Labeling of central projections of primary afferents in adult rats: a comparison between biotinylated dextran amine, neurobiotin and Phaseolus vulgaris-leucoagglutinin

Enhancement of Neural Stem Cell Proliferation in Rats with Spinal Cord Injury by a Combination of Repetitive Transcranial Magnetic Stimulation (rTMS) and Human Umbilical Cord Blood Mesenchymal Stem Cells (hUCB-MSCs)

BACKGROUND This study was designed to explore the combined effects of repetitive transcranial magnetic stimulation (rTMS) and human umbilical cord blood mesenchymal stem cells (hUCB-MSCs) transplantation on neural stem cell proliferation in rats with spinal cord injury (SCI). MATERIAL AND METHODS SCI was induced in 90 rats by laminectomy at T10. Fifteen rats each were treated with 0.5 Hz rTMS or 10 Hz rTMS or underwent hUCB-MSC transplantation; 15 each were treated with 0.5 Hz rTMS+hUCB-MSCs or 10 Hz rTMS+hUCB-MSCs; and 15 were untreated (control group). The Basso, Beattie, and Bresnahan (BBB) scores and motor evoked potentials (MEPs) were measured, and all rats underwent biotin dextran-amine (BDA) tracing of the corticospinal tract (CST). The levels of expression of neural stem cell proliferation related proteins, including BrdU, nestin, Tuj1, Ng2+ and GFAP, were measured, and the levels of bFGF and EGF determined by Western blotting. RESULTS BBB scores and MEPs were increased after rTMS and hUCB-MSC transplantation, while histologically determined SCI-induced neuron apoptosis was attenuated. The numbers of BDA-positive fibers and Brdu-, nestin- and Tuj1-positive cells were markedly increased and the numbers of Ng2+- and GFAP-positive cells were markedly decreased following treatment with rTMS alone or rTMS plus hUCB-MSC transplantation. The levels of expression of bFGF and EGF were significantly upregulated following rTMS treatment and hUCB-MSC transplantation. Higher performance was observed after combined treatment with rTMS and hUCB-MSC transplantation than after either alone. CONCLUSIONS The combination of rTMS treatment and hUCB-MSC transplantation could attenuate SCI-induced neural stem cell apoptosis and motor dysfunction in rats.

3D printing collagen/chitosan scaffold ameliorated axon regeneration and neurological recovery after spinal cord injury

Spinal cord injury (SCI) is a disaster that can cause severe motor, sensory, and functional disorders. Implanting biomaterials have been regarded as hopeful strategies to restore neurological function. However, no optimized scaffold has been available. In this study, a novel 3D printing technology was used to fabricate the scaffold with designed structure. The composite biomaterials of collagen and chitosan were also adopted to balance both compatibility and strength. Female Sprague-Dawley rats were subjected to a T8 complete-transection SCI model. Scaffolds of C/C (collagen/chitosan scaffold with freeze-drying technology) or 3D-C/C (collagen/chitosan scaffold with 3D printing technology) were implanted into the lesion. Compared with SCI or C/C group, 3D-C/C implants significantly promoted locomotor function with the elevation in Basso-Beattie-Bresnahan (BBB) score and angle of inclined plane. Decreased latency and increased amplitude were observed both in motor-evoked potential and somatosensory-evoked potential in 3D-C/C group compared with SCI or C/C group, which further demonstrated the improvement of neurological recovery. Fiber tracking of diffusion tensor imaging (DTI) showed the most fibers traversing the lesion in 3D-C/C group. Meanwhile, we observed that the correlations between the locomotor (BBB score or angle of inclined plane) and the DTI parameters (fractional anisotropy values) were positive. Although C/C implants markedly enhanced biotin dextran amine (BDA)-positive neural profiles compared with SCI group, rats implanted with 3D-C/C scaffold displayed the largest degree of BDA profiles regeneration. Collectively, our 3D-C/C scaffolds demonstrated significant therapeutic effects on rat complete-transected spinal cord model, which provides a promising and innovative therapeutic approach for SCI. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1898-1908, 2019.

A brand-new generation of fluorescent nano-neural tracers: biotinylated dextran amine conjugated carbonized polymer dots

We develop a novel fluorescent nano-neural tracer: BDA–CPDs, which can be anterogradely transported within the peripheral nervous system of rats.

