Title

 

Authors

A. Cavallero¹, E. Gardella¹, C. Brullo², E. Canepa³, O. Bruno², J. Prickaerts⁴, L.A. Parker⁵, R. Ricciarelli³ and E. Fedele¹. 
 
¹Dept. of Experimental Medicine, Pharmacology and Toxicology Section; ²Dept. of Pharmaceutical Sciences; ³Dept. of Experimental Medicine, Section of General Pathology, University of Genoa, Italy. ⁴Department of Psychiatry and Neuropsychology, School for Mental Health and Neuroscience, University of Maastricht, the Netherlands. ⁵Department of Psychology, University of Guelph, Canada.

 

Abstract

A large body of evidence indicates that pharmacological blockade of cerebral cAMP degradation by PDE4s results in memory-enhancing effects. Therefore, PDE4 inhibitors could be useful in pathologies characterized by cognitive deficits, such as Alzheimer’s disease (AD). Despite their clinical relevance, however, PDE4 inhibitors (i.e. rolipram) produce severe emetic effects. In the present study, we have evaluated the in vivo effects of GEBR-7b, a novel selective PDE4D inhibitor, on hippocampal cAMP levels, memory performances and emetic-like behaviour in rodents. Moreover, since it has been suggested that PDE4D inhibitors might be useful in AD and that Aβ peptides could modulate memory processes by positively or negatively influencing hippocampal LTP, we have also investigated the effects of GEBR-7b on Aβ levels both in vivo and in vitro. Hippocampal cAMP levels have been measured in unanaesthetised freely moving rats by intracerebral microdialysis, memory performances have been assessed using object recognition tests, the taste reactivity test and the xylazine/ketamine anaesthesia test have been used to evaluate the emetic potential of the drug. Aβ levels have been measured in hippocampal tissues and cultured N2a cells by ELISA. Rolipram has been used as comparator drug. In the hippocampus, retrodialysis administration of GEBR-7b increased extracellular cAMP. At 30 µM, the drug slightly, though not significantly, augmented extracellular cAMP, whereas a significant and constant 40-50% increase was observed at 100 µM. Infusion of rolipram also increased extracellular cAMP, but the effect was more pronounced (50-60% at 30 µM and 150-200% at 100 µM). In the memory tasks, GEBR-7b was effective in improving rat performances both in the object location and in the object recognition tests at the doses of 0.001 and 0.003 mg/kg. Similar results were obtained with mice in the object location test. Rolipram also improved memory in these tests but at doses that were 3 to 30 times higher than those required for GEBR-7b. In the taste reactivity test, GEBR-7b did not induce emetic-like behaviour in rats (conditioned gaping) at doses up to 0.3 mg/kg, whereas rolipram showed significant effects. In the xylazine/ketamine test, rolipram reduced the duration of anaesthesia in mice already at the dose effective in the memory test (0.03 mg/kg), whereas GEBR-7b was devoid of any effect up to 0.1 mg/kg. GEBR-7b showed a significant reduction of anaesthesia only at the dose of 0.3 mg/kg. Finally, GEBR-7b did not affect Aβ 40 and Aβ 42 levels in hippocampal tissues, following systemic administration of 0.003 or 0.1 mg/kg, or in N2a cultured cells exposed to 0.01-1 µM of the drug. Our data show that GEBR-7b increases hippocampal cAMP and improves memory functions at doses that do not cause emesis-like behaviour in rodents. These findings further support the idea that selective PDE4D inhibitors could represent innovative cognitive-enhancing drugs devoid of side-effects characteristic of non selective PDE4 blockers.