Occurrence of secretogranin II in the prolactin-immunoreactive neurons of the rat lateral hypothalamus: an in situ hybridization and immunocytochemical study

Physiology of the orexinergic/hypocretinergic system: a revisit in 2012

The neuropeptides orexins and their G protein-coupled receptors, OX(1) and OX(2), were discovered in 1998, and since then, their role has been investigated in many functions mediated by the central nervous system, including sleep and wakefulness, appetite/metabolism, stress response, reward/addiction, and analgesia. Orexins also have peripheral actions of less clear physiological significance still. Cellular responses to the orexin receptor activity are highly diverse. The receptors couple to at least three families of heterotrimeric G proteins and other proteins that ultimately regulate entities such as phospholipases and kinases, which impact on neuronal excitation, synaptic plasticity, and cell death. This article is a 10-year update of my previous review on the physiology of the orexinergic/hypocretinergic system. I seek to provide a comprehensive update of orexin physiology that spans from the molecular players in orexin receptor signaling to the systemic responses yet emphasizing the cellular physiological aspects of this system.

Orexin/hypocretin neurons: chemical phenotype and possible interactions with melanin-concentrating hormone neurons

We showed earlier that a specific neuron population of the rat lateral hypothalamus, differing from the codistributed melanin-concentrating hormone (MCH) neurons, express both dynorphin (DYN) and secretogranin II (SgII) genes. We demonstrated later that this population corresponds in fact to the newly identified orexin/hypocretin (OX/Hcrt) neurons. In the present study, by revisiting the chemical phenotype of these neurons, we confirm that all of them contain DYN B- and SgII-immunoreactive materials. The roles played by these peptide/protein in OX/Hcrt neurons are still unclear. Double immunocytochemical stainings highlight putative somasomatic, axosomatic and axodendritic contacts between OX/Hcrt and MCH neurons. Adding OX/Hcrt to the culture medium of hypothalamic slices from 8-day-old rats results either in a significant increase of MCH mRNA after 24 h survival or a strong fall after 10 days culture. These results taken together suggest that OX/Hcrt can directly and/or indirectly affect MCH expression, and that both OX/Hcrt and MCH neuron populations interact to respond in a coordinated manner to central and peripheral signals.

The hypocretin/orexin ligand-receptor system: implications for sleep and sleep disorders

The molecules originally described as the hypocretins and subsequently as the orexins were initially implicated in the control of food intake. Recent observations implicate this newly-described neurotransmitter system in the sleep disorder narcolepsy and, potentially, in the regulation of normal sleep processes. This article reviews the research that led to the isolation of the hypocretin/orexin peptides, their receptors and the activity of these molecules as we currently understand them. A model is proposed in which the cells that make these peptides might be involved in arousal state control.

Insulin-induced hypoglycemia increases preprohypocretin (orexin) mRNA in the rat lateral hypothalamic area

The recent identification of two peptides named hypocretins (Hcrt), and expressed in neurons of the rat tuberal lateral hypothalamus (LHA) previously detected by an ovine prolactin antiserum, led us to revisit some experimental procedures intented to understand the physiological roles of these neurons. In the present study, rats received intraperitoneal injections of insulin and/or glucose. Immunocytochemical observations and quantitation of in situ hybridization signals pointed out a clear stimulation of Hcrt neurons following the sole injection of insulin in hypoglycemic but not in hyperglycemic conditions. This result, together with the robust appetite boosting effect of Hcrt reported elsewhere, suggests the involvement of Hcrt neurons in the control of food intake.

Preprohypocretin (orexin) and prolactin-like immunoreactivity are coexpressed by neurons of the rat lateral hypothalamic area

Recently, two new neuropeptides named hypocretins I and II (Hcrt) have been described in the rat lateral hypothalamus. The distribution of the neurons expressing these new peptides is similar to that of neurons described in the literature as prolactin (PRL) immunoreactive neurons and specifically labeled by an antiserum (AS) raised against ovine prolactin (oPRL). In this study, we report that every neuron labeled by immunohistochemistry or in situ hybridization for Hcrt is also labeled by the oPRL-AS, and conversely. Furthermore, the labeling of the oPRL-AS is inhibited when this AS is pre-incubated with the 104-109 fragment of the preproHcrt, thus indicating that the oPRL-AS is recognizing an epitope carried by this particular fragment of the preproHcrt. Our previous experimental work on these neurons showed that they may be involved in energy metabolism and water balance, which is in agreement with the current literature about Hcrt functions.

