RETROGRADE DEGENERATION OF THALAMIC NEURONS IN THE MEDIODORSAL NUCLEUS AFTER NEONATAL AND ADULT ASPIRATION LESIONS OF THE MEDIAL PREFRONTALCORTEX IN THE RAT - IMPLICATIONS FOR MECHANISMS OF FUNCTIONAL RECOVERY

The behavioural consequences of neonatal lesions of the frontal cortex are limited as compared with similar lesions performed in adulthood. The present study has investigated, using unbiased quantitative method s with randomized systematic sampling, the total neuronal cell numbers in the mediodorsal nucleus of the thalamus after aspiration lesions o f the medial prefrontal cortex performed in neonatal and in adult rats . It was found that the reduction in total cell numbers after neonatal prefrontal cortex lesions was similar to that found after adult corte x lesions. in neonatally lesioned animals the neuronal cell density wa s significantly increased by 13%, whereas in adult lesioned animals it was unchanged. On the other hand, the volume of the mediodorsal nucle us was reduced by 27% in neonatally, and 20% in adult lesioned animals . Total neuronal cell, number of the mediodorsal nucleus was significa ntly decreased in neonatally as well as in adult lesioned rats, by 14% and 21%, respectively. These findings are discussed in the light of t he previously proposed role of the thalamus as a neural substrate of f unctional sparing after neonatal cortical lesions.