Regulation of glutamic acid decarboxylase of chick and rat retina cells by GABA and excitatory amino acids

REGULATION OF GLUTAMIC ACID DECARBOXYLASE OF CHICK AND RAT RETINA CELLS BY GABA AND EXCITATORY AMINO ACIDS^*

OLGA MARIA M.S. DE ALMEIDA^1,2, NELSON L. DOS SANTOS¹, JAN NORA HOKOǹ, EDNA N. YAMASAKI, PATRÍCIA F. GARDINO¹AND FERNANDO G. DE MELLO¹

¹

²Instituto de Biologia Roberto Alcantara Gomes, Universidade Estadual do Rio de Janeiro, 20551-030 Rio de Janeiro, RJ, Brazil.

GABA added to cultured retina cells inhibits the expression of glutamic acid decarboxylase (GAD) molecules and activity. In the present work using chick and rat retina cultures, we studied the mechanism of GAD regulation by GABA and excitatory amino acids (EAAs) using immunohistochemical and Page-immunoblot detection of the enzyme, as well as by measuring the enzyme activity. Retina cells were obtained from chick embryos () and aggregate cultures were prepared. For organotypical cultures chick embryos (), post-hatched chick and rat () retinas were used. The treatment with GABA (1 to 20mM) fully prevented the expression of the enzyme in aggregate and organotypical cultures from retinas of chick embryos, as compared to the control tissue. A substantial reduction of GAD was also observed in organotypical cultures of post-hatched chicken and rat retinas. Even though a clear reduction of GAD was observed, part of the enzyme was relatively resistant to the regulation by GABA. The GABA effect was not mimiced by THIP, baclofen or CACA, agonists of GABAa, b, and c receptors, respectively. These drugs did not significantly affect GAD immunoreactivity or activity as compared to control groups. The pre-treatment with NNC-711 a potent GABA carrier inhibitor, prevented GAD reduction induced by GABA and caused a 50% reduction in the inhibition of GAD activity promoted by GABA. These results together indicate that GAD regulation by GABA does not involve known GABAergic receptors activation and requires the transmitter uptake by GABAergic cells. The treatment of aggregates, pre-exposed to GABA, with glutamate or kainate induced an intense GAD-like immunoreactivity in several cell bodies but not in plexus areas. Immunoblotting analysis revealed an immunoreactive fraction corresponding to a protein in the region of 67kD. However GAD activity was not detected in these aggregates. The lack of activity could be a consequence of the production of NO induced by EAAs. In fact treatment of aggregates or retina homogenates with SNAP (but not with its inactive form) induced a reduction of more than 60% in GAD activity. — ( June 27, 2000 ).

Publication Dates

• Publication in this collection
  05 Oct 2000
• Date of issue
  Sept 2000