Phosphodiesterase type 5 (PDE5) inhibition improves object recognition memory: Indications for central and peripheral mechanisms

Abstract

A promising target for memory improvement is phosphodiesterase type 5 (PDE5), which selectively hydrolyzes cyclic guanosine monophosphate (cGMP). In rodents, PDE5 inhibitors (PDE5-Is) have been shown to improve memory performance in many behavioral paradigms. However, it is questioned whether the positive effects in animal studies result from PDE5 inhibition in the central nervous system or the periphery. Therefore, we studied the effects of PDE5 inhibition on memory and determined whether compound penetration of the blood-brain barrier (BBB) is required for this activity. Two selective PDE5-Is, vardenafil and UK-343,664, were tested in the object recognition task (ORT) in both a MK-801- and scopolamine-induced memory deficit model, and a time-delay model without pharmacological intervention. Compounds were dosed 30 min before the learning trial of the task. To determine if the PDE5-Is crossed the BBB, their concentrations were determined in plasma and brain tissue collected 30 min after oral administration. Vardenafil improved object recognition memory in all three variants of the ORT. UK-343,664 was ineffective at either preventing MK-801-induced memory disruption or time-dependent memory decay. However, UK-343,664 attenuated the memory impairment of scopolamine. Vardenafil crossed the BBB whereas UK-343,664 did not. Further, co-administration of UK-343,664 and scopolamine did not alter the brain partitioning of either molecule. This suggests that the positive effect of UK-343,664 on scopolamine-induced memory decay might arise from peripheral PDE5 inhibition. The results herein suggest that there may be multiple mechanisms that mediate the efficacy of PDE5 inhibition to improve memory performance in tasks such as the ORT and that these involve PDE5 located both within and outside of the brain. To further elucidate the underlying mechanisms, the cellular and subcellular localization of PDE5 needs to be determined.

Introduction

With the seminal findings of Barad, Bourtchouladze, Winder, Golan, and Kandel (1998) on the role of PDE4 in the regulation of long term potentiation (LTP) and learning and memory, there has grown a broad interest in the phosphodiesterases (PDEs) as molecular targets to treat neuropsychiatric dysfunction (e.g. Halene and Siegel, 2007, Menniti et al., 2006). PDEs are enzymes that inactivate the second messenger molecules cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). There are eleven families of PDEs, distinguished by molecular properties, substrate specificity, and regulation (Bender & Beavo, 2006). These enzymes are expressed in unique and overlapping patterns throughout the body, and in particular in the CNS (Lakics, Karran, & Boess, 2010). Both cAMP and cGMP play important roles in regulating processes of neuroplasticity, including LTP, that form the neurophysiological bases of learning and memory (Bliss and Collingridge, 1993, Frey et al., 1993, Son et al., 1998). Pharmacological inhibitors of different PDE families offer means to modulate discreet cyclic nucleotide signaling pathways involved in different aspects of learning and memory, possibly for therapeutic benefit.

A PDE family of particular interest with regard to pharmacological targeting is PDE type 5 (PDE5). PDE5 inhibitors (PDE5-Is), including sildenafil, tadalafil, and vardenafil, are the first PDE-Is to achieve widespread clinical use, especially for the treatment of conditions caused by vascular dysfunction. PDE5 is specific for the metabolic inactivation of cGMP. The enzyme is expressed at high levels in smooth muscle of discreet vascular beds, where PDE5 inhibition causes increases in smooth muscle cGMP which results in vascular relaxation. Initially, PDE5-Is were of interest to reduce systemic blood pressure but were not sufficiently efficacious for commercial development. However, PDE5 is expressed at high levels in the vascular bed of the penis and these compounds found therapeutic utility and commercial success in the treatment of male erectile dysfunction. PDE5 is also expressed at high levels in the lung, and PDE5-Is are now being used successfully to treat pulmonary hypertension. It has also been found that PDE5 inhibition regulates vascular tone in the prostate and there is growing interest in the use of PDE5-Is for treatment of benign prostatic hyperplasia. Thus, the success of PDE5-Is derives from the fundamental role of the enzyme in the regulation of vascular tone, with disease utility tied to enzyme localization in specific vascular beds (for a review see also Ghofrani et al., 2006, Puzzo et al., 2008).

