Ongoing and stimulus-evoked activity of sympathetically correlated neurons in the intermediate zone and dorsal horn of acutely spinalized rats

We have shown previously that in the acutely spinalized anesthetized rat the activities of many dorsal horn interneurons (DHN) at the T(10) level are correlated positively with both ongoing and stimulus-evoked renal sympathetic nerve activity (RSNA) and therefore may belong to networks generating RSNA after acute, cervical, spinal transection. In the present study, we recorded from both DHN and interneurons in the intermediate zone (IZN) of the T(10) spinal segment in acutely C(1)-transected, chloralose-anesthetized, artificially respired rats. The activities of a similar percentage of IZN and DHN were correlated positively with ongoing RSNA, but the peaks of spike-triggered averages of RSNA based on the activity of IZN were larger, relative to dummy averages, than spike-triggered averages of RSNA based on the activity of DHN. Sympathetically correlated DHN and IZN differed in their responses to noxious somatic stimuli. Most correlated DHN had relatively simple somatic fields; they were excited by noxious stimulation of the T(10) and nearby dermatomes and inhibited by stimulation of more distal dermatomes. As we have shown previously, the excitatory and inhibitory fields of these neurons were very similar to fields that, respectively, excited and inhibited RSNA. On the other hand, the somatic fields of 50% of sympathetically correlated IZN were significantly more complex, indicating a difference between either the inputs or the processing properties of IZN and DHN. Sympathetically correlated IZN and DHN also differed in their responses to colorectal distension (CRD), a noxious visceral stimulus. CRD increased RSNA in 11/15 rats and increased the activity of most sympathetically correlated T(10) IZN. On the other hand, CRD decreased the activity of a majority of sympathetically correlated T(10) DHN. These observations suggest that the same stimulus may differentially affect separate, putative, sympathoexcitatory pathways, exciting one and inhibiting the other. Thus the magnitude and even the polarity of responses to a given stimulus may be determined by the modality and location of the stimulus, the degree to which multiple pathways are affected by the stimulus, and the ongoing activity of presympathetic neurons, at multiple rostrocaudal levels, before stimulation. A multipathway system may explain the variability in autonomic responses to visceral and somatic stimuli exhibited in spinally injured patients.

The effects of ropivacaine on sodium currents in dorsal horn neurons of neonatal rats

We used a whole cell patch clamp technique to study the effects of ropivacaine on rat dorsal horn neurons. Under voltage clamp, ropivacaine (10-400 microM) produced a dose-dependent inhibition of sodium current. From a holding potential (V(h)) of -80 mV, sodium currents evoked by test pulses to 0 mV were inhibited by ropivacaine with a mean drug concentration required to produce 50% current inhibition (IC(50)) value of 117.3 microM, which was more than the value of the bupivacaine (IC(50) 53.7 microM). The inhibition effect of ropivacaine was also voltage-dependent. Current evoked from a V(h) of -60 mV was inhibited by ropivacaine with a mean IC(50) value of 74.3 microM, which was less than that obtained at the V(h) of -80 mV. The inhibition effect of ropivacaine on sodium current was use dependent. Repeated activation by a train of depolarizing pulses (5 Hz, 20 ms) increased the inhibitory effects of ropivacaine. The ratio amplitudes of the 20th to the first pulse were 91.2% and 71.1%, respectively, in the absence and presence of ropivacaine (50 microM). Ropivacaine also produced a significant hyperpolarizing shift of 11 mV in the steady-state inactivation curve of sodium current. The inhibition of ropivacaine on the sodium channel may contribute to the mechanism of action of local anesthetics during epidural and spinal anesthesia.

Actions of endomorphins on synaptic transmission of Adelta-fibers in spinal cord dorsal horn neurons

The effects of endogenous mu-opioid ligands, endomorphins, on Adelta-afferent-evoked excitatory postsynaptic currents (EPSCs) were studied in substantia gelatinosa neurons in spinal cord slices. Under voltage-clamp conditions, endomorphins blocked the evoked EPSCs in a dose-dependent manner. To determine if the block resulted from changes in transmitter release from glutamatergic synaptic terminals, the opioid actions on miniature excitatory postsynaptic currents (mEPSCs) were examined. Endomorphins (1 microM) reduced the frequency but not the amplitude of mEPSCs, suggesting that endomorphins directly act on presynaptic terminals. The effects of endomorphins on the unitary (quantal) properties of the evoked EPSCs were also studied. Endomorphins reduced unitary content without significantly changing unitary amplitude. These results suggest that in addition to presynaptic actions on interneurons, endomorphins also inhibit evoked EPSCs by reducing transmitter release from Adelta-afferent terminals.

