Acetylcholinesterase-containing neurons of layer VIb in immature neocortex: possible component of an early formed intrinsic cortical circuit.

In this ontogenetic study the neurons of layer VIb of rodent somatosensory cortex have been characterized using acetylcholinesterase (AchE) histochemistry, Golgi, and electron microscopic techniques. Already, at birth, the neurons were found to be AchE-rich. They contain presumed AchE reaction product within their granular endoplasmic reticulum. These cells send fine axons upwards towards the subpial layer where they terminate in a dense, AchE-rich, fiber plexus. In chronically undercut cortex, AchE staining persists in layer VIb neurons and in the subpial fiber plexus. These observations continue to support the view that there is an intrinsic neuronal circuit connecting layer VI with I in immature neocortex. The findings also raise the possibility that the putative circuit is cholinergic.