Biomedical subjects

A R Light

Publications and source records attributed to A R Light.

At least 37 records · Page 2Linked to original sources

PubMed1993

Ultrastructural morphology, synaptic relationships, and CGRP immunoreactivity of physiologically identified C-fiber terminals in the monkey spinal cord.

The spinal cord terminations of two electrophysiologically identified single C-fibers (one identified as a C-nociceptor) were intra-axonally labeled with horseradish peroxidase and analyzed with both light and electron microscopy. Serial section ultrastructural analysis and postembedding immunocytochemical techniques for calcitonin gene-related peptide (CGRP), substance P (SP), and GABA were used to study the synaptology, and neuropeptide content. All C-terminal synapses were in laminae I and II. The terminals sampled (n = 73) from these two C-fibers rarely established glomerular synaptic complexes, but rather, simple terminals, usually measuring 1-4 microns in length and 1-3 microns in diameter. They most often established 1 or 2 (range 1 to 5) quite large asymmetric axodendritic synaptic contacts. Postsynaptic structures included dendritic spines and shafts with and without vesicles. C-terminals were filled with small round synaptic vesicles (45-60 nm) and also contained variable numbers of large dense-core vesicles (LDCVs, 80-110 nm). LDCVs inside identified C-terminals frequently displayed CGRP immunoreactivity. We were unable to detect SP immunoreactivity inside our sample of C-fiber LDCVs. C-terminals were never found postsynaptic to other profiles. Thus, the C-fiber terminals sampled in this study have simple synaptology, do not receive presynaptic control and contain CGRP immunoreactivity. They differ greatly from the terminals of A delta nociceptors studied previously by our group that had glomerular endings, often received presynaptic input and did not contain CGRP immunoreactivity. This suggests the existence of different processing mechanisms, at the level of the first synapse, for nociceptive inputs arriving to lamina I and II through different types of primary afferents.

Animals↗

PubMed1993

Physiological and morphological characteristics of spinal neurons projecting to the parabrachial region of the cat.

Neurons in the lumbosacral, superficial spinal dorsal horn in the cat were recorded extra- and intracellularly, using dorsal root stimulation as a search stimulus. Isolated neurons were tested for antidromic activation from the contra- and ipsilateral parabrachial region. Seventy-one nociceptive-specific neurons, 11 innocuous cooling neurons, and 8 multireceptive neurons were antidromically activated from the lateral parabrachial region. The receptive fields and response properties were typical of other lamina I and lamina II neurons, in that the receptive fields were usually discrete and relatively small, and the responses ranged from sluggish and decrementing to brisk and augmenting with afterdischarge. The conduction velocity to the parabrachial region averaged 3.7 m/sec for the nociceptive-specific neurons, 3.9 m/sec for the innocuous cooling neurons, and 13.5 m/sec for the multireceptive neurons. Intracellularly labeled neurons were mostly medium to large Waldeyer-like neurons in lamina I. Some had axon collaterals that distributed varicosities in laminae I, II, and V. These data indicate that a slowly conducting nociceptive-specific and thermoreceptive pathway exists between the superficial dorsal horn and the parabrachial region at the pontine-midbrain junction.

Afferent Pathways↗

PubMed1992

Serotoninergic medullary raphespinal projection to the lumbar spinal cord in the rat: a retrograde immunohistochemical study.

Classically the raphespinal system has been regarded as a serotoninergic system; inhibition of spinal nociceptive transmission produced by stimulation of the medullary raphe nuclei is mediated partially by spinal serotoninergic receptors. However, recent evidence suggests that the raphe nuclei are not homogeneous populations of serotoninergic cells. The objective of the present study was to re-examine, in the rat, the serotoninergic raphespinal projection to the lumbar spinal cord, and to determine the relative contribution of serotoninergic raphespinal neurons to the total population of raphespinal neurons. Microinjections of wheat-germ agglutinin horseradish peroxidase conjugate coupled to colloidal gold into the lumbar spinal cord resulted in the retrograde labeling of 53% and 59% of the serotoninergic neurons in the raphe nuclei and in the para-raphe zone, respectively. Conversely, 47% and 28% of the retrogradely labeled neurons in the raphe and para-raphe zone, respectively, demonstrated serotonin-like immunoreactivity. Thus, contrary to previous reports, the present results suggest 1) that only about half of the serotoninergic neurons in the raphe nuclei and in the surrounding para-raphe zone project to the lumbar spinal cord, and 2) that a large proportion of the neurons in the raphe nuclei (53%) and in the surrounding para-raphe zone (72%) that project to the lumbar spinal cord are not serotoninergic.

