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

It has repeatedly been suggested that galanin acts within the anterior pituitary (AP) in an auto/paracrine manner to modulate luteinizing hormone (LH) release. Except for one recent report in the rat, evidence for this notion is absent. The purpose of this study was to investigate in the pig the effects of galanin on LH and growth hormone (GH) release and to evaluate putative local effects using various AP culture systems (monolayer, perifusion, reaggregates). Independent of age galanin dose dependently (0.05, 0.2, 1 microM) stimulated basal but not gonadotropin-releasing hormone (GnRH; > or =0.01 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 microM) tested C7 proved to have agonistic effects, whereas M40 and M15 (galantide) were ineffective in blocking galanin (0.2 microM)-induced LH secretion or affecting basal or GnRH-induced LH release. M32 [galanin (1-13) NPY (25-36) amide] inhibited (p < or = 0.05) GnRH-induced LH release at doses of > or =2 microM, an effect which could be totally compensated by 1 microM galanin. However, the neuropeptide (NPY) antagonist BIBP 3226 (1 microM) partially overcame the effect 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 specific well-characterized galanin antiserum diluted 1:20 or 1:50 attenuated GnRH-induced LH release (p < or = 0.05). However, an NPY antiserum (also affinity purified and at the same dilution) used as control unexpectedly inhibited GnRH (and galanin)-induced LH release as well, thus suggesting that attenuation of GnRH-induced LH release by galanin antiserum might be at least partly nonspecific. Furthermore 96-hour exposure of AP reaggregates to two types of porcine preprogalanin antisense oligodeoxynucleotides neither affected basal nor GnRH-induced LH release. In line with the failure to unequivocally prove paracrine effects of galanin, concentrations of galanin in AP cultures and AP culture medium were very low (< or =2 pg galanin/10(5) AP cells). In conclusion the present study provides 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 mechanisms within the anterior pituitary. Our findings do not provide convincing evidence to support the notion that intrapituitary galanin is involved in the fine tuning of LH secretion, at least in the preovulatory pig.