Mechanism of the inhibitory effect of imipramine on the Na+-dependent transport of L-glutamic acid in rat intestinal brush-border membrane

Doxepin and imipramine but not fluoxetine reduce the activity of the rat glutamate transporter EAAT3 expressed in Xenopus oocytes

Background

Many researchers have suggested that the glutamatergic system may be involved in the effects of antidepressant therapies. We investigated the effects of doxepin, imipramine, and fluoxetine on the excitatory amino acid transporter type 3 (EAAT3).

Methods

EAAT3 was expressed in Xenopus oocytes by injection of EAAT3 mRNA. Membrane currents were recorded after application of L-glutamate (30 μM) in the presence or absence of various concentrations of doxepin, imipramine, and fluoxetine. To study the effects of protein kinase C (PKC) activation on EAAT3 activity, oocytes were pre-incubated with phorbol 12-myristate-13-acetate (PMA) before application of imipramine and doxepin.

Results

Doxepin at 0.063–1.58 μM significantly decreased EAAT3 activity. Imipramine reduced EAAT3 activity in a concentration-dependent manner at 0.16–0.95 μM. However, fluoxetine did not affect EAAT3 activity, and PMA increased EAAT3 activity. At 0.32 μM, imipramine caused an equivalent decrease in EAAT3 activity in the presence or absence of PMA. However, 0.79 μM doxepin did not abolish the enhancement of EAAT3 activity by PMA.

Conclusions

We showed that doxepin and imipramine, but not fluoxetine, inhibited EAAT3 activity at clinically relevant concentrations. This reveals a novel mechanism of action for doxepin and imipramine; that they increase glutamatergic neurotransmission. PKC may be involved in the effects of doxepin on EAAT3, but is not involved in the effects of imipramine at the concentrations studied.

Characterization of the tritium-labeled analog of L-threo-beta-benzyloxyaspartate binding to glutamate transporters

L-Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. Termination of glutamate receptor activation and maintenance of low extracellular glutamate concentrations are primarily achieved by glutamate transporters (excitatory amino acid transporters 1-5, EAATs1-5) located on both the nerve endings and the surrounding glial cells. To identify the physiological roles of each subtype, subtype-selective EAAT ligands are required. In this study, we developed a binding assay system to characterize EAAT ligands for all EAAT subtypes. We recently synthesized novel analogs of threo-beta-benzyloxyaspartate (TBOA) and reported that they blocked glutamate uptake by EAATs 1-5 much more potently than TBOA. The strong inhibitory activity of the TBOA analogs suggested that they would be suitable to use as radioisotope-labeled ligands, and we therefore synthesized a tritiated derivative of (2S,3S)-3-{3-[4-ethylbenzoylamino]benzyloxy}aspartate ([3H]ETB-TBOA). [3H]ETB-TBOA showed significant high-affinity specific binding to EAAT-transfected COS-1 cell membranes with each EAAT subtype. The Hill coefficient for the Na+-dependence of [3H]ETB-TBOA binding revealed a single class of noncooperative binding sites for Na+, suggesting that Na+ binding in the ligand binding step is different from Na+ binding in the substrate uptake process. The binding was displaced by known substrates and blockers. The rank order of inhibition by these compounds was consistent with glutamate uptake assay results reported previously. Thus, the [3H]ETB-TBOA binding assay will be useful to screen novel EAAT ligands for all EAAT subtypes.

Inhibitory Effects of Basic Drugs on the Sodium-Dependent Transport of L-Alanine via System B0 in the Small Intestine

To elucidate the mechanism of the interaction of basic drugs with Na(+)-dependent L-alanine absorption from the small intestine, we investigated the effect of imipramine on the transport of L-alanine via system B(0), which is thought to be one of the main Na(+)-dependent systems for intestinal absorption of short-chain neutral amino acids, including L-alanine. The uptake of L-alanine by cells that express hATB(0) (human amino acid transporter B(0)) was inhibited in the presence of imipramine. The Eadie-Hofstee plot showed that the inhibition was not competitive with the substrate (L-alanine) but competitive with Na(+). When rat intestinal brush border membrane vesicles were used, several basic drugs had inhibitory effects. Inhibition was also observed in intestinal absorption evaluated by an in situ single-pass perfusion technique. However, the potencies of inhibition were different. The potency of inhibition was dependent on the lipophilicity of the drugs.

The kinetic mechanism of the glutamate-aspartate carrier in rat intestinal brush-border membrane vesicles: the role of potassium

The sodium dependent transport system for L-glutamate and L-aspartate localized in the apical part of rat enterocytes has previously been kinetically characterized (Prezioso, G., and Scalera, V. (1996). Biochim. Biophys. Acta 1279, 144-148). In this paper the mechanism by which the potassium cation specifically activates the L-glutamate-sodium cotransport process is investigated. Potassium has been found to act as an activator when it is present inside the membrane vesicles, while its presence outside is ineffective, and the effect is saturable. The kinetic parameters with respect to sodium and glutamate have been compared in the presence and in the absence of the activator. The results indicate that the ordered sodium-sodium glutamate mechanism is not altered by potassium, and that the activation is probably exerted on both the rate determining steps of the transport process. It is proposed that (1) a specific binding site for potassium is present on the inside hydrophilic part of the membrane carrier, (2) the binding of the effector accelerates the intramembrane rearrangement steps of both the disodium glutamate-carrier complex and the free carrier, (3) the affinity of the carrier is lowered with respect to sodium whereas it is increased for glutamate, and (4) K+ antiport is not performed by this carrier.