Anatomical evidence for differential rates of maturation of the medial dorsal nucleus projections to prefrontal cortex in rats

Abstract

Functional maturation of the prefrontal system has been studied historically through the use of the lesion-behavior method. More recently an anatomical index of functional maturation has been used which stresses the importance of the onset (⩾72h) of stable long-lasting degeneration argyrophilia following an experimental lesion. If these two indices of development assess a similar process (neural maturation), then findings utilizing the two techniques should converge. We tested this proposition by examining the age of onset of stable degeneration argyrophilia in the thalamic projections to sulcal and medial prefrontal cortex in rats. The behavioral data from the effects of lesions of prefrontal cortex in infant rats suggest that the sulcal prefrontal system matures earlier than the medial prefrontal system. Electrolytic lesions were produced in the medial dorsal nucleus of the thalamus at either 13, 20 or 25 days of age. The subjects in each group were divided into a short survival time group (24 h) or a long survival time group (72 h). The tissue was then stained by the method of Fink-Heimer (procedure 1). In the 13-day 72-h survival group there was a sharp distinction in the presence of stable degeneration between the two prefrontal subfields. In the sulcal prefrontal cortex stable degeneration was found in 11 of 13 brains, whereas stable degeneration was found in medial prefrontal cortex in only 4 of 13 brains. Further, in 3 of the 4 brains demonstrating stable degeneration in medial prefrontal cortex, the degeneration was seen at threshold levels in only a single section. By 20 days of age stable degeneration was found in both subfields in all brains. These data indicate a strong correspondence between anatomical (stable degeneration) and the findings of lesion-behavior studies of functional maturation and offer further evidence that the two subfields of prefrontal cortex mature at different rates postnatally.