Organization of anterogradely labeled spinocervical tract terminations in the lateral cervical nucleus of the cat

Lateral cervical nucleus projections to periaqueductal gray matter in cat

The midbrain periaqueductal gray matter (PAG) integrates the basic responses necessary for survival of individuals and species. Examples are defense behaviors such as fight, flight, and freezing, but also sexual behavior, vocalization, and micturition. To control these behaviors the PAG depends on strong input from more rostrally located limbic structures, as well as from afferent input from the lower brainstem and spinal cord. Mouton and Holstege (2000, J Comp Neurol 428:389-410) showed that there exist at least five different groups of spino-PAG neurons, each of which is thought to subserve a specific function. The lateral cervical nucleus (LCN) in the upper cervical cord is not among these five groups. The LCN relays information from hair receptors and noxious information and projects strongly to the contralateral ventroposterior and posterior regions of thalamus and to intermediate and deep tectal layers. The question is whether the LCN also projects to the PAG. The present study in cat, using retrograde and anterograde tracing techniques, showed that neurons located in the lateral two-thirds of the LCN send fibers to the lateral part of the PAG, predominantly at rostrocaudal levels A0.6-P0.2. This part of the PAG is known to be involved in flight behavior. A concept is put forward according to which the LCN-PAG pathway alerts the animal about the presence of cutaneous stimuli that might represent danger, necessitating flight. J. Comp. Neurol. 471:434-445, 2004.

The raccoon lateral cervical nucleus: mediolateral organization of GABA-positive and GABA-negative neurons and fibers

In the lateral cervical nucleus (LCN) of the cat, GABA-immunoreactive neurons and substance P-immunoreactive fibers are concentrated in the medial part of the nucleus, whereas in the monkey LCN no preferential locations have been identified. In raccoons, substance P-immunoreactive fibers display a distribution pattern similar to that in cats. However, the presence and distribution of GABA-immunoreactive neurons in the raccoon LCN has not been examined, and it is therefore not known whether raccoons are similar to cats or primates in this respect. Thus, in the present study, the raccoon LCN was examined for the presence and distribution of GABA-immunoreactive cells with respect to their numbers, locations, and sizes. The distribution of GABA-positive fibers and varicosities within the LCN was also investigated. The results of measurements of cross-sectional areas of LCN neurons indicate a trend toward decreasing cell size along the dorsolateral to medial axis of the raccoon LCN. Compared to neurons of the centrally located ventromedial division, neurons are statistically significantly larger in the dorsolateral division and smaller in the medial division of the nucleus. Cell counts in post-embedding-stained semithin sections through the nucleus revealed an average of 8,700 neurons per LCN. Approximately 4% of LCN neurons are GABA-immunoreactive. These neurons are small and most (80%) of them are located in the medial third of the LCN. In contrast, GABA-immunoreactive fibers and varicosities are present in about equal density throughout the raccoon LCN. Thus, the distributions of GABA-immunoreactive neurons and neuron sizes in the raccoon LCN conform closely to those in cats. Together with previous observations in cats and raccoons, the present findings support the notion that these small GABA-immunoreactive neurons may be local circuit inhibitory neurons and indicate the presence of a mediolateral segregation that may be of fundamental importance for the functional organization of the carnivore LCN.

Immunocytochemical evidence for vesicular storage of glutamate in cat spinocervical and cervicothalamic tract terminals

The densities of synaptic vesicles and gold particles, signaling fixed glutamate, were examined in spinocervical and cervicothalamic tract terminals. Statistically significant positive correlations between these parameters were detected in both terminal populations, whereas presumed inhibitory profiles displayed insignificant or negative correlations. These findings indicate a vesicular storage of glutamate in spinocervical and cervicothalamic tract terminals, and thus provide further evidence for glutamate as a neurotransmitter in the spinocervicothalamic pathway.

Catecholaminergic innervation of the lateral cervical nucleus: a correlated light and electron microscopic analysis of tyrosine hydroxylase-immunoreactive axons in the cat

The organization of catecholamine-containing axons in the cat lateral cervical nucleus was examined by immunocytochemical methods using a specific tyrosine hydroxylase antiserum. Light microscopic examination revealed numerous tyrosine hydroxylase-immunoreactive axons and varicosities throughout this nucleus, and some of these structures were found in contact with neuronal cell bodies. Correlated ultrastructural analysis showed that these varicosities were synaptic boutons which formed symmetric synaptic junctions with dendrites and somata. This evidence suggests that catecholamines exert a postsynaptic action upon neurons within the lateral cervical nucleus.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Compartmentation of glutamate and glutamine in the lateral cervical nucleus: further evidence for glutamate as a spinocervical tract neurotransmitter

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.

