Visual Neuroscience

Table of Contents - 2003 - Volume 20, Issue 05  


Activation of glutamate transporters in rods inhibits presynaptic calcium currents

KATALIN  RABL  a1 p1 , ERIC J.  BRYSON  a1 and WALLACE B.  THORESON  a1 a2 c1
a1 Department of Ophthalmology, University of Nebraska Medical Center, Omaha
a2 Department of Pharmacology, University of Nebraska Medical Center, Omaha

Article author query
rabl k   [Google Scholar] 
bryson ej   [Google Scholar] 
thoreson wb   [Google Scholar] 
 

Abstract

We found that L-glutamate (L-Glu) inhibits L-type Ca2+ currents (ICa) in rod photoreceptors. This inhibition was studied in isolated rods or rods in retinal slices from tiger salamander using perforated patch whole cell recordings and Cl−-imaging techniques. Application of L-Glu inhibited ICa by [similar]20% at 0.1 mM and [similar]35% at 1 mM. L-Glu also produced an inward current that reversed around ECl. The metabotropic glutamate receptor (mGluR) agonists t-ADA (Group I), DCG-IV (Group II), and L-AP4 (Group III) had no effect on ICa. However, the glutamate transport inhibitor, TBOA (0.1 mM), prevented L-Glu from inhibiting ICa. D-aspartate (D-Asp), a glutamate transporter substrate, also inhibited ICa with significantly more inhibition at 1 mM than 0.1 mM. Using Cl− imaging, L-Glu (0.1–1 mM) and D-Asp (0.1–1 mM) were found to stimulate a Cl− efflux from terminals of isolated rods whereas the ionotropic glutamate receptor agonists NMDA, AMPA, and kainate and the mGluR agonist, 1S,3R-ACPD, did not. Glutamate-evoked Cl− effluxes were blocked by the glutamate transport inhibitors TBOA and DHKA. Cl− efflux inhibits Ca2+ channel activity in rod terminals (Thoreson et al. (2000), Visual Neuroscience 17, 197). Consistent with the possibility that glutamate-evoked Cl− efflux may play a role in the inhibition, reducing intraterminal Cl− prevented L-Glu from inhibiting ICa. In summary, the results indicate that activation of glutamate transporters inhibits ICa in rods possibly as a consequence of Cl− efflux. The neurotransmitter L-Glu released from rod terminals might thus provide a negative feedback signal to inhibit further L-Glu release.

(Received April 14 2003)
(Accepted August 28 2003)


Key Words: Photoreceptor; L-type calcium channel; Retina; Glutamate transporter; Tiger salamander.

Correspondence:
c1 Address correspondence and reprint requests to: Wallace B. Thoreson, Ophthalmology Department, University of Nebraska Medical Center, 985540 Nebraska Medical Center, Omaha, NE 68198-5540, USA. E-mail: WBTHORES@UNMC.EDU
p1 On leave from the Department of General Zoology and Neurobiology, University of Pécs, Pécs, Hungary