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 (ERK1/2) phosphorylation is critical for neuronal and behavioural functions; in particular, phosphorylated ERK1/2 (pERK1/2) expression in the nucleus accumbens (Acb) of the rat is stimulated by addictive drugs with the exception of morphine, which decreases accumbal ERK1/2 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 ERK1/2 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 ERK1/2 phosphorylation in the Acb, the behavioural effects of morphine were evaluated in a place-conditioning apparatus and ERK1/2 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 pERK1/2 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 pERK1/2 expression but not of morphine-elicited CPP, between RLA and RHA rats and sustain the observation that changes in pERK1/2 expression in the Acb are not a requisite for the reinforcing effects of morphine.
['Rosas|Michela|M|', 'Porru|Simona|S|', 'Sabariego|Marta|M|', 'Piludu|Maria Antonietta|MA|', 'Giorgi|Osvaldo|O|', 'Corda|Maria G|MG|', 'Acquas|Elio|E|']
D000818:Animals D001362:Avoidance Learning D016739:Behavior, Addictive D001923:Brain Chemistry D003216:Conditioning, Operant D007150:Immunohistochemistry D020935:MAP Kinase Signaling System D008297:Male D009020:Morphine D009294:Narcotics D009714:Nucleus Accumbens D010766:Phosphorylation D051381:Rats D012054:Reinforcement, Psychology
Morphine Nucleus accumbens Psychogenetic selection RLA rats Conditioned place preference (CPP) RHA rats Extracellular signal-regulated kinase (ERK) Phosphorylated ERK (pERK)