A possible role of neuropeptide Y as a mediator of undernutrition to the hypothalamic gonadotropin-releasing hormone pulse generator in goats

Estrogen receptor-α signaling in tanycytes lies at the crossroads of fertility and metabolism

BACKGROUND

Estrogen secretion by the ovaries regulates the hypothalamic-pituitary-gonadal axis during the reproductive cycle, influencing gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) secretion, and also plays a role in regulating metabolism. Here, we establish that hypothalamic tanycytes-specialized glia lining the floor and walls of the third ventricle-integrate estrogenic feedback signals from the gonads and couple reproduction with metabolism by relaying this information to orexigenic neuropeptide Y (NPY) neurons.

METHODS

Using mouse models, including mice floxed for Esr1 (encoding estrogen receptor alpha, ERα) and those with Cre-dependent expression of designer receptors exclusively activated by designer drugs (DREADDs), along with viral-mediated, pharmacological and indirect calorimetric approaches, we evaluated the role of tanycytes and tanycytic estrogen signaling in pulsatile LH secretion, cFos expression in NPY neurons, estrous cyclicity, body-weight changes and metabolic parameters in adult females.

RESULTS

In ovariectomized mice, chemogenetic activation of tanycytes significantly reduced LH pulsatile release, mimicking the effects of direct NPY neuron activation. In intact mice, tanycytes were crucial for the estrogen-mediated control of GnRH/LH release, with tanycytic ERα activation suppressing fasting-induced NPY neuron activation. Selective knockout of Esr1 in tanycytes altered estrous cyclicity and fertility in female mice and affected estrogen's ability to inhibit refeeding in fasting mice. The absence of ERα signaling in tanycytes increased Npy transcripts and body weight in intact mice and prevented the estrogen-mediated decrease in food intake as well as increase in energy expenditure and fatty acid oxidation in ovariectomized mice.

CONCLUSIONS

Our findings underscore the pivotal role of tanycytes in the neuroendocrine coupling of reproduction and metabolism, with potential implications for its age-related deregulation after menopause.

SIGNIFICANCE STATEMENT

Our investigation reveals that tanycytes, specialized glial cells in the brain, are key interpreters of estrogen signals for orexigenic NPY neurons in the hypothalamus. Disrupting tanycytic estrogen receptors not only alters fertility in female mice but also impairs the ability of estrogens to suppress appetite. This work thus sheds light on the critical role played by tanycytes in bridging the hormonal regulation of cyclic reproductive function and appetite/feeding behavior. This understanding may have potential implications for age-related metabolic deregulation after menopause.

Raphe glucose-sensing serotonergic neurons stimulate KNDy neurons to enhance LH pulses via 5HT2CR: rat and goat studies

Dysfunction of central serotonergic neurons is known to cause depressive disorders in humans, who often show reproductive and/or glucose metabolism disorders. This study examined whether dorsal raphe (DR) serotonergic neurons sense high glucose availability to upregulate reproductive function via activating hypothalamic arcuate (ARC) kisspeptin neurons (= KNDy neurons), a dominant stimulator of gonadotropin-releasing hormone (GnRH)/gonadotropin pulses, using female rats and goats. RNA-seq and histological analysis revealed that stimulatory serotonin-2C receptor (5HT2CR) was mainly expressed in the KNDy neurons in female rats. The serotonergic reuptake inhibitor administration into the mediobasal hypothalamus (MBH), including the ARC, significantly blocked glucoprivic suppression of luteinizing hormone (LH) pulses and hyperglycemia induced by intravenous 2-deoxy-D-glucose (2DG) administration in female rats. A local infusion of glucose into the DR significantly increased in vivo serotonin release in the MBH and partly restored LH pulses and hyperglycemia in the 2DG-treated female rats. Furthermore, central administration of serotonin or a 5HT2CR agonist immediately evoked GnRH pulse generator activity, and central 5HT2CR antagonism blocked the serotonin-induced facilitation of GnRH pulse generator activity in ovariectomized goats. These results suggest that DR serotonergic neurons sense high glucose availability to reduce gluconeogenesis and upregulate reproductive function by activating GnRH/LH pulse generator activity in mammals.

