Effects on Cell Viability 633Experiments conducted on cultured neurons have also provided evidence for
a cytoprotective action of cannabinoids, although variable results have been ob-
tained regarding the possible involvement of cannabinoid receptors. A neuropro-
tective action of some cannabinoid agonists in a model of glutamate excitotoxic
death was first reported by Skaper et al. (1996) using cerebellar granule neurons.
WIN 55,212-2 and CP 55,940 were later shown to protect hippocampal neurons
from presynaptically evoked glutamate excitotoxicity by a CB 1 -mediated process
(Shen and Tayer 1998). Since then, other reports have shown that cannabinoids
protect cultured neurons against glutamatergic excitotoxicity via CB 1 receptor ac-
tivation (e.g. Abood et al. 2001; Hampson and Grimaldi 2001) or independently
of the CB 1 receptor (e.g. Nagayama et al. 1999; Sinor et al. 2000). In this respect,
various cannabinoids with phenolic structure exert a CB 1 -independent neuropro-
tective effect owing to their intrinsic antioxidant properties (Hampson et al. 1998;
Chen and Buck 2000; Marsicano et al. 2002).
3.1.1
Mechanism of Action
Although the precise molecular mechanism for cannabinoid-mediated neuropro-
tection its not fully understood, probably the best-described mechanism is the
cannabinoid inhibitory action on glutamate release, which would protect cells
from excitotoxicity (Mechoulam et al. 2002; van der Stelt et al. 2002; Freund et
al. 2003). Glutamatergic transmission is regulated by cannabinoids basically at
the presynaptic level through inhibition of glutamate release as a consequence of
their inhibitory effect on Ca2+influx through different types of voltage-sensitive
channels, including N- and P/Q-type channels (Howlett et al. 2002; Freund et al.
2003). Modulation of presynaptic K+channels may also contribute to this cannabi-
noid action. Other cannabinoid effects that may contribute to neuroprotection
include, for example, inhibition of nitric oxide production and proinflammatory
cytokine release by activated microglial cells (Mechoulam et al. 2002), attenu-
ation of endothelin 1-induced brain vasoconstriction (Mechoulam et al. 2002),
and stimulation of brain-derived neurotrophic factor expression (Marsicano et al.
2003).
The observation that anandamide (Di Marzo et al. 1994) and 2-arachidonoyl-
glycerol (Stella et al. 1997) biosyntheses are activated by Ca2+may constitute
a mechanism of feedback regulation, with the generated cannabinoids preventing
excessive Ca2+influx by their inhibitory action on Ca2+channels. Several brain
insults induce a selective increase in certain endocannabinoid species, further sug-
gesting a protective role for these molecules. For example, traumatic brain injury
enhances 2-arachidonoylglycerol levels (Panikashvili et al. 2001), glutamatergic
excitotoxicity increases anandamide levels (Hansen et al. 2001; Marsicano et al.
2003), and focal cerebral ischaemia raises palmitoylethanolamide levels (Franklin
et al. 2003). The biological meaning for this apparent endocannabinoid-species
selectivity is, however, unknown.