Septate-like junctions in the normal male rat pituitary gland

Potassium channel subunit Kv3.2 and the water channel aquaporin-4 are selectively localized to cerebellar pinceau

The pinceau is a cerebellar structure formed by descending GABA-ergic basket cell axonal terminals converging on the initial axonal segment of Purkinje cell. Although basket cells exert a powerful inhibitory influence on the output of the cerebellar cortex, the function and mode of action of the pinceau are not understood because the majority of basket cell axons fail to make identifiable synaptic contacts with the Purkinje cell axon. Several proteins were previously reported to cluster specifically in this area, including a number of voltage-activated potassium channel subunits. In this study, we used immunohistochemistry, electron microscopy, and electron tomography to examine the ultrastructural localization of a novel voltage-gated potassium channel subunit, Kv3.2, in the pinceau. We found strong, selective localization of Kv3.2 to basket cell axons. Additionally, because potassium buffering is often conducted through water channels, we studied the extent of a brain-specific water channel, aquaporin-4 (AQP4), using confocal and electron microscopy. As expected, we found AQP4 was heavily localized to astrocytic processes of the pinceau. The abundance of potassium channels and AQP4 in this area suggests rapid ionic dynamics in the pinceau, and the unusual, highly specialized morphology of this region implies that the structural features may combine with the molecular composition to regulate the microenvironment of the initial segment of the Purkinje cell axon.

Synaptoid contacts between gland cells of the anterior pituitary of the rat

BACKGROUND

There is evidence suggesting cross-talk among gland cells of the anterior pituitary. We had reported a rare form of synaptoid contact between corticotrophs in the anterior pituitary of the dog. We then found similar synaptoid contacts with different characteristics in the rat, as described in the present article.

METHODS

Male Sprague-Dawley rats were used. The anterior pituitaries were prepared for ultrastructural study of substance P immunoreactivity of the anterior pituitary. Routine preembedding immunohistochemical staining was conducted, the sections were embedded in Epon 812 (Serva Feinbiochemica, Heidelberg, New York), and ultrathin sections were prepared.

RESULTS

In the anterior pituitary of the rat, synaptoid contacts were found between corticotrophs and lactotrophs. They appeared very close to typical synapses in the central nervous system, aside from evident weakness of presynaptic density.

CONCLUSIONS

The presence of synaptoid contacts suggests a form of cross-talk between the gland cells in the anterior pituitary of the rat.

The axonal membrane protein Caspr, a homologue of neurexin IV, is a component of the septate-like paranodal junctions that assemble during myelination

We have investigated the potential role of contactin and contactin-associated protein (Caspr) in the axonal-glial interactions of myelination. In the nervous system, contactin is expressed by neurons, oligodendrocytes, and their progenitors, but not by Schwann cells. Expression of Caspr, a homologue of Neurexin IV, is restricted to neurons. Both contactin and Caspr are uniformly expressed at high levels on the surface of unensheathed neurites and are downregulated during myelination in vitro and in vivo. Contactin is downregulated along the entire myelinated nerve fiber. In contrast, Caspr expression initially remains elevated along segments of neurites associated with nascent myelin sheaths. With further maturation, Caspr is downregulated in the internode and becomes strikingly concentrated in the paranodal regions of the axon, suggesting that it redistributes from the internode to these sites. Caspr expression is similarly restricted to the paranodes of mature myelinated axons in the peripheral and central nervous systems; it is more diffusely and persistently expressed in gray matter and on unmyelinated axons. Immunoelectron microscopy demonstrated that Caspr is localized to the septate-like junctions that form between axons and the paranodal loops of myelinating cells. Caspr is poorly extracted by nonionic detergents, suggesting that it is associated with the axon cytoskeleton at these junctions. These results indicate that contactin and Caspr function independently during myelination and that their expression is regulated by glial ensheathment. They strongly implicate Caspr as a major transmembrane component of the paranodal junctions, whose molecular composition has previously been unknown, and suggest its role in the reciprocal signaling between axons and glia.