Effects of cutaneous nerve and intraspinal conditioning of C-fibre afferent terminal excitability in decerebrate spinal rats

Elucidating afferent-driven presynaptic inhibition of primary afferent input to spinal laminae I and X

Although spinal processing of sensory information greatly relies on afferent-driven (AD) presynaptic inhibition (PI), our knowledge about how it shapes peripheral input to different types of nociceptive neurons remains insufficient. Here we examined the AD-PI of primary afferent input to spinal neurons in the marginal layer, lamina I, and the layer surrounding the central canal, lamina X; two nociceptive-processing regions with similar patterns of direct supply by Aδ- and C-afferents. Unmyelinated C-fibers were selectively activated by electrical stimuli of negative polarity that induced an anodal block of myelinated Aβ/δ-fibers. Combining this approach with the patch-clamp recording in an ex vivo spinal cord preparation, we found that attenuation of the AD-PI by the anodal block of Aβ/δ-fibers resulted in the appearance of new mono- and polysynaptic C-fiber-mediated excitatory postsynaptic current (EPSC) components. Such homosegmental Aβ/δ-AD-PI affected neurons in the segment of the dorsal root entrance as well as in the adjacent rostral segment. In their turn, C-fibers from the L5 dorsal root induced heterosegmental AD-PI of the inputs from the L4 Aδ- and C-afferents to the neurons in the L4 segment. The heterosegmental C-AD-PI was reciprocal since the L4 C-afferents inhibited the L5 Aδ- and C-fiber inputs, as well as some direct L5 Aβ-fiber inputs. Moreover, the C-AD-PI was found to control the spike discharge in spinal neurons. Given that the homosegmental Aβ/δ-AD-PI and heterosegmental C-AD-PI affected a substantial percentage of lamina I and X neurons, we suggest that these basic mechanisms are important for shaping primary afferent input to the neurons in the spinal nociceptive-processing network.

Primary afferent-driven presynaptic inhibition of C-fiber inputs to spinal lamina I neurons

Presynaptic inhibition of primary afferent terminals is a powerful mechanism for controlling sensory information flow into the spinal cord. Lamina I is the major spinal nociceptive projecting area and monosynaptic input from C-fibers to this region represents a direct pathway for transmitting pain signals to supraspinal centers. Here we used an isolated spinal cord preparation to show that this pathway is under control of the afferent-driven GABAergic presynaptic inhibition. Presynaptic inhibition of C-fiber input to lamina I projection and local-circuit neurons is mediated by recruitment of Aβ-, Aδ- and C-afferents. C-fiber-driven inhibition of C-fibers functions as a feedforward mechanism, by which the homotypic afferents control sensory information flow into the spinal cord and regulate degree of the primary nociceptive afferent activation needed to excite the second order neurons. The presynaptic inhibition of C-fiber input to lamina I neurons may be mediated by both synaptic and non-synaptic mechanisms, and its occurrence and extent are quite heterogeneous. This heterogeneity is likely to be reflective of involvement of lamina I neurons in diverse circuitries processing specific modalities of sensory information in the superficial dorsal horn. Thus, our results implicate both low- and high-threshold afferents in the modulation of C-fiber input into the spinal cord.

Origins of antidromic activity in sensory afferent fibers and neurogenic inflammation

Neurogenic inflammation results from the release of biologically active agents from the peripheral primary afferent terminal. This release reflects the presence of releasable pools of active product and depolarization-exocytotic coupling mechanisms in the distal afferent terminal and serves to alter the physiologic function of innervated organ systems ranging from the skin and meninges to muscle, bone, and viscera. Aside from direct stimulation, this biologically important release from the peripheral afferent terminal can be initiated by antidromic activity arising from five anatomically distinct points of origin: (i) afferent collaterals at the peripheral-target organ level, (ii) afferent collaterals arising proximal to the target organ, (iii) from mid-axon where afferents lacking myelin sheaths (C fibers and others following demyelinating injuries) may display crosstalk and respond to local irritation, (iv) the dorsal root ganglion itself, and (v) the central terminals of the afferent in the dorsal horn where local circuits and bulbospinal projections can initiate the so-called dorsal root reflexes, i.e., antidromic traffic in the sensory afferent.

The transition from naïve to primed nociceptive state: A novel wind-up protocol in mice

Wind-up (WU) is a progressive, frequency-dependent facilitation of spinal cord neurons in response to repetitive nociceptive stimulation of constant intensity. We identified a new WU-associated phenomenon in naïve mice (not exposed to noxious stimulation immediately prior to WU stimulation), which were subjected to a novel experimental protocol composed of three consecutive trains of WU stimulation. The 1st train produced a typical linear 'wind-up' curve as expected following a repeating series of stimuli; in addition, this 1st train sensitized ('primed') the nociceptive system so that the responses to two subsequent trains (inter-train interval of 10 min) were significantly amplified compared with the response to the 1st train. We named this augmented response potentiation-of-windup, or "PoW". The PoW phenomenon appears to be centrally mediated, as the augmented response was suppressed by administration of an NMDA receptor antagonist (MK-801) and by cutting the spinal cord. Furthermore, the PoW protocol is accompanied by enhanced pain behavior. The 'priming' effect of the 1st train could be mimicked by exposure to natural noxious stimuli prior to the PoW protocol. Presumably, the PoW phenomenon has not been previously reported due to a procedural reason: typically, WU protocols have been executed in 'primed' rather than naïve animals, i.e., animals exposed to nociceptive stimulation prior to the actual WU recording. Our findings indicate that the PoW paradigm can distinguish between 'naïve' and 'primed' states, suggesting its use as a tool for the assessment of central sensitization.

Novel aspects of spinal cord evoked potentials (SCEPs) in the evaluation of dorso-ventral and lateral mechanical impacts on the spinal cord

OBJECTIVES

The aim of the current study was to mimic mechanical impacts on the spinal cord by manifesting the effects of dorsoventral (DVMP) and lateral (LMP) mechanical pressure on neural activity to address points to be considered during surgery for different purposes, including spinal cord decompression.

APPROACHES

Spinal cords of anesthetized rats were compressed at T13. Different characteristics of axons, including vulnerability, excitability, and conduction velocity (CV), in response to promptness, severity, and duration of pressure were assessed by spinal cord evoked potentials (SCEPs). Real-time SCEPs recorded at L4-5 revealed N1, N2, and N3 peaks that were used to represent the activity of injured sensory afferents, interneurons, and MN fibers. The averaged SCEP recordings were fitted by trust-region algorithm to find the equivalent Gaussian and polynomial equations.

MAIN RESULTS

The pyramidal and extrapyramidal pathways possessed CVs of 3-11 and 16-80 m s(-1), respectively. DVMP decreased the excitability of myelinated neural fibers in antidromic and orthodromic pathways. The excitability of fibers in extrapyramidal and pyramidal pathways of lateral corticospinal (LCS) and anterior corticospinal (ACS) tracts decreased following LMP. A significant drop in the amplitude of N3 and its conduction velocity (CV) revealed higher susceptibility of less-myelinated fibers to both DVMP and LMP. The best parametric fitting model for triplet healthy spinal cord CAP was a six-term Gaussian equation (G6) that fell into a five-term equation (G5) at the complete compression stage.

