Modification of serotonergic immunoreactive pattern in the dorsal horn of the rat spinal cord following dorsal root rhizotomy

Anatomy and physiology of phrenic afferent neurons

Large-diameter myelinated phrenic afferents discharge in phase with diaphragm contraction, and smaller diameter fibers discharge across the respiratory cycle. In this article, we review the phrenic afferent literature and highlight areas in need of further study. We conclude that 1) activation of both myelinated and nonmyelinated phrenic sensory afferents can influence respiratory motor output on a breath-by-breath basis; 2) the relative impact of phrenic afferents substantially increases with diaphragm work and fatigue; 3) activation of phrenic afferents has a powerful impact on sympathetic motor outflow, and 4) phrenic afferents contribute to diaphragm somatosensation and the conscious perception of breathing. Much remains to be learned regarding the spinal and supraspinal distribution and synaptic contacts of myelinated and nonmyelinated phrenic afferents. Similarly, very little is known regarding the potential role of phrenic afferent neurons in triggering or modulating expression of respiratory neuroplasticity.

Lack of analgesic efficacy of spinal ondansetron on thermal and mechanical hypersensitivity following spinal nerve ligation in the rat

The balance between descending inhibition and facilitation is thought to be disturbed in chronic pain states. Increased facilitation by spinally released serotonin has been suggested by demonstration that mechanically evoked neuronal responses of wide dynamic range neurons are inhibited by 5-HT3 receptor antagonists in rats following spinal nerve ligation (SNL) but not sham operation. Despite these physiologic data, the effects of spinal 5-HT3 receptor blockade on behavioral hypersensitivity and neurochemical alterations in spinal serotonergic system have not been thoroughly investigated following spinal nerve ligation in the rat. To test this, we acutely injected intrathecal ondansetron in rats between 14 and 30 days after SNL and assessed effects on thermal and mechanical hypersensitivity. We also determined the density of serotonergic nerve fibers, serotonin content and the levels of 5-HT3 receptors within the spinal cord at this time point. Intrathecal ondansetron (1, 3, 10, 30, and 100microg) produced no effect on behavioral measures of thermal or mechanical hypersensitivity whereas intrathecal morphine (1microg) and gabapentin (200microg) partially reversed thermal and mechanical hypersensitivity following SNL. In addition, SNL did not alter the density of serotonergic fibers or 5-HT3 receptor immunoreactivity or spinal tissue content of 5-HT within the dorsal horn. These results do not support anatomic plasticity of descending serotonergic pathways or tonic 5-HT3 receptor activity in maintaining hypersensitivity after nerve injury and in contrast to previous studies fail to demonstrate an anti-hypersensitivity effect of intrathecal injection of the 5-HT3 receptor antagonist ondansetron. Importantly, behavioral measures of mechanical hypersensitivity assess threshold responses whereas physiological studies of mechanically evoked neuronal responses involve application of suprathreshold stimuli. Thus, suprathreshold or more intense stimuli may be necessary to recruit descending serotonergic facilitatory drive required to observe the inhibitory effects of ondansetron on spinal neuronal excitability and behavioral hypersensitivity.

Distribution and localization of 5-HT(1A) receptors in the rat lumbar spinal cord after transection and deafferentation

