Interacting appetite-regulating pathways in the hypothalamic regulation of body weight.

Various aspects of the complex spatio-temporal patterning of hypothalamic signaling that leads to the development of synchronized nocturnal feeding in the rat are critically examined. Undoubtedly, as depicted in Fig. 7, a distinct ARN in the hypothalamus is involved in the control of nocturnal appetite. At least four basic elements operate within this ARN. These are: 1) A discrete appetite-driving or orexigenic network of NPY, NE, GABA, GAL, EOP, and orexin transduces and releases appetite-stimulating signals. 2) Similarly, anorexigenic signal-producing pathways (e.g., CRH, GLP-1, alpha MSH, and CART) orchestrate neural events for dissipation of appetite and to terminate feeding, possibly by interrupting NPY efflux and action at a postsynaptic level within the hypothalamus. It is possible that some of these may represent the physiologically relevant "off" switches under the influence of GABA alone, or AgrP alone, or in combination with NPY released from the NPY-, GABA-, and AgrP-coproducing neurons. 3) Recent evidence shows that neural elements in the VMN-DMN complex tonically restrain the orexigenic signals during the intermeal interval; the restraint is greatly aided by leptin's action via diminution of orexigenic (NPY) and augmentation of anorexigenic (GLP-1, alpha MSH, and CART) signals. Since interruption of neurotransmission in the VMN resulted in hyperphagia and development of leptin resistance, it seems likely that the VMN is an effector site for the restraint exercised by leptin. The daily rhythms in leptin synthesis and release are temporally dissociable because the onset of daily rise in leptin gene expression in adipocytes precedes that in leptin secretion. Nevertheless, these rhythms are in phase with daily ingestive behavior because the peak in circulating leptin levels occurs during the middle of the feeding period. These observations, coupled with the fact that circulating levels of leptin are directly related to adiposity, pose a new challenge for elucidating the precise role of leptin in daily patterning of feeding in the rat. 4) A neural timing mechanism also operates upstream from the ARN in the daily management of energy homeostasis. Although the precise anatomical boundaries are not clearly defined, this device is likely to be composed of a group of neurons that integrate incoming internal and external information for the timely onset of the drive to eat. Evidently, this network operates independently in primates, but it is entrained to the circadian time keeper in the SCN of rodents. Apart from its role in the onset of drive to eat, the circadian patterns of gene expression of NPY, GAL, and POMC denote independent control of the timing device on the synthesis and availability for release of orexigenic signals. The VMN-DMN-PVN complex is apparently an integrated constituent of the timing mechanism in this context, because lesions in each of these sites result in loss of regulated feeding. The accumulated evidence points to the PVN and surrounding neural sites within this framework as the primary sites of release and action of various orexigenic and anorexigenic signals. A novel finding is the identification of the interconnected wiring of the DMN-mPVN axis that may mediate leptin restraint on NPY-induced feeding. The chemical phenotypes of leptin and NPY target neurons in this axis remain to be identified. These multiple orexigenic and anorexigenic pathways in the hypothalamic ARN appear to represent redundancy, a characteristic of regulated biological systems to provide a "fail-safe" neural mechanism to meet an organism's constant energy needs for growth and maintenance. Within this formulation, the coexisting orexigenic signals (NPY, NE, GAL, GABA, and AgrP) represent either another level of redundancy or it is possible that these signals operate within the ARN as reinforcing agents to varying degrees under different circumstances. (ABSTRACT TRUNCATED)

Resistance to adenovirally induced hyperleptinemia in rats. Comparison of ventromedial hypothalamic lesions and mutated leptin receptors.

Leptin regulates appetite and body weight via hypothalamic targets, but it can act directly on cultured pancreatic islets to regulate their fat metabolism. To obtain in vivo evidence that leptin may act peripherally as well as centrally, we compared the effect of adenovirally induced hyperleptinemia on food intake, body weight, and islet fat content in ventromedial hypothalamic-lesioned (VMHL) rats, sham-lesioned (SL) controls, and Zucker Diabetic Fatty (ZDF) rats in which the leptin receptor is mutated. Infusion with recombinant adenovirus containing the rat leptin cDNA increased plasma leptin by approximately 20 ng/ml in VMHL and ZDF rats but had no effect on their food intake, body weight, or fat tissue weight. Caloric matching of hyperphagic VMHL rats to SL controls did not reduce their resistance to hyperleptinemia. Whereas prediabetic ZDF rats had a fourfold elevation in islet fat, in VMHL rats islet fat was normal and none of them became diabetic. Isolated islets from ZDF rats were completely resistant to the lipopenic action of leptin, while VMHL islets exhibited 50% of the normal response; caloric matching of VMHL rats to SL controls increased leptin responsiveness of their islets to 92% of controls. We conclude that leptin regulation of adipocyte fat requires an intact VMH but that islet fat content is regulated independently of the VMH.

Viral vectors as probes to decipher brain circuitry for weight control.

