Pharmacological differentiation of presynaptic inhibitory alpha-adrenoceptors and opiate receptors in the cat nictitating membrane.

1 The action of morphine, naturally occurring and synthetic opiate peptides on [3H]-noradrenaline release induced by nerve stimulation was studied in the isolated nerve muscle preparation of the cat nictitating membrane under experimental conditions in which the alpha-presynaptic receptors were blocked by phentolamine 1 microM. 2 Morphine and the naturally occurring peptides: [Met5]-enkephalin, [Leu5]-enkephalin and beta-endorphin reduced 3H-transmitter overflow and responses to nerve stimulation from the cat nictitating membrane, effects which were completely antagonized by naloxone 0.3 microM. The relative order of potency for the inhibition of the stimulation-induced 3H-transmitter overflow at the level of the IC50 (microM) was as follows: [Met5]-enkephalin (0.020 microM) greater than or equal to [Leu5]-enkephalin (0.036 microM) > morphine (0.3 microM) > beta-endorphin (1 microM). 3 The synthetic opiate pentapeptides: BW 180 C (Tyr-D-Ala-Gly-Phe-D-Leu), and BW834 C (Tyr-D-Ala-Gly-pClPhe-DLeu), which are resistant to enzymatic degradation were more potent than the enkephalins in reducing the stimulation-evoked transmitter overflow from the cat nictitating membrane. On the other hand, the tetrapeptide BW832 C, which lacks the D-leucine terminal of BW180 C l was less potent than the enkephalins in inhibiting neurotransmission. 4 In the presence of phenoxybenzamine 1 microM, 3H-transmitter overflow was increased 8 fold and the inhibition of neurotransmission by methionine-enkephalin was not affected. Exposure to phenoxybenzamine 10 microM increased [3H]-noradrenaline overflow 15 fold and antagonized the effects of methionine enkephalin on transmitter release. 5 In the cat nictitating membrane the inhibitory presynaptic opiate receptors are different from the presynaptic alpha-autoreceptors which regulate the release of noradrenaline elicited by nerve depolarization through a negative feed-back mechanism.