Substance P and the ethanol-evoked changes in the independent motivated behavioral reactions of rabbits

Ethanol (0.5 g/kg) administered intravenously led to alterations in central mechanisms of feeding and escape, elicited by threshold electrical stimulation either of lateral or of ventromedial hypothalamic centers of the rabbit. Subsequent intravenous injection of substance P (30 mcg/kg) restored the excitability of the ventromedial hypothalamus and facilitatory effects on this motivational center of the midbrain reticular formation. The restoration of both inhibitory influences of the dorsal hippocampus and facilitatory ones of the midbrain reticular formation on the excitability of the lateral hypothalamus was also observed after SP administration. Data obtained suggest that oligopeptides could be used to increase the tolerance to ethanol or to cure the negative acute effects of alcohol in motivated behaviours.     

Substance P in the central mechanisms of the escape reaction

The role of substance P in the central mechanisms of escape reaction elicited by electrical stimulation of the ventromedial hypothalamus was investigated in chronic experiments on rabbits. Intravenous injection of substance P (30 micrograms/kg) led to a short-term (less than 10 min) increase in the threshold of stimulation of the ventromedial hypothalamus and to more stable (up to 1.5 h) disorders of the hippocampal-hypothalamic relations. After substance P injection the inhibitory effects of the dorsal hippocampus but not the facilitating influences of the midbrain reticular formation on excitability of the hypothalamic motivation center were found to be lacking. Disorders of the central mechanisms of escape reaction after substance P injection correlated with new patterns of the main EEG rhythms in different areas of the brain cortex in response to the ascending excitations of the limbic-midbrain structures. Interpreting the mechanisms of substance P involvement in escape reaction the authors point to the ability of the given peptide to interact with different transmitter systems of the brain and opiate receptors and to alter the brain blood circulation.     

Noradrenergic and GABAergic systems in the medial hypothalamus are activated during hypoglycemia

Noradrenergic and GABAergic systems in the medial hypothalamus influence plasma glucose and may be activated during glucoprivation. Microdialysis probes were placed into the ventromedial nucleus (VMH), lateral hypothalamus (LHA), and paraventricular nucleus (PVH) of male Sprague-Dawley rats to monitor extracellular concentrations of norepinephrine (NE) and GABA. During systemic hypoglycemia, induced by insulin (1.0 U/kg), NE concentrations increased in the VMH (P < 0.05) and PVH (P = 0.06) in a bimodal fashion during the first 10 min and 20-30 min after insulin administration. In the VMH, GABA concentrations increased (P < 0.05) in a similar manner as NE. Extracellular NE concentrations in the LHA were slightly lower (P = 0.13), and GABA levels remained at baseline. The increases in NE and GABA in the VMH were absent during euglycemic clamp; however, NE in the PVH still increased, reflecting a direct response to hyperinsulinemia. On the basis of these data, we propose that the activity of noradrenergic afferents to the medial hypothalamus is increased during hypoglycemia and influences the activity of local GABAergic systems to activate appropriate physiological compensatory mechanisms.     

Specificity and neural sites of action of anisomycin in the reduction or facilitation of female sexual behavior in rats

These experiments were designed to investigate the role of neuronal protein synthesis in the hormonal activation of female sexual behavior using intracranial implants of the protein synthesis inhibitor, anisomycin. In the first experiment, female rats receiving bilateral cannulae implants in the medial preoptic area (POA), septal region (SEPT), ventromedial hypothalamus (VMH), or midbrain central gray (CG) were injected with 2.5 micrograms estradiol benzoate (EB), followed 48 hr later by 500 micrograms progesterone (P). Females receiving anisomycin in the VMH at the time of EB injection had lower levels of lordosis and darting compared to tests without anisomycin. Sexual behavior was unaffected in females receiving anisomycin implants in the POA, SEPT, or CG. In a second experiment, we replicated the finding that anisomycin could attenuate lordotic responsivity when placed in the VMH of female rats injected with 2.5 micrograms EB and 500 micrograms P. In addition, we found that POA implants of anisomycin could facilitate lordosis in females given a low dose of EB (1.25 microgram) plus 500 micrograms P. In a third experiment, we assessed the effects of anisomycin application to the VMH or POA of female rats receiving estradiol (E; diluted 1:250 with cholesterol) implants in the VMH and systemic P. Treatment of the VMH with anisomycin prior to E in the VMH suppressed lordotic responding, whereas anisomycin application to the POA prior to E in the VMH had no effect on lordosis. The results of these experiments suggest that reducing protein synthesis in the region of the VMH disrupts the action of estrogen on the VMH, and that the facilitative action of anisomycin in the POA of female rats requires more estrogen treatment than threshold stimulation of the VMH alone.     