SIGNIFICANCE

The spinal cord is more susceptible to dorsoventral than lateral mechanical pressures, and this should be considered in spinal cord operations. SCEPs have shown promising capabilities for evaluating the severity of SCI and thus can be applied for diagnostic or prognostic intraoperative monitoring (IOM).

Cortical presynaptic control of dorsal horn C-afferents in the rat

Lamina 5 sensorimotor cortex pyramidal neurons project to the spinal cord, participating in the modulation of several modalities of information transmission. A well-studied mechanism by which the corticospinal projection modulates sensory information is primary afferent depolarization, which has been characterized in fast muscular and cutaneous, but not in slow-conducting nociceptive skin afferents. Here we investigated whether the inhibition of nociceptive sensory information, produced by activation of the sensorimotor cortex, involves a direct presynaptic modulation of C primary afferents. In anaesthetized male Wistar rats, we analyzed the effects of sensorimotor cortex activation on post tetanic potentiation (PTP) and the paired pulse ratio (PPR) of dorsal horn field potentials evoked by C–fiber stimulation in the sural (SU) and sciatic (SC) nerves. We also explored the time course of the excitability changes in nociceptive afferents produced by cortical stimulation. We observed that the development of PTP was completely blocked when C-fiber tetanic stimulation was paired with cortex stimulation. In addition, sensorimotor cortex activation by topical administration of bicuculline (BIC) produced a reduction in the amplitude of C–fiber responses, as well as an increase in the PPR. Furthermore, increases in the intraspinal excitability of slow-conducting fiber terminals, produced by sensorimotor cortex stimulation, were indicative of primary afferent depolarization. Topical administration of BIC in the spinal cord blocked the inhibition of C–fiber neuronal responses produced by cortical stimulation. Dorsal horn neurons responding to sensorimotor cortex stimulation also exhibited a peripheral receptive field and responded to stimulation of fast cutaneous myelinated fibers. Our results suggest that corticospinal inhibition of nociceptive responses is due in part to a modulation of the excitability of primary C–fibers by means of GABAergic inhibitory interneurons.

Fast synaptic inhibition in spinal sensory processing and pain control

The two amino acids GABA and glycine mediate fast inhibitory neurotransmission in different CNS areas and serve pivotal roles in the spinal sensory processing. Under healthy conditions, they limit the excitability of spinal terminals of primary sensory nerve fibers and of intrinsic dorsal horn neurons through pre- and postsynaptic mechanisms, and thereby facilitate the spatial and temporal discrimination of sensory stimuli. Removal of fast inhibition not only reduces the fidelity of normal sensory processing but also provokes symptoms very much reminiscent of pathological and chronic pain syndromes. This review summarizes our knowledge of the molecular bases of spinal inhibitory neurotransmission and its organization in dorsal horn sensory circuits. Particular emphasis is placed on the role and mechanisms of spinal inhibitory malfunction in inflammatory and neuropathic chronic pain syndromes.

Facilitation and inhibition of withdrawal reflexes following repetitive stimulation: electro- and psychophysiological evidence for activation of noxious inhibitory controls in humans

A systematic evaluation of nociceptive withdrawal reflexes and pain rating was undertaken in order to explore the mechanisms underlying temporal summation of repetitive electrocutaneous stimulation in healthy individuals (n=12; age=27.5+/-1.5 years). Five-second subreflex threshold (RT) electrocutaneous stimulation at different frequencies (single stimulus, 5, 10, and 20 Hz) and intensities (0.6RT and 0.8RT) was applied on the dorsum of the foot, and the withdrawal reflex from the ipsilateral biceps femoris muscle was measured. The subjects scored the pain intensity on a visual analogue scale (0-100 mm) for the beginning, the middle and the end phase of the 5 s series of stimulation, and the respective averaged reflex size was calculated. The reflex size increased at stimulus frequencies 10 Hzx0.8RT and 20 Hzx0.8RT as compared with 5 Hzx0.8RT (SNK, P<0.05), and by an increase in current intensity from 0.6RT to 0.8RT (SNK, P<0.05). Pain intensity increased with the increase in the current intensity from 0.6RT to 0.8RT (SNK, P<0.05). Profound activation of inhibition following electrocutaneous pain stimuli was demonstrated by reduction in pain intensity and reflex size during the last second as compared with the first second at 0.6RT current intensity (SNK, P<0.05). The pain intensity peaked between 5 and 10 Hz (P<0.05) and was reduced at 20 Hz for current intensities at 0.8RT (P<0.05). This study provides evidence for both frequency dependent central integration of the repetitive electrocutaneous stimuli and activation of a pain inhibitory system by psychophysical and electrophysiological means, demonstrating the delicate balance between neuronal facilitation and inhibition in the human pain system.

Effect of excitatory and inhibitory agents and a glial inhibitor on optically-recorded primary-afferent excitation

The effects of GABA, excitatory amino-acid receptors antagonists and a glial metabolism inhibitor on primary-afferent excitation in the spinal dorsal horn were studied by imaging the presynaptic excitation of high-threshold afferents in cord slices from young rats with a voltage-sensitive dye. Primary afferent fibers and terminals were anterogradely labeled with a voltage-sensitive dye from the dorsal root attached to the spinal cord slice. Single-pulse stimulation of C fiber-activating strength to the dorsal root elicited compound action potential-like optical responses in the superficial dorsal horn. The evoked presynaptic excitation was increased by the GABA_A receptor antagonists picrotoxin and bicuculline, by glutamate receptor antagonists D-AP5 and CNQX, and by the glial metabolism inhibitor mono-fluoroacetic acid (MFA). The increase in presynaptic excitation by picrotoxin was inhibited in the presence of D-AP5, CNQX and MFA. Presynaptic modulation in the central terminal of fine primary afferents by excitatory and inhibitory amino acids may represent a mechanism that regulates the transmission of pain.

Substance P in nociceptive sensory neurons

Substance P is contained within a subpopulation of nociceptive primary sensory neurons that project to the superficial dorsal horn of the spinal cord. Stimulation of the peripheral processes of primary afferent fibres at intensities that activate A delta and C fibres elicits a pronounced release of substance P from the cat spinal cord in vivo. Experiments with the neurotoxins capsaicin and 5,6-dihydroxytryptamine have shown that substance P release from the spinal cord in vivo derives largely from afferent fibres. Intrathecal perfusion of the cat spinal cord with morphine abolishes the nerve-evoked release of substance P while capsaicin produces a dramatic increase in peptide release. Prolonged treatment of rats with capsaicin depletes substance P from the dorsal horn and results in reduced sensitivity to noxious peripheral stimuli. The duration of the somatic action potential recorded from cultured sensory neurons is known to be decreased by enkephalin and is prolonged by capsaicin. The acute effects of both morphine and capsaicin on substance P may be mediated by an interaction with voltage-sensitive ion channels on the sensory neuron. These observations suggest that nociceptive input to the dorsal horn can be regulated by drugs that interact directly with substance P-containing sensory terminals.

