Progestins' effects on sexual behaviour of female rats and hamsters involving D1 and GABA(A) receptors in the ventral tegmental area may be G-protein-dependent

In the ventral tegmental area (VTA), progestins have actions involving dopamine type 1-like receptors (D(1)) and gamma-aminobutyric acid (GABA)(A)/benzodiazepine receptor complexes (GBRs) for lordosis. Evidence suggests that D(1) and GBRs can have G-protein-mediated effects. We investigated if, in the VTA, inhibiting G-proteins prevents D(1)- and/or GBR-mediated increases in progestin-facilitated lordosis. Hamsters, with bilateral guide cannulae to the VTA, received systemic E(2) (10 microg) at hour 0 and progesterone (P, 250 microg) at hour 45. At hour 48, hamsters were pre-tested for lordosis and infused with the G-protein inhibitor, guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S, 50 microM/side), or 10% DMSO saline vehicle. Thirty minutes after initial infusions, hamsters were re-tested and then immediately infused with the D(1) agonist, SKF38393 (100 ng/side), the GBR agonist, muscimol (100 ng/side), or saline vehicle. Hamsters were post-tested for lordosis 30 min later. For rats, E(2) (10 microg) priming at hour 0 was followed by lordosis pre-testing at hour 44. After pre-testing, rats received infusions of GDP-beta-S or vehicle, followed by infusions of SKF38393, muscimol, or vehicle and then infusions of the neurosteroid, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP, 100 or 200 ng/side), or beta-cyclodextrin vehicle. Rats were tested immediately after each infusion of SKF38393, muscimol or vehicle, as well as 10 and 60 min after 3alpha,5alpha-THP or vehicle infusions. Inhibiting G-proteins, in the VTA, reduced the ability of systemic P or intra-VTA SKF38393 or muscimol to facilitate lordosis of E(2)-primed hamsters. Blocking G-proteins, in the VTA, prevented SKF38393-, muscimol- and/or 3alpha,5alpha-THP-mediated increases in lordosis of E(2)-primed rats. Thus, progestins' actions in the VTA for lordosis that involve D(1) and/or GBRs may also include recruitment of G-proteins.     

GABA(A), D1, and D5, but not progestin receptor, antagonist and anti-sense oligonucleotide infusions to the ventral tegmental area of cycling rats and hamsters attenuate lordosis.

In hamsters, progesterone (P) in the hypothalamus and ventral tegmental area (VTA) is necessary for receptivity; in rats, hypothalamic P induces receptivity and midbrain P further enhances it. How P exerts its effects in the VTA on lordosis is of interest because few estrogen-induced P receptors (PRs) have been identified there. Sexual receptivity of rats and hamsters is enhanced when P's actions in the VTA are restricted to the membrane and when the gamma-aminobutyric acid (GABA)A agonist, muscimol, is infused into the VTA, but attenuated with infusions of the GABA(A) antagonist, bicuculline. The dopamine (DA) agonist. SKF38393, rapidly enhances receptivity when infused intravenously; this effect can be blocked by both DA receptor (DR) and PR antagonists. This study investigated the importance of PRs, glutamic acid decarboxylase (GAD), the enzyme responsible for GABA production, GABA(A) receptors (GBRs), and DRs in the VTA of cycling rats and hamsters for the expression of lordosis. Proestrous and diestrous animals implanted with bilateral VTA cannulae were pre-tested for receptivity, infused with either an antagonist (RU38486 (20 microg), bicuculline (100 ng), SCH23390 (100 ng)), anti-sense oligonucleotide (against PR (250 ng), GAD (500 ng), D1 (500 ng), D5 (250 ng)), or control infusions to each cannulae and re-tested. Vehicle and scrambled oligonucleotides were infused as controls and elicited similar effects. Antagonists of GBRs and DRs significantly reduced lordosis on post-tests compared to the PR antagonist and control conditions in rats and hamsters. Lordosis was significantly reduced, compared to controls, only by anti-sense oligonucleotides for GAD and D1- and D5-DR subtypes. These data suggest that in the VTA GABAergic and dopaminergic neurons may be more important in the mediation of sexual receptivity than neurons containing intracellular PRs.     

