Physiological and morphological identification of hypothalamic magnocellular neuroendocrine cells in goldfish preoptic nucleus

1. Intracellular recordings were made from antidromically identified neurones in the goldfish preoptic nucleus and Procion Yellow was ejected from the recording pipette, marking these magnocellular neuroendocrine cells diffusely, for histological identification. 2. In confirmation of earlier studies these preoptic neuroendocrine cells showed resting membrane potentials of 47 mV, action potentials up to 85 mV, action potentials of long duration (3·9 msec) occurring in two steps, long-lasting hyperpolarizing after-potentials and orthodromic driving from olfactory input. 3. Magnocellular neuroendocrine cells exhibited all-or-none jumps to shorter antidromic latencies as pituitary stimulus strength increased, suggesting multiple branching of the `axones' either near the preoptic nucleus or within the neural lobe. 4. I find three morphological types of neuroendocrine cells throughout the magnocellular part of the preoptic nucleus. Cell Type I is a large (37 μm), multipolar neurone, 48 μm from the ependyma, with fine `dendrites' projecting into the lateral hypothalamus and within the preoptic nucleus, with multiple branched `axones'. Cell Type II is a large (31 μm), multipolar neurone, 24 μm from the ependyma, with a coarse `dendrite' to the ependyma and fine `dendrites' within the preoptic nucleus, with limited `axonal' branching. Cell Type III is a small (18 μm), multipolar neurone, 46 μm from the ependyma, with fine `dendritic' processes distributed within the preoptic nucleus, with limited `axonal' branching. 5. I conclude that magnocellular neuroendocrine cells show electrical membrane properties of other central neurones with both physiological and morphological evidence for multiple `axonal' branching. The three identifiable neuroendocrine cell types ( I, II, III ) are distributed widely within the anatomical limits of the preoptic nucleus, pars magnocellularis, with each type receiving `specific' input connexions and with unique output pathways. I suggest that these three types of neuroendocrine cells may be related to the `cellular' secretion of `specific' neurohypophysial hormones and neurophysins.