Characterization of serotonergic receptors in rabbit, porcine and human conjunctivae

Purpose. To characterize the serotonin (5-HT) receptors linked to the modulation of adenylyl cyclase activity in rabbit, porcine and human conjunctivae. Methods. Serotonin receptor-subtype expression was examined using reverse transcription-polymerase chain reaction (RT-PCR) and receptor subtype-specific polyclonal anti-bodies for the immunofluorescent labeling of conjunctival cryosections. In addition, measurements of the effects of serotonergics on the short-circuit current (I_sc) across rabbit and porcine conjunctivae were contrasted. Results. RT-PCR assays indicated the expression of 5-HT_1B and 5-HT_1D receptors, subtypes negatively coupled to adenylyl cyclase, in the rabbit conjunctiva. This approach also suggested the co-expression of 5-HT_1B, 5-HT _1D, 5-HT_1F, 5-HT₄ and 5-HT₇ mRNA's in the porcine conjunctiva, and 5-HT_1D, 5-HT_1F and 5-HT₇ in the human conjunctiva. Since the 5-HT₄ and 5-HT₇ receptors are positively linked to adenylyl cyclase, these results implied that the porcine and human tissues exhibited subtypes both positively and negatively linked to the enzyme. However, immunohistochemical observations, using currently available antibodies solely localized the 5-HT₇ moiety in the porcine and human epithelia, suggested that the 1B/1D forms may be minor elements. Consistent with this prospect, 5-HT was a stimulant of the transepithelial I_sc across the porcine conjunctiva, an opposite response from earlier findings that demonstrated inhibitory effects by 5-HT on the rabbit Isc, which are now explained by the localization of the 1B/1D receptors in the rabbit stratified epithelium. Conclusions. The 5-HT receptors expressed by mammalian conjunctivae are not identical. In terms of 5-HT receptor expression, the porcine tissue may be a more appropriate model for human, than is the rabbit, in that 5-HT may serve as a secretagogue in the human epithelium.