5-HT1A receptor full and partial agonists and 5-HT2C (but not 5-HT3) receptor antagonists increase rates of punished responding in rats

Drugs with different intrinsic activity at 5-HT_1A receptors and antagonists at 5-HT 2A 2C and 5-HT₃ receptors were studied for their ability to increase the rates of punished operant responding in rats. Like chlordiazepoxide (5 and 10 mg/kg) and diazepam (1.25 and 2.5 mg/kg), 0.125 mg/kg 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT_1A receptor agonist, and 5 and 10 mg/kg ipsapirone, a partial agonist at these receptors, increased the rates of punished responding, whereas (S)-WAY 100135, a 5-HT_1A receptor antagonist, had no effect at doses from 1 to 10 mg/kg. 8-OH-DPAT and ipsapirone, like benzodiazepines, significantly reduced unpunished responding. The 5-HT 2A 2C receptor antagonists ritanserin (2 mg/kg), mianserin (8 mg/kg), and mesulergine (0.1 mg/kg) significantly increased the rates of punished responding, whereas 0.5-2 mg/kg ketanserin, that has higher affinity for 5-HT_2A than 5-HT_2C receptors, had no effect. Antagonists, at 5-HT₃ receptors such as ondansetron (0.001-0.1 mg/kg) and tropisetron (0.001-0.1 mg/kg), had no effect on punished or unpunished responding. The results show that agents acting as full or partial agonists at 5-HT_1A receptors and blockers of postsynaptic 5-HT_2C receptors have anxiolytic-like effects in a model of punished operant responding, whereas antagonists at 5-HT_1A and 5-HT₃ receptors have no such effect.