Intracranial dialysis and microinfusion studies suggest that morphine may act in the ventromedial hypothalamus to inhibit female rat sexual behavior.

The present investigation examined the neural sites and mechanisms of opiate inhibition of female sexual behavior. Systemic administration of morphine (10 mg/kg) significantly reduced ovarian steroid-induced estrous behavior in female rats. This behavioral inhibition was prevented when the opiate receptor antagonist naloxone (5 mg/kg) was administered 30 min prior to morphine. Bilateral infusion of morphine directly into the ventromedial hypothalamus (VMH) also inhibited hormone-dependent estrous behavior for at least 2 hr. Furthermore, naloxone infusion into the VMH 20 min before behavior testing reduced the inhibitory effects of systemically administered morphine on lordosis. These results suggest that morphine may inhibit female sexual behavior by acting directly on the VMH, the primary site at which ovarian steroids facilitate this behavior. In a separate experiment we used in vivo brain microdialysis to test the hypothesis that morphine inhibits lordosis by interfering with norepinephrine (NE) neurotransmission in the VMH. In control rats, the onset of mating was associated with increased NE release in the VMH. Morphine-treated animals displayed neither behavioral estrus nor elevated NE release from the VMH when tested with stimulus males. These data are consistent with the hypothesis that morphine suppresses NE release in the VMH. Nevertheless, mechanisms other than or in addition to attenuation of hypothalamic NE release may contribute to the inhibitory effects of morphine on lordosis.     

Inhibition of luteinizing hormone secretion by localized administration of estrogen, but not dihydrotestosterone, is enhanced in the ventromedial hypothalamus during feed restriction in the young wether

The ability of steroids to inhibit LH secretion is enhanced during undernutrition. To identify potential hypothalamic sites at which this enhancement may occur, we examined LH secretion in feed-restricted or fed young wethers treated with locally administered metabolites of testosterone. In experiment 1, microimplants containing crystalline estradiol-17beta (E) or cholesterol were administered via chronic guide tubes directed to the preoptic area (POA) or ventromedial hypothalamus (VMH) in fed or feed-restricted wethers. E treatment in the VMH decreased LH pulse frequency, pulse amplitude, and mean LH concentration in feed-restricted, but not fed, wethers. E may act in the POA to suppress LH under feed restriction, but definite conclusions cannot be drawn because of steroid-independent effects of feed restriction on LH pulse frequency. In experiment 2, the effect of dihydrotestosterone (DHT) in the VMH was determined. DHT administration to the VMH did not alter LH secretion in either feed-restricted or fed wethers. Thus the VMH is one site wherein E negative feedback is enhanced during feed restriction in the wether. In contrast, we found no evidence for enhanced responsiveness to androgen negative feedback within the VMH of feed-restricted wethers. We suggest that increased sensitivity within the VMH to E, but not to DHT, is important for suppressing LH secretion in undernourished male sheep.     

