Two kinds of opiate receptor.

Studies with azidomorphine derivatives have revealed that some of them, particularly N-cyclopropylmethylnorazidomorphine (CAM), stimulate some opiate receptors, while inhibit the others. The opiate receptors stimulated by CAM are called opiate A receptors, while those antagonized by CAM are called opiate B receptors. Opiate receptors are located at nerve terminals and upon stimulation decrease the release of a neurotransmitter. Opiate A receptors are most probably located at cholinergic nerve terminals, are present in the guinea pig ileum, mouse vas deferens and in the brain. Their stimulation leads to constipation and mental clouding. Opiate B receptors located on adrenergic nerve terminals are present in the cat nictitating membrane and in the brain. Their stimulation produces analgesia, depression of coughing and respiration, catalepsy, and mental clouding.