Effects of cortical and thalamic lesions upon primary afferent terminations, distributions of projection neurons, and the cytochrome oxidase pattern in the trigeminal brainstem complex.

Early postnatal lesions of the primary somatosensory cortex alter the vibrissa-related cytochrome oxidase (CO) pattern in nucleus principalis (PrV) of the rat's trigeminal (V) brainstem complex (Erzurumlu and Ebner, '88: Dev. Brain Res. 44:302-308). At present, the reason for this change is not clear. It may be that the corticotrigeminal projection is necessary for the maintenance of vibrissa-related patterns in PrV. However, it is also possible that the loss of the normal pattern of CO activity reflects a change in the organization of brainstem cells resulting from transneuronal retrograde degeneration. In order to address this question, we made lesions of either the primary somatosensory cortex (S-I) or ventrobasal thalamus (VB) in newborn rats and directly assayed distribution of V primary afferents by transganglionic transport of horseradish peroxidase and V-thalamic neurons by retrograde transport of either fluorogold or true blue. Neonatal cortical and thalamic lesions produced no qualitative change in the distribution of primary afferent terminals in either PrV or V subnucleus interpolaris (SpI) beyond that which could be attributed to shrinkage of the brainstem resulting from retrograde degeneration. Most importantly, the "patchy" pattern of terminations observed in normal rats remained apparent in the brain-damaged animals. The normal distribution of V-thalamic neurons in PrV was disrupted by both cortical and thalamic lesions. These cells are normally patterned in a way that matches the distribution of primary afferent terminals and thus that of the mystacial vibrissae. This was not the case in the neonatally brain-damaged rats. Taken together, these results are consistent with the conclusion that neonatal cortical and thalamic lesions disrupt the normal CO pattern in PrV primarily because of their effects upon the patterning of brainstem cells. The present findings demonstrate further that clustering of primary afferents does not require a normal complement of postsynaptic neurons.