2 PARALLEL SIGNALING PATHWAYS COUPLE 5HT(1A)-RECEPTORS TO N-TYPE AND L-TYPE CALCIUM CHANNELS IN C-LIKE RAT DORSAL-ROOT GANGLION-CELLS

The coupling of serotonin receptors to Ca2+ channels was studied in a subpopulation of acutely isolated rat dorsal root ganglion (DRG) cell bodies (type 1 DRG cells), which have membrane properties similar to C -type nociceptive sensory neurons. In these cells, serotonin (5HT) inh ibited high-threshold Ca2+ channel current and decreased action potent ial duration. The inhibitory effects of 5HT and the 5HT(1A) agonist 8- OH-DPAT were shown to be antagonized by the 5HT(1A) antagonists spiper one and pindolol, respectively, indicating involvement of a 5HT(1A) re ceptor. Several observations suggest that 5HT(1A) receptors couple to N- and L-type Ca2+ channels by two different signaling pathways in typ e 1 DRG cells. The inhibition of Ca2+ channel currents produced by 10 mu M 5HT occurred in two phases, an initial slowing of current activat ion rate (kinetic slowing), which was complete within 10 a, and a simu ltaneous reduction in steady state current amplitude (steady state inh ibition), which peaked in similar to-1 min. The kinetic slowing, but n ot steady state inhibition, was reversed by a positive prepulse to +70 mV (prepulse). Blockade of N-type Ca2+ channels selectively reduced t he kinetic slowing and its reversal by prepulses. Chelation of intrace llular Ca2+ or blockade of L-type Ca2+ channels selectively reduced th e steady state inhibition. Recordings using the cell-attached patch co nfiguration suggest that steady state inhibition required a component that was diffusible in the cytosol, while kinetic slowing occurred via a membrane delimited pathway. The application of 5HT to the fell body outside the patch pipette reduced macroscopic Ca2+ channel currents i n 33% of small-diameter DRG cells tested, indicating the participation of a cytosolic diffusible component. Application of 5HT (a membrane i mpermeant compound) outside the patch pipette produced steady state in hibition only, whereas similar application of membrane permeant 5HT(1A ) agonists, 8-OH-DPAT or 5-methoxy-N,N-dimethyl-tryptamine, produced k inetic slowing and steady state inhibition. Together these data sun es t that 5HT(1A), receptors couple negatively to Ca2+ channels via two p athways: a membrane-delimited pathway that couples to N-channels and a ctuates voltage-sensitive kinetic slowing and a pathway dependent on a cytosolic diffusible component and free intracellular Ca2+, which cou ples to L channels and actuates steady state inhibition.