12 R.G. Pertwee
selectivity since, in contrast to established CB 1 receptor agonists, many other
classes of centrally active agent lack activity in at least one of the tests (Wiley and
Martin 2003). This feature of the tetrad assay was particularly important when
it was first devised, as selective CB 1 receptor antagonists had still to be devel-
oped. Now that such antagonists are available (Sect. 3.2), there is less need for
a bioassay with CB 1 receptor selectivity. Some non-CB 1 receptor ligands do show
activity in all four tetrad tests. These include stearoylethanolamide (Maccarrone
et al. 2002), the anandamide analogue, O-2093 (Di Marzo et al. 2002), metabolites
of anandamide (reviewed in Pertwee and Ross 2002) and certain anti-psychotic
agents (Wiley and Martin 2003). Moreover, although the endocannabinoid anan-
damide shows cannabimimetic activity in the mouse tetrad assay, it is only an-
tagonized by SR141716A when protected from enzymic hydrolysis (reviewed in
Pertwee and Ross 2002). However, other CB 1 receptor agonists do show suscep-
tibility to antagonism by SR141716A in this bioassay (reviewed in Howlett et al.
2002).
Other in vivo bioassays for CB 1 receptor agonists include the dog static ataxia
test, the monkey behavioural test, the rat catalepsy test and the drug discrimination
test,whichisusuallycarriedoutwithmonkeys,ratsorpigeons(reviewedinHowlett
et al. 2002; Martin et al. 1995). The potencies shown by some cannabinoids in drug
discrimination experiments performed with rats have been found to correlate
well with their psychoactive potencies in humans (Balster and Prescott 1992). In
vivo bioassays that provide measures of other CB 1 receptor-mediated effects in
animals, for example changes in memory, have also been developed (reviewed
in Howlett et al. 2002; see also the chapter by Riedel and Davies, this volume).
However, these have not been used widely for characterizing novel cannabinoid
receptor ligands. Methods for evaluating cannabinoids in humans have also been
developed (Howlett et al. 2002).
2.4 CannabinoidReceptorKnockoutMice......................
One important advance has been the development of transgenic CB 1 –/–,CB 2 –/–and
CB 1 –/–/CB 2 –/–mice that lack CB 1 ,CB 2 or both CB 1 and CB 2 receptors (reviewed
in Howlett et al. 2002; see also the chapters by Abood and by Valverde et al., this
volume). The availability of such animals provides a useful additional method
for establishing whether or not responses to test compounds are CB 1 and/or CB 2
receptor mediated and, indeed, an important means of detecting the presence of
new types of cannabinoid receptor (Sect. 4.1). Cannabinoid receptor knockout
mice are also being used to help determine the physiological roles of CB 1 and CB 2
receptors.