Type 4 Phosphodiesterase Plays Different Integrating Roles in Different Cellular Domains in Pyramidal Cortical Neurons

We investigated the role of phosphodiesterases (PDEs) in the integration of cAMP signals and protein kinase A (PKA) activity following beta-adrenergic stimulation, by carrying out real-time imaging of male mouse pyramidal cortical neurons expressing biosensors to monitor cAMP levels (Epac1-camps and Epac2-camps300) or PKA activity (AKAR2). In the soma, isoproterenol (ISO) increased the PKA signal to approximately half the maximal response obtained with forskolin, with a characteristic beta(1) pharmacology and an EC50 of 4.5 nM. This response was related to free cAMP levels in the submicromolar range. The specific type 4 PDE (PDE4) inhibitor rolipram had a very small effect alone, but strongly potentiated the PKA response to ISO. Blockers of other PDEs had no effect. PDE4 thus acts as a brake in the propagation of the beta(1)-adrenergic signal from the membrane to the bulk somatic cytosol. The results for a submembrane domain were markedly different, whether recorded with a PKA-sensitive potassium current related to the slow AHP or by two-photon imaging of small distal dendrites. The responses to ISO were stronger than in the bulk cytosol. This is consistent with the cAMP/PKA signal being strong at the membrane, as shown by electrophysiology, and favored in cellular domains with a high surface area to volume ratio, in which this signal was detected by imaging. Rolipram alone also produced a strong cAMP/PKA signal, revealing tonic cAMP production. PDE4 thus appears as a crucial integrator with different physiological implications in different subcellular domains.