LOCAL, SEGMENTAL AND SUPRASPINAL INTERACTION WITH A DORSOLATERAL SPINAL CUTANEOUS AFFERENT SYSTEM

Distinct brain mechanisms support spatial vs temporal filtering of nociceptive information

The role of endogenous analgesic mechanisms has largely been viewed in the context of gain modulation during nociceptive processing. However, these analgesic mechanisms may play critical roles in the extraction and subsequent utilization of information related to spatial and temporal features of nociceptive input. To date, it remains unknown if spatial and temporal filtering of nociceptive information is supported by similar analgesic mechanisms. To address this question, human volunteers were recruited to assess brain activation with functional magnetic resonance imaging during conditioned pain modulation (CPM) and offset analgesia (OA). CPM provides one paradigm for assessing spatial filtering of nociceptive information while OA provides a paradigm for assessing temporal filtering of nociceptive information. CPM and OA both produced statistically significant reductions in pain intensity. However, the magnitude of pain reduction elicited by CPM was not correlated with that elicited by OA across different individuals. Different patterns of brain activation were consistent with the psychophysical findings. CPM elicited widespread reductