Neuropeptide K suppresses feeding in the rat

Tachykinins, new players in the control of reproduction and food intake: A comparative review in mammals and teleosts

In vertebrates, the tachykinin system includes tachykinin genes, which encode one or two peptides each, and tachykinin receptors. The complexity of this system is reinforced by the massive conservation of gene duplicates after the whole-genome duplication events that occurred in vertebrates and furthermore in teleosts. Added to this, the expression of the tachykinin system is more widespread than first thought, being found beyond the brain and gut. The discovery of the co-expression of neurokinin B, encoded by the tachykinin 3 gene, and kisspeptin/dynorphin in neurons involved in the generation of GnRH pulse, in mammals, put a spotlight on the tachykinin system in vertebrate reproductive physiology. As food intake and reproduction are linked processes, and considering that hypothalamic hormones classically involved in the control of reproduction are reported to regulate also appetite and energy homeostasis, it is of interest to look at the potential involvement of tachykinins in these two major physiological functions. The purpose of this review is thus to provide first a general overview of the tachykinin system in mammals and teleosts, before giving a state of the art on the different levels of action of tachykinins in the control of reproduction and food intake. This work has been conducted with a comparative point of view, highlighting the major similarities and differences of tachykinin systems and actions between mammals and teleosts.

The anorexigenic effect of neuropeptide K in chicks involves the paraventricular nucleus and arcuate nucleus of the hypothalamus

Neuropeptide K (NPK) induces satiety in birds and mammals. We demonstrated that in birds this effect was associated with the hypothalamus, but beyond this little is known in any species regarding the central mechanism of action. Thus, this study was designed to identify hypothalamic molecular mechanisms associated with the food intake-inhibiting effects of NPK in chicks. In Experiment 1, intracerebroventricular (ICV) injection of 1.0 and 3.0 nmol of NPK reduced food intake and we identified an effective dose for microinjection. In Experiment 2, food intake was reduced when NPK was microinjected into the PVN. In Experiment 3, whole hypothalamus was collected from chicks at 1 h post-ICV NPK injection. The abundance of corticotropin-releasing factor (CRF) and agouti-related peptide (AgRP) mRNA was reduced in NPK-injected chicks. In Experiment 4, within the isolated paraventricular nucleus (PVN) there was less CRF mRNA, and within the arcuate nucleus (ARC) there was less AgRP mRNA, in NPK- than vehicle-treated chicks at 1 h post-injection. We conclude that there are first order neurons for NPK that reside within the PVN, and the anorexigenic effect of NPK is associated with a decrease in AgRP in the ARC.

Hypothalamic mechanisms associated with neuropeptide K-induced anorexia in Japanese quail (Coturnix japonica)

Central administration of neuropeptide K (NPK), a 36-amino acid peptide, is associated with anorexigenic effects in rodents and chickens. The mechanisms underlying the potent anorexigenic effects of NPK are still poorly understood. Thus, the aim of the present study was to identify the hypothalamic nuclei and neuropeptides that mediate anorexic effects of NPK in 7 day-old Japanese quail (Coturnix japonica). After a 6 h fast, intracerebroventricular (ICV) injection of NPK decreased food and water intake for 180 min post-injection. Quail injected with NPK had more c-Fos immunoreactive cells in the arcuate nucleus (ARC), lateral hypothalamus, and paraventricular nucleus (PVN) compared to the birds that were injected with the vehicle. In the ARC of NPK-injected quail, there was decreased neuropeptide Y (NPY), NPY receptor sub-type 1, and agouti-related peptide mRNA, and increased CART, POMC, and neurokinin receptor 1 mRNA. NPK-injected quail expressed greater amounts of corticotropin-releasing factor (CRF), CRF receptor sub-type 2, melanocortin receptors 3 and 4, and urocortin 3 mRNA in the PVN. In conclusion, results provide insights into understanding NPK-induced changes in hypothalamic physiology and feeding behavior, and suggest that the anorexigenic effects of NPK involve the ARC and PVN, with increased CRF and melanocortin and reduced NPY signaling.

