Metabotropic glutamate receptors (mGluRs) modulate neuronal function via different transduction mechanisms that are either dependent or independent on G-protein function. Here we investigated, using whole cell patch-clamp recordings in combination with fluorimetric measurements of intracellular calcium concentration ([Ca(2+)](i)), the metabolic pathways involved in the responses induced by group I mGluRs in dopamine neurons of the rat midbrain. The inward current and the [Ca(2+)](i) increase caused by the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 100 microM) were permanently activated and subsequently abolished in cells loaded with the nonhydrolizable GTP-analogue GTP-gamma-S (600 microM). In addition, when GDP-beta-S (600 microM) was dialyzed into the cells to produce the blockade of the G proteins, the DHPG-dependent responses were reduced. When the tissue was bathed with the phospholipase C inhibitor 1-[6[[(17 beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]exyl]-1H-pyrrole-2,5-dione (10 microM), the DHPG-induced calcium transients slightly diminished but the associated inward currents were not affected. Interestingly, a substantial depression of the DHPG-induced inward current and transient increase of [Ca(2+)](i) was caused by the protein tyrosine kinase inhibitors tyrphostin B52 (40 microM) and 4',5,7-trihydroxyisoflavone (genistein; 40 microM), whereas genistein's inactive analogue 4',5,7-trihydroxyisoflavone-7-glucoside (40 microM) was ineffective. The blockade of the Src family of tyrosine kinase by 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (20 microM), mitogen-activated protein kinase by 2'-amino-3' methoxyflavone (50 microM), and protein kinase C by staurosporine (1 microM) had no effect on the cellular responses caused by DHPG. The mGluR5-selective antagonist 2-methyl-6-(phenylethynyl)-pyridine (10--100 microM) did not affect the actions of DHPG. Thus our results indicate that the responses, mainly mediated by mGluRs1 in dopamine neurons, are activated by intracellular mechanisms coupled to G proteins and regulated by tyrosine kinases.