Conversion of testosterone to estradiol may not be necessary for the expression of mating behavior in male Syrian hamsters (Mesocricetus auratus).

Male sexual behavior is mediated in part by androgens, but in several species, mating is also influenced by estradiol formed locally in the brain by the aromatization of testosterone. The role of testosterone aromatization in the copulatory behavior of male Syrian hamsters is unclear because prior studies are equivocal. Therefore, the present study tested whether blocking the conversion of testosterone to estradiol would inhibit male hamster sexual behavior. Chronic systemic administration of the nonsteroidal aromatase inhibitor Fadrozole (2.0 mg/kg/day) for 5 or 8 weeks did not significantly increase mount latency or reduce mount frequency, intromission frequency, ejaculation frequency, or anogenital investigation relative to levels shown by surgical controls. However, Fadrozole effectively inhibited aromatase activity, as evidenced by the suppression of estrogen-dependent progesterone receptor immunoreactivity in the male hamster brain. The JZB39 anti-progesterone receptor antibody labeled significantly more neurons in brains of sham-treated hamsters than in brains of Fadrozole-treated hamsters. These data suggest that aromatization of testosterone to estradiol is not necessary for normal mating behavior in Syrian hamsters.

GABAergic regulation of light-induced c-Fos immunoreactivity within the suprachiasmatic nucleus.

Analysis of the photic induction of c-Fos immunoreactivity (-ir) within the suprachiasmatic nucleus (SCN) has proven to be a powerful tool with which to study the neurochemical mechanisms involved in phase shifting the circadian clock. Some systemically administered GABAergic drugs inhibit light-induced phase shifts and c-Fos-ir, whereas others inhibit light-induced phase shifts without affecting c-Fos-ir. More recently, we have found that injection of GABAergic drugs directly into the SCN region can have dramatically different effects on light-induced phase shifts than following their systemic administration. The present study investigated the effects of GABA(A) and GABA(B) agonists and antagonists injected into the SCN region on c-Fos-ir within the SCN. Microinjection of either a GABA(A) agonist, muscimol, or a GABA(B) agonist, baclofen, into the SCN region significantly reduced light-induced c-Fos-ir within the SCN when administered before light exposure at circadian time (CT) 13.5 or CT 19. In contrast, microinjection of a GABA(A) antagonist, bicuculline, but not a GABA(B) antagonist, CGP-35348, into the SCN region increased light-induced c-Fos-ir within the SCN when administered before light exposure at CT 13.5 or CT 19. These data indicate that GABAergic agonists and antagonists injected directly into the SCN region alter light-induced Fos-ir in a manner similar to their effects on light-induced phase shifts. Comparison of these data with previous studies examining the effects of systemically administered GABAergic drugs suggests that GABA(B)-active drugs have similar effects whether given systemically or within the SCN, but that GABA(A)-active drugs have more complex effects on c-fos induction and have multiple sites of action.

GABA(A) and GABA(B) agonists and antagonists alter the phase-shifting effects of light when microinjected into the suprachiasmatic region.

GABAergic drugs have profound effects on the regulation of circadian rhythms. The present study evaluated the effects of microinjections of GABAergic drugs into the suprachiasmatic region in hamsters on phase shifts induced by light and by microinjection of a cocktail containing vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI) and gastrin-releasing peptide (GRP). The phase-advancing effects of light at circadian time (CT) 19 were significantly reduced by microinjection of GABA(A) or GABA(B) agonists into the SCN, but were not altered by microinjection of GABA(A) or GABA(B) antagonists. Microinjection of a GABA(B) agonist also reduced the phase-delaying effects of light at CT 13.5-14 while a GABA(B) antagonist increased the phase delays caused by light. Neither GABA(B) drug altered the phase delays produced by microinjection of a peptide cocktail containing VIP, PHI, GRP. These data indicate that changes in GABA(A) or GABA(B) activity within the SCN can alter the phase-shifting effects of light on circadian rhythms and support a role for GABA in gating photic input to the circadian clock.

Bicuculline increases and muscimol reduces the phase-delaying effects of light and VIP/PHI/GRP in the suprachiasmatic region.