Gamma aminobutyric acid mediates ventromedial hypothalamic mechanisms controlling the execution of lordotic responses in the female rat

To study the participation of gamma-aminobutyric acid (GABA) in the control of sexual behavior in the female rat, the effect of GABA and picrotoxin injections in the ventromedial hypothalamic nucleus (VMN) upon lordosis frequency and multiunit spike activity (MUSA) was determined. Infusion of 100 micrograms GABA into conscious rats reduced lordotic responsiveness within 15 min after injection; with a similar time course, the same dose markedly reduced MUSA in urethane anesthetized rats. Forty-five min after injection lordotic responsiveness recuperated to preinjection levels; at this time MUSA showed a rebound increase in neuron firing frequency. The possible relation between ventromedial hypothalamic neuronal activity and capacity for lordotic responses was further tested with injections of a local anesthetic: 1 microliter of 2% Xylocaine infused into the VMN produced similar results, suppressing MUSA and lordotic responsiveness for ca. 45 min beginning immediately after injection. Microinjections of GABA antagonist picrotoxin had the opposite effects: 0.1 microgram increased MUSA and lordotic responsiveness at 5 and 45 min; however at 20 min, when MUSA was at its highest, lordosis frequency was not elevated. Injections of solvent had no consistent effects on either measure. Two conclusions many be tentatively drawn from these data: (a) the VMN is the origin of a neural signal which exerts a moment-to-moment gating control on the execution of lordosis, and (b) the generation and/or the output of this signal is under the control of a GABAergic hypothalamic mechanism which normally exerts an inhibitory effect on the display of lordotic responses.     

Interaction between glutamate and GABA on H-noradrenaline release from rat hypothalamus

Glutamate has been shown to stimulate noradrenaline (NA) release from hypothalamic nerve terminals. In the present study, we evaluated the possible interaction between the excitatory amino acid glutamate and gamma-aminobutyric acid (GABA), an inhibitory transmitter, on noradrenaline (NA) release from mediobasal hypothalamus (MBH) of adult male rats. Hypothalamic slices loaded in vitro with ³H-NA were superfused and exposed to glutamate, N-methyl- -aspartic acid (NMDA), or kainate (KA). We found that ³H-NA release evoked by the excitatory amino acids glutamate and NMDA was dramatically decreased by GABA. The facilitatory effects of NMDA and KA were prevented concentration-dependently by the GABA_B receptor antagonist 2-hydroxy saclofen which restored the NMDA effect. In addition, beclofen blocked K⁺-induced ³H-NA release. Activation of GABA_A receptors by muscimol and THIP was ineffective. In conclusion, glutamate and GABA, through GABA_B receptors, may interact to modulate NA release from the rat mediobasal hypothalamus.     

In vivo evidence for progesterone dependent decreases in serotonin release in the hypothalamus and midbrain central grey: relation to the induction of lordosis

The effects of progesterone (P) on serotonin (5HT) overflow in the ventromedial hypothalamus (VMH), preoptic area (POA) and midbrain central grey (MCG) were studied using in vivo microdialysis. Ovariectomized rats, pretreated with 5 μg estradiol, were anesthetized with chloral hydrate and stereotaxically implanted with dialysis probes directed towards one of the respective brain sites. Extracellular 5HT levels stabilized 3 to 5 h following probe implantation. Under stable baseline conditions, perfusion of 1 μM tetrodotoxin through the dialysis probe resulted in a 60–65% reduction in 5HT overflow in the brain areas studied. In experiments testing the effect of P on 5HT overflow, rats were subcutaneously injected with 0.5 mg P or propylene glycol vehicle. Samples were analyzed for 5HT at 20 min intervals for 4 h after treatment. Perfusate levels of 5HT were not significantly changed in the VMH, POA or MCG in vehicle-treated rats. Similarly, P treatment failed to significantly alter 5HT overflow in the POA. In the VMH, perfusate levels of 5HT were significantly reduced 60 min after P treatment. Decreases in perfusate 5HT levels were detected 20 min after P in the MCG. The decreases in 5HT overflow measured in the VMH and MCG following P treatment persisted for the remainder of the sampling period with the exception of 1 time point in the VMH. The results provide in vivo evidence for P-influenced decreases in 5HT release in the VMH and MCG. The rapid decrease in extracellular 5HT in the MCG suggests that this effect may represent a non-genomic action of P. These results are discussed in relation to the role of 5HT in the regulation of lordosis behavior.     

