Endocannabinoids and excitotoxicity: lessons from hypoglossal motoneurons

Brainstem hypoglossal motoneurons (HMs) exclusively innervate tongue muscles and are severely damaged in the neurodegenerative disease called amyotrophic lateral sclerosis (ALS). One mechanism leading to such cell death is proposed to be glutamate-mediated excitotoxic stress. HMs are particularly vulnerable to excitotoxicity due to their expression of calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors and scarcity of intracellular Ca2+ binding proteins like parvalbumin and calbindin. Indeed, blocking glutamate uptake in medullary slices can lead to pathological bursting and motoneuron damage. The endocannabinoid system is widely distributed in the brain and is believed to be an important regulator of synaptic transmission. Several studies reported neuroprotection mediated by the endocannabinoid system in such pathological insults like brain ischemia, traumatic brain injury or excitotoxicity. Moreover, in ALS animal models, up-regulation of the endocannabinoid system has been detected, suggesting it can play a role during disease development. Thus, detailed information on how the endocannabinoid system can affect cells during pathological insults like excitotoxicity is a valuable asset for future investigations of novel therapy approaches for ALS. The objective of this work was to investigate the effect of modulation of the endocannabinoid system during excitotoxic stress in hypoglossal motoneurons in vitro. Thin medullary slices (for electrophysiology and viability assay) or whole brainstem isolates (for Western Blot) from postnatal Wistar rats were used. Each slice/brainstem containing hypoglossal nuclei was transferred to a recording/incubation chamber and superfused with oxygenated Krebs solution. Excitotoxic stress was evoked by application of DL-TBOA (DL-threo-β-benzyloxyaspartic acid, 50 μM), a potent and selective inhibitor of excitatory amino acid transporters, with consequent build-up of extracellular glutamate. It was observed that modulation of endocannabinoid CB1 receptor (CB1R) function affected TBOA-evoked bursting, an event previously correlated with TBOA toxicity. Co-application of the endocannabinoid anandamide (AEA, 10 μM), a CB1R agonist, with TBOA resulted in lowered probability of the occurrence of pathological bursting, whereas co-application of the CB1R antagonist AM251 (10 μM) disrupted TBOA-induced bursts, leading to their “fragmentation”. Furthermore, AEA significantly decreased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) isolated by co-application of bicuculline and strychnine (10 μM and 0.4 μM, respectively) and caused occurrence of biphasic activity in spontaneous inhibitory postsynaptic currents (sIPSCs; isolated by co-application of DNQX and APV at 10 μM and 50 μM, respectively) in some of the recorded cells. AM251 caused a decrease in the frequency of sIPSCs, but during application of bicuculline and strychnine it evoked activity which partly resembled bursting observed during TBOA application. Moreover, co-application of AEA with TBOA significantly decreased the number of damaged propidium iodide-positive cells with respect to counterstained Hoechst 33342-positive cells, which suggests a protective effect of this CB1R agonist against TBOA-induced toxicity. In addition, Western blot analysis showed a significant increase in CB1R protein levels after only 4 hours of TBOA incubation, indicating that the endocannabinoid system is activated during this excitotoxic insult. We suggest that a likely role of the endocannabinoid system in our brainstem preparation is to counteract the effects and consequences of elevated glutamate levels in the extracellular compartment.