Multiple inputs of GABA-immunoreactive neurons in the cuneate nucleus of the rat

Curcumin promotes microglial M2 polarization and suppresses chronic constriction: Injury-induced neuropathic pain in a rat model of peripheral neuropathy

OBJECTIVES

Glia (i.e., astrocyte and microglia) activation in the central nervous system plays a critical role in developing neuropathic pain. Microglia can be activated into proinflammatory (M1) and anti-inflammatory (M2) phenotypes. Switching microglial polarization from M1 to M2 phenotypes represents a novel therapeutic strategy for neuropathic pain. Curcumin has been widely used for its anti-inflammatory and immunomodulatory effects. This study investigated effects of curcumin on astrocyte activation and microglia polarization in the cuneate nucleus (CN) and development of neuropathic pain behavior after chronic constriction injury (CCI) of the median nerve.

METHODS

Rats were fed with curcumin once daily at a dose of 40, 80, or 120 mg/kg 30 min before and until 7 d after median nerve CCI. Subsequently, mechanical allodynia and thermal hyperalgesia were evaluated using von Frey filaments and plantar tests, respectively. The levels of astrocyte marker, monoclonal glial fibrillary acidic protein; microglia marker, ionized calcium-binding adapter molecule 1; M1 marker, CD86; and M2 marker, CD206 in the cuneate nucleus were determined. Enzyme-linked immunosorbent assay was applied to measure cytokine concentrations.

RESULTS

Curcumin administration dose-dependently reduced mechanical allodynia and thermal hyperalgesia and decreased monoclonal glial fibrillary acidic protein and ionized calcium-binding adapter molecule 1 immunoreactivity in the ipsilateral cuneate nucleus after CCI. On ultrastructural observation, curcumin treatment was associated with fewer features of activated astrocytes and microglia. Furthermore, CCI rats given curcumin exhibited a decline in CD86 immunoreactivity and proinflammatory cytokine levels but an increase in CD206 immunoreactivity and release of anti-inflammatory cytokines.

CONCLUSIONS

In our findings, curcumin switches microglial phenotypes from M1 to M2 by suppressing astrocytic activation, reducing proinflammatory cytokine release, promoting anti-inflammatory cytokine production, and contributing to relief of neuropathic pain.

Modulation of tactile feedback for the execution of dexterous movement

[Figure: see text].

Melatonin reduces neuropathic pain behavior and glial activation through MT2 melatonin receptor modulation in a rat model of lysophosphatidylcholine-induced demyelination neuropathy

In this study, we investigated whether melatonin treatment prevents development of neuropathic pain via suppression of glial mitogen-activated protein kinases (MAPKs) activation in the cuneate nucleus (CN) in a lysophosphatidylcholine (LPC)-induced median nerve demyelination neuropathy model. Rats were fed orally with melatonin once a day at a dose of 37.5, 75, or 150 mg/kg 30 min before until 3 days after LPC treatment. Subsequently, behavioral tests were conducted on these animals, and immunohistochemistry and immunoblotting were used for qualitative and quantitative analysis of glia and MAPKs, including ERK, JNK, and p38, activation. Enzyme-linked immunosorbent assays were applied to measure pro-inflammatory cytokine responses. Furthermore, intra-CN microinjection of S26131 (MT₁ receptor antagonist), 4P-PDOT (MT₂ receptor antagonist), or prazosin (MT₃ receptor antagonist) were performed to investigate the association between melatonin receptor subtypes and effects of melatonin on demyelination neuropathy. LPC treatment of the median nerve induced a significant increase in glial fibrillary acidic protein (GFAP; an astrocyte marker) and ED1 (an activated microglia marker) immunoreactivity in the ipsilateral CN and led to development of neuropathic pain behavior. Inspection of GFAP-immunoreactive astrocytes revealed that astrocytic hypertrophy, but not proliferation, contributed to increased GFAP immunoreactivity. Double immunofluorescence showed that both GFAP-immunoreactive astrocytes and ED1-immunoreactive microglia co-expressed p-ERK, p-JNK, and p-p38 immunoreactivity. Melatonin administration dose-dependently reduced neuropathic pain behavior, decreased glial and MAPKs activation, and diminished the release of pro-inflammatory cytokines in the ipsilateral CN after LPC treatment. Furthermore, 4P-PDOT, but not S26131 or prazosin, antagonized the therapeutic effects of melatonin. In conclusion, administration of melatonin, via its cognate MT₂ receptor, inhibited activation of glial MAPKs, production of pro-inflammatory cytokines, and development of demyelination-induced neuropathic pain behavior.

