Hyperpolarization-activated inward currents contribute to spontaneous electrical activity and CO2/H+ sensitivity of cultivated neurons of fetal rat medulla

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

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.

Two-pore domain k(+) channels and their role in chemoreception

A number of tandem P-domain K(+)- channels (K(2)P) generate background K(+)-currents similar to those found in enteroreceptors that sense a diverse range of physiological stimuli including blood pH, carbon dioxide, oxygen, potassium and glucose. This review presents an overview of the properties of both cloned K(2)P tandem-P-domain K-channels and the endogenous chemosensitive background K-currents found in central chemoreceptors, peripheral chemoreceptors, the adrenal gland and the hypothalamus. Although the identity of many of these endogenous channels has yet to be confirmed they show striking similarities to a number of K(2)P channels especially those of the TASK subgroup. Moreover these channels seem often (albeit not exclusively) to be involved in pH and nutrient/metabolic sensing.

Carbon dioxide and pH effects on temperature-sensitive and -insensitive hypothalamic neurons

The preoptic-anterior hypothalamus (POAH) controls body temperature, and thermoregulatory responses are impaired during hypercapnia. If increased CO2 or its accompanying acidosis inhibits warm-sensitive POAH neurons, this could provide an explanation for thermoregulatory impairment during hypercapnia. To test this possibility, extracellular electrophysiological recordings determined the effects of CO2 and pH on the firing rates of both temperature-sensitive and -insensitive neurons in hypothalamic tissue slices from 89 male Sprague-Dawley rats. Firing rate activity was recorded in 121 hypothalamic neurons before, during, and after changing the CO2 concentration aerating the tissue slice chamber or changing the pH of the solution bathing the tissue slices. Increasing the aeration CO2 concentration from 5% (control) to 10% (hypercapnic) had no effect on most (i.e., 69%) POAH temperature-insensitive neurons; however, this hypercapnia inhibited the majority (i.e., 59%) of warm-sensitive neurons. CO2 affected similar proportions of (non-POAH) neurons in other hypothalamic regions. These CO2 effects appear to be due to changes in pH since the CO2-affected neurons responded similarly to isocapnic acidosis (i.e., normal CO2 and decreased pH) but were not responsive to isohydric hypercapnia (i.e., increased CO2 and normal pH). These findings may offer a neural explanation for some heat-related illnesses (e.g., exertional heat stroke) where impaired heat loss is associated with acidosis.

Functional significance of HCN2/3-mediated I(h) in striatal cells at early developmental stages

Hyperpolarization-activated cAMP-gated cation currents (I(h)) were recently linked to pre- and postnatal developmental processes in several brain regions, including the ventral telencephalon. To evaluate the role of I(h) in striatal development, we used short-term cultured cells from the lateral ganglionic eminence at embryonic day 14 (E14) and postnatal days 1-3 (P1-3) as well as the embryonic striatal progenitor cell line ST14A. Western blot analysis of the I(h) underlying subunit proteins HCN1-4 revealed strong HCN2 expression in proliferating ST14A cells and weak expression in postmitotic ST14A cells and in cells from the developing brain. We also found HCN3 expression only in ST14A cells at both proliferative and nonproliferative stages but not in short-term cultured striatal cells. In all cases, HCN1 and HCN4 transcripts were below the detection level. Despite the selective protein expression, RT-PCR analysis showed stable expression of HCN2-4 but not HCN1 mRNA in all short-term-cultured striatal cells and in the ST14A cell line. Consistent with the strong protein expression, an I(h) was recorded with features of an HCN2-mediated current in ST14A cells at the proliferative stage and in short-term-cultured E14 cells. Of particular importance is that we detected no currents upon hyperpolarization in the ST14A cells at the nonproliferative stage when only HCN3 protein was present. These results suggest the potential importance of ST14A cells in defining the molecular mechanisms regulating I(h) expression and function.