Multidisciplinary research has recently identified an intrinsic appetite-regulating network (ARN) in the hypothalamus. The idea that viruses could help to chart this complex network has gained impetus owing to a combination of our improved understanding of virology and of genetic engineering. Recently, three groups have employed viral vectors as probes to: (1) trace the inflow of sensory information from the neocortex and limbic systems to the ARN; (2) trace the outflow of information from the ARN to the sympathetic nervous system to monitor adiposity and energy expenditure; and (3) decipher the mechanisms underlying leptin resistance, which is responsible for environmentally based obesity.

The effect of hyperprolactinemia produced by transplantable pituitary MtTW15 tumor cells in male rats on hypothalamic luteinizing hormone-releasing hormone release in vitro: effects of naloxone and K+.

Although hyperprolactinemia has been reported to decrease reproductive function in male rats, the mechanism of these effects is not fully understood. We examined the effects of chronic hyperprolactinemia and castration on the LHRH content of the medial basal hypothalamus (MBH) and on the basal and evoked in vitro release of LHRH from the MBH-preoptic area (POA). Adult Wistar-Furth male rats were inoculated with MtTW15 tumor fragments; 3 weeks later half of the rats were castrated. Hyperprolactinemic (H) and normoprolactinemic (N) rats were decapitated 2 weeks later to measure MBH LHRH and serum PRL and LH levels. Elevated PRL levels (greater than 2 micrograms/ml) resulted in significantly increased MBH LHRH stores. Castration caused a 57% depletion of MBH LHRH in N rats; in castrated H rats the MBH LHRH content was also reduced by 40%, a significantly lesser extent. Although serum LH levels in intact H rats were only slightly reduced, postcastration LH hypersecretion was significantly attenuated. In a parallel study, the LHRH release rate was assessed by in vitro perfusion of the MBH-POA. The basal LHRH release rates of intact N and H rats were similar. Castrated N rats released LHRH at a reduced rate (50%; P less than 0.01), whereas in castrated H rats the LHRH release rate was reduced by 20%, which corresponded with the partial depletion of the MBH LHRH content in these rats. To examine the possibility of opiate involvement, LHRH release evoked by two consecutive naloxone (NAL) infusions (1 mg/ml for 30 min) was studied. The two NAL infusions resulted in two similar significant increments of LHRH in the MBH-POA of intact N and H rats. However, castration produced different effects on the NAL-induced LHRH release. First, the second NAL pulse was not effective in stimulating LHRH release from the MBH-POA of N and H castrated rats. Further, the first NAL infusion elicited a significant increase in LHRH output from the MBH-POA of N and H castrated rats, but it was significantly lower in comparison with that in their respective intact counterparts. In addition, the NAL-induced LHRH response was higher from the MBH-POA of castrated H than that in castrated N rats. These studies show that neither basal nor evoked LHRH output in vitro is affected by severe and chronic hyperprolactinemia produced by MtTW15 pituitary tumor cells in intact male rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Discriminative effects of testosterone on hypothalamic luteinizing hormone-releasing hormone levels and luteinizing hormone secretion in castrated male rats: analyses of dose and duration characteristics.

We have previously reported that in castrated male rats gonadal steroids can raise LHRH levels in the medial basal hypothalamus (MBH). However, there was an important dissociation between episodic LH release and the MBH LHRH response to 17 beta-estradiol. In the present study we have examined the effects of varying the dose of testosterone (T) on the MBH LHRH levels, episodic LH release pattern, and pituitary responsiveness to LHRH; in addition we have determined the duration of T exposure required to elicit the MBH LHRH response. Results show that low serum levels of T (413-638 pg/ml) were just as effective as higher concentrations (1337-1776 pg/ml) in raising the MBH LHRH levels at 96 h; the minimum duration of exposure to T was 72 h. However, whereas higher T levels (greater than 1 ng/ml) suppressed LH release coincident with elevated LHRH levels, low T concentrations did not alter LH release contemporaneous with the MBH LHRH elevations. Analysis of the episodic LH secretion pattern disclosed that these low concentrations of T changed neither the number of pulses per 3 h per rat, pulse amplitude, nor the interpulse interval. On the other hand, as T concentrations were increased, pulse amplitude fell in a dose-related manner while other components of episodic LH secretion remained unchanged. Despite these differential effects of T on the LH release pattern, pituitary sensitivity to exogenous LHRH was reduced by the low as well as high levels of T. These studies show that 1) T can activate those intracellular neurosecretory events that are involved in augmented LHRH supply to the nerve terminals in the median eminence; 2) T can stimulate LHRH accumulation in the MBH by mechanisms that may not involve inhibition of LHRH release; and 3) higher T levels do not alter LH pulse frequency but depress only the pulse amplitude that may result from decrements in the amounts of LHRH released with each neural episode and depressed pituitary sensitivity to LHRH.

Circadian periodicities of serum androgens, progesterone, gonadotropins and luteinizing hormone-releasing hormone in male rats: the effects of hypothalamic deafferentation, castration and adrenalectomy.

