Collaterals of spinothalamic cells in the rat

The tracts, cytoarchitecture, and neurochemistry of the spinal cord

The human spinal cord can be described using a range of nomenclatures with each providing insight into its structure and function. Here we have comprehensively reviewed the key literature detailing the general structure, configuration of tracts, the cytoarchitecture of Rexed's laminae, and the neurochemistry at the spinal segmental level. The purpose of this review is to detail current anatomical understanding of how the spinal cord is structured and to aid researchers in identifying gaps in the literature that need to be studied to improve our knowledge of the spinal cord which in turn will improve the potential of therapeutic intervention for disorders of the spinal cord.

Encoding of cutaneous stimuli by lamina I projection neurons

ABSTRACT

Lamina I of the dorsal horn, together with its main output pathway, lamina I projection neurons, has long been implicated in the processing of nociceptive stimuli, as well as the development of chronic pain conditions. However, the study of lamina I projection neurons is hampered by technical challenges, including the low throughput and selection biases of traditional electrophysiological techniques. Here we report on a technique that uses anatomical labelling strategies and in vivo imaging to simultaneously study a network of lamina I projection neurons in response to electrical and natural stimuli. Although we were able to confirm the nociceptive involvement of this group of cells, we also describe an unexpected preference for innocuous cooling stimuli. We were able to characterize the thermal responsiveness of these cells in detail and found cooling responses decline when exposed to stable cold temperatures maintained for more than a few seconds, as well as to encode the intensity of the end temperature, while heating responses showed an unexpected reliance on adaptation temperatures.

How the parcellation theory of comparative forebrain specialization emerged from the Division of Neuropsychiatry at the Walter Reed Army Institute of Research

The Golgi method gave birth to modern neuroscience. The Nauta method, developed in a novel Army think tank at the Walter Reed Army Medical Center, was the next major breakthrough before neuroscience emerged as a separate discipline. Dr. Walle Nauta's (1916-1994) method allowed for the first time the ability to trace interneuronal connections accurately to their termination. The think tank, created by Dr. David Rioch (1900-1985), provided a unique intellectual environment for interdisciplinary neuroscience research, the first of its kind. Rioch hired exceptional senior faculty and recruited outstanding young investigators who were drafted into the Army, typically after finishing their M.D.s or Ph.D.s, and were interested in brain research. Many of these young investigators went on to illustrious careers in neuroscience. I worked with Walle Nauta at a time when the technique was first being applied to nonmammalian vertebrate brains. Along with other Army draftees, I was encouraged to pursue my own research interests. This led me on a quest to understand interspecific variability of connections in relation to evolution and ontogeny of the brain. By 1980, I had found that the variability of all known connections could be explained by a theory to the effect that new structures such as the neocortex were not formed by one system invading another and mingling, as Clarence Luther Herrick (1858-1904) had proposed, but by selective proliferation and differentiation sometimes involving the select loss of connections to reduce cross-modality interference as in the case of the parcellation and differentiation of cortical areas. The resulting parcellation theory predicts that elements of a primordial neocortex existed from the beginning of vertebrate evolution and did not originate by an invasion of nonolfactory modalities into the olfactory lobe, as commonly believed before the introduction of the Nauta method. This theory would not have been created if it were not for the brilliant environment that was Walter Reed in the 1960s.

Post-stroke pain caused by peripheral sensory hypersensitization after transient focal cerebral ischemia in rats

The mechanisms underlying central post-stroke pain are not well understood and there is no satisfactory treatment. Here, in a rat model of stroke, we measured nociceptive threshold using current stimulation of primary afferent neurons in both hind paws. Male Wistar rats underwent middle cerebral artery occlusion (MCAO) for 50 min. Nociceptive thresholds for Aβ, Aδ and C fiber stimulation (at 2000, 250, and 5 Hz, respectively, using a Neurometer), and neurological deficits, were measured for 23 days after MCAO. Sensory thresholds in both hind paws were significantly lower in MCAO model rats than in control rats for 23 days after MCAO, with the greatest difference seen in Aδ fibers and the smallest in C fibers. Brain infarct area was measured histologically, and the correlation between neurological deficit and infarct size was examined. Neurological deficits were worse in animals with larger infarcts. Furthermore, correlations were observed between infarct size, neurological deficit, and sensory threshold of Aδ fibers 1 day after MCAO. These findings indicate that rats develop hyperalgesia after MCAO and that sensory abnormalities in Aδ fibers after cerebral ischemia may play an important role in post-stroke pain.

