The lack of a significant number of differences in gene expression between the MSA and W groups of the present study, such that the number of gene expression differences (with p-values less than 0.01) was less than that expected by chance (i.e., less than 1% of the total number of genes), appears to disagree with a recent genomic study examining inbred P rats in a 1-hr operant ethanol self-administration procedure (Rodd et al., 2008). These authors reported that inbred P rats responding for ethanol displayed a significant number (> 200) of gene expression differences, in the ACB, compared to a water control group, when animals were killed the day after the last operant session. This lack of agreement suggests that multiple factors, other than temporal (i.e., time-of-day) conditioning and ethanol alone, are influencing the number of gene expression differences between the effects of MSA sessions of oral self-administration of ethanol per day and daily sessions of operant ethanol self-administration, compared with their respective controls. Some of these factors may include the role of Pavlovian and instrumental conditioning or lever pressing
