Involvement of reduced acetylcholine release in Δ⁹-tetrahydrocannabinol-induced impairment of spatial memory in the 8-arm radial maze

Abstract

To clarify the mechanism by which Δ⁹-tetrahydrocannabinol, a major psychoactive component of marijuana, impairs spatial memory in the 8-arm radial maze in rats via the cholinergic system, we used two acetylcholinesterase inhibitors, physostigmine and tetrahydroaminoacridine. Moreover, we examined the effect of Δ⁹-tetrahydrocannabinol on acetylcholine release in the frontal cortex and dorsal and ventral hippocampus using in vivo microdialysis. Physostigmine (0.01–0.05 mg/kg, i.p.) and tetrahydroaminoacridine (1–5 mg/kg, p.o.) improved the impairment of spatial memory induced by Δ⁹-tetrahydrocannabinol (6 mg/kg, i.p.) in the 8-arm radial maze. Δ⁹-tetrahydrocannabinol (6 mg/kg, i.p.) produced a significant decrease in acetylcholine release in the dorsal hippocampus as assessed by microdialysis. Moreover, tetrahydroaminoacridine at a dose of 1 mg/kg, which improved the impairment of spatial memory, reversed the decrease in acetylcholine release induced by Δ⁹-tetrahydrocannabinol in the dorsal hippocampus during 60–120 min after the Δ⁹-tetrahydrocannabinol injection. These findings suggest that inhibition of the cholinergic pathway by reduced acetylcholine release is one of the means by which Δ⁹-tetrahydrocannabinol impairs spatial memory in the 8-arm radial maze.

Introduction

Δ⁹-tetrahydrocannabinol has been recognized as the major psychoactive component of marijuana. It has been demonstrated to induce hypothermia, analgesia, hypoactivity, inhibition of prolactin secretion, and stimulation of adrenocorticotrophic hormone release. It has also been shown to impair immediate memory, short-term memory, and spatial cognition in humans [4], [5], [18], [26] Δ⁹-tetrahydrocannabinol has also been reported to impair the performance of delayed matching-to-sample tasks in rhesus monkeys [28], [29]. In rodents, Δ⁹-tetrahydrocannabinol impaired the performance of delayed matching-to-sample tasks [12], non-match-to-position tasks [17], and delayed alternation tasks in the T-maze [22] and radial maze [15], [21]. These impaired performances are thought to be working memory deficits because these tasks can be used to evaluate the working memory. In addition, these working memory impairments were attenuated by the cannabinoid CB₁ receptor antagonist, N-(piperidine-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A) [[17], [22], Litchman et al., 1996]. Similarly, the synthetic cannabinoids, 1R,3R,4R)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl) cyclohexan-1-ol (CP-55,940) and R-(+)-[2, 3-Dihydro-5-methyl-3[morpholinyl)methyl]-pyrrolo[1,2,3]-1, 4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate (WIN-55,212-2) impaired performances in a radial arm maze [14] and non-matching-to-position tasks [10]. Moreover, SR141716A alone enhanced spatial memory in a delayed task in a radial maze [16] and short-term olfactory memory in social recognition tests [25]. We have previously reported that Δ⁹-tetrahydrocannabinol impaired spatial memory in a standard task of an 8-arm radial maze, and its impairment was improved by SR141716A. Moreover, Δ⁹-tetrahydrocannabinol selectively impaired working memory in reference and working memory tasks of an 8-arm radial maze in which the food was baited at 4 of 8 arms. These findings suggest that Δ⁹-tetrahydrocannabinol may produce selective impairment of working memory through cannabinoid CB₁ receptors [20]. We also reported that Δ⁹-tetrahydrocannabinol can be used as a pharmacological tool to evaluate the effects of cognitive enhancers on learning and memory deficits in rats [13]. Thus, these findings suggest that cannabinoid CB₁ receptors play an important role in learning and memory, particularly working memory.

Δ⁹-tetrahydrocannabinol and synthetic cannabinoid CB₁ receptor agonists have been reported to reduce acetylcholine release in vivo from the rat hippocampus [3], [22] and frontal cortex [3], [6], and to inhibit electrically-evoked [¹⁴C] acetylcholine release in vitro from hippocampal and striatal slices [7], [8]. These findings suggest that cannabinoid-induced impairment of learning and memory might be related to a reduction in acetylcholine neurotransmission [6]. On the other hand, cannabinoids are known to increase acetylcholine release in the hippocampus and cortex [1], [2], and Lichman and Martin [15] reported that physostigmine (0.06–0.24 mg/kg, i.p.) failed to improve Δ⁹-tetrahydrocannabinol-induced impairment of spatial memory in the 8-arm radial maze. These results are not consistent with the above mechanism of cholinergic dysfunction for cannabinoid-induced impairment of learning and memory.

The purpose of this study was to clarify the involvement of cholinergic system dysfunction in Δ⁹-tetrahydrocannabinol-induced impairment of spatial memory in the 8-arm radial maze in rats. We therefore examined the effects of two acetylcholinesterase inhibitors, physostigmine and tetrahydroaminoacridine, on the Δ⁹-tetrahydrocannabinol-induced impairment of spatial memory. Moreover, we examined the effects of Δ⁹-tetrahydrocannabinol on acetylcholine release in the frontal cortex and dorsal and ventral hippocampus using microdialysis in freely moving rats.