The Damaging Effect of Systemic Injection of Monosodium Glutamate (MSG) on the Development of Normal and Form‐Deprived Eyes of the Chick

Glutamate has been implicated in ocular development. The present study aims to characterize the physical changes that occur as a result of systemic intake of monosodium glutamate (MSG). As MSG has also been suggested to play a part in myopia, its effect on form‐deprived eyes was also investigated. Form‐deprivation myopia (FDM) was induced by placing a black goggle over one eye of each chick. Neonatal chicks were injected with daily MSG (0.1, 1 and 10 mg/day) or saline control for 14 days intraperitoncally (n = 6–7 in all cases). The results show that MSG significantly reduced the eye weight (EW), ocular length (OL), and equatorial diameter (ED) of the normal (i.e. with MSG treatment but not form‐deprived) ocular growth (mean ± s.e.m, MSG dose in parenthesis): EW, 0.90 ± 0.02 vs. 0.81 ± 0.05 (0.1), 0.78 ± 0.04 (1), 0.83 ± 0.04 (10); OL, 1.02 ± 0.03 vs. 0.94 ± 0.03 (0.1), 0.95 ± 0.01 (1), 0.97 ± 0.02 (10); ED, 1.36 ± 0.01 vs. 1.30 ± 0.02 (0.1), 1.28 ± 0.02 (1), 1.33 ± 0.03 (10). As a result of FDM, these measurements of the eye tend to increase as well as result in thinning of the retina and thickening of the sclera. However, under MSG treatment, no significant increase in these parameters with form‐deprived chicks occurred. In fact, a gradual downward trend was observed. Histological measurements show that both the normal and myopic retinal and scleral layers were thinner than the controls (i.e. no MSG or FDM) at MSG‐1 and 10 mg/day (thickness ± s.e.m, MSG dose in parenthesis): control retina: 97 ± 12 vs. 77 ± 3 (1) and 78 ± 2 (10); FDM retina: 83 ± 16 vs. 74 ± 4 (1) and 75 ± 3 (10); control sclera: 44 ± 7 vs. 29 ± 2 (1) and 36 ± 3 (10); FDM sclera; 41 ± 5 vs. 31 ± 2 (1) and 32 ± 2 (10). However, MSG # injections of 0.1 mg/day caused a significant increase in both normal and myopic retinal and scleral thickness compared to the controls stated above: MSG retina, 140 ± 14 and FDM retina, 157 ± 15; MSG sclera, 55 ± 7 and FDM sclera, 63 ± 8. The results suggest that MSG at high concentrations can cause a reduction in ocular growth probably due to its related cytotoxicity and the subsequent cellular destruction with or without inherent FDM. The increase in retinal and scleral thickness at low dose of MSG may be due to intracellular swelling as has been reported in other studies. The damaging effect of MSG to the retina of both goggled and open eyes may be sufficient to cause the eye to become myopic as images will thus be projected in front of the retina. Despite the low dosage used, it still caused significant ocular damages, which suggests that the amount of MSG intake must be viewed with caution.