Ventromedial Hypothalamus Of Female Rats Research Articles

Glutamate release in the ventromedial hypothalamus of the female rat during copulation: Modulation by estradiol

Binding of glutamate or its ionotropic receptor agonists in the ventromedial hypothalamus (VMH) of female rats inhibits both appetitive and consummatory aspects of sexual behavior. Because vaginocervical stimulation activates glutamate neurons in the VMH, and administration of estradiol benzoate (EB) and progesterone (P) delays this effect, the present study examined the effects of hormonal priming on glutamate release within the VMH of female rats paired with sexually vigorous males. Ovariectomized, sexually experienced rats were implanted with guide cannula aimed at the ventrolateral VMH, through which microdialysis probes were inserted prior to testing. Females were assigned randomly to one of three hormone treatment conditions: EB+P, EB alone, or the oil vehicle. Testing was conducted over 5h, including a 120-min period of habituation to the testing chamber, a 60-min period of baseline sample collection, and a 120-min period during which a sexually vigorous male was introduced into the testing chamber. Dialysates were collected every 20min during the test and were analyzed for glutamate using HPLC. Females primed with oil had large and significant increases in glutamate release from baseline once the male was introduced to the chamber. Treatment with EB alone decreased glutamate release in response to male cues. Although treatment with EB+P did not differ significantly from EB alone, the degree of reduced glutamate release was less than with EB alone. These results indicate that priming with EB reduces glutamate transmission in the VMH in response to male cues. Taken together with our previous findings, estradiol blunts the activation of glutamate neurons in the VMH thus allowing female rats to copulate.

GnRH-immunoreactive fiber changes with unilateral amygdala-kindled seizures

Purpose: To assess whether unilateral amygdala seizures are associated with a change in the number and lateral distribution of gonadotropin releasing hormone (GnRH)-staining fibers in the ventromedial hypothalamus of female rats. Methods: The study compared three groups of female rats: (1) amygdala seizures induced by focal injection of kainic acid (KA); (2) saline injected controls; and (3) naı̈ve controls. The animals were sacrificed at 4 weeks in the diestrus phase. GnRH fibers were counted in the ventromedial hypothalamus and compared among groups. Results: GnRH fiber counts were significantly lower in KA than saline and naı̈ve animals ipsilaterally but not contralaterally. Conclusions: This finding may support a potential mechanism by which (1) temporolimbic epilepsy may promote the development of reproductive endocrine disorders and (2) the laterality of the epilepsy may influence the particular nature of the reproductive endocrine disorder.

Molecular analysis of estrogen induction of preproenkephalin gene expression and its modulation by thyroid hormones

Estrogen receptors (ER) and thyroid hormone receptors (TR) are ligand-dependent nuclear transcription factors. Estrogen-induced preproenkephalin (PPE) gene expression in the hypothalamus is directly related to estrogen-induced lordosis behavior in the rat. In the present study, we showed that the PPE mRNA level in the ventromedial hypothalamus of female rats was significantly decreased by ovariectomy. This decrease was reversed by estrogen replacement in a dose- and time-dependent manner. Using transient transfection and electrophoretic mobility shift assays (EMSA), functional estrogen response elements (ERE) were identified between −437 and −145 base pairs (bp) of the rat PPE gene promoter region. Two ERE-like elements are present between −405 and −364 of the rat PPE gene promoter, which bind ERα as demonstrated by EMSA. Estrogen produced a dose-dependent increase in CAT activity in cotransfection assays with ERα expression vector and a 437PPE-CAT reporter construct containing 437 bp of the rat PPE gene promoter and the CAT reporter gene. This estrogen-induced PPE promoter activity was inhibited by liganded-TR in transient cotransfection assays. Analysis of DNA–protein interactions by EMSA revealed that both ERα and TR (α1 and β1) could bind to the EREs in the rat PPE gene promoter. Furthermore, estrogen induction of PPE mRNA in the ventromedial hypothalamus of the ovariectomized female rat was significantly attenuated by concomitant administration of triiodothyronine. These results suggest that estrogen regulation of the hypothalamic PPE gene expression is mediated through an estrogen-receptor complex directly interacting with the functional EREs in its promoter region; and that this estrogen effect can be modified by thyroid hormones.

