Modulation of the discriminative stimulus properties of cocaine by 5-HT1B and 5-HT2C receptors.

The present study assessed compounds displaying affinity for 5-HT1A, 5-HT1B, 5-HT2A and 5-HT2C receptors for their ability to substitute for, enhance or antagonize the discriminative stimulus effects of cocaine (10 mg/kg) in rats. In substitution tests, the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT; 0.2-1.6 mg/kg), the 5-HT1A/B receptor agonists RU 24969 (0.25-2 mg/kg) and CGS 12066B (2-16 mg/kg), the 5-HT1B/2C receptor agonists m-chlorophenylpiperazine (mCPP; 0.25-2 mg/kg) and m-trifluoromethylphenylpiperazine (TFMPP; 0.125-2 mg/kg), the 5-HT2A/2C receptor agonist 1-(4-bromo-2,5-dimethoxyphenyl)-2-aminopropane ([+/-]-DOB; 0.0625-0.5 mg/kg), the 5-HT2C receptor agonist MK 212 (0.25-1 mg/kg) and the nonselective 5-HT receptor agonist quipazine (1-8 mg/kg) engendered 30% to 70% cocaine-appropriate responding. The DA receptor antagonists SCH 23390 (0.025 or 0.05 mg/kg) and haloperidol (0.125 or 0.25 mg/kg) failed to block the partial substitution of RU 24969 (1 mg/kg) for cocaine. In combination tests, a fixed dose of either quipazine (4 mg/kg), RU 24969 (0.25, 0.5 or 1 mg/kg) or TFMPP (0.5 mg/kg) but not 8-OH-DPAT (0.4 mg/kg) or CGS 12066B (16 mg/kg) produced a leftward shift in the cocaine dose-response curve (0.625-5 mg/kg). In contrast, coadministration of either mCPP (0.25-2 mg/kg) or MK 212 (0.125-2 mg/kg) plus a dose of cocaine (5 mg/kg) that produced > 85% cocaine-appropriate responding when given alone partially antagonized cocaine; mCPP (1 mg/kg) also produced a rightward shift in the cocaine dose-response curve. Neither 8-OH-DPAT (0.2-1.6 mg/kg), (+/-)-DOB (0.125-0.5 mg/kg) nor quipazine (2-8 mg/kg) blocked the cocaine stimulus. The 5-HT1A receptor antagonist NAN 190 (0.2-0.8 mg/kg), the 5-HT2A/2C receptor antagonist LY 53857 (0.5-4 mg/kg) and the 5-HT/DA receptor antagonist pirenperone (0.5-4 mg/kg) neither substituted for nor enhanced cocaine; however, pirenperone but not NAN 190 or LY 53857 partially blocked the cocaine (10 mg/kg) response. Although 5-HT receptor compounds do not substitute for cocaine, several 5-HT receptor agonists (i.e., the indole derivative RU 24969 and the arylpiperazines mCPP, MK 212, TFMPP and quipazine), but not antagonists, differentially modulate the stimulus effects of cocaine.(ABSTRACT TRUNCATED AT 400 WORDS)