5HT₄Receptors Couple Positively to Tetrodotoxin-Insensitive Sodium Channels in a Subpopulation of Capsaicin-Sensitive Rat Sensory Neurons

Abstract

The distribution of tetrodotoxin (TTX)-sensitive and -insensitive Na ϩ currents and their modulation by serotonin (5HT) and prostaglandin E 2 (PGE 2 ) 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 (Cardenas et al., 1995). Types 1 and 2 DRG cells expressed TTX-insensitive Na ϩ currents, whereas types 3 and 4 DRG cells exclusively expressed TTX-sensitive Na ϩ currents. Application of 5HT (1-10 M) increased TTX-insensitive Na ϩ currents in type 2 DRG cells but did not affect Na ϩ currents in type 1, 3, or 4 DRG cells. The 5HT receptor involved resembled the 5HT 4 subtype. It was activated by 5-methoxy-N,Ndimethyltryptamine (10 M) but not by 5-carboxyamidotryptamine (1 M), (ϩ)-8-hydroxydipropylaminotetralin (10 M), or 2-methyl-5HT (10 M), and was blocked by ICS 205-930 with an EC 50 of ϳ2 M but not by ketanserin (1 M). PGE 2 (4 or 10 M) also increased Na ϩ currents in varying portions of cells in all four groups.The effect of 5HT and PGE 2 on Na ϩ 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 ϩ current, however, was not mimicked by 8-chlorophenylthio-cAMP (200 M), suggesting the possibility that cAMP was not involved.The data suggest that the 5HT-and PGE 2 -mediated increase in Na ϩ current may be involved in hyperesthesia in different but overlapping subpopulations of nociceptors. Key words: serotonin; PGE 2 ; capsaicin; nociceptor; tetrodotoxin; cAMP; dorsal root ganglion; 5HT 4 receptorPrimary hyperesthesia is thought to be a consequence of the release of proinflammatory mediators in the vicinity of nociceptor endings. Proinflammatory agents such as serotonin (5HT), prostaglandins, and adenosine are derived from a number of sources (Cooper and Sessle, 1992). Interactions between such agents and endings of thinly myelinated and unmyelinated nociceptive afferents induce activity, decrease threshold, and increase suprathreshold mechanothermal reactivity (Handwerker, 1976;Kumazawa and Mizumura, 1980;Mense, 1981;Martin et al., 1987;Schaible and Schmidt, 1988;Lang et al., 1990;Grubb et al., 1991;Herbert and Schmidt, 1992).Contributions to hyperesthesia by proinflammatory agents are possibly achieved by modulation of membrane currents involved in the initiation and repolarization of action potentials in nociceptive endings, as well as induction of edema in the surrounding matrix (Cooper, 1993). The study of acutely isolated dorsal root ganglion (DRG) cell bodies may be usef ul in determining the roles of various ion channels in the actions of proinflammatory agents on nociceptor f unction. A number of studies have shown that afferent cell bodies exhibit properties of nociceptor endings in vitro. These include expression of currents sensitive to proinflammatory mediators as well as features of peripheral transduction mechanisms (Baccaglini and Hogan...

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

“…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.…”

“…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).…”

Acute p38-Mediated Modulation of Tetrodotoxin-Resistant Sodium Channels in Mouse Sensory Neurons by Tumor Necrosis Factor-α

“…؉ currents in mouse DRG neurons Previous studies suggest that TTX-R Na ϩ currents and TNF receptors are colocalized with TRPV1 and other nociceptive markers in a subpopulation of DRG neurons (Cardenas et al, 1995(Cardenas et al, , 1997Li et al, 2004). To minimize variability in our recordings, we chose to record from an electrophysiologically identified subclass of DRG neurons that is likely to include neurons expressing both TTX-R Na ϩ channels and TNF receptors.…”

A Transient Receptor Potential Vanilloid 4-Dependent Mechanism of Hyperalgesia Is Engaged by Concerted Action of Inflammatory Mediators

“…PGE 2 and 5-HT, in particular, also induce hyperalgesia via the cAMP/PKA pathway when administered alone (Taiwo et al, 1989(Taiwo et al, , 1992Cardenas et al, 1997;Aley and Levine, 1999). Therefore, we hypothesized that the increase in cAMP induced by either PGE 2 or 5-HT alone is insufficient to activate TRPV4-dependent mechanisms of hyperalgesia; instead, simultaneous action of multiple inflammatory mediators would be required to achieve sufficient activation of the cAMP pathway to engage TRPV4.…”