||Clinical studies indicate that genetically determined personality traits, such as sensation seeking and impulsivity, are associated with increased drug addiction liability. Thus, in spite of the fact that for instance a prolonged ethanol exposure is an essential precondition to develop the disease, genetic factors account for about 50-60% of the risk of developing alcoholism. These observations have stimulated the use of selected strains and lines of rodents as models to investigate genetically determined differences in behavioral and neurochemical traits related to addictive disorders. Among those, the Roman high (RHA)- and low (RLA)-avoidance rats are selectively bred for rapid vs poor acquisition of active avoidance, respectively, and differ markedly in emotional reactivity, coping style, and behavioral and neurochemical responses to a number of drugs of abuse such as morphine, ethanol and psychostimulants. On the other hand, one of the most efficacious approaches to model several aspects of alcoholism is represented by the use of rat lines selectively bred for high ethanol preference or excessive ethanol drinking that represent well-suited experimental models to investigate the biological bases of alcoholism by virtue of their predictive, face, and construct validity. In particular, Sardinian alcohol-preferring (sP) rats constitute one of these rat lines selectively bred for excessive ethanol intake. Thus, when given a choice between 10% (v/v) ethanol and water, under the standard, home cage 2-bottle regimen, with unlimited access for 24 h/day, sP rats display a clear preference for the ethanol solution and daily consume 6 to 7 g/kg of pure ethanol. Among the kinases and transcription factors that have been deeply investigated in the recent years for their involvement in the molecular basis of addiction, the extracellular signalregulated kinase (ERK) occupies a strategic position, both at the cytoplasmic and nuclear level. In fact, activation of ERK is critical for memory formation and consolidation in different tasks such as conditioned taste aversion and fear conditioning, but it also appears critical in synaptic plasticity, as well as in synapse formation and function. Activation of ERK is also critical for long-term potentiation, a phenomenon of long-lasting synaptic plasticity that allows the characterization of the role of transduction molecules in the processes of learning and formation of long-term memories. In addition, in mature neurons, activation of ERK is involved in the mechanism of action of addictive drugs and its expression in dopaminerich terminal areas plays a critical role in several of their psychopharmacological effects, including their ability to sustain acquisition of conditioned place preference. However, some controversial findings still remain on the effects of morphine on ERK phosphorylation in the nucleus accumbens. Similarly, the issue of genetically determined differences among pairs of rat lines selectively bred for rapid vs slow acquisition of active avoidance (RHA vs RLA) and for high vs poor ethanol preference or excessive ethanol drinking (sP vs sNP) has never been systematically investigated. Hence, in order to gain insights on the differential sensitivity in terms of activated ERK in brain structures of the mesolimbic system and extended amygdala of such rat lines, the present study was devoted to investigate these issues by assessing the ability of morphine to activate ERK phosphorylation in the shell (AcbSh) and core (AcbC) of the nucleus accumbens (Acb) of Roman RHA and RLA, Sprague-Dawley and Wistar rats as well as in CD-1 mice and C57BL6J mice. This study was also aimed at comparing the effects of acute cocaine administration in Roman rats, and of ethanol administration in sP vs sNP rats, on ERK phosphorylation in Acb, prelimbic (PrL) and infralimbic (IL) prefrontal cortex (PFCx) and nuclei of the extended amygdala (BSTL, CeA and BLA). The results of these experiments demonstrate 1) that morphine significantly decreased Acb pERK expression, as determined by counting the number of pERK positive cells/area, in RLA, Sprague-Dawley and Wistar rats, 2) failed to either decrease or increase this measure in Acb of RHA rats and 3) significantly increased pERK expression in both CD-1 and C57BL6J mice. Ethanol (1 g/kg) significantly increased pERK immunoreactivity in AcbSh and AcbC of sP but not sNP rats. Conversely, ethanol failed to affect pERK expression in PrL and IL PFCx as well as in BSTL and CeA of both sP and sNP rats. In RHA but not RLA rats, cocaine (5 mg/kg) increased pERK in the IL PFCx and AcbSh, two areas involved in its acute effects, but did not modify pERK in the PrL PFCx and AcbC, which mediate the chronic effects of cocaine. These findings suggest that the lack of stimulation of pERK expression observed in RHA rats after morphine administration may be of considerable importance and significance, given the enhanced responsiveness of these animals to the neurochemical and behavioral effects of morphine; these findings also confirm previously reported differential effects of morphine on ERK phosphorylation in the Acb of rats vs mice and call for further experiments to elucidate the significance of the property of morphine to either increase and decrease pERK expression in rodents’ AcbSh and AcbC. The results also show that ethanol activated ERK differentially in Acb and PFCx of sP and sNP rats and indicate that selective breeding of these rat lines resulted in a double dissociation of the effects of acute ethanol on ERK phosphorylation in brain regions critical for ethanol’s psychopharmacological effects. Furthermore, these results extend to pERK expression previous findings on the greater sensitivity to acute cocaine of RHA vs RLA rats and confirm the notion that genetic factors influence the differential responses to addictive drugs. In conclusion, these findings support the view that the genetically selected rat lines represent a valid model to investigate the neural basis of the individual vulnerability to drug addiction and that pERK phosphorylation in nuclei of extended amygdala represents a valuable dynamic marker of drug-induced neuronal activation .