Title

Insular cortex of the Syrian golden hamster: Architectonics and characterization of the nNOS-containing neurons

Date of Completion

January 1998

Keywords

Biology, Anatomy|Biology, Neuroscience|Biology, Animal Physiology

Degree

Ph.D.

Abstract

The Syrian golden hamster (Mesocricetus auratus) is a well-established experimental model for gustatory studies. Investigations of the processing of taste information at the cerebral cortical level, however, lag behind those at peripheral and brainstem levels. In part, this may be due to the lack of knowledge of the organization of the insular cortex of the hamster. To address this deficiency, a classical cytoarchitecture and myelin atlas of the insular cortex was prepared dividing the insula into three zones, and providing explicit definitions, photomicrographs and drawings for each subzone at different cortical levels. To provide potentially functional correlations for the cytoarchitectural boundaries, a histochemical atlas of the hamster insula was started, using the NADPH diaphorase (NADPHd) and Timm-Danscher stains. The NADPHd reaction stained two populations of neurons: Type I were revealed in Golgi-like detail and Type II were stained only by a stippling of the perikarya. About 85% of Type I cells were found in layers V and VI, but Type II cells and a diffuse blue band of staining were confined within layers II and III. Endothelial cells were also stained. Under appropriate conditions the NADPHd staining may reveal the presence of nitric oxide synthase (NOS) the enzyme that synthesizes NO. Histochemical and immunohistochemical studies showed that Type I staining was primarily attributable to nNOS, and endothelial cell staining was attributable to eNOS, and that all other staining (including Type II cells and the diffuse blue band) was not primarily due to NOS, but to a dicumarol-sensitive enzyme. Type I cells (i.e. "nNOS containing") were typically aspinous or sparsely spinous bitufted or multipolar, but not bipolar, neurons, and possessed dendrites that frequently crossed cell layers and that extended for long distances in the rostrocaudal axis. Type I cells possessed long, horizontally-oriented axons, resembling basket cell axons, and these could cross cytoarchitectural boundaries. All Type I neurons contained both NPY and somatostatin (unlike basket cells), and most likely all contain GAD, and thus are inhibitory interneurons. The results are discussed with relation to the possible roles of the insular cortex in gustatory processing, and of nNOS-containing neurons in cortical processing. ^