Vol. 301, Issue 2, 441-450, May 2002

Modulation of the Rat Hippocampal Dinucleotide Receptor by Adenosine Receptor Activation

Miguel Díaz-Hernández, M. Fátima Pereira, Jesús Pintor, Rodrigo A. Cunha, J. A. Ribeiro and María Teresa Miras-Portugal

Departamento de Bioquímica, Facultad de Veterinaria, Universidad Complutense de Madrid, Madrid, Spain (M.D.H., J.P., M.T.M.P.); Laboratório de Neurociências, Faculdade de Medicina, Universidade de Lisboa, Portugal (M.F.P., J.A.R.); and Centro de Neurociências de Coimbra, Portugal (R.A.C.)

Diadenosine pentaphosphate (Ap₅A) and ATP stimulate an intracellular free calcium concentration ([Ca²⁺]_I) increase in rat hippocampal synaptosomes via different receptors as demonstrated by the lack of cross-desensitization between Ap₅A and ATP responses. The ATP response was inhibited by P2 receptor antagonists and not by the dinucleotide receptor antagonist, diinosine pentaphosphate (Ip₅I). In contrast, the Ap₅A response was inhibited by Ip₅I but not by P2 receptor antagonists. Studies in single hippocampal synaptic terminals showed that 31% of them responded to Ap₅A by a [Ca²⁺]_i increase. Adenosine receptors (A₁, A_2A, and A₃) were also present in isolated terminals as demonstrated by immunohistochemistry. The activation of A₁ or A_2A receptors by specific agonists changed the sigmoid concentration-response curve for Ap₅A (EC₅₀ = 33.5 ± 4.5 µM) into biphasic curves. When the high-affinity adenosine receptors A₁ or A_2A were activated, the Ap₅A dose-response curves showed a high-affinity component with EC₅₀ values of 41.1 ± 1.9 pM and 99.9 ± 10.2 nM, respectively. The low-affinity component showed EC₅₀ values of 17.1 ± 0.8 and 21.6 ± 1.4 µM for A₁ and A_2A receptor activation, respectively. However, the adenosine A₃ receptor activation induced a right shift of the dinucleotide concentration-response curve, showing an EC₅₀ value of 331.4 ± 54.6 µM. In addition, in the presence of the A_2A agonist, the Ap₅A calcium influx responses were increased up to 300% of the control values. These results clearly demonstrate that the activation of presynaptic adenosine receptors is able to modulate the dinucleotide response in hippocampal nerve terminals.