Δ⁹-Tetrahydrocannabinol Acts as a Partial Agonist to Modulate Glutamatergic Synaptic Transmission between Rat Hippocampal Neurons in Culture

Abstract

Delta9-tetrahydrocannabinol (Delta9-THC) is the principal psychoactive ingredient in marijuana. We examined the effects of Delta9-THC on glutamatergic synaptic transmission. Reducing the extracellular Mg++ concentration bathing rat hippocampal neurons in culture to 0.1 mM elicited a repetitive pattern of glutamatergic synaptic activity that produced intracellular Ca++ concentration spikes that were measured by indo-1-based microfluorimetry. Delta9-THC produced a concentration-dependent inhibition of spike frequency with an EC50 of 20 +/- 4 nM and a maximal inhibition of 41 +/- 3%. Thus, Delta9-THC was potent, but had low intrinsic activity. Delta9-THC (100 nM) inhibition of spiking was reversed by 300 nM N-piperidino-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide (SR 141716), indicating that the inhibition was mediated by CB1 cannabinoid receptors. Delta9-THC attenuated the inhibition produced by a full cannabinoid receptor agonist, (+)-[2, 3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo-[1,2,3-de]-1, 4-benzoxazin-6-yl](1-napthalenyl)methanone monomethanesulfonate (Win 55212-2), indicating that Delta9-THC is a partial agonist. The effect of Delta9-THC on synaptic currents was also studied. 6-Cyano-2,3-dihydroxy-7-niroquiinoxaline (CNQX)-sensitive excitatory postsynaptic currents were recorded from cells held at -70 mV in the whole-cell configuration of the patch-clamp and elicited by presynaptic stimulation with an extracellular electrode. Win 55212-2 and Delta9-THC inhibited excitatory postsynaptic current (EPSC) amplitude by 96 +/- 2% and 57 +/- 4%, respectively. Excitatory postsynaptic current amplitude was reduced to 75 +/- 5% in the presence of both drugs, demonstrating that Delta9-THC is a partial agonist. The psychotropic effects of Delta9-THC may result from inhibition of glutamatergic synaptic transmission. The modest physical dependence produced by Delta9-THC as well as its lack of acute toxicity may be due to the ability of the drug to reduce, but not block, excitatory neurotransmission.

Cannabidiol inhibits synaptic transmission in rat hippocampal cultures and slices via multiple receptor pathways

“…As THC and WIN 55,212-2 both decreased synaptic activity in agreement with previous studies (Shen and Thayer, 1999;Ohno-Shosaku et al, 2005;Straiker and Mackie, 2005;Bajo et al, 2009), this indicates that the CBD-mediated effects are not an artefact of our culture preparation. This was further verified by observations that CBD inhibited basal synaptic transmission and increased the pairedpulse facilitation ratio in acute hippocampal slices.…”

“…In contrast, CBD was without effect on the resting membrane potential (Vm) at all concentrations tested (0.1 mM, 0.2 Ϯ 2.1 mV from a control Vm of -60.7 Ϯ 2.1 mV; 1 mM, 1.5 Ϯ 1.6 mV from a control Vm of -57.6 Ϯ 1.6 mV; 10 mM, 0.3 Ϯ 0.7 from a control Vm of -62.0 Ϯ 3.7 mV; all n = 5 and P > 0.05). Similar reductions in spontaneous AP frequency were observed following the application of the CB1 agonist, WIN 55,212-2 (0.1 mM Figure 1B) and the CB1 partial agonist, THC (10 mM, Figure 1B), as has been shown previously (Shen and Thayer, 1999;Bajo et al, 2009;Roloff and Thayer, 2009). To determine if the effects of CBD were mediated by a Gai/o G-protein coupled receptor (GPCR), experiments were performed on cultures treated with Pertussis toxin (PTX, 200 ng·mL -1 , 18 h).…”

Receptor-dependent formation of endogenous cannabinoids in cortical neurons

“…The possible role of 2-arachidonylglycerol as a negative feedback regulator of N-methyl-D-aspartate-mediated responses, suggested by these results, is reinforced by the ability of cannabinoid drugs to reduce glutamate transmis-Ž . sion Shen et al, 1996;Shen and Thayer, 1999 , inhibit Ž long-term potentiation Collins et al, 1994;Terranova et . al., 1995;Misner and Sullivan, 1999 , and alleviate gluta-Ž .…”

From Phytocannabinoids to Cannabinoid Receptors and Endocannabinoids: Pleiotropic Physiological and Pathological Roles Through Complex Pharmacology

“…In some cases, protection is due to a direct effect on neuronal cells, while in others, it results from effects on nonneuronal elements within the brain. Mechanisms include modulation of excitatory glutamatergic transmissions and synaptic plasticity (288,358,772,788); modulation of immune responses and the release of inflammatory mediators (104); modulation of excitability, N-methyl-D-aspartate (NMDA) receptors, gap junctions, and [Ca 2ϩ ] i (321, 919); and antioxidant properties (323). ⌬ 9 -THC protects primary cultured neurons against kainate-mediated toxicity in a CB1-dependent manner (2) and exerts neuroprotective effects in an ouabain-induced rat model of in vivo excitotoxicity (870).…”