Synaptic degeneration in rat visual cortex after neonatal administration of methylmercury.

Neonatal rats received subcutaneous injections of methylmercuric chloride in physiological saline (5 mg Hg/kg/day) beginning on postnatal day 5 and continuing until a given animal demonstrated a persistent loss of body weight during 48 h. Clinical signs of visual impairment were observed in all methylmercury-treated animals within 2 to 3 days of the initial weight loss (postnatal days 19 to 23). Four animals exhibited myoclonic jerking of the hind limbs, and one of these animals demonstrated a generalized motor seizure. Electron microscopy of visual cortex in the nonconvulsive animals revealed a selective degeneration of axon terminals forming symmetric synapses. Degenerating neurons were concentrated in layer IV with a more diffuse distribution in other cortical laminae. Dendritic spines were not found on degenerating dendritic profiles, although apparently normal axon terminals were observed to form synapses on degenerating dendritic shafts. These morphologic criteria suggest that aspinous or sparsely-spinous stellate neurons, the inhibitory GABAergic interneurons of the neocortex, are selectively impaired in methylmercury-induced lesions of the developing visual cortex.