The serotonergic system is highly plastic, capable of adapting to changing afferent information in diverse mammalian systems. We hypothesized that removing supraspinal and/or peripheral input would play an important role in defining the distribution of one of the most prevalent serotonergic receptors, the 5-HT(1A) receptor (R), in the spinal cord. We investigated the distribution of this receptor in response to a complete thoracic (T7-T8) spinal cord transection (eliminating supraspinal input), or to spinal cord isolation (eliminating both supraspinal and peripheral input) in adult rats. Using two antibodies raised against either the second extracellular region (ECL(2)) or the third intracellular region (ICL(3)) of the 5-HT(1A)R, we compared the 5-HT(1A)R levels and distributions in specific laminae of the L3-L5 segments among the control, spinal cord-transected, and spinal cord-isolated groups. Each antibody labeled different populations of 5-HT(1A)R: ECL(2) labeled receptors in the axon hillock, whereas ICL(3) labeled receptors predominantly throughout the soma and proximal dendrites. Spinal cord transection increased the number of ECL(2)-positive cells in the medial region of laminae III-IV and lamina VII, and the mean length of the labeled axon hillocks in lamina IX. The number of ICL(3)-labeled cells was higher in lamina VII and in both the medial and lateral regions of lamina IX in the spinal cord-transected compared to the control group. In contrast, the length and number of ECL(2)-immunolabeled processes and ICL(3)-immunolabeled cells were similar in the spinal cord-isolated and control groups. Combined, these data demonstrate that the upregulation in 5-HT(1A)R that occurs with spinal cord transection alone is dependent on the presence of sensory input.

Olfactory ensheathing cells: their potential use for repairing the injured spinal cord

STUDY DESIGN

The literature concerning the potential use of olfactory ensheathing cells for repairing damaged spinal cord was reviewed.

OBJECTIVE

To engender a better understanding of the role that olfactory ensheathing cells play in spinal cord regeneration.

SUMMARY OF BACKGROUND DATA

Intraspinal transplants (e.g., fetal neuronal cells, progenitor stem cells, and olfactory ensheathing cells) have been used to restore intraspinal circuitry or to serve as a "bridge" for damaged axons. Among these transplants, olfactory ensheathing cells provide a particularly favorable substrate for spinal axonal regeneration because these cells can secrete extracellular molecules and neurotrophic factors and have the ability to migrate into gliotic scar tissue, an important attribute that might be associated with high potential for axonal regeneration.

METHODS

Recent advances using centrally and peripherally derived olfactory ensheathing cells to promote spinal cord regeneration were reviewed.

RESULTS

Both centrally and peripherally derived olfactory ensheathing cells can lead to a degree of functional and anatomic recovery after spinal cord injury in adult animals.

CONCLUSION

Olfactory ensheathing cells from olfactory lamina propria in the nose are among the best transplants for "bridging" descending and ascending pathways in damaged spinal cord.

Activation of locomotion in adult chronic spinal rats is achieved by transplantation of embryonic raphe cells reinnervating a precise lumbar level

Traumatic lesions of the spinal cord yield a loss of supraspinal control of voluntary locomotor activity, although the spinal cord contains the necessary circuitry to generate the basic locomotor pattern. In spinal rats, this network, known as central pattern generator (CPG), was shown to be sensitive to serotonergic pharmacological stimulation. In previous works we have shown that embryonic raphe cells transplanted into the sublesional cord of adult rats can reinnervate specific targets, restore the lesion-induced increase in receptor densities of neurotransmitters, promote hindlimb weight support, and trigger a locomotor activity on a treadmill without any other pharmacological treatment or training. With the aim of discriminating whether the action of serotonin on CPG is associated to a specific level of the cord, we have transplanted embryonic raphe cells at two different levels of the sublesional cord (T9 and T11) and then performed analysis of the kinematic and EMG activity synchronously recorded during locomotion. Locomotor performances were correlated to the reinnervated level of the cord and compared to that of intact and transected nontransplanted animals. The movements expressed by T11 transplanted animals correspond to a well defined locomotor pattern comparable to that of the intact animals. On the contrary, T9 transplanted animals developed limited and disorganized movements as those of nontransplanted animals. The correlation of the locomotor performances with the level of reinnervation of the spinal cord suggests that serotonergic reinnervation of the L1-L2 level constitutes a key element in the genesis of this locomotor rhythmic activity. This is the first in vivo demonstration that transplanted embryonic raphe cells reinnervating a specific level of the cord activate a locomotor behavior.

