of the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD) or oil. Masculine and feminine copulatory behaviors in response to testosterone or estradiol were later examined in both sexes. Males and females showed high mounting and thrusting in response to testosterone, but only males reliably showed ejaculatory behavior. Conversely, males never showed feminine copulatory behaviors in response to estradiol. Sex differences in these behaviors were not affected by neonatal ATD, but ATD-treated females received fewer mounts and thrusts than controls, possibly indicating reduced attractiveness to males. In other groups of subjects, neonatal ATD demasculinized males' tyrosine hydroxylase expression in the anteroventral periventricular preoptic area, and estrogen receptor alpha expression in the medial preoptic area. Thus, although sexual behavior in both sexes of prairie voles is highly masculinized, aromatase during neonatal life is necessary only for females' femininity. Furthermore, copulatory behavior potentials and at least some aspects of brain development in male prairie voles are dissociable by their requirement for neonatal aromatase.     

Gonadal steroid receptors colocalize with central nervous system neurons projecting to the rat prostate gland

Mating-induced Fos-immunoreactive (-ir) cells are colocalized with androgen receptors (AR), estrogen receptors (ER), or both in limbic and hypothalamic areas known to mediate male rat mating behavior. A steroid-responsive neural network might govern copulatory behavior in male laboratory rats that is analogous to the network described in female rats that governs the lordosis response. This hypothesized network in males may synchronize and coordinate sexual behavioral responses with physiological responses of the genitals and the internal organs of reproduction. Therefore, the pseudorabies virus (PRV; Bartha strain), a transneuronal, viral retrograde tract tracer, was microinjected into the prostate gland to label this network. After 7 days, brains from infected animals were processed for immunohistochemical labeling of AR, ER, and PRV. The majority of PRV-ir cells exhibited either AR or ER immunoreactivity in the medial preoptic area, median preoptic nucleus, bed nucleus of stria terminalis, hypothalamic paraventricular nucleus, and zona incerta, areas known to play roles in male rat mating behavior. Other structures such as the central tegmental field/subparafascicular nucleus of the thalamus, central nucleus of the amygdala, and medial amygdala, also important in the display of male copulatory behavior, were less reliably labeled. Collectively, a steroid receptor-containing neuronal circuit, largely contained in the diencephalon, was revealed that likely is involved in the autonomic control of the prostate gland and the consummatory aspects of male rat mating behavior.     

Role of the medial preoptic area in sexual behaviour of the male rat: a study using repeated cycloheximide infusions

The effect of cycloheximide (CHX), an inhibitor of proteosynthesis, on sexual behaviour was studied in adult male rats in which it was infused into the medial preoptic area of the hypothalamus (MPOA). Sexual interaction took place under control and modification of the precopulatory behaviour of the female. Among the various bilaterally infused amounts of CHX--20 micrograms in 1 microliter, 40 micrograms in 1 microliter, 80 micrograms in 2 microliters--the effect of the largest dose corresponded to a hypothetical state of copulatory readiness of the males; 2 h after administration the males were not capable of initiating copulatory behaviour with a passively receptive female, but did so successfully with a highly soliciting female. The dependence of copulatory readiness of the males on the precopulatory behaviour of the females was confirmed repeatedly after this dose of CHX. At the same time, the precopulatory activity of the males towards a passively receptive female was unimpaired. The effect of CHX was reversible; 48 h after infusion the males displayed high copulatory readiness. Only half the males (n = 7) given bilateral infusions of 80 micrograms CHX fulfilled criterion of copulatory performance. Histological control demonstrated that the MPOA was affected bilaterally by CHX infusion in 10 males out of 14. The results are discussed from the aspect of participation of the MPOA in the regulation of male sexual behaviour. The method allows changes in sexual behaviour to be studied in the same individual in a chronic experiment.     

