Verification of the disector method for counting neurons, with comments on the empirical method

The empirical and disector methods are unbiased sampling methods for determining numbers of neurons. The present study verifies and thus calibrates these methods by determining true numbers of ganglion cells in serial reconstructions and then using each method to estimate the same populations. The empirical method gives accurate counts but is laborious (inefficient). Five separate disector analyses, distinguished by height (h), were done for each ganglion. The findings are: (1) that the estimates are consistently low when h is minimal (reference and look-up sections are adjacent), but (2) the estimates are accurate when h is greater (one to four sections intervene between reference and look-up sections). We ascribe the difficulties with the first disector to "lost" or "invisible" caps. We emphasize that we would not have known of the problem unless we verified our counts. If there is suspicion that difficulties with profile recognition might occur, we recommend that serial sections of an appropriately chosen sample of tissue be prepared and 500-1,000 neurons (or, more generally, particles) be reconstructed. Then the method of choice can be used on the issue of choice to make certain of the necessary accuracy before proceeding with the main study.

Neuroma formation and numbers of axons in a rat model of experimental peripheral neuropathy

Two weeks following chronic partial constriction of rat sciatic nerve, the perineurium was disrupted and a neuroma had formed at the constriction site in all nerves (n = 5). Axon counts demonstrated an 84-99% and a 62-84% decrease in myelinated and unmyelinated axons respectively, distal to the lesion. Distally, the majority of surviving myelinated axons had diameters of less than 5.0 microns. There was considerable disparity in fiber loss from animal to animal, but similar behavioral changes were demonstrated by all animals. These results are discussed with reference to previously published data and possible mechanisms underlying the behavioral manifestations of this neuropathy model.

Distribution of the growth associated protein GAP-43 in the central processes of axotomized primary afferents in the adult rat spinal cord; presence of growth cone-like structures

GAP-43-immunolabelled structures were visualized by electron microscopy in the adult rat L4-L5 superficial dorsal horn 2 weeks after sciatic nerve transection. The majority of immunolabelled elements were unmyelinated axons, but some synaptic terminals and myelinated axons also labelled. The labelled unmyelinated axons were commonly located in prominent bundles which on serial section analysis could be followed into larger single trunks. These enlargements contain many organelles and give rise to smaller processes, which is compatible with their being growth cones. Sciatic nerve transection may result, therefore, in central regenerative processes which reorganize the neuropil and contribute to the decreased sensibility and pain that follows peripheral nerve section.

Do neuropeptides in the dorsal horn change if the dorsal root ganglion cell death that normally accompanies peripheral nerve transection is prevented?

Peripheral nerve section causes the death of dorsal root ganglion cells and changes in neuroactive peptides in the dorsal horn of the spinal cord. The relationship between these 2 events has not been previously studied, however. One approach would be to prevent sensory cell death and then determine changes in peptide immunoreactivity. To do this, transected rat sciatic nerve stumps were placed in an impermeable silicone tube for one month. The tube was then removed and after 30 additional days the cells were counted. The data indicate that no cell death occurred. We conclude that the sensory cells are first saved due to some factor present in the tube, and then after 30 days, the cells become independent of the tube and its contents. In these same animals, all of the peptides we examined were significantly changed. Four of the peptides, calcitonin gene-related peptide (CGRP), substance P (SP), cholecystokinin octapeptide (CCK) and galanin (GAL) were significantly depleted in the medial L4-L5 superficial dorsal horn, and vasoactive intestinal polypeptide (VIP) was significantly increased. We conclude that there are major changes in spinal peptide systems following peripheral nerve transection even if there is no accompanying death of sensory neurons. Thus we suggest that dramatic central changes in peptide immunoreactivity following peripheral nerve transection are independent of the sensory cell death that usually occurs in response to this injury.

Absence of neurogenesis of adult rat dorsal root ganglion cells

Recently, an age-related increase in the number of dorsal root ganglion (DRG) cells was reported in adult rats. This suggests neurogenesis of adult primary afferent neurons, which would be an extremely important phenomenon if it occurred. Other evidence is not compatible with this idea, however, so the issue is not settled. The primary point of contention concerns the counts of DRG cells in relation to age. In our opinion, these disagreements arise, at least in part, because different counting methods give different results for the same material. Thus, any method for determining DRG cell numbers should be calibrated. We previously calibrated some of the common methods used to count DRG cells and found that an empirical method gave accurate cell counts. In the present study, we have used this method and asked whether an age-related increase in the number of lumbar DRG cells can be demonstrated in adult rats. Our data indicate that DRG cell numbers remain essentially constant from 3 to 22 months of age. Most ancillary evidence is consistent with the hypothesis that mammalian DRG cell numbers do not change during adult life. Thus, we feel that the evidence does not support the hypothesis that there is neurogenesis of adult rat primary afferent neurons.

