Divergent Sensitivities to Drugs of Abuse: Neurochemical and Neuroanatomical Characterization of the Roman Rats

Nomes un petit percentatge dentre tots els individus que experimenten amb drogues dabus esdevenen addictes. La vulnerabilitat per a desenvolupar un trastorn addictiu esta relacionada amb trets de personalitat impulsiva o amb apetencia per la novetat. Les soques de rata Romanes, geneticament seleccionades per alta (RHA) o baixa (RLA) adquisicio de levitacio activa en dos sentits, son un model de laboratori valid de les divergencies en apetencia per la novetat i substancies gratificants que sobserven en humans. A mes a mes, aquestes soques de rata difereixen en la funcionalitat del sistema dopaminergic. Se sap que les rates RHA beuen etanol voluntariament mentre que les rates RLA mostren aversio. En aquesta tesis, les rates Romanes sempren com a model de vulnerabilitat addictiva. Lobjectiu de la tesis ha consistit en entendre els mecanismes neurobiologics que sustenten les diferencies de vulnerabilitat addictiva entre les dues soques de rata Romanes. El treball sha dividit en 3 fases experimentals. En primer lloc, es va avaluar la resposta conductual de les rates Romanes a la injeccio duna dosi baixa detanol. Tal i com sobserva en humans amb elevat risc desdevenir alcoholics, les rates RHA van ser menys sensibles als efectes conductuals daquesta dosi. En la segona fase, es varen estudiar els cervells de rates Romanes naive per tal de caracteritzar varies dianes moleculars del sistema dopaminergic i neuropeptids relacionats: es van quantificar diferents subtipus de receptor de dopamina per mitja de la tecnica dautoradiografia de receptors aixi com els nivells dexpressio dARNm per diferents neuropeptids mitjancant la tecnica dhibridacio in situ. Comparades amb les rates RLA, les rates RHA presenten majors nivells de receptors D1 i D3 aixi com tambe dARNm per la DYN a nivell de lescorca del nucli accumbens (NAc). En canvi, presenten menors nivells dexpressio del receptor D3 a nivell de les illes de Calleja. A mes a mes, quan sadministra un agonista D3, les rates RLA mostren major inhibicio de lactivitat locomotora i supressio de lexpressio del gen NGFI-A a la Calleja magna (mesurat per mitja de tecniques dhibridacio in situ) que les rates RHA. Aquests resultats afegeixen noves evidencies de les diferencies en la funcionalitat del sistema dopaminergic i potser representen leix central de lentramat neurobiologic subjacent a les diferencies en apetencia per la novetat i preferencia per a les drogues dabus com letanol entre les dues soques. En la tercera fase, es va estudiar el fenomen de sensibilitzacio conductual induit per amfetamina en les rates Romanes i es van elaborar mapes dactivitat neuronal per mitja de tecniques dhibridacio in situ amb cinc gens dexpressio immediata. La sensibilitzacio conductual es un model de plasticitat conductual i neuronal induida per lus cronic de drogues. Les rates RHA que van rebre un tractament cronic amb amfetamina van mostrar sensibilitzacio conductual i un augment en lexpressio dels gens de secretogranina i PSD95 al corus del NAc. Aquestes troballes es discuteixen en el context daltres models de laboratori dus cronic de drogues. Daltra banda, les rates RLA no van mostrar sensibilitzacio conductual pero en canvi van mostrar adaptacions neuronals que poden estar relacionades amb la manca de sensibilitzacio. A mes a mes, les rates RLA que van rebre amfetamina per primer cop van mostrar activacio del nucli central de lamigdala (CeA). Tambe es va detectar activacio del CeA en ratolins que van rebre un tractament amb etanol i naltrexona, farmac que sempra a la practica clinica per tal de prevenir les recaigudes en alcoholics. Aquesta troballa suggereix que el CeA pot ser una estructura del cervell rellevant per tal de frenar el desenvolupament dels trastorns addictius. Hundreds of millions of people experiment with drugs of abuse, but only a small percentage of them become addicted. Vulnerability to develop addiction has been associated with impulsivity or novelty-seeking. The Roman rats, genetically selected for high (RHA) or low (RLA) active avoidance acquisition in the two-way shuttle box, appear to be a valid laboratory model of divergent novelty and substance-seeking profiles and differ in the functionality of the dopaminergic system. So far, it is known that RHA rats drink ethanol voluntarily whereas RLA rats show aversion to it. In the present thesis, the Roman rats have been used as a model of differences in vulnerability to addiction. The aim of the thesis was to understand the neurobiological mechanisms that underlie such differences in vulnerability between the two Roman rat strains. The work has been divided in 3 experimental blocks. First, we studied the behavioral response to an injection of a low dose of ethanol in the Roman rats. Like those humans that have higher risk to develop alcoholism, RHA rats were less sensitive to the effects of low-dose of ethanol. Second, brains of naive Roman rats were studied in order to characterize several molecular targets of the dopaminergic system and related neuropeptides: dopamine receptor subtypes were quantified by means of receptor autoradiography and mRNA coding for neuropeptides were quantified using in situ hybridization histochemistry. When compared to RLA rats, RHA showed higher binding of D1 and D3 dopamine receptor subtypes and DYN mRNA expression in the nucleus accumbens (NAc) shell, although they showed lower basal binding of D3 receptors in the Calleja islands. Moreover, a challenge with a D3 agonist resulted in greater inhibition of locomotor activity as well as supression of NGFI-A mRNA as measured with in situ hybridization in the Calleja magna in RLA rats when compared to RHA rats. These results provide further evidences of the differences in dopamine function between the Roman strains and may represent the neurobiological core of the divergences in novelty-seeking and preference for addictive drugs such as ethanol. Third, behavioral sensitization, a model of behavioral and neuronal plasticity secondary to chronic drug use, was also studied in the Roman rats. Neuronal activity maps with 5 different immediate early genes were made by means of in situ hybridization. Amphetamine pre-treated RHA rats showed behavioral sensitization and increased secretogranin and PSD-95 in the NAc core which is suggestive of increased glutamatergic activity at this site. These findings are discussed in the context of the laboratory models of chronic drug use. Pretreatment with amphetamine in RLA rats did not result in behavioral sensitization but induced neuronal adaptations that may be related to the lack of this phenomenon. Moreover, RLA rats that experienced amphetamine for the first time showed activation of the central nucleus of the amygdala (CeA). Activation of the CeA was also seen in mice receiving ethanol and naltrexone, a drug used to prevent relapse in alcoholics. These findings suggest that the CeA may be a relevant brain structure in preventing drug addiction.