Ventromedial hypothalamic stimulation accelerates norepinephrine turnover in brown adipose tissue of rats

To establish a functional link between the ventromedial hypothalamus (VMH) and brown adipose tissue (BAT), effects of electrical stimulation of the VMH and the lateral hypothalamus (LH) on norepinephrine (NE) turnover in the interscapular BAT were examined in rats. Stimulation of the VMH elicited about 3-fold increase in the rate of NE turnover in BAT, whereas stimulation of the LH had no appreciable effects. The effect of VMH stimulation was abolished after sympathetic ganglion blockade or by surgical sympathetic denervation of BAT. It was concluded that there is a sympathetic nerve-mediated connection between the VMH and BAT, and that stimulation of the VMH induces metabolic activation and heat production in BAT through an increase in sympathetic nerve activity.     

Substance P and effects of ethanol on the central mechanisms of the escape reaction in rabbits

Substance P (SP) effects on the central mechanisms of escape reaction, elicited by threshold electrical stimulation of the ventromedial hypothalamus were investigated in rabbits pretreated with ethanol (0.5 g/kg). SP (30 micrograms/kg) was demonstrated to normalize in 71.4% of cases the excitability of the ventromedial hypothalamus which was decreased by ethanol and restored in 83.3% of cases the facilitatory effects of the midbrain reticular formation in escape reactions. However, SP was ineffective in the restoration of the inhibitory effects of the dorsal hippocamp on the excitability of the ventromedial hypothalamus that was obvious in intact animals. Partial normalizing effect of SP on escape reaction in rabbits after previous ethanol administration can be accounted for by the fact that both undecapeptide and ethanol are similar in their realization of central effects such as an interaction with the same brain neurotransmitters, interference with neuronal enzyme processes and reactions with opiate receptors.     

Suppression of gastric acid secretion by intracisternal bombesin does not require the ventromedial hypothalamus

Intracisternal administration of the tetradecapeptide peptide bombesin suppresses gastric acid release. Other studies have shown that the ventromedial hypothalamus (VMH) may have an inhibitory role in gastric regulation. To determine if the inhibition of gastric acid secretion by intracisternally administered bombesin is mediated by the ventromedial hypothalamus, bombesin was injected intracisternally in rats with ventromedial hypothalamic lesions. Neither anterior nor posterior VMH lesions altered the effects of bombesin on gastric acid, concentration, volume, total output, or on serum gastrin. The bombesin-induced rise in gastric pH was very mildly attenuated by both lesions. The previous finding of enhanced gastric acid secretion after anterior VMH lesions was confirmed. The results suggest that the VMH is not crucial in the bombesin-induced inhibition of acid secretion.     

Rate of fall in blood glucose and recurrent hypoglycemia affect glucose dynamics and noradrenergic activation in the ventromedial hypothalamus

Noradrenergic activity in the ventromedial hypothalamus (VMH) is increased and activates a sympathoadrenal response during hypoglycemia. How the rate at which hypoglycemia develops affects local glucose concentrations and norepinephrine (NE) release was evaluated by placing microdialysis probes into the VMH of male Sprague-Dawley rats receiving insulin (20 mU·kg(-1)·min(-1)) and variable glucose infusions. During a first episode of hypoglycemia, interstitial glucose concentrations in the VMH generally declined at the same rate as plasma glucose; however, the faster hypoglycemia developed, the greater the magnitude of the initial NE release in the VMH (r(2) = 0.72, P < 0.001). Following recurrent episodes of hypoglycemia, VMH glucose decreased at a slower rate than plasma glucose, and the initial NE release was attenuated at the same rates of blood glucose decline. The plasma glucose threshold for the initial NE release in VMH was similar for all groups (～3.23 mM); however, the VMH glucose threshold was stimulated and was lower when blood glucose declined more slowly (0.86 ± 0.06 vs. 1.06 ± 0.04 mmol/l, P < 0.01). The timing of the initial increase in NE release in VMH corresponded with an increase in plasma epinephrine during the first episode of hypoglycemia but not following recurrent hypoglycemia. Although a decrease in VMH glucose concentration is required for noradrenergic activation in VMH, there does not appear to be a set glucose threshold within the VMH for activation of this response.     

