Reduced Ia-afferent-mediated Hoffman reflex in streptozotocin-induced diabetic rats

In addition to reduced nerve conduction velocity, diabetic neuropathic patients often exhibit a reduction in the amplitude of the compound muscle action potential elicited by stimulation of the Ia-afferent-mediated reflex pathway (Hoffman or H wave) that can contribute to diminished or absent tendon reflexes. In contrast to nerve conduction velocity deficits, changes in H-wave amplitudes have not been reproduced in diabetic animal models. Using electrophysiological techniques developed for repeated recordings in individual animals, we report H-wave deficits in streptozotocin (STZ)-treated insulin-dependent diabetic rats. After 4 weeks of diabetes induced by STZ treatment, a 47% reduction in the H-wave amplitude was demonstrated by recording compound muscle action potentials in foot muscles after stimulation of Ia afferents. Interestingly, we also demonstrate that the H-wave amplitude gradually recovers to a 26% deficit after 12 weeks of experimental diabetes. The recovery of the H wave in STZ-treated rats distinguishes this deficit mechanistically from other STZ-induced electrophysiological changes and may model a similar recovery of the H wave reported in diabetic patients.

Consistent repeated M- and H-Wave recording in the hind limb of rats

Sensory and motor conduction velocities calculated from latencies of H reflexes and M waves in rat hind limbs have been used to assess experimental peripheral neuropathy. Amplitudes and latencies vary with recording location, and are seldom assessed directly. Using subcutaneous electrodes on the foot, we recorded consistent M waves and H reflexes while stimulating the sciatic or tibial nerve. The late wave disappeared when dorsal roots were cut, verifying that it was an H reflex. However, stimulus-response characteristics differed from those in humans: (a) the threshold was often higher than for M waves; (b) stimulus intensity eliciting a maximum H-reflex amplitude (Hmax) was often higher than adequate for a maximum M-wave amplitude; and (c) the amplitudes of H reflexes stimulated with intensities supramaximal for the M wave were over 90% of Hmax. H reflexes and M waves recorded repeatedly in rats can be useful in assessing sensory and motor function in models of neuropathy, using amplitudes as well as conduction velocities.

Neuronal injury increases retrograde axonal transport of the neurotrophins to spinal sensory neurons and motor neurons via multiple receptor mechanisms

We investigated the retrograde axonal transport of 125I-labeled neurotrophins (NGF, BDNF, NT-3, and NT-4) from the sciatic nerve to dorsal root ganglion (DRG) sensory neurons and spinal motor neurons in normal rats or after neuronal injury. DRG neurons showed increased transport of all neurotrophins following crush injury to the sciatic nerve. This was maximal 1 day after sciatic nerve crush and returned to control levels after 7 days. 125I-BDNF transport from sciatic nerve was elevated with injection either proximal to the lesion or directly into the crush site and after transection of the dorsal roots. All neurotrophin transport was receptor-mediated and consistent with neurotrophin binding to the low-affinity neurotrophin receptor (LNR) or Trk receptors. However, transport of 125I-labeled wheat germ agglutinin also increased 1 day after sciatic nerve crush, showing that increased uptake and transport is a generalized response to injury in DRG sensory neurons. Spinal cord motor neurons also showed increased neurotrophin transport following sciatic nerve injury, although this was maximal after 3 days. The transport of 125I-NGF depended on the expression of LNR by injured motor neurons, as demonstrated by competition experiments with unlabeled neurotrophins. The absence of TrkA in normal motor neurons or after axotomy was confirmed by immunostaining and in situ hybridization. Thus, increased transport of neurotrophic factors after neuronal injury is due to multiple receptor-mediated mechanisms including general increases in axonal transport capacity.

