Nocturnal rotational behavior in rats: further neurochemical support for a two-population model

The relationship between circling behavior and the concentrations of dopamine (DA), serotonin, and their metabolites in corpus striatum was investigated in rats. We have previously reported evidence indicating that in both sexes there are two kinds, or populations, of rats: those with their turning biases directed away from (Contra greater than Ipsi rats), and those with their turning biases directed towards (Ipsi greater than Contra rats), the side containing the striatum with the greater dopaminergic innervation. In the present experiment rats were classified according to whether the contralateral or ipsilateral striatum contained the greater dopamine concentration. Whereas the ipsilateral striata were found to contain the same concentrations of dopamine, the contralateral sides were found to differ significantly; and the difference between the contralateral and ipsilateral dopamine concentrations was significantly correlated with the contralateral, but not the ipsilateral, dopamine concentration. These results are identical to those we previously reported using the Vmax for dopamine uptake in vitro as the measure for striatal dopaminergic innervation. As an initial attempt to determine what neurochemical mechanisms might underlie the differences between the "Contra greater than Ipsi" and "Ipsi greater than Contra" rats, it was found that dopamine turnover, as measured by the ratios of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid to DA, was higher in the striata of the latter group than in the former group. The present results are discussed in terms of their support for the two-population model, and in relation to previous work on behavioral and neurochemical asymmetry in rats.