Calcitonin gene-related peptide immunoreactivity and afferent receptive properties of dorsal root ganglion neurones in guinea-pigs.

To establish the afferent receptive properties of lumbosacral dorsal root ganglion (DRG) neurones that express calcitonin gene-related peptide (CGRP), intracellular recordings were made with fluorescent dye-filled electrodes in deeply anaesthetised young guinea-pigs. After determination of neuronal functional properties, dye was injected into the soma. CGRP-like immunoreactivity (CGRP-LI) was examined on histological sections of dye-marked neurones. Fourteen of 34 C-fibre neurones showed CGRP-LI. These included 10/21 C-fibre nociceptive neurones. All C-polymodal nociceptors in glabrous (n = 4) but none in hairy skin (n = 4) were positive. Positive C-fibre high threshold mechanoreceptive (HTM) units had receptive fields in dermal or deeper tissue. Four (n = 6) unresponsive or unidentified C-fibre units were positive. Neither C-fibre cooling sensitive (n = 4) nor C-fibre low threshold mechanoreceptive (LTM) units (n = 3) had CGRP-LI. Six of 23 A-fibre nociceptive cells were positive including one Aalpha/beta unit. Three of these positive cells had epidermal and three had dermal/deep receptive fields. Three of 36 A-fibre LTM units exhibited CGRP-LI; all were Aalpha/beta-fibre G hair units. All glabrous skin and muscle spindle units and in hairy skin slowly adapting and field units, and some G-hair units lacked CGRP-LI. CGRP-LI stained fibres were found in tissues containing receptive fields of positive DRG neurones: glabrous skin, near hair follicles and in skeletal muscle. A few substance P-labelled neurones did not exhibit CGRP-LI and vice versa. Thus CGRP expression was detected in under half the nociceptive neurones, was not limited to nociceptive neurones and apart from receptive properties was also related to location/depth in the tissues of a DRG neurone's peripheral terminals.

Correlations between neuronal morphology and electrophysiological features in the rodent superficial dorsal horn.

Relationships between the morphology of individual neurones of the spinal superficial dorsal horn (SDH), laminae I and II, and their electrophysiological properties were studied in spinal cord slices prepared from anaesthetized, free-ranging hamsters. Tight-seal, whole-cell recordings were made with pipette microelectrodes filled with biocytin to establish electrophysiological characteristics and to label the studied neurones. Neurones were categorized according to location and size of the somata, the dendritic and axonal pattern of arborization, spontaneous synaptic potentials, evoked postsynaptic currents, pattern of discharge to depolarizing pulses and current-voltage relationships. Data were obtained for 170 neurones; 13 of these had somata in lamina I and 157 in lamina II. Stimulation of the segmental dorsal root evoked a prompt excitatory response in almost every neurone sampled (161/166) with nearly 3/4 displaying putative monosynaptic EPSCs. The majority of neurones (133/170) fitted one of several distinctive morphological categories. To a considerable extent, neurones with a common morphological configuration and neurite disposition shared electrophysiological characteristics. Five of the 13 lamina I neurones were relatively large with extensive dendritic arborization in the horizontal dimension and a prominent axon directed ventrally and contralaterally. These presumptive ventrolateral projection neurones differed structurally and electrophysiologically from the other lamina I neurones, which had ipsilateral, locally arborizing axons and/or branches entering the dorsal lateral funiculus. One hundred and twenty lamina II neurones fitted one of five morphological categories: islet, central, medial-lateral, radial or vertical. Central cells were further divided into three groups on functional features. We conclude that the spinal SDH comprises many types of neurones whose morphological characteristics are associated with specific functional features implying diversity in functional organization of the SDH and in its role as a major synaptic termination for thin primary afferent fibres.

Sensory fibers of the pelvic nerve innervating the Rat's urinary bladder.

