Cannabinoid receptor stimulation is anti-inflammatory and improves memory in old rats

Abstract

The number of activated microglia increase during normal aging. Stimulation of endocannabinoid receptors can reduce the number of activated microglia, particularly in the hippocampus, of young rats infused chronically with lipopolysaccharide (LPS). In the current study we demonstrate that endocannabinoid receptor stimulation by administration of WIN-55212-2 (2 mg/kg day) can reduce the number of activated microglia in hippocampus of aged rats and attenuate the spatial memory impairment in the water pool task. Our results suggest that the action of WIN-55212-2 does not depend upon a direct effect upon microglia or astrocytes but is dependent upon stimulation of neuronal cannabinoid receptors. Aging significantly reduced cannabinoid type 1 receptor binding but had no effect on cannabinoid receptor protein levels. Stimulation of cannabinoid receptors may provide clinical benefits in age-related diseases that are associated with brain inflammation, such as Alzheimer's disease.

Introduction

Microglial cells play a pivotal role as immune effectors in the central nervous system and may participate in the initiation and progression of neurological disorders, such as Alzheimer's disease (AD), Parkinson's disease and multiple sclerosis by releasing cytotoxic proteins, reactive oxygen species or complement (Akiyama et al., 2000, Kim and de Vellis, 2005). Recent evidence suggests that the aging process induces morphological changes in microglia (Streit, 2006), and treatment with NSAIDs does not reduce microglia activation in old rats, in contrast to their effectiveness in young rats (Hauss-Wegrzyniak et al., 1999), thus raising the possibility of microglial senescence (Perry et al., 1993, Streit, 2006).

The endocannabinoid system may regulate many aspects of the brain's inflammatory response, including the release of pro-inflammatory cytokines and modulation of microglial activation (Klein, 2005, Marchalant et al., 2007). The endocannabinoid system is comprised of two G-protein-coupled receptors designated as CB1 and CB2 (Pertwee, 2005); although not all endocannabinoid effects can be explained only by these two receptors (Begg et al., 2005). CB1 receptors are expressed in the brain and are responsible for most of the behavioral effects of exogenous cannabinoids (Pertwee, 2005, Tsou et al., 1998). CB2 receptors are expressed by immune and hematopoietic cells peripherally (Begg et al., 2005), and may be expressed on neurons in the brainstem and the brain (Benito et al., 2003, Gong et al., 2006, Onaivi et al., 2006, Van Sickle et al., 2005) although their presence in the brain is controversial (Munro et al., 1993). Two endogenous ligands for these receptors, arachydonylethanolamine and 2-arachidonoylglycerol (Stella, 2004), influence immune responses by inhibiting cytokine release and other anti-inflammatory actions (Klein et al., 2003, Klein, 2005). In vitro, microglia expresses CB receptors and release cytokines in response to exposure to LPS or beta-amyloid protein (Facchinetti et al., 2003, Ramirez et al., 2005, Sheng et al., 2005). Astrocytes may also synthesize and release endocannabinoids (Walter et al., 2002). Although stimulation of CB1 receptors, e.g. by administration of Δ⁹-tetrahydrocannabinol, can impair performance in rats, mice or monkeys (Castellano et al., 2003), we previously demonstrated that stimulation of the CB1/2 receptors using a low dose of (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)-pyrrolo[1,2,3-de]-1,4benzoxazin-6-yl]-1-naphthalenyl-methanone mesylate (WIN-55212-2) significantly reversed the LPS-induced microglia activation in young rats without attenuating the neuroinflammation-induced performance impairment observed in the water pool task (Marchalant et al., 2007). Because normal aging is associated with increased levels of microglial activation, in the current study, we investigated for anti-inflammatory and memory enhancing effects of CB1/2 receptor stimulation in normal aged rats.