Gliogenic LTP spreads widely in nociceptive pathways.

Learning and memory formation involve long-term potentiation (LTP) of synaptic strength. A fundamental feature of LTP induction in the brain is the need for coincident pre- and postsynaptic activity. This restricts LTP expression to activated synapses only (homosynaptic LTP) and leads to its input specificity. In the spinal cord, we discovered a fundamentally different form of LTP that is induced by glial cell activation and mediated by diffusible, extracellular messengers, including d-serine and tumor necrosis factor (TNF), and that travel long distances via the cerebrospinal fluid, thereby affecting susceptible synapses at remote sites. The properties of this gliogenic LTP resolve unexplained findings of memory traces in nociceptive pathways and may underlie forms of widespread pain hypersensitivity.

Properties of spinal lamina III GABAergic neurons in naïve and in neuropathic mice.

BACKGROUND: Nerve injury leads to Aß-fibre-mediated mechanical allodynia that is in part due to an impaired GABAergic inhibition in the spinal cord dorsal horn. The properties and function of GABAergic neurons in spinal cord lamina III, an area where low-threshold mechanosensitive Aß-fibres terminate are, however, largely unknown. METHODS: We used transgenic mice, which express enhanced green fluorescent protein (EGFP) under control of the promoter GAD67. The morphology and neurochemical characteristics of GABAergic, EGFP-expressing neurons were characterized. We assessed active and passive membrane properties of spinal lamina III GABAergic neurons in naïve animals and animals with a chronic constriction injury (CCI) of the sciatic nerve. RESULTS: EGFP-expressing neurons in lamina III were predominantly islet cells (47%), whereas non-EGFP-expressing neurons were largely inverted stalked cells (40%). EGFP-expressing neurons accounted for about 25% of GABAergic neurons in lamina III. Forty-four percent co-expressed glycine, 10% neuronal nitric oxide synthase and 3% co-expressed parvalbumin. We found costaining with protein kinase CßII in 42% of EGFP-expressing neurons but no expression of protein kinase Cγ. Membrane properties and excitability of EGFP-and non-EGFP-expressing neurons from naïve and neuropathic animals were indistinguishable. The most frequent firing pattern was tonic firing (naïve: 35%, neuropathic: 37%) followed by gap firing (naïve: 33%, neuropathic: 25%). Delayed, initial burst and single-spike firing patterns made up the remainder in both groups. CONCLUSION: A change in membrane excitability or discharge pattern of this group of lamina III GABAergic neurons is unlikely the cause for mechanical allodynia in animals with CCI.

[Risk assessment in pain therapy]. / Risikoabschätzung in der Schmerztherapie.

Analgesic therapy is not without risk. However, the risk of most analgesic interventions is minor compared to the risk of the inadequate treatment of pain and insufficient treatment may lead to chronic pain.A correct diagnosis should be the basis of any specific treatment of pain disorders. Only a diagnosis which implicates a multi-disciplinary assessment and which considers both the pathoanatomical, functional and biopsychosocial dysfunctions can lead to an adequate therapeutic intervention. Furthermore, therapeutic planning should include the personal needs of the patient and should have realistic aims.Pharmacological treatment is guided by the WHO pain ladder. The risks of the relevant substance groups must be considered. NSAIDs (non-steroidal anti-inflammatory drugs) which are included in all steps of the WHO pain ladder carry specific risks for the gastrointestinal, cardiovascular and renal systems and are contraindicated in many patients in need of analgesic therapy, e.g. in many elderly patients. Opioids which are recommended at steps 2 and 3 of the WHO pain ladder have less organ toxicity but they are still used reluctantly. Coanalgetics, especially antidepressants bear specific risks and the discussion on suicide rates under antidepressant medication is ongoing.Invasive methods such as the intrathecal application of analgesics are valuable procedures if the indication is correct and the treating physician has sufficient experience. Pain therapy is essential and the risks of the procedures are manageable. Considering the current knowledge on the mechanisms of pain sensitisation, the lack of adequate pain control can lead to chronic pain with severe consequences for the patient.