Neonatal spinal injury induces de novo projections of primary afferents to the lumbosacral intermediolateral nucleus in rats

•

Neonatal spinal injury induces dextran amine-labeled primary afferent projections to the sacral intermediolateral nucleus.

•

Dextran amine-labeled afferent fibers form varicose terminals on the parasympathetic preganglionic neurons.

•

Dextran amine tends to be incorporated preferentially in dorsal root ganglion neurons with myelinated fibers.

•

De novo projections of myelinated afferents might contribute to the recovery of micturition following neonatal spinal injury.

Complete spinal transection in adult rats results in poor recovery of hind limb function and severe urinary bladder dysfunction. Neonatal rats with spinal cord transection, however, exhibit spontaneous and significant recovery of micturition control. A previous study in which biotinylated-dextran amine (BDA) was used as an anterograde tracer demonstrated that primary afferent fibers from the fifth lumbar dorsal root ganglion (DRG) project more strongly and make more terminals in the ventral horn after neonatal spinal cord transection at the mid-thoracic level. In the present study, we injected BDA into the sixth lumbar (L6) DRG of neonatally spinalized rats to label primary afferent fibers that include visceral afferents. The labeled fibers projected to the intermediolateral nucleus (IML) in the intermediate zone on ipsilateral side of the L6 spinal segment, whereas no projections to the IML were observed in sham-operated or intact rats. The BDA-labeled fibers of neonatally spinalized rats formed varicose terminals on parasympathetic preganglionic neurons in the IML. These findings suggest that some primary afferent projections from the L6 DRG to the IML appear after neonatal spinal cord transection, and these de novo projections might contribute to the recovery of autonomic function such as micturition following spinal cord injury in the neonatal stage.

Central projections and connections of lumbar primary afferent fibers in adult rats: effectively revealed using Texas red-dextran amine tracing

Signals from lumbar primary afferent fibers are important for modulating locomotion of the hind-limbs. However, silver impregnation techniques, autoradiography, wheat germ agglutinin-horseradish peroxidase and cholera toxin B subunit-horseradish peroxidase cannot image the central projections and connections of the dorsal root in detail. Thus, we injected 3-kDa Texas red-dextran amine into the proximal trunks of L₄ dorsal roots in adult rats. Confocal microscopy results revealed that numerous labeled arborizations and varicosities extended to the dorsal horn from T₁₂–S₄, to Clarke's column from T₁₀–L₂, and to the ventral horn from L_1–5. The labeled varicosities at the L₄ cord level were very dense, particularly in laminae I–III, and the density decreased gradually in more rostral and caudal segments. In addition, they were predominately distributed in laminae I–IV, moderately in laminae V–VII and sparsely in laminae VIII–X. Furthermore, direct contacts of lumbar afferent fibers with propriospinal neurons were widespread in gray matter. In conclusion, the projection and connection patterns of L₄ afferents were illustrated in detail by Texas red-dextran amine-dorsal root tracing.

Anterograde and retrograde tracing with high molecular weight biotinylated dextran amine through thalamocortical and corticothalamic pathways

Biotinylated dextran amine (BDA) has been used for neural pathway tracing in the central nervous system for many decades, in which high molecular weight BDA appeared to be transported predominantly in the anterograde direction and less in the retrograde direction. In the current study, we reexamined the properties of neural labeling with high molecular weight BDA through a reciprocal neural pathway between thalamus and somatosensory cortex. After injection of BDA into the ventral posteromedial nucleus of thalamus (VPM) in the rat, the BDA labeling was sequentially examined on somatosensory cortex at 3, 5, 7, 10, and 14 survival days. Both of anterogradely labeled axonal terminals and retrogradely labeled neuronal cell bodies were observed simultaneously on the somatosensory cortex. With the increasing of survival times after injection, morphological changes occurred on the labeled axonal arbors and neuronal dendrites, in which the high quality of BDA labeling appeared on the tenth survival day. These results indicate that high molecular weight BDA is not only a sensitive anterograde tracer but also an excellent retrograde marker to be used for tracing through thalamocortical and corticothalamic pathways. And the detailed structure of neural labeling with BDA similar to Golgi-like resolution can be obtained at optimal survival times of animals after the injection of high molecular weight BDA.

