5HT4 Receptors Couple Positively to Tetrodotoxin-insensitive Sodium Channels in a Subpopulation of Capsaicin-sensitive Rat Sensory Neurons

The distribution of tetrodotoxin (TTX)-sensitive and -insensitive Na+ currents and their modulation by serotonin (5HT) and prostaglandin E2 (PGE2) was studied in four different types of dorsal root ganglion (DRG) cell bodies (types 1, 2, 3, and 4), which were previously identified on the basis of differences in membrane properties (). Types 1 and 2 DRG cells expressed TTX-insensitive Na+ currents, whereas types 3 and 4 DRG cells exclusively expressed TTX-sensitive Na+ currents. Application of 5HT (1-10 microM) increased TTX-insensitive Na+ currents in type 2 DRG cells but did not affect Na+ currents in type 1, 3, or 4 DRG cells. The 5HT receptor involved resembled the 5HT4 subtype. It was activated by 5-methoxy-N,N-dimethyltryptamine (10 microM) but not by 5-carboxyamidotryptamine (1 microM), (+)-8-hydroxydipropylaminotetralin (10 microM), or 2-methyl-5HT (10 microM), and was blocked by ICS 205-930 with an EC50 of approximately 2 microM but not by ketanserin (1 microM). PGE2 (4 or 10 microM) also increased Na+ currents in varying portions of cells in all four groups. The effect of 5HT and PGE2 on Na+ currents was delayed for 20-30 sec after exposure to 5HT, suggesting the involvement of a cytosolic diffusible component in the signaling pathway. The agonist-mediated increase in Na+ current, however, was not mimicked by 8-chlorophenylthio-cAMP (200 microM), suggesting the possibility that cAMP was not involved. The data suggest that the 5HT- and PGE2-mediated increase in Na+ current may be involved in hyperesthesia in different but overlapping subpopulations of nociceptors.