Possible sources and sites of action of the nitric oxide involved in synaptic plasticity at spinal lamina I projection neurons.

The synaptic long-term potentiation between primary afferent C-fibers and spinal lamina I projection neurons is a cellular model for hyperalgesia [Ikeda H, Heinke B, Ruscheweyh R, Sandkühler J (2003) Synaptic plasticity in spinal lamina I projection neurons that mediate hyperalgesia. Science 299:1237-1240]. In lamina I neurons with a projection to the periaqueductal gray, this long-term potentiation is dependent on nitric oxide. In the present study, we used immunohistochemistry to detect possible sources and sites of action of the nitric oxide necessary for the long-term potentiation at lamina I spino-periaqueductal gray neurons in rats. None of the three isoforms of the nitric oxide synthase was expressed in a significant number of lamina I spino-periaqueductal gray neurons or primary afferent C-fibers (as evaluated by staining of their cell bodies in the dorsal root ganglia). However, endothelial and inducible nitric oxide synthase were found throughout the spinal cord vasculature and neuronal nitric oxide synthase was present in a number of neurons in laminae II and III. The nitric oxide target soluble guanylyl cyclase was detected in most lamina I spino-periaqueductal gray neurons and in approximately 12% of the dorsal root ganglion neurons, all of them nociceptive as evaluated by coexpression of substance P. Synthesis of cyclic 3',5'-guanosine monophosphate upon stimulation by a nitric oxide donor confirmed the presence of active guanylyl cyclase in at least a portion of the spino-periaqueductal gray neuronal cell bodies. We therefore propose that nitric oxide generated in neighboring neurons or blood vessels acts on the spino-periaqueductal gray neuron and/or the primary afferent C-fiber to enable long-term potentiation. Lamina I spino-parabrachial neurons were stained for comparison and yielded similar results.

Low dose of S+-ketamine prevents long-term potentiation in pain pathways under strong opioid analgesia in the rat spinal cord in vivo.

BACKGROUND: micro-Opioid receptor (MOR) agonists are strong antinociceptive drugs. Low, but not high doses of the MOR agonist fentanyl prevent synaptic long-term potentiation (LTP) in pain pathways. Block of spinal N-methyl-D-aspartate (NMDA) receptors prevent central sensitization. Here we tested whether the NMDA receptor antagonist S(+)-ketamine reduces C-fibre-evoked potentials and prevents induction of LTP despite high doses of fentanyl. METHODS: C-fibre-evoked field potentials were recorded in the superficial laminae I/II of the rat lumbar spinal cord. High-frequency stimulation (HFS) was applied to the sciatic nerve at C-fibre strength to induce LTP. S(+)-ketamine 5 mg kg(-1) was given 1 h before or after HFS. S(+)-ketamine 5 mg kg(-1) and fentanyl as a bolus (40 microg kg(-1)) followed by an infusion (96 microg kg(-1) h(-1)) were given before HFS to test the action of the combination of these drugs. RESULTS: HFS potentiated C-fibre-evoked field potentials to mean 173 (sem 15)% of control (n=7) for at least 1 h. Low-dose S(+)-ketamine given before HFS blocked the induction of LTP. S(+)-ketamine given after HFS had no effect on the maintenance of LTP. Low-dose S(+)-ketamine prevented induction of LTP under fentanyl-infusion. CONCLUSIONS: Low-dose S(+)-ketamine does not affect C-fibre-evoked potentials alone but blocks LTP induction in pain pathways. In contrast, high doses of opioids strongly reduce C-fibre-evoked potentials, but do not fully prevent LTP induction. In this animal study the combination of S(+)-ketamine with fentanyl reveals both a reduction of C-fibre-evoked potentials and prevention of LTP and seem therefore a better choice for perioperative pain management compared with the sole administration.

Pre- and postsynaptic contributions of voltage-dependent Ca2+ channels to nociceptive transmission in rat spinal lamina I neurons.

