Differential modulation of voltage-activated conductances by intracellular and extracellular cyclic nucleotides in leech salivary glands

Effects of ATP and derivatives on neuropile glial cells of the leech central nervous system

We investigated the effects of ATP (adenosine 5'-triphosphate) and derivatives on leech neuropile glial cells, focusing on exposed glial cells. ATP dose-dependently depolarized or hyperpolarized neuropile glial cells in situ as well as exposed neuropile glial cells. These potential shifts varied among cells and repetitive ATP application did not change their amplitude, duration or direction. In exposed neuropile glial cells, ATP most frequently induced a Na(+)-dependent depolarization and decreased the input resistance. The agonist potency ATP > ADP (adenosine 5'-diphosphate) > AMP (adenosine 5'-monophosphate) > adenosine indicates that P2 purinoceptors mediate this depolarization. The P2Y agonist 2-methylthio-ATP mimicked the ATP-induced depolarization, whereas the P2Y antagonist PPADS (pyridoxal-phosphate-6-azophenyl-2', 4'-disulphonic acid) reduced it. P2X agonists were without effect. Because the P1 antagonist 8-SPT (8-(p-sulphophenyl)-theophylline) also depressed ATP-induced depolarizations and some ATP-insensitive glial cells responded to adenosine, we suggest coexpression of metabotropic P2Y and P1 purinoceptors. The ATP-induced depolarization requires activation of Na(+) channels or nonselective cation channels, whereas the ATP-induced hyperpolarization indicates activation of K(+) channels. ATP also increased the intracellular Ca(2+) concentration ([Ca(2+)](i)), that is independent of Ca(2+) influx but reflects intracellular Ca(2+) release possibly triggered by IP(3) formation. ADP and AMP also increased [Ca(2+)](i), but were less efficient than ATP; adenosine and 2-methylthio-ATP did not affect [Ca(2+)](i). In view of the mobilization of intracellular Ca(2+), ATP is clearly different from other leech neurotransmitters, because it enables intracellular Ca(2+) signaling without causing prominent changes in glial membrane potential. Thus disturbance of the extracellular microenvironment and the demand for metabolic energy are minimized.