The Journal of Neurophysiology Vol. 87 No. 6 June 2002, pp. 2867-2879
Copyright ©2002 by the American Physiological Society

Sigma Receptors Inhibit High-Voltage-Activated Calcium Channels in Rat Sympathetic and Parasympathetic Neurons

Department of Pharmacology and Therapeutics, University of South Florida College of Medicine, Tampa, Florida 33612

Zhang, Hongling and Javier Cuevas. Sigma Receptors Inhibit High-Voltage-Activated Calcium Channels in Rat Sympathetic and Parasympathetic Neurons. J. Neurophysiol. 87: 2867-2879, 2002. Studies on the expression and cellular function of sigma receptors in autonomic neurons were conducted in neonatal rat intracardiac and superior cervical (SCG) ganglia. Individual neurons from SCG and intracardiac ganglia were shown to express transcripts encoding the sigma-1 receptor using single-cell RT-PCR techniques. The relationship between sigma receptors and calcium channels was studied in isolated neurons of these ganglia under voltage-clamp mode using the perforated-patch configuration of the whole cell patch-clamp recording technique. Bath application of sigma receptor agonists was shown to rapidly depress peak calcium channel currents in a reversible manner in both SCG and intracardiac ganglion neurons. The inhibition of barium (I_Ba) currents was dose-dependent, and half-maximal inhibitory concentration (IC₅₀) values for haloperidol, ibogaine, (+)-pentazocine, and 1,3-Di-O-tolylguanidin (DTG) were 6, 31, 61, and 133 µM, respectively. The rank order potency of haloperidol > ibogaine > (+)-pentazocine > DTG is consistent with the effects on calcium channels being mediated by a sigma-2 receptor. Preincubation of neurons with the irreversible sigma receptor antagonist, metaphit, blocked DTG-mediated inhibition of Ca²⁺ channel currents. Maximum inhibition of calcium channel currents was 95%, suggesting that sigma receptors block all calcium channel subtypes found on the cell body of these neurons, which includes N-, L-, P/Q-, and R-type calcium channels. In addition to depressing peak Ca²⁺ channel current, sigma receptors altered the biophysical properties of these channels. Following sigma receptor activation, Ca²⁺ channel inactivation rate was accelerated, and the voltage dependence of both steady-state inactivation and activation shifted toward more negative potentials. Experiments on the signal transduction cascade coupling sigma receptors and Ca²⁺ channels demonstrated that neither cell dialysis nor intracellular application of 100 µM guanosine 5'-O-(2-thiodiphosphate) trilithium salt (GDP--S) abolished the modulation of I_Ba by sigma receptor agonists. These data suggest that neither a diffusible cytosolic second messenger nor a G protein is involved in this pathway. Activation of sigma receptors on sympathetic and parasympathetic neurons is likely to modulate cell-to-cell signaling in autonomic ganglia and thus the regulation of cardiac function by the peripheral nervous system.