Anatomical evidence of pruriceptive trigeminothalamic and trigeminoparabrachial projection neurons in mice

Itch is relayed to higher centers by projection neurons in the spinal and medullary dorsal horn. We employed a double-label method to map the ascending projections of pruriceptive and nociceptive trigeminal and spinal neurons. The retrograde tracer fluorogold (FG) was stereotaxically injected into the right thalamus or lateral parabrachial area (LPb) in mice. Seven days later, mice received intradermal (id) microinjection of histamine, chloroquine, capsaicin, or vehicle into the left cheek. Histamine, chloroquine, and capsaicin intradermally elicited similar distributions of Fos-positive neurons in the medial aspect of the superficial medullary and spinal dorsal horn from the trigeminal subnucleus caudalis to C2. Among neurons retrogradely labeled from the thalamus, 43%, 8%, and 22% were Fos-positive following id histamine, chloroquine, or capsaicin. Among the Fos-positive neurons following pruritic or capsaicin stimuli, ∼1-2% were retrogradely labeled with FG. Trigeminoparabrachial projection neurons exhibited a higher incidence of double labeling in the superficial dorsal horn. Among the neurons retrogradely labeled from LPb, 36%, 29%, and 33% were Fos positive following id injection of histamine, chloroquine, and capsaicin, respectively. Among Fos-positive neurons elicited by id histamine, chloroquine, and capsaicin, respectively, 3.7%, 4.3%, and 4.1% were retrogradely labeled from LPb. The present results indicate that, overall, relatively small subpopulations of pruriceptive and/or nociceptive neurons innervating the cheek project to thalamus or LPb. These results imply that the vast majority of pruritogen- and algogen-responsive spinal neurons are likely to function as interneurons relaying information to projection neurons and/or participating in segmental nocifensive circuits.

Effect of spinal manipulation thrust magnitude on trunk mechanical activation thresholds of lateral thalamic neurons

OBJECTIVES

High-velocity low-amplitude spinal manipulation (HVLA-SM), as performed by doctors who use manual therapy (eg, doctors of chiropractic and osteopathy), results in mechanical hypoalgesia in clinical settings. This hypoalgesic effect has previously been attributed to alterations in peripheral and/or central pain processing. The objective of this study was to determine whether thrust magnitude of a simulated HVLA-SM alters mechanical trunk response thresholds in wide dynamic range (WDR) and/or nociceptive specific (NS) lateral thalamic neurons.

METHODS

Extracellular recordings were carried out in the thalamus of 15 anesthetized Wistar rats. Lateral thalamic neurons having receptive fields, which included the lumbar dorsal-lateral trunk, were characterized as either WDR (n=22) or NS (n=25). Response thresholds to electronic von Frey (rigid tip) mechanical trunk stimuli were determined in 3 directions (dorsal-ventral, 45° caudalward, and 45° cranialward) before and immediately after the dorsal-ventral delivery of a 100-millisecond HVLA-SM at 3 thrust magnitudes (control, 55%, 85% body weight).

RESULTS

There was a significant difference in mechanical threshold between 85% body weight manipulation and control thrust magnitudes in the dorsal-ventral direction in NS neurons (P=.01). No changes were found in WDR neurons at either HVLA-SM thrust magnitude.

CONCLUSIONS

This study is the first to investigate the effect of HVLA-SM thrust magnitude on WDR and NS lateral thalamic mechanical response threshold. Our data suggest that, at the single lateral thalamic neuron level, there may be a minimal spinal manipulative thrust magnitude required to elicit an increase in trunk mechanical response thresholds.

Bilateral central pain sensitization in rats following a unilateral thalamic lesion may be treated with high doses of ketamine

BACKGROUND

Central post-stroke pain is a neuropathic pain condition caused by a vascular lesion, of either ischemic or hemorrhagic origin, in the central nervous system and more precisely involving the spinothalamocortical pathway responsible for the transmission of painful sensations. Few animal models have been developed to study this problem. The objectives of this study were to evaluate different modalities of pain in a central neuropathic pain rat model and to assess the effects of ketamine administered at different doses. Animals were evaluated on the rotarod, Hargreaves, Von Frey and acetone tests. A very small hemorrhage was created by injecting a collagenase solution in the right ventral posterolateral thalamic nucleus. Following the establishment of the neuropathy, ketamine was evaluated as a therapeutic drug for this condition.

