Substance P and serotonin act synergistically to activate a cation permeability in a neuronal cell line

Both substance P and, to a lesser degree, serotonin activate cation permeability in neuroblastoma x glioma hybrid cells, as determined by measurement of [14C]guanidinium uptake. Serotonin potentiates the action of substance P by shifting the concentration-effect curve of substance P to the left. The EC50 value for the synergistic effect of serotonin was around 0.3 microM. Dopamine and noradrenaline displayed comparable activity, albeit only at 50 and 130 times higher concentrations, respectively. The order of potency of various substance P-analogues was not changed by serotonin, indicating that the specificity of the substance P site on the hybrid cells was not affected by serotonin. Various other neurotransmitters and peptides had no effect on the response of the hybrid cells to substance P. The serotonin receptor interacting with the substance P receptor may be classified as a 5-HT3-receptor since methysergide, cimetidine, and ketanserin were ineffective, but two inhibitors specific for 5-HT3-receptors, ICS 205-930 (3 alpha-tropanyl-1H-indole-3-carboxylic acid ester) and MDL 72222 (1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,5-dichlorobenzoate), blocked the effect of serotonin at nanomolar concentrations. However, the two serotonin antagonists might also be blocking the ion permeability, since at higher concentrations they fully inhibited the stimulation of guanidinium uptake by substance P or by substance P plus serotonin. The synergism between substance P and serotonin on the hybrid cells offers the opportunity to study the mechanism of interaction of neurotransmitter receptors on a permanent neuronal cell line.