Action of enkephalin analogues and morphine on brain acetylcholine release: differential reversal by naloxone and an opiate pentapeptide.

1 Methionine (Met)-enkephalin, leucine (Leu)-enkephalin and their synthetic analogues were tested for effects on the spontaneous release of cortical acetylcholine (ACh) in vivo. The ability of naloxone to reverse the action of enkephalins on ACh release was compared with its action against morphine. An enkephalin analogue, structurally related to Met-enkephalin, was tested for opiate antagonistic activity in ACh release experiments. 2 Intraventricular administration of Met-enkephalin, Leu-enkephalin, D-Ala2-Met5-enkephalinamide (DALA) and D-Ala2-D-Leu5-enkephalin (DALEU) produced a dose-related inhibition of cortical ACh release. Met- and Leu-enkephalin were very similar both in their potency and the time course of their action on ACh release. Both DALA and DALEU were more potent and had a longer duration of action than Leu-enkephalin. Systemic injections of two pentapeptides, D-Met2-Pro5-enkephalinamide and D-Ala2-MePhe4-Met5(O)-ol-enkephalin (33,824), produced a sustained inhibition of cortical ACh release. 3 Naloxone, administered systemically following the depression of ACh release induced by either intraventricular injections of enkephalins (DALA or DALEU), or systemic injections of enkephalins (D-Met2-Pro5-enkephalinamide or 33,824), reversed this depression and restored the release to baseline levels. The effect of D-Met2-Pro5-enkephalinamide on the release of ACh was reversed by naloxone with difficulty. Naloxone also reversed the inhibitory effect of systemic morphine and this reversal was associated with a large overshoot of ACh release. The latter was never observed in the enkephalin experiments. 4 Intraventricular injection of the pentapeptide, D-Ala2-D-Ala3-Met5-enkephalinamide (TAAPM), at doses that did not influence the basal ACh release, blocked or reversed the inhibitory effect of morphine on this release. This peptide did not block the effect of the non-opiate, chlorpromazine, under similar conditions. In two experiments TAAPM failed to reverse the inhibition of ACh release produced by systemically injected enkephalin, D-Met2-Pro5-enkephalinamide. 5 Effects of morphine and enkephalin on ACh release are discussed in terms of their action on difference opiate receptors.