Differences in pharmacological properties of dopamine release between the substantia nigra and striatum: An in vivo electrochemical study

The properties of dopamine (DA) release in the rat substantia nigra (SN) an d striatum were investigated using high-speed chronoamperometric recordings in brain slices. In both brain regions, a 2-min bath superfusion with 30 m M KCl produced robust DA-like electrochemical signals, with the mean amplit ude of the signal being >10-fold greater in the striatum than the SN. The r eproducibility of the response was confirmed by a second stimulus (S2)/firs t-stimulus (S1) ratio of >0.8 in both regions. The bath application of tetr odotoxin significantly reduced the S2/S1 ratio in both the striatum and SN, implicating the requirement for voltage-sensitive sodium channels in the D A-release process. However, the application of cadmium chloride, a nonselec tive blocker of voltage-sensitive calcium channels, reduced the S2/S1 ratio only in the striatum and not within the SN. Moreover, removal of Ca2+ from the buffer did not significantly affect release within the SN, despite a > 85% reduction in release within the striatum. In addition, although the D-2 receptor antagonist sulpiride enhanced the S2/S1 ratio in the striatum, no effect of this agent was seen in the SN. Finally, the application of d-amp hetamine produced DA-like electrochemical signals in both the striatum and SN. However, the amplitude of the d-amphetamine-evoked response, relative t o the KCl-evoked release, was much smaller in the striatum than in the SN. Taken together, these data support the hypothesis that differences in the m echanism or mechanisms of release exist between somatodendritic and axonal elements within the nigrostriatal pathway.