Behavioral and neurochemical assessment of the role of ERK pathway in the psychopharmacological effects of ethanol, caffeine and of their interaction

The intracellular signaling cascades constitute the means by which addictive substances induce the remodeling of the circuits involved in motivated behaviors which underlie the learning processes and the development of memories at the basis of the progression of addiction. A molecule protagonist in such signaling cascades is the Extracellular signal-Regulated Kinase (ERK), a member of the mitogen-activated protein kinase, that constitutes an important biochemical factor common to many cellular functions. The abundant expression of ERK in brain areas of the addiction circuits emphasizes the relevance of these kinases in modulating behavioral functions mediated by these circuits. Thus, the general aim of the present doctoral thesis was to study the role of ERK in terms of protein kinase expression and behavioral responses induced by ethanol, caffeine and their association. The first and second chapters examined the involvement of protein kinases ERK in different aspects of ethanol-induced place conditioning. Specifically, in the first chapter we used the MEK inhibitor SL327 to study how the blockade of this cascade could affect the acquisition and expression of Conditioned Place Preference (CPP) and Conditioned Place Aversion (CPA) elicited by ethanol. The second chapter explored the results of the pharmacological relationship between caffeine and ethanol in ethanol-elicited CPP and CPA. In this chapter we also investigated the expression of pERK as a result of 1) the acute administration of both substances and 2) the presentation of stimuli positively (CPP) or negatively (CPA) conditioned to ethanol. The last chapter examines the pharmacological relationship between caffeine and ethanol through the analysis of horizontal and vertical locomotion and evaluated whether there was reciprocal cross-sensitization with respect to these effects. We also examined the phosphorylation of DARPP-32(Thr75), another factor of intracellular signaling cascade, and of ERK in the nucleus accumbens. The findings of these studies revealed that MEK/ERK pathway is differentially involved in distinct phases of associative learning behavior expressed in the CPP and CPA elicited by ethanol. Moreover, our data disclose that ERK activation takes place differentially in distinct brain regions depending on the motivational significance of the conditioned stimulus. Furthermore, we demonstrated that caffeine significantly impaires ethanol-elicited place conditioning (both CPP and CPA) and prevents ethanol-induced pERK expression in several brain areas with different activation patterns depending on the brain area examined. Our observations also revealed that caffeine and ethanol affect horizontal and vertical locomotion in different manner and without undergoing cross-sensitization. Finally, caffeine prevents ethanol-elicited pERK expression in the nucleus accumbens whereas there were no effects on pDARPP-32(Thr75). Taken together the results of the present thesis offer new insights into the complexity of the involvement of ERK cascade in the acquisition and expression of associative learning and provide new information about the antagonistic interaction between caffeine and ethanol expressed in place conditioning and locomotor activation.