Cellular and subcellular distribution of substance P receptor immunoreactivity in the dorsal vagal complex of the rat and cat: A light and electron microscope study

Immunoreactivity for the substance P receptor (NK1 receptor) has been investigated by light and electron microscopy in the dorsal vagal complexes of adult rats and cats. The general pattern of NK1 immunoreactivity was similar for both rat and cat. Numerous NK1-immunoreactive neurons were present in the area postrema, the nucleus of the solitary tract, and the dorsal motor nucleus of the vagus nerve. The density of labelled neurons differed between the subnuclei of the nucleus of the solitary tract. Overall, the efferent neurons of the dorsal motor nucleus of the vagus nerve highly expressed NK1 when compared to neurons in the nucleus of the solitary tract. The results are discussed with reference to the viscerotopic organisation of the dorsal vagal complex. Ultrastructural analysis demonstrated that NK1 immunoreactivity was present only at the membrane surface of somatic and dendritic profiles of neurons. No labelling was found in axon terminals, axons, or glial processes. NK1 immunoreactivity, as revealed by a preembedding immunogold technique in serial ultrathin sections, was preferentially located at nonsynaptic sites. A semiquantitative study suggested that the density of NK1 receptors is statistically higher at membrane sites free of any contact (synaptic or not) with axon terminals. The subcellular localisation of NK1 immunoreactivity was similar for neurons of both rat and cat. These results suggest that in the dorsal vagal complex, substance P might act on NK1 receptors through a process of volume transmission.