Axotomy upregulates the anterograde transport and expression of brain-derived neurotrophic factor by sensory neurons

In addition to the known retrograde transport of neurotrophins, it is now evident that endogenous brain-derived neurotrophic factor (BDNF) is transported in the anterograde direction in peripheral and central neurons. We used a double-ligation procedure that distinguishes between anterograde and retrograde flow to quantify the anterograde transport of endogenous neurotrophins and neuropeptides in the peripheral nervous system before and after axotomy. BDNF accumulation proximal to the ligation (anterograde transport) was twice that distal to the ligation (retrograde direction). Anterograde transport of nerve growth factor and neurotrophin-3 was not evident. Furthermore, BDNF anterograde transport increased 3.5-fold within 24 hr after sciatic nerve injury or dorsal rhizotomy. Anterograde transport of substance P and calcitonin gene-related peptide decreased after peripheral nerve lesion, demonstrating that there was no generalized increase in anterograde transport. To determine the source of the anterogradely transported BDNF, we performed in situ hybridization in a variety of tissues before and after axotomy. Expression of BDNF mRNA in proximal nerve segments did not change with treatment, showing that the increased accumulation of BDNF was not a result of increased local synthesis. BDNF mRNA and protein were expressed by dorsal root ganglion sensory neurons but not by motor neurons. BDNF mRNA expression was increased 1 d after nerve injury, and BDNF protein was also increased twofold to threefold, suggesting that sensory neurons are the major contributing source of the increased BDNF traffic in the sciatic nerve. Our results suggest that increased anterogradely transported BDNF plays a role in the early neuronal response to peripheral nerve injury at sites distal to the cell body.

Physiological characterization of Taxol-induced large-fiber sensory neuropathy in the rat

The cancer chemotherapeutic agent Taxol (paclitaxel) causes a dose-related peripheral neuropathy in humans. We produced a dose-dependent large-fiber sensory neuropathy, without detrimental effects on general health, in mature rats by using two intravenous injections 3 days apart. Tests of other dosing schedules demonstrated the dependence of the severity of the neuropathy and of animal health on both the dose and the frequency of dosing. Pathologically, severe axonal degeneration and hypomyelination were observed in sections of dorsal roots, whereas ventral roots remained intact. Electrophysiologically, H-wave amplitudes in the hindlimb and amplitudes of predominantly sensory compound nerve action potentials in the tail were reduced. These effects persisted for at least 4 months after treatment. Motor amplitudes were not affected, but both motor and sensory conduction velocities decreased. The ability of rats to remain balanced on a narrow beam was impaired, indicating proprioceptive deficits. Muscle strength, measured by hindlimb and forelimb grip strength, and heat nociception, measured by tail-flick and hindlimb withdrawal tests, were not affected by Taxol. This model of Taxol-induced neuropathy in mature rats, with minimal effects on general health, parallels closely the clinical syndrome observed after Taxol treatment in humans.

Time course of degenerative and regenerative changes in the dorsal horn in a rat model of peripheral neuropathy

The time course of histochemical changes in the dorsal horn of rats subjected to an experimental peripheral neuropathy has been examined. Qualitative and quantitative analyses of the changes in dorsal horn staining were made for soybean agglutinin (SBA)-binding glycoconjugates, the soluble lectins RL-14.5 and RL-29, the growth-associated protein (GAP)-43, and the neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP). These analyses were made at various time points after chronic constriction of the sciatic nerve. Quantitative analysis indicated that staining density increased in the normal territories stained for SBA-binding glycoconjugates, RL-14.5, RL-29, and GAP-43 on the neuropathic side compared with the control side. In addition, there was an extension of the territories stained for SBA-binding glycoconjugates and RL-29 ipsilateral to the injury. The peak increases occurred at 14 or 28 days, followed by a decrease toward control levels by 70 days. In contrast, the staining density for SP in the ipsilateral dorsal horn decreased at 3 and 5 days and reached a peak decrease at 14 days. Then, the staining for SP returned toward control values. The staining for CGRP was unchanged at all time points examined. Dorsal rhizotomies ipsilateral to the nerve injury in neuropathic rats indicated that the increases in staining were attributable to changes in primary afferent neurons. These data suggest that peripheral neuropathy causes complex degenerative and regenerative changes in the central branches of primary afferent neurons. The associated synaptic reorganization may contribute to the sensory abnormalities that accompany peripheral neuropathy.