ATP facilitates spontaneous glycinergic IPSC frequency at dissociated rat dorsal horn interneuron synapses

1. The ATP action on spontaneous miniature glycinergic inhibitory postsynaptic currents (mIPSCs) was investigated in rat substantia gelatinosa (SG) neurons mechanically dissociated from the 2nd layer of the dorsal horn in which their presynaptic glycinergic nerve terminals remained intact. 2. ATP reversibly facilitated the frequency of the mIPSCs in a concentration-dependent manner without affecting their amplitude distribution. The ATP agonist, 2-methylthioATP (2MeSATP), mimicked the ATP action, while another ATP receptor agonist, alphabeta-methylene-ATP (alpha,beta-meATP), had no effect on mIPSCs. 3. The ATP receptor antagonists, suramin (1 x 10-6 M) and pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (1 x 10-5 M), completely blocked the facilitatory effect of ATP on glycine release (102.0 +/- 11.2 % and 99.3 +/- 16.2 %, n = 6, respectively) without altering the current amplitude distributions. 4. N-Ethylmaleimide (NEM), a sulphydryl alkylating agent, suppressed the inhibitory effect of adenosine on mIPSC frequency (111.2 +/- 13. 3 %, n = 4) without altering the current amplitude distribution. However, ATP still facilitated the mIPSC frequency (693.3 +/- 245.2 %, n = 4) even in the presence of NEM. 5. The facilitatory effect of ATP (1 x 10-5 M) on mIPSC frequency was not affected by adding 1 x 10-4 M Cd2+ to normal external solution but was eliminated in a Ca2+-free external solution. 6. These results suggest that ATP enhances glycine release from nerve terminals, presumably resulting in the inhibition of SG neurons which conduct nociceptive signals to the CNS. This presynaptic P2X-type ATP receptor may function to prevent excess excitability in SG neurons, thus preventing an excessive pain signal and/or SG cell death.

The difference in temporal distribution of c-Fos immunoreactive neurons between the medullary dorsal horn and the trigeminal subnucleus oralis in the rat following experimental tooth movement

The difference in temporal distribution of c-Fos-immunoreactivity (Fos-IR) was assessed in the medullary dorsal horn (MDH) and in the dorsomedial part of the trigeminal subnucleus oralis (Vodm) following experimental tooth movement of the rat maxillary molars. The number of MDH c-Fos-immunoreactive neurons increased bilaterally at 2 h and decreased markedly by 12 h, and then increased again with a small peak at 48 h. In contrast, Vodm c-Fos expression was not up-regulated until 12 h, but increased in number after 24 h, which increase lasted until 72 h. These findings indicate that experimental tooth movement induced nociceptive c-Fos response in a biphasic manner. Furthermore, the later response appeared after 24 h, and lasted for a few days, mainly manifested in the Vodm during experimental tooth movement.

Differential effects of MK-801, a N-methyl-D-aspartate non-competitive antagonist, on the dorsal horn neuron hyperactivity and hyperexcitability in neuropathic rats

N-methyl-D-aspartate (NMDA) involvement in altered spinal neuron activity following peripheral nerve injury has been investigated in rats with chronic constriction of the sciatic nerve. Extracellular single neuron recordings were performed, in anesthetized, paralyzed rats, from the sciatic spinal cord segments (lumbar, L5-L6) ipsilateral to the constriction, and the effect of iontophoresized MK-801, an NMDA receptor non-competitive antagonist, was tested on baseline hyperactivity and hyperresponsiveness to noxious stimulation. The results show that baseline activity was unaffected whereas the noxious evoked responses were significantly modified, there being amplitude reduction and after-discharges suppression. The different role of NMDA in the abnormal pain states related to the abnormal neuronal activities is discussed.

