The Extraneuronal Transporter for Monoamine Transmitters Exists in Cells Derived from Human Central Nervous System Glia

Abstract

From studies on sympathetically innervated peripheral tissues it is well known that both neuronal and nonneuronal transport systems contribute to the inactivation of released monoamine transmitters. The close proximity between synapses and glia cell processes in the CNS leads to the so far unresolved question whether non-neuronal transporters are involved in the inactivation of centrally released monoamine transmitters such as noradrenaline, dopamine and 5-hydroxytryptamine. 1-Methyl-4-phenylpyridinium (MPP⁺) is a prototypical substrate of the extraneuronal monoamine transporter (uptake₂). [³H]MPP+ was found to accumulate in various human glioma cell lines. [³H]MPP⁺transport was characterized in more detail in HTZ146 human glioma cells. The K_i values of various compounds for the inhibition of initial rates of [³H]MPP⁺ transport into HTZ146 cells were closely correlated with the known K_i values for the inhibition of the extraneuronal monoamine transporter (P < 0.001, r= 0.991, n= 7). The rank order of inhibitory potencies was decynium 22 > corticosterone > cyanine 863 > O-methylisoprenaline > quinine > clonidine > quinidine. [³H]MPP⁺ accumulation was investigated not only in various CNS tumour cell lines but also in primary cultures of human astrocytes and rat cerebral cortex slices. In all tested experimental systems, accumulation was sensitive to cyanine-related inhibitors of the extraneuronal monoamine transporter. These findings suggest that the extraneuronal monoamine transporter exists in glia cells. Furthermore, it was shown that MPP⁺ is able to make use of the extraneuronal monoamine transporter not only to enter but also to leave glia cells. This finding suggests that the extraneuronal monoamine transporter may play a key role in the mechanism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity.