1 Department of Clinical Medicine - Translational Neuropsychiatry Unit, Department of Clinical Medicine, Health, Aarhus University2 Centre for Psychiatric Research, Faculty of Health Sciences, Aarhus University, Aarhus University3 Department of Clinical Medicine - Translational Neuropsychiatry Unit, Department of Clinical Medicine, Health, Aarhus University
Background: It is well established that cyclic adenosine monophosphate (AMP) signalling via cAMP-dependent protein kinase (PKA) within neurons plays an important role in depression and antidepressant treatment. However, the importance of several newly discovered targets that function independently from PKA, such as exchange protein activated by cAMP (Epac), remains unexplored in this regard. Objectives: In this study we used a cAMP analogue that inhibits PKA but not Epac (Rp-8-Br-cAMPS), to explore the modifying actions of these two targets on immobility in the forced swim test (FST) and cerebellar CREB phosphorylation in rats. In addition, we assessed central cAMP and cGMP levels and investigated the involvement of cGMP-dependent protein kinase (PKG) on any observed effects by using a selective PKG inhibitor (Rp-8-Br-PET-cGMPS). Methods: Rats were implanted with an i.c.v. guide cannula to the right lateral ventricle and were allowed 7 days for recovery. Animals were subjected to a 15 minute pre-swim followed by a 5 min test swim 24 hours later. Three 5 μl infusions of vehicle (Ringer’s solution) or drug solution (Rp-8-Br-cAMPS (100 nmol), Rp-8-Br-PET-cGMPS (1 nmol) or both) were administered at 24, 6 and 1 hour before the final swim. Rats were decapitated and their brains dissected immediately following the test. Western blotting was used for the measurement of total and phosphorylated CREB levels in the cerebellum, whereas low pH ELISA kits (Sigma) were used for the measurement of cAMP and cGMP levels in hippocampus, frontal cortex and cerebellum regions. Results: Rp-8-Br-cAMPS potently reduced immobility in the FST and increased the phosphorylation of CREB in the cerebellum, effects that were unaltered by the co-administration of Rp-8-Br-PET-cGMPS. Furthermore, Rp-8-Br-cAMPS increased the accumulation of cAMP and cGMP in the hippocampus, frontal cortex and cerebellum of these rats. Conclusion: Together, these results suggest that PKA may not be the only mediator of cAMP-induced antidepressant activity, but that other mechanism(s) may play at least an equally important role in this regard. Based on the pharmacodynamic profile of Rp-8-Br-cAMPS, the relatively recently discovered Epac messenger molecule emerges as a likely candidate to fulfil this role. Therefore, this study identifies Epac as a promising novel antidepressant target.