FUNCTIONAL COUPLING OF HUMAN METABOTROPIC GLUTAMATE-RECEPTOR HMGLU(1D) - COMPARISON TO SPLICE VARIANTS HMGLU(1A) AND HMGLU(1B)

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 cal cium with a rank order of potency typical of a group I mGlu receptor ( quisqualate > L-glutamate > (S)-dihydroxyphenylglycine > (1S,3R)-1-ami nocyclopentane-1,3-dicarboxylic acid (1S,3R -ACPD)). These responses w ere reduced by the antagonist(+)-alpha-methyl-4-carboxyphenylglycine ( MCPG), by pretreatment with pertussis toxin and phorbol eater, and by removal of extracellular calcium. In transiently transfected HEK293 ce lls, the hmGlu(1b) and -(1d) receptors increased inositol monophosphat e (IP) production only in the presence of glutamate, whereas hmGlu(1a) coupled even in the absence of agonist. This was not due to differenc es in receptor expression levels as assessed by immunoblotting. Adenyl ate cyclase activity in HEK293 cells expressing the hmGlu(1) variants was neither stimulated nor inhibited by glutamate. In A9 cells hmGlu(1 a)-mediated calcium/fluo-3 fluorescence was sensitive to depletion of intracellular calcium stores by thapsigargin, but the hmGlu(1d) respon se was resistant. Thus, hmGlu(1d) receptors can be distinguished from hmGlu(1a) by their lack of agonist-independent coupling and their depe ndence on extracellular calcium. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.