166 V. Marzo et al.
detected in vascular endothelial cells. This putative receptor mediates the local
vasodilator (but not the systemic hypotensive) effects of AEA, and is blocked by
both cannabidiol and a synthetic analogue, O-1918 (Jarai et al. 1999; Offertaler et
al. 2003). It is coupled to guanylyl cyclase and p42/44 mitogen-activated protein
kinase and protein kinase B/Akt. Interestingly, this novel endothelial receptor
seems to be activated also by NADA (O’Sullivan et al. 2004). A receptor sensitive
to abn-cbd has been proposed to mediate microglial cell migration (Walter et
al. 2003), but this site of action, unlike the one in endothelial cells, was also
activated by 2-AG.- “Saturated NAE receptors”: one other GPCR, forN-palmitoylethanolamine, has
been proposed to explain some of the analgesic and anti-inflammatory actions
ofthisAEAcongener(Calignanoetal.1998).AreceptorforN-palmitoylethanol-
amine has been proposed to occur also in microglial cells (Franklin et al. 2003)
and shown to be different from that proposed to mediate the central effects of
another saturated AEA congener,N-stearoylethanolamine (Maccarrone et al.
2002b).
In addition, several channels that transport Ca2+and K+across the cell mem-
brane are targeted directly by sub-micromolar concentrations of AEA (Di Marzo
et al. 2002b). These are:
–TASK-1K+channels (Maingret et al. 2001), which are inhibited by AEA.
–T-typeCa2+channels (Chemin et al. 2001), which are also blocked by AEA,
apparently acting at an intracellular site.- Vanilloid TRPV1 receptors, the sites of action of capsaicin, the pungent com-
ponent of “hot” red peppers (Szallasi and Blumberg 1999), which in contrast
are activated by AEA and NADA (Zygmunt et al. 1999; Smart et al. 2000; Huang
et al. 2002). In this case the effect clearly requires the activation of an intracel-
lular domain of the protein (De Petrocellis et al. 2001; Jordt and Julius 2002), a
mechanism that explains the significantly higher potency with which AEA and
NADA induce TRPV1-mediated currents when injected directly into the neuron
(Premkumar et al. 2004; Evans et al. 2004).
In heterologous expression systems, the potency of AEA for inducing typical
TRPV1-mediated effects (e.g. cation currents, Ca2+-influx and cell depolarization)
is at least fivefold lower than its average potency at CB 1 receptors. However, recent
data (recently reviewed by Di Marzo et al. 2001a; 2002a; Ross 2003; van der Stelt and
Di Marzo 2004) indicate that the potency of AEA and NADA at TRPV1 receptors
increases by up to 10- to 15-fold in some pathological states. In fact, the number of
TRPV1-mediated pharmacological effects, in vitro and in vivo, being reported in
theliteratureforAEAisincreasingbytheday.Arecentstudyshowedthatelevated
levels of endocannabinoids acting at TRPV1 cause ileitis in toxin A-treated rats
(McVey et al. 2003). Evidence for a role for AEA and TRPV1 in store-operated Ca2+-
entry into sensory neurons has also been found (M. van der Stelt and V. Di Marzo,
manuscript in preparation). Furthermore, as AEA often exerts opposing actions,
dependingonwhetheritactsonCB 1 or TRPV1 receptors, blockade of CB 1 receptors