Stimulation of porcine pituitary luteinizing hormone release by galanin: Putative auto/paracrine regulation

It has repeatedly been suggested that galanin acts within the anterior pitu itary (AP) in an auto/paracrine manner to modulate luteinizing hormone ((LH ) release. Except for one recent report in the rat, evidence for this notio n is absent. The purpose of this study was to investigate in the pig the ef fects of galanin on LH and growth hormone (GH) release and to evaluate puta tive local effects using various AP culture systems (monolayer, perifusion, reaggregates). Independent of age galanin dose dependently (0.05, 0.2, 1 m uM) stimulated basal but not gonadotropin-releasing hormone (GnRH; greater than or equal to0.01 nM)-induced LH release. Neither basal nor GH-releasing hormone (GHRH)stimulated GH release was affected at any age. Of 4 galanin receptor antagonists (0.2, 1 muM) tested C7 proved to have agonistic effect s, whereas M40 and M15 (galantide) were ineffective in blocking galanin (0. 2 muM)-inclucecl LH secretion or affecting basal or GnRH-induced LH release . M32 [galanin (1-13) NPY (25-36) amide] inhibited (p less than or equal to 0.05) GnRH-induced LH release at doses of greater than or equal to2 muM, a n effect which could be totally compensated by 1 muM galanin. However, the neuropeptide (NPY) antagonist BIBP 3226 (1 muM) partially overcame the effe ct of M32 (M32 is known to also bind to NPY receptors and NPY is inhibitory in the pig). In further studies using APs from preovulatory gilts a specif ic well-characterized galanin antiserum diluted 1:20 or 1:50 attenuated GnR H-induced LH release (p less than or equal to 0.05). However, an NPY antise rum (also affinity purified and at the same dilution) used as control unexp ectedly inhibited GnRH (and galanin)-induced LH release as well, thus sugge sting that attenuation of GnRH-induced LH release by galanin antiserum migh t be at least partly nonspecific. Furthermore 96-hour exposure of AP reaggr egates to two types of porcine preprogalanin antisense oligodeoxynucleotide s neither affected basal nbr GnRH-induced LH release. In line with the fail ure to unequivocally prove paracrine effects of galanin, concentrations of galanin in AP cultures and AP culture medium were very low (less than or eq ual to2 pg galanin/10(5) AP cells). In conclusion the present study provide s some evidence to ascribe a hypophysiotropic role to galanin in regulating LH but not GH secretion in the pig. The study also points to the critical role of appropriate controls when trying to prove auto/paracrine control me chanisms within the anterior pituitary. Our findings do not provide convinc ing evidence to support the notion that intrapituitary galanin is involved in the fine tuning of LH secretion, at least in the preovulatory pig.