1 Department of Clinical Medicine - Translational Neuropsychiatry Unit, Department of Clinical Medicine, Health, Aarhus University2 Department of Clinical Medicine, Health, Aarhus University3 Department of Clinical Medicine - Translational Neuropsychiatry Unit, Department of Clinical Medicine, Health, Aarhus University
Enhanced cAMP signalling is associated with antidepressant activity  and is attributed to the activation of cAMP-dependent protein kinase (PKA) and the subsequent phosphorylation of cAMP-response element binding protein (CREB) . However, to our knowledge only a single previous study has demonstrated antidepressant-like activity following the direct activation of PKA . In this project we critically evaluate this notion by investigating the mood-altering actions of a PKA inhibitor, Rp-8-Br-cAMPS, in the rat forced swim test (FST) while correlating these results with the cAMP concentration in several brain areas. Furthermore, we investigated the effect of a cGMP-dependent protein kinase (PKG) inhibitor, Rp-8-Br-PET-cGMPS, and measured cGMP levels given the significant cross-talk mechanisms between the cAMP and cGMP signalling pathways. Male Sprague Dawley rats were anaesthetised and implanted with a guide cannula just above the right lateral ventricle to allow for intracerebroventricular (i.c.v.) delivery of drug or vehicle solutions. Rats received three i.c.v. infusions during a 24 hour period before the final test. The standard FST protocol was used and immobility was scored as readout for depression-like behaviour. cAMP and cGMP levels were measured in the frontal cortex, hippocampus and cerebellum by using commercially available ELISA kits. All data were analysed using one-way analyses of variance (ANOVA) followed by Dunnett’s multiple comparison tests. Rp-8-Br-cAMPS alone and in combination with Rp-8-Br-PET-cGMPS reduced immobility in the FST [F(3,15) = 13.09, p = 0.0002]. Rp-8-Br-PET-cGMPS alone did not significantly alter immobility. The concentration of cAMP was increased by Rp-8-Br-cAMPS alone as well as in combination with Rp-8-Br-PET-cGMPS in the hippocampus [F(3,14) = 13.15, p = 0.0002], frontal cortex [F(3,14) = 9.04, p = 0.0014] and cerebellum [F(3,15) = 6.06, p = 0.0065]. Rp-8-Br-PET-cGMPS significantly increased cAMP levels only in the frontal cortex. The concentration of GMP was increased by Rp-8-Br-cAMPS alone and in combination with Rp-8-Br-PET-cGMPS in the hippocampus [F(3,14) = 5.39, p = 0.011] and cerebellum [F(3,14) = 8.14, p = 0.0022] but not in the frontal cortex [F(3,15) = 2.60, p = 0.091]. Rp-8-Br-PET-cGMPS did not significantly alter the cGMP levels in any of brain regions studied. This study demonstrates for the first time an antidepressant-like response following the direct inhibition of PKA. This result may be explained either by PKA-dependent mechanisms, for example the disinhibition of a variety of G-protein coupled receptor subtypes (e.g. adrenergic-, dopaminergic- and metabotropic glutamate receptors), or by cAMP-mediated, PKA-independent mechanisms such as the activation of exchange protein directly activated by cAMP (Epac) 1 and 2 that subsequently activates Rap-1, a member of the Ras family of small GTPases with a variety of important physiological functions. Nonetheless, the data from this study challenge the general notion that the antidepressant-like activity induced by elevated cAMP is predominantly mediated via the activation of PKA, and emphasise the complexity of cAMP signalling mechanisms in antidepressant action.