COMT protein expression was investigated as a
COMT protein prostaglandin endoperoxide synthase was investigated as a potential mechanism by which tolcapone may differentially affect male and female P and Wistar rats. COMT protein levels in the PFC were lower in P rats compared to Wistars but female P rats expressed greater levels of COMT in the PFC relative to males. Engleman et al. (2006) reported that P rats exhibited lower extracellular DA levels relative to Wistars which may result in a down regulation of COMT expression. Importantly, strain differences in PFC DA tone were only previously assessed in male rats (Engleman et al., 2006), making it unclear whether or not this strain difference is present in females. The current studies assessed levels of COMT protein but not enzymatic activity of COMT. While allelic variations in COMT have been modeled in mice (Papaleo et al., 2008, Risbrough et al., 2014), to date no such rat models exist. However, mice with increased COMT activity show deficits in attentional processes (Papaleo et al., 2008) and response inhibition (Simpson et al., 2014) similar to individuals with AUD (Noel and Van der Linden, 2007, Vollstadt-Klein et al., 2012) and P rats (Beckwith and Czachowski, 2014, Beckwith and Czachowski, 2016, Linsenbardt et al., 2016). Additionally, polymorphisms of the COMT gene are characterized by differences in enzyme activity (Chen et al., 2004) and these differences are associated with alterations in executive functioning (Egan et al., 2001, Farrell et al., 2012). Thus, greater enzymatic activity of COMT in P rats may explain reduced cortical DA tone and COMT protein levels relative to Wistars. Similar to observations in clinical populations (Farrell et al., 2012, Mattay et al., 2003), animals with greater enzymatic activity may show increased sensitivity to pharmacological compounds which target PFC DA and PFC-mediated behaviors. Differences in COMT metabolism of DA may underlie the behavioral differences reported here, but future studies are needed to fully explore these possibilities. The efficacy of tolcapone in male rats for each of the reinforcers tested here may be associated with differences in PFC function. It has been hypothesized that the PFC encodes, among other things, subjective value (Kable & Glimcher, 2007). Therefore, the efficacy of tolcapone in the PSSD task may be mediated by each animal\'s appraisal of its respective reinforcer. This is consistent with the non-ethanol specific effects of tolcapone on consummatory behavior in P rats which may be due to the perceived value of reward in these animals which show excessive reward consumption for both ethanol and sucrose (McCane et al., 2014). In fact, Shnitko and Robinson (2014) recently reported statistically indistinguishable cue-evoked striatal DA activity in ethanol- and sucrose-reinforced rats. The authors posited that these results may indicate that both solutions were equally reinforcing, evident by similar operant responding in both groups (Shnitko & Robinson, 2014). Cue-evoked DA release is consistently reported (Brown et al., 2011, Fotros et al., 2013) and is hypothesized to play a role in encoding of salient stimuli (Berridge & Robinson, 1998). Enhancement of DA tone may shift the signal to noise ratio (Servan-Schreiber, Printz, & Cohen, 1990), dampening the effects of stimulus presentation on subsequent behavior. Under this hypothesis, in the current experiments, a tolcapone-mediated suppression of cue salience may lead to a reduction in reward seeking and consumption. Subjective value assigned to a reinforcing substance would influence the salience that one attributes to cues predictive of said substance. Importantly, the nonspecific nature of tolcapone\'s effects on reinforcers in general suggests that COMT inhibition may possess clinical utility in treatment of addiction disorders broadly. Given innate differences in PFC DA tone between P and Wistar rats (Engleman et al., 2006), coupled with our findings that COMT expression differs by strain and sex in the PFC, we hypothesize that tolcapone exerts its effects primarily on cortical catecholamines. However, high COMT expression in the hippocampus has also been reported (Matsumoto et al., 2003) and tolcapone affects hippocampal DA and behavior in hippocampus-mediated tasks (Laatikainen et al., 2012, Laatikainen et al., 2013). Furthermore, hippocampal DA has been hypothesized to contribute to attribution of incentive salience (Fotros et al., 2013). Thus in the present experiments, it is unclear to what extent hippocampal DA levels may contribute to tolcapone-mediated changes in drinking behaviors. Experiments that seek to tease apart the involvement of DA transmission in the hippocampus versus PFC in ethanol-motivated responding are thus warranted.