Trait anxiety and environmental enrichment as determining factors on the development and functioning of the reward and aversion circuitry in rats made dependent on alcohol or morphine

Abstract

The biological basis of the emotional states linked to reward and aversion are still very poorly understood. Classical formulations have associated the mesocorticolimbic system with the modulation of emotional responses induced by reward stimuli. From this system stand out the nucleus accumbens, the ventral tegmental area and the prefrontal cortex. Moreover, other brain areas, such as amygdala, the periaqueductal gray, the locus coeruleus and hypothalamus have been implicated in aversive processes. In previous studies from our laboratory we have demonstrated consistently that the neuronal modulation of negative emotional states induced by withdrawal from drugs of abuse and those promoted by fear stimuli appear to overlap in the brainstem structures. Recent studies, however, have suggested the importance of telencephalic structures as biological substrates for pleasure as well as aversion, particularly the nucleus accumbens. In this context, we must point out the role of environmental enrichment as an attenuator agent on both the deleterious effects of stress on cognition and the ability of drugs of abuse to lead the individual to addiction. With regard to this point, it is well-known that exposure to an enriched environment during development improves learning in a variety of behavioral tests, and promotes a clear "protective" effect on vulnerability to drug use. It is also known that the great majority of individuals who experience the effects of a drug of abuse do not become dependent. With regard to this poinst, it has been stated that one of the most important aspects to be elucidated in the neurobiology of drug addiction is to determine which factors associated with individual differences, determine the vulnerability of the individual to the effects of drugs of abuse. The genetic characteristics are known to act strongly in disorders related to substance use. However, non-genetic factors such as social context and the type of environment in which the individual is raised also appear to influence, critically, their vulnerability to the reinforcing effects of a drug. (AU)