Substance P-like and serotonin-like immunoreactivity in the lateral cervical nucleus of the raccoon

The distribution of substance P and serotonin in the lateral cervical nucleus (LCN) of the raccoon was examined by light microscopic immunohistochemistry. Substance P-immunoreactive fibers were found to be clustered in the ventromedial part of the LCN, whereas only few such fibers appeared in the dorsolateral part of the nucleus. This organization is closely similar to that previously observed in the cat, and provides further evidence for an anatomic and functional segregation along the transverse axis of the LCN in carnivores. In some sections, substance P-positive fibers were found primarily in areas of the ventromedial LCN containing small neurons, indicating that such fibers may be involved in functions of the LCN associated with nociceptive projection neurons and/or local circuit neurons. The raccoon LCN also received a relatively sparse innervation of serotonin-positive fibers that were distributed throughout the nucleus, an organization similar to that previously observed in the cat. The functional role of the serotonergic fibers is unclear. However, their presence suggests that descending influences on transmission in the spinocervicothalamic pathway, in addition to the well-documented descending control of spinocervical tract neurons, may be present also at the level of the LCN.

Lamina I spinocervical tract terminations in the medial part of the lateral cervical nucleus in the cat

The terminations of spinocervical tract fibers in the lateral cervical nucleus (LCN) of the cat were examined with anterogradely transported Phaseolus vulgaris leucoagglutinin (PHA-L) in order to analyze their organization relative to the most medial part and the main body (the lateral two-thirds) of the LCN, which have differential projections and physiological characteristics. Iontophoretic injections of PHA-L in laminae I-V of the spinal dorsal horn yielded dense labeling in somatotopically appropriate regions of the main body of the LCN, and, as seen previously with horseradish peroxidase, additional terminations were present in the medial LCN after injections at either cervical or lumbar spinal levels. The morphological characteristics of the PHA-L labeling in these two parts of the LCN were different. Terminations in the lateral LCN consisted of dense clusters of thick fibers bearing large numbers of boutons. The terminal axons in the medial part of the LCN displayed a reticulated network of longitudinally oriented, fine fibers with well-spaced varicosities. Some of the fine fibers in the medial LCN appeared to be collaterals of thicker fibers that terminated in the lateral LCN. Injections of PHA-L that were restricted to lamina I resulted in terminal labeling only in the medial LCN. The labeling was more sparse than that observed in the medial LCN after larger dorsal horn injections but displayed the same morphological characteristics. Lamina I terminations were seen in the medial LCN after cervical or lumbar injections on both the ipsilateral and contralateral sides. The PHA-L observations were corroborated by the presence of many retrogradely labeled lamina I cells at both cervical and lumbar spinal levels, following injections of cholera toxin subunit b or rhodamine-labeled microspheres in the medial LCN. In addition, double-immunofluorescent labeling for PHA-L and substance P was performed in a few cases, since substance P immunoreactivity is present in fibers in the medial LCN and also in cell bodies in lamina I; however, very few spinocervical fibers displayed immunoreactivity for both antigens. These observations indicate that the medial part of the LCN receives input from lamina I neurons, and probably from lamina III-V neurons as well, at cervical and lumbar spinal levels. The lamina I input to the medial LCN provides a basis for the small population of nociceptive neurons that differentiate the medial LCN. The lamina I input could also be responsible for the general inhibition of lateral LCN neurons by wide-field noxious stimulation, via activation of GABAergic interneurons in the medial LCN.(ABSTRACT TRUNCATED AT 400 WORDS)

Thalamically projecting cells of the lateral cervical nucleus in monkey

The number, location, and morphology of thalamically projecting lateral cervical nucleus (LCN) cells were determined in monkey using retrograde transport of wheatgerm agglutinin-conjugated horseradish peroxidase. These data were compared to the total population of LCN neurons as determined by Nissl stain. In 4 Macaca fascicularis and one Saimiri sciureus the average size of the thalamic projection from LCN was found to be 506 +/- 94 cells contralateral to the injections. Thalamically projecting LCN neurons were located between the lower medulla and the third cervical segment; approximately 90% of these cells were in the first two cervical segments. Morphologic analysis of thalamically projecting LCN cells showed that they were smaller in size, and more oblong in shape in caudal regions of the nucleus. In 3 macaques, the average total number of LCN cells was determined to be 1617 +/- 908 on one side, in Nissl material. In these Nissl-stained preparations LCN neurons were found as far caudal as the fourth cervical segment; 68% were located in the first two cervical segments. Hence, thalamically projecting LCN neurons in the monkey are located in the rostral portion of the nucleus and comprise about one-third of the total population. Comparison of these data with reports in the literature imply that, unlike the cat, the major projection from LCN in monkeys is to the mesencephalon rather than to the thalamus.

