Galanin enhancement of gonadotropin-releasing hormone-stimulated luteinizing hormone secretion in female rats is estrogen dependent.

The hypothalamic peptide GnRH is the primary neuroendocrine signal regulating pituitary LH in females. The neuropeptide galanin is cosecreted with GnRH from hypothalamic neurons, and in vitro studies have demonstrated that galanin can act at the level of the pituitary to directly stimulate LH secretion and also augment GnRH-stimulated LH secretion. Several lines of evidence have suggested that the hypophysiotropic effects of galanin are important for the generation of preovulatory LH surges. To determine whether the pituitary actions of galanin are enhanced by the preovulatory steroidal milieu, LH responses to galanin administration (with or without GnRH) were examined in: 1) ovariectomized (OVX); 2) OVX, estrogen (E)-primed; and 3) OVX, E- and progesterone-treated female rats. Results from the study indicate that galanin enhances GnRH-stimulated LH secretion only in the presence of E (in OVX, E-primed, or E- and progesterone-treated rats). Galanin alone does not directly stimulate LH secretion under any of the steroid conditions examined. In the absence of gonadal steroids (OVX rats), galanin inhibits GnRH-stimulated LH secretion. These findings suggest that the primary pituitary effect of galanin is to modulate GnRH-stimulated LH secretion, and that the potentiating effects of galanin occur only in the presence of E.

Testosterone-dependent effects of galanin on pituitary luteinizing hormone secretion in male rats.

Galanin is a 29-amino-acid peptide that colocalizes with GnRH in hypothalamic neurons. High concentrations of galanin are present in portal vessel blood of both male and female rats, and galanin receptors are present on gonadotropes in both sexes. Results from studies of female rats indicate that galanin acts at the level of the pituitary to directly stimulate LH secretion and also to enhance GnRH-stimulated LH secretion. The effects of galanin on pituitary LH secretion in male rats are relatively uncharacterized; thus, the present in vivo study was conducted 1). to examine the ability of galanin to affect basal or GnRH-stimulated LH secretion in male rats and 2). to determine whether the effects of galanin on LH secretion in male rats are testosterone-dependent. All three doses of galanin used (1, 5, and 10 micro g/pulse) significantly enhanced GnRH-stimulated LH secretion in intact male rats. Only the highest dose of galanin directly stimulated LH secretion (without GnRH coadministration) in intact males. Galanin did not directly stimulate LH secretion or enhance GnRH-stimulated LH secretion in castrated male rats. In fact, the highest dose of galanin inhibited GnRH-stimulated LH secretion in castrated males. Upon testosterone replacement, the ability of galanin to directly stimulate LH secretion and to enhance GnRH-stimulated LH secretion was restored in castrated males. These results suggest a role for galanin in the regulation of LH release in male rats and demonstrate that testosterone upregulates the ability of the pituitary to respond to the stimulatory effects of galanin.