GALANIN IS A PHYSIOLOGICAL REGULATOR OF SPONTANEOUS PULSATILE SECRETION OF GROWTH-HORMONE IN THE MALE-RAT

To determine whether galanin (GAL), a 29- amino acid neuropeptide, plays a role in the physiological regulation of the pulsatile secretion of GH and PRL in the male rat, secretory patterns of both hormones were studied in freely moving animals after GAL passive immunoneutralization. Adult male Sprague-Dawley rats were equipped with iv and intracerebroventricular catheters. After 7 days, 3 µl of a specific GAL antiserum (GAL-AS) or normal rabbit serum (NRS; controls) were infused in the third ventricle of 10 rats, 25 and 1 h before the animals were bled every 15 min for 6 h (1000-1600 h). Plasma GH and PRL concentrations were measured by RIA, and the hormonal secretory patterns were analyzed by the PULSAR program. Control rats, treated with NRS, displayed typical GH secretion, with pulses of high amplitude (167 ± 27 ng/ml) and low frequency (2.4 ± 0.2 pulses/6 h), separated by periods of low trough levels (3.8 ± 0.6 ng/ml). Rats treated with GALAS had altered pulsatile GH secretion. Pulse height was markedly reduced (77 ± 15 ng/ml; P < 0.01 vs. controls), and peak frequency was higher (3.6 ± 0.5 pulses/6 h; P < 0.05), while GH baseline levels and integrated GH secretion over the 6-h sampling period remained unaltered. Injection of rat GH-releasing hormone (1 µg/rat, iv) caused a similar GH stimulation in both groups of rats, as determined by the peak GH response at 5 min (368 ± 112 vs. 342 ± 81 ng/ml) or by the integrated GH response over 1 h (5.13 ± 1.30 vs. 4.77 ± 1.15 /µg-min/ml in NRS- and GAL-AS-treated rats, respectively; P < 0.05). In contrast to GH, pulsatile secretion of PRL was not affected by the GAL-AS treatment. These results indicate that GAL is a physiological regulator of spontaneous pulsatile secretion of GH, but not PRL, in the male rat. The influence of GAL on GH secretion appears to be exerted within the hypothalamus, mainly by a stimulation of GRF secretion. However, the changes in GH pulse frequency observed after GAL immunoneutralization suggest that GAL might also influence the somatostatin inhibitory tone. © 1990 by The Endocrine Society.