Characterization of structure-activity relationships for serotonin receptors coupled to adenylate cyclase.

Abstract

The need to develop selective compounds for 5-HT receptors requires further elucidation of structure-activity relationships involved with receptor recognition and activation. In particular, there is great interest in the involvement of the 5-HT₁(A) receptor in therapeutic treatment of conditions such as anxiety, depression and hypertension. Despite previous attempts to define the pharmacophore for 5-HT₁(A) receptors, the structural requirements for 5-HT₁(A) receptor activity remain largely unexplored. In fact, no selective 5-HT₁(A) antagonists currently exist. Binding assays provide important information concerning receptor affinity, but do not reflect functional consequences of receptor interaction. One of the functional correlates of the 5-HT₁(A) receptor is the inhibition of forskolin-stimulated adenylate cyclase (FSC). The objective of this dissertation was to assess the merits of using the FSC assay as a functional screen to determine 5-HT₁(A) intrinsic activity of novel compounds. From the variety of compounds tested, it was apparent that the FSC assay can be used as an in vitro functional screen for assessing 5-HT₁(A) agonistic and antagonistic, as well as partial agonistic, activity. All putative 5-HT₁(A) agonists tested in the FSC assay had potencies (EC₅₀) that were less than their respective 5-HT₁(A) binding affinities (Kᵢ). The use of pindolol to block 5-HT₁(A) receptors, and therefore cause a rightward shift in any observed inhibition curve, provided qualitative evidence of 5-HT₁(A) interaction. Quantitative evaluation of 5-HT₁(A) activity had to be approached cautiously, due to possible complications arising from non-5-HT₁(A)-mediated inhibition. The data for a series of enanatiomers of 8-OH-DPAT and its analogs supported a recently proposed model for the 5-HT₁(A) agonist pharmacophore. Also, within the enantiomeric pairs exhibiting stereoselectivity, the compound with the higher 5-HT₁(A) affinity also possessed greater 5-HT₁(A) agonistic activity. Within a series of tetrahydropyridylindoles (THPI), the 4-THPI analogs generally had both greatest 5-HT₁(A) affinity and agonistic activity, although there were exceptions to this trend. Several compounds, whose structures were based on the small molecules of the 5-HT₁(A) agonist 8-hydroxy-2-(di-n-propylaminotetralin) (8-OH-DPAT) and 5-HT itself, demonstrated potential antagonistic activity in the FSC assay. Further analysis of an extended series of these compounds in the FSC assay is required to establish specific conclusions concerning the 5-HT₁(A) pharmacophore.