Ascending afferents to the lateral cervical nucleus are enriched in glutamate-like immunoreactivity: a combined anterograde transport-immunogold study in the cat

To investigate whether glutamate (Glu) may be a transmitter in terminals of ascending afferents to the lateral cervical nucleus (LCN), these terminals were identified by anterograde transport of wheatgerm agglutinin-horseradish peroxidase from the spinal cord, and their content of Glu-like immunoreactivity (Glu-LI) was assessed at the ultrastructural level by the immunogold technique. The gold particle density over the peroxidase-positive terminals of the spinocervical tract (SCT) was significantly higher (by a factor of 2.44) than over a reference terminal population containing flattened or pleomorphic vesicles. Further, LCN neurons were densely labeled by the Glu antiserum, although the gold particle density over neuronal cell bodies was not as high as in the SCT terminals. Previous investigations have shown enrichment of Glu-LI in putative glutamatergic terminals in other parts of the CNS. Hence, the present observations indicate that Glu may be a transmitter in the synapses between SCT terminals and LCN neurons. The cell body labeling in the LCN is more difficult to interpret because of possible interference of metabolic pools of glutamate.

Serotonergic innervation of the lateral cervical nucleus: an immunohistochemical study in cats and monkeys (Aotus trivirgatus)

A serotonergic input to the lateral cervical nucleus of cats and monkeys (Aotus trivirgatus) was demonstrated with immunohistochemical methods. In both species, the lateral cervical nucleus was found to contain a network of serotonin-immunoreactive fibers. However, the density of labeled fibers was greater in the monkeys than in the cats. Most labeled fibers were thin and had irregularly spaced varicosities. Electron microscopic examination showed that labeled varicosities were in apposition with dendrites, neuronal somata and unlabeled terminals, but synapses were rare. The results demonstrate that the lateral cervical nucleus receives a serotonergic innervation, as is the case with other somatosensory relay structures such as the spinal dorsal horn and the dorsal column nuclei. The presence of a serotonergic innervation suggests that the transmission of somatosensory information through the lateral cervical nucleus is modulated by a descending pathway. However, its effect on the response properties of neurons in the lateral cervical nucleus is unknown.

Substance P-like immunoreactivity in the lateral cervical nucleus of the owl monkey (Aotus trivirgatus): a comparison with the cat and rat

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.

Synaptic connections of GABA-containing boutons in the lateral cervical nucleus of the cat: an ultrastructural study employing pre- and post-embedding immunocytochemical methods

The lateral cervical nucleus receives input from the spinocervical tract and projects to the thalamus and mesencephalon. The organization of this nucleus was examined using two immunocytochemical methods. Pre-embedding immunolabelling was performed using an antibody against glutamate decarboxylase, and post-embedding immunogold-reaction was performed with an antibody to glutaraldehyde-fixed GABA. Light microscopic analysis of material reacted for glutamate decarboxylase revealed that punctate structures were present throughout the nucleus and were associated with large cells in the dorsolateral region of the nucleus. Electron microscopy demonstrated that the punctate structures were synaptic boutons which formed symmetrical synaptic junctions with dendrites and somata of cells in the nucleus. The ultrastructural preservation of material prepared for the post-embedding immunogold technique was superior to that prepared for pre-embedding immunostaining. Positively labelled synaptic boutons exhibited high colloidal gold density and, like those prepared for the pre-embedding method, formed symmetrical synaptic junctions with dendrites and somata of neurons. Labelled boutons were densely packed with irregularly-shaped synaptic vesicles. They displayed characteristics which were distinct from those unlabelled boutons. Boutons, revealed by both immunolabelling methods, were not observed to form synaptic associations with other axon terminals and were presynaptic to dendrites and somata only. Therefore, it is probable that such boutons are responsible for postsynaptic inhibition of cells in the nucleus. In view of this evidence, it is concluded that the lateral cervical nucleus is not simply a relay but is actively involved in processing sensory information.

GABA-immunoreactive neurons and terminals in the lateral cervical nucleus of the cat

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.