Comparative inhibition of excitatory neurotransmission by N-Acetylcysteine and L-cysteine in bovine isolated retina

Abstract

Purpose: We have evidence that L-cysteine, the substrate for biosynthesis of hydrogen sulfide (H₂S) can regulate potassium (K⁺)-evoked glutamate release from bovine isolated retina. However, the effect of the precursor for L-cysteine, N-acetyl cysteine (NAC) on excitatory neurotransmission has not been fully elucidated. In the present study, we compared the mechanisms by which L-cysteine and NAC regulate K⁺-evoked [³H] D-aspartate release and glutamate-induced neurotoxicity in bovine retina.

Methods: Isolated neural retina were incubated in oxygenated Krebs solution containing 200 nM of [³H] D-aspartate and then prepared for studies of neurotransmitter release. The MTT assay was used to assess retinal neuron survival.

Results: Both L-cysteine (0.1 µM to 10 µM) and NAC (10 µM to 1 mM) attenuated K⁺-induced [³H] D-aspartate release in a concentration-dependent manner. At an equimolar concentration of 10 µM, L-cysteine and NAC inhibited evoked neurotransmitter release by 54.3% (p < 0.001) and 8.3%, respectively. Whereas, the CBS inhibitor, aminooxyacetic acid (AOA; 3mM) and the K_ATP channel blocker,glibenclamide (300 µM) had no effect on K⁺-induced [³H] D-aspartate release, they completely reversed the inhibitory effect of L-cysteine (1 µM to 10 µM). Interestingly, glibenclamide had no effect on NAC-induced inhibition of K⁺-induced [³H] D-aspartate release while AOA partially reversed this response. Both L-cysteine (1 mM) and NAC (1 µM) partially reversed glutamate (12 mM)-induced neuron degeneration by 31.1% (p<0.05) and 18.4%, respectively.

Conclusions: Both L-cysteine and NAC regulate excitatory neurotransmission in bovine retina by separate mechanisms.

Keywords: 518 excitatory neurotransmitters • 616 neurotransmitters/neurotransmitter systems • 688 retina