Vol. 288, Issue 2, 820-826, February 1999

Electrophysiological Comparison of 5-Hydroxytryptamine_1A Receptor Antagonists on Dorsal Raphe Cell Firing¹

Lynn P. Martin, David M. Jackson, Carin Wallsten and Barbara L. Waszczak

Department of Pharmaceutical Sciences, Northeastern University, Boston, Massachusetts (L.P.M., B.L.W.); and Department of Pharmacology, Astra Arcus AB, Preclinical R&D, Södertälje, Sweden (D.M.J., C.W.)

Single-unit recording studies were undertaken in chloral hydrate-anesthetized rats to compare the effects on dorsal raphe cell firing of several putative 5-hydroxytryptamine (HT)_1A receptor antagonists, including WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide), p-MPPI (4-(2-methoxyphenyl)1-[2'-[N-(2''-pyridinyl)-p-iodobenzamido]ethyl]piperazine), and two newly described 5-HT_1A receptor antagonists, NDL-249 [(R)-3-(N-propylamino)-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxamide] and NAD-299 [(R)-3-N,N-dicyclobutylamino-8-fluoro-3,4-dihydro-2H-1-benzopyran-5-carboxamide]. Consistent with a 5-HT_1A receptor antagonist profile, pretreatment with an approximately equimolar (0.02-0.03 µmol/kg) i.v. dose of each compound caused a significant rightward shift in the dose-response curve for 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin]. Antagonist potency was clearly highest for NAD-299 and WAY 100635, which caused shifts roughly 3 times greater than those for either p-MPPI or NDL-249 (ED₅₀ for 8-OH-DPAT, 1.3 ± 0.3 µg/kg; after NAD-299, 18.2 ± 1.0 µg/kg; after WAY 100635, 16.9 ± 2.9 µg/kg; after NDL-249, 6.0 ± 1.2 µg/kg; after p-MPPI, 4.7 ± 1.1 µg/kg). In separate studies, each of the antagonists was administered alone in increasing cumulative doses to evaluate whether they possessed intrinsic agonist activity in this system. At doses below 0.01 µmol/kg, none of the drugs altered firing by more than ±20% basal rates. At higher doses (>0.1 µmol/kg), WAY 100635, NDL-249, and NAD-299 caused a dose-dependent suppression of dorsal raphe cell firing (ED₅₀ = 0.6 ± 0.2, 0.7 ± 0.3, and 0.9 ± 0.4 µmol/kg, respectively). However, the ED₅₀ values for inhibition by these drugs were roughly 30 times higher than the doses that antagonized effects of 8-OH-DPAT. Moreover, the inhibition by all three antagonists (but not 8-OH-DPAT) was readily reversed by d-amphetamine (3.2 mg/kg i.v.), a releaser of norepinephrine, suggesting that these effects were likely due to alpha adrenergic receptor blockade rather than to 5-HT_1A receptor agonism. Thus, it was concluded that WAY 100635, NAD-299, NDL-249, and p-MPPI all fulfill criteria as 5-HT_1A receptor antagonists lacking intrinsic efficacy in the dorsal raphe system. The newly described compound NAD-299 exhibits antagonist potency comparable to that of WAY 100635 in this electrophysiological assay.