The present study investigated the effects of gamma-amino butyric acid (GABA)A-active drugs on the ability of light or coadministration of vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI), and gastrin-releasing peptide (GRP) to phase delay hamster activity rhythms. Microinjection of the GABAA agonist, muscimol, significantly (p < .01) reduced the phase-delaying effect of light administered at circadian time (CT) 13.5. By contrast, microinjection of the GABAA antagonist, bicuculline, significantly (p < .01) increased the phase-delaying effect of light administered at CT 13.5. Microinjection of muscimol or bicuculline into the suprachiasmatic nucleus (SCN) produced little or no effect on circadian phase when no light pulses were provided. Coadministration of muscimol with VIP/PHI/GRP significantly (p < .01) reduced the phase-delaying effect of VIP/PHI/GRP, whereas administration of bicuculline with VIP/PHI/GRP significantly (p < .05) increased the phase-delaying effect of these peptides. These data indicate that changes in GABAA activity within the SCN can modulate the phase-delaying effects of light and VIP/PHI/GRP during the early portion of subjective night.

Neural circuitry controlling vasopressin-stimulated scent marking in Syrian hamsters (Mesocricetus auratus).

In Syrian hamsters, arginine vasopressin (AVP) plays a critical role in the control of a form of scent marking called flank marking. Microinjection of AVP into the medial preoptic-anterior hypothalamus (MPOA-AH), lateral septal nucleus (LS), bed nucleus of the stria terminalis (BNST), and the periaqueductal gray (PAG) stimulates high levels of flank marking. Microinjection of an antagonist of the V1a-AVP receptor into sites such as the MPOA-AH inhibits expression of flank marking. The purpose of the present study was to investigate the neural circuit controlling flank marking by localizing the induction of Fos protein in response to the microinjection of AVP, a V1a-AVP antagonist (AVPA) or saline into the MPOA-AH. Immediately after microinjection, hamsters were placed in a clean cage and their behavior was videotaped for 10 minutes. Ninety minutes after the behavioral experiment hamsters were perfused and their brains removed for subsequent immunocytochemical localization of Fos protein. The number of Fos-positive neurons was significantly greater in the BNST, PAG, and central amygdala (Ce) following the microinjection of AVP than following the microinjection of either AVPA or saline. In AVP-injected animals, the number of Fos-labeled cells in the Ce, PVN, and PAG increased with increased frequency of either flank marking or flank gland grooming. These data support the hypothesis that neurons within the MPOA-AH, BNST, and PAG play an important role in the control of flank marking and suggest that the Ce may be a previously unrecognized part of this neural circuit.

Ovarian hormones alter the behavioral response of the medial preoptic anterior hypothalamus to arginine-vasopressin.

In Syrian hamsters (Mesocricetus auratus) arginine-vasopressin (AVP) within the medial preoptic-anterior hypothalamus (MPOA-AH) plays a critical role in the control of a hormone-dependent behavior called flank marking. The present study investigated whether ovarian hormones influence flank marking by altering the response of the MPOA-AH to AVP. The amount of flank marking stimulated by microinjection of AVP (9 microM in 200 nl saline) into the MPOA-AH varied significantly over the 4 days of the estrous cycle with the lowest levels of flank marking observed on estrus. A second experiment demonstrated that administration of progesterone significantly reduced AVP-stimulated flank marking in estradiol-treated ovariectomized hamsters. These data support the hypothesis that the changing levels of estradiol and progesterone during the estrous cycle influence flank marking by altering the sensitivity or response of the MPOA-AH to AVP.

Analysis of the phase shifting effects of gastrin releasing peptide when microinjected into the suprachiasmatic region.

The role of gastrin releasing peptide (GRP) in phase shifting circadian rhythms was investigated. Microinjection of GRP into the suprachiasmatic region (SCN) region produced only small changes in the phase of free-running circadian activity rhythms in hamsters housed in constant darkness. However, co-administration of GRP with vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) produced significantly larger phase delays than GRP alone. These data support the hypothesis that GRP interacts with VIP and PHI within the SCN to induce phase shifts of circadian rhythms.

Role of oxytocin in the hypothalamic regulation of sexual receptivity in hamsters.

