Kohei Inamura

TRPA1 regulates gastrointestinal motility through serotonin release from enterochromaffin cells

Serotonin (5-hydroxytryptamine; 5-HT) is abundantly present throughout the gastrointestinal tract and stored mostly in enterochromaffin (EC) cells, which are located on the mucosal surface. 5-HT released from EC cells stimulate both intrinsic and extrinsic nerves, which results in various physiological and pathophysiological responses, such as gastrointestinal contractions. EC cells are believed to have the ability to respond to the chemical composition of the luminal contents of the gut; however, the underlying molecular and cellular mechanisms have not been identified. Here, we demonstrate that the transient receptor potential (TRP) cation channel TRPA1, which is activated by pungent compounds or cold temperature, is highly expressed in EC cells. We also found that TRPA1 agonists, including allyl isothiocyanate and cinnamaldehyde, stimulate EC cell functions, such as increasing intracellular Ca2+ levels and 5-HT release, by using highly concentrated EC cell fractions and a model of EC cell function, the RIN14B cell line. Furthermore, we showed that allyl isothiocyanate promotes the contraction of isolated guinea pig ileum via the 5-HT3 receptor. Taken together, our results indicate that TRPA1 acts as a sensor molecule for EC cells and may regulate gastrointestinal function.

Molecular cloning and characterization of Kv6.3, a novel modulatory subunit for voltage‐gated K+ channel Kv2.11

We report identification and characterization of Kv6.3, a novel member of the voltage‐gated K+ channel. Reverse transcriptase‐polymerase chain reaction analysis indicated that Kv6.3 was highly expressed in the brain. Electrophysiological studies indicated that homomultimeric Kv6.3 did not yield a functional voltage‐gated ion channel. When Kv6.3 and Kv2.1 were co‐expressed, the heteromultimeric channels displayed the decreased rate of deactivation compared to the homomultimeric Kv2.1 channels. Immunoprecipitation studies indicated that Kv6.3 bound with Kv2.1 in co‐transfected cells. These results indicate that Kv6.3 is a novel member of the voltage‐gated K+ channel which functions as a modulatory subunit.

Disruption of the Ether-à-go-go K+ Channel Gene BEC1/KCNH3 Enhances Cognitive Function

The K+ channel, one of the determinants for neuronal excitability, is genetically heterogeneous, and various K+ channel genes are expressed in the CNS. The therapeutic potential of K+ channel blockers for cognitive enhancement has been discussed, but the contribution each K+ channel gene makes to cognitive function remains obscure. BEC1 (KCNH3) is a member of the K+ channel superfamily that shows forebrain-preferential distribution. Here, we show the critical involvement of BEC1 in cognitive function. BEC1 knock-out mice performed behavioral tasks related to working memory, reference memory, and attention better than their wild-type littermates. Enhanced performance was also observed in heterozygous mutants. The knock-out mice had neither the seizures nor the motor dysfunction that are often observed in K+ channel-deficient mice. In contrast to when it is disrupted, overexpression of BEC1 in the forebrain caused the impaired performance of those tasks. It was also found that altering BEC1 expression could change hippocampal neuronal excitability and synaptic plasticity. The results indicate that BEC1 may represent the first K+ channel that contributes preferentially and bidirectionally to cognitive function.

Neurochemical and neuropharmacological characterization of ASP2905, a novel potent selective inhibitor of the potassium channel KCNH3

ABSTRACT KCNH3 (BEC1) is a member of the ether‐à‐go‐go (KCNH) family of voltage‐gated K+ channels. The aim of this study was to determine the pharmacological profiles in vitro and in vivo of a KCNH3 inhibitor N‐(4‐fluorophenyl)‐N′‐phenyl‐N′′‐(pyrimidin‐2‐ylmethyl)‐1,3,5‐triazine‐2,4,6‐triamine (ASP2905). We analyzed the effects of ASP2905 on channel activity in vitro and its neuropharmacological properties in young and aged rats as well as in mice. ASP2905 potently inhibited potassium currents in CHO cells expressing KCNH3 (IC50 = 9.0 nM). In contrast, ASP2905 (≤ 10 &mgr;M) minimally bound with low affinities to 55 transmembrane proteins. ASP2905 (0.1 &mgr;M, 1 &mgr;M) decreased the frequency of spontaneous inhibitory postsynaptic currents in cultured rat hippocampal neurons. In mice, ASP2905 reversed the disruption of spontaneous alternation behavior induced by MK‐801 and scopolamine (minimum effective dose of ASP2905: 0.0625 mg/kg, po). ASP2905 ameliorated the cognitive deficits of aged rats in step‐through passive avoidance (0.0313 and 0.0625 mg/kg, po) and Morris water‐maze tasks (0.01 mg/kg, po) and effectively penetrated the brain. The mean plasma and brain concentrations of ASP2905 reached their maxima (Cmax = 0.399 ng/ml and 1.77 ng/g, respectively) 1 h after a single oral administration and then decreased (t1/2 = 1.5–1.6 h) (brain plasma ratio = 2.7–4.9). The present study suggests that ASP2905 is a selective, orally administered inhibitor of KCNH3, which can enhance cognitive performance.