Electrophysiological analysis of dopamine cells from the substantia nigra pars compacta of circling rats.

Extracellular single unit recordings were obtained from dopamine cells in the substantia nigra pars compacta during forced locomotion on a circular turntable treadmill. Stainless steel wire electrodes, 18 microns diameter, insulated with Parylene C were used. During the entire recording session the rat was in the treadmill apparatus. The device was stopped while a cell was being sought. A cell was identified as dopaminergic by a frequency of 3 to 10 Hz and a biphasic or triphasic action potential of greater than 2 ms in duration. An attempt was made to record from cells under the following conditions: animal at rest, animal turning in one direction, at rest again, turning in the opposite direction and finally, at rest. If the cell was still firing after these recordings, haloperidol was injected i.p. to see that the presumed dopamine cell increased its firing rate. A cell was held for all the observations in 4 animals. In an additional 10 rats, recordings were made before, during and after movement in one direction. Three animals were recorded only before and during movement. In 6 of the total of 17 animals haloperidol was administered. Results showed that firing patterns of cells in awake animals were similar to those reported from dopamine cells of anesthetized rats. During either contralateral or ipsilateral turning the firing frequency and burst activity significantly increased. These results indicate that the activity of dopamine cells in substantia nigra is increased bilaterally during circling.