Robust changes of afferent-induced excitation in the rat spinal dorsal horn after conditioning high-frequency stimulation

We investigated the neuronal plasticity in the spinal dorsal horn and its relationship with spinal inhibitory networks using an optical-imaging method that detects neuronal excitation. High-intensity single-pulse stimulation of the dorsal root activating both A and C fibers evoked an optical response in the lamina II (the substantia gelatinosa) of the dorsal horn in transverse slices of 12- to 25-day-old rat spinal cords stained with a voltage-sensitive dye, RH-482. The optical response, reflecting the net neuronal excitation along the slice-depth, was depressed by 28% for more than 1 h after a high-frequency conditioning stimulation of A fibers in the dorsal root (3 tetani of 100 Hz for 1 s with an interval of 10 s). The depression was not induced in a perfusion solution containing an NMDA antagonist, DL-2-amino-5-phosphonovaleric acid (AP5; 30 microM). In a solution containing the inhibitory amino acid antagonists bicuculline (1 microM) and strychnine (3 microM), and also in a low Cl(-) solution, the excitation evoked by the single-pulse stimulation was enhanced after the high-frequency stimulation by 31 and 18%, respectively. The enhanced response after conditioning was depotentiated by a low-frequency stimulation of A fibers (0.2-1 Hz for 10 min). Furthermore, once the low-frequency stimulation was applied, the high-frequency conditioning could not potentiate the excitation. Inhibitory transmissions thus regulate the mode of synaptic plasticity in the lamina II most likely at afferent terminals. The high-frequency conditioning elicits a long-term depression (LTD) of synaptic efficacy under a greater activity of inhibitory amino acids, but it results in a long-term potentiation (LTP) when inhibition is reduced. The low-frequency preconditioning inhibits the potentiation induction and maintenance by the high-frequency conditioning. These mechanisms might underlie robust changes of nociception, such as hypersensitivity after injury or inflammation and pain relief after electrical or cutaneous stimulation.

Effects of midazolam in the spinal nerve ligation model of neuropathic pain in rats

Potential changes in the spinal GABAergic activity after nerve injury were studied by comparing the effects of systemic administration of the benzodiazepine midazolam on the noxious evoked responses of dorsal horn in rats with spinal nerve ligation of neuropathy and control animals. The tight ligation of the L(5) and L6 spinal nerves was performed in adult male Sprague-Dawley rats and resulting mechanical and cold allodynia were assessed with von Frey hairs and the acetone drop test. Single unit extracellular recordings of dorsal horn neurones were performed 15-18 days after the surgery under halothane anaesthesia using transcutaneous electrical stimulation of the receptive field at three times the C-fibre threshold. The rats in the spinal nerve ligation group, but not in the sham-operated control group developed mechanical and cold allodynia. Subcutaneous administration of midazolam 0.1-3.0 mg/kg reduced the Adelta-fibre evoked activity in a dose-related manner in all study groups, but the C-fibre evoked activity was significantly reduced only in the spinal nerve ligation group. The inhibitory effects of s.c. midazolam were significantly reversed by i.t. administration of flumazenil, suggesting a spinal site of action. Midazolam reduced C-fibre evoked firing significantly more in the spinal nerve ligation model than in the non-operated or sham controls. These results indicate changes in the spinal GABAergic system in the neuropathic animals and could be of importance in the development of new treatments for neuropathic pain.

Physiological properties of the lamina I spinoparabrachial neurons in the rat

Single-unit extracellular recordings of spino-parabrachial (spino-PB) neurons (n = 53) antidromically driven from the contralateral parabrachial (PB) area were performed in the lumbar cord in anesthetized rats. All the spino-PB neurons were located in the lamina I of the dorsal horn. Their axons exhibited conduction velocities between 2.8 and 27.8 m/s, in the thin myelinated fibers range. They had an extremely low spontaneous activity (median = 0. 064 Hz) and a small excitatory receptive field (</=2 toes or pads). They were all activated by both peripheral A (mainly Adelta) and C fibers after intense transcutaneous electrical stimulation. Their discharge always increased in response to noxious natural stimuli of increasing intensities. The great majority (75%) of spino-PB neurons were nociceptive specific, i.e., they were excited only by noxious stimuli. The remaining (25%) still were excited primarily by noxious stimuli but also responded moderately to innocuous stimuli. Almost all spino-PB neurons (92%, 49/53) were activated by both mechanical and heat noxious stimuli. Among them, 35% were in addition moderately activated by noxious cold (thresholds between +20 and -10 degrees C). Only (8%, 4/53) responded exclusively to noxious heat. Spino-PB neurons clearly encoded the intensity of mechanical (n = 39) and thermal (n = 38) stimuli in the noxious range, and most of the individual stimulus-response functions were monotonic and positive up to 40/60 N. cm(-2) and 50 degrees C, respectively. For the mechanical modality, the mean threshold was 11.5 +/- 1.25 N. cm(-2) (mean +/- SE), the response increased almost linearly with the logarithm of the pressure between 10 and 60 N. cm(-2), the mean p(50) (pressure evoking 50% of the maximum response) and the maximum responsiveness were: 30 +/- 2.4 N. cm(-2) and 40.5 +/- 5 Hz, respectively. For the thermal modality, the mean threshold was 43.6 +/- 0.5 degrees C, the mean curve had a general sigmoid aspect, the steepest portion being in the 46-48 degrees C interval, the mean t(50) and the maximum responsiveness were: 47.4 +/- 0.3 degrees C and 40 +/- 4.4 Hz, respectively. Most of the spino-PB neurons tested (13/16) had their noxiously evoked responses clearly inhibited by heterotopic noxious stimuli. The mean response to noxious stimuli during heterotopic stimuli was 31.7 +/- 6.1% of the control response. We conclude that the nociceptive properties of the lamina I spino-PB neurons are reflected largely by those of PB neurons that were suggested to be involved in autonomic and emotional/aversive aspects of pain.