Carotid chemoreceptor afferent projections to leptin receptor containing neurons in nucleus of the solitary tract

Neurons expressing the leptin receptor (Ob-R) exist within the caudal nucleus of the solitary tract (NTS). Additionally, afferent neurons expressing the Ob-R have been identified within the nodose ganglion and NTS. Furthermore, systemic injections or focal injections of leptin directly into NTS potentiate the response of NTS neurons to carotid chemoreceptor activation. However, the distribution of carotid body afferents in relation to Ob-R containing neurons within NTS is not known. In this study, chemoreceptor afferent fibers were labeled following microinjection of the anterograde tract tracer biotinylated dextran amine (BDA) into the carotid body or petrosal/nodose ganglion of Wistar rats. After a survival period of 10-14 days, the NTS was processed for BDA and Ob-R immunoreactivity. Afferent axons originating in the carotid body were found to project to the lateral (Slt), gelantinosa (Sg), and medial (Sm) subnuclei of the NTS complex. A similar, but more robust distribution of BDA labeled fibers was observed in the NTS complex after injections into the petrosal/nodose ganglion. Carotid body BDA labeled fibers were observed in close apposition to Ob-R immunoreactive neurons in the region of Slt, Sg and Sm. In addition, a small number of carotid body afferents were found to contain both BDA and express Ob-R-like immunoreactivity within the regions of Slt, Sg and Sm. Taken together, these data suggest that leptin may modulate carotid chemoreceptor function not only through direct effects on NTS neurons, but also through a direct effect on carotid body primary afferent fibers that innervate NTS neurons.

Electrophysiological and morphological evidence for synchronized GnRH pulse generator activity among Kisspeptin/neurokinin B/dynorphin A (KNDy) neurons in goats

Neurons in the arcuate nucleus (ARC) that concomitantly express kisspeptin, neurokinin B (NKB) and dynorphin A are termed KNDy neurons and are likely candidates for the intrinsic gonadotropin-releasing hormone (GnRH) pulse generator. Our hypothesis is that KNDy neurons are functionally and anatomically interconnected to generate discrete neural signals that govern pulsatile GnRH secretion. Our goal was to address this hypothesis using electrophysiological and anatomical experiments in goats. Bilateral electrodes targeting KNDy neurons were implanted into ovariectomized goats, and GnRH pulse generator activity, represented by characteristic increases in multiple-unit activity (MUA volleys), was measured. Spontaneous and pheromone- or senktide (an NKB receptor agonist)-induced MUA volleys were simultaneously recorded from both sides of the ARC. An anterograde tracer, biotinylated dextran amine (BDA), was also injected unilaterally into the ARC of castrated male goats, and the distribution of fibers containing both BDA and NKB was examined using dual-labeling histochemistry. The results showed that MUA volleys, regardless of origin (spontaneous or experimentally induced), occur simultaneously between the right and left sides of the ARC. Tract tracing indicated that axons projecting from NKB neurons in the ARC were directly apposed to other NKB neuronal cells located bilaterally in the ARC. These results demonstrate that GnRH pulse generator activity occurs synchronously between both sides of the ARC in goats and that KNDy neurons are bilaterally interconnected in the ARC via NKB-containing fibers. Taken together, the results suggest that KNDy neurons form a neuronal circuit to synchronize burst activity among KNDy neurons and thereby generate discrete neural signals that govern pulsatile GnRH secretion.

Diverse sensory inputs permit priming in the acidic saline model of hyperalgesia

BACKGROUND

The rodent acidic saline model of hyperalgesia uses repeat injections of acidic saline in the right lateral gastrocnemius muscle, spaced five days apart, to induce a persistent decrease in hindpaw withdrawal thresholds. The objective of this study was to determine if alternate injection sites would permit development of hyperalgesia.

METHODS

The location of the first muscle injection was varied between 3 groups of rats to include the right lateral gastrocnemius, the right medial gastrocnemius or the left lateral gastrocnemius. All second injections were placed in the right lateral gastrocnemius.