Much attention has been given to the pelvic nerve afferent innervation of the urinary bladder; however, reports differ considerably in descriptions of afferent receptor types, their conduction velocities, and their potential roles in bladder reflexes and sensation. The present study was undertaken to do a relatively unbiased sampling of bladder afferent fibers of the pelvic nerve in adult female rats. The search stimulus for units to be studied was electrical stimulation of both the bladder nerves and the pelvic nerve. Single-unit activity of 100 L(6) dorsal root fibers, activated by both pelvic and bladder nerve stimulation, was analyzed. Sixty-five units had C-fiber and 35 units had Adelta-fiber conduction velocities. Receptive characteristics were established by direct mechanical stimulation, filling of the bladder with 0.9% NaCl at a physiological speed and by filling the bladder with solutions containing capsaicin, potassium, or turpentine oil. The majority (61) of these fibers were unambiguously excited by bladder filling with 0.9% NaCl and were classified as mechanoreceptors. All mechanoreceptors with receptive fields on the body of the bladder had low pressure thresholds (

Causalgia, pathological pain, and adrenergic receptors.

Control of expression of molecular receptors for chemical messengers and modulation of these receptors' activity are now established as ways to alter cellular reaction. This paper extends these mechanisms to the arena of pathological pain by presenting the hypothesis that increased expression of alpha-adrenergic receptors in primary afferent neurons is part of the etiology of pain in classical causalgia. It is argued that partial denervation by lesion of peripheral nerve or by tissue destruction induces a change in peripheral nociceptors, making them excitable by sympathetic activity and adrenergic substances. This excitation is mediated by alpha-adrenergic receptors and has a time course reminiscent of experimental denervation supersensitivity. The change in neuronal phenotype is demonstrable after lesions of mixed nerves or of the sympathetic postganglionic supply. Similar partial denervations also produce a substantial increase in the number of dorsal root ganglion neurons evidencing the presence of alpha-adrenergic receptors. The hypothesis proposes the increased presence of alpha-adrenergic receptors in primary afferent neurons to result from an altered gene expression triggered by cytokines/growth factors produced by disconnection of peripheral nerve fibers from their cell bodies. These additional adrenergic receptors are suggested to make nociceptors and other primary afferent neurons excitable by local or circulating norepinephrine and epinephrine. For central pathways, the adrenergic excitation would be equivalent to that produced by noxious events and would consequently evoke pain. In support, evidence is cited for a form of denervation supersensitivity in causalgia and for increased expression of human alpha-adrenergic receptors after loss of sympathetic activity.

Pain.

Advances in our understanding of the activation of peripheral damage-sensing neurons (nociceptors) over the past year have been complemented by electrophysiological and imaging studies of central nervous system pain-related centres. The manipulation of gene expression in a reversible and cell type specific way combined with imaging and electrophysiological studies holds promise for helping us to identify the spatial and molecular substrates of pain perception with increasing precision and gives hope for improved analgesic therapies.

Expression of alpha2-adrenergic receptors in rat primary afferent neurones after peripheral nerve injury or inflammation.

1. Immunocytochemistry with polyclonal antibodies directed against specific fragments of intracellular loops of alpha2A- and alpha2C-adrenergic receptors (alpha2A-AR, alpha2C-AR) was used to explore the possibility that expression of these receptors in dorsal root ganglion (DRG) neurones of rat alters as a result of peripheral nerve injury or localized inflammation. 2. Small numbers of neurones with positive alpha2A-AR immunoreactivity (alpha2A-AR-IR) were detected in DRG from normal animals or contralateral to nerve lesions. In contrast, after complete or partial sciatic nerve transection the numbers of ipsilateral L4 and L5 DRG somata expressing alpha2A-AR-IR sharply increased (>5-fold). There was no discernible change in the number of DRG neurones exhibiting alpha2A-AR-IR innervating a region in association with localized chemically induced inflammation. 3. After nerve injury, double labelling with Fluoro-Gold, a marker of retrograde transport from transected fibres, or by immunoreactivity for c-jun protein, an indicator of injury and regeneration, suggested that many of the neurones expressing alpha2A-AR-IR were uninjured by the sciatic lesions. 4. In general the largest proportionate increase in numbers of neurones labelled by alpha2A-AR-IR after nerve lesions appeared in the medium-large diameter range (31-40 microm), a group principally composed of cell bodies of low threshold mechanoreceptors. The number of small diameter DRG neurones labelled by alpha2A-AR-IR, a category likely to include somata of nociceptors, also increased but proportionately less. 5. Relatively few DRG neurones exhibited alpha2C-AR-IR; this population did not appear to change after either nerve lesions or inflammation. 6. These observations are considered in relation to effects of nerve injury on excitation of primary afferent neurones by sympathetic activity or adrenergic agents, sympathetically related neuropathy and reports of sprouting of sympathetic fibres in DRG.

