Selective enhancement of main olfactory input to the medial amygdala by GnRH

Abstract

In male hamsters mating behavior is dependent on chemosensory input from the main olfactory and vomeronasal systems, whose central pathways contain cell bodies and fibers of gonadotropin-releasing hormone (GnRH) neurons. In sexually naive males, vomeronasal organ removal (VNX), but not main olfactory lesions, impairs mating behavior. Intracerebroventricular (i.c.v.)-GnRH restores mating in sexually naive VNX males and enhances medial amygdala (Me) immediate-early gene activation by chemosensory stimulation. In sexually experienced males, VNX does not impair mating and i.c.v.-GnRH suppresses Me activation. Thus, the main olfactory system is sufficient for mating in experienced-VNX males, but not in naive-VNX males. We investigated the possibility that GnRH enhances main olfactory input to the amygdala in naive-VNX males using i.c.v.-GnRH and pharmacological stimulation (bicuculline/D,L-homocysteic acid mixture) of the main olfactory bulb (MOB). In sexually naive intact males there was a robust increase of Fos protein expression in the anteroventral medial amygdala (MeAv) with MOB stimulation, but no effect of GnRH. There was no effect of stimulation or GnRH in posterodorsal medial amygdala (MePd). In naive-VNX animals, GnRH increased Fos in MeAv and MePv. Only combined MOB stimulation and i.c.v.-GnRH produced a significant increase in Fos in the dorsal (reproduction-related) portion of MeP (MePd). When the animals were sexually experienced before VNX, a condition in which GnRH does not enhance mating, i.c.v.-GnRH combined with MOB stimulation suppressed Fos expression in MePd. This suggests a more selective effect of GnRH on olfactory input in MePd than elsewhere in medial amygdala of VNX males.

Figure 1. Activation of the main and accessory olfactory systems by different types of stimuli

MOE/MOB = main olfactory epithelium/bulb, VNO/AOB = vomeronasal organ/accessory olfactory bulb, PC = piriform cortex, ACN = anterior cortical nucleus of the amygdala, MeA = anterior medial amygdala, MeAv, d = anterior medial amygdala ventral, dorsal portion, respectively, MeP = posterior medial amygdala, MePv, d = posterior medial amygdala ventral, dorsal portion, respectively. Conspecific, reproduction-related stimuli activate MeA and MePd.

Figure 2

Schematic of areas of interest in all 3 experiments. A. The anterior medial amygdala (MeA) is subdivided into dorsal (MeAd) and ventral (MeAv) portions. B. The posterior medial amygdala (MeP) is subdivided into dorsal (MePd) and ventral (MePv) portions. The anterior cortical nucleus (ACN) and piriform cortex (Pir Ctx) were counted in both sections and averaged (see Experimental Procedures). Modified from Morin and Wood (2001). MeA sections correspond to Fig. 26, AP −1.2 mm from bregma; and MeP sections correspond to Fig. 28; AP −1.8 mm from bregma in the Morin and Wood atlas.

Figure 3

In sexually-naïve intact males chemical stimulation of the MOB yielded significantly more Fos expression in MeA, MeP, the ventral subdivisions of each nucleus (MeAv/MePv) and in the piriform cortex (Pir). (* = significantly more than control, Holm-Sidak posthoc tests after 2-way ANOVA, p<.001). Analysis of the IEG expression in the accessory olfactory bulbs indicates that the strong effect of stimulation in the medial amygdala is not due to cross-stimulation of the AOB. The cannulated bulbs of the stimulated and unstimulated animals did not have significantly different amounts of Fos expression in the AOB, data not shown.

Figure 4

In sexually-naïve VNX males there was significantly more Fos expression in the GnRH-treated animals compared to saline (*= statistically significant main effect within area, 2-way ANOVA, p=.005). There was an additional enhancement of Fos expression in MePd of animals that were treated with GnRH and stimulated (a = statistically significant over all other groups within MePd, Holm-Sidak post-hoc analysis, p=.002).

Figure 5

In sexually-experienced VNX males there is an overall increase in Fos expression in all groups, consistent with enhanced access of olfactory information to medial amygdala. The depression seen in MePd with GnRH and stimulation was not significant in this analysis. In piriform cortex (Pir) there was significant effect of GnRH with more Fos expression in animals that also received stimulation (* = statistically significant main effect, 2-way ANOVA, p<.05).

Figure 6

Fos expression in the posterodorsal division of the medial amygdala (MePd) is here normalized to the baseline (icv-saline/MOB-saline control group) within each experiment. MePd is known to be involved in responses to chemosensory input relevant to reproductive behavior. Among the animals that received icv-GnRH and MOB-stimulation, there was a statistically significant effect of combined MOB-stimulation and GnRH between groups of animals (p<.05; Naïve-VNX>Naïve-intact>Experienced-VNX (3-way ANOVA with Holm-Sidak post-hoc analysis). a= significantly greater than icv-GNRH Naïve-intact group. b= significantly greater than icv-GNRH experienced-VNX group.