HYPOTHALAMIC GALANIN-IMMUNOREACTIVE NEURONS PROJECTING TO THE POSTERIOR LOBE OF THE RAT PITUITARY - A COMBINED RETROGRADE TRACING AND IMMUNOHISTOCHEMICAL STUDY

To identify the galanin‐immunoreactive neurons projecting to the posterior lobe of the pituitary in the rat hypothalamus, a retrograde tracer (complex of wheat germ agglutinin‐enzymatically inactive horseradish peroxidase‐colloidal gold) was injected into the posterior lobe of the pituitary. Sections of the hypothalamus were treated with a combination of silver enhancement of retrogradely transported tracer and immunohistochemistry of galanin. Of the total number of hypothalamic cells doubly labeled with retrograde tracing and galanin‐immunostaining, 56–60% were found in the supraoptic nucleus, 18–23% in the retrochiasmatic nucleus, 8–10% in the lateral magnocellular portion of the paraventricular nucleus. The ratio of (number of doubly labeled cells/number of galanin‐immunoreactive cells) in each of the above regions was similar to the ratio of (number of retrogradely labeled cells/number of Nissl‐stained cells) in the supraoptic nucleus. Of all retrogradely labeled cells in the hypothalamus, 51–56% also contained galaninlike immunoreactivity. In conclusion: (1) galanin‐immunoreactive fibers in the posterior lobe of the pituitary originate mainly in the supraoptic nucleus, retrochiasmatic nucleus, and lateral magnocellular portion of the paraventricular nucleus, (2) most of galanin‐immunoreactive cells in these regions project to the posterior lobe of the pituitary, and (3) about half the neurons constituting the hypothalamo‐neurohypophyseal system contain galaninlike immunoreactivity. Copyright © 1990 Wiley‐Liss, Inc.