In the nervous system, NMDA receptors (NMDARs) participate in neurotransmission and modulate the viability of neurons. In contrast, little is known about the role of NMDARs in pancreatic islets and the insulin-secreting beta cells whose functional impairment contributes to diabetes mellitus. Here we found that inhibition of NMDARs in mouse and human islets enhanced their glucose-stimulated insulin secretion (GSIS) and survival of islet cells. Further, NMDAR inhibition prolonged the amount of time that glucose-stimulated beta cells spent in a depolarized state with high cytosolic Ca(2+) concentrations. We also noticed that, in vivo, the NMDAR antagonist dextromethorphan (DXM) enhanced glucose tolerance in mice, and that in vitro dextrorphan, the main metabolite of DXM, amplified the stimulatory effect of exendin-4 on GSIS. In a mouse model of type 2 diabetes mellitus (T2DM), long-term treatment with DXM improved islet insulin content, islet cell mass and blood glucose control. Further, in a small clinical trial we found that individuals with T2DM treated with DXM showed enhanced serum insulin concentrations and glucose tolerance. Our data highlight the possibility that antagonists of NMDARs may provide a useful adjunct treatment for diabetes.

Astringency is an everyday sensory experience best described as a dry mouthfeel typically elicited by phenol-rich alimentary products like tea and wine. The neural correlates and cellular mechanisms of astringency perception are still not well understood. We explored taste and astringency perception in human subjects to study the contribution of the taste as well as of the trigeminal sensory system to astringency perception. Subjects with either a lesion or lidocaine anesthesia of the Chorda tympani taste nerve showed no impairment of astringency perception. Only anesthesia of both the lingual taste and trigeminal innervation by inferior alveolar nerve block led to a loss of astringency perception. In an in vitro model of trigeminal ganglion neurons of mice, we studied the cellular mechanisms of astringency perception. Primary mouse trigeminal ganglion neurons showed robust responses to 8 out of 19 monomeric phenolic astringent compounds and 8 polymeric red wine polyphenols in Ca(2+) imaging experiments. The activating substances shared one or several galloyl moieties, whereas substances lacking the moiety did not or only weakly stimulate responses. The responses depended on Ca(2+) influx and voltage-gated Ca(2+) channels, but not on transient receptor potential channels. Responses to the phenolic compound epigallocatechin gallate as well as to a polymeric red wine polyphenol were inhibited by the Gαs inactivator suramin, the adenylate cyclase inhibitor SQ, and the cyclic nucleotide-gated channel inhibitor l-cis-diltiazem and displayed sensitivity to blockers of Ca(2+)-activated Cl(-) channels.

Nineteen GABA A receptor (GABA A R) subunits are known in mammals with only a restricted number of functionally identified native combinations. The physiological role of ␤1-subunit-containing GABA A Rs is unknown. Here we report the discovery of a new structural class of GABA A R positive modulators with unique ␤1-subunit selectivity: fragrant dioxane derivatives (FDD). At heterologously expressed ␣1␤x␥2L (x-for 1,2,3) GABA A R FDD were 6 times more potent at ␤1-versus ␤2-and ␤3-containing receptors. Serine at position 265 was essential for the high sensitivity of the ␤1-subunit to FDD and the ␤1N286W mutation nearly abolished modulation; vice versa the mutation ␤3N265S shifted FDD sensitivity toward the ␤1-type. In posterior hypothalamic neurons controlling wakefulness GABA-mediated whole-cell responses and GABAergic synaptic currents were highly sensitive to FDD, in contrast to ␤1-negative cerebellar Purkinje neurons. Immunostaining for the ␤1-subunit and the potency of FDD to modulate GABA responses in cultured hypothalamic neurons was drastically diminished by ␤1-siRNA treatment. In conclusion, with the help of FDDs we reveal a functional expression of ␤1-containing GABA A Rs in the hypothalamus, offering a new tool for studies on the functional diversity of native GABA A Rs. ␥-Aminobutyric acid (GABA),4 the major inhibitory neurotransmitter in the brain, mediates inhibition via GABA A receptors (GABA A R), heteropentameric proteins constructed from subunits derived from several related gene families with six ␣-, three ␤-, three ␥-, one ␦-, one ⑀-, one -, and one -subunit in mammals. In addition 3 rho ()-subunits contribute to what have been called "GABA C receptors" (1). According to the current model of the GABA A R structure the GABA-binding pocket is formed at the ␣/␤-subunit interface, whereas the benzodiazepine (BZ)-binding pocket is located at the ␣/␥ interface (2) with the subunits arranged pseudo-symmetrically around the ion channel in the sequence ␥-␤-␣-␤-␣ anticlockwise when viewed from the synaptic cleft (3).Functional receptor compositions are restricted in their number and delineated on the basis of several criteria such as (i) capability of selected subunits to form a heteropentamer with defined pharmacological properties, (ii) a similar pharmacological fingerprint must be found in native receptors, and (iii) immunohistochemical co-localization of these subunits must be demonstrated at synaptic or extrasynaptic sites (1). Only few subunit combinations are currently accepted as "identified" native GABA A R subtypes with ␤1-containing receptors not among them (1) mainly because subunit-selective pharmacological tools are missing.In total, the GABA A R incorporates more than ten distinct modulatory binding sites targeted by anticonvulsive, antiepileptic, sedative, hypnotic, and anxiolytic compounds belonging to chemically different structural classes (4 -7) with some of them showing receptor type-specific actions. Benzodiazepine (BZ)-site agonists discriminate ␥2-containing GABA A Rs from recombi...

Transient receptor potential vanilloid-3 (TRPV3) is a thermo-sensitive ion channel expressed in skin keratinocytes and in a variety of neural cells. It is activated by warmth as well as monoterpenoids including camphor, menthol, dihydrocarveol and 1,8-cineol. TRPV3 is described as a putative nociceptor and previous studies revealed sensitization of the channel during repeated short-term stimulation with different agonists. In the present investigation TRPV3 was transiently expressed in either Xenopus oocytes or HEK293 cells. Whole-cell voltage-clamp techniques were used to characterize the behavior of TRPV3 when challenged with different agonists. Similarly, a human keratinocyte-derived cell line (HaCaT cells) was used to monitor the behavior of native TRPV3 when challenged with different agonists. We report here that prolonged exposure (5-15 minutes) of monoterpenoids results in agonist-specific desensitization of TRPV3. Long-term exposure to camphor and 1,8-cineol elicits desensitizing currents in TRPV3 expressing oocytes, whereas the non-terpenoid agonist 2-APB induces sustained currents. Agonist-specific desensitization of endogenous TRPV3 was also found in HaCaT cells, which may be taken as a representative for the native system. Terpenoids have a long history of use in therapeutics, pharmaceuticals and cosmetics but knowledge about underpinning molecular mechanisms is incomplete. Our finding on agonist-induced desensitization of TRPV3 by some monoterpenoids displays a novel mechanism through which TRP channels could be functionally modulated. Therefore, we conclude that desensitization of TRPV3 channels might be the molecular basis of action for some of the medicinal properties of camphor and 1,8-cineol.