Presynaptic P2X receptors facilitate inhibitory GABAergic transmission between cultured rat spinal cord dorsal horn neurons

The superficial layers of the spinal cord dorsal horn (DH) express P2X2, P2X4, and P2X6 subunits entering into the formation of ionotropic (P2X) receptors for ATP. Using a culture system of laminae I-III from neonatal rat DH, we show that ATP induced a fast nonselective cation current in 38% of the neurons (postsynaptic effect). ATP also increased the frequency of miniature IPSCs (mIPSCs) mediated by GABA(A) receptors or by glycine receptors in 22 and 9%, respectively, of the neurons tested (presynaptic effect) but had no effect on glutamatergic transmission. The presynaptic effect of ATP on GABAergic transmission was not significantly affected by thapsigargin (1 microM) but was completely dependent on Ca(2+) influx. Presynaptic and postsynaptic effects were inhibited by suramin, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, and reactive blue and were not reproduced by uridine 5'-triphosphate (UTP) or adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-S), suggesting the implication of ionotropic P2X rather than of metabotropic P2Y receptors. alphabeta-methylene-ATP (100 microM) did not reproduce the effects of ATP. ATP reversibly increased the amplitude of electrically evoked GABAergic IPSCs and reduced paired-pulse inhibition or facilitation without affecting IPSC kinetics. This effect was preferentially, but not exclusively, observed in neurons coreleasing ATP and GABA. We conclude that in cultured DH neurons, the effects of ATP are mediated by P2X receptors having a pharmacological profile dominated by the P2X2 subunit. The presynaptic receptors might underlie a modulatory action of ATP on a subset of GABAergic interneurons involved in the spinal processing of nociceptive information.

Visualization of lamina I of the dorsal horn in live adult rat spinal cord slices

The superficial dorsal horn of the spinal cord, particularly lamina I, plays a key role in the integration and relay of pain related sensory input. To study the physiology of lamina I neurons in slices, a clear delineation of this layer can be greatly advantageous. Yet, it has remained difficult to distinguish this layer in live tissue in conventional transverse spinal slices because of its very narrow thickness at the edge of the dorsal horn. We describe here the criteria we used to delineate lamina I in live tissue using gradient contrast videomicroscopy in 400 microm-thick parasagittal spinal cord slices from adult rats (30-60-day-old). Because of the longitudinal orientation of the neurons in this layer, the resulting distinctive reticulated appearance of lamina I made it possible to readily distinguish it from lamina II. The usefulness of this distinguishing parameter is demonstrated by our ability to contrast synaptic properties of neurons in lamina I from those in lamina II. Complete morphological identification of lamina I neurons however also requires visualization of the cell in the horizontal plane. To maintain compatibility with the parasagittal slice, we used 3D reconstructions from confocal images of the recorded neurons. Rotation of the neuron in space allowed for its morphological characterization in all three planes (horizontal, parasagittal, and transverse). This approach therefore presents optimal conditions for systematic electrophysiological recording from visually identified lamina I neurons.

The ontogeny of endomorphin-1- and endomorphin-2-like immunoreactivity in rat brain and spinal cord

Endomorphin-1 and endomorphin-2 are recently described peptides with high affinity and specificity for the mu opioid receptor. They are believed to be the endogenous ligands for that receptor. We describe the maturation of the endomorphins in brain and spinal cord using recently characterized antibodies to each. Endomorphin-1-like immunoreactivity was examined in brain, focusing on the periaqueductal gray of the midbrain and the diagonal band of Broca; endomorphin-2-like immunoreactivity is reported for the medulla and spinal cord. In these regions, and in all other regions studied but not described in this paper, the endomorphins were not seen at birth or at 3 days of age. Staining was present in 7-day-old and older animals. At these early ages, the pattern and density of staining are not fully developed, but appear complete by 21 days of age. The results suggest that both endomorphin-1 and endomorphin-2 develop relatively late compared to other opioid peptides.

