Somatostatin-like immunoreactivity in rat main olfactory bulb: extent of coexistence with neuropeptide Y-, tyrosine hydroxylase- and vitamin D-dependent calcium binding protein-like immunoreactivities.

A double-labeling immunofluorescence colocalization technique was used to examine the extent of coexistence of somatostatin (SOM)-like immunoreactivity with neuropeptide Y (NPY)-, tyrosine hydroxylase (TH)- and vitamin D-dependent calcium binding protein (D-CaBP)-like immunoreactivities in neurons of the rat main olfactory bulb. SOM-like immunoreactivity (SOM-I) was distributed within restricted populations of periglomerular neurons and deep short-axon cells, and rarely within superficial short-axon cells at the glomerular layer/external plexiform layer (GL/EPL) border region. Double-labeling analysis revealed that all of the SOM-I deep and superficial short-axon cells also contained NPY-I. Colocalization of SOM-I and TH-I or of SOM-I and D-CaBP-I was infrequently observed within periglomerular neurons. The rare SOM-I short-axon cells at the GL/EPL border always exhibited D-CaBP-I. These results demonstrate virtual complete coexistence of SOM and NPY in short-axon neurons of the main olfactory bulb. With a few exceptions, however, bulbar SOM neurons appear to constitute subclasses of periglomerular cells immunohistochemically distinct from those containing TH or D-CaBP.