Oxytocin (OXT) has been implicated in the control of a variety of social and reproductive behaviors in several species. The purpose of the present study was to test the hypothesis that OXT activity within the medial preoptic-anterior hypothalamus (MPOA-AH) and the ventromedial hypothalamus (VMH) plays a critical role in the expression of sexual receptivity in Syrian hamsters. The first 2 experiments investigated whether OXT would stimulate sexual receptivity in female hamsters in a dose-dependent manner. A 3rd experiment investigated whether sexual receptivity would be inhibited when endogenous OXT activity was blocked. Microinjection of OXT into the MPOA-AH or the VMH induced sexual receptivity in a dose-dependent manner in ovariectomized (OVX) hamsters primed with estradiol. Microinjection of a selective OXT antagonist, d(CH2)5[Tyr(Me)2Thr4,Tyr-NH29] ornithine vasotocin into the MPOA-AH or the VMH significantly reduced the levels of sexual receptivity exhibited by OVX hamsters administered estradiol and progesterone. These findings support the hypothesis that OXT activity in the MPOA-AH and the VMH plays an important role in the regulation of sexual receptivity in hamsters.

Neuropeptide Y receptor distribution and regulation in the suprachiasmatic nucleus of the Syrian hamster (Mesocricetus auratus).

The suprachiasmatic nucleus (SCN) receives a direct photic projection from the retina, the retinohypothalamic tract (RHT), and an indirect photic projection from the intergeniculate leaflet of the thalamus, the geniculohypothalamic tract (GHT). The primary neurochemical signal in the GHT appears to be neuropeptide Y (NPY), and several lines of evidence indicate that NPY may be involved in determining the response of the SCN to light. The purpose of the present study was (i) to localize NPY binding sites in the hamster SCN and to compare the distribution of these binding sites with the terminal field of the RHT and (ii) to determine if SCN levels of NPY binding change during the day. RHT fibers, defined using the carbocyanine dye DiI, were localized primarily within the ventrolateral region of the SCN. The distribution of NPY receptors, as visualized by 125I-peptide YY (PYY) binding, overlapped the projection field of the RHT. Specific binding of 125I-PYY was significantly greater in the ventrolateral SCN than in the medial SCN. To determine whether NPY binding changes during the day, the levels of 125I-PYY in the SCN were determined 4 h before the onset of darkness, 1 h before the onset of darkness and 2 h after the onset of darkness in hamsters housed in a 14:10 light-dark cycle. The levels of binding at 4 and 1 h before dark onset were significantly lower than 2 h after the onset of darkness. In contrast, no significant differences were observed in 125I-PYY binding over these same sampling intervals in either the medial preoptic area or the lateral hypothalamus.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatostatin mRNA levels within the periventricular nucleus are regulated by estradiol and insulin-like growth factor-I in immature female rats.

Adolescent growth is regulated by developmental increases in growth hormone (GH) secretion. Although the hypothalamic release of GH-releasing hormone (GHRH) stimulates and the release of SRIF inhibits GH secretion, it is not known how these regulatory mechanisms change developmentally. In addition, GH secretion is facilitated during maturation by increases in peripheral estradiol and may be inhibited, via a negative feedback mechanism, by insulin-like growth factor-I (IGF-I). It is not clear whether these act through the hypothalamic regulation of GHRH and somatostatin (SRIF). In order to further understand the regulation of GH secretion during development, the present study determined how estradiol and IGF-I altered SRIF mRNA in the hypothalamus in immature female rats. The working hypotheses were that estradiol would decrease SRIF mRNA accounting, in part, for its positive effect on GH release and IGF-I would increase SRIF mRNA representing a negative feedback mechanism regulating GH secretion. Preprosomatostatin (ppSRIF) mRNA levels within the periventricular nucleus (PeVN) were measured with in situ hybridization in ovariectomized female rat pups (n = 5 per group). Infusions were delivered sc via either a Silastic capsule (oil, 10 or 60 micrograms/ml estradiol) or an osmotic minipump (acetic acid, 120 or 240 micrograms IGF-I/day). Following ovariectomy on Day 21, animals were treated for either 1 or 7 days beginning on Day 24 of age. A total of 18 treatment groups were evaluated, including control, estradiol alone, IGF-I alone, and estradiol and IGF-I combined at both doses and treatment durations.(ABSTRACT TRUNCATED AT 250 WORDS)

The control of circadian rhythms and the levels of vasoactive intestinal peptide mRNA in the suprachiasmatic nucleus are altered in spontaneously hypertensive rats.

