Increased number of unmyelinated fibers in the ventral root after peripheral neurectomy in adult rat

Does the right side know what the left is doing?

Following peripheral-nerve lesions there are well-documented events that affect the contralateral nonlesioned structures. These contralateral effects are qualitatively similar to those occurring at the ipsilateral side, but are usually smaller in magnitude and have a briefer time course. It is unclear whether the findings are an epiphenomenon or serve a biological purpose, but in either case the existence of these effects implies the presence of unrecognized signalling mechanisms that link the two sides of the body. Strong circumstantial evidence argues against a peripheral mechanism (for example, via circulating factors) and in favour of a central mechanism, in particular signalling via the system of commissural interneurons that is present in spinal cord and brainstem. While an altered pattern of activity in this system might underlie the phenomenon, there are several reasons for proposing that the changes depend upon chemical signals, possibly growth factors. Because of its relative easy access for experimental manipulation, the spinal cord could serve as a model system to study these transmedian signalling systems.

The role of capsaicin-sensitive muscle afferents in fatigue-induced modulation of the monosynaptic reflex in the rat

1. The role of group III and IV afferent fibres of the lateral gastrocnemious muscle (LG) in modulating the homonymous monosynaptic reflex was investigated during muscle fatigue in spinalized rats. 2. Muscle fatigue was induced by a series of increasing tetanic electrical stimuli (85 Hz, 600 ms) delivered to the LG muscle nerve. Series consisted of increasing train numbers from 1 to 60. 3. Potentials from the spinal cord LG motor pool and from the ventral root were recorded in response to proprioceptive afferent stimulation and analysed before and during tetanic muscle activations. Both the pre- and postsynaptic waves showed an initial enhancement and, after a '12-train' series, an increasing inhibition. 4. The enhancement of the responses to muscle fatiguing stimulation disappeared after L3-L6 dorsal root section, while a partial reflex inhibition was still present. Conversely, after section of the corresponding ventral root, there was only a reduction in the inhibitory effect. 5. The monosynaptic reflex was also studied in animals in which a large number of group III and IV muscle afferents were eliminated by injecting capsaicin (10 mM) into the LG muscle. As a result of capsaicin treatment, the fatigue-induced inhibition of the pre- and postsynaptic waves disappeared, while the response enhancement remained. 6. We concluded that the monosynaptic reflex inhibition, but not the enhancement, was mediated by those group III and IV muscle afferents that are sensitive to the toxic action of capsaicin. The afferents that are responsible for the response enhancement enter the spinal cord through the dorsal root, while those responsible for the inhibition enter the spinal cord through both the ventral and dorsal roots.

Ganglionic axons in motor roots and pia mater

In addition to motor axons and preganglionic axons, ventral roots contain unmyelinated or thin myelinated sensory axons and postganglionic sympathetic axons. It has been said that ventral roots channel sensory axons to the CNS. However, it now seems that these axons end blindly, shift to the pia or loop and return towards the periphery and that these units reach the CNS via dorsal roots. Sensory ventral root axons project from a variety of somatic or visceral receptors; some of them are third branches of dorsal root afferents and some seem to lack a CNS projection. Many ventral root afferents contain substance P (SP) and/or calcitonin gene-related peptide (CGRP). These fibres are not affected by neonatal capsaicin treatment and they cannot induce radicular or pial extravasation. Some thin ventral root axons are sympathetic and relate to blood vessels. Afferents containing SP and/or CGRP and sympathetic axons also occur in the spinal pia mater. The sensory axons mediate pain. They might also have vasomotor, tissue-regulatory and/or mechanoreceptive functions. The motor roots of cranial nerves IV, VI and XI contain unmyelinated axons arranged like in ventral roots outside the autonomic outflow. However, the motor root of cranial nerve V channels some unmyelinated axons into the CNS. The occurrence of thin axons in ventral roots and pia mater changes during development and ageing. After peripheral nerve injury, ipsilateral ventral roots and pia are invaded by new sensory and postganglionic sympathetic axons.

Postnatal development and aging of the rat ventral root L5: electron microscopic and immunohistochemical studies

The ventral root L5 of neonatal and adult rats has been used in many experimental studies on ganglionic C-fibers. Since the normal presence of such axons in L5 roots from animals of different ages is unknown, the results of these studies cannot be appropriately interpreted. In the present study we examine L5 ventral roots from developing and aging rats in this respect. Electron microscopic examination revealed that C-fibers occur in neonatal roots. The adult proportion has been established at day 30. Immunohistochemical analysis showed that thin ganglionic fibers with substance P/calcitonin gene-related peptide- or tyrosine hydroxylase-like immunoreactivity in the L5 root and the spinal pia mater seem to increase postnatally from low levels at birth. In roots from aged rats, myelinated fibers with a variety of aberrant features occur in normal numbers. The occurrence of unmyelinated axons is elevated. The increased presence of fibers with calcitonin gene-related peptide- or tyrosine hydroxylase-like immunoreactivity in aged roots indicates that the extra unmyelinated fibers may represent motor sprouts and sympathetic fibers, respectively. We conclude that the rat ventral root L5 contains a variable number of putative sensory and sympathetic axons at all ages.