Neurotrophin-3 administration attenuates deficits of pyridoxine-induced large-fiber sensory neuropathy

Chronic treatment of adult rats for 2-3 weeks with high doses of pyridoxine (vitamin B6) produced a profound proprioceptive loss, similar to that found in humans overdosed with this vitamin or treated with the chemotherapeutic agent cisplatin. Pyridoxine toxicity was manifest as deficits in simple and precise locomotion and sensory nerve function and as degeneration of large-diameter/large-fiber spinal sensory neurons. As assessed quantitatively in a beam-walking task and by EMG recording of H waves evoked by peripheral nerve stimulation, coadministration of the neurotrophic factor neurotrophin-3 (NT-3; 5-20 mg . kg-1 . d-1, s.c.) during chronic pyridoxine treatment largely attenuated the behavioral and electrophysiological sequelae associated with pyridoxine toxicity. Furthermore, NT-3 administration prevented degeneration of sensory fibers in the dorsal column of the spinal cord. These data are consistent with the evidence that NT-3 is a target-derived neurotrophic factor for muscle sensory afferents and suggest that pharmacological doses of NT-3 may be beneficial in the treatment of large-fiber sensory neuropathies.

Intracellular Ca2+ stores in chick cerebellum Purkinje neurons: ontogenetic and functional studies

The molecular composition of intracellular Ca2+ stores in developing chicken cerebellum Purkinje neurons from embryonic day 11 (E11) to posthatching day 2 (P2) was studied by immunocytochemistry using specific antibodies for three molecular constituents, the receptor (R) and/or channel sensitive to inositol 1,4,5-trisphosphate (IP3), Ca(2+)-adenosinetriphosphatase (ATPase), and calsequestrin (CS). CS, IP3R, and Ca(2+)-ATPase were first detected by light-microscopic immunofluorescence in migrating Purkinje cells at E11-E12 and throughout late phases of embryonic development. Ontogenesis of CS, IP3R, and Ca(2+)-ATPase accompanied well-defined stages of cerebellum histogenesis and cytogenesis and was accomplished before hatching. High-resolution immunogold electronmicroscopy revealed that, at E18-P1, CS was still largely distributed to the endoplasmic reticulum (ER) lumen and began to be segregated to ER subcompartments (calciosomes) only by P2, whereas the IP3R was concentrated into ER cisternal stacks as early as E18. Both ionotropic and metabotropic plasma membrane receptors were present in dissociated single chicken Purkinje cells from E16 onward, as indicated by measurements of membrane currents (whole cell recording mode) and of cytoplasmic Ca2+ transients monitored with the cell-trappable fluorescent indicator fura 2-acetoxymethyl ester, respectively. Cytoplasmic Ca2+ transients were detected after either activation of glutamate metabotropic receptors, i.e., evidence of IP3-sensitive Ca2+ channels, or application of caffeine, i.e., evidence of ryanodine-sensitive Ca2+ channels. Intracellular Ca2+ stores appear to be functional during embryonic development.

The efferent projections of the periaqueductal gray in the rat: a Phaseolus vulgaris-leucoagglutinin study. I. Ascending projections

This study has examined the ascending projections of the periaqueductal gray in the rat. Injections of Phaseolus vulgaris-leucoagglutinin were placed in the dorsolateral or ventrolateral subregions, at rostral or caudal sites. From either region, fibers ascended via two bundles. The periventricular bundle ascended in the periaqueductal and periventricular gray matter. At the posterior commissure level, this bundle divided into a dorsal component that terminated in the intralaminar and midline thalamic nuclei, and a ventral component that supplied the hypothalamus. The ventral bundle formed in the deep mesencephalic reticular formation and supplied the ventral tegmental area, substantia nigra pars compacta, and the retrorubral field. The remaining fibers were incorporated into the medial forebrain bundle. These supplied the lateral hypothalamus and forebrain structures, including the preoptic area, the nuclei of the diagonal band, and the lateral division of the bed nucleus of the stria terminalis. The dorsolateral subregion preferentially innervated the centrolateral and paraventricular thalamic nuclei and the anterior hypothalamic area. The ventrolateral subregion preferentially innervated the parafascicular and central medial thalamic nuclei, the lateral hypothalamic area, and the lateral division of the bed nucleus of the stria terminalis. Although the dorsolateral and ventrolateral subregions gave rise to differential projections, the projections from both the rostral and caudal parts of either subregion were similar. This suggests that the dorsolateral and ventrolateral subregions are organized into longitudinal columns that extend throughout the length of the periaqueductal gray. These columns may correspond to those demonstrated in recent physiological studies.

