A long ascending pathway of enkephalin-like immunoreactive spinoreticular neurons in the rat

Homologies of spinal ascending nociceptive pathways between rats and macaques: can we transpose to human? A review and analysis of the literature

PURPOSE

Due to the difficulty of using neural tracers in humans, knowledge of the nociceptive system's anatomy is mainly derived from studies in animals and mainly in rats. The aim of this study was to investigate the morphological differences of the ascending spinal nociceptive pathways between the rat and the macaque monkey; in order to evaluate the variability of this anatomy during phylogenesis, and thus to know if the anatomical description of these pathways can be transposed from the rat to the human.

METHODS

A review and analysis of the literature were performed. The criteria used for comparison were: origins, pathways, their terminations in target structures, and projections from target structures of ascending spinal nociceptive pathways. The monkey was used as an intermediate species for comparison because of the lack of data in humans. The hypothesis of transposition of anatomy between rat and human was considered rejected if differences were found between rat and monkey.

RESULTS

An anatomical difference in termination was found for the spino-annular or spino-periaqueductal grey (spino-PAG) pathway and transposition of its anatomy from rat to human was rejected. No difference was found in other pathways and the transposition of their anatomy from rat to human was therefore, not rejected.

CONCLUSION

This work highlights the conservation of most of the ascending spinal nociceptive pathways' anatomy between rat and monkey. Thus, the possibility for a transposition of their anatomy between rat and human is not rejected.

The tracts, cytoarchitecture, and neurochemistry of the spinal cord

The human spinal cord can be described using a range of nomenclatures with each providing insight into its structure and function. Here we have comprehensively reviewed the key literature detailing the general structure, configuration of tracts, the cytoarchitecture of Rexed's laminae, and the neurochemistry at the spinal segmental level. The purpose of this review is to detail current anatomical understanding of how the spinal cord is structured and to aid researchers in identifying gaps in the literature that need to be studied to improve our knowledge of the spinal cord which in turn will improve the potential of therapeutic intervention for disorders of the spinal cord.

Distribution of glycine/GABA neurons in the ventromedial medulla with descending spinal projections and evidence for an ascending glycine/GABA projection

The ventromedial medulla (VM), subdivided in a rostral (RVM) and a caudal (CVM) part, has a powerful influence on the spinal cord. In this study, we have identified the distribution of glycine and GABA containing neurons in the VM with projections to the cervical spinal cord, the lumbar dorsal horn, and the lumbar ventral horn. For this purpose, we have combined retrograde tracing using fluorescent microspheres with fluorescent in situ hybridization (FISH) for glycine transporter 2 (GlyT2) and GAD67 mRNAs to identify glycinergic and/or GABAergic (Gly/GABA) neurons. Since the results obtained with FISH for GlyT2, GAD67, or GlyT2 + GAD67 mRNAs were not significantly different, we concluded that glycine and GABA coexisted in the various projection neurons. After injections in the cervical cord, we found that 29% ± 1 (SEM) of the retrogradely labeled neurons in the VM were Gly/GABA (RVM: 43%; CVM: 21%). After lumbar dorsal horn injections 31% ± 3 of the VM neurons were Gly/GABA (RVM: 45%; CVM: 12%), and after lumbar ventral horn injections 25% ± 2 were Gly/GABA (RVM: 35%; CVM: 17%). In addition, we have identified a novel ascending Gly/GABA pathway originating from neurons in the area around the central canal (CC) throughout the spinal cord and projecting to the RVM, emphasizing the interaction between the ventromedial medulla and the spinal cord. The present study has now firmly established that GABA and glycine are present in many VM neurons that project to the spinal cord. These neurons strongly influence spinal processing, most notably the inhibition of nociceptive transmission.

