GABAA receptor activation in the lateral septum reduces the expression of conditioned defeat and increases aggression in Syrian hamsters

Abstract

Exposure to social stressors can cause profound changes in an individual's physiology and behavior. In Syrian hamsters, even a single social defeat results in conditioned defeat, which includes an abolishment of territorial aggression and the emergence of high levels of submissive behavior. The purpose of the current study was to determine whether the lateral septum (LS) is a component of the putative neural circuit underlying conditioned defeat. Experiment 1 explored the possibility that plasticity in the LS is necessary for the induction of conditioned defeat. Infusions of the protein synthesis inhibitor, anisomycin, prior to defeat training, however, failed to alter conditioned defeat during testing on the following day, suggesting that synaptic plasticity in the LS is not critical for defeat-induced suppression of aggression. Experiment 2 tested whether the LS is necessary for the expression of conditioned defeat. Infusions of the GABA(A) agonist muscimol into the LS prior to testing significantly increased aggression and decreased submission in previously defeated animals suggesting that the LS is an important component of the neural circuit mediating the expression of both aggression and submission in conditioned defeat. Experiment 3 examined whether the effects of muscimol on aggression were dependent on prior social defeat. Non-defeated animals receiving muscimol infusions prior to testing with a non-aggressive intruder displayed significantly more aggression than did hamsters receiving control injections. Thus, these data suggest that the activation of GABA(A) receptors in the LS increases aggression regardless of whether or not a hamster has previously experienced social defeat.

Figure 1

Histological reconstructions of injection sites for animals receiving infusions of muscimol into the LS in Experiments 1 (A), 2 (B) and 3 (C). Black dots represent injection sites in the LS. Drawings are adapted from Morin and Wood (2001).

Figure 2

Total duration (mean ± SEM) of behaviors exhibited during the 10-min test with a non-aggressive intruder (NAI). Animals received bilateral infusions of anisomycin or saline into the LS immediately prior to conditioned defeat training. There were no significant differences between drug groups.

Figure 3

Total duration (mean ± SEM) of behaviors exhibited during the 10-min test with a non-aggressive intruder (NAI). Animals received bilateral infusions of muscimol or saline into the LS 5 min prior to behavioral testing. * indicates significant differences compared to vehicle controls (p < 0.005).

Figure 4

Total duration (mean ± SEM) of behaviors exhibited by non-defeated animals during the 10-min test with a non-aggressive intruder (NAI). Animals received bilateral infusions of muscimol or saline into the LS 5 min prior to behavioral testing. * indicates significantly different than saline controls (p < 0.05) and # indicates significantly different than saline controls (p < 0.001).