Variation in serotonergic inhibition of calcium channel currents in four types of rat sensory neurons differentiated by membrane properties

Abstract

1. Rat dorsal root ganglion (DRG) cell bodies were screened according to action potential (AP) duration, capsaicin sensitivity, expression of IH, IA, and N-, L-, and T-type Ca2+ channel currents. AP duration was measured at half of total amplitude at a membrane potential of -60 mV. Sensitivity to capsaicin was defined as production of an inward current at a holding potential (HP) of -60 mV by 1 microM capsaicin. IH was evoked by a 787-ms hyperpolarization to -110 mV from an HP of -60 mV. IA was evoked by repolarization to -60 mV after a 787-ms hyperpolarization to -110 mV. High-threshold Ca2+ channel current was evoked by a depolarization to -10 or 0 mV from an HP of -60 mV, and L- and N-type Ca2+ channel current was fractionated using selective Ca2+ channel blockers (nimodipine and omega-conotoxin GVIA). T-type Ca2+ channel current was evoked by a depolarization to -40 mV from an HP of -90 mV. Ninety-seven of the 116 DRG cells studied fit closely into one of four categories based on expression of the above characteristics. These four categories, referred to as types 1-4, are described below. 2. Type 1 DRG cells (soma diameter 24.6 +/- 0.5 microns, mean +/- SE; n = 34) had long-duration APs (average = 9.8 ms) with a prominent shoulder on the falling limb and were capsaicin sensitive. Significant IH or IA was not expressed. High-threshold Ca2+ channel current was on average 28% omega-conotoxin GVIA sensitive (N-type) and 46% nimodipine sensitive (L-type); 26% was resistant to both blockers (resistant). T-type Ca2+ channel currents averaged 245 pA. 3. Type 2 DRG cells (soma diameter 25.2 +/- 0.9 microns, n = 19) had short-duration APs (average = 2.9 ms) with a small shoulder on the falling limb and were capsaicin sensitive. IH was negligible but IA averaged 184 pA. High-threshold Ca2+ channel current averaged 42% N-type, 23% L-type, and 35% resistant. T-type Ca2+ channel currents averaged 47 pA. 4. Type 3 DRG cells (soma diameter 18.6 +/- 0.8 microns, n = 21) had short-duration APs (average = 1.8 ms) and were insensitive to capsaicin. IA was not expressed but IH averaged 147 pA. High-threshold Ca2+ channel current averaged 27% N-type, 44% L-type, and 29% resistant. T-type Ca2+ channel currents averaged 306 pA. 5. Type 4 DRG cells (soma diameter 33.9 +/- 0.4 microns, n = 23) had short-duration APs (average = 1.1 ms) and were capsaicin insensitive. IA was not expressed but IH averaged 810 pA. High-threshold Ca2+ channel current was 16% N-type, 4% L-type, and 80% resistant. T-type Ca2+ channel currents averaged 4,031 pA. 6. There was a large variation in the inhibition of high-threshold Ca2+ channel currents by serotonin (5-HT) and (+)8-OH-DPAT in type 1 DRG cells versus types 2-4. On average, 5-HT (10 microM) inhibited high-threshold Ca2+ channel current by an average of 42% in type 1 DRG cells, compared with 15%, 18%, and 7% inhibition in types 2-4, respectively. Similarly, (+)8-OH-DPAT (1 microM) inhibited high-threshold Ca2+ channel current by an average of 35% in type 1 DRG cells, compared with 5%, 8%, and 3% inh...

Roles of Tetrodotoxin (TTX)-Sensitive Na+Current, TTX-Resistant Na+Current, and Ca2+Current in the Action Potentials of Nociceptive Sensory Neurons

“…Judging by the completeness of block of the current at the end of the pulse, the mixture of blockers was sufficient to block essentially all of the calcium current. This is consistent with previous observations that a large fraction of the HVA calcium current in small DRG cells is L-and N-type current (Scroggs and Fox, 1992b;Cardenas et al, 1995), and the results suggest that the remaining HVA current in small-diameter DRG neurons is carried mainly by P/Q-type channels (Mintz et al, 1992). The subtracted current (Fig.…”

“…This mibefradil-sensitive current was always much smaller than either the TTX-R or the HVA calcium current, contributing Ϫ0.07 to Ϫ0.97 pC/pF (mean, Ϫ0.30 Ϯ 0.31 pC/pF). This is consistent with previous observations of relatively small T-type currents in type 1 and type 2 DRG neurons (Cardenas et al, 1995). Even after mibefradil addition, a small (less than Ϫ50 pA peak amplitude) net inward current occasionally remained.…”

“…There were no systematic differences between capsaicin-sensitive and capsaicin-insensitive cells in action potential shapes, passive membrane properties, or characteristics of sodium and calcium currents, so results were pooled. The correspondence of the characteristics of the cells we studied with type 1 and 2 cells, together with the presence of TTX-R current (in all but one cell), is consistent with their identification as nociceptors corresponding to C-type and A␦-type fibers (Cardenas et al, 1995(Cardenas et al, , 1997. It is possible that a few cells that we studied with small but detectable I h would fall into the type 7 classification in the scheme of Petruska et al (2000), also believed to correspond with a type of nociceptor.…”

“…This cell had an unusually narrow action potential (1.0 msec at half-maximal amplitude), was not sensitive to 500 nM capsaicin, did not express I A , and expressed a large I h current. These observations all suggest that the anomalous cell was likely a type 3 or 4 cell (Cardenas et al, 1995).…”

“…The long-lasting AHPs probably reflect a combination of slowly decaying potassium conductances together with long membrane time constants. Cardenas et al (1995Cardenas et al ( , 1997 and Petruska et al (2000) have developed a system to classify acutely dissociated DRG neurons into distinct groups on the basis of a combination of properties, including cell diameter, action potential characteristics, and level of expression of ionic currents such as the hyperpolarizationactivated cation current I h , and the transient potassium current I A . Virtually all of the cells we studied corresponded well with the types of neurons designated types 1 and 2 in their systems in having long-duration action potentials with shoulders and longlasting AHPs as well as expressing little or no I h , with an average density of 1.4 Ϯ 1.5 pA/pF (n ϭ 52).…”