Specific peripheral sensory neurons respond to increases in extracellular osmolality but the mechanism responsible for excitation is unknown. Here we show that small increases in osmolality excite isolated mouse dorsal root ganglion (DRG) and trigeminal ganglion (TG) neurons expressing the cold-sensitive TRPM8 channel (transient receptor potential channel, subfamily M, member 8). Hyperosmotic responses were abolished by TRPM8 antagonists, and were absent in DRG and TG neurons isolated from Trpm8−/− mice. Heterologously expressed TRPM8 was activated by increased osmolality around physiological levels and inhibited by reduced osmolality. Electrophysiological studies in a mouse corneal preparation demonstrated that osmolality regulated the electrical activity of TRPM8-expressing corneal afferent neurons. Finally, the frequency of eye blinks was reduced in Trpm8−/− compared with wild-type mice and topical administration of a TRPM8 antagonist reduced blinking in wild-type mice. Our findings identify TRPM8 as a peripheral osmosensor responsible for the regulation of normal eye-blinking in mice.

TRPV1 is a well-characterised channel expressed by a subset of peripheral sensory neurons involved in pain sensation and also at a number of other neuronal and non-neuronal sites in the mammalian body. Functionally, TRPV1 acts as a sensor for noxious heat (greater than ~42 °C). It can also be activated by some endogenous lipid-derived molecules, acidic solutions (pH < 6.5) and some pungent chemicals and food ingredients such as capsaicin, as well as by toxins such as resiniferatoxin and vanillotoxins. Structurally, TRPV1 subunits have six transmembrane (TM) domains with intracellular N- (containing 6 ankyrin-like repeats) and C-termini and a pore region between TM5 and TM6 containing sites that are important for channel activation and ion selectivity. The N- and C- termini have residues and regions that are sites for phosphorylation/dephosphorylation and PI(4,5)P2 binding, which regulate TRPV1 sensitivity and membrane insertion. The channel has several interacting proteins, some of which (e.g. AKAP79/150) are important for TRPV1 phosphorylation. Four TRPV1 subunits form a non-selective, outwardly rectifying ion channel permeable to monovalent and divalent cations with a single-channel conductance of 50-100 pS. TRPV1 channel kinetics reveal multiple open and closed states, and several models for channel activation by voltage, ligand binding and temperature have been proposed. Studies with TRPV1 agonists and antagonists and Trpv1 (-/-) mice have suggested a role for TRPV1 in pain, thermoregulation and osmoregulation, as well as in cough and overactive bladder. TRPV1 antagonists have advanced to clinical trials where findings of drug-induced hyperthermia and loss of heat sensitivity have raised questions about the viability of this therapeutic approach.

Transient receptor potential (TRP) ion channels in peripheral sensory neurons are functionally regulated by hydrolysis of the phosphoinositide PI(4,5)P2 and changes in the level of protein kinase mediated phosphorylation following activation of various G protein coupled receptors. We now show that the activity of TRPM3 expressed in mouse dorsal root ganglion (DRG) neurons is inhibited by agonists of the Gi-coupled µ opioid, GABA-B and NPY receptors. These agonist effects are mediated by direct inhibition of TRPM3 by Gβγ subunits, rather than by a canonical cAMP mediated mechanism. The activity of TRPM3 in DRG neurons is also negatively modulated by tonic, constitutive GPCR activity as TRPM3 responses can be potentiated by GPCR inverse agonists. GPCR regulation of TRPM3 is also seen in vivo where Gi/o GPCRs agonists inhibited and inverse agonists potentiated TRPM3 mediated nociceptive behavioural responses.DOI: http://dx.doi.org/10.7554/eLife.26138.001

than likely, the success of surufatinib is attributable to both its novel mechanism of action and favourable business decisions around its development. In addition, we do not yet know how surufatinib will fit into the complex treatment algorithm for NETs. Evidence regarding sequence of therapies is a crucial need in the field, although might be impractical to study in prospective clinical trials. We also need to study mechanisms of resistance to tyrosine kinase inhibitors, develop and validate predictive biomarkers, understand reasons for heterogeneity in objective response, and identify better quantitative radiological response criteria in the setting of angiogenesis inhibition. Last, but not least, we must also keep patients with NETs at the core of how we think about optimal treatment strategies. Given the chronicity of well differentiated NETs, these patients will experience an accumulation of toxicities over years that include non-trivial drug side-effects, particularly with tyrosine kinase inhibitors.PLK reports receiving research funding to her institution from Advanced Accelerator Applications, Brahms (Thermo-Fisher Scientific), Ipsen, Lexicon Pharmaceuticals, and Xencor; and reports serving on scientific advisory board meetings for and receiving honorarium from Advance Accelerator Applications and Ipsen.

Transient receptor potential melastatin 3 (TRPM3) is a nonselective cation channel that is inhibited by G␤␥ subunits liberated following activation of G␣ i/o protein-coupled receptors. Here, we demonstrate that TRPM3 channels are also inhibited by G␤␥ released from G␣ s and G␣ q. Activation of the G s-coupled adenosine 2B receptor and the G q-coupled muscarinic acetylcholine M1 receptor inhibited the activity of TRPM3 heterologously expressed in HEK293 cells. This inhibition was prevented when the G␤␥ sink ␤ARK1-ct (C terminus of ␤-adrenergic receptor kinase-1) was coexpressed with TRPM3. In neurons isolated from mouse dorsal root ganglion (DRG), native TRPM3 channels were inhibited by activating G s-coupled prostaglandin-EP2 and G q-coupled bradykinin B2 (BK2) receptors. The G i/o inhibitor pertussis toxin and inhibitors of PKA and PKC had no effect on EP2-and BK2-mediated inhibition of TRPM3, demonstrating that the receptors did not act through G␣ i/o or through the major protein kinases activated downstream of G-protein-coupled receptor (GPCR) activation. When DRG neurons were dialyzed with GRK2i, which sequesters free G␤␥ protein, TRPM3 inhibition by EP2 and BK2 was significantly reduced. Intraplantar injections of EP2 or BK2 agonists inhibited both the nocifensive response evoked by TRPM3 agonists, and the heat hypersensitivity produced by Freund's Complete Adjuvant (FCA). Furthermore, FCA-induced heat hypersensitivity was completely reversed by the selective TRPM3 antagonist ononetin in WT mice and did not develop in Trpm3 ؊/؊ mice. Our results demonstrate that TRPM3 is subject to promiscuous inhibition by G␤␥ protein in heterologous expression systems, primary neurons and in vivo, and suggest a critical role for this ion channel in inflammatory heat hypersensitivity.