2-AG and anandamide enhance hippocampal long-term potentiation via suppression of inhibition

Abstract

It is widely accepted that exogenous cannabinoids can impair short-term memory and cognition in humans and other animals. This is likely related to the inhibition of long-term potentiation (LTP), a form of synaptic plasticity, by the global and sustained activation of CB1 cannabinoid receptors in the presence of exogenous agonists. Conversely, the temporally and spatially restricted release of endogenous cannabinoid (eCB) ligands may enhance synaptic plasticity in a synapse-specific manner. We examined the role of eCB signaling in LTP by recording field excitatory postsynaptic potentials (fEPSPs) in the CA1 stratum radiatum in hippocampal slices from juvenile mice. LTP was induced either electrically, by theta burst stimulation (TBS), or pharmacologically, by treatment for 15 min with a solution designed to increase intracellular cAMP (chem-LTP). A stable and long-lasting potentiation in fEPSP slope following TBS was significantly reduced by blocking cannabinoid receptor activation with CB1 receptor antagonists. Chem-LTP caused a sustained 2-fold increase in fEPSP slope and was also blocked by CB1 receptor antagonists. TBS-LTP was partially reduced by inhibiting the synthesis of the endogenous ligands 2-arachidonylglycerol (2-AG) and anandamide. A similar effect was observed with chem-LTP. Blocking inhibitory synapses completely prevented the effect of CB1 receptor antagonists or inhibition of eCB synthesis on TBS-LTP and chem-LTP. These results indicate that simultaneous activation of CB1 receptors by 2-AG and anandamide enhances TBS-induced and pharmacologically-induced LTP, and this effect is mediated by the suppression of inhibition at GABAergic synapses.

Keywords: 2-arachichodonyl glycerol; GABAergic inhibition; LTP (long term potentiation); anandamide (AEA); endocannabinoid (eCB); hippocampus.

Figure 1

LTP expression is inhibited by CB1 receptor blockade. (A) Individual example of LTP induced by one train of TBS containing 10 bursts delivered at minute 0. Top: Example fEPSP sweeps from baseline (BL) and 60 min post-TBS. Scale bars: 0.2 mV/20 ms. (B) Group data for TBS-LTP under control conditions (n = 10) or in the presence of the CB1 receptor blockers NESS 0327 (n = 7) or SR141716A (n = 6). (C) Individual example of chem-LTP induced by 15 min exposure to the induction cocktail (red symbols, -15–0 min). Top: Example sweeps from baseline (BL), during induction, and 60 min post-induction. Scale bars: 0.2 mV/20 ms. (D) Group data for chem-LTP under control conditions (n = 12) or in the presence of the CB1 receptor blockers NESS 0327 (n = 8) or SR141716A (n = 6).

Figure 2

Blocking eCB synthesis impairs LTP expression. (A) Example time courses of TBS-LTP induced by one train of TBS delivered at minute 0 in the presence or absence of eCB synthesis inhibitors. (B) Group data for the magnitude of TBS-LTP at 60 min under various conditions. Ordinary one-way ANOVA, F_(8,53) = 14.52, p < 0.0001. *p < 0.05 compared to control by Dunnett’s multiple comparison test. (C) Example time courses of chem-LTP induced by 15 min exposure to the induction cocktail (−15–0 min) in the presence or absence of eCB synthesis inhibitors. (D) Group data for the magnitude of chem-LTP at 60 min under various conditions. Ordinary one-way ANOVA, F_(8,62) = 9.1, p < 0.0001. *p < 0.05 compared to control by Dunnett’s multiple comparison test.

Figure 3

eCB modulation of TBS-LTP requires GABAergic transmission. (A) Example time courses of TBS-LTP induced by one train of TBS delivered at minute 0 in control conditions or in the presence of picrotoxin (PTX) or PTX plus the CB1 receptor antagonist NESS 0327. (B) Example time courses of TBS-LTP induced by one train of TBS in the presence of PTX or PTX plus the DAG-lipase inhibitor DO34. (C) Group data for the magnitude of TBS-LTP at 60 min post-induction under various conditions. Control data repeated from Figure 2.

Figure 4

eCB modulation of chem-LTP requires GABAergic transmission. (A) Example time courses of chem-LTP in control conditions or in the presence of picrotoxin (PTX) or PTX plus the CB1 antagonist NESS 0327. (B) Example time courses of chem-LTP in the presence of PTX or PTX plus the eCB synthesis inhibitors DO34 and LEI-401. (C) Group data for the magnitude of chem-LTP at 60 min post-induction under various conditions. Control data repeated from Figure 2.