Effects of chronic electrode implantation on dopaminergic neurons in vivo

The present study evaluated the effects of chronic electrode implantation on stimulus-dependent increases of the dopamine (DA) metabolite dihydroxyphenylacetic acid (DOPAC) in relationship to a well characterized in vivo model which used electrical stimulation from acute electrode placements in the nigro-striatal pathway. Five days after bipolar electrodes were implanted into the nigro-striatal pathway, non-contingent electrical stimulation (100 microA, 25 Hz, 1.5 msec duration, 20 min session) did not change DA or DOPAC concentrations in the caudate nucleus, nucleus accumbens, or olfactory tubercles, whereas the same stimulation from acute electrode placements causes significant ipsilateral increases in caudate DOPAC. Although DOPAC concentrations did not change when these chronically implanted electrodes were stimulated, similar chronic electrode placement supported intracranial self-stimulation (ICSS). In order to examine the effects of self-stimulation on DOPAC concentrations, five ICSS test groups were established for comparison: implanted only, trained only, minimum response rate, 50% maximum response rate and maximum response rate. Following a 50 min test session, a comparison of either DA, or DOPAC concentrations across the different ICSS conditions revealed no change for the caudate nucleus, nucleus accumbens or olfactory tubercles. Likewise, there was no change between the stimulated and unstimulated sides within each ICSS group. When a comparison was made between implanted only and maximal ICSS response rate groups for changes in DA or DOPAC concentrations in the frontal cortex, no differences were found. Apparently, chronic electrode implantation abolished the ability to electrically stimulate nigro-striatal dopaminergic neurons under non-contingent conditions, and the relationship between dopaminergic neurons and ICSS appears to be indirect in nature.