A quantitative comparison of the efferent projections of the anterior and posterior subdivisions of the medial amygdala in female mice

Highlights

• The anterior and posterior Me differentially innervate downstream targets.

• The MeP, but not the MeA, densely innervates the BNST and PMCo.

• The MeA, but not the MeP, densely innervates the HDB and mOT.

• Circuits from the Me to the mOT may underlie innate approach behaviors.

Abstract

In rodents, many aspects of sociosexual behavior are mediated by chemosignals released by opposite-sex conspecifics. These chemosignals are relayed via the main (MOS) and accessory olfactory systems (AOS) to the medial amygdala (Me). The Me is subdivided into anterior (MeA) and posterior (MeP) subnuclei, and lesions targeting these regions have different effects on proceptive courtship behaviors in female mice. Differential behavioral effects of MeA vs. MeP lesions could reflect a difference in the projections of neurons located in these Me subnuclei. To examine this question, we injected female mice with the anterograde tracer, Fluoro-Ruby into either the MeA or MeP and quantified labeled puncta in 11 forebrain target sites implicated in courtship behaviors using confocal fluorescence microscopy. We found that the MeP more densely innervates the medial and intermediate regions of the posterior bed nucleus of the stria terminalis (pBNST) and the posteromedial cortical amygdala (PMCo), while the MeA more densely innervates the horizontal diagonal band of Broca (HDB) and the medial olfactory tubercle (mOT), a region that may be a component of the circuitry responsible for olfactory-mediated motivated behaviors.

Introduction

Female mice prefer to seek out and investigate pheromonal odors released by male conspecifics, even in the absence of prior experience with those odors. These chemosignals are detected and processed by two separate olfactory systems, the main (MOS) and accessory (AOS) olfactory systems, which detect primarily volatile and non-volatile body odor cues, respectively. Both the MOS and AOS contribute to the normal display of sociosexual behaviors in female mice (Keller et al., 2006a, Keller et al., 2006b, Martel and Baum, 2009b). Female urinary stimuli that activate either the MOS or the AOS are sufficient to establish a conditioned place preference in male mice, suggesting that either olfactory system suffices to mediate the rewarding effects of opposite-sex odors (Korzan et al., 2013).

The medial amygdala (Me) is a critical node of convergent inputs from the MOS and AOS. Both the main and accessory olfactory bulbs send direct projections to the Me; Me-projecting mitral cells in the ventromedial portion of the main olfactory bulb of female mice are preferentially activated by opposite-sex urinary odors, suggesting that this circuit may relay sexually relevant olfactory information to the Me (Kang et al., 2009). The Me includes anterior, posterodorsal and posteroventral subdivisions, and it has been proposed that these subnuclei play distinct roles in the processing of conspecific body odors. In male Syrian hamsters, the anterior Me (MeA) acts as a “chemosensory filter” that distinguishes between opposite-sex and same-sex odorants (Maras and Petrulis, 2006), while the posterodorsal Me (MePD), which includes a dense population of steroid receptor-expressing neurons (Wood and Newman, 1993), enhances male hamsters' attraction toward female hamster body odorants. More recently, we found that in estrous female mice, lesions targeting only the MeP eliminated females' motivation to preferentially approach urinary odors from testes-intact, as opposed to castrated males, whereas lesions of either the MeA or MeP reduced females' lordotic responses to male mounts (DiBenedictis et al., 2012).

We hypothesized that the ability of MeP, but not MeA, lesions to disrupt the normal preference of female mice for male chemosignals would be reflected in differences in the downstream projection targets of these two amygdalar subnuclei. Two recent studies examined differences in the efferent projections of Me subdivisions in mice (Pardo-Bellver et al., 2012, Usunoff et al., 2009). One of these studies (Pardo-Bellver et al., 2012) used a small number of subjects (n=2 in one group) with no quantitative index of the strength of efferent Me projections; the other study (Usunoff et al., 2009) examined the MePD but provided no assessment of MeA projections. Here, we largely confirmed and extended the results of Pardo-Bellver et al. (2012) in outbred (Swiss Webster) female mice using a method of analysis that allowed for a quantitative assessment of MeA vs. MeP projections to downstream forebrain projection targets. We found that the MeA and MeP differentially target several forebrain sites implicated in motivated behaviors, including the posterior BNST (pBNST) and medial olfactory tubercle (mOT).