Grouping of somatosensory neurons in the spinal cord and the gracile nucleus of the rat by cluster analysis

Regional Hyperexcitability and Chronic Neuropathic Pain Following Spinal Cord Injury

Spinal cord injury (SCI) causes maladaptive changes to nociceptive synaptic circuits within the injured spinal cord. Changes also occur at remote regions including the brain stem, limbic system, cortex, and dorsal root ganglia. These maladaptive nociceptive synaptic circuits frequently cause neuronal hyperexcitability in the entire nervous system and enhance nociceptive transmission, resulting in chronic central neuropathic pain following SCI. The underlying mechanism of chronic neuropathic pain depends on the neuroanatomical structures and electrochemical communication between pre- and postsynaptic neuronal membranes, and propagation of synaptic transmission in the ascending pain pathways. In the nervous system, neurons are the only cell type that transmits nociceptive signals from peripheral receptors to supraspinal systems due to their neuroanatomical and electrophysiological properties. However, the entire range of nociceptive signaling is not mediated by any single neuron. Current literature describes regional studies of electrophysiological or neurochemical mechanisms for enhanced nociceptive transmission post-SCI, but few studies report the electrophysiological, neurochemical, and neuroanatomical changes across the entire nervous system following a regional SCI. We, along with others, have continuously described the enhanced nociceptive transmission in the spinal dorsal horn, brain stem, thalamus, and cortex in SCI-induced chronic central neuropathic pain condition, respectively. Thus, this review summarizes the current understanding of SCI-induced neuronal hyperexcitability and maladaptive nociceptive transmission in the entire nervous system that contributes to chronic central neuropathic pain.

Impaired expression of neuronal nitric oxide synthase in the gracile nucleus is involved in neuropathic changes in Zucker Diabetic Fatty rats with and without 2,5-hexanedione intoxication

These studies examined the influence of 2,5-hexanedione (2,5-HD) intoxication on expression of neuronal nitric oxide synthase (nNOS) in the brainstem nuclei in Zucker Diabetic Fatty (ZDF) vs. lean control (LC) rats. Functional neuropathic changes were also investigated following axonal damage and impaired axonal transport induced by the treatment. Animals were intoxicated by i.p. injection of 2,5-HD plus unilateral administration of 2,5-HD over the sciatic nerve. The mechanical thresholds and withdrawal latencies to heat and cold stimuli on the foot were measured at baseline and after intoxication. The medulla sections were examined by nNOS immunohistochemistry and NADPH-diaphorase histochemistry at the end of the treatments. The mechanical thresholds and withdrawal latencies were significantly decreased while nNOS immunostained neurons and NADPH-diaphorase positive cells were selectively reduced in the gracile nucleus at baseline in ZDF vs. LC rats. NADPH-diaphorase reactivity and nNOS positive neurons were increased in the ipsilateral gracile nucleus in LC rats following 2,5-HD intoxication, but its up-regulation was attenuated in ZDF rats. These results suggest that diabetic and chemical intoxication-induced nNOS expression is selectively reduced in the gracile nucleus in ZDF rats. Impaired axonal damage-induced nNOS expression in the gracile nucleus is involved in neuropathic pathophysiology in type II diabetic rats.

Mapping sensory circuits by anterograde transsynaptic transfer of recombinant rabies virus

Primary sensory neurons convey information from the external world to relay circuits within the CNS, but the identity and organization of the neurons that process incoming sensory information remains sketchy. Within the CNS, viral tracing techniques that rely on retrograde transsynaptic transfer provide a powerful tool for delineating circuit organization. Viral tracing of the circuits engaged by primary sensory neurons has, however, been hampered by the absence of a genetically tractable anterograde transfer system. In this study, we demonstrate that rabies virus can infect sensory neurons in the somatosensory system, is subject to anterograde transsynaptic transfer from primary sensory to spinal target neurons, and can delineate output connectivity with third-order neurons. Anterograde transsynaptic transfer is a feature shared by other classes of primary sensory neurons, permitting the identification and potentially the manipulation of neural circuits processing sensory feedback within the mammalian CNS.