RESULTS

Histopathological observations showed a well localized lesion with neuronal necrosis and astrocytosis following the collagenase injection that was localized within the VPL. No significant change in motor coordination was observed following surgery in either the saline or collagensae groups. In the collagenase group, a significant decrease in mechanical allodynia threshold was observed. A sporadic and transient cold allodynia was also noted. No thermal hyperalgesia was seen following the collagenase injection. Ketamine was then tested as a potential therapeutic drug. A significant decrease in motor coordination was seen only following the administration of 25 mg/kg of ketamine in both groups. An alleviation of mechanical allodynia was achieved only with the high ketamine dose. The minimal effective ketamine serum concentration (150 ng/mL) was only achieved in animals that received 25 mg/kg.

CONCLUSIONS

An intrathalamic hemorrhage induced a bilateral mechanical allodynia in rats. Cold hyperalgesia was observed in 60% of these animals. Mechanical allodynia was alleviated with high doses of ketamine which corresponded with therapeutic plasmatic concentrations.

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.

Evolution and ontogeny of neural circuits

Recent studies on neural pathways in a broad spectrum of vertebrates suggest that, in addition to migration and an increase in the number of certain select neurons, a significant aspect of neural evolution is a “parcellation” (segregation-isolation) process that involves the loss of selected connections by the new aggregates. A similar process occurs during ontogenetic development. These findings suggest that in many neuronal systems axons do not invade unknown territories during evolutionary or ontogenetic development but follow in their ancestors' paths to their ancestral targets; if the connection is later lost, it reflects the specialization of the circuitry.

The pattern of interspecific variability suggests (1) that overlap of circuits is a more common feature in primitive (generalized) than in specialized brain organizations and (2) that most projections, such as the retinal, thalamotelencephalic, corticotectal, and tectal efferent ones, were bilateral in the primitive condition. Specialization of these systems in some vertebrate groups has involved the selective loss of connections, resulting in greater isolation of functions. The parcellation process may also play an important role in cell diversification.

The parcellation process as described here is thought to be one of several underlying mechanisms of evolutionary and ontogenetic differentiation.

Collateral projections of neurons in laminae I, III, and IV of rat spinal cord to thalamus, periaqueductal gray matter, and lateral parabrachial area

Projection neurons in lamina I, together with those in laminae III–IV that express the neurokinin 1 receptor (NK1r), form a major route through which nociceptive information reaches the brain. Axons of these cells innervate various targets, including thalamus, periaqueductal gray matter (PAG), and lateral parabrachial area (LPb), and many cells project to more than one target. The aims of this study were to quantify projections from cervical enlargement to PAG and LPb, to determine the proportion of spinothalamic neurons at lumbar and cervical levels that were labelled from PAG and LPb, and to investigate morphological differences between projection populations. The C7 segment contained fewer lamina I spinoparabrachial cells than L4, but a similar number of spino-PAG cells. Virtually all spinothalamic lamina I neurons at both levels were labelled from LPb and between one-third and one-half from PAG. This suggests that significant numbers project to all three targets. Spinothalamic lamina I neurons differed from those labelled only from LPb in that they were generally larger, were more often multipolar, and (in cervical enlargement) had stronger NK1r immunoreactivity. Most lamina III/IV NK1r cells at both levels projected to LPb, but few were labelled from PAG. The great majority of these cells in C7 and over one-fourth of those in L4 were spinothalamic, and at each level some projected to both thalamus and LPb. These results confirm that neurons in these laminae have extensive collateral projections and suggest that different neuronal subpopulations in lamina I have characteristic patterns of supraspinal projection. J. Comp. Neurol. 515:629–646, 2009.

