Physical and Functional Interaction between 5-HT₆ Receptor and Nova-1

5-HT₆ receptor (5-HT₆R) is implicated in cognitive dysfunction, mood disorder, psychosis, and eating disorders. However, despite its significant role in regulating the brain functions, regulation of 5-HT₆R at the molecular level is poorly understood. Here, using yeast two-hybrid assay, we found that human 5-HT₆R directly binds to neuro-oncological ventral antigen 1 (Nova-1), a brain-enriched splicing regulator. The interaction between 5-HT₆R and Nova-1 was confirmed using GST pull-down and co-immunoprecipitation assays in cell lines and rat brain. The splicing activity of Nova-1 was decreased upon overexpression of 5-HT₆R, which was examined by detecting the spliced intermediates of gonadotropin-releasing hormone (GnRH), a known pre-mRNA target of Nova-1, using RT-PCR. In addition, overexpression of 5-HT₆R induced the translocation of Nova-1 from the nucleus to cytoplasm, resulting in the reduced splicing activity of Nova-1. In contrast, overexpression of Nova-1 reduced the activity and the total protein levels of 5-HT₆R. Taken together, these results indicate that when the expression levels of 5-HT₆R or Nova-1 protein are not properly regulated, it may also deteriorate the function of the other.

Differential Effects of Quercetin and Quercetin Glycosides on Human α7 Nicotinic Acetylcholine Receptor-Mediated Ion Currents

Quercetin is a flavonoid usually found in fruits and vegetables. Aside from its antioxidative effects, quercetin, like other flavonoids, has a various neuropharmacological actions. Quercetin-3-O-rhamnoside (Rham1), quercetin-3-O-rutinoside (Rutin), and quercetin-3-(2(G)-rhamnosylrutinoside (Rham2) are mono-, di-, and tri-glycosylated forms of quercetin, respectively. In a previous study, we showed that quercetin can enhance α7 nicotinic acetylcholine receptor (α7 nAChR)-mediated ion currents. However, the role of the carbohydrates attached to quercetin in the regulation of α7 nAChR channel activity has not been determined. In the present study, we investigated the effects of quercetin glycosides on the acetylcholine induced peak inward current (I(ACh)) in Xenopus oocytes expressing the α7 nAChR. I(ACh) was measured with a two-electrode voltage clamp technique. In oocytes injected with α7 nAChR copy RNA, quercetin enhanced I(ACh), whereas quercetin glycosides inhibited I(ACh). Quercetin glycosides mediated an inhibition of I(ACh), which increased when they were pre-applied and the inhibitory effects were concentration dependent. The order of I(ACh) inhibition by quercetin glycosides was Rutin≥Rham1>Rham2. Quercetin glycosides-mediated I(ACh) enhancement was not affected by ACh concentration and appeared voltage-independent. Furthermore, quercetin-mediated I(ACh) inhibition can be attenuated when quercetin is co-applied with Rham1 and Rutin, indicating that quercetin glycosides could interfere with quercetin-mediated α7 nAChR regulation and that the number of carbohydrates in the quercetin glycoside plays a key role in the interruption of quercetin action. These results show that quercetin and quercetin glycosides regulate the α7 nAChR in a differential manner.

Effect of Acute and Chronic Electroconvulsive Shock on 5-Hydroxytrypamine 6 Receptor Immunoreactivity in Rat Hippocampus

Electroconvulsive shock (ECS) induces not only an antidepressant effect but also adverse effects such as amnesia. One potential mechanism underlying both the antidepressant and amnesia effect of ECS may involve the regulation of serotonin (5-hydroxytryptamine) 6 (5-HT6) receptor, but less is known about the effects of acute ECS on the changes in 5-HT6 receptor expression in the hippocampus. In addition, as regulation of 5-HT receptor expression is influenced by the number of ECS treatment and by interval between ECS treatment and sacrifice, it is probable that magnitude and time-dependent changes in 5-HT6 receptor expression could be influenced by repeated ECS exposure. To explore this possibility, we observed and compared the changes of 5-HT6 receptor immunoreactivity (5-HT6 IR) in rat hippocampus at 1, 8, 24, or 72 h after the treatment with either a single ECS (acute ECS) or daily ECS for 10 days (chronic ECS). We found that acute ECS increased 5-HT6 IR in the CA1, CA3, and granule cell layer of hippocampus, reaching peak levels at 8 h and returning to basal levels 72 h later. The magnitude and time-dependent changes in 5-HT6 IR observed after acute ECS were not affected by chronic ECS. These results demonstrate that both acute and chronic ECS transiently increase the 5-HT6 IR in rat hippocampus, and suggest that the magnitude and time-dependent changes in 5-HT6 IR in the hippocampus appear not to be influenced by repeated ECS treatment.

Effects of Protopanaxatriol-Ginsenoside Metabolites on Rat N-Methyl-D-Aspartic Acid Receptor-Mediated Ion Currents

Ginsenosides are low molecular weight glycosides found in ginseng that exhibit neuroprotective effects through inhibition of N-methyl-D-aspartic acid (NMDA) receptor channel activity. Ginsenosides, like other natural compounds, are metabolized by gastric juices and intestinal microorganisms to produce ginsenoside metabolites. However, little is known about how ginsenoside metabolites regulate NMDA receptor channel activity. In the present study, we investigated the effects of ginsenoside metabolites, such as compound K (CK), protopanaxadiol (PPD), and protopanaxatriol (PPT), on oocytes that heterologously express the rat NMDA receptor. NMDA receptor-mediated ion current (INMDA) was measured using the 2-electrode voltage clamp technique. In oocytes injected with cRNAs encoding NMDA receptor subunits, PPT, but not CK or PPD, reversibly inhibited INMDA in a concentration-dependent manner. The IC50 for PPT on INMDA was 48.1+/-4.6 microM, was non-competitive with NMDA, and was independent of the membrane holding potential. These results demonstrate the possibility that PPT interacts with the NMDA receptor, although not at the NMDA binding site, and that the inhibitory effects of PPT on INMDA could be related to ginseng-mediated neuroprotection.

