Rhythms of glutamic acid decarboxylase mRNA in the suprachiasmatic nucleus.

The purpose of the present study was to determine whether there is a rhythm in glutamic acid decarboxylase (GAD) message in the suprachiasmatic nucleus (SCN) of rats housed in a light:dark cycle. The mRNAs encoding two isoforms of GAD (i.e., GAD65 and GAD67) were examined using in situ hybridization histochemistry. Computerized image analysis of film autoradiographs revealed that GAD65 mRNA was significantly higher in the light than it was in the dark. GAD67 mRNA levels were lower overall and did not decrease significantly in the dark. Following emulsion autoradiography, silver grain counts over individual SCN cells indicated that GAD65 mRNA was highest in the dorsomedial hypothalamus during the light. These data suggest that GAD mRNA varies rhythmically in the SCN and that mRNA levels are regulated differently within SCN subdivisions during the light:dark cycle.

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)

Vasopressin and sex differences in hamster flank marking.

Vasopressin (AVP) within the medial preoptic-anterior hypothalamic continuum (MPOA-AH) plays an essential role in the control of flank marking in Syrian hamsters. Sex differences are found in the scent marking of many mammalian species, including hamsters. The first two experiments tested the hypothesis that sex differences in flank marking are the result of sex differences in the availability of AVP for release in several CNS sites. No support for this hypothesis was provided because neither immunohistochemical analysis nor radioimmunoassay of tissue punches revealed sex differences in AVP immunoreactivity in the MPOA-AH or other sites likely to be involved in flank marking. The third experiment, which tested the hypothesis that sex differences in flank marking are the result of sex differences in the sensitivity or response of the MPOA-AH to AVP, found no sex differences in the amount of flank marking stimulated by microinjection of AVP in the MPOA-AH. These data provide no support for the hypothesis that sex differences in vasopressinergic activity are responsible for sex differences in flank marking.

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.

Microinjection of arginine-vasopressin into the periaqueductal gray stimulates flank marking in Syrian hamsters (Mesocricetus auratus).

Syrian hamsters can communicate using a distinctive form of scent marking called flank marking. Vasopressin-sensitive neurons within the medial preoptic-anterior hypothalamic continuum (MPOA-AH) play a critical role in the control of this form of olfactory communication. Extrahypothalamic regions may also mediate hamster flank marking. Since the MPOA-AH and the periaqueductal gray (PAG) are reciprocally connected, the present study investigated whether PAG neurons are involved in the control of flank marking. The first study found that microinjection of vasopressin, but not oxytocin or saline, into the PAG induced high levels of flank marking in male (n = 8) and female (n = 5) hamsters (P less than 0.01). The second study demonstrated that microinjection of vasopressin into the PAG stimulated flank marking in a dose-dependent manner in both male (n = 7) and female (n = 11) hamsters (P less than 0.01). These data suggest that vasopressin-responsive neurons within the periaqueductal gray participate in the control of hamster flank marking.

Norepinephrine inhibits vasopressin-stimulated flank marking in the Syrian hamster by acting within the medial preoptic-anterior hypothalamus.

Syrian hamsters exhibit a form of scent marking behavior called flank marking. Flank marking, which is elicited during social contact with other hamsters and by the odors of other hamsters, communicates socially important information such as mate choice and dominance status. Vasopressinergic activity within the medial preoptic-anterior hypothalamic continuum (MPOA-AH) is essential for the expression of flank marking. In female hamsters, an inverse relationship exists between the expression of flank marking and sexual receptivity during the 4-day estrous cycle. Since norepinephrine (NE) appears to facilitate sexual receptivity, the present study investigated whether NE might have an inhibitory effect on flank marking by acting on Vasopressinergic activity within the MPOA-AH. Microinjection of 9.0 µM arginine vasopressin (AVP) into the MPOA-AH stimulated high levels of flank marking. Microinjection of 9.0 µM AVP combined with NE in concentrations of 4.0, 0.4 or 0.2 nM, drastically reduced or eliminated flank marking. In contrast, AVP in combination with 0.09, 0.04 or 0.004 nM NE produced no significant reductions in flank marking. In addition, microinjection of 9.0 µM AVP, in combination with epinephrine (4.0 nM), but not dopamine (4.0 nM), serotonin (4.0 nM) or neuropeptide Y (900 µM), significantly reduced AVP-induced flank marking. In male hamsters, microinjection of NE (4.0 nM) combined with AVP (9.0 µM) into the MPOA-AH was not found to inhibit AVP-stimulated flank marking. These results suggest that NE is involved in regulating the expression of flank marking during the estrous cycle by acting on Vasopressinergic activity within the MPOA-AH.

Connections between the pontine central gray and the ventromedial hypothalamus are essential for lordosis in female rats.

Lesions and knife cuts were used to study central gray (CG) and ventromedial hypothalamic (VMH) mediation of sexual receptivity in female rats. Lesions of the midbrain-pontine CG eliminated lordosis in female rats. Bilateral sagittal knife cuts that bracketed the rostral pontine CG also eliminated lordosis, and an experiment with the retrograde tracer Fluoro-Gold confirmed the effectiveness of these cuts in severing the lateral connections linking the VMH and the CG. Finally, females with a unilateral hypothalamic cut combined with a contralateral CG transection almost never showed lordosis. Each cut, at a different level for each side of the brain, transected axons linking the VMH and the CG. The demonstration that this combination eliminated lordosis provides new evidence that the lateral connections between the VMH and the CG are essential for the display of sexual receptivity in female rats.

Preoptic lesions increase the display of lordosis by male rats.

Male rats do not normally show feminine patterns of sexual behavior even when injected with the ovarian hormones estrogen and progesterone. We find that brain lesions which damage the preoptic-anterior hypothalamic continuum augment the display of lordosis in hormone-treated male rats. The most effectively feminizing brain lesions are ones which bilaterally destroy a substantial portion of the medial preoptic area encompassing the sexually dimorphic nucleus of the preoptic area (SDN-POA). Males with particularly large preoptic lesions are receptive following estrogen treatment and show a progesterone facilitation of receptivity. In this respect, they cannot be behaviorally distinguished from females. Thus, axons originating in and/or passing through the preoptic area apparently inhibit the display of feminine sexual behaviors in males. Preoptic development and lordosis are each predictably affected by perinatal stimulation by testicular hormones, and hormone-stimulated preoptic development may form the neurological basis for some of the defeminizing effects of perinatal hormonal exposure. Our results raise the possibility that the site of this behavioral defeminization is the SDN-POA.