Thiamine pyrophosphatase activity in the axonal smooth endoplasmic reticulum of neurosecretory neurons

Neurosecretory cells of the supraoptic-neurohypophysial system of normal mice were investigated with the use of the cytochemical reaction for thiamine pyrophosphatase (TPPase) at the ultrastructural level. In the hypothalamic perikarya dense lead percipitates occur within the cisterns of the mature face of the Golgi apparatus, these being the cisterns that give rise to neurosecretory granules (NSG). Smooth endoplasmic reticulum is occasionally confluent with TPPase-positive Golgi cisterns. Along axons, within swellings, and within terminals distinct profiles of TPPase-positive tubules and cisterns are revealed, apparently part of a network of axonal smooth endoplasmic reticulum (AER). Some NSG appear to be confluent with AER. NSG with TPPase-positive tubular protrusions (likely vestiges of AER) are seen. Apart from reaction product (lead precipitate), the AER often contains an electron dense substance optically similar to that of NSG. TPPase-containing AER is often associated with mitochondria. Profiles of electron-lucent, precipitate-free tubules and cisterns are occasionally seen alongside reactive AER. Optimal TPPase activity in the AER occurs at pH 7.0--7.4, whereas in the Golgi complex intense marking is in the range of pH 6.0--8.5. A faint peppering of precipitate occasionally appears in the AER in controls (incubation medium without substrate), but neither in density nor in extent is this comparable to the reaction product seen after incubation in the presence of TPP. Preliminary comparison has been made between the AER revealed by the TPPase reaction, and that visualized after heavy metal impregnation according to the method of Alonso and Assenmacher (1978a). The nature of the close association between NSG and AER, and the possible roles of this membrane system in neurosecretory cells is discussed.