Functional MRI of the cervical spinal cord during noxious and innocuous thermal stimulation in the alpha-chloralose- and halothane-anesthetized rat

Patterns of neuronal activity in the spinal cord using functional magnetic resonance imaging during noxious (48 degrees C) and innocuous (40 degrees C) thermal stimulation of the rat forepaw were examined. The patterns of functional activity elicited by thermal stimuli were compared in alpha-chloralose- and halothane-anesthetized rats. Although the locations of active pixels were similar during both types of stimulation, the mean percentage signal change was higher during noxious stimulation in both anesthetic groups. Ipsilateral dorsal horn activity was evident during both noxious and innocuous stimulation in all animals. The greatest consistency of ipsilateral dorsal horn activity occurred at the C3 to C5 spinal cord segments in all groups. Consistent contralateral dorsal horn activity appeared in segments C6 to C8 in all groups. C-fos immunohistochemical staining confirmed the presence of neural activity in the spinal cords of all animals.

Morphology and electrophysiological properties of hamster spinal dorsal horn neurons that express VGLUT2 and enkephalin

The excitatory amino acid glutamate mediates transmission at spinal synapses, including those formed by sensory afferent fibers and by intrinsic interneurons. The identity and physiological properties of glutamatergic dorsal horn neurons are poorly characterized despite their importance in spinal sensory circuits. Moreover, many intrinsic spinal glutamatergic synapses colocalize the opioid peptide enkephalin (ENK), but the neurons to which they belong are yet to be identified. Therefore, we used immunohistochemistry and confocal microscopy to investigate expression of the VGLUT2 vesicular glutamate transporter, an isoform reported in nonprimary afferent spinal synapses, and ENK in electrophysiologically identified neurons of hamster spinal dorsal horn. VGLUT2 immunoreactivity was localized in restricted fashion to axon varicosities of neurons recorded from laminae II-V, although the occurrence of immunolabeling in individual varicosities varied widely between cells (39 +/- 36%, n = 31 neurons). ENK colocalized with VGLUT2 in up to 77% of varicosities (17 +/- 21%, n = 21 neurons). The majority of neurons expressing VGLUT2 and/or ENK had axons with dense local terminations or projections consistent with propriospinal functions. VGLUT2 and ENK labeling were not correlated with cellular morphology, intrinsic membrane properties, firing patterns, or synaptic responses to sensory afferent stimulation. However, VGLUT2 expression was significantly higher in neurons with depolarized resting membrane potential. The results are new evidence for a population of dual-function dorsal horn interneurons that might provide another mechanism for limiting excitation within dorsal horn circuits during periods of strong sensory activation.

Evidence for the presence of both peroxisome proliferator-activated receptors alpha and beta in the rat spinal cord

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear receptor superfamily. Different subtypes of PPARs (alpha, beta, and gamma) have been described. Their distinct physiological functions depend on their differential ligand activation profiles but also on their specific tissue expression. Previous studies have described their presence in the central nervous system. However, their expression in the adult rat spinal cord in normal physiological conditions has never been investigated. We demonstrated by using reverse-transcription-polymerase chain reaction, and Western blotting, the mRNA and protein expression of PPARalpha and PPARbeta, but not PPARgamma in cervical, thoracic, and lumbar segments of the spinal cord. Using immunohistochemistry, we also showed for the first time the specific cellular distribution of these transcription factors in the different segments of the spinal cord. In the gray matter, the distribution of PPARalpha was homogenous whereas PPARbeta was specifically localized in motoneurons and in medial part of laminae IV, V, VI, VII, VIII, and X. These latter areas are known as nociceptive afferent pathways to supra-spinal structures such as the medulla reticular nucleus and the thalamus. In the white matter, PPARalpha was localized exclusively in astrocytes while PPARbeta was present in oligodendrocytes. The possible functions of PPARalpha and PPARbeta expressed in both white and gray matters of the spinal cord will be discussed but need further studies.

