A study of metabotropic glutamate receptor- and calcium release- mechanisms in cultured dorsal root ganglion neurones
Electrophysical recordings were carried out using the activation Ca2+-dependent chloride (ICl(Ca)) and Ca2+-dependent non-selective cation (ICAN) currents as a physiological index of raised intracellular calcium. Increases in cytoplasmic calcium were subsequently demonstrated following the isolation of mGluR activation and intracellular photo-release of 3',5'-cyclic guanosine monophosphate. These responses were blocked by increasing the intracellular Ca2+ buffering capacity of the neurones, but were insensitive to the application of a modified 'calcium-free' extracellular recording solution. They were also sensitive to intracellular ryanodine and were prevented by depletion of the intracellular calcium stores by pretreatment with caffeine (5 mM). cGMP, but no mGluR events, were also sensitive to a calmodulin antagonist and nicotinamide. Direct evidence for increases in intracellular calcium following mGluR activation was provided using Fura-2 imaging techniques. These responses were found to be sensitive to a specific Group I mGluR antagonist, 1-aminoindan-1,5-dicarboyxlic acid. The presence of subtype specific mGluR mRNA was also investigated showing the presence of both Group I and Group II mGluRs, although this may be affected by the developmental stage of the DRG neurones. These results thus indicate a potential novel mechanism of intracellular calcium release from a ryanodine-sensitive store involving cGMP and the possible production of cyclic adenosine 5'-phosphoribose. Following mGluR activation a similar pathway of calcium release may exist although complex interactions with other pathways involving inositol 1,4,5 trisphosphate may occur. The multiple modulatory effects of mGluR activation on VSCC provides an additional mechanism by which glutamate may regulate neuronal excitability.