Efferent projections of the medial preoptic nucleus and medial hypothalamus in the pigeon

Abstract

The efferent projections of the medial preoptic nucleus (POM), anterior-medial hypothalamic area (AM) and the posteromedial hypothalamic nucleus (PMH) in the pigeon were traced by the autoradiographic technique. Similar and differential connections were noted from these regions. Projections from POM and AM-PMH were traced to nucleus (n.) septalis lateralis, n. dorsomedialis thalami, n. dorsolateralis anterior thalami (pars ventralis), posterior hypothalamic and medial mammillary areas, area ventralis tegmenti (Tsai), central gray of midbrain and n. intercollicularis and substantia grisea periventricularis of the midbrain. The density of silver grains in these regions differed with POM and AM-PMH injections. Other projections were observed exclusively from only 1 or 2 of the nuclear regions injected. Connections from POM and the rostral part of AM were seen to the median eminence, neurohypophysis and the nucleus of anterior pallial commissure. Only cells of the anterior part of AM project fibers to n. septalis medialis. In the hypothalamus, projections from POM are concentrated in the periventricular region and in the preoptic-hypophyseal tract in the extreme lateral hypothalamus, while AM-PMH projections are heaviest in the medial hypothalamus and lateral preoptic area. A major difference in the connections of PMH from POM is the more substantial PMH projection to the midbrain. A prominent projection courses dorsolaterally and posteriorly from PMH toward n. ovoidalis and splits into 2 pathways: a lateral pathway which heavily innervates n. intercollicularis and the periventricular gray and a ventrolateral projection to the midbrain tegmentum. The projections described above provide anatomical substrates for neuroendocrine, autonomic and behavioral functions.