ON THE ROLE OF NEUROPEPTIDE-Y RECEPTORS OF THE Y2-TYPE IN THE CONTROL OF HYPOTHALAMIC CATECHOLAMINERGIC MECHANISMS AND NEUROENDOCRINE FUNCTION - CENTRAL EFFECTS OF THE NPY FRAGMENT (13-36)

The effects of intracerebroventricular injections (i.v.t.) of increasing doses of porcine neuropeptide Y (13-36) [pNPY (13-36); 7.5-1250 pmol/rat] on catecholamine (CA) levels and turnover in discrete hypothalamic regions and on neuroendocrine function have been studied in the unanaesthetized, unrestrained male rat. Histofluorometrical methods in combination with tyrosine hydroxylase (TH) inhibition was used for determination of CA levels and utilization. The serum levels of the adenohypophyseal hormones prolactin, luteinizing hormone and growth hormone as well as corticosterone serum levels were determined. The pNPY (13-36) fragment in low doses (25 pmol) produced a reduction of CA utilization, probably mainly of dopamine (DA), in the medial and lateral palisade zone (MPZ and LPZ) of the median eminence. At the highest dose (1250 pmol) pNPY (13-36) instead reduced DA levels and increased DA utilization in LPZ without significantly affecting CA levels and utilization in the MPZ. Only NA nerve terminals of the anterior periventricular hypothalamic were affected by pNPY (13-36), showing a depletion of NA after doses of 75-250 pmol and a significant increase in NA utilization after the highest dose (1250 pmol/rat). pNPY (13-36) (75-750 pmol/rat) produced a reduction of corticosterone serum levels, and after the highest doses of pNPY (13-36) (750 and 1250 pmol/rat) the prolactin serum levels were significantly reduced. The growth hormone serum levels were significantly reduced after a low dose (25 pmol/rat) of pNPY (13-36). An increase of luteinizing hormone serum levels was observed after many of the high doses but only became significant with 750 pmol/rat. These data give indications for a complex regulation by NPY receptors of the Y2 type of neuroendocrine function and of the tuberoinfundibular DA neurons and the NA nerve terminals of the posterior periventricular hypothalamic nucleus in unanaesthetized male rats. An inhibitory role of NPY receptors of the Y2 type appears to exist in the central control of corticosterone, prolactin and growth hormone secretion in the adult male rat while a stimulating role exists in control of luteinizing hormone secretion. Although mainly non-CA mechanisms appears to be involved, it seems possible that the biphasic regulation of the tuberoinfundibular neurons by NPY receptors of the Y2 type may contribute to their inhibitory regulation of prolactin and stimulatory regulation of luteinizing hormone secretion. Based on a comparison with pNPY (1-36) it seems possible that NPY Y1 receptors instead are involved mainly in the stimulatory control of corticosterone and of prolactin secretion and the inhibitory control of luteinizing hormone secretion.