Selective presence of a giant saccular organelle in the axon initial segment of a subpopulation of layer V pyramidal neurons.

Recently it has been shown that a giant saccular organelle (GSO) of unknown function is present in the axon initial segment (AIS) of an uncharacterized population of pyramidal cells of the rodent neocortex. Using tract-tracing methods and immunocytochemistry, in the present study we show that in rodents this GSO is present in the AIS of subpopulations of layer V pyramidal neurons projecting to various subcortical, non-thalamic targets, including the spinal cord. GSO-containing neurons express SMI32 and some of them are under the control of the Thy-1 gene promoter. In addition, our results demonstrate that the GSO expresses the inositol 1,4,5-triphosphate receptor 1 (IP3R1) and the sarco (endo) plasmic reticulum Ca(2+) ATPase 2, both in rodent and human neocortex. These results indicate the involvement of the GSO in the regulation of Ca(2+) levels in the AIS in a particular subpopulation of layer V neurons that give rise to subcortical non-thalamic descending projections.