1 The influence of
sodium ion substitutes on the
5-hydroxytryptamine (5-HT)-induced flux of the organic
cation [14C]
guanidinium through the
ion channel of the mouse
5-HT3 receptor and on the competition of
5-HT with the selective
5-HT3 receptor antagonist [3H]
GR 65630 was studied, unless stated otherwise, in mouse
neuroblastoma N1E-115 cells. 2 Under physiological conditions (135 mm
sodium),
5-HT induced a concentration-dependent [14C]
guanidinium influx with an EC50 (1.3 microm) similar to that in electrophysiological studies. 3 The stepwise replacement of
sodium by increasing concentrations of the organic
cation hydroxyethyl trimethylammonium (
choline) concentration dependently caused both a rightward shift of the
5-HT concentration-response curve and an increase in the maximum effect of
5-HT. Complete replacement of
sodium resulted in a 34-fold lower potency of
5-HT and an almost two times higher maximal response. A low potency of
5-HT in
choline buffer was also observed in other
5-HT3 receptor-expressing rodent cell lines (NG 108-15 or NCB 20). 4 Replacement of Na+ by Li+ left the potency and maximal effects of
5-HT almost unchanged. Replacement by tris (hydroxymethyl)
methylamine (Tris),
tetramethylammonium (TMA) or N-methyl-d-glucamine (NMDG) caused an increase in maximal response to
5-HT similar to that caused by
choline. The potency of
5-HT was only slightly reduced by Tris, to a high degree decreased by TMA (comparable to the decrease by
choline), but not influenced by NMDG. 5 The potency of
5-HT in inhibiting [3H]
GR65630 binding to intact cells was 35-fold lower when
sodium was completely replaced by
choline, but remained unchanged after replacement by NMDG. 6 The results are compatible with the suggestion that
choline competes with
5-HT for the
5-HT3 receptor; the increase in maximal response may be partly due to a
choline-mediated delay of the 5-HT-induced desensitization. For studies of 5-HT-evoked [14C]
guanidinium flux through
5-HT3 receptor channels, NMDG appears to be an 'ideal'
sodium substituent since it increases the signal-to-noise ratio without interfering with
5-HT binding.