cGMP signaling cascades are prominently represented in the control of neuronal function including the pre- and post-synaptic mediation of different forms of neuroplasticity (Kleppisch and Feil, 2009, Son et al., 1998). Thus, it is of interest whether PDE5 is involved in the regulation of one or more such neuronal signaling cascades and whether PDE5-Is may impact cognitive function. In fact, there are now a number of reports of PDE5-Is improving learning and memory performance in animals. This includes improved memory for novel objects in the object recognition task (ORT) (Domek-Lopacinska and Strosznajder, 2008, Prickaerts et al., 2005, Rutten et al., 2007, Rutten et al., 2009, van Donkelaar et al., 2008), avoidance learning (Baek et al., 2011, Baratti and Boccia, 1999, Boccia et al., 2011, Devan et al., 2004) and complex maze learning in rodents (Devan et al., 2004) (for an overview see Reneerkens, Rutten, Steinbusch, Blokland, & Prickaerts, 2009). Particularly interesting is the finding that chronic treatment with a PDE5-I caused a long lasting improvement in memory function and reduced plaque load in a mouse model of the amyloid deposition of Alzheimer’s disease (AD) (Puzzo et al., 2009), although the decrease in amyloid burden has not been replicated in a more recent study (Cuadrado-Tejedor et al., 2011). However, PDE5 mRNA and protein is expressed at only low levels or not observed in forebrain regions thought to mediate learning and memory function in these types of tasks (see Section 4). This raises the question of localization of the PDE5 target and mechanism of action that accounts for the pro-cognitive effects of PDE5-Is. Previously, we reported that cognitive enhancing doses of PDE5-Is do not affect cerebral blood flow and glucose utilization, indicating that such vascular effects do not account for pro-cognitive efficacy (Rutten et al., 2009). In the present study, we take another approach to localize the PDE5 target involved in the pro-cognitive action of PDE5 inhibitors by investigating whether PDE5-Is must cross the blood-brain barrier (BBB) to improve memory for novel objects.

We examined the efficacy of UK-343,664, a PDE5-I that is assumed to only poorly cross the BBB (Abel et al., 2001, Walker et al., 2001), to improve memory in three variants of the ORT: a 24 h delay interval where memory degrades over time or a 1 h interval where memory is disrupted by administration of scopolamine or MK-801. The scopolamine-induced memory deficit model is a widely used cognitive impairment model (Klinkenberg & Blokland, 2010). This anti-cholinergic agent has shown to impair memory in several behavioral tests including the ORT (e.g. (Rutten et al., 2006, Schreiber et al., 2007). After scopolamine, the MK-801-induced memory deficit model is the second most commonly used deficit model for preclinical cognition research (van der Staay, Rutten, Erb, & Blokland, 2011). MK-801 is an N-methyl-D-aspartate (NMDA) receptor antagonist that disrupts in particular short-term memory (STM) and attention processes (e.g. Boess et al., 2004, van der Staay et al., 2008, Zhang et al., 2000), thereby causing cognitive deficits affiliated to schizophrenia (Kiss et al., 2010, Moghaddam and Jackson, 2003, Vardigan et al., 2010). Therefore one has to be aware that the effects of MK-801 on a memory performance cannot be seen separately from STM and attention processes. The dose of MK-801 used in the present study is known to impair memory in rodents, but without causing sensorimotor impairments, motivational effects and/or signs of intoxication (van der Staay et al., 2011). We compare the efficacy of UK-343,664 in these assays to that of vardenafil, which is assumed to more readily cross the BBB (Prickaerts et al., 2004) and which has previously been reported to be efficacious in the ORT (Prickaerts et al., 2004, Rutten et al., 2007, Rutten et al., 2009, van Donkelaar et al., 2008). These results are discussed with regard to the localization of the PDE5 target(s) for these compounds and the role of central and peripheral mechanisms in modulating object recognition memory.