Group I mGluRs coupled to G proteins are regulated by tyrosine kinase in dopamine neurons of the rat midbrain

Group I mGluRs coupled to G proteins are regulated by tyrosine kinase in dopamine neurons of the rat midbrain. J Neurophysiol 85: 2490–2497, 2001. 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 ([Ca21]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 [Ca21]i increase caused by the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 100 mM) were permanently activated and subsequently abolished in cells loaded with the nonhydrolizable GTP-analogue GTP-g-S (600 mM). In addition, when GDP-b-S (600 mM) 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[[(17b)-3-methoxyestra-1,3,5(10)-trien-17- yl]amino]exyl]-1H-pyrrole-2,5-dione (10 mM), 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 [Ca21]i was caused by the protein tyrosine kinase inhibitors tyrphostin B52 (40 mM) and 49,5,7-trihydroxyisoflavone (genistein; 40 mM), whereas genistein’s inactive analogue 49,5,7-trihydroxyisoflavone-7-glucoside (40 mM) 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 mM), mitogen-activated protein kinase by 29-amino-39 methoxyflavone (50 mM), and protein kinase C by staurosporine (1 mM) had no effect on the cellular responses caused by DHPG. The mGluR5-selective antagonist 2-methyl-6-(phenylethynyl)-pyridine (10–100 mM) 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.