Rostrocaudal and laminar distribution of spinothalamic neurons in the high cervical spinal cord of the cat

In the cat, retrogradely labelled spinothalamic tract cells were found in contralateral medial laminae VI and lateral laminae VII-VIII and ipsilateral lateral laminae VII-VIII and medial laminae VI of the C1, C2, C3 and C4 segments. The proportion of cells in each ipsilateral laminae was similar in each segment but the proportion in contralateral medial laminae VI decreased while the proportion in contralateral laminae VII-VIII increased caudally.     

Inhibition of primate spinothalamic tract neurons by stimulation in ipsilateral or contralateral ventral posterior lateral (VPLc) thalamic nucleus

The effects of stimulation in the ventral posterior lateral (VPLc) thalamic nuclei on the activity of primate spinothalamic tract neurons were investigated. All 19 cells studied were strongly inhibited by conditioning trains of stimuli delivered to either ipsilateral or contralateral VPLc. Both background discharges and activity evoked by innocuous or noxious cutaneous stimulation were inhibited.     

Identification of spinothalamic tract cells in fresh, unfixed rat spinal cord

Diamidino yellow dihydrochloride (DY . 2HCl) can reliably label spinothalamic tract cells both in the perfused, fixed and in the fresh, unfixed rat spinal cord. The major advantages of DY . 2HCl as a retrograde marker are that it leaks from the cells very slowly and can be conveniently identified in unfixed neurons by fluorescence microscopy. DY . 2HCl will be a useful marker for identification of spinothalamic tract cells that are to be studied in vitro.     

Observations on the synaptology of intracellularly injected spinothalamic tract neurons in the cat

Single spinothalamic tract cells in the deep lumbar group have been injected with horseradish peroxidase and their dendritic trees examined on the electron microscope. Large dendrites are surrounded by rosettes of from 5 to 12 synaptic terminals. Most terminals contain round vesicles but some terminals contain flattened clear vesicles and others contain membranous tubes or cisterns. Large (mean diameter 100 nm) round, dense-core vesicles are found in both terminals containing clear round vesicles and terminals containing flattened vesicles. As yet none of those terminals on STT cell dendrites have been found participating in axo-axonic synapses, suggesting that presynaptic inhibition or facilitation plays no role in modulating the responses of these cells to presynaptic activity.     

Serotonin innervation of physiologically identified lamina I projection neurons

Physiologically characterized lamina I projection neurons, including antidromically activated spinomesencephalic and spinothalamic tract cells, were intracellularly stained with HRP and then processed and examined for serotonin-immunoreactive contacts. We observed cells with both high and low densities of contacts from serotonergic axons. Serotonin contacts were found on both nociceptive-specific and wide-dynamic-range projection neurons. The density of contacts did not appear to correlate with any physiological characteristic.     

TrkB expression and phospho-ERK activation by brain-derived neurotrophic factor in rat spinothalamic tract neurons

Brain-derived neurotrophic factor (BDNF) is a neurotrophin implicated in the phenomena of synaptic plasticity in the adult. It is found in terminals of nociceptive primary afferents. Following a pain-related stimulus, it is released in the spinal cord, where it activates its high-affinity receptor TrkB, leading to the phosphorylation of the mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase (ERK). A large body of evidence suggests that BDNF has a positive neuromodulatory effect on glutamate transmission in the spinal cord. However, none of these studies examined anatomically whether projection neurons known to be involved in transmission of nociceptive inputs express BDNF's receptor. Because the spinothalamic tract (STT) is a well-characterized pathway for its role in the transfer and integration of sensory and nociceptive informations, this study in rats aimed to 1) determine whether neurons of the STT pathway express the TrkB receptor, 2) establish the rostrocaudal and laminar distribution of STT-TrkB neurons in the whole spinal cord, and 3) test the potential functionality of TrkB expression in these cells by investigating the ability of BDNF to activate the MAP kinase ERK. Using tract tracing coupled to immunofluorescent labeling for TrkB, we observed that in all levels of the spinal cord most STT neurons were immunoreactive for TrkB. Furthermore, microinjections of BDNF into the spinal cord or release of endogenous BDNF by intraplantar injection of capsaicin activated ERK phosphorylation in TrkB-containing STT neurons. These data suggest an important role for BDNF in nociception as an activator of spinothalamic projection neurons.     