RESULTS

As reported by others, placing both injections in the right lateral gastrocnemius produced a significant reduction in paw withdrawal thresholds 24 hours after the second injection (p < 0.05). Relocating the first injection to the right medial gastrocnemius or the left lateral gastrocnemius also produced significant reductions in paw withdrawal thresholds (p < 0.05 for both). Hyperalgesia was also observed if the first muscle injection was replaced with a systemic injection of lipopolysaccharide. Further experiments tested whether glia cells may contribute to the priming process. Pretreatment with minocycline prior to the first injection completely blocked the development of hyperalgesia but was ineffective if injected before the second injection.

CONCLUSIONS

These data indicate that anatomically diverse peripheral stimuli can converge within the central nervous system to produce hyperalgesia.

Macrophage presence is essential for the regeneration of ascending afferent fibres following a conditioning sciatic nerve lesion in adult rats

BACKGROUND

Injury to the peripheral branch of dorsal root ganglia (DRG) neurons prior to injury to the central nervous system (CNS) DRG branch results in the regeneration of the central branch. The exact mechanism mediating this regenerative trigger is not fully understood. It has been proposed that following peripheral injury, the intraganglionic inflammatory response by macrophage cells plays an important role in the pre-conditioning of injured CNS neurons to regenerate. In this study, we investigated whether the presence of macrophage cells is crucial for this type of regeneration to occur. We used a clodronate liposome technique to selectively and temporarily deplete these cells during the conditioning phase of DRG neurons.

RESULTS

Retrograde and anterograde tracing results indicated that in macrophage-depleted animals, the regenerative trigger characteristic of pre-conditioned DRG neurons was abolished as compared to injury matched-control animals. In addition, depletion of macrophage cells led to: (i) a reduction in macrophage infiltration into the CNS compartment even after cellular repopulation, (ii) astrocyte up-regulation at rostral regions and down-regulation in brain derived neurotrophic factor (BDNF) concentration in the serum.

CONCLUSION

Activation of macrophage cells in response to the peripheral nerve injury is essential for the enhanced regeneration of ascending sensory neurons.

Pathways from the ventral hippocampus and caudal amygdala to forebrain regions that regulate sensorimotor gating in the rat

The neural substrates regulating sensorimotor gating in rodents are studied in order to understand the basis for gating deficits in clinical disorders such as schizophrenia. N-methyl-D-aspartate (NMDA) infusion into the ventral temporal lobe, including caudal parts of the ventral hippocampal region and amygdala, has been shown to disrupt sensorimotor gating in rats, as measured by prepulse inhibition (PPI) of startle. One working model is that reduced PPI after infusion of NMDA into this region is mediated via its efferents to ventral forebrain structures, i.e. medial prefrontal cortex (mPFC) and nucleus accumbens. Yet, PPI-disruptive effects persist after lesions of the precommissural fornix, the principal output pathway of the hippocampal formation. Here, we aimed to characterize non-fornical forebrain projections from this region that might mediate the PPI-disruptive effects of the ventral temporal lobe. Electrolytic lesions of the precommissural fornix in male Sprague-Dawley rats were followed by infusions of fluorogold into the mPFC or by infusions of biotinylated dextan amine into the ventral temporal lobe. Projections from the ventral subiculum and CA1 regions of the ventral hippocampus to the mPFC and accumbens core and shell were interrupted by fornix lesions. Projections to the mPFC and accumbens from other regions of the ventral temporal lobe, particularly the lateral entorhinal cortex and the embedded olfactory and vomeronasal parts of the caudal amygdala, survived fornix lesions. These additional projections coursed rostrally through the amygdala and emerged via the stria terminalis, interstitial nuclei of the posterior limb of the anterior commissure, and the ventral amygdalofugal pathway. PPI-regulatory portions of the ventral temporal lobe innervate the accumbens and mPFC via multiple routes. It remains to be determined which of these non-fornical projections may be responsible for the persistent regulation of PPI after fornix lesions.

