HCN channels contribute to serotonergic modulation of ventral surface chemosensitive neurons and respiratory activity

Abstract

Chemosensitive neurons in the retrotrapezoid nucleus (RTN) provide a CO2/H(+)-dependent drive to breathe and function as an integration center for the respiratory network, including serotonergic raphe neurons. We recently showed that serotonergic modulation of RTN chemoreceptors involved inhibition of KCNQ channels and activation of an unknown inward current. Hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels are the molecular correlate of the hyperpolarization-activated inward current (Ih) and have a high propensity for modulation by serotonin. To investigate whether HCN channels contribute to basal activity and serotonergic modulation of RTN chemoreceptors, we characterize resting activity and the effects of serotonin on RTN chemoreceptors in vitro and on respiratory activity of anesthetized rats in the presence or absence of blockers of KCNQ (XE991) and/or HCN (ZD7288, Cs(+)) channels. We found in vivo that bilateral RTN injections of ZD7288 increased respiratory activity and in vitro HCN channel blockade increased activity of RTN chemoreceptors under control conditions, but this was blunted by KCNQ channel inhibition. Furthermore, in vivo unilateral RTN injection of XE991 plus ZD7288 eliminated the serotonin response, and in vitro serotonin sensitivity was eliminated by application of XE991 and ZD7288 or SQ22536 (adenylate cyclase blocker). Serotonin-mediated activation of RTN chemoreceptors was blocked by a 5-HT7-receptor blocker and mimicked by a 5-HT7-receptor agonist. In addition, serotonin caused a depolarizing shift in the voltage-dependent activation of Ih. These results suggest that HCN channels contribute to resting chemoreceptor activity and that serotonin activates RTN chemoreceptors and breathing in part by a 5-HT7 receptor-dependent mechanism and downstream activation of Ih.

Activation state of the hyperpolarization-activated current modulates temperature-sensitivity of firing in locus coeruleus neurons from bullfrogs

“…5, E and F). This result implies that 1) Cs ϩ is reasonably selective antagonist of I h in LC neurons, and 2) similar to other studies (17,36), relatively small increases in I h at the base of its activation curve can influence the firing frequency, presumably through membrane depolarization (Figs. 5, D-F).…”

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).…”

“…(B) pH sensitivity of RTN neurons in vivo ( Hz/ pH) is unchanged by bath application of 5 μM 5-HT (raw data for one neuron at left) (from (Mulkey et al, 2007a)). (C) Blocking K V 7 channels (with 10 μM XE991) and HCN channels (with 50 μM ZD7288) essentially eliminated 5-HT effects on firing rate in a CO 2 -sensitive rat RTN neuron in vitro ; arrows indicate current injection to reset baseline firing (from (Hawkins et al, 2015)). (D) Serotonergic neurons of egr-2 lineage innervate most lower brainstem and spinal cord regions implicated in respiratory control (C1, C1 adrenergic neurons; DRN, dorsal raphe; MRN, median raphe; LC, locus coeruleus; DH, dorsal horn; see Figure 1 for other abbreviations) (redrawn from (Brust et al, 2014)).…”

Activation of 5-hyrdoxytryptamine 7 receptors within the rat nucleus tractus solitarii modulates synaptic properties

“…LP 44 has greater affinity and specificity for 5-HT 7 R than AS 19, therefore LP 44 was used for the majority of experiments (Brenchat et al, 2010, 2009; Leopoldo et al, 2011). Concentrations of LP 44 were based on literature and preliminary experiments (Costa et al, 2012; Hawkins et al, 2015). Blockade of 5-HT 7 R was accomplished by use of the selective antagonist SB 269970 [1 μM (Thomas et al, 2000)], whereas 5-HT 1A Rs were blocked by the antagonist WAY 100135 (10 μM, Ostrowski et al, 2014).…”