Effects of electroacupuncture Zusanli (ST36) on food intake and expression of POMC and TRPV1 through afferents-medulla pathway in obese prone rats

OBJECTIVE

The purpose of this study was to determine the effects of electroacupuncture (EA) ST36 on food intake and body weight in obese prone (OP) rats compared to obese resistant (OR) strain on a high fat diet. The influences of EA on mRNA levels of pro-opiomelanocortin (POMC), transient receptor potential vanilloid type-1 (TRPV1), and neuronal nitric oxide synthase (nNOS) were also examined in the medulla regions and ST36 skin tissue.

METHODS

Advanced EA ST36 was conducted in two sessions of 20 min separated by an 80 min interval for 7 days. Food intake and body weight were recorded in conscious rats every day. Real time PCR was conducted in the micropunches of the medulla regions and skin tissues at the end of the treatment.

RESULTS

Food intake and body weight were significantly reduced by advanced EA ST36 in OP rats, but slightly decreased in OR strain and sham-EA rats. Advanced EA ST36 produced a marked increase in POMC mRNA level in the nucleus tractus solitarius (NTS) and hypoglossal nucleus (HN) regions. TRPV1 and nNOS mRNAs were simultaneously increased in the NTS/gracile nucleus regions and in the ST36 skin regions by the EA treatment in OP rats.

CONCLUSIONS

We conclude that advanced EA ST36 produces an up-regulation of anorexigenic factor POMC production in the NTS/HN, which inhibits food intake and reduces body weight. EA-induced expression of TRPV1-nNOS in the ST36 and the NTS/gracile nucleus is involved in the signal transduction of EA stimuli via somatosensory afferents-medulla pathways.

Electroacupuncture Zusanli (ST36) on Release of Nitric Oxide in the Gracile Nucleus and Improvement of Sensory Neuropathies in Zucker Diabetic Fatty Rats

The purpose of these studies was to examine the effects of electroacupuncture (EA) Zusanli (ST36) on release of nitric oxide (NO) in the gracile nucleus (GN) and determine if functional neuropathic changes were modified by EA ST36-induced NO in the nucleus in Zucker diabetic fatty (ZDF) rats. The foot withdrawal responses to mechanical, thermal and cold stimuli were measured before and after EA stimulation. A microdialysis probe was implanted in the GN and dialysate samples were collected 20 min before, during and after EA ST36. Total nitrate and nitrite (NO_x⁻) concentrations in the samples were quantified by using chemiluminescence. The baseline dialysate NO_x⁻ concentrations in the GN were decreased in ZDF rats compared to lean control (LC) rats (P < .05). In ZDF rats, dialysate NO_x⁻ releases in the GN were markedly increased during EA ST36, whereas in LC rats, the releases were moderately enhanced at 20–40 min after EA ST36. The withdrawal latencies to mechanical, cold and thermal stimuli were significantly improved 20 min after EA ST36 both in LC and ZDF rats, but not altered by non-acupoint stimulation. The withdrawal latencies to EA ST36 were further potentiated by 3-morpholinyl-sydnoneimine and inhibited by N^G-Propyl-l-arginine infused into the GN in ZDF rats (P < .05). These results show that EA ST36 increases NO release in the GN, and NO in the nucleus modifies withdrawal latencies to mechanical, cold, and thermal nociception stimuli. Data suggest that EA ST36 induces NO release in the GN, which contributes to improvement of sensory neuropathies in rats.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Electroacupuncture modulates cortical activities evoked by noxious somatosensory stimulations in human

