Post-weaning social isolation in male mice leads to abnormal aggression and disrupted network organization in the prefrontal cortex: Contribution of parvalbumin interneurons with or without perineuronal nets

Adverse social experiences during childhood increase the risk of developing aggression-related psychopathologies. The prefrontal cortex (PFC) is a key regulator of social behavior, where experience-dependent network development is tied to the maturation of parvalbumin-positive (PV+) interneurons. Maltreatment in childhood could impact PFC development and lead to disturbances in social behavior during later life. However, our knowledge regarding the impact of early-life social stress on PFC operation and PV+ cell function is still scarce. Here, we used post-weaning social isolation (PWSI) to model early-life social neglect in mice and to study the associated neuronal changes in the PFC, additionally distinguishing between the two main subpopulations of PV+ interneurons, i.e. those without or those enwrapped by perineuronal nets (PNN). For the first time to such detailed extent in mice, we show that PWSI induced disturbances in social behavior, including abnormal aggression, excessive vigilance and fragmented behavioral organization. PWSI mice showed altered resting-state and fighting-induced co-activation patterns between orbitofrontal and medial PFC (mPFC) subregions, with a particularly highly elevated activity in the mPFC. Surprisingly, aggressive interaction was associated with a higher recruitment of mPFC PV+ neurons that were surrounded by PNN in PWSI mice that seemed to mediate the emergence of social deficits. PWSI did not affect the number of PV+ neurons and PNN density, but enhanced PV and PNN intensity as well as cortical and subcortical glutamatergic drive onto mPFC PV+ neurons. Our results suggest that the increased excitatory input of PV+ cells could emerge as a compensatory mechanism for the PV+ neuron-mediated impaired inhibition of mPFC layer 5 pyramidal neurons, since we found lower numbers of GABAergic PV+ puncta on the perisomatic region of these cells. In conclusion, PWSI leads to altered PV-PNN activity and impaired excitatory/inhibitory balance in the mPFC, which possibly contributes to social behavioral disruptions seen in PWSI mice. Our data advances our understanding on how early-life social stress can impact the maturing PFC and lead to the development of social abnormalities in adulthood.

Differential Roles of the Two Raphe Nuclei in Amiable Social Behavior and Aggression - An Optogenetic Study

Serotonergic mechanisms hosted by raphe nuclei have important roles in affiliative and agonistic behaviors but the separate roles of the two nuclei are poorly understood. Here we studied the roles of the dorsal (DR) and median raphe region (MRR) in aggression by optogenetically stimulating the two nuclei. Mice received three 3 min-long stimulations, which were separated by non-stimulation periods of 3 min. The stimulation of the MRR decreased aggression in a phasic-like manner. Effects were rapidly expressed during stimulations, and vanished similarly fast when stimulations were halted. No carryover effects were observed in the subsequent three trials performed at 2-day intervals. No effects on social behaviors were observed. By contrast, DR stimulation rapidly and tonically promoted social behaviors: effects were present during both the stimulation and non-stimulation periods of intermittent stimulations. Aggressive behaviors were marginally diminished by acute DR stimulations, but repeated stimulations administered over 8 days considerably decreased aggression even in the absence of concurrent stimulations, indicating the emergence of carryover effects. No such effects were observed in the case of social behaviors. We also investigated stimulation-induced neurotransmitter release in the prefrontal cortex, a major site of aggression control. MRR stimulation rapidly but transiently increased serotonin release, and induced a lasting increase in glutamate levels. DR stimulation had no effect on glutamate, but elicited a lasting increase of serotonin release. Prefrontal serotonin levels remained elevated for at least 2 h subsequent to DR stimulations. The stimulation of both nuclei increased GABA release rapidly and transiently. Thus, differential behavioral effects of the two raphe nuclei were associated with differences in their neurotransmission profiles. These findings reveal a surprisingly strong behavioral task division between the two raphe nuclei, which was associated with a nucleus-specific neurotransmitter release in the prefrontal cortex.

Social Learning Requires Plasticity Enhanced by Fluoxetine Through Prefrontal Bdnf-TrkB Signaling to Limit Aggression Induced by Post-Weaning Social Isolation

Escalated or abnormal aggression induced by early adverse experiences is a growing issue of social concern and urges the development of effective treatment strategies. Here we report that synergistic interactions between psychosocial and biological factors specifically ameliorate escalated aggression induced by early adverse experiences. Rats reared in isolation from weaning until early adulthood showed abnormal forms of aggression and social deficits that were temporarily ameliorated by re-socialization, but aggression again escalated in a novel environment. We demonstrate that when re-socialization was combined with the antidepressant fluoxetine, which has been shown to reactivate juvenile-like state of plasticity, escalated aggression was greatly attenuated, while neither treatment alone was effective. Early isolation induced a permanent, re-socialization-resistant reduction in Bdnf expression in the amygdala and the infralimbic cortex. Only the combined treatment of fluoxetine and re-socialization was able to recover Bdnf expression via epigenetic regulation. Moreover, the behavior improvement after the combined treatment was dependent on TrkB activity. Combined treatment specifically strengthened the input from the ventral hippocampus to the mPFC, suggesting that this pathway is an important mediator of the beneficial behavioral effects of the combined psychosocial and pharmacological treatment of abnormal aggression. Our findings suggest that synergy between pharmacological induction of plasticity and psychosocial rehabilitation could enhance the efficacy of therapies for pathological aggression.

Enduring abolishment of remote but not recent expression of conditioned fear by the blockade of calcium-permeable AMPA receptors before extinction training

RATIONALE

Calcium-permeable (GluA2 subunit-free) AMPA receptors (CP-AMPAR) play prominent roles in fear extinction; however, no blockers of these receptors were studied in tests relevant to extinction learning so far.

METHODS

The CP-AMPAR antagonist IEM-1460 was administered once before extinction trainings, which were started either 1 or 28 days after fear conditioning (FC). We used a mild extinction protocol that durably decreased but did not abolish conditioned fear. The messenger RNA (mRNA) expression of GluA1 and GluA2 subunits were investigated at both time points in the ventromedial prefrontal cortex (vmPFC) and amygdala.

RESULTS

IEM-1460 transiently facilitated extinction 1 day after conditioning, but learned fear spontaneously recovered 4 weeks later. When the extinction protocol was applied 28 days after training, IEM-1460 enhanced extinction memory, moreover abolished conditioned fear for at least a month. The expression of GluA1 and GluA2 mRNAs was increased at both time points in the vmPFC. In the basolateral and central amygdala, the GluA1/GluA2 mRNA ratio increased, suggesting a shift towards the preponderance of GluA1 over GluA2 expression.

CONCLUSIONS

AMPAR blockade lastingly enhanced the extinction of remote but not recent fear memories. Time-dependent changes in AMPA receptor subunit mRNA expression may explain the differential effects of CP-AMPAR blockade on recent and remote conditioned fear, further supporting the notion that the mechanisms maintaining learned fear change over time. Our findings suggest clinical implications for CP-AMPAR blockers, particularly for acquired anxieties (e.g., post-traumatic stress disorder) which have a slow onset and are durable.