Effect of streptozotocin-induced diabetes on the activity of calcium channels in rat dorsal horn neurons

We have previously found that spinal dorsal horn neurons from streptozotocin-diabetic rats, an animal model for diabetes mellitus, show the prominent changes in the mechanisms responsible for [Ca2+]i regulation. The present study aimed to further characterize the effects of streptozotocin-induced diabetes on neuronal calcium homeostasis. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in Fura-2AM-loaded dorsal horn neurons from acutely isolated spinal cord slices using fluorescence technique. We studied Ca2+ entry through plasmalemmal Ca2+ channels during potassium (50 mM KCl)-induced depolarization. The K+-induced [Ca2+]i elevation was inhibited to a different extent by nickel ions, nifedipine and omega-conotoxin suggesting the co-expression of different subtypes of plasmalemmal voltage-gated Ca2+ channels. The suppression of [Ca2+]i transients by Ni2+ (50 microM) was the same in control and diabetic neurons. On the other hand, inhibition of [Ca2+]i transients by nifedipine (50 microM) and omega-conotoxin (1 microM) was much greater in diabetic neurons compared with normal animals. These data suggest that under diabetic conditions the activity of N- and L- but not T-type voltage-gated Ca2+ channels substantially increased in dorsal horn neurons.

Synaptic reorganization in the substantia gelatinosa after peripheral nerve neuroma formation: aberrant innervation of lamina II neurons by Abeta afferents

Intracellular recording and extracellular field potential (FP) recordings were obtained from spinal cord dorsal horn neurons (laminae I-IV) in a rat transverse slice preparation with attached dorsal roots. To study changes in synaptic inputs after neuroma formation, the sciatic nerve was sectioned and ligated 3 weeks before in vitro electrophysiological analysis. Horseradish peroxidase labeling of dorsal root axons indicated that Abeta fibers sprouted into laminae I-II from deeper laminae after sciatic nerve section. FP recordings from dorsal horns of normal spinal cord slices revealed long-latency synaptic responses in lamina II and short-latency responses in lamina III. The latencies of synaptic FPs recorded in lamina II of the dorsal horn after sciatic nerve section were reduced. The majority of monosynaptic EPSPs recorded with intracellular microelectrodes from lamina II neurons in control slices were elicited by high-threshold nerve stimulation, whereas the majority of monosynaptic EPSPs recorded in lamina III were elicited by low-threshold nerve stimulation. After sciatic nerve section, 31 of 57 (54%) EPSPs recorded in lamina II were elicited by low-threshold stimulation. The majority of low-threshold EPSPs in lamina II neurons after axotomy displayed properties similar to low-threshold EPSPs in lamina III of control slices. These results indicate that reoccupation of lamina II synapses by sprouting Abeta fibers normally terminating in lamina III occurs after sciatic nerve neuroma formation. Furthermore, these observations indicate that the lamina II neurons receive inappropriate sensory information from low-threshold mechanoreceptor after sciatic nerve neuroma formation.

Silent NMDA receptor-mediated synapses are developmentally regulated in the dorsal horn of the rat spinal cord

In vitro whole cell patch-clamp recording techniques were utilized to study silent pure-N-methyl-D-aspartate (NMDA) receptor-mediated synaptic responses in lamina II (substantia gelatinosa, SG) and lamina III of the spinal dorsal horn. To clarify whether these synapses are present in the adult and contribute to neuropathic pain, transverse lumbar spinal cord slices were prepared from neonatal, naive adult and adult sciatic nerve transected rats. In neonatal rats, pure-NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) were elicited in SG neurons either by focal intraspinal stimulation (n = 15 of 20 neurons) or focal stimulation of the dorsal root (n = 2 of 7 neurons). In contrast, in slices from naive adult rats, no silent pure-NMDA EPSCs were recorded in SG neurons following focal intraspinal stimulation (n = 27), and only one pure-NMDA EPSC was observed in lamina III (n = 23). Furthermore, in rats with chronic sciatic nerve transection, pure-NMDA EPSCs were elicited by focal intraspinal stimulation in only 2 of 45 SG neurons. Although a large increase in Abeta fiber evoked mixed alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and NMDA receptor-mediated synapses was detected after sciatic nerve injury, Abeta fiber-mediated pure-NMDA EPSCs were not evoked in SG neurons by dorsal root stimulation. Pure-NMDA receptor-mediated EPSCs are therefore a transient, developmentally regulated phenomenon, and, although they may have a role in synaptic refinement in the immature dorsal horn, they are unlikely to be involved in receptive field plasticity in the adult.