A noninvasive high-resolution imaging technique of cerebral electric activities has been developed to directly link scalp potential measurement with the magnetic resonance images of the subjects, which is very helpful for the elucidation of the cortical processing following various stimulations. Here, we used a 64-channel Neuroscan ESI-128 system to explore the specific cortical activities elicited by electroacupuncture (EA) acupoint in normal volunteers and the modulatory effect of EA on cortical activities evoked by noxious somatosensory stimulation. A specific later-latency somatosensory-evoked potential (SEP, P150) located in bilateral anterior cingulated cortex was observed after EA acupoint but not non-acupoint. Two pain-specific SEP components (P170 and N280), located in bilateral suprasylvian operculum and anterior cingulated cortex respectively were observed following painful median nerve stimulation. Binding EA acupoint with painful median nerve stimulation, the amplitudes of P170 and N280 appeared to be attenuated significantly, 2D topography exhibited tremendous decrease of cortical activation between 120 ms and 296 ms in latency, and visual analogue scale (VAS) changes also showed a similar pattern to the change of amplitude. The bilateral anterior cingulated cortex recruited following acupoint stimuli might, to some extent, suggest that EA has the specific physiological effects. Decrease of pain-induced cortical activation by EA acupoint was considered to be mainly due to an interaction of the signals in anterior cingulated cortex ascending from the pain stimulation and EA.

Upregulation of Group I metabotropic glutamate receptors in neurons and astrocytes in the dorsal horn following spinal cord injury

Of the glutamate receptor types, the metabotropic glutamate receptors (mGluRs) are G proteins coupled and can initiate a number of intracellular pathways leading to hyperexcitability of spinal neurons. In this study, we tested the expression of mGluRs to determine which cell types might contribute to sustained neuronal hyperexcitability in the lumbar enlargement with postoperative day (POD) 7 (early), 14 (late), and 30 (chronic phase) following spinal cord injury (SCI) by unilateral hemisection at T13 in Sprague-Dawley rats. Expression was determined by confocal analyses of immunocytochemical reaction product of neurons (NeuN positive) and astrocytes (GFAP positive) in the dorsal horn on both sides of the L4 segment. Neurons were divided into two sizes: small (<20 microm) and large (>35 microm), for physiological reasons. We report a significant increase of mGluR(1) expression in large and small neurons of the dorsal horn on both sides of the cord in late and chronic phases when compared to control sham groups. Expression of mGluR(2/3) significantly increased in large neurons on the ipsilateral (hemisected) side in the late phase. Expression of mGluR(5) significantly increased in large neurons in early, late, and chronic phases. In addition, mGluR(1) and mGluR(5) expression after hemisection was significantly increased in astrocytes in early, late, and chronic phases; whereas mGluR(2/3) did not display any significant changes. In conclusion, our data demonstrate long-term changes in expression levels of Group I mGluRs (mGluR(1) and mGluR(5)) in both neurons and astrocytes in segments below a unilateral SCI. Thus, permanent alterations in dorsal horn receptor expression may play important roles in transmission of nociceptive responses in the spinal cord following SCI.

Responses of neuronal nitric oxide synthase expression in the brainstem to electroacupuncture Zusanli (ST 36) in rats

Recent studies have reported that l-arginine-derived nitric oxide (NO) in the gracile nucleus modifies the hypotensive responses to electroacupuncture (EA) stimulation of Zusanli (ST 36). The purpose of this study was to examine the influence of EA stimulation of ST 36 on neuronal NO synthase (nNOS) expression in the brainstem nuclei in rats. EA stimulation of ST 36 and a non-acupoint was performed using 3 Hz of stimulation for 10 s every 2 min for a period of 120 min in rats anesthetized with ketamine. Rats in the sham-treated group received surgery and EA needles were placed into the acupoints without performing the stimulation. After 2-h stimulation and sham treatment, animals were perfused with 4% paraformaldehyde. Sections of rat medulla were examined by immunolabeling with a polyclonal antibody directed against nNOS. The brainstem nuclei were also visualized by NADPH-diaphorase histochemistry, a marker of nNOS activity. nNOS expression and NADPH-diaphorase reactivity were quantified by using a microscope with reticule grid to count the number of positive cells over a nucleus. Unilateral EA stimulation of ST 36 in rats caused increases in nNOS immunostained cells in the rostral region of the ipsilateral gracile nucleus, but was not altered in the contralateral gracile nucleus compared with sham-treated rats (P < 0.05, n = 6-7). NADPH-diaphorase-positive cells were also increased in the ipsilateral gracile nucleus of rats with EA stimulation. nNOS immunostaining and NADPH-diaphorase-positive neurons were significantly increased in both ipsilateral and contralateral sides of the medial nucleus tractus solitarius (mNTS) in rats receiving EA ST 36 compared with sham-treated animals (P < 0.05). nNOS immunostaining and NADPH-diaphorase reactivity was neither altered in the gracile nucleus and mNTS of non-acupoint stimulated rats nor other brainstem nuclei in rats with EA ST 36. These results show that nNOS immunoreactivity and NADPH-diaphorase reactivity are consistently increased in the gracile nucleus and the mNTS by EA ST 36. We conclude that EA ST 36 induces nNOS expression in the gracile nucleus and mNTS, and enhanced nNOS-NO in the nuclei may modify central cardiovascular regulation, which contribute to hypotensive effects of acupuncture.