Invasion of the rat ventral root L5 by putative sympathetic C-fibers after neonatal sciatic nerve crush

The present study examines the occurrence of C-fibers in lumbar ventral roots after sciatic nerve crush in neonatal and adult rats. Electron microscopic analysis showed that the number of C-fibers in the ventral root L5 increased significantly on the lesion side after neonatal but not adult sciatic nerve crush and that the number of C-fibers was higher in the ventral root L5 on the unoperated side compared to this root in normal control rats. In order to determine whether the new C-fibers in the L5 root on the lesion side are sensory or sympathetic we made immunohistochemical studies on roots from neonatally crushed rats. We found that there was no obvious lesion side/contralateral side or operated rat/control rat difference with respect to the occurrence and general configuration of axons with substance P-, calcitonin gene-related peptide- or vasoactive intestinal polypeptide-like immunoreactivity. However, the occurrence of axons with tyrosine hydroxylase-like immunoreactivity appeared clearly higher in the ventral root L5 on the lesion side compared to the unoperated side in neonatally crushed rats. Moreover, these axons seemed to be more numerous also in the ventral root L5 on the unoperated side compared to normal control rats. No lesion side/contralateral side or operated rat/control rat differences were seen in the ventral root L4. We propose that the ventral root L5 is invaded by putative sympathetic C-fibers after sciatic nerve crush lesions in newborn rats.

Substance P-, calcitonin gene-related peptide, growth-associated protein-43, and neurotrophin receptor-like immunoreactivity associated with unmyelinated axons in feline ventral roots and pia mater

The spinal pia mater receives a rich innervation of small sensory axons via the ventral roots. Other sensory axons enter the ventral roots but end blindly or turn abruptly in hairpin loop-like formations and continue in a distal direction. In the present study, the content of substance P (SP)-, calcitonin gene-related peptide (CGRP)-, growth-associated protein (GAP-43)-, and low-affinity neurotrophin receptor protein (p75NGFr)-like immunoreactivity (-LI) associated with these different types of sensory axons was assessed with light and electron microscopic immunohistochemical techniques. In addition, the binding of antibodies against synthetic peptides representing unique sequences of residues in the products of the trk and trkB protooncogenes was analyzed. These genes encode membrane spanning proteins, which have been shown to constitute specific high affinity binding sites for several members of the nerve growth factor family of neurotrophic factors. The results of the present study imply that the ventral root afferents comprise several different types of sensory axons, which all contain SP-, CGRP-, GAP-43-, and p75NGFr-like immunoreactivities. In addition, at least some of the presumed sensory fiber bundles in ventral roots and the pia mater were immunoreactive for the trkB gene product. Moreover, leptomeningeal cells and nonneuronal cells of the ventral roots were shown to bind antibodies to both the trk and trkB gene products. The ventral root afferents seem to share their immunohistochemical pattern with pain-transducing axons at some other locations, such as the tooth pulp. The contents of SP- and CGRP-LI in sensory axons that reach the central nervous system (CNS) through the ventral root indicate that ventral root afferents may be involved in sensory mechanisms, such as the ventral root pain reaction, as well as in the control of the pial blood vessels. The demonstration of GAP-43 and neurotrophin receptor-immunoreactivities associated with unmyelinated fibers in ventral roots and the pia mater is discussed in relation to previous reports on postnatal plasticity in these axonal populations.

Immunohistochemical evidence for sprouting of ventral root afferents after neonatal sciatic neurectomy in the rat

The present study demonstrates that afferent fibers in the rat ventral root sprout after transection of the neonatal sciatic nerve. The key data are that the number of unmyelinated fibers increased dramatically in the L5 ventral roots and about half of these unmyelinated fibers were labeled with antibody to growth associated phosphoprotein (GAP-43), suggesting that they are regenerating axons. Furthermore, these fibers survived in the distal stump of the acutely cut ventral root, suggesting that they are afferent fibers. The results provide direct evidence for sprouting of afferent fibers in the ventral root induced by a neonatal peripheral nerve lesion.

Evidence for invasion of regenerated ventral root afferents into the spinal cord of the rat subjected to sciatic neurectomy during the neonatal period

Sectioning the sciatic nerve of experimental animals at the neonatal stage triggers growth of afferent fibers in the ventral root. The present study examined the possibility that the regenerating fiber terminals grow into the spinal cord. The sciatic nerve on one side was cut in neonatal rats. After the rats were fully grown, either an electrophysiological or a histochemical study was performed. The results of electrophysiological experiments showed that stimulation of certain loci in the L5 spinal cord evoked antidromic potentials in the L5 ventral root with a long latency. Various evidence suggests that the long latency potentials are due to activation of C fibers. These C-fiber potentials were on average bigger and were elicited from more numerous loci on the side ipsilateral to the sciatic nerve lesion than on the contralateral side. Furthermore, stimulation of the spinal cord of unoperated normal rats rarely evoked such potentials. For the histochemical study, horseradish peroxidase (HRP) was injected into the L5 spinal cord after cutting the L4-L6 dorsal roots. A lot more cells in the L5 dorsal root ganglion (DRG) on the side ipsilateral to the sciatic nerve lesion were labeled with HRP transported retrogradely through the L5 ventral root than on the contralateral side. Control experiments showed that few DRG cells are labeled with HRP in normal unoperated rats. The combined results of the electrophysiological and histochemical studies suggest invasion of ventral root afferents into the spinal cord, given enough postoperative time. It is not known whether or not these terminals make functional synaptic contacts in the spinal cord.