Firing properties of substantia nigra dopaminergic neurons in freely moving rats

Single unit recordings were obtained from putative dopaminergic neurons in the substantia nigra of awake, freely moving rats. The cells exhibited waveforms, range of firing rates and types of firing patterns identical to those of identified DA neurons of anesthetized or paralyzed rats. Two firing patterns were observed: single spike activity and a bursting mode with spikes of progressively diminished amplitude and increased duration within each burst. The degree of burst firing varied considerably among the cells and individual cells sometimes switched from one pattern of firing (e.g. predominantly single spike) to another (e.g. bursting), although the determinants of these transitions are, at this time, unclear. Putative DA neurons were inhibited by i.v. apomorphine and excited by i.v. haloperidol. Haloperidol also reversed the apomorphine-induced inhibition of firing. Inhibitions and excitations were associated with a reduction and elevation, respectively, in burst firing. The effects of the two drugs were identical to their effects in immobilized rats. In several cases, a putative DA neuron was observed to fire all of its spikes in near coincidence with at least one other cell with identical electrophysiological characteristics. This form of interaction (i.e. presumed electrical coupling) between DA cells is only rarely observed in anesthetized or paralyzed rats and may play a significant role in the normal functioning of the nigrostriatal DA system.