Thalamic control of dopaminergic functions in the caudate-putamen of the rat--I. The influence of electrical stimulation of the parafascicular nucleus on dopamine utilization.

A neurochemical response of four dopamine-rich brain regions to unilateral electrical stimulation of the parafascicular thalamic nucleus was examined in the halothane-anaesthetized rat. Tissue concentrations of dopamine and its two major metabolites, 3,4-dihydroxyphenylacetic acid and 4-hydroxy-3-methoxyphenylacetic acid, were assayed by a high performance liquid chromatographic technique in samples of caudate-putamen complex, nucleus accumbens, prefrontal cortex and substantia nigra. The ratios of metabolite to parent amine concentrations were taken as indices of dopamine utilization. Halothane anaesthesia alone evoked significant bilateral increases of dopamine utilization in every brain region studied. Electrical stimulation of one parafascicular nucleus produced further bilateral elevations of dopamine utilization in the caudate-putamen complex without altering these parameters in the substantia nigra. In the prefrontal cortex, however, thalamic stimulation resulted in significant bilateral decreases of dopamine utilization. Electrical stimulation of cortical or other thalamic areas did not evoke this regional pattern of dopamine utilization. It is argued that these indices of dopamine utilization together serve as reliable indicators of synaptic dopamine release and it is concluded that the parafascicular thalamus is capable of facilitating dopaminergic neurotransmission in the caudate-putamen by a mechanism that is probably independent of changes in dopamine cell firing rate. An anatomical analysis suggests that a thalamo-cortical-striatal route is most likely to mediate this function.