Purine receptor-mediated endocannabinoid production and retrograde synaptic signalling in the cerebellar cortex.
OBJECTIVE
Presynaptic CB₁ cannabinoid receptors can be activated by endogenous cannabinoids (endocannabinoids) synthesized by postsynaptic neurones. The hypothesis of the present work was that activation of calcium-permeable transmitter-gated ion channels in postsynaptic neurones, specifically of P2X purine receptors, can lead to endocannabinoid production and retrograde synaptic signalling.


METHODS
GABAergic inhibitory postsynaptic currents (IPSCs) were recorded with patch-clamp techniques in Purkinje cells in mouse cerebellar slices. Purine receptors on Purkinje cells were activated by pressure ejection of ATP from a pipette.


RESULTS
ATP evoked an inward current in Purkinje cells, most likely due to P2X receptor activation. The ATP-evoked currents were accompanied by currents via voltage-gated calcium channels. ATP suppressed electrical stimulation-evoked IPSCs and miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin, and these effects were prevented by the CB₁ antagonist rimonabant and the calcium chelator BAPTA (applied into the Purkinje cell). ATP also suppressed mIPSCs when voltage-gated calcium channels were blocked by cadmium, and intracellular calcium stores were depleted by thapsigargin. However, ATP failed to suppress mIPSCs when the extracellular calcium concentration was zero.


CONCLUSIONS
ATP elicits CB₁ receptor-dependent retrograde synaptic suppression, which is probably mediated by an endocannabinod released by the postsynaptic neurone. An increase in intracellular calcium concentration in the postsynaptic neurone is necessary for this retrograde signalling. We propose that ATP increases the calcium concentration by two mechanisms: calcium enters into the neurone via the P2X receptor ion channel and the ATP-evoked depolarization triggers voltage-gated calcium channels.
Institut für Experimentelle und Klinische Pharmakologie und Toxikologie, Albert-Ludwigs-Universität, Freiburg i. Br., Germany.
Flora E
FE
Kovacs
Peter
P
Illes
Bela
B
Szabo
D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000378:metabolism; D000818:Animals; D020746:Calcium Channels, L-Type / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects; D063385:Cannabinoid Receptor Modulators / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002525:Cerebellar Cortex / Q000187:drug effects / Q000378:metabolism*; D063388:Endocannabinoids; D066298:In Vitro Techniques; D007700:Kinetics; D051379:Mice; D018408:Patch-Clamp Techniques; D058913:Purinergic Agonists / Q000494:pharmacology; D011689:Purkinje Cells / Q000187:drug effects / Q000378:metabolism; D043884:Receptor, Cannabinoid, CB1 / Q000037:antagonists & inhibitors; D011983:Receptors, Purinergic / Q000378:metabolism*; D058469:Receptors, Purinergic P2X / Q000378:metabolism; D026941:Sodium Channel Blockers / Q000494:pharmacology; D055366:Synaptic Potentials / Q000187:drug effects; D009435:Synaptic Transmission / Q000187:drug effects
D020746:Calcium Channels, L-Type; D063385:Cannabinoid Receptor Modulators; D063388:Endocannabinoids; D058913:Purinergic Agonists; D043884:Receptor, Cannabinoid, CB1; D011983:Receptors, Purinergic; D058469:Receptors, Purinergic P2X; D026941:Sodium Channel Blockers; D000255:Adenosine Triphosphate