Recent advances in the electrophysiological characterization of 5-HT3 receptors.

5-HT3 receptors are ligand-gated, cation-selective ion channels, mediating membrane depolarization and neuronal excitation. Established and potential therapeutic applications of selective 5-HT3 receptor antagonists, coupled with the localization of this receptor subtype within discrete areas of the CNS, have resulted in an intensification of research in this area. In this review, Jeremy Lambert and colleagues summarize recent developments in the electrophysiological characterization of 5-HT3 receptors, and comment upon the unresolved issue of 5-HT3 receptor heterogeneity.

An electrophysiological investigation of the properties of 5-HT3 receptors of rabbit nodose ganglion neurones in culture.

1. The biophysical and pharmacological properties of 5-hydroxytryptamine (5-HT)-evoked currents in rabbit nodose ganglion neurones in culture have been determined by use of the whole-cell and outside-out membrane patch recording modes of the patch-clamp technique. 2. In 49% of cells investigated the bath application of 10(-5) M 5-HT at negative holding potentials elicited an inward current. The whole-cell response to 5-HT reversed in sign (E5-HT) at approximately -2 mV and exhibited inward rectification. 3. The influence of various ion substitutions upon E5-HT established that the 5-HT-evoked current is mainly mediated by a mixed Na+, K+ cation conductance with little or no contribution from Cl- ions. The omission of Ca2+ and Mg2+ from the extracellular solution enhanced the amplitude of the 5-HT-induced current. 4. On isolated outside-out membrane patches, the bath application of 10(-6) M 5-HT induced single channel currents with a chord conductance of approximately 17 pS at -70 mV and an average slope conductance of 19 pS over the range -100 to -40 mV. The 5-HT-induced single channels exhibited modest inward rectification and were reduced in frequency, but not amplitude, by the 5-HT3 receptor antagonist metoclopramide (10(-6) M). 5. The bath application of 5-HT (3 x 10(-7)-3 x 10(-5) M) to whole cells voltage clamped at -60 mV produced dose-dependent inward currents which were mimicked by 2-methyl-5-HT and 1-phenylbiguanide with equipotent molar ratios, relative to 5-HT, of 2.5 and 32 respectively. 6. Whole-cell inward currents produced by the local pressure application of 5-HT (10(-5) M) were unaffected by 10(-6) M methysergide, 10(-6) M ketanserin or 10(-6) M citalopram, but were concentration-dependently antagonized by the selective 5-HT3 receptor antagonists tropisetron (IC50 = 4.6 x 10(-11) M) ondansetron (IC50 = 5.7 x 10(-11) M), and bemesetron (IC50 = 3.3 x 10(-10) M). The response to 5-HT was also blocked by the non-selective antagonists metoclopramide (IC50 = 1.2 x 10(-8) M), cocaine (IC50 = 8.3 x 10(-8) M) and (+)-tubocurarine (IC50 = 1.6 x 10(-7) M).

Ketamine potentiates 5-HT3 receptor-mediated currents in rabbit nodose ganglion neurones.

The interaction of ketamine with the 5-HT3 receptor of rabbit nodose ganglion neurones is described. Ketamine (3-30 microM) enhanced 5-HT3 receptor-mediated currents recorded under voltage-clamp conditions. This action did not appear to be related to the known effect of ketamine of inhibiting 5-HT uptake.

Physiological and pharmacological properties of 5-HT3 receptors--a patch clamp-study.

Whole cell and patch clamp techniques were used to investigate the properties of 5-HT3 receptors of a murine neuroblastoma cell line (N1E-115) and adult rabbit nodose ganglion neurones. In addition, some preliminary results from guinea-pig nodose ganglion neurones are presented. In such cells, voltage-clamped at -60 mV, 5-HT (10 microM) induced an inward current associated with a conductance increase. The results of ion substitution experiments suggest that the 5-HT activated ion channel is permeable to both Na+ and K+ ions with a permeability ratio (PNa/PK) of 0.94 and 0.92 for rabbit nodose ganglion cells and N1E-115 cells respectively. On outside out membrane patches excised from rabbit nodose ganglion neurones, 5-HT (1 microM) activated clearly discernible single channel currents with a conductance of 16.6 +/- 0.7 pS (n = 4). In contrast, fluctuation analysis of 5-HT induced whole cell currents suggests that the single channel conductance of N1E-115 cells is only 0.3 pS, a value some 50 fold lower. The 5-HT-induced whole cell currents recorded from all three preparations were antagonised by the selective 5-HT3 receptor antagonist ondansetron (GR38032F) and by the less selective agents metoclopramide, cocaine and (+)-tubocurarine. However, these preparations demonstrate a differential sensitivity to some antagonists. In particular, (+)-tubocurarine was a potent antagonist in N1E-115 cells (IC50 = 0.85 nM) but was approximately 200 fold (IC50 = 156 nM) and 1200 fold (IC50 = 10 microM) less potent in rabbit and guinea-pig nodose ganglion neurones respectively. Additionally, a novel effect of ketamine (10 microM) to potentiate the 5-HT-induced current of rabbit nodose ganglion neurones is described.

Antagonism of 5-HT3 receptor mediated currents in murine N1E-115 neuroblastoma cells by (+)-tubocurarine.

5-HT3 receptor-mediated membrane currents were recorded from voltage-clamped clonal N1E-115 neuroblastoma cells. The inward current response to ionophoretically applied serotonin (5-HT) was reversibly antagonised by the 5-HT3 receptor antagonists GR 38032F and metoclopramide with IC50 values of 0.2 nM and 14 nM, respectively. Low concentrations of (+)-tubocurarine [+)-Tc) also blocked the response to 5-HT (IC50 = 0.8 nM), but other nicotinic cholinoceptor antagonists (gallamine, vecuronium and trimetaphan) were ineffective when applied at a relatively high concentration (1 microM). Blockade by (+)-Tc was neither voltage- nor use-dependent, suggesting that (+)-Tc does not block 5-HT-activated ion channels in N1E-115 cells.