Effects of monoamines on interneurons in four spinal reflex pathways from group I and/or group II muscle afferents

Effects of locally applied serotonin (5-HT) and noradrenaline (NA) were tested on extracellularly recorded responses of single spinal interneurons in deeply anaesthetized cats. These effects were tested on: (i) interneurons mediating reciprocal inhibition from group Ia afferents; (ii) interneurons mediating non-reciprocal inhibition from group Ia and Ib afferents; (iii) intermediate zone interneurons co-excited by group I and II afferents; and (iv) dorsal horn interneurons excited by group II afferents. Effects of monoamines were tested on responses evoked at latencies compatible with monosynaptic coupling. Responses evoked by group Ia and/or Ib muscle afferents were facilitated in all of the tested interneurons both by NA and 5-HT. Responses evoked by group II muscle afferents were depressed in the majority of the interneurons but were facilitated in some of them. 5-HT depressed these responses in all dorsal horn interneurons and in one subpopulation of intermediate zone interneurons, while it facilitated them in another subpopulation of intermediate zone interneurons. NA depressed them in all intermediate zone interneurons and in one subpopulation of dorsal horn interneurons, while it facilitated them in another subpopulation of dorsal horn interneurons. The results of this study lead to the conclusions that: (i) modulation of synaptic actions of muscle spindle and tendon organ afferents on spinal interneurons by 5-HT and NA is related to both the type of the afferent and the functional type of the interneuron; and that (ii) 5-HT and NA counteract each others' actions on some interneuronal types but mutually enhance them on the others.

Functional regeneration of sensory axons into the adult spinal cord

The arrest of dorsal root axonal regeneration at the transitional zone between the peripheral and central nervous system has been repeatedly described since the early twentieth century. Here we show that, with trophic support to damaged sensory axons, this regenerative barrier is surmountable. In adult rats with injured dorsal roots, treatment with nerve growth factor (NGF), neurotrophin-3 (NT3) and glial-cell-line-derived neurotrophic factor (GDNF), but not brain-derived neurotrophic factor (BDNF), resulted in selective regrowth of damaged axons across the dorsal root entry zone and into the spinal cord. Dorsal horn neurons were found to be synaptically driven by peripheral nerve stimulation in rats treated with NGF, NT3 and GDNF, demonstrating functional reconnection. In behavioural studies, rats treated with NGF and GDNF recovered sensitivity to noxious heat and pressure. The observed effects of neurotrophic factors corresponded to their known actions on distinct subpopulations of sensory neurons. Neurotrophic factor treatment may thus serve as a viable treatment in promoting recovery from root avulsion injuries. I

Chronic, selective forebrain responses to excitotoxic dorsal horn injury

Intraspinal injection of the AMPA/metabotropic receptor agonist quisqualic acid (QUIS) results in excitotoxic injury which develops pathological characteristics similar to those associated with ischemic and traumatic spinal cord injury (SCI) (R. P. Yezierski et al., 1998, Pain 75: 141-155; R. P. Yezierski et al., 1993, J. Neurotrauma 10: 445-456). Since spinal injury can lead to partial or complete deafferentation of ascending supraspinal structures, it is likely that secondary to the disruption of spinal pathways these regions could undergo significant reorganization. Recently, T. J. Morrow et al. (Pain 75: 355-365) showed that autoradiographic estimates of regional cerebral blood flow (rCBF) can be used to simultaneously identify alterations in the activation of multiple forebrain structures responsive to noxious formalin stimulation. Accordingly, we examined whether excitotoxic SCI produced alterations in the activation of supraspinal structures using rCBF as a marker of neuronal activity. Twenty-four to 41 days after unilateral injection of QUIS into the T12 to L3 spinal segments, we found significant increases in the activation of 7 of 22 supraspinal structures examined. As compared to controls, unstimulated SCI rats exhibited a significant bilateral increase in rCBF within the arcuate nucleus (ARC), the hindlimb region of S1 cortex (HL), parietal cortex (PAR), and the thalamic posterior (PO), ventral lateral (VL), ventral posterior lateral (VPL), and ventral posterior medial (VPM) nuclei. All structures showing significantly altered rCBF are associated with the processing of somatosensory information. These changes constitute remote responses to injury and suggest that widespread functional changes occur within cortical and subcortical regions following injury to the spinal cord.