Calibration of methods for determining numbers of dorsal root ganglion cells

Numbers of primary afferent neurons underlie important generalizations concerning the organization of primary sensory systems. A major difficulty, however, is that different investigators do not agree on the neuronal counts. The problem, in our opinion, is that the various methods used to determine these numbers do not provide the same results. Thus to be certain that a method provides accurate counts, calibration is necessary. To do this, true numbers of ganglion cells were determined by serially reconstructing significant parts of four rat lumbar dorsal root ganglion cell populations. Then 6 commonly used methods of counting neurons were used to determine neuron numbers for these same populations. The data indicate that the empirical method, using the modifications recommended in this paper, estimates numbers of neurons with the needed accuracy whereas the other 5 do not. Thus, of the tested counting procedures, the empirical method is recommended. If other methods are to be used, they should also be calibrated.

Changes in calcitonin gene-related peptide immunoreactivity in the rat dorsal horn following electrical stimulation of the sciatic nerve

Immunoreactive staining for calcitonin gene-related peptide (CGRP) in the rat superficial dorsal horn (L4-L5 segments) was examined following electrical stimulation of the sciatic nerve. Electrical stimulation for twenty minutes at an intensity to elicit both A and C fiber volleys resulted in a significant decrease in CGRP immunoreactivity in the medial two-thirds of the dorsal horn on the stimulated side as compared to the non-stimulated side of the spinal cord. The change was most pronounced in the medial area of the dorsal horn, with a 54% decrease in immunostaining density compared to the non-stimulated side. In contrast, when the sciatic nerve was stimulated for the same period at an intensity that stimulates only A fibers, there was no significant difference in the density of immunoreactive staining in the dorsal horn between the non-stimulated and stimulated sides of the spinal cord. These results suggest that activity in a specific population of primary afferent fibers causes a dramatic depletion of CGRP in the dorsal horn.

Percentages of dorsal root axons immunoreactive for galanin are higher than those immunoreactive for calcitonin gene-related peptide in the rat

The present study shows that 28% of the myelinated and 27% of the unmyelinated axons in the L5 and S1 rat dorsal roots are immunolabeled for galanin. By contrast only 10% of the myelinated and 15% of the unmyelinated axons are immunolabeled for calcitonin gene-related peptide, which is the numerically predominant primary afferent peptide marker for dorsal root ganglion cells. Thus galanin, because of its presence in so many primary afferent fibers, emerges as an important primary afferent marker. In addition, since our data also show that galanin is present predominantly in unmyelinated and fine myelinated sensory axons, a hypothesis is that it is particularly concerned with the transmission of noxious information.

Denervation-induced intraspinal synaptogenesis of calcitonin gene-related peptide containing primary afferent terminals

The purpose of the present study is to provide evidence that chronic spinal denervation leads to an increase in numbers of synaptic terminals from a specific population of primary afferent fibers. Rats were unilaterally deafferented for 35 days (chronic denervation) by dorsal rhizotomies performed from T2 to T8 and T10 to L5, which isolates or spares the T9 root. The contralateral T9 root was spared by similar surgery 5 days (acute denervation) prior to sacrifice. The survival time on the chronic side presumably allows sprouting of T9 primary afferents to occur, whereas the time on the acute side does not. The terminals were labeled with calcitonin gene-related peptide (CGRP), which is a compound that labels a specific population of primary afferent fibers and terminals, and stereological methods were used to determine the numbers of immunolabeled terminals in laminae I and IIo on the chronic and acute sides of the T9 spinal cord. The findings are that the chronic side had approximately twice as many terminals as the acute side. This difference is statistically significant. These findings are compatible with the hypothesis that chronic denervation leads to synaptogenesis from surviving primary afferent fibers.

