Serotonin-2 (5-HT2) receptor-mediated signal transduction in human ciliary muscle cells: role in ocular hypotension.
OBJECTIVE
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(2A2C) 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 alpha-methyl-5-HT (EC50=63+/-17 nM), 5-HT (EC50=85+/-16 nM), (R)-DOI (EC50=165+/-47 nM), and 5-methoxy alpha-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; alpha-methyl-5-HT=36+/-11 nM; (R)-DOI=120 nM; 5-HT=130+/-36 nM; MK-212=470 nM; mCPP>1 microM; BW723C86=1766 nM. The agonist-induced [Ca2+]i mobilization in h-CM cells was potently blocked by the 5-HT2A-selective antagonist M-100907 (IC50=1.2+/-0.4 nM) but less potently by the antagonists for 5-HT2B (RS-127445, IC50>10 microM) and 5-HT2C (RS-102221, IC50=5.8+/-2.3 microM) 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-HT2A 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, alpha-methyl-5HT, and AL-34662.
Sharif
Najam A
NA
Ophthalmology Discovery Research, Alcon Research, Ltd., Fort Worth, TX 76134-2099, USA. naj.sharif@alconlabs.com
Kelly
Curtis R
CR
Crider
Julie Y
JY
Davis
Terry L
TL
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001345:Autoradiography; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D018929:Cell Culture Techniques; D002478:Cells, Cultured; D002924:Ciliary Body / Q000166:cytology / Q000378:metabolism*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D032342:Muscle Cells / Q000378:metabolism*; D009798:Ocular Hypertension / Q000209:etiology / Q000378:metabolism; D011485:Protein Binding; D044348:Receptors, Serotonin, 5-HT2 / Q000096:biosynthesis*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction
D044348:Receptors, Serotonin, 5-HT2; D002118:Calcium