Functional coupling of human metabotropic glutamate receptor hmGlu 1d: comparison to splice variants hmGlu 1a and - 1b

Abstract

Functional coupling of the human mGlu 1 splice variants was examined by heterologous expression. In cells stably (CHO) or transiently (A9) expressing the hmGlu 1d receptor, agonists elevated intracellular calcium with a rank order of potency typical of a group I mGlu receptor (quisqualate> l-glutamate>( S)-dihydroxyphenylglycine>(1 S,3 R)-1-aminocyclopentane-1,3-dicarboxylic acid (1 S,3 R -ACPD)). These responses were reduced by the antagonist (+)- α-methyl-4-carboxyphenylglycine (MCPG), by pretreatment with pertussis toxin and phorbol ester, and by removal of extracellular calcium. In transiently transfected HEK293 cells, the hmGlu 1b and - 1d receptors increased inositol monophosphate (IP) production only in the presence of glutamate, whereas hmGlu 1a coupled even in the absence of agonist. This was not due to differences in receptor expression levels as assessed by immunoblotting. Adenylate cyclase activity in HEK293 cells expressing the hmGlu 1 variants was neither stimulated nor inhibited by glutamate. In A9 cells hmGlu 1a-mediated calcium/fluo-3 fluorescence was sensitive to depletion of intracellular calcium stores by thapsigargin, but the hmGlu 1d response was resistant. Thus, hmGlu 1d receptors can be distinguished from hmGlu 1a by their lack of agonist-independent coupling and their dependence on extracellular calcium.