Do neuronal signals regulate potassium flow in glial cells? Evidence from an invertebrate central nervous system

Experiments were conducted with the aid of intracellular microelectrodes to study physiological properties of neuropile glial cells in the central nervous system of the medicinal leech. The results showed significant contributions of both K+ and Cl- ions to the membrane potential. The transmitter substance 5-hydroxytryptamine increased the K+ conductance of the cell membrane. On the basis of these experiments, a model for potassium homeostasis in leech neuropile is suggested, according to which excess K+ ions in the extracellular space lead to passive KCl fluxes across the glial cell membrane and the transmitter 5-hydroxytryptamine induces a K+ release from glial cells into the extracellular space. Since 5-hydroxytryptamine is known to be an inhibitory transmitter in the leech central nervous system, this release will occur in regions with inactive neurons, which may be specially well suited for neuronal reaccumulation of K+ ions.