Neurobiology of Acupuncture: Toward CAM

It has long been accepted that acupuncture, puncturing and scraping needles at certain points on the body, can have analgesic and anesthetic effects, as well as therapeutic effects in the treatment of various diseases. This therapy, including acupuncture anesthesia, has drawn the attention of many investigators and become a research subject of international interest around the world. Numerous studies have demonstrated that the nervous system, neurotransmitters, endogenous substances and Jingluo (meridians) may respond to needling stimulation and electrical acupuncture. An abundance of information has now accumulated concerning the neurobiological mechanisms of acupuncture, in relation to both neural pathways and neurotransmitters/hormonal factors that mediate autonomic regulation, pain relief and other therapeutics. Early studies demonstrated that the analgesic effects of electroacupuncture (EA) are mediated by opioid peptides in the periaqueductal gray. Recent evidence shows that nitric oxide plays an important role in mediating the cardiovascular responses to EA stimulation through the gracile nucleus-thalamic pathway. Other substances, including serotonin, catecholamines, inorganic chemicals and amino acids such as glutamate and alpha-aminobutyric acid (GABA), are proposed to mediate certain cardiovascular and analgesic effects of acupuncture, but at present their role is poorly understood. The increased interest in acupuncture health care has led to an ever-growing number of investigators pursuing research in the processes of the sense of needling touch, transduction of needling stimulation signals, stimulation parameters and placebos. In this Review, the evidence and understanding of the neurobiological processes of acupuncture research have been summarized with an emphasis on recent developments of nitric oxide mediating acupuncture signals through the dorsal medulla-thalamic pathways.

Nitric oxide in the gracile nucleus mediates depressor response to acupuncture (ST36)

The purpose of these studies was to determine the role of gracile nucleus and the effects of l-arginine-derived nitric oxide (NO) synthesis in the nucleus on the cardiovascular responses to electroacupuncture (EA) stimulation of "Zusanli" (ST36). Arterial blood pressure and heart rate were monitored during EA stimulation of ST36 following microinjections of agents into gracile nucleus. EA ST36 produced depressor and bradycardiac responses in anesthetized Sprague-Dawley rats. The cardiovascular responses to EA ST36 were blocked by bilateral microinjection of lidocaine into gracile nucleus. Microinjection of L-arginine into gracile nucleus facilitated the hypotensive and bradycardiac responses to EA ST36. The cardiovascular responses to EA ST36 were attenuated by bilateral microinjection of neuronal NO synthase (nNOS) antisense oligos into gracile nucleus. Microinjection of nNOS sense oligos into gracile nucleus did not alter the cardiovascular response to EA ST36. The results demonstrate that a blockade of neuronal conduction in the gracile nucleus inhibits the cardiovascular responses to EA ST36. The hypotensive and bradycardiac responses to EA ST36 are modified by influences of L-arginine-derived NO synthesis in the gracile nucleus. We conclude that NO plays an important role in mediating the cardiovascular responses to EA ST36 through gracile nucleus.

