Time resolved dopamine overflow from synaptosomes and chopped striatal tissue with rapid superfusion.

Overflow of dopamine has been measured with a rapid superfusion apparatus in an attempt to obtain a system in which overflow is a measure of the primary release process. The tissue samples employed, chopped tissue and synaptosomes, were prepared from rat striatum. The superfusion system employed an on-line amperometric detector to provide temporal information. In addition, liquid chromatography with electrochemical detection was used for identification and quantification of dopamine. Dopamine release could be induced from both samples by exposure to K+ in the presence of Ca2+. The presence of pargyline (0.1 mM) did not significantly affect overflow from either sample. In addition, dopamine stores could be replenished in both samples by exposure to 0.5 microM DA, an effect blocked by amphetamine, nomifensine, and amfonelic acid. However, overflow from synaptosomes showed considerably less distortion from interactions of released substances with the tissue than from chopped tissue. The temporal profile of overflow was more rapid and uptake inhibitors did not affect overflow during depolarization. Since overflow from synaptosomes appears to be more closely related to release, the temporal response of this preparation to K+ stimulations was examined in more detail. A linear relation between dopamine overflow and log (K+) was obtained with 3-s exposures to K+. In contrast, a sigmoidal relationship was obtained with 30-s exposures. Thus, the data support the concept that depolarization of nerve terminals by K+ is a biphasic process that can be temporally resolved.