Differences in Ca2+ channels governing generation of miniature and evoked excitatory synaptic currents in spinal laminae I and II.

Many neurons of spinal laminae I and II, a region concerned with pain and other somatosensory mechanisms, display frequent miniature "spontaneous" EPSCs (mEPSCs). In a number of instances, mEPSCs occur often enough to influence neuronal excitability. To compare generation of mEPSCs to EPSCs evoked by dorsal root stimulation (DR-EPSCs), various agents affecting neuronal activity and Ca2+ channels were applied to in vitro slice preparations of rodent spinal cord during tight-seal, whole-cell, voltage-clamp recordings from laminae I and II neurons. The AMPA/kainate glutamate receptor antagonist CNQX (10-20 microM) regularly abolished DR-EPSCs. In many neurons CNQX also eliminated mEPSCs; however, in a number of cases a proportion of the mEPSCs were resistant to CNQX suggesting that in these instances different mediators or receptors were also involved. Cd2+ (10-50 microM) blocked evoked EPSCs without suppressing mEPSC occurrence. In contrast, Ni2+ (

Relationship of substance P to afferent characteristics of dorsal root ganglion neurones in guinea-pig.

1. The relationship between the afferent properties and substance P-like immunoreactivity (SP-LI) of L6 and S1 dorsal root ganglion (DRG) neuronal somata was examined in anaesthetized guinea-pigs. Glass pipette microelectrodes filled with fluorescent dyes were used to make intracellular recordings and to label DRG somata. The dorsal root conduction velocity (CV) and the afferent receptive properties of each unit were categorized according to criteria established in other species. Categories included a variety of low threshold mechanoreceptive classes, innocuous thermoreceptive and several nociceptive classes. Nociceptive units were further subdivided on the basis of CV and the locus of the receptive field (superficial cutaneous, deep cutaneous or subcutaneous). 2. SP-LI was determined using the avidin-biotin complex method and the relative staining intensity determined by image analysis. The possible significance of labelling intensity is discussed. Clear SP-LI appeared in twenty-nine of 117 dye-labelled neurones. All SP-LI positive units with identified receptive properties were nociceptive but not all categories of nociceptors were positive. The intensity of SP-LI labelling varied, often systematically, in relation to afferent properties. There was a tendency for nociceptive neurones with slower CVs and/or smaller cell bodies to show SP-LI. 3. Nineteen of fifty-one C fibre neurones showed SP-LI. Fewer than half the C polymodal nociceptors (CPMs) were positive. The most intensely labelled units were the deep cutaneous nociceptors and some of the CPMs in glabrous skin. C low threshold mechanoreceptors and cooling-sensitive units did not show SP-LI. 4. Ten of sixty-six A fibre neurones exhibited SP-LI, including eight of sixteen A delta nociceptors and two of fifteen A alpha/beta nociceptors. A fibre neurones exhibiting SP-LI included seven of eight deep cutaneous mechanical nociceptors and some superficial cutaneous mechano-heat nociceptors of hairy skin. In contrast, none of twenty superficial cutaneous A high threshold mechanoreceptor units or the thirty-five A fibre low threshold units (D-hair and other units) showed detectable SP-LI. 5. We conclude that SP-LI labelling in guinea-pig DRG neurones is related to (a) afferent receptive properties, (b) the tissue in which the peripheral receptive terminals are located, (c) the CV and (d) the soma size.