Tachykinin-1 in the central nervous system regulates adiposity in rodents

Ghrelin is a circulating hormone that targets the central nervous system to regulate feeding and adiposity. The best-characterized neural system that mediates the effects of ghrelin on energy balance involves the activation of neuropeptide Y/agouti-related peptide neurons, expressed exclusively in the arcuate nucleus of the hypothalamus. However, ghrelin receptors are expressed in other neuronal populations involved in the control of energy balance. We combined laser capture microdissection of several nuclei of the central nervous system expressing the ghrelin receptor (GH secretagoge receptor) with microarray gene expression analysis to identify additional neuronal systems involved in the control of central nervous system-ghrelin action. We identified tachykinin-1 (Tac1) as a gene negatively regulated by ghrelin in the hypothalamus. Furthermore, we identified neuropeptide k as the TAC1-derived peptide with more prominent activity, inducing negative energy balance when delivered directly into the brain. Conversely, loss of Tac1 expression enhances the effectiveness of ghrelin promoting fat mass gain both in male and in female mice and increases the susceptibility to diet-induced obesity in ovariectomized mice. Taken together, our data demonstrate a role TAC1 in the control energy balance by regulating the levels of adiposity in response to ghrelin administration and to changes in the status of the gonadal function.

The leucokinin pathway and its neurons regulate meal size in Drosophila

BACKGROUND

Total food intake is a function of meal size and meal frequency, and adjustments to these parameters allow animals to maintain a stable energy balance in changing environmental conditions. The physiological mechanisms that regulate meal size have been studied in blowflies but have not been previously examined in Drosophila.

RESULTS

Here we show that mutations in the leucokinin neuropeptide (leuc) and leucokinin receptor (lkr) genes cause phenotypes in which Drosophila adults have an increase in meal size and a compensatory reduction in meal frequency. Because mutant flies take larger but fewer meals, their caloric intake is the same as that of wild-type flies. The expression patterns of the leuc and lkr genes identify small groups of brain neurons that regulate this behavior. Leuc-containing presynaptic terminals are found close to Lkr neurons in the brain and ventral ganglia, suggesting that they deliver Leuc peptide to these neurons. Lkr neurons innervate the foregut. Flies in which Leuc or Lkr neurons are ablated have defects identical to those of leucokinin pathway mutants.

CONCLUSIONS

Our data suggest that the increase in meal size in leuc and lkr mutants is due to a meal termination defect, perhaps arising from impaired communication of gut distension signals to the brain. Leucokinin and the leucokinin receptor are homologous to vertebrate tachykinin and its receptor, and injection of tachykinins reduces food consumption. Our results suggest that the roles of the tachykinin system in regulating food intake might be evolutionarily conserved between insects and vertebrates.

Anorexigenic effects of central neuropeptide K are associated with hypothalamic changes in juvenile Gallus gallus

The central mechanisms that mediate neuropeptide K (NPK) associated anorexia are poorly understood in any species, and information in this area of avian biology is totally lacking. Thus, the effects of intracerebroventricular NPK treatment were studied in Cobb-500 chicks (Gallus gallus). In Experiment 1, NPK caused decreased feed intake, but did not affect water intake or whole blood glucose concentration. In Experiment 2, NPK-treated chicks had increased c-Fos immunoreactivity in the parvicellular division of the paraventricular nucleus and arcuate nucleus. The lateral hypothalamus, ventromedial hypothalamus, dorsomedial hypothalamus, periventricular nucleus, magnocellular division of the paraventricular nucleus, and the superchiasmatic nucleus were not affected by NPK treatment. In Experiment 3, the number of feed pecks, exploratory pecks, jumps, escape attempts, and distance moved were decreased, while time spent standing was increased. None of the NPK-treated chicks sat or entered deep rest. In Experiment 4, blockage of corticotrophin releasing factor receptors did not affect NPK-induced anorexia. Thus, we conclude that NPK is a regulator of chick appetite and the effects may be mediated directly in the arcuate nucleus and parvicellular division of the paraventricular nucleus.

