Differential Autoreceptor Control of Somatodendritic and Axon Terminal Dopamine Release in Substantia Nigra, Ventral Tegmental Area, and Striatum

Abstract

Dopamine (DA) is released from somatodendritic sites of neurons in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA), where it has neuromodulatory effects. The aim of this study was to evaluate the role of D 2 autoreceptor inhibition in the regulation of this somatodendritic release in each region. Fast cyclic voltammetry at carbon fiber microelectrodes was used to measure electrically evoked DA release in vitro. Furthermore, we compared D 2 regulation of somatodendritic release with the more familiar axon terminal release in caudate putamen (CPu) and nucleus accumbens (NAc). Evoked DA release was TTX-sensitive at all sites. There was significant D 2 autoinhibition of DA release in SNc; however, this mechanism was two- to threefold less powerful, as compared with axon terminal release in CPu. In contrast to SNc, somatodendritic release in VTA was not under significant D 2 receptor control, whereas release in the respective axon terminal region (NAc) was controlled strongly by autoinhibition. Thus, these data indicate that, first, autoinhibition via D 2 receptors consistently plays a less significant role in the control of somatodendritic than axon terminal DA release, and, second, even at the level of somatodendrites themselves, D 2 autoinhibition displays marked regional variation. In the light of previous data indicating that DA uptake processes are also less active in somatodendritic than in terminal regions, these results are interpreted as indicating that DA transmission is regulated differently in somatodendritic zones, as compared with axon terminals, and thus may have different functional consequences.