Transient outwardly rectifying A currents are involved in the firing rate response to altered CO₂ in chemosensitive locus coeruleus neurons from neonatal rats

Abstract

The effect of hypercapnia on outwardly rectifying currents was examined in locus coeruleus (LC) neurons in slices from neonatal rats [postnatal day 3 (P3)-P15]. Two outwardly rectifying currents [4-aminopyridine (4-AP)-sensitive transient current and tetraethyl ammonium (TEA)-sensitive sustained current] were found in LC neurons. 4-AP induced a membrane depolarization of 3.6 ± 0.6 mV (n = 4), while TEA induced a smaller membrane depolarization of 1.2 ± 0.3 mV (n = 4). Hypercapnic acidosis (HA) inhibited both currents. The maximal amplitude of the TEA-sensitive current was reduced by 52.1 ± 4.5% (n = 5) in 15% CO2 [extracellular pH (pHo) 7.00, intracellular pH (pHi) 6.96]. The maximal amplitude of the 4-AP-sensitive current was reduced by 34.5 ± 3.0% (n = 6) in 15% CO2 (pHo 7.00, pHi 6.96), by 29.4 ± 6.8% (n = 6) in 10% CO2 (pHo 7.15, pHi 7.14), and increased by 29.0 ± 6.4% (n = 6) in 2.5% CO2 (pHo 7.75, pHi 7.35). 4-AP completely blocked hypercapnia-induced increased firing rate, but TEA did not affect it. When LC neurons were exposed to HA with either pHo or pHi constant, the 4-AP-sensitive current was inhibited. The data show that the 4-AP-sensitive current (likely an A current) is inhibited by decreases in either pHo or pHi. The change of the A current by various levels of CO2 is correlated with the change in firing rate induced by CO2, implicating the 4-AP-sensitive current in chemosensitive signaling in LC neurons.

Microglial Acid Sensing Regulates Carbon Dioxide-Evoked Fear

“…Coronal SFO slices (300 μm) were used for whole-cell patch clamp recordings of CO 2 -evoked neuronal firing as described (31). The chemosensitive response of a neuron was determined by measuring the change in firing rate in response to a hypercapnic acidotic solution of aCSF equilibrated with either 7.…”

Anatomical and functional connections between the locus coeruleus and the nucleus tractus solitarius in neonatal rats

“…We have previously shown with cNTS neurons that about 40–50% of cNTS neurons from neonatal rats will not change their firing rate in response to this level of hypercapnia (non-chemosensitive neurons) and about 40–50% will increase their firing rate in response to hypercapnia (chemosensitive neurons), while the firing rate in the remaining neurons is reduced upon exposure to 15% CO 2 (Conrad et al , 2009). Similarly, 15% CO 2 results in an increased firing rate in 70–80% of LC neurons from neonatal rats with the remainder not responding to hypercapnia (Li and Putnam, 2013). Both cNTS and LC neurons also respond with increased firing rate to hypercapnia of 10% and 7–8% (Dean et al , 1990; Oyamada et al ., 1998; Li and Putnam, 2013) and LC neurons respond with a decreased firing rate in response to hypocapnia (2.5% CO 2 ) (Li and Putnam, 2013).…”

“…Similarly, 15% CO 2 results in an increased firing rate in 70–80% of LC neurons from neonatal rats with the remainder not responding to hypercapnia (Li and Putnam, 2013). Both cNTS and LC neurons also respond with increased firing rate to hypercapnia of 10% and 7–8% (Dean et al , 1990; Oyamada et al ., 1998; Li and Putnam, 2013) and LC neurons respond with a decreased firing rate in response to hypocapnia (2.5% CO 2 ) (Li and Putnam, 2013). Thus the electrophysiology experiments as designed here allow for the differentiation of whether a neuron is chemosensitive or not.…”

A HCO3−-dependent mechanism involving soluble adenylyl cyclase for the activation of Ca2+ currents in locus coeruleus neurons

“…In LC neurons, it is likely that sAC is activated by the increase in intracellular HCO 3 − induced by hypercapnia. Most studies of the cellular signaling pathways in chemosensitive neurons emphasize changes in pH and the role of pH-sensitive ion channels (Pineda & Aghajanian, 1997; Xu et al, 2000; Bayliss et al, 2001; Wiemann & Bingmann, 2001; Bradley et al, 2002; Filosa et al, 2002; Putnam et al, 2004; Cui et al, 2011; Li and Putnam, 2013). Our findings suggest that changes of intracellular HCO 3 − are an additional important signal associated with the chemosensitive response to hypercapnia.…”

“…Numerous other studies have focused on the ability of changes of intracellular or extracellular pH during hypercapnia to alter the activity of ion channels (Putnam et al, 2004). Chemosensitive LC neurons have been shown to contain a variety of pH-sensitive channels, including inward rectifying K + channels (Pineda & Aghajanian, 1997), transient A currents and delayed rectifying K + currents (Li & Putnam, 2013), TASK channels (Bayliss et al, 2001) and TRP channels (Cui et al, 2011). Acidification alters these channels in such a way that LC neurons depolarize and increase their firing rate.…”