Effects of morphine on place conditioning and ERK1/2 phosphorylation in the nucleus accumbens of psychogenetically selected Roman low- and high-avoidance rats

RATIONALE

Extracellular signal-regulated kinase (ERK_1/2) phosphorylation is critical for neuronal and behavioural functions; in particular, phosphorylated ERK_1/2 (pERK_1/2) expression in the nucleus accumbens (Acb) of the rat is stimulated by addictive drugs with the exception of morphine, which decreases accumbal ERK_1/2 phosphorylation in the Sprague-Dawley and Wistar rats. The psychogenetically selected Roman low- (RLA) and high-avoidance (RHA) rats differ behaviourally and neurochemically in many responses to addictive drugs. In particular, morphine elicits a greater increment in locomotor activity and in dopamine transmission in the Acb of RHA vs RLA rats. However, the effects of morphine on place conditioning (conditioned place preference (CPP)) and ERK_1/2 phosphorylation in the Roman lines remain unknown.

OBJECTIVES AND METHODS

To characterize in the Roman lines the reinforcing properties of morphine (i.e. morphine-elicited CPP acquisition) and the relationship between these properties and its effects on ERK_1/2 phosphorylation in the Acb, the behavioural effects of morphine were evaluated in a place-conditioning apparatus and ERK_1/2 phosphorylation was assessed by immunohistochemistry in the shell and core subregions of the Acb of rats both acutely administered with morphine or undergoing conditioning.

RESULTS

Morphine elicited CPP in both Roman lines and decreased pERK_1/2 expression in the Acb of RLA but not RHA rats. Such decrease was prevented by conditioning.

CONCLUSIONS

These findings indicate that the selective breeding of the Roman lines has generated a divergence, in terms of morphine-elicited pERK_1/2 expression but not of morphine-elicited CPP, between RLA and RHA rats and sustain the observation that changes in pERK_1/2 expression in the Acb are not a requisite for the reinforcing effects of morphine.