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)