Immunohistochemical distribution of enkephalin, substance P, and somatostatin in the brainstem of the leopard frog, Rana pipiens

The brainstems of frogs contain many of the neurochemicals that are found in mammals. However, the clustering of nuclei near the ventricles makes it difficult to distinguish individual cell groups. We addressed this problem by combining immunohistochemistry with tract tracing and an analysis of cell morphology to localize neuropeptides within the brainstem of Rana pipiens. We injected a retrograde tracer, Fluoro-Gold, into the spinal cord, and, in the same frog, processed adjacent sections for immunohistochemical location of antibodies to the neuropeptides enkephalin (ENK), substance P (SP), and somatostatin (SOM). SOM+ cells were more widespread than cells containing immunoreactivity (ir) to the other substances. Most reticular nuclei in frog brainstem contained ir to at least one of these chemicals. Cells with SOM ir were found in nucleus (n.) reticularis pontis oralis, n. reticularis magnocellularis, n. reticularis paragigantocellularis, n. reticularis dorsalis, the optic tectum, n. interpeduncularis, and n. solitarius. ENK-containing cell bodies were found in n. reticularis pontis oralis, n. reticularis dorsalis, the nucleus of the solitary tract, and the tectum. The midbrain contained most of the SP+ cells. Six nonreticular nuclei (griseum centrale rhombencephali, n. isthmi, n. profundus mesencephali, n. interpeduncularis, torus semicircularis laminaris, and the tectum) contained ir to one or more of the substances but did not project to the spinal cord. The descending tract of V, and the rubrospinal, reticulospinal, and solitary tracts contained all three peptides as did the n. profundus mesencephali, n. isthmi, and specific tectal layers. Because the distribution of neurochemicals within the frog brainstem is similar to that of amniotes, our results emphasize the large amount of conservation of structure, biochemistry, and possibly function that has occurred in the brainstem, and especially in the phylogenetically old reticular formation.

Ascending projections from the area around the spinal cord central canal: A Phaseolus vulgaris leucoagglutinin study in rats

A single small iontophoretic injection of Phaseolus vulgaris leucoagglutinin labels projections from the area surrounding the spinal cord central canal at midthoracic (T6-T9) or lumbosacral (L6-S1) segments of the spinal cord. The projections from the midthoracic or lumbosacral level of the medial spinal cord are found: 1) ascending ipsilaterally in the dorsal column near the dorsal intermediate septum or the midline of the gracile fasciculus, respectively; 2) terminating primarily in the dorsal, lateral rim of the gracile nucleus and the medial rim of the cuneate nucleus or the dorsomedial rim of the gracile nucleus, respectively; and 3) ascending bilaterally with slight contralateral predominance in the ventrolateral quadrant of the spinal cord and terminating in the ventral and medial medullary reticular formation. Other less dense projections are to the pons, midbrain, thalamus, hypothalamus, and other forebrain structures. Projections arising from the lumbosacral level are also found in Barrington's nucleus. The results of the present study support previous retrograde tract tracing and physiological studies from our group demonstrating that the neurons in the area adjacent to the central canal of the midthoracic or lumbosacral level of the spinal cord send long ascending projections to the dorsal column nucleus that are important in the transmission of second-order afferent information for visceral nociception. Thus, the axonal projections through both the dorsal and the ventrolateral white matter from the CC region terminate in many regions of the brain providing spinal input for sensory integration, autonomic regulation, motor and emotional responses, and limbic activation.

An immunohistochemical investigation of the opioid cell column in lamina X of the male rat lumbosacral spinal cord

Tri-color immunohistochemistry was employed to examine enkephalin-like immunoreactive neurons in lamina X of the rat lumbosacral spinal cord. Serial coronal sections from levels L1 to S3 were examined. A rostral group of large (40-50 microm diameter), pyramidal-shaped enkephalin-like immunoreactive neurons were shown from levels L1 to L4-5. Essentially all of these neurons were also immunoreactive for galanin and cholecystokinin. A second enkephalin-like immunoreactive cell group, extending from L5 to approximately the S2-3 level, contained smaller (20-30 microm diameter), ovoid-shaped perikaryia. Approximately 75% of these enkephalin-like immunoreactive neurons were also immunoreactive for neuropeptide Y. Neurotensin-immunoreactivity was also present in this area, having varying amounts of co-localization with these other two peptides. These results demonstrate that the lumbosacral opioid cell column in lamina X is not a neurochemically homogenous structure.

