Potassium channels contribute to basic neuronal excitability and modulation. Here, we examined expression patterns of the voltage-gated potassium channel Kv1.4, the nociceptive transduction channels TRPV1 and TRPV2 as well as the putative anti-nociceptive cannabinoid receptor CB1 by immunofluorescence double-labelings in sections of rat dorsal root ganglia (DRGs). Kv1.4, TRPV1 and CB1 were each detected in about one third of neurons (35.7±0.5%, 29.4±1.1% and 36.4±0.5%, respectively, mean diameter 19.1±0.3 microm). TRPV2 was present in 4.4±0.4% of all neurons that were significantly larger in diameter (27.4±0.7 microm; P < 0.001). Antibody double-labeling revealed that the majority of Kv1.4-positive neurons co-expressed TRPV1 (73.9±1.5%) whereas none expressed TRPV2. The largest overlap was found with CB1 (93.1±0.1%). CB1 expression resembled that seen for Kv1.4 since the majority of neurons expressing CB1-protein also expressed TRPV1 (69.4± 6.5%) but not TRPV2 (0.6±0.3%). When CB1-mRNA was detected using in situ hybridizations an additional subset of larger neurons was labeled including 82.4±17.7% of the TRPV2 expressing neurons. However, co- localization of Kv1.4 with CB1-mRNA (92%, mean diameter: 18.5 microm) was essentially the same as with CB1-protein. The almost complete overlap of CB1 and Kv1.4 in nociceptive DRG neurons suggests a functional synergistic action between Kv1.4 and CB1. The potassium channel may have two important roles in nociception. As the molecular basis of A-type current it could be involved in the control of repetitive discharges at peripheral terminals and as a downstream signal transduction site of CB1 in the control of presynaptic transmitter release at central terminals.

As an endogenous agonist at the cannabinoid receptor CB1 and the capsaicin-receptor TRPV1, anandamide may exert both anti- and pronociceptive actions. Therefore we studied the effects of anandamide and other activators of both receptors on changes in free cytosolic calcium ([Ca²⁺]_i) in acutely dissociated small dorsal root ganglion neurons (diameter: < or =30 microm). Anandamide (10 microM) increased [Ca²⁺]_i in 76% of the neurons. The EC(50) was 7.41 microM, the Hill slope was 2.15 ± 0.43 (mean ± SE). This increase was blocked by the competitive TRPV1-antagonist capsazepine (10 microM) and in Ca²⁺-free extracellular solution. Neither exclusion of voltage-gated sodium channels nor additional blockade of voltage-gated calcium channels of the L-, N-, and/or T-type, significantly reduced the anandamide-induced [Ca²⁺]_i increase or capsaicin-induced [Ca²⁺]_i transients (0.2 microM). The CB1-agonist HU210 (10 microM) inhibited the anandamide- induced rise in [Ca²⁺]_i. Conversely, the CB1-antagonist AM251 (3 microM) induced a leftward shift of the concentration-response relationship by approximately 4 microM (P < 0.001; Hill slope, 2.17 ± 0.75). Intracellular calcium transients in response to noxious heat (47 degrees C for 10 s) were highly correlated with the anandamide-induced [Ca²⁺]_i increases (r = 0.84, P < 0.001). Heat-induced [Ca²⁺]_i transients were facilitated by preincubation with subthreshold concentrations of anandamide (3 microM), an effect that was further enhanced by 3 microM AM251. Although anandamide acts on both TRPV1 and CB1 receptors in the same nociceptive DRG neurons, its pronociceptive effects dominate. Anandamide triggers an influx of calcium through TRPV1 but no intracellular store depletion. It facilitates the heat responsiveness of TRPV1 in a calcium-independent manner. These effects of anandamide differ from those of the classical exogenous TRPV1- agonist capsaicin and suggest a primarily modulatory mode of action of anandamide.