Erythropoietin reduces nerve demyelination, neuropathic pain behavior and microglial MAPKs activation through erythropoietin receptors on Schwann cells in a rat model of peripheral neuropathy

Neuroprotective effects of erythropoietin (EPO) on peripheral nerve injury remain uncertain. This study investigated the efficacy of EPO in attenuating median nerve chronic constriction injury (CCI)-induced neuropathy. Animals received an intraneural injection of EPO at doses of 1,000, 3,000, or 5,000 units/kg 15 min before median nerve CCI. Afterwards, the behavioral and electrophysiological tests were conducted. Immunohistochemistry and immunoblotting were used for qualitative and quantitative analysis of microglial and mitogen-activated protein kinases (MAPKs), including p38, JNK, and ERK, activation. Enzyme-linked immunosorbent assay and microdialysis were applied to measure pro-inflammatory cytokine and glutamate responses, respectively. EPO pre-treatment dose-dependently ameliorated neuropathic pain behavior, decreased microglial and MAPKs activation, and diminished the release of pro-inflammatory cytokines and glutamate in the ipsilateral cuneate nucleus after CCI. Moreover, EPO pre-treatment preserved myelination of the injured median nerve on morphological investigation and suppressed injury-induced discharges. We also observed that EPO receptor (EPOR) expression was up-regulated in the injured nerve after CCI. Double immunofluorescence showed that EPOR was localized to Schwann cells. Furthermore, siRNA-mediated knockdown of EPOR expression eliminated the therapeutic effects of EPO on attenuating the microglial and MAPKs activation, pro-inflammatory cytokine responses, injury discharges, and neuropathic pain behavior in CCI rats. In conclusion, binding of EPO to its receptors on Schwann cells maintains myelin integrity and blocks ectopic discharges in the injured median nerve, that in the end contribute to attenuation of neuropathic pain via reducing glutamate release from primary afferents and inhibiting activation of microglial MAPKs and production of pro-inflammatory cytokines.

Neurosteroid Allopregnanolone Suppresses Median Nerve Injury–induced Mechanical Hypersensitivity and Glial Extracellular Signal–regulated Kinase Activation through γ-Aminobutyric Acid Type A Receptor Modulation in the Rat Cuneate Nucleus

Background

Mechanisms underlying neuropathic pain relief by the neurosteroid allopregnanolone remain uncertain. We investigated if allopregnanolone attenuates glial extracellular signal-regulated kinase (ERK) activation in the cuneate nucleus (CN) concomitant with neuropathic pain relief in median nerve chronic constriction injury (CCI) model rats.

Methods

We examined the time course and cellular localization of phosphorylated ERK (p-ERK) in CN after CCI. We subsequently employed microinjection of a mitogen-activated protein kinase kinase (ERK kinase) inhibitor, PD98059, to clarify the role of ERK phosphorylation in neuropathic pain development. Furthermore, we explored the effects of allopregnanolone (by mouth), intra-CN microinjection of γ-aminobutyric acid type A receptor antagonist (bicuculline) or γ-aminobutyric acid type B receptor antagonist (phaclofen) plus allopregnanolone, and allopregnanolone synthesis inhibitor (medroxyprogesterone; subcutaneous) on ERK activation and CCI-induced behavioral hypersensitivity.

Results

At 7 days post-CCI, p-ERK levels in ipsilateral CN were significantly increased and reached a peak. PD98059 microinjection into the CN 1 day after CCI dose-dependently attenuated injury-induced behavioral hypersensitivity (withdrawal threshold [mean ± SD], 7.4 ± 1.1, 8.7 ± 1.0, and 10.3 ± 0.8 g for 2.0, 2.5, and 3.0 mM PD98059, respectively, at 7 days post-CCI; n = 6 for each dose). Double immunofluorescence showed that p-ERK was localized to both astrocytes and microglia. Allopregnanolone significantly diminished CN p-ERK levels, glial activation, proinflammatory cytokines, and behavioral hypersensitivity after CCI. Bicuculline, but not phaclofen, blocked all effects of allopregnanolone. Medroxyprogesterone treatment reduced endogenous CN allopregnanolone and exacerbated nerve injury-induced neuropathic pain.

Conclusions

Median nerve injury-induced CN glial ERK activation modulated the development of behavioral hypersensitivity. Allopregnanolone attenuated glial ERK activation and neuropathic pain via γ-aminobutyric acid type A receptors. Reduced endogenous CN allopregnanolone after medroxyprogesterone administration rendered rats more susceptible to CCI-induced neuropathy.

