Phorbol ester uncouples adenosine inhibition of presynaptic Ca2+ transients at hippocampal synapses.
Synaptic transmission involves Ca2+ influx at presynaptic terminals. Adenosine receptors inhibit transmission, and this effect can be abolished by activation of PKC with phorbol esters. Whether protein kinase C (PKC) acts via alterations in Ca2+ entry at the presynaptic terminal is unknown. In the present study, we recorded the presynaptic Ca2+ transients (preCa(delta)) in hippocampal stratum radiatum, using fluorescence photometry. The calcium dye Fura-2 AM was used to load the Schaffer collateral/commissural tract and its terminals. Tetrodotoxin (TTX)-sensitive Na+ channels and Cd2+-sensitive, high-voltage activated Ca2+ channels (HVACCs) were required to elicit the preCa(delta). Application of the phorbol ester phorbol-12,13-dibutyrate (PDBu) abolished the adenosine inhibition of both preCa(delta) and the field excitatory postsynaptic potentials (fEPSPs). PDBu consistently potentiated fEPSPs, and also increased preCa(delta) in a large majority of the slices examined. Regardless of whether potentiation was observed, PDBu always prevented adenosine inhibition of preCa(delta). In contrast, the inactive phorbol ester, 4alpha-phorbol, did not alter adenosine inhibition of preCa(delta), indicating that PKC activation is necessary for the occurrence of the observed effects. Our findings suggest that PKC activation abolishes adenosine's inhibitory effect on synaptic activity involving presynaptic Ca2+ entry.
Stocca
Gabriella
G
Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA. gstocca@mail.nih.gov
Lovinger
David M
DM
D000241:Adenosine / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D016257:Fura-2; D006624:Hippocampus / Q000187:drug effects / Q000201:enzymology*; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D009434:Neural Pathways / Q000187:drug effects / Q000201:enzymology*; D009924:Organ Culture Techniques; D010703:Phorbol Esters / Q000494:pharmacology; D010783:Photometry; D017729:Presynaptic Terminals / Q000187:drug effects / Q000201:enzymology*; D011493:Protein Kinase C / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D018047:Receptors, Purinergic P1 / Q000187:drug effects / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology
D015220:Calcium Channels; D010703:Phorbol Esters; D018047:Receptors, Purinergic P1; D013779:Tetrodotoxin; D011493:Protein Kinase C; D000241:Adenosine; D002118:Calcium; D016257:Fura-2