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Characterization of 5-HT-sensitive potassium conductances in neonatal rat facial motoneurones in vitro

P. M. Larkman and J. S. Kelly

1. The properties of the 5-HT-sensitive K⁺ conductance of neonatal rat facial motoneurones were examined in brainstem slices using whole-cell patch-clamp techniques.

2. In a small proportion of motoneurones, 5-hydroxytryptamine (5-HT) evoked an inward current mediated solely by a decrease in K⁺ conductance. The reversal potential (V_5-HT) was dependent on the external K⁺ concentration and the 5-HT-evoked current (I_5-HT) displayed a linear current-voltage (I-V) relationship.

3. In the remaining motoneurones, the 5-HT-evoked decrease in K⁺ conductance could only be observed in isolation once a concomitant 5-HT-mediated enhancement of the hyperpolarization-activated current, I_h, had been abolished with the I_h blocker, ZD-7288.

4. External Cs⁺ also abolished the I_h-mediated component of I_5-HT but, in addition, blocked part of the 5-HT-sensitive K⁺ current. At potentials hyperpolarized to V_5-HT, Cs⁺ voltage dependently blocked I_5-HT while at potentials depolarized to V_5-HT, I_5-HT was largely unaffected. Ba²⁺ and Rb⁺ had identical actions to Cs⁺ on the 5-HT-sensitive K⁺ current.

5. The Ba²⁺-, Rb⁺- and Cs⁺-sensitive component of the 5-HT-sensitive K⁺ current inwardly rectified with a reversal potential that was dependent on the K⁺ equilibrium potential (E_K).

6. Replacing external Na⁺ with N-methyl-D-glucamine, blocking Ca²⁺ entry, or preventing an increase in intracellular [Ca²⁺] with BAPTA, all failed to alter I_5-HT at potentials depolarized to E_K.

7. I_5-HT at depolarized potentials was reversibly blocked by 4-aminopyridine (4 mM) but not tetraethylammonium chloride (30 mM) and did not show inactivation during depolarizing voltage pulses (1·5 s duration).

8. The results suggest that, in addition to enhancing I_h, 5-HT modulates two distinct K⁺ conductances in neonatal rat facial motoneurones. The actions of Cs⁺, Ba²⁺ and Rb⁺ support the involvement of a member of the inwardly rectifying family of K⁺ channels while the other K⁺ channel may belong to the voltage-gated family.

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