Endocannabinoids Modulate N-Type Calcium Channels and G-Protein-Coupled Inwardly Rectifying Potassium Channels via CB1 Cannabinoid Receptors Heterologously Expressed in Mammalian Neurons

Abstract

Endocannabinoids may serve as retrograde messengers to inhibit neurotransmitter release during depolarization-induced suppression of inhibition (DSI) or excitation (DSE). We therefore tested whether endocannabinoids inhibit N-type voltage-dependent Ca 2ϩ channels by activating G i/o -protein-coupled CB1 cannabinoid receptors (CB1R)-a possible mechanism underlying DSI/ DSE. Three putative endocannabinoids [2-arachidonylglycerol (2-AG), 2-arachidonyl glycerol ether (2-AGE), and anandamide (AEA)] and the cannabimimetic aminoalkylindole WIN 55,212-2 (WIN) inhibited whole-cell Ca 2ϩ currents in rat sympathetic neurons previously injected with cDNA encoding a human CB1R. Agonistmediated Ca 2ϩ current inhibition was blocked by a selective CB1R antagonist [SR141716A, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboximide hydrochloride] and pertussis toxin (PTX) pretreatment. The rank order of potency was WIN (IC 50 ϭ 2 nM) Ͼ 2-AGE (350 nM) ϳ 2-AG (480 nM) Ͼ AEA (ϳ3 M), with each agonist displaying similar efficacy (approximately 50% maximal inhibition). Increasing CB1R expression level significantly enhanced AEA potency. AEA (10 M) also inhibited Ca 2ϩ channels in a voltageindependent, CB1R-independent, and PTX-insensitive manner, whereas 2-AG and 2-AGE were devoid of this activity. All three endocannabinoids activated G-protein-coupled inwardly rectifying potassium (GIRK) channels, GIRK1/4, heterologously expressed in sympathetic neurons. These results suggest a mechanism by which endocannibinoids might influence presynaptic function.

Dual Modulation of Endocannabinoid Transport and Fatty Acid Amide Hydrolase Protects against Excitotoxicity

“…In addition, the CB 1 receptors are involved in other repair systems including brain-derived neurotrophic factor that is dependent on the ERK/MAPK pathway (Derkinderen et al, 2003;Khaspekov et al, 2004) and phosphatidylinositol 3-kinase (PI3K) that makes up a potential FAK/PI3K/ERK repair pathway (Galve-Roperh et al, 2002;Liu et al, 2003). It has also been suggested that activated CB 1 receptors lead to the inhibition of voltage-sensitive calcium channels (Mackie and Hille, 1992;Guo and Ikeda, 2004), perhaps contributing to reductions in excitotoxic progression.…”

Retrograde Endocannabinoid Signaling in a Postsynaptic Neuron/Synaptic Bouton Preparation from Basolateral Amygdala

“…The fact that this preparation contains few postsynaptic processes minimizes space-clamp errors that could alter quantification of synaptic modulation and plasticity. The observation that CB 1 agonist-induced synaptic depression was completely reversible in this preparation allowed us to accurately determine the time course of the effects of receptor activation alone and will facilitate comparison with studies of CB 1 modulation of ion channels in single-cell preparations (Mackie and Hille, 1992;Guo and Ikeda, 2004). We were also able to examine GABA A receptor function by applying exogenous GABA in this preparation, which is difficult to accomplish under well controlled pharmacological conditions in brain slices.…”

Modulation of pain transmission by G-protein-coupled receptors

“…The CB1 receptor agonists CP55,940 and ACEA inhibit the increase in the intracellular Ca 2+ concentration evoked by depolarization or capsaicin (Khasabova et al, 2004;Sagar et al, 2005), and this effect is limited primarily to medium-and large-sized DRG neurons (Khasabova et al, 2004). Furthermore, CB1 agonists activate GIRK channels expressed in cell lines and sympathetic neurons (Guo and Ikeda, 2004). The antinociceptive effect of WIN55,212-2 is reduced in GIRK2 knockout mice (Blednov et al, 2003), suggesting that the analgesic action of cannabinoid receptor agonists is at least partially dependent on their effect on GIRK channels.…”