GABA-mediated oxytocinergic inhibition in dorsal horn neurons by hypothalamic paraventricular nucleus stimulation

In anaesthetized rats, we tested whether the unit activity of dorsal horn neurons that receive nociceptive input is modulated by electrical stimulation of the hypothalamic paraventricular nucleus (PVN). An electrophysiological mapping of dorsal horn neurons at L3-L4 let us choose cells responding to a receptive field located in the toes region of the left hindpaw. Dorsal horn neurons were classified according to their response properties to peripheral stimulation. Wide Dynamic Range (WDR) cells responding to electrical stimulation of the peripheral receptive field and presenting synaptic input of Adelta, Abeta, and C-fibers were studied. Suspected interneurons that are typically silent and lack peripheral receptive field responses were also analyzed. PVN electrical stimulation inhibits Adelta (-55.0+/-10.2%), C-fiber (-73.1+/-6.7%), and post-discharge (-75.0+/-8.9%) peripheral activation in WDR cells, and silent interneurons were activated. So, this last type of interneuron was called a PVN-ON cell. In WDR cells, the inhibition of peripheral responses caused by PVN stimulation was blocked by intrathecal administration of a specific oxytocin antagonist or bicuculline. However, PVN-ON cell activation was blocked by the same specific oxytocin antagonist, but not by bicuculline. Our results suggest that PVN stimulation inhibits nociceptive peripheral-evoked responses in WDR neurons by a descending oxytocinergic pathway mediated by GABAergic PVN-ON cells. We discuss our observation that the PVN electrical stimulation selectively inhibits Adelta and C-fiber activity without affecting Abeta fibers. We conclude that Adelta and C-fibers receive a presynaptic inhibition mediated by GABA.

John Eccles' studies of spinal cord presynaptic inhibition

Presynaptic inhibition is one of many areas of neurophysiology in which Sir John Eccles did pioneering work. Frank and Fuortes first described presynaptic inhibition in 1957. Subsequently, Eccles and his colleagues characterized the process more fully and showed its relationship to primary afferent depolarization. Eccles' studies emphasized presynaptic inhibition of the group Ia monosynaptic reflex pathway but also included group Ib, II and cutaneous afferent pathways, and the dorsal column nuclei. Presynaptic inhibition of the group Ia afferent pathway was demonstrated by depression of monosynaptic excitatory postsynaptic potentials and inhibition of monosynaptic reflex discharges. Primary afferent depolarization was investigated by recordings of dorsal root potentials, dorsal root reflexes, cord dorsum and spinal cord field potentials, and tests of the excitability of primary afferent terminals. Primary afferent depolarization was proposed to result in presynaptic inhibition by reducing the amplitude of the action potential as it invades presynaptic terminals. This resulted in less calcium influx and, therefore, less transmitter release. Presynaptic inhibition and primary afferent depolarization could be blocked by antagonists of GABA(A) receptors, implying a role of interneurons that release gamma aminobutyric acid in the inhibitory circuit. The reason why afferent terminals were depolarized was later explained by a high intracellular concentration of Cl(-) ions in primary sensory neurons. Activation of GABA(A) receptors opens Cl(-) channels, and Cl(-) efflux results in depolarization. Another proposed mechanism of depolarization was an increase in extracellular concentration of K(+) following neural activity. Eccles' work on presynaptic inhibition has since been extended in a variety of ways.

Medial-olivocochlear-efferent inhibition of the first peak of auditory-nerve responses: evidence for a new motion within the cochlea

Despite the insights obtained from click responses, the effects of medial-olivocochlear (MOC) efferents on click responses from single-auditory-nerve (AN) fibers have not been reported. We recorded responses of cat single AN fibers to randomized click level series with and without electrical stimulation of MOC efferents. MOC stimulation inhibited (1) the whole response at low sound levels, (2) the decaying part of the response at all sound levels, and (3) the first peak of the response at moderate to high sound levels. The first two effects were expected from previous reports using tones and are consistent with a MOC-induced reduction of cochlear amplification. The inhibition of the AN first peak, which was strongest in the apex and middle of the cochlea, was unexpected because the first peak of the classic basilar-membrane (BM) traveling wave receives little or no amplification. In the cochlear base, the click data were ambiguous, but tone data showed particularly short group delays in the tail-frequency region that is strongly inhibited by MOC efferents. Overall, the data support the hypothesis that there is a motion that bends inner-hair-cell stereocilia and can be inhibited by MOC efferents, a motion that is present through most, or all, of the cochlea and for which there is no counterpart in the classic BM traveling wave.

Segmental noxious versus innocuous electrical stimulation for chronic pain relief and the effect of fading sensation during treatment

It is not clear whether segmental innocuous stimulation has a stronger analgesic effect than segmental noxious stimulation for chronic pain and whether the fading of current sensation during treatment interferes with the analgesic effect, as suggested by the gate control theory. Electrical stimulation (by way of Interferential Current) applied at the pain area (segmental) was administered to 4 groups of patients with osteoarthritis (OA) knee pain. Two groups were administered with noxious stimulation (30% above pain threshold) and two with innocuous stimulation (30% below pain threshold). In each group half of the patients received a fixed current intensity while the other half raised the intensity continuously during treatment whenever fading of sensation was perceived. Group 5 and 6 received sham stimulation and no treatment, respectively. The outcome measures were: chronic pain intensity, morning stiffness, range of motion (ROM), pain threshold and % pain reduction. Both noxious and innocuous stimulation significantly decreased chronic pain (P<0.001) and morning stiffness (P<0.01) and significantly increased pain threshold (P<0.001) and ROM (P<0.001) compared with the control groups. Nevertheless, noxious stimulation decreased pain intensity (P<0.05) and increased pain threshold (P<0.001) significantly more than innocuous stimulation. No differences in treatment outcomes were found between adjusted and unadjusted stimulation. (a) Interferential current is very effective for chronic OA knee pain, (b) segmental noxious stimulation produces a stronger analgesic effect than segmental innocuous stimulation, (c) the fading of sensation during treatment, does not decrease the analgesic effect. Possible mechanisms explaining the findings are discussed.

Enhanced Wind-Up of the C-Fiber-Mediated Nociceptive Flexor Reflex Movement Following Painful Diabetic Neuropathy in Mice

We examined wind-up of the nociceptive flexor withdrawal responses in diabetic mice that had developed tactile allodynia after treatment with streptozotocin (STZ). In control and STZ-treated mice, simultaneous activation of Adelta- and C-fibers by electrical stimuli at C-fiber intensity delivered to the ventral aspect of the toe elicited a biphasic withdrawal reflex composed of short- and long-latency movements of the ipsilateral hind paw that were respectively mediated by activation of Adelta- and C-fibers. There were no significant differences between control and diabetic mice in the activation threshold of each reflex movement or the amplitude of reflexes elicited by various stimulus intensities. However, a repetitive conditioning stimulus (CS) elicited significantly greater wind-up of the C-fiber-mediated movement and early saturation of wind-up in diabetic mice. In both control and diabetic mice, the CS elicited no or occasionally slight wind-up of the A delta-fiber-mediated movement. Moreover, post-CS facilitation, which reflects the prolonged excitability increase, was observed in both Adelta-fiber- and C-fiber-mediated movements of control mice, whereas significant post-CS facilitation was only obtained in the C-fiber-mediated movement of diabetic mice, which may reflect supraspinal descending influences. Such changes in the excitability of spinal neurons in diabetic mice may represent some aspect of painful diabetic neuropathy.