Microelectrode arrays for stimulation of neural slice preparations.

A planar 6 x 6 array of iridium electrodes with four reference electrodes has been developed for use with neural tissue preparations. Precise knowledge of the relative locations of the array elements allows for spatial neurophysiological analyses. The 10 microns diameter platinized iridium electrodes on a 100 microns pitch have been used to stimulate acutely prepared slices of spinal cord from free-ranging rodents. An intracellular recording from a single neuron in the substantia gelatinosa (SG) using the whole-cell, tight-seal technique allowed low noise, high resolution studies of excitatory or inhibitory electrical responses of a given neuron to inputs from the primary afferent fibers or from stimulation by individual electrodes of the array. The resulting maps of responses provide an indication of the interconnectivity of neural processes. The pattern emerging is that of limited interconnectivity in the SG from areas surrounding a recorded neuron but with strong excitatory or inhibitory effects from those oriented in a longitudinal (rostral-caudal) direction relative to the neuron. The observations to date suggest the neurons of the SG are arranged in sets of independent networks, possibly related to sensory modality and input from particular body regions.

Effects of partial nerve injury on the responses of C-fiber polymodal nociceptors to adrenergic agonists.

The effects of partial division of the great auricular nerve of adult rabbits were evaluated on the responsiveness of cutaneous C-fiber polymodal nociceptors (CPMs) to sympathetic stimulation (SS), close-arterial injections of epinephrine (EPI) and other alpha-adrenergic agonists. In normal unanesthetized rabbits, the two ears were usually at the same temperature. Two to 4 weeks after partial nerve lesions, however, the operated ear was cooler by 1-3 degrees C in the majority of animals, suggestive of increased vasoconstriction and possible denervation supersensitivity. Neither SS nor EPI (50 ng) excited CPM units (n = 23) from intact anesthetized animals. In contrast, 14-27 days after partial nerve lesions, SS (8 out of 38 units) and EPI (12 out of 38 units) were excitatory for a class of CPMs. There was notable variability in the response of different units and of a given unit between first and second trials. Responses consisted of 1-22 impulses for SS and 1-23 impulses for EPI in the 60 s following a trial. Arterial occlusion did not activate responsive units, suggesting that the excitation was not caused by vascular or temperature changes. Selective alpha2-adrenoceptor blockade with yohimbine (0.6-1.0 mg/kg i.v.) or rauwolscine (1.0 mg/kg i.v.) reversibly antagonized the effects of SS and EPI. EPI-responsive units were also excited by norepinephrine (50 ng) and guanabenz (10 microg) but not by clonidine (3 microg) or B-HT 933 (3 microg). The results suggest that circulating EPI, acting via an alpha-adrenoceptor subtype, can play a part in the development and/or maintenance of aberrant pain syndromes such as causalgia and other sympathetically related dystrophies.

Sympathectomy induces adrenergic excitability of cutaneous C-fiber nociceptors.

1. The effects of ipsilateral removal of the superior cervical ganglion on the subsequent responsiveness of C-fiber polymodal nociceptors (CPMs) of the ear to close-arterial injections of norepinephrine (NE) were evaluated in adult, anesthetized rabbits. 2. In normal unanesthetized rabbits, the two ears were usually at the same temperature. Immediately after the ganglionectomy, the ipsilateral ear was warmer; however, at the time of electrophysiological recordings (4-23 days) the majority of animals had the ipsilateral ear cooler by > or = 1 degree C, suggestive of denervation supersensitivity. 3. NE (50 ng) did not activate any CPMs (n = 28) from intact animals. 4. Seven of 22 CPMs recorded from sympathectomized ears were activated by NE (50 ng). The responses varied considerably but typically consisted of 2-4 impulses in the 60 s after the NE injection. In some instances, repetitive activity continued for many minutes. Such prolonged discharge differs from the adrenergic responses seen after partial nerve damage. 5. The induction of adrenergic excitability in CPMs by sympathectomy is suggested to be a counterpart to postsympathectomy neuralgia in human beings and a possible part of the mechanism leading to sympathetically related pain states.

