Differential effects of 5-HT agonists and antagonists on the repeated acquisition and performance of response sequences in monkeys

As a means of characterizing the role of 5-HT1A and 5-HT2A receptors in learning, 5-hydroxytryptamine (5-HT) agonists and antagonists with selective affinities for each receptor subtype (i.e. 8-hydroxy-dipropylaminotetralin (8-OH-DPAT), (-)-4-(dipropylamino)-1,3,4,5-tetrahydrobenz-(c,d,)indole-6-carboxamide (LY228729), (+/-)-1-(4-iodo-2,5-dimeth-oxyphenyl)-2-aminopropane hydrochloride (DOI), 4-iodo-N-[2- [4-(methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinyl-benzamide hydrochloride (p-MPPI), N-[2- [4- (2-methoxyphenyl)-1-piperazinyl] ethyl] -N-2-pyridinyl-cyclohexanecarboxamide maleate (WAY-100635), 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyllpiperazine hydrobromide (NAN-190) and ritanserin) were administered to monkeys responding under a multiple schedule of repeated acquisition and performance. In addition, a selective 5-HT1A receptor agonist (8-OH-DPAT) was administered in combination with a 5-HT2A receptor antagonist (ritanserin) to examine any potential interactions between the two 5-HT receptor subtypes. When administered alone, 8-OH-DPAT (0.1-3.2mg/kg), LY228729 (0.32-3.2 mg/kg) and DOI (0.018-3.2 mg/kg) dose-dependently decreased overall response rate in both schedule components, and generally increased the percentage of errors in the acquisition components at doses lower than those that increased the percentage of errors in the performance components. At the doses of each drug tested (i.e. 0.1 or 0.32 mg/kg), both p-MPPI and WAY-100635 antagonized the disruptive effects of 8-OH-DPAT, by shifting the dose-effect curves for overall response rate and the percentage of errors to the right. In contrast, ritanserin (0.32 or 1mg/kg) had little or no effect on the disruptions produced by 8-OH-DPAT, but it effectively antagonized the rate-decreasing and error-increasing effects produced by the 5-HT2A agonist DOI. Administration of the 5-HT1A antagonists WAY-100635 and NAN-190 alone produced dose-dependent rate-decreasing effects, but the effects on accuracy of responding in the acquisition components differed from those of the 5-HT1A agonists (8-OH-DPAT and LY228729), in that they did not produce an increase in the percentage of errors. Together, these results suggest that 5-HT is capable of disrupting learning in monkeys through actions at both the 5-HT1A and 5-HT2A receptors, and that 5-HT2A receptor antagonism does not unilaterally modify the effects produced by 5-HTA1A receptor activation.