While our understanding of the nanoscale architecture of anterograde synaptic transmission
is rapidly expanding, the qualitative and quantitative molecular principles underlying
distinct mechanisms of retrograde synaptic communication remain elusive. We show that
a particular form of tonic cannabinoid signaling is essential for setting target cell–dependent
synaptic variability. It does not require the activity of the two major endocannabinoid-producing
enzymes. Instead, by developing a workflow for physiological, anatomical, and molecular
measurements at the same unitary synapse, we demonstrate that the nanoscale stoichiometric
ratio of type 1 cannabinoid receptors (CB 1 Rs) to the release machinery is sufficient
to predict synapse-specific release probability. Accordingly, selective decrease of
extrasynaptic CB 1 Rs does not affect synaptic transmission, whereas in vivo exposure
to the phytocannabinoid Δ 9 -tetrahydrocannabinol disrupts the intrasynaptic nanoscale
stoichiometry and reduces synaptic variability. These findings imply that synapses
leverage the nanoscale stoichiometry of presynaptic receptor coupling to the release
machinery to establish synaptic strength in a target cell–dependent manner.
