NON-COMPETITIVE METABOTROPIC GLUTAMATE 1 RECEPTOR ANTAGONISTS BLOCK ACTIVITY OF SLOWLY ADAPTING TYPE I MECHANORECEPTOR UNITS IN THE RAT SINUS HAIR FOLLICLE P. M. B. CAHUSAC a * AND S. C. MAVULATI b a Department of Psychology, University of Stirling, Stirling FK9 4LA, Scotland b Department of Biological and Biomedical Sciences, Glasgow Cale- donian University, Cowcaddens Road, Glasgow G4 0BA, Scotland Abstract—Previous studies suggested that Group I metabo- tropic glutamate (mGlu) receptors play a role in mechano- transduction processes of slowly adapting type I mechanore- ceptors. Using an isolated rat sinus hair follicle preparation we tested a range of compounds. Surprisingly, only non-com- petitive mGlu1 receptor antagonists produced profound and long-lasting depression of mechanically evoked firing. 6-Amino- N-cyclohexyl-N,3-dimethylthiazolo[3,2-]benzimidazole-2- carboxamide hydrochloride (YM-298198) had an IC 50 of 8.7 M (95% CI 5.7 to 13.2 M), representing the most potent known blocker of type I mechanoreceptors. The derivative 6-amino-N- cyclohexyl-3-methylthiazolo[3,2-]benzimidazole-2-carboxamide hydrochloride (desmethyl YM-298198) had a comparable po- tency. Another compound 7-(hydroxyimino)cyclopropa[b]- chromen-1a-carboxylate ethyl ester (CPCCOEt) had a similar depressant effect, although it was less potent with an approx- imate IC 50 of 100 M. Between three and seven times the concentration of CPCCOEt and YM-298198 respectively was required to produce similar depressions in slowly adapting type II units. No depression, and some weak excitatory effects, were observed using the following ligands: the competitive mGlu1 receptor antagonist -amino-5-carboxy-3-methyl-2-thio- pheneacetic acid (3-MATIDA) (300 M), the phosphoserine phosphatase inhibitor DL-2-amino-3-phosphonopropionic acid (DL-AP3) (2 mM), non-competitive mGlu5 receptor antagonists 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine; (S)-3,5-DHPG, (S)- 3,5-dihydroxyphenylglycine (MTEP) (10 M) and 2-methyl-6- (phenylethynyl)pyridine hydrochloride (MPEP) (100 M), the mGlu1 receptor agonist (S)-3,5-dihydroxyphenylglycine ((S)- 3,5-DHPG) (500 M), and the mGlu5 receptor agonist (RS)-2- chloro-5-hydroxyphenylglycine (CHPG) (1 mM). The results suggest that the non-competitive mGlu1 receptor antago- nists are not acting at conventional mGlu1 receptors but at other binding sites, possibly those directly associated with mechanogated channels or on any of a number of indirect biochemical pathways. YM-298198 and related compounds may prove to be useful ligands to identify mechanosensitive channel proteins. The selective interference of type I units may provide further evidence that Merkel cells are mechano- transducers. Finally such compounds may deliver insights or treatments for Merkel cell carcinoma. Crown Copyright © 2009 Published by Elsevier Ltd on behalf of IBRO. All rights reserved. Key words: Merkel nerve endings, mechanogated channels, metabotropic glutamate receptors, slowly adapting mech- anoreceptors. Understanding the molecular basis of mechanotransduc- tion is an important goal in neuroscience (Kung, 2005; Lumpkin and Caterina, 2007; Sackin, 1995). Slowly adapt- ing type I mechanoreceptors in vertebrates are identified with Merkel nerve endings (Iggo and Muir, 1969; Wood- bury and Koerber, 2007). It has been suggested that Mer- kel cells are mechanotransducers that signal to adjacent nerve endings (Iggo and Findlater, 1984). Merkel nerve endings contain all the essential components for a gluta- matergic synapse (Haeberle et al., 2004; Hitchcock et al., 2004; Nunzi et al., 2004), and certain glutamate receptor blockers reduce mechanically evoked responses (Cahusac et al., 2005; Fagan and Cahusac, 2001). There are several possible sources of glutamate, including Merkel cells and their endings, reviewed by (Cahusac, 2008). Group I metabo- tropic glutamate (mGlu) receptors and their protein sub- strates are expressed by Merkel cells ( Tachibana et al., 2003). 6-Amino-N-cyclohexyl-N,3-dimethylthiazolo[3,2-]benzimid- azole-2-carboxamide hydrochloride (YM-298198) was de- veloped as a high-affinity mGlu1 receptor antagonist (Ko- hara et al., 2005), and is active in the nM range. This, and the structurally related compound 7-(hydroxyimino)cyclo- propa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt), are distinct from phenylglycine-based competitive mGlu receptor ligands. Both YM-298198 and CPCCOEt are non- competitive in that they block the receptor without affecting the binding affinity for glutamate (Litschig et al., 1999). YM-298198 is a potent blocker of the mGlu1 receptor- mediated slow EPSP elicited in Purkinje cells by parallel fiber activation (Fukunaga et al., 2007a). Using these and other compounds active at mGlu receptors we sought to clarify the possible role of mGlu receptors in Merkel cell function and hence mechanotransduction in rat sinus hair follicle slowly adapting type I (St I) mechanoreceptors. Our findings, though unexpected, may provide further impetus *Corresponding author. Tel: +44-0-1786-467661; fax: +44-0-1786- 467641. E-mail address: p.m.b.cahusac@stir.ac.uk (P. M. B. Cahusac). Abbreviations: CHPG, (RS)-2-chloro-5-hydroxyphenylglycine; COV, coefficient of variation; CPCCOEt, 7-(hydroxyimino)cyclopropa[b]chromen- 1a-carboxylate ethyl ester; desmethyl YM-298198, 6-amino-N-cyclohexyl- 3-methylthiazolo[3,2-]benzimidazole-2-carboxamide hydrochloride; DL-AP3, DL-2-amino-3-phosphonopropionic acid; mGlu, metabotropic glutamate; MPEP, 2-methyl-6-(phenylethynyl)pyridine hydrochloride; MTEP, 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine; SIF, synthetic in- terstitial fluid; (S)-3,5-DHPG, (S)-3,5-dihydroxyphenylglycine; St I, si- nus hair follicle slowly adapting type I; St II, sinus hair follicle slowly adapting type II; TEA, tetraethylammonium; YM-298198, 6-amino-N- cyclohexyl-N,3-dimethylthiazolo[3,2-]benzimidazole-2-carboxamide hydrochloride; 3-MATIDA, -amino-5-carboxy-3-methyl-2-thiophene- acetic acid. Neuroscience 163 (2009) 933–941 0306-4522/09 $ - see front matter. Crown Copyright © 2009 Published by Elsevier Ltd on behalf of IBRO. All rights reserved. doi:10.1016/j.neuroscience.2009.07.015 933