Iminoglycine transport system in synaptosomes and its interaction with enkephalins

Abstract

Evidence is presented which suggests that proline, pipecolic acid, and glycine are accumulated by a common transport system in rat brain cortical synaptosomes and synaptosomal plasma membrane vesicles. This system is Na+ dependent and appears to be similar to the iminoglycine transport system present in renal tubules and in renal brush border membranes. The opioid pentapeptides Leu- and Met-enkephalin specifically inhibit the uptake of these three imino/amino acids, presumably by interaction with a nonopioid receptor, since the inhibition is not affected by the opiate antagonist naloxone and occurs with des-tyrosyl enkephalins as well as with the intact pentapeptides.

Interaction of enkephalins and des-tyrosyl-enkephalins with synaptosomal plasma membrane vesicles: enkephalin binding and inhibition of proline transport

“…The characteristics of the inhibition of vesicular proline uptake by enkephalins were identical qualitatively with those observed with intact synaptosomes (Rhoads et al, 1983b). The inhibition occurred with the enkephalins and des-tyrosylenkephalins but not with morphine or opioid peptides containing the enkephalin peptide sequences.…”

“…The properties of the uptake and release of proline in brain slices, synaptosomes, and plasma membrane vesicles derived from synaptosomes are similar to those of known neurotransmitter amino acids (Peterson & Raghupathy, 1972;Bennett et al, 1972;Balcar & Johnston, 1974;Mulder & Snyder, 1974;Balcar et al, 1976;Rhoads et al, 1982a-c). Inhibition of synaptosomal proline uptake by enkephalins was not blocked by the classical opioid antagonist naloxone, and the des-tyrosyl derivatives of the enkephalins inhibited to the same degree as the intact pentapeptides (Rhoads et al, 1983b). These results suggested that the inhibition of proline uptake is due to a nonopioid property of the enkephalins and may, therefore, be mediated by a new subclass of enkephalin receptor, perhaps uniquely coupled to the proline carrier.…”

“…Another presynaptic function of nerve terminals involves the reuptake of released neurotransmitters, especially amino acid neurotransmitters, via high-affinity, Na+-dependent transport systems (Iversen, 1971). We recently reported (Rhoads et al, 1983b) that the high-affinity, Na+-dependent transport system for proline in isolated nerve terminals (synaptosomes) prepared from rat cerebral cortex was inhibited by enkephalins. In contrast, the uptake of amino acid neurotransmitters such as y-aminobutyric acid and glutamic acid was not altered in the presence of enkephalins.…”

“…This required that bacitracin (100 pg/mL) be added to the incubation mixtures to retard proteolytic degradation of the [3H]Leu-enkephalin. Binding was determined in buffer A or in buffer B (Rhoads et al, 1983b) containing 130 mM NaC1, 5 mM MgC12, 5 mM KC1, 1 mM EGTA, and 10 mM MOPS, pH 7.4. This latter buffer was routinely used in synaptosomal amino acid transport studies.…”

Tachykinins and calcitonin gene-related peptide enhance release of endogenous glutamate and aspartate from the rat spinal dorsal horn slice

“…Thus, another potential mechanism for the enhancement of the basal release of Glu and of Asp by tachykinins and CGRP is via the electrogenic transport system, which is Ca2+ independent and would be expected to "release" Glu and Asp whenever cells are depolarized. It is of interest that the Gly uptake system is inhibited by the peptides, leu-and met-enkephalin (Rhoads et al, 1984).…”