DEVELOPMENTALLY TRANSIENT EXPRESSION OF ACETYLCHOLINESTERASE WITHIN CORTICAL PYRAMIDAL NEURONS OF THE RAT-BRAIN

Using a histochemical method for the visualization of cholinesterase a ctivity in neurons, we have observed developmentally transient express ion of acetylcholinesterase (AChE) in cortical pyramidal neurons of th e rat brain. Depending on the extent of the deposition of AChE reactio n product, several types of cortical neurons could be visualized. We d esignated neurons with moderate-to-high staining intensity as AChE(H) and neurons with relatively lower staining intensity as AChE(L). At bi rth (PO), very little AChE activity was found within cortical neurons. Between P1-P4, there was a gradual emergence of AChE-stained cortical neurons. At this stage, the majority of these neurons were of the ACh E(L) type. At P5-P7 we observed an abrupt increase in AChE-stained cor tical neurons. The number and the staining intensity of these neurons was at a peak at P8-P10. At this age range, the majority of these neur ons were of the AChE(H) variety and displayed morphological characteri stics of cortical pyramidal neurons. At P11-P15, there was an abrupt d ecrease in the number of AChE(H) neurons. After P15, the density and s taining intensity of cortical AChE-positive (cholinergic) axons gradua lly increased. Nevertheless, AChE(L) pyramidal neurons were detected t hrough these fibers up to P21. At P21, a dense plexus of AChE-positive axons was observed in all cortical areas while very little AChE react ion product was visible in pyramidal neurons, and this pattern continu ed into adult life. When the adult cortex was denervated from its AChE -positive axons by lesions of the nucleus basalis magnocellularis, man y AChE(L) pyramidal neurons were uncovered. However, virtually no AChE (H) cortical pyramidal neurons were observed in such preparations. Dev elopmentally transient AChE activity within these cortical pyramidal n eurons might contribute to their maturation and establishment of synap tic connections.