Brief calcium transients evoked by glutamate receptor agonists in rat dorsal horn neurons: fast kinetics and mechanisms.
1. The calcium indicator dye, indo-1, was used to analyse the receptor-specific mechanisms of intracellular calcium ion ([Ca2+]i) responses evoked by excitatory amino acid (EAA) stimulation of dorsal horn neurons. Measurements of somal changes in [Ca2+]i were made on a subsecond time scale under conditions designed to allow membrane potential to mediate interactions between agonist-gated channels and voltage-gated calcium channels (VGCCs). 2. Voltage-gated calcium channels were activated in a receptor-independent manner using elevated extracellular [K+]. The concentration-dependence of K(+)-evoked [Ca2+]i transients was steep and variable among cells, with a mean maximal [Ca2+]i response of 1400 nM and a rapid maximal rate of rise. These data indicate that VGCCs provide a high-capacity route for Ca2+ entry that is very sensitive to small changes in membrane potential. 3. Stimulation of non-NMDA receptors using the non-desensitizing agonist kainate also evoked large [Ca2+]i responses (mean, 840 nM) that were predominantly due to indirect activation of VGCCs. However, in 60% of neurons tested, a component of the [Ca2+]i transient evoked by kainate at concentrations above 10 microM was not blocked by the potent VGCC blocker, lanthanum (La3+). The La(3+)-resistant [Ca2+]i responses to kainate rose exponentially, required extracellular Ca2+, and were caused neither by evoked release of EAA transmitters nor by reversal of Na(+)-Ca2+ exchange. These responses may be mediated by a Ca(2+)-permeable conformation of non-NMDA receptors and can also be evoked by quisqualate, (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and glutamate. 4. Non-NMDA receptors were activated in a desensitizing manner using quisqualate or AMPA. Quisqualate evoked small [Ca2+]i transients (210 nM) with a slow rate of rise. Typically, above 3 microM quisqualate, the size of the responses decreased, reflecting desensitization of the receptor. Responses to quisqualate were blocked by removal of extracellular Ca2+ indicating that mobilization of intracellular Ca2+ stores does not occur in the majority of dorsal horn neurons. However, trans-(+-)-1-amino-1,3-cyclopentane dicarboxylic acid (trans-ACPD) was occasionally able to evoke modest Ca2+ release. 5. Activation of the Ca(2+)-permeable NMDA receptors evoked [Ca2+]i transients that were large (780 nM), with a moderate rate of rise, and that generally achieved a maximum amplitude at NMDA concentrations around 300 microM. 6. Glutamate was used to examine [Ca2+]i responses to the activation of mixed EAA receptor subtypes by an endogenous ligand.(ABSTRACT TRUNCATED AT 400 WORDS)
Reichling
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DB
Department of Physiology and Cellular Biophysics, Columbia University, College of Physicians and Surgeons, New York, NY 10032.
MacDermott
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AB
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D002478:Cells, Cultured; D003515:Cycloleucine / Q000031:analogs & derivatives / Q000494:pharmacology; D005260:Female; D005971:Glutamates / Q000494:pharmacology; D018698:Glutamic Acid; D015640:Ion Channel Gating / Q000187:drug effects; D007608:Kainic Acid / Q000494:pharmacology; D007700:Kinetics; D007811:Lanthanum / Q000494:pharmacology; D016202:N-Methylaspartate / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009498:Neurotoxins / Q000494:pharmacology; D010783:Photometry; D011188:Potassium / Q000494:pharmacology; D011247:Pregnancy; D017729:Presynaptic Terminals / Q000187:drug effects; D016318:Quisqualic Acid / Q000494:pharmacology; D051381:Rats; D017470:Receptors, Glutamate / Q000187:drug effects*; D016194:Receptors, N-Methyl-D-Aspartate / Q000187:drug effects; D013116:Spinal Cord / Q000166:cytology* / Q000187:drug effects; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid / Q000494:pharmacology
D015220:Calcium Channels; D005971:Glutamates; D009498:Neurotoxins; D017470:Receptors, Glutamate; D016194:Receptors, N-Methyl-D-Aspartate; D003515:Cycloleucine; C036361:1-amino-1,3-dicarboxycyclopentane; D018698:Glutamic Acid; D016202:N-Methylaspartate; D007811:Lanthanum; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; D016318:Quisqualic Acid; D011188:Potassium; D007608:Kainic Acid; D002118:Calcium