Oxytocin antagonist disrupts male mouse medial amygdala response to chemical-communication signals

Abstract

The male mouse medial amygdala is an important site for integration of main and accessory olfactory information. Exposure to biologically relevant chemical signals from the same species (conspecific) results in a general pattern of immediate early gene (IEG) expression in medial amygdala different from that elicited by chemical signals from other species (heterospecific), of no demonstrable biological relevance. The neuropeptide oxytocin (OT) in the medial amygdala has been shown to be necessary for social recognition. In the present set of experiments, male mice with i.c.v. cannulae were injected with either PBS (vehicle control) or oxytocin antagonist (OTA) (1 ng in 1 μl PBS) and exposed to conspecific (female mouse urine) and heterospecific (steer urine and worn cat collar) chemical stimuli. Similarly to our previous report with intact male mice [Samuelsen and Meredith (2009a) Brain Res 1263:33-42], PBS-injected mice exhibited different immediate early gene (IEG) expression patterns in the medial amygdala according to the biological relevance of the chemical stimuli. However, OTA injection eliminates the increase in IEG expression in the medial amygdala to any of the tested conspecific or heterospecific stimuli. Importantly, OTA injection disrupts avoidance of an unfamiliar predator odor, worn cat collar. Here we suggest that the disruption of social recognition behavior in male mice with altered OT receptor activity results from an inability of the medial amygdala to process relevant conspecific (and heterospecific) chemosensory signals.

Figure 1. The relative duration of investigation (RDI) of conspecifics

The RDI for male mice injected i.c.v. with PBS or OTA and exposed in trial two to the either a DIFF female or the SAME female from trial one. The data are presented as RDI ± SEM (Ferguson 2001; see Experimental Procedures). Male mice injected with either PBS or OTA and exposed to a DIFF female showed no change in investigation. However, male mice injected i.c.v. with PBS and exposed to the SAME female exhibited a significant decrease in investigation. OTA injection i.c.v. prevents the formation of a social memory for the SAME female and the male mouse investigates the SAME female as much as in the first trial.

Figure 2. Categorical response to chemical signals in the medial amygdala after PBS or OTA i.c.v injection

(A) In PBS mice, all stimuli elicited increased FRAs expression (mean number of nuclei ± SEM) overall in MeA, but only the biologically relevant stimuli: the conspecific stimulus, fMU, and the heterospecific predator odor, CC, increased FRAs expression overall in MeP. In OTA mice, no stimuli increased FRAs expression in either MeA or MeP. (B) Among the subdivisions of anterior and posterior medial amygdala, all stimuli increased FRAs expression in MeAv and MeAd in PBS-injected mice. In MePv, both fMU and CC increased FRAs expression, but in MePd only fMU significantly increased FRAs expression. In OTA-injected mice, no stimuli increased FRAs expression in any subdivision of medial amygdala. * indicates a significant difference from both PBS-injected control mice and their stimulus-exposed counterparts; + indicates a significant difference between PBS- and OTA-injected mice exposed to the same stimulus. 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 (MeA); Panels C, D, E are approximately at this level (1.06 mm posterior to bregma); (B) Posterior medial amygdala (MeP); Panels F, G, H are approximately at this level (1.58 mm posterior to bregma). Representative sections showing FRAs-labeled nuclei in anterior medial amygdala (C, D) and posterior medial amygdala (F,G) in PBS-injected mice exposed to (C, F) clean control collar or to (D,G) worn cat collar (CC). E and H show the reduced FRAs expression in MeA and MeP of an OTA-injected mouse exposed to CC.

Figure 4. Behavioral response of PBS- and OTA-injected mice to conspecific and heterospecific chemical signals

(A) PBS mice exposed to SU spent significantly more time (mean seconds ± SEM) than control investigating the stimulus. OTA-injected mice exposed to CC spent significantly more time investigating the stimulus than both control and PBS-injected mice investigating CC. OTA-injected mice groomed significantly more in the presence of a control collar than PBS-injected mice exposed to control. Also, OTA-injected mice exposed to CC spent significantly more time grooming than PBS-injected mice in the presence of CC. (B) OTA-injected mice exposed to fMU contact the stimulus significantly more than both control exposed PBS-injected mice and PBS-injected mice exposed to fMU. Only PBS-injected mice exposed to CC showed significantly more stretch-attend behavior to the stimulus than control or their OTA-injected counterparts. * indicates significant difference from control; + indicates significant difference between PBS- and OTA-injected mice exposed to same stimulus. Refer to the results section for p values.