A survey of spinal dorsal horn neurones encoding the spatial organization of withdrawal reflexes in the rat

Action-based sensory encoding in spinal sensorimotor circuits

The concept of a modular organisation of the spinal withdrawal reflex circuits has proven to be fundamental for the understanding of how the spinal cord is organised and how the sensorimotor circuits translate sensory information into adequate movement corrections. Recent studies indicate that a task-related body representation is engraved at the network level through learning-dependent mechanisms involving an active probing procedure termed 'somatosensory imprinting' during development. It was found that somatosensory imprinting depends on the tactile input that is associated with spontaneous movements that occur during sleep and results in elimination of erroneous connections and establishment of correct connections. In parallel studies it was found that the strength of the first order tactile synapses in rostrocaudally elongated zones in the adult dorsal horn in the lower lumbar cord is related to the modular organisation of the withdrawal reflexes. Hence, the topographical organisation of the tactile input to this spinal area seems to be action-based rather than a simple body map as previously thought. Far from being innate and adult like at birth, the adult organisation seems to emerge from an initial 'floating' and diffuse body representation with many inappropriate connections through profound activity-dependent rearrangements of afferent synaptic connections. It is suggested that somatosensory imprinting plays a key role in the self-organisation of the spinal cord during development.

Spinal interneurons that are selectively activated during fictive flexion reflex

Behavioral choices in invertebrates are mediated by a combination of shared and specialized circuitry, including neurons that are inhibited during competing behaviors. Less is known, however, about the neural mechanisms of behavioral choice in vertebrates. The spinal cord can appropriately select among several types of limb movements, including limb withdrawal (flexion reflex), scratching, and locomotion, and thus is conducive to examination of vertebrate mechanisms of behavioral choice. Flexion reflex can interrupt and reset the rhythm of scratching and locomotion, suggesting that a combination of shared and specialized circuitry contributes to these behaviors, but little is known about the interneurons involved. Here, I used in vivo intracellular recording and dye injection to identify a group of spinal interneurons that are strongly activated during fictive flexion reflex but inhibited during fictive scratching and fictive swimming. These flexion-selective interneurons are typically rhythmically hyperpolarized during fictive scratching and fictive swimming. This hyperpolarization can be maximal during the ipsilateral hip flexor bursts of rhythmic limb motor patterns, although these cells are strongly activated during the ipsilateral hip flexor bursts of fictive flexion reflex. Thus, these interneurons are relatively specialized for fictive limb withdrawal, rather than contributing to the hip flexor phase of multiple types of limb movements. These flexion-selective cells are physiologically and morphologically distinguishable from a recently described group of spinal interneurons (transverse interneurons) that are strongly activated during both fictive flexion reflex and fictive scratching. Thus, spinal interneurons with distinct behavioral roles may to some extent be morphologically distinguishable.

Parieto-frontal interactions, personal space, and defensive behavior

In the monkey brain, two interconnected cortical areas have distinctive neuronal responses to visual, tactile, and auditory stimuli. These areas are the ventral intraparietal area (VIP) and a polysensory zone in the precentral gyrus (PZ). The multimodal neurons in these areas typically respond to objects touching, near, or looming toward the body surface. Electrical stimulation of these areas evokes defensive-like withdrawing or blocking movements. These areas have been suggested to participate in a range of functions including navigation by optic flow, attention to nearby space, and the processing of object location for the guidance of movement. We suggest that a major emphasis of these areas is the construction of a margin of safety around the body and the selection and coordination of defensive behavior. In this review, we summarize the physiological properties of these brain areas and discuss a range of behavioral phenomena that might be served by those neuronal properties, including the ducking and blocking reactions that follow startle, the flight zone of animals, the personal space of humans, the nearby, multimodal attentional space that has been studied in humans, the withdrawal reaction to looming visual stimuli, and the avoidance of obstacles during self-motion such as locomotion or reaching.

Spinal dorsal horn neuronal responses to myelinated versus unmyelinated heat nociceptors and their modulation by activation of the periaqueductal grey in the rat

The aim of this study was to further understand the central processing of inputs arising from unmyelinated and myelinated nociceptors by (i) determining the response characteristics of Class 2 dorsal horn neurones to preferential activation of C- and A-fibre heat nociceptors, and (ii) investigating the control exerted by the dorsolateral/lateral region of the midbrain periaqueductal grey (DL/L-PAG) on C- and A-fibre-evoked responses of these neurones. The use of different rates of skin heating to preferentially activate unmyelinated (C-fibre; 2.5 degrees C s(-1)) versus myelinated (A-fibre; 7.5 degrees C s(-1)) heat nociceptors revealed that, in response to C-nociceptor activation, Class 2 neurones encode well only over the first 5 degrees C above threshold, and that at higher temperatures responses decline. In contrast, responses to A-nociceptor activation are linear and encode skin temperature over more than 10 degrees C, and almost certainly into the tissue-damaging range. PAG stimulation raised thresholds and decreased significantly the magnitude of responses to A- and C-nociceptor activation. However, differences were revealed in the effects of descending control on the relationships between skin temperature and neuronal firing rate; the linear relationship that occurred over the first 5 degrees C of slow rates of skin heating was no longer evident, whereas that to fast rates of skin heating was maintained over the entire range, albeit shifted to the right. These data indicate that the sensori-discriminative information conveyed in A-fibre nociceptors is maintained and that the information from C-nociceptors is lost in the presence of descending control from the DL/L-PAG. The data are discussed in relation to the role of the DL/L-PAG in mediating active coping strategies.

Topography of spinal neurons active during hindlimb withdrawal reflexes in the decerebrate cat

There exists a spatial organization of receptive fields and a modular organization of the flexion withdrawal reflex system. However, the three dimensional location and organization of interneurons interposed in flexion reflex pathways has not been systematically examined. We determined the anatomical locations of spinal neurons involved in the hindlimb flexion withdrawal reflex using expression of the immediate early gene c-fos and the corresponding FOS protein. The flexion withdrawal reflex was evoked in decerebrate cats via stimulation of the tibial or superficial peroneal nerve. Animals that received stimulation had significantly larger numbers of cells expressing FOS-like immunoreactivity (42.7+/-2.3 cells/section, mean+/-standard error of the mean) than operated unstimulated controls (18.6+/-1.4 cells/section). Compared with controls, cells expressing FOS-like immunoreactivity were located predominantly on the ipsilateral side, in laminae IV-VI, at L6 and rostral L7 segments, and between 20% and 60% of the distance from the midline to the lateral border of the ventral gray matter. Labeled neurons resulting from tibial nerve stimulation were medial to neurons labeled following superficial peroneal nerve stimulation in laminae I-VI, but not VII. The mean mediolateral positions of labeled neurons from both nerves shifted medially as the transverse plane in which they were viewed was moved from rostral to caudal and as the coronal plane in which they were viewed was moved from dorsal to ventral. The mediolateral separation between populations of labeled cells was consistent with primary afferent projections and the location of reflex encoders. This topographical segregation corresponding to different afferent inputs is a possible anatomical substrate for a modular organization of the flexion withdrawal reflex system.

