Circling behaviour induced by electrical stimulation of the medial forebrain bundle, importance of stimulus...

Unilateral electrical stimulation of the substantia nigra or the ventral tegmental area produced postural asymmetry and increased locomotor activity respectively. Concurrent stimulation of the efferent pathways of these two areas (medial forebrain bundle) resulted in contraversive circling behaviour that was dependent upon the current intensity and frequency of the stimuli. The application of biphasic electrical pulses minimised the damage to the brain site stimulated; no decrease in circling intensity over time, nor spontaneous circling after administration of amphetamine or apomorphine without stimulation was observed. The contraversive circling behaviour was induced by activation of the ascending dopaminergic pathways as revealed by the close correlation between the site of stimulation and the localisation of this pathway, its antagonism by haloperidol and its abolishment by pretreatment with reserpine and alpha-methyl-p-tyrosine. Apomorphine likewise inhibited the electrical stimulation-induced circling behaviour. These results are discussed with regard to the influence of the stimulation parameters and the dopaminergic processes involved.