In vivo effects of phosphodiesterase inhibition on basal cyclic guanosine monophosphate levels in the prefrontal cortex, hippocampus and cerebellum of freely moving rats

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Abstract
We have characterized the various phosphodiesterases (PDE) that degrade cyclic GMP in the prefrontal cortex, hippocampus, and cerebellum using the microdialysis technique to measure in vivo extracellular cyclic GMP in awake rats. The following PDE blockers were used (100 and 1,000 μM): 8‐methoxymethyl‐IBMX (8‐MM‐IBMX), erythro‐9‐(2‐hydroxy‐3‐nonyl)adenine (EHNA), milrinone, rolipram, and zaprinast. For solubility reasons, sildenafil was tested only at 100 μM. All drugs were administered locally in the brain regions through the dialysis probe. At 100 μM, 8‐MM‐IBMX enhanced the cyclic nucleotide extracellular levels in the prefrontal cortex and hippocampus but not in the cerebellum; EHNA and milrinone were active only in the hippocampus; rolipram was devoid of any effect; zaprinast and sildenafil were effective in all three brain areas. At 1 mM, 8‐MM‐IBMX, milrinone, and zaprinast increased extracellular cyclic GMP in all the brain regions examined, EHNA became active also in the prefrontal cortex and rolipram showed a significant effect only in the cerebellum. This is the first in vivo functional study showing that, in cortex, PDE1, ‐2, and ‐5/9 degrade cGMP, with PDE9 probably playing a major role; in hippocampus, PDE5/9 and PDE1 are mainly involved and seem almost equally active, but PDE2 and ‐3 also contribute; in cerebellum, PDE5/9 are the main cGMP hydrolyzing enzymes, but also PDE1 and ‐4 significantly operate.