Retrograde analyses of spinothalamic projections in the macaque monkey: Input to ventral posterior nuclei

The distribution of retrogradely labeled spinothalamic tract (STT) neurons was analyzed in monkeys following variously sized injections of cholera toxin subunit B (CTb) in order to determine whether different STT termination sites receive input from different sets of STT cells. This report focuses on STT input to the ventral posterior lateral nucleus (VPL) and the subjacent ventral posterior inferior nucleus (VPI), where prior anterograde tracing studies identified scattered STT terminal bursts and a dense terminal field, respectively. In cases with small or medium-sized injections in VPL, labeled STT cells were located almost entirely in lamina V (in spinal segments consistent with the mediolateral VPL topography); few cells were labeled in lamina I (<8%) and essentially none in lamina VII. Large and very large injections in VPL produced marked increases in labeling in lamina I, associated first with spread into VPI and next into the posterior part of the ventral medial nucleus (VMpo), and abundant labeling in lamina VII, associated with spread into the ventral lateral (VL) nucleus. Small injections restricted to VPI labeled many STT cells in laminae I and V with an anteroposterior topography. These observations indicate that VPL receives STT input almost entirely from lamina V neurons, whereas VPI receives STT input from both laminae I and V cells, with two different topographic organizations. Together with the preceding observation that STT input to VMpo originates almost entirely from lamina I, these findings provide strong evidence that the primate STT consists of anatomically and functionally differentiable components.

Inhibition of glutamate uptake in the spinal cord induces hyperalgesia and increased responses of spinal dorsal horn neurons to peripheral afferent stimulation

Glutamate is a primary excitatory neurotransmitter in the mammalian CNS. Glutamate released from presynaptic neurons is cleared from the synaptic cleft passively by diffusion and actively by glutamate transporters. In this study, the role of glutamate transporters in sensory processing in the spinal cord has been investigated in behavioral, in vivo and in vitro experiments. Intrathecal application of a non-selective glutamate transport inhibitor, L-trans-pyrrolidine-2,4-dicarboxylic acid (10 microl of 100 microM solution) induced hypersensitivity to peripheral mechanical and thermal stimuli. Topical application of L-trans-pyrrolidine-2,4-dicarboxylic acid (100 microM) onto the dorsal surface of the L3-L6 spinal cord increased spontaneous activities, innocuous and noxious stimulus-evoked responses and after-discharges of wide dynamic range neurons in the L4-5 spinal segments. Whole cell recordings made from superficial dorsal horn neurons in an isolated whole spinal cord from newborn rats (2-3 weeks old) revealed that bath-applied L-trans-pyrrolidine-2,4-dicarboxylic acid (100 microM) produced partial membrane depolarization, increased spontaneous action potentials with decreased neuronal membrane resistance and time constant, but without significant changes of capacitance. Finally, the amplitude and duration of primary afferent evoked-excitatory postsynaptic currents recorded from neurons in the substantia gelatinosa in the spinal slices from young adult rats (6-8 weeks old) were increased in the presence of L-trans-pyrrolidine-2,4-dicarboxylic acid (100 microM). This study indicates that glutamate transporters regulate baseline excitability and responses of dorsal horn neurons to peripheral stimulation, and suggests that dysfunction of glutamate transporters may contribute to certain types of pathological pain.

Parieto-frontal interactions, personal space, and defensive behavior

In the monkey brain, two interconnected cortical areas have distinctive neuronal responses to visual, tactile, and auditory stimuli. These areas are the ventral intraparietal area (VIP) and a polysensory zone in the precentral gyrus (PZ). The multimodal neurons in these areas typically respond to objects touching, near, or looming toward the body surface. Electrical stimulation of these areas evokes defensive-like withdrawing or blocking movements. These areas have been suggested to participate in a range of functions including navigation by optic flow, attention to nearby space, and the processing of object location for the guidance of movement. We suggest that a major emphasis of these areas is the construction of a margin of safety around the body and the selection and coordination of defensive behavior. In this review, we summarize the physiological properties of these brain areas and discuss a range of behavioral phenomena that might be served by those neuronal properties, including the ducking and blocking reactions that follow startle, the flight zone of animals, the personal space of humans, the nearby, multimodal attentional space that has been studied in humans, the withdrawal reaction to looming visual stimuli, and the avoidance of obstacles during self-motion such as locomotion or reaching.

Heterosynaptic long-term depression of craniofacial nociception: divergent effects on pain perception and blink reflex in man

Noxious low-frequency stimulation (LFS) of presynaptic nerve fibers induces long-term depression (LTD) of synaptic transmission. In vitro studies suggest a sole homosynaptic effect. Consequently, the present study addressed the hypothesis that LTD of craniofacial nociception in man is mediated by a homosynaptic mechanism. Nociceptive supraorbital afferents were excited by electric pulses via a concentric electrode in ten healthy volunteers. The electrically evoked bilateral blink reflex (BR) was recorded from both orbicularis oculi muscles by surface electrodes. The BR was evoked in blocks of ten electric stimuli each (0.1 Hz) with an interblock interval of 8 min. Conditioning noxious LFS (1 Hz, 20 min) was applied via concentric electrode either to the same site as BR test stimuli (ipsilateral) or to the corresponding contralateral forehead area (contralateral). LFS and test stimulus intensities corresponded to about threefold the pain threshold. After three baseline stimulus blocks, either conditioning ipsilateral or contralateral LFS were applied or stimulation was interrupted for 20 min as a control task. Afterwards, test stimulation blocks were continued for 40 min. Each volunteer participated in all three sessions on different days. Noxious LFS induced LTD of the BR independently from the side of conditioning stimulation. Pain perception decreased after ipsilateral LFS but not after contralateral LFS. The bilateral effect of noxious LFS on the BR provides evidence for heterosynaptic LTD based on bilateral projections of supraorbital nerve afferents onto spinal trigeminal nuclei. The divergent effect on pain perception may be due to a preferential contralateral projection of nociceptive afferents onto reflex interneurons but not onto trigeminothalamic projection neurons.

Localization and connectivity in spinal interneuronal networks: the adduction-caudal extension-flexion rhythm in the frog

We have previously reported that focal intraspinal N-methyl-d-aspartate (NMDA) iontophoresis in the frog elicits a motor output, which is organized in terms of its constituent isometric force directions at the ipsilateral ankle and its topography. Furthermore, the associated EMG patterns can be reconstructed as the linear combinations of seven muscle synergies, labeled A to G. We now focus on one of the most common NMDA-elicited outputs, the adduction-caudal extension-flexion rhythm, and examine the relationship between the different force phases in terms of synergies and topography. Two distinct EMG patterns produce caudal extensions, and only one of the two patterns is used at most sites. The key synergy combinations for the two patterns are B + e and D + c (strongest synergies capitalized). These two patterns map at distinct locations in the lumbar cord. Within individual sites rhythms, we find linkages among the synergies used to produce adductions, the onsets of flexions after caudal extensions, and the synergy pattern producing the caudal extensions. For example, the synergy composition of adductions at B + e caudal extension sites is dominated by E + b and at D + c caudal extension sites by C + d. The two types of adductions map at distinct locations, situated between the two caudal extension regions. Specifically the linked patterns of caudal extension-adduction interleave rostrocaudally in a CE2-ADD1-ADD2-CE1 sequence, where 1 and 2 refer to the two pattern types. The implications of this topography and connectivity with respect to motor systems organization and behaviors are discussed.

Gradual enlargement of human withdrawal reflex receptive fields following repetitive painful stimulation

Dynamic changes in the topography of the human withdrawal reflex receptive fields (RRF) were assessed by repetitive painful stimuli in 15 healthy subjects. A train of five electrical stimuli was delivered at a frequency of 3 Hz (total train duration 1.33 s). The train was delivered in random order to 10 electrode sites on the sole of the foot. Reflexes were recorded from tibialis anterior, soleus, vastus lateralis, biceps femoris, and iliopsoas (IL). The RRF changes during the stimulus train were assessed during standing with even support on both legs and while seated. The degree of temporal summation was depending on stimulation site. At the most sensitive part of the RRF, a statistically significant increase in reflex size was seen after two stimuli while four stimuli were needed to observe reflex facilitation at less sensitive electrode sites. Hence, the region from which reflexes could be evoked using the same stimulus intensity became larger through the train, that is, the RRF was gradually expanding. Reflexes evoked by stimuli four and five were of the same size. No reflex facilitation was seen at other stimulus sites outside the RRF. In all muscles except in IL, the largest reflexes were evoked when the subjects were standing. In the ankle joint, the main withdrawal pattern consisted of plantar flexion and inversion when the subjects were standing while dorsi-flexion was prevalent in the sitting position. Up to 35 degrees of knee and hip flexion were evoked often leading to a lift of the foot from the floor during standing. In conclusion, a gradual expansion of the RRF was seen in all muscles during the stimulus train. Furthermore, the motor programme task controls the reflex sensitivity within the reflex receptive field and, hence, the sensitivity of the temporal summation mechanism.

