The selective PDE5 inhibitor, sildenafil, improves object memory in Swiss mice and increases cGMP Levels in hippocampal slices

doi:10.1016/j.bbr.2005.04.021

Behavioural Brain Research

Volume 164, Issue 1, 14 October 2005, Pages 11-16

https://doi.org/10.1016/j.bbr.2005.04.021 Get rights and content

Abstract

Previous studies have shown memory enhancing effects of phosphodiesterase type 5 (PDE5) inhibitors in rats. However, differences in nitric oxide (NO)-mediated cyclic GMP (cGMP) signaling in the hippocampus have been described between rats and mice. In the present study we investigated the memory enhancing effects of the PDE5 inhibitor, sildenafil on memory performance in Swiss mice using the object recognition task. Sildenafil (0.3, 1 and 3 mg/kg) was administered orally directly after the first trial. The memory for the objects was retested 24 h later when mice show no memory for the familiar object. Sildenafil improved the object discrimination performance of Swiss mice at a dose of 1 mg/kg. Hippocampal slices of Swiss mice incubated with sildenafil (10 μM) increased cGMP levels in varicosities in the CA3 region of the hippocampus and a number of short, thin fibers. Addition of DEA/NO, an NO donor (10 μM), in the presence of sildenafil (10 μM) strongly increased cGMP immunoreactivity of varicosities in the CA3 region. Double immunostaining of cGMP with the presynaptic marker synaptophysin did not reveal any co-localization of these markers under any circumstance. Taken together, inhibition of PDE5 improves object recognition memory in mice. Furthermore, a postsynaptic role of cGMP could be involved in this respect.

Introduction

Recently, evidence has been accumulating that cyclic GMP (cGMP) plays a role in memory processes, e.g. [3], [14]. Accordingly, inhibition of the cGMP hydrolysing phosphodiesterase type 5 (PDE5) may be an important tool to improve early consolidation of memory (see [15]).

In vitro experiments demonstrated that the selective PDE5 inhibitors zaprinast, vardenafil and sildenafil increase NO-mediated cGMP accumulation in the dorsal hippocampus of rats as assessed with radioimmunoassay and immunocytochemistry [5], [14], [20].

All these experiments have been performed in rat tissues. Recently, we observed that in the mouse hippocampus slice preparation cGMP immunoreactivity was already observed in astrocytes without stimulation of a NO donor [21]. Under similar conditions, such a pattern of cGMP immunoreactivity has never been observed in the rat hippocampus. Thus, there are differences in the cellular localisation and activity of NO-mediated cGMP signalling between mouse and rat hippocampus. Nevertheless, the expression pattern of PDE 5 mRNA were not different between the species [21].

Considering the differences in NO-mediated cGMP signalling mentioned above, the aim of the present study was to examine the memory-enhancing properties of PDE5 inhibitors in mice. For the first time the object recognition task (ORT) was used to study the effects of PDE5 inhibition, i.e. sildenafil, on the delay-dependent forgetting in mice.

In addition to the behavioural study, we evaluated the effects of PDE5 inhibition on cGMP immunoreactivity (cGMP-IR) in hippocampal slices using cGMP immunocytochemistry in a separate experiment. Since NO, as a retrograde messenger, is assumed to mediate a presynaptic effect [1], [18], we also analyzed whether cGMP immunoreactivity co-localized with the presynaptic marker synaptophysin.

Section snippets

Animals

All experimental procedures were approved by the local ethical committee of the Maastricht University for animal experiments and met governmental guidelines. Ten 6-month-old male Swiss mice (Charles River, NL) were used for behavioral testing and four mice were used for immunocytochemistry. The animals were housed individually in standard Makrolon cages on sawdust bedding in an air-conditioned room (about 20 °C). The mice were not housed in the same room in which they were tested. A radio, which

Effects of sildenafil treatment in the ORT

The results of treatment with sildenafil are summarized in Table 2. Additional analysis of measures of location preference (left or right side of arena) or object preference showed no differences between conditions (data not shown). In T1 there was no difference between treatment sessions on the total level of exploration (e₁: F(3,27) = 2.92, n.s.). However, the total level of exploration 24 h after T1 was different between treatment conditions (e₂: F(3,27) = 5.51, P < 0.01). Post-hoc comparisons

Behavioral studies

As shown in a previous study, mice can be tested in the ORT and the relative discrimination index (d₂) can be used as a measure of object memory in mice [17]. The present study investigated the effect of the selective PDE5 inhibitor sildenafil on the object recognition task in mice. The levels of exploration did not differ between treatment conditions in the first trial. However, in the second trial the level of exploration for the vehicle condition was lower than for the highest dose condition

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Research reportThe selective PDE5 inhibitor, sildenafil, improves object memory in Swiss mice and increases cGMP Levels in hippocampal slices

Abstract

Introduction

Section snippets

Animals

Effects of sildenafil treatment in the ORT

Behavioral studies

Neuroscience

J Chem Neuroanat

Acta Histochem

Eur J Pharmacol

Eur J Pharmacol

Neurochem Int

Eur J Pharmacol

Brain Res

Brain Res

Neuron

Presynaptic role of cGMP-dependent protein kinase during long-lasting potentiation

J Neurosci

Research report
The selective PDE5 inhibitor, sildenafil, improves object memory in Swiss mice and increases cGMP Levels in hippocampal slices