Ontogenic development of prolactin immunoreactive neurons in the rat lateral hypothalamus

The present study investigated the ontogenic expression of a prolactin-like substance (oPRL-ir) in rat hypothalamus from embryonic day (E) 17 to postnatal day (P) 29. By immunocytochemistry, the oPRL-ir peptide was only detected from P3. As in adults, labeled neurons were found exclusively in the lateral hypothalamic area. By in situ hybridization, with a cocktail of oligonucleotides complementary to the PRL mRNA, no labeling was observed in the hypothalamus, although dense labeling was obtained over the pituitary. With reverse-transcriptase polymerase chain reaction, a 408 bp band, presumably corresponding to an oPRL mRNA, was detected from PO in the LHA, but also in other brain regions. These results suggest that the oPRL-ir neurons do not contain oPRL. The nature of the oPRL-ir peptide is still unknown, but its late onset of expression may be related to its putative involvement in feeding behavior.

Polyethylene glycol-induced hypovolemia affects the expression of MCH mRNA, but not dynorphin or secretogranin II mRNAs, in the rat lateral hypothalamus

Two prominent neuron populations of the rat lateral hypothalamus express genes encoding respectively the prepromelanin-concentrating hormone (MCH) or dynorphin (DYN) and secretogranin II (SGII). Their roles remain hypothetical in mammals. In the present study, we examined the changes in MCH, DYN and SGII gene expression in dehydrated rats compared to controls. Dehydration was obtained by subcutaneous injection of polyethylene glycol (PEG) resulting in a large reduction of the extracellular fluid volume. Using competitive semi-quantitative RT-PCR and in situ hybridization methods, PEG-injected animals showed a significant increase of MCH mRNA level but no variation of DYN and SGII mRNA levels. These results confirm previous observations suggesting that intra- and extracellular dehydration challenges affect different regulation circuits; they indicate that both neuron populations could be involved in the maintenance of body fluid homeostasis, directly, or indirectly, as integrators of various information leading to goal-oriented behaviour.

Changes in rat melanin-concentrating hormone and dynorphin messenger ribonucleic acids induced by food deprivation

Melanin-concentrating hormone (MCH) and dynorphin genes are expressed in two discrete neuron populations of the rat lateral hypothalamus. Their roles remain hypothetical in mammals. In order to analyse changes in MCH and dynorphin gene expression, a multiplex competitive semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) was developed to assay their mRNAs. This technique was used to examine MCH and dynorphin mRNA content in 24-h and 48-h food-deprived rats compared to controls. A two-fold induction of dynorphin mRNA by 24 h, followed by a sharp decrease at 48 h were observed. A moderate increase in MCH mRNA content was noticed by 24 h; 48 h of fasting restored the control levels.

Immunocytochemical detection of the neurokinin B receptor (NK3) on melanin-concentrating hormone (MCH) neurons in rat brain

The presence of the neurokinin B receptor (NK3 receptor) in the rat lateral hypothalamus and the zona incerta was previously reported. The aim of the present study was to define its cellular localization in these areas. Investigations, coupling immunocytochemical and in situ hybridization techniques, focussed on two neuron populations: the melanin-concentrating hormone (MCH) neurons and a population of neurons recognized by an ovine prolactin antiserum (PRL-ir neurons). While PRL-ir neurons did not exhibit NK3 immunoreactivity, 57% +/- 6% of MCH neurons were strongly stained by the NK3 antiserum. These results suggest that neurokinin B is involved in the regulation of MCH neuron activity via the NK3 receptor; they provide new bases for further investigations on MCH role in the control of food and water intake.

Alteration of dynorphin and secretogranin II in the prolactin immunoreactive neurons of the rat lateral hypothalamus upon osmotic stimulation

The prolactin immunoreactive neurons of the rat lateral hypothalamus were previously reported to express the dynorphin and secretogranin II genes. In the present study, the response of these neurons to osmotic challenge was immunocytologically investigated by using prolactin, dynorphin, secretogranin II and c-Fos antisera. In addition, the mRNA levels for secretogranin II and dynorphin were compared by in situ hybridization in controls and salt-loaded rats. For this model of chronic hyperosmolality, the prolactin and c-Fos immunoreactivities were not stimulated by salt drinking, but dynorphin and secretogranin II immunoreactivities as well as mRNA levels for dynorphin and secretogranin II significantly increased in the lateral hypothalamus. We suggest that the prolactin-immunoreactive neurons may be involved in the regulation of water homeostasis.