Continuous acceleration of neural activity of the GnRH pulse generator during chronic peripheral infusion of neurokinin 3 receptor agonist in goats

Secretion of pulsatile gonadotropin-releasing hormone (GnRH) is essential for reproduction. Kisspeptin neurons in the arcuate nucleus (ARC), which coexpress neurokinin B (NKB) and its receptor (NK3R), are believed to be components of the GnRH pulse generator that regulates pulsatile GnRH secretion. We examined the effects of peripheral infusion of senktide, an NK3R selective agonist, on GnRH pulse generator activity by monitoring multiple unit activity (MUA) in the goat ARC. Previous studies have shown that characteristic increases in MUA (MUA volleys) reflect GnRH pulse generator activity. Senktide was infused intravenously or intravaginally for 2 h while recording MUA. Both infusions significantly increased the MUA volley frequency compared with the control. These results demonstrate that peripherally administered senktide acts centrally to sustainably accelerate the neural activity of the GnRH pulse generator throughout the infusion period. This suggests the possibility of practical applications of NK3R agonists for improving reproductive activity in farm animals.

Peripheral administration of SB223412, a selective neurokinin-3 receptor antagonist, suppresses pulsatile luteinizing hormone secretion by acting on the gonadotropin-releasing hormone pulse generator in estrogen-treated ovariectomized female goats

Accumulating evidence suggests that KNDy neurons located in the hypothalamic arcuate nucleus (ARC), which are reported to express kisspeptin, neurokinin B, and dynorphin A, are indispensable for the gonadotropin-releasing hormone (GnRH) pulse generation that results in rhythmic GnRH secretion. The aims of the present study were to investigate the effects of peripheral administration of the neurokinin 3 receptor (NK3R/TACR3, a receptor for neurokinin B) antagonist, SB223412, on GnRH pulse-generating activity and pulsatile luteinizing hormone (LH) secretion in ovariectomized Shiba goats treated with luteal phase levels of estrogen. The NK3R antagonist was infused intravenously for 4 h {0.16 or 1.6 mg/(kg body weight [BW]·4 h)} during which multiple unit activity (MUA) in the ARC was recorded, an electrophysiological technique commonly employed to monitor GnRH pulse generator activity. In a separate experiment, the NK3R antagonist (40 or 200 mg/[kg BW·day]) was administered orally for 7 days to determine whether the NK3R antagonist could modulate pulsatile LH secretion when administered via the oral route. Intravenous infusion of the NK3R antagonist significantly increased the interval of episodic bursts of MUA compared with that of the controls. Oral administration of the antagonist for 7 days also significantly prolonged the interpulse interval of LH pulses. The results of this study demonstrate that peripheral administration of an NK3R antagonist suppresses pulsatile LH secretion by acting on the GnRH pulse generator, suggesting that NK3R antagonist administration could be used to modulate reproductive functions in ruminants.

Peripheral administration of κ-opioid receptor antagonist stimulates gonadotropin-releasing hormone pulse generator activity in ovariectomized, estrogen-treated female goats