Activation of voltage-dependent Ca2+ channels (VDCCs) is critical for neurotransmitter release, neuronal excitability and postsynaptic Ca2+ signalling. Antagonists of VDCCs can be antinociceptive in different animal pain models. Neurons in lamina I of the spinal dorsal horn play a pivotal role in the processing of pain-related information, but the role of VDCCs to the activity-dependent Ca2+ increase in lamina I neurons and to the synaptic transmission between nociceptive afferents and second order neurons in lamina I is not known. This has now been investigated in a lumbar spinal cord slice preparation from young Sprague-Dawley rats. Microfluorometric Ca2+ measurements with fura-2 have been used to analyse the Ca2+ increase in lamina I neurons after depolarization of the cells, resulting in a distinct and transient increase of the cytosolic Ca2+ concentration. This Ca2+ peak was reduced by the T-type channel blocker, Ni2+, by the L-type channel blockers, nifedipine and verapamil, and by the N-type channel blocker, omega-conotoxin GVIA. The P/Q-type channel antagonist, omega-agatoxin TK, had no effect on postsynaptic [Ca2+]i. The NMDA receptor channel blocker D-AP5 reduced the Ca2+ peak, whereas the AMPA receptor channel blocker CNQX had no effect. Postsynaptic currents, monosynaptically evoked by electrical stimulation of the attached dorsal roots with C-fibre and Adelta-fibre intensity, respectively, were reduced by N-type channel blocker omega-conotoxin GVIA and to a much lesser extent, by P/Q-type channel antagonist omega-agatoxin TK, and the L-type channel blockers verapamil, respectively. No difference was found between unidentified neurons and neurons projecting to the periaqueductal grey matter. This is the first quantitative description of the relative contribution of voltage-dependent Ca2+ channels to the synaptic transmission in lamina I of the spinal dorsal horn, which is essential in the processing of pain-related information in the central nervous system.

Reduction of glycine receptor-mediated miniature inhibitory postsynaptic currents in rat spinal lamina I neurons after peripheral inflammation.

Peripheral inflammation may induce long-lasting sensitization in the central nociceptive system. Neurons in lamina I of the spinal dorsal horn play a pivotal role in the integration and relay of pain-related information. In rats we studied whether changes in passive and active membrane properties and/or alteration of glycine receptor-mediated inhibitory control of spinal lamina I neurons may contribute to central sensitization in a model of peripheral long-lasting inflammation (complete Freund's adjuvant, hindpaw). Spontaneously occurring glycine receptor-mediated miniature inhibitory postsynaptic currents (GlyR-mediated mIPSCs) were recorded in lumbar spinal lamina I neurons. Miniature IPSC rise, decay kinetics and mean GlyR-mediated mIPSC amplitude were not affected by peripheral inflammation. The mean frequency of GlyR-mediated mIPSCs of lamina I neurons ipsilateral to the inflamed hindpaw was, however, significantly reduced by peripheral inflammation when compared with neurons from noninflamed animals. Principal passive and active membrane properties and firing patterns of spinal lamina I neurons were not changed by inflammation. These results indicate that long-lasting peripheral inflammation leads to a reduced glycinergic inhibitory control of spinal lamina I neurons by a presynaptic mechanism.

Bidirectional actions of nociceptin/orphanin FQ on A delta-fibre-evoked responses in rat superficial spinal dorsal horn in vitro.

The present study investigated the modulatory actions of nociceptin/orphanin FQ on excitatory glutamatergic transmission in spinal dorsal horn. In transverse spinal cord slices with an attached dorsal root, mono- and polysynaptic A delta-fibre-evoked extracellular field potentials were recorded from superficial dorsal horn. Nociceptin/orphanin FQ showed bidirectional effects on monosynaptic transmission with a potentiation at lower concentrations (100-300 nM) and a dose-dependent depression at higher concentrations (1-3 microM). The polysynaptic field potential was dose-dependently depressed by nociceptin/orphanin FQ (100 nM-3 microM). None of the actions of nociceptin/orphanin FQ was reversed by the non-specific opioid receptor antagonist naloxone, the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovaleric acid or the peptide nocistatin. The bidirectional actions of nociceptin/orphanin FQ on the monosynaptic field potential may provide an in vitro model for the bidirectional actions of nociceptin/orphanin FQ in behavioural studies showing hyperalgesia at low doses of intrathecal nociceptin/orphanin FQ and analgesia at higher doses.