Selective enhancement of shock avoidance responding elicited by GABA blockade in the posterior hypothalamus of rats

Recent studies have shown that blockade of gamma-aminobutyric acid (GABA) in the posterior hypothalamus in anesthetized rats elicits cardio-respiratory stimulation similar to that seen in emotional defense reactions and, in conscious rats, locomotor arousal suggesting a flight response. The present study was conducted in order to test the hypothesis that the behavioral effects elicited by GABA blockade in the posterior hypothalamus were the results of disinhibiting a mechanism whose activation selectively enhances reactivity to aversive stimuli. Male rats were trained on a Sidman shock avoidance schedule (RS20:SS10) as well as a food-reinforced approach schedule (VI 1). Under anesthesia, guide cannulae were stereotaxically implanted bilaterally in the posterior hypothalamus at sites where microinjection of the GABA antagonist, bicuculline methiodide (BMI) 25 ng, increased heart rate. After recovery, rats were tested in both the avoidance and VI 1 schedules after hypothalamic microinjection of saline, BMI 25 ng, and the GABA agonist, muscimol 25 ng. Microinjection of BMI significantly increased the avoidance responses but had no effect on the approach responses. Muscimol decreased both the avoidance and approach responses. When microinjected into the lateral hypothalamic area, BMI had no effect on the response rates in either schedule while muscimol decreased the approach responding only. Therefore, GABA blockade at the discrete area of the posterior hypothalamic nucleus appears to elicit a selective enhancement of avoidance responses. These results suggest that an endogenous GABAergic system in the posterior hypothalamus may tonically inhibit a constellation of autonomic, locomotor and motivational responses that are necessary for some types of defense reaction.     

Leptin inhibits norepinephrine efflux from the hypothalamus in vitro: role of gamma aminobutyric acid

Leptin, a hormone secreted by adipocytes, produces a number of central and neuroendocrine effects, the mechanisms behind which are not completely understood. Hypothalamic norepinephrine (NE) is involved in many of the neuroendocrine effects that are associated with leptin. Therefore, we hypothesized that leptin could affect hypothalamic NE activity to bring about its central and neuroendocrine effects. Because gamma aminobutyric acid (GABA) is known to affect the release of NE, we also tested the possibility that leptin-induced changes in NE could be mediated through GABA. The mediobasal hypothalami from adult male rats were incubated in an in vitro incubation system for four consecutive incubation periods of 60 min each at 37 degrees C in Krebs Ringers Henseleit (KRH) solution in an atmosphere of 95% O(2) and 5% CO(2.) After determining the basal release, the hypothalami were challenged with 0, 0.1, 1 or 10 nm of leptin, bicuculline (a GABA-A receptor antagonist; 10 microM) and bicuculline (10 microM) +10 nM of leptin during the second incubation period. Residual effects of leptin were measured in the third incubation where tissues were incubated with KRH alone, and the viability of tissues was determined in the fourth incubation when tissues were exposed to high K(+) KRH. NE levels in the incubation medium were measured using high-performance liquid chromatography with electrochemical detection (HPLC-EC). Leptin inhibited NE efflux from the hypothalamus in a dose-dependent manner. Moreover, incubation of hypothalami with 10 nM of leptin and bicuculline, a completely blocked the leptin-induced decrease in NE efflux. These results demonstrate for the first time that leptin could act directly on the hypothalamus to inhibit NE efflux through GABA. It was concluded that leptin could probably produce its central and neuroendocrine effects by modulating NE and GABA levels in the hypothalamus.     

Effects of serotonin on the hypothalamic-pituitary GABAergic system

The effects of serotonin (5-HT) and its precursor, 5-hydroxytryptophan (5-HTP) on the GABAergic system in the mediobasal hypothalamus (MBH) and the anterior pituitary were studied. The IP administration of 5-HTP produced a transient increase (only at 45 min after the injection) in glutamate decarboxylase activity (GAD) of MBH and in GABA concentration in anterior pituitary. Besides, 5-HTP administration increased the in vitro evoked GABA release from MBH. The administration of 5-HTP to hypophysectomized rats partially reversed the inhibitory effects of hypophysectomy on GABA concentration in MBH. We also examined the direct effect of 5-HT on some parameters on the hypothalamic GABAergic system. The presence of 5-HT in the incubation medium increased GAD activity and evoked GABA release from MBH. These observations indicate that the serotoninergic stimulation of the hypothalamic GABAergic system could be a direct effect which may, at least partially, be independent of the feedback mechanism induced by prolactin on the GABAergic neurons. The serotoninergic increase of prolactin secretion could be accomplished through stimulation of the hypothalamic GABAergic transmission.     

