Differential distribution of muscarinic cholinergic and putative nicotinic cholinergic receptors within the hypothalamo-neurohypophysial system of the rat.

Binding of the muscarinic cholinergic receptor probe [3H]quinuclidinylbenzilate ([3H]QNB) and the putative nicotinic receptor probe [125I]alpha-bungarotoxin ([125I]alpha BTX) to vasopressin (VP) and oxytocin (OT) neuroendocrine cells was investigated with a combination of quantitative receptor binding, autoradiography and immunocytochemistry. A single high-affinity site was labelled by [3H]QNB in the hypothalamus and pituitary (KD = 0.76-1.44 X 10(-10) M) with a mean hypothalamic density of 213 fmol/mg protein compared with only 56 fmol/mg protein in the pituitary. Analysis of autoradiographic silver grains from [3H]QNB binding revealed a relative absence of binding associated with magnocellular VP and OT cell groups in the hypothalamus. The median eminence and neural lobe of the pituitary contained low levels of [3H] QNB binding, which, however, were the highest within the hypothalamo-neurohypophysial system. The ligand [125I]alpha BTX binds with both a high and low affinity to sites within the hypothalamus and pituitary (high-affinity KD = 0.77-1.03 X 10(-10) M). In the hypothalamus the density of high-affinity binding sites (25 fmol/mg protein) is approximately 2.5 times greater than in the pituitary. In contrast to [3H]QNB, high-affinity binding of [125I]alpha BTX was found to be highly concentrated within the supraoptic nucleus, nucleus circularis, and the magnocellular areas of the paraventricular nucleus. Autoradiographic silver grains were distributed over both VP and OT immunoreactive neurons and processes. Binding within the neural lobe was very low. These data suggest that the cholinergic regulation of VP and OT release may occur via nicotinic cholinergic receptors at the level of the magnocellular cell bodies and predominantly via muscarinic cholinergic receptors within the neural lobe.