Pulsatile gonadotropin-releasing hormone (GnRH) secretion is indispensable for reproduction in mammals. Kisspeptin neurons in the hypothalamic arcuate nucleus (ARC), referred to as KNDy neurons because of the coexpression of neurokinin B and dynorphin A, are considered as components of the GnRH pulse generator that produces rhythmic GnRH secretion. The present study aimed to investigate if peripheral administration of PF-4455242, a κ-opioid receptor (KOR, a dynorphin A receptor) antagonist, facilitates pulsatile luteinizing hormone (LH) secretion and GnRH pulse generator activity in estrogen-treated ovariectomized Shiba goats to determine the possibility of using KOR antagonists to artificially control ovarian activities. PF-4455242 was intravenously infused for 4 h (1 or 10 μmol/kg body weight/4 h) or as a single subcutaneous injection (1 or 10 μmol/kg body weight). In a separate experiment, the same KOR antagonist (10 μmol/kg body weight/4 h) was intravenously infused during the recording of multiple unit activity (MUA) in the ARC that reflects the activity of the GnRH pulse generator to test the effects of KOR antagonist administration on GnRH pulse generator activity. Intravenous infusion and single subcutaneous injection of the KOR antagonist significantly increased the frequency of LH pulses compared with controls. Intravenous infusion of KOR antagonist also significantly increased the frequency of episodic bursts in the MUA. The present study demonstrates that peripherally administered KOR antagonist stimulates pulsatile LH secretion by acting on the GnRH pulse generator, and peripheral administration of PF-4455242 can be used to facilitate pulsatile LH secretion, which in turn facilitates ovarian activities in farm animals.

Return of postpartum ovarian activity in dairy goats supplemented with different levels of energy

This study aimed to evaluate the return of post partum ovarian activity in dairy goats supplemented with different levels of energy during the dry period in the semi-arid region of Pernambuco. Twenty-four animals were allocated into four treatments of six animals each: Control, consisting of goats that had access to pasture and only received 4 kg of fresh palm, and three groups with isoproteic supplementation (20% Crude Protein), ranging in TDN content of 65%, 75% and 85%. The daily milk yield showed a linear growth behavior (P <0.05) since day 35th postpartum, when the animals recovered the weight gain and body condition score after 14 and 21 days postpartum, respectively, with slight weight gain in 75% and 85% treatments. Among the reproductive parameters, uterine involution and return to estrus activity showed a decrease in the number of days associated with the higher energy levels (P <0.05). The emergence of the 1st follicles > 2 mm occurred around 33 days postpartum and the maximum diameter of the pre-ovulatory follicle was 5,41mm. These results were not influenced by treatments (P> 0.05). We concluded that the supplementation with different levels of energy promoted a positive effect, as increased productive parameters and reduced the number of days for the reestablishment of the reproductive parameters postpartum; for instance, uterine involution, return to estrus and consequently return of ovarian activity postpartum.

Effects of intravenous administration of neurokinin receptor subtype-selective agonists on gonadotropin-releasing hormone pulse generator activity and luteinizing hormone secretion in goats

Recent evidence suggests that neurokinin B (NKB), a member of the neurokinin (tachykinin) peptide family, plays a pivotal role in gonadotropin-releasing hormone (GnRH) pulse generation. Three types of neurokinin receptors (NKRs), NK1R, NK2R and NK3R, are found in the brain. Although NKB preferentially binds to NK3R, other NKRs are possibly also involved in NKB action. The present study examined the effects of intravenous administration of the NKR subtype-selective agonists GR73632 (NK1R), GR64349 (NK2R), and senktide (NK3R) on GnRH pulse generator activity and luteinizing hormone (LH) secretion. Multiple-unit activity (MUA) was monitored in ovariectomized goats (n = 5) implanted with recording electrodes. Characteristic increases in MUA (MUA volleys) were considered GnRH pulse generator activity. Although three NKR agonists dose-dependently induced an MUA volley and an accompanying increase in LH secretion, the efficacy in inducing the volley markedly differed. As little as 10 nmol of senktide induced an MUA volley in all goats, whereas a dose of 1000 nmol was only effective for the NK1R and NK2R agonists in two and four goats, respectively. When the treatment failed to evoke an MUA volley, no apparent change was observed in the MUA or LH secretion. Similar effects of the NK2R and NK3R agonists were observed in the presence of estradiol. The results demonstrated that NK3R plays a predominant role in GnRH pulse generation and suggested that the contributions of NK1R and NK2R to this mechanism may be few, if any, in goats.

