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Combined Retrograde Tracing and Immunocytochemical Identification of Luteinizing Hormone-Releasing Hormone- and Somatostatin-Containing Neurons Projecting to the Median Eminence of the Rat^*

Functional Morphology and Reproductive Neuroendocrinology Sections, Laboratory of Molecular and Integratiue Neuroscience, National Institute of Environmental Health Sciences, National Institutes of Health Research Triangle Park, North Carolina 27709
the Department of Anatomy, University Medical School Pecs, Hungary
the Department of Cell Biology and Anatomy, University of North Carolina Chapel Hill, North Carolina 27599

Address all correspondence and requests for reprints to: Istvan Merchenthaler, M.D., Mail Drop C4–07, National Institute of Environmental Health Sciences, National Institutes of Health, P.O. Box 12233, Research Triangle Park, North Carolina 27709.

Abstract

LHRH and somatostatin or somatotropinrelease 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.

Footnotes

^* This work was supported in part by research grants from the NSF (INT-8602688) and the Hungarian Academy of Sciences (AKA 429427 and OTKA 204).

Received May 18, 1989.

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