Activation of group I metabotropic glutamate receptors induces long-term depression at sensory synapses in superficial spinal dorsal horn.

Low-frequency stimulation of primary afferent Adelta-fibers can induce long-term depression of synaptic transmission in rat superficial spinal dorsal horn. Here, we have identified another form of long-term depression in superficial spinal dorsal horn neurons that is induced by specific group I but not group II metabotropic glutamate receptor (mGluR) agonists. Synaptic strength between Adelta-fibers and dorsal horn neurons was examined by intracellular recordings in a spinal cord-dorsal root slice preparation from young rat. In the presence of bicuculline and strychnine, bath application of (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid ((1S,3R)-ACPD) or the specific group I mGluR agonist (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG) but not the specific group II mGluR agonist (2S,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl)glycine (DCG-IV) for 20 min produced an acute and a long-term depression of synaptic strength. Bath application of the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovaleric acid did not affect these depressions by (1S,3R)-ACPD. After pre-incubation of slices with pertussis toxin, a G-protein inhibitor, (1S,3R)-ACPD still induced acute and long-term depressions. The phospholipase C inhibitor U73122 stereoselectively blocked the induction of long-term depression without affecting acute synaptic inhibition. This study demonstrates that, in the spinal cord, direct activation of group I mGluRs that are coupled to phospholipase C through pertussis toxin-insensitive G-proteins induces a long-term depression of synaptic strength. This may be relevant to the processing of sensory information in the spinal cord, including nociception.

Induction of homosynaptic long-term depression at spinal synapses of sensory a delta-fibers requires activation of metabotropic glutamate receptors.

The synaptic strength between primary afferent Adelta-fibers, many of which convey pain-related information, and second order neurons in the spinal dorsal horn can be depressed for prolonged periods of time in a use- and N-methyl-D-aspartate receptor-dependent fashion. Here, we have used a transverse spinal cord slice-dorsal root preparation of young rat to characterize the nature of this form of long-term depression and the role of metabotropic glutamate receptors. Dorsal roots were bisected and intracellular recordings were made from lamina II neurons with independent excitatory synaptic inputs from both dorsal root halves. Conditioning stimulation of one dorsal root half (1 Hz, 900 pulses) induced long-term depression that was specific for the stimulated pathway, i. e. homosynaptic in nature. The induction of long-term depression was prevented by non-selective group I and group II mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine, by selective group I receptor antagonist (S)-4-carboxyphenylglycine and by selective group II mGluR antagonist (RS)-alpha-methylserine-O-phosphate monophenyl ester. Group III mGluR antagonist (RS)-alpha-methylserine-O-phosphate was ineffective. Short-term depression was not affected by any of these antagonists.Thus, a homosynaptic form of long-term depression exists at putative nociceptive synapses in the spinal dorsal horn and its induction requires the activation of both group I and II metabotropic glutamate receptors.

[Nociception in newborn and premature babies]. / Nozizeption bei Früh- und Neugeborenen.

BACKGROUND: The somatosensory system of preterms and newborns differs substantially from adults. These differences are of considerable preclinical and clinical interest. Maturation of A- and C-fibre synaptic connections in the dorsal horn and development of descending inhibition from the brainstem all take place postnatally in the rat. In early stages of development there is no definite spatial separation in the dorsal horn between the nociceptive and the non-nociceptive system. In preterms but not in adults non-noxious stimuli can induce central sensitization. Many neurotransmitters and signalling molecules involved in pain pathways are expressed early in the developing nervous system but do not reach adult levels for a considerable period. More important, receptors are frequently transiently overexpressed or expressed in areas during development where they are not seen in the adult and may have a different functional profile. The descending pain inhibitory system that provides an important protection against central sensitization develops later than the ascending nociceptive system. Thus, during a critical period of time the immature nociceptive system is highly vulnerable. For example, neonatal circumcision in the absence of analgesia results in increased pain responses during subsequent routine vaccination months later. CONCLUSIONS: In view of the changing nature of neonatal somatosensory and pain pathways and the vulnerability of the developing nervous system to alterations in sensory stimulation it is important that preterms and newborns need the care of a specialist for prevention and treatment of pain to avoid suffer and long-term changes in the nervous system.

