MORPHOLOGICAL AND PHARMACOLOGICAL EVIDENCE FOR NEUROPEPTIDE Y-GALANININTERACTION IN THE RAT HYPOTHALAMUS

Galanin (GAL) and neuropeptide Y (NPY) have been shown to play importa nt roles in the regulation of pituitary hormone secretion, as well as ingestive and sexual behaviors, by acting within the hypothalamus. Whi le the mechanism of action of these regulatory peptides is under inten sive investigation, less attention has been paid to the possible inter action between them in influencing these central regulatory processes. Because NPY and GAL augment pituitary gonadotropin release, the prese nt study was undertaken to evaluate the nature of morphological and fu nctional relationships between these excitatory hypothalamic peptiderg ic systems. Double immunolabeling for NPY and GAL was carried out on v ibratome sections taken from the hypothalamus of colchicine-pretreated female rats. Avidin-biotin peroxidase technique and a dark blue diami nobenzidine reaction was used to visualize NPY profiles, while the GAL neurons were labeled with a light brown diaminobenzidine reaction usi ng either the avidin-biotin peroxidase or the peroxidase antiperoxidas e technique. Light microscopic examination of the immunostained materi al showed that in the arcuate nucleus, paraventricular nucleus, suprao ptic nucleus, anterior hypothalamus, and medial preoptic area, an abun dant network of NPY-immunoreactive axone surrounded GAL-immunostained cells. Numerous dark blue NPY-containing putative boutons were observe d in close proximity to GAL-immunolabeled cell bodies and dendrites. C orrelated light and electron microscopic examination revealed that mos t of the immunoreactive NPY axon terminals established synaptic connec tions with GAL-expressing cells. Synaptic connections were most freque ntly found in the medial preoptic area and in the magnocellular region of the paraventricular nucleus and arcuate nucleus. Fewer connections were observed in the supraoptic nucleus. These morphological observat ions demonstrate the existence of a strong NPY input to hypothalamic G AL neurons, thereby suggesting a modulatory role for NPY in monitoring GAL release. To evaluate the functional relevance of this anatomical relationship, the effects of intraventricular injection of a GAL recep tor antagonist, galantide, were examined on NPY-induced LH release in ovarian steroid-primed ovariectomized rats. As expected, intraventricu lar injection of NPY readily stimulated LH release. Although, while on its own, galantide was ineffective in altering basal LH release, it m arkedly attenuated the NPY-induced LH response, thereby suggesting tha t GAL released in response to NPY administration may, in part, mediate the excitatory effects of NPY. These experimental results, taken toge ther with the morphological observations, document the involvement of an NPY --> GAL signaling modality in the release of gonadotropins and, likewise, raise the possibility of a similar signaling process in the release of other pituitary hormones and elicitation of behavioral eff ects attributed to NPY and GAL.