Docosahexaenoic acid confers analgesic effects after median nerve injury via inhibition of c-Jun N-terminal kinase activation in microglia

The c-Jun N-terminal kinase (JNK) in the central nervous system plays a critical role in the processing of neuropathic pain. Docosahexaenoic acid (DHA), a predominant omega-3 polyunsaturated fatty acid in the central nervous system, has a neuroprotective efficacy. In this study, we examined the relationships between JNK activation in the cuneate nucleus (CN) and behavioral hypersensitivity after chronic constriction injury (CCI) of the median nerve. We further investigated the effects of DHA administration on JNK activation and development of hypersensitivity. Using immunohistochemistry and immunoblotting, low levels of phosphorylated JNK (p-JNK) were detected in the CN of sham-operated rats. As early as 1 day after CCI, p-JNK levels in the ipsilateral CN were significantly increased and peaked at 7 days. Double-immunofluorescence labeling with cell-specific markers showed that p-JNK immunoreactive cells coexpressed OX-42, a microglia activation marker, suggesting the expression of p-JNK in the microglia. Microinjection of SP600125, a JNK inhibitor, into the CN 1 day after CCI attenuated injury-induced behavioral hypersensitivity in a dose-dependent manner. Furthermore, animals received intravenous injection of DHA at doses of 100, 250 or 500 nmol/kg 30 min after median nerve CCI. DHA treatment decreased p-JNK and OX-42 levels, diminished the release of proinflammatory cytokines and improved behavioral hypersensitivity following CCI. In conclusion, median nerve injury-induced microglial JNK activation in the CN modulated development of behavioral hypersensitivity. DHA has analgesic effects on neuropathic pain, at least in part, by means of suppressing a microglia-mediated inflammatory response through the inhibition of JNK signaling pathway.

Sleep deprivation aggravates median nerve injury-induced neuropathic pain and enhances microglial activation by suppressing melatonin secretion

STUDY OBJECTIVES

Sleep deprivation is common in patients with neuropathic pain, but the effect of sleep deprivation on pathological pain remains uncertain. This study investigated whether sleep deprivation aggravates neuropathic symptoms and enhances microglial activation in the cuneate nucleus (CN) in a median nerve chronic constriction injury (CCI) model. Also, we assessed if melatonin supplements during the sleep deprived period attenuates these effects.

DESIGN

Rats were subjected to sleep deprivation for 3 days by the disc-on-water method either before or after CCI. In the melatonin treatment group, CCI rats received melatonin supplements at doses of 37.5, 75, 150, or 300 mg/kg during sleep deprivation. Melatonin was administered at 23:00 once a day.

PARTICIPANTS

Male Sprague-Dawley rats, weighing 180-250 g (n = 190), were used.

MEASUREMENTS

Seven days after CCI, behavioral testing was conducted, and immunohistochemistry, immunoblotting, and enzyme-linked immunosorbent assay were used for qualitative and quantitative analyses of microglial activation and measurements of proinflammatory cytokines.

RESULTS

In rats who underwent post-CCI sleep deprivation, microglia were more profoundly activated and neuropathic pain was worse than those receiving pre-CCI sleep deprivation. During the sleep deprived period, serum melatonin levels were low over the 24-h period. Administration of melatonin to CCI rats with sleep deprivation significantly attenuated activation of microglia and development of neuropathic pain, and markedly decreased concentrations of proinflammatory cytokines.

CONCLUSIONS

Sleep deprivation makes rats more vulnerable to nerve injury-induced neuropathic pain, probably because of associated lower melatonin levels. Melatonin supplements to restore a circadian variation in melatonin concentrations during the sleep deprived period could alleviate nerve injury-induced behavioral hypersensitivity.

AMPA and GABA(A/B) receptor subunit expression in the cuneate nucleus of adult squirrel monkeys during peripheral nerve regeneration

The primate somatosensory neuroaxis provides an excellent model system with which to investigate adult neural plasticity. Here, we report immunohistochemical staining data for AMPA and GABAA/B receptor subunits in the cuneate nucleus of adult squirrel monkeys 1 and 5 months after median nerve compression. This method of nerve injury allowed the investigation of the way in which patterns of receptor correlates change during peripheral nerve regeneration. These results are compared to cortical data collected within the same animals. As observed in the cortex, the pattern of subunit staining in the brainstem 1 month after nerve compression suggests that the sensory deprived nucleus enters a state of reorganization. That is, the expression of GluR2/3 AMPA receptor subunits is significantly increased, while GABA α1 and GABABR1b receptor subunits are significantly decreased. Five months after nerve injury, the pattern of subunit expression is again very similar to that observed in the infragranular layers of cortex. At this later time we observe a significant increase in GluR2/3 and GABABR1a, with no change in GABAAα1, and a significant decrease in GABABR1b. Together these results suggest that during reorganization and recovery from injury the brainstem and cortex are governed by homogeneous mechanisms of plasticity.

