The hypothalamus and the control of energy homeostasis: different circuits, different purposes.

The hypothalamus regulates many aspects of energy homeostasis, adjusting both the drive to eat and the expenditure of energy in response to a wide range of nutritional and other signals. It is becoming clear that various neural circuits operate to different degrees and probably serve specific functions under particular conditions of altered feeding behaviour. This review will discuss this functional diversity by illustrating hypothalamic neurones that express neuropeptide Y (NPY), the melanocortin-4 receptor (MC4-R) and the orexins. NPY neurones in the arcuate nucleus (ARC) release NPY, a powerful inducer of feeding and obesity, in the paraventricular nucleus (PVN) and the lateral hypothalamic area (LHA). ARC-NPY neurones are inhibited by leptin and insulin and become overactive when levels of these hormones fall during undernutrition. They may function physiologically to protect against starvation. With disruption of the inhibitory leptin signals due to gene mutations, the NPY neurones are overactive, which contributes to hyperphagia and obesity in the ob/ob and db/db mice and fa/fa Zucker rat. The MC4-R is activated by alpha-melanocyte-stimulating hormone [alpha-MSH; a cleavage product of pro-opiomelanocortin (POMC), which is expressed in the other ARC neurones] and inhibits feeding. This effect is antagonised by agouti gene-related peptide (AGRP), which is coexpressed by the ARC-NPY neurones only. Activation of MC4-R, possibly mediated by blockade of AGRP release, appears to restrain overeating of a palatable diet. This response may be programmed by a transient rise in leptin soon after presentation of palatable food, and rats that fail to do this will overeat and become obese. Orexin-A and -B (corresponding to hypocretins 1 and 2) are expressed in specific LHA neurones. These have extensive reciprocal connections with many areas involved in appetite control, including the nucleus of the solitary tracts (NTS), which relays vagal afferent satiety signals from the viscera. Orexin neurones also have close anatomical connections with LHA glucose-sensitive neurones. Orexin-A induces acute feeding but does not cause obesity. Orexin neurones are stimulated by hypoglycaemia partly via the NTS and inhibited by food ingestion. These neurones may therefore be involved in the severe hyperphagia of hypoglycaemia and short-term control of feeding.

Effect of a selective neuropeptide Y Y(2) receptor antagonist, BIIE0246 on neuropeptide Y release.

We have examined the selective neuropeptide Y Y(2) receptor antagonist, (S)-N(2)-[[1-[2-[4-[(R,S)-5,11-dihydro-6(6h)-oxodibenz[b, e]azepin-11-yl]-1-piperazinyl]-2-oxoethyl]cyclopentyl]acetyl ]-N-[2-[1 ,2-dihydro-3,5(4H)-dioxo-1,2-diphenyl-3-H-1,2, 4-triazol-4-yl]ethyl]-argininamid (BIIE0246) on neuropeptide release from rat hypothalamic slices in vitro. BIIE0246 prevented neuropeptide Y-(13-36)-induced reduction in basal and K(+)-stimulated neuropeptide Y release. Addition of BIIE0246 alone enhanced K(+)-stimulated neuropeptide release, without affecting basal release. These data are consistent with anatomical and functional studies suggesting a pre-synaptic role for neuropeptide Y Y(2) receptors in regulating rat hypothalamic neuropeptide Y release in the rat.

Regulation of neuropeptide Y release from hypothalamic slices by melanocortin-4 agonists and leptin.

We have examined the regulation of the orexigenic neurotransmitter, NPY, in hypothalamic slices of rat brain to discover whether the leptin or melanocortin receptor-4 (MCR-4) agonists, which act as satiety signals, can influence the release of this neurotransmitter. Basal and potassium-stimulated NPY release from hypothalamic slices was not significantly altered by the addition of recombinant murine leptin. However, the melanocortin-4 agonists, alpha-MSH and MT-II, significantly inhibited potassium-stimulated NPY release (p < 0.01) without significantly altering basal NPY release. However, the MCR-4 antagonist, agouti-related protein, did not significantly alter either basal or stimulated NPY release. In conclusion, hypothalamic NPY release can be attenuated by MCR-4 agonists, but not by leptin, suggesting that the activation of MCR-4 receptors leading to satiety can also further inhibit food intake through an inhibition of orexigenic NPYergic activity.

Effect of cytokines on hypothalamic neuropeptide Y release in vitro.

Studies involving altered energy balance states in rodents have demonstrated that hypothalamic neuropeptide Y (NPY) activity is strongly activated in states of negative energy balance, such as periods of dietary restriction or starvation. However, in cancer cachexia, when there is a significant reduction in body weight as a result of appetite loss, leading to loss in fat and lean tissue mass, there is no augmentation in the activity of the hypothalamic NPY system. Therefore, we have examined whether cytokines, interleukin (IL)-1, IL-1beta, IL-6, and tumor-necrosis factor-alpha (TNF-alpha; cachectin), which are elevated in cancer patients, can attenuate NPY release from hypothalamic slices in vitro. None of the cytokines altered either the basal or stimulated NPY release from the hypothalamic slices. However, we were able to measure a significant reduction in potassium-stimulated NPY release (-60%) by using the nonselective voltage-dependent calcium channel blocker NiCl (30 microM) without any effect on basal release, as a positive control. Therefore, we suggest that the failure to activate the hypothalamic NPY system in states of cancer cachexia cannot be attributed to a cytokine-induced reduction in neurotransmitter release.

Regulation of neuropeptide Y release by neuropeptide Y receptor ligands and calcium channel antagonists in hypothalamic slices.

Neuropeptide Y (NPY) is an important regulator of energy balance in mammals through its orexigenic, antithermogenic, and insulin secretagogue actions. We investigated the regulation of endogenous NPY release from rat hypothalamic slices by NPY receptor ligands and calcium channel antagonists. High-potassium stimulation (60 mM) of the slices produced a calcium-dependent threefold increase in NPY release above basal release. The Y2 receptor agonists NPY(13-36) and N-acetyl[Leu28,Leu31]NPY(24-36), the Y4 agonist rat pancreatic polypeptide (rPP), and the Y4/Y5 agonist human pancreatic polypeptide (hPP) significantly reduced both basal and stimulated NPY release. NPY(13-36)-induced reduction of NPY release could be partially prevented in the presence of the weak Y2 antagonist T4-[NPY(33-36)]4, whereas the hPP- and rPP-induced inhibition of release was not affected by the Y5 antagonist CGP71683A or the Y1 antagonist BIBP3226. The selective Y1, Y2, and Y5 antagonists had no effect on either basal or potassium-stimulated release when administered alone. The calcium channel inhibitors omega-conotoxin GVIA (N-type), omega-agatoxin TK (P/Q-type), and omega-conotoxin MVIIC (Q-type) all significantly inhibited potassium-stimulated NPY release, without any effect on basal release, whereas nifedipine had no effect on either basal or stimulated release. Addition of both omega-conotoxin GVIA and omega-agatoxin TK together completely inhibited the potassium-stimulated release. In conclusion, we have demonstrated that NPY release from hypothalamic slices is calcium-dependent, involving N-, P-, and Q-type calcium channels. NPY release is also inhibited by Y2 agonists and rPP/hPP, suggesting that Y2 and Y4 receptors may act as autoreceptors on NPY-containing nerve terminals.