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Department of Pharmacological and Physiological Sciences, University of Chicago, IL 60637.

1. Single inwardly rectifying K+ channel currents were recorded from acutely isolated adult serotonergic dorsal raphe (DR) neurones using the cell-attached and outside-out patch clamp configuration. 2. Four equally spaced conductance levels were observed in both outside-out and cell-attached patch recordings with conductance levels averaging 11, 21, 30 and 40 pS. Larger conductance openings (50-120 pS) were seen less frequently. 3. When using 136 [K+]0 the single channel I-V relation was linear in the range 0 mV to -100 mV in all cases. 4. Transitions between the various conductance levels were observed, as were apparent direct opening and closing to each individual conductance level. Furthermore openings of 11, 21 and 30 pS were observed in almost all the patches. These results suggest that the different-sized events result from substrates of a single channel rather than several different channels with different conductances. 5. Unitary K+ channel current probability of opening, recorded in cell-attached patch, was unchanged after 5-hydroxytryptamine (5-HT) was added to the bath outside the patch pipette which suggests that no easily diffusible second messenger was involved. 6. The single K+ channel activity, however, was increased on average by 670% following the addition of 5-HT to the bath when recording channel activity in the outside-out configuration. Usually all K+ channel subconductance levels increased in activity but the largest increases occurred in the events with 30 and 40 pS conductance. 7. These results suggest that 5-HT enhances the probability of opening of the resting K+ channel activity, which can open to several levels of conductance, and that no new channel or freely diffusible second messenger is involved in the response.

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