A drug discrimination analysis of the actions of novel serotonin1A receptor ligands in the rat using the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin.

In the present study, using a two-lever drug discrimination procedure, we characterized the effects of a series of chemically-diverse, novel 5-HT1A receptor agonists and antagonists in rats trained to discriminate the serotonin (5-HT)1A receptor agonist, 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT) (0.31 mg/kg, i.p.) from saline. In analogy to the 5-HT1A receptor agonists, ipsapirone (a pyrimidinylpiperazine) and flesinoxan (a benzodioxane), the arylpiperazine derivatives, WY-48,723 and WY-50,324, as well as the methoxynaphtylpiperazines, S 14506 and S 14671, substituted 100% for the discriminative stimulus (DS) effects of 8-OH-DPAT, with the latter two displaying remarkable potency [50 effective doses (ED50s): 1.25, 0.34, 0.05, 0.69, 0.009 and 0.006 mg/kg, s.c., respectively]. In contrast, an additional pyrimidinylpiperazine, zalospirone, failed to fully mimick the training drug (maximal effect: 40%) even at a dose markedly disrupting response rates (2.5 mg/kg, s.c.). The potency of agonists in generalizing to 8-OH-DPAT correlated significantly (P < .05) with their affinity at rat hippocampal 5-HT1A receptors in vitro (r = .78), and with their potency to induce hypothermia in the rat (r = .96). S 15535 and S 15931, novel benzodioxopiperazines possessing mixed 5-HT1A autoreceptor agonist/postsynaptic 5-HT1A receptor antagonist properties, antagonized (approximately 75%) the discriminative stimulus (DS) properties of 8-OH-DPAT (ED50s: 6.9 and 0.97 mg/kg, s.c., respectively), although their structural analogue, S 14489, reduced by only 50% the action of 8-OH-DPAT. The 5-HT1A receptor antagonists, (+/-)-pindolol (-70%; ED50: 0.65 mg/kg, s.c.), (-)-alprenolol (-67%; ED50: 7.1) and NAN-190 (-80%; ED50: 1.5), all blocked the 8-OH-DPAT DS. Likewise, several novel antagonists at 5-HT1A autoreceptors and postsynaptic 5-HT1A receptors; the benzoisothiazolpiperazine, SDZ 216-525 (-83%; ED50: 0.64), the aryloxyalkylamine, (-)-tertatolol (-83%; ED50: 7.7) and the methoxyphenylpiperazine, (+)-WAY 100,135 (-80%; ED50: 17.0), antagonized the 8-OH-DPAT cue. Antagonist potency correlated significantly with affinity at 5-HT1A receptors (r = .83) and potency for antagonism of 8-OH-DPAT-induced hypothermia (r = .83). Antagonists showed only variable and not significant (P > .05) generalization rates (13-50%) when tested alone.(ABSTRACT TRUNCATED AT 400 WORDS)

S 14671: a naphtylpiperazine 5-hydroxytryptamine1A agonist of exceptional potency and high efficacy possessing antagonist activity at 5-hydroxytryptamine1C/2 receptors.

The interaction at 5-hydroxytryptamine (5-HT) receptors of the novel naphtylpiperazine, S 14671 (1-[2-(2-thenoylamino)ethyl]-4[1-(7- methoxynaphtyl)]piperazine), was compared to that of the 5-HT1A ligands, 8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), WY 50,324 [N-(29(4-(2-pyrimidinyl)-1-piperazinyl)ethyl)tricyclo(3.3.1.1(3,7) )- decane-1-carboxamide], (+)-flesinoxan, buspirone and BMY 7378 [(8-[2-[4-(2-methoxyphenyl)- 1-piperazinyl]ethyl]-8-azaspirol[-4-]-decane-7,9-dione 2HCl]. S 14671 showed a very high affinity for 5-HT1A sites (pKi, 9.3) as compared to the reference ligands (pKi values, 9.2, 8.7, 8.7, 7.9 and 8.7, respectively). S 14671 bound in an apparently competitive manner and, in distinction to the reference compounds, possessed a Hill Coefficient (1.4) significantly superior to 1. Although showing low affinity at 5-HT1B and 5-HT3 sites, S 14671 displayed significant affinity at both 5-HT1C and 5-HT2 sites; pKi, 7.8 in each case. Furthermore, S 14671 acted as an antagonist of 5-HT-stimulated phosphoinositide turnover in rat choroid plexus (5-HT1C) and cortex (5-HT2). In vivo, upon s.c. administration, S 14671 acted as a high efficacy agonist in models of 5-HT1A receptor-mediated activity: induction of flat-body posture, spontaneous tail-flicks, hypothermia and corticosterone secretion and inhibition of morphine-induced antinociception. In every test, S 14671 was the most potent compound: it was active at doses as low as 5 micrograms/kg s.c. Relative potency across all tests was S 14671 greater than 8-OH-DPAT greater than WY 50,324 greater than (+)-flesinoxan greater than buspirone with BMY 7378 too weak for comparison to be meaningful. The action of S 14671 in 5-HT1A tests was blocked by BMY 7378 and the 5-HT1A antagonist, (-)-alprenolol, but unaffected by the 5-HT1C/2 antagonist, ritanserin, and the 5-HT3 antagonist, ondansetron. Activation of postsynaptic 5-HT1A receptors was confirmed in 5,7-dihydroxytryptamine-lesioned rats, in which the potency of S 14671 to elicit spontaneous tail-flicks was potentiated. Activation of presynaptic receptors was demonstrated by inhibition of the electrical activity of the dorsal raphe nucleus with the following order of relative potency: S 14671 greater than 8-OH-DPAT greater than WY 50,324 greater than BMY 7378 greater than buspirone. Spiperone, which acts as a pure 5-HT1A antagonist at raphe 5-HT1A receptors, blocked the action of S 14671. In conclusion, S 14671 is a structurally novel ligand manifesting high efficacy and exceptional potency at both pre- and postsynaptic 5-HT1A receptors.(ABSTRACT TRUNCATED AT 400 WORDS)