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Previous findings have indicated the presence of local circuit neurons in the lateral cervical nucleus (LCN). An immunohistochemical study with gamma-aminobutyric acid (GABA) antiserum was therefore performed both to investigate whether GABA-immunoreactive neurons are present in the LCN, and if so, to compare their characteristics with those previously assigned to probable internuncial neurons in the nucleus. The fine structure and synaptology of GABA-positive boutons in the LCN were also studied. Transversely cut sections from the upper cervical spinal cord of three cats were processed for GABA immunohistochemistry with the free-floating PAP technique. On light microscopic examination immunoreactive neurons were observed within the ventromedial half of the LCN. Their total number was estimated to be 42.5 +/- 11.7 in the entire LCN on one side of the cervical spinal cord, but this may have been an underestimation, as the penetration by the antisera was limited. The labeled neurons were small and had a relatively large nucleus and a low bouton covering ratio. In their number, localization, and ultrastructural appearance the GABA-positive neurons closely resembled the population of neurons previously suggested to be local circuit neurons. Immunoreactive bouton-sized puncta were scattered throughout the LCN. Ultrastructural examination showed labeled terminals with a mean sectional area of 0.85 micron 2 and a relatively high density of synaptic vesicles. The vast majority of GABA-positive terminals were in contact with dendrites and only a minority had synaptic contact with cell bodies. No axoaxonal synapses were observed. The GABA-positive boutons probably derive at least partly from the observed GABA-positive neurons, but there is also a possibility of extrinsic GABAergic input.
Previous findings have indicated the presence of local circuit neurons in the lateral cervical nucleus (LCN). An immunohistochemical study with gamma-aminobutyric acid (GABA) antiserum was therefore performed both to investigate whether GABA-immunoreactive neurons are present in the LCN, and if so, to compare their characteristics with those previously assigned to probable internuncial neurons in the nucleus. The fine structure and synaptology of GABA-positive boutons in the LCN were also studied. Transversely cut sections from the upper cervical spinal cord of three cats were processed for GABA immunohistochemistry with the free-floating PAP technique. On light microscopic examination immunoreactive neurons were observed within the ventromedial half of the LCN. Their total number was estimated to be 42.5 +/- 11.7 in the entire LCN on one side of the cervical spinal cord, but this may have been an underestimation, as the penetration by the antisera was limited. The labeled neurons were small and had a relatively large nucleus and a low bouton covering ratio. In their number, localization, and ultrastructural appearance the GABA-positive neurons closely resembled the population of neurons previously suggested to be local circuit neurons. Immunoreactive bouton-sized puncta were scattered throughout the LCN. Ultrastructural examination showed labeled terminals with a mean sectional area of 0.85 micron 2 and a relatively high density of synaptic vesicles. The vast majority of GABA-positive terminals were in contact with dendrites and only a minority had synaptic contact with cell bodies. No axoaxonal synapses were observed. The GABA-positive boutons probably derive at least partly from the observed GABA-positive neurons, but there is also a possibility of extrinsic GABAergic input.
The location of substance P (SP) in the lateral cervical nucleus (LCN) of monkeys (Aotus trivirgatus), cats, and rats was investigated with immunohistochemical methods. Light microscopic analysis showed that SP-positive fibers and terminals are evenly distributed throughout the LCN of the monkey and rat, whereas the SP labeling in the LCN of the cat is concentrated in the medial part of the nucleus, with only very sparse labeling in the lateral part. Electron microscopic examination of the monkey LCN revealed the presence of SP-like immunoreactivity within terminal boutons and unmyelinated axons. The SP-positive boutons are in synaptic contact with dendrites and, occasionally, cell bodies; they contain densely packed, clear, round synaptic vesicles, as well as dense-core vesicles. The distribution of SP-like immunoreactivity in the LCN of monkeys, cats, and rats is similar to that of nociceptive-responsive neurons demonstrated in electrophysiological experiments. The possible role of the SP-containing fibers in the transmission of nociceptive information through the LCN is discussed.
Previous observations indicate that spinocervical tract terminals contain relatively high levels of glutamate. To examine whether these high glutamate levels are likely to represent a neurotransmitter pool or an elevated metabolic pool, the distributions of glutamate- and glutamine-like immunoreactivities were examined in adjacent immunogold-labeled sections of the lateral cervical nucleus. Spinocervical tract terminals were identified by anterograde transport of horseradish peroxidase and wheat germ agglutinin-horseradish peroxidase conjugate from the spinal cord. Spinocervical tract terminals were found to contain significantly higher levels of glutamate-like immunoreactivity than other examined tissue compartments (large neuronal cell bodies, terminals with pleomorphic vesicles, astrocytes, and average tissue level). In contrast, the highest levels of glutamine-like immunoreactivity were detected in astrocytes. The different analyzed tissue elements formed three groups with respect to glutamate:glutamine ratios: one high ratio group including spinocervical tract terminals, a second group with intermediate ratios consisting of neuronal cell bodies and terminals containing pleomorphic synaptic vesicles, and a third low ratio group including astrocytes. Our findings indicate the presence of a compartmentation of glutamate and glutamine in the lateral cervical nucleus, similar to that postulated in biochemical studies of the central nervous system. The results also show that spinocervical tract terminals have high glutamate: glutamine ratios, similar to those previously observed in putative glutamatergic terminals in the cerebellar cortex. Thus, spinocervical tract terminals display biochemical characteristics that would be expected of glutamatergic terminals and the present findings therefore provide further evidence for glutamate as a spinocervical tract neurotransmitter.
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