Neuroinflammation Contributes to High Salt Intake-Augmented Neuronal Activation and Active Coping Responses to Acute Stress.

High dietary salt intake increases risk of stress-related neuropsychiatric disorders. Here, we explored the contribution of high dietary salt intake-induced neuroinflammation in key stress-responsive brain regions, the hypothalamic paraventricular nucleus and basolateral amygdala, in promoting exaggerated neuronal activation and coping behaviors in response to acute psychogenic stress. Mice that underwent high dietary salt intake exhibited increased active stress coping behaviors during and after an acute swim stress, and these were reduced by concurrent administration of minocycline, an inhibitor of microglial activation, without affecting body fluid hyperosmolality caused by high dietary salt intake. Moreover, minocycline attenuated high dietary salt intake-induced increases of paraventricular nucleus tumor necrosis factor-α, activated microglia (ionized calcium-binding adaptor molecule 1), and acute swim stress-induced neuronal activation (c-Fos). In the basolateral amygdala, similar effects were observed on ionized calcium-binding adaptor molecule 1+ and c-Fos+ counts, but not tumor necrosis factor-α levels. These data indicate that high dietary salt intake promotes neuroinflammation, increasing recruitment of neurons in key stress-associated brain regions and augmenting behavioral hyper-responsivity to acute psychological stress.

Effects of Peripheral Immune Challenge on In Vivo Firing of Basolateral Amygdala Neurons in Adult Male Rats.

Peripheral inflammation often causes changes in mood and emergence of depressive behavior, and is characterized by a group of physical manifestations including lethargy, malaise, listlessness, decreased appetite, anhedonia, and fever. These behavioral changes are induced at the molecular level by pro-inflammatory cytokines like interleukin (IL)-1ß, IL-6 and TNF-α. The basolateral amygdala (BLA) is a key brain region involved in mood and may mediate some of the behavioral effects of inflammation. However, it is unknown whether peripheral inflammatory state affects the activity of BLA neurons. To test this, adult male Sprague-Dawley rats were treated with IL-1ß (1 µg, intraperitoneal (i.p.)), and behavioral and electrophysiological measures were obtained. IL-1ß reduced locomotion in the open-field test and also reduced home-cage mobility, consistent with features of sickness-like behavior. Using in vivo single-unit extracellular electrophysiological recordings from anesthetized rats, we found that spontaneous BLA neuronal firing was acutely (<30 min) increased after IL-1ß, followed by a return to baseline level, particularly in the basal nucleus of the BLA complex. To verify and expand on effects of peripheral inflammation, we tested whether another, long-lasting inflammagen also changes BLA neuronal firing. Lipopolysaccharide (250 µg/kg, i.p.) increased BLA firing rate acutely (<30 min) and persistently. The findings demonstrate a rapid effect of peripheral inflammation on BLA activity and suggest a link between BLA neuronal firing and triggering of behavioral consequences of peripheral inflammation. These findings are a first step toward understanding the neuronal basis of depressive behavior caused by acute peripheral inflammation.

Reduced Microglial Activity and Enhanced Glutamate Transmission in the Basolateral Amygdala in Early CNS Autoimmunity.

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 behaviors is poorly understood. To identify CNS changes associated with these behaviors, we focused on the basolateral amygdala (BLA) because of its central role in the regulation of emotional behavior. Whole-cell recordings were performed in the principal neurons of the BLA in early EAE, before demyelination, T-cell invasion, and motor dysfunction. EAE female mice displayed increased frequency of mEPSCs, with no alteration in amplitude or evoked EPSC paired-pulse ratio compared with 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 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 "deactivation" 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 experimental autoimmune encephalomyelitis (an animal model of multiple sclerosis) in the basolateral amygdala, a nucleus important in the types of behavioral changes we have previously described. These electrophysiological and morphological effects occurred without significant elevation of local inflammatory cytokines or local demyelination. Unexpectedly, in the context of inflammatory state, we found that microglia were "deactivated." 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.

Modulation of Binge-like Ethanol Consumption by IL-10 Signaling in the Basolateral Amygdala.

Excessive ethanol consumption alters the neuroimmune system and particularly impacts the cytokine milieu of the CNS. Cytokine dysregulation has been shown to underlie addictive-like behaviors including alcohol abuse; however, many studies focus primarily on the proinflammatory cytokine profile during alcohol dependence. The current study furthers this research by determining the impact of excessive ethanol consumption on interleukin-10 (IL-10) and interleukin-4 (IL-4) activity in a model of non-dependent binge consumption called the "drinking in the dark" (DID) paradigm. Furthermore, the ability of IL-10 to modulate ethanol consumption was tested using site-directed pharmacology. Immunohistochemistry analyses determined that ethanol decreased IL-10 by 50 % in the basolateral amygdala (BLA) but had no effect on IL-4. Neither IL-10 nor IL-4, however, were altered in the central amygdala (CEA). Enzyme linked immunosorbent assays confirmed that IL-10 was decreased in the amygdala but not in the serum, suggesting changes of this cytokine with the DID paradigm are restricted to the central nervous system. Finally, bilateral infusions of IL-10 into the BLA, but not CeA, reduced binge-like drinking and corresponding blood ethanol concentrations without impacting either locomotor activity or anxiety-like behavioral correlates. Together, these data support the idea that alcohol abuse dysregulates specific anti-inflammatory cytokines; however, ameliorating alcohol-induced effects on cytokines, like IL-10, may prove to be an effective therapy in curbing excessive consumption.