Afferent Pathways↗

PubMed1992

The effect of stimulus duration on noxious-stimulus induced c-fos expression in the rodent spinal cord.

C-fos is a proto-oncogene that is expressed within some neurons following depolarization. The protein product, fos, has been proposed as an anatomical marker for neuronal activity following noxious peripheral stimulation. However, the literature on noxious-stimulus induced fos expression contains several puzzling observations on the time course and laminar distribution of neuronal labeling within the spinal cord. This study has analyzed the effect of stimulus duration on the expression of fos-like immunoreactivity (FLI) within the spinal cord of anesthetized rats. In order to examine the time course of fos expression following brief periods of stimulation, we required a type of stimulus that was intense enough to activate nociceptors but that did not produce tissue damage. We have therefore employed pulsed, high intensity electrical stimulation, with stimulus durations ranging from 3 s to 24 h. The results indicate that stimulus duration has a profound effect upon the number of labeled cells, the intensity of neuronal labeling, the laminar pattern of FLI, and the time course of fos expression. Brief stimulation periods induce relatively few and relatively lightly labeled neurons, located predominantly within the most superficial laminae of the dorsal horn. Maximal immunoreactivity appears approximately 2 h after stimulation has ceased, and disappears within hours. Continuous stimulation produces many more labeled cells, darker labeling, and FLI within both dorsal and ventral laminar regions. Maximal FLI is seen after approximately 4.5 h of continuous stimulation, with reduction in the number of labeled cells thereafter. These data indicate that the results of any study employing c-fos as a marker for neuronal activity may be affected by the duration of the exciting stimulus.

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PubMed1992

Synaptic interactions between GABA-immunoreactive profiles and the terminals of functionally defined myelinated nociceptors in the monkey and cat spinal cord.

This study analyzes the synaptic interactions between the central terminals of A delta high threshold mechanoreceptors (A delta HTMs) and GABA-immunoreactive profiles. A delta HTM primary afferents from three monkeys and one cat were electrophysiologically identified and intracellularly labeled with HRP, and their terminal arborizations in laminae I and II of the sacrocaudal spinal cord were studied at the ultrastructural level. GABA-immunoreactive profiles in relation to A delta HTM terminals were demonstrated using postembedding colloidal gold techniques. Monkey A delta HTM terminals (n = 131) usually constituted the central element of synaptic glomeruli; they established large asymmetric synaptic contacts with 1-13 dendrites (modal value 2-4) and were surrounded by 0-6 peripheral axon terminals (modal value 2-3). The large majority (around 85%) of the peripheral axon terminals were GABA immunoreactive. They were found presynaptic to the A delta HTM terminal and/or to dendrites postsynaptic to the primary afferent terminal. Furthermore, all peripheral axon terminals found presynaptic to the A delta HTM terminals showed GABA immunoreactivity. Within a single A delta HTM fiber, this synaptic arrangement was found in 20-60% of its boutons. In addition, 28% of the postsynaptic dendritic profiles displayed weak GABA immunoreactivity. Some of them contained vesicles; however, only in a few cases did we observe synapses between a GABA-immunoreactive vesicle-containing dendrite and a dendritic profile postsynaptic to an A delta HTM terminal. Similar synaptology and interactions with GABA-immunoreactive profiles were displayed by the terminals of the single cat A delta HTM fiber studied. Our data support the hypothesis that GABA-containing neurons use both presynaptic and/or postsynaptic mechanisms to exert a powerful control, presumably inhibitory, over the transmission of nociceptive information between A delta HTM afferents and second-order neurons in monkey and cat spinal cord. Our results also imply that GABA may be released within the synaptic glomeruli formed by A delta HTM terminals either by local dendrites or by axon terminals. We discuss the possibility that these GABAergic synapses can be driven by inputs from both primary afferents and/or descending systems to modulate the transmission of nociceptive sensory information.

