br Methods br Results br Discussion There are

Methods
Results
Discussion There are two particularly important facets of the present ERK findings relevant to drug conditioning. One is the strong ERK signal observed in the medial prefrontal cortex and the nucleus accumbens during the immediate post-trial interval following the final conditioning test selectively in the immediate apomorphine post-trial group that was the only group that manifested a conditioned response. The second is that the group that had undergone the same conditioning induction protocol and received the same apomorphine post-trial treatments but delayed by 15 min. in the post-trial treatment phase of the experiment exhibited extinction of the conditioned response along with the loss of the ERK response. Altogether, these findings are consistent with an ERK activation effect elicited by the conditioned apomorphine behavior in this paradigm of drug conditioning. In that, the immediate but not the delayed apomorphine post-test treatment maintained the conditioned response the results are also in line with the proposition that immediate post-trial apomorphine administration can interact with the post-trial state initiated by the conditioned response and impact the re-consolidation process. In several previous reports [10,13,33,34] we have suggested that post-trial dopaminergic drug treatments administered during consolidation/reconsolidation, in which dopamine systems have been activated, can modulate this post-trial dopamine trace and consequently increase/decrease the incentive/salience value of the association. As a corollary, the loss of an ERK response in the dopamine target areas following extinction of the conditioned response in the apomorphine post-trial delay group in the present experiment suggests that the immediate post-trial apomorphine treatments would be ineffective if they were subsequently administered after extinction in that there would be no post-trial dopamine activation trace to modify. A determination of this possibility awaits direct experimental validation. The findings of increased ERK in the medial prefrontal cortex in our previous apomorphine context specific sensitization and conditioning studies [26,27] and the present results appear supportive of evidence that dopamine projection sites contribute to dopamine effects upon learning and memory processes [[35], [36], [37], [38], [39], [40]]. In the search for the identity of cellular and molecular changes in the CRT 0066101 areas pertinent to the sensitization effects of repeated psychostimulant drug treatments, ERK has generated significant interest [[41], [42], [43], [44], [45], [46], [47]]. Furthermore, psychostimulant drugs such as cocaine produce an increased ERK response in striatal dopaminergic projection areas [[47], [48], [49], [50]], including the nucleus accumbens [31], frontal cortex [51] and amygdala [44]. It also has been reported [52] that cocaine increases dendritic spine growth within hours of cocaine exposure in the frontal cortex. This observation is in line with an earlier report [53] that the MAPK/ERK pathway in mediating cell surface signals to gene expression effects is relevant to possible growth effects such as neurite growth. Notwithstanding the impact of drugs such as cocaine on neurite growth, these findings have not differentiated drug effects elicited in non-drug tests by external cues versus drug-evoked effects such as sensitization. In the present study, the ERK changes were observed under non-drug conditions immediately following a brief 5 min conditioning test and were selective to conditioned drug cues and no longer evident following extinction. Thus, the ERK effects in this test protocol were not indicative of lasting neural changes but rather appeared to reflect increased neural activity in brain systems activated by the conditioned drug cues. In our previous studies, we found that the apomorphine treatment we used when given acutely induced widespread ERK increases but with repeated treatments that induced context specific sensitization the ERK increases were more selective to dopamine terminal areas [26]. In that apomorphine is a D1/D2 agonist it would be expected that the primary effects would be found in dopamine terminal areas. In the present study, we focused the ERK measurements on the dopamine projection areas of the mPFC and NAc and it is not surprising that ERK increases associated with the conditioned response were found in both brain structures in that the unconditioned drug response was induced by a stimulant dose of the D1/D2 agonist apomorphine. Interestingly, using a different conditioning model we were able to show that apomorphine administered immediately post-trial at an auto-receptor dose level (0.05 mg/kg) that induces behavioral inhibition generates a conditioned inhibitory response [24]. In light of the association of an increase in ERK activity in dopamine projection areas with an apomorphine conditioned stimulant response it would be important to determine whether a conditioned apomorphine inhibitory response is associated with decreased ERK response in dopamine projection areas. It will also be important in future studies to extend the present experimental protocol to conditioned drug effects induced by drugs such as cocaine and morphine and to broaden the array of brain structures and neurotransmitter systems sampled.