Repetitive Painful Stimulation Produces an Expansion of Withdrawal Reflex Receptive Fields in Humans

The aims of the present study were to investigate whether temporal summation of the nociceptive withdrawal reflex depends on the stimulation site on the sole of the human foot, and to characterize the reflex receptive fields (RRF) of lower limb muscles to repetitive stimulation. The cutaneous RRFs were assessed in 15 subjects in sitting position by recording the EMG from five lower leg muscles and the kinematic responses (ankle, knee, and hip joints) to repetitive painful electrical stimulation. The stimulus consisted of a series of five stimuli (frequency: 3 Hz) delivered randomly at 10 different sites on the sole of the foot. The size of the reflexes increased generally between the first and the second stimulus, however, the increment depended on the stimulation site. In tibialis anterior, the RRF covered the distal sole of the foot and gradually expanded during the stimulus train. No expansion toward the heel area was detected. In soleus, the reflexes were facilitated after the second stimulus at all sites and remained in this state until the last stimulus. In vastus lateralis, biceps femoris, and iliopsoas a gradual expansion of the RRF was seen, resulting in RRFs covering the lateral, distal foot, and part of the proximal foot (iliopsoas). Knee and hip flexion were evoked at all sites. Ankle dorsiflexion was evoked at the distal foot, while ankle plantarflexion was evoked at the heel. The enlargement of the RRF reflects spinal temporal summation leading to gradually stronger reflex responses. The degree of temporal summation was dependent on stimulation site. The facilitation of the withdrawal reflex responses due to repetitive stimulation might have potential applications in the rehabilitation engineering field, where these reflexes could be used to assist gait of patients with central nervous system injuries.

The organization of motor responses to noxious stimuli

Withdrawal reflexes are the simplest centrally organized responses to painful stimuli, making them popular models for the study of nociception. Until recently, it was believed that withdrawal was a single reflex response involving excitation of all flexor muscles in a limb with concomitant inhibition of extensors. However, recent findings suggest that withdrawal reflexes are tailored to produce the most appropriate movement according the site at which the stimulus is applied, which could require extensors to act as the primary movers. This idea is supported by new evidence obtained from the direct measurement of limb movements, although these data indicate that differentiation of withdrawal reflexes is most readily seen from stimuli applied to the plantar surface of the foot. Injurious stimuli augment the protective function of reflexes by enhancing (sensitizing) reflexes that protect the injured site and inhibiting those reflexes that might exacerbate the insult. The areas from which a reflex can be sensitized closely match those from which the reflex itself can be evoked, provided that the spinal cord is intact. If descending pathways are interrupted, sensitization can be evoked from a much wider area. Thus, the exact movement made in a withdrawal reflex is determined by the location of the evoking stimulus and whether the reflex sensitized or inhibited after an injury depends on the relationship between the site of the injury and the movement made by the reflex. The factors should be borne in mind when designing experiments in which reflexes are used as the end point in studies of nociception.

Lamina I, but not Lamina V, Spinothalamic Neurons Exhibit Responses That Correspond With Burning Pain

Single-unit recordings from monkey spinothalamic tract (STT) neurons reveal that the responses of polymodal nociceptive lamina I STT neurons correspond with the profile of burning pain elicited in human subjects by repeated brief-contact heat. In contrast, lamina V wide-dynamic-range (WDR) neurons show a significantly different response pattern. This finding indicates that burning pain is signaled by modality-selective lamina I neurons and not convergent lamina V WDR neurons.

Sensitization of dorsal root reflexes in vitro and hyperalgesia in neonatal rats produced by capsaicin

The maturation of dorsal root reflexes (DRRs) in lumbar roots was characterized in neonatal rats at 1, 2 and 3 weeks after birth using an in vitro isolated spinal cord preparation with attached dorsal roots and dorsal root ganglia (DRG). Changes of DRRs in rats of increasing age were also tested by administration of capsaicin to the DRG and related to spinal mechanisms of hyperalgesia by defining the behavioral responses of neonatal rats to intradermal capsaicin. DRRs evoked by stimulating the adjacent root in 1 week old rats are characterized by highly desynchronized waveforms with power spectra concentrated at frequencies greater than 200 Hz. DRRs in 1 week old rats show very little change in amplitude or area with increasing afferent stimulation strength. In contrast DRRs in 2 and 3 weeks old rats are highly synchronized with power concentrated at frequencies less than 100 Hz and show a graded increase in amplitude and area with increasing stimulus strength. The recovery of DRR amplitude in a paired pulse stimulus protocol is faster in 1 week rats than in 2 or 3 weeks old rats. Finally, DRRs in 2 and 3 week old rats show increased amplitude and area following application of capsaicin to the DRG of the stimulating root whereas those in 1 week old rats do not. These changes parallel the behavioral responses of neonatal rats as 2 and 3 weeks old rats show secondary mechanical hyperalgesia following intradermal capsaicin, but 1 week old rats do not. Our data indicate that the spinal circuitry for DRRs in the neonatal period undergoes rapidly dynamic development in the rat. This development is sufficiently rapid that mechanisms of spinal sensitization induced by capsaicin can be studied in rats 2 weeks old and older.

Properties of an Adult Spinal Sensorimotor Circuit Shaped Through Early Postnatal Experience

During development, information about the three-dimensional shape and mechanical properties of the body is laid down in the synaptic connectivity of sensorimotor systems through adaptive mechanisms. This functional adaptation occurs through alteration of connection properties. Here, we characterize the differences between strong and weak connections in the nociceptive withdrawal reflex in adult decerebrate spinal rats, representing the preserved end product of the developmental adaptation process. Stronger excitatory reflex connections from the skin onto a muscle had relatively higher gain in their input-output relations, shorter onset latencies (up to ∼150 ms) and lower trial-to-trial variability in relation to response amplitude (SD ∼ mean1/2) than weaker pathways. Although inhibitory and excitatory nociceptive receptive fields of a muscle overlap to some degree, the results indicate that the inhibitory input is not a major determinant of the gain distribution within the excitatory receptive field and vice versa. The N-methyl-d-aspartate (NMDA) receptor antagonist, d-2-amino-5-phosphonovalerate (0.1–1 μg), applied topically on the spinal cord reduced the gain, whereas the response amplitude was mainly reduced by an absolute number by the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor antagonist, 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (1–10 μg). The results indicate that NMDA receptors have a critical role in gain regulation in the nociceptive withdrawal reflex system. It is suggested that after normal postnatal experience-dependent adaptation, the number of connections from a given skin site onto the reflex encoding interneurons is a major determinant of the difference in gain.

Mental stress inhibits pain perception and heart rate variability but not a nociceptive withdrawal reflex

AIM

Do distraction from- or attention to sural nerve stimulation affect pain, heart rate variability, and a spinal withdrawal reflex?

