Convergent effects of multiple modulators on two-pore-domain 'leak' potassium channels in respiratory-related neurons

The neuronal membrane conductance at rest is dominated by channels that preferentially conduct potassium ions. These resting or 'leak' K⁺ channels drive the membrane potential toward the potassium equilibrium potential, away from spike threshold and provide a shunt conductance to diminish voltage responses to synaptic currents. Interestingly, a number of modulators act to dynamically regulate neuronal excitability by increasing or decreasing activity of these channels.

Although resting K⁺ channels represent a major target for neuromodulators, the molecular basis for this class of channels has remained elusive. Recently, however, a novel gene family of putative leak potassium (KCNK) channels was identified by molecular cloning [1,2].

Among those, a subgroup of so-called TASK channels generate currents with a unique constellation of properties. Thus, in heterologous systems, TASK-1 and TASK-3 currents are persistent and time-independent, and they display a weak rectification in asymmetric physiological K⁺ conditions that obeysconstant field predictions for an open, K⁺-selective pore (ie, they are instantaneous open-rectifiers). Moreover, cloned TASK channel currents are inhibited by extracellular protons in a physiological pH range and activated by inhalation anesthetics at clinically relevant concentrations [1,2].

We have used histochemical and whole cell electrophysiological approaches in vitro, taking advantage of the unique properties of TASK channels, to establish functional expression of native TASK currents in brainstem neurons, including those associated with respiration. As described below, we find that TASK channels underlie a pH-, anesthetic- and transmitter-sensitive K⁺ current in respiratory-related motoneurons; they also contribute to pH-sensitive responses in presumptive respiratory chemoreceptor neurons of the locus coeruleus (LC) and medullary raphe.

Using in situ hybridization, we found high levels of TASK-1 and TASK-3 mRNA in cranial and spinal motoneurons and accordingly, hypoglossal motoneurons expressed a pH-sensitive K⁺ current under voltage clamp with kinetic and voltage-dependent properties of TASK channels (ie, instantaneous open-rectification). In addition, this motoneuronal pH-sensitive, open-rectifier K⁺ current was inhibited by a number of neurotransmitters (serotonin, norepinephrine, SP, TRH) and activated by halothane and sevoflurane with an EC₅₀ identical to that of their anesthetic effects.

By combining in situ hybridization with immunohistochemistry, we found that TASK channel transcripts are expressed in cate-cholaminergic LC neurons and serotonergic raphe neurons, although at moderate levels. In those aminergic cells, pH-sensitive currents appear to involve multiple ionic mechanisms. Nevertheless, a contribution from native TASK channels was readily revealed by taking advantage of their combined pH- and halothane-sensitivity; thus, the pH-sensitive halothane-induced K⁺ current in LC and raphe neurons had the properties of an instantaneous, open rectifier.

In summary, TASK channels represent a molecular substrate for convergent effects of multiple modulatory mechanisms. By virtue of their pH-sensitivity and cell-type expression in respiratory-related neurons, TASK channels may contribute to integrated central respiratory responses to alterations in brain acid-base status; this could involve effects on chemoreceptor neurons in LC and raphe – and/or directly on respiratory motoneurons. Inhibition of TASK channels by transmitters associated with behavioral arousal may provide an excitatory bias to motoneurons, and support well known state-dependent differences in motor activity; TASK channel activation in motoneurons and aminergic brainstem neurons likely contributes to immobilizing and hypnotic anesthetic effects.