Brain-derived neurotrophic factor improves long-term potentiation and cognitive functions after transient forebrain ischemia in the rat.

We investigated the effect of brain-derived neurotrophic factor (BDNF) on hippocampal long-term potentiation (LTP) and cognitive functions after global cerebral ischemia in the rat. After four-vessel occlusion, BDNF was administered via an osmotic minipump continuously over 14 days intracerebroventricularly. Electrophysiological experiments were performed 14 days after cerebral ischemia. Test stimuli and tetanization were delivered to the Schaffer collaterals of the hippocampus and field excitatory postsynaptic potentials (fEPSP) were recorded in the CA1 region. Cognitive impairment was analyzed repeatedly with a passive avoidance test, a hole-board test, and with an activity center on the same animal. In sham-operated animals, LTP was consistantly induced after delivering a tetanus (increase of initial slope of fEPSP to 173 +/- 12% of baseline; n = 6). After transient forebrain ischemia LTP could not be induced (117 +/- 4% of baseline; n = 7). In ischemic animals treated with BDNF, LTP could be induced (168 +/- 28% of baseline; n = 8). Transient forebrain ischemia resulted in a significant decrease in spatial discrimination performance but not of associative memory. The ratios for working memory (WM) and reference memory (RM) 15 days after ischemia were lower in the ischemic rats (n = 10) than in the sham-operated control animals (n = 10; WM: 22 +/- 6 vs 72 +/- 7; RM: 30 +/- 7 vs 72 +/- 5). Postischemic intracerebroventricular BDNF infusion increased both WM (63 +/- 4; n = 10) and RM (58 +/- 5; n = 10). The spontaneous locomotor activity did not differ significantly in the three groups. These data indicate a protective effect of BDNF for synaptic transmission and cognitive functions after transient forebrain ischemia.

Inhibition of caspases prevents cell death of hippocampal CA1 neurons, but not impairment of hippocampal long-term potentiation following global ischemia.

An essential role for caspases in programmed neuronal cell death has been demonstrated in various in vitro studies, and synthetic caspase inhibitors have recently been shown to prevent neuronal cell loss in animal models of focal cerebral ischemia and traumatic brain injury, respectively. The therapeutic utility of caspase inhibitors, however, will depend on preservation of both structural and functional integrity of neurons under stressful conditions. The present study demonstrates that expression and proteolytic activity of caspase-3 is up-regulated in the rat hippocampus after transient forebrain ischemia. Continuous i.c.v. infusion of the caspase inhibitor N-benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethyl ketone significantly attenuated caspase-3-like enzymatic activity, and blocked delayed cell loss of hippocampal CA1 neurons after ischemia. Administration of N-benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethyl ketone, however, did not prevent impairment of induction of long-term potentiation in post-ischemic CA1 cells, suggesting that caspase inhibition alone does not preserve neuronal functional plasticity.

Long-term depression of C-fibre-evoked spinal field potentials by stimulation of primary afferent A delta-fibres in the adult rat.

Long-term potentiation (LTP) of spinal C-fibre-evoked field potentials can be induced by brief electrical stimulation of afferent C-fibres, by natural noxious stimulation of skin or by acute nerve injury. Here, we report that in urethane anaesthetized, adult rats prolonged high frequency burst stimulation of the sciatic nerve at Adelta-fibre strength produced long-term depression (LTD) of C-fibre-evoked field potentials, and also depressed the increased amplitudes of C-fibre-evoked field potentials recorded after LTP had been established (depotentiation). Electrical stimulation of Abeta-fibres failed to induce LTD or depotentiation. In spinalized rats, prolonged Adelta-fibre conditioning stimulation induced LTP rather than LTD of C-fibre-evoked field potentials. Thus, tonic descending inhibition may determine the direction of plastic changes in C-fibre-mediated synaptic transmission. Spinal application of the N-methyl-D-aspartic acid receptor antagonist D-APV blocked induction of LTD in intact rats and LTP in spinalized rats. The presently described LTD and the depotentiation of established LTP of C-fibre-evoked field potentials in spinal dorsal horn may underlie some forms of prolonged analgesia induced by peripheral nerve stimulation procedures.