MATERIAL AND METHODS

In 26 male volunteers, electrical stimulation at the distal cutaneous receptive field of the sural nerve elicited pain and a nociceptive withdrawal reflex. Intensity of pain was rated on a numeric rating scale. Electromyographic reflex responses were measured from biceps femoris muscle. Cardiac autonomic function was estimated by heart rate variability measures and was expressed in the time domain as mean of RR-intervals for normal heart beats (mean-RR) and standard deviation of all normal RR-intervals (SD-NN) and, in the frequency domain, where pure vagal activity was assessed by high frequency power (0.15-0.4 Hz). Low frequency power (0.04-0.15 Hz) reflects both parasympathetic and sympathetic control. Effect parameters were recorded before and during random distraction and attention. Distraction from sural nerve stimulation was induced by a mental arithmetic test, paced auditory serial addition task (PASAT), while attention was induced by concentrating on painful foot stimulation.

RESULTS

Paced auditory serial addition task decreased mean-RR and SD-NN, frequency domain parameters, as well as pain (P<0.001). In contrast, PASAT did not change the spinal withdrawal reflex. Attention did not affect any effect parameter.

CONCLUSION

Distraction by PASAT altered autonomic activity and inhibited pain but failed to affect withdrawal reflex responses, while attention had no effect on either parameter. Psychological distraction and attention may have different effects on noxious evoked pain perception and autonomic activity. Pain relief during PASAT probably involves supraspinal mechanisms.

Multi-modal induction and assessment of allodynia and hyperalgesia in the human oesophagus

BACKGROUND AND AIMS

Experimental pain models based on single stimuli have to some degree limited visceral pain studies in humans. Hence, the aim of this study was to investigate the effect of multi-modal visceral pain stimuli of the oesophagus in healthy subjects before and after induction of visceral hyperalgesia. We used a multi-modal psychophysical assessment regime and a neurophysiological method (nociceptive reflex) for the characterisation of the experimentally induced hyperalgesia.

METHODS

A probe for multi-modal (cold, warm, electrical, and mechanical) visceral stimulation was positioned in the lower part of the oesophagus in eleven healthy subjects. Mechanical stimuli were applied as distensions with a bag, which also had electrodes mounted for electrical stimulation. Thermal stimulation with temperatures from 0 to 60 degrees C was applied with re-circulating water in the bag. To assess the interaction between visceral and somatic pathways, the nociceptive withdrawal reflex to electrical stimuli at the ankle was measured with and without simultaneous mechanical oesophageal distension to painful levels. Finally, the oesophageal sensitisation was induced by perfusion with hydrochloric acid. Multimodal responses (pain threshold, stimulus response function, size of nociceptive reflex, and referred pain areas) were assessed before and after the induced hyperalgesia.

RESULTS

The multi-modal psychophysical responses and reflex sizes were assessed twice before sensitisation, and the parameters were reproducible. Sensitisation of the oesophagus resulted in hyperalgesia to electrical and mechanical stimuli (29 and 35% decrease in pain threshold) and allodynia to cold and warmth stimuli (11% increase in sensory rating). After sensitisation, the referred pain area to mechanical stimuli increased more than 300% with a change in the localisation of the referred pain to all stimuli, and the amplitude of nociceptive reflex increased 100%, all indicating the presence of central hyperexcitability.

CONCLUSIONS

Visceral hyperalgesia/allodynia can be induced experimentally and assessed quantitatively by the newly introduced multi-modal psychophysical assessment approach. The significant changes of the experimentally evoked referred pain patterns and of the nociceptive reflex evoked from a distant somatic structure indicate that even short-lasting visceral hyperalgesia can generate generalised sensitisation.

Pain mechanisms: labeled lines versus convergence in central processing

The issue of whether pain is represented by specific neural elements or by patterned activity within a convergent somatosensory subsystem has been debated for over a century. The gate control theory introduced in 1965 denied central specificity, and since then most authors have endorsed convergent wide-dynamic-range neurons. Recent functional and anatomical findings provide compelling support for a new perspective that views pain in humans as a homeostatic emotion that integrates both specific labeled lines and convergent somatic activity.

Absence of local sign withdrawal in chronic human spinal cord injury

Local sign withdrawal, a reflex to direct the limb away from noxious cutaneous stimuli, is thought to be indicative of a modular organization of the spinal cord. To assess the integrity of such an organization of the spinal cord in chronic human spinal cord injury (SCI), we tested the electromyogram (EMG) and joint torque responses to cutaneous stimuli applied to 6 locations of the leg in 10 SCI volunteers and 3 spinal-intact controls. The 6 locations included the medial arch of the foot, the second metatarsal, the dorsum, the region over the sural nerve at the lateral malleolus, and the anterior and posterior aspects of the lower leg. Although spinal-intact subjects demonstrated local sign withdrawal, the data from SCI subjects indicated that an invariant flexion response pattern was produced regardless of stimulus location. Ankle dorsiflexion and hip flexion were produced in all subjects at all locations and no difference in the ratio of hip:ankle torques could be detected for the 6 test locations. A windup-crossover test, employing a sequence of 6 stimuli at 1-s intervals was used to assess whether common neuronal pathways were responsible for the loss of modular organization. An additional 10 SCI volunteers were tested using stimuli in which the stimulus location was switched between the 2nd and 3rd stimulus of the test sequence. The response to the crossover stimulus more closely resembled the response to the 3rd stimulus of a windup sequence than a response without conditioning stimuli. These results indicate that increased excitability produced by windup at one stimulus site is maintained at the 2nd site. This observation suggests that deep dorsal horn neurons, typically associated with musculotopic mapping, may be reorganized in chronic spinal cord injury.

Intrathecal substance p-saporin attenuates operant escape from nociceptive thermal stimuli

Destruction of neurons in the superficial dorsal horn that express substance P receptor (NK-1R) has been reported to block development of behavioral hypersensitivity following peripheral sensitization of nociceptors. Baseline sensitivity was not altered in these rat models that assessed innate reflex responses (i.e. hind-paw withdrawal to thermal or mechanical stimulation). In the present study, we evaluated effects of intrathecal substance P-saporin (SP-sap), a toxin selective for cells expressing NK-1R, on operant escape responses of rats to thermal stimulation. For comparison, lick/guard reflex testing was performed. Injection of a modest dose (175 ng) of SP-sap into the lumbar subarachnoid space produced a partial loss of lamina I/II NK-1R-expressing dorsal horn neurons but did not affect NK-1R-expressing neurons in deeper laminae. Lick/guard responses to 0.3, 44 or 47 degrees C were not affected after SP-sap treatment, but escape responses to these temperatures were significantly attenuated. Three hours after application of mustard oil to the dorsal surface of both hind paws, escape from 44 degrees C was enhanced for controls but not SP-sap-treated rats. Lick/guard responses were enhanced by mustard oil for both SP-sap and control animals. Administration of morphine (1.0 mg/kg, s.c.) before testing decreased escape responding at 47 degrees C for both controls and SP-sap rats. Thus, partial loss of NK-1R-expressing neurons in the superficial dorsal horn attenuated thermal nociceptive sensitivity and prevented secondary hyperalgesia when studied with an operant algesia assay, in contrast to innate reflexes which were less sensitive to modification by intrathecal SP-sap.

