CCL2-ethanol interactions and hippocampal synaptic protein expression in a transgenic mouse model

Abstract

Chronic exposure to ethanol produces a number of detrimental effects on behavior. Neuroadaptive changes in brain structure or function underlie these behavioral effects and may be transient or persistent in nature. Central to the functional changes are alterations in the biology of neuronal and glial cells of the brain. Recent data show that ethanol induces glial cells of the brain to produce elevated levels of neuroimmune factors including CCL2, a key innate immune chemokine. Depending on the conditions of ethanol exposure, the upregulated levels of CCL2 can be transient or persistent and outlast the period of ethanol exposure. Importantly, results indicate that the upregulated levels of CCL2 may lead to CCL2-ethanol interactions that mediate or regulate the effects of ethanol on the brain. Glial cells are in close association with neurons and regulate many neuronal functions. Therefore, effects of ethanol on glial cells may underlie some of the effects of ethanol on neurons. To investigate this possibility, we are studying effects of chronic ethanol on hippocampal synaptic function in a transgenic mouse model that expresses elevated levels of CCL2 in the brain through enhanced glial expression, a situation know to occur in alcoholics. Both CCL2 and ethanol have been reported to alter synaptic function in the hippocampus. In the current study, we determined if interactions are evident between CCL2 and ethanol at the level of hippocampal synaptic proteins. Two ethanol exposure paradigms were used; the first involved ethanol exposure by drinking and the second involved ethanol exposure in a paradigm that combines drinking plus ethanol vapor. The first paradigm does not produce dependence on ethanol, whereas the second paradigm is commonly used to produce ethanol dependence. Results show modest effects of both ethanol exposure paradigms on the level of synaptic proteins in the hippocampus of CCL2 transgenic mice compared with their non-transgenic littermate controls, consistent with ethanol-CCL2 interactions. No evidence of toxic effects of CCL2 or CCL2-ethanol interactions was observed. Taken together, these results support the idea that ethanol induced astrocyte production of CCL2 can result in neuroadaptive changes that interact with the actions of ethanol.

Age-related differences in anxiety-like behavior and amygdalar CCL2 responsiveness to stress following alcohol withdrawal in male Wistar rats

“…In agreement with research demonstrating that cytokines are increased in brain by stress (Madrigal et al 2010; Vecchiarelli et al 2016), substitution of the two initial withdrawals of the CIA protocol with intracerebroventricular (ICV) (1 week apart) of the chemokine (C-C motif) ligand (CCL2) before the final 5 days of the alcohol diet enhanced anxiety-like behavior during withdrawal from the single final 5-day alcohol exposure versus 5 days of ethanol diet (Breese et al 2008; Knapp et al 2011). In accord with this finding, elevation of CCL2 alone altered hippocampal synaptic function (Nelson et al 2011) and elevation of CCL2 in combination with alcohol altered hippocampal synaptic structure (Gruol et al 2014). CCL2 and CCR2 knockout mice were shown to have altered alcohol drinking behavior (Blednov et al 2005), a finding that further supports the idea that CCL2 influences neural function underlying behavior.…”

Transgenic mice with increased astrocyte expression of CCL2 show altered behavioral effects of alcohol

“…For example, high doses of CCL2 were most effective in studies using CCL2 infusion. Our ELISA studies of hippocampus and cerebellum indicate that tissue levels of CCL2 are considerably lower (~1.5 ng/ml) (Gruol et al, 2014) than the most effective doses of CCL2 (e.g., 2 μg/d) in the infusion experiments. Differences in the brain regions exposed to CCL2, and cellular accessibility may also be important differences between the two studies.…”

“…Further studies will be needed to test this possibility. There was no genotypic difference in the level of mGluR2/3 in the hippocampus from alcohol naïve CCL2-tg and non-tg mice (Gruol et al, 2014). …”

“…Group II mGluR regulation of long-term depression (LTD) in the hippocampus has been shown to play a key role in spatial learning and memory (Altinbilek and Manahan-Vaughan, 2007). Results from our Western blot studies using an antibody that detects both mGluR2 and mGluR3 (mGluR2/3) showed that mGluR2/3 levels were reduced in hippocampus from both 2BC drinking (non-dependent) and CIE/2BC treated (dependent) CCL2-tg mice compared to non-tg mice of the same alcohol treatment group (Gruol et al, 2014). Studies have shown that mGluR2/3 mRNA or protein is downregulated in other brain regions by CIE (Meinhardt et al, 2013; Barker et al, 2016) and that mGluR2 mRNA is downregulated in the cortex of post-mortem alcohol-dependent humans (Meinhardt et al, 2013), consistent with the ability of alcohol to affect expression of this protein.…”

The addicted brain: understanding the neurophysiological mechanisms of addictive disorders

“…Alcohol has also been shown to engage the immune pathway. Gruol et al ( 2014 ) used a transgenic mouse model overexpressing the immune cytokine CCL2 to determine if elevated levels of CCL2 interact with the effects of ethanol in the hippocampus and found that elevated levels of CCL2 protected against the effects of acute ethanol on synaptic plasticity (i.e., LTP).…”