Conditioning as a critical determinant of sensitization induced by psychomotor stimulants

It is apparent that stimuli associated with psychomotor stimulants as well as opiates acquire the ability to elicit motor behaviors that probably reflect the acquisition and operation of incentive motivational processes. Such conditioning also appears to be a critical determinant of behavioral sensitization seen with repetitive administration of these agents. The conditioning of motor excitation to stimuli associated with psychomotor stimulants follows the principles of classical conditioning and is relatively long lasting. Dopaminergic mechanisms appear to be involved in the acquisition of such conditioned behaviors, since neuroleptics are effective blockers of the process. Dopaminergic blockade probably disrupts conditioning through several different mechanisms including attenuation of the conditioned and unconditioned excitatory properties of the CS and blockade of the US. DA blockade prevents stimuli associated with psychomotor stimulants from acquiring and subsequently generating positive affective motivational states that are reflected by increases in motoric output. While dopamine appears to be necessary for the formation of conditioned motor excitation, it is not critically involved in the expression of the conditioned effects. This seems to suggest that DA may serve only to modulate the formation of motivationally significant associations but is not involved in the expression of conditioned drug effects that may be mediated through DA-independent pathways. The amygdala and nucleus accumbens are two structures in the CNS involved in the acquisition of conditioned motor excitation. Interestingly, both of these brain regions are the recipients of mesolimbic DA input. Dopamine probably plays different roles in each region during the conditioning process. In the amygdala, mesolimbic DA may serve to modulate processes that attach emotional significance to environmental stimuli; further, DA may play a role in determining which stimuli gain access to structures afferent to the amygdala, including the nucleus accumbens. Dopamine in the nucleus accumbens, on the other hand, serves to determine which limbic inputs gain access to the motor pathways. In this way, DA in the nucleus accumbens may translate the motivational determinants of behavior that are mediated by limbic structures into biologically relevant actions. Understanding the mechanisms that determine the conditioning of drug effects to associated stimuli also has possible relevance for elucidating processes underlying addictive behaviors. For example, it has been proposed (Stewart et al. 1984) that the acquisition of incentive motivational properties by stimuli associated with drugs determines the craving in addicts.(ABSTRACT TRUNCATED AT 400 WORDS)