Brain regulation of feeding behavior and food intake in fish

In mammals, the orexigenic and anorexigenic neuronal systems are morphologically and functionally connected, forming an interconnected network in the hypothalamus to govern food intake and body weight. However, there are relatively few studies on the brain control of feeding behavior in fish. Recent studies using mammalian neuropeptides or fish homologs of mammalian neuropeptides indicate that brain orexigenic signal molecules include neuropeptide Y, orexins, galanin and beta-endorphin, whereas brain anorexigenic signal molecules include cholecystokinin, bombesin, corticotropin-releasing factor, cocaine- and amphetamine-regulated transcript, and serotonin. Tachykinins may also have an anorectic action in fish. The brain hypothalamic area is associated with regulation of food intake, while sites outside the hypothalamus are also involved in this function. There is correlation between short-term changes in serum growth hormone levels and feeding behavior, although possible mechanisms integrating these functions remain to be defined.

Preprotachykinin gene expression in goldfish brain: sexual, seasonal, and postprandial variations

Recently, we described the complete nucleotide sequence of gamma-preprotachykinin (gamma-PPT) mRNA and the deduced amino acid sequence of the precursor on the basis of molecular cloning and sequence analysis of cDNA from goldfish brain. In the present study, gamma-PPT gene expression in the brain of goldfish was examined using quantitative Northern blot analysis. The results showed that the gamma-PPT gene is highly but differentially expressed in the olfactory bulbs, hypothalamus, and posterior brain regions. There are sexual dimorphism and seasonal variations in gamma-PPT gene expression. In addition, the postprandial changes in gamma-PPT gene expression in the olfactory bulbs and hypothalamus suggest that tachykinin peptides are involved in regulation of feeding behavior in goldfish.

Modulation of the hypothalamo-pituitary-gonadal axis and the pineal gland by neurokinin A, neuropeptide K and neuropeptide gamma

Modulation of the hypothalamo-pituitary-gonadal axis and the pineal gland by neurokinin A, neuropeptide K, and neuropeptide gamma. PEPTIDES 1999. Neurokinin A (NKA), neuropeptide K (NPK) and neuropeptide gamma (NPG) are members of the family of tachykinins, and act preferentially on NK-2 tachykinin receptors. These peptides are widely distributed and are potent stimulators of smooth muscle contraction, especially in the respiratory and gastrointestinal tract. They also induce vasodilatation and plasma extravasation. Through their effects on the vascular tone, they are also potential regulators of the blood flow and therefore of the function of many organs and tissues. Tachykinins have been demonstrated to influence the secretory activity of endocrine cells, and they may have a physiological role as regulators of endocrine functions. A number of reports have indicated that NPK, NKA and NPG act on the hypothalamo-pituitary gonadal axis to regulate functions related to reproduction. Therefore, we thought that, at this point, it was important to review the available evidence suggesting the role of these tachykinins on reproductive functions by effects exerted at 3 different levels of regulation: the hypothalamus, the anterior pituitary and the gonads. These 3 tachykinin peptides were reported to have effects on reproductive functions, acting on the control of the secretion of gonadotropin and prolactin at the level of the hypothalamo-pituitary axis, and on the steroid secretion by the testes and the ovaries. Acting on the hypothalamus, tachykinins, mainly NPK, were reported to inhibit LH secretion, but this effect is dependent on the presence of gonadal steroids. On the anterior pituitary gland, however, tachykinins were shown to stimulate LH and prolactin secretion, and this effect is also dependent on the presence of gonadal steroids. Tachykinin concentrations in the hypothalamus and pituitary are regulated by steroid hormones. In the hypothalamus, estrogens and testosterone increase tachykinin concentration. In the anterior pituitary gland, estradiol and thyroid hormones markedly depress tachykinin concentrations. Ovariectomy and exposure to short photoperiods significantly increase anterior pituitary tachykinins in the Siberian hamster. In the pineal gland, SP and NK-1 receptors are present and, more recently, the presence of NKA and probably also NPK was demonstrated. Castration and steroid replacement modified the content of tachykinins in the pineal gland. The removal of the superior cervical ganglia was followed by an increase in NKA content in the pineal gland. These results suggest that gonadal steroids may influence tachykinins in the pineal gland. In the gonads, tachykinins stimulated the secretory activity of Sertoli cells, but inhibited testosterone secretion by Leydig cells. There are very few reports on the role of tachykinins in the ovary, but some of them indicated that these peptides are present in some of the ovarian structures, and they may affect the secretion of ovarian steroids. Thus, NKA, NPK and NPG appear to have a modulatory role, mainly acting as paracrine factors, on the hypothalamo-pituitary-gonadal axis.

