NIGROSTRIATAL DOPAMINERGIC CELL-ACTIVITY IS UNDER CONTROL BY SUBSTANTIA-NIGRA OF THE CONTRALATERAL BRAIN SIDE - ELECTROPHYSIOLOGICAL EVIDENCE

The biochemical balance between right- and left-ascending DA systems is an essential factor to regulate behavioral lateralization. However, there is no electrophysiological evidence for the regulation of interhemispheric DA systems. In the present paper we report electrophysiological evidence supporting the hypothesis that the A9 DA cells are under control of the contralateral substantia nigra. The activity of more than 80% of the A9 cells recorded was affected by contralateral SN stimulation. This is a very high proportion because the previously reported response of A9 cells to ipsilateral caudate stimulation is proportionally lower. The potency of a stimulus, estimated as the number of action potentials induced or inhibited by each electrical stimulation of the contralateral substantia nigra or by the percentage of modification in the number of action potentials induced or inhibited in relation to the spontaneous potential expected, was also high. The response to the contralateral stimulation was complex. Fifty-four percent of all the DA cells studied showed more than a single response. Forty-four percent showed at least one stimulation and at least one inhibition. Because 1) the percentage of cells with at least one stimulation (70%) was higher than the percentage of cells with at least one inhibition (56%), and 2) the potency of stimulations was higher than the inhibition potency, the present data provide evidence that contralateral control of A9 cells is mainly excitatory. The percentage of cells activated by contralateral stimulation was high, between 30 ms and 220 ms and between 400 ms and 700 ms. The probability of inhibition was higher than the probability of activation between 10 ms and 30 ms and between 230 and 380 ms. We concluded that A9 DA cells have an important regulatory input from the contralateral brain side. The right-left relationship reported here could be the critical factor allowing a dual brain system (A9 cells) to have only one action on behavior.