Postnatal development of transmitter systems: Sexual differentiation of the gabaergic system and effects of muscimol

The development and sex differences of the central nervous GABAergic system were examined by measuring GABA (gamma-aminobutyric acid) in discrete brain nuclei of the hypothalamus, as well as the nigrostriatal and the limbic systems of male and female rats on the day of birth and on days 5, 10, and 15. The highest concentrations were found in the hypothalamic and nigrostriatal nuclei; the lowest in the limbic system. Sex differences were observed only on day 10 in the medial preoptic area, with GABA being higher in males than in females; and also in the substantia nigra, where female GABA levels were higher than male. These results suggested an involvement of GABA in the sexual differentiation of the brain. As a control, concentrations of the GABA precursor glutamate were determined. No sex differences in glutamate concentrations were found in any brain region during the first 15 days postnatally. Since a GABA mimetic substance applied during the critical period of brain differentiation could disturb the development of the GABAergic system, the consequences of a perinatal treatment with the GABA agonist muscimol were investigated. Significant reduction of GABA concentrations by muscimol were observed in the hypothalamic and nigrostriatal systems at specific times postnatally. On day 5, GABA concentrations were diminished only in the medial preoptic area, then on day 10, in the anterior hypothalamus and the substantia nigra, and still later, on day 15, in the caudato putamen. In contrast, the effects of muscimol on glutamate concentrations could be observed over a longer postnatal period. Glutamate was already diminished on day 5 in 7 areas of the hypothalamic, nigrostriatal, and limbic systems.     

Effect of progesterone on monoamine turnover in the brain of the estrogen-primed rat

The effect of progesterone (P) on monoamine levels and turnover was evaluated in 8 brain nuclei in estrogen-primed rats. Animals were subcutaneously (SC) injected with P or vehicle 21 hours after SC treatment with 5 micrograms of estradiol benzoate (EB). EB-primed animals treated with P showed high levels of lordosis behavior and an LH surge three hours later. Initial concentrations of norepinephrine (NE), dopamine (DA), serotonin (5HT) and 5-hydroxyindole acetic acid were determined in EB-saline treated controls 3 hours after P or vehicle. NE and DA turnover was estimated from the exponential decline of these amines 2 hours after IP injection of alpha-methyl-p-tyrosine (5 hours after P or vehicle). The accumulation of 5HT 20 min following IP injection of pargyline was used as an index of 5HT turnover. P did not affect the initial NE, 5HT or 5HIAA concentrations in any of the brain nuclei studied, but decreased DA content in the arcuate-median eminence region (Ar-ME). The DA rate constant was elevated in the nucleus of the diagonal band of Broca and the DA turnover rate was decreased in the Ar-ME. In the periventricular region (PVE, anterior hypothalamic level) the NE turnover rate (K, pg/microgram protein/hr) and rate constant (k, hr-1) decreased following P treatment. Progesterone treatment decreased the accumulation of 5HT in the ventromedial hypothalamus (VMN, pars lateralis) and the dorsal midbrain central grey (MCG). Progesterone effects on monoamine turnover were not found in the lateral septal, medial preoptic, anterior hypothalamic or dorsal raphe nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)     