Effects of 17beta-estradiol on responses of viscerosomatic convergent thalamic neurons in the ovariectomized female rat

Ovarian hormones have been shown to exert multiple effects on CNS function and viscerosomatic convergent activity. Ovariectomized (OVX) female rats were used in the present study to examine the long-term effects of proestrus levels of 17beta-estradiol (EB) delivered by a 60-day time-released subcutaneous pellet on the response properties of viscerosomatic convergent thalamic neurons. In addition, avoidance thresholds to mechanical stimulation for one of the convergent somatic territories, the trunk, was assessed using an electro-von Frey anesthesiometer before and at the end of the 6-wk post-OVX/implant period prior to the terminal electrophysiological experiments, which were done under urethane anesthesia. Rats implanted with an EB-containing pellet, relative to placebo controls, demonstrated 1) altered thalamic response frequencies and thresholds for cervix and vaginal but not colon stimulation; 2) some response variations for just the lateral group of thalamic subnuclei; and 3) altered thalamic response frequencies and thresholds for trunk stimulation. Thalamic response thresholds for trunk pressure in EB versus placebo rats were consistent with the avoidance thresholds obtained from the same groups. In addition, EB replacement affected visceral and somatic thresholds in opposite ways (i.e., reproductive-related structures were less sensitive to pressure, whereas somatic regions showed increased sensitivity). These results have obvious reproductive advantages (i.e., decreased reproductive organ sensitivity for copulation and increased trunk sensitivity for lordosis posturing), as well as possible clinical implications in women suffering from chronic pelvic pain syndromes and/or neuropathic pain.

A quantitative study of spinothalamic neurons in laminae I, III, and IV in lumbar and cervical segments of the rat spinal cord

The major ascending outputs from superficial spinal dorsal horn consist of projection neurons in lamina I, together with neurons in laminae III–IV that express the neurokinin 1 receptor (NK1r) and have dendrites that enter the superficial laminae. Some neurons in each of these populations belong to the spinothalamic tract, which conveys nociceptive information via the thalamus to cortical areas involved in pain. A projection from the cervical superficial dorsal horn to the posterior triangular nucleus (PoT) has recently been identified. PoT is at the caudal end of the thalamus and was not included in injection sites in many previous retrograde tracing studies. We have injected various tracers (cholera toxin B subunit, Fluoro-Gold, and fluorescent latex microspheres) into the thalamus to estimate the number of spinothalamic neurons in each of these two populations, and to investigate their projection targets. Most lamina I and lamina III/IV NK1r-immunoreactive spinothalamic neurons in cervical and lumbar segments could be labeled from injections centered on PoT. Our results suggest that there are 90 lamina I spinothalamic neurons per side in C7 and 15 in L4 and that some of those in C7 only project to PoT. We found that 85% of the lamina III/IV NK1r-immunoreactive neurons in C6 and 17% of those in L5 belong to the spinothalamic tract, and these apparently project exclusively to the caudal thalamus, including PoT. Because PoT projects to second somatosensory and insular cortices, our results suggest that these are major targets for information conveyed by both these populations of spinothalamic neurons.

Termination zones of functionally characterized spinothalamic tract neurons within the primate posterior thalamus

The primate posterior thalamus has been proposed to contribute to pain sensation, but its precise role is unclear. This is in part because spinothalamic tract (STT) neurons that project to the posterior thalamus have received little attention. In this study, antidromic mapping was used to identify individual STT neurons with axons that projected specifically to the posterior thalamus in Macaca fascicularis. Each axon was located by antidromic activation at low stimulus amplitudes (<30 microA) and was then surrounded distally by a grid of stimulating points in which 500-microA stimuli were unable to activate the axon antidromically, thereby indicating the termination zone. Several nuclei within the posterior thalamus were targets of STT neurons: the posterior nucleus, suprageniculate nucleus, magnocellular part of the medial geniculate nucleus, and limitans nucleus. STT neurons projecting to the ventral posterior inferior nucleus were also studied. Twenty-five posterior thalamus-projecting STT neurons recorded in lumbar spinal cord were characterized by their responses to mechanical, thermal, and chemical stimuli. Sixteen of 25 neurons were recorded in the marginal zone and the balance was located within the deep dorsal horn. Thirteen neurons were classified as wide dynamic range and 12 as high threshold. One-third of STT neurons projecting to posterior thalamus responded to noxious heat (50 degrees C). Two-thirds of those tested responded to cooling. Seventy-one percent responded to an intradermal injection of capsaicin. These data indicate that the primate STT transmits noxious and innocuous mechanical, thermal, and chemical information to multiple posterior thalamic nuclei.