Serum testosterone (T), dihydrotestosterone (DHT), progesterone (P), LH, FSH and luteinizing hormone-releasing hormone (LHRH) levels and the LHRH content in the medial basal hypothalamus (MBH) and preoptic area (POA) were estimated during a 24-h period. Circadian rhythms, temporally unrelated to each other, were evident in serum FSH and LHRH and the MBH LHRH content; serum LH fluctuated randomly. In addition, serum T and DHT demonstrated a parallel circadian pattern while serum T and P appeared inversely related. Serum P rhythm persisted in long-term castrate males. Without adversely affecting serum LH, anterior hypothalamic deafferentation abolished both serum T and P rhythms, whereas adrenalectomy obliterated only the serum T periodicity. These studies show that the POA-MBH complex and the adrenals play important roles in the circadian regulation of testicular secretions.

Opiate-induced hypersensitivity to testosterone feedback: pituitary involvement.

We have examined the mode of morphine's (M) action to increase the sensitivity of castrated male rats to the inhibitory feedback action of testosterone (T) on LH release. In castrated rats, sc implantation of M pellets or 5-mm long T-filled capsules (T5) failed to suppress LH release, but a combination of M and T5 drastically decreased serum LH levels. Likewise, while treatment with a higher dose of T (30-mm long implant, sc) suppressed LH release, combined treatment with M and T30 produced a further suppression of LH levels. We have now assessed the in vitro release rate of LHRH from the medial basal hypothalamus-preoptic area of castrated rats treated with M and/or T as well as the in vivo pituitary LH response to LHRH challenge in similarly treated rats. Interestingly, the in vitro basal and naloxone-induced LHRH release from the medial basal hypothalamus-preoptic area of the six groups of rats was similar, regardless of whether LH levels were in the high castrate or low basal range. On the other hand, M treatment greatly attenuated LH release in vivo in response to LHRH challenge (10pmol-1pmol) [corrected] in T-treated rats. In fact, LH increments in response to 1pmol [corrected] LHRH, seen in control, T5, and T30 groups, were abolished by additional M treatment of T-treated rats. This in vitro assessment of LHRH release suggests that the drastic decrease in LH release in T-plus M-treated rats may not be due to impaired LHRH release, but, rather, be due in part to reduced pituitary responsiveness to intermittent endogenous LHRH signals. The reduced pituitary responsiveness to LHRH in T-plus M-treated rats may be a consequence of either a direct pituitary effect of opiates in conjunction with T or augmented action of hypothalamic neurohumoral agents which may inhibit LH release on their own or antagonize the LH-releasing action of LHRH at the level of pituitary gonadotrophs.

Inhibition of neuropeptide Y (NPY)-induced feeding and c-Fos response in magnocellular paraventricular nucleus by a NPY receptor antagonist: a site of NPY action.

Neuropeptide Y (NPY) is one of the important endogenous orexigenic peptides. In these studies we employed c-Fos immunostaining and a selective NPY Y1 receptor antagonist to identify the site of action of NPY in the hypothalamus. The results showed that intracerebroventricular administration of NPY stimulated feeding and increased immunostaining of c-Fos, a product of the immediate early gene c-fos, in several hypothalamic sites, including the dorsomedial nucleus, the supraoptic nucleus, and the two subdivisions of the paraventricular nucleus (PVN), the parvocellular PVN, and magnocellular PVN (mPVN). Intracerebroventricular administration of 1229U91, a selective NPY Y1 receptor antagonist, affected neither food intake nor c-Fos-like immunoreactivity (FLI) in these hypothalamic sites. Co-administration of NPY and NPY Y1 receptor antagonist inhibited NPY-induced food intake by 48%, but failed to affect NPY-induced FLI in the supraoptic nucleus, dorsomedial nucleus, and parvocellular PVN. However, this combined treatment decreased FLI by 46% in the mPVN (P < 0.05). These results showed that whereas NPY can stimulate FLI in several hypothalamic sites, the selective NPY Y1 antagonist suppressed NPY-induced FLI only in the mPVN. Thus, these findings lend credence to the view that a subpopulation of Y1 receptor-containing neurons in the mPVN in part mediate stimulation of feeding by NPY.

Interleukin-1 inhibits the ovarian steroid-induced luteinizing hormone surge and release of hypothalamic luteinizing hormone-releasing hormone in rats.

Interleukin-1 (IL-1), a polypeptide cytokine secreted by activated macrophages, has been postulated as a chemical messenger between the immune and endocrine systems. IL-1-immunopositive neurons and fibers have been visualized in the human and rat hypothalamus, and IL-1 receptors are present in the rat brain. We have examined the effects of human recombinant IL-1 (alpha- and beta-subtypes) on LH release in vivo and hypothalamic LHRH release in vitro. Ovariectomized rats were primed with estradiol benzoate, and progesterone was injected 48 h later to elicit a LH surge in the afternoon. IL-1 alpha and IL-1 beta were injected either intracerebroventricularly (icv) via a preimplanted cannula in the third ventricle of the brain or iv. Systemic injection of IL-1 alpha or IL-1 beta (58.8 pmol at 1300 and 1500 h) failed to influence the afternoon LH surge seen in saline-injected control rats. However, IL-1 beta (1.76 pmol) administered icv at 1300 and 1500 h or a single icv injection at 1300 h blocked the progesterone-induced LH surge. Similar icv injections of IL-1 alpha also significantly suppressed the afternoon LH surge compared to that in saline-injected control rats. However, IL-1 alpha was relatively less effective than the beta-subtype, since the LH surge was detected in some rats. To ascertain whether suppression of the LH surge was due to inhibition of LHRH release, the medial basal hypothalamus-preoptic area of estradiol benzoate-progesterone-treated ovariectomized rats was incubated with and without IL-1. Both IL-1 alpha and IL-1 beta, at concentrations of 0.1 nM and higher, significantly suppressed LHRH release in vitro from the medial basal hypothalamus-preoptic area. In contrast, IL-1 (10 nM) was completely ineffective in suppressing LHRH release from the microdissected median eminence. These results demonstrated an overall inhibitory effect of icv IL-1 on the LHRH-LH axis and suggest that suppression of the steroid-induced LH surge by IL-1 may primarily be due to inhibition of LHRH release at hypothalamic sites located within the blood-brain barrier.