Changes in sensory processing in the spinal dorsal horn accompany vincristine-induced hyperalgesia and allodynia

Abnormal sensation and pain are major dose-limiting factors in cancer chemotherapy with vincristine. In this study, we have adapted a model of this condition by using repeated daily intraperitoneal injections of vincristine in rats. Mechanical allodynia and hyperalgesia without change in responses to thermal stimuli were first observed following 5-8 days of vincristine treatment (0.1mg/kg/day) and then persisted throughout the remainder of the treatment interval (2-3 weeks). Electrophysiological recording from wide dynamic range (WDR) neurons in the lumbar (L4-L5) spinal dorsal horn in hyperalgesic rats demonstrated significantly increased spontaneous activity and after-discharges to noxious mechanical stimuli (von Frey filaments with a bending force greater than 58.02mN, skin compression 1.3 and 3N, 1mm(2)), increased acute A- and C-fiber responses, after-discharges and abnormal 'wind-up' to electrical stimuli (5mA, 2ms) at 0.1Hz applied across the receptive field. These results suggest a state of central sensitization develops in spinal WDR neurons with repeated vincristine treatment that contributes to the spontaneous pain and hyperalgesia seen in patients and the hyperresponsiveness to sensory stimuli seen in animals treated with vincristine.

Simultaneous recordings of wind-up of paired spinal dorsal horn nociceptive neuron and nociceptive flexion reflex in rats

To study the sensory-motor interaction of spinal processing underlying the neuronal mechanisms of the nociceptive flexion reflex (NFR) and its temporal facilitation, 16 spinal dorsal horn (DH) wide-dynamic-range (WDR) neurons and paired 16 single motor units (SMU) from the gastrocnemius soleus muscle (GS) were simultaneously recorded using extracellular single unit and electromyographic techniques in spinal, halothane-anesthetized rats. The paired DH WDR neuron and GS SMU showed a parallel increase in the firing rate and duration of spike responses to noxious pinch stimuli applied to their common cutaneous receptive field (cRF) on the ipsilateral hind paw skin. Innocuous brush or pressure evoked no, or less, firing in the SMU but evoked a graded increase in spike responses in the simultaneously-recorded WDR neuron. Moreover, both pressure and noxious pinch stimuli evoked a short-lasting after-discharge (for several min) in the WDR neuron but without any after-discharge in the simultaneously-recorded SMU. The paired WDR neuron and SMU also showed a parallel basal response (termed as early and late components according to latency), after-discharge and wind-up of the late response to repetitively applied supra-threshold electrical stimulation (intensity: >1.5 T, duration: 1 ms and frequency: 1 Hz for 15 s). Linear regression and cross-correlation histogram analyses showed that the DH WDR neuron had a significant correlation with the simultaneously-recorded SMU and they were functionally located in the spinally-organized NFR circuitry via polysynaptic connections. Systemic administration of fentanyl, an opioid receptor agonist, resulted in a parallel, naloxone-reversible suppression of both basal late response component and wind-up response in both WDR neuron and SMU paired; however, fentanyl suppressed only the early response of the SMU without any effect on that of the DH WDR neuron. The present results provide new direct evidence showing an essential role of spinal DH WDR neurons in the mediation of spinally-organized NFR as well as its temporal facilitation (wind-up). Based on these data, the spinal DH WDR neuron seems to function as a signal discriminator or frequency encoder of multireceptive primary afferent impulses that may determine excitable level of motor output and the occurrence of a behavioral NFR via polysynaptic connections. Consequently, the spinal WDR neuron-mediated NFR and its temporal facilitation are likely to be modulated by spinal endogenous opioid peptides via opioid receptors on the nociceptive sensory components of the spinally-organized NFR circuitry.

Spinal sensorimotor transformation: relation between cutaneous somatotopy and a reflex network

The projection of primary afferents onto spinal interneurons constitutes the first step in sensorimotor transformations performed by spinal reflex systems. Despite extensive studies on spinal somatotopy, uncertainties remain concerning the extent and significance of representational overlap and relation to spinal reflex circuits. To address these issues, the cutaneous projection from the hindpaw and its relation to the topography of lamina V neurons encoding withdrawal reflex strength ("reflex encoders") was studied in rats. Thin and coarse primary afferent terminations in laminas II and III-IV, respectively, were mapped by wheat germ agglutinin-horseradish peroxidase and choleragenoid tracing. The functional weights of these projections were characterized by mapping nociceptive and tactile field potentials and compared with the topography of reflex encoders. Both anatomical and physiological data indicate that thin and coarse skin afferent input is spatially congruent in the horizontal plane. The representation of the hindpaw in the spinal cord was found to be intricate, with a high degree of convergence between the projections from different skin sites. "Somatotopic disruptions" such as the representation of central pads medial to that of the digits were common. The weight distribution of the cutaneous convergence patterns in laminas III-IV was similar to that of lamina V reflex encoders. This suggests that the cutaneous convergence and features such as somatotopic disruptions have specific relations to the sensorimotor transformations performed by reflex interneurons in the deep dorsal horn. Hence, the spinal somatotopic map may be better understood in light of the topography of such reflex systems.

The whole body receptive field of dorsal horn multireceptive neurones

Multireceptive neurones are found in the spinal dorsal horn and may be projection neurones and/or interneurones for polysynaptic reflexes. The cutaneous receptive field of a multireceptive neurone exhibits a gradient of sensitivity with the centre responding to any mechanical stimulus, including hair movements and light touch, while the periphery responds only to noxious stimuli. These neurones also receive signals from viscera, muscles and joints. This convergence of inputs means that multireceptive neurones are continuously capturing all the information from both the interface with the external environment (the skin) and the internal milieu (the viscera, muscles, etc.). This information constitutes a 'basic somaesthetic activity' that could help the somatosensory system build a 'global representation of the body'. In addition to be seen as a global entity, the output of multireceptive neurones should be understood in dynamic terms since the size of the peripheral fields of the individual neurones may change, as a result of the plasticity of both excitatory and inhibitory segmental processes. Furthermore, the activity of these neurones can be inhibited from most of the remaining parts of the body via supraspinal mechanisms. These diffuse noxious inhibitory controls (DNIC) are triggered by peripheral A delta- and C-fibres, involve brain structures confined to the caudal-most part of the medulla including the subnucleus reticularis dorsalis (SRD) and are mediated by descending pathways in the dorsolateral funiculi. A painful focus that both activates a segmental subset of neurones and inhibits the remaining population can seriously disrupt this basic activity, resulting in the distortion of the body representation in favour of the painful focus, which becomes pre-eminent and (relatively) oversized.

Modular organisation and spinal somatosensory imprinting

The withdrawal reflex system has been extensively used as a model system for studies of pain related mechanisms, sensorimotor integration, learning and memory. For a long time, this system was assumed to be organised as a flexion reflex system. However, recent studies indicate that this system has a modular organisation, each module performing a detailed and functionally adapted sensorimotor transformation related to the withdrawal efficacy of its output muscle(s). Each module appears to be a self-organising circuitry that uses sensory feedback on single muscle contractions to adjust its synaptic organisation during development. These findings and their implications for the understanding of higher motor functions as well as clinical aspects will be discussed.

Reflex receptive fields for human withdrawal reflexes elicited by non-painful and painful electrical stimulation of the foot sole

OBJECTIVES

Human withdrawal reflex receptive fields (RRFs) were assessed for 4 different electrical stimulus intensities, ranging from below the pain threshold (PTh) to up to two times the PTh intensity (0.8x, 1.2x, 1.6x, and 2.0xPTh).

METHODS

Thirteen subjects participated, and the reflexes were recorded in a sitting position. The stimuli were delivered in random order to 12 positions distributed over the foot sole. Tibialis anterior (TA), gastrocnemius medialis (GM), vastus lateralis (VL), and biceps femoris (BF) reflexes were recorded. Further, knee and ankle joint angle changes were recorded.