Brain serotonin and catecholamine responses to repeated stress in rats

This study was designed to compare the effects of single and repeated administration of a discrete 2-min restraint stress on serotonin (5-HT) and catecholamine neuron activity in various regions of rat brain. A single 2-min restraint stress significantly increased the 5-hydroxyindoleacetic acid (5-HIAA) and 5-HT responses in hypothalamus and cerebral cortex and the 5-HIAA response in brainstem. A second 2-min restraint stress applied 90 min after the initial stress did not appreciably alter the steady-state concentrations of 5-HIAA and 5-HT nor did it produce any further changes in the 5-HIAA and 5-HT responses compared to those seen following a single stress in these 3 brain regions. In addition, the synthesis rate of 5-HT in anterior hypothalamus, posterior hypothalamus, hippocampus and brainstem was not altered by a second stress applied 90 min after the initial stress. In contrast, a second 2-min restraint stress applied 30 or 60 min after the initial stress significantly increased the 5-HIAA concentration in hypothalamus, cerebral cortex and brainstem. Also, the synthesis rate of 5-HT was greater following application of a second stress at 30 min than following either a single stress or a second stress applied at 90 min. Following application of a single 2-min restraint stress the hypothalamic concentration of norepinephrine (NE) was significantly decreased at 5 min after onset of the stress and returned to prestress levels by 15 min; the hypothalamic dopamine (DA) concentration was significantly increased at 30 min after the onset of the stress, while the hypothalamic epinephrine (EPI) concentration remained unchanged. A second 2-min restraint stress applied at 30 min markedly lowered NE concentrations in whole and mediobasal hypothalamus but not in laterobasal hypothalamus, and the NE concentrations remained decreased for a period lasting at least 60 min; there was a significant decrease in the hypothalamic EPI concentration 60 min after application of the second stress at 30 min. In addition, the synthesis rate of catecholamines was significantly greater in anterior but not in posterior hypothalamus after application of a second stress 30 min after the initial stress than following either a single stress or a second stress applied at 90 min. Negative correlations were demonstrated between increased synthesis rates of both hypothalamic 5-HT and anterior hypothalamic catecholamines and decreased corticosterone response to single and repeated stress.(ABSTRACT TRUNCATED AT 400 WORDS)     

Increased GABAergic transmission in medial hypothalamus facilitates lordosis but has the opposite effect in preoptic area

The role of gamma-aminobutyric acid (GABA) in mediation of lordosis in the rat has been unclear. We report here that GABA plays a dual role in the mediation of lordosis and has differential effects in the medial hypothalamus (MH) and preoptic area/anterior hypothalamus continuum (POA-AH). Bilateral infusion of the GABAA antagonist bicuculline into the MH of cannulated females primed with estradiol benzoate and progesterone (EB + P) resulted in a marked and transient inhibition of ongoing lordosis. A similar pattern of inhibition was seen in females treated with EB only. In contrast, infusion of the same dose of bicuculline into the POA-AH of sexually receptive females had no effect on lordosis whereas infusion of the GABAA agonist muscimol into this site resulted in a short-term inhibition of lordosis. Furthermore, when females were treated with subthreshold doses of EB + P to induce a low level of lordosis responding, infusion of muscimol into the MH resulted in a significant enhancement of lordosis; infusion of bicuculline into the POA-AH also enhanced lordosis responding as compared to saline-infused controls. These data indicate that increased GABAergic neurotransmission in the MH facilitates lordosis whereas increased GABAergic activity in the POA-AH inhibits this behavior.     

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

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 μg of estradiol benzoate followed 44 h later by 200 μg 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.     

A GABAergic mechanism in the posterior hypothalamus modulates baroreflex bradycardia

Several laboratories have shown that electrical stimulation in the posterior hypothalamus inhibits the baroreceptor reflex. However, the results of these studies are difficult to interpret since it is not known if the attenuation of the baroreflex results from activation of axons of passage or from stimulation of hypothalamic cell bodies. The purpose of this study was to determine the effects of chemical stimulation of posterior hypothalamic neurons upon the baroreflex. Arterial baroreceptors were activated by increasing the pressure in an isolated carotid sinus in anesthetized cats and by an increased arterial pressure following intravenous injection of phenylephrine in both anesthetized cats and rats. The baroreceptor reflex was evaluated before and after a GABA antagonist (picrotoxin) was microinjected into the posterior hypothalamus. The bradycardia, but not the depressor response, elicited by increasing carotid sinus pressure was attenuated after unilateral microinjections of picrotoxin into the posterior hypothalamus. In addition, the heart rate response to a phenylephrine-evoked rise in arterial pressure was reduced after picrotoxin was microinjected in both the cats and the rats. Microinjection of a GABA agonist (muscimol) into the same hypothalamic site returned resting heart rate and arterial pressure to levels seen prior to picrotoxin. These results show that the depression of the bradycardia produced by hypothalamic stimulation results from activation of cell bodies in the posterior hypothalamus. This hypothalamic effect upon the baroreflex bradycardia may involve a GABAergic mechanism.     

