Modulation of electrically evoked acetylcholine release through cannabinoid cb 1 receptors: evidence for an endocannabinoid tone in the human neocortex

Abstract

Cannabinoids are known to inhibit neurotransmitter release in the CNS through CB 1 receptors. The present study compares the effects of synthetic cannabinoids on acetylcholine (ACh) release in human and mice neocortex. We further investigated a possible endocannabinoid tone on CB 1 receptors in human neocortex caused by endogenous agonists like anandamide or 2-arachidonylglycerol. Brain slices, incubated with [ 3H]-choline, were superfused and stimulated electrically under autoinhibition-free conditions to evoke tritium overflow assumed to represent ACh release. The first series of experiments was performed with 26 pulses, 60 mA, at 0.1 Hz. In mice neocortical slices, the cannabinoid receptor agonist WIN55212-2 decreased ACh release (pIC 50=6.68, I max=67%). In the human neocortex the concentration-response curve of WIN55212-2 was bell-shaped and flat ( I max observed approximately 30%). The estimated maximum possible inhibition, however, was much larger: I max derived=79%. Lec, the negative logarithm (lg) of the biophase concentration of endocannabinoids in ‘WIN55212-2 units,’ was −6.52, the pK d of WIN55212-2 was 7.47. The CB 1 receptor antagonist/inverse agonist SR141716 enhanced ACh release in the human neocortex (by 38%) and prevented the inhibitory effect of WIN55212-2. The concentration-response curve of WIN55212-2 was changed in its shape including a shift to the right due to the presence of SR141716. A pA 2 of this antagonist between 11.60 and 11.18 was obtained. SR141716 alone had no effect in mice neocortical slices. A partial agonist without inverse agonistic activity, O-1184, enhanced ACh release in the human neocortex. The endocannabinoid uptake-inhibitor AM404 decreased ACh release in human, but not in mice, neocortical slices. Change of the stimulation parameters (eight trains of pseudo-one-pulse bursts (4 pulses, 76 mA, 100 Hz), spaced by 45 s intervals) led to a stronger inhibitory effect of WIN55212-2, and abolished the disinhibitory effect of SR141716 and O-1184. The results show that activation of CB 1 cannabinoid receptors leads to inhibition of ACh release in the human and mouse neocortex. The endocannabinoid tone is high in the human, but not in the mouse neocortex and is dependent on neuronal activity. SR141716 acts as a competitive CB 1 receptor antagonist.