Glutamate-mediated [Ca²⁺]_c dynamics in spontaneously firing dopamine neurons of the rat substantia nigra pars compacta

The mechanism by which glutamate regulates the cytosolic free Ca²⁺ concentration ([Ca²⁺]_c) in spontaneously firing dopamine neurons is not clear. Thus we have investigated the glutamate-mediated [Ca²⁺]_c dynamics in the acutely isolated dopamine neurons from the rat substantia nigra pars compacta by measuring [Ca²⁺]_c and spontaneously occurring action potentials (SAPs). The freshly isolated dopamine neurons showed tetrodotoxin (TTX)-sensitive spontaneous firing of 2-3 Hz and the resting [Ca²⁺]_c decreased with abolition of the SAPs. The level of [Ca²⁺]_c was affected by the spontaneous firing rate. In the presence of the Na⁺ channel antagonist, TTX (0.5 µM), glutamate increased [Ca²⁺]_c by activating different glutamate receptors depending on the glutamate concentration used. Addition of glutamate at low concentrations (<3 µM) raised [Ca²⁺]_c mainly by activating metabotropic glutamate receptors (mGluR), whereas at high concentrations (>10 µM) it raised [Ca²⁺]_c mainly by activating AMPA/kainate receptors. The contribution of NMDA receptors to the glutamate-mediated [Ca²⁺]_c rises was largest at intermediate concentrations of glutamate. Activation of mGluR elicited a Ca²⁺ release from intracellular Ca²⁺ stores and continuous Ca²⁺ influx out of the cell. The spontaneous firing activities were highly enhanced by submicromolar levels of glutamate and abolished at levels above 10 µM. From these results, we conclude that at low glutamate concentrations the [Ca²⁺]_c in the dopamine neurons is mainly governed by mGluR and the firing activities, whose rate is regulated at submicromolar glutamate concentrations, but at higher glutamate concentrations [Ca²⁺]_c is dominantly affected by AMPA/kainate receptors.