Effects of capsaicin applied to a peripheral nerve on the responses of primate spinothalamic tract cells

Capsaicin is a neurotoxin that appears to affect unmyelinated nociceptive sensory fibers selectively. We examined the effects of capsaicin applied topically to the sural nerve on peripheral nerve volleys and on the responses of neurons belonging to the spinothalamic tract (STT) in the monkey. The responses examined included those following electrical stimulation of the sural nerve and also those produced by more natural forms of noxious and innocuous stimuli applied to the skin. Capsaicin (1% solution) applied onto the sural nerve for 15 min resulted in a reduction of the sizes of A delta- and C-fiber afferent volleys. These changes paralleled the reduction of A- and C-fiber responses of the STT cells elicited by electrical stimulation of the sural nerve. During capsaicin application onto the sural nerve, the background activity of STT cells increased for 5-10 min. After capsaicin treatment, the responses of STT cells to innocuous mechanical stimuli applied to the cutaneous receptive field were increased, whereas the responses to noxious mechanical stimuli were decreased. However, topical capsaicin application almost eliminated the responses of STT cells to noxious heat stimuli. The results of the present study suggest that topical capsaicin application onto a peripheral nerve produces a transient nociceptive response followed by a decrease in sensitivity to noxious stimuli, particularly to noxious heat. These changes are due to conduction block of the nerve fibers at the site of capsaicin application.     

Comparison of response properties of dorsal and ventral spinocerebellar tract neurons to a physiological stimulus

The response characteristics of dorsal spinocerebellar tract (DSCT) neurons and ventral spinocerebellar tract (VSCT) neurons to the cutaneous inputs applied to footpads were studied in the cat. Three different wave forms were used: step displacement of varying amplitudes (0.1-3.5 mm); constant amplitude ramps with different slopes (5-120 mm/s); and constant amplitude sinusoidal displacements of varying frequencies (1-20 Hz). Both DSCT and VSCT neurons responded phasically to cutaneous stimuli of different wave forms. The phasic responses were related to both the amplitude and velocity of the peripheral stimulus. However, the responses of DSCT neurons were graded over only a very narrow, low range of stimulus intensities, whereas the responses of VSCT neurons were graded over a larger range of skin indentation up to 3 mm. Only the DSCT neurons exhibited some length sensitivity to ramp stimuli, and only DSCT neurons were activated repetitively by periodic stimuli. These results suggest both DSCT and VSCT can transmit exteroceptive information but respond selectively to different features of these stimuli.     

Quantitative analysis of spinothalamic tract neurons in adult and developing mouse

Understanding the development of nociceptive circuits is important for the proper treatment of pain and administration of anesthesia to prenatal, newborn, and infant organisms. The spinothalamic tract (STT) is an integral pathway in the transmission of nociceptive information to the brain, yet the stage of development when axons from cells in the spinal cord reach the thalamus is unknown. Therefore, the retrograde tracer Fluoro‐Gold was used to characterize the STT at several stages of development in the mouse, a species in which the STT was previously unexamined. One‐week‐old, 2‐day‐old and embryonic‐day‐18 mice did not differ from adults in the number or distribution of retrogradely labeled STT neurons. Approximately 3,500 neurons were retrogradely labeled from one side of the thalamus in each age group. Eighty percent of the labeled cells were located on the side of the spinal cord contralateral to the injection site. Sixty‐three percent of all labeled cells were located within the cervical cord, 18% in thoracic cord, and 19% in the lumbosacral spinal cord. Retrogradely labeled cells significantly increased in diameter over the first postnatal week. Arborizations and boutons within the ventrobasal complex of the thalamus were observed after the anterograde tracer biotinylated dextran amine was injected into the neonatal spinal cord. These data indicate that, whereas neurons of the STT continue to increase in size during the postnatal period, their axons reach the thalamus before birth and possess some of the morphological features required for functionality. J. Comp. Neurol. 518:3193–3204, 2010. © 2010 Wiley‐Liss, Inc.     