Effects of L-arginine-derived nitric oxide synthesis on cardiovascular responses to stimulus-evoked somatosympathetic reflexes in the gracile nucleus

The purpose of these studies was to determine the role of gracile nucleus (Gr) and the effects of L-arginine-derived nitric oxide (NO) synthesis in the nucleus on the cardiovascular responses to somatosympathetic reflexes (SSR). Electrical stimulation of sural and tibial nerve to evoke excitatory and inhibitory SSR was carried out in anesthetized Sprague-Dawley rats. Arterial blood pressure and heart rate were monitored during stimulus-evoked SSR following microinjections of the agents into Gr. Cardiovascular responses to electrical stimulation of the sural and tibial nerves were blocked by microinjection of lidocaine into Gr. The hypertensive and tachycardiac responses to stimulation of the sural nerve were attenuated by bilateral microinjection of L-arginine into Gr, but enhanced by the presence of nNOS antisense oligodeoxynucleotides (oligos) in the area. Microinjection of L-arginine into Gr facilitated the hypotensive and bradycardic responses to stimulation of the tibial nerve while pretreatment with nNOS antisense oligos into Gr attenuated the tibial stimulation evoked inhibitory SSR. The stimulus-evoked responses were not altered by microinjection of nNOS sense oligos into Gr. The results show that the cardiovascular responses to stimulus-evoked SSR were inhibited by the presence of a blockade of neuronal conduction in the Gr. L-Arginine-derived NO synthesis in the Gr attenuates the cardiovascular responses to stimulus-evoked excitatory SSR and facilitates the responses to inhibitory SSR. We conclude that NO in the Gr plays an inhibitory role in the central cardiovascular control through SSR regulation.

Separate, parallel sensory and hedonic pathways in the mammalian somatosensory system

We propose that separate sensory and hedonic representations exist in each of the primary structures of the somatosensory system, including brainstem, thalamic and cortical components. In the dorsal horn of the spinal cord, the hedonic representation, which consists primarily of nociceptive-specific, wide dynamic range, and thermoreceptive neurons, is located in laminae I and II, while the sensory representation, composed primarily by low-threshold and wide dynamic range neurons, is found in laminae III through V. A similar arrangement is found in the caudal spinal trigeminal nucleus. Based on the available anatomical and electrophysiological data, we then determine the corresponding hedonic and sensory representations in the area of the dorsal column nuclei, ventrobasal and posterior thalamic complex, and cortex. In rodent primary somatosensory cortex, a hedonic representation can be found in laminae Vb and VI. In carnivore and primate primary and secondary somatosensory cortical areas no hedonic representation exists, and the activities of neurons in both areas represent the sensory aspect exclusively. However, there is a hedonic representation in the posterior part of insular cortex, bordering on retroinsular cortex, that receives projections from two thalamic areas in which hedonics are represented. The functions of the segregated components of the system are discussed, especially in relation to the subjective awareness of pain.

Inflammatory pain reduces correlated activity in the dorsal column nuclei

Persistent pain can result in sensitization of neurons in the spinal cord dorsal horn and produce physiological changes in sites such as the thalamus, that receive projections from the dorsal horn. Although the dorsal column nuclei receive both primary afferent input and projections from the dorsal horn, their participation in persistent pain states is relatively unexplored, perhaps because they play a limited role in acute, cutaneous nociception. We have used a model of inflammatory pain to examine the physiological properties of dorsal column nucleus neurons during persistent pain. We used this model in order to minimize direct damage to large myelinated primary afferents that project directly to the dorsal column nuclei. Inflammation was produced by injection of complete Freund's adjuvant into one hindpaw in rats, and neurons in the gracile nucleus were recorded 2-8 days later. Inflammation resulted in increased responsiveness to nociceptive (pinch) stimulation and increased incidence of afterdischarge firing 2-3 days after injection. Spontaneous activity was increased 6-8 days after injection. Inflammation decreased the strength of correlated firing in neuron pairs that shared common inputs, but did not affect the strength of monosynaptic interactions between neurons. These results suggest that the dorsal column nuclei can participate in persistent pain processes. Based on their anatomical connections, the dorsal column nuclei may contribute to thalamic changes during persistent pain as well as to supraspinal centers that modulate pain transmission in the spinal cord.