The glial cell line-derived neurotrophic factor family receptor components are differentially regulated within sensory neurons after nerve injury

Glial cell line-derived neurotrophic factor (GDNF) has potent trophic effects on adult sensory neurons after nerve injury and is one of a family of proteins that includes neurturin, persephin, and artemin. Sensitivity to these factors is conferred by a receptor complex consisting of a ligand binding domain (GFRalpha1-GFRalpha4) and a signal transducing domain RET. We have investigated the normal expression of GDNF family receptor components within sensory neurons and the response to nerve injury. In normal rats, RET and GFRalpha1 were expressed in a subpopulation of both small- and large-diameter afferents projecting through the sciatic nerve [60 and 40% of FluoroGold (FG)-labeled cells, respectively]. GFRalpha2 and GFRalpha3 were both expressed principally within small-diameter DRG cells (30 and 40% of FG-labeled cells, respectively). Two weeks after sciatic axotomy, the expression of GFRalpha2 was markedly reduced (to 12% of sciatic afferents). In contrast, the proportion of sciatic afferents that expressed GFRalpha1 increased (to 66% of sciatic afferents) so that virtually all large-diameter afferents expressed this receptor component, and the expression of GFRalpha3 also increased (to 66% of sciatic afferents) so that almost all of the small-diameter afferents expressed this receptor component after axotomy. There was little change in RET expression. The changes in the proportions of DRG cells expressing different receptor components were mirrored by alterations in the total RNA levels within the DRG. The changes in GFRalpha1 and GFRalpha2 expression after axotomy could be largely reversed by treatment with GDNF.

Spontaneous activity of rat dorsal horn cells in spinal segments of sciatic projection following transection of sciatic nerve or of corresponding dorsal roots

Natural forms of stimulation were used to compare the spontaneous and evoked activity of dorsal horn neurons in three groups of rats: controls with no surgical lesion, rats with transection of the sciatic nerve and rats with transection of the dorsal roots at the same segmental level. In control rats, cells encountered in the dorsal horn were classified according to their peripheral field as tactile specific, convergent tactile and nociceptive, nociceptive, or movement driven. In 20 control animals, only 20% of the 140 cells with a peripheral field were spontaneously active. After sciatic nerve transection made on the side of recording a few days previously (18 rats), all of the 141 cells studied showed spontaneous activity, only 69 of them having a peripheral field. After dorsal root transections a few days previously (nine rats), 25 spontaneously active cells were found in the dorsal horn ipsilateral to the section, none with a peripheral field. Spontaneous activities of cells without a peripheral field were separated into three types as a function of bursting pattern, which were similar following both types of transection. The spontaneous activity shown by dorsal horn cells without peripheral fields following dorsal root transection precludes attribution of spontaneous spiking in such cells to abnormal input from the periphery, and shows that abnormal activity can develop in deafferented dorsal horn cells themselves. A possible role played by this spontaneous activity in deafferentation pain is considered.

Changes in calcium signalling in dorsal horn neurons in rats with streptozotocin-induced diabetes

Intracellular calcium signalling was studied in the dorsal horn from neurons of rats with streptozotocin-induced diabetes versus control animals. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in Fura-2 acetoxymethyl ester-loaded dorsal horn neurons from acutely isolated spinal cord slices using a fluorescence technique. The recovery of depolarization-induced [Ca2+]i increase was delayed in diabetic neurons compared with normal animals. In normal neurons, [Ca2+]i after the end of KCl depolarization recovered to the basal level monoexponentially with a time constant of 8.0+/-0.5 s (n = 23), while diabetic neurons showed two exponentials in the [Ca2+]i recovery. The time constants of these exponentials were 7.2+/-0.5 and 23.0+/-0.6 s (n = 19), respectively. The amplitude of calcium release from caffeine-sensitive endoplasmic reticulum calcium stores became significantly smaller in diabetic neurons. The amplitudes of [Ca2+]i transients evoked by 30 mM caffeine were 268+/-29 nM (n = 13) and 31+/-9 nM (n = 17) in control and diabetic neurons, respectively. We conclude that streptozotocin-induced diabetes is associated with prominent changes in the mechanisms responsible for [Ca2+]i regulation, which presumably include a slowdown of Ca2+ elimination from the cytoplasm by the endoplasmic reticulum.