Characterization and function of TWIK-related acid sensing K+ channels in a rat nociceptive cell

We examined the properties of a proton sensitive current in acutely dissociated, capsaicin insensitive nociceptive neurons from rat dorsal root ganglion (DRG). The current had features consistent with K(+) leak currents of the KCNK family (TASK-1, TASK-3; TWIK-related acid sensing K(+)). Acidity and alkalinity induced inward and outward shifts in the holding current accompanied by increased and decreased whole cell resistance consistent with a K(+) current. We used alkaline solutions to open the channel and examine its properties. Alkaline evoked currents (AECs; pH 10.0-10.75), reversed near the K(+) equilibrium potential (-74 mV), and were suppressed 85% in 0 mM K(+). AECs were insensitive to Cs(+) (1 mM) and anandamide (1 microM), but blocked by Ba(++) (1 mM), quinidine (100 microM) or Ruthenium Red (10 microM). This pharmacology was identical to that of rat TASK-3 and inconsistent with that of TASK-1 or TASK-2. The TASK-like AEC was not modulated by PKA (forskolin, kappa opioid agonists U69593 and GR8696, somatostatin) but was inhibited by PKC activator phorbol-12-myristate-13 acetate (PMA). When acidic solutions were used, we were able to isolate a Ba(++) and Ruthenium Red insensitive current that was inhibited by Zn(++). This Zn(++) sensitive component of the proton sensitive current was consistent with TASK-1. In current clamp studies, acidic pH produced sensitive changes in resting membrane potential but did not influence excitability (pH 7.2-6.8). In contrast, Zn(++) produced substantial changes in excitability at physiological pH. Alkaline solutions produced hyperpolarization followed by proportional burst discharges (pH 10.75-11.5) and increased excitability (at pH 7.4). In conclusion, multiple TASK currents were present in a DRG nociceptor and differentially contributed to distinct discharge mechanisms.

Hyperpolarization-activated (Ih) conductances affect brainstem auditory neuron excitability

Blockade of the hyperpolarization-activated cyclic-nucleotide-gated mixed-cationic conductance (I(h)) by ZD7288 markedly reduces excitability of neurons in the superior olivary complex (SOC), in vivo. Following pressure ejection application of 100 microM ZD7288, extracellular recorded single unit responses of 47/47 SOC neurons to monaural or binaural pure tone best frequency (BF) stimuli (30 dB above threshold) decreased by 49.7+/-19%, and background activity decreased by 56.3+/-18.1%. Pressure ejection of the vehicle did not affect excitability. The dose- and time-dependence of ZD7288 (10-100 microM) effects are consistent with specific blockade of I(h) currents. SOC neuron responses to pressure-ejected glutamate were also decreased following application of 100 microM ZD7288 by 76.7+/-28.0%, which suggests a predominant direct effect of ZD7288 on auditory cell excitability. The considerable variability in the magnitude of ZD7288 effects between cells was only partially accounted for by greater effects on neurons with BFs greater than 16 kHz. Therefore, I(h) channels significantly contribute to auditory brainstem neuron excitability, affecting their response level to acoustic stimuli. The variability in the ZD7288 reduction in excitability and its variation with the BF of units could be an indication of regulation and plasticity in neuronal encoding of sounds.

Maintenance of sympathetic tone by a nickel chloride-sensitive mechanism in the rostral ventrolateral medulla of the adult rat

In urethane-anaesthetised artificially ventilated Sprague-Dawley rats, bilateral microinjection of the divalent cation nickel chloride (Ni(2+); 50 mM, 50 nl) into the rostral ventrolateral medulla elicited a dramatic inhibition of splanchnic sympathetic nerve activity (-44+/-6%) and a marked depressor response (-35+/-7 mmHg). Selective blockade of high-voltage activated Ca(2+) channels with omega-agatoxin IVA (P/Q-type), omega-conotoxin GVIA (N-type) and nifedipine (L-type) did not decrease arterial pressure or splanchnic sympathetic nerve activity when injected separately into the rostral ventrolateral medulla, or combined with kynurenate. Injection of caesium chloride or ZD 7288, a blocker of the hyperpolarization-activated cation current, into the rostral ventrolateral medulla had no effect on arterial pressure or splanchnic sympathetic nerve activity. Bilateral microinjection of nickel chloride into the caudal ventrolateral medulla/pre-Bötzinger complex elicited small increases in splanchnic sympathetic nerve activity (+17+/-13%) and arterial pressure (+12+/-4 mmHg). These were substantially smaller than those evoked by blockade of glutamatergic receptors or high-voltage activated Ca(2+) channels in this area. Injection of kynurenate or high-voltage activated Ca(2+) channel blocker, but not Ni(2+), in this area evoked respiratory termination. The results indicate the existence of a distinct mechanism maintaining the tonic activity of rostral ventrolateral medulla presympathetic neurons that is different from that maintaining the tonic activity in the caudal ventrolateral medulla/pre-Bötzinger region. We conclude that ion channels that are sensitive to Ni(2+), but are insensitive to high-voltage activated (L, P/Q, N) Ca(2+) channel blockers, and are located postsynaptically on the presympathetic rostral ventrolateral medulla neurons are responsible for the tonic activity of the presympathetic neurons in rostral ventrolateral medulla. These channels could well be the low-voltage-activated (or T-type) Ca(2+) channels although other conductances cannot be conclusively excluded.

