1 Department of Clinical Medicine - Translational Neuropsychiatry Unit, Department of Clinical Medicine, Health, Aarhus University2 Department of Clinical Medicine, Health, Aarhus University3 Centre for Psychiatric Research, Faculty of Health Sciences, Aarhus University, Aarhus University4 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.