Morpho-physiologic characteristics of dorsal subicular network in mice after pilocarpine-induced status epilepticus

The goal of this study was to examine the morpho-physiologic changes in the dorsal subiculum network in the mouse model of temporal lobe epilepsy using extracellular recording, juxtacellular and immunofluorescence double labeling, and anterograde tracing methods. A significant loss of total dorsal subicular neurons, particularly calbindin, parvalbumin (PV) and immunopositive interneurons, was found at 2 months after pilocarpine-induced status epilepticus (SE). However, the sprouting of axons from lateral entorhinal cortex (LEnt) was observed to contact with surviving subicular neurons. These neurons had two predominant discharge patterns: bursting and fast irregular discharges. The bursting neurons were mainly pyramidal cells, and their dendritic spine density and bursting discharge rates were increased significantly in SE mice compared with the control group. Fast irregular discharge neurons were PV-immunopositive interneurons and had less dendritic spines in SE mice when compared with the control mice. When LEnt was stimulated, bursting and fast irregular discharge neurons had much shorter latency and stronger excitatory response in SE mice compared with the control group. Our results illustrate that morpho-physiologic changes in the dorsal subiculum could be part of a multilevel pathologic network that occurs simultaneously in many brain areas to contribute to the generation of epileptiform activity.

Versatile, high-resolution anterograde labeling of vagal efferent projections with dextran amines

None of the anterograde tracers used to label and investigate vagal preganglionic neurons projecting to the viscera has proved optimal for routine and extensive labeling of autonomic terminal fields. To identify an alternative tracer protocol, the present experiment evaluated whether dextran conjugates, which have produced superior results in the CNS, might yield widespread and effective labeling of long, fine-caliber vagal efferents in the peripheral nervous system. The dextran conjugates that were evaluated proved reliable and versatile for labeling the motor neuron pool in its entirety, for single- and multiple-labeling protocols, for both conventional and confocal fluorescence microscopy, and for permanent labeling protocols for brightfield microscopy of the projections to the gastrointestinal (GI) tract. Using a standard ABC kit followed by visualization with DAB as the chromagen, Golgi-like labeling of the vagal efferent terminal fields in the GI wall was achieved with the biotinylated dextrans. The definition of individual terminal varicosities was so sharp and detailed that it was routinely practical to examine the relationship of putative vagal efferent contacts (by the criteria of high magnification light microscopy) with the dendritic and somatic architecture of counterstained neurons in the myenteric plexus. Overall, dextran conjugates provide high-definition labeling of an extensive vagal motor pool in the GI tract, and offer considerable versatility when multiple-staining protocols are needed to elucidate the complexities of the innervation of the gut.

Plasticity of lumbosacral propriospinal neurons is associated with the development of autonomic dysreflexia after thoracic spinal cord transection

Complete thoracic (T) spinal cord injury (SCI) above the T6 level typically results in autonomic dysreflexia, an abnormal hypertensive condition commonly triggered by nociceptive stimuli below the level of SCI. Overexpression of nerve growth factor in the lumbosacral spinal cord induces profuse sprouting of nociceptive pelvic visceral afferent fibers that correlates with increased hypertension in response to noxious colorectal distension. After complete T4 SCI, we evaluated the plasticity of propriospinal neurons conveying visceral input rostrally to thoracic sympathetic preganglionic neurons. The anterograde tracer biotinylated dextran amine (BDA) was injected into the lumbosacral dorsal gray commissure (DGC) of injured/nontransected rats immediately after injury (acute) or 2 weeks later (delayed). At 1 or 2 weeks after delayed or acute injections, respectively, a higher density (P < 0.05) of BDA(+) fibers was found in thoracic dorsal gray matter of injured vs. nontransected spinal cords. For corroboration, fast blue (FB) or cholera toxin subunit beta (CTb) was injected into the T9 dorsal horns 2 weeks postinjury/nontransection. After 1 week transport, more retrogradely labeled (P < 0.05) DGC propriospinal neurons (T13-S1) were quantified in injured vs. nontransected cords. We also monitored immediate early gene c-fos expression following colorectal distension and found increased (P < 0.01) c-Fos(+) cell numbers throughout the DGC after injury. Collectively, these results imply that, in conjunction with local primary afferent fiber plasticity, injury-induced sprouting of DGC neurons may be a key constituent in relaying visceral sensory input to sympathetic preganglionic neurons that elicit autonomic dysreflexia after high thoracic SCI.

