Properties of the peripheral opiate receptors in the cat nictitating membrane

The noradrenaline overflow and the contractile response elicited by nerve stimulation of the muscle were inhibited by 1 micron morphine in the cat nictitating membrane. This concentration of morphine did not modify the response of the muscle to exogenous noradrenaline. The inhibitory effect of morphine was increased by low Na+ (50 mM), whereas the capacity of naloxone as antagonist to morphine was higher with 150 mM than with 50 mM Na+. These results suggest that the peripheral opiate receptors which interact with noradrenergic neurotransmission could show a sodium allosteric transformation similar to that described for the brain opiate receptor. The effect of morphine was enhanced by manganese ion in the presence of normal Na+. The responses of the cat nictitating membrane to nerve stimulation were not altered in the presence of the protein modifying reagent DTNB [5,5'-dithiobis-(2-nitrobenzoic acid)] but the effect of morphine on the adrenergic neurotransmission was diminished by DTNB with 150 mM Na+. It is postulated that the affinity of the ligands for presynaptic receptors which regulate adrenergic neurotransmission might be modified during the physiological changes in ion concentration which accompany nerve depolarization.