Neurotoxic effects of neonatal injections of monosodium L-glutamate (L-MSG) on the retinal ganglion cell layer of the golden hamster: anatomical and functional consequences on the circadian system.

In rodents, daily injection of neurotoxic monosodium L-glutamate (MSG) during the postnatal period induces retinal lesions, optic nerve degeneration with an alteration of visual pathway and an absence of the b-wave in the electroretinogram. Despite this damage, electrophysiological responses subsist in the lateral geniculate bodies and synchronization of circadian rhythms to the light/dark cycle can still occur. Using two formal properties of the circadian system (entrainment and phase-shift by light), we assessed the functionality of retinal projections to the circadian clock in MSG-treated hamsters. Displaced amacrine and ganglion cell populations were quantified and retinal terminals in the suprachiasmatic nuclei were estimated. Animals received daily doses of glutamate during the first ten days after birth according to two protocols. The two treatments similarly destroyed 56% of the overall population of the ganglion cell layer: 30% of displaced amacrine and 89% of ganglion cells. Surviving ganglion neurons (7,500 cells) were evenly distributed across the entire retina except in one area of high cell density located in the temporoventral quadrant. Retinal projections of the "image-forming" pathway were drastically reduced in the dorsal lateral geniculate bodies, less in their ventral part. The "nonimage-forming" pathway was also affected since the volume of labeled terminals in the suprachiasmatic nuclei was reduced by one-half to one-third. Nevertheless, treated hamsters exhibited a free-running locomotor activity rhythm after several months in constant darkness, could be entrained by the light/dark cycle and phase-shifted by light pulses. These results suggest that a damaged retinohypothalamic tract can still assume the photic entrainment of the circadian clock.