Intrapancreatic axonal hyperbranching of dorsal root ganglia neurons in chronic pancreatitis model rats and its relation to pancreatic pain

OBJECTIVES

Increase in number of intrapancreatic nerve bundles has been implicated in the generation of persistent pain in chronic pancreatitis. To examine the origin of these nerve fibers and the mechanisms linking neural morphological change to pain generation, we used neuronal tracing techniques in combination with immunohistochemistry in spontaneous chronic pancreatitis in the Wistar Bonn/Kobori (WBN/Kob) rats.

METHODS

For retrograde tracing, horseradish peroxidase was injected into the pancreas, and labeled neurons in the sensory ganglia were counted. For anterograde tracing, biotinylated dextran amine was injected into the dorsal root ganglia (DRGs), and labeled intrapancreatic sensory fibers were histochemically assessed. For assessment of pain generation, we evaluated c-Fos-positive neurons in the spinal dorsal horn and behavioral changes of the animals.

RESULTS

In WBN/Kob rats, the numbers of horseradish peroxidase-labeled neurons were decreased in the DRGs, and the numbers of biotinylated dextran amine-labeled intrapancreatic nerve fibers and terminals were increased. Biotinylated dextran amine-labeled nerve fibers contained growth-associated protein 43. The number of c-Fos-positive neurons in the dorsal horn was also increased and was correlated with intrapancreatic growth-associated protein 43 immunoreactivity. Grooming behavior was reduced in WBN/Kob rats, and this reduction was facilitated by exocrine stimulation.

CONCLUSIONS

Axonal branching in DRG neurons innervating the pancreas increases in WBN/Kob rats, and these morphological changes are likely involved in pain generation in chronic pancreatitis.

Projections of the second cervical dorsal root ganglion to the cochlear nucleus in rats

Physiological, anatomical, and clinical data have demonstrated interactions between somatosensory and auditory brainstem structures. Spinal nerve projections influence auditory responses, although the nature of the pathway(s) is not known. To address this issue, we injected biotinylated dextran amine into the cochlear nucleus or dorsal root ganglion (DRG) at the second cervical segment (C2). Cochlear nucleus injections retrogradely labeled small ganglion cells in C2 DRG. C2 DRG injections produced anterograde labeling in the external cuneate nucleus, cuneate nucleus, nucleus X, central cervical nucleus, dorsal horn of upper cervical spinal segments, and cochlear nucleus. The terminal field in the cochlear nucleus was concentrated in the subpeduncular corner and lamina of the granule cell domain, where endings of various size and shapes appeared. Examination under an electron microscope revealed that the C2 DRG terminals contained numerous round synaptic vesicles and formed asymmetric synapses, implying depolarizing influences on the target cell. Labeled endings synapsed with the stalk of the primary dendrite of unipolar brush cells, distal dendrites of presumptive granule cells, and endings containing pleomorphic synaptic vesicles. These primary somatosensory projections contribute to circuits that are hypothesized to mediate integrative functions of hearing.

Brain projections from the medullary dorsal reticular nucleus: an anterograde and retrograde tracing study in the rat

In the last 15 years a role has been ascribed for the medullary dorsal reticular nucleus as a supraspinal pain modulating area. The medullary dorsal reticular nucleus is reciprocally connected with the spinal dorsal horn, is populated mainly by nociceptive neurons and regulates spinal nociceptive processing. Here we analyze the distribution of brain projections from the medullary dorsal reticular nucleus using the iontophoretic administration of the anterograde tracer biotinylated-dextran amine and the retrograde tracer cholera toxin subunit B. Fibers and terminal boutons labeled from the medullary dorsal reticular nucleus were located predominately in the brainstem, although extending also to the forebrain. In the medulla oblongata, anterograde labeling was observed in the orofacial motor nuclei, inferior olive, caudal ventrolateral medulla, rostral ventromedial medulla, nucleus tractus solitarius and most of the reticular formation. Labeling at the pons-cerebellum level was present in the locus coeruleus, A5 and A7 noradrenergic cell groups, parabrachial and deep cerebellar nuclei, whereas in the mesencephalon it was located in the periaqueductal gray matter, deep mesencephalic, oculomotor and anterior pretectal nuclei, and substantia nigra. In the diencephalon, fibers and terminal boutons were found mainly in the parafascicular, ventromedial, and posterior thalamic nuclei and in the arcuate, lateral, posterior, peri- and paraventricular hypothalamic areas. Telencephalic labeling was consistent but less intense and concentrated in the septal nuclei, globus pallidus and amygdala. The well-known role of the medullary dorsal reticular nucleus in nociception and its pattern of brain projections in rats suggests that the nucleus is possibly implicated in the modulation of: (i) the ascending nociceptive transmission involved in the motivational-affective dimension of pain; (ii) the endogenous supraspinal pain control system centered in the periaqueductal gray matter-rostral ventromedial medulla-spinal cord circuitry; (iii) the motor reactions associated with pain.

