The vomeronasal organ is required for the male mouse medial amygdala response to chemical-communication signals, as assessed by immediate early gene expression

Abstract

Many species use chemical signals to convey information relevant to social and reproductive status between members of the same species (conspecific), but some chemical signals may also provide information to another species (heterospecific). Both of these types of complex chemical signals may be detected by the vomeronasal organ, which sends projections to the accessory olfactory bulb and on to the medial amygdala. Previous reports in hamster and mouse suggest that the medial amygdala sorts this complex chemosensory information categorically, according to its biological relevance (salience). In the present set of experiments, male mice having undergone vomeronasal removal surgery (VNX) or a sham-operation (SHAM) were exposed to conspecific (male and female mouse urine) or heterospecific (hamster vaginal fluid and worn cat collar) chemical stimuli. Similarly to our previous report with intact male mice [Samuelsen and Meredith (2009) Brain Res 1263:33-42], SHAM mice exhibit different immediate early gene (IEG) expression patterns in the medial amygdala dependent upon the biological relevance of the chemical stimuli. However, regardless of biological relevance, vomeronasal organ removal eliminates all responses in the medial amygdala to any of the chemical stimuli. Interestingly, VNX also disrupts the avoidance of (an unfamiliar) predator odor, worn cat collar. Here we show that the medial amygdala response to the tested chemical signals is dependent upon an intact vomeronasal organ.

Figure 1

Representative coronal sections (20μm) through the nose of (A) SHAM and (B) VNX mice. (A) The dashed box denotes the intact VNO. (B) The dashed box shows the bilateral removal of the VNO.

Figure 2

Categorical response to chemical signals in the medial amygdala after SHAM or VNX surgery. In SHAM mice, all stimuli increased FRAs expression (mean number of nuclei ± SEM) in MeAv and MeAd, but only the conspecific stimuli (mMU and fMU) increased FRAs expression in MePv and MePd. In VNX-operated mice, no stimuli increased FRAs expression in any subdivision of medial amygdala. * indicates a significant difference from both SHAM-control mice (exposed to clean swabs) and from VNX mice exposed to the stimulus. + indicates a significant difference from control. Refer to the results section for p values.

Figure 3

Representative coronal sections (40 μM) showing outlines within which FRAs-immunoreactive nuclei were counted. (A, B) Ventrolateral forebrain structures of the coronal sections used to measure IEG response in medial amygdala (from the Paxinos and Franklin (2003) mouse brain atlas): (A) At the level of anterior medial amygdala; Panels C, D, E are approximately at this level (1.06 mm posterior to bregma); (B) Posterior medial amygdala; Panels F, G, H are approximately at this level (1.58 mm posterior to bregma). C, D; F, G: Representative sections showing FRAs-labeled nuclei in anterior medial amygdala (C, D) and posterior medial amygdala (F, G) in SHAM-operated mice exposed to (C, F) clean swabs or to (D, G) female mouse urine (fMU). E and H show the reduced FRAs expression the medial amygdala of a mouse exposed to fMU after bilateral vomeronasal organ removal.

Figure 4

SHAM and VNX mouse behavioral response to conspecific and heterospecific chemical signals. (A) VNX mice exposed to fMU spent significantly more time (mean seconds ± SEM) than control investigating the stimulus swabs. (B) There were no differences in other measured behaviors. * indicates significant difference from control. Refer to the results section for p values.

Figure 5

IEG expression in the medial amygdala in response to predator stimuli after SHAM or VNX surgery. SHAM mice exposed to cat collar (CC) worn for two weeks had increased FRAs expression in both anterior medial amygdala subdivisions, MeAv and MeA. CC exposure results in a robust increase in MePv, but no difference in MePd. In VNX mice, this stimulus did not increase FRAs expression in any medial amygdala subdivision. * Significantly different from control. Refer to the results section for p values. (mean number of nuclei ± SEM).

Figure 6

SHAM and VNX mouse behavioral response to predator odor. (A) VNX mice exposed to CC, spent significantly more time investigating the stimulus compared to both control and SHAM CC, (B) but contacted them a similar number of times. + indicates significant difference from control and SHAM CC. Refer to the results section for p values, and the discussion for interpretation of SHAM animals’ apparent avoidance of worn cat collar.