Influence of hypergravity on the development of monoaminergic systems in the rat spinal cord

We have investigated in this study the influence of a moderate hypergravity (1.8 G) on the development of monoaminergic projections to the spinal cord in the rat. Pregnant dams and their offspring were submitted to hypergravity from day 11 of gestation to postnatal day 15. Some animals were sacrificed at birth, other at postnatal day 15 and other after 8 months of normal gravity. In newborn animals, a substantial delay of the development of monoaminergic projections to the spinal cord was evidenced. In 15 days and 8 months animals, the pattern of innervation appeared anarchic, with numerous dystrophic profiles, mainly of serotonergic system. Ultrastructural examination of serotonergic projections revealed a paucity of synapses, and the frequent enveloping of serotonergic boutons by thin astrocytic profiles. We conclude that rats submitted to hypergravity during the critical period of onset of monoaminergic projections to the spinal cord are affected durably in the organization and the ultrastructure of these projections. Future studies are directed to the functional analysis of hypergravity animals, and to the influence of microgravity on the same system.

Cervical dorsal rhizotomy enhances serotonergic innervation of phrenic motoneurons and serotonin-dependent long-term facilitation of respiratory motor output in rats

We tested the hypothesis that spinal plasticity elicited by chronic bilateral cervical dorsal rhizotomy (C3-C5; CDR) has functional implications for respiratory motor control. Surgery was performed on rats (CDR or sham-operated) 26 d before phrenic motoneurons were retrogradely labeled with cholera toxin. Rats were killed 2 d later, and their spinal cords were harvested and processed to reveal the cholera toxin-labeled phrenic motoneurons and serotonin-immunoreactive terminals. The number of serotonin-immunoreactive terminals within 5 micrometer of labeled phrenic motoneuron soma and primary dendrites increased 2.1-fold after CDR versus sham-operation. Time-dependent phrenic motor responses to hypoxia were compared among CDR, sham-operated, and control rats. Anesthetized, paralyzed, vagotomized, and artificially ventilated rats were exposed to three, 5 min episodes of isocapnic hypoxia (FiO2 = 0.11), separated by 5 min hyperoxic intervals (FiO2 = 0.5). One hour after hypoxia, a long-lasting, serotonin-dependent enhancement of phrenic motor output (long-term facilitation) was observed in both sham and control rats. After CDR, long-term facilitation was 108 and 163% greater than control and sham responses, respectively. Pretreatment of CDR rats with a 5-HT2 receptor antagonist (ketanserin tartrate, 2 mg/kg, i.v.) before episodic hypoxia prevented long-term facilitation and revealed a modest (-28 +/- 13%; p < 0.05) long-lasting depression of phrenic motor output. The results indicate that CDR: (1) increases serotonergic innervation of the phrenic motor nucleus; and (2) augments serotonin-dependent long-term facilitation of phrenic motor output. These results further suggest a form of plasticity based on changes in the capacity for neuromodulation.

The contribution of supraspinal, peripheral and intrinsic spinal circuits to the pattern and magnitude of Fos-like immunoreactivity in the lumbar spinal cord of the rat withdrawing from morphine

