Rolipram reverses scopolamine-induced and time-dependent memory deficits in object recognition by different mechanisms of action

Abstract

In this study, the effect of the selective phosphodiesterase type 4 (PDE4) inhibitor rolipram on memory performance was investigated using the object recognition task. First, three doses of rolipram (0.01, 0.03 or 0.1 mg/kg) were tested with a 24 h delay between the learning (T1) and the test (T2) trial. Doses of rolipram were injected at different time points (30 min before T1, immediately after T1 or 3 h after T1). In a second experiment, the effects of rolipram (0.03, 0.1 or 0.3 mg/kg) were tested in combination with scopolamine (0.1 mg/kg) applying a 1 h delay between trials. Both substances were administered 30 min before T1. Using a 24 h interval, rolipram showed an improvement in long-term memory performance when injected 3 h after T1 at a dose of 0.03 mg/kg. Further, rolipram reversed the scopolamine-induced short-term memory deficit at a dose of 0.1 mg/kg. Although the improved memory performance in both conditions is likely to be explained by elevated cAMP levels, two separate working mechanisms might explain these effects.

Introduction

Cyclic adenosine monophosphate (cAMP) plays an important role in intracellular signaling (Bailey, Bartsch, & Kandel, 1996) and in processes of neuroplasticity such as long-term potentiation (LTP). Hippocampal LTP has been proposed to be a neurophysiological correlate of memory (Bliss & Collingridge, 1993). LTP consists of two phases, an early phase (E-LTP), which does not require gene transcription and lasts less than 3 h and a late phase (L-LTP) depending on gene transcription and protein synthesis and which lasts longer (>3 h). The postsynaptic cAMP/protein kinase A (PKA)/cAMP responsive element binding protein (CREB) pathway, as well as the postsynaptic cGMP/PKG/CREB pathway, are involved in L-LTP (Frey et al., 1993, Impey et al., 1996, Lu and Hawkins, 2002, Lu et al., 1999, Slack and Walsh, 1995).

Rolipram is a selective inhibitor of the phosphodiesterase type 4 (PDE4) enzyme, which hydrolyzes cAMP. Thus, PDE4 inhibition enhances the intracellular availability of cerebral cAMP in the absence of direct stimulation of neurotransmitter receptors (Silvestre, Fernandez, & Palacios, 1999). Studies investigating PDE4 function using rolipram indicate that cAMP is involved in neural processes that underlie learning and memory (Bach et al., 1999, Barad et al., 1998, Zhang and O’Donnell, 2000b). Accordingly, a substantial amount of studies have shown a positive effect of PDE4 inhibition on spatial memory (Bach et al., 1999, Gong et al., 2004, Nagakura et al., 2002), inhibitory avoidance learning (Egawa et al., 1997, Ghelardini et al., 2002, Imanishi et al., 1997, Randt et al., 1982, Zhang et al., 2000a, Zhang and O’Donnell, 2000b), contextual fear conditioning (Barad et al., 1998, Gong et al., 2004) and object recognition memory (Bourtchouladze et al., 2003). Moreover, several studies have shown an involvement of PDE4 inhibitors in working and reference memory (Egawa et al., 1997, Zhang et al., 2000a, Zhang et al., 2004, Zhang and O’Donnell, 2000b).

Previous work showed that intra-hippocampal infusion of 8Br-cAMP improved memory performance when injected 3 or 6 h after one-trial training, but not when injected directly after the training trial (Bernabeu et al., 1997, Bernabeu et al., 1996, Prickaerts et al., 2002a). There is additional evidence that the late phase of memory consolidation is modulated by a cAMP/PKA signaling pathway in the hippocampus (Bernabeu et al., 1997). To our knowledge, all studies that claim to improve memory performance via the cAMP/PKA pathway administered rolipram before the training trial. However, the late phase of memory consolidation, in which the cAMP/PKA pathway is involved, occurs approximately 3 h after training. At this time point cAMP levels are probably minimal due to its metabolic clearance (T_1/2 = 1–3 h, Krause & Kuhne, 1988). Therefore, another mechanism of action is probably responsible for the memory enhancing effects of rolipram when injected before T1. However, to our knowledge there is no profound evaluation of the effects of cAMP on early versus late consolidation. Furthermore, most cAMP evaluating studies have investigated the effects of cAMP via rolipram treatment in a scopolamine-induced short-term memory deficit model. Therefore, we decided to test the effects of rolipram on long-term memory performance by administering the drug at different time points.

To study the memory enhancing properties of rolipram, we investigated its effects in an object recognition task (ORT) (Ennaceur & Delacour, 1988). The ORT is a memory test that relies on a rat’s innate explorative behavior. In the first trial, the animal is presented with two objects. In the second trial, after a variable delay, one familiar and one new object are presented. In both trials, the amount of time spent exploring the objects is recorded. If an animal has memory of the objects presented in trial 1 (T1), it will explore the novel object more in the second trial (T2). Depending on the time of treatment, drug effects on acquisition, consolidation or retrieval of object memory can be tested (Prickaerts, Sik, van der Staay, de Vente, & Blokland, 2005). Treatments before the first trial influence acquisition or early consolidation processes, whereas treatments after T1 will affect early or late consolidation processes. The present study investigated the effects of different doses of rolipram on different long-term memory processes, i.e., acquisition (by injecting the drug 30 min before T1), early (by injecting the drug immediately after T1) and late (by injecting the drug 3 h after T1) consolidation, by using a 24 h delay. In addition, rolipram was tested in the scopolamine model for short-term memory deficits using a 1 h delay. This was done to replicate previous findings and for comparison of different dose–response curves, i.e., long-term vs. short-term memory. Of note, the effects of rolipram on the scopolamine deficit model were never before examined in the object recognition task.