Cationic modulation of 5-HT2 and 5-HT3 receptors in rat sensory neurons: the role of K, Ca and Mg

The effects of changes in external K⁺, Ca²⁺, and Mg²⁺ concentrations on 5-HT₂- and 5-HT₃ receptor-mediated depolarizations of the resting membrane potential in rat dorsal root ganglion (DRG) cells was studied. In cells exhibiting a 5-HT₂-mediated response, 5-HT and α-methyl 5-HT depolarized the resting membrane potential (RMP) and increased the slope of the current–voltage (I/V) relationship. The equilibrium potential (E_r) for the depolarization was linearly related to the logarithm of the K⁺]_o, indicating the depolarization resulted from a decrease in resting K⁺ conductance. In a subpopulation of large-diameter acutely dissociated DRG neurons recorded from using the whole-cell patch-clamp configuration, 5-HT produced an inward shift in the current required to hold cells at −60 mV. This inward shift in holding current was associated with a reduction in membrane conductance and reversed near E_k. This data suggests that the 5-HT₂ receptor-mediated depolarization and increase in R_in seen in intact DRG preparation is produced by blockade of an outward K⁺ leak current. Increases in K⁺]_o reduced the increase in R_in and depolarization induced by 5-HT with 50% inhibition of the depolarization occurring at 8.3 mM of K⁺]_o. Half-normal Ca²⁺ (1.2 mM) produced a downward shift of the 5-HT concentration–response curve, reducing the maximal response by 40%, with minimal effect on the half-maximal response. Mg²⁺ ions did not affect this 5-HT response. In cells exhibiting a 5-HT₃ receptor response, 5-HT and 2-methyl-5-HT produced depolarization with decreased R_in. The E_r for this depolarizing response (−30.2±1.8 mV) became less negative (−11.5 mV) in 10 mM K⁺]_o with minimal effect on the amplitude of the depolarization. In Na⁺-free superfusate, the 5-HT-induced depolarization was converted to hyperpolarization. This indicated the 5-HT₃ response increased a mixed Na⁺/K⁺ conductance. Elevated Ca²⁺ or Mg²⁺ markedly reduced the 5-HT₃ response. Incubation with 3.5 mM Ca²⁺ shifted the 5-HT concentration–response curve downward and to the right, decreasing the maximal response by 49% and increasing the EC₅₀ by 10-fold. Elevated Mg²⁺ produced similar effects. In cells where both 5-HT₂- and 5-HT₃-mediated responses could be demonstrated, the elevation of K⁺ or the reduction of Ca²⁺ converted a 5-HT₂ response to a 5-HT₃ response. The above data suggest that elevation of K⁺]_o or reduction of Ca²⁺]_o produced by rapid firing rates of sensory neurons will favor the expression of 5-HT₃ responses over 5-HT₂ responses.