To date, two cannabinoid receptors, CB1 and CB2,
have been cloned. The CB1 receptor has been found in a
variety of tissues, particularly in the brain. CB2 receptor
mRNA is mainly expressed in the immune system, though one
group has found it in mouse cerebellum. Previous immunostaining
studies in our lab demonstrated the presence of CB1 receptors
in the retina though little evidence exists for the presence
of CB2. The putative endogenous ligand for CB2 has been
found in retina, however, suggesting that further study
of CB2 in retina is warranted. Because glutamate is toxic
to retinal ganglion cells in glaucoma and activation of
CB2 receptors may be able to protect neurons from glutamate-induced
death, we examined the expression of CB2 mRNA in adult
rat retina in order to better understand possible neuroprotective
mechanisms relevant to glaucoma. Using in situ
hybridization, we demonstrated that CB2 cannabinoid receptor
messenger RNA was clearly expressed in the adult rat retina,
including the somas of retinal ganglion cells. Antisense
cRNA probe detected strong signals in the retinal ganglion
cell layer, the inner nuclear layer, and the inner segments
of photoreceptor cells. Using reverse transcription polymerase
chain reaction (RT-PCR) in both rat and mouse tissue, we
obtained an RT-PCR product with the same sequence as that
reported for CB2 in the GenBank database, thus confirming
the presence of CB2 mRNA in retina. The presence of CB2
in retina provides new evidence for the presence of CB2
in the central nervous system (CNS) and an excellent model
for its study.
Inhibition of Adult Rat Retinal Ganglion Cells by D1-Type Dopamine Receptor Activation