Withdrawal from morphine evokes increases in Fos-like immunoreactivity in the spinal cord, particularly in the superficial dorsal horn, laminae I/II. To determine the origin of the increased Fos-like immunoreactivity, we selectively targeted central or peripheral opioid receptors with naloxone-methiodide, an antagonist that does not cross the blood-brain barrier, or induced withdrawal after eliminating possible sources of input to the superficial dorsal horn. To induce tolerance, we implanted rats with morphine or placebo pellets (75 mg, six pellets over three days). On day 4, withdrawal was precipitated and after 1 h, the rats were killed, their spinal cords removed and 50 microm transverse sections of the spinal cord immunoreacted with a rabbit polyclonal antiserum directed against the Fos protein. In placebo-pelleted rats, none of the different procedures, viz. spinal transection, unilateral dorsal rhizotomy (L4-S2), neonatal capsaicin treatment or direct intrathecal opioid antagonist injection, induced expression of the Fos protein. However, both spinally transected and rhizotomized withdrawing animals showed significant increases in Fos-like immunoreactivity in laminae I/II, compared to intact withdrawing rats. Neonatal treatment with capsaicin, which eliminates C-fibres, did not alter Fos-like-immunoreactivity. Selective withdrawal of morphine from peripheral opioid receptors by naloxone-methiodide did not induce Fos-like immunoreactivity in the lumbar spinal cord greater than that recorded in nonwithdrawing rats. However, intrathecal injection of naloxone-methiodide increased Fos-like immunoreactivity in laminae I/II and the ventral horn to a greater extent than did subcutaneous injection of naloxone. We hypothesize that the increased Fos expression after systemic withdrawal in spinally-transected rats results from a loss of descending inhibitory control that is activated during withdrawal. The increase in withdrawal-induced Fos-like immunoreactivity after rhizotomy may be secondary to loss of inhibitory controls exerted by large diameter primary afferents or to deafferentation-induced reorganization in the dorsal horn. Since capsaicin did not alter the magnitude of Fos-like immunoreactivity in withdrawing rats, we conclude that hyperactivity of opioid receptor-laden C-fibres is not a necessary contributor to the withdrawal-induced increase in Fos-like immunoreactivity in laminae I and II. Taken together with the results recorded after intrathecal injection of naloxone-methiodide in tolerant rats, we conclude that the pattern of lumbar spinal cord Fos expression following systemic withdrawal is primarily a consequence of increased activity in opioid receptor-containing circuits intrinsic to the dorsal horn and that the magnitude of Fos expression is normally dampened by supraspinal and primary afferent-derived inhibitory inputs.

Biphasic response of spinal GABAergic neurons after a lumbar rhizotomy in the adult rat

The expression of gamma-aminobutyric acid (GABA) and of the isoforms of the enzyme involved in its synthesis, glutamic acid decarboxylase (GAD), is modified in several rat brain structures in different injury models. The aim of the present work was to determine whether such plasticity of the GABAergic system also occurred in the deafferented adult rat spinal cord, a model where a major reorganization of neural circuits takes place. GABAergic expression following unilateral dorsal rhizotomy was studied by means of non-radioactive in situ hybridization to detect GAD67 mRNA and by immunohistochemistry to detect GAD67 protein and GABA. Three days following rhizotomy the number of GAD67 mRNA-expressing neurons was decreased in the superficial layers of the deafferented horn, while GABA immunostaining of axonal fibres located in this region was highly increased. Seven days after lesion, on the other hand, many GAD67 mRNA-expression neurons were bilaterally detected in deep dorsal and ventral layers, this expression being correlated with the increased detection of GAD67 immunostained somata and with the reduction of GABA immunostaining of axons. GABA immunostaining was frequently found to be associated with reactive astrocytes that exhibited intense immunostaining for glial fibrillary acidic protein (GFAP) but remained GAD67 negative. These results indicate that degeneration of afferent terminals induces a biphasic response of GABAergic spinal neurons located in the dorsal horn and show that many spinal neurons located in deeper regions re-express GAD67, suggesting a possible participation of the local GABAergic system in the reorganization of disturbed spinal networks.

Functional properties of cells in rat trigeminal subnucleus interpolaris following local serotonergic deafferentation

