Research reportInvolvement of the cerebellar adenosine A1 receptor in cannabinoid-induced motor incoordination in the acute and tolerant state in mice
Introduction
Cannabinoids are known to impair motor function in humans and laboratory animals [1]. We have previously demonstrated that Δ9-tetrahydrocannabinol (Δ9-THC), the main psychoactive component of marijuana, impairs motor coordination through a cerebellar CB1 receptor mechanism in mice [7]. This behavioral effect of Δ9-THC was not surprising due to previous evidence of dense CB1 receptor localization in the cerebellum and the well-known function of the cerebellum in coordinating movements [19].
Autoradiographical studies have demonstrated a presynaptic localization of CB1 receptors on cerebellar granule cell axonal terminals in the molecular layer of the cerebellum [19], [23]. It is likely that these receptors inhibit glutamate release from the neuronal terminal upon activation by cannabinoid agonists such as CP55,940 and anandamide [19]. There is also evidence for localization of CB1 receptors on the inhibitory basket cells and excitatory climbing fiber terminals which synapse onto Purkinje cell bodies [23], [33]. Several inhibitory presynaptic receptors appear to be co-localized with CB1 receptors such as the GABAB, adenosine A1 and κ opioid. As for these receptors, the CB1 receptors are Gi/o protein-coupled to inhibit adenylate cyclase and modulate the conductance of Ca2+ and K+ channels [1], [12], [17], [20], [21]. In vitro studies have demonstrated additive GTPase activity when GABAB and A1 agonists are co-incubated [35] and also when CB1 and GABAB agonists are co-incubated [26]. It was concluded that these receptors may couple to their own pool of G-proteins but converge to inhibit common adenylate cyclase enzymes [26]. Such an interaction is plausible between CB1 and A1 receptors.
The purpose of our study was to expand on our previous work which demonstrated a functional interaction between CB1 and A1 receptors in the cerebellum of male CD-1 mice using motor incoordination as the behavioral test. We have recently demonstrated accentuation of Δ9-THC-induced motor incoordination by the adenosine A1 selective agonist, N6-cyclohexyladenosine (CHA), through a cerebellar mechanism [7]. The dose of CHA utilized was too low by itself to induce motor incoordination but significantly accentuated the motor incoordination by intracerebellar (ICB) Δ9-THC when co-administered. Furthermore, ICB administration of the selective A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), significantly attenuated the motor incoordination induced by Δ9-THC.
The purpose of the present investigation was to establish whether the synthetic cannabinoid agonist, CP55,940, shares the same interaction with the adenosine A1 receptor system when microinjected directly into the mouse cerebellum. We also set out to extend present and previous research by hypothesizing that tolerance to intracerebellar cannabinoid-induced motor incoordination occurs following repeated systemic administration. As further support of the adenosine A1 receptor involvement in cannabinoid-induced motor incoordination, we tested for a cross-tolerance effect with the A1 selective agonist, CHA, by daily systemic dosing for 3 days with CHA followed by an ICB CP55,940 challenge. We suggest that the acute and chronic interactions between drugs acting through CB1 and A1 receptors may result from anatomical co-localization and overlap in their signal transduction cascade.
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Animals
Male CD-1 mice were purchased from Charles River Labs (Raleigh, NC). The mice were 5–6 weeks old and weighed between 23 and 28 g at the time of the behavioral experiments. The mice were maintained in a housing facility under controlled humidity and temperature (23–25°C) and kept on a 12:12-h light/dark cycle. Each animal was housed in its own individual plastic cage. Mice had free access to water and commercial mouse chow. All experiments were evaluated and approved by the Animal Use and Care
Rotorod experiments
The doses of CP55,940 used in the present experiments (15 and 20 μg) were selected based on a previous dose–response analysis [13]. The 15-μg dose was established to be the median dose in production of motor incoordination. The 20-μg dose produced a more pronounced motor incoordination and was therefore deemed appropriate for use in the tolerance experiments with systemic CP55,940.
Fig. 1 shows the effects of ICB microinjection of CHA and DPCPX on CP55,940-induced motor incoordination. A
Discussion
The results of this investigation support the hypothesis of an adenosinergic modulation of CP55,940-induced motor incoordination. An acute interaction was noted following ICB microinjection of an A1 receptor selective agonist (CHA) and/or antagonist (DPCPX) prior to ICB CP55,940. Furthermore, repeated systemic injection of CHA resulted in cross-tolerance to CP55,940.
CHA accentuated the motor incoordination produced by CP55,940 at all three evaluation times (see Fig. 1). It should be noted that
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