KCNQ Channels Determine Serotonergic Modulation of Ventral Surface Chemoreceptors and Respiratory Drive

Abstract

Chemosensitive neurons in the retrotrapezoid nucleus (RTN) regulate breathing in response to CO2/H+ changes. Their activity is also sensitive to neuromodulatory inputs from multiple respiratory centers, and thus they serve as a key nexus of respiratory control. However, molecular mechanisms that control their activity and susceptibility to neuromodulation are unknown. Here, we show in vitro and in vivo that KCNQ channels are critical determinants of RTN neural activity. In particular, we find that pharmacological block of KCNQ channels (XE991, 10 μM) increased basal activity and CO2-responsivness of RTN neurons in rat brain slices; whereas KCNQ channel activation (retigabine 2–40 μM) silenced these neurons. Interestingly, we also find that KCNQ and apamin sensitive SK channels act synergistically to regulate firing rate of RTN chemoreceptors; simultaneous blockade of both channels led to a increase in CO2-responsivness. Furthermore, we also show that KCNQ channels but not SK channels are downstream effectors of serotonin modulation of RTN activity in vitro. In contrast, inhibition of KCNQ channel did not prevent modulation of RTN activity by Substance P or TRH; previously identified neuromodulators of RTN chemoreception. Importantly, we also show that KCNQ channels are critical for RTN activity in vivo. Inhibition of KCNQ channels lowered the CO2 threshold for phrenic nerve discharge in anesthetized rats and decreased the ventilatory response to serotonin in awake and anesthetized animals. Given that serotonergic dysfunction may contribute to respiratory failure, our findings suggest KCNQ channels as a new therapeutic avenue for respiratory complications associated with multiple neurological disorders.

Neural Control of Breathing and CO2 Homeostasis

“…Hypercapnia enhances the respiratory activation induced by optogenetic stimulation of raphe obscurus neurons, even though that particular group of serotonergic neurons is not CO 2 -responsive (Brust et al, 2014; Depuy et al, 2011). Serotonin excites RTN neurons, in part, via 5-HT 2 -mediated inhibition of a K V 7 channel current and 5-HT 7 -mediated activation of HCN channels (Figures 2A, 3A–C) (Hawkins et al, 2015; Hawryluk et al, 2012), even though the pH-sensitivity of RTN neurons is unchanged by serotonin (Figure 3A,B) or by pharmacologically modulating K V 7 and/or HCN channels (Hawryluk et al, 2012; Mulkey et al, 2007a). However, both 5-HT exposure and direct K V 7/HCN channel modulation in the RTN can shift the CO 2 threshold for respiration and enhance respiratory output at physiological pH or PCO 2 levels (Hawryluk et al, 2012; Mulkey et al, 2007a).…”

“…Serotonin excites RTN neurons, in part, via 5-HT 2 -mediated inhibition of a K V 7 channel current and 5-HT 7 -mediated activation of HCN channels (Figures 2A, 3A–C) (Hawkins et al, 2015; Hawryluk et al, 2012), even though the pH-sensitivity of RTN neurons is unchanged by serotonin (Figure 3A,B) or by pharmacologically modulating K V 7 and/or HCN channels (Hawryluk et al, 2012; Mulkey et al, 2007a). However, both 5-HT exposure and direct K V 7/HCN channel modulation in the RTN can shift the CO 2 threshold for respiration and enhance respiratory output at physiological pH or PCO 2 levels (Hawryluk et al, 2012; Mulkey et al, 2007a). Thus, changes in RTN neuron excitability elicited by serotonin, independent of pH sensing per se , can be manifest as altered threshold or gain of the ventilatory response to CO 2 ; this may also be true for other neurons that contribute, directly or indirectly, to the respiratory chemoreflex.…”

“…These differences may be technical and trivial (e.g., for slices or isolated cells: low temperature recordings, tissue immaturity, neuronal damage; for unit recordings in vivo : effects of anesthesia), but they might also reflect other important mechanisms at play in vivo . For example, the RTN response to CO 2 may be facilitated in vivo by neuromodulators (Hawryluk et al, 2012) or inputs from additional CO 2 -activated CNS neurons. These quantitative differences may also reflect the role of astrocytes (Figure 4) (Erlichman and Leiter, 2010; Gourine et al, 2010; Wenker et al, 2010).…”

Regulation of Breathing and Autonomic Outflows by Chemoreceptors

“…Also, CO 2 chemosensitivity is restored to normal in a mouse model of Rhett syndrome by the simple addition of citalopram, a serotonin-selective reuptake blocker that also causes a brain-wide increase in serotonin concentration (436). Finally, serotonin depolarizes RTN neurons, an action that is likely to enhance the HCVR (170, 299) (Fig. 6H).…”

Sudden unexpected death in epilepsy: Fatal post-ictal respiratory and arousal mechanisms

“…However, it is known that these genes are also expressed in the brain, and it is possible they play a role in control of breathing. Recently, KCNQ channels were reported to play a central role in the detection of CO 2 in the NTS (Hawryluk et al, 2012). Many other LQTS genes that cause cardiac dysfunction are also expressed at high levels in the brain and are present in respiratory nuclei.…”