Activation of Ca2+-Dependent Currents in Dorsal Root Ganglion Neurons by Metabotropic Glutamate Receptors and Cyclic ADP-Ribose Precursors

Crawford, Jane H., John F. Wootton, Guy R. Seabrook, and Roderick H. Scott. Activation of Ca2+-dependent currents in dorsal root ganglion neurons by metabotropic glutamate receptors and cyclic ADP-ribose precursors. J. Neurophysiol. 77: 2573–2584, 1997. Cultured dorsal root ganglion neurons were voltage clamped at −90 mV to study the effects of intracellular application of nicotinamide adenine dinucleotide (βNAD+), intracellular flash photolysis of caged 3′,5′-cyclic guanosine monophosphate (cGMP), and metabotropic glutamate receptor activation. The activation of metabotropic glutamate receptors evoked inward Ca2+-dependent currents in most cells. This was mimicked both by intracellular flash photolysis of the caged axial isomer of cGMP [P-1-(2-nitrophenyl)ethyl cGMP] and intracellular application of βNAD+. Whole cell Ca2+-activated inward currents were used as a physiological index of raised intracellular Ca2+ levels. Extracellular application of 10 μM glutamate evoked the activation of Ca2+-dependent inward currents, thus reflecting a rise in intracellular Ca2+ levels. Similar inward currents were also activated after isolation of metabotropic glutamate receptor activation by application of 10 μM glutamate in the presence of 20 μM 6-cyano-7-nitroquinoxaline-2,3-dione and 20 μM dizocilpine maleate (MK 801), or by extracellular application of 10 μM trans-(1 S,3 R)-1-amino-1,3-cyclopentanedicarboxylic acid. Intracellular photorelease of cGMP, from its caged axial isomer, in the presence of βNAD+ was also able to evoke similar Ca2+-dependent inward currents. Intracellular application of βNAD+ alone produced a concentration-dependent effect on inward current activity. Responses to both metabotropic glutamate receptor activation and cGMP were suppressed by intracellular ryanodine, chelation of intracellular Ca2+ by bis-( o-aminophenoxy)- N,N,N′,N′-tetraacetic acid, and depletion of intracellular Ca2+ stores, but were insensitive to the removal of extracellular Ca2+. Therefore both cGMP, possibly via a mechanism that involves βNAD+ and/or cyclic ADP-ribose, and glutamate can mobilize intracellular Ca2+ from ryanodine-sensitive stores in sensory neurons.