| Rats suppress intake of a gustatory conditioned stimulus (CS) when intake of the CS is paired with either an appetitive or an aversive unconditioned stimulus (US). A conditioned taste aversion (CTA) describes the reduction in CS intake that occurs when a CS is paired with an aversive, illness inducing US (such as x-rays or LiCl). In appetitive conditioning, an anticipatory contrast effect develops when rats decrease intake of a saccharin CS following once daily pairings with a rewarding US such as sucrose. This latter effect occurs because the less preferred saccharin CS is devalued as it comes to predict availability of the more rewarding sucrose US. Finally, suppressed intake of a gustatory CS also occurs when the CS is paired with a drug of abuse. While this effect has been interpreted as a CTA for 30 years, recent research indicates that the suppressive effects of drugs on CS intake are likely the result of the rewarding, rather than the aversive, properties of the drug. The neural mechanisms by which drugs of abuse might be compared with and ultimately come to override natural rewards remain unclear. These mechanisms are of great interest, as substance abusers reduce social, occupational, and recreational activities as a result of compulsive drug use. The present set of experiments addressed this question by using two strains of rats (Lewis and Fischer 344) with known differences in brain neurochemistry, known differences in responsiveness to natural rewards and drugs of abuse, and with an apparent differential sensitivity to reward comparison (i.e., sucrose and cocaine-induced suppression of saccharin intake). We employed behavioral and lesion techniques to examine the nature of the difference in responding between the Lewis and Fischer rats, the generality of the strain difference, and the underlying neural substrates mediating the difference in behavior. Specifically, we evaluated basic taste responses in brief access, responses to dopaminergic agonists, the involvement of PKA in the NAc, responses to opiates, and the role of the gustatory thalamus in these phenomena. Our findings indicate that the two strains exhibit differences in reward comparison behavior and self-administration behavior, but these differences indicate that Lewis rats are not “addiction prone”. (Abstract shortened by UMI.)... |
