Mapping of contraversive and ipsiversive circling responses to ventral tegmental and substantia nigra electrical stimulation

Abstract

Circling responses to ventral mesencephalic electrical stimulation were studied over a range of stimulation sites and a range of stimulation frequencies. Contraversive circling was seen with 62% of the sites stimulated; positive sites were found in the ventral tegmental area, the medial lemniscus, and the zona compacta and zona reticulata of the substantia nigra; frequency thresholds were in the range of 15–60 Hz. Ipsiversive circling was seen with 30% of the sites stimulated; these sites tended to be in the region of nigral dopamine cell bodies, but this correlation was not perfect; some ipsiversive circling sites were found in zona reticulata, and some were found dorsal to zona compacta. Ipsiversive circling had high frequency thresholds, in the range of 100–150 Hz, and generally had longer latencies than those for contraversive circling. In one-third of the cases where ipsiversive circling was seen with high frequency stimulation, contraversive circling was obtained with lower frequency stimulation at the same site. In these cases contraversive circling was seen first, with short latency at low frequencies. As stimulation frequency was raised, the period of contraversive circling became shorter and the animals then stopped and reversed direction. The dispersion of positive sites rules out the suggestion that there are simple medial-lateral differences in the direction of circling elicited by nigral stimulation, and the dispersion of sites and the frequency response of the effects suggest that neither direction of circling results from direct depolarization of the dopaminergic cells themselves.