84 M.E. Abood
ent conformations of the CB 1 receptor, which in turn can activate select G proteins
(Glass and Northup 1999; Griffin et al. 1998; Kearn et al. 1999; Mukhopadhyay et al.
2000; Selley et al. 1996). This selectivity appears to be driven by distinct molecular
interactions that occur between the different classes of cannabinoid compounds
and the receptor proteins. These data indicate that receptor “subtypes” may also
be observed as a result of activation of distinct second messenger pathways that
produce different physiological responses.
This chapter will focus on the molecular biology of the G protein-coupled
cannabinoid receptors.
2
General Structure and Distribution
Two cannabinoid receptors have been identified to date; the CB 1 receptor is local-
ized predominantly in the central nervous system (CNS), whereas the CB 2 receptor
is located primarily in the immune system. The CB 1 receptor cDNA was isolated
from a rat brain library by a homology screen for GPCRs and its identity confirmed
by transfecting the clone into CHO cells and demonstrating cannabinoid-mediated
inhibition of adenylyl cyclase (Matsuda et al. 1990). Initial identification of the lig-
and for this “orphan receptor” involved the screening of many candidate ligands,
including opioids, neurotensin, angiotensin, substance P, and neuropeptide Y,
among others, until cannabinoids were found to act via this molecule. In cells
transfected with the clone, CP 55,940,∆^9 -THC and other psychoactive cannabi-
noids,butnotcannabidiol(whichlacksCNSactivity)werefoundtoinhibitadenylyl
cyclase, whereas in untransfected cells no such response was found. Furthermore,
the rank order of potency for inhibition of adenylyl cyclase in transfected cells
correlated well with cell lines previously shown to possess cannabinoid-inhibited
adenylyl cyclase activity. Distribution of the expression of CB 1 mRNA also paral-
leled that of cannabinoid receptor binding in rat brain. Analysis of the primary
amino acid sequence of the CB 1 receptor predicts seven transmembrane (TM) do-
main regions, typical of GPCRs. Bramblett et al. (1995) have constructed a model
of the cannabinoid receptor. A representation of the CB 1 receptor based on their
modelisshowninFig.1.
The CB 2 receptor was also isolated by its homology to other GPCRs, using a
PCR-based approach in myeloid cells (Munro et al. 1993). The human CB 2 receptor
cDNA was isolated from the human promyelocytic cell line, HL60. The clone has
44% amino acid sequence identity overall with the CB 1 clone, and percentage
similarity rises to 68% in the TM domains. The amino acid residues conserved
between CB 1 and CB 2 are shaded in Fig. 1. The localization of the CB 2 receptor
appears to be mainly in the periphery: in the spleen and in low levels in adrenal,
heart, lung, prostate, uterus, pancreas, and testis and in cells of immune origin,
including microglia in the CNS (Munro et al. 1993; Galiegue et al. 1995; Walter
et al. 2003). An alignment of human CB 1 and CB 2 isshowninFig.2.Usingthe
numbering scheme of Ballesteros and Weinstein (Ballesteros and Weinstein 1995),
each amino acid is given a number that begins with the helix number followed by