Melatonin reduces median nerve injury-induced mechanical hypersensitivity via inhibition of microglial p38 mitogen-activated protein kinase activation in rat cuneate nucleus

In this study, we examined the relationships between p38 mitogen-activated protein kinase (MAPK) activation in the cuneate nucleus (CN) and behavioral hypersensitivity after chronic constriction injury (CCI) of the median nerve. We further investigated effects of melatonin administration and pinealectomy on p38 MAPK activation and development of hypersensitivity. Using immunohistochemistry and immunoblotting, low levels of phosphorylated p38 (p-p38) MAPK were detected in CN of normal rats. As early as 1 day after CCI, p-p38 MAPK levels in the ipsilateral CN were significantly increased (1.4 ± 0.2-fold, P < 0.05), which reached a maximum at 7 days (5.1 ± 0.4-fold, P < 0.001). Double immunofluorescence labeling with cell-specific markers showed that p-p38 MAPK immunoreactive cells co-expressed OX-42, a microglia activation maker, suggesting the expression of p-p38 MAPK in microglia. Microinjection of SB203580, a p38 MAPK inhibitor, into the CN 1 day after CCI attenuated injury-induced behavioral hypersensitivity in a dose-dependent manner. Furthermore, animals received melatonin treatment at daily doses of 37.5, 75, 150, or 300 mg/kg from 30 min before until 3 days after CCI. Melatonin treatment dose-dependently attenuated p-p38 MAPK levels, release of pro-inflammatory cytokines, and behavioral hypersensitivity following CCI; conversely, pinealectomy that resulted in a reduction in endogenous melatonin levels exacerbated these effects. In conclusion, median nerve injury-induced microglial p38 MAPK activation in the CN modulated development of behavioral hypersensitivity. Melatonin supplementation eased neuropathic pain via inhibition of p38 MAPK signaling pathway; contrarily, reducing endogenous blood melatonin levels by pinealectomy promoted phosphorylation of p38 MAPK and made rats more vulnerable to nerve injury-induced neuropathic pain.

Changes in GABA and GABA(B) receptor expressions are involved in neuropathy in the rat cuneate nucleus following median nerve transection

This study examined the relationship between changes in GABA transmission and behavioral abnormalities after median nerve transection. Following unilateral median nerve transection, the percentage of GABA-like immunoreactive neurons in the cuneate nucleus and that of GABA(B) receptor-like immunoreactive neurons in the dorsal root ganglion in the injured side decreased and reached a nadir at 4 weeks after median nerve transection. Four weeks after bilateral median nerve transection and intraperitoneal application with saline, baclofen (2 mg kg⁻¹), or phaclofen (2 mg kg⁻¹) before unilateral electrical stimulation of the injured median nerve, we investigated the level of neuropeptide Y release and c-Fos expression in the stimulated side of the cuneate nucleus. The neuropeptide Y release level and the number of c-Fos-like immunoreactive neurons in the baclofen group were significantly attenuated, whereas those in the phaclofen group had increased compared to the saline group. These findings indicate that median nerve transection reduces GABA transmission, promoting injury-induced neuropeptide Y release and consequently evoking c-Fos expression in cuneate nucleus neurons. Furthermore, this study used the CatWalk method to assess behavioral abnormalities in rats following median nerve transection. These abnormalities were reversed by baclofen treatment. Overall, the results suggest that baclofen treatment block neuropeptide Y release, subsequently lessening c-Fos expression in cuneate neurons and consequently attenuating neuropathic signal transmission to the thalamus.

Nitric oxide implicates c-Fos expression in the cuneate nucleus following electrical stimulation of the transected median nerve

In this study, we investigated whether nitric oxide (NO) modulated injury-induced neuropeptide Y (NPY) releasing and c-Fos expression in the cuneate nucleus (CN) after median nerve transection (MNT). We first examined the temporal changes of neuronal nitric oxide synthase (nNOS) expression in the dorsal root ganglion (DRG) and CN after MNT. Following MNT, the amounts of nNOS-like immunoreactive (nNOS-LI) neurons in the DRG and CN significantly increased as compared with those of the sham-operated rats. Furthermore, 4 weeks after MNT, the increases of nNOS-LI neurons in the DRG and CN were attenuated by pre-emptive lidocaine treatment in a dose-dependent manner. Finally, 4 weeks after MNT, pre-stimulation administration of L-NAME (N (ω)-Nitro-L: -arginine methyl ester) or 7-NI (7-nitroindazole) suppressed the amount of NPY release from the stimulated terminals and thus attenuated c-Fos expression in the CN. Our data implied that NO would modulate neuronal activity in the DRG and CN both after MNT.

