Synaptic pruning during adolescence shapes adult social behavior in both males and females.

Evolutionarily conserved, peer-directed social behaviors are essential to participate in many aspects of human society. These behaviors directly impact psychological, physiological, and behavioral maturation. Adolescence is an evolutionarily conserved period during which reward-related behaviors, including social behaviors, develop via developmental plasticity in the mesolimbic dopaminergic "reward" circuitry of the brain. The nucleus accumbens (NAc) is an intermediate reward relay center that develops during adolescence and mediates both social behaviors and dopaminergic signaling. In several developing brain regions, synaptic pruning mediated by microglia, the resident immune cells of the brain, is important for normal behavioral development. We previously demonstrated that during adolescence, in rats, microglial synaptic pruning shapes the development of NAc and social play behavior in males and females. In this report, we hypothesize that interrupting microglial pruning in NAc during adolescence will have persistent effects on male and female social behavior in adulthood. We found that inhibiting microglial pruning in the NAc during adolescence had different effects on social behavior in males and females. In males, inhibiting pruning increased familiar exploration and increased nonsocial contact. In females, inhibiting pruning did not change familiar exploration behavior but increased active social interaction. This leads us to infer that naturally occurring NAc pruning serves to reduce social behaviors toward a familiar conspecific in both males and females.

Circulating ovarian hormones interact with protein interacting with C kinase (PICK1) within the medial prefrontal cortex to influence cocaine seeking in female mice.

Protein interacting with C kinase 1 (PICK1) is an AMPA receptor binding protein that works in conjunction with glutamate receptor interacting protein (GRIP) to balance the number of GluA2-containing AMPARs in the synapse. In male mice, disrupting PICK1 in the medial prefrontal cortex (mPFC) leads to a decrease in cue-induced cocaine seeking and disrupting GRIP in the mPFC has the opposing effect, consistent with other evidence that removal of GluA2-containing AMPARs potentiates reinstatement. However, PICK1 does not seem to play the same role in female mice, as knockdown of either PICK1 or GRIP in the mPFC leads to similar increases in cue-induced cocaine seeking. These previous findings indicate that the role of PICK1 in the prefrontal cortex is sex specific. The goal of the current study was to examine whether ovarian hormones contribute to the effect of prefrontal PICK1 knockdown on reinstatement of cocaine seeking. While we replicated the increased cue-induced cocaine seeking in prefrontal PICK1 knockdown sham mice, we did not see any difference between the GFP control mice and PICK1 knockdowns following ovariectomy. However, this effect was driven primarily by an increase in cocaine seeking in ovariectomized GFP control mice while there was no effect ovariectomy in PICK1 knockdown mice. Taken together, these findings suggest that circulating ovarian hormones interact with the effects of PICK1 on cue-induced reinstatement.

Adolescent social isolation induced alterations in nucleus accumbens glutamate signalling.

Exposure to adversity during early childhood and adolescence increases an individual's vulnerability to developing substance use disorder. Despite the knowledge of this vulnerability, the mechanisms underlying it are still poorly understood. Excitatory afferents to the nucleus accumbens (NAc) mediate responses to both stressful and rewarding stimuli. Understanding how adolescent social isolation alters these afferents could inform the development of targeted interventions both before and after drug use. Here, we used social isolation rearing as a model of early life adversity which we have previously demonstrated increases vulnerability to cocaine addiction-like behaviour. The current study examined the effect of social isolation rearing on presynaptic glutamatergic transmission in NAc medium spiny neurons in both male and female mice. We show that social isolation rearing alters presynaptic plasticity in the NAc by decreasing the paired-pulse ratio and the size of the readily releasable pool of glutamate. Optogenetically activating the glutamatergic input from the ventral hippocampus to the NAc is sufficient to recapitulate the decreases in paired-pulse ratio and readily releasable pool size seen following electrical stimulation of all NAc afferents. Further, optogenetically inhibiting the ventral hippocampal afferent during electrical stimulation eliminates the effect of early life adversity on the paired-pulse ratio or readily releasable pool size. In summary, we demonstrate that social isolation rearing leads to alterations in glutamate transmission driven by projections from the ventral hippocampus. These data suggest that targeting the circuit from the ventral hippocampus to the nucleus accumbens could provide a means to reverse stress-induced plasticity.

Sex-specific role for prefrontal cortical protein interacting with C kinase 1 in cue-induced cocaine seeking.

Disruption of prefrontal glutamate receptor interacting protein (GRIP), which anchors GluA2-containing AMPA receptors (AMPARs) into the synaptic membrane, potentiates cue-induced cocaine seeking in both males and females. Protein interacting with C kinase 1 (PICK1) plays an opposing role to that of GRIP, removing AMPARs from the synapse. Consistent with our hypothesis that disruption of PICK1 in the mPFC would lead to a decrease in addiction-like behaviour, we found that conditional deletion of PICK1 in the mPFC attenuates cue-induced cocaine seeking in male mice. However, prefrontal PICK1 deletion had the opposite effect in females, leading to an increase in cue-induced reinstatement of cocaine seeking. We did not see any effects of PICK1 knockdown on sucrose taking or seeking, suggesting the sex-specific effects do not generalise to natural reinforcers. These findings suggest the role of PICK1 in the prefrontal cortex of females may not be consistent with its accepted role in males. To determine whether these sex differences were influenced by gonadal hormones, we gonadectomised a cohort of males and found that removal of circulating androgens eliminated the effect of prefrontal PICK1 knockdown. As there was no effect of gonadectomy on its own on any of the behavioural measures collected, our results suggest that androgens may be involved in compensatory downstream effects of PICK1 knockdown. Taken together, these results highlight the need for consideration of sex as a biological variable when examining mechanisms underlying all behaviours, as convergent sex differences can reveal different mechanisms where behavioural sex differences do not exist.

Impaired microglia-mediated synaptic pruning in the nucleus accumbens during adolescence results in persistent dysregulation of familiar, but not novel social interactions in sex-specific ways.

Evolutionarily conserved, peer-directed social behaviors are essential to participate in many aspects of human society. These behaviors directly impact psychological, physiological, and behavioral maturation. Adolescence is an evolutionarily conserved period during which reward-related behaviors, including social behaviors, develop via developmental plasticity in the mesolimbic dopaminergic 'reward' circuitry of the brain. The nucleus accumbens (NAc) is an intermediate reward relay center that develops during adolescence and mediates both social behaviors and dopaminergic signaling. In several developing brain regions, synaptic pruning mediated by microglia, the resident immune cells of the brain, is important for normal behavioral development. In rats, we previously demonstrated that microglial synaptic pruning also mediates NAc and social development during sex-specific adolescent periods and via sex-specific synaptic pruning targets. In this report, we demonstrate that interrupting microglial pruning in NAc during adolescence persistently dysregulates social behavior towards a familiar, but not novel social partner in both sexes, via sex-specific behavioral expression. This leads us to infer that naturally occurring NAc pruning serves to reduce social behaviors primarily directed toward a familiar conspecific in both sexes, but in sex-specific ways.