Plant, synthetic and endogenous cannabinoids can modulate the excitability of sensory neurones, and this may be of benefit in the treatment of pain disorders ( Ross et al., 2004) . The endocannabinoid, anandamide (arachidonoyl ethanolamide; AEA) has dual effects, enhancing or inhibiting depolarisation-activated Ca²⁺ influx in cultured dorsal root ganglion (DRG) neurones. However, only the inhibitory responses to AEA were sensitive to pertussis toxin pre-treatment suggesting distinct mechanisms of action ( Evans et al., 2004) .

In this study we have used cultured DRG neurones from 2-day old Sprague-Dawley rats, the whole cell patch clamp recording method and fura-2 Ca²⁺ imaging to investigate the possible modulation of K⁺ conductances by AEA. AEA (1 µM; with 0.01 % DMSO) reduced the burst firing behaviour of a sub-population (~20 %) of DRG neurones and caused a slowing of the repolarising phase of action potentials. AEA (1 µM) inhibited voltage-activated K⁺ currents evoked from a holding potential of –70 mV by 100 ms voltage step commands to +30 mV by 24 ± 6 % (n=7; P<0.03) and partial recovery was seen 10-30 minutes after removing the drug perfusion pipette from the bath. Pre-treatment for 16-24 hrs with pertussis toxin (500 ng/ml) did not attenuate AEA-evoked inhibition of K⁺ currents, the inhibition being 30 ± 5 %; n=11. Co-application of AEA with the cannabinoid receptor antagonist, SR141716A (100 nM) also failed to attenuate the inhibition of K⁺ currents by AEA, the inhibition being 38 ± 8 %; n=5. Application of SR141716A alone had no significant effect on K⁺ currents (n=7). The action of AEA was not mimicked by non-hydrolysable methanandamide, which failed to modulate the K⁺ current (n=5). This indicated that AEA metabolites may play a critical role in the modulation of K⁺ conductances in DRG neurones. Additional evidence supporting this hypothesis was that pre-treating neurones with the fatty acid amide hydrolase (FAAH) inhibitor phenylmethylsulfonyl fluoride (PMSF; 10 µM) attenuated the effects of AEA. Under these conditions the mean K⁺ current amplitude was not significantly different from control. Consistently, a primary metabolite of AEA, arachidonic acid (1 µM) also inhibited K⁺ currents by 26 ± 5% (n=11).

These data indicate that, in addition to the established CB₁ receptor-mediated modulation of K⁺ conductances, AEA also displays properties that are consistent with the production of active metabolites.

Evans, R., Scott, R. & Ross, R. (2004) Multiple actions of anandamide on neonatal rat cultured sensory neurones. Br. J. Pharmacol.,141, 1223-1233.
Ross, R., Evans, R. & Scott, R. (2004) Cannabinoids and sensory neurones. Current Neuropharmacology. 2 , 59-73.

This work was funded by The Wellcome Trust and the University of Aberdeen.