Dorsal horn NMDA receptor function is changed after peripheral inflammation

The N-methyl-D-aspartic acid (NMDA) receptor antagonist D, L-2-amino-5-phosphonopentanoic acid (AP5) caused a stronger inhibition of wind-up in single wide dynamic range (WDR) neurons after carrageenan inflammation compared with control neurons without inflammation in the receptive field. This indicates that even a short period (2.5 h) of inflammation induces changes in the function of NMDA receptors. The drug effect was also studied in separate control experiments with few wind-up inducing stimulus trains and little nociceptive input prior to baseline recordings. In these control experiments all evoked responses were reduced by the drug, but the wind-up was significantly increased. A wind-up increase after NMDA receptor antagonism has been reported in two previous studies. Thus, other mechanisms than NMDA receptor stimulation may be more important for the wind-up in not sensitized dorsal horn neurons. As for long-term potentiation, it seems that NMDA receptor antagonists have an increased effect after sensitization. Thus, sensitized and not sensitized dorsal horn neurons may respond differently to an NMDA receptor active drug. In rats nerve stimulation and halothane anaesthesia induced larger evoked responses to afferent stimulation than cutaneous stimulation and urethane anaesthesia, the AP5 effect was however similar.

Brief and prolonged effects of Lissauer tract stimulation on dorsal horn cells

Increased excitability of dorsal horn neurones may play a critical role in producing some pain states and there is evidence that the excitability of neurones lying throughout the dorsal horn is subject to regulation by cells in its most superficial laminae. This paper examines the effect on dorsal horn cell receptive fields and excitability of the specific activation of Lissauer's tract, a tract containing propriospinal axons which arise from cells in the substantia gelatinosa and which project to the substantia of neighbouring spinal segments. Experiments were carried out on anaesthetised spinal rats in the L3-4 spinal segments with microelectrode stimulation on the surface of the Lissauer tract (LT) and microelectrode recording of single cells or small groups of cells that responded to gentle brushing on the skin. Single shocks or brief trains of low-level stimuli to the LT produced a characteristic long-latency dorsal root potential (DRP) on the L3 dorsal root and a brief burst of firing in superficial cells with no stimulation of primary afferents. Generally, this was accompanied by no excitation of deeper dorsal horn cells but commonly by a period of inhibition, often followed by facilitation. We then turned to the effect of long periods (30-90min) of continual LT stimulation because we had seen hints of prolonged facilitation of the deeper cells after periods of such stimulation. Trains of 5 stimuli separated by 2ms and repeated every 200ms were used with individual pulses of 200 micros duration and less than 10 microA amplitude. This resulted in a shift of the effect on deep cells from primarily inhibition to mainly facilitation. We then examined in detail the effect of these long periods of LT stimulation on the size of receptive fields (RFs) of dorsal horn cells first on single units and then by repeated mapping of the RFs of small groups of cells. Control periods of 60min with no LT stimulation produced no significant RF changes but 30, 60 or 90min of LT stimulation produced successively greater expansions of RFs. When the LT stimulus was turned off after 1h, the RFs remained expanded for at least 2h. The spike height of these cells remained unchanged. The effect was not influenced by the NMDA antagonist MK801 but was imitated by the GABA(A) antagonist picrotoxin. It is concluded that the prolonged expansion of RFs could not be produced by modulation of descending control since the animals had spinal transections. Neither was it dependent on an NMDA-sensitive mechanism. With these data it is not possible to conclude whether the mechanism is pre-synaptic, post-synaptic or both. It is proposed that the most likely explanation is a decrease in the normal tonic inhibition operated in part by a GABA dependent mechanism. This phenomenon may play a role in prolonged pathological states of increased spinal cord excitability.

Transmission of chronic nociception by spinal neurons expressing the substance P receptor

Substance P receptor (SPR)-expressing spinal neurons were ablated with the selective cytotoxin substance P-saporin. Loss of these neurons resulted in a reduction of thermal hyperalgesia and mechanical allodynia associated with persistent neuropathic and inflammatory pain states. This loss appeared to be permanent. Responses to mildly painful stimuli and morphine analgesia were unaffected by this treatment. These results identify a target for treating persistent pain and suggest that the small population of SPR-expressing neurons in the dorsal horn of the spinal cord plays a pivotal role in the generation and maintenance of chronic neuropathic and inflammatory pain.