Median preoptic nucleus projections to vasopressin-containing neurones of the supraoptic nucleus in sheep. A light and electron microscopic study.

It has been postulated that osmoreceptors are situated in either or both of two components of the lamina terminalis, the subfornical organ (sfo) and organum vasculosum laminae terminalis (ovlt) and that information from these sites may be relayed to the hypothalamus directly or via a synapse in the median preoptic nucleus (mnpo). We have investigated the nature of projections from the mnpo to vasopressin (AVP)-containing neurones in the hypothalamus. Microinjections of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) have been made into the mnpo and supraoptic nucleus (son) of the sheep. These injections indicated that in the sheep, as in the rat, the mnpo shares a reciprocal innervation with the sfo and ovlt. Furthermore, the most extensive efferent outflow of the mnpo is to the son, with lesser projections directed to the pvn and other hypothalamic sites. When examined at the electron microscopic level, fibres projecting from the mnpo to the son were found to form synapses with immunocytochemically identified AVP neurones. It is suggested that this pathway is one of the major routes by which information from putative osmoreceptors in the lamina terminalis is conveyed to AVP neurones in the hypothalamus.