FUNCTIONAL-PROPERTIES OF A CLONED 5-HYDROXYTRYPTAMINE IONOTROPIC RECEPTOR SUBUNIT - COMPARISON WITH NATIVE MOUSE RECEPTORS

1. A comparative study of the whole-cell and single-channel properties of cloned and native mouse 5-hydroxytryptamine ionotropic receptors ( 5-HT3) was undertaken using mammalian cell lines expressing the cloned 5-HT3 receptor subunit A (5-HT(3)R-A), superior cervical ganglia (SCG ) neurones and N1E-115 cells. 2. No pharmacological difference was fou nd in the sensitivity to the agonists 5-HT and 2-methyl-5-HT, or to th e antagonists d-tubocurare and 3-tropanyl-3,5-dichlorobenzoate (MDL-72 222). 3. Current-voltage (I-V) relationships of whole-cell currents sh owed inward rectification in the three preparations. Rectification was stronger both in cells expressing the 5-HT(3)R-A subunit and in N1E-1 15 cells when compared with SCG neurones. 4. No clear openings could b e resolved in 5-HT-activated currents in patches excised from cells ex pressing the 5-HT(3)R-A subunit or N1E-115 cells. Current fluctuation analysis of whole-cell and excised-patch records revealed a slope cond uctance of 0.4-0.6 pS in both preparations. Current-voltage relationsh ips of these channels showed strong rectification that fully accounted for the whole-cell voltage dependence. 5. In contrast, single channel s of about 10 pS were activated by 5-KT in patches excised from SCG ne urones. The weak voltage dependence of their conductance did not accou nt completely for the rectification of whole-cell currents. A lower un itary conductance (3.4 pS) was inferred from whole-cell noise analysis . 6. We conclude that the receptor expressed from the cloned cDNA is i ndistinguishable from the 5-HT3 receptor of N1E-115 cells, suggesting an identical structure for these two receptors. The higher conductance and different voltage dependence of the 5-HT3 receptor in SCG neurone s might indicate the participation of an additional subunit in the str ucture of native ganglionic 5-HT3 receptors. Homo-oligomeric 5-HT(3)R- A channels may also be present as suggested by the lower conductance e stimated by whole-cell noise analysis.