The topographical organization of neurons in the rat medial frontal, insular and olfactory cortex projecting to the solitary nucleus, olfactory bulb, periaqueductal gray and superior colliculus

Abstract

In 19 rats two different retrograde tracers (Fast Blue, Diamidino Yellow, Rhodamine-labeled latex microspheres, or wheat germ agglutinin conjugated with HRP) were injected into the solitary nucleus (NTS) and either the olfactory bulb (OB), periaqueductal gray (PAG) or superior colliculus (SC). The pattern of retrogradely labeled neurons in the medial frontal, insular and olfactory cortices was examined to determine the topographical organization of the cell populations projecting to these subcortical targets and the extent to which they overlapped. In the medial frontal cortex (MFC) SC projections originated most dorsally, while NTS and OB projections originated most ventrally and exhibited slight overlap. PAG projections originated from virtually the entire MFC and overlapped with cells projecting to the OB, NTS and SC. These results are consistent with the role of dorsal MFC as the rat's frontal eye field and the ventral MFC as a visceral motor area. Laterally, in the insular cortex there was virtually complete overlap between cells projecting to the NTS and PAG. The extensive overlap of PAG projections with NTS projections medially and laterally and with SC projections medially suggests the PAG is involved in a variety of brain visceral and somatic functions. In the piriform cortex there was overlap between cells projecting to the OB and cells projecting to the SC; the cells projecting to the SC were located in the endopiriform nucleus, and may provide a substrate for orienting responses to odors.