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PubMed1991

The terminations of single, physiologically identified, somatosensory, corticospinal tract axons in the lumbar spinal cord of the cat.

1. Two hundred and twelve corticospinal axons were identified by stimulation in the hindlimb representation in area 3b of the somatosensory cortex and were recorded in the left dorsolateral funiculus of the spinal cord of the cat. The mean conduction velocity was 38 m/s, range 9-113 m/s. 2. Electrical stimulation of the receptive field evoked discharge in corticospinal axons with a mean latency of 36 ms (range 9-100 ms). 3. One hundred nine of the 212 recorded axons were successfully intra-axonally labeled by iontophoretic injection of horseradish peroxidase, with the mean length of labeled axon being 4.8 mm. Seventy-three of the labeled axons issued no collaterals, and 36 issued at least one labeled collateral into the spinal gray matter along the labeled portion of the parent axon. 4. Most labeled axons issued only one labeled collateral per spinal cord segment. Fourteen collaterals from 10 units were labeled well enough to permit reconstruction of their terminal arborizations. 5. Most terminal collaterals were oriented rostrocaudally and terminated in laminae V, VI, and VII. Most collaterals terminated within large mediolateral extents of the gray matter with no apparent topographic organization. 6. No collaterals terminated in laminae I or II or within the motoneuron pools, and no apparent correlation was found between their anatomic and physiological characteristics.

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PubMed1990

Effects of iontophoresed opioids on physiologically characterized laminae I and II dorsal horn neurons in the cat spinal cord.

The objective of the present study was to determine in vivo the effects of opioids applied locally via microiontophoresis on physiologically characterized laminae I and II dorsal horn neurons in the cat spinal cord. Experiments were performed on pentobarbital-anesthetized or decerebrate, spinalized cats. The effects of morphine (MOR), [D-Ala2,methyl-Phe4, Gly-ol5]enkephalin (DAGO) and naloxone (NALO) on spontaneous- and D,L-homocysteic acid-evoked unit activity were examined for 94 laminae I and II dorsal horn neurons. MOR, DAGO and NALO produced mixed effects (i.e., excitation or inhibition) on unit activity; however, the majority of cells examined (67%) were inhibited. Whether MOR, DAGO or NALO exerted excitatory or inhibitory influences on unit activity did not depend on the modality of the neuron (66% of high threshold units, 56% of multireceptive units and 69% of low threshold units were inhibited; some cells of each modality also were excited). NALO also was found rarely to be neutral; 75% of the cells examined were inhibited and 16% excited by NALO. Since NALO was found to itself have significant effects on laminae I and II dorsal horn unit activity, the ability to use NALO as an opioid antagonist to MOR and DAGO was confounded.

Animals↗

PubMed1990

Electrical stimulation in the medullary nucleus raphe magnus inhibits noxious heat-evoked fos protein-like immunoreactivity in the rat lumbar spinal cord.

Noxious heat applied to the footpad evokes the expression of fos protein-like immunoreactivity in the rat spinal cord lumbar dorsal horn. Electrical stimulation in the medullary nucleus raphe magnus (NRM) while not evoking fos-like immunoreactivity itself, reduced significantly the number of neurons (to 50% of control) in the dorsal horn demonstrating fos protein-like immunoreactivity in response to noxious heating of the footpad. Thus descending projections from the medullary NRM appear to have direct inhibitory effects on dorsal horn neurons that receive nociceptive input from cutaneous thermal nociceptors.

Animals↗

PubMed1990

Termination patterns of serotoninergic medullary raphespinal fibers in the rat lumbar spinal cord: an anterograde immunohistochemical study.

