Axonal transport of receptors: coexistence with neurotransmitter and recycling

Receptor sites move along the axon of neurones by means of fast transport mechanisms. In rat sciatic nerves, the accumulation of muscarinic receptors above a ligature was linear up to 24 hr after the operation and exactly paralleled that of dopamine-beta-hydroxylase. After 6-hydroxydopamine treatment, there was a marked reduction of these receptor sites in the rat sciatic nerve and the spleen. Fractionation of dog splenic nerves by differential and isopycnic centrifugation enabled us to show that the muscarinic receptors in sympathetic nerves are associated with noradrenaline and dopamine-beta-hydroxylase in synaptic vesicles. Receptor and neurotransmitter may thus coexist in the same subcellular organelle. Opiate receptors measured in vitro and in vivo with 3H-lofentanil in the rat vagus nerve were found to accumulate on both sides of the ligature. After capsaicin treatment this accumulation was markedly reduced suggesting that the opiate receptors in the vagus are mainly associated with substance P neurones. Double ligature experiments suggest the existence of a recycling phenomenon in the perikaryon for vesicles containing muscarinic receptors and dopamine-beta-hydroxylase. This may represent a pathway to convey informational molecules from the extracellular to the intracellular compartment of a neurone.