RESULTS

The strongest reflexes were seen in the TA compared with the other 3 muscles. Dorsi-flexion dominated distal to the talocrural joint corresponding to the TA receptive field area. An expansion of the RRF for the TA and GM was seen when increasing the stimulus intensity from 0.8xPTh to 1.2xPTh and from 1.2xPTh to 1.6xPTh, indicating a gradually increasing reflex threshold towards the border, where TA contraction is inappropriate in a withdrawal reaction. For the BF and VL, the borders of the RRF areas were not detected. By integrating the reflex size within the RRF (i.e. the reflex volume), gradually increasing reflexes for increasing stimulus intensity were seen in all 4 muscles tested, most clearly in the TA and GM. The subjective pain intensity correlated to the reflex volume for the TA, GM, and BF.

CONCLUSIONS

In conclusion, the highest reflex sensitivity was seen in the centre of the RRF, while the stimulus intensity needed for eliciting a reflex increased towards the receptive field border. Within the RRF, stronger reflexes were evoked for increasing stimulus intensity. The limit in the size of the receptive field size for the TA and GM supports a modular withdrawal reflex organisation.

Encoding of noxious stimulus intensity by putative pain modulating neurons in the rostral ventromedial medulla and by simultaneously recorded nociceptive neurons in the spinal dorsal horn of rats

Neurons in the nucleus raphe magnus and adjacent structures of the rostral ventromedial medulla (RVM) are involved in the control of nociceptive transmission. In the RVM the so-called on-cells are excited, and the so-called off-cells are inhibited, by noxious stimuli applied almost anywhere on the body surface, thus showing that they receive information from spinal and trigeminal nociceptive neurons. In deeply anesthetized rats, recordings were made from RVM neurons that resembled on- and off-cells (herein called putative on- and off-cells) in order to investigate (1) how they encode the intensity of thermal noxious stimuli (46--56 degrees C) applied to a hindpaw, and (2) how their encoding properties relate to those of simultaneously recorded spinal neurons. In 49 of 98 cases, a graded increase in the stimulus temperature caused a monotonic decrease in the response latency of putative on-cells, putative off-cells and spinal neurons, while the response discharge rate monotonically increased for putative on-cells and spinal neurons and decreased for putative off-cells. In the majority of simultaneous recordings of RVM and spinal neurons, the latency and discharge rate of the putative on- or off-cell were highly correlated with the latency and discharge rate of the spinal neuron, and the stimulus/response slopes were similar. These results show that putative on- and off-cells can encode the stimulus intensity in terms of response latency and discharge rate, and suggest that such encoding closely reflects spinal neuronal encoding. This may be relevant for the transmission and modulation of pain information by RVM neurons.

Effects of intravenous and intrathecal sufentanil on a C-fibre reflex elicited by a wide range of stimulus intensities in the rat

A C-fibre reflex elicited by electrical stimulation within the territory of the sural nerve was recorded from the ipsilateral biceps femoris muscle in anaesthetised, intact rats, and in anaesthetised rats whose brains had been transected at the level of the obex. The temporal evolution of the response was studied by recording recruitment curves built with stimulus intensities from 0 to 10 times threshold. Both i.v. and i.t. sufentanil resulted in dose-dependent depressions of the reflex. Increasing the stimulus intensity from 1.5 to 10 times threshold resulted in an increase in the ED(50) from 0.58 (0.40-0.86) to 2.40 (1.87-3.31) microgram/kg for i.v. sufentanil and from 0.64 (0.46-0.79) to 1.63 (1.29-3.31) microgram/kg for i.t. sufentanil. With increasing stimulus intensity, the dose-response curves showed a progressive shift to the right, but this shift was only slight with the highest intensity stimuli. The ratios for the ED(50)s for i.v. to i.t. sufentanil were near 1. Following i.v. administration, sufentanil also facilitated the C-fibre reflex and produced tonic inter-stimulus discharges. They disappeared after the i.v. injection of naloxone. In the obex-transected rats, the depressive effect of sufentanil increased, while the facilitations and tonic inter-stimulus discharges disappeared. These findings suggest that the analgesic effects of i.v. ant i.t. sufentanil are similar, probably because sufentanil is highly soluble in lipids. Sufentanil-induced facilitations relate to supraspinal actions on motor controls and/or on the descending control of nociceptive transmission.

Interaction between cutaneous and muscle afferent activity in polysynaptic reflex pathways: a human experimental study

Interactions between the input from cutaneous and nociceptive muscle afferents in polysynaptic reflex pathways were investigated in man. Interaction was tested by evoking reflexes before, during, and after a period of muscle pain induced by intramuscular injection of hypertonic saline. Muscle pain was induced either in the ankle flexor (tibialis anterior, TA) or in the extensor (soleus, SOL) muscles by injection of 1 ml hypertonic saline. Electrical skin stimulation (1.1 x initial reflex threshold) at the dorsum of the foot over the tarsal joint was used to elicit cutaneo-muscular polysynaptic reflexes in the knee flexor semitendinosus (ST). The injected hypertonic saline evoked a robust muscle pain (the subjects made a continuous score of the muscle pain on a 0-10 cm VAS scale, and the mean VAS area was 1229+/-251 cm x s and lasting 390+/-30 s). In five of 12 subjects, the infusion of hypertonic saline into TA evoked referred pain to the dorsal aspect of the ankle. A significant inhibition (17+/-8.2%, P<0.05) of the ST-reflex by pain in SOL was observed. Pain in TA facilitated (92+/-36%, P<0.05) the short-latency part (50-70 ms post stimulation) of the reflex. The muscle pain did not modulate the perceived sensory intensity of the electrical stimuli. The findings indicate an interaction of input from thin muscle afferents and cutaneous group A-fibre afferents in polysynaptic segmental reflex pathways, which seems to depend on the location of the muscle pain.

Developmental tuning in a spinal nociceptive system: effects of neonatal spinalization

Recent studies indicate a modular organization of the nociceptive withdrawal reflex system. Each module has a characteristic receptive field, closely matching the withdrawal movement caused by its effector muscle. In the rat, the strength of the sensory input to each module is tuned during the first postnatal weeks, i.e., erroneous spinal connections are depressed, and adequate connections are strengthened. To clarify if this tuning is dependent on supraspinal structures, the effect of a complete neonatal spinal cord transection on the postnatal tuning of withdrawal reflexes was studied. The nociceptive receptive fields of single hindlimb muscles and compound withdrawal reflexes were examined in decerebrate unanesthetized and awake rats, respectively. Noxious thermal CO(2) laser stimulation was used to evoke reflex responses. Neonatal spinal cord transection resulted in a disrupted reflex organization in the adult rat, resembling that previously found in neonatal rats. The receptive fields of single hindlimb muscles exhibited abnormal distribution of sensitivity not matching the withdrawal action of the effector muscles. Likewise, the composite nocifensive movements, as documented in the awake rat, often resulted in erroneous movements toward the stimulus. It is concluded that withdrawal reflexes do not become functionally adapted in rats spinalized at birth. These findings suggest a critical role for supraspinal systems in the postnatal tuning of spinal nociceptive systems.

Modular organization of human leg withdrawal reflexes elicited by electrical stimulation of the foot sole

Human withdrawal reflex receptive fields were determined for leg muscles by randomized, electrical stimulation at 16 different positions on the foot sole. Tibialis anterior, gastrocnemius medialis, peroneus longus, soleus, rectus femoris, and biceps femoris reflexes, and ankle joint angle changes were recorded from 14 subjects in sitting position. Tibialis anterior reflexes were evoked at the medial, distal foot and correlated well with ankle dorsal flexion. Gastrocnemius medialis reflexes were evoked on the heel and correlated with plantar flexion. Stimulation on the distal, medial sole resulted in inversion (correlated best with tibialis anterior activity), whereas stimulation of the distal, lateral sole evoked eversion. Biceps femoris reflexes were evoked on the entire sole followed by a small reflex in rectus femoris. A detailed withdrawal reflex organization, in which each lower leg muscle has its own receptive field, may explain the ankle joint responses. The thigh activity consisted primarily of flexor activation.