Distribution of the postsynaptic dorsal column projection in the cuneate nucleus of monkeys

Cells in the spinal cord that are postsynaptic to primary afferent fibers project to the dorsal column nuclei in the postsynaptic dorsal column pathway. The projection of cells in the cervical spinal cord of monkeys to the cuneate nucleus has been reported to avoid pars rotunda of that nucleus, the part that contains the somatotopic representation of the ipsilateral hand. We used the sensitive anterograde tracer Phaseolus vulgaris leucoagglutinin to reexamine this projection. We made multiple iontophoretic injections into the cervical enlargements of three monkeys (two Macaca fascicularis and one Macaca mulatta). Control injections were made in the contralateral dorsal columns of one of these and in the dorsal roots of a fourth animal (M. fascicularis) to test for transport by fibers of passage. After 28-39 days, the animals were deeply anesthetized and perfused, and the tissue was processed for immunohistochemical detection of the label. In all cases (excluding control injections), labeled fibers and varicosities were distributed widely in the ipsilateral cuneate and external cuneate nuclei, including pars rotunda. The dorsal column nuclei ipsilateral to control injections contained no label or only very few poorly labeled fibers, indicating that labeling through fibers of passage did not contribute importantly to the results. This study indicates that the postsynaptic projection to the cuneate nucleus is widespread and includes pars rotunda. Such projections may contribute to transmission of information originating in nociceptors through the dorsal column-medial lemniscal system to the ventrobasal thalamus.

Responses of neurons in the gracile nucleus of cats to innocuous and noxious stimuli: basic characterization and antidromic activation from the thalamus

1. Responses to innocuous and noxious mechanical and thermal stimuli were recorded from 90 neurons in the gracile nucleus of anesthetized cats. Cells were tested by antidromic activation for projections to the contralateral ventrobasal thalamus. 2. Cells were characterized broadly by their responses to mechanical stimuli as 1) responding only to tapping (16%), 2) fast-adapting to low-intensity mechanical stimuli (33%), or 3) slowly adapting (51%; most with a fast-adapting component to their responses). All fast-adapting cells and those slowly adapting cells that were tested with noxious heat were further categorized on the basis of their patterns of firing and responses to stimuli. These plus the tap-responsive cells comprised a more restricted sample of 76 categorized cells. 3. Many (22) slowly adapting cells responded to noxious heat (69% of tested slowly adapting cells; 29% of all categorized cells), either on the first application (9 cells) or after sensitization (13 cells), indicating input originating in nociceptors. Nearly all of these (21) responded more to intense pressure than to innocuous pressure. The majority of slowly adapting cells not responsive to noxious heat (5 of 8) or not tested with it (8 of 12) also responded more to intense than to innocuous pressure, suggesting possible input originating in nociceptors. Most cells that responded to noxious heat also had both rapidly and slowly adapting responses with low thresholds. Many were recorded in the range of the cluster region of the gracile nucleus. 4. Cells antidromically activated from the thalamus projected to the rostral part of the ventral posterior lateral nucleus, regardless of their physiological category, and included many with nociceptive input. Latencies of antidromic activation were shorter at more caudal locations in the gracile nucleus, indicating higher conduction velocities to the thalamus. Responses of antidromically activated cells to low-intensity phasic stimuli tended to be greater than those of cells not antidromically activated. 5. Background activity of the neurons was low, most firing at less than one spike/s. Antidromically activated cells had higher background activity than cells not antidromically activated. 6. The results indicate a greater proportion and more widespread distribution of cells with nociceptive input in the cat gracile nucleus than has been previously recognized. Many of these projected to the ventrobasal thalamus, showing that information originating in nociceptors can reach the thalamus through a dorsal column-medial lemniscal pathway in cats.

Changes in lectin, GAP-43 and neuropeptide staining in the rat superficial dorsal horn following experimental peripheral neuropathy

The density and distribution of reactivity for two lectins (soybean agglutinin (SBA) and RL-29), growth associated protein-43 (GAP-43) and the neuropeptides substance P and calcitonin gene-related peptide were analyzed in the spinal cord dorsal horn of rats with an experimental peripheral neuropathy. Twenty-eight days postsurgery, the density of label for RL-29 and GAP-43 was increased in laminae I and II on the experimental compared to the control side. In contrast, the density of neuropeptide label was decreased in the same region. Furthermore, on the experimental side, the distribution of both SBA and RL-29 reactivity was increased, extending into lamina III. We hypothesize that the increases in density and distribution of reactivity for the lectins and GAP-43, as well as the decreases in neuropeptide reactivity, reflect injury-induced regenerative changes in primary afferent terminals.