Induction of long-term potentiation at spinal synapses by noxious stimulation or nerve injury.

Use-dependent long-term potentiation of synaptic strength (LTP) is an intensively studied model for learning and memory in vertebrates. Induction of LTP critically depends on the stimulation parameters of presynaptic fibres with synchronous high-frequency bursts being most effective at many central synapses. It is, however, not known whether naturally occurring discharge patterns may induce LTP and whether LTP has any biological function in sensory systems. Here we have investigated the LTP of excitatory synaptic transmission between primary afferent C-fibres, many of which are nociceptors, and neurons in rat superficial spinal dorsal horn. LTP that lasted for 4-6 h could not only be induced by electrical stimulation of sural nerve but also by natural stimulation of heat-, mechano- or chemosensitive nociceptors in the skin or by acute nerve injury. Maintenance of LTP was not affected when afferent nerves were cut 1 h or 5 min after noxious skin stimulation, indicating that an ongoing afferent barrage is not required. Natural noxious stimuli induced LTP in animals which were spinalized but were ineffective in intact animals. Thus, induction of LTP is suppressed by tonically active supraspinal descending systems. We conclude that the natural non-synchronized discharge patterns that are evoked by noxious stimulation may induce LTP and that this new form of LTP may be an underlying mechanism of afferent induced hyperalgesia.

Activation of spinal N-methyl-D-aspartate or neurokinin receptors induces long-term potentiation of spinal C-fibre-evoked potentials.

The use-dependent increase in synaptic strength between primary afferent C-fibres and second-order neurons in superficial spinal dorsal horn may be an important cellular mechanism underlying central hyperalgesia. This long-term potentiation can be blocked by antagonists of the N-methyl-D-aspartate subtype of glutamate receptor, the neurokinin 1 or the neurokinin 2 receptor. We have tested here whether activation of these receptors by superfusion of the spinal cord with corresponding agonists in the absence of presynaptic activity is sufficient to induce long-term potentiation. In urethane anaesthetized rats C-fibre-evoked field potentials were elicited in superficial laminae of lumbar spinal cord by electrical stimulation of the sciatic nerve. In rats with intact spinal cord, controlled superfusion of the spinal cord at recording segments for 60 min with N-methyl-D-aspartate, substance P or neurokinin A never induced long-term potentiation. Spinal superfusion with a mixture of N-methyl-D-aspartate, substance P and neurokinin A also failed to induce long-term potentiation in four rats tested. In spinalized rats, however, long-term potentiation was induced by either N-methyl-D-aspartate (at 10 microM, to 173 +/- 16% of control) substance P (at 10 microM, to 176 +/- 13% of control) or by neurokinin A (at 1 microM, to 198 +/- .20% of control). The induction of long-term potentiation by N-methyl-D-aspartate, substance P or neurokinin A was blocked by intravenous application of the receptor antagonists dizocilpine maleate (0.5 mg/kg), RP67580 (2 mg/kg) or SR48968 (0.2 mg/kg), respectively. Thus, activation of N-methyl-D-aspartate or neurokinin receptors may induce long-lasting plastic changes in synaptic transmission in afferent C-fibres and this effect may be prevented by tonic descending inhibition.

Corelease of two fast neurotransmitters at a central synapse.

It is widely accepted that individual neurons in the central nervous system release only a single fast transmitter. The possibility of corelease of fast neurotransmitters was examined by making paired recordings from synaptically connected neurons in spinal cord slices. Unitary inhibitory postsynaptic currents generated at interneuron-motoneuron synapses consisted of a strychnine-sensitive, glycine receptor-mediated component and a bicuculline-sensitive, gamma-aminobutyric acid (GABA)A receptor-mediated component. These results indicate that spinal interneurons release both glycine and GABA to activate functionally distinct receptors in their postsynaptic target cells. A subset of miniature synaptic currents also showed both components, consistent with corelease from individual synaptic vesicles.

