Different neuron populations in the feline lateral cervical nucleus: A light and electron microscopic study with the retrograde axonal transport technique

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Abstract
A comparative light and electron microscopic study was made of the two major neuron populations in the feline lateral cervical nucleus (LCN) that have an ascending projection–namely, the cervicothalamic and cervicomesencephalic neurons. The small population of neurons in the LCN that remained unlabelled after extensive injections into all known areas of LCN termination was also investigated. The intra‐axonal retrograde tracing technique was employed and three different tracers were used: native horseradish peroxidase, horseradish peroxidase conjugated to wheat germ agglutinin, and cholera toxin subunit B (CTb). The diaminobenzidine and tetramethylbenzidine methods were used for the peroxidase reaction, and an immunocytochemical staining method with monoclonal antibodies was employed to localize CTb. The light microscopic cell count was performed on plasticembedded semithin (2 μm) sections, and a section‐embedding technique was used for sampling regions of interest for electron microscopy. The total number of neurons in the LCN was calculated to be 8,300, which is higher than numbers reported from other studies on frozen sections. It was concluded from control experiments that many neurons fail to counterstain in frozen sections. The average proportion of labelled cervicothalamic neurons was 94%, which accords with earlier reports. The frequency of labelled neurons projecting to the midbrain was 25–49%, depending on the method used. The highest number of labelled neurons was found with the immunocytochemical method with use of CTb as a tracer. In the electron microscopic study, the cell area, form factor, nuclear area, bouton covering ratio, mean appositional length of boutons, and bouton density were measured. There were no significant differences in these respects between the cervicothalamic and cervicomesencephalic neurons, but the unlabelled neurons differed significantly from the labelled ones, with a smaller somal area, a lower bouton covering ratio, and a larger relative nuclear area. The ascending neurons were scattered over the entire LCN, but the neurons that remained unlabelled after the extensive injections was found in its ventromedial part. It is suggested that these are interneurons.In the light microscopic study of the animals injected with CTb, the ventromedial part of the LCN displayed labelled structures resembling terminals. Electron microscopic examination of the sections revealed boutons synapsing on unlabelled neurons and dendrites. Occasionally labelled neurons and dendrites were also contacted. These terminals, which were found both after thalamic and tectal injections, probably belong to the recurrent axon collaterals of the ascending neurons.

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