Aromatase activity in the rat brain: Hormonal regulation and sex differences

The intracellular conversion of testosterone to estradiol by the aromatase enzyme complex is an important step in many of the central actions of testosterone. In rats, estrogen given alone, or in combination with dihydrotestosterone, mimics most of the behavioral effects of testosterone, whereas treatment with antiestrogens or aromatase inhibitors block facilitation of copulatory behavior by testosterone. We used a highly sensitive in vitro radiometric assay to analyze the distribution and regulation of brain aromatase activity. Studies using micropunch dissections revealed that the highest levels of aromatase activity are found in an interconnected group of sexually dimorphic nuclei which constitutes a neural circuit important in the control of male sexual behavior. Androgen regulated aromatase activity in many diencephalic nucleic, including the medial preoptic nucleus, but not in the medial and cortical nuclei of the amygdala. Additional genetic evidence for both androgen-dependent and -independent control of brain AA was obtained by studies of androgen-insensitive testicular-feminized rats. These observations suggest that critical differences in enzyme responsiveness are present in different brain areas. Within several nuclei, sex differences in aromatase induction correlated with differences in nuclear androgen receptor concentrations suggesting that neural responsiveness to testosterone is sexually differentiated. Estradiol and dihydrotestosterone acted synergistically to regulate aromatase activity in the preoptic area. In addition, time-course studies showed that estrogen treatment increased the duration of nuclear androgen receptor occupation in the preoptic area of male rats treated with dihydrotestosterone. These results suggest possible ways that estrogens and androgens may interact at the cellular level to regulate neural function and behavior.     

Estradiol Mediates Effects of Testosterone on Vasotocin Immunoreactivity in the Adult Quail Brain

In adult male quail, the activation of sexual behavior by testosterone (T) is mediated at the cellular level by the interaction of T metabolites with intracellular steroid receptors. In particular, the aromatization of T into an estrogen plays a key limiting role. Nonaromatizable androgens such 5alpha-dihydrotestosterone (DHT) synergize with estradiol (E2) to activate the behavior. Given that the density of vasotocin (VT) immunoreactive structures is increased by T in adult male quail and that VT injections affect male behavior, we wondered whether the expression of VT is also affected by T metabolites such as E2 and DHT. We analyzed here, in castrated male quail, the effects of a treatment with T, E2, DHT, or E2 + DHT on sexual behavior and brain VT immunoreactivity. The restoration by T of the VT immunoreactivity in the medial preoptic nucleus, bed nucleus striae terminalis, and lateral septum of castrated male quail could be fully mimicked by a treatment with E2. The androgen DHT had absolutely no effect on the VT immunoreactivity in these conditions and, at the doses used here, DHT did not synergize with E2 to enhance the density of VT immunoreactive structures. These effects of T metabolites in the brain were not fully correlated with their effects on the activation of male copulatory behavior, suggesting that the increase in VT expression in the brain does not represent a necessary step for the activation of behavior. Although VT expression in the medial preoptic nucleus and bed nucleus striae terminalis is often tightly correlated with the expression of male copulatory behavior, VT presumably does not represent simply one step in the biochemical cascade of events that is induced by T in the brain and leads to the expression of male sexual behavior.     

Selective contributions of the medial preoptic nucleus to testosterone-dependant regulation of the paraventricular nucleus of the hypothalamus and the HPA axis

Previous data have consistently demonstrated an inhibitory effect of androgens on stress-induced hypothalamic-pituitary-adrenal (HPA) responses. Several brain regions may influence androgen-mediated inhibition of the HPA axis, including the medial preoptic area. To test the role of the medial preoptic nucleus (MPN) specifically, we examined in high- and low-testosterone-replaced gonadectomized rats bearing discrete bilateral lesions of the MPN basal and stress-induced indexes of HPA function, and the relative levels of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) mRNA in the amygdala. High testosterone replacement decreased plasma adrenocorticotropin hormone (ACTH) and paraventricular nucleus (PVN) Fos responses to restraint exposure in sham- but not in MPN-lesioned animals. AVP-, but not CRH-immunoreactivity staining in the external zone of the median eminence was increased by testosterone in sham animals, and MPN lesions blocked this increment in AVP. A similar interaction between MPN lesions and testosterone occurred on AVP mRNA levels in the medial nucleus of the amygdala. These findings support an involvement of MPN projections in mediating the AVP response to testosterone in both the medial parvocellular PVN and medial amygdala. We conclude that the MPN forms part of an integral circuit that mediates the central effects of gonadal status on neuroendocrine and central stress responses.     

Central neural pathways for angiotensin-induced thirst.