Electrical and chemical stimulation given in the ventral medullary raphe nuclei inhibits spinal nociceptive reflexes and spinal nociceptive transmission; serotoninergic receptors have been demonstrated to partially mediate that inhibition. In the present study, the termination patterns of raphespinal fibers in the rat lumbar spinal cord demonstrating serotonin-like immunoreactivity were examined by using the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in combination with immunohistochemistry. Fibers and terminations from the ventral medullary raphe nuclei (raphe magnus and raphe pallidus) demonstrating both PHA-L- and serotonin-like immunoreactivity were identified in all laminae of the dorsal horn and the ventral horn. Networks of large fibers, characterized by large boutons, and which did not demonstrate serotonin-like immunoreactivity, were identified in deeper laminae of the dorsal horn. The heterogeneous morphology of raphespinal fibers identified in the dorsal horn suggests that these fibers also may be heterogeneous in neurochemistry and function. Medial medullary sites outside the raphe nuclei were found to innervate the ventral horn and all laminae of the dorsal horn, with the exception of lamina I. Descending fibers and terminations also demonstrating serotonin-like immunoreactivity were identified in deep laminae (III, IV, V, VI) of the dorsal horn and in the ventral horn. Similarly, large fiber networks were identified which did not demonstrate serotonin-like immunoreactivity.

Animals↗

PubMed1989

Intraspinal course of descending serotoninergic pathways innervating the rodent dorsal horn and lamina X.

Although numerous studies have discussed serotoninergic pathways projecting from the brainstem to the spinal cord, the intraspinal course of these descending axons remains a matter of debate. One of the difficulties with traditional immunohistochemical methods is that the interpretation of results has been subjective. We have therefore employed a computer-assisted image-processing system to provide objective measurements of serotonin immuno-reactivity within the rodent dorsal horn and lamina X caudal to unilateral thoracic cordotomy in order to establish the intraspinal course of descending serotoninergic fibers. Lesion analysis combined with computer-determined optical density measurements, high-power microscopic examination, and camera lucida drawings demonstrated that the predominant serotoninergic pathway terminating within the dorsal horn courses unilaterally within the dorsal portion of the dorsolateral funiculus. The thoracic dorsal horn may receive a slightly greater contribution from the contralateral side than the lumbar dorsal horn. Lamina X receives serotoninergic innervation both from fibers descending within the ipsilateral dorsolateral funiculus and from more ventrally located ipsilateral pathways, and it also contains serotonin derived from other sources.

Animals↗

PubMed1989

Synaptic ultrastructure of functionally and morphologically characterized neurons of the superficial spinal dorsal horn of cat.

Recordings of neuronal unitary discharges evoked by primary afferent input were made in the superficial part of the spinal cord's dorsal horn, the marginal zone and substantia gelatinosa (also known as laminae I and II), using fine micropipette electrodes filled with HRP. After physiological characterization with respect to primary afferent input, HRP was injected intracellularly iontophoretically into the recorded neuron. Following histochemical processing, the neurons so delineated were studied at the light and electron microscopic levels. No clear relationship between function and either general cellular configuration or synaptic ultrastructure appeared in these analyses, although the concentration of dendritic distribution could be related to the nature of primary afferent excitation. Nocireceptive cells had dendrites mostly branching and ending in lamina I and IIo, while the dendrites of innocuous mechanoreceptive cells arborized principally in lamina II and III. Glomerular synaptic complexes (large, complex arrays of axonic and dendritic profiles with synaptic interconnections) were found to contact a few neurons of both the nocireceptive and mechanoreceptive classes. All neurons received large numbers of simple axonic contacts (small axonic boutons with only 1 or 2 synaptic contacts with a single postsynaptic profile). A degree of specificity in the presynaptic articulations appeared to be reflected by the observations that (1) nocireceptive neurons were never found to receive synaptic contacts from boutons which resembled the known ultrastructure of peripheral innocuous mechanoreceptors, and (2) mechanoreceptive neurons were never seen to receive synaptic contacts from boutons which resembled the known ultrastructure of primary afferent nocireceptors. The axons of the labeled neurons of both nocireceptive and mechanoreceptive classes terminated in simple axonic synapses. All classes of neurons participated in dendrodendritic contacts; however, only some mechanoreceptive neurons had dendrites containing vesicles that were presynaptic to other profiles. No nocireceptive neurons, regardless of gross configuration, were found to have vesicles in their dendrites, but 3 nocireceptive neurons received synapses from presynaptic dendritic profiles.

Afferent Pathways↗

PubMed1988

Direct projection of the corticospinal tract to the superficial laminae of the spinal cord in the rat.