Cutaneous polymodal receptors: characteristics and plasticity.

The cutaneous sensory units labeled C-fiber polymodal nociceptors have a broadly coherent set of responsive characteristics. These include; (a) elevated thresholds to mechanical stimulation and to heat; (b) excitation by irritant and algesic chemicals; and (c) sensitization by injury or algesic substances. These characteristics and the match between the signals produced by C-polymodal nociceptors to pain-causing stimuli and human reports of pain indicate a probable causal connection. Nevertheless, there are indications that this population of sensory units may contain functionally-distinct subtypes. Some human C-polymodal nociceptors have been reported to be excited by histamine at low concentrations, whereas much of the population lacks such responsiveness. Further, in vitro studies of the effects of non-steroidal anti-inflammatory agents and low pH on sensitization suggest distinctions in the responsiveness of different elements whose general characteristics place them into the C-polymodal category. The enhanced responsiveness of C-polymodal nociceptors after heat stimulation or exposure to acidity has a probable relationship to the primary hyperalgesia produced after injury to hairy skin or in the presence of inflammation. Furthermore, the alterations of C-polymodal nociceptor characteristics after partial nerve injury and sympathectomy imply a change in phenotype of neurons spared by denervation and are suggestive of a possible relationship to sympathetically related pain and post-sympathetic neuralgias. These evidences of plasticity in responsiveness of a set of sense organs putatively associated with cutaneous pain represent lessons in the adaptability of biological mechanisms, and clues to the pathophysiology of pain.

ATP modulation of synaptic transmission in the spinal substantia gelatinosa.

Actions of adenosine triphosphate (ATP) on neurons of the substantia gelatinosa (SG) were evaluated in spinal cord slices using tight-seal, whole-cell recordings. Bath-applied ATP activated a fast inward current and potentiated both glutamate-induced and synaptically evoked currents by acting through a purinergic receptor with the pharmacology of the P2 type. ATP also induced a delayed slow outward current and depressed synaptic currents that appeared to result from hydrolysis of ATP to adenosine. The inhibitory actions had features suggesting mediation by a P1-like purinergic receptor. Suramin, a putative P2 antagonist, inhibited ATP-induced fast inward currents but did not suppress synaptic currents evoked by dorsal root stimulation. It was concluded that in the SG, ATP released in synaptic regions acts as a synaptic modulator by augmenting excitatory amino acid actions and possibly by also producing a secondary adenosine inhibition.

Effects of nerve injury on sympathetic excitation of A delta mechanical nociceptors.

1. The effects of sympathetic stimulation and close arterial injection of norepinephrine were tested on cutaneous myelinated-fiber (A delta) mechanical nociceptors [high-threshold mechanoreceptors-(MyHTMs)] from normal and from partially transsected nerves. 2. Neither sympathetic stimulation nor close arterial injection of norepinephrine (200 ng) excited MyHTMs (18) recorded from the uninjured great auricular nerve of adult rabbits. 3. MyHTMs (58) conducting across the site of partial cut lesions, made 2 to 28 days previously, had threshold and responsiveness to mechanical stimuli, receptive field organization, and absence of background discharge typical of such elements in normal nerve. 4. Four MyHTMs recorded from the injured nerves were excited by sympathetic stimulation and/or norepinephrine injection but only one gave more than two impulses within 60 s to either form of stimulation. 5. The meagerness of the sympathetic and adrenergic excitation of MyHTMs after nerve injury contrasts with that observed under similar conditions for C-fiber polymodal nociceptors. Therefore, induction of adrenergic responsiveness in nociceptors after partial denervation in cutaneous MyHTMs appears to be less important for mechanisms related to pathogenic pain than alterations in certain C-fiber nociceptors.