Neuroanatomy of the pain system and of the pathways that modulate pain

We review many of the recent findings concerning mechanisms and pathways for pain and its modulation, emphasizing sensitization and the modulation of nociceptors and of dorsal horn nociceptive neurons. We describe the organization of several ascending nociceptive pathways, including the spinothalamic, spinomesencephalic, spinoreticular, spinolimbic, spinocervical, and postsynaptic dorsal column pathways in some detail and discuss nociceptive processing in the thalamus and cerebral cortex. Structures involved in the descending analgesia systems, including the periaqueductal gray, locus ceruleus, and parabrachial area, nucleus raphe magnus, reticular formation, anterior pretectal nucleus, thalamus and cerebral cortex, and several components of the limbic system are described and the pathways and neurotransmitters utilized are mentioned. Finally, we speculate on possible fruitful lines of research that might lead to improvements in therapy for pain.

Localization of enkephalin immunoreactivity in the spinal cord of the long-tailed ray Himantura fai

Enkephalin-like immunoreactivity (ENK-LI) was found throughout the spinal cord of the long-tailed ray Himantura fai. The densest ENK-LI was in the superficial portion of lamina A of the dorsal horn. Lamina B and the deeper parts of lamina A contained radially oriented, labelled fibres. Laminae C, D, and E contained many longitudinally oriented fascicles which were surrounded by a reticulum of transversely oriented, labelled fibres, some of which projected into the ventral and lateral funiculi. Labelled fibres were found in the dorsal commissure and around the central canal, but the later did not cross the midline. One-third of all enkephalinergic cells were found throughout laminae A and B, while two-thirds were located in the medial half of C, D, and E. Occasionally a labelled cell was located in the lateral funiculus. The ventral horn (laminae F and G) contained many enkephalinergic fibres but no labelled nuclei. A few dorsal column axons contained ENK-LI. In the lateral funiculus there were two groups of labelled axons, a superficial, dorsolateral group, and a deeper group, occupying a crescent-shaped region. The ventral funiculus also contained many labelled axons. The central projection of the dorsal root passed through the substantia gelatinosa and divided into rostrally and caudally projecting fascicles within lamina C. The root, and these fascicles, both lacked ENK-LI. In contrast, the fascicles in laminae D and E did contain enkephalinergic fibres. The origin of the various fibre systems and the role of enkephalin in the regulation of sensory processing and motor output are discussed.

Immunohistochemical evidence that Met-enkephalin and GABA coexist in some neurones in rat dorsal horn

A pre-embedding immunohistochemical method to detect Met-enkephalin was combined with postembedding immunohistochemistry with GABA and glycine antisera, in order to determine whether or not Met-enkephalin coexisted with either of these inhibitory transmitters in neuronal cell bodies within the superficial dorsal horn of the rat. The distribution of immunostaining with the three antisera was similar to that which has been described previously. Of 74 enkephalin-immunoreactive neurones in laminae II and III, 51 were immunoreactive with the GABA antiserum and 23 were not. All of the neurones which were not GABA-immunoreactive were located in lamina II. None of the enkephalin-immunoreactive cells showed glycine-like immunoreactivity. These results suggest that enkephalin is present both in GABAergic neurones and in neurones which do not contain GABA within the rat superficial dorsal horn. It is likely that the population of neurones immunoreactive with both enkephalin and GABA antisera includes lamina II islet cells and that the population which were enkephalin-immunoreactive but not GABA-immunoreactive includes stalked cells. In addition, this latter group may correspond to those cells which possess both enkephalin- and substance P-like immunoreactivity and which have been described previously in this area.