The dorsolateral corticospinal tract in mice: an alternative route for corticospinal input to caudal segments following dorsal column lesions

In rodents, the main contingent of corticospinal tract (CST) axons descends in the ventral part of the dorsal column. There is, however, a contingent of CST axons that descends in the dorsolateral column (the "dorsolateral corticospinal tract," or DLCST). Here, we define some of the features of the DLCST by tracing CST projections following injections of biotinylated dextran amine into the sensorimotor cortex, assessing the distribution of DLCST axons and terminal arborizations in intact mice and in mice in which the main contingent of CST axons in the dorsal column had been transected. Axons of the DLCST diverge from the main tract at the pyramidal decussation, gather in fascicles in the dorsolateral gray matter below the spinomedullary junction, and project in a gradual trajectory laterally toward the dorsolateral column over the first few cervical segments. DLCST axons then project along the dorsolateral column to sacral levels, giving rise to collaterals that project into the gray matter. Labeled DLCST axons were most abundant in cervical segments, where they were often collected in fascicles, and progressively decreased in number in more caudal segments. Tracing of DLCST axons in mice with selective lesions of the dorsal column revealed that DLCST axons arborize extensively throughout the dorsal and ventral horns and that the overall territory that the DLCST axons invade is similar to the territory innervated by the CST axons in the main tract. Some DLCST axon arbors with varicosities are seen near large neurons in the ventral horn (presumed motoneurons). Substantial numbers of DLCST axons project across the midline to the gray matter on the contralateral side. Thus, the DLCST provides an alternate route for CST input to caudal segments, which is of particular relevance for studies of CST distribution and function following partial spinal cord injuries.     

Responses of rat dorsal horn neurons to natural stimulation and to iontophoretically applied norepinephrine

Extracellular recordings were obtained of 177 neurons throughout the lumbar spinal dorsal horn of urethane- or halothane-anesthetized rats. These neurons all responded to iontophoretically applied L-glutamate and their responses to natural stimulation of the ipsilateral hindlimb were characterized. Iontophoretically applied norepinephrine was tested on 94 of these neurons. Fifty-one neurons were inhibited and 22 were excited. Norepinephrine produced a biphasic inhibitory/excitatory effect on nine neurons. Norepinephrine was exclusively inhibitory on superficial dorsal horn neurons that responded only to innocuous brush and touch and on neurons in the nucleus proprius that responded to brush, touch, and noxious skin pinch. Norepinephrine excited some superficial brush/touch/pinch neurons and produced short inhibitions that were followed by prolonged excitations of some nucleus proprius neurons that responded only to noxious skin pinch. Neurons in the base of the dorsal horn that responded to low-threshold proprioceptive stimulation were excited by norepinephrine. Both the inhibitory and excitatory effects of norepinephrine were stereoselective, but they were not blocked by receptor subtype-selective antagonists. Desensitization to norepinephrine occurred for 30% of the neurons. This study demonstrates that the inhibitory effects of norepinephrine on rat dorsal horn neurons are not restricted to neurons that are responsive to noxious stimuli and that some of these neurons are primarily excited by norepinephrine. The excitatory effects of norepinephrine on low-threshold proprioceptive neurons may contribute to norepinephrine's known enhancement of spinal flexor reflex activity.     

