Discriminative stimulus effects of 8-OH-DPAT in pigeons: antagonism studies with the putative 5-HT1A receptor antagonists BMY 7378 and NAN-190.

Pigeons were trained to discriminate 0.3 mg/kg of the 5-HT1A receptor agonist 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) from saline. RU 24969 (5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole), at doses of 5.6-10 mg/kg, and eltoprazine (5.6 mg/kg), both mixed 5-HT1A/B agonists, substituted completely for 8-OH-DPAT, whereas 3.0-10 mg/kg of the 5-HT1B/C agonist TFMPP (1-(m-trifluromethylphenyl)piperazine) and 0.1-3.0 of the 5-HT3 antagonist MDL 72222 (3-tropanyl-3,5-dichlorobenzoate) yielded only saline-appropriate responses. Substitution for 8-OH-DPAT by eltoprazine and RU 24969, which does not occur in rats, provides in vivo support for the suggestion that the absence of a 5-HT1B receptor in the pigeon allows more complete expression of 5-HT1A-mediated effects. BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)]8-azaspirol-[4.5]- decane-7,9-dione) attenuated the 8-OH-DPAT stimulus at doses from 1.0 to 10 mg/kg but, when administered alone, also resulted in approximately 40% 8-OH-DPAT-appropriate responding at the highest dose. NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalamido)butyl)-piperazine (0.3-3.0 mg/kg) produced a dose-dependent and complete antagonism of the 8-OH-DPAT-discriminative stimulus; administered alone NAN-190 resulted only in saline-key responding. NAN-190 also reversed the rate-decreasing effects of higher doses of 8-OH-DPAT. The beta-adrenoceptor antagonist (+/-)-pindolol (5.6-17 mg/kg) antagonized the discriminative stimulus effects of lower 8-OH-DPAT doses but was unable to block the effects of higher doses of 8-OH-DPAT. Prazosin (1.0-10 mg/kg), which like NAN-190, is an alpha 1-antagonist, neither substituted for nor blocked the discriminative stimulus effects of 8-OH-DPAT. These results suggest that NAN-190 is an effective 5-HT1A receptor antagonist in this procedure with pigeons, with no indication of agonist actions, whereas BMY 7378 and pindolol are best characterized as partial 5-HT1A receptor agonists.