title: Two parallel signaling pathways couple 5HT1A receptors to N- and L-type calcium channels in C-like rat dorsal root ganglion cells.
abstract: 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) inhibited high-threshold Ca2+ channel current and decreased action potential duration. The inhibitory effects of 5HT and the 5HT1A agonist 8-OH-DPAT were shown to be antagonized by the 5HT1A antagonists spiperone and pindolol, respectively, indicating involvement of a 5HT1A receptor. Several observations suggest that 5HT1A receptors couple to N- and L-type Ca2+ channels by two different signaling pathways in type 1 DRG cells. The inhibition of Ca2+ channel currents produced by 10 microM 5HT occurred in two phases, an initial slowing of current activation rate (kinetic slowing), which was complete within 10 s, and a simultaneous reduction in steady state current amplitude (steady state inhibition), which peaked in approximately 1 min. The kinetic slowing, but not steady state inhibition, was reversed by a positive prepulse to +70 mV (prepulse). Blockade of N-type Ca2+ channels selectively reduced the kinetic slowing and its reversal by prepulses. Chelation of intracellular Ca2+ or blockade of L-type Ca2+ channels selectively reduced the steady state inhibition. Recordings using the cell-attached patch configuration 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 cell body outside the patch pipette reduced macroscopic Ca2+ channel currents in 33% of small-diameter DRG cells tested, indicating the participation of a cytosolic diffusible component. Application of 5HT (a membrane impermeant compound) outside the patch pipette produced steady state inhibition only, whereas similar application of membrane permeant 5HT1A agonists, 8-OH-DPAT or 5-methoxy-N,N-dimethyl-tryptamine, produced kinetic slowing and steady state inhibition. Together these data suggest that 5HT1A receptors couple negatively to Ca2+ channels via two pathways: a membrane-delimited pathway that couples to N-channels and actuates voltage-sensitive kinetic slowing and a pathway dependent on a cytosolic diffusible component and free intracellular Ca2+, which couples to L channels and actuates steady state inhibition.
authors.affiliation: Department of Anatomy and Neurobiology, College of Medicine, University of Tennessee, Memphis 38163, USA.
authors.forename: C G
authors.initials: CG
authors.lastname: Cardenas
authors.forename: L P
authors.initials: LP
authors.lastname: Del Mar
authors.forename: R S
authors.initials: RS
authors.lastname: Scroggs
mesh_terms: D000818:Animals; D015220:Calcium Channels / Q000145:classification / Q000502:physiology*; D046508:Culture Techniques; D005727:Ganglia, Spinal / Q000502:physiology*; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D009412:Nerve Fibers / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology; D009619:Nociceptors / Q000502:physiology*; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D011985:Receptors, Serotonin / Q000502:physiology*; D044263:Receptors, Serotonin, 5-HT1; D015398:Signal Transduction / Q000502:physiology*
chemical_list: D015220:Calcium Channels; D011985:Receptors, Serotonin; D044263:Receptors, Serotonin, 5-HT1