Anesthesia during hormone administration abolishes the estrogen induction of preproenkephalin mRNA in ventromedial hypothalamus of female rats

Estrogen treatment increases preproenkephalin (PPE) mRNA levels in the ventromedial nucleus of the hypothalamus (VMH). Roy et al. (Brain Res., 337 (1985) 163-166) discovered that anesthesia during estrogen priming could reduce female rat sexual receptivity. In the present study we tested whether the action of estrogen to induce PPE gene expression in the VMH could be similarly affected by anesthesia. By quantitative in situ hybridization and slot-blot analysis techniques we found a 1.8-fold increase in PPE mRNA levels in the VMH after 1 hour of estrogen treatment in ovariectomized (OVX) Sprague-Dawley female rats. Anesthetizing the rats with pentobarbital for 1 h during the exposure to estrogen blocked the estrogen induction of PPE mRNA in the VMH. By way of contrast no changes in the PPE mRNA levels were observed in the caudate putamen. A similar trend was seen using chloral hydrate. It appears that neuronal activity is required for the early phase of estrogen induction of PPE mRNA levels in the VMH. This in turn could be correlated with changes in female sociosexual behaviors.

Sex differences in and effects of estrogen on oxytocin receptor messenger ribonucleic acid expression in the ventromedial hypothalamus

In situ hybridization techniques were used in the present study to detect hypothalamic expression of the oxytocin receptor (OR) gene. Binding studies have localized OR to various brain regions and have detected a high density of receptors in the ventromedial hypothalamus (VMH), a documented target of estrogen action. This study was designed to compare levels of OR messenger RNA (mRNA) in the VMH of male and female rats and to study the effects of estrogen treatment on mRNA levels in the VMH of female rats. After cloning a rat OR gene from a genomic testes library, a probe was generated for use in in situ hybridization assays to evaluate sex differences in OR mRNA expression in the VMH. In addition, ovariectomized females were treated with estrogen, and VMH OR mRNA expression was compared with that in ovariectomized or intact females. The results of these studies showed that male rats expressed higher levels of OR mRNA in the VMH than females. Estrogen-treated ovariectomized females exhibited significantly greater expression in the VMH than either oil-treated or intact females. These results support binding studies that have shown oxytocin binding in the VMH to be regulated by gonadal steroids and suggest that estrogen may either directly or indirectly regulate transcription of the OR gene.

Steroid priming promotes oxytocin-induced norepinephrine release in the ventromedial hypothalamus of female rats

In vivo microdialysis was used to detect norepinephrine (NE) release in the ventromedial hypothalamus of estradiol (E 2)- or E 2 plus progesterone (P)-treated female rats injected with 1.0 IU of oxytocin (OXY). Dialysates were collected before and after OXY administration on 3 consecutive days and analyzed for NE content by high performance liquid chromatography with electrochemical detection. After the last sample was collected on day 1, animals were injected with 3 μg E 2 benzoate or oil. On day 3, E 2-primed animals received 200 μg of P and control females received oil prior to OXY administration. OXY administration did not induce NE release on day 1. When OXY was administered to animals that received E 2 approximately 20 h earlier, increased release of NE was not consistently seen. In contrast, E 2-primed animals that received P on day 3 displayed significant increases in the release of NE after OXY administration compared to their own basal levels and to NE levels in control animals. To distinguish whether E 2 priming is sufficient to promote OXY-induced release of NE without the addition of P, NE content of VMH dialysates in a second group of animals was examined following exposure to vehicle or E 2 alone. When OXY was administered 24 or 48 h after estrogen priming, only 1 of 4 E 2-primed females had modestly elevated dialysate NE levels. To evaluate the interactions between OXY and NE in the regulation of reproductive behavior, lordosis responses were observed in hormone-primed female rats receiving systemic injections of OXY, the α 1-adrenoceptor antagonist prazosin, or both OXY and prazosin. OXY enhanced lordosis behavior in females primed with subthreshold doses of E 2 and P. Prazosin abolished lordosis behavior in rats primed with behaviorally effective doses of E 2 and P and significantly inhibited lordosis in steroid-primed females given OXY. These data suggest that after priming with both E 2 and P together, but not with E 2 alone, OXY may facilitate lordosis behavior through activation of NE transmission.

