Uptake/internalization of Met-enkephalin by brain synaptosomes

In this manuscript, we provide evidence for the uptake of met-enkephalin by brain neurons. Tritiated met-enkephalin was accumulated by a crude synaptosomal fraction of rat brain, in the presence of peptidase inhibitors. Characterization of this process in striatum and mediobasal hypothalamus showed incubation time- and temperature-dependence and inhibition in part by several metabolic inhibitors. 3H-met-enkephalin uptake could be abolished in a dose-dependent manner by increasing concentrations of unlabelled met-enkephalin. Preincubation with naloxone resulted in up to 50% reduction of 3H-met-enkephalin uptake, suggesting a partial dependence on an opiate receptor interaction. 3H-met-enkephalin uptake was significantly reduced by freezing and thawing of the tissue preparation and completely abolished by addition of detergent or colchicine. The process showed dependence on substrate concentration, but was not saturable. Kinetic analysis of the 3H-met-enkephalin uptake revealed that the overall process best fit a transport model postulating the presence of a high-affinity, saturable uptake mechanism together with a second nonsaturable one. The uptake showed regional variation in brain with a distribution that closely paralleled those reported for met-enkephalin-like immunoreactivity and opiate receptor density. The precise significance of this 3H-met-enkephalin accumulation in brain remains to be determined. Whether this process represents 'reuptake' in the classical sense for termination of action, or internalization by some other process remains to be shown.