The anterograde transport of both wheat germ agglutinin conjugated to horseradish peroxidase and the kidney bean lectin Phaseolus vulgaris leucoagglutinin was utilized to investigate the projection of primary sensorimotor corticospinal tract axons to the superficial laminae of the spinal dorsal horn in the rat. Both methods yielded qualitatively similar patterns of connectivity. Corticospinal tract axons were found to terminate within all laminae on the side contralateral to the injection site. Labeling was most dense within laminae III and IV and medial portions of laminae I, II, and V in the cervical and lumbar enlargements. Labeling in the ventral horn, though present, was relatively less dense. P. vulgaris leucoagglutinin-labeled axons within laminae I and II exhibited boutons en passant and terminaux; many of these axons also terminated or were collaterals of axons that terminated in deeper dorsal horn laminae. Results are discussed with reference to the somatotopic organization of the spinal cord and to a possible role for the cortex in the modulation of nociception within the spinal cord.

Animals↗

PubMed1988

Morphology and ultrastructure of physiologically identified substantia gelatinosa (lamina II) neurons with axons that terminate in deeper dorsal horn laminae (III-V).

In order to determine their local circuit function, we have examined physiologically characterized, intracellularly labeled neurons in laminae I and II with light and electron microscopes. Single neurons in the spinal substantia gelatinosa (lamina II) of the cat and monkey were recorded intracellularly and characterized physiologically. Following characterization, the neurons were labeled with horseradish peroxidase that was iontophoretically ejected from the recording micropipette. After fixation and sectioning, histochemical reaction allowed visualization of the neuron soma, dendrites, and axon. The four nociceptive neurons reported here (three from cats and one from a monkey) had axons that distributed terminal collaterals to deeper laminae of the spinal cord, including laminae III, IV, and V. Electron microscopy of the axons demonstrated that the parent axons were myelinated and that the terminal collaterals established synaptic contact with neurons in the deeper laminae. These results suggest that some substantia gelatinosa neurons relay nociceptive information to neurons in deeper regions of the spinal dorsal horn via myelinated axons.

Animals↗

PubMed1988

Normal anatomy and physiology of the spinal cord dorsal horn.

The dorsal horn of the spinal cord receives afferent input from innocuous primary afferent neurons via collaterals from the dorsal columns. This input is integrated and relayed primarily by neurons in laminae III-VI. Dorsal horn neurons which encode innocuous inputs project to the medulla and the cervical spinal cord via the dorsal columns and the dorsolateral funiculus. Nociceptive primary afferent neurons enter the spinal dorsal horn via collaterals from Lissauer's tract. Nociceptive input is integrated and relayed by neurons in laminae I, II and V which project to the reticular formation and thalamus via the anterolateral tract.

Action Potentials↗

PubMed1986

The effects of focal stimulation in nucleus raphe magnus and periaqueductal gray on intracellularly recorded neurons in spinal laminae I and II.

Single neurons in spinal laminae I and II of cats were recorded intracellularly while stimulating in nucleus raphe magnus (NRM) and periaqueductal gray (PAG) with monopolar tungsten microelectrodes. Brain stem stimulation inhibited about one-half of the nociceptive-specific neurons, whereas the other half was unaffected. Brain stem stimulation inhibited about one-half of the multireceptive neurons, but the other half was excited and then inhibited. Brain stem stimulation inhibited about one-third of the low-threshold neurons, one-half was excited then inhibited, and one-fifth showed no effect. In all classes of neurons, the inhibition was produced by an inhibitory postsynaptic potential (IPSP) that began with a latency of approximately 25 ms and lasted approximately 400 ms following a single stimulus. The IPSP occurred with a small conductance increase and was reversed by hyperpolarizing currents applied to the cell. These data indicate that NRM and PAG modulated laminae I and II neurons via a postsynaptic mechanism. The conduction velocity of this descending pathway was calculated to range from 6.1 to 66.6 m/s with an average of 13.8 m/s. These data also indicate heterogeneity in the pathway, since some neurons were inhibited, whereas other neurons were excited then inhibited by descending stimulation. Finally, these data indicate specificity in these descending pathways since nearly one-half of neurons that had low-threshold inputs were excited by brain stem stimulation, whereas nearly all nociceptive-specific neurons were either inhibited or unaffected.

