S09-5 Gonadotropin-inhibitory hormone in birds: possible modes of action

Gonadotropin inhibitory hormone (GnIH) inhibits gonadotropin release in vitro in a dose-dependent manner (Tsutsui et al., 2000). In the present study, we investigated the relative neural distributions of GnRH and GnIH, and in vivo activity of exogenous GnIH in song sparrows (Melospiza melodia) and house sparrows (Passer domesticus). Using immunocytochemistry (ICC), GnIH-containing neurons were localized in both species in the paraventricular nucleus, the GnIHcontaining fibers emanating from those neurons to multiple brain locations, including the median eminence and brainstem. Double-label ICC with light microscopy and fluorescent ICC with confocal microscopy indicate a high probability of colocalization of GnIH and GnRH (-I and -II) neurons and fibers within the avian brain. Thus, GnIH might be acting at the level of the hypothalamus as well as at the anterior pituitary to regulate gonadotropin release. In an in vivo experiment, cGnRH-I alone or a cGnRH-I/GnIH cocktail was injected intravenously into song sparrows. Blood was collected at 2, 5 and 10 minutes post-injection. GnIH rapidly (within 2 minutes) attenuated the GnRH-induced rise in plasma luteinizing hormone. As GnRHII is involved with regulation of reproductive behaviors, GnIH may play a role in regulating them as well. In sum, GnIH is (1) present in photoperiodic songbird species, (2) has the potential to regulate gonadotropin release at more than one level, and (3) might also be involved in the regulation of reproductive behaviors.