Dissociation of hypothalamic noradrenergic activity and sympathoadrenal responses to recurrent hypoglycemia

This study evaluated whether attenuation of sympathoadrenal responses to recurrent hypoglycemia is mediated by diminished noradrenergic activity in the hypothalamus. Male Sprague-Dawley rats received either once daily insulin (1.0 units/kg) injections or an equal administration of saline for 3 days. Both groups received an administration of insulin on the fourth day, during which blood glucose and plasma catecholamines were determined, and extracellular norepinephrine (NE) in the ventromedial hypothalamus (VMH) or paraventricular hypothalamic nucleus (PVN) was monitored with microdialysis. The peak response of plasma epinephrine to insulin-induced hypoglycemia (nadir approximately 3.2 mmol/l) was significantly reduced during the fourth hypoglycemic episode (774 +/- 134 pg/ml) compared with the first episode (2,561 +/- 410 pg/ml, P < 0.001). Baseline levels of extracellular NE were elevated approximately 25% (P = 0.07) in the VMH and approximately 46% (P = 0.03) in the PVN after multiple hypoglycemic episodes. There was no difference in noradrenergic activity during the first or fourth hypoglycemic episode in either brain area. The reduced sympathoadrenal output after recurrent hypoglycemia is likely postsynaptic from hypothalamic NE release or is mediated via a collateral pathway.     

GABA influences the development of the ventromedial nucleus of the hypothalamus.

The region that becomes the ventromedial nucleus of the hypothalamus (VMH) is surrounded by cells and fibers containing immunoreactive gamma-aminobutyric acid (GABA) by embryonic day 13 (E13), several days before the nucleus emerges in Nissl stains. As GABA plays many roles during neural development, we hypothesized that it influences VMH development, perhaps by providing boundary information for migrating neurons. To test this hypothesis we examined the VMH in embryonic mice in which the beta3 subunit of the GABA(A)-receptor, a receptor subunit that is normally highly expressed in this nucleus, was disrupted by gene targeting. In beta3 -/- embryos the VMH was significantly larger, and the distribution of cells containing immunoreactive estrogen receptor-alpha was expanded compared to controls. Using in vitro brain slices from wild-type C57BL/6J mice killed at E15 we found that treatment with the GABA(A) antagonist bicuculline increased the number of cells migrating per video field analyzed in the VMH. In addition, treatment with either bicuculline or the GABA(A) agonist muscimol altered the orientation of cell migration in particular regions of this nucleus. These data suggest that GABA is important for the organization of cells during VMH formation.     

Prior hypoglycemia enhances glucose responsiveness in some ventromedial hypothalamic glucosensing neurons

Antecedent insulin-induced hypoglycemia (IIH) reduces adrenomedullary responses (AMR) to subsequent bouts of hypoglycemia. The ventromedial hypothalamus [VMH: arcuate (ARC) + ventromedial nuclei] contains glucosensing neurons, which are thought to be mediators of these AMR. Since type 1 diabetes mellitus often begins in childhood, we used juvenile (4- to 5-wk-old) rats to demonstrate that a single bout of IIH (5 U/kg sc) reduced plasma glucose by 24% and peak epinephrine by 59% 1 day later. This dampened AMR was associated with 46% higher mRNA for VMH glucokinase, a key mediator of neuronal glucosensing. Compared with neurons from saline-injected rats, ventromedial nucleus glucose-excited neurons from insulin-injected rats demonstrated a leftward shift in their glucose responsiveness (EC50 = 0.45 and 0.10 mmol/l for saline and insulin, respectively, P = 0.05) and a 31% higher maximal activation by glucose (P = 0.05), although this maximum occurred at a higher glucose concentration (saline, 0.7 vs. insulin, 1.5 mmol/l). Although EC50 values did not differ, ARC glucose-excited neurons had 19% higher maximal activation, which occurred at a lower glucose concentration in insulin- than saline-injected rats (saline, 2.5 vs. insulin, 1.5 mmol/l). In addition, ARC glucose-inhibited neurons from insulin-injected rats were maximally inhibited at a fivefold lower glucose concentration (saline, 2.5 vs. insulin, 0.5 mmol/l), although this inhibition declined at >0.5 mmol/l glucose. These data suggest that the increased VMH glucokinase after IIH may contribute to the increased responsiveness of VMH glucosensing neurons to glucose and the associated blunting of the AMR.     

