Behavioral effects of 5-HT receptor ligands in the aversive brain stimulation, elevated plus-maze and learned helplessness tests.
In order to illustrate the use of animal models in the study of the anxiolytic and antidepressant properties of drugs acting on 5-HT receptors, a series of experiments is described. With electrical stimulation of the midbrain central gray (CG), an aversive area of the brain, the 5-HT-1 receptor antagonist propranolol raised the aversive threshold in a dose-dependent way, following its microinjection into the CG. This antiaversive effect of propranolol, which is similar to that of benzodiazepine anxiolytics, was prevented by microinjection into the same brain site of the 5-HT-2 receptor blocker ritanserin. Ritanserin itself and the 5-HT-1A receptor ligand ipsapirone caused either little or no effect. In another animal model of anxiety, the elevated plus-maze, intra-CG propranolol also caused an anxiolytic-like effect, antagonized by ritanserin, indicating a 5-HT mediation. However, systemically injected isamoltane, a congener of propranolol, was ineffective in the elevated plus-maze, whereas ipsapirone caused an anxiolytic effect. Ritanserin was again inactive. Finally, both ipsapirone as well as another 5-HT-1A receptor ligand BAY R 1531, given IP, reversed the learning deficit resulting from exposure to uncontrollable foot-shocks, an effect characteristic of antidepressant drugs.