Evidence that nitric oxide may mediate the ovarian steroid-induced luteinizing hormone surge: involvement of excitatory amino acids.

The involvement of excitatory N-methyl-D-aspartate (NMDA) receptors in the hypothalamic control of pituitary LH secretion is well recognized. Recent evidence shows that nitric oxide (NO), a free radical gas, may act as neurotransmitter in the brain, and its efflux is stimulated by activation of NMDA receptors. Studies were undertaken to determine whether NO is involved in the hypothalamic release of LHRH and in the LH surge induced by progesterone (P) in estrogen-primed ovariectomized rats. Rats were ovariectomized and 2 weeks later received estradiol benzoate (30 micrograms sc) at 1000 h. Two days later, P was injected at 1000 h to potentiate the estradiol benzoate-induced LH surge in the afternoon. Serial blood samples were collected at hourly intervals from 1400-1800 h via an intraatrial cannula implanted the day before P injection. Additionally, at various times before onset of the LH surge at 1400 h, the rats were injected sc with one of three inhibitors of NO synthase, the enzyme that generates NO. Control, saline-injected rats showed unambiguous LH surges in the afternoon. However, either a single injection at 1000 h of NG-methyl-L-arginine (20 mg/kg) or three injections at 1000, 1200, and 1400 h of either Nw-nitro-L-arginine methyl ester (NAME, 40 mg/kg) or Nw-nitro-L-arginine (60 mg/kg) to inhibit NO efflux markedly suppressed the P-induced LH surge in the afternoon. To ascertain whether suppression of LH surge was due to blockade of hypothalamic LHRH release, a series of in vitro studies were performed in steroid-primed rats. First we examined the effects of sodium nitroprusside (NPS), a compound that spontaneously generates and releases NO. NPS increased basal and KCl-induced LHRH release in vitro from the medial basal hypothalamus-preoptic area and median eminence fragments. No direct effect of NO at the pituitary level was seen, since NPS did not alter basal or LHRH-induced LH in vitro release from hemipituitaries. In addition, we tested the effects of NAME on NMDA-induced LHRH release in vitro from the median eminence-arcuate nucleus fragments. As expected, NMDA alone (50 mM) induced a significant increase in LHRH release. Addition of NO synthase inhibitor, NAME (1 or 10 mM) to suppress NO efflux, significantly diminished the NMDA-induced LHRH release.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that hypothalamic neuropeptide Y secretion decreases in aged male rats: implications for reproductive aging.

The decrease in circulating testosterone and LH titer that occurs in aged male rats may in part be a consequence of decreased excitatory neurochemical signals that promote the episodic discharge of LHRH from the hypothalamus. In view of evidence that neuropeptide Y (NPY) stimulates the release and potentiates the action of LHRH on LH secretion, the present studies investigated age-related changes in the concentrations of NPY in individual hypothalamic nuclei and in the ability of hypothalamic tissues to release NPY in vitro. Compared with tissues of 2.5-month-old male rats, medial basal hypothalamic tissues of 13-month-old rats released significantly less NPY in response to K+ depolarization. In contrast, K+-evoked LHRH release from the same tissues was unimpaired. Aged male rats also exhibited markedly reduced concentrations of NPY in the median eminence and in the arcuate, medial preoptic, suprachiasmatic, paraventricular, dorsomedial, and ventromedial hypothalamic nuclei, which are sites of NPY perikarya and nerve terminal networks. These neurochemical changes occurred in association with decreased serum testosterone and LH levels. These findings demonstrate a widespread age-related decline in NPY levels and release in the hypothalamus, which may be responsible for the reduction in testosterone secretion and may contribute to the decline in reproductive function in aged male rats.

Testosterone raises neuropeptide-Y concentration in selected hypothalamic sites and in vitro release from the medial basal hypothalamus of castrated male rats.