Mediodorsal thalamic afferents to layer III of the rat prefrontal cortex: synaptic relationships to subclasses of interneurons

The mediodorsal nucleus of the thalamus (MD) represents the main subcortical structure that projects to the prefrontal cortex (PFC) and it regulates key aspects of the cognitive functions of this region. Within the PFC, GABA local circuit neurons shape the activity patterns and hence the "memory fields" of pyramidal cells. Although the connections between the MD and PFC are well established, the ultrastructural relationships between projecting fibers from the MD and different subclasses of GABA cells in the PFC are not known. In order to address this issue in the rat, we examined MD axons labeled by tract-tracing in combination with immunogold-silver to identify different calcium-binding proteins localized within separate populations of interneurons. Electron micrographic examination of PFC sections from these animals revealed that MD terminals made primarily asymmetric synapses onto dendritic spines and less commonly onto dendritic shafts. Most of the dendrites receiving MD synaptic input were immunoreactive for parvalbumin (ParV), whereas MD synapses onto dendrites labeled for calretinin or calbindin were less frequent. We also observed that some MD terminals were themselves immunoreactive for calcium-binding proteins, again more commonly for ParV. These results suggest that the MD exerts a dual influence on PFC pyramidal cells: direct inputs onto spines and an indirect influence mediated via synapses onto each subclass of interneurons. The apparent preferential input to ParV cells endows MD afferents with a strong indirect inhibitory influence on pyramidal neuron activity by virtue of ParV cell synapses onto soma, proximal dendrites, and axon initial segments.

The anterograde and retrograde axonal transport of biotinylated dextran amine and biocytin in the nervous system of teleosts

Biotinylated dextran amine (BDA) and biocytin are well transported both retrogradely and anterogradely. Both tracers have stable molecular structure for long-term storage and examination, and their visualizations can be realized by simple histochemical reactions. Therefore, the BDA and biocytin are widely used in neuroanatomical studies as the tract-tracing markers. The results obtained by BDA and biocytin applications to various areas of the nervous system in teleosts were qualitatively identical, and the retrogradely and anterogradely labeled structures could be clearly identified with reference to the counter-staining. Iontophoretic injections or crystal insertions resulted in filling of cell bodies, dendrites and terminals in the core of injection side, revealing morphological details of the local and distant somata, dendritic arborizations and axonal terminals. However, biocytin exhibited superior to BDA in anterograde transport, and could label very thin axons, axonal collaterals and terminal ramifications. In contrast, retrograde transport of BDA was superior to that of biocytin, and resulted in more complete dendritic filling of retrograde labeled neurons including dendritic arborizations and spines.

Olfactory ensheathing cell grafts have minimal influence on regeneration at the dorsal root entry zone following rhizotomy

The effectiveness of grafts of olfactory ensheathing cells (OECs) as a means of promoting functional reconnection of regenerating primary afferent fibers was investigated following dorsal root injury. Adult rats were subjected to dorsal root section and reanastomosis and at the same operation a suspension of purified OECs was injected at the dorsal root entry zone and/or into the sectioned dorsal root. Regeneration of dorsal root fibers was then assessed after a survival period ranging from 1 to 6 months. In 11 animals, electrophysiology was used to look for evidence of functional reconnection of regenerating dorsal root fibers. However, electrical stimulation of lesioned dorsal roots failed to evoke detectable cord dorsum or field potentials within the spinal cord of any of the animals examined, indicating that reconnection of regenerating fibers with spinal cord neurones had not occurred. In a further 11 rats, immunocytochemical labeling and biotin dextran tracing of afferent fibers in the lesioned roots was used to determine whether regenerating fibers were able to grow into the spinal cord in the presence of an OEC graft. Although a few afferent fibers could be seen to extend for a limited distance into the spinal cord, similar minimal in-growth was seen in control animals that had not been injected with OECs. We therefore conclude that OEC grafts are of little or no advantage in promoting the in-growth of regenerating afferent fibers at the dorsal root entry zone following rhizotomy.