Noxious Stimuli Evoke a Biphasic Flexor Reflex Composed of Aδ-Fiber-Mediated Short-Latency and C-Fiber-Mediated Long-Latency Withdrawal Movements in Mice

The nociceptive flexor reflex was studied in mice, focusing in particular on movement. Electrical stimuli delivered to the ventral aspect of the toe through a pair of needle electrodes inserted subcutaneously elicited a biphasic withdrawal reflex that was composed of short- and long-latency movements of the ipsilateral hind paw. The first response had a lower activation threshold compared with the second movement. Similar biphasic responses were observed in the afferent volley recorded from the sciatic nerve as well as in the electromyographic activity recorded from the femoris biceps muscle. Tetrodotoxin, applied over the sciatic nerve, abolished the first movement, but the second response was preserved, revealing that the first movement was elicited by the activation of myelinated Adelta-fibers, whereas the second movement was mediated by unmyelinated C-fibers. Thus, simultaneous activation of Adelta- and C-fibers leads to separate, but sequentially occurring withdrawal movements of the hind paw in mice. Systemic administration of morphine suppressed the withdrawal reflex, which was attributable to a preferential reduction of the Adelta-fiber-mediated short-latency component. This method will be useful for understanding how Adelta-fiber- and C-fiber-mediated nociceptive reflexes are affected by drugs in the whole animal system.

Role of primary afferents in spinal cord stimulation-induced vasodilation: characterization of fiber types

Selected patients with peripheral vascular disease can be treated with spinal cord stimulation (SCS) to improve blood flow in the limbs. However, the mechanisms producing these effects remain unclear. The present study was designed to investigate if SCS produces cutaneous vasodilation via antidromic activation of the unmyelinated C-fibers and/or the small myelinated fibers. SCS was applied to anesthetized rats with a ball electrode at the L2-L3 spinal level. In Protocol 1, effects of capsaicin were examined. Blood flow changes in the hindpaw induced by SCS were measured in the footpad with laser Doppler flowmeters. Topical application of capsaicin (1%) on the tibial nerve did not affect SCS-induced vasodilation at 30 and 60% of motor threshold (MT). However, the duration of vasodilation induced by SCS at 90% MT and at 10 times MT was significantly reduced after capsaicin application on the tibial nerve. In Protocol 2, antidromic compound action potentials (CAPs) of the tibial nerve were recorded in response to SCS. CAPs of the large and the small myelinated afferent fibers were observed in response to SCS at all intensities. However, even with SCS at ten times MT, CAPs of C-fibers could not be detected in the tibial nerve. In Protocol 3, antidromic CAPs of the dorsal root were measured in response to SCS. Antidromic CAPs of C-fibers in dorsal roots were evoked by SCS at >or=90% of MT. It is concluded that SCS-induced vasodilation at <or=60% of MT may be mediated via only the myelinated fibers, whereas vasodilation at >or=90% of MT may also involve antidromic activation of some unmyelinated C-fibers.

Only minor spinal motor reflex effects from feline group IV muscle nociceptors

The contribution of group III and IV muscle nociceptors activated by injection of KCl or bradykinin into the muscle artery (i.a.) of the gastrocnemius-soleus muscle to spinal motor reflex pathways was investigated in high spinal cats. Group I-III fibres were completely blocked by TTX, leaving group IV-fibre conduction intact. Thus, effects from i.a. KCl or bradykinin injection persisting after TTX were attributed to TTX resistant group IV fibres while the contribution of group III fibres was approximately defined by the difference between those effects and the control effects before TTX. Confirming former findings the chemical activation of group III and IV muscle afferents induced distinct reflex facilitation of the flexor posterior biceps semitendinosus and inhibition of the extensor quadriceps. After the block of all myelinated fibres by TTX the same stimuli induced only minor reflex effects mediated by the persistently conducting TTX resistant group IV afferents. It is concluded that the main functional meaning of group IV muscle afferents, which respond preferentially with a higher threshold to mechanical stimuli, is probably less related to reflex motor control than that of group III afferents.

Functional expression of AMPA receptors on central terminals of rat dorsal root ganglion neurons and presynaptic inhibition of glutamate release

No direct evidence has been found for expression of functional AMPA receptors by dorsal root ganglion neurons despite immunocytochemical evidence suggesting they are present. Here we report evidence for expression of functional AMPA receptors by a subpopulation of dorsal root ganglion neurons. The AMPA receptors are most prominently located near central terminals of primary afferent fibers. AMPA and kainate receptors were detected by recording receptor-mediated depolarization of the central terminals under selective pharmacological conditions. We demonstrate that activation of presynaptic AMPA receptors by exogenous agonists causes inhibition of glutamate release from the terminals, possibly via primary afferent depolarization (PAD). These results challenge the traditional view that GABA and GABA(A) receptors exclusively mediate PAD, and indicate that PAD is also mediated by glutamate acting on presynaptically localized AMPA and kainate receptors.

GABA(A) and 5-HT(3) receptors are involved in dorsal root reflexes: possible role in periaqueductal gray descending inhibition

The dorsal root reflex (DRR) is a measure of the central excitability of presynaptic inhibitory circuits in the spinal cord. Activation of the periaqueductal gray (PAG), a center for descending inhibition of spinal cord nociceptive transmission, induces release of variety of neurotransmitters in the spinal cord, including GABA and serotonin (5-HT). GABA has been shown to be involved in generation of DRRs. In this study, pharmacological agents that influence DRRs and their possible mechanisms were investigated. DRRs were recorded in anesthetized rats from filaments teased from the cut central stump of the left L(4) or L(5) dorsal root, using a monopolar recording electrode. Stimulating electrodes were placed either on the left sciatic nerve or transcutaneously in the left foot. Animals were paralyzed and maintained by artificial ventilation. Drugs were applied topically to the spinal cord. A total of 64 units were recorded in 34 Sprague-Dawley rats. Peripheral receptive fields were found for nine of these units. In these units, DRRs were evoked by brush, pressure, and pinch stimuli. Nine units were tested for an effect of electrical stimulation in the periaqueductal gray on the DRRs. In eight cases, DRR responses were enhanced following PAG stimulation. The background activity was 4.2 +/- 1.9 spikes/s (mean +/- SE; range: 0-97.7; n = 57). The responses to agents applied to the spinal cord were (in spikes/s): artificial cerebrospinal fluid, 7.1 +/- 3.6 (range: 0-86.9; n = 25); 0.1 mM GABA, 16.8 +/- 8.7 (range: 0-191.0; n = 22); 1.0 mM GABA, 116.0 +/- 26.5 (range: 0.05-1001.2; n = 50); and 1.0 mM phenylbiguanide (PBG), 68.1 +/- 25.3 (range: 0-1,073.0; n = 49). Bicuculline (0.5 mM, n = 27) and ondansetron (1.0 mM, n = 10) blocked the GABA and PBG effects, respectively (P < 0.05). Significant cross blockade was also observed. It is concluded that GABA(A) receptors are likely to play a key role in the generation of DRRs, but that 5-HT(3) receptors may also contribute. DRRs can be modulated by supraspinal mechanisms through descending systems.