Paradoxical inhibition of nociceptive neurons in the dorsal horn of the rat spinal cord during a nociceptive hindlimb reflex

Nociceptive-specific and multireceptive neurons in the lumbar dorsal horn are excited by noxious stimuli applied to the hindpaw and inhibited by noxious stimuli applied to distant body regions. Given that at least a subset of these neurons are part of the circuit for nociceptive reflexes, inhibition of nociceptive-specific and multireceptive neurons should inhibit nociceptive reflexes. Unfortunately, previous attempts to test this hypothesis have been inconclusive because of methodological differences between electrophysiological and behavioral experiments. The present study overcame this problem by recording neural and reflex activity simultaneously. Rats were anesthetized with halothane and surgically prepared for single-unit recording from the lumbar dorsal horn. Hindpaw heat caused a burst of activity that reliably preceded hindpaw withdrawal in 10 nociceptive-specific and 17 multireceptive neurons. A distant noxious stimulus (tail in 50 degrees C water or ear pinch) inhibited the evoked activity of both nociceptive-specific and multireceptive neurons and simultaneously changed the topography of the hindpaw reflex from flexion to extension without altering reflex latency. The present data are consistent with previous reports of inhibition of nociceptive-specific and multireceptive neurons during application of a distant noxious stimulus. However, inhibition of nociceptive-specific and multireceptive neurons concomitant with a shift in the hindlimb reflex from flexion to extension suggests that these neurons are part of the circuit for flexor reflexes specifically. Presumably, lateral inhibition from the flexor to extensor circuit allows for the release of hindlimb extension when neurons in the flexion circuit are inhibited by a distant noxious stimulus. Such a system reduces the chance of injury by allowing for withdrawal reflexes to a single noxious stimulus and escape reactions, such as running and jumping, to multiple noxious stimuli.

Asymmetries in sensory pathways from skin to motoneurons on each side of the body determine the direction of an avoidance response in hatchling Xenopus tadpoles

1. When swimming is initiated by tail stimulation in hatchling Xenopus tadpoles, the first trunk contraction is usually on the opposite side and directs the animal away from the stimulus. We have investigated how asymmetries in the skin sensory pathways mediate this response. 2. In alpha-bungarotoxin-immobilized tadpoles, intracellular recordings were made of responses to ipsilateral (ISS) and contralateral skin stimulation (CSS) in thirty-two presumed motoneurons. ISS evokes an inhibitory postsynaptic potential (IPSP) followed by an excitatory postsynaptic potential (EPSP) whereas CSS only evokes an EPSP. Blocking the short latency IPSP evoked by ISS with strychnine reduced the difference in spike latency on the two sides but spikes still occurred first to CSS. 3. Motoneuron EPSPs evoked by ISS and CSS were therefore recorded during microperfusion of strychnine to block the short latency IPSP. We found: (a) the CSS-EPSPs have lower threshold, larger amplitude at a given intensity of stimulus, faster rising phase, and shorter latencies than those of ISS-EPSPs; (b) the ISS-EPSP onset latencies were longer than CSS-EPSPs and became shorter as the stimulus intensity increased while those of CSS-EPSPs remained little changed. At high stimulus intensities, EPSPs caused by CSS and ISS became similar; and (c) onset latencies of ISS-EPSPs had higher variance than those of CSS-EPSPs. However, this difference was reduced as the stimulus intensity was increased. 4. Since motoneuron EPSP onset latencies varied with stimulus intensity, we proposed that the pathway from the opposite side had stronger synapses from afferents to sensory interneurons. To test this proposal we built a neuronal population model of the spinal pathway from skin afferents, via sensory interneurons to ipsilateral and contralateral motoneurons incorporating this asymmetry. Inhibition was omitted from the model. 5. Simulated motoneuron EPSPs in response to skin stimulation on each side of the body showed the major asymmetries found experimentally. If the distribution and axonal projections of the interneurons in the two sensory pathways were made the same these differences remained. However, if the synaptic strength from sensory afferents onto interneurons projecting to the two sides were made equal, the difference between the two sides were lost. 6. We propose that the sensory pathway to contralateral motoneurons has more effective excitation from afferents to sensory interneurons which leads to these motoneurons firing first. At higher stimulus strengths, when population recruitment can blur these subtle differences in excitation between the two sides, inhibition normally plays a significant role to ensure that most first responses are still contralateral.

Direct comparison of heat-evoked activity of nociceptive neurons in the dorsal horn with the hindpaw withdrawal reflex in the rat

Although the sensory coding of nociceptive neurons in the dorsal horn has been studied extensively, surprisingly little is known about how these neurons contribute to nociceptive reflexes. The objective of the present study was to examine the characteristics of dorsal horn neurons capable of initiating hindpaw withdrawal. To this end, neural and reflex activity were measured simultaneously in response to noxious radiant heat applied to the hindpaw in lightly anesthetized rats. Subsets of both multireceptive (MR; 52/95) and nociceptive-specific (NS; 19/46) neurons showed a consistent burst of activity that preceded the reflex. However, when compared with NS neurons, MR neurons as a group were: more likely to be active before the reflex (55 vs. 41%); more active before the reflex (31 vs. 23 Hz); and active earlier (2.8 vs. 2.3 s before the reflex). Subsets of MR neurons were active before the reflex regardless of receptive field size or location in the dorsal horn. In contrast, NS neurons with small receptive fields or those located outside of superficial laminae were rarely active before the reflex and thus unlikely to be part of the reflex circuit. These results suggest that current classification schemes, in particular MR and NS categories, cannot be used as the sole criterion to predict involvement in nociceptive reflexes. However, simultaneous measurement of neural and reflex activity provides an opportunity to determine the characteristics of nociceptive neurons involved in withdrawal reflexes.

Nociceptive masseter inhibitory reflexes evoked by laser radiant heat and electrical stimuli

Electrical stimulation of the mental nerve evokes two suppression periods SP1 and SP2 in masseter muscle activity bilaterally. In order to investigate a possible nociceptive origin of the suppression periods, we compared the reflex responses evoked by electrical stimulation and by selective activation of nociceptors in hairy skin using painful infrared laser stimuli. The SP was elicited during more than 90% maximal voluntary contraction. Thresholds for detection, pain, and SP in the mental nerve area were determined by the method of limits. A suppression period was evoked by laser stimuli in nine of ten subjects bilaterally. The mean onset latency was 46.9 ms, the mean duration 58.9 ms. The electrical threshold of SP1 (9 mA) was 7.7 x I(0), about 20% smaller than I(P), and significantly higher than I(SP2) (4.7 mA). The onset latencies and durations were 11.7 ms and 21 ms for SP1, and 45 ms and 42.7 ms for SP2 (stimulus intensity 2 x I(P)). The mean difference in onset latencies between laser SP and electrically evoked SP1 was 35.1 +/- 6.2 ms, which closely matches the nociceptor response latency to a laser heat pulse. Based on the threshold and the onset latency we conclude that at least SP1 and laser SP are nociceptive in origin and mediated by group III fibers.

Nocifensive reflex thresholds in rats: measures of central nervous system effects of barbiturates

PURPOSE

To characterize the pharmacodynamic relationships between plasma pentobarbitone and thiopentone concentrations and nocifensive reflexes during emergence from anaesthesia.