Differential L-glutamate responsiveness among superficial dorsal horn neurons.

1. Intracellular recordings were made from 128 superficial dorsal horn (laminae I and II) neurons in slice preparations of the lumbosacral spinal cord obtained from young hamsters. Stimulation of the segmental dorsal root evoked postsynaptic potentials in all neurons. The average transmembrane resting potential was -61 +/- 1 mV (mean +/- SE; n = 123). The mean action potential amplitude was 75 +/- 1 mV (n = 105) with a duration at half peak of 1.1 +/- 0.1 ms (n = 102). The mean input resistance of these neurons was 72 +/- 4 M omega (n = 125). These values are comparable to those reported in other studies on neurons of this region using penetrating microelectrodes. 2. Bath application of N-methyl-D-aspartate (NMDA; 50 microM) depolarized 67 of 71 (94%) of the tested neurons. Superfusion with the non-NMDA amino acid agonists DL-alpha-amino-3-hydroxy-5-methyl-4- isoxazole propionic acid (AMPA; 20 microM) and kainate (KA; 50 microM) depolarized all tested neurons by > 10 mV. On the other hand, only 13 of 67 (19%) tested neurons were depolarized > 4 mV by superfusion solutions containing 3 mM L-glutamate (Glu). L-Aspartate at 3 mM depolarized three out of seven neurons by > 4 mV and appeared to be equally as effective as Glu. 3. The non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) substantially attenuated the AMPA- and KA-induced depolarizations and partially attenuated the NMDA-induced depolarizations. The NMDA antagonist 3 [(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP; 50 microM) reversibly blocked the NMDA-induced depolarization in all tested neurons. Glu-induced depolarization was unaffected by CNQX but was attenuated by CPP in three of three tested neurons. These observations indicate that some of the Glu-induced depolarization was mediated by NMDA receptors. 4. CNQX reversibly attenuated excitatory postsynaptic potentials (EPSPs) produced by primary afferent activity in A delta- and C-fibers whereas CPP suppressed only the late EPSP components. Therefore in the neurons sampled, synaptic responses evoked from primary afferent fibers appear to be mediated by both non-NMDA and NMDA receptors. 5. The glutamate uptake inhibitors, L-trans-pyrrolidine-2,4-dicarboxylate (L-trans PDC; 50 microM; n = 6) and threo-3-hydroxy-D-aspartate (1 mM; n = 1) did not have a consistent effect upon Glu action background discharge, RN or Vm in Glu-unresponsive neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Cutaneous sensory receptors.

The range of sensations elicitable from the skin is wide. We review the complex and diverse nature of cutaneous sense organs and the way these cutaneous receptors function as transducers of information from the skin. The discussion touches on the afferent properties of various classes of cutaneous receptors, the conduction velocity of their peripheral nerve fibers, the morphology of the receptor terminal, and the mode of sensation evoked. A brief view of associated features of dorsal root ganglion neurons is also provided.

Adenosine inhibition of synaptic transmission in the substantia gelatinosa.