The overlap of spinothalamic and dorsal column nuclei projections in the ventrobasal complex of the rat thalamus: a double anterograde labeling study using light microscopy analysis

Projections from the spinal cord and the dorsal column nuclei (DCN) to the ventrobasal complex of the thalamus (VB) were studied in the rat by using double anterograde labeling strategy. This strategy was based on the injection of 3H-leucine into the DCN and of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the spinal cord and their subsequent transport. Adjacent 30-micron-thick sections were then processed differentially for autoradiography or for HRP by using tetramethyl benzidine (TMB) as a chromogen. Similar areas of the ventrobasal complex were labeled, in adjacent sections, after a large injection of 3H-leucine into the DCN and when wheat germ agglutinin-HRP had been injected in any part of the spinal cord. If, however, a small injection of the radioactive tracer was centered in the gracile nucleus and compared with an injection of WGA-HRP placed in the lumbar enlargement of the cord, the rostral and dorsal portions of the lateral VB were labeled from both sources. On the other hand, if tritiated leucine was injected into the cuneate nucleus, and WGA-HRP placed in the cervical enlargement, then the caudal and ventral portions of the lateral VB demonstrated overlap of both labels. The present results show that, in the rat, areas of termination of both the spinothalamic tract and the lemniscal pathway originating from the DCN overlap in the lateral VB. This overlap is somatotopically organized, thus indicating that the same area of the VB receives somatic inputs from one particular part of the body through both pathways. These results are discussed in comparison to those of comparable studies performed in the cat and in the monkey and with reference to the electrophysiological data that have demonstrated that, in the rat VB, neurons responding to noxious stimulation are intermingled with neurons exclusively responding to non-noxious stimulation.     

Protection of corticospinal tract neurons after dorsal spinal cord transection and engraftment of olfactory ensheathing cells

Transplantation of olfactory ensheathing cells (OECs) into the damaged rat spinal cord leads to directed elongative axonal regeneration and improved functional outcome. OECs are known to produce a number of neurotrophic molecules. To explore the possibility that OECs are neuroprotective for injured corticospinal tract (CST) neurons, we transplanted OECs into the dorsal transected spinal cord (T9) and examined primary motor cortex (M1) to assess apoptosis and neuronal loss at 1 and 4 weeks post-transplantation. The number of apoptotic cortical neurons was reduced at 1 week, and the extent of neuronal loss was reduced at 4 weeks. Biochemical analysis indicated an increase in BDNF levels in the spinal cord injury zone after OEC transplantation at 1 week. The transplanted OECs associated longitudinally with axons at 4 weeks. Thus, OEC transplantation into the injured spinal cord has distant neuroprotective effects on descending cortical projection neurons.     

Distribution of trigeminohypothalamic and spinohypothalamic tract neurons displaying substance P receptor-like immunoreactivity in the rat

Primary afferent neurons containing substance P (SP) are apparently implicated in the transmission of noxious information from the periphery to the central nervous system, and SP released from primary afferent neurons acts on second-order neurons with the SP receptor (SPR). In the rat, nociceptive information reached the hypothalamus not only through indirect pathways but also directly through trigeminohypothalamic and spinohypothalamic pathways. Thus, in the present study, the distribution pattern of trigeminohypothalamic and spinohypothalamic tract neurons showing SPR-like immunoreactivity (SPR-LI) was examined in the rat by a retrograde tract-tracing method combined with immunofluorescence histochemistry for SPR. A substantial number of trigeminal and spinal neurons with SPR-LI were retrogradely labeled with Fluoro-Gold (FG) injected into the hypothalamic regions. These neurons were distributed mainly in lamina I of the medullary and spinal dorsal horns, lateral spinal nucleus, regions around the central canal of the spinal cord, and the lateral aspect of the deep part of the spinal dorsal horn. A number of SPR-LI neurons in the spinal parasympathetic nucleus were labeled with FG injected into the area around the paraventricular hypothalamic nucleus. Some SPR-LI neurons in the lateral spinal nucleus and the lateral aspect of the deep part of the spinal dorsal horn were also labeled with FG injected into the septal region. On the basis of the distribution areas of SPR-LI trigeminal and spinal neurons projecting to the hypothalamic and septal regions, it is likely that these neurons are involved in the transmission of somatic and/or visceral noxious information. J. Comp. Neurol. 378:508–521, 1997. © 1997 Wiley-Liss, Inc.     

