Purinergic receptors and their activation by endogenous purines at perisynaptic glial cells of the frog neuromuscular junction

Glial cells are closely associated with synapses and are modulated by neurotransmitters released during synaptic transmission. At many synapses, ATP is released during synaptic transmission and is involved in cell-cell signaling. Since glial cells have purinoceptors, it is possible that ATP mediates synaptic neuron-glia signaling. This work aims at determining which types of purinoceptors are present on perisynaptic Schwann cells, the perisynaptic glial cells at the frog neuromuscular junction, and test their sensitivity to endogenous purines by monitoring the relative changes of intracellular Ca2+. Local application of ATP induced the release of Ca2+ from internal stores. Adenosine induced Ca2+ responses that were blocked by A1 receptor antagonists and mimicked by an A1 receptor agonist and were caused by the release of Ca2+ from internal stores via a pertussis toxin-sensitive G-protein. A2 receptor antagonists had no effect on Ca2+ responses induced by adenosine. Me-S-ATP, an ATP analog, triggered Ca2+ release from internal stores via a pertussis toxin-sensitive G-protein, consistent with the activation of P2Y receptors. L-AMP-PCP, another ATP analog, induced Ca2+ entry mainly through L-type Ca2+ channels by a pertussis toxin-insensitive mechanism, consistent with the activation of P2X receptors. Blockade of adenosine receptors did not affect glial Ca2+ responses induced by nerve evoked transmitter release. However, blockade of ATP receptors reduced the size and increased the delay of the responses. Hence, purinoceptors are present on the perisynaptic Schwann cells and are activated by endogenous ATP released during synaptic transmission.