METHODS

Forty-nine rats were studied. Plasma barbiturate concentrations were measured with high performance liquid chromatography. Nocifensive reflexes were assessed with the hindlimb withdrawal latency (WL) to heat and the somatic motor response threshold (SMRT) to tail pressure. In Protocol I, SMRT, WL, sedation, and the presence of paw-licking and the righting reflex were assessed in unrestrained rats before and every 10 min for two hours after an intraperitoneal injection of pentobarbitone (30 mg.kg-1). Plasma pentobarbitone kinetics were determined in a separate group of rats. In Protocol II, SMRT and drug concentrations were measured concurrently in partially restrained animals before and for 35 min after a computer-controlled i.v. bolus of thiopentone. In Protocol III the SMRT-plasma thiopentone relationship was determined during increasing and decreasing plasma thiopentone concentrations.

RESULTS

Enhancement of both nocifensive reflexes was observed in the unrestrained animals. Enhancement of SMRT was maximal [175% (153-197) of control values] at a mean plasma thiopentone concentration of 11 (9-13) micrograms.ml-1. The SMRT-plasma thiopentone curve showed a mean efflux-influx difference in plasma thiopentone concentration of 4(2.3-5.7) micrograms.ml-1.

CONCLUSIONS

Barbiturate-associated nocifensive reflex enhancement occurs in unrestrained animals with both thermal and pressure stimuli. The SMRT-plasma thiopentone concentration relationship during emergence from anaesthesia was similar to that observed previously during induction. The thiopentone plasma concentration-SMRT plot showed an equilibrium delay similar to that previously described by others for thiopentone at an electroencephalographic effect site.

Developmental adaptation of rat nociceptive withdrawal reflexes after neonatal tendon transfer

Nociceptive withdrawal reflexes (NWRs) were studied in adult rats in which the movement patterns produced by single muscles had been altered by neonatal tendon transfer. NWRs evoked by cutaneous noxious mechanical and thermal (CO2-laser) stimulation were recorded using electromyography in a decerebrate spinal preparation. The sensitivity distribution within the receptive fields of the NWRs of the extensor digitorum longus and the peronei muscles exhibited changes corresponding to the altered movement patterns. No detectable change of NWRs was found in normal muscles whose receptive fields overlapped that of the modified muscle. Furthermore, NWRs of muscles that regained an essentially normal function after neonatal tendon transfer did not differ from normal. It is proposed that a developmental experience-dependent mechanism, which takes into account the hindlimb movement pattern caused by contraction of single muscles, underlies the functionally adapted organization of adult NWRs.

Developmental adaptation of withdrawal reflexes to early alteration of peripheral innervation in the rat

1. In adult decerebrate spinal rats whose plantar nerves (PLN) had been transected at either postnatal day 1 (P1) or P21 the nociceptive withdrawal reflexes (NWR) of musculi extensor digitorum longus (EDL), peroneus longus (PER) and semitendinosus (ST) were characterized with respect to receptive field (RF) organization, magnitude and time course, using electromyography. Thermal (short CO2 laser pulses) and mechanical (calibrated pinch) stimulation were used. The innervation patterns in normal and lesioned adult rats were assessed by acute nerve lesions. 2. The spatial organization of the mean mechano- and thermonociceptive RFs of all the muscles studied was similar to normal in both P1- and P21-lesioned rats, although in some P21-lesioned rats atypical EDL RFs were encountered. 3. In P1-lesioned rats thermo-NWR of PER and EDL had normal magnitudes, while mechano-NWR were reduced. In P21-lesioned rats both thermo- and mechano-NWR of these muscles had reduced magnitudes. Except for thermo-NWR of ST in P1-lesioned rats, which were increased, NWR of ST had normal magnitudes in both P1- and P21-lesioned rats. The time course of thermonociceptive NWR of the muscles studied were near normal in both P1- and P21-lesioned rats. 4. Acute nerve lesions in adult P1-lesioned rats revealed an essentially abolished contribution to NWR from the PLN. Instead, the contribution to NWR from other hindpaw nerves, such as the superficial and deep peroneal nerves, was dramatically increased. By contrast, in P21-lesioned rats, the regenerated PLN contributed significantly to the NWR. 5. It is concluded that despite profound alterations of plantar hindpaw innervation induced by early PLN transection the cutaneous nociceptive input to NWR attained an essentially normal spatial organization. An experience-dependent mechanism is suggested to be instrumental in adapting the reflex connectivity to the peripheral innervation.

Postnatal development of the nociceptive withdrawal reflexes in the rat: a behavioural and electromyographic study

1. The postnatal development of nociceptive withdrawal reflexes was studied. In awake intact rats, forelimb, hindlimb and tail reflexes were recorded on videotape. In decerebrate spinal rats, electromyography (EMG) was used to record nociceptive withdrawal reflexes in musculi extensor digitorum longus (EDL), peronei, gastrocnemius-soleus (G-S) and biceps posterior-semitendinosus (BP-ST). Thermal (short-lasting CO2 laser pulses) and mechanical stimulation were used. 2. In adults, nociceptive withdrawal reflexes were typically well directed and reflex pathways to single hindlimb muscles had functionally adapted receptive fields. By contrast, at postnatal day (P) 1-7, the nociceptive withdrawal reflexes were often inappropriate, sometimes producing movements towards the stimulation, and EMG recordings revealed unadapted variable receptive fields. With increasing age, the nociceptive withdrawal reflexes progressively became well directed, thus producing localized withdrawal. Both withdrawal movements and spatial organization of the receptive fields were adult-like at P20-25. 3. Up to P25, reflex thresholds were more or less constant in both intact awake rats and spinal decerebrate rats, except in G-S in which no nociceptive withdrawal reflexes were evoked from P20 on. After P25, mechanical, but not thermal, thresholds increased dramatically. 4. EMG recordings revealed that during the first three postnatal weeks, the latency of the CO2 laser-evoked nociceptive withdrawal reflexes decreased significantly in peronei and BP-ST, but not in EDL, and thereafter increased significantly in peronei, BP-ST and EDL. The magnitude of the nociceptive withdrawal reflexes in these muscles increased markedly between P7 and P20 and showed little change thereafter. 5. Possible mechanisms underlying the postnatal tuning of the nociceptive withdrawal reflexes are discussed.

Cutaneous inhibitory receptive fields of withdrawal reflexes in the decerebrate spinal rat

1. The inhibitory cutaneous input to the withdrawal reflex pathways to single hindlimb muscles was investigated in decerebrate spinal rats (n = 53) using electromyography. 2. Withdrawal reflexes in the peronei, extensor digitorum longus and tibialis anterior muscles of the leg were strongly inhibited by conditioning mechanical, thermal (CO2 laser) and intracutaneous electrical stimulation of specific skin areas. By contrast, withdrawal reflexes in the biceps posterior-semitendinosus muscles of the thigh could only be weakly inhibited by conditioning skin stimulation. 3. Powerful inhibition of withdrawal reflexes in single lower leg muscles was elicited from the ipsilateral hindpaw plantar area, which would move towards the stimulation on contraction in the respective muscle. In addition, weak nociceptive inhibition was evoked from the corresponding skin areas on the contralateral hindlimb and, in some muscles, the tail. 4. The ipsilateral inhibitory and excitatory receptive fields of the withdrawal reflexes in single muscles overlapped somewhat. On stimulation of these transitional areas the reflex responses were preceded by a short-lasting inhibition. 5. Graded mechanical and thermal stimulation demonstrated prominent inhibitors effects from nociceptive receptors. Weak inhibitory effects were elicited by innocuous mechanical stimulation, suggesting a weak contribution from low threshold mechanoreceptors. Latency measurements indicated an inhibitory input from both myelinated and unmyelinated fibres. 6. In conclusion, the withdrawal reflex pathways receive a powerful nociceptive inhibitory input through spinal pathways. The movement-related organization of this input suggests that it serves to prevent inappropriate withdrawal reflexes.