Nerve injury-induced changes in GABA(A) and GABA(B) sub-unit expression in area 3b and cuneate nucleus of adult squirrel monkeys: further evidence of developmental recapitulation

The primate somatosensory system provides an excellent model system with which to investigate adult neural plasticity. Here, we report immunohistochemical staining data for the GABA(A) α1, GABA(B)R1a, and GABA(B)R1b receptor subunits in somatosensory area 3b, and cuneate nucleus one week after median nerve compression in adult squirrel monkeys. We find a significant decrease in GABA(A) α1 subunit staining across all cortical layers and within both soma and neuropil of the deprived cortical and brainstem regions. The GABA(B) staining showed an opposing shift in deprived regions, with a significant increase in presynaptic GABA(B)R1a staining, and a significant decrease in postsynaptic GABA(B)R1b staining in deprived regions of the cortex and brainstem. These changes in receptor subunit expression generate patterns that are very similar to those reported in the neonate. Furthermore, the similarities between brainstem and cortical expression suggest conserved forms of adult plasticity in these two regions. Taken together these results, along with the results from our previous paper investigating AMPA subunit expression in these same animals, support the hypothesis that deprived neurons enter a previously hidden state of developmental recapitulation that serves to prime the brain for NMDA receptor mediated receptive field reorganization.

Cortical influences on rapid brainstem plasticity

Cortical contributions to brainstem plasticity in the somatosensory system are poorly understood. Tactile receptive fields (RFs) of brainstem dorsal column nuclei (DCN) neurons rapidly enlarge when peripheral inputs are disrupted by local anesthetic blocks with lidocaine (LID). Cortical inputs appear to influence this plasticity because enlargements have been shown to be greater when cortical inputs are disrupted. Like disruptions of peripheral inputs, disruptions of DCN inhibition by DCN administration of the GABAA receptor antagonist bicuculline methiodide (BMI) also cause rapid enlargements of DCN RFs when cortical inputs are intact. These findings leave questions about interactions between cortical inputs, DCN inhibition, and DCN RF plasticity. To study potential interactions, the present experiments evaluated RF sizes of DCN tactilely responsive neurons in anesthetized rats following DCN microinjection of BMI when cortical inputs were acutely disrupted or intact. These tests were also supplemented by subsequent LID tests to directly compare post-BMI and post-LID effects on the same RF. BMI caused DCN RF enlargements when cortical inputs were disrupted or intact; however, enlargements after cortical input disruption were greater than when cortical inputs were intact. Following RF enlargement and retraction after BMI, LID often caused a second enlargement of the same RF, across skin that partially matched skin involved in the enlargement after BMI. This occurred when cortical inputs were disrupted or intact. We hypothesize that cortical inputs are not required for BMI and LID to initiate partially matching enlargements in individual DCN tactile RFs, however, cortical inputs constrain magnitudes of these enlargements.

The chemo- and somatotopic architecture of the Galago cuneate and gracile nuclei

The pattern of peripheral nerve inputs into the dorsal column nuclei, cuneate and gracile, was investigated in the prosimian Galago garnetti. The major findings were, that there is a greater segregation of the inputs from the fingers/hand within the cuneate compared with input form the toes/foot within the gracile. In both nuclei, cell clusters can be identified as cytochrome oxidase dense blotches, reactive also for the activity-dependent enzyme nitric oxide synthase. In the cuneate, cell clusters were apparent as six main cytochrome oxidase/nitric oxide synthase-reactive ovals arranged in a medial to lateral sequence. In contrast in the gracile, a higher degree of parcellation was noted and several cytochrome oxidase/nitric oxide synthase blotches were distributed along the rostrocaudal axis of the nucleus. This different architecture parallels differences in the organization of the inputs from the hand and from the foot. In the cuneate, cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals from the glabrous and hairy skin of digits d1 to d5 segregated in each of the five most lateral cytochrome oxidase/nitric oxide synthase blotches. Afferents from the thenar, palmar pads and hypothenar overlapped with those from digit 1, digit 2 to digit 4 and digit 5, respectively. Inputs from wrist arm and shoulder were segregated in the most medial blotch. In the gracile, multiple foci of cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals were observed upon injections of single sites in the toes or plantar pads. Although in multiple foci, inputs from different toes segregated from one another as well. Terminals from the plantar pads appeared to converge on the same cytochrome oxidase/nitric oxide synthase blotches targeted by inputs from the toes. In both the cuneate and the gracile, cytochrome oxidase/nitric oxide synthase blotches also presented intense immunoreactivity for GABA, calbindin, parvalbumin, and brain derived neurotrophic factor. Finally, in the cuneate the cell cluster region presented similarities in prosimian galagos and four species of New World monkeys, whereas it appeared more differentiated and complex in the Old Word macaque monkeys. In conclusion, the different pattern of segregation of the inputs from the hand and from the foot can be related to the different metabolic organization of the cuneate and of the gracile, respectively.

