Dendritic calcium transients in the leech giant glial cell in situ.

Glial cells have been shown to respond to neuronal activity with changes in the membrane potential and the intracellular Ca2+ concentration. In order to get closer to glial structures associated with neuronal synapses, we have now looked at Ca2+ signalling in the glial processes ("glial dendrites") in response to neurotransmitters and neuronal activity. Single giant glial cells in situ of isolated ganglia of the leech Hirudo medicinalis were filled iontophoretically with the Ca(2+)-sensitive dyes Oregon green 488 BAPTA-1 or Fluo-3. Relative Ca(2+)-dependent fluorescence changes in response to bath and focal application of the ionotropic glutamate receptor agonist kainate (50 microM) and of 5-hydroxytryptamine (5-HT, 100 microM) were recorded in glial dendrites, using confocal laser scanning microscopy. The amplitudes of the [Ca2+]i transients in the dendritic processes were 2-4 times larger than those recorded in the cell body. Electrical stimulation of a nerve root (20 Hz for 15 s) elicited [Ca2+]i transients in glial dendrites (n = 32) that were reduced by the ionotropic glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; n = 14). The results demonstrate that neuronal activity can evoke [Ca2+]i transients not only in glial cell bodies but also in glial dendrites, where these transients display regional variation. This may reflect local release of neurotransmitters like glutamate and 5-hydroxytryptamine and/or regional differences in the density of glial receptors.