Substance P innervation of the rat and cat thalamus. II. Cells of origin in the spinal cord

Evidence in the preceding paper suggests that fibers and terminals immunopositive for substance P (SP) in somatosensory thalamic nuclei are part of the spinothalamic tract (STT). In this paper, more direct evidence on this point is provided by immunocytochemistry for SP on the cervical spinal cord, alone or combined with the retrograde transport of colloidal gold-labeled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase (WGAapoHRP-Au). In cats and rats pretreated with colchicine and/or anterolateral chordotomy (to increase SP content in cell bodies), many small to large cell bodies are SP-immunopositive especially in laminae I and V, but also in more ventral laminae of the upper cervical cord. SP neurons are also present in the dorsolateral funiculus (in the lateral spinal nucleus, LSN, in rats) but not in the lateral cervical nucleus or in the internal basilar nucleus. In both species there is a considerable degree of overlap in the distribution of SP-positive neurons and that of STT neurons. SP immunocytochemistry in rats after WGAapoHRP-Au injection in the somatosensory thalamus reveals SP-positive STT neurons in LSN, in lamina I and in lamina V, and, to a lesser extent, in more ventral laminae. These results demonstrate that SP is a marker and/or neuromediator for some STT neurons. Together with the evidence discussed in the preceding paper, the results also suggest that SP-positive neurons may be involved in the transmission of nociceptive input.

Ultrastructural localization of substance P, met-enkephalin, and somatostatin immunoreactivity in lamina X of the primate spinal cord

The ultrastructural localization of substance P (SP), met-enkephalin (MENK), and somatostatin (SS) in the lamina X area surrounding the central canal of the macaque monkey was examined by the indirect peroxidase-antiperoxidase method. The most common synaptic terminals in lamina X were simple terminals (S) with small rounded or pleomorphic clear vesicles; one to two dense-core vesicles were occasionally also present. These were found on soma, dendrites, and dendritic spines, in all regions of lamina X. A second class of terminal with round or oval clear vesicles was glomerular (G) in shape, with scalloped edges, and contained many mitochondria. These large terminals had several synaptic contacts onto dendrites, spines, and small terminals and were found mainly in the lateral region. The third class (L) contained small clear vesicles and several vesicles with large, dense cores (100-125 nm), and also contacted dendrites, mainly lateral to the canal. The fourth class of terminal (D) contained small clear vesicles and several vesicles with small, dense cores (75-100 nm); these contacted dendrites and somata in all areas. Very few terminals with flat vesicles were identified. There was an unequal distribution of immunoreactivity among the several terminal classes identified in lamina X. Most SP terminals were S terminals, but SP L terminals were also common; few were D terminals. MENK terminals were usually either S terminals or D terminals; L terminals were rarely MENK positive. SS terminals were commonly D terminals or S terminals; L terminals were also rarely SS positive. Only SP terminals were identified as G terminals. Synaptic targets of SP, MENK, and SS terminals were most commonly dendrites. In addition to unlabelled neurons, peptidergic neurons and their processes were also synaptic targets of terminals containing the same peptide. The distributions of these peptides in primate lamina X differ from that of the same peptides in primate superficial dorsal horn. These differences are important, in consideration of some of the parallels that may be drawn between the lamina X area and the superficial dorsal horn; both areas have high concentrations of the same peptides, receive nociceptive primary afferents, and contain spinothalamic and other projection neurons. Nevertheless, comparison of the distribution of immunoreactivity among terminal classes indicates that neurochemical organization at the ultrastructural level is quite distinct in each of the two areas. This may also reflect other roles of the lamina X area, including its involvement in visceral functions, although it would be expected that this element might be less prominent at the cervical levels we investigated.

