Uptake of [3H]Glycine by Synaptosomes of Channel

Abstract

It is known that channel catfish erythrocytes can take up glycine by several distinct transport systems. Further, glycine is an inhibitory neurotransmitter in mammalian brain and spinal cord. Consequently, the uptake of [(3)H]glycine by catfish brain was investigated and found to be a saturable process, dependent on the presence of Na(++) and Cl(--) and sensitive to temperature. A kinetic analysis of transport was performed at 22C. This showed that a high-affinity system existed which exhibited a K(m) of 5.1 (+/- 2. 1) microM. Several structural analogues of glycine were capable of inhibiting uptake in a competitive manner. The most effective inhibitor was sarcosine (IC(50) 5 36 microM). Uptake was also able to be inhibited by harmaline, a drug known to interfere with Na(+)-dependent transport processes. It is concluded that glycine transport by channel catfish brain has much in common with transport by mammalian nervous tissue which is carried out by the membrane carriers GLYT1 and GLYT2. On the other hand, synaptosomal transport differs somewhat from glycine transport by channel catfish erythrocytes.

Kinetics of Binding of [³H]Glycine to Transport Proteins in Channel Catfish Brain

“…These proteins belong to a family of Na + -coupled carriers that possess 12 hydrophobic plasma membrane-spanning regions [16]. We have previously reported that channel catfish brain has the ability to transport glycine by a system that can be stimulated by both Na + and Cl - [10]. That system had a K t of 5.1 ÌM, a similar value to the K d we now report.…”

“…The energy for this active transport system is derived from the Na + gradient across the plasma membrane produced by the action of the sodium pump [12]. In the present study, we have measured the binding of [ 3 H]glycine to particles prepared from channel catfish brain and concluded that our observations represent binding to the transporters that previously we have shown to be responsible for [ 3 H]glycine uptake into synapsomes from these fish [10].…”

“…Previous experiments have demonstrated the presence of a secondary active transport system for glycine in synaptosomes prepared from channel catfish brain [10]. The uptake of neurotransmitters from the synaptic gap is a common mechanism for the removal of a neurotransmitter from the vicinity of the relevant receptor.…”

“…Both synaptosomes from brain and red cells isolated from catfish have been shown to transport glycine by an Na + -dependent, active uptake process [9,10]. This transport process resembles system Gly [11].…”

Membrane Glycine Transport Proteins

“…This uptake of glycine required Na + ions and bore the hallmark of a secondary active process reminiscent of system Gly. A similar glycine transport system has been examined in the brain of channel catfish [1,82]. Just over a decade ago, Ló pez-Corcuera et al [52] purified a glycine transporter from pig brainstem with a molecular weight of 100 kD.…”