Although neuropeptide-Y (NPY)-containing neurons are widely distributed in the hypothalamus, castration decreased NPY concentrations only in the median eminence (ME), arcuate nucleus (ARC), and ventromedial nucleus (VMN). We have now examined the effects of testosterone (T) replacement in 2-week castrated male rats on NPY levels in hypothalamic and preoptic area regions and in vitro NPY release in three experiments. In the first experiment we studied the effect of T on NPY concentration in castrated rats. Two-week castrated rats were implanted sc with T-filled or empty Silastic capsules 30 mm in length. Ten days later rats were killed, and NPY levels were measured by RIA in microdissected sites. T implants raised serum T levels to the range found in gonad-intact rats and decreased serum LH levels to the basal range. Further, of the six brain sites examined, significant increases in NPY concentrations occurred selectively in the ME, ARC, and VMN of T-implanted rats. In the second experiment, the ability of T to reverse the effect of castration on NPY levels compared to those in intact (sham) rats was assessed. Again, castration decreased NPY levels in the ME, ARC, and VMN only, and replacement of physiological levels of T restored NPY levels approximately 100%, 127%, and 74% in the ARC, VMN, and ME, respectively. In the third experiment, the effect of castration and T implants (30-mm T capsules for 10 days) to 2-week castrated rats on the in vitro release of NPY from medial basal hypothalamus (MBH) was assessed. Basal NPY release was not significantly changed after castration and T replacement. However, in response to a 30-min pulse of KCl (45 mM) NPY release from the MBH of castrated rats was significantly reduced compared to that in intact and T-replaced castrated rats. These studies show that castration decreases and T replacement restores NPY levels selectively in three hypothalamic sites, viz. ME, ARC, and VMN, and KCl-induced NPY release from the MBH in vitro is decreased after castration and restored by T replacement, thereby suggesting that a local subset of androgen-concentrating neurons may regulate NPY levels and release in a site-specific manner. Further, these results are in line with our emerging view that gonadal steroids modulate neurosecretion not only of LHRH, but also of other functionally linked regulatory peptides.

Evidence for the existence of distinct central appetite, energy expenditure, and ghrelin stimulation pathways as revealed by hypothalamic site-specific leptin gene therapy.

To identify the specific hypothalamic sites in which leptin acts to decrease energy intake and/or increase energy expenditure, recombinant adeno-associated virus vector-encoding leptin was microinjected bilaterally into one of four hypothalamic sites in female rats. Leptin transgene expression in the ventromedial nucleus and paraventricular nucleus induced comparable decreases in daily food intake (FI; 18-20%) and body weight (BW; 26-29%), accompanied by drastic reductions in serum leptin (81-97%), insulin (92-93%), free fatty acids (35-36%), and normoglycemia. Leptin transgene expression in the arcuate nucleus (ARC) decreased BW gain (21%) and FI (11%) to a lesser range, but the metabolic hormones were suppressed to the same extent. Leptin transgene expression in the medial preoptic area (MPOA) decreased BW and metabolic hormones without decreasing FI. Finally, leptin transgene expression in all four sites augmented serum ghrelin and thermogenic energy expenditure, as shown by uncoupling protein-1 mRNA expression in brown adipose tissue. Proopiomelanocortin gene expression in the ARC was up-regulated by leptin expression in all four sites, but neuropeptide Y gene expression in the ARC was suppressed by leptin transgene expression in the ARC but not in the MPOA. Thus, whereas leptin expression in the paraventricular nucleus, ventromedial nucleus, or ARC suppresses adiposity and insulin by decreasing energy intake and increasing energy expenditure, in the MPOA it suppresses these variables by increasing energy expenditure alone.

Progesterone-induced changes in hypothalamic luteinizing hormone-releasing hormone and catecholamines: differential effects of pentobarbital.

The effects of pentobarbital (Pnt) treatment on the progesterone (P)-induced afternoon increase in the medial basal hypothalamic (MBH) LHRH and serum LH and FSH levels in ovariectomized estradiol benzoate-primed rats were studied. Pnt injection before P blocked the afternoon rise in serum gonadotropins but failed to alter the increase in the MBH LHRH levels. Moreover, when Pnt was injected 150 min after P, the MBH LHRH content continued to rise to levels 25-37% above those seen in control rats. Analyses of LHRH concentrations in discrete hypothalamic nuclei revealed that the Pnt-induced accumulation was confined mainly to the median eminence, with a small increase in the suprachiasmatic nuclei region. P administration increased the MBH norepinephrine activity and concurrently decreased dopamine activity. Pnt was ineffective in suppressing the MBH LHRH response in these rats, but drastically reduced norepinephrine and accelerated dopamine turnovers in the MBH. These studies show 1) no definitive cause and effect relationship of the increments in MBH LHRH either with LH release (or LHRH release) or with changes in hypothalamic catecholamines induced by P treatment, and 2) that the striking rise in the MBH LHRH levels in estradiol benzoate-primed rats may represent formation of new immunoreactive LHRH predominantly in the median eminence region.

Evidence that stimulation of two modalities of pituitary luteinizing hormone release in ovarian steroid-primed ovariectomized rats may involve neuropeptide Y Y1 and Y4 receptors.