Urethane modification of EEG-like activity and acoustically evoked field potentials recorded from deep nuclei.

Average acoustic-evoked responses (AAER) and EEGs were recorded from the lateral hypothalamus (LH), ventromedial hypothalamus (VMH), medial forebrain bundle (MFB) and reticular formation (RF) of freely behaving rats before and after various doses (0.2, 0.4, 0.8, 1.2 g/5g) of the anesthetic drug, urethane. The effect of a surgical level of urethane (1.2 g/5g) over time (8--14 h) was also studied. Urethane produced a dose-dependent slowing in the EEG frequency. The effect of 1.2 g/kg over time was a bimodal depression of the EEG frequency. Three components of the AAER were recorded consistently and evaluated in terms of amplitude. With increasing doses, the early component (P2) was depressed in the LH2, VMH and MFB, but not in the RF which remained at control values or higher. The later components (N2 and P2) were depressed in all four structures. This suggests that urethane may be blocking the integrative function of the RF, as well as in the thalamus, resulting in lower sensory input to higher CNS structures.     

Effects of ghrelin on neuronal activity in the ventromedial nucleus of the hypothalamus in infantile rats: An in vitro study

Ghrelin is an endogenous ligand for the growth hormone (GH) secretagogue (GHS) receptor (GHS-R) and a potent stimulant for GH secretion even in infantile rats before puberty. Although the ventromedial nucleus of the hypothalamus (VMH) might be a site of action for ghrelin to induce GH release, the electrophysiological effect of ghrelin on VMH neurons in infantile rats remains to be elucidated. Thus, the purpose of the present study was to investigate the effect of ghrelin on VMH neurons using hypothalamic slices of infantile rats. Ghrelin excited a majority of VMH neurons in a concentration-dependent manner. VMH neurons that were excited by GH releasing peptide-6 (GHRP-6), a synthetic GHS, were also excited by ghrelin and vice versa. Repeated application of ghrelin to the same VMH neuron decreased progressively the excitatory responses depending on the number of times it was administered. The excitatory effect of ghrelin on VMH neurons in normal artificial cerebrospinal fluid (ACSF) persisted in low Ca²⁺-high Mg²⁺ ACSF. The present results indicate that (1) ghrelin excites a majority of VMH neurons dose-dependently and postsynaptically and (2) the excitatory effects of ghrelin are mimicked by GHRP-6 and desensitized by repeated applications of ghrelin.     

Changes in the substance P content of the blood and hypothalamus during experimental emotional stress

A study was made of the content of substance P in the blood and hypothalamus of Wistar rat brain in acute and chronic emotional stress and after intraperitoneal injection of substance P in a dose of 250 mg/kg. A possibility was demonstrated of inducing long-term changes in the content of substance P in the hypothalamus after a single injection. Exposure to a single 24-hour stress was followed by an increase in the substance P content in the hypothalamus.     

Analysis of the effect of increasing electrodermal stimulation on the somatovegetative reactions evoked by stimulation of the ventromedial hypothalamus

The influence of increasing electrodermal stimulation (EDS) on the dynamics of the somatovegetative reactions evoked by electrical stimulation of negative emotiogenic regions of the hypothalamus was studied. EDS produced a blocking effect on the somatovegetative reactions evoked by stimulation of the ventromedial hypothalamus (VMH). Pretreatment with naloxone prevented the effects of EDS. Injection of beta-endorphine in a dose of 10 micrograms to the lateral ventricules of the animal brain also blocked the somatovegetative reactions during VMH stimulation. Injection of beta-endorphine in doses of 50-100 micrograms enhanced and prolonged the somatovegetative reactions evoked by VMH stimulation. Elevated arterial blood pressure, pronounced bradycardia, extrasystoles, muscle tremor, and pathologic respiration were recorded. These disorders were completely reversed by EDS. It is assumed that both opiate peptides and their receptors are involved in the mechanism of the somatovegetative reactions evoked by VMH stimulation, experiencing the influence of EDS.     