Vasoactive intestinal peptide (VIP) has been localized within the suprachiasmatic nucleus of the hypothalamus (SCN) and appears to play an important role in the entrainment of circadian rhythms with the light-dark (LD) cycle. The spontaneously hypertensive rat (SHR), an inbred strain used extensively in research on primary hypertension, has significantly more VIP mRNA in its brain than normotensive Wistar-Kyoto control (WKY) rats. Because VIP levels are abnormally high in SHR rats the present study examined whether the mechanisms controlling circadian rhythms are also altered in SHR rats. When entrained to a 24 h LD cycle, SHR rats began their wheel-running rhythm approximately 1.5 h earlier than WKY controls. SHR rats re-entrained to a phase delay in the LD cycle more slowly than did WKY rats, but tended to re-entrain to a phase advance more rapidly. The free-running period of SHR rats in both constant light and constant dark was significantly shorter than that of WKY rats. In SHR rats, phase delays produced by 1-h pulses of light were less than one-half the magnitude of the delays seen in WKY rats; however, the phase advances were nearly twice that of WKY rats. Using in situ hybridization, the SCN levels of mRNA encoding VIP were found to be significantly greater in SHR rats, but the mRNA levels of another peptide important for entrainment, gastrin releasing peptide, did not differ between SHR and WKY rats. These data indicate that the mechanisms controlling circadian rhythms in SHR rats differ significantly from those controlling rhythms in WKY rats and that VIP mRNA is significantly elevated within the SCN of SHR rats. The role of VIP in the entrainment of circadian rhythms is discussed.

Estradiol increases the behavioral response to arginine vasopressin (AVP) in the medial preoptic-anterior hypothalamus.

Flank marking, a form of hamster scent marking controlled by arginine vasopressin (AVP) in the medial preoptic-anterior hypothalamus (MPOA-AH), is altered by circulating levels of gonadal hormones. We hypothesized that gonadal hormones influence flank marking either by altering the availability of AVP for release in the MPOA-AH or by altering the sensitivity or responsiveness of MPOA-AH neurons to AVP. We examined the levels of AVP immunoreactivity (AVP-IR) over the estrous cycle in the MPOA-AH and other areas. Arginine vasopressin immunoreactivity did not vary in the areas examined as a function of the stage of the estrous cycle. In Experiment 2 we measured flank marking after MPOA-AH microinjection of AVP in ovariectomized hamsters receiving estradiol or empty Silastic capsules. Hamsters implanted with estradiol capsules marked significantly more in response to AVP than did hamsters receiving no hormone replacement. These results support the hypothesis that estradiol influences flank marking by altering the sensitivity or responsiveness of the MPOA-AH or its efferents to AVP. Additionally, we observed an unexpected effect of AVP in estradiol-treated hamsters. After microinjection with 90 microM AVP, lordosis occurred spontaneously in 60% of the hamsters even though no male was present. We suggest that female hamsters may be a useful model to further investigate the role of AVP and AVP-like peptides in female sexual behavior.

Sex differences in odor-stimulated flank marking in the golden hamster (Mesocricetus auratus).

To determine whether sex differences exist in the frequency of odor-stimulated flank marking, intact male and female hamsters were exposed to the recently vacated home cages of male stimulus hamsters for a 10-min test on 4 consecutive days. Females were found to mark at significantly higher levels than males. To investigate the role of gonadal hormones in the sex differences in flank marking, gonadectomized male and female hamsters were implanted with Silastic capsules containing estradiol or testosterone. Females exhibited twofold higher levels of odor-stimulated flank marking than males, and the amount of flank marking was significantly higher when the hamsters were administered testosterone than when they were administered estradiol. These data demonstrate that sex differences exist in the frequency of flank marking stimulated by the odors of male hamsters, and that these sex differences do not appear to result from the typical sex-specific patterns of circulating levels of estradiol and testosterone.

Disruption of human circadian and cognitive regulation following a discrete hypothalamic lesion: a case study.

We report a patient with rostral hypothalamic damage that significantly disrupted temporal patterning of the sleep-wake cycle, body temperature, and cognitive and behavioral functioning. The findings suggest that the suprachiasmatic region of the hypothalamus is important for the circadian control of human behavior, and that circadian organization may be essential for normal cognitive functioning.

Interaction of colocalized neuropeptides: functional significance in the circadian timing system.