Regulation of gonadotropin secretion by monitoring energy availability

Nutrition is a principal environmental factor influencing fertility in animals. Energy deficit causes amenorrhea, delayed puberty, and suppression of copulatory behaviors by inhibiting gonadal activity. When gonadal activity is impaired by malnutrition, the signals originating from an undernourished state are ultimately conveyed to the gonadotropin-releasing hormone (GnRH) pulse generator, leading to suppressed secretion of GnRH and luteinizing hormone (LH). The mechanism responsible for energetic control of gonadotropin release is believed to involve metabolic signals, sensing mechanisms, and neuroendocrine pathways. The availabilities of blood-borne energy substrates such as glucose, fatty acids, and ketone bodies, which fluctuate in parallel with changes in nutritional status, act as metabolic signals that regulate the GnRH pulse generator activity and GnRH/LH release. As components of the specific sensing system, the ependymocytes lining the cerebroventricular wall in the lower brainstem integrate the information derived from metabolic signals to control gonadotropin release. One of the pathways responsible for the energetic control of gonadal activity consists of noradrenergic neurons from the solitary tract nucleus in the lower brainstem, projecting to the paraventricular nucleus of the hypothalamus. Further studies are needed to elucidate the mechanisms underlying energetic control of reproductive function.

A population of kisspeptin/neurokinin B neurons in the arcuate nucleus may be the central target of the male effect phenomenon in goats

Exposure of females to a male pheromone accelerates pulsatile gonadotropin-releasing hormone (GnRH) secretion in goats. Recent evidence has suggested that neurons in the arcuate nucleus (ARC) containing kisspeptin and neurokinin B (NKB) play a pivotal role in the control of GnRH secretion. Therefore, we hypothesized that these neurons may be the central target of the male pheromone. To test this hypothesis, we examined whether NKB signaling is involved in the pheromone action, and whether ARC kisspeptin/NKB neurons receive input from the medial nucleus of the amygdala (MeA)—the nucleus suggested to relay pheromone signals. Ovariectomized goats were implanted with a recording electrode aimed at a population of ARC kisspeptin/NKB neurons, and GnRH pulse generator activity, represented by characteristic increases in multiple-unit activity (MUA) volleys, was measured. Pheromone exposure induced an MUA volley and luteinizing hormone (LH) pulse in control animals, whereas the MUA and LH responses to the pheromone were completely suppressed by the treatment with an NKB receptor antagonist. These results indicate that NKB signaling is a prerequisite for pheromone action. In ovariectomized goats, an anterograde tracer was injected into the MeA, and possible connections between the MeA and ARC kisspeptin/NKB neurons were examined. Histochemical observations demonstrated that a subset of ARC kisspeptin/NKB neurons receive efferent projections from the MeA. These results suggest that the male pheromone signal is conveyed via the MeA to ARC kisspeptin neurons, wherein the signal stimulates GnRH pulse generator activity through an NKB signaling-mediated mechanism in goats.

Intermittent fasting dietary restriction regimen negatively influences reproduction in young rats: a study of hypothalamo-hypophysial-gonadal axis

Nutritional infertility is very common in societies where women fail to eat enough to match their energy expenditure and such females often present as clinical cases of anorexia nervosa. The cellular and molecular mechanisms that link energy balance and central regulation of reproduction are still not well understood. Peripheral hormones such as estradiol, testosterone and leptin, as well as neuropeptides like kisspeptin and neuropeptides Y (NPY) play a potential role in regulation of reproduction and energy balance with their primary target converging on the hypothalamic median eminence-arcuate region. The present study was aimed to explore the effects of negative energy state resulting from intermittent fasting dietary restriction (IF-DR) regimen on complete hypothalamo-hypophysial-gonadal axis in Wistar strain young female and male rats. Significant changes in body weight, blood glucose, estrous cyclicity and serum estradiol, testosterone and LH level indicated the negative role of IF-DR regimen on reproduction in these young animals. Further, it was elucidated whether serum level of metabolic hormone, leptin plays a mechanistic role in suppressing hypothalamo-hypophysial-gonadal (HPG) axis via energy regulators, kisspeptin and NPY in rats on IF-DR regimen. We also studied the effect of IF-DR regimen on structural remodeling of GnRH axon terminals in median eminence region of hypothalamus along with the glial cell marker, GFAP and neuronal plasticity marker, PSA-NCAM using immunostaining, Western blotting and RT-PCR. Together these data suggest that IF-DR regimen negatively influences reproduction in young animals due to its adverse effects on complete hypothalamus-hypophysial-gonadal axis and may explain underlying mechanism(s) to understand the clinical basis of nutritional infertility.

