ATTENUATION OF 5-HT1A AND 5-HT2 BUT NOT 5-HT1C RECEPTOR MEDIATED BEHAVIOR IN RATS FOLLOWING CHRONIC TREATMENT WITH 5-HT RECEPTOR AGONISTS, ANTAGONISTS OR ANTI-DEPRESSANTS

The effects of chronic (10 days) treatment with serotonin (5-HT) receptor agonists on 5-HT1A receptor mediated lower lip retraction (LLR), 5-HT1C receptor mediated penile erections (PE) or 5-HT2 receptor mediated head shakes (HS) were studied in rats. It was found that the 5-HT1A and 5-HT2 receptor mediated behaviour could be attenuated after chronic treatment, whereas 5-HT1C receptor mediated behaviour remained unchanged. The ED50 for the 5-HT1A receptor mediated, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT)-induced LLR showed an increase from 0.07 mg/kg in placebo pretreated rats to 0.13 in 8-OH-DPAT (1 mg/kg/day) pretreated rats. The number of 5-HT2 receptor mediated (+/-)-1-2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.46 and 1 mg/kg)-induced HS was significantly reduced (67% and 50%, respectively) after 10 days' pretreatment with DOI (1 mg/kg/day). In the same animals the number of 5-HT1C receptor mediated PE was increased. Ten days' pretreatment with MK 212 (0.46 mg/kg/day) failed to affect MK 212 (0.22 and 0.46 mg/kg/)-induced PE. In addition, the effects of chronic treatment with some antidepressants were studied. The monoamine oxidase (MAO) inhibitor tranylcypromine (4 mg/kg/day) given for 10 days caused an increase in the ED50 for 8-OH-DPAT induced LLR (ED50 values were 0.06 and 0.14 mg/kg, respectively, in placebo - and tranylcypromine - pretreated rats) and attenuated MK 212 (0.22 and 0.46 mg/kg)-induced PE. Chronic treatment with mianserin (10 mg/kg/day), a tetracyclic antidepressant with 5-HT1C and 5-HT2 receptor antagonistic properties, did not change PE induced by MK 212, but caused an increase of PE induced by DOI and a decrease of DOI-induced HS. Ten days' pretreatment with the 5-HT re-uptake inhibitor Org 6997 (5 mg/kg/day) had no effect on MK 212-induced PE. The results demonstrate that 5-HT1A and 5-HT2 but not 5-HT1C receptor mediated behaviour can be attenuated by chronic treatment with agonists for these receptors. The 5-HT1C receptor mediated behaviour remains unchanged in response to chronic agonist treatment. Chronic treatment with anti-depressants have differential effect on these behaviours. The possible implication for the mechanism of action of antidepressants is discussed.