The action of maternal separation on glutamate receptors and neuroinflammation

Abstract

Early life adversity can contribute to the propensity for developing neuropsychiatric diseases, including anxiety and substance abuse disorders. In rodents, early stressors such as repeated maternal separation (MS) impact the composition of ionotropic glutamate receptors in the prefrontal cortex (PFC) and nucleus accumbens (NAc). Cortical and limbic regions are evidenced to be involved in maintaining anxiety-like behaviors and drug-cue association after cocaine-induced conditioned place preference (CPP). α-amino- 3-hydroxy- 5-methyl-4- isoxazolepropionic acid receptors (AMPARs) are ubiquitously present glutamate-gated ion channels that are crucial for proper PFC and NAc neuronal communication. Mounting evidence suggests that neuro-immune signaling molecules, such as the pro-inflammatory cytokine TNF, regulate AMPAR subunit composition; increased TNF levels are reported to reduce GluA2-positive AMPARs. Since MS has been reported to induce neuro-immune changes across development, it is likely that these alterations impact AMPAR subunit composition and can therefore confer vulnerability to later insults. In Study 1, we examined the contribution of early postnatal MS and subsequent immune insult at adolescence on the selective loss of GluA2 subunits in the PFC and NAc of male and female rats, via a neuroimmune pathway. We investigated whether MS alone, or in concert with a later immune challenge of lipopolysaccharide (LPS), increased TNF expression in the PFC. We report that MS leads to increased adolescent PFC and NAc TNF expression in males. Additionally, a history of MS sensitized PFC TNF production in response to LPS in both males and females. In the male PFC, MS alone decreased GluA2 mRNA and protein at the membrane, while LPS further reduced mRNA but not protein expression. Within the female PFC, rearing under MS conditions did not affect GluA2 gene or protein expression. However, LPS did reduce GluA2 mRNA, but not protein, in MS females. In the NAc, male and female GluA2 mRNA was unaffected by MS, while LPS treatment reduced both variables. MS males had reduced GluA2 protein, and there was no effect in females. Interestingly, MS did not induce normal anxiety-like behavior in the open field test (OFT), but LPS did induce an expected sickness behavior in the animals. Taken together, these results suggest that decreased GluA2 subunit expression is mediated by neuro-immune activity through increased TNF, particularly in males. These data lend support for the role of a secondary insult and MS in changing neuro-immune activity and affecting the functional composition of PFC and NAc AMPARs in a sex- and region-specific manner. The aim of Study 2 was to test the specific role of soluble TNF in MS-induced GluA2 loss and cocaine-induced CPP with biologic disruption of TNF signaling. TNF gene expression was elevated in both PFC and NAc of MS males, but not females. GluA2 expression was reduced in both regions in MS male rats, and systemic treatment with either ibudilast - a phosphodiesterase inhibitor, or XPro1595 - a blood-brain-barrier (BBB)-permeable blocker of soluble TNF - reversed such loss. MS males also formed greater preference for a cocaine-paired environment, which returned to baseline levels after XPro1595 administration. These data suggest a sex-specific mechanistic link between TNF signaling and changes in GluA2 expression and drug-cue conditioning, thereby providing further evidence for a role of MS and neuro-immune activity in cortical and striatal AMPAR changes. Moreover, manipulation of the TNF signaling pathway represents a novel approach for influencing response to rewarding effects of drug use. Cumulatively, these studies provide novel insight into the sex-specific role of MS- induced neuroinflammation on glutamate receptor composition, and subsequent effects on behavior. Furthermore, our data suggests that MS may impact females via a pathway distinct from those observed in males. This could therefore confer sex-specific resiliency in certain behaviors, particularly during adolescence.