Background  The enteric nervous system (ENS) possesses extensive synaptic connections which integrate information and provide appropriate outputs to coordinate the activity of the gastrointestinal tract. The regulation of enteric synapses is not well understood. Cannabinoid (CB)₁ receptors inhibit the release of acetylcholine (ACh) in the ENS, but their role in the synapse is not understood. We tested the hypothesis that enteric CB₁ receptors provide inhibitory control of excitatory neurotransmission in the ENS.

Methods  Intracellular microelectrode recordings were obtained from mouse myenteric plexus neurons. Interganglionic fibers were stimulated with a concentric stimulating electrode to elicit synaptic events on to the recorded neuron. Differences between spontaneous and evoked fast synaptic transmission was examined within preparations from CB₁ deficient mice (CB₁^−/−) and wild‐type (WT) littermate controls.

Key Results  Cannabinoid receptors were colocalized on terminals expressing the vesicular ACh transporter and the synaptic protein synaptotagmin. A greater proportion of CB₁^−/− neurons received spontaneous fast excitatory postsynaptic potentials than neurons from WT preparations. The CB₁ agonist WIN55,212 depressed WT synapses without any effect on CB₁^−/− synapses. Synaptic activity in response to depolarization was markedly enhanced at CB₁^−/− synapses and after treatment with a CB₁ antagonist in WT preparations. Activity‐dependent liberation of a retrograde purine messenger was demonstrated to facilitate synaptic transmission in CB₁^−/− mice.

Conclusions & Inferences  Cannabinoid receptors inhibit transmitter release at enteric synapses and depress synaptic strength basally and in an activity‐dependent manner. These actions help explain accelerated intestinal transit observed in the absence of CB₁ receptors.