Individual differences in Pavlovian autoshaping of lever pressing in rats predict stress-induced corticosterone release and mesolimbic levels of monoamines

Pavlovian autoshaping CRs are directed and reflexive consummatory responses targeted at objects repeatedly paired with rewarding substances. To evalua te the hypothesis that autoshaping may provide an animal learning model of vulnerability to drug abuse, this study relates individual differences in l ever-press autoshaping CR performance in rats to stress-induced corticoster one release and tissue monoamine levels in the mesolimbic dopamine tract. L ong-Evans rats (n = 14) were given 20 sessions of Pavlovian autoshaping tra ining wherein the insertion of a retractable lever CS was followed by the r esponse-independent presentation of food US. Large between-subjects differe nces in lever-press autoshaping CR performance were observed, with group hi gh CR frequency (n = 5) performing many more lever press CRs than group low CR frequency (n = 9). Tail-blood samples were obtained before and after th e 20th autoshaping session, then 24 h later the rats were sacrificed and di ssection yielded tissue samples of nucleus accumbens (NAC), prefrontal cort ex (PFC), caudate putamen (CP), and ventral tegmental area (VTA). Serum lev els of postsession corticosterone were elevated in group high CR frequency. HPLC revealed that group high CR frequency had higher tissue levels of dop amine and DOPAC in NAG, lower levels of DOPAC/DA turnover in CPI and lower levels of 5-HIAA and lower 5-HIAA/5-HT turnover in VTA. The neurochemical p rofile of rats that perform more autoshaping CRs share some features of vul nerability to drug abuse. (C) 2000 Elsevier Science Inc.