Effects of two selective phosphodiesterase type 5 inhibitors, sildenafil and vardenafil, on object recognition memory and hippocampal cyclic GMP levels in the rat.

ABSTRACT

The present study investigated the effects of two cyclic GMP-specific phosphodiesterase enzyme type 5 inhibitors, sildenafil and vardenafil, on the memory performance in the object recognition task. Both compounds were given per orally (1, 3 and 10 mg/kg sildenafil; 0.1, 0.3, 1 and 3 mg/kg vardenafil) immediately after the exposure to two identical objects. The memory for the objects was tested 24 h later. Vehicle-treated rats spent equal times exploring a new and the familiar object demonstrating that they did not remember the familiar one. However, sildenafil improved the object discrimination performance of the rats with a high discrimination performance at a dose of 3 mg/kg. Rats treated with vardenafil also showed an improved object discrimination performance. Compared with sildenafil, vardenafil appeared to be even more potent in this respect since it already produced a high discrimination performance at a dose of 0.3 mg/kg. The effects of both compounds on cyclic GMP and cyclic AMP accumulation were studied in rat hippocampal slices incubated in vitro. Cyclic GMP levels were increased after incubation with the highest concentration of 100 microM vardenafil (together with 0.1 mM sodium nitroprusside), although no changes in cyclic GMP levels were detected after incubation with different concentrations of sildenafil. Both compounds had no effect on cyclic AMP levels. Additional cyclic GMP immunocytochemistry showed that incubation with vardenafil (in the presence of sodium nitroprusside) resulted in a concentration-dependent staining of cyclic GMP. Staining was predominantly found in neuronal fibres in the hippocampal CA2/CA3 region. It was already detected at a concentration of 0.1 microM vardenafil. Also positive fibres were detected after incubation with sildenafil but at a higher concentration of 10 microM. Taken together, these results suggest that inhibition of phosphodiesterase enzyme type 5 improves object recognition memory. This effect might be explained by increased levels of central cyclic GMP.