Evidence is reviewed implicating the preoptic region in angiotensin-induced thirst. The most responsive area according to results obtained with behavioral, electrophysiological, and autoradiographic mapping techniques is at the caudal border of the medial preoptic region and rostral border of the anterior hypothalamus. The neural pathway from this preoptic site for angiotensin-induced thirst extends along the medial forebrain bundle through the midlateral hypothalamus to the paramedial midbrain tegmentum and to an area ventrolateral to the central gray. Lesions of this pathway in the midlateral hypothalamus and rostral midbrain significantly attenuated drinking induced by microinjections of angiotensin II into the preoptic area but did not disrupt water intake induced by microinjections of angiotensin II into the subfornical organ or cerebral ventricles. Although the efferent pathways from angiotensin-receptive sites in the subfornical organ and cerebral ventricles are unknown, it appears from these observations that the medial forebrain bundle is not involved. Lesions of the medial forebrain bundle-lateral hypothalamus also do not disrupt drinking induced by microinjections of hypertonic saline into the preoptic region although lesions placed 1 mm further lateral do. Since fat lateral hypothalamic lesions are without effect on drinking induced by centrally administered angiotensin II, this suggests that intracellular and extracellular thirst signals are subserved by separate neural pathways in the hypothalamus.     

Mating-induced expression of c-fos in the male Syrian hamster brain: Role of experience,pheromones, and ejaculations

This study was designed to investigate the effects of pheromonal cues and specific behaviors within the male copulatory sequence on c-fos expression in the medial nucleus of the amygdala (Me), the bed nucleus of the stria terminalis (BNST), and the medial preoptic area (MPOA) of the Syrian hamster brain. Sexually experienced male hamsters were placed into clean testing arenas and were either: 1) left alone as handled controls; 2) exposed to female hamster vaginal secretion (FHVS) on cotton swabs; or mated to various end points of copulation with a sexually receptive female: 3) five intromissions, 4) one ejaculation, 5) five ejaculations, or 6) long intromissions. A seventh group of sexually naive control males 7) was left alone in the arena. The brains of these males were compared to those of the sexually experienced controls to determine whether exposure to cues associated with prior sexual experience could alter c-fos expression. In males exposed only to FHVS, Fos immunoreactivity (Fos-ir) increased within the posterodorsal Me, the anterodorsal part of the posteromedial BNST, and the magnocellular medial preoptic nucleus (MPNmag). Following one ejaculation, Fos-ir increased within the caudal posterodorsal Me, the dorsolateral MPOA, and the paraventricular nucleus of the hypothalamus. After multiple ejaculations, additional labeling was observed within the posteroventral part of the posteromedial BNST, the medial preoptic nucleus (MPN), the central tegmental field, and in cell clusters of the caudal posterodorsal Me and rostral posteromedial BNST. Fos-ir also increased within the posterodorsal Me, MPN, and MPNmag in sexually experienced control males exposed to the empty test chamber compared to sexually naive males exposed to an identical chamber. These results demonstrate that the mating-induced pattern of neuronal activation in sexually experienced males is dependent upon multiple factors, including prior sexual experience in the testing environment, investigation of FHVS, and the number of ejaculations achieved. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 481–501, 1997     

Organizing effects of sex steroids on brain aromatase activity in quail

Preoptic/hypothalamic aromatase activity (AA) is sexually differentiated in birds and mammals but the mechanisms controlling this sex difference remain unclear. We determined here (1) brain sites where AA is sexually differentiated and (2) whether this sex difference results from organizing effects of estrogens during ontogeny or activating effects of testosterone in adulthood. In the first experiment we measured AA in brain regions micropunched in adult male and female Japanese quail utilizing the novel strategy of basing the microdissections on the distribution of aromatase-immunoreactive cells. The largest sex difference was found in the medial bed nucleus of the stria terminalis (mBST) followed by the medial preoptic nucleus (POM) and the tuberal hypothalamic region. A second experiment tested the effect of embryonic treatments known to sex-reverse male copulatory behavior (i.e., estradiol benzoate [EB] or the aromatase inhibitor, Vorozole) on brain AA in gonadectomized adult males and females chronically treated as adults with testosterone. Embryonic EB demasculinized male copulatory behavior, while vorozole blocked demasculinization of behavior in females as previously demonstrated in birds. Interestingly, these treatments did not affect a measure of appetitive sexual behavior. In parallel, embryonic vorozole increased, while EB decreased AA in pooled POM and mBST, but the same effect was observed in both sexes. Together, these data indicate that the early action of estrogens demasculinizes AA. However, this organizational action of estrogens on AA does not explain the behavioral sex difference in copulatory behavior since AA is similar in testosterone-treated males and females that were or were not exposed to embryonic treatments with estrogens.     