Adelta and C primary afferents convey dorsal root reflexes after intradermal injection of capsaicin in rats

Antidromic activity was recorded in anesthetized rats from single afferent fibers in the proximal ends of cut dorsal root filaments at the L(4-6) level and tested for responses to acute cutaneous inflammation produced by intradermal injection of capsaicin. This antidromic activity included low-frequency spontaneous firing and dorsal root reflex (DRR) discharges evoked by applying von Frey hairs to the skin of the foot. DRRs could be recorded from both small myelinated (Adelta) and unmyelinated (C) afferent fibers, as well as from large myelinated (Abeta) fibers. After capsaicin was injected intradermally into the plantar skin of the foot, a significant enhancement of DRR activity was seen in Adelta and C fibers but not in Abeta fibers, and this increase lasted for approximately 1 h. This study supports the hypothesis that centrally mediated antidromic activity in Adelta and C primary afferent fibers contributes to the development of neurogenic inflammation, presumably by release of inflammatory substances in the periphery.

Contribution of TTX-resistant C-fibres and Adelta-fibres to nociceptive flexor-reflex and non-flexor-reflex pathways in cats

The contribution of Adelta-fibres and C-fibres activated by noxious heat stimulation of the central pad of the foot to nociceptive spinal flexor reflex pathways (FRA-type) and to nociceptive excitatory reflex pathways to foot extensors (non-FRA type) was investigated in high spinal cats. A-fibres were completely blocked by tetrodotoxin (TTX), leaving C-fibre conduction intact. Thus, effects persisting after TTX were attributed to nociceptive C-fibres while the contribution of nociceptive Adelta-fibres was defined by the difference between those effects and the control effects before TTX. The initial action of noxious stimulation on both types of reflex action was mediated predominantly by Adelta-fibres, while the later action was mainly mediated by C-fibres. In two (out of seven) experiments Adelta-fibres exerted a significant inhibitory influence on the C-fibre action in FRA pathways, but such an inhibitory interaction between the two fibre groups was absent in the non-FRA reflex pathways. The technique of TTX application at the peripheral nerve proved to be a reliable method for a long-lasting selective investigation of C-fibre effects. The results revealed that both Adelta- and C-fibres contributed to nociceptive FRA and non-FRA reflex pathways.

Acute pain mechanisms

The systems activated by tissue-injuring stimuli are complex. The nociceptive primary afferents have little spontaneous activity under normal conditions; however, after tissue injury, they display longlasting, ongoing activity. This results, in part, because the injury elicits the release of active factors that sensitize or excite the peripheral nerve terminal. A threshold that is lowered to the extent that body temperature and the pressure of edema are adequate stimuli results in spontaneous pain. This phenomenon is mediated by a variety of blood-borne active factors released during plasma extravasation, by agents released from local inflammatory cells, and by neurotransmitters released from the peripheral terminals of the primary afferent fibers themselves. Well-defined projections into the dorsal horn convey the "pain message" to at least two well-defined populations of neurons: those that are nociceptive specific and those that display an intensity-linked discharge over a range of stimuli from innocuous to noxious. Convergence from various fiber types, modalities, and end organs permits the encoding of afferent traffic with respect to intensity and location. The convergence of axons from somatic and visceral structures reflects the mechanism for the so-called "referred pain state." Most importantly, these dorsal horn systems have a dynamic component in addition to the hard-wiring; their output can be regulated both up and down. The up-regulation provides the basis for much of the facilitated processing that is believed to account for a significant percentage of the postinjury pain state. The facilitated state has a unique pharmacology, with the underlying mechanisms reflecting a cascade of actions that starts with the NMDA receptor and proceeds through the spinal release of intermediaries, such as prostaglandins and nitric oxide. Conversely, the ability to down-regulate the dorsal horn stimulus response function accounts for the powerful control exerted by a wide variety of diverse factors, including the spinal delivery of opioid and nonopioid analgesics and the "endogenous analgesia system." These linkages reflect the complexity of the encoding mechanisms that transduce the tissue injury into the behavioral sequela known as pain. This article also emphasizes that, although considerable progress has been made in the past decade, the current pace of research promises greater insights.

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.

Mechanisms of touch-evoked pain (allodynia): a new model

In this paper we review the current neurophysiological models of touch-evoked pain and present a new proposal that addresses the mechanisms of allodynia. The new model is based on the notion that A-beta mechanoreceptors can gain access to nociceptive neurones by means of a presynaptic link, at central level, between low threshold mechanoreceptors and nociceptors. We propose that the excitation of nociceptors provoked by a peripheral injury activates the spinal interneurones that mediate primary afferent depolarization (PAD) between low threshold mechanoreceptors and nociceptors. As a consequence of the increased and persistent barrage driving these neurones their excitability is increased such that, when activated by low threshold mechanoreceptors from areas surrounding the injury site, they produce a very intense PAD in the nociceptive afferents which is capable of generating spike activity. This activation would be conducted antidromically in the form of dorsal root reflexes (DRRs) but would also be conducted forward activating the second order neurones normally driven by nociceptors. The sensory consequence of this mechanism is pain evoked by the activation of low threshold mechanoreceptors from an area surrounding an injury site (allodynia).

Strength-duration curve of conductive spinal cord evoked potentials in cats

Strength-duration curves of the ascending and descending conductive spinal cord potentials (SCEPs) in cats were obtained using constant current stimuli. For the formulation of numeric indices of excitability, the rheobase is defined as the minimal current strength below which response cannot occur even if the current continues, and the chronaxie is defined as the minimal duration of a current required to evoke the potential at twice the rheobase strength. The chronaxies and rheobases were calculated from the constructed strength-duration curves. The purpose of this study is to produce strength-duration curves and to evaluate the utility of chronaxies and rheobases for SCEPs. This study showed the following results: (1) there was a hyperbolic relationship between stimulus strength and stimulus duration at threshold values, similar to that seen in peripheral nerves; (2) the ascending and descending tracts of SCEP were mediated through the same pathway (based on the similar chronaxies and rheobases); (3) following spinal cord compression the chronaxie and rheobase increased significantly (P < 0.05), which is similar to peripheral nerve disturbance. However, the rheobase decreased significantly following slight spinal cord compression (P < 0.05) and systemic cooling (P < 0.01), and the strength-duration curve shifted showing a tendency towards decrease of the galvanic threshold, therefore, amplitude augmentation with slight compression and with decrease in temperature seems to contribute to the reduction of the threshold. The strength-duration curve, the chronaxie and the rheobase may be useful in assessing spinal cord function.

