ABSTRACT
Enkephalins are used as synaptic transmitters in some parts of both the central and peripheral nervous systems. Like all other peptide neurotransmit ters, they are stored in dense-cored secretory granules made by the Golgi apparatus, and transported to the nerve terminal, where they are released by exocytosis when high-frequency action potentials reach the nerve terminal. Enkephalins act on μ-and δ-opioid receptors on target cells, which are seven transmembrane domain, G-protein linked receptors. Most often, the target cells for enkephalinergic neurons in the CNS are involved in pain pathways and acti vation of μ-and δ-opioid receptors result in a 'dampening' of pain sensation. Drugs that act on opioid receptors, such as morphine and heroin, have the same analgesic effect. Unlike other transmitters, no specific presynaptic uptake processes for the enkephalins or other peptide neurotransmitters exist. They are degraded by the action of synaptic peptidases into shorter, inactive peptides, or their constituent amino acids.