The suprachiasmatic nucleus (SCN), which appears to act as a circadian clock, contains a subpopulation of local circuit neurons in which vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI), and gastrin releasing peptide (GRP) are colocalized. To determine whether VIP, PHI, and GRP interact within the SCN to produce a signal important for circadian control, the behavioral and cellular effects of coadministration of these neuropeptides were investigated. Coadministration of VIP, PHI, and GRP within the SCN mimicked the phase-delaying effects of light on circadian control following in vivo microinjection and activated SCN single units recorded in vitro. These behavioral and cellular effects of coadministration of VIP, PHI, and GRP were significantly greater than administration of VIP, PHI, or GRP alone or coadministration of any 2 of these peptides. These data illustrate a new mechanism whereby multiple, colocalized neuropeptides interact in a functionally significant manner, and indicate that the interaction of VIP, PHI, and GRP may be involved in the regulation of circadian rhythms by the SCN.

Day-night variation in prepro vasoactive intestinal peptide/peptide histidine isoleucine mRNA within the rat suprachiasmatic nucleus.

Neurons within the suprachiasmatic nuclei of the hypothalamus (SCN) appear to function as a circadian clock that controls the timing of many physiological systems. The SCN contain several chemically distinct neuronal subpopulations, including a large group of interneurons within the ventrolateral SCN that exhibit co-localizable immunoreactivity for both vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI). The purpose of the present study was to determine whether VIP/PHI neurons within the rat SCN exhibit rhythmicity in the cellular levels of the messenger RNA encoding the precursor from which both VIP and PHI are derived. Using both quantitative in situ and solution hybridization prepro-VIP/PHI mRNA levels early in the dark phase were demonstrated to be significantly higher than those 5 h after the onset of the daily light period. Since no statistically reliable (P greater than 0.05) day-night variation was observed in the levels of prepro-VIP/PHI mRNA within cortex, these data suggest that the rhythmicity in prepro-VIP/PHI mRNA is an intrinsic property of VIP/PHI-containing SCN neurons, or rhythmically driven by local synaptic events within the SCN.

Coordination of hamster lordosis and flank marking behavior: role of arginine vasopressin within the medial preoptic-anterior hypothalamus.

The role of arginine vasopressin (AVP) within the medial preoptic area-anterior hypothalamus (MPOA-AH) in the neural coordination of lordosis and flank marking was investigated. AVP, but not saline, microinjected into the MPOA-AH of ovariectomized hamsters not given hormone replacement therapy stimulated high levels of flank marking when tested with a sexually experienced male, or when tested alone. In contrast, AVP microinjected into the MPOA-AH of ovariectomized hamsters given estradiol benzoate (EB) and progesterone did not stimulate flank marking or inhibit lordosis during tests with a sexually experienced male. However these same females exhibited high levels of flank marking in response to AVP when tested alone. A second experiment demonstrated that progesterone was not required for inhibition of AVP-induced flank marking in ovariectomized females given EB replacement and tested with sexually experienced males. The present study provides no evidence that AVP acts within the MPOA-AH to inhibit hamster lordosis, but demonstrates that ovarian hormones and male social contact block the induction of flank marking by AVP microinjected into the MPOA-AH. These data suggest that one component in the neural coordination of lordosis and flank marking is inhibition of the response of the MPOA-AH to AVP.

Ovarian hormones influence odor stimulated flank marking behavior in the hamster (Mesocricetus auratus).

The effect of ovarian hormones on the frequency of female flank marking in response to the odors of male hamsters was examined. The amount of flank marking differed significantly (p less than 0.01) among the days of the estrous cycle, with the lowest levels of flank marking occurring on estrus. Ovariectomy (OVX) eliminated the 4 day rhythm, and significantly (p less than 0.01) reduced the total amount of flank marking. Implantation of Silastic capsules containing estradiol benzoate (EB) in OVX hamsters increased flank marking by approximately two-fold when compared to hamsters implanted with blank Silastic capsules. In a second series of experiments, OVX hamsters given EB flank marked significantly (p less than 0.01) more than those with blank capsules. Subcutaneous injection of progesterone (P) into OVX-EB treated hamsters 6 hr prior to testing significantly reduced flank marking when compared to OVX-EB treated hamsters given oil. No differences were observed in flank marking between groups of OVX hamsters with blank capsules and injected with P or oil. In summary, these data indicate that the four day rhythm of odor stimulated flank marking in intact female hamsters may be the result of the 4 day fluctuations of estrogen and progesterone that occur during the estrous cycle.