Primary afferent depolarization of myelinated fibres in the joint and interosseous nerves of the cat

1. Changes in the excitability of the intraspinal terminals of fibres in the posterior knee joint and interosseous nerves were used as a measure of primary afferent depolarization (PAD) which is associated with presynaptic inhibition of transmission from afferent fibres. These were estimated from changes in the intensity of electrical stimuli required to activate the fibres in 50% of trials. In order to avoid the inclusion of group I muscle afferents which contaminate the joint and interosseal nerves, the analysis was restricted to fibres conducting at less than 75 m s-1 and/or displaying patterns of PAD which differed from those of group Ia and Ib muscle afferents in lower lumbar segments of anaesthetized cats. PAD was evoked by electrical stimulation of ipsilateral hindlimb nerves. 2. PAD of fibres in the posterior knee joint nerve was induced from group I (Ia and Ib) and group II muscle afferents and cutaneous afferents but not by stimulation of the joint or the interosseous nerves. The most effective stimuli were those applied to the superficial peroneal, sural, quadriceps and posterior biceps and semitendinosus nerves. 3. PAD of fibres in the interosseous nerve was also induced by stimulation of group I (Ia and Ib) and group II muscle afferents and cutaneous afferents and, in addition, by stimulation of joint and interosseous nerves. The most effective stimuli were those applied to the superficial peroneal, quadriceps, flexor digitorum longus and posterior biceps and semitendinosus nerves. 4. Individual fibres of the joint and the interosseous nerves were depolarized by only some of the conditioning stimuli. Even the most effective stimuli did not produce PAD in all of the fibres tested. Individual fibres of the joint and the interosseous nerves were depolarized by diverse combinations of afferents of different functional types and of different peripheral nerves. The differences in the sources of PAD were not associated with the conduction velocities and hence are unlikely to be related to differences in the receptor origin of the tested fibres. The diversity in the sources of PAD of individual fibres is interpreted as providing a high degree of differentiation in the control of transmission from receptors in joints and interosseal membranes.

Cocaine: effect on spinal projection neurons in the rat

In the anesthetized rat, systemic cocaine at an analgesic dose (25 mg/kg, IP) had no general influence on spontaneous activity in nociceptive spinoreticular tract neurons or in rostrally projecting low-threshold mechanoreceptive neurons of the spinal dorsal horn. Peripherally evoked activity was moderately (mean: 30%) reduced in 43% (6/14) of the spinoreticular tract neurons, whereas in 50% of them there was no marked change in evoked activity by cocaine. Evoked activity was slightly reduced in 25% (2/8) of the low-threshold mechanoreceptive neurons and not changed in 75% of them. The suppressive effect of a distant conditioning noxious stimulus on responses to spinoreticular tract neurons was enhanced by cocaine in 3/6 of the spinoreticular tract neurons. Primary afferent terminal excitability of A-fibers was slightly increased following cocaine as indicated by the increased amplitude (mean: 24%) of the antidromically evoked compound action potential recorded from the sural nerve. It is concluded that the previously shown marked enhancement of spontaneous activity in the bulborecticular formation is generated supraspinally. Thus, while cocaine-induced analgesia appears to be due primarily to supraspinal mechanisms, the present results suggest that spinal mechanisms are also involved possibly presynaptically.

Electrical stimulation of primary afferent A fibres does not reduce substance P release in the dorsal horn of the cat

Antibody microprobes were used to measure the release of immunoreactive substance P in the dorsal horn of anaesthetised cats during noxious mechanical or thermal stimulation of the hind limb. Concomitant electrical stimulation of the ipsilateral tibial or sural nerve at intensities sufficient to excite only A alpha beta or additionally A delta primary afferent fibres did not reduce the release of substance P evoked by noxious stimuli. The results suggest that segmental inhibition produced in the dorsal horn by electrical stimulation of peripheral nerves is not mediated by presynaptic inhibition of substance P release from nociceptive primary afferent fibres.

Effect of peripheral electrical stimulation on measures of tooth pain threshold and oral soft tissue comfort in children

The effect of peripheral electrical stimulation on tooth pain threshold and comfort of oral soft tissue was evaluated in 30 children, using a double-blind, crossover, study design. Tooth pain threshold was measured before and after 8 minutes of electrical or sham stimulation. Comfort of oral soft tissue during placement of a rubber dam clamp was evaluated 3 minutes after electrical or sham stimulation was begun. Comfort level was rated by each subject and by the investigator, using a visual analog scale (VAS). Heart rate was measured before and immediately following placement of the clamp. Electrical stimulation significantly increased tooth pain threshold and reduced the cardiovascular stress response without altering comfort levels during placement of the clamp. These findings suggest that peripheral electrical stimulation applied in a pediatric dental setting alters sensory aspects of pain but not affective aspects of comfort.

Absence of tonic supraspinal control of substance P release in the substantia gelatinosa of the anaesthetized cat

Antibody microprobes were used to measure immunoreactive substance P (irSP) release in the substantia gelatinosa of the lower lumbar spinal cord of barbiturate-anaesthetized cats. Release of irSP was produced by noxious peripheral stimuli. Such release was not altered by blocking spinal conduction at the first lumbar segment by cooling or transecting the spinal cord. The results suggest that the release of irSP from the central terminals of nociceptors is not subject to tonic supraspinal inhibition.

Effects of midbrain stimulation and iontophoretic application of serotonin, noradrenaline, morphine and GABA on electrical thresholds of afferent C- and A-fibre terminals in cat spinal cord

We have used the single-fibre excitability testing method to investigate whether electrical stimulation in midbrain periaqueductal gray or lateral reticular formation, as well as intraspinal iontophoretic application of the suspected inhibitory neurotransmitters serotonin (5-hydroxytryptamine), noradrenaline, the opiate morphine, or gamma-aminobutyric acid (GABA), exert presynaptic actions at the central terminals of cutaneous afferent unmyelinated or myelinated fibres. Thresholds to antidromically excited 42 single unmyelinated and 18 myelinated fibres in the sural nerve by intraspinal microstimulation were determined before and during periaqueductal gray or lateral reticular formation stimulation (3 100 ms trains/s at 100 Hz; 100-900 microA) or intraspinal iontophoretic application (40-300 nA) of 5-hydroxytryptamine, noradrenaline, morphine or GABA from a multibarrel micropipette. Periaqueductal gray or lateral reticular formation stimulation had mixed effects on unmyelinated and myelinated fibre thresholds, with most threshold measurements within +/- 10% of control. There was a tendency for thresholds to increase more during periaqueductal gray than lateral reticular formation stimulation. Thresholds of unmyelinated fibres were predominantly raised during iontophoretic application of 5-hydroxytryptamine (20/29 fibres), noradrenaline (10/13) and morphine (15/21), while GABA had mixed effects; thresholds of nearly all myelinated fibres were raised by each drug. Both periaqueductal gray or lateral reticular formation stimulation and application of 5-hydroxytryptamine, noradrenaline or morphine tended to raise thresholds in the majority of the 53 unmyelinated and myelinated fibres tested. Methodological problems in interpreting the physiological significance of these results for presynaptic modulation are discussed.

