VERTICALIZATION OF BEHAVIOR ELICITED BY DOPAMINERGIC MOBILIZATION IS QUALITATIVELY DIFFERENT BETWEEN C57BL 6J AND DBA/2J MICE/

Behavioral effects of dopaminergic stimulation were evaluated in C57BL /6J mice and compared to the effects occurring in DBA/2J mice, an inbr ed strain with reduced densities of striatal dopamine receptors. Effec ts of apomorphine (0.5-64 mg/kg) alone and in combination with cocaine (30 mg/kg) were assessed using a time-sampling technique that classif ied climbing and leaning in separate categories. Locomotion was also a ssessed in a separate experiment. Climbing occurred in DBA/2J mice onl y at doses of apomorphine that were 16 times higher than the smallest effective dose in C57BL/6J mice; nevertheless, relative to baseline va lues, effects were fairly comparable. By contrast, whereas DBA/2J mice showed dose-dependent leaning under apomorphine, C57BL/6J mice exhibi ted little leaning even at doses not producing climbing, and only afte r the highest apomorphine dose was leaning significantly increased. Ap omorphine was equipotent in inducing gnawing across strains, although somewhat less efficacious in DBA/2J mice. When given alone, cocaine pr oduced significant climbing, but not leaning or gnawing, in either str ain. Whereas cocaine potentiated apomorphine-induced climbing and gnaw ing in both strains, apomorphine-induced leaning was not consistently changed by cocaine in either strain. These effects were not indirectly due to hyperkinesia, since neither apomorphine alone nor apomorphine and cocaine in combination was stimulant; apomorphine alone reduced lo comotor activity and attenuated cocaine-induced hyperkinesia. The pres ent data do not support a unitary, purely quantitative, account of ins ensitivity to dopaminergic stimulation based upon low densities of str iatal dopamine receptors in DBA/2J mice. Rather, this constellation of results is suggestive of qualitative interstrain dissimilarities in d opaminergic responsiveness that could reflect organizational differenc es in receptor populations.