Regional differences in evoked dopamine efflux in brain slices of rat anterior and posterior caudate putamen

Summary

Fast cyclic voltammetry using carbon fibre microelectrodes in rat brain slices, was used to investigate regional differences in electrically-evoked dopamine (DA) efflux at 10 different sites in the anterior caudate putamen (aCPu) and 10 sites in the posterior caudate putamen (pCPu). For each site DA overflow was evoked by both single pulse (1P) stimulation and by trains of 25 pulses applied at a frequency of 50 Hz (25P/50 Hz). Peak DA efflux evoked by 1P was about 58% greater in the aCPu (0.19 μmol/l DA) than in the pCPu (0.12 μmol/l DA), but showed no mediolateral variation in either region. Peak DA efflux evoked by 25P/50 Hz relative to 1P efflux also varied between the two regions; the aCPu contained predominantly low ratio (25P/50 HZ: 1P) sites ranging from 1.47 to 3.71, whereas in the pCPu these ratios were higher, ranging from 2.73 to 9.40, and were particularly high in the dorsomedial region of the pCPu. Efflux detected in low ratio sites of the aCPu showed little dependence on the frequency (10 to 500 Hz), or the number of pulses (5 to 20) in a train. By contrast DA efflux evoked in high ratio sites of the pCPu responded in a pulse and frequency dependent manner, the maximum ratio (approximately 8 times 1P) being at 20P/20 Hz. Interestingly the frequency response relationship obtained in the pCPu resembled the profile observed in the nucleus accumbens (NAc).

Voltammetric evidence and experiments with selective reuptake blockers indicated that only DA was measured in our studies and 5-HT did not significantly contribute to the frequency dependent pattern of efflux detected in high ratio sites of the pCPu, where striatal 5-HT concentrations are highest. Experiments with the selective D₂ receptor antagonists metoclopramide or (−)sulpiride revealed that under our experimental conditions, DA efflux in the aCPu was not modulated by DA autoreceptor activation. By contrast, autoreceptor modulation did occur in high ratio sites of the pCPu at stimulations lasting longer than approximately 1000 ms.

These observations support the concept that the caudate putamen is heterogeneously organised with respect to the frequency characteristics of evoked DA release. The factors controlling frequency dependent release under these conditions may be a function of A10 innervation, since high ratio release sites occur in areas where the density of such innervation is greatest, for example, the dorsomedial pCPu. This is supported by the observation that high ratio release sites are also found in the NAc, which receives dopaminergic fibres predominantly from an A10 region. However, the involvement of different regionally distributed transmitters acting on presynaptic receptors involved in the regulation of dopamine release, or differences between nerve terminals in striosomes and matrix, cannot be excluded.