Effects of electrical stimulation of medial olivocochlear neurons on ipsilateral and contralateral cochlear responses

Recent anatomical evidence has cast doubt on the interpretation of the neural elements involved in past experiments in which efferents were electrically stimulated. To separate effects produced by medial olivocochlear (MOC) efferents from effects produced by lateral olivocochlear (LOC) efferents, MOC efferents were selectively stimulated by an electrode in the region of the MOC cell bodies in cats. For comparison, efferents were also stimulated with an electrode in the fourth ventricle (OCB stimulation, previously called COCB stimulation). MOC stimulation and fourth-ventricle OCB stimulation both produced qualitatively similar results bilaterally in that auditory-nerve compound action potential (N1) and endocochlear potential were reduced, and cochlear microphonic (CM) was increased. Both efferent-induced changes were affected in similar ways by changes in shock parameters, and were blocked by strychnine. At low sound levels, the decrease in N1 amplitude was approximately equivalent to a shift (decrease) in sound level but the change in N1 latency was not. The ratio of the CM increase to the N1 sound-level shift was independent of shock level or location. MOC stimulation typically produced an N1 sound-level shift of 11-16 dB in the contralateral ear and 4-7 dB in the ipsilateral ear. The ratio of these shifts almost equals the ratio of MOC neurons which had cell bodies on the stimulating-electrode side. Previous results reported by others with 'UOCB stimulation' now seem attributable to excitation of uncrossed MOC efferents rather than to excitation of uncrossed LOC efferents as previously thought. There is no effect reported in the literature or seen by us which can definitely be attributed to LOC neurons. Fourth-ventricle OCB stimulation typically produced an N1 sound-level shift in both ears of 19-22 dB which is approximately the sum of the crossed and uncrossed MOC shifts. Considering also that many uncrossed-MOC fibers course close to the midline (i.e. near the stimulating electrode), it seems likely that fourth-ventricle OCB stimulation excites both crossed and uncrossed MOC efferents. Referring to such stimulation in the cat as 'COCB stimulation' is therefore inaccurate and may lead to wrong conclusions about the functional role of various components of the olivocochlear fibers.

Prolonged C-fibre mediated facilitation of the flexion reflex in the rat is not due to changes in afferent terminal or motoneurone excitability

A standard suprathreshold mechanical stimulus applied to the hindpaw of decerebrate-spinal rats produces a discharge in hamstring flexor alpha-motoneurones which is stable for hours, provided no tissue injury is produced. Tissue injury results, however, in a decrease of threshold and an increase in the responsiveness of the reflex. This reflex hypersensitivity can be mimicked by brief (20 s) low frequency (1 Hz) conditioning stimuli to muscle or cutaneous nerves, if C-fibres are recruited. The prolonged post-conditioning facilitation of the flexion reflex by C-afferent volleys is now shown to be independent of changes in the excitability of the test afferent terminals in the dorsal horn and of the motoneurones. The hypersensitivity is therefore due to changes in the interneurones that link cutaneous nociceptive afferents with flexor motoneurones.

The primary afferent depolarizing action of kainate in the rat

Dorsal roots (L3-L7) isolated from immature (1-9 day old) rats were depolarized selectively by kainate (1-100 microM). L-Glutamate (25-100 microM), but not L-aspartate, mimicked the action of kainate. N-methylaspartate had no activity on these preparations and quisqualate was thirty times less active than kainate. Depolarizations evoked by L-glutamate (100-1000 microM) faded rapidly in the presence of L-glutamate. Depolarizations evoked by kainate were depressed during the fade induced by L-glutamate. Certain electrically evoked C-fibre volleys in dorsal roots or leg nerves of rats at any age were selectively depressed or abolished in the presence of kainate. The effect of kainate was more selective than that of gamma-aminobutyric acid or capsaicin. Prolonged treatment of dorsal roots with kainate did not appear to be deleterious to C-fibres. It is suggested that certain primary afferent C-fibres possess kainate receptors which may be activated physiologically by L-glutamate released at their central terminations.

Pharmacology of amino acid receptors on vertebrate primary afferent nerve fibres

Structure-activity of primary afferent depolarising action (PAD) mediated by gamma-aminobutyrate (GABA) analogues suggests a difference between subsynaptic receptors located at fibre terminations within the dorsal horn and axonal receptors which are distributed throughout non-synaptic regions. The interaction of the bicuculline-sensitive GABA receptor (GABA A) ionophore complex with barbiturates and benzodiazepines suggests that at least three binding sites are required to explain the independent GABA-mimetic, GABA-potentiating and picrotoxin-reversing effects of such agents. Difficulties with explanation of the depressant effects of baclofen on spinal transmission, in terms of the bicuculline-resistant GABA (GABA B) receptor hypothesis, are mentioned. Glutamate-induced PAD of low threshold afferents is mediated indirectly through release of potassium. However, such terminals possess receptors (possibly autoreceptors for L-glutamate), activated by (+)2-amino-4-phosphonobutyrate, which cause depression of transmitter release. Primary afferent C-fibres possess receptors which are selectively activated by kainate and which mediate picrotoxin-resistant PAD. Such receptors may be involved in the presynaptic conditioning of C-fibre transmitter release. The peripheral terminals of vestibular primary afferents, in amphibia, possess excitatory amino acid receptors which are probably activated by the transmitter released from hair cells.

Functional and topographical properties of field potentials evoked in rat dorsal horn by cutaneous C-fibre stimulation

Extracellular field potentials in the lumbosacral dorsal horn evoked by stimulation of cutaneous C fibres in the sural nerve were explored in the halothane-anaesthetized rat. C-fibre-evoked field potentials were prominent in lamina II and lamina V of the dorsal horn. These potentials had a latency of 80-130 ms and a duration of more than 200 ms. A peak in the C-fibre-evoked field potential, termed the CI potential, with a median latency of 120 ms, range 105-150 ms, was distinguished in lamina II. The time from onset to peak of the CI potential was, on average, 13 ms when all C fibres were activated. The amplitude of the CI potential in lamina II was directly proportional to the amplitude of the C-fibre-evoked nerve volley, whereas the relation between the C-fibre nerve volley and the C-fibre-evoked field potential in lamina V was non-linear. A selective block of A fibres did not influence the amplitude of these field potentials. Following stimulation of C afferent fibres in the medial sural nerve, at frequencies higher than 0.1 Hz, the CI potential in lamina II, but not the C-fibre-evoked field potential in lamina V, was increased. There was no concomitant change of the A-fibre-evoked field potentials. The magnitude of the potentiation of the CI potential was dependent both on the frequency and the number of stimuli. Mean percentage potentiation was 200%, range 150-300%, after seventy stimulations at a frequency of 1.0 Hz. During the stimulation the CI potential increased monotonically. The decay of the potentiation of the CI potential was well described by two exponentially declining phases. There was a positive correlation between the size of the time constants of the decay and the number of stimuli during conditioning. Following noxious radiant heat (temperature 50-55 degrees C) applied to a restricted part of the skin (area 20-30 mm2) within the receptive field of the medial sural nerve for 10-20 s, the CI potential was potentiated by 50-130%. The duration of this potentiation was 3-15 min. This potentiation was somatotopically organized. By contrast, there was no effect on the amplitude of the CI potential following innocuous skin stimulation (slowly moving contact, brushing the skin, warmth 40 degrees C).(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative study of GABA-mediated depolarizations of lumbar A delta and C primary afferent neurones of the rat

The distribution of GABA receptors on various categories of primary afferents was studied by means of intracellular recordings from rat dorsal root ganglion neurones. Cells were identified on the basis of their conduction velocity and classified as A delta and C neurones. Transient applications of GABA led to a decrease of membrane resistance and a concomitant depolarization. Maximal GABA-induced responses were weaker in C than in A delta and A beta cells. Smaller conductance changes in C cells suggest a lower density of GABAA receptors, and the heterogeneity of the "membrane potential/response amplitude" relationship indicate that the ionic mechanisms underlying GABA-induced responses may not be uniform on all primary afferents; this is supported by the wide range of reversal potential values recorded under voltage-clamp conditions in A delta cells.

