Activation of mGluR5 modulates GABAA receptor function in retinal amacrine cells
Amacrine cells in the vertebrate retina receive glutamatergic input from bipolar cells and make synapses onto bipolar cells, ganglion cells, and other amacrine cells. Recent studies indicate that amacrine cells express metabotropic glutamate receptors (mGluRs) and that signaling within the inner plexiform layer (IPL) of the retina might be modulated by mGluR activity. This study tests the hypothesis that activation of mGluR5 modulates GABAA receptor function in retinal amacrine cells. Whole cell voltage-clamp recordings were combined with pharmacology to establish the identity of the ionotropic GABA receptors expressed in primary cultures of chick amacrine cells and to determine how mGluR5 activity affected the behavior of those receptors. Application of GABA (20 μM) produced currents that reversed at -58.2 ± 0.9 mV, near the predicted Cl- reversal potential of -59 mV. The GABAA receptor antagonist, bicuculline (50 μM), completely blocked the GABA-gated currents. cis-4-Aminocrotonic acid (CACA; 100 μM), a GABAC receptor agonist, produced small currents that were not blocked by the GABAC antagonist, (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid (TPMPA; 20 μM), but were completely blocked by bicuculline. These results indicate that cultured amacrine cells express GABAA receptors exclusively. Activating mGluR5 with (RS)-2-chloro-5-hydroxyphenylglycine (CHPG; 300 μM) enhanced GABA-gated currents by 10.0 ± 1.5%. Buffering internal Ca2+ with BAPTA (10 mM) blocked the CHPG-dependent enhancement. Activation of PKC with the cell-permeable PKC activators (-)-7-octylindolactam V, phorbol 12-myristate 13 acetate (PMA), or 1-oleoyl-2-acetyl-sn-glycerol (OAG) also enhanced GABA-gated currents in a dose-dependent manner. Preactivation of PKC occluded the mGluR5-dependent enhancement, and inhibition of Ca-dependent PKC isotypes with Gö6976 (35 nM) suppressed the effects of mGluR5 activation, suggesting that mGluR5 and PKC are part of the same pathway. To determine if mGluR5-dependent enhancement occurred at synaptic GABAA receptors, postsynaptic currents were recorded in the presence of CHPG. On average, the mean amplitudes of the quantal events were increased by about 18% when mGluR5 was activated. These results indicate that activation of mGluR5 enhances GABA-gated current in cultured amacrine cells in a manner that is both Ca2+- and PKC-dependent. These results support the possibility that glutamate released from bipolar cells can modulate the function of GABAergic amacrine cells and alter signaling in the inner plexiform layer.