1. We studied adenosine's action on synaptic transmission from primary afferent fibers to neurons of the substantia gelatinosa (SG) using tight-seal whole cell recordings in transverse slices of hamster spinal cord. Adenosine had two actions, hyperpolarization of the postsynaptic membrane and depression of the excitatory postsynaptic currents (EPSCs) evoked by dorsal root stimulation. 2. Under voltage clamp adenosine elicited a sustained outward current at a holding potential of -70 mV. The outward current was blocked by a combination of intracellular cesium and tetraethylammonium, an effect characteristic of potassium channels. The adenosine-induced current reversed at -97 +/- 6 (SD) mV, close to the potassium equilibrium potential. These observations suggest that adenosine activates a potassium conductance in SG neurons so as to inhibit primary afferent synaptic transmission postsynaptically. 3. Adenosine reduced the miniature EPSC frequency without significantly changing the amplitude. In contrast, the glutamate receptor competitive antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) substantially reduced the amplitudes of miniature EPSCs while producing a much smaller effect on the miniature frequency than adenosine. In evoked EPSCs adenosine reduced unitary content without reducing unitary amplitude. The effects on both miniature and evoked EPSCs suggest that adenosine inhibits synaptic currents by suppressing presynaptic transmitter release. 4. EPSCs evoked by dorsal root stimuli were subdivided into monosynaptic and polysynaptic categories. Adenosine at superfusion concentrations of 20-300 microM suppressed all polysynaptic EPSCs. Less than half of monosynaptic EPSCs were inhibited, usually those evoked by the slowest-conducting primary afferents. These observations were interpreted to indicate that a principal action of adenosine in SG is on interneuronal communication.

Synaptic mediation from cutaneous mechanical nociceptors.

1. Responses of dorsal horn neurons to cutaneous mechanical stimulation were studied in an in vitro preparation of hamster spinal cord with partially intact innervation from an isolated patch of hairy skin. Stable extracellular and intracellular recordings were obtained from cells with different mechanoreceptive properties similar to those reported for other species in vivo. Analyses were made of the intracellular responses of 25 dorsal horn neurons activated selectively by mechanical stimulation to the skin patch. 2. Bath application of the broad spectrum, excitatory amino acid (EAA) receptor antagonist, kynurenic acid (1 mM) blocked excitation of 7 of 8 high-threshold mechanoreceptive units by either cutaneous nerve volleys or mechanical stimulation of the skin. This concentration of kynurenic acid suppressed peripherally evoked responses in 8 of 14 neurons responsive to innocuous mechanical stimuli. 3. High-threshold mechanoreceptive neurons of the superficial dorsal horn exhibited one of three distinctive patterns of postsynaptic potentials in response to electrical stimulation of cutaneous afferent fibers: 1) a simple fast excitatory postsynaptic potential (EPSP), 2) a fast EPSP with a prolonged decay phase lasting between 100 and 1,000 ms, and 3) a multiphasic response dissociable on the basis of stimulus strength consisting of a fast EPSP followed by a hyperpolarizing inhibitory postsynaptic potential (IPSP) (duration 80-100 ms). Gentle mechanical stimuli initiated inhibition from areas adjacent to the high-threshold mechanically excitatory field; this suggests that membrane hyperpolarization in these neurons was evoked by input from low-threshold mechanoreceptors. 4. Bath application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM), a competitive EAA antagonist selective for non-N-methyl-D-aspartate (non-NMDA) receptor subtypes, substantially or completely (56-100%) suppressed EPSPs evoked from cutaneous afferent fibers in high-threshold mechanoreceptive neurons. CNQX also decreased the membrane depolarization, the frequency of EPSPs, and the frequency of action potentials evoked by mechanical stimulation of the receptive field. 5. CNQX (10 microM) or kynurenic acid (1 mM) had considerably weaker effects on IPSPs than on EPSPs evoked from the periphery in superficial dorsal horn neurons. IPSP amplitudes were unchanged by these agents in some neurons and decreased by only 20-25% in others. 6. We conclude that L-glutamate acting on non-NMDA receptors mediates fast synaptic excitation of superficial dorsal horn neurons from peripheral mechanical nociceptors with myelinated fibers. Furthermore, the observations imply either an agent other than L-glutamate or one acting at different membrane receptors is a synaptic mediator for other peripheral afferent units including some activated by innocuous mechanical stimuli.