The organisation of the stress response, and its relevance to chiropractors: a commentary

The stress response is a natural reaction by the body, against potentially harmful stimuli to enhance the chance for survival. Persistent activation of the stress response can cause changes to homeostatic mechanisms. The study of stress neurophysiology, in the evaluation of the manifestation of disease in the body, suggests that these chronic changes have detrimental effects on sub cortical structures. Furthermore, there is much scientific support for the notion that chronic activation of supraspinal systems will lead to maladaptation of homeostatic mechanisms, causing the impairment of processes within the body, and ultimately leading to visceral disorders. The chiropractic profession for many years has alluded to chronic change of neurophysiological pathways as a potential explanation of visceral disorders, but the profession has typically described these in terms of somatovisceral or viscerosomatic reflex activity. Change in supraspinal neurophysiological efferent activity is increasingly being used to explain "stress" related disease. The chiropractic profession should consider investigating such stress responses by conducting spinal manipulative therapy trials that evaluate supraspinal effects of manipulation. Such research may help elucidate key mechanisms associated with the change of visceral disorders noted by some chiropractors following manipulative therapy.

C1-C3 spinal cord projections to periaqueductal gray and thalamus: a quantitative retrograde tracing study in cat

By far, the strongest spinal cord projections to periaqueductal gray (PAG) and thalamus originate from the upper three cervical segments, but their precise organization and function are not known. In the present study in cat, tracer injections in PAG or in thalamus resulted in more than 2400 labeled cells, mainly contralaterally, in the first three cervical segments (C1-C3), in a 1:4 series of sections, excluding cells in the dorsal column and lateral cervical nuclei. These cells represent about 30% of all neurons in the entire spinal cord projecting to PAG and about 45% of all spinothalamic neurons. About half of the C1-C3 PAG and C1-C3 thalamic neurons were clustered laterally in the ventral horn (C(1-3vl)), bilaterally, with a slight ipsilateral preponderance. The highest numbers of C(1-3vl)-PAG and C(1-3vl)-thalamic cells were found in C1, with the greatest density rostrocaudally in the middle part of C1. A concept is put forward that C(1-3vl) cells relay information from all levels of the cord to PAG and/or thalamus, although the processing of specific information from upper neck muscles and tendons or facet joints might also play a role.

Afferent pain pathways: a neuroanatomical review

Painful experience is a complex entity made up of sensory, affective, motivational and cognitive dimensions. The neural mechanisms involved in pain perception acts in a serial and a parallel way, discriminating and locating the original stimulus and also integrating the affective feeling, involved in a special situation, with previous memories. This review examines the concepts of nociception, acute and chronic pain, and also describes the afferent pathways involved in reception, segmental processing and encephalic projection of pain stimulus. The interaction model of the cerebral cortex areas and their functional characteristics are also discussed.

Axon terminals possessing α2C-adrenergic receptors densely innervate neurons in the rat lateral spinal nucleus which respond to noxious stimulation

The lateral spinal nucleus (LSN) in the rat spinal cord contains projection neurons that are densely innervated by peptidergic varicosities which probably originate from spinal interneurons. The alpha2C-adrenoceptor (alpha2C-AR) is present on axon terminals in this nucleus and therefore norepinephrine is likely to modulate input to LSN neurons. We investigated the involvement of LSN neurons in nociceptive transmission and their relationship with axons that possess alpha2C-ARs. Double-labeling immunostaining experiments showed that alpha2C-ARs are present on axon terminals of excitatory and inhibitory interneurons that frequently contain colocalised peptides. Electron microscopy revealed that alpha2C-AR terminals are presynaptic to dendrites and somata of LSN neurons and predominantly form asymmetric synapses. We retrogradely labeled LSN neurons that project to the caudal ventrolateral medulla and combined this with induction of c-Fos expression by peripheral noxious thermal stimulation along with immunolabelling for the alpha2C-AR and the substance P (neurokinin-1) receptor. This enabled us to identify neuronkinin-1 projection neurons in the LSN that express c-Fos and to determine if such cells receive contacts from alpha2C-AR terminals. The results show that some LSN neurons are activated by noxious stimulation and that this input is likely to be modulated by norepinephrine acting on alpha2C-ARs which are present on axon terminals that are presynaptic to LSN neurons.

