Serotonin-2 (5-HT 2 ) Receptor-Mediated Signal Transduction in Human Ciliary Muscle Cells: Role in Ocular Hypotension

Purpose and Methods: The aim of this study was to characterize the serotonin (5-hydroxytryptamine; 5-HT) receptors that mediate phosphoinositide (PI) hydrolysis and intracellular Ca²⁺ ([Ca²⁺]_i) mobilization in isolated cells of human ciliary muscle (h-CM) from multiple donors using a variety of agonists and antagonists. An additional aim was to visualize the mRNAs and receptor binding sites for 5-HT₂ receptors in human ciliary body (h-CB), CM, and other tissues by reverse transcriptase polymerase chain reaction and quantitative autoradiography techniques, respectively, and to correlate with ocular hypotensive activity of such compounds.

Results: CBs isolated from several donor eyes revealed the presence of 5-HT_2A–2C receptor mRNAs. [³H]-5-HT and [³H]-ketanserin autoradiography on sections of human eyes revealed a high density of 5-HT₂ receptor binding sites in the iris, ciliary epithelium, and longitudinal CM. In isolated h-CM cells, the agonists α-methyl-5-HT (EC₅₀ = 63 ± 17 nM), 5-HT (EC₅₀ = 85 ± 16 nM), (R)-DOI (EC₅₀ = 165 ± 47 nM), and 5-methoxy α-methyl tryptamine (EC₅₀ = 1200 ± 270 nM) differentially stimulated PI turnover. These agonists also mobilized [Ca²⁺]_i in h-CM cells with the following potencies (EC₅₀s): 5-methoxy-tryptamine = 42 ± 11 nM; α-methyl-5-HT = 36 ± 11 nM; (R)-DOI = 120 nM; 5-HT = 130 ± 36 nM; MK-212 = 470 nM; mCPP > 1 µM; BW723C86 = 1766 nM. The agonist-induced [Ca²⁺]_i mobilization in h-CM cells was potently blocked by the 5-HT_2A-selective antagonist M-100907 (IC₅₀ = 1.2 ± 0.4 nM) but less potently by the antagonists for 5-HT_2B (RS-127445, IC₅₀ > 10 µM) and 5-HT_2C (RS-102221, IC₅₀ = 5.8 ± 2.3 µM) receptors.

Conclusions: In conclusion, h-CB, h-CM, and CM cells express mRNAs and proteins for 5-HT₂ receptor subtypes, of which the predominant functionally active subtype is the 5-HT_2A receptor, as defined by agonist and antagonist activities. These receptors may be responsible for mediating the intraocular pressure reduction observed in recent literature with a number of 5-HT₂ agonists, such as (R)-DOI, α-methyl-5HT, and AL-34662.