Physiological characterization of spinohypothalamic tract neurons in the lumbar enlargement of rats

1. Ninety-six neurons in the lumbar enlargement of urethananesthetized rats were antidromically activated from the contralateral hypothalamus. The antidromic stimulating electrode was moved systematically within the hypothalamus until antidromic activation could be produced with currents of less than or equal to 50 microA (18.6 +/- 10.8 microA; mean +/- SD). The points at which antidromic activation thresholds were lowest were found in several regions of the hypothalamus but were concentrated in the optic tract and the supraoptic decussation. 2. The recording locations of 79 spinohypothalamic tract (SHT) neurons were marked and recovered. Twenty-nine were located in the superficial dorsal horn (SDH), 42 in the deep dorsal horn (DDH), 4 in the intermediate zone, and 2 in the gray matter surrounding the central canal. Two additional marks were located in the dorsal lateral funiculus (DLF). 3. The responses of 46 SHT neurons were examined during innocuous and noxious mechanical stimulation of their receptive fields. Forty-eight percent of recorded SHT neurons responded to both innocuous and noxious stimuli (wide dynamic range, WDR) and 39% responded only to noxious stimuli (high threshold, HT). Therefore 87% of SHT neurons responded preferentially or exclusively to noxious mechanical stimulation. Nine percent of SHT neurons responded exclusively to innocuous manipulation of joints and muscles. Four percent of SHT neurons responded only to innocuous tactile stimul (low threshold, LT). WDR, HT, and LT neurons were recorded widely throughout the dorsal horn; no relationship was found between the locations of recording sites in the dorsal horn and the response types of the neurons. SHT neurons that responded to stimulation of muscle, tendon, or joint were recorded deep in the gray matter. 4. The effects of heating the receptive fields were determined for 25 SHT neurons. Fourteen (56%) responded to thermal stimuli. Six (43%) of the responsive neurons responded at low frequencies to innocuous warming (38-41 degrees C) but more vigorously to noxious (greater than or equal to 45 degrees C) heating. The other eight responded only to noxious heat. Eighteen percent (3/17) of tested SHT neurons were activated by noxious cooling of their receptive fields. 5. Cutaneous receptive fields of most recorded SHT neurons were small, typically involving areas as small as two or three toes on the ipsilateral hindlimb; the largest receptive fields covered the entire paw. These findings indicate that relatively precise information about the location of innocuous and noxious stimuli is conveyed directly to the hypothalamus by SHT neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Distributions of spinothalamic, spinohypothalamic, and spinotelencephalic fibers revealed by anterograde transport of PHA-L in rats

Fibers projecting from several levels of the spinal cord to the diencephalon and telencephalon were labeled anterogradely with Phaseolus vulgaris leucoagglutinin injected iontophoretically. Labeled fibers in the thalamus confirmed projections previously observed. In addition, many labeled fibers were seen in the hypothalamus and in telencephalic areas not generally recognized previously as receiving such projections. In the hypothalamus, these areas included the lateral hypothalamus (including the medial forebrain bundle), the posterior hypothalamic area, the dorsal hypothalamic area, the dorsomedial nucleus, the paraventricular nucleus, the periventricular area, the suprachiasmatic nucleus, and the lateral and medial preoptic areas. In the telencephalon, areas with labeled fibers included the ventral pallidum, the globus pallidus, the substantia innominata, the basal nucleus of Meynert, the amygdala (central nucleus), the horizontal and vertical limbs of the diagonal band of Broca, the medial and lateral septal nuclei, the bed nucleus of the stria terminalis, the nucleus accumbens, infralimbic cortex, and medial orbital cortex. These results suggest that somatosensory, possibly including visceral sensory, information is carried directly from the spinal cord to areas in the brain involved in autonomic regulation, motivation, emotion, attention, arousal, learning, memory, and sensory-motor integration. Many of these areas are associated with the limbic system.

