5-Hydroxytryptamine activates a barium-sensitive, cAMP-mediated potassium conductance in the leech giant glial cell

G protein activation by uncaging of GTP-gamma-S in the leech giant glial cell

Glial cells can be activated by neurotransmitters via metabotropic, G protein-coupled receptors. We have studied the effects of 'global' G protein activation by GTP-gamma-S on the membrane potential, membrane conductance, intracellular Ca(2+) and Na(+) of the giant glial cell in isolated ganglia of the leech Hirudo medicinalis. Uncaging GTP-gamma-S (injected into a giant glial cell as caged compound) by moderate UV illumination hyperpolarized the membrane due to an increase in K+ conductance. Uncaging GTP-gamma-S also evoked rises in cytosolic Ca(2+) and Na+, both of which were suppressed after depleting the intracellular Ca(2+) stores with cyclopiazonic acid (20 micromol l(-1)). Uncaging inositol-trisphosphate evoked a transient rise in cytosolic Ca(2+) and Na+ but no change in membrane potential. Injection of the fast Ca(2+) chelator BAPTA or depletion of intracellular Ca(2+) stores did not suppress the membrane hyperpolarization induced by uncaging GTP-gamma-S. Our results suggest that global activation of G proteins in the leech giant glial cell results in a rise of Ca(2+)-independent membrane K+ conductance, a rise of cytosolic Ca(2+), due to release from intracellular stores, and a rise of cytosolic Na+, presumably due to increased Na+/Ca(2+) exchange.

5-Hydroxytryptamine-mediated increase in glutamate uptake by the leech giant glial cell

The clearance of synaptically released glutamate is one of the pivotal functions of glial cells. We have studied the role of 5-hydroxytryptamine (5-HT, 30 microM), a neurotransmitter and neurohormone in the leech central nervous system with a versatile action spectrum, on the efficacy of glial glutamate uptake. The activity of the glutamate uptake carrier in the giant glial cell in isolated ganglia of Hirudo medicinalis was monitored by measuring the membrane current and the change in the intracellular Na(+) concentration (Na(+) (i)) as induced by the glutamate carrier substrate D-aspartate (D-asp, 1 mM). 5-HT increased the D-asp-induced current (EC(50) at 5 microM) and rise in Na(+) (i), an effect which was mimicked by the membrane-permeable cyclic nucleotide analogue dibutyryl-cyclic AMP (db-cAMP). The adenylyl cyclase inhibitor SQ 22,536 and the protein kinase A antagonist Rp-cAMP inhibited the effect of 5-HT. Blocking the G protein in the giant glial cell by injecting GDP-beta-S suppressed the effect of 5-HT, but not the effect of db-cAMP, on the D-asp-induced current. Our results suggest that 5-HT enhances the glial uptake of glutamate via cAMP- and PKA-mediated pathway.

Inwardly rectifying potassium channels (Kir) in central nervous system glia: a special role for Kir4.1 in glial functions

Glia in the central nervous system (CNS) express diverse inward rectifying potassium channels (Kir). The major function of Kir is in establishing the high potassium (K+) selectivity of the glial cell membrane and strongly negative resting membrane potential (RMP), which are characteristic physiological properties of glia. The classical property of Kir is that K+ flows inwards when the RMP is negative to the equilibrium potential for K+ (E(K)), but at more positive potentials outward currents are inhibited. This provides the driving force for glial uptake of K+ released during neuronal activity, by the processes of "K+ spatial buffering" and "K+ siphoning", considered a key function of astrocytes, the main glial cell type in the CNS. Glia express multiple Kir channel subtypes, which are likely to have distinct functional roles related to their differences in conductance, and sensitivity to intracellular and extracellular factors, including pH, ATP, G-proteins, neurotransmitters and hormones. A feature of CNS glia is their specific expression of the Kir4.1 subtype, which is a major K+ conductance in glial cell membranes and has a key role in setting the glial RMP. It is proposed that Kir4.1 have a primary function in K+ regulation, both as homomeric channels and as heteromeric channels by co-assembly with Kir5.1 and probably Kir2.0 subtypes. Significantly, Kir4.1 are also expressed by oligodendrocytes, the myelin-forming cells of the CNS, and the genetic ablation of Kir4.1 results in severe hypomyelination. Hence, Kir, and in particular Kir4.1, are key regulators of glial functions, which in turn determine neuronal excitability and axonal conduction.

Calcium signaling in invertebrate glial cells

Calcium signaling studies in invertebrate glial cells have been performed mainly in the nervous systems of the medicinal leech (Hirudo medicinalis) and the sphinx moth Manduca sexta. The main advantages of studing glial cells in invertebrate nervous systems are the large size of invertebrate glial cells and their easy accessibility for optical and electrophysiological recordings. Glial cells in both insects and annelids express voltage-gated calcium channels and, in the case of leech glial cells, calcium-permeable neurotransmitter receptors, which allow calcium influx as one major source for cytosolic calcium transients. Calcium release from intracellular stores can be induced by metabotropic receptor activation in leech glial cells, but appears to play a minor role in calcium signaling. In glial cells of the antennal lobe of Manduca, voltage-gated calcium signaling changes during postembryonic development and is essential for the migration of the glial cells, a key step in axon guidance and in stabilization of the glomerular structures that are characteristic of primary olfactory centers.