Stimulation of Porcine Pituitary Luteinizing Hormone Release by Galanin: Putative Auto/Paracrine Regulation

Abstract

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 µM) stimulated basal but not gonadotropin-releasing hormone (GnRH; ≧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 µM) tested C7 proved to have agonistic effects, whereas M40 and M15 (galantide) were ineffective in blocking galanin (0.2 µM)-induced LH secretion or affecting basal or GnRH-induced LH release. M32 [galanin (1–13) NPY (25–36) amide] inhibited (p ≤ 0.05) GnRH-induced LH release at doses of ≧2 µM, an effect which could be totally compensated by 1 µM galanin. However, the neuropeptide (NPY) antagonist BIBP 3226 (1 µM) 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 ≤ 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 (≤2 pg galanin/10⁵ 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.

Immunoreactive Galanin Expression in Ovine Gonadotropin‐Releasing Hormone Neurones: No Effects of Gender or Reproductive Status

“…Studies on bovine (13), porcine (14) and rat (45) pituitary fragments suggests that galanin can either induce LH secretion alone or potentiate GnRH‐induced LH secretion (8). Exogenous galanin administration has also been shown to stimulate LH secretion or enhance GnRH‐induced LH release in the monkey (46) and rat (8) but not in humans (47).…”

Neuropeptides in anterior pituitary development

“…The regulation of proliferation and of lactotroph secretory activity by galanin has been confirmed by studies on animals using genetic alterations of the galanin gene (Wynick et al, 1998b). Relationships have also been reported with secretion of LH (Lopez et al, 1991(Lopez et al, , 1993Sahu et al, 1994;Arvat et al, 1995;Giustina et al, 1995;Xu et al, 1996;Elsaesser, 2001;Todd et al, 1998), GH (mainly in man, where the human form of galanin (hGAL) is a 30-amino acid peptide) (Carey et al, 1993;Chan et al, 1996;Cremagnani et al, 1996;Davis et al, 1987;Gabriel et al, 1988;Giustina et al, 1993Giustina et al, , 1994Giustina et al, , 1995Hulting et al, 1991;Loche et al, 1989;Maiter et al, 1990;Moore et al, 1994;Murakami et al, 1989;Nishiki et al, 1997;Ottlecz et al, 1986;Sato et al, 1991;Spencer et al, 1994;Todd et al, 2000), and ACTH (Cimini et al, 1993;Cimini, 1996;Hooi et al, 1990b;Malendowicz et al, 1994c).…”

Peptides: Basic determinants of reproductive functions

“…In contrast, treatment with galanin antiserum increase the LH and FSH secretion [154]. Although galanin has an inhibitory effect on rat LH secretion, it stimulates LH secretion in the porcine [39]. Since the energy normally consumed for ovulatuar function is senseless during pregnancy, the enhanced galanin activity may contribute to inhibition of the pre-ovulatory LH surge [99].…”