Citation

Emotional dysfunction is common in multiple sclerosis (MS) patients and in mouse models of MS, including experimental autoimmune encephalomyelitis (EAE); however, the etiology of these behaviours is poorly understood. To identify CNS changes associated with these behaviours, we focused on the the basolateral amygdala (BLA) because of its central role in the regulation of emotional behaviour. Whole-cell recordings were carried out in the principal neurons of the BLA in early EAE, prior to demyelination, T cell invasion and motor dysfunction. EAE female mice displayed increased frequency of miniature excitatory postsynaptic currents (EPSC), with no alteration in amplitude or evoked EPSC paired-pulse ratio compared to controls. We found an increase in the AMPA-NMDA ratio and dendritic spine density, indicating increased numbers of glutamatergic synapses. We saw similar electrophysiological changes in BLA principal neurons after microglia were either inactivated (minocycline) or depleted (Mac1-Saporin) in the BLA. Microglia regulate synapses through pruning, directed by complement protein 3 (C3) expression. C3 was downregulated in the BLA in EAE. Ultrastructural analysis of microglia revealed more complex ramifications and a reduced extracellular digestion of cellular elements. We also observed reduced IBA-1 and CD68 staining and lack of proinflammatory cytokine expression in the amygdala. Thus, early EAE is a state of microglial “de-activation” associated with reduced synaptic pruning. This contrasts with the prototypic microglial activation commonly associated with inflammatory CNS disease. Additionally, these data support a role for the acquired immune system to influence both neuronal and microglial function in early CNS autoimmunity.Significance Statement: Microglia help regulate synaptic homeostasis, but there has been little evidence for how this might be important in neuroinflammatory diseases. The data from this study reveal increased synaptic activity and spine density in early stages of EAE (an animal model of multiple sclerosis) in the basolateral amygdala, a nucleus important in the types of behavioural changes we have previously described. These electrophysiological and morphological effects occurred without any significant elevation of local inflammatory cytokines or local demyelination. Unexpectedly, in the context of inflammatory state, we found that microglia were “de-activated”. This study provides strong evidence for a link between microglial activity and synaptic function; the conclusions contrast with the generally accepted view that microglia are activated in inflammatory disease. Copyright 2018 the authors.