The neuropeptide Y Y1 receptor is a somatic receptor on dorsal root ganglion neurons and a postsynaptic receptor on somatostatin dorsal horn neurons.

Using indirect immunofluorescence, neuropeptide Y Y1 receptor (Y1 receptor)-like immunoreactivity (LI) was localized close to the plasmalemma of small neurons in lumbar dorsal root ganglia (DRGs) and neurons in the inner lamina II of the lumbar spinal cord of the rat. Using confocal microscopy, colocalization of Y1 receptor-LI and transferrin receptor-LI, a marker for endosomes and coated vesicles, was observed in dot-like structures along the plasmalemma. Under the electron microscope, Y1 receptor-LI was localized in coated vesicles and endosomes, in the membrane of tubular cisternae, sometimes connected to multivesicular bodies, and in the plasmalemma. These complex distribution patterns may reflect receptor turnover and internalization processes. In the lamina II of the spinal dorsal horn, Y1 receptor-LI was localized in the plasmalemma of neurons without any apparent association with paramembrane structures, as described above for the DRG neurons. Many dendrites were Y1 receptor-positive, and some of them made synaptic contacts with unstained axonal terminals. In general, Y1 receptor-LI was localized in the membrane outside the postsynaptic density. Double-immunofluorescence staining showed that most Y1 receptor-immunoreactive neurons in lamina II contained somatostatin-LI. Both in DRG and dorsal horn neurons, the Y1 receptor thus seems to represent a postjunctional/postsynaptic receptor.     

Calcium-binding proteins, parvalbumin- and calbindin-D 28k-immunoreactive neurons in the rat spinal cord and dorsal root ganglia: a light and electron microscopic study

The distribution of two calcium-binding proteins, parvalbumin (PV) and calbindin-D 28K (CaBP), was studied by the peroxidase-anti-peroxidase immunohistochemical method at the light and electron microscopic level in the rat spinal cord and dorsal root ganglia. The possible coexistence of these two proteins was also investigated. PV-positive neurons were revealed in all layers of the spinal cord, except lamina I, which was devoid of labelling. Most of the PV-positive cells were found in the inner layer of lamina II, lamina III, internal basilar nucleus, central gray region, and at the dorsomedial and ventromedial aspects of the lateral motor column in the ventral horn. Neuronal processes intensely stained for PV sharply delineated inner lamina II. With the electron microscope most of them appeared to be dendrites, but vesicle containing profiles were also found in a smaller number. CaBP-positive neurons appeared to be dispersed all over the spinal gray matter. The great majority of them were found in laminae I, II, IV; the central gray region; the intermediolateral nucleus; and in the ventral horn just medial to the lateral motor column. Laminae I and II were densely packed with CaBP-positive punctate profiles that proved to be dendrites and axons in the electron microscope. A portion of labelled neurons in lamina IV and on the ventromedial aspect of the lateral motor column in the ventral horn disclosed both PV- and CaBP-immunoreactivity. All of the funiculi of the spinal white matter contained a large number of fibres immunopositive for both PV and CaBP. The highest density of CaBP-positive fibres was found in the dorsolateral funiculus, which was also densely packed with PV-positive fibres. PV-positive fibres were even more numerous in the dorsal part of the dorsal funiculus. The territory of the gracile funiculus in the brachial cord and that of the pyramidal tract in its whole extent were devoid of labelled fibres. In the thoracic cord, the dorsal nucleus of Clarke received a large number of PV-positive fibres. Dorsal root ganglia displayed both PV- and CaBP-immunopositivity. The cell diameter distribution histogram of PV-positive neurons disclosed two peaks--one at 35 microns and the other at 50 microns. CaBP-positive cells in the dorsal root ganglia corresponded to subgroups of small and large neurons with mean diameters of 25 microns and 45 microns, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)