Journal club 2014-06-27

Ligand determinants of fatty acid activation of the pronociceptive ion channel TRPA1

William John Redmond1, Liuqiong Gu2, Maxime Camo1, Peter McIntyre2,3 and Mark Connor1

1 Australian School of Advanced Medicine, Macquarie University, NSW, Australia
2 Department of Pharmacology, University of Melbourne, Parkville, Victoria, Australia
3 Health Innovations Research Institute and School of Medical Sciences, RMIT University, Melbourne,
Victoria, Australia

ABSTRACT

Background and purpose. Arachidonic acid (AA) and its derivatives are important modulators of cellular signalling. The transient receptor potential cation channel subfamily A, member 1 (TRPA1) is a cation channel with important functions in mediating cellular responses to noxious stimuli and inflammation. There is limited information about the interactions between AA itself and TRPA1, so we investigated the effects of AA and key ethanolamide and amino acid/neurotransmitter deriva- tives of AA on hTRPA1.

Experimental approach. HEK 293 cells expressing hTRPA1 were studied by mea- suring changes in intracellular calcium ([Ca]i) with a fluorescent dye and by stan- dard whole cell patch clamp recordings.
Key results. AA (30 μM) increased fluorescence in hTRPA1 expressing cells by 370% (notional EC50 13 μM). The covalent TRPA1 agonist cinnamaldehyde

(300 μM) increased fluorescence by 430% (EC50, 11 μM). Anandamide (230%) and N -arachidonoyl tyrosine (170%) substantially activated hTRPA1 at 30 μM, how- ever, N -arachidonoyl conjugates of glycine and taurine were less effective while
N -acyl conjugates of 5-HT did not affect hTRPA1. Changing the acyl chain length or the number and position of double bonds reduced fatty acid efficacy at hTRPA1. Mutant hTRPA1 (Cys621, Cys641 and Cys665 changed to Ser) could be activated by AA (100 μM, 40% of wild type) but not by cinnamaldehyde (300 μM). Conclusions and implications. AA is a more potent activator of TRPA1 than its ethanolamide or amino acid/neurotransmitter derivatives and acts via a mechanism distinct from that of cinnamaldehyde, further underscoring the likelyhood of multi- ple pharmacologically exploitable sites on hTRPA1.

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Journal club 2014-06-13

Neuron. 2014 May 7;82(3):573-86. doi: 10.1016/j.neuron.2014.02.046. Epub 2014 Apr 10.

Dynorphin acts as a neuromodulator to inhibit itch in the dorsal horn of the spinal cord.

Kardon AP1, Polgár E2, Hachisuka J1, Snyder LM1, Cameron D2, Savage S2, Cai X1, Karnup S1, Fan CR3, Hemenway GM3, Bernard CS3, Schwartz ES4,Nagase H5, Schwarzer C6, Watanabe M7, Furuta T8, Kaneko T8, Koerber HR1, Todd AJ9, Ross SE10.

Abstract

Menthol and other counterstimuli relieve itch, resulting in an antipruritic state that persists for minutes to hours. However, the neural basis for this effect is unclear, and the underlying neuromodulatory mechanisms are unknown. Previous studies revealed that Bhlhb5(-/-) mice, which lack a specific population of spinal inhibitory interneurons (B5-I neurons), develop pathological itch. Here we characterize B5-I neurons and show that they belong to a neurochemically distinct subset. We provide cause-and-effect evidence that B5-I neurons inhibit itch and show that dynorphin, which is released from B5-I neurons, is a key neuromodulator of pruritus. Finally, we show that B5-I neurons are innervated by menthol-, capsaicin-, and mustard oil-responsive sensory neurons and are required for the inhibition of itch by menthol. These findings provide a cellular basis for the inhibition of itch by chemical counterstimuli and suggest that kappa opioids may be a broadly effective therapy for pathological itch.

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