The Journal of Neurophysiology Vol. 85 No. 6 June 2001, pp. 2490-2497
Copyright ©2001 by the American Physiological Society

Group I mGluRs Coupled to G Proteins Are Regulated by Tyrosine Kinase in Dopamine Neurons of the Rat Midbrain

 ¹Istituto di Ricovero e Cura a Carattere Scientifico Fondazione S. Lucia, 00179 Rome; and  ²Clinica Neurologica, Università di Tor Vergata, 00133 Rome, Italy

Tozzi, Alessandro, Ezia Guatteo, Luigi Caputi, Giorgio Bernardi, and Nicola B. Mercuri. 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 ([Ca²⁺]_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 [Ca²⁺]_i increase caused by the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 100 µM) were permanently activated and subsequently abolished in cells loaded with the nonhydrolizable GTP-analogue GTP--S (600 µM). In addition, when GDP--S (600 µM) 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)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]exyl]-1H-pyrrole-2,5-dione (10 µM), 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 [Ca²⁺]_i was caused by the protein tyrosine kinase inhibitors tyrphostin B52 (40 µM) and 4',5,7-trihydroxyisoflavone (genistein; 40 µM), whereas genistein's inactive analogue 4',5,7-trihydroxyisoflavone-7-glucoside (40 µM) 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 µM), mitogen-activated protein kinase by 2'-amino-3' methoxyflavone (50 µM), and protein kinase C by staurosporine (1 µM) had no effect on the cellular responses caused by DHPG. The mGluR5-selective antagonist 2-methyl-6-(phenylethynyl)-pyridine (10-100 µM) 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.