κ and δ -opioid receptor agonists differentially inhibit striatal dopamine and acetylcholine release

At least three different families of endogenous opioid peptides, the enkephalins, endorphins and dynorphins, are present in the mammalian central nervous system (CNS). Immuno-cytochemical studies have demonstrated their localization in neurones 1–8 , which supports the view that these peptides may have a role as neurotransmitters or neuromodulators. However, the target cells and cellular processes acted upon by the opioid peptides are still largely unknown. One possible function of neuropeptides, including the opioid peptides, may be presynaptic modulation of neurotransmission in certain neuronal pathways, for example, by inhibition or promotion of neurotransmitter release from the nerve terminals 9–12 . Here we report that dynorphin and some benzomorphans potently and selectively inhibit the release of (radiolabelled) dopamine from slices of rat corpus striatum, by activating κ -opioid receptors. In contrast, [Leu 5 ]enkephalin and [D-Ala 2 , D-Leu 5 ]enkephalin selectively inhibit acetylcholine release by activating δ -opioid receptors.