Computerized ultrastructural analysis of the shape of the active synaptic zones in rat spinal cord

Active synaptic zones are cytoplasmic specializations that indicate where synaptic transmission occurs. We have used computerized three-dimensional reconstructions from serial ultrathin sections to define certain features of the geometry of these zones in mammalian spinal cord. Our main finding is that the active zones in the dorsal portion of the spinal cord can be placed in one of two categories with respect to curvature: (1) uncurved or slightly curved and (2) very curved. The very curved category is associated with simple axodendritic type synapses in which the axonal terminal arises from primary afferent fibers.

The receptive part of the primary afferent axon is most vulnerable to systemic capsaicin in adult rats

The present study shows that systemic capsaicin in adult rats results in a significant loss of axons in the subepidermal nerve plexus of the posterior leg but no loss of axons in the sural nerves of these same animals. These data are interpreted as indicating that the receptive part of the peripheral sensory axon is destroyed but that the cell body and most of the peripheral axon remains intact. Thus we suggest that the receptive part of the peripheral sensory axon is the most vulnerable part of the primary afferent neuron to capsaicin in these animals. These findings may explain the observation that adult rats treated with systemic capsaicin are deficient in their responses to certain painful stimuli but usually do not show obvious signs of primary afferent neuron death. We also suggest that as the dose of capsaicin is increased the whole neuron dies. It remains to be determined if the peripheral damage reported here is related to the striking loss of primary afferent markers in the dorsal horn that is also seen after this treatment.

Long ascending unmyelinated primary afferent axons in the rat dorsal column: immunohistochemical localizations

The present study demonstrates a significant number of calcitonin gene-related peptide (CGRP) immunolabeled unmyelinated axons in the C3 fasciculus gracilis of the rat. Approximately 88% of these axons are lost following dorsal rhizotomies from midthoracic levels caudally. Assuming that CGRP labels primary afferent axons, these findings support the hypothesis that there are significant numbers of long ascending unmyelinated primary afferent axons in the dorsal columns of the rat. If these findings can be generalized, they may have clinical import.

Ascending unmyelinated primary afferent fibers in the dorsal funiculus

The primary purpose of the present study is to obtain evidence as to the destination of the recently discovered unmyelinated primary afferent fibers in the mammalian dorsal funiculus. To do this rat dorsal roots were transected unilaterally from segments T8 or T9 caudally, and the numbers of axons were determined in the C3 fasciculus gracilis in normal animals and from both sides of the rhizotomied animals. In addition, C3 fasciculus gracilis counts were done in animals that had complete T6 or T10 spinal transections. The data indicate that there is an 80% loss of unmyelinated axons ipsilaterally and a 60% loss contralaterally in the fasciculus gracilis of the rhizotomied animals. These findings are interpreted as indicating that a significant fraction of the unmyelinated fibers in the fasciculus gracilis ascend, presumably to the nucleus gracilis in the brain stem, and also that a significant number of these fibers branch. We also provide evidence for contralateral myelinated primary afferent fiber projection in the fasciculus gracilis and show that the myelinated primary afferent fibers seem to be a more diverse population than the unmyelinated primary afferent fibers in the C3 fasciculus gracilis.

Preservation of sensory cells by placing stumps of transected nerve in an impermeable tube

It is known that transection of a major peripheral nerve results in the loss of a significant number of sensory cells whose axons travel in that nerve. The present study confirms this observation and shows that placement of the stump of such a transected nerve into an impermeable tube prevents this loss. We further show that this preservation does not depend on axonal regeneration. Further experiments to define the phenomenon and to obtain beginning insight into mechanisms are discussed. If these findings can be generalized to humans, they may have clinical significance.

Peptide immunoreactivity of unmyelinated primary afferent axons in rat lumbar dorsal roots

The present study demonstrates calcitonin gene-related peptide (CGRP), somatostatin (SOM), bombesin (BOM), and substance P (SP) at the electron microscopic level in lumbar dorsal root axons of normal rats. The highest percentages of labeled axons were for CGRP (14%) and then, in descending order, for SP (8.6%), SOM (6.8%), and BOM (3.1%). The labeled axons were exclusively unmyelinated for SP, SOM, and BOM, and predominantly unmyelinated for CGRP. These data are consistent with the data for labeled sensory cell bodies for these same compounds. We emphasize that these peptides were immunocytochemically visualized in the dorsal roots without experimental manipulation, such as colchicine or dorsal root ligation. Quantitative sampling of this type can be used to assay changes in response to physiological stimuli in numbers of sensory axons that contain identifiable concentrations of these peptides.