Neuropeptide gamma: a mammalian tachykinin endowed with potent antidipsogenic action in rats

Neuropeptide gamma (NP gamma) is a 21 aminoacid peptide belonging to the tachykinin (TK) family and including neurokinin A (NKA) in its C-terminal sequence. NP gamma possesses higher affinity than NKA for central NK-2 receptors; it shows lower affinity for NK-1 receptors, however, it potently stimulates salivary secretion, which is mediated by NK-1 receptor activation. Pulse intracerebroventricular (pICV) injection of TKs selectively inhibits water intake in rats. Our studies have suggested that NK-1 receptors may mediate the inhibition of angiotensin II-induced drinking, while NK-2 receptors that of drinking induced by cell dehydration. The present study evaluated the effect of pICV injections of NP gamma on water intake in rats. The injection of NP gamma, 8-250 ng/rat, markedly inhibited angiotensin II-induced drinking, and its effect was blocked by the NK-1 receptor antagonist WIN 62577. NP gamma potently inhibited also drinking induced by SC hypertonic NaCl load or water deprivation. The threshold dose for these effects was 31 ng/rat. Also carbachol-induced drinking was inhibited, but at higher doses. On the other hand, NP gamma did not modify food intake in food deprived rats or 0.1% saccharin intake in water and food sated rats, at the same doses effective on drinking. Present findings support the idea that TKs selectively inhibit water intake in rats and are in keeping with our hypothesis that NK-1 and NK-2 receptors mediate, respectively, inhibition of angiotensin II- and cell dehydration-induced drinking.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of interleukin 1 beta on the hypothalamic tachykinin, neurokinin A

Interleukin-1 beta (IL-1 beta) is a pleiotropic cytokine that appears to be an integral component of the bidirectional signalling between the immune and central nervous systems. It is produced in the hypothalamus and has been shown to inhibit the hypothalamo-pituitary-gonadal axis and to activate the hypothalamo-pituitary-adrenal axis. IL-1 beta is reported to up-regulate the tachykinin, substance P (SP), in the peripheral nervous system. We have recently observed that members of the hypothalamic tachykinin family including SP, neurokinin A (NKA) and two N-terminal extended forms of NKA (neuropeptides kappa and gamma), inhibit hypothalamic LHRH and pituitary LH release and stimulate adrenal corticosterone secretion. The similarity in the endocrine effects of the tachykinins and the cytokine prompted us to test the hypothesis that IL-1 beta may stimulate the hypothalamic tachykinins, which would then mediate the neuroendocrine effects of IL-1 beta. First, the effects of IL-1 beta on the in vitro release of NKA-like immunoreactivity (NKA-li) from the hypothalamus was examined. Addition of 10 nM IL-1 beta significantly increased NKA-li release from the hypothalami of castrated rats, but not from the hypothalami of intact rats. To identify the site of IL-1 beta action, the effects of intraventricular IL-1 beta (100 ng) on NKA-li levels in various hypothalamic sites of intact and castrated rats were examined. The results showed that IL-1 beta increased NKA-li selectively in the median eminence (ME) and arcuate nucleus (ARC) of castrated rats only.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide K stimulates corticosterone release in the rat

We have investigated the effects of the tachykinin, neurokinin A (NKA) and N-terminally extended forms NPK and NP gamma on plasma levels of corticosterone. Both peripheral and central injections of these three NK-2 receptor agonists stimulated adrenal corticosterone release in gonad-intact and castrated male rats. A comparison of their effects revealed that NPK was relatively more potent than NKA and NP gamma. These results suggest that tachykinins may act at both central and peripheral sites to stimulate corticosterone release and that NKA and NP gamma may represent degradation forms of the larger, biologically effective tachykinin, NPK.

Galanin inhibits sexual behavior in male rats

Intracerebroventricular injection of galanin potently inhibited (0.5 micrograms/rat) or completely suppressed (5.0 micrograms/rat) copulatory activity in sexually experienced male rats, without producing any other obvious behavioral deficit. It is suggested that galanin, known to potently stimulate feeding behavior, may be involved in the inverse modulation of feeding and sexual behaviors.

