Dopamine high-affinity transport site topography in rat brain: major differences between dorsal and ventral striatum.

Investigations were conducted to determine the topography of the high-affinity dopamine uptake process within the rat striatum. [3H]Dopamine uptake into crude synaptosomes prepared from micropunch samples was found to be two- to three-fold higher in dorsal caudate-putamen relative to nucleus accumbens septi. In contrast, the concentrations of dopamine in the two regions were equivalent. The recognition site associated with high-affinity dopamine uptake was labeled using [3H]mazindol, and the binding of this ligand was also found to be two- to three-fold higher in homogenates from dorsal caudate-putamen samples relative to nucleus accumbens septi. Regional differences in uptake of [3H]dopamine or binding of [3H]mazindol were shown to be due to variations in Vmax or Bmax, not to differences in apparent affinity. Autoradiography of [3H]mazindol binding in rat striatum revealed a decreasing density of the site along the dorsal-to-ventral axis, with the highest binding occurring in the dorsolateral caudate-putamen, lower binding in the ventral caudate-putamen, and lowest levels in the septal pole of the nucleus accumbens septi. Quantification showed that the extent of this gradient was two-fold. Further autoradiographic studies revealed less striatal heterogeneity in the pattern of binding of [3H]ketanserin, another radioligand associated with the striatal dopaminergic innervation but not linked to the dopamine uptake process of the plasma membrane. The findings suggest that the dopaminergic fibers of the ventral striatum, especially the medial nucleus accumbens septi, may be relatively lacking in their capacity for dopamine uptake following its release. This organization may result in regional differences in the time-course of of extraneuronal dopamine following transmitter release and may render the dopamine-containing terminals of the ventral striatum less susceptible to the degenerative influences of neurotoxins that are incorporated by the high-affinity dopamine uptake process.