A large body of evidence indicates that neuropeptide Y (NPY) is involved in stimulation of basal and cyclic release of hypothalamic LHRH and pituitary LH. To identify the NPY receptor subtypes that mediate the excitatory effects of NPY in these two modalities of LH release, we studied the effects of 1229U91, a selective Y1 receptor antagonist and Y4 receptor agonist, in two experimental paradigms that reproduce the two modalities of LH secretion in steroid-primed ovariectomized (OVX) rats. Rats were ovariectomized and implanted with a permanent cannula into the lateral cerebroventricle. In the first experiment, rats received estradiol benzoate (EB, 30 microg/rat) on day 5, followed 2 days later with progesterone (2 mg/rat) at 1000 h to induce an afternoon LH surge. 1229U91 (30 microg/3 microl) or vehicle (control) was injected intracerebroventricularly into these rats either once at 1300 h or twice (15 microg/injection) at 1100 and 1200 h. Blood samples were collected before progesterone injection at 1000 h and at hourly intervals from 1300 -1800 h via an intrajugular cannula implanted on the previous day. In control rats, serum LH levels rose significantly at 1400 h, and these high levels were maintained until 1700 h. After two injections of 1229U91, LH levels displayed a tendency to rise at 1300-1400 h, as in controls, but thereafter, decreased rapidly below the control range. In the second experiment, the acute effect of 1229U91 on LH release was evaluated in OVX rats pretreated with EB alone. Saline alone or saline containing 1, 3, 10, or 30 microg 1229U91 was injected intracerebroventricularly at 1000 h, and the effects on LH release were analyzed at 10, 20, 30, and 60 min. 1229U91 elicited a dose-dependent stimulation of LH release, with maximal response (950% of basal levels) occurring at 10 min after the 30-microg dose; elevated levels were maintained for 1 h. Because 1229U91 is a potent Y4 agonist with some affinity for Y5 receptors, these results raised the possibility that activation of Y4/Y5 receptors by 1229U91 may augment LH release. Therefore, we examined the effects of icv administration of rat pancreatic polypeptide, a Y4-selective agonist, and [D-Trp32]-NPY, a Y5 agonist on LH release in EB-primed rats. Rat pancreatic polypeptide (0.5-2 microg/rat) stimulated LH release in a dose-related manner, and peak levels (280% of basal levels) were seen at 10-20 min; the response evoked by a higher dose (10 microg) was smaller than that induced by 0.5 or 2 microg. [D-Trp32]-NPY was relatively less effective, because only the highest (10-microg) dose elicited a modest stimulation (244% of basal levels). These results demonstrate that 1229U91, a Y1 antagonist and Y4 agonist, evokes two types of responses; it suppresses the protracted ovarian steroid-induced LH surge, and acutely, it also stimulates LH. These results imply that normally two different types of NPY receptors may mediate the stimulation of LH release. Because 1229U91 is a Y1 receptor antagonist, inhibition of the steroid-induced LH surge by 1229U91 suggests that Y1 receptors may mediate the cyclic release of LH. On the other hand, acute stimulation of LH by 1229U91 implies that the Y4 agonist-like activity of 1229U91 may mediate the basal release of LH and that either NPY or a yet-to-be-identified endogenous Y4 receptor agonist may activate Y4 receptors in the hypothalamus to stimulate LH release.

Evidence that gonadal steroids modulate nitric oxide efflux in the medial preoptic area: effects of N-methyl-D-aspartate and correlation with luteinizing hormone secretion.

Several lines of evidence suggest that nitric oxide (NO) is involved in the neuroendocrine control of reproductive function. This study was undertaken to determine 1) NO activity in the medial preoptic area (MPOA) where LHRH- and NO synthase-containing neurons are coextensive; 2) whether N-methyl-D-aspartate (NMDA) receptor activation, which stimulates LHRH release, augments NO activity in the MPOA; and 3) whether NO activation in the MPOA underlies the steroid dependency of NMDA-induced pituitary LH release. As extracellular levels of cGMP in discrete brain sites are a reliable index of basal and stimulated activity of NO, extracellular cGMP levels in the MPOA of freely moving, awake rats were measured by microdialysis in the current study. In the first experiment, the MPOA of intact and castrated male rats were microdialyzed with artificial cerebrospinal fluid at a rate of 5 microliters/min. The basal level of cGMP efflux was determined from the initial seven samples collected at 20-min intervals. The NO response to a single i.v. injection of NMDA (10 mg/kg) or saline was assessed in the next five samples. In the second experiment, the basal and NMDA-evoked NO effluxes in the MPOA of ovariectomized (ovx) and estrogen-treated ovx rats were examined. Results showed that in both sexes, the absence of gonadal steroids resulted in significantly lower basal cGMP levels. Additionally, the cGMP response to NMDA was steroid dependent. Whereas in castrated rats it failed to affect cGMP efflux, NMDA in intact male rats promptly raised cGMP levels at 20 min, and these elevated levels were maintained through the duration of the experiment. This NMDA-induced cGMP response, observed selectively in intact rats, was also associated with stimulation of plasma LH levels. In female rats, NMDA similarly enhanced MPOA cGMP efflux and pituitary LH secretion in estradiol benzoate-treated, but not in oil-treated, ovx rats. The NMDA receptor antagonist D,L-amino-5-phosphoropentanoic acid and the NO synthase inhibitor, N omega-nitro-L-arginine, completely blocked the NMDA-induced cGMP and LH responses, thereby demonstrating the specificity of the NMDA receptor --> NO line of communication probably operating in the MPOA in the control of pituitary LH release. Therefore, these results show that gonadal steroids augment basal as well as NMDA-induced MPOA cGMP efflux in male and female rats. It is likely that facilitation of NO/cGMP activity in the MPOA may underlie the steroid dependency of NMDA-evoked LH hypersecretion in the rat.