Identification of synaptic connections in neural ensembles by graphical models.

A method for the identification of direct synaptic connections in a larger neural net is presented. It is based on a conditional correlation graph for multivariate point processes. The connections are identified via the partial spectral coherence of two neurons, given all others. It is shown how these coherences can be calculated by inversion of the spectral density matrix. In simulations with GENESIS, we discuss the relevance of the method for identifying different neural ensembles including an excitatory feedback loop and networks with lateral inhibitions.

Characterization of long-term potentiation of C-fiber-evoked potentials in spinal dorsal horn of adult rat: essential role of NK1 and NK2 receptors.

Impulses in afferent C fibers, e.g., during peripheral trauma, may induce plastic changes in the spinal dorsal horn that are believed to contribute to some forms of hyperalgesia. The nature of lasting changes in spinal nociception are still not well understood. Here we characterized the long-term potentiation (LTP) of spinal field potentials with a negative focus in superficial spinal dorsal horn evoked by supramaximal electrical stimulation of the sciatic nerve in urethan-anesthetized adult rats. The field potentials studied in this work had high thresholds (>/=7 V, 0.5 ms), long latencies (90-130 ms), and long chronaxy (1.1 ms) and were not abolished by muscle relaxation and spinalization. Thus they were evoked by afferent C fibers. In response to 1-Hz stimulation of afferent C fibers, amplitudes of C-fiber-evoked field potentials remained constant, whereas number of action potentials of some dorsal horn neurons increased progressively (wind-up). In all 25 rats tested, high-frequency, high-intensity stimulation (100 Hz, 30-40 V, 0.5 ms, 400 pulses given in 4 trains of 1-s duration at 10-s intervals) always induced LTP (to approximately 200% of control), which consistently lasted until the end of recording periods (4-9 h). This tetanic stimulation also significantly decreased mean threshold of C-fiber-evoked field potentials. The C-fiber volley, which was recorded simultaneously in sural nerve, was, however, not affected by the same tetanic stimulation. High-frequency, low-intensity stimulation (100 Hz, 3 V, 0.5 ms) never induced LTP in six rats tested. At an intermediate frequency, high-intensity stimulation (20 Hz, 40 V, 0.5 ms, 400 pulses given in 4 trains of 5 s at 10-s intervals) induced LTP in four out of six rats, which lasted until end of recording periods (3-6 h). In the remaining two rats, no LTP was induced. Low-frequency, high-intensity stimulation (2 Hz, 30-40 V, 0.5 ms, 400 pulses) induced LTP that lasted for 2-8 h in four out of five rats. Intravenous application of neurokinin 1 (NK1) or neurokinin 2 (NK2) receptor antagonist RP 67580 (2 mg/kg, n = 5) or SR 48968 (0.3 mg/kg, n = 5) 30 min before high-frequency, high-intensity stimulation blocked the induction of LTP in all rats tested. In contrast, the same dose of their inactive enantiomers RP 68651 (n = 5) or SR 48965 (n = 5) did not affect the induction of LTP. Spinal superfusion with RP 67580 (1 microM) from 30 min before to 30 min after high-frequency, high-intensity stimulation blocked induction of LTP in all five rats tested. Spinal application of SR 48968 (10 nM) prevented LTP in five out of seven rats. However, when spinal superfusions with RP 67580 (1 microM, n = 3) or SR 48968 (10 nM, n = 3) were started 1 h after high-frequency, high-intensity stimulation, established LTP was not affected. Thus the activation of neurokinin receptors is necessary for the induction but not for the maintenance of LTP of C-fiber-evoked field potentials in spinal dorsal horn. This model may be useful to study plastic changes in spinal cord induced by peripheral C-fiber stimulation. The LTP of C-fiber-evoked field potentials may be a mechanism underlying some forms of hyperalgesia.