Regulation of gonadotropin-releasing hormone neurons by glucose

Reproduction is influenced by energy balance, but the physiological pathways mediating their relationship have not been fully elucidated. As the central regulators of fertility, gonadotropin-releasing hormone (GnRH) neurons integrate numerous physiological signals, including metabolic cues. Circulating glucose levels regulate GnRH release and might in part mediate the effects of negative energy balance on fertility. Existing evidence suggests that neural pathways originating in the hindbrain, as well as in the hypothalamic feeding nuclei, transmit information concerning glucose availability to GnRH neurons. Here we review recent evidence suggesting that GnRH neurons might directly sense changes in glucose availability by a mechanism involving AMP-activated protein kinase. These findings expand our understanding of how metabolic signaling in the brain regulates reproduction.

Kisspeptin directly excites anorexigenic proopiomelanocortin neurons but inhibits orexigenic neuropeptide Y cells by an indirect synaptic mechanism

The neuropeptide kisspeptin is necessary for reproduction, fertility, and puberty. Here, we show strong kisspeptin innervation of hypothalamic anorexigenic proopiomelanocortin (POMC) cells, coupled with a robust direct excitatory response by POMC neurons (n > 200) to kisspeptin, mediated by mechanisms based on activation of a sodium/calcium exchanger and possibly opening of nonselective cation channels. The excitatory actions of kisspeptin on POMC cells were corroborated with quantitative PCR data showing kisspeptin receptor GPR54 expression in the arcuate nucleus, and the attenuation of excitation by the selective kisspeptin receptor antagonist, peptide 234. In contrast, kisspeptin inhibits orexigenic neuropeptide Y (NPY) neurons through an indirect mechanism based on enhancing GABA-mediated inhibitory synaptic tone. In striking contrast, gonadotropin-inhibiting hormone (GnIH and RFRP-3) and NPY, also found in axons abutting POMC cells, inhibit POMC cells and attenuate the kisspeptin excitation by a mechanism based on opening potassium channels. Together, these data suggest that the two central peptides that regulate reproduction, kisspeptin and GnIH, exert a strong direct action on POMC neurons. POMC cells may hypothetically serve as a conditional relay station downstream of kisspeptin and GnIH to signal the availability of energy resources relevant to reproduction.

Male effect pheromone tickles the gonadotrophin-releasing hormone pulse generator

In sheep and goats, the primer pheromone produced by the male induces out-of-seasonal ovulation in anoestrous females, the so-called 'male effect.' Because the initial endocrine event following reception of the pheromone is the stimulation of pulsatile luteinising hormone (LH) secretion, the central target of the pheromone is considered to be the putative gonadotrophin-releasing hormone (GnRH) pulse generator. Using electrophysiological techniques to record multiple-unit activity (MUA) in close proximity to kisspeptin neurones in the arcuate nucleus (ARC) of Shiba goats, we found that bursts (volleys) of MUA occur at regular intervals, and repetitive bursts are invariably associated with discrete pulses of LH, suggesting that the ARC kisspeptin neurones may be the intrinsic source of the GnRH pulse generator. A brief exposure of female goats to the pheromone immediately elicited an instantaneous rise in MUA, which is followed by an MUA volley and an accompanying LH pulse, indicating that the pheromone signal is transmitted to a subset of the ARC kisspeptin neurones to activate them. Because it has been suggested that the neurokinin B and dynorphin coexpressed in those neurones play critical roles in generating rhythmic bursts, they may be involved in the intracellular pheromone actions that are responsible for inducing the GnRH pulse.

