Conditioned effects of ethanol on the immune system

Abstract

Several studies indicate that the immune system can be subjected to classical conditioning. Acute ethanol intoxication significantly modulates several pro-inflammatory cytokines (e.g. interleukins-1 and 6 [IL-1β and IL-6, respectively] and tumor necrosis factor alpha [TNFα])) in several brain areas, including amygdala (AMG), paraventricular nucleus (PVN), and hippocampus (HPC). It is unknown, however, whether cues associated with ethanol can elicit conditioned alterations in cytokine expression. The present study analyzed, in male Sprague-Dawley rats, whether ethanol-induced changes in the central cytokine response may be amenable to conditioning. In Experiments 1 and 2, the rats were given one or two pairings between a distinctive odor (conditional stimulus, CS) and the post-absorptive effects of a high (3.0 or 4.0 g/kg, Experiments 1 and 2, respectively) ethanol dose. Neither of these experiments revealed conditioning of IL-6, IL-1β, or TNFα, as measured via mRNA levels. Yet, re-exposure to the lemon-odor CS in Experiment 1 significantly increased C-Fos levels in the PVN. In Experiment 3, the rats were given four pairings between an odor CS and a moderate ethanol dose (2.0 g/kg), delivered intraperitoneally (i.p.) or intragastrically (i.g.). Re-exposure to the odor CS significantly increased IL-6 levels in HPC and AMG, an effect only evident in paired rats administered ethanol i.p. Overall, this study suggests that ethanol exposure can regulate the levels of IL-6 at HPC and AMG via classical conditioning mechanisms. These ethanol-induced, conditioned alterations in cytokine levels may ultimately affect the intake and motivational effects of ethanol. Impact statement This study examines, across three experiments, whether odor cues associated with ethanol exposure can condition changes in cytokine expression. The analysis of ethanol-induced conditioning of immune responses is a novel niche that can help understand the transition from social drinking to alcohol abuse and dependence. Ethanol-induced conditioning of the immune system could likely exacerbate neuroinflammation and drug-related toxicity, which in turn may facilitate further engagement in ethanol intake. The main new finding of the present study was that, after four pairings of ethanol's unconditioned effects and a distinctive odor, the latter CS increased IL-6 levels in HPC and AMG. This suggests that ethanol's effects upon IL-6 in HPC and AMG may come under conditioned control, particularly after repeated pairings between distinctive odor cues and ethanol's effects. This article advances our knowledge of conditioned increases in cytokine responses, which should help understand the mechanisms underlying alcohol use, abuse, and relapse.

Conditioning the neuroimmune response to ethanol using taste and environmental cues in adolescent and adult rats

“…We can only speculate as to the reasons underlying the relative lack of significant, conditioned immune, effects in Experiment 1. It should be noted, however, that a similar lack of significant results was observed in our prior 22 study after using one or two-conditioning trials. Differences in the experimental designs and procedures hinder a direct comparison, yet it seems that ethanol-conditioned immune responses in adults may require repeated CS-US pairings to be acquired.…”

“…Group sizes were determined based on power analyses performed on a comparable data set in which conditioned effects of ethanol were previously reported. 22 Specifically, power analysis of IL-6 gene expression data in a context conditioning procedure with n ¼ 6-8 per group and a ¼ 0.05, power was 0.67 with an effect size (gp2) of 0.20, suggesting use of 8-10 rats per group would provide adequate power. Thus, studies presented here were performed with starting n ¼ 12 per group to account for occasional attrition due, for instance, to inadequate sampling of the CS (sucrose) on the day of training as per our a priori exclusion criteria described above.…”

“…This behavioral response was associated with a complex pattern of neuroimmune responses, which exhibited regional and ontogenetic variability. In Experiment 1, we examined the adult HPC, a site in which we previously noted cytokine conditioning by ethanol, 22 as well as the NTS and PbN, sites involved in CTA conditioning and in brain-to-immune system communication. 37 IL-6 was not affected by the conditioning, although paired animals exhibited a significant increase of IjBa-a reporter of NfjB activity that, in our previous work, was consistently associated with IL-6 expression.…”

“…avoidance of the taste CS) in tandem with the potential neuroimmune adaptations. We chose to examine areas previously shown to be sensitive to conditioning (HPC, AMG, see Gano et al 22 ), as well as those (nucleus of the solitary tract [NTS], parabrachial nucleus [PbN]) implicated in the circuitry underlying conditioned taste aversion (CTA) responses, 23 predicting that these sites would be most likely to display conditioned changes in neuroimmune gene expression.…”

Altered brain activity during withdrawal from chronic alcohol is associated with changes in IL-6 signal transduction and GABAergic mechanisms in transgenic mice with increased astrocyte expression of IL-6

“…Chronic alcohol consumption (13 g/kg per day for 3 weeks) using a two-bottle choice drinking paradigm also increased levels of IL-6 (mRNA and protein) in the mouse brain (Zhang et al, 2014). Interestingly, a recent study showed that alcohol-induced changes in the level of mRNA for several cytokines, including IL-6, in the brain of rats could be modulated by behavioral conditioning, such that alcohol pairing with a conditioned stimulus resulted in a conditioned cytokine response (Gano et al, 2017).…”