A quantitative and morphological study of projection neurons in lamina I of the rat lumbar spinal cord

In the rat lumbar spinal cord the major supraspinal targets for lamina I projection neurons are the caudal ventrolateral medulla (CVLM), lateral parabrachial area (LPb) and periaqueductal grey matter (PAG). In this study we have estimated the number of lamina I neurons retrogradely labelled from each of these sites in the L4 segment, as well as the proportion that can be labelled by injecting different tracers into two separate sites. Our results suggest that this segment contains approximately 400 lamina I projection neurons on each side, and that approximately 85% of these can be labelled from either the CVLM or the LPb on the contralateral side. Around 120 lamina I cells in L4 project to the PAG, and over 90% of these cells can also be labelled from the CVLM or LPb. Most lamina I neurons projecting to CVLM or LPb are located in the contralateral dorsal horn, but in each case some cells were found to have bilateral projections. We also examined horizontal sections to investigate morphology and the expression of the neurokinin 1 (NK1) receptor in cells labelled from CVLM, LPb or PAG. There were no consistent morphological differences between these groups, however, while cells with strong or moderate NK1 receptor-immunostaining were labelled from LPb or CVLM, they seldom projected to the PAG. These results suggest that many lamina I cells project to more than one site in the brain and that those projecting to PAG may represent a distinct subclass of lamina I projection neuron.

Differentiation of lamina I spinomedullary and spinothalamic neurons in the cat

We characterized spinomedullary neurons that project to the ventrolateral portion of the medulla that receives lamina I terminations in two sets of experiments in the cat. First, their distribution was examined using single unilateral iontophoretic injections of cholera toxin subunit B. The injection sites were characterized by microelectrode recordings from nociceptive- and thermoreceptive-specific units, indicative of lamina I input. The spinomedullary neurons were symmetrically distributed bilaterally, predominantly (63-69%) in lamina I but also in laminae V-VIII and the thoracic lateral horn (intermediolateral cell column). In horizontal sections, spinomedullary lamina I neurons included all three main morphological types described earlier. Second, spinomedullary and spinothalamic neurons were compared in retrograde double-labeling experiments. Different combinations of tracers were injected in the right thalamus and the left or right ventrolateral medulla (guided by recordings). The numbers of spinomedullary and spinothalamic neurons on the left side were comparable, and the segmental and laminar distributions were similar, except that a greater proportion of spinomedullary neurons originated from thoracic segments. However, the proportion of double-labeled neurons was consistently approximately 1%, indicating that spinomedullary and spinothalamic pathways arise from separate subpopulations. Spinomedullary neurons were more ventrally located within lamina I than spinothalamic neurons. A significantly greater proportion of spinomedullary neurons had fusiform somata (49% vs. 36%). These observations indicate that lamina I is the major source of spinal input to this portion of the ventrolateral medulla, that the projection includes several morphological types of inputs, and that this projection is distinct from the spinothalamic projection. These findings are consistent with the concept that lamina I projections constitute an ascending homeostatic afferent pathway relating the physiological condition of the body.

Differential effects of chronic spinal hemisection on somatic and visceral inputs to caudal brainstem

The medullary reticular formation (MRF) receives convergent inputs from multiple somatic and pelvic/visceral territories. The effects of chronic 30-day lateral hemisections at T8 on the responses of single MRF neurons to noxious mechanical stimulation of both hindpaws was examined in urethane-anesthetized male rats. Neuronal responses on both sides of the MRF to pinching of the hindpaw on the side opposite the lesion (intact-side) were found either to be completely absent or if present, weak (i.e. hindpaw was hyposensitive). The presence or absence of intact-side responses appeared to be dependent on the lesion extent. In contrast, bilateral MRF responses to pinching the lesion-side hindpaw were present; however, responses were greater in magnitude (lower thresholds) relative to surgical sham controls suggesting hypersensitivity. Responses to lesion-side hindpaw stimulation on both sides of the MRF indicated that whereas the ascending projections are primarily crossed below the level of lesion, they are both crossed and uncrossed above. These findings are in contrast with our previous data on ascending projections from the bilaterally organized male urogenital tract. The results presented for the hindpaws correlate with clinical observations of patients with similar incomplete spinal cord injuries (Brown-Séquard syndrome).

Ascending projections from the area around the spinal cord central canal: A Phaseolus vulgaris leucoagglutinin study in rats