Neurokinin B and dynorphin A in kisspeptin neurons of the arcuate nucleus participate in generation of periodic oscillation of neural activity driving pulsatile gonadotropin-releasing hormone secretion in the goat

Gonadotropin-releasing hormone (GnRH) neurons in the basal forebrain are the final common pathway through which the brain regulates reproduction. GnRH secretion occurs in a pulsatile manner, and indirect evidence suggests the kisspeptin neurons in the arcuate nucleus (ARC) serve as the central pacemaker that drives pulsatile GnRH secretion. The purpose of this study was to investigate the possible coexpression of kisspeptin, neurokinin B (NKB), and dynorphin A (Dyn) in neurons of the ARC of the goat and evaluate their potential roles in generating GnRH pulses. Using double and triple labeling, we confirmed that all three neuropeptides are coexpressed in the same population of neurons. Using electrophysiological techniques to record multiple-unit activity (MUA) in the medial basal hypothalamus, we found that bursts of MUA occurred at regular intervals in ovariectomized animals and that these repetitive bursts (volleys) were invariably associated with discrete pulses of luteinizing hormone (LH) (and by inference GnRH). Moreover, the frequency of MUA volleys was reduced by gonadal steroids, suggesting that the volleys reflect the rhythmic discharge of steroid-sensitive neurons that regulate GnRH secretion. Finally, we observed that central administration of Dyn-inhibit MUA volleys and pulsatile LH secretion, whereas NKB induced MUA volleys. These observations are consistent with the hypothesis that kisspeptin neurons in the ARC drive pulsatile GnRH and LH secretion, and suggest that NKB and Dyn expressed in those neurons are involved in the process of generating the rhythmic discharge of kisspeptin.

Central insulin-like growth factor 1 receptors play distinct roles in the control of reproduction, food intake, and body weight in female rats

Estradiol and progesterone induction of the LH surge in ovariectomized female rats requires concurrent activation of brain insulin-like growth factor 1 (IGF1) receptors. The present study determined whether brain IGF1 receptor signaling is required for estrous cyclicity in gonadally intact female rats. A selective IGF1 receptor antagonist (JB-1) or vehicle was continuously administered into the third ventricle by osmotic minipumps. Following surgical placement of the minipumps, all rats temporarily reduced food intake, lost weight, and suspended estrous cycles. Control rats resumed cycles within a few days and exhibited compensatory hyperphagia until they returned to presurgical body weight. Animals receiving JB-1 had severely delayed or absent estrous cycles, failed to show rebound feeding, and regained body weight more slowly. Vehicle-infused animals pair fed to JB-1-treated rats had even lower body weights but resumed estrous cycles sooner than those given drug alone. Chronic infusion of IGF1 alone had no effect on any of these parameters, but coinfusion of IGF1 with the antagonist completely reversed JB-1 effects on food intake and estrous cyclicity and partially reversed the effects on body weight. There were no significant differences in the expression of galanin-like peptide (Galp) or Kiss1 mRNA in the arcuate or periventricular hypothalamic area of control and JB-1-treated animals at a time point when food intake and estrous cycles were different between controls and JB-1-treated rats. These data suggest that brain IGF1 signaling is necessary for normal estrous cycles as well as compensatory hyperphagia and that IGF1 modulation of the reproductive axis is not secondary to reduced food intake.

