VIP- and PHI-immunoreactivity in olfactory centers of the adult cat

The purpose of the study was to determine the morphology and distribution of vasoactive intestinal polypeptide- and peptide histidine isoleucine-immunoreactive (VIP- and PHI-ir) neurons and innervation patterns in the main and accessory olfactory bulb, anterior olfactory nucleus, and piriform cortex of the adult cat. In these centers, VIP- and PHI-immunoreactive material are present in the same neuronal types, respectively, therefore summarized as VIP/PHI-ir neurons. In the main olfactory bulb, the majority of VIP/PHI-ir neurons are localized in the external plexiform layer. These neurons give rise to two or more locally branching axons. They form boutons on mitral and external tufted cell bodies. According to the morphology and location, we have classified these neurons as Van Gehuchten cells. Some VIP/PHI-ir neurons are present in the glomerular layer. They have small somata and give rise to dendrites branching exclusively into glomeruli. We have classified these neurons as periglomerular cells. In the granule cell layer, neurons with long apical dendrites and one locally projecting axon are present. In the accessory olfactory bulb, VIP/PHI-ir neurons are localized in the mixed external/mitral/internal plexiform layer. They represent Van Gehuchten cells. In the anterior olfactory nucleus and piriform cortex, VIP/PHI-ir bipolar basket neurons are present. They are localized mainly in layers II/III. These neurons are characterized by a bipolar dendritic pattern and by locally projecting axons forming basket terminals on large immunonegative cell somata. Because of their common morphological features, we summarize them as the retrobulbar VIP/PHI-ir interneuron population. The PHI-ir neurons display the same morphology as the VIP-ir cells. However, they are significantly lower in number with a ratio of VIP-ir to PHI-ir cells about 2:1 in the main and accessory olfactory bulb and in the anterior olfactory nucleus. By contrast, in the piriform cortex the ratio is about 1:1.