Differential effects of serotonin 5-HT1A receptor agonists on the discriminative stimulus effects of the 5-HT2A receptor agonist 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane in rats and rhesus monkeys

Abstract

Although many drugs act by indirectly stimulating multiple receptors (e.g., reuptake inhibitors), relatively little is known about interactions between agonism at different receptors. This study compared the effect of serotonin (5-HT)(1A) receptor agonists with the discriminative stimulus effects of the 5-HT(2A) receptor agonist 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) in rats and rhesus monkeys. Eight rats discriminated 0.56 mg/kg i.p. DOM and responded under a fixed ratio (FR) 10 schedule of food presentation, whereas three rhesus monkeys discriminated 0.32 mg/kg s.c. DOM and responded under an FR 5 schedule of stimulus shock termination. DOM and the 5-HT(2A) receptor agonists 2,5-dimethoxy-4-n-propylthiophenethylamine (2C-T-7) and dipropyltryptamine (DPT), but not the 5-HT(1A) receptor agonists 8-hydroxy-2-(di-n-propylamino) tetralin hydrochloride (8-OH-DPAT) and 3-chloro-4-fluorophenyl-(4-fluoro-4-([(5-methyl-6-methylaminopyridin-2-ylmethyl) amino) methyl] piperidin-1-yl) methanone (F13714), occasioned responding on the DOM-associated lever in rats and monkeys. Both 8-OH-DPAT and F13714 attenuated the discriminative stimulus effects of DOM in monkeys but not in rats; these effects of 8-OH-DPAT and F13714 were prevented by the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridyl)cyclohexanecarboxamide (WAY 100635). DPT and 2C-T-7 enhanced the discriminative stimulus effects of DOM in rats and monkeys in an additive manner. Taken together, the results suggest that the DOM discriminative stimulus is pharmacologically similar and mediated by 5-HT(2A) receptors in rats and monkeys; however, the ability of 5-HT(1A) receptor agonists to modify the effects of DOM is markedly different between these species. These results indicate possible differences in the neurobiology of 5-HT systems that could be important for studying drugs that have multiple mechanisms of action (e.g., reuptake inhibitors that indirectly stimulate multiple receptors).

Fig. 1.

Discriminative stimulus effects and effects on response rate for F13714, 8-OH-DPAT, DOM, 2C-T-7, and DPT in monkeys (n = 3; left) and rats (n = 8; right) discriminating DOM. Abscissae, dose in milligrams per kilogram of body weight; V, vehicle. Ordinates, mean (±S.E.M.) percentage of responses on the DOM-associated lever (top) and mean (±S.E.M.) rate of responding in responses per second (bottom). See Materials and Methods for other details.

Fig. 2.

Discriminative stimulus effects and effects on response rate for DOM administered alone and in combination with different doses of 8-OH-DPAT in monkeys (left) and rats (right). See Fig. 1 and Materials and Methods for other details.

Fig. 3.

Discriminative stimulus effects and effects on response rate for DOM administered alone and in combination with different doses of F13714 in monkeys (left) and rats (right). See Fig. 1 and Materials and Methods for other details.

Fig. 4.

Discriminative stimulus effects and effects on response rate for DOM administered alone, with 8-OH-DPAT or F13714 (same data as plotted in Figs. 2 and 3, respectively), and with 8-OH-DPAT or F13714 and WAY 100635. See Fig. 1 and Materials and Methods for other details.

Fig. 5.

Top, discriminative stimulus effects of DOM administered alone and in combination with different doses of DOM (left), 2C-T-7 (center), and DPT (right) in monkeys. Bottom, same data replotted as isobolograms, with the ED₅₀ values for DOM plotted on the ordinates and the ED₅₀ values of the pretreatment drugs plotted on the abscissae. See Fig. 1 and Materials and Methods for other details.

Fig. 6.

Top, discriminative stimulus effects of DOM administered alone and in combination with different doses of DOM (left), 2C-T-7 (center), and DPT (right) in rats. Bottom, same data replotted as isobolograms, with the ED₅₀ values of DOM plotted on the ordinates and the ED₅₀ values of the pretreatment drugs plotted on the abscissae. See Fig. 1 and Materials and Methods for other details.