Lesions confined to the ventromedial hypothalamus decrease the frequency of coital contacts in female rats

Ovariectomized female rats received either bilateral radiofrequency lesions of the ventromedial nucleus of the hypothalamus (VMH) or control treatments and were tested for copulatory activity following either estrogen (E) alone, or E plus progesterone (P) administration. In separate experiments the females were tested in two testing apparatuses both of which allowed the test females to control their contacts with sexually active males. One of the testing apparatuses also allowed the females to control their contacts with sexually inactive males and ovariectomized females. Females receiving VMH lesions engaged in fewer coital contacts with sexually active males than sham-operated females in the E plus P condition. Lesioned females also tended to spend less time with sexually active males than did sham operates in both the E and E plus P hormonal conditions. The VMH-lesioned females did not differ from the sham-operated females in the ability to display lordosis during the coital contacts or the frequency and duration of visits to the inactive males or ovariectomized females. The sham-operated females did have some transitory alterations in copulatory behavior in comparison to unoperated control females.     

Sexually dimorphic estrogen receptor α mRNA expression in the preoptic area and ventromedial hypothalamus of green anole lizards

Estradiol (E2) is important in activation of male reproductive behaviors, and masculinizes morphology of associated brain regions in a number of mammalian and avian species. In contrast, it is testosterone, rather than its metabolites, that is the most potent activator of male sexual behavior in green anole lizards. As in other vertebrate groups, however, E2 is critical for receptivity in females of this species. Aromatase, the enzyme which converts testosterone to E2, is more active in the male than female green anole brain, and appears to be actively regulated on a seasonal basis, suggesting some role for E2 in males. This study was designed to enhance our understanding of potential E2 actions by localizing and quantifying relative levels of estrogen receptor-alpha (ERα) mRNA in forebrain regions involved in masculine and feminine behaviors in anoles. These areas include the preoptic area (POA), ventromedial amygdala (AMY) and ventromedial hypothalamus (VMH). In situ hybridization was conducted in adult males and females collected during both breeding and non-breeding seasons. ERα mRNA was expressed in each brain region across sexes and seasons. However, expression was up to 3 times greater in the VMH compared to the POA and AMY. In the POA and VMH, expression was higher in females compared to males, independent of season. The increased receptor expression in females is consistent with E2 playing a larger role in female than male reproductive behaviors.     

Estradiol pulses induce progestin receptors selectively in substance P-immunoreactive neurons in the ventrolateral hypothalamus of female guinea pigs

Low doses of estradiol, administered as pulses, are as effective as higher doses for priming ovariectomized (OVX) guinea pigs to display progesterone-facilitated lordosis. High doses of estradiol, administered by constant-release implants, induce progestin receptors in many substance P-immunoreactive (SP-IR) neurons in the ventrolateral hypothalamus (VLH), a site at which estradiol primes OVX guinea pigs to respond behaviorally to progesterone. To test the hypothesis that behaviorally effective estradiol pulses induce progestin receptors selectively in substance P-containing neurons in the VLH, OVX females received estradiol implants 1 week prior to perfusion, or two pulses of estradiol- 17β, injected 39 and 11 h before perfusion. Colchicine was administered intracerebroventricularly prior to perfusion. No significant differences were observed in the total number of progestin receptor-immunoreactive (PR-IR) or substance P-immunoreactive cells in the VLH and VLH/ventromedial hypothalamus (VMH), respectively, of females receiving the two estradiol treatments. However, the percentage of PR-IR cells in the VLH also immunoreactive for SP was significantly higher in the estradiol pulse-treated (53%), than in the estradiol capsule-implanted animals (36%). These data suggest that behaviorally effective estradiol pulses induce progestin receptors selectively in substance P-containing neurons in the VLH and are consistent with the hypothesis that substance P is involved in progesterone-facilitated lordosis in guinea pigs.