UNITARY PROPERTIES OF POTASSIUM CHANNELS ACTIVATED BY 5-HT IN ACUTELYISOLATED RAT DORSAL RAPHE NEURONS

1. Single inwardly rectifying K+ channel currents were recorded from a cutely isolated adult serotonergic dorsal raphe (DR) neurones using th e cell-attached and outside-out patch clamp configuration. 2. Four equ ally spaced conductance levels were observed in both outside-out and c ell-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+]o the single channel I-V relation w as linear in the range 0 mV to -100 mV in all cases. 4. Transitions be tween the various conductance levels were observed, as were apparent d irect opening and closing to each individual conductance level. Furthe rmore openings of 11, 21 and 30 pS were observed in almost all the pat ches. These results suggest that the different-sized events result fro m substates of a single channel rather than several different channels with different conductances. 5. Unitary K+ channel current probabilit y of opening, recorded in cell-attached patch, was unchanged after 5-h ydroxytryptamine (5-HT) was added to the bath outside the patch pipett e which suggests that no easily diffusible second messenger was involv ed. 6. The single K+ channel activity, however, was increased on avera ge by 670% following the addition of 5-HT to the bath when recording c hannel activity in the outside-out configuration. Usually all K+ chann el subconductance levels increased in activity but the largest increas es occurred in the events with 30 and 40 pS conductance. 7. These resu lts suggest that 5-HT enhances the probability of opening of the resti ng K+ channel activity, which can open to several levels of conductanc e, and that no new channel or freely diffusible second messenger is in volved in the response.