Apomorphine-susceptible rats and apomorphine-unsusceptible rats differ in the tyrosine hydroxylase-immunoreactive network in the nucleus accumbens core and shell.

Individual variability in behavioural responses to stressors such as novelty and drugs of abuse is a well-known phenomenon in both animals and man. These individual differences are largely associated with differences in dopamine transmission in mesolimbic areas such as the nucleus accumbens. Apomorphine-susceptible (APO-SUS) rats and apomorphine-unsusceptible (APO-UNSUS) rats serve as a valid animal model for individual differences and these two types of rat differ in a number of behavioural, physiological, endocrinological and pharmacological parameters. In order to study the differences in the catecholaminergic network in the nucleus accumbens, possibly underlying at least some of the differences between the two types of rat, we quantified the extent of the tyrosine-hydroxylase immunoreactive (TH-IR) network and the number of TH-IR varicosities in subareas of the nucleus accumbens core and shell in naive rats. This study shows that the nucleus accumbens of APO-SUS rats has a more extensive fibre network and more varicosities than the nucleus accumbens of APO-UNSUS rats, and that the subarea of the shell contains more varicosities than the subarea of the core. These data provide a basis for further studying the structural and neurochemical properties of the nucleus accumbens contributing to individual differences in response to stressors such as novelty and drugs of abuse.