Responses of nitric oxide synthase expression in the gracile nucleus to sciatic nerve injury in young and aged rats

Neuronal nitric oxide synthase (nNOS) is induced in dorsal root ganglion neurons following axotomy in young rats, and is also increased in the gracile nucleus neurons of intact aged rats. The present study examined the influence of sciatic nerve axotomy on nNOS expression in the gracile nucleus in young compared to aged rats. The unilateral transection of the sciatic nerve was performed in young (4 months) and old (24 months) Fischer rats. Sections of rat medulla obtained 14 days after axotomy were immunolabelled using a polyclonal antibody directed against nNOS and stained by nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry, a marker of nNOS activity. In young rats, unilateral axotomy produced increased NADPHd containing neurons in the rostral region and the caudal region of the ipsilateral gracile nucleus compared to the side with intact sciatic nerve. In old rats, the NADPHd containing neurons in the ipsilateral gracile nucleus were moderately increased by axotomy over the age changes seen in the contralateral side. Similar results were obtained with nNOS immunoreactivity in young rats, but more cells were seen with NADPHd staining compared to nNOS immunostaining in old rats. The results suggest that unilateral sciatic axotomy causes an increase in nNOS expression in the ipsilateral gracile nucleus of young rats, which is still seen in old rats as an increase over normal aging changes.

Behavioral analysis of diffuse noxious inhibitory controls (DNIC): antinociception and escape reactions

'Diffuse noxious inhibitory controls' or DNIC is the inhibition of multireceptive neurons in the dorsal horn of the spinal cord that results when a noxious stimulus is applied to a region of the body remote from the neuron's excitatory receptive field. Although this phenomenon is well-documented, the behavioral consequences of DNIC are not clear. The present study was undertaken to determine how nocifensor withdrawal reflexes evoked by a noxious stimulus are altered by application of a second noxious stimulus to a distant part of the body. The tail flick or hindpaw withdrawal reflex of lightly anesthetized (0.6-1.0% halothane) rats was measured before, during and after another appendage was placed in water ranging in temperature from 45 to 54 degrees C. When the forepaw or hindpaw was placed in water exceeding 49 degrees C the tail flick reflex to acute noxious radiant heat was inhibited. In contrast, noxious conditioning stimuli, regardless of temperature or location, had no effect on the latency for hindpaw withdrawal evoked by an acute noxious stimulus, but did produce a change in reflex topography from flexion to extension. These results, along with previous research on DNIC, suggest that intense noxious stimuli: (1) inhibit the tail flick reflex via inhibition of multireceptive neurons in the dorsal horn; (2) disinhibit hindpaw extensor motoneurons by inhibiting the activity of multireceptive neurons involved in hindlimb flexion; and (3) reduce pain sensation by inhibiting multireceptive neurons projecting to the brain (see Model in Discussion).

Electrophysiological evidence for the antinociceptive effect of transcutaneous electrical stimulation on mechanically evoked responsiveness of dorsal horn neurons in neuropathic rats

Using a rat model of peripheral neuropathy induced by a tight ligation of L5-6 spinal nerves, the effects of transcutaneous electrical stimulation on the mechanical responses of wide dynamic range (WDR) dorsal horn neurons were investigated. The responses of the WDR neurons to both the brush and pinch stimuli were found to be enhanced in the neuropathic rats compared to those in the normal rats. These enhanced responses were depressed by low-frequency and high-intensity transcutaneous electrical stimulation (2 Hz, 4-5 mA) applied to the somatic receptive field. The durations of the depressive effects on the brush responses ranged between 30 and 45 min and those on the pinch responses were 60-90 min. These results imply that the transcutaneous electrical stimulation used here produces an antinociceptive effect via a depressive action on the enhanced mechanical responsiveness of the spinal neurons in this rat model of peripheral neuropathy.