Comparison of met-enkephalin, dynorphin A, and neurotensin immunoreactive neurons in the cat and rat spinal cords: II. Segmental differences in the marginal zone

This study examined the number of met-enkephalin, dynorphin A 1-8, and neurotensin immunoreactive (IR) neurons in the marginal zone (lamina I) at one thoracic (T8:cat,T9:rat), one midlumbar (L5:cat,L4:rat), and one lower lumbar or sacral (S1:cat,L6:rat) spinal cord segment in the cat and rat. Marginal zone IR neurons ranged 10-70 microns in diameter in cats and 10-50 microns in rats and were flattened, pyramidal, fusiform, or polygonal in morphology. Immunoreactive neurons for each peptide in both species were found in the marginal zone at all spinal levels, but with a differential segmental distribution. The average number of IR neurons per 50-microns section generally was lowest in thoracic cord and greatest in lower lumbar/sacral cord for all peptides. For enkephalin and dynorphin, the estimated total number of IR neurons per segment and number of IR neurons per volume (mm3) generally were lowest in the midlumbar segments and highest in the thoracic and lower lumbar/sacral cord. For neurotensin, the estimated total number of neurons per segment remained lowest in the thoracic and largest in the lower lumbar/sacral cord. The number of neurotensin IR neurons per volume was equal in the thoracic and midlumbar cord, but remained highest at lower lumbar/sacral levels. The IR neurons quantified in this study may be interneurons or may serve as supraspinal projection neurons. The large number of IR neurons observed in segments receiving a relatively large visceral afferent input suggests that some of these neurons may be involved in visceral sensory processing. In addition, the segmental distribution of the IR neurons indicates that physiological and pharmacological studies on the effects of opioid and/or neurotensin peptides should be interpreted in light of the spinal segment(s) investigated.

Induction of the gene encoding pro-dynorphin by experimentally induced arthritis enhances staining for dynorphin in the spinal cord of rats

The response of dynorphinergic neurons in the lumbosacral spinal cord of the rat to chronic arthritic inflammation was studied by the combined use of biochemical and immunohistochemical procedures. In polyarthritic rats, in which all four limbs showed a swelling, inflammation and hyperalgesia, a pronounced elevation was seen in the level of messenger ribonucleic acid encoding prodynorphin (pro-enkephalin B) in the lumbosacral spinal cord. In addition, the levels of immunoreactive dynorphin A1-17, a primary gene product of this precursor, were greatly increased. This activation was reflected in a striking intensification of the immunohistochemical staining of both dynorphin and alpha/beta-neo-endorphin, a further major product of pro-dynorphin. In control animals perikarya were stained exceedingly rarely and encountered only in laminae I and II. Stained fibres and varicosities were seen throughout the dorsal and ventral gray matter, being most concentrated in laminae I, II, IV and V of the dorsal horn and dorsolateral to the central canal. In polyarthritic rats, fibres and varicosities were much more intensely stained throughout the cord, particularly in laminae I/II, IV and V and dorsolateral to the central canal. Many strongly-stained perikarya could be seen: these comprised many small diameter cells in laminae I and II, and some large diameter marginal neurons and large diameter cells, heterogenous in appearance, in the deeper laminae IV and V. Monolaterally inflamed rats injected in the right hind-paw showed pathological changes only in this limb. Correspondingly, in unilateral inflammation, an elevation in immunoreactive dynorphin was seen exclusively in the right dorsal horn and the above-described intensification of staining for dynorphin and neo-endorphin was seen only in this quadrant. This reveals the neuroanatomical specificity of the response. Thus, in the lumbosacral cord of the rat, pro-dynorphin neurons are most preponderant in laminae I, II, IV and V. A pronounced intensification of the immunohistochemical staining of these neurons is seen in chronic arthritis. Furthermore, there is a parallel elevation in the levels of messenger ribonucleic acid encoding pro-dynorphin and of its primary products dynorphin and neo-endorphin. These findings demonstrate an enhancement in the functional activity of spinal cord localized dynorphin neurons in the response to chronic arthritic inflammation.