The Serotonin-6 Receptor as a Novel Therapeutic Target

Serotonin (5-hydroxytryptamine, 5-HT) is an important neurotransmitter that is found in both the central and peripheral nervous systems. 5-HT mediates its diverse physiological responses through 7 different 5-HT receptor families: 5-HT1, 5-HT2, 5-HT3, 5-HT4, 5-HT5, 5-HT6, and 5-HT7 receptors. Among them, the 5-HT6 receptor (5-HT6R) is the most recently cloned serotonin receptor and plays important roles in the central nervous system (CNS) and in the etiology of neurological diseases. Compared to other 5-HT receptors, the 5-HT6R has been considered as an attractive CNS therapeutic target because it is expressed exclusively in the CNS and has no known isoforms. This review evaluates in detail the role of the 5-HT6R in the physiology and pathophysiology of the CNS and the potential usefulness of 5-HT6R ligands in the development of therapeutic strategies for the treatment of CNS disorders. Preclinical studies provide support for the use of 5-HT6R ligands as promising medications to treat the cognitive dysfunction associated with Alzheimer's disease, obesity, depression, and anxiety.

An inhibitory compound against the interaction between Galpha(s) and the third intracellular loop region of serotonin receptor subtype 6 (5-HT(6)) disrupts the signaling pathway of 5-HT(6)

Serotonin receptor subtype 6 (5-HT(6)) is a neurotransmitter receptor, which is involved in various brain functions such as memory and mood. It mediates signaling via the interaction with a stimulatory G-protein. Especially, the third intracellular loop (iL3) of 5-HT(6) and the alpha subunit of stimulatory G protein (Galpha(s)) are responsible for the signaling process of 5-HT(6). Chemical compounds that could inhibit the interaction between the iL3 region of 5-HT(6) and Galpha(s) were screened from a chemical library consisted of 5,600 synthetic compounds. One of the identified compounds bound to Galpha(s) and effectively blocked the interaction between Galpha(s) and the iL3 region of 5-HT(6). The identified compound was further shown to reduce the serotonin-induced accumulation of cAMP in 293T cells transformed with 5-HT(6) cDNA. It also lowered the Ca2+ efflux induced by serotonin in cells expressing 5-HT(6) and chimeric Galpha(s5/q). These results indicate that the interaction between the iL3 of 5-HT(6) and Galpha(s) can be exploited for screening of regulatory compounds against the signaling pathway of 5-HT(6).

The role of K+ channels on spontaneous action potential in rat clonal pituitary GH3 cell line

The types of K+ channel which determine the pattern of spontaneous action potential (SAP) were investigated using whole-cell variation of patch clamp techniques under current- and voltage-clamp recording conditions in rat clonal pituitary GH3 cells. Heterogeneous pattern of SAP activities was changed into more regular mode with elongation of activity duration and afterhyperpolarization by treatment of TEA (10 mM). Under this condition, exposure of the class III antiarrhythmic agent E-4031 (5 micrometer) to GH3 cells hardly affected SAP activities. On the other hand, the main GH3 stimulator thyrotropin-releasing hormone (TRH) still produced its dual effects (transient hyperpolarization and later increase in SAP frequency) in the presence of TEA. However, addition of BaCl2 (2 mM) in the presence of TEA completely blocked SAP repolarization process and produced membrane depolarization in all tested cells. This effect was observed even in TEA-untreated cells and was not mimicked by higher concentration of TEA (30 mM). Also this barium-induced membrane depolarization effect was still observed after L-type Ca2+ channel was blocked by nicardipine (10 micrometer). These results suggest that barium-sensitive current is important in SAP repolarization process and barium itself may have some depolarizing effect in GH3 cells.

Effects of cholecystokinin octapeptide on neuronal activities in the rat nucleus tractus solitarius

Cholecystokinin (CCK) is a gastrointestinal hormone which plays an important role in satiety and gastric motility. It is also widely distributed throughout the central nervous system, where it appears to be involved in the central control of anxiety, feeding behavior and nociception. Two distinct CCK receptor types, CCKA and CCKB, have been found in the brain. Both CCK receptors coexist in the rat nucleus tractus solitarius (NTS), which is the primary center for the coordination of peripheral and central activities related to gastrointestinal, cardiovascular and respiratory functions. In order to study ionic actions of CCK on each type of receptor, we investigated the effects of CCK-8S on neurons located in the NTS of the rat using whole-cell patch-clamp recordings in brainstem slices. Application of CCK-8S, under current clamp, produced a membrane depolarization accompanied by action potential firing. This CCK-evoked excitation was dose-dependent (10 nM ~ 10 micrometer) and observed in more than 60% of NTS neurons. Under voltage clamp conditions, CCK-8S induced an inward current with a notably increased spontaneous excitatory synaptic activity. However, CCK-8S did not significantly change the amplitude of pharmacologically isolated and evoked EPSP(C)s. Using selective CCKA and CCKB receptor antagonists, we observed two different effects of CCK-8S, which suggest CCKA receptor-mediated inhibitory and CCKB receptor-mediated excitatory effects in the NTS. These results may help to explain the ability of CCK to modulate gastrointestinal and other reflex systems in the NTS.