Activation of melanocortin receptors accelerates the gonadotropin-releasing hormone pulse generator activity in goats

The present study aims to elucidate whether the central melanocortin receptors [melanocortin-3 and -4 receptors (MC3/4-R)] are involved in regulating GnRH pulse generator activity in female goats. The GnRH pulse generator activity was electrophysiologically assessed at the intervals of characteristic increases in multiple-unit activity (MUA volleys) in the mediobasal hypothalamus. In ovariectomized goats, all doses (0.02, 0.2 and 2 nmol) of MT II, an MC3/4-R agonist, injected into the lateral ventricle significantly shortened MUA volley intervals. The duration of the period during which MT II accelerated MUA volleys was positively correlated with the dose of MT II injected. The stimulatory effect of MT II on the GnRH pulse generator activity was attenuated in the presence of estrogen. Intracerebroventricular injection of SHU9119, an MC3/4-R antagonist, significantly prolonged MUA volley intervals at 1 nmol. MT II (0.2 nmol)-induced acceleration of MUA volleys was partially blocked by the antagonism of MC3/4-R with pre-administered SHU9119 (1 nmol). The present findings demonstrate that MC3/4-R are involved in maintaining GnRH pulse generator activity in goats.

Central injection of senktide, an NK3 receptor agonist, or neuropeptide Y inhibits LH secretion and induces different patterns of Fos expression in the rat hypothalamus

Arcuate neurokinin B (NKB) neurons express estrogen receptor-alpha and are strongly modulated by gonadal steroids. Although numerous studies suggest that NKB neurons participate in the reproductive axis, there is no information on the regulation of luteinizing hormone (LH) secretion by NKB or its receptor, NK3. In the present study, we determined if central injection of senktide, a selective NK3 receptor agonist, would alter serum LH in ovariectomized, estrogen-primed rats. The effects of senktide were compared to neuropeptide Y (NPY), a well-characterized modulator of LH secretion. Saline, senktide, or NPY was injected into the lateral ventricle of unanesthetized rats and serial blood samples were collected for LH radioimmunoassay. The rats were sacrificed 90 min after injection and the brains were removed and processed for Fos immunocytochemistry. A significant inhibition of serum LH was observed from 30 to 90 min after injection of senktide relative to saline controls. In the senktide-injected rats, the inhibition of serum LH was accompanied by increased Fos expression in the medial preoptic area and arcuate nucleus--two reproductive control centers. Senktide also induced Fos in the paraventricular nuclei (PVN) and supraoptic nuclei (SON). Injection of NPY also inhibited serum LH but increased Fos expression only in the PVN and SON. This study provides the first demonstration of alterations in LH secretion by an NK3 receptor agonist. These data, combined with the induction of Fos in medial preoptic and arcuate neurons, strongly support the hypothesis that NKB neurons play a role in the regulation of gonadotropin secretion.

Further evidence for the role of glucose as a metabolic regulator of hypothalamic gonadotropin-releasing hormone pulse generator activity in goats

The present study examined the relative importance of blood glucose vs. free fatty acids as a metabolic signal regulating GnRH release as measured electrophysiologically by multiple-unit activity (MUA) in the arcuate nucleus/median eminence region in ovariectomized, estradiol-treated goats. MUA was recorded before, during, and after: 1) cellular glucoprivation by peripheral infusion of 2-deoxy-d-glucose (2DG; 25, 50, and 75 mg/kg.h, iv); 2) peripheral hypoglycemia in response to various doses (15-195 mU/kg.h, iv) of insulin infusion; and 3) cellular lipoprivation induced by peripheral infusion of sodium mercaptoacetate (MA; 2.4 mg/kg.h alone or combined with 25 mg/kg.h of 2DG, iv), and effects on the interval of characteristic increases in MUA (MUA volleys) were examined. Infusion of the highest dose of 2DG increased the mean interval between MUA volleys, whereas the lower doses of 2DG had no effect on volley interval. The MUA volley intervals lengthened as insulin-induced hypoglycemia became profound. There was a negative correlation between MUA volley intervals and blood glucose concentrations during insulin infusion, and coinfusion of glucose with insulin returned the MUA volley interval to a normal frequency. Infusion of MA alone or MA with 2DG did not increase MUA volley intervals. These findings demonstrate that glucose availability, but not fatty acids, regulates the GnRH pulse generator activity in the ruminant. Glucose is considered a key metabolic regulator that fine-tunes pulsatile GnRH release.