Action Potentials↗

PubMed1986

Intracellular staining study of the feline cuneate nucleus. II. Thalamic projecting neurons.

Morphological and physiological features of thalamic projecting neurons in the middle region of the cuneate nucleus of cats (from obex to 4 mm below it) have been studied, using intracellular recording and iontophoresis of horseradish peroxidase. All cuneothalamic neurons in the present sample) responded to movement of hairs on wrist, paw, or digits. However, approximately 50% of the neurons could be activated by other types of stimulation (e.g., light or maintained pressure on the skin, movement of claws, etc.). No clear differences were apparent in the physiological responses correlated with the varied dendritic pattern of stained neurons. Dendritic arborizations of most cuneothalamic neurons were more extensive than assumed previously, from Golgi impregnated material. As a consequence, only a few neurons have dendrites ramifying within a restricted region--i.e., corresponding to a typical cluster of the middle cuneate nucleus. Dendrites extending in various directions and spanning a distance up to 500 microns provide cuneothalamic neurons with the ability to receive input from relatively widespread areas. Collateral branches of axons of cuneothalamic neurons were observed in 50% of the stained neurons. Most of these collaterals terminated ventrally within the cuneate nucleus. Extensive collateral arborizations were observed in the dorsal as well as the ventral cuneate. These results, together with those reported in the previous paper, suggest complex interactions of afferent inputs on cuneothalamic neurons. In particular, such neurons are likely to be influenced by convergent input from different receptor classes and, because of their axonal collaterals, probably affect the excitability of other neurons, projecting or intrinsic, in their immediate vicinity or in other nuclear regions.

Afferent Pathways↗

PubMed1985

The spinal terminations of single, physiologically characterized axons originating in the pontomedullary raphe of the cat.

Single myelinated axons were recorded in the dorsolateral funiculus of the cat and physiologically characterized as descending from the midline medulla or midline pons. Following further physiological characterization (e.g., conduction velocity, adequate stimulus, receptive field, activation by stimulation of periaqueductal gray), the axons were labeled with horseradish peroxidase that was iontophoretically ejected from the recording micropipette. Histochemical reaction allowed visualization of the stained axons and their arborizations in the spinal gray matter. The conduction velocities of the sampled axons ranged from 7.3 to 117.2 m/second with a mean of 35.5 m/second. However, unmyelinated axons could not be sampled with the technique employed here. Descending axons could be divided into two groups: (1) those which terminated in laminae I, II, V, and X, and (2) those which terminated in laminae V, VII, and X. Axons from both groups had myelinated parent axons, were activated by periaqueductal gray stimulation, and responded to noxious pinch of their receptive field. Terminal collaterals from both groups of axons were generally transversely oriented. These results suggest heterogeneous functions for these descending axons which may include modulation of nociceptive input to higher centers.

Animals↗

PubMed1985

The ultrastructure and synaptic connections of the spinal terminations from single, physiologically characterized axons descending in the dorsolateral funiculus from the midline, pontomedullary region.

Single axons descending in the spinal, dorsolateral funiculus which were directly activated by stimulating in or near nucleus raphe magnus (nRM) in the rostral medulla were iontophoretically injected with horseradish peroxidase (HRP). Labeled axons and terminal arborizations in the spinal cord were demonstrated by diaminobenzidine histochemistry following fixation. Sections were processed for electron microscopy and embedded between coverslips. Following examination with the light microscope, selected boutons were sectioned in ultrathin series and examined with the electron microscope. Two arborization patterns were observed with the light microscope. One terminated dorsally in laminae I, II, V, and X, while the other terminated ventrally in laminae V, VII, and X. At the ultrastructural level, boutons predominantly contacted dendrites and occasionally contacted cell somas. Ventrally projecting axons had boutons characterizable as Gray's type I. Some dorsally projecting axons also had Gray's type I boutons, while other axons had Gray's type II boutons. Thus, these descending axons may be heterogeneous in function. No differences in physiological characteristics were found between axons with Gray's type I boutons versus axons with Gray's type II boutons.

Animals↗