Activation of metabotropic glutamate receptors decreases a high-threshold calcium current in spiking neurons of the Xenopus retina.

Two types of spiking neuron were identified among acutely dissociated neurons from the Xenopus retina by their responses to a depolarizing current step: single spikers and multiple spikers. In culture, multiple spikers had perikaryal diameters > 15 microns, whereas single spikers had smaller somata, 5-10 microns in diameter. Using a conventional whole-cell patch-clamp technique, both T- and L-type calcium currents were identified in multiply spiking cells whereas only an L-type current was present in singly spiking cells. The metabotropic glutamate receptor (mGluR) agonist trans-(1S-3R)-1-amino-1,3-cyclopentane-dicarboxylic acid (trans-ACPD) significantly decreased the L-type calcium current by 46 +/- 3% (mean +/- S.E.M.) in both types of cell but had only a minor effect on the T-type current in multiply spiking neurons. In the presence of 50 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 100 microM quisqualate (a potent mGluR1/5 agonist) decreased the L-type calcium current by 47 +/- 9% but had no effect on the T-type current. The selective mGluR4/6/7 agonist (+/-) 2-amino-4-phosphonobutyric acid (L-AP4, 100 microM), and the mGluR2/3 agonist (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG1, 100 microM) decreased the L-type calcium current by 12 +/- 3% and 14 +/- 2%, respectively. The inhibition of calcium current by trans-ACPD was reduced when the patch pipette contained the G-protein inhibitor, GDP beta S. The presence of the G-protein activator GTP gamma S in the patch pipette irreversibly reduced the L-type calcium current, but was without effect on the T-type current. Heparin applied intracellularly significantly reduced the inhibitory effect of quisqualate, indicating an involvement of the inositol triphosphate (IP3) pathway in the mGluR-induced reduction of calcium current. Replacement of internal EGTA with BAPTA significantly reduced the inhibitory effect of quisqualate. In contrast, internal application of cAMP did not prevent an inhibition of calcium current by quisqualate. Thus, the mechanism by which calcium current is inhibited by mGluR seems not to involve an intracellular cAMP cascade. Our findings indicate that activation of mGluR1/5 results in the inhibition of a high-threshold calcium current. This process is mediated by the activation of a G-protein and is consistent with inhibition occurring by an IP3-stimulated release of internal calcium.