The interaction of antidepressant drugs with central and peripheral (enteric) 5‐HT3 and 5‐HT4 receptors
Authors: Lucchelli A. , Santagostino‐Barbone M.G. , Barbieri A. , Candura S.M. , Tonini M. 
Published: July 2012

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‐HT3 and 5‐HT4 receptors.
Clomipramine, paroxetine and fluoxetine inhibited [3H]‐DAU 6215 binding to 5‐HT3 recognition sites in NG 108‐15 cells with IC50 values in the range 1.3–4 μm. Litoxetine had an IC50 of 0.3 μm. The specific binding of [3H]‐GR 113808 to 5‐HT4 recognition sites in pig striatal membranes was inhibited by all four antidepressants with negligible potency (IC50 values ≥ 20 μm).
In whole ileal segments, concentration‐response curves to 5‐HT were biphasic, with the high‐ and low‐potency phases involving 5‐HT4 and 5‐HT3 receptors, respectively. Curves to 2‐methyl‐5‐hydroxytryptamine (2‐methyl‐5‐HT: a 5‐HT3 receptor agonist) and 5‐methoxytryptamine (5‐MeOT: a 5‐HT4 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 nm).
Fluoxetine (0.1–1 μm) and litoxetine (0.3–3 μm) antagonized both the high‐ and low‐potency phases of the 5‐HT curve. Schild analysis for the low‐potency phase yielded pA2 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 μm), fluoxetine markedly inhibited the 5‐HT response maximum. Clomipramine (10–300 nm) inhibited, by a mechanism independent of concentration, both phases of the 5‐HT curve with a reduction of the maximum response. Paroxetine (1 μm) was ineffective on the high‐potency phase, but caused a rightward shift of the low‐potency phase (pKB: 6.1 ± 0.01).
Responses to 2‐methyl‐5‐HT were inhibited by 1 μm fluoxetine (pKB: 5.4 ± 0.02). Like clomipramine (30 and 100 nm), litoxetine (1 and 3 μm) produced rightward displacements of 2‐methyl‐5‐HT‐induced contractions, which were virtually independent of antidepressant concentration (pKB values: 6.0 ± 0.02 and 5.5 ± 0.01, respectively). At higher concentrations, fluoxetine (3 μm) and clomipramine (300 nm) markedly reduced the 2‐methyl‐5‐HT response maximum. Paroxetine (1 μm) was ineffective.
Responses to 5‐MeOT were shifted to the right by fluoxetine (0.1–1 μm) and litoxetine (1 and 3 μm) in a concentration‐dependent manner. At higher concentrations, fluoxetine (3 μm) markedly reduced the 5‐MeOT response maximum, an effect also observed with 100 and 300 nm clomipramine. Paroxetine (1 μm) was ineffective.
In unstimulated LMMPs, the excitatory effects evoked by 5‐HT, 2‐methyl‐5‐HT and 5‐MeOT and the antagonism produced by 300 nm 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.
In conclusion, our results show that clomipramine, fluoxetine, paroxetine and litoxetine possess low to moderate potency/affinity at both central and peripheral (enteric) 5‐HT3 receptors. In contrast, all four antidepressants are virtually ineffective at central 5‐HT4 receptors. Inhibition of 5‐HT4 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‐HT3 and 5‐HT4 receptors may be relevant for both potential therapeutic action and adverse effects at gastrointestinal level.