Whole-cell patch-clamp recordings were made from neurons in the trigeminal nucleus caudalis and trigeminal ganglion, in vitro, to investigate the cellular actions of the endogenous cannabinoid, anandamide. Anandamide has been shown to act through both the cannabinoid receptor 1 (CB1) and the vanilloid receptor 1 (VR1). Anandamide (30 μm) caused a 54 % increase in the rate of miniature excitatory post-synaptic currents (mEPSCs), without affecting their amplitude. The effect of anandamide was blocked by the VR1 antagonist capsazepine (20 μm), but not by the CB1-specific antagonist AM251 (3 μm). Application of the VR1 receptor agonist capsaicin (300 nm) caused a 4200 % increase in the mEPSC rate. In dissociated trigeminal ganglion neurons, both anandamide and capsaicin caused an outward current in neurons that were voltage clamped at +40 mV. The maximal outward current produced by anandamide (EC50, 10 μm) was 45 % of that produced by capsaicin (10 μm). Co-application of the VR1 antagonist capsazepine (30 μm) completely reversed the effects of both capsaicin and anandamide. The anandamide transport inhibitor, AM404 (30 μm) caused a 40 % increase in mEPSC rate in the slice preparation and an outward current in dissociated neurons. The latter current was reversed by the VR1 antagonist iodoresiniferatoxin (1 μm). The fatty acid amide hydrolase (FAAH) inhibitors phenylmethylsulfonyl fluoride (PMSF) (20 μm) and OL53 (1 μm) did not enhance the effect of anandamide in either the slice or dissociated neuron preparations. These results suggest that within the superficial medullary dorsal horn, anandamide (30 μm) acts presynaptically to enhance the release of glutamate via activation of the VR1 receptor.
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