Evidence that Fluoro-Gold can be transported avidly through fibers of passage

Small iontophoretic injections of the retrograde tracer Fluoro-Gold were restricted to the dorsal columns in the cervical enlargement of 6 rats. Large numbers of neurons were labeled in the lumbosacral dorsal horn in each rat. In the most effective case, more than 1800 neurons were labeled in alternate sections through nine examined segments. Many neurons were also labeled in lumbosacral dorsal root ganglia of all cases. This study, in contrast to previous reports, indicates that Fluoro-Gold can be transported avidly by axons passing through, but not terminating in, injection sites.

Convergence of heterotopic nociceptive information onto neurons of caudal medullary reticular formation in monkey (Macaca fascicularis)

1. Recordings were made in anesthetized monkeys from neurons in the medullary reticular formation (MRF) caudal to the obex. Responses of 19 MRF neurons to mechanical, thermal, and/or electrical stimulation were examined. MRF neurons exhibited convergence of nociceptive cutaneous inputs from widespread areas of the body and face. 2. MRF neurons exhibited low levels of background activity. Background activity increased after periods of intense cutaneous mechanical or thermal stimulation. Nearly all MRF neurons tested failed to respond to heterosensory stimuli (flashes, whistle sounds), and none responded to joint movements. 3. MRF neurons were excited by and encoded the intensity of noxious mechanical stimulation. Responses to stimuli on contralateral limbs were greater than those to stimuli on ipsilateral limbs. Responses were greater to stimuli on the forelimbs than to stimuli on the hindlimbs. 4. MRF neurons responded to noxious thermal stimulation (51 degrees C) of widespread areas of the body. Mean responses from stimulation at different locations were generally parallel to those for noxious mechanical stimulation. Responses increased with intensity of noxious thermal stimulation (45-50 degrees C). 5. MRF neurons responded with one or two peaks of activation to percutaneous electrical stimulation applied to the limbs, the face, or the tail. The differences in latency of responses to stimulating two locations along the tail suggested that activity was elicited by activation of peripheral fibers with a mean conduction velocity in the A delta range. Stimulation of the contralateral hindlimb elicited greater responses, with lower thresholds and shorter latencies, than did stimulation of the ipsilateral hindlimb. 6. Electrophysiological properties of monkey MRF neurons resembled those of neurons in the medullary subnucleus reticularis dorsalis (SRD) in the rat. Neurons in the caudal medullary reticular formation could play a role in processing nociceptive information. Convergence of nociceptive cutaneous input from widespread areas of the body suggests that MRF neurons may contribute to autonomic, affective, attentional, and/or sensory-motor processes related to pain.

Cells of origin of the spinohypothalamic tract in the rat

We recently demonstrated that large numbers of neurons in the spinal cord of rats project directly to the hypothalamus. In the present study, we used the retrograde tracer Fluoro-Gold (FG) to examine this projection more completely. In the first series of studies, we attempted to label the entire population of spinal cord neurons that project to the hypothalamus. Injections that virtually filled the hypothalamus on one side without spreading into any other diencephalic area labeled a large number of neurons (estimated to be more than 9,000 in the case with the most neurons labeled) bilaterally at all levels of the spinal cord. Approximately 60% of the labeled neurons were contralateral to the injection. The greatest number of labeled neurons was found within the deep dorsal horn. Many were also found within the lateral spinal nucleus, the superficial dorsal horn, and the gray matter surrounding the central canal. A small number of labeled cells was located in the intermediate zone and ventral horn of the spinal gray matter. Labeled neurons were distributed bilaterally within the sacral parasympathetic nucleus and trigeminal nucleus caudalis. Injections of FG restricted to the medial hypothalamus labeled neurons within the spinal cord in a distribution similar to that produced by injections that filled the hypothalamus. However, fewer neurons were labeled in the spinal cord (estimated to be more than 6,200) and trigeminal nucleus caudalis. Injections of FG restricted to the lateral hypothalamus also labeled fewer neurons (approximately 3,300) than did injections that filled the hypothalamus. In these cases, also, the pattern of labeled neurons within the spinal cord was similar to that produced by injections within either medial or both medial and lateral hypothalamus. However, few neurons were labeled in the sacral parasympathetic nucleus following injections into the lateral hypothalamus. These findings show the distribution of a large number of spinal cord neurons that project directly to medial or lateral hypothalamic regions that are involved in autonomic, neuroendocrine, and emotional responses to somatosensory stimulation, including painful stimuli.