L-arginine/nitric oxide amplifies the magnitude and duration of the luteinizing hormone surge induced by estrogen: involvement of neuropeptide Y.

Although estrogen can induce LH hypersecretion in ovariectomized (ovx) rats, the magnitude and duration of the LH surge are invariably far less than those of the preovulatory LH surge in cycling rats. Recently, the crucial roles of hypothalamic nitric oxide (NO) and neuropeptide Y (NPY) in the induction of LHRH and the LH surge have been recognized. In this study, we have tested the hypothesis that low grade stimulatory feedback effects of 17 beta-estradiol (E2) on LH secretion in ovx rats is due to a deficit in NO signaling. As the NO substrate, L-arginine (L-Arg), can enhance NO-dependent physiological responses, the effects of L-Arg on the LH surge in E2-primed ovx rats was studied. Ovx rats bearing permanent cannulas in the lateral cerebroventricle received 15-mm long SILASTIC capsules filled with E2 (300 micrograms/ml oil, s.c.) at 1000 h. Two days later, saline (SAL) or L-Arg (0.1 or 1 nmol in 5 microliters SAL) or D-Arg (1.0 nmol) was injected intracerebroventricularly every hour from 1100-1400. Blood samples were collected at 1100 h and at hourly intervals from 1400-1800 h via intraatrial cannulae implanted the day before. Whereas an expected moderate LH surge occurred in SAL-injected control rats, L-Arg, but not D-Arg, greatly augmented the magnitude and duration of the LH surge, with peak values attaining the range normally seen on proestrus. Furthermore, L-Arg (1 or 10.0 mM) and the NO donor, sodium nitroprusside (1 or 10.0 mM), stimulated the basal and K(+)-induced in vitro release of LHRH and NPY from the hypothalami of ovarian steroid-primed ovx rats. As NPY is stimulatory to LHRH release, we next tested the possibility that L-Arg/NO may initially stimulate NPY release in the hypothalamus, which, in turn, elicits LHRH discharge. E2-treated ovx rats receiving intracerebroventricular L-Arg were injected with antisense or missense oligodeoxynucleotides (oligos) to NPY-messenger RNA. Antisense, not missense, NPY oligos blocked the L-Arg-induced potentiation of the LH surge. These results revealed for the first time the neurochemical cause underlying the E2 feedback action and show that a deficiency in either L-Arg itself or L-Arg-based NO signaling is responsible for the low grade LH surge in ovx rats treated with estrogen alone. Additionally, the L-Arg-dependent potentiation of the LH surge may involve increased signaling in the NO and NPYergic systems, resulting in optimal LHRH hypersecretion from the hypothalamus.

Neuropeptide Y stimulates feeding but inhibits sexual behavior in rats.

The effects of neuropeptide Y (NPY), a tyrosine-rich peptide found in the rat brain, on feeding and sexual behavior were studied in male and female rats. Intraventricular (ivt) injections of NPY during the final hours of the light period induced feeding in a dose-related manner. While the lowest dose tested (0.02 nM) was without effect, higher doses (0.12, 0.47, 2.3 nM) uniformly elicited feeding with a latency of about 15 min in male rats. With the most effective dose, 0.47 nM, the increased food intake was due to an increased local eating rate. In contrast, the pattern of feeding behavior after a related peptide, rat pancreatic polypeptide (rPP), was quite different and less impressive. During the first hour, only one ivt dose of rPP (0.45 nM) evoked an increase in food intake, due to an increased time spent eating. Further, the effects of NPY on food intake were greater during the nocturnal period. Interestingly, increased food intake in nocturnal tests (4 h) was due solely to augmented intake during the first 60 min after ivt administration. In mating tests, initiated 2 h after the onset of darkness and 10 min after ivt administration of peptide, all but the lowest dose of NPY (0.01 nM) drastically suppressed ejaculatory behavior. Most rats treated with higher doses of NPY (0.02, 0.12, or 0.47 nM) mounted and intromitted only a few times before the cessation of sexual activity, and elongated latencies to the initial mount and intromission were observed. In contrast to the dramatic NPY-induced suppression of ejaculatory behavior, rPP (0.11 and 0.45 nM) was without effect on copulatory behavior. To substantiate further that the impairment of sexual behavior seen in NPY-treated rats was not due to an attenuated sexual ability, an additional experiment was performed. Penile reflexes, including erection, were monitored 10 min after ivt injection of NPY (0.12 nM), rPP (0.11 nM), or saline. No effect of NPY or rPP was observed on the proportion of rats showing erection or latency to initial erection, or in the number of erections per test. In fact, a slight facilitation of penile dorsiflexion responses was seen after NPY. These findings suggest that NPY selectively depresses sexual motivation in the male rat. In ovariectomized female rats responding to estrogen plus progesterone with a good level of sexual receptivity (lordosis quotient greater than 70), ivt saline and 0.01 nM NPY were without effect on sexual behavior.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropeptide-Y concentration in microdissected hypothalamic regions and in vitro release from the medial basal hypothalamus-preoptic area of streptozotocin-diabetic rats with and without insulin substitution therapy.

