Efferent projection of the nucleus praeopticus medialis to the median eminence in rats.

Efferent projection from the medial preoptic nucleus, especially to the median eminence, was studied in the rat by light and electron microscopy, in combination with 3H-leucine labelling and electrical coagulation of the nucleus. In the light microscopy autoradiographs of the brain, an ipsilateral distribution of silver grains was followed caudally and dorsally in the periventricular area, into the triangular area between the ventromedial nucleus and the arcuate nucleus, and, further, into the paraventricular and supraoptic nuclei and median eminence. After electrical coagulation of the medial preoptic area, electron microscopy showed numerous degenerated nerve terminals in the external layer of the median eminence. Occasionally, cored vesicles (90 nm mean diameter with a core of 70 nm mean diameter) and clear vesicles (40-50 nm diameter) were encountered in the axoplasm of the degenerated nerve terminals. These findings indicate that the axons of the nerve cells of the medial preoptic nucleus extend caudally, terminating in the external layer of the median eminence.