Tracing developing pathways in the brain: a comparison of carbocyanine dyes and cholera toxin b subunit

The present study examined the efficiency of fluorescent carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylinodocarbocyanine perchlorate and cholera toxin B subunit in tracing the crossed tectal projection to the nucleus rotundus of the thalamus (tectorotundal pathways) of paraformaldehyde-fixed and living chick embryos. The tracers were injected into the optic tectum under three experimental conditions (carbocyanine postfix, carbocyanine in vivo, and cholera toxin B subunit in vivo) and the anterograde transport of the nucleus rotundus was monitored and compared. In the carbocyanine postfix method, small crystals of carbocyanine dye were inserted into the tectum of paraformaldehyde-fixed embryos. A 6-month post-insertion period was required to label the crossed tectorotundal pathway. Results showed that tectal neurons did not begin to innervate the ipsilateral nucleus rotundus until embryonic day 9 and the contralateral nucleus rotundus until embryonic day 17. This slow progression of labeling through the crossed tectal projection resulted in significant contrast of the labeling between the ipsilateral and contralateral nuclei rotundus. In the carbocyanine in vivo method, a small volume of carbocyanine dye solution was injected into the tectum of living embryos. A 8- to 12-h survival period was sufficient enough to label the tectorotundal pathway. By embryonic day 8, the labeled axons terminated in the ipsilateral nucleus rotundus and the crossed tectorotundal projection was first detected by embryonic day 10. Similarly, in the cholera toxin B subunit in vivo method, a small volume of cholera toxin B subunit solution was injected into the tectum of living embryos. After a 6- to 10-h survival period, heavily labeled axons were found to innervate bilaterally the nucleus rotundus by embryonic day 8. This appeared to be the earliest schedule for detecting the crossed tectorotundal projection, compared with that of both the postfix and in vivo methods of carbocyanine dye. Based on the differences in the detectability of the crossed tectorotundal projection between the postfix and in vivo methods, the present data suggest that the former method is of limited purpose for labeling tectal collaterals during embryogenesis. Moreover, given the rapid transport rate and absence of photobleaching, which is often seen when using carbocyanine dye, the cholera toxin B subunit in vivo method appears to be the tracer of choice for investigating embryonic pathways.

Differential effects of neurotrophins on neuronal survival and axonal regeneration after spinal cord injury in adult rats

Spinal cord injury (SCI) induces retrograde cell death in descending pathways, which can be prevented by long-term intrathecal infusion of neurotrophins (Novikova et al. [2000] Eur J Neurosci 12:776-780). The present study investigates whether the same treatment also leads to improved regeneration of the injured tracts. After cervical SCI in adult rats, a peripheral nerve graft was attached to the rostral wall of the lesion cavity. The animals were treated by local application into the cavity of Gelfoam soaked in (1) phosphate buffered saline (untreated controls) or (2) a mixture of the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) (local treatment), or by intrathecal infusion of BDNF + NT-3 for (3) 2 weeks (short-term treatment) or (4) 5-8 weeks (long-term treatment). Despite a very strong survival effect, long-term treatment failed to stimulate ingrowth of descending tracts into the nerve graft. In comparison with untreated controls, the latter treatment also caused 35% reduction in axonal sprouting of descending pathways rostral to the lesion site and 72% reduction in the number of spinal cord neurons extending axons into the nerve graft. Local and short-term treatments neither prevented retrograde cell death nor enhanced regeneration of descending tracts, but induced robust regeneration of spinal cord neurons into the nerve graft. These results indicate that the signal pathways promoting neuronal survival and axonal regeneration, respectively, in descending tracts after SCI respond differently to neurotrophic stimuli and that efficient rescue of axotomized tract neurons is not a sufficient prerequisite for regeneration.