Increase in delta, but not mu, receptors in MSG-treated rats.

Neonatal treatment of rats with Monosodium Glutamate (MSG) has been demonstrated to destroy cell bodies of neurons in the arcuate nucleus including the brain beta-endorphin (B-END) system. The effects on opiate receptors of the loss of B-END is unknown. Seven to nine month old rats treated with MSG on the first two postnatal days and litter matched untreated control rats were decapitated and their brains dissected into several regions. Opiate receptor assays were carried out with [3H] morphine (mu receptor ligand) and [3H] DADL (delta receptor ligand) for each brain region for both MSG-treated and control rats simultaneously. Scatchard plot analyses showed a selective increase in delta receptors in the thalamus only. No corresponding change in mu receptors in the thalamus was found. The cross-competition IC50 data supported this conclusion, showing a loss in the potency of morphine in displacing [3H] DADL in the thalamus of MSG treated rats.