Avian intrapulmonary chemoreceptor discharge rate is increased by anion exchange blocker 'DIDS'

Avian intrapulmonary chemoreceptors (IPC) are neurons that sense lung P(CO(2)) and provide phasic feedback for the control of breathing in birds. To try to understand mechanisms of CO(2) transduction and intracellular pH regulation in IPC, the anion exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) was used to block transmembrane Cl(-)/HCO(3)(-) transport. Single-unit IPC discharge rates were measured at steady intrapulmonary CO(2) levels and during step changes in CO(2) in 15 anesthetized, unidirectionally ventilated adult mallard ducks (Anas platyrhynchos). Measurements were repeated after giving 50, 100 and 200 micromol/kg cumulative i.v. dosages of DIDS. Mean IPC discharge rates at steady (tonic) P(CO(2)) levels were significantly increased by 100 and 200 micromol/kg DIDS, but not by 50 micromol/kg DIDS. Mean dynamic (phasic) IPC responses to CO(2) steps were not significantly affected by DIDS. Results indicate that the DIDS-sensitive Cl(-)/HCO(3)(-) membrane exchanger is involved with tonic CO(2) signal transduction in IPC. However, because some individual IPC were unaffected by DIDS, yet still altered their discharge rate with CO(2), additional mechanisms besides the Cl(-)/HCO(3)(-) exchange are probably required for CO(2) chemotransduction in IPC.

A bradycardiac agent ZD7288 blocks the hyperpolarization-activated current (I(h)) in retinal rod photoreceptors

Recently it has been reported that "I(f) channel blockers", which block the hyperpolarization-activated inward current (I(f)) in heart sino atrial node cells, also block the hyperpolarization-activated inward current (I(h)) in other tissues. Here we compared the effects of one of these agents, ZD7288 [4-(N-ethyl-N-phenylamino)-1, 2-dimethyl-6-(methylamino) pyrimidinium chloride], with those of Cs(+) on I(h) in amphibian rod photoreceptors using patch clamp and intracellular recordings. ZD7288 strongly inhibited I(h) in newt rod photoreceptors in a concentration-dependent manner (1-100 microM). ZD7288 exerted a blocking action on the conductance of I(h) with no alteration of its gating properties, and the blocking action of I(h) was not use-dependent. At concentrations as low as 1 microM, ZD7288 markedly enhanced the hyperpolarizing membrane responses of frog rod photoreceptors to bright light and delayed the response recovery, indicating that ZD7288 is highly selective for I(h). The apparent effect of the drug was slow in onset and irreversible, suggesting that ZD7288 act at a cytosolic site on the I(h) channel. These observations also confirm the involvement of I(h) in accelerating the response recovery process from deep membrane hyperpolarization induced by bright light in rod cells.

CO(2) inhibits specific inward rectifier K(+) channels by decreases in intra- and extracellular pH

Hypercapnia has been shown to affect cellular excitability by modulating K(+) channels. To understand the mechanisms for this modulation, four cloned K(+) channels were studied by expressing them in Xenopus oocytes. Exposures of the oocytes to CO(2) for 4-6 min produced reversible and concentration-dependent inhibitions of Kir1.1 and Kir2.3 currents, but had no effect on Kir2.1 and Kir6.1 currents. Intra- and extracellular pH (pH(i), pH(o)) dropped during CO(2) exposures. The inhibition of Kir2.3 currents was mediated by reductions in both intra- and extracellular pH, whereas the suppression of Kir1.1 resulted from intracellular acidification. In cell-free excised inside-out patches with cytosolic-soluble factors washed out, a decrease in pH(i) produced a fast and reversible inhibition of macroscopic Kir2.3 currents. The degree of this inhibition was similar to that produced by hypercapnia when compared at the same pH(i) level. Exposure of cytosolic surface of patch membranes to a perfusate bubbled with 15% CO(2) without changing pH failed to inhibit the Kir2.3 currents. These results therefore indicate that (1) hypercapnia inhibits specific K(+) channels, (2) these inhibitions are caused by intra- and extracellular protons rather than molecular CO(2), and (3) these effects are independent of cytosol-soluble factors.

