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The l-opioid receptor regulates reward derived from both drug use and natural experiences, including social interaction, through actions in the nucleus accumbens. Here, we studied nucleus accumbens microcircuitry and social behavior in male and female mice with heterozygous genetic knockout of the l-opioid receptor (Oprm1 1/2 ). This genetic condition models the partial reduction of l-opioid receptor signaling reported in several neuropsychiatric disorders. We first analyzed inhibitory synapses in the nucleus accumbens, using methods that differentiate between medium spiny neurons (MSNs) expressing the D1 or D2 dopamine receptor. Inhibitory synaptic transmission was increased in D2-MSNs of male mutants, but not female mutants, while the expression of gephyrin mRNA and the density of inhibitory synaptic puncta at the cell body of D2-MSNs was increased in mutants of both sexes. Some of these changes were more robust in Oprm1 1/2 mutants than Oprm1 2/2 mutants, demonstrating that partial reductions of l-opioid signaling can have large effects. At the behavioral level, social conditioned place preference and reciprocal social interaction were diminished in Oprm1 1/2 and Oprm1 2/2 mutants of both sexes. Interaction with Oprm1 mutants also altered the social behavior of wild-type test partners. We corroborated this latter result using a social preference task, in which wild-type mice preferred interactions with another typical mouse over Oprm1 mutants. Surprisingly, Oprm1 2/2 mice preferred interactions with other Oprm1 2/2 mutants, although these interactions did not produce a conditioned place preference. Our results support a role for partial dysregulation of l-opioid signaling in social deficits associated with neuropsychiatric conditions.
Toddes et al. (Thu,) studied this question.