Combined retrograde tracing and immunocytochemical identification of luteinizing hormone-releasing hormone- and somatostatin-containing neurons projecting to the median eminence of the rat.

LHRH and somatostatin or somatotropin-release inhibiting factor (SRIF) are produced by neurons whose cell bodies are located in telencephalic and diencephalic regions in the rat. Many, but not all, of these neurons project to the external zone of the median eminence (ME), where the peptides are released from the nerve terminals into hypophysial portal vessels. In the present study, we identified these neurons by in vivo injection of a retrograde tracer, the lectin wheat germ agglutinin (WGA), into the external zone of the ME. Subsequently, colchicine was given into the lateral ventricle 10-24 h after the WGA injection. The animals were killed 24-48 h after the WGA injection. Vibratome sections of the brains were stained for both WGA and LHRH or SRIF with a dual immunocytochemical technique. Approximately 70% of the LHRH neurons in the septum and the anterior hypothalamus and about 70% of the SRIF neurons in the medial preoptic area, the anterior periventricular area, and the paraventricular nucleus were double labeled, indicating that they projected to the ME. None of the SRIF neurons in the ventromedial and arcuate nuclei were labeled with WGA. Double labeled LHRH cells were either smooth and fusiform or spiny. WGA-accumulating LHRH or SRIF perikarya were intermixed with single labeled LHRH or SRIF cells, which apparently did not project to the ME. The results indicate that there are at least two populations of LHRH neurons in the preoptic-septal region and two populations of SRIF neurons in the medial preoptic and anterior periventricular areas and the paraventricular nucleus of the rat brain: one with access to the portal capillaries of the ME and, therefore, functionally related to the regulation of the pituitary, and another without access to portal capillaries, perhaps functionally related to intracerebral neurotransmission or modulation. Moreover, some hypophysiotropic LHRH and SRIF neurons may have axon collaterals reaching multiple targets within the central nervous system.