Effects of Various Types of Hypothalamic Deafferentation on Luteinizing Hormone Pulses in Ovariectomized Rats

The pulsatile luteinizing hormone (LH) secretion in chronically ovariectomized rats bearing various types of hypothalamic deafferentation was examined. Ovariectomized rats were subjected to complete, anterolateral or anterior hypothalamic deafferentation and bled every 6 min for 3 h through an indwelling atrial cannula 5 days after the brain surgery. Another group of ovariectomized animals was subjected to posterior-anterior hypothalamic deafferentation (PAD), which cut off the anterior part of the arcuate nucleus from the mediobasal hypothalamus, and bled 1, 3 or 5 weeks after the deafferentation. Coronal sections of the brain were immunostained with anti-LH-releasing hormone (LHRH) serum. The pulsatile LH secretion was observed in rats bearing anterior, anterolateral or complete hypothalamic deafferentation and these types of deafferentation did not affect the frequency of LH pulses. The mean LH level during the 3-h sampling period and the amplitude of LH pulses decreased as the incision extended postero-laterally. Rats bearing PAD showed an irregular fluctuating pattern in plasma LH concentration 1 week after PAD. Parameters of LH pulses were restored with time after PAD. This suggests that the system generating LHRH pulses severed by PAD had been reorganized. LHRH-immunopositive neuronal fibres were found in the external layer of the median eminence in the rats bearing any type of deafferentation. The present results suggest that the frequency of LH pulses could be controlled by the LHRH pulse generator, which consists of non-LHRH neurons and is located in the mediobasal hypothalamus.     

Gonadal influences on spinal cord and brain monoamines in male rats

Concentrations of norepinephrine (NE), dopamine (DA), 3,4-dihydroxy phenylacetic acid (DOPAC), serotonin (HT) and 5-hydroxyindoleacetic acid (HIAA) were measured by high-performance liquid chromatography-electrical detection (HPLC-ED) in homogenates of lumbosacral spinal cord, mediobasal hypothalamus and cerebral cortex of male rats. The effects of castration and testosterone propionate (TP) (20 micrograms/day X 2 days) were compared. Castrated animals had the highest levels of DA and DOPAC in the cerebral cortex and NE, HT and HIAA in the spinal cord, as well as decreased hypothalamic DOPAC. Testosterone treatment returned spinal cord monoamine concentrations to intact control levels. These findings point to the spinopetal monoaminergic pathways as sensitive targets for androgen action.     

Activation of a subpopulation of orexin/hypocretin-containing hypothalamic neurons by GABAA receptor-mediated inhibition of the nucleus accumbens shell, but not by exposure to a novel environment

Gamma-amino butyric acid (GABA)A receptor stimulation in the nucleus accumbens shell produces intense hyperphagia in rats and increases Fos expression in the lateral hypothalamus. To explore the involvement of hypothalamic orexin/hypocretin- or melanin concentrating hormone-immunoreactive neurons in this effect, the GABAA agonist, muscimol (0, 50 ng), was infused directly into the nucleus accumbens shell of rats; 90 min later, their brains were collected and subsequently processed for immunohistochemistry. A group exposed to a novel environment was included to evaluate the specificity of Fos expression changes with regard to general arousal. Alternating sections through the hypothalamus were double-stained for orexin/hypocretin-Fos or melanin concentrating hormone-Fos combinations. Intra-accumbens shell muscimol treatment significantly increased the percentage of orexin/hypocretin-containing neurons expressing Fos in the lateral, but not medial, portion of the perifornical/lateral hypothalamic area. Regardless of treatment condition, greater percentages of orexin/hypocretin-containing neurons in the medial portion of the hypothalamus expressed Fos relative to cells located more laterally. None of the manipulations increased Fos expression in melanin concentrating hormone-immunoreactive neurons. Muscimol treatment also markedly increased Fos expression in the arcuate nucleus, which connects reciprocally to the lateral/perifornical hypothalamic area. Thus, orexin/hypocretin-containing neurons in lateral sectors of the hypothalamus, along with cells in the arcuate nucleus, display phasic increases in Fos expression after an orexigenic pharmacological manipulation of the nucleus accumbens shell, but to a lesser degree after the heightened arousal associated with exposure to a novel environment.     