A single small iontophoretic injection of Phaseolus vulgaris leucoagglutinin labels projections from the area surrounding the spinal cord central canal at midthoracic (T6-T9) or lumbosacral (L6-S1) segments of the spinal cord. The projections from the midthoracic or lumbosacral level of the medial spinal cord are found: 1) ascending ipsilaterally in the dorsal column near the dorsal intermediate septum or the midline of the gracile fasciculus, respectively; 2) terminating primarily in the dorsal, lateral rim of the gracile nucleus and the medial rim of the cuneate nucleus or the dorsomedial rim of the gracile nucleus, respectively; and 3) ascending bilaterally with slight contralateral predominance in the ventrolateral quadrant of the spinal cord and terminating in the ventral and medial medullary reticular formation. Other less dense projections are to the pons, midbrain, thalamus, hypothalamus, and other forebrain structures. Projections arising from the lumbosacral level are also found in Barrington's nucleus. The results of the present study support previous retrograde tract tracing and physiological studies from our group demonstrating that the neurons in the area adjacent to the central canal of the midthoracic or lumbosacral level of the spinal cord send long ascending projections to the dorsal column nucleus that are important in the transmission of second-order afferent information for visceral nociception. Thus, the axonal projections through both the dorsal and the ventrolateral white matter from the CC region terminate in many regions of the brain providing spinal input for sensory integration, autonomic regulation, motor and emotional responses, and limbic activation.

Interconnections between the neuroendocrine hypothalamus and the central autonomic system. Geoffrey Harris Memorial Lecture, Kitakyushu, Japan, October 1998

Tract-tracing techniques in combination with immunohistochemistry and in situ hybridization were used in intact and operated rats (hypothalamic lesions, transections of neuronal pathways) to localize and characterize neuronal connections between the hypothalamus and autonomic centers. Viscerosensory and somatosensory signals which relay in the spinal cord and the medulla oblongata reach the hypothalamus through various catecholaminergic and noncatecholaminergic neuronal pathways. Vice versa, the hypothalamus influences autonomic activities through humoral and neurohumoral pathways. Descending hypothalamic efferents carry feedback signals to viscerosensory and brainstem catecholaminergic neurons and regulatory inputs to parasympathetic (dorsal vagal nucleus) and sympathetic (thoracolumbar intermediolateral cell column) preganglionic neurons. These fibers arise mainly from neurons of the paraventricular, arcuate, perifornical, and dorsomedial nuclei and the lateral hypothalamus. The major neuroanatomical observations are the following: (1) pathways between the hypothalamus and autonomic centers are bidirectional: the ascending and descending fibers may use the same avenues; (2) the descending axons are mainly peptidergic (CRF, vasopressin, oxytocin, somatostatin, enkephalin, POMC, and cANP), while the ascending fibers are both peptidergic (enkephalin, NPY, neurotensin, dynorphins) and catecholaminergic; (3) descending hypothalamic axons terminate directly on the sensory, preganglionic, and catecholaminergic neurons in the medulla and the spinal cord; (4) hypothalamic projections to the autonomic centers are always bilateral; (5) while medullary autonomic and catecholaminergic fibers innervate hypothalamic neurons directly, spinohypothalamic axons are relayed on neurons in the lateral hypothalamus.

The role of ascending neuronal pathways in stress-induced release of noradrenaline in the hypothalamic paraventricular nucleus of rats

Central catecholaminergic pathways carrying pain-related signals to the hypothalamic paraventricular nucleus (PVN) were investigated in laboratory rats. Four per cent formalin injected subcutaneously was employed as a stressful stimulus. Neuronal activity in brainstem catecholaminergic and paraventricular neurones was assessed by Fos immunohistochemistry. Stress-induced noradrenaline (NE) release from nerve terminals in the PVN was measured in extracellular fluid by in-vivo microdialysis. Within 30 min, formalin elicited a four- to sixfold increase in plasma ACTH and corticosterone concentrations and intense Fos-like activity was seen in the superficial zones of the lumbar spinal cord ipsilateral to the side of the formalin injection. In brainstem catecholaminergic neurones, the PVN, and midline thalamic nuclei, formalin-induced Fos-immunopositivity was equally present in the ipsi- and contralateral sides of the injection. An immediate elevation (4-5 times higher than baseline levels) of NE levels was measured in both the right and left PVN after a formalin injection into the right paw. Unilateral surgical transections at the medulla-spinal cord junction failed to affect formalin-induced elevations in NE levels in the PVN independently of the side of the formalin injection or the knife cut. Thus, this observation clearly shows that fibres carrying pain-evoked signals ascend bilaterally from the spinal cord to the brainstem and forebrain. Hemisections of the medulla oblongata between the level of A1-A2 NE cell groups and the locus coeruleus reduced but did not eliminate formalin-induced NE release from the PVN ipsilateral to the knife cut. This effect was independent of the side of the formalin injection. In the contralateral PVN, high and similar NE levels were measured in response to a formalin injection into the right or the left leg. The present study indicates that formalin-induced pain signals are carried by sensory fibres to the ipsilateral spinal cord. From there, axons of different dorsal horn neurones reach noradrenergic cells on both sides of the medulla oblongata. The majority of noradrenergic fibers ascend on the same side and innervate the ipsilateral PVN. Since formalin administration resulted in a moderate elevation of NE levels in the PVN on the operated side, the role of other ascending noradrenergic (from the locus coeruleus) or noncatecholaminergic fibres that could modulate NE release from the PVN should be considered.

