Cholinergic location of delta opioid receptors in canine atria and sinoatrial node

Although delta opioid receptors (DORs) are associated with ischemic preconditioning and with vagal transmission in the sinoatrial (SA) node and atria, their precise location remains unconfirmed. DORs and the autonomic markers vesicular choline transporter (VAChT) and tyrosine hydroxylase (TH) were labeled in tissue slices and synaptosomes from canine atrium and SA node. Synapsin I verified the neural character of labeled structures. Acetylcholine and norepinephrine content indicated both cholinergic and adrenergic synaptosomes were present. VAChT and TH signals indicated more than 80% of synapsin positive synaptosomes were cholinergic and less than 8% were adrenergic. DORs co‐localized 75–85% with synapsin in tissue slices of atrium and SA node. Co‐localization was equally strong (85%) for nodal synaptosomes but less so for atrial synaptosomes (57%). Co‐localization between DOR and VAChT was 75–85% regardless of the source. DOR/TH overlap was uniformly low (8–17%). Western blots of synaptosomal extracts confirmed two DOR bands at molecular weights corresponding to reports for DOR monomers and dimers. DOR abundance was greater in nodal synaptosomes than in atrial synaptosomes due to more monomers in the SA node. The preferential association of DORs with cholinergic nerve terminals supports the hypothesis that post‐ganglionic prejunctional DORs regulate local vagal transmission within the heart.