The melanocortin agonist Melanotan-II reduces the orexigenic and adipogenic effects of neuropeptide Y (NPY) but does not affect the NPY-driven suppressive effects on the gonadotropic and somatotropic axes in the male rat

Neuropeptide Y (NPY) is a strong orexigenic neurotransmitter also known to modulate several neuroendocrine axes. alpha-Melanocyte-stimulating hormone (MSH) is an essential anorectic neuropeptide, acting on hypothalamic MC3/4 receptor subtypes. When given as an intracerebroventricular bolus injection, Melanotan-II (MT-II), a non selective MC receptor agonist, inhibits feeding, suppresses the NPY orexigenic action, and reduces basal insulinaemia. We evaluated the effects of a 7-day central infusion of MT-II (15 nmol/day) given either alone or in association with NPY (5 nmol/day) in male Sprague-Dawley rats. MT-II produced almost full anorexia for 1-2 days but then feeding gradually returned to normal despite continued MT-II infusion. When coinfused with NPY, MT-II also produced the same initial anorectic episode but then maintained feeding to upper normal levels, thus cancelling the hyperphagia driven by NPY. Whereas NPY infusion produced a doubling of fat pad weight, MT-II reduced adiposity by a factor of two compared to pair-fed rats, and vastly curtailed the NPY-driven increase in fat pad weight. MT-II infusion also significantly curtailed the NPY-induced rise in insulin and leptin secretions. NPY infusion significantly inhibited hypothalamic pro-opiomelanocortin mRNA expression, most likely cancelling the alpha-MSH anorectic activity. As expected from previous studies, chronic NPY infusion strongly inhibited both the gonadotropic and somatotropic axes, and coinfusion of MT-II did not reverse these NPY-driven effects, in sharp contrast with that seen for the metabolic data. MT-II infusion alone had little effect on these axes. In conclusion, chronic MT-II infusion generated a severe but transient reduction in feeding, suggesting an escape phenomenon, and clearly reduced fat pad size. When coinfused with NPY, MT-II was able to cancel most of the NPY effects on feeding, but not those on the neuroendocrine axes. It appears therefore that, as expected, NPY and alpha-MSH closely interact in the control of feeding, whereas the neural pathways by which NPY affects growth and reproduction are distinct and not sensitive to MC peptide modulation.

Central cholecystokinin-octapeptide accelerates the activity of the hypothalamic gonadotropin-releasing hormone pulse generator in goats

To clarify central actions of cholecystokinin-octapeptide (CCK-8) on reproduction, effects of an intracerebroventricular (i.c.v.) administration of CCK-8 on the activity of the gonadotropin-releasing hormone (GnRH) pulse generator were examined in ovariectomized (OVX) goats in the absence or presence of oestradiol. Goats were chronically fitted with recording electrodes in the mediobasal hypothalamus, and electrophysiological manifestations of the GnRH pulse generator were monitored as characteristic increases in the multiple-unit activity (MUA volleys). In OVX goats, a bolus i.c.v. injection of as little as 0.01 nmol of CCK-8 induced a MUA volley with a short latency, which resulted in a significant decrease in the post-treatment volley interval compared to that in the saline injected control. Administration of higher doses of CCK-8 (0.1 and 2 nmol) did not further accelerate the occurrence of the MUA volley, but stimulatory effects were observed for a longer period than that after the 0.01 nmol injection. When goats were treated with oestradiol, while a bolus i.c.v. injection of 0.01 nmol CCK-8 had no effect, an injection of 0.1 nmol of the peptide significantly decreased the post-treatment volley interval. On continuous i.c.v. infusion of CCK-8 at 3 nmol per 200 micro l/h for 3 h, MUA volleys with shorter intervals than those in the control were successively induced without any apparent change in basal plasma luteinizing hormone levels in OVX goats. These results demonstrate that central CCK-8 strongly accelerates the activity of the GnRH pulse generator in goats.