"Is there a cannabinoid autoreceptor? And if so what is the cannabinoid autoreceptor?"
(i.e. is the cannabinoid autoreceptor CB1, CB2 or is it one of the G protein coupled receptors, GPR18, GPR55 or GPR119?)
An autoreceptor is a receptor located presynaptically that sends back negative feedback that inhibits the synthesis of the neurotransmitter to which the autoreceptor belongs and this negative feedback also inhibits the release of the neurotransmitter from storage vesicles into the synaptic cleft.
The CB1 and CB2 receptors are respectively called the first and second cannabinoid receptors and are a pair of G protein coupled receptors.
GPR18, GPR55 and GPR119 are G protein coupled receptors that are also activated by some lipid messengers including a few of the endocannabinoids and their related compounds.