The inhibition of the cage-leaving response--a model for studies of the serotonergic neurotransmission in the rat.

Abstract

It was observed that rats that had been given drugs that enhance serotonergic neurotransmission, e.g. the serotonin releasing compounds p-chloroamphetamine (PCA) and fenfluramine, the MAO-A inhibitors and serotonin releasing agents amiflamine and alpha-ethyltryptamine and the serotonin agonists 5-methoxy-N, N-dimethyltryptamine (5-MeODMT), 8-hydroxy-2-(di-n-propylamino) tetraline (8-OH-DPAT), m-chlorophenyl piperazine (m-CPP) and 5-methoxy-3 (1,2,3,6-tetrahydropyridin-4-yl)1H-indole (RU 24969), did not leave their home-cages when the grid-covers were removed in contrast to normal rats who almost immediately left the cages. The association between the serotonin neurotransmission and the inhibitory effect of PCA on the cage-leaving response was indicated by the findings that 1. Serotonin uptake inhibitors (alaproclate and citalopram) antagonized the effect of PCA. 2. High, neurotoxic doses of PCA antagonized the effect of PCA when tested one week after the former administration. The serotonin uptake inhibitor zimeldine counteracted the effect of neurotoxic PCA. 3. Depletion of brain serotonin with p-chlorophenylalanine counteracted the effect of acute PCA. 4. Repeated treatment of rats for 7 days with zimeldine, amiflamine, alpha-ethyltryptamine or clorgyline plus a low dose of PCA counteracted the effect of acute PCA probably due to a functional down-regulation at postsynaptic receptors. Clorgyline or a low dose of PCA by themselves had no effect. 5. Compounds interacting with dopamine or noradrenaline mechanisms, e.g. alpha-methyltyrosine, N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP 4), pimozide, remoxipride and prazosin did not antagonize the effect of PCA nor did (+)-amphetamine inhibit the cage-leaving response. None of the serotonin receptor antagonists (cinanserin, ketanserin, metergoline, methysergide, metitepine, mianserin, pirenperone) blocked the inhibition of the cage-leaving response produced by PCA, indicating that the receptors involved may not be of the S1- and S2- types. Observation of the cage-leaving response may be a valuable technique in studies of drugs that enhance the serotonin neurotransmission in the rat brain.