Oxytocin effects on lordosis frequency and lordosis duration following infusion into the medial pre-optic area and ventromedial hypothalamus of female rats

When infused into the medial preoptic area of ovariectomized female rats treated with estradiol benzoate (3 μg) and progesteron (150 μg), oxytocin was found to increase the lordosis frequency and the lordosis duration. Oxytocin administered into the ventromedial hypothalamus increased the lordosis duration but not the lordosis frequency. In the mesencephalic central grey, oxytocin did not significantly affect lordosis. Oxytocin in the ventromedial hypothalamus facilitated lordosis duration at doses which were ineffective in the medial preoptic area. This contrasts with the pattern of results observed for lordosis frequency. These results lend support to the hypothesis that the lordosis-facilitating actions of oxytocin in the ventromedial hypothalamus are due to a permissive effect of progesterone and that the ventromedial hypothalamus and medial pre-optic area may control different aspects of lordosis.

Hormonal Activation of Female Sexual Behavior is Accompanied by Hypothalamic Norepinephrine Release

Abstract The present study employed the intracranial microdialysis technique to measure norepinephrine release in the ventrolateral dendritic fields of the ventromedial hypothalamus of freely-moving animals before and during ovarian steroid (estradiol and progesterone) activation of female sexual behavior (lordosis). One day after implantation of a dialysis probe, animals were injected with 3 mug of estradiol benzoate followed 44 h later by 200 mug of progesterone. Introduction of a male rat 4 h after progesterone treatment was correlated with dramatic increases in extracellular norepinephrine levels measured in dialysates of the ventrolateral ventromedial hypothalamus of female rats which displayed high levels of lordosis behavior. In contrast, female rats given the same steroid treatment but which did not show lordosis responses did not have elevated norepinephrine levels in their dialysates. Moreover, animals that received an estrogen antagonist concurrently with the estrogen treatment had neither an increase in ventromedial hypothalamic levels of norepinephrine during behavior testing nor did they display lordosis. These results indicate a close relationship among ovarian steroids, noradrenergic transmission in the ventromedial hypothalamus, and the expression of female sexual behavior.

Localized Behavioral Effects of Tritiated Estradiol Implants in the Ventromedial Hypothalamus of Female Rats*

In an attempt to localize estrogen-sensitive brain sites sufficient to prime feminine sexual behavior, 30-gauge cannulae containing approximately 9 ng high specific activity [3H]-estradiol were implanted unilaterally or bilaterally into the ventromedial region of the hypothalamus of ovariectomized rats. Tests for lordosis behavior were conducted on days 3 and 6 postimplantation, 4–6 h after progesterone treatment. Brain, pituitary, and uterine tissue were then sampled for radioactivity. Cannulae delivered, on the average, 3 ng [3H]estradiol during 8 days. Radioactivity was localized almost exclusively to the hypothalamus or hemihypothalamus in which the implant was placed. This was true regardless of whether tissue radioactivity or radioactivity in cell nuclei was analyzed. In the bilateral implant experiments, average hypothalamic [3H]estradiol radioactivity amounted to 56 pg/rat in tissue and 220 fg in purified cell nuclei. The latter is 4% of the estimated estrogen eceptor capacity of cell nuclei of the hypothalamus. In the unilateral implant experiments, 10 of 24 rats responded at least once; the mean lordosis quotients were 38 for the first test (number responding, 7) and 61 for the second test (number responding, 8). Of 6 rats which received blank cannulae, none exhibited lordosis behavior after progesterone treatment. In the bilateral implant experiments, 6 of 11 rats responded at least once with a mean lordosis quotient for responders of 60 on the first test (n = 4) and 52 on the second test (n = 6). The results indicate that estrogenic stimulation of a subset of hypothalamic cells, representing a small percentage of the estradiol-concentrating capacity of the hypothalamus, can prime behavioral responsiveness.

The ventromedial hypothalamus and aggressive behaviors

Drug injections into the ventromedial hypothalamus of female rats which in previous experiments had been shown to facilitate avoidance behavior and disinhibit punished operant responding did not modify aggressive behaviors in several situations. This contrasts with previous observations that lesions in this area affect aggressive as well as avoidance and punished behaviors.