Amitriptyline inhibits the activity of the rat glutamate transporter EAAT3 expressed in Xenopus oocytes

Objectives: Evidence suggests that glutamatergic systems may be involved in the pathophysiology of major depression and the mechanism of action of antidepressants. We have investigated the effects of amitriptyline, a tricyclic antidepressant, on the activity of the excitatory amino acid transporter type 3 (EAAT3), a protein that can regulate extracellular glutamate concentrations in the brain. Methods: EAAT3 was expressed in Xenopus oocytes. Using a two-electrode voltage clamp, membrane currents were recorded after application of 30 μM L-glutamate in the presence or absence of various concentrations of amitriptyline or after application of various concentrations of L-glutamate in the presence or absence of 0.64 μM amitriptyline. Key findings: Amitriptyline concentration-dependently reduced EAAT3 activity. This inhibition reached statistical significance at 0.38-1.27 μM amitriptyline. Amitriptyline 0.64 μM reduced the pharmacokinetic parameter V_max, but did not affect the pharmacokinetic parameter Km, of EAAT3 for L-glutamate. The amitriptyline inhibition disappeared after a 4-min washout. Phorbol-12-myristate-13-acetate, a protein kinase C activator, increased EAAT3 activity. However, 0.64 μM amitriptyline induced a similar degree of decrease in EAAT3 activity in the presence or absence of phorbol-12-myristate-13-acetate. Conclusions: Our results suggested that amitriptyline at clinically relevant concentrations reversibly reduced EAAT3 activity via decreasing its maximal velocity of glutamate transporting function. The effects of amitriptyline on EAAT3 activity may have represented a novel site of action for amitriptyline to increase glutamatergic neurotransmission. Protein kinase C may not have been involved in the effects of amitriptyline on EAAT3.