Co-localization of substance P and Met-enkephalin-Arg6-Gly7-Leu8 in the intraspinal neurons of the rat, with special reference to the neurons in the substantia gelatinosa

A double-labeling immunofluorescence technique was employed to investigate the co-localization of the functionally antagonistic neuropeptides, substance P and enkephalins, within intraspinal neurons of the rat. Anti-Met-enkephalin-Arg6-Gly7-Leu8 (Enk-8) antiserum was used as a marker of the preproenkephalin A neuron system. The observations were focused on the lumbar spinal cord. Co-localization was most prominent within neurons in the substantia gelatinosa, in which more than 95% of substance P-like immunoreactivity neurons showed Enk-8-like immunoreactivity. These double-labeled cells corresponded to 45% of Enk-8-like immunoreactive neurons in the same area. This suggests that SP/Enk-8 interaction occurs at the axon terminals of the substantia gelatinosa neurons. In deeper layers of the dorsal horn (laminae III, IV), only 14% and 6% of SP-like immunoreactive and Enk-8-like immunoreactive neurons were double labeled, respectively. Co-localization was also observed in neurons located in the laminae I, V, VII and X, suggesting concomitant involvement of these peptides in a variety of spinal cord functions.

Lamina X of primate spinal cord: distribution of five neuropeptides and serotonin

The distribution of substance P, somatostatin, cholecystokinin, vasoactive intestinal polypeptide, enkephalin and serotonin in axons, terminals and neurons was compared in the area surrounding the central canal (lamina X) at five representative levels of the monkey spinal cord, using peroxidase-antiperoxidase immunocytochemistry. Immunoreactive neurons containing each of the neurochemicals were identified. At the cervical, thoracic and lumbar levels the area lateral to the canal had dense terminal fields immunoreactive for each neurochemical. The dorsal commissural region, the pericanal area, and the ventral commissural area were supplied by some but not all of the substances. In the lower thoracic cord innervation extended into the dorsal midline area and into the ventromedial commissural region. In contrast, in the sacral cord, the dorsal commissural region could be subdivided on the basis of innervation, and the lateral region was densely supplied by only cholecystokinin and serotonin, while the sacral ventral commissure and the pericanal area were supplied by all six neurochemicals. The immunocytochemical mappings were compared with published maps of functional classes of neurons and with the distribution of primary afferents and descending fibers in lamina X. The dense peptidergic and serotonergic innervation in the lateral area and the dorsal commissural area corresponded particularly with the location of projection neurons and primary afferents described in other studies.

neuropeptides in long ascending spinal tract cells in the rat: evidence for parallel processing of ascending information

A study has been made of the involvement of spinal peptidergic neurons in ascending tracts at lumbar-sacral levels in rats, by combining the retrograde transport of a protein-gold complex with immunocytochemistry. Ten neuropeptides have been considered for their presence in the cells of origin of the following six ascending tracts, including some involved in pain transmission: the spinosolitary tract, the medial and lateral spinoreticular tracts, the spinomesencephalic tract, the spinothalamic tract and the postsynaptic dorsal column tract. Although there was overlap in the distribution of several of the types of peptidergic cells and some ascending tract cells only a very small percentage of long ascending tract cells were found to contain neuropeptides. Most (90%) of those peptidergic ascending tract cells, however, were clearly congregated in two distinct spinal regions: the lateral spinal nucleus and the region surrounding the central canal (including lamina X). Ascending tract cells in both of these regions contained a wide variety of neuropeptides. Immunoreactivities for a total of seven different peptides were seen. The lateral spinal nucleus had the highest percentage of neuropeptide containing ascending tract cells; cells of all the four populations of peptidergic neurons lying in this region were involved in supraspinal projections; they stained for vasoactive intestinal polypeptide, bombesin, substance P or dynorphin and their axons projected in the spinomesencephalic, spinoreticular and spinosolitary tracts. The region surrounding the central canal contained bombesin-, enkephalin-, cholecystokinin- and somatostatin-immunoreactive ascending tract cells; these cells were found at the origin of the spinothalamic, spinomesencephalic, spinoreticular and spinosolitary tracts. In this region only the cells staining for substance P were not involved in supraspinal projections. The peptidergic ascending tract cells in other spinal regions were few; they were found in either lamina I or lateral part of lamina V. Ascending tract lamina I cells reacted for dynorphin or vasoactive intestinal polypeptide and their axons projected in the spinosolitary and spinomesencephalic tracts. Ascending tract lamina V cells reacted for somatostatin and were found at the origin of the medial component of the spinoreticular tract. It is proposed that peptidergic ascending tract cells form minor but distinct subgroups within each ascending tract. Each of the ascending tracts are divisible into peptide- and nonpeptide-containing groups of cells which convey information in a parallel fashion.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical identification of long ascending peptidergic neurons contributing to the spinoreticular tract in the rat

