Cocaine and neuronal uptake in the canine saphenous vein

Abstract

This study was designed to investigate the effects of the neuronal uptake inhibitor, cocaine on the adrenergic neuroeffector interaction in the canine saphenous vein. Tissues were incubated with 3H-noradrenaline in control solution or in presence of the cocaine. The tissue content of 3H-noradrenaline and its metabolites was determined after the incubation. As the concentration of cocaine in the incubation medium increased, gradually less 3H-noradrenaline and DOPEG were detected in the tissue, while the content of DOMA, NMN, MOPEG and, in particular that of VMA increased; comparable results were obtained with high concentrations of cocaine and desmethylimipramine (DMI). Helical strips of canine saphenous veins were incubated with 3H-noradrenaline and mounted for isometric tension recording and for measurement of the efflux of labelled transmitter and its metabolites. Cocaine, but not DMI, slightly increased the spontaneous efflux of DOPEG, suggesting that cocaine enters the nerve terminals and displaces noradrenaline from its storage sites. During electrical stimulation, cocaine at 3 x 10-5 mol/l increased the contractile response and the overflow of 3H-noradrenaline, DOMA, NMN and MOPEG and decreased the appearance of DOPEG. Similar results were obtained with DMI (10-6 mol/l) except that it did not increase the overflow of DOMA and MOPEG. During electrical stimulation in presence of DMI, cocaine did not affect the contractile response and decreased the appearance of intact labelled transmitter. Electrical stimulation, cocaine and DMI did not affect the overflow of VMA. The present experiments indicate that in the canine saphenous vein: (1) DOPEG is formed intraneuronally, but DOMA, MOPEG, NMN and VMA extraneuronally; (2) VMA is retained in the tissue much longer than the other metabolites; (3) determination of total 3H-content after incubation with 3H-noradrenaline in presence of inhibitors of neuronal uptake underestimates the degree of inhibition of the neuronal amine carrier; and (4) the quantification of the effect of cocaine on the neuronal uptake of released transmitter is complicated by several other actions of the drug (local anesthetic properties, displacement of stored transmitter, activation of effector cells) and that of the effect of DMI by its inhibitory effect on monoamine oxidase, in particular at extraneuronal sites.