ACTH fragments in compensation mechanisms of self-stimulation after ablation of the septal region in rabbits

Unilateral lesions of septal region, which partially included commissure anterior, commissural nuclei, fornix and apical parts of medial and lateral preoptic area, inhibited hypothalamic self-stimulation in rabbits. Intraventricular injection of ACTH (4-10) and ACTH (1-24) evoked the restoration of self-stimulation behaviour.     

Sterility and delayed vaginal opening following electrolytic lesions placed neonatally in medial preoptic area in female rats.

Electrolytic lesions were placed in the preoptic area or in the middle and posterior hypothalamus of female rats within 48 hrs after birth. Vaginal opening was delayed significantly in rats having the medial preoptic area (mPOA) destructed as compared to sham-operated animals and those given lesions in the other parts of the hypothalamus. Only rats with the mPOA completely destructed developed the anovulatory syndrome. In these animals, unilateral ovariectomy was not followed by enlargement of its fellow. The present results are not in agreement with those of previous workers that electrolytic destructions of the diencephalon induced precocious puberty. The difference may be ascribable to difference in size and locus of lesions.     

Noradrenergic afferents and receptors in the medial preoptic area: neuroanatomical and neurochemical links between the regulation of sleep and body temperature

Several studies have shown the importance of the medial preoptic area in the regulation of sleep-wakefulness and of body temperature. The medial preoptic area has a rich noradrenergic innervation, coming mostly from the lateral tegmental noradrenergic system. The accumulating evidences show that the noradrenergic afferents to the medial preoptic area are involved in the induction of sleep. This hypnogenic mechanism operates through the postsynaptic alpha1 and alpha2-adrenergic receptors. Noradrenergic afferents are also involved in the thermoregulatory mechanisms, and the activation of these fibers brings about a fall in body temperature. Though the body temperature changes are brought about by the same receptor subtypes as those involved in hypnogenesis, observations suggest the possibility of separate sets of noradrenergic afferents in the medial preoptic area for sleep regulation and thermoregulation. In this review, we present the compelling evidences, which showed that the noradrenergic afferents of the medial preoptic area bring about a fall in body temperature and other thermoregulatory behavioral alterations associated with sleep.     

Projection from the ventral bed nucleus of the stria terminalis to the retrorubral field in rats and the effects of cells in these areas on mating in male rats versus gerbils

This research identified the rat counterpart of the lateral cell group of the sexually dimorphic area (SDA) found in medial preoptic area (MPOA) gerbil of gerbils. The lateral SDA (lSDA) is critical for mating in male gerbils and contains most of the SDA cells projecting to the retrorubral field (RRF), a projection that is also important for mating. Therefore, to locate the counterpart of the lateral SDA, we traced the inputs to the rat RRF, which were dense in the ventral part of the bed nucleus of the stria terminalis (BST). To determine if the ventral BST or its projection to the RRF affects mating in male rats, we disrupted them bilaterally by placing cell-body lesions bilaterally in the ventral BST or unilaterally there and in the contralateral RRF. We also studied the effects of RRF lesions in both rats and gerbils. Bilateral ventral BST lesions, which left the medial preoptic nucleus intact, produced persistent and severe mating deficits. Disconnecting the ventral BST from the RRF also had long-lasting, but less severe, consequences. RRF lesions produced only temporary mating deficits in rats, but virtually eliminated mating in gerbils. The recovery of mating in rats after RRF, but not ventral BST, lesions, and the intermediate effects of disconnecting these areas from each other suggest that the ventral BST may contain mating-related projection neurons other than those projecting to the RRF or that its RRF-projecting cells send collaterals to another site. In either case, the pedunculopontine tegmental nucleus or raphe nuclei may be involved.     

Microinjections of growth hormone-releasing factor into the medial preoptic area/suprachiasmatic nucleus region of the hypothalamus stimulate food intake in rats

The role of the suprachiasmatic nucleus/medial preoptic area region of the hypothalamus in the expression of rat hypothalamic growth hormone-releasing factor-induced feeding in the rat was examined. Rats were tested for their 90-min food intake following microinjections of growth hormone-releasing factor (0.0, 0.01, 0.1 or 1.0 pmol) aimed at the suprachiasmatic nucleus/medial preoptic area region. It was found that growth hormone-releasing factor dose-dependently stimulated food intake with the 1.0 pmol dose being the most effective, increasing food intake by approximately 200%. Injections outside the suprachiasmatic nucleus/medial preoptic area region were ineffective. These data are taken to suggest that the suprachiasmatic nucleus/medial preoptic area region of the hypothalamus is important for the central stimulatory effects of growth hormone-releasing factor on feeding.