Dinucleotide receptor modulation by protein kinases (protein kinases A and C) and protein phosphatases in rat brain synaptic terminals

The diadenosine polyphosphates, diadenosine tetraphosphate and diadenosine pentaphosphate (Ap(5)A), can activate an ionotropic dinucleotide receptor that induces Ca2+ transients into synaptosomes prepared from rat brain. This receptor, also termed the P-4, purinoceptor, is sensitive only to adenine dinucleotides and is insensitive to ATP. Studies on the modulatory role of protein kinase A (PKA), protein kinase C (PKC), and protein phosphatases on the response of diadenosine polyphosphate receptors were performed by measuring the changes in the intracellular Ca2+ levels with fura-2. Activation and inhibition of PKA were carried out by means of forskolin and the PKA inhibitory peptide (PKA-IP), respectively. The Ap(5)A response was inhibited by forksolin to 35% of control values, but PKA-IP induced an increase of 37%. The effect of PKC activation was similar to that observed for PKA. PKC stimulation with phorbol 12,13-dibutyrate produced an inhibition of 67%, whereas the PKC inhibitors staurosporine and PKC inhibitory peptide enhanced the responses elicited by Ap(5)A to 40% in both cases. Protein phosphatase inhibitors diminished the responses elicited by Ap(5)A to 17% in the case of okadaic acid, to 50% for microcystin, and to 45% in the case of cyclosporin A. Thus, the activity of dinucleotide receptors in rat brain synaptosomes appears to be modulated by phosphorylation/dephosphorylation. These processes could be of physiological significance in the control of transmitter release from neurons that are postsynaptic to nerves that release diadenosine polyphosphates.