8-hydroxy-2 (di-n-propylamino) tetraline (8-OH-DPAT) and 5-methoxy- 3(1,2,3,6-tetrahydro-4-pyridinyl)1H indole succinate (RU 24969), two agonists on the putative serotonin 1A and serotonin 1B receptors, were used for exploring the role of these sites in the inhibitory effect of serotonin (5-HT) on feeding. In free-feeding rats, 2.5- 5 mg/kg RU 24969 significantly reduced food intake while doses of 8-OH-DPAT ranging from 0.125 to 0.5 mg/kg increased eating. The effects of the highest doses were associated with hyperlocomotion and hyperreactivity for RU 24969 and a typical motor syndrome (flat body posture and forepaw treading) for 8-OH-DPAT. The motor syndrome caused by 0.5 mg/kg 8-OH-DPAT was much more obvious in food-deprived rats in which food intake was also markedly reduced. RU 24969 1.25 and 5 mg/kg reduced food intake by food-deprived rats and caused hyperlocomotion not different from that in free-feeding animals. Pretreatment with metergoline (2 mg/kg i.p.) prevented the effect of 5 mg/kg RU 24969 on food intake by food-deprived rats but had no effect on the reduction of eating caused by 0.5 mg/kg 8-OH-DPAT. The motor syndrome caused by 8-OH-DPAT was not changed by metergoline but the hyperlocomotion caused by RU 24969 was potentiated. Haloperidol (0.1 mg/kg i.p.) completely blocked the hyperlocomotion but did not change the reduction of food intake caused by RU 24969 in food- deprived rats. It is suggested that the putative serotonin 1B receptors specifically mediate the inhibitory effect of 5-HT on feeding whereas serotonin 1A sites act by enhancing eating only in free-feeding animals.
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