The Oxytocin Receptor Studies And Research On Expressions Of Neurotransmitters' Receptors In Oxytocin- And Vasopressin-containing Neurons In Rat Hypothalamus

Oxytocin (OT) , a nonapeptide, is mainly synthesized in the supraoptic nucleus (SO) and paraventricular nucleus (Pa) of mammals' hypothalamus, and secreted into peripheral blood throughneurohypophysis when stimulated to act on the target tissues like mammary gland or myometrium. Hence, the milk ejection or myometrium contraction is induced. Besides the central nervous system (CNS), OT expresses in the reproductive organs like ovary, testis and epididymis to take a part in the modulations on the development of germ cells. OT also expresses in the thymus, adrenal gland, and vascular system and may exert certain physiological roles there.Customarily, the classic oxytocic and milk-ejecting roles of OT are emphasized. However, recent researches revealed that, OT can play some other roles like modulating maternal behaviour, pair-bonding social communication, and memory. Under many kinds of stresses, the oxytocinergic neurons in CNS are activated and OT is released into peripheral blood and extracellular spaces of neurocytes. Many evidences have proved that the OT elevation in the brain after stresses is related to the modulation on hypothalamus-pitutary-adrenal gland (HPA) axis.All the OT functions mentioned above are induced by the oxytocin receptor (OTR), which is a G-protein coupled seven transmembrane protein. When OT binds to OTR, the signal transduction pathways like phospholipase C-protein kinase C route can be activated, and the intracellular Ca2+ concentraions increased, which may trigger some biological functions. We subcloned the carboxyl fragment from human OTR coding sequence and express it in the prokaryotic system. We also subcloned the total mouse OTR coding sequence and did the transient eukaryotic transfection experiment in the COS-7 cells.The OT- and arginine vasopressin (AVP)-containing neurons comprise the magnocellular part of Pa, and nearly the total SO. They localize very ajacently and distinctively. As to the neuropeptidesynthesis, the oxytocinergic neurons specifically express OT and the vasopressinergic neurons specifically express AVP. There exist many kinds of neurotransmitters receptors and neuromodulators receptors on the 2 kinds of neurons, and neurites from a variety of origins can innervate them. By this means, multiple neurotransmitters and neuromodulators can quickly regulate the magnocellular neurons. Through immunocytochemistry study, we found that the 3 and/or a5 subunits of nicotinic acetylcholine receptor (N-AChR) are predominantly expressed in the oxytocinergic neurons. Our finding is quite different from the conclusion of the British scholar Dr. Mason's work in 1980s. We also found in this study that, the a3 and/or 5 subunits of N-AChR can also be expressed on the axons of oxytocinergic neurons, and these immunoreactive axons could be seen projecting to the median eminence and further to the posterior end of neurohypophysis. In the hypothalamus of the 7-day postnatal pup rats, the expression patterns of a3 and/or a5 subunits of N-AChR are similar to that in adult rats. Under hyperosmotic stress in adult rats, the predominant expression pattens of a3 and/or a5 subunits did not change. However, in SO, the oxytocinergic, N-AChR-immunoreactive dendrites increased after hyperosmotic challenge.Interestingly, we found in rat hypothalamic magnocellular neurons by immunocytochemistry that, the substance P receptor (SPR) expression appeared extremely similiar to that of a3 and/or a5 subunits of N-AChR and they are expressed predominantly on the oxytocinergic neurons cell bodies and axons. We also did the pre-enbedding ABC immunoelectronmicroscopy to study the SPR expression on the urtrastructures of the oxytocinergic neurons in Pa and SO.