Interaction of convergent pathways that inhibit N-type calcium currents in sensory neurons.
Norepinephrine and GABA inhibit omega-conotoxin GVIA-sensitive (N-type) calcium current in embryonic sensory neurons by separate pathways. We have investigated the mechanisms that limit the modulation of N current by varying the level of activation for a single pathway or simultaneously activating multiple pathways. Calcium currents were measured with tight-seal, whole-cell recording methods. Simultaneous application of the two transmitters at saturating concentrations produced a larger inhibition of the current than either transmitter by itself, but the maximal inhibition was not linearly additive. Maximal, direct activation of GTP-binding proteins by intracellular application of guanosine 5'-(3-O-thio)-triphosphate (GTP gamma S) resulted in a similar limit to the inhibition; furthermore, GTP gamma S did not enhance the maximal inhibition produced by co-application of transmitters. Interventions downstream in the modulatory pathway (e.g. direct activation of protein kinase C or inhibition of protein phosphatases) were also unable to alter the maximal limit for inhibition. These results suggest that transmitter-mediated inhibition is not limited by receptor number, levels of G-protein or protein kinase C activation, or degree of phosphorylation; rather, the extent of inhibition may be limited by the structural properties of the N channels themselves.
Diversé-Pierluissi
M
M
Department of Physiology and Neuroscience, Tufts University School of Medicine, Boston, MA 02111, USA.
Dunlap
K
K
D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D002642:Chick Embryo; D004594:Electrophysiology; D004988:Ethers, Cyclic / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism; D005727:Ganglia, Spinal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D009475:Neurons, Afferent / Q000378:metabolism*; D009638:Norepinephrine / Q000502:physiology; D019319:Okadaic Acid; D018408:Patch-Clamp Techniques; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors; D011493:Protein Kinase C / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D005680:gamma-Aminobutyric Acid / Q000502:physiology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D004988:Ethers, Cyclic; D019319:Okadaic Acid; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D005680:gamma-Aminobutyric Acid; D011493:Protein Kinase C; D010749:Phosphoprotein Phosphatases; D019204:GTP-Binding Proteins; D009638:Norepinephrine