Differential olivocochlear projections from lateral versus medial zones of the superior olivary complex

An anterograde tracer (35S-methionine) was injected unilaterally in the superior olivary complex (SOC) at regions previously demonstrated by retrograde labeling to contain olivocochlear (OC) cell bodies. Quantitative analysis of cochlear autoradiographs from these cats demonstrates that there are two OC systems. The lateral OC system has cell bodies lateral to the medial superior olivary nucleus (MSO) and projects to the inner hair cell (IHC) region bilaterally (mostly ipsilaterally). The medial OC system has cell bodies medial, ventral, and anterior to the MSO and projects to the outer hair cell (OHC) region bilaterally (mostly contralaterally). A single medial OC neuron innervates many small patches of OHCs with substantial gaps between the patches. Medial OC neurons also appear to project to the IHC region to a small extent. A review of the literature with the medial-lateral division of OC efferents in mind reveals many differences between these two systems. In particular, lateral OC axons are unmyelinated and innervate the dendrites of radial afferent fibers under IHCs, whereas medial OC axons are myelinated and directly innervate OHCs. Although both systems appear to be cholinergic, the lateral OC system also shows met-enkephalin-like immunoreactivity. The synapses of the medial OC system are formed in development before those of the lateral OC system and they degenerate more slowly after the OC axons are cut. The many differences between these two OC systems suggest that they are functionally separate systems.

Ultrastructure of chemically defined neuron systems in the dorsal horn of the monkey. II. Methionine-enkephalin immunoreactivity

Enkephalinergic axons and terminals were identified by the PAP immunohistochemical method in lamina I (marginal zone) and lamina IIO (outer substantia gelatinosa) in the dorsal horn of the monkey spinal cord. Synaptic profiles with enkephalin-like immunoreactivity (MELI) contained clear, round, vesicles, sometimes a few large granular vesicles, and usually formed asymmetrical contacts. MELI terminals forming synaptic contacts with various sizes of dendrites and with dendritic spines were the most common type of relationship found; axosomatic contacts were few. Additionally, two types of complexes were observed in which an MELI terminal formed a specialized apposition with an unlabelled terminal. The contact often resembled a synapse and in most cases the MELI terminal was suspected to be presynaptic. One complex consisted of a MELI terminal apposing the LGV type terminal (containing large granular vesicles), which in turn was presynaptic to a dendrite. (The identity of the LGV terminal could not be determined, but it had some characteristics similar to those described for substance P terminals and for a class of primary afferents in the monkey dorsal horn). The other type of complex consisted of a MELI terminal apposing an R-type terminal (containing small, round, clear vesicles) which was in turn presynaptic to a dendrite. Often, the MELI terminal also formed a synapse onto the same dendrite. The axodendritic, axospinous and axosomatic contacts of MELI terminals in the superficial dorsal horn may produce some of the depressive postsynaptic-like effects of enkephalin iontophoresis onto dorsal horn neurons. In these cases the responses of dorsal horn neurons to both low threshold and nociceptive primary afferents is suppressed. However, the opiate receptor-dependent PAD of C-fibers observed in the dorsal horn may be mediated by the MELI complexes formed with LGV and R terminals found in lamina I.

Chronic peripheral nerve section diminishes the primary afferent A-fibre mediated inhibition of rat dorsal horn neurones

The inhibitory effect of A-primary afferent activity on A- and C-evoked activity in dorsal horn convergent neurones has been investigated in the decerebrate spinal rat. A-afferent conditioning stimuli produce a powerful inhibition of the C-evoked activity in the majority of units recorded in lamina 5 but were almost without effect on the C-evoked activity in units recorded within the substantia gelatinosa (laminae 1 and 2). The ability of an A-volley to inhibit the response to a C-volley begins immediately after the arrival of the A-volley and lasts for 50-70 ms. Conditioning A-stimuli also inhibit the A-evoked activity of dorsal horn neurones, the inhibition lasting up to 125 ms. Unlike the effect of A-conditioning stimuli on C-responses, which was restricted to units in lamina 5, the A-volleys inhibited the response of both substantia gelatinosa and lamina 5 units. In rats with chronically sectioned sciatic nerves (7-14 days) both A on A and A on C inhibitions were significantly diminished in spite of intact afferent volleys and postsynaptic activity. In neurones activated by stimulation of the sectioned nerve, the A-conditioning stimuli either failed to produce an inhibition or produced a weak and shorter effect. These results are discussed in terms of the possible functional significance of A-afferent mediated inhibition.

The contralateral input to the dorsal horn of the spinal cord in the decerebrate spinal rat

Input from the contralateral limb and tail was examined in the lumbar dorsal horn of decerebrate spinal rats. Fifty-three cells were recorded from laminae 4, 5 and 6 and classified according to their ipsilateral response to natural and electrical stimulation. Twenty-nine (54%) of these cells were found to have inhibitory contralateral fields. This inhibition was evoked by noxious pinching or heating of the skin. In most cases the inhibitory field was a mirror image of the excitatory ipsilateral field although it also often included the tail. Activity evoked by natural and electrical stimulation as well as spontaneous activity was inhibited by contralateral skin stimulation. Noxious specific and wide dynamic range cells displayed these fields but low threshold mechanoreceptive cells did not. Twenty-six cells (49%) received direct short-latency excitatory input from the contralateral sciatic nerve; this correlated well with the presence of contralateral fields. Trains of stimuli applied to the contralateral sciatic nerve at A delta- and C-fibre strength resulted in inhibition of the cell whereas trains of A beta strength had no effect. The results demonstrate the existence of segmental contralateral control over dorsal horn cell activity, not involving supraspinal pathways.

Do opioid peptides mediate a presynaptic control of C-fibre transmission in the rat spinal cord?

A presynaptic inhibitory role for opioid peptides in the control of C-fibre-evoked activity in the dorsal horn has been investigated. The excitability of C-fibre terminals in the dorsal horn of decerebrate spinal rats was tested using intraspinal terminal stimulation and recording the size of the antidromic C wave from the dorsal roots. Naloxone (1-2 mg/kg) failed to alter the baseline terminal excitability of the C-fibres, but reduced the increase in terminal excitability produced by A-fibre afferent conditioning stimuli. The inhibition of postsynaptic C-evoked activity in lamina 5 cells produced by A-afferent fibre conditioning stimuli was also reduced by naloxone. This effect may reflect the reversal of an opioid-mediated presynaptic inhibition, although blockade of a direct postsynaptic inhibitory action could also be involved.