Experimental diabetes adversely affects hypothalamic control of gonadotropin secretion and sex behavior and induces hyperphagia accompanied by severe body weight loss. Neuropeptide-Y (NPY) stimulates pituitary gonadotropin release, inhibits sexual behavior, and stimulates robust feeding in rats by acting at different sites in the hypothalamus. Therefore, we tested the hypothesis that altered hypothalamic NPY neurosecretion may mediate the constellation of effects observed in streptozotocin-induced diabetic (STZ-D) rats. Adult male rats were made diabetic by a single injection of STZ (50 mg/kg). Five months later, in vitro NPY release from the hypothalamic fragment encompassing the medial basal hypothalamus and preoptic area and NPY concentrations in seven hypothalamic sites were assessed. Basal NPY release was not significantly changed after STZ treatment. However, in response to a 30-min pulse of KCl (45 mM), NPY release from the medial basal hypothalamus-preoptic area of STZ-D rats was significantly increased compared to that in age-matched controls. In the STZ-D rats, NPY concentrations in six of the seven microdissected nuclei, including those mediating control of pituitary gonadotropin, sexual, and feeding behaviors, were increased compared to control values. In an additional study similar increments in NPY concentrations in the hypothalamic sites were observed 6 months after STZ treatment. The effects of insulin on NPY levels in microdissected hypothalamic sites in STZ-treated and BB diabetic rats was next assessed. One group of rats was treated with STZ, and the other group of rats was additionally treated with insulin (6 U/kg.day) for 3 months after development of diabetes with STZ. Again, STZ treatment alone, even for 3 months, increased NPY levels in all seven nuclei, including the suprachiasmatic nuclei. Insulin therapy completely prevented the STZ-induced increments in NPY levels in all hypothalamic sites, and the blood glucose level was 233 +/- 22 mg/dl in insulin-treated STZ-D rats and 496 +/- 6 mg/dl in untreated STZ-D rats. Similarly, NPY concentrations in five of the seven nuclei were unchanged in spontaneously diabetic BB rats (blood glucose, 435 +/- 67 mg/dl) maintained on insulin (5-8 U/kg.day). These results demonstrate that STZ-D rats have a widespread increase in NPY levels in hypothalamic sites, and there is an increase in the evoked release of NPY from the hypothalamus.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional heterogeneity in neuropeptide-Y-producing cells in the rat brain as revealed by testosterone action.

Despite the widespread distribution of neuropeptide-Y (NPY) in various hypothalamic sites, castration reduced and testosterone (T) replacement restored NPY levels selectively in the median eminence (ME), arcuate nucleus (ARC), and ventromedial nucleus (VMN). Since androgen-concentrating cells and NPY-producing cells display overlapping distribution in the ARC and brain stem (BS), we assessed the participation of BS NPY cells in the steroid-dependent site-specific effects on NPY levels. The BS projections to the hypothalamus were severed by bilateral neural transection (BNT) with a knife lowered on either side of the sagittal sinus to the depth of the dorsal tegmentum in the mesencephalon in intact, castrated, or castrated rats that additionally received sc T implants to maintain physiological T levels. Two weeks later, NPY concentrations in microdissected hypothalamic sites and serum LH and T levels were quantitated by RIA. Castration decreased and T replacement increased NPY concentrations in only three sites, such as the VMN, ARC, and ME. In response to BNT in gonadally intact rats, a different site-specific response of NPY levels was observed; NPY levels decreased in the ME, as seen after castration, and, additionally, decreased in the medial preoptic area, paraventricular nucleus, and dorsomedial nucleus, suggesting that BS NPY neurons innervate these four sites. When castration and BNT were performed simultaneously, a combined regional response was evident. NPY levels decreased in six sites, including two sites (ARC and VMN) that normally respond to castration alone, three sites (medial preoptic area, paraventricular nucleus, and dorsomedial nucleus) that normally respond to BNT alone, and the ME, the only site that showed reduction of NPY levels of similar magnitude after either castration or BNT, but the response of combined surgery was not additive. This observation suggested that gonadal steroids act outside the hypothalamus to raise ME NPY levels, and therefore, BNT in intact rats impaired the effectiveness of steroids. To test this hypothesis, the effects of physiological T replacement in the castrate plus BNT group were studied. We observed that whereas T replacement readily raised NPY levels in the VMN and ARC, it was completely ineffective in the third T-dependent site, the ME. Collectively, these findings revealed a functional heterogeneity among NPY-producing cells in response to T. Apparently, there are two distinct neural sites in the rat brain where T acts to exert a site-specific stimulatory effect on NPY in the hypothalamus.(ABSTRACT TRUNCATED AT 400 WORDS)