Tropisetron increases the inhibitory effect of mild restraint on lordosis behavior of hormonally primed, ovariectomized rats

Ovariectomized rats, hormonally primed with 10 μg estradiol benzoate and 500 μg progesterone are resistant to the lordosis-inhibiting effects of a 5 min restraint experience. However, modulation of the serotonergic (5-HT) system alters this resistance to stress. In the following experiment, ovariectomized Fischer inbred rats were hormonally primed with 10 μg estradiol benzoate and 500 μg progesterone. The effect of 5 min restraint on sexual behavior was examined after bilateral hypothalamic infusion or intraperitoneal (ip) treatment with the 5-HT(3) receptor antagonist, 3-tropanylindole-3-carboxylate hydrochloride (tropisetron). Infusion with 50 or 100 ng tropisetron inhibited lordosis behavior. When rats were infused with 10 or 25 ng tropisetron, rats showed normal lordosis behavior. However, when infusion with 10 or 25 ng tropisetron was combined with 5 min restraint, lordosis behavior was inhibited. These findings are consistent with prior work that has implicated hypothalamic serotonin in control of lordosis behavior and in the effect of mild restraint on the behavior. In contrast to the effects of the intracranial infusion, intraperitoneal injection with 1.0 or 2.0 mg/kg tropisetron did not amplify the effects of restraint.     

WAY100635 and female rat lordosis behavior

Sexually receptive proestrous rats with bilateral cannulae in the ventromedial nucleus of the hypothalamus (VMN) were infused with 200 ng of (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) or with 8-OH-DPAT plus varying concentrations (200 to 2000 ng) of the 5-HT1A receptor antagonist, N-[2[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY100635). 8-OH-DPAT inhibited lordosis behavior within 15 min of the infusion and every dose of WAY100635 prevented the inhibition. When non-sexually receptive, ovariectomized rats, hormonally primed with 0.5 microg estradiol benzoate and 500 microg progesterone, were infused with WAY100635 (400 to 2000 ng), the 5-HT1A receptor antagonist did not facilitate lordosis responding. These findings support earlier findings that activation of 5-HT1A receptors in the mediobasal hypothalamus inhibits lordosis behavior. However, they further demonstrate that tonic activation of 5-HT1A receptors is not responsible for the absence of sexual receptivity in suboptimally hormonally primed ovariectomized rats.     

The Effect of Gonadal Steroids on Vasoactive Intestinal Peptide Concentration and Release from Mediobasal Hypothalamus and the Anterior Pituitary Gland

The effects of chronic administration of sex steroids on the content of vasoactive intestinal peptide (VIP) in the mediobasal hypothalamus and anterior pituitary were studied in adult rats. Gonadectomy had no effect on VIP concentration in the mediobasal hypothalamus or anterior pituitary gland. Estradiol benzoate (1 μg/100 g body wt/day) administered for 10 days decreased mediobasal hypothalamus VIP concentration of ovariectomized rats whereas it produced no change in mediobasal hypothalamus VIP content of orchidectomized rats. Testosterone propionate (100 μg/100 g body wt/day) administration decreased mediobasal hypothalamus VIP content in both sexes. Estradiol administration caused an increase whereas testosterone treatment resulted in a decrease in anterior pituitary VIP levels in both sexes. The effect of chronic administration of the sex steroids on VIP release from the mediobasal hypothalamus and anterior pituitary was also investigated. Estradiol increased evoked VIP release from the mediobasal hypothalamus and decreased mediobasal hypothalamus VIP content whereas testosterone decreased both mediobasal hypothalamus release and content. Chronic treatment with estradiol enhanced anterior pituitary VIP release and content while testosterone decreased both parameters studied. The data indicate that anterior pituitary VIP content is under the control of gonadal hormones and that the increased anterior pituitary VIP found after estradiol administration may be due to an augmented release from the mediobasal hypothalamus and probably an increase in anterior pituitary VIP synthesis.