We have previously demonstrated increases in serotonin (5-HT) content and immunoreactivity within spinal trigeminal subnucleus interpolaris (SpVi) that are correlated with the functional changes observed in this subnucleus following adult infraorbital nerve (ION) transection. To assess the possible functional significance of this change, we have examined the influence of 5-HT afference upon the normal response properties of cells in SpVi. We employed local depletion of the transmitter, using 5,7-dihydroxtryptamine (5,7-DHT), in combination with extracellular single-cell recording. Chromatographic methods revealed a 97.6% depletion of 5-HT 24 hr after neurotoxin injection. Immunocytochemical procedures revealed depletion of 5-HT throughout SpVi. Physiological recordings were made from 403 SpVi cells in 5,7-DHT-injected rats and 387 cells in vehicle-injected rats. All recordings were made 19-27 hr after injection. Horseradish peroxidase (HRP) deposits from the recording electrode were used to mark recording tracks. 5-HT depletion did not influence receptive field (RF) location, size, or continuity, or the dynamic response characteristics of SpVi cells. It did, however, (1) alter the probability that certain types of somatosensory receptor surfaces would activate local-circuit neurons, and (2) influence the rate of firing of spontaneously active SpVi cells. There was a significant increase in the proportion of vibrissa-sensitive cells with infraorbital RF components, and a concurrent decrease in the proportion of guard-hair-sensitive cells. It therefore appears that 5-HT input to SpVi is necessary for some mechanoreceptive features of the normal functional organization of this area. These functional changes were interesting in that they were opposite to those found following adult ION transection, which increases 5-HT within SpVi. Thus, changes in 5-HT central afference to SpVi that follow ION damage may be responsible for at least one type of functional change observed following this peripheral lesion.

Serotonergic terminals express a growth associated protein (GAP-43) in the adult rat spinal cord

Dual colour immunofluorescence has been used to compare the distribution of serotonin (5-hydroxytryptamine, 5-HT) and GAP-43 in the adult rat. GAP-43 immunostaining was observed in all spinal cord regions containing 5-HT immunoreactivity. 5-HT and GAP-43 double labelled fibres and varicosities were present and were most evident around motoneurones, in lamina X, and in the intermediolateral cell column. Single labelled GAP-43 fibres and varicosities were also observed and were the dominant population in the dorsal horn and in certain fibre tracts. We conclude that the 5-HT system is one of a small number of spinal cord systems that express high levels of GAP-43 in the adult.

Effects of neonatal adrenalectomy on 5-HT and TRH-like immunoreactivity in the rat spinal cord

This study investigated the effects of neonatal unilateral adrenalectomy on the serotonin- (5-HT) and thyrotropin-releasing hormone-like immunoreactivities (TRH-LI) in the intermediolateral cell column (IML) of the rat spinal cord where the sympathetic preganglionic neurons innervating the adrenal medulla are located. The density of the innervation was measured by a computer-assisted image analysis. Two weeks after the lesion, only one rat (1/3) showed a 30% decrease of 5-HT-LI in the ipsilateral IML, although no modifications were observed for TRH-LI. At 1 month, we observed a variable decrease of 19% to 30% for either 5-HT- or TRH-LI in the ipsilateral IML. However, one animal (1/3) showed nonsignificant modifications. At 3 months, we showed a 17% mean loss of 5-HT-LI and TRH-LI in the ipsilateral IML. However, the decreases of 5-HT-LI and TRH-LI did not always appear similar. These results appear different from those obtained after neonatal removal of the superior cervical ganglion.

Time course of serotonergic afferent plasticity within rat spinal trigeminal nucleus following infraorbital nerve transection

High-performance liquid chromatography with electrochemical detection (HPLC-ED) and immunocytochemistry were used to examine the time course of serotonergic afferent plasticity within trigeminal subnucleus interpolaris (SpVi) following infraorbital nerve (ION) transection in adult rats. Biochemical analysis was also performed in trigeminal subnucleus caudalis (SpVc) to examine the possibility of transient lesion-induced changes in this region. No significant changes in serotonin (5-HT) or 5-hydroxyindoleacetic acid (5-HIAA) concentration, or in density of 5-HT-immunoreactive (5-HTIR) axonal varicosities were observed in either subnucleus on the lesioned side, up to 51 days following ION cut. However, at 76-79 days post-lesion, a significant increase in 5-HT concentration was again demonstrated within SpVi.