Pharmacological Actions of Cannabinoids 7Table 1.Typical dissociation constant (KD) values of radiolabelled ligands at cannabinoid receptor CB 1 and
CB 2 binding sites
Radioligand Source of membranes Receptor KD(nM) Reference(s)
[^3 H]SR141716A Rat braina rCB 1 0.19–1.20 For references,
Guinea-pig forebrain g-pCB 1 1.24 see Pertwee 1999a
[^123 I]AM251 Rat cerebellum rCB 1 0.
[^3 H]R-(+)-WIN55212 Rat cerebellum rCB 1 1.89, 4.67, 8.
Guinea-pig forebrain g-pCB 1 2.
Cultured cellsb rCB 1 2.
Cultured cellsb hCB 1 16.2, 11.
Cultured cellsb hCB 2 3.7, 3.
[^3 H]HU-210 Rat brain minus brain stem rCB 1 0.
(HU-243) Cultured cellsb hCB 2 0.
[^3 H]CP55940 Cultured cellsb hCB 1 0.4 to 3.3 For references,
Cultured cellsb rCB 1 4seePertwee
Rat braina rCB 1 0.07 to 2.3 1997, 1999a
Mouse whole brain mCB 1 3.
Cultured cellsb hCB 2 0.2 to 7.
Cultured cellsb mCB 2 0.
[^3 H]CP55940 Rat cerebellum rCB 1 2.37 Mauler et al. 2002
Human cerebral cortex hCB 1 1.
Cultured cellsb hCB 1 1.
Cultured cellsb hCB 2 4.
[^3 H]BAY 38-7271 Rat cerebellum rCB 1 1.84 Mauler et al. 2002
Human cerebral cortex hCB 1 2.
Cultured cellsb hCB 1 2.
Cultured cellsb hCB 2 4.g-pCB 1 ,Guinea-pigCB 1 receptors; hCB 1 and hCB 2 , human cannabinoid receptors; mCB 1 and mCB 2 , mouse
cannabinoid receptors; rCB 1 and rCB 2 , rat cannabinoid receptors.
aWhole brain or a discrete area.
bCells transfected with CB 1 or CB 2 receptors.
receptor types. It is noteworthy, therefore, that CB 1 or CB 2 selectivity can still be
achieved in displacement assays with the non-selective radiolabelled ligands by
using membranes obtained from cannabinoid receptor-free cultured cells that have
been transfected with CB 1 or CB 2 receptors or membranes obtained from brain
(CB 1 -rich) or spleen (CB 2 -rich). Some care is needed in interpreting binding data
obtained with brain or spleen membranes. Thus, whilst there is little evidence that
CB 2 receptors are expressed by central neurons, these receptors are expressed by
microglial cells (Howlett et al. 2002). Similarly, although it is mainly CB 2 receptors
that are present in spleen, this tissue also expresses some CB 1 receptors (reviewed
in Howlett et al. 2002; Pertwee 1997). Moreover, there is growing evidence for
the presence in brain and other tissues of non-CB 1 , non-CB 2 cannabinoid recep-