Leech giant glial cell: Functional role in a simple nervous system

The giant glial cell in the central nervous system of the leech Hirudo medi cinalis has been the subject of a series of studies trying to link its phys iological properties with its role in neuron-glia interactions. Isolated ve ntral cord ganglia of this annelid offer several advantages for these studi es. First, single giant glial cells can easily be identified and are quite accessible to electrophysiological and microfluorometric studies. Second, o nly two giant macroglial cells are located in the neuropil of each ganglion , rendering them well suited for studying neuron-glia interactions. Third, many neurons can be identified and are well known with respect to their phy siology and their roles in controlling simple behaviors in the leech. This review briefly outlines the major recent findings gained by studying this p reparation and its contributions to our knowledge of the functional role of glia in nervous systems. Emphasis is directed to glial responses during ne uronal activity and to the analysis of intracellular Ca2+ and H+ transients mediated by neurotransmitter receptors and ion-driven carriers. Among its numerous properties, the leech giant glial cell prominently expresses a lar ge K+ conductance, voltage-dependent Ca2+ channels, ionotropic non-NMDA glu tamate receptors, and an electrogenic, reversible Na+-HCO3- cotransporter. (C) 1999 Wiley-Liss, Inc.