The distribution and characterization of NADPH-d/NOS-IR neurons in the rat cuneate nucleus

Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and nitric oxide synthase (NOS) immunohistochemistry have been used to characterize the nitric oxide (NO)-containing neurons in the rat cuneate nucleus. The present results showed that NADPH-d-positive/NOS-immunoreactive (-IR) neurons were distributed in the entire rostrocaudal extent of the nucleus. In the caudal region (approximately 1-2 mm caudal to the obex), NADPH-d/NOS-IR neurons were aggregated along the dorsal area of the nucleus notably in the lateral aspect. When traced rostrally, labeled neurons were progressively reduced and the cells were randomly distributed. The labeled neurons varied from round, ovoid to spindle-shaped with a mean profile area of about 140.1+/-1.7 microm(2) (n=720). They made up 7-10% of the neuronal population in the cuneate nucleus. By immunoelectron microscopy, the immunoreaction product was deposited throughout the cytoplasm extending from the soma to the proximal and distal dendrites. Results of NADPH-d staining paralleled that of NOS immunohistochemistry. Furthermore, NADPH-d reactivity and NOS-IR were colocalized in the same neurons following double labeling. Using NADPH-d histochemistry along with anti-gamma-aminobutyric acid (GABA) and -glycine postembedding immunolabeling for identification of GABA- and glycine-IR neurons, respectively, about 33% of the NADPH-d-positive neurons contained both GABA and glycine, 26% of them contained only glycine, while 41% of them showed neither GABA nor glycine labeling. Cuneothalamic neurons (CTNs) were identified by injecting the retrograde tracer Fluorogold (FG) into the ventrobasal complex of the thalamus. Numerous FG-labeled neurons were present in the contralateral cuneate nucleus, but none were reactive for NADPH-d. The present results suggest that approximately 60% of the NADPH-d/NOS-IR neurons in the cuneate nucleus are interneurons containing GABA and/or glycine.

Afferent synaptic contacts on glycine-immunoreactive neurons in the rat cuneate nucleus

This study was aimed to clarify whether the primary afferent terminals (PATs), GABAergic terminals, and glutamatergic terminals made direct synaptic contacts with glycine-IR neurons in the cuneate nucleus of rats. In this connection, injection of the anterograde tracer WGA-HRP into brachial plexus, antiglycine preembedding immunoperoxidase, and anti-GABA, along with antiglutamate postembedding immunogold labeling, were used to identify the PATs, glycine-IR neurons, GABA-IR terminals, and glutamate-IR terminals, respectively. The present results showed that HRP-labeled PATs, immunoperoxidase-labeled glycine-IR terminals, immunogold-labeled GABA-IR, and glutamate-IR terminals made axodendritic synaptic contacts with immunoperoxidase-labeled glycine-IR neurons. The latter three presynaptic elements also formed axosomatic synapses with glycine-IR neurons. Statistical analysis has shown that the minimum diameter of the glycine-IR dendrites postsynaptic to the above-mentioned four presynaptic elements did not differ significantly. In addition, the synaptic ratio of the glutamate-IR terminals on the glycine-IR dendrites was higher than that of GABA-IR terminals. The synaptic ratio of the GABA-IR terminals on glycine-IR dendrite was in turn higher than that of the PATs and glycine-IR terminals. It is suggested that the PATs and glutamate-IR terminals on the glycine-IR neurons may be involved in subsequent postsynaptic inhibition for spatial precision of lateral inhibition. On the other hand, the GABA-IR and glycine-IR terminals which make synaptic contacts with the dendrites of glycine-IR neurons may provide a putative means for disinhibition or facilitation to maintain the baseline neuronal activity in the rat cuneate nucleus. The results of quantitative analysis suggest that glutamate act as the primary excitatory neurotransmitter, while GABA, when compared with glycine, may serve as a more powerful inhibitory neurotransmitter on glycine-IR neurons in the rat cuneate nucleus.

