POSTNATAL-DEVELOPMENT OF CORTICAL ACETYLCHOLINESTERASE-RICH NEURONS IN THE RAT-BRAIN - PERMANENT AND TRANSIENT PATTERNS

The development of acetylcholinesterase (AChE) activity within cortica l neurons of the rat brain was investigated using a histochemical meth od. The fate of these neurons in later stages of development was studi ed in animals in which AChE within cortical axons (mostly cholinergic) had been depleted by lesions of the cholinergic neurons of the basal forebrain or by injections of diisopropyl fluorophosphate, We designat ed neurons with medium to high intensity of reaction product as AChE(H ) and neurons with a low intensity of reaction product as AChE(L). Fou r groups of AChE(H) cortical neurons were detected: (1) AChE(H) Cajal- Retzius cells were present in layer I at birth (PO) and decreased stea dily in number until none could be detected at P17 or thereafter. (2) AChE(H) neurons within layer VI and underlying white matter were prese nt at PO, peaked in number and staining intensity at P8-P9, showed a m oderate decrease in number at P11-P13 and a further decrease into adul thood. (3) AChE(H) polymorphic intracortical neurons appeared at P3-P4 in deep cortical layers and by P9 were present in layers II-VI. They continued to increase in number through P11-P14 at which time they dis played the adult pattern and were found in all cortical areas. (4) A l arge population of AChE(H) pyramidal neurons appeared at P1-P4, peaked at P8-P10 and was no longer visible at P21. In the adult cerebral cor tex, few pyramidal neurons displayed AChE activity and these were almo st always of the AChE(L) type. These results indicate that the AChE wi thin cortical neurons is developmentally regulated and that the conten t of this enzyme helps to differentiate cortical neurons into distinct populations. The transient expression of AChE activity within cortica l neurons suggests a role for this enzyme in the development of the ce rebral cortex. (C) 1995 Academic Press, Inc.