MODULAZIONE DI MECCANISMI DI PLASTICITA' NEURONALE DOPO SOMMINISTRAZIONE ACUTA O CRONICA DI COCAINA

The major aim of this thesis was to assess the role of the neurotrophin BDNF and the imemdiate early gene Arc in the mechanism of action of cocaine. To pursue this purpose we exposed the animals to various drug paradigms. We first injected the animals with different doses of cocaine and sacrificed the animals at different time points. These experiments revealed that acute injection of cocaine produced a finely tuned, time-dependent and regional-selective expression profile of both BDNF and Arc suggesting that they indie partecipate in the rapid action of the psychostimulant. The analysis of repente exposure of the animals to cocaine revealed that precursor (pro-) and mBDNF protein forms were markedly reduced 2 h and 72 h post-injection in the prefrontal cortex. Interestingly, in the striatum we found that repeated cocaine injection increased pro-BDNF levels without altering the mature form of the neurotrophin, thereby suggesting that cocaine differently affects BDNF transcription and translation in a region-selective manner, but might also alter neurotrophin processing. These data further support the notion that the corticostriatal network is highly vulnerable to the effects of cocaine, and suggest that abnormal regulation of BDNF expression could contribute, at least in part, to the functional defects observed in drug abusers. We then decided to evaluate whether repente stress, a major risk factor for cociane relapse, might alter the pattern of BDNF and Arc expression following acute administration of cocaine. we provide evidence that repeated stress prevents cocaine-induced activation of BDNF expression and signaling in rat prefrontal cortex. A single injection of cocaine up-regulates BDNF expression in sham (i.e. unstressed) rats but not in repeatedly stressed rats. Similarly, the expression as well as trafficking of the high affinity BDNF receptor trkB promoted by the psychostimulant is impaired in chronically-stressed rats challenged with cocaine. Moreover, among the different intracellular signaling pathways that can be activated by the neurotrophin, i.e. ERK1/2-, Akt- and PLCγ-pathway, we found that cocaine is able to selectively activate the ERK1/2 pathway in sham animals, but not in rats exposed to repeated stress. These data demonstrate that stress globally interferes with BDNF-mediated signaling responses to cocaine challenge, providing key insights into the molecular basis of stress-cocaine interaction and indicating the critical role of the prefrontal cortex in mediating such interaction. To follow up our studies we decided to employ a new administration paradimg of cocaine in order to separate the direct pharmacological effects of cocaine from those associated with active drug selfadministration. To this end, we employed a yoked control-operant paradigm and sacrificed the animals after a single intravenous (i.v.) cocaine self-administration session. Animals self-administering cocaine (SA, 0.25 mg/0.1 ml saline per infusion, 2-h session) did more active lever-presses than yoked-cocaine (YC) and yoked-vehicle animals (YV). This goal-oriented behaviour was accompanied by a selective increase in Arc mRNA levels in the medial prefrontal cortex. There were no changes in the expression of the other genes in this brain region. mRNA levels of Arc and Zif268 in striatum and Zif268 in the nucleus accumbens markedly increased both in SA and YC animals; but there was no change in the expression of FGF-2 and BDNF. No changes were observed in hippocampus, hypothalamus, frontal cortex, and midbrain in SA and YC animals compared to YV animals in any of the genes. These findings demonstrate that a single session of cocaine i.v. self-administration is sufficient to shape rat behaviour towards goal-directed behaviours and selectively up-regulate Arc expression in mPFC (of SA animals), providing the first evidence that the function of prefrontal cortex is already profoundly influenced by the first voluntary cocaine exposure.