In the present study, we examined the peptidergic content of lumbar spinoreticular tract neurons in the colchicine-treated rat. This was accomplished by combining the retrograde transport of the fluorescent dye True Blue with the immunocytochemical labeling of neurons containing cholecystokinin-8, dynorphin A1-8, somatostatin, substance P or vasoactive intestinal polypeptide. After True Blue injections into the caudal bulbar reticular formation, separate populations of retrogradely labeled cells were identified as containing cholecystokinin-like, dynorphin-like, substance P-like or vasoactive intestinal polypeptide-like immunoreactivity. Retrogradely labeled somatostatin-like neurons were not identified in any of the animals examined. Each population of double-labeled cells showed a different distribution in the lumbar spinal cord. The highest yield of double-labeling occurred for cholecystokinin, with 16% of all intrinsic cholecystokinin-like neurons containing True Blue. These double labeled neurons were found predominantly at the border between lamina VII and the central canal region. About 11% of intrinsic vasoactive intestinal polypeptide-like neurons in the lumbar spinal cord were retrogradely labeled from the bulbar reticular formation. These neurons were found mostly in the lateral spinal nucleus, with only a few double-labeled cells located deep in the gray matter. Dynorphin-like double-labeled neurons were localized predominantly near the central canal; a smaller population was also seen in the lateral spinal nucleus. It was found that double-labeled dynorphin-like neurons made up 8% of all intrinsic dynorphin-like neurons. Retrogradely-labeled substance P-like neurons were rare; the few double-labeled neurons were found in the lateral spinal nucleus and lateral lamina V. These findings suggest a significant role for spinal cord peptides in long ascending systems beyond their involvement in local circuit physiology.

Combined axonal transport tracing and immunocytochemistry for mapping pathways of peptide-containing nerves in the peripheral nervous system

The various combinations of axonal transport tracing and immunocytochemistry used for mapping pathways of peptide-containing nerves, and in particular those of the peripheral nervous system, are reviewed. The advantages and disadvantages of these methods are discussed. The applications and results presented illustrate the future potential value of this approach.

Neurochemical identification of afferents onto spinomedullary neurons in the rat spinal cord central gray matter

The synaptic relationship between spinal cord central gray projection neurons and immunocytochemically identified afferents in the rat were examined at the light microscopic level using the combined techniques of retrogradely transported True blue and serotonin (5-HT), enkephalin (ENK), and substance P (SP) immunocytochemistry. At L4-L6, numerous retrogradely labeled neurons could be identified around the central canal after large bulbar injections of True blue. Of these projection neurons, 75% were apposted by 5-HT varicosities, 57% by ENK varicosities and 58% by SP varicosities. Hemisection of the spinal cord produced a marked reduction in the amount of 5-HT immunoreactivity and the number of putative 5-HT contacts observed on neurons of the spinal cord central gray. A small decrease in SP immunoreactivity and putative contacts was seen after dorsal rhizotomy. Neither rhizotomy nor hemisection produced discernable changes in ENK immunofluorescence. Based on the distributions of 5-HT, ENK and SP in the spinal cord, we suggest that a more precise delineation of lamina X in the rat can be made according to immunocytochemical rather than strictly morphological criteria.