Rhythmic neuronal interactions and synchronization in the rat dorsal column nuclei

Single-unit and multiunit activities were recorded from dorsal column nuclei of anesthetized rats in order to study the characteristics of the oscillatory activity expressed by these cells and their neuronal interactions. On the basis of their firing rate characteristics in spontaneous conditions, two types of dorsal column nuclei cell have been identified. Low-frequency cells (74%) were silent or displayed a low firing rate (1.9+/-0.48 spikes/s), and were identified as thalamic-projecting neurons because they were activated antidromically by medial lemniscus stimulation. High-frequency cells (26%) were characterized by higher discharge rates (27.2+/-5.1 spikes/s). None of them was antidromically activated by medial lemniscus stimulation. Low-frequency neurons showed a non-rhythmic discharge pattern spontaneously which became rhythmic under sensory stimulation of their receptive fields (48% of cases; 4.8+/-0.23Hz). All high-frequency neurons showed a rhythmic discharge pattern at 13.8+/- 0.68Hz either spontaneously or during sensory stimulation of their receptive fields. The shift predictor analysis indicated that oscillatory activity is not phase-locked to the stimulus onset in either type of cell, although the stimulus can reset the phase of the rhythmic activity of high-frequency cells. Cross-correlograms between pairs of low-frequency neurons typically revealed synchronized rhythmic activity when the overlapping receptive fields were stimulated. Rhythmic synchronization of high-frequency discharges was rarely observed spontaneously or under sensory stimulation. High-frequency neuronal firing could be correlated with the low-frequency neuronal activity or more often with the multiunit activity during sensory stimulation. Moreover, the presence of oscillatory activity modulated the sensory responses of dorsal column nuclei cells, favoring their responses. These findings indicate that thalamic-projecting and non-projecting neurons in dorsal column nuclei exhibited distinct oscillatory characteristics. However, both types of neuron may be entrained into an oscillatory rhythmic pattern when their overlapping receptive fields are stimulated, suggesting that in those conditions the dorsal column nuclei generate a populational oscillatory output to the somatosensory thalamus which could modulate and amplify the effectiveness of the somatosensory transmission.

Somatostatin-IR neurons are a major subpopulation of the cuneothalamic neurons in the rat cuneate nucleus

This study was aimed to localize and characterize the somatostatin-immunoreactive (SOM-IR) neurons in the rat cuneate nucleus (CN). By immuno-histochemistry, the SOM-IR neurons, which were widely distributed in the nucleus, were round, spindle or multiangular in shape (mean area = 226.1 +/ -3.1 microm(2), n = 1016). By electron microscopy, the neurons shared all the ultrastructural features of the cuneothalamic neurons (CTNs) which showed a slightly indented nucleus and a fairly rich cytoplasm containing well-developed Golgi apparatuses and rough endoplasmic reticulum (rER). The SOM immunoreaction product filled the cytoplasm of the neurons extending from the soma to the proximal and distal dendrites, which were postsynaptic to unlabeled boutons. In addition to soma and dendrites, SOM-IR boutons were also identified which made axodendritic synaptic contacts with SOM-IR dendrites. The SOM-IR neurons were characterized by using anti-SOM pre-embedding immunolabeling coupled with horseradish peroxidase (HRP) retrograde method, or SOM immunolabeling along with anti-glutamate, gamma-aminobutyric acid (GABA) or glycine post-embedding immunolabeling for identification of CTNs, glutamate-IR, GABA-IR and glycine-IR neurons, respectively. It was shown that more then 80% of the CTNs contained SOM and, furthermore, they contained glutamate but not GABA or glycine. On the basis of present findings, it is suggested the majority of the SOM-IR neurons in the rat CN are CTNs and that they may be involved in modulation of somatosensory synaptic transmission.

Cuneothalamic relay neurons are postsynaptic to glycine-immunoreactive terminals in the rat cuneate nucleus

This study was aimed to clarify whether the cuneothalamic relay neurons (CTNs) in the rat cuneate nucleus contained glycine or whether the neurons were modulated directly by presynaptic glycine-IR terminals. For this purpose, retrograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) and immunoperoxidase labelling for glycine have been used to ascertain if the CTNs in the rat are glycine-immunoreactive (glycine-IR). Our results have shown that the WGA-HRP-labelled CTNs (mean area = 318 +/- 6.5 microm(2)) were not reactive for glycine. Glycine immunoreactivity, however, was localized in some small-sized neurons (mean area = 210 +/- 6.2 microm(2)) and axon terminals associated with the CTNs. The synaptic organization between the glycine-IR terminals and CTNs was further analyzed using anti-glycine postembedding immunogold labelling. By electron microscopy, the immunogold-labelled glycine-IR terminals containing pleomorphic synaptic vesicles formed symmetrical synaptic contacts with the dendrites, dendritic spines, and somata of CTNs. Quantitative estimation showed that the mean ratios of glycine-IR terminals to total terminals associated with the soma, proximal dendrites and distal dendrites of the CTN were 49.5, 45.2, and 45.8%, respectively. The higher incidence of glycine-IR terminals on the soma, however, was not significantly different from that of the proximal and distal dendrites. Notwithstanding the above, this study has shown a large number of glycine-IR terminals making direct synaptic contacts with CTNs, suggesting that glycine is one of the important neurotransmitters involved in postsynaptic inhibition on the cuneothalamic relay neurons to modulate incoming somatosensory information from forelimb areas in the rat.

