January 1999, Volume 126, Issue 1, Pages 333 - 341

Original Article

Mechanisms involved in the metabotropic glutamate receptor-enhancement of NMDA-mediated motoneurone responses in frog spinal cord Alice M. Holohean1,3, John C. Hackman1,2,3 & Robert A. Davidoff1,3,4 1Neurophysiology Laboratory, Veterans Affairs Medical Center, PO Box 016960, University of Miami School of Medicine, Miami, Florida 33101, U.S.A     2Spinal Cord Pharmacology Laboratory, Veterans Affairs Medical Center, PO Box 016960, University of Miami School of Medicine, Miami, Florida 33101, U.S.A.     3Department of Neurology (D4-5), PO Box 016960, University of Miami School of Medicine, Miami, Florida 33101, U.S.A.     4Author for correspondence at: Department of Neurology (D4-5), P.O. Box 016960, University of Miami School of Medicine, Miami, Florida 343101, U.S.A. E-mail: rdavidoff@mednet.med. miami.edu

Keywords

N-methyl-D-aspartate receptors;   frog;   spinal cord motoneurones;   trans-(±)-1-amino-1,3-cyclopentanedicarboxylic acid;   metabotropic glutamate receptors;   G-proteins;   open channel block;   Mg2+ ions;   Ca2+ ions;   calmodulin

Abstract

1   The metabotropic glutamate receptor (mGluR) agonist trans-(±)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) (10 - 100 µM) depolarized isolated frog spinal cord motoneurones, a process sensitive to kynurenate (1.0 mM) and tetrodotoxin (TTX) (0.783 µM). 2   In the presence of NMDA open channel blockers [Mg2+; (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK801); 3,5-dimethyl-1-adamantanamine hydrochloride (memantine)] and TTX, trans-ACPD significantly potentiated NMDA-induced motoneurone depolarizations, but not -amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA)- or kainate-induced depolarizations. 3   NMDA potentiation was blocked by (RS)--methyl-4-carboxyphenylglycine (MCPG) (240 µM), but not by -methyl-(2S,3S,4S)--(carboxycyclopropyl)-glycine (MCCG) (290 µM) or by -methyl-(S)-2-amino-4-phosphonobutyrate (L-MAP4) (250 µM), and was mimicked by 3,5-dihydroxyphenylglycine (DHPG) (30 µM), but not by L(+)-2-amino-4-phosphonobutyrate (L-AP4) (100 µM). Therefore, trans-ACPD's facilitatory effects appear to involve group I mGluRs. 4   Potentiation was prevented by the G-protein decoupling agent pertussis toxin (3 - 6 ng ml-1, 36 h preincubation). The protein kinase C inhibitors staurosporine (2.0 µM) and N-(2-aminoethyl)-5-isoquinolinesulphonamide HCl (H9) (77 µM) did not significantly reduce enhanced NMDA responses. Protein kinase C activation with phorbol-12-myristate 13-acetate (5.0 µM) had no effect. 5   Intracellular Ca2+ depletion with thapsigargin (0.1 µM) (which inhibits Ca2+/ATPase), 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetracetic acid acetyl methyl ester (BAPTA-AM) (50 µM) (which buffers elevations of [Ca2+]i), and bathing spinal cords in nominally Ca2+-free medium all reduced trans-ACPD's effects. 6   The calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide (W7) (100 µM) and chlorpromazine (100 µM) diminished the potentiation. 7   In summary, group I mGluRs selectively facilitate NMDA-depolarization of frog motoneurones via a G-protein, a rise in [Ca2+]i from the presumed generation of phosphoinositides, binding of Ca2+ to calmodulin, and lessening of the Mg2+-produced channel block of the NMDA receptor.

Received 28 May 1998; Revised 29 September 1998; Accepted 5 October 1998

^© Macmillan Publishers Ltd 1999