Diffusible factors from rat arcuate nucleus and Broca's diagonal band nucleus increase size and neurite outgrowth, respectively, of cultured melanin-concentrating hormone containing neurons

Functional expression of P2 purinoceptors in a primary neuroglial cell culture of the rat arcuate nucleus

The arcuate nucleus (ARC) plays an important role in the hypothalamic control of energy homeostasis. Expression of various purinoceptor subtypes in the rat ARC and physiological studies suggest a modulatory function of P2 receptors within the neuroglial ARC circuitry. A differentiated mixed neuronal and glial microculture was therefore established from postnatal rat ARC, revealing neuronal expression of ARC-specific transmitters involved in food intake regulation (neuropeptide Y (NPY), proopiomelanocortin (POMC), tyrosine hydroxylase (TH)). Some NPYergic neurons cosynthesized TH, while POMC and TH expression proved to be mutually exclusive. Stimulation with the general purinoceptor agonists 2-methylthioadenosine-5'triphosphate (2-MeSATP) and ATP but not the P2X1/P2X3 receptor subtype agonist α,β-methyleneadenosine-5'triphosphate (α,β-meATP) induced intracellular calcium signals in ARC neurons and astrocytes. Some 5-10% each of 2-MeSATP responsive neurons expressed POMC, NYP or TH. Supporting the calcium imaging data, radioligand binding studies to hypothalamic membranes showed high affinity for 2-MeSATP, ATP but not α,β-meATP to displace [α-(35)S]deoxyadenosine-5'thiotriphosphate ([(35)S]dATPαS) from P2 receptors. Repetitive superfusion with equimolar 2-MeSATP allowed categorization of ARC cells into groups with a high or low (LDD) degree of purinoceptor desensitization, the latter allowing further receptor characterization. Calcium imaging experiments performed at 37°C vs. room temperature showed further reduction of desensitization. Agonist-mediated intracellular calcium signals were suppressed in all LDD neurons but only 25% of astrocytes in the absence of extracellular calcium, suggestive of metabotropic P2Y receptor expression in the majority of ARC astrocytes. The highly P2Y1-selective receptor agonists MRS2365 and 2-methylthioadenosine-5'diphosphate (2-MeSADP) activated 75-85% of all 2-MeSATP-responsive ARC astrocytes. Taking into consideration the high potency to dose-dependently stimulate ARC cells of the LDD group, the high affinity for rat P2X(1-3) and low affinity for rat P2X4, P2X7 and P2Y receptor subtypes except P2Y1 and P2Y13, the agonist 2-MeSATP primarily acted upon P2X2 and P2Y1 purinoceptors to trigger intracellular calcium signaling in ARC neurons and astrocytes.

Lithium increases melanin-concentrating hormone mRNA stability and inhibits tyrosine hydroxylase gene expression in PC12 cells

Melanin-concentrating hormone (MCH) is a cyclic peptide involved in the regulation of food-intake behaviour and stress response in mammals. Expression of the MCH gene predominates in hypothalamic neurons. Mechanisms governing the regulation of expression of MCH gene in established cell lines were not explored yet. Here, we analysed the actions of nerve growth factor (NGF), dexamethasone, forskolin and lithium on MCH mRNA levels in the PC12 pheochromocytoma cell line. We compared them with those observed on tyrosine hydroxylase (TH) mRNA, constitutively expressed in PC12 cells, and neurotensin (NT) mRNA, taken as a control. In untreated cells, MCH RNA species of high molecular weight were found. Exposure of cells at a combination of NGF and lithium resulted in decreased expression of these MCH RNAs and in the transient production of mature MCH mRNA. Strikingly, after short exposure of PC12 cells to NGF, lithium per se elicited a marked increase in MCH mRNA levels whilst it exerted a potent inhibitory action on TH mRNA expression. Detailed investigations revealed that lithium enhanced MCH mRNA expression through post-transcriptional mechanisms whereas it regulated TH gene expression mainly at the level of transcription. These results demonstrate that lithium, an agent widely used for treatment of manic depressive illness, can exert an opposite effect on MCH and TH mRNA production in PC12 cells. The MCH gene system in NGF-treated PC12 cells provides a good opportunity for studying the effect of lithium on gene expression at post-transcriptional levels in a neuron-like cellular model.

Regulation of gonadotropin-releasing hormone gene expression in vivo and in vitro

The pulsatile release of gonadotropin-releasing hormone (GnRH) into the portal vasculature is responsible for the maintenance of reproductive function. Levels of GnRH decapeptide available for this process can be regulated at transcriptional, posttranscriptional, and posttranslational levels. In the immortalized neuronal GT1 cell lines which synthesize and secrete GnRH, regulation of GnRH biosynthesis has been studied using activators of the protein kinase A (PKA), protein kinase C (PKC), and calcium second messenger systems. These substances, while stimulating GnRH release, cause a universal inhibition of all biosynthetic indices measured to date, including decreases in transcription of the proGnRH gene, GnRH mRNA levels, mRNA stability, and translational efficiency. In contrast, in the animal, the mechanism for the regulation of GnRH gene expression appears to be primarily posttranscriptional, since changes in GnRH mRNA levels often occur in the absence of changes in GnRH primary transcript levels an index of GnRH gene transcription. For example, GnRH mRNA levels increase in response to stimulation with glutamate analogs, while GnRH primary transcript levels are unchanged. However, parallel changes in GnRH mRNA and primary transcript have been observed on proestrus prior to the LH/GnRH surge, suggesting that the regulation of GnRH mRNA levels in vivo involves a complex interplay of transcriptional and posttranscriptional processes.

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.

Melanotropic peptides in the mammalian brain: the melanin-concentrating hormone

Melanin-concentrating hormone (MCH) has been identified in neurons of the mammalian brain. This review summarizes some current information regarding the cell biology of this neuropeptide and the topography of MCH-immunoreactive (-IR) neurons in several species including mouse, rat, hamster, guinea pig, rabbit, dog and monkey; and atlas of MCH-IR neurons in the hypothalamus and subthalamus of the brain of guinea pig is presented. Based upon the location of this MCH cell group, it is hypothesized that they may be functionally involved in circuits of extrapyramidal motor systems from striatal centers to the thalamus and cerebral cortex and to the midbrain and spinal cord.