Distribution of synaptic boutons at the axon initial segment of principal neurons of cortico-cortical projection

摘要

Depending on cell subtypes, axon initial segment (AIS) membrane can be enriched with particular distributions of ion channels. It appears that the presence of Na_v1.6 channels, targeted preferentially to distal AIS, is crucial to originate the axon potential of the cell. AIS plasticity (which can compromise either AIS length or AIS position) is coupled with changes in neuronal excitability in brainstem nuclei of the avian auditory pathway and in mammalian hippocampus. Yet basic parameters such as the length and diameter of the AIS and, of no lesser importance, the number and distribution of boutons synapsing the AIS membrane remains largely unknown for distinct subpopulations of cerebral cortical principal cells. This paper presents a precis of data on synaptic boutons apposed to AIS membrane of principal cells sited in layer 3 and layer 5 of secondary visual cortex and extending their axon projection ipsilaterally and contralaterally to primary visual cortex of the rat. Data were obtained by using correlated light microscopy (retrograde track-tracing labelling) and electron microscopy (AIS reconstruction of labelled cells). Bouton-innervation of AIS was found to be unrelated to size of AIS. Layer 3 cells and associative ipsilateral projection cells significantly had the most bouton-innervated AIS; infragranular cells and coinmissural projection cells through the callosum significantly had the less bouton-innervated AIS. Notwithstanding the axon projection, the distribution of synaptic boutons along AIS was found to be steadily decreasing from axon hillock to axon proper in all layer 5 cells (n = 6): however, the distribution at layer 3 cell AIS was irregular and unsteady from cell to cell (n = 6). We conclude that neurons of layer 3 still may be subdivided in subgroups within the groups of cortico-cortical ipsilateral or contralateral projection we have considered in this study. These subgroups might possess undisclosed structural and functional cell features associated to permanent or transitory plastic synaptic-bouton distributions along the AIS.