The interaction of antidepressant drugs with central and peripheral (enteric) 5‐HT 3 and 5‐HT 4 receptors

1 A combined study of receptor binding in central neuronal cell membranes and functional responses in isolated segments of guinea‐pig small intestine allowed characterization of the interaction of four antidepressant drugs with central and peripheral 5‐HT 3 and 5‐HT 4 receptors. 2 Clomipramine, paroxetine and fluoxetine inhibited [ 3 H]‐DAU 6215 binding to 5‐HT 3 recognition sites in NG 108‐15 cells with IC 50 values in the range 1.3–4 μ m . Litoxetine had an IC 50 of 0.3 μ m . The specific binding of [ 3 H]‐GR 113808 to 5‐HT 4 recognition sites in pig striatal membranes was inhibited by all four antidepressants with negligible potency (IC 50 values ≥ 20 μ m ). 3 In whole ileal segments, concentration‐response curves to 5‐HT were biphasic, with the high‐ and low‐potency phases involving 5‐HT 4 and 5‐HT 3 receptors, respectively. Curves to 2‐methyl‐5‐hydroxytryptamine (2‐methyl‐5‐HT: a 5‐HT 3 receptor agonist) and 5‐methoxytryptamine (5‐MeOT: a 5‐HT 4 receptor agonist) were monophasic. All antidepressants were used at concentrations lacking anticholinoceptor properties, as demonstrated in both electrically stimulated longitudinal muscle‐myenteric plexus preparations (LMMPs) and in unstimulated LMMPs following addition of acetylcholine (100 n m ). 4 Fluoxetine (0.1–1 μ m ) and litoxetine (0.3–3 μ m ) antagonized both the high‐ and low‐potency phases of the 5‐HT curve. Schild analysis for the low‐potency phase yielded pA 2 estimates of 6.6 ± 0.3 (Schild slope of 1.1) and of 6.6 ± 0.1 (Schild slope of 1.1), respectively. At higher concentrations (3 μ m ), fluoxetine markedly inhibited the 5‐HT response maximum. Clomipramine (10–300 n m ) inhibited, by a mechanism independent of concentration, both phases of the 5‐HT curve with a reduction of the maximum response. Paroxetine (1 μ m ) was ineffective on the high‐potency phase, but caused a rightward shift of the low‐potency phase (p K B : 6.1 ± 0.01). 5 Responses to 2‐methyl‐5‐HT were inhibited by 1 μ m fluoxetine (p K B : 5.4 ± 0.02). Like clomipramine (30 and 100 n m ), litoxetine (1 and 3 μ m ) produced rightward displacements of 2‐methyl‐5‐HT‐induced contractions, which were virtually independent of antidepressant concentration (p K B values: 6.0 ± 0.02 and 5.5 ± 0.01, respectively). At higher concentrations, fluoxetine (3 μ m ) and clomipramine (300 n m ) markedly reduced the 2‐methyl‐5‐HT response maximum. Paroxetine (1 μ m ) was ineffective. 6 Responses to 5‐MeOT were shifted to the right by fluoxetine (0.1–1 μ m ) and litoxetine (1 and 3 μ m ) in a concentration‐dependent manner. At higher concentrations, fluoxetine (3 μ m ) markedly reduced the 5‐MeOT response maximum, an effect also observed with 100 and 300 n m clomipramine. Paroxetine (1 μ m ) was ineffective. 7 In unstimulated LMMPs, the excitatory effects evoked by 5‐HT, 2‐methyl‐5‐HT and 5‐MeOT and the antagonism produced by 300 n m clomipramine were comparable to those obtained in whole ileal segments. This suggests that 5‐HT contained in the mucosa of whole preparations does not interfere with agonist‐induced contractile responses and with the inhibitory effect of antidepressant drugs. 8 In conclusion, our results show that clomipramine, fluoxetine, paroxetine and litoxetine possess low to moderate potency/affinity at both central and peripheral (enteric) 5‐HT 3 receptors. In contrast, all four antidepressants are virtually ineffective at central 5‐HT 4 receptors. Inhibition of 5‐HT 4 receptor‐mediated ileal contractions by fluoxetine, litoxetine and clomipramine may result from allosteric antagonism or, more likely, from post‐receptor blockade of second messenger generation. The interaction of antidepressants with central and peripheral 5‐HT 3 and 5‐HT 4 receptors may be relevant for both potential therapeutic action and adverse effects at gastrointestinal level.