Rats and marmosets respond differently to serotonin agonists and antagonists

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Abstract
The actions of the serotonin precursor 5-hydroxytryptophan (5-HTP), the agonist 5-methoxy-N,N-dimetyltryptamine (MeODMT) and quipazine (QPZ) and the antagonists cyproheptadine, methysergide and metergoline, were studied in the rat and in the common marmoset (Callithrix jacchus). The precursor and agonists elicited head shakes, forepaw padding, splayed hindlimbs, tremor and Straub tail in the rat. However, head shakes were not observed after MeODMT and Straub tail was not observed after QPZ. Carbidopa plus 5-HTP potentiated only head shakes, while tranylcypromine (TCP) plus 5-HTP potentiated all the behaviors above. In the marmoset, the action of these drugs elicited drowsiness, teeth chattering, ataxia, vomiting and decreased motor activity, although vomiting was not elicited by MeODMT and ataxia and drowsiness by QPZ. Although TCP plus 5-HTP potentiated all these behaviors, carbidopa plus 5-HTP was not effective. Rats treated with the antagonists (1.0, 5.0 and 10 mg/kg doses) did not show any of these behaviors, but marmosets treated with the same drugs developed “drowsiness”, vomiting, and decreased motor activity; nonetheless, cyproheptadine (5.0 and 10 mg/kg doses) did not elicit “drowsiness”, while increasing motor activity and the number of head shakes. Pretreatment of marmosets with these antagonists blocked only teeth chattering elicited by MeODMT (4.0 mg/kg) and QPZ (10 mg/kg). Pretreatment with haloperidol, p-chlorophenylalanine and α-methyl-P-tyrosine had no effect. The data obtained show that rats and marmosets present differential behavioral responses to the 5-HT drugs used. Marmosets have a richer behavioral repertoire and show greater sensitivity to 5-HT agonists and antagonists. The data suggest that this animal might become a useful biological model for the study of central 5-HT systems.