Neurochemical evidence of functional A10 dopamine terminals innervating the ventromedial axis of the neostriatum: in vitro voltammetric data in rat brain slices.

The neostriatum (CPu) and nucleus accumbens (NAc) receive their primary dopamine (DA) afferents from the A9 and A10 cell groups, respectively. Anatomical evidence has, however, shown that some clusters of A10 afferents innervate the medial (periventricular) aspect of the CPu14. The present study used fast cyclic voltammetry (FCV) at carbon fibre microelectrodes to locate and measure the size of these clusters on the basis of diagnostic differences in DA efflux in A9 and A10 terminal regions. All experiments were conducted in CPu or NAc slices superfused with oxygenated artificial CSF at 32 degrees C. Carbon fibre microelectrodes were placed 80 microns below the slice surface and bipolar stimulating electrodes were located 200 microns away. Except in experiments where the stimulation frequency, pulse width or number of pulses were investigated, DA efflux was evoked using 0.1 ms, 10 mA pulses applied singly (1p) or in 20 pulse trains (20p) at 50 Hz and monitored using FCV. The CPu was first mapped on the basis of the ratio of 20p:1p DA efflux. The CPu consisted mainly (64%) of low ratio (< 3) sites while the NAc core comprised exclusively high ratio (> 6) loci. Population analysis revealed a small percentage (10%) of striatal sites with high (> 6) ratios. These high ratio sites matched the reported distribution of A10 afferent clusters, being found almost entirely along the ventromedial axis of the CPu. Individual clusters of high ratio sites (20p:1p ratios > 6), 'mapped' on the basis of evoked 20p DA efflux, were found to be irregular in outline and around 500 microns across. In order to characterise the clusters further, the influence of stimulation frequency, train duration (number of pulses) and pulse width on DA efflux were examined. Peak DA efflux in the clusters and NAc occurred at 50 Hz while the striatal matrix had a flat frequency response. Both clusters and NAc showed a similar dependence of DA efflux on the number of pulses in the stimulus train. In the CPu matrix, DA efflux was less dependent on the number of pulses. In the striatal matrix, increasing the stimulation pulse width enhanced DA efflux on trains more than on single pulses while, in both NAc and striatal clusters, there was no preferential effect on trains. The medial location of these clusters within the CPu more closely matches the anatomical distribution of the A10 afferents of Gerfen et al. than the more evenly dispersed striosomes.(ABSTRACT TRUNCATED AT 400 WORDS)