Developmental switch in neuropeptide Y and melanocortin effects in the paraventricular nucleus of the hypothalamus.

Homeostatic regulation of energy balance in rodents changes dramatically during the first 3 postnatal weeks. Neuropeptide Y (NPY) and melanocortin neurons in the arcuate nucleus, a primary energy homeostatic center in adults, do not fully innervate the paraventricular nucleus (PVN) until the third postnatal week. We have identified two classes of PVN neurons responsive to these neuropeptides, tonically firing neurosecretory (NS) and burst-firing preautonomic (PA) cells. In neonates, NPY could inhibit GABAergic inputs to nearly all NS and PA neurons, while melanocortin regulation was minimal. However, there was a dramatic, age-dependent decrease in NPY responses specifically in the PA neurons, and a 3-fold increase in melanocortin responses in NS cells. These age-dependent changes were accompanied by changes in spontaneous GABAergic currents onto these neurons. This primarily NPYergic regulation in the neonates likely promotes the positive energy balance necessary for growth, while the developmental switch correlates with maturation of homeostatic regulation of energy balance.

The distribution and mechanism of action of ghrelin in the CNS demonstrates a novel hypothalamic circuit regulating energy homeostasis.

The gastrointestinal peptide hormone ghrelin stimulates appetite in rodents and humans via hypothalamic actions. We discovered expression of ghrelin in a previously uncharacterized group of neurons adjacent to the third ventricle between the dorsal, ventral, paraventricular, and arcuate hypothalamic nuclei. These neurons send efferents onto key hypothalamic circuits, including those producing neuropeptide Y (NPY), Agouti-related protein (AGRP), proopiomelanocortin (POMC) products, and corticotropin-releasing hormone (CRH). Within the hypothalamus, ghrelin bound mostly on presynaptic terminals of NPY neurons. Using electrophysiological recordings, we found that ghrelin stimulated the activity of arcuate NPY neurons and mimicked the effect of NPY in the paraventricular nucleus of the hypothalamus (PVH). We propose that at these sites, release of ghrelin may stimulate the release of orexigenic peptides and neurotransmitters, thus representing a novel regulatory circuit controlling energy homeostasis.

Serotonin activates the hypothalamic-pituitary-adrenal axis via serotonin 2C receptor stimulation.

The dynamic interplay between serotonin [5-hydroxytryptamine (5-HT)] neurotransmission and the hypothalamic-pituitary-adrenal (HPA) axis has been extensively studied over the past 30 years, but the underlying mechanism of this interaction has not been defined. A possibility receiving little attention is that 5-HT regulates upstream corticotropin-releasing hormone (CRH) signaling systems via activation of serotonin 2C receptors (5-HT(2C)Rs) in the paraventricular nucleus of the hypothalamus (PVH). Through complementary approaches in wild-type rodents and 5-HT(2C)R-deficient mice, we determined that 5-HT(2C)Rs are necessary for 5-HT-induced HPA axis activation. We used laser-capture PVH microdissection followed by microarray analysis to compare the expression of 13 5-HTRs. Only 5-HT(2C)R and 5-HT(1D)R transcripts were consistently identified as present in the PVH, and of these, the 5-HT(2C)R was expressed at a substantially higher level. The abundant expression of 5-HT(2C)Rs in the PVH was confirmed with in situ hybridization histochemistry. Dual-neurohistochemical labeling revealed that approximately one-half of PVH CRH-containing neurons coexpressed 5-HT(2C)R mRNA. We observed that PVH CRH neurons consistently depolarized in the presence of a high-affinity 5-HT(2C)R agonist, an effect blocked by a 5-HT(2C)R antagonist. Supporting the importance of 5-HT(2C)Rs in CRH neuronal activity, genetic inactivation of 5-HT(2C)Rs produced a downregulation of CRH mRNA and blunted CRH and corticosterone release after 5-HT compound administration. These findings thus provide a mechanistic explanation for the longstanding observation of HPA axis stimulation in response to 5-HT and thereby give insight into the neural circuitry mediating the complex neuroendocrine responses to stress.

Multiple NPY receptors Inhibit GABA(A) synaptic responses of rat medial parvocellular effector neurons in the hypothalamic paraventricular nucleus.

We have recently shown that NPY and alpha-melanocyte-stimulating hormone, which potently induce or inhibit feeding, respectively, have opposing modulatory actions on GABAergic synapses in the medial parvocellular region of the paraventricular hypothalamic nucleus (mpPVN). Because this action might underlie the effects of NPY on feeding, we have examined the pharmacology of NPY responses using electrophysiological recordings. Focal electrical stimulation within the PVN elicited a GABA(A) synaptic response in some mpPVN neurons, which was reversibly inhibited by NPY in a concentration-dependent manner (EC(50) = 28 nM). NPY did not alter the response to the GABA(A) agonist, muscimol. Agonist responses to NPY analogs were not consistent with a single NPY receptor subtype; the most subtype selective agonists were less effective than the more broadly selective ones. Antagonist blockade of individual receptor subtypes partly inhibited NPY action, while fully blocking effects of selective agonists. Combining Y1 and Y5 antagonists blocked actions of NPY entirely, but the Y2 antagonist also completely blocked actions of NPY in some neurons. NPY inhibits GABA(A) synaptic transmission onto mpPVN neurons, but this can be mediated by three different NPY receptors. Controversy regarding the receptor or receptor subtypes involved in NPY-mediated feeding may arise from the multiple NPY receptors present.

NPY presynaptic actions are reduced in the hypothalamic mpPVN of obese (fa/fa), but not lean, Zucker rats in vitro.

1. Neuropeptide Y (NPY) profoundly enhances feeding when injected intracerebroventricularly, or directly into hypothalamic nuclei, such as the paraventricular nucleus (PVN). Paradoxically, NPY has a reduced action on feeding in obese Zucker rats relative to lean Zucker rats, although the obese rats have much higher levels of hypothalamic NPY expression. GABAergic inputs to a subpopulation of medial parvocellular PVN (mpPVN) neurons are sensitive to NPY. Here, we tested the hypothesis that the blunted eating response to NPY observed in obese Zucker rats will be reflected in a reduced NPY action at mpPVN GABAergic synapses. 2. 'Blind' whole-cell patch-clamp recordings made from mpPVN neurons in acute brain slices of lean and obese Zucker rats revealed GABAergic inhibitory postsynaptic currents (IPSC) responses which were inhibited by NPY. While the maximum response in the obese Zucker rats was significantly less than in lean Zucker or Sprague-Dawley rats, there was no difference in the EC(50). 3. Experiments using blocking concentrations of Y(1)- or Y(5)-receptor antagonists revealed no differences between lean and obese Zucker rats in the contributions of either of these receptors to the total NPY response in mpPVN. 4. NPY is less effective at the mpPVN GABA synapse in obese than in lean Zucker rats. This is not associated with a change in the proportion of Y(1) or Y(5) receptors mediating the NPY response, and is consistent with the downregulation of NPY receptors or a reduction in receptor-effector coupling, and with the reduced sensitivity of obese rats to NPY.