Hyperpolarization-activated current, Ih, in inspiratory brainstem neurons and its inhibition by hypoxia

A hyperpolarization-activated current, Ih, is often implied in pacemaker-like depolarizations during rhythmic oscillatory activity. We describe Ih in the isolated respiratory centre of immature mice (P6-P11). Ih was recorded in 15% (22/146) of all inspiratory neurons examined. The mean half-maximal Ih activation occurred at -78 mV and the reversal potential was -40 mV. Ih was inhibited by Cs+ (1-5 mM) and by organic blockers N-ethyl-1,6-dihydro-1, 2-dimethyl-6-(methylimino)-N-phenyl-4-pyrimidinamine (ZD 7288; 0.3-3 microM) and N,N'-bis-(3,4-dimethylphenylethyl)-N-methylamine (YS 035, 3-30 microM), but not by Ba2+ (0.5 mM). The organic Ih blockers did not change the inspiratory bursts recorded from the XIIth nerve and synaptic drives in inspiratory neurons. Hypoxia reversibly inhibited Ih but, in the presence of organic blockers, the hypoxic reaction remained unchanged. We conclude that although Ih channels are functional in a minority of inspiratory neurons, Ih does not contribute to respiratory rhythm generation or its modulation by hypoxia.

Modulation of the hyperpolarization-activated cation current of rat thalamic relay neurones by intracellular pH

1. Properties of the hyperpolarization-activated cation current (Ih) were investigated in thalamocortical neurones of an in vitro slice preparation of the rat ventrobasal thalamic complex (VB) before and during changes of pipette pH (pHp), intracellular pH (pHi) and bath pH (pHb) using the whole-cell patch-clamp technique and fluorescence ratio imaging of the pH indicator 2',7'-bis(carboxyethyl)-5(and -6)-carboxyfluorescein (BCECF). 2. Recording of Ih with predefined pHp revealed significant shifts in the voltage dependence of Ih activation (V ) of 4-5 mV to more positive values for a pHp of 7.5 and 2-3 mV to more negative values for a pHp of 6.7 as compared to control values (pHp = 7.1). 3. Application of the weak acid lactate (20 mM), which produced a slow monophasic intracellular acidification, induced a reversible negative shift of V of up to 3 mV. Application of 20 mM TMA, which caused a distinct intracellular alkalinization, shifted V to 4-5 mV more positive values. 4. In slices bathed in Hepes-buffered saline, no significant pHo dependence of Ih was observed. Changing pHo by altering the extracellular [HCO3-] in the presence of constant pCO2 also revealed no significant pHo dependence of Ih. 5. Rhythmic stimulation of thalamocortical neurones with repetitive depolarizing pulse trains caused an intracellular acidification, which reversibly decreased the amplitude and time course of activation of Ih. 6. The results of the present study indicate that shifts in pHi result in a significant modulation of the gating properties of Ih channels in TC neurones. Through this mechanism activity-dependent shifts in pHi may contribute to the up- and downregulation of Ih.

Identification of a critical motif responsible for gating of Kir2.3 channel by intracellular protons

Protons are involved in gating Kir2.3. To identify the molecular motif in the Kir2.3 channel protein that is responsible for this process, experiments were performed using wild-type and mutated Kir2. 3 and Kir2.1. CO2 and low pHi strongly inhibited wild-type Kir2.3 but not Kir2.1 in whole cell voltage clamp and excised inside-out patches. This CO2/pH sensitivity was completely eliminated in a mutant Kir2.3 in which the N terminus was substituted with that in Kir2.1, whereas a similar replacement of its C terminus had no effect. Site-specific mutations of all titratable residues in the N terminus, however, did not change the CO2/pH sensitivity. Using several chimeras generated systematically in the N terminus, a 10-residue motif near the M1 region was identified in which only three amino acids are different between Kir2.3 and Kir2.1. Mutations of these residues, especially Thr53, dramatically reduced the pH sensitivity of Kir2.3. Introducing these residues or even a single threonine to the corresponding positions of Kir2.1 made the mutant channel pH-sensitive. Thus, a critical motif responsible for gating Kir2.3 by protons was identified in the N terminus, which contained about 10 residues centered by Thr53.