Lemniscal recurrent and transcortical influences on cuneate neurons

Intracellular recordings were obtained from cuneate neurons of chloralose-anesthetized, paralysed cats to study the synaptic responses induced by electrical stimulation of the contralateral medial lemniscus. From a total of 178 cells sampled, 109 were antidromically fired from the medial lemniscus, 82 of which showed spontaneous bursting activity. In contrast, the great majority (58/69) of the non-lemniscal neurons presented spontaneous single spike activity. Medial lemniscus stimulation induced recurrent excitation and inhibition on cuneolemniscal and non-lemniscal cells. Some non-lemniscal neurons were activated by somatosensory cortex and inhibited by motor cortex stimulation. Some other non-lemniscal cells that did not respond to medial lemniscus stimulation in control conditions were transcortically affected by stimulating the medial lemniscus after inducing paroxysmal activity in the sensorimotor cortex. These findings indicate that different sites in the sensorimotor cortex can differentially influence the sensory transmission through the cuneate, and that the distinct available corticocuneate routes are selected within the cerebral cortex. From a total of 92 cells tested, the initial effect induced by low-frequency stimulation of the sensorimotor cortex was inhibition on most of the cuneolemniscal neurons (32/52) and excitation on the majority of the non-lemniscal cells (25/40). The fact that a substantial proportion of cuneolemniscal and non-lemniscal cells was excited and inhibited, respectively, suggests that the cerebral cortex may potentiate certain inputs by exciting and disinhibiting selected groups of cuneolemniscal cells. Finally, evidence is presented demonstrating that the tendency of the cuneolemniscal neurons to fire in high-frequency spike bursts is due to different mechanisms, including excitatory synaptic potentials, recurrent activation through lemniscal axonal collaterals, and via the lemnisco-thalamo-cortico-cuneate loop.A corticocuneate network circuit to explain the results is proposed.

Synaptic relationships between corticocuneate terminals and glycine-immunoreactive neurons in the rat cuneate nucleus

The present study describes the ultrastructural synaptic relationships between corticocuneate terminals (CCTs) and glycine-immunoreactive (glycine-IR) neurons in the cuneate nucleus of rats using anterograde tract-tracing of wheatgerm agglutinin conjugated with horseradish peroxidase (WGA-HRP) and anti-glycine immunoperoxidase labeling methods. The HRP-labeled CCTs made axodendritic synapses preferentially in the ventral part of the cuneate nucleus near the obex. In a total of 182 CCTs surveyed, 14 of them made direct synaptic contacts with immunoperoxidase-labeled glycine-IR dendrites. The present results suggest that cortical modulation on the sensory transmission of cuneate nucleus may be mediated through glycine-IR neurons.

Morphometric study of glycine-immunoreactive neurons and terminals in the rat cuneate nucleus

The distribution of glycine-immunoreactive (glycine-IR) neurons and their associated axon terminals in the rat cuneate nucleus was studied using antiglycine postembedding immunoperoxidase labelling and immunogold staining, respectively. The immunoperoxidase-labelled glycine-IR neurons were widely distributed in the entire rostrocaudal extent of the nucleus. They made up 30.8% (9671/31368) of the neurons surveyed. Quantitative evaluation showed that the percentage of glycine-IR neurons in the caudal level was significantly higher than that in the middle and rostral levels. The glycine-IR neurons were small cells (mean area = 198+/-1.9 microm2, n = 2862) with ovoid or spindle-shaped somata. Statistical analysis showed that the size of the glycine-IR neurons in the rostral level was significantly smaller than that in the middle and caudal levels. Immunogold labelled glycine-IR terminals which contained predominantly pleomorphic synaptic vesicles were mostly small (mean area = 1.24+/-0.03 microm2, n = 286) and they constituted 24.7% (286/1158) of the total terminals surveyed. They formed axodendritic, axosomatic and axoaxonic synapses with unlabelled elements. It is suggested from this study that glycine is one of the major neurotransmitters involved in the depression of synaptic transmission in the cuneate nucleus.