Central effects of neuropeptide K on water and food intake in the rat

The present study investigated the effect on water and food intake in the rat of the intracerebroventricular (ICV) injection of neuropeptide K (NPK), the N-terminally extended form of neurokinin A. NPK inhibited water deprivation-induced water intake even at 31.2 ng/rat. At higher doses, it inhibited also water intake induced by ICV angiotensin II or by subcutaneous hypertonic NaCl, and food-associated drinking, the threshold dose being 125 ng/rat. In response to 125 ng/rat, food intake following 16 h food deprivation was not reduced. NPK inhibited food intake only at 500 ng/rat, a dose that evoked excessive grooming in treated animals. Thus NPK is a potent inhibitor of water deprivation-induced drinking and at higher doses it exerts a general antidipsogenic effect towards several dipsogenic determinants, without affecting food intake. On the other hand, it inhibits food intake only at high doses, 500 ng/rat or more, but this inhibition might be just related to the intense grooming evoked. The effects of NPK on ingestive behavior are markedly different from those of neurokinin A, which selectively inhibits osmotic drinking and food-associated drinking. These differences suggest that NPK itself may exert its effects on the central nervous system, not necessarily through the conversion to neurokinin A.

Effects of various tachykinins on pituitary LH secretion, feeding, and sexual behavior in the rat

Our investigations of the four tachykinines tested have shown that NPK characteristically evoked a spectrum of biological effects in male and female rats. NPK suppressed pituitary LH release by inhibiting the release of hypothalamic LHRH, presumably by activation of NK-2 tachykinin receptor subtypes. However, NPK may also act at the level of gonadotrophs to stimulate LH release in male rats. Central injection of NPK rapidly disrupted copulatory behavior in sexually active male rats. NPK also suppressed feeding, but, in this case, peripheral injections were more effective than central injections. Taken together, these observations strongly imply that NPK may be an inhibitory messenger molecule in the hypothalamic control of reproduction, sexual, and feeding behaviors.

Neuropeptide K (NPK) suppresses copulatory behavior in male rats

Recent studies show that neuropeptide K (NPK), a member of the tachykinin family of neuropeptides, is found in various hypothalamic sites implicated in the control of gonadotropin secretion, food intake and sexual behavior. Since our previous studies showed that NPK inhibited feeding and gonadotropin secretion in rats, we have now assessed the effects of NPK on sexual behavior in male rats. Copulatory behavior was recorded subsequent to injection of different doses of NPK either into the third ventricle of the brain or intraperitoneally in sexually experienced male rats. We observed that intraventricular (IVT) administration of 0.125 nmol NPK produced only a slight effect on behavior as indicated by a significant increase in intromission latency. However, a four-fold higher dose of 0.5 nmol of NPK completely eliminated copulatory behavior. These rats displayed no overt locomotor deficit but ignored the receptive females, rarely approaching them during the test period. Only two mounts and no intromissions were observed in 6 rats during the test period. In contrast, neither the same dose of 0.5 nmol nor a higher dose of 3.14 nmol injected intraperitoneally produced any discernible effect on sexual performance. These results show that NPK acts centrally to inhibit sexual behavior in male rats and that hypothalamic NPK may be an important part of the neural circuit that regulates reproduction and related behaviors.

The tachykinin NH2-senktide inhibits alcohol intake in alcohol-preferring rats

The present study evaluated the effect of the intracerebroventricular injection of the tachykinins, substance P, neurokinin A and [Asp5.6,MePhe8]substance P(5-11) (also referred to as NH2-senktide), on the alcohol intake of genetically selected, alcohol-preferring rats. Animals were offered both water and 8% ethanol 2 h/day; tachykinins were administered just before access to fluids. Neurokinin A and substance P did not modify alcohol intake at doses up to 1000 and 2000 ng/rat, respectively. On the other hand, NH2-senktide potently suppressed alcohol intake at doses of 31.2-500 ng/rat. At the same doses, however, it did not significantly affect water intake. This finding suggests that its effect on alcohol intake might be rather selective and not due to general impairment of the behavior. Activation of tachykinin NK-3 receptors, for which NH2-senktide is a highly selective agonist, produces angiotensin II release in the brain; however, the effect of NH2-senktide on alcohol intake is probably not mediated by angiotensin II, as suggested by the fact that it is not modified by captopril pretreatment.