Postsynaptic dorsal column pathway of the rat. III. Distribution of ascending afferent fibers

The distribution in the dorsal column nuclei (DCn) of post-synaptic dorsal column (PSDC) fibers was examined in rats following injections of Phaseolus vulgaris leucoagglutinin (PHA-L) in the spinal cord. Lemniscal neurons in the DCn were retrogradely labeled in the same animals by injecting the thalamus with Fluoro-Gold. In some experiments, primary afferent fibers were also labeled by injecting dorsal root ganglia with choleragenoid-conjugated HRP. Injections of PHA-L into the cervical enlargement labeled many fibers and varicosities throughout most of the ipsilateral cuneate nucleus. Labeled fibers were also present in the external cuneate and internal basilar nuclei. Injections of PHA-L into thoracic cord labeled fibers and varicosities in the medial cuneate and lateral gracile nuclei, as well as the external cuneate nucleus. Injections into the lumbar enlargement labeled fibers and varicosities throughout most of the gracile nucleus. Injections in sacral cord labeled fibers in the most medial part of the gracile nucleus. Dense labeling of PSDC fibers was found in areas with high densities of retrogradely labeled lemniscal neurons and areas with high densities of primary afferent fibers. In all regions of the DCn and in the external cuneate nucleus, fibers and varicosities labeled for PHA-L were seen in apposition to retrogradely labeled lemniscal cells. The distribution of postsynaptic afferent fibers in the DCn of the rat and its relationship to lemniscal neurons and primary afferent fibers contrast sharply with these features in cats.

Studies of peptidergic input to the lateral spinal nucleus

Lesions were made to interrupt potential sources of peptidergic input to the lateral spinal nucleus (LSn) in rats. Rhizotomies and spinal transections, as well as lesions of the lateral funiculus, failed to reduce immunohistochemical staining for substance P, dynorphin, Met-enkephalin, somatostatin and FMRF-amide in the LSn at lumbar levels. Thus, all examined peptidergic afferent input to the LSn appears to originate locally within the spinal cord.

PHA-L can be transported anterogradely through fibers of passage

Iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) were restricted to the dorsal columns in the spinal cords of 9 rats. In 5 of these, labeling was found in the dorsal column nuclei, indicating that fibers of passage can transport PHA-L. However, labeled fibers were less dense and less intensely stained than those seen following similar injections that involved the dorsal horn.

Direct somatosensory projections from the spinal cord to the hypothalamus and telencephalon

Somatosensory input to the hypothalamus has been thought to ascend via an indirect, multisynaptic pathway. However, we have antidromically identified nociceptive spinal cord neurons that project directly to the lateral hypothalamus in rats. Retrograde tracers injected into the lateral hypothalamus labeled many spinal neurons bilaterally within the marginal zone, the lateral reticulated area, the lateral spinal nucleus, and the area surrounding the central canal. An anterograde tracer injected into these areas of the spinal cord labeled fibers and terminals in the lateral hypothalamus and, surprisingly, in a number of telencephalic areas. These findings demonstrate a direct somatosensory projection from the spinal cord to the hypothalamus and several telencephalic regions.

Postsynaptic dorsal column pathway of the rat. II. Evidence against an important role in nociception

The response characteristics of neurons at the origin of the postsynaptic dorsal column (PSDC) pathway were determined in unanesthetized, decerebrated, spinalized rats. Sixty-four percent of PSDC neurons responded only to innocuous mechanical stimuli. Thirty-six percent responded to innocuous stimuli but were more powerfully activated by noxious pinch. Ninety-three percent of the tested PSDC neurons were not activated by any of several intensities of sustained, repeated noxious heating of their receptive fields. The failure of pinch-responsive PSDC cells to respond to thermal stimulation, even in sensitized skin, suggests that they do not receive a functionally significant input from C polymodal nociceptors, heat nociceptors, or mechanical-heat nociceptors. We conclude, therefore, that the postsynaptic dorsal column pathway is not importantly involved in nociception in the rat.