Aspartate immunoreactive axons in normal rat L4 dorsal roots

The present study demonstrates that approximately 15% of the unmyelinated axons and 4% of the myelinated axons in the rat L4 dorsal root are immunostained for the excitatory amino acid aspartate. Thus these primary afferent axons contain enough of the antigen to be labeled. This is the first report that high concentrations of aspartate characterize a subpopulation of dorsal root axons. This allows the suggestion that aspartate is a candidate transmitter for primary afferent neurons. We emphasize that these axons are demonstrated in otherwise normal animals so that changes in percentages of labeled axons in response to various stimuli are not complicated by manipulations usually necessary to demonstrate immunoreactive compounds in the cell body.

Changes in dorsal horn synaptic disc numbers following unilateral dorsal rhizotomy

The present study estimates the numbers of synaptic discs and numbers of degenerating synaptic terminals in laminae I-IV of the rat S2 dorsal horn ipsi- and contralateral to unilateral dorsal rhizotomy. These data allow us to estimate the loss of synapses of primary afferents and to correlate this loss with the rate of axon disappearance in the proximal stump of a transected S2 dorsal root. Our first findings are that 47% of the ipsilateral synapses and 27% of the contralateral synapses disappear within a day following unilateral rhizotomy. Conclusions are that the predominant synaptic population in this part of the rat spinal cord is of primary afferent origin and that there is an extensive bilateral projection of the dorsal root fibers. The contralateral projection is confirmed by the appearance of numerous degenerating terminals on the contralateral side. We also find that synaptic loss and appearance of degenerating terminals occur relatively synchronously in laminae I-IV. Finally we find that the time course of the synaptic loss correlates primarily with the disappearance of unmyelinated fibers in the proximal stump of the transected dorsal root.

Glutamate immunoreactivity in rat dorsal root axons

Approximately 8.5% of the unmyelinated and 2.5% of the myelinated primary afferent axons in lumbar dorsal roots of normal rats are immunostained for glutamate. Thus unmyelinated fibers are the predominantly immunostained population under the conditions of our experiments. The mean size of the unmyelinated fibers is greater at L6 than at L4 and L2. The meaning of this is not clear, but it may imply that pelvic visceral afferents are slightly larger than afferents from other areas. We emphasize that the immunostained axons can be demonstrated in otherwise normal animals, so changes in the percentages of labeled axons in response to various stimuli will be of interest.

Numbers of synapses in laminae I-IV of the rat dorsal horn

The present study determines numerical densities (NVsyn) and total numbers of synaptic discs in laminae I-IV of the rat S2 dorsal horn. Previous methods for NVsyn have the advantage of being relatively simple, but these assume that the discs are round, flat, and of uniform size. In our material, serial reconstructions indicate that these assumptions are not met. Accordingly we use a stereological method that is not as dependent on these assumptions. This method is to divide the surface density of the discs by the mean surface area of a disc (NVsyn = SVsyn/Ssyn). We refer to this as a reconstruction method because synaptic discs are reconstructed from serial sections. We also calculate numerical densities by several previously used standard methods, and the findings are similar but not identical. We find that numerical density and total synaptic numbers are smallest in lamina I, and densities and total numbers are not significantly different when lamina II is compared to laminae III and IV. Thus the intense labeling of terminals with certain compounds that characterize lamina I and II does not imply an increase in total synaptic numbers or in synaptic density. In addition there is a general increase in synaptic densities and numbers as one proceeds from lamina I to lamina IV. Another point is that the numerical density of synapses in the dorsal horn is approximately that of the cerebral cortex. These data will serve as a basis from which to judge the effects of denervations and other manipulations that purportedly change synaptic numbers.

Organization of calcitonin gene-related peptide-immunoreactive terminals in the primate dorsal horn

The present paper is concerned with the arrangement of axons and synaptic terminals immunostained for calcitonin gene-related peptide (CGRP), a primary afferent marker, in the primate (Macaca fascicularis) dorsal horn. The CGRP axons and terminals are uniformly distributed in laminae I and II outer (o) but they are concentrated laterally and distributed intermittently in the reticulated region of lamina V. A prominent bundle of labeled axons is seen in the sacral cord dorsal to the central canal. Emphasis is given to the relation of CGRP-immunoreactive terminals to other terminals, both labeled and unlabeled, in laminae I and IIo. In this regard, adjacent CGRP-immunoreactive terminals are often united by puncta adhaerentia. Of particular interest is the observation that CGRP-immunoreactive terminals can be found presynaptic to other terminals which sometimes resemble central primary afferent endings. In addition CGRP-immunoreactive terminals end on other CGRP terminals. Both findings suggest that primary afferent terminals interact synaptically with other primary afferent terminals.

