Characteristic Response to Chemosensory Signals in GABAergic Cells of Medial Amygdala Is Not Driven by Main Olfactory Input

Abstract

Chemosensory stimuli from same species (conspecific) and different species (heterospecific) elicit categorically different immediate-early gene (IEG) response patterns in medial amygdala in male hamsters and mice. All heterospecific stimuli activate anterior medial amygdala (MeA) but only especially salient heterospecific stimuli, such as those from predators activate posterior medial amygdala (MeP). We previously reported that characteristic patterns of response in separate populations of cells in MeA and MeP distinguish between different conspecific stimuli. Both gamma aminobutyric acid (GABA)-immunoreactive (ir) cells and GABA-receptor-ir cells make this distinction. Here, using zinc sulfate lesions of the main olfactory epithelium, we show evidence that main olfactory input does not contribute to the characteristic patterns of response in GABA-ir cells of male hamster amygdala, either for conspecific or heterospecific stimuli. Some GABAergic cells are output neurons carrying information from medial amygdala to behavioral executive regions of basal forebrain. Thus, the differential response to different conspecific signals can lead to differential activation of downstream circuits based on nonolfactory input. Finally, we show that an intact vomeronasal organ is necessary and sufficient to produce the characteristic patterns of response to conspecific and heterospecific chemosensory stimuli in hamster medial amygdala. Although main olfactory input may be critical in species with less prominent vomeronasal input for equivalent medial amygdala responses, work presented here suggests that hamster medial amygdala uses primarily vomeronasal input to discriminate between important unlearned conspecific social signals, to distinguish them from the social signals of other species, and may convey that information to brain circuits eliciting appropriate social behavior.

Keywords: GABA; IEG; chemosignal; circuit; lesion; vomeronasal.

Figure 1.

Olfactory lesions: FRAs expression in GABA(+) and GABA(−) cells. Double-label FRAs(+)/GABA(+) cells in vomeronasal and olfactory projection areas and mICNc for animals treated with intranasal infusion of zinc sulfate to ablate olfactory epithelium (OLFX) or with intranasal saline, as control. Asterisks without brackets indicate a significant difference in FRAs expression compared with animals with the same intranasal treatment but exposed to clean-swab control stimulation. Asterisks with associated brackets indicate significant differences in FRAs expression between saline-treated and OLFX animals exposed to the same stimulus. (A, B) Vomeronasal projection areas: (A) MeA; (B) MeP: the patterns of expression are essentially identical for OLFX and SAL animals and similar to those for intact untreated animals (Westberry and Meredith, 2016), suggesting olfactory input is not necessary to produce the characteristic patterns of response. In MeA, there was a small but significant main effect of treatment such that OLFX animals had slightly lower FRAs expression overall, but no individual stimuli showed a significant difference with treatment. Heterospecific stimuli significantly activated neither GABA(+) nor GABA(−) cells in MeP. (C) mICNc: there were no significant differences between OLFX and saline-treated animals. Almost all cells in mICNc are GABA-ir. HVF significantly suppressed GABA(+) cells in both saline-treated and OLFX animals (#). FRAs expression in mICNc GABA(+) cells was increased in all other groups, that is, except those exposed to HVF, but the increases were not sufficient for statistical significance. (D, E) Olfactory projection areas: (D) ACN; (E) PC; unlike vomeronasal projection area response, there was a significant difference between FRAs expression in OLFX and saline-treated animals for all stimuli, including CS controls. In OLFX animals, there was no activation of FRAs above the (OLFX) control level for either cell phenotype with any stimulus, in either ACN or PC. FRAs expression for saline-treated animals was similar to that for intact untreated animals (Meredith and Westberry 2004), although increases in FRAs expression did not reach significant levels for all stimuli in ACN or for any in PC.

Figure 2.

Vomeronasal organ removal (VNX). There was no overall response to tested stimuli after VNX. When all 3 areas (MeA, MeP, and mICN) and all stimuli (CS, HVF, mFGS, and mMU) were analyzed in a common 2-way RM ANOVA, there were no significant main effects or interactions. However, 1-way ANOVAs for each brain area indicated a possible small differential response not evident in the overall analysis, indicated by asterisks in parentheses. In intact or OLFX animals, all conspecific stimuli activate MeP, but after VNX, only HVF did so—suggesting the categorical pattern seen in intact (and OLFX) animals was disrupted after VNX in naive males.

Figure 3.

Investigation time for stimulus swabs. (A) Intact animals investigate scented-swab stimuli significantly more than unscented CS control swabs but not significantly differently between individual stimuli. Thus, differential FRAs response to different stimuli cannot generally be attributed to different investigation times. (B) VNX animals also investigate scented swabs. Investigation time for mFGS was not significantly different from control swabs or from other stimuli tested. (C, D) Investigation of scented swabs in saline-treated and OLFX animals was similar, and similar to that in intact animals, despite behavioral anosmia in OLFX animals. Asterisks = different from CS.

Figure 4.

Mating test. Animals from OLFX and saline-treated groups were unimpaired in a 5-min test for mating behavior, indicating (for OLFX animals) a functional vomeronasal organ. VNX animals were significantly impaired in mating behavior except when pre-exposed to HVF (see Results). Asterisks indicate a significant difference from VNX. Mating tests were given at a too short latency to result in changes in FRAs expression.