Amisulpride is a potent 5-HT7 antagonist: relevance for antidepressant actions in vivo

Abstract

Rationale: Amisulpride is approved for clinical use in treating schizophrenia in a number of European countries and also for treating dysthymia, a mild form of depression, in Italy. Amisulpride has also been demonstrated to be an antidepressant for patients with major depression in many clinical trials. In part because of the selective D(2)/D(3) receptor antagonist properties of amisulpride, it has long been widely assumed that dopaminergic modulation is the proximal event responsible for mediating its antidepressant and antipsychotic properties.

Objectives: The purpose of these studies was to determine if amisulpride's antidepressant actions are mediated by off-target interactions with other receptors.

Materials and methods: We performed experiments that: (1) examined the pharmacological profile of amisulpride at a large number of central nervous system (CNS) molecular targets and, (2) after finding high potency antagonist affinity for human 5-HT(7a) serotonin receptors, characterized the actions of amisulpride as an antidepressant in wild-type and 5-HT(7) receptor knockout mice.

Results: We discovered that amisulpride was a potent competitive antagonist at 5-HT(7a) receptors and that interactions with no other molecular target investigated in this paper could explain its antidepressant actions in vivo. Significantly, and in contrast to their wild-type littermates, 5-HT(7) receptor knockout mice did not respond to amisulpride in two widely used rodent models of depression, the tail suspension test and the forced swim test.

Conclusions: These results indicate that 5-HT(7a) receptor antagonism, and not D(2)/D(3) receptor antagonism, likely underlies the antidepressant actions of amisulpride.

Figure 1

Amisulpride (▲) and chlorpromazine (■) versus [³H]LSD competition binding at 5-HT_7a receptors. Amisulpride competes effectively against the high affinity 5-HT_7a antagonist chlorpromazine at cloned h5-HT_7a receptors, suggesting that it binds with high affinity to 5-HT_7a receptors.

Figure 2

Amisulpride (▲) and 5-HT (■) versus [³H]5-CT competition binding at 5-HT_7a receptors. The comparatively low affinity of amisulpride for [³H]5-CT, a 5-HT_7a agonist with high intrinsic activity that preferentially binds high affinity sites, in contrast with its higher affinity for [³H]LSD, a very weak partial agonist that labels primarily low affinity antagonist binding sites, suggests that amisulpride is an antagonist at 5-HT_7a receptors.

Figure 3

The modified Gaddum/Schild equation-fitted dose response data for 5-HT activation of 5-HT_7a receptors in the presence of 0, 1, 3, 10, 30, 100, and 300 nM amisulpride. All dose response data generated in the presence of all seven amisulpride concentrations were fitted to generate the curves shown: 0, 10, 30, and 100 nM amisulpride. There appears to be a parallel, rightward shift of the dose response curves in the presence of increasing concentrations of amisulpride, suggesting that amisulpride is a competitive antagonist at 5-HT_7a receptors.

Figure 4

Dose-response effect of amisulpride on the immobility profile of 5-HT₇^+/+ (□) and 5-HT₇^−/− (■) mice in (upper panel) the tail suspension test and (lower panel) the forced swim test. Amisulpride shows a dose-dependent antidepressant efficacy in both depression models by reducing immobility time in 5-HT₇^+/+ but not 5-HT₇^−/− mice. Values are mean ± SEM. n = 6 animals per genotype per treatment group. *p < 0.05, **p < 0.01, ***p<0.001 between the genotypes; †p < 0.05, †††p < 0.001 within a genotype compared to control; two-way ANOVA followed by Bonferroni's post-hoc test.