Inhibition of the potassium current IK(SO), in cerebellar granule cells, by the inhibitors of MEK1 activation, PD 98059 and U 0126

Prokineticin 2 influences subfornical organ neurons through regulation of MAP kinase and the modulation of sodium channels

Prokineticin 2 (PK2) is a neuropeptide that acts as a signaling molecule regulating circadian rhythms in mammals. We have previously reported PK2 actions on subfornical organ (SFO) neurons, identifying this circumventricular organ as a target at which PK2 acts to influence autonomic control (Cottrell GT, and Ferguson AV. J. Neurosci. 24: 2375–2379, 2004). In this study, we have examined the cellular mechanisms by which PK2 increases the excitability of SFO neurons. Whole cell patch recordings from dissociated rat SFO neurons demonstrated that the mitogen-activated protein (MAP) kinase inhibitor PD-98059 prevented PK2-induced depolarization and decreases in delayed rectifier K+ current. PK2 also increased intracellular Ca2+ concentration ([Ca2+]i) in 39% of dissociated SFO neurons (mean increase = 20.8 ± 5.5%), effects that were maintained in the presence of thapsigargin but abolished by both nifedipine, or the absence of extracellular Ca2+, suggesting that PK2-induced [Ca2+]i transients resulted from Ca2+ entry through voltage-gated Ca2+ channels. Voltage-clamp recordings showed that PK2 was without effects on Ca2+ currents evoked by voltage ramps, suggesting that PK2-induced Ca2+ influx was secondary to PK2-induced increases in action potential frequency, an hypothesis supported by data showing that tetrodotoxin abolished effects of PK2 on [Ca2+]i. These observations suggested PK2 modulation of voltage-gated Na+ currents, a possibility confirmed by voltage-clamp experiments showing that PK2 increased the amplitude of both transient and persistent Na+ currents in 29% of SFO neurons (by 34 and 38%, respectively). These data indicate that PK2 influences SFO neurons through the activation of a MAP kinase cascade, which, in turn, modulates Na+ and K+ conductances.

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.

What are the roles of the many different types of potassium channel expressed in cerebellar granule cells?

Potassium (K) channels have a key role in the regulation of neuronal excitability. Over a hundred different subunits encoding distinct K channel subtypes have been identified so far. A major challenge is to relate these many different channel subunits to the functional K currents observed in native neurons. In this review, we have concentrated on cerebellar granule neurons (CGNs). We have considered each of the three principal super families of K channels in turn, namely, the six transmembrane domain, voltage-gated super family, the two transmembrane domain, inward-rectifier super family and the four transmembrane domain, leak channel super family. For each super family, we have identified the subunits that are expressed in CGNs and related the properties of these expressed channel subunits to the functional currents seen in electrophysiological recordings from these neurons. In some cases, there are strong molecular candidates for proteins underlying observed currents. In other cases the correlation is less clear. We show that at least 26 potassium channel alpha subunits are moderately or strongly expressed in CGNs. Nevertheless, a good empirical model of CGN function has been obtained with just six distinct K conductances. The transient KA current in CGNs, seems due to expression of Kv4.2 channels or Kv4.2/4.3 heteromers, while the KCa current is due to expression of large-conductance slo channels. The G-protein activated KIR current is probably due to heteromeric expression of KIR3.1 and KIR3.2. Perhaps KIR2.2 subunits underlie the KIR current when it is constitutively active. The leak conductance can be attributed to TASK-1 and or TASK-3 channels. With less certainty, the IK-slow current may be due to expression of one or more members of the KCNQ or EAG family. Lastly, the delayed-rectifier Kv current has as many as six different potential contributors from the extensive Kv family of alpha subunits. Since many of these subunits are highly regulated by neurotransmitters, physiological regulators and, often, auxiliary subunits, the resulting electrical properties of CGNs may be highly dynamic and subject to constant fine-tuning.