GABA modulates presynaptic signalling mediated by dinucleotides on rat synaptic terminals.

Diadenosine pentaphosphate (Ap(5)A) elicits Ca(2+) transients in isolated rat midbrain synaptic terminals acting through specific ionotropic dinucleotide receptors. The activation of GABA(B) receptors by baclofen changes the sigmoidal concentration-response curve for Ap(5)A (EC(50) = 44 microM) into biphasic curves. Thus, when GABA(B) receptors are activated, the curve shows a high-affinity component in the picomolar range (EC(50) = 77 pM) and a low-affinity component in the micromolar range (EC(50) = 17 microM). In addition, in the presence of GABA or baclofen, Ap(5)A calcium responses are increased up to 50% over the control values. Saclofen, a specific antagonist of GABA(B) receptors, blocks the potentiatory effect of baclofen. As occurs with Ap(5)A, GABA(B) receptors are also capable to modulate diguanosine pentaphosphate (Gp(5)G)-induced calcium responses. The combination of immunocytochemical and microfluorimetric techniques carried out on single synaptic terminals have shown that in the presence of baclofen, 64% of the terminals responding to 100 microM Ap(5)A are also able to respond to 100 nM Ap(5)A. This value is close to the percentage of synaptic terminals responding to Ap(5)A and labeled with the anti-GABA(B) receptor antibody (69%). The activity of cyclic AMP-dependent protein kinase (PKA) seems to be involved in the potentiatory effect of GABA(B) receptors on Ap(5)A calcium responses, because PKA activation by forskolin or dibutiryl cyclic AMP blocks the potentiatory effect of baclofen, whereas PKA inhibition facilitates calcium signaling mediated by Ap(5)A. These results demonstrate that the activation of presynaptic GABA(B) receptors is able to modulate dinucleotide responses in synaptic terminals.