Developmentally transient expression of acetylcholinesterase within cortical pyramidal neurons of the rat brain

Using a histochemical method for the visualization of cholinesterase activity in neurons, we have observed developmentally transient expression of acetylcholinesterase (AChE) in cortical pyramidal neurons of the rat brain. Depending on the extent of the deposition of AChE reaction product, several types of cortical neurons could be visualized. We designated neurons with moderate-to-high staining intensity as AChE_H and neurons with relatively lower staining intensity as AChE_L. At birth (P0), 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 AChE_L type. At P5-P7 we observed an abrupt increase in AChE-stained cortical 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 neurons were of the AChE_H variety and displayed morphological characteristics of cortical pyramidal neurons. At P11-P15, there was an abrupt decrease in the number of AChE_H neurons. After P15, the density and staining intensity of cortical AChE-positive (cholinergic) axons gradually increased. Nevertheless, AChE_L pyramidal neurons were detected through these fibers up to P21. At P21, a dense plexus of AChE-positive axons was observed in all cortical areas while very little AChE reaction product was visible in pyramidal neurons, and this pattern continued into adult life. When the adult cortex was denervated from its AChE-positive axons by lesions of the nucleus basalis magnocellularis, many AChE_L pyramidal neurons were uncovered. However, virtually no AChE_H cortical pyramidal neurons were observed in such preparations. Developmentally transient AChE activity within these cortical pyramidal neurons might contribute to their maturation and establishment of synaptic connections.