Intraspinal sprouting after administration of nerve growth factor antibodies to neonatal rats

Previous observations show that neonatal removal of nerve growth factor results in an increased number of fine dorsal root axons in the adult animal. The present study shows that the same treatment results in 49% more axons in the adult tract of Lissauer, which is an area of spinal white matter that contains predominantly fine primary afferent axons. These data are consistent with the idea that postnatal removal of NGF leads to increased numbers of fine primary afferent axons which then pass into the white matter of the spinal cord and there make synaptic contacts. If so, this would be a useful preparation for studies on the mechanisms by which newly formed sensory axons can be induced to enter the spinal cord postnatally.

Evidence for unmyelinated sensory fibres in the posterior columns in man

The human posterior columns are generally described as a myelinated fibre pathway. The present study demonstrates that more than 25% of the component axons are unmyelinated. Many of these unmyelinated axons are labelled by antibodies to calcitonin gene-related peptide (CGRP), a substance found in sensory cells, axons and terminals. On this basis we suggest that there are significant numbers of unmyelinated primary afferent axons in the human posterior columns. These results indicate that unmyelinated sensory axons are more widespread than previously thought, and that they should be taken into account when considering stimulation of the posterior columns to relieve pain.

Conditioning lesions of peripheral nerves change regenerated axon numbers

The present study investigates the effects of conditioning lesions on regenerated axon numbers in tributary nerves after a test lesion. If a rat sciatic nerve is crushed 7 and 14 days prior to a test crush, the numbers of regenerated myelinated axons 8 weeks later in the sural nerve (SN) and nerve to the medial gastrocnemius (NMG) are increased, both over normal and over numbers after a single crush. If the lesions are only separated by 2 days, however, the numbers are similar to the numbers after a single crush. Thus conditioning occurs, but a minimum time between crushes is necessary for the effects of conditioning to be manifest. If the intervals between lesions are 14 days, the numbers are similar to those after the 7-day intervals. Moving each successive crush proximally or distally does not change regenerated myelinated axon numbers. Thus increasing the time between lesions after conditioning occurs, at least within the constraints of our paradigm, does not change regenerated axon numbers and the location of the lesion has relatively little bearing on the numbers of axons that regenerate. These findings allow us to change axonal numbers in these tributary nerves in a predictable way, and they are also compatible with the hypothesis that conditioning results from priming of the cell body rather than changes in the environment of the regenerating axons.

Calcitonin gene-related peptide immunostained axons provide evidence for fine primary afferent fibers in the dorsal and dorsolateral funiculi of the rat spinal cord

The hypothesis being tested in the present paper is that there are large numbers of fine primary afferent axons in the dorsal and dorsolateral funiculi of the lumbar spinal cord of the rat. The data show numerous calcitonin gene-related peptide labeled fine myelinated and unmyelinated axons in these funiculi. Approximately 95% of the labeled axons disappear after dorsal rhizotomy. Accordingly, the hypothesis is confirmed. Thus it is becoming apparent that fine primary afferent fibers are more widely distributed in spinal white matter than had been previously recognized. Implications are that it is not possible to find areas in the spinal white matter that contain only large myelinated sensory axons and that significant numbers of fine primary afferent fibers will be lost even if lesions are restricted to the dorsal funiculus. The sizable population of fine myelinated primary afferent axons in the dorsal funiculus is emphasized. An obvious question, suggested by significant differences in average diameters of the axons in the different pathways, is whether there are differences in the types of information carried by the fine afferent fibers in their different locations in the white matter of the lumbar cord.

Propriospinal fibers in the white matter of the cat sacral spinal cord

The propriospinal system, which consists of those neurons completely contained within the spinal cord, is important because it underlies much spinal behavior. To provide quantitative data on this system, the present study determines numbers of axons in the isolated S2 cat spinal cord and compares these figures with the normal. The conclusion is that 60% of the fibers in the spinal cord at this location are propriospinal. Findings of particular interest are that the great majority of unmyelinated propriospinal axons are found in the dorsal part of the lateral funiculus, and that there are large numbers of descending myelinated fibers in the dorsal funiculi. These data will serve as a basis for evaluating axon numbers that follow various experimental regimens purporting to result in neural sprouting.