A role for spinal lamina I neurokinin-1-positive neurons in cold thermoreception in the rat

Lamina I neurons of the spinal cord convey specific nociceptive activity to the brain. A subpopulation of lamina I cells bears substance P receptors (neurokinin-1) and recent studies have shown that these neurons encode for the intensity of noxious peripheral stimulation. Here, we report that cool thermal stimuli, applied to the hindpaw of anaesthetized rats, induce Fos expression in lamina I neurokinin-1 neurons that is graded with respect to the intensity of the thermal stimulus. Thus, as the temperature of the stimulus was reduced, both the total number of neurokinin-l-positive neurons expressing Fos and the proportion of Fos nuclei present within neurokinin-1 cells showed a significant increase. These data show that lamina I neurokinin-1 cells encode the intensity of noxious cooling of the skin. In laminae III and IV, although there was no correlation between neurokinin-1 cell activation and stimulus intensity, the total Fos count in these layers was inversely related to the depth of cooling. Thus, neurons in laminae III and IV may also play a role in thermoreception.

Complete compensation in skilled reaching success with associated impairments in limb synergies, after dorsal column lesion in the rat

Each of the dorsal columns of the rat spinal cord conveys primary sensory information, by way of the medullary dorsal column nucleus, to the ventrobasal thalamus on the contralateral side; thus the dorsal columns are an important source of neural input to the sensorimotor cortex. Damage to the dorsal columns causes impairments in synergistic proximal or whole-body movements in cats and distal limb impairments in primates, particularly in multiarticulated finger movements and tactile foviation while handling objects, but the behavioral effects of afferent fiber lesions in the dorsal columns of rodents have not been described. Female Long-Evans rats were trained to reach with a forelimb for food pellets and subsequently received lesions of the dorsomedial spinal cord at the C2 level, ipsilateral to their preferred limb. Reaching success completely recovered within a few days of dorsal column lesion. Nevertheless, a detailed analysis of high-speed video recordings revealed that rotatory limb movements (aiming, pronation, supination, etc.) were irreversibly impaired. Compensation was achieved with whole-body and alternate limb movements. These results indicate the following: (1) in the absence of the dorsal columns, other sensorimotor pathways support endpoint success in reaching; (2) sensory input conveyed by the dorsal columns is important for both proximal and distal limb movements used for skilled reaching; and (3) detailed behavioral analyses in addition to endpoint measures are necessary to completely describe the effects of dorsal column lesions.

Branching and/or collateral projections of spinal dorsal horn neurons

Branching and/or collateral projections of spinal dorsal horn neurons is a common phenomenon. Evidence is presented for the existence of STTm/STTl, STTc/STTi, STT/SMT, STT/SRT, SCT/DCPS, SST/DCPS, SCT/SST, STT/SHT, STeT/SHT, STeTs and other doubly or multiply projecting spinal neurons that have been anatomically and physiologically identified and named based on the locations of the cells of origin and their terminations in the brain. These newly discovered spinal projection neurons are characterized by a single cell body and branched axons and/or collaterals that project to two or more target areas in the brain. These novel populations of neurons seem to be a fuzzy set of spinal projection neurons that function as an intersection set of the corresponding single projection spinal neurons and to be at an intermediate stage phylogenetically. Identification strategies are discussed, and general concluding remarks are made in this review.

Regeneration of ascending spinal axons in goldfish

Regeneration of descending spinal cord tracts occur spontaneously in adult goldfish. Very little information is available regarding the fate of ascending fibers. Using Dextran amines as a tracer, we studied the normal and regenerated ascending axonal projection patterns in adult goldfish brain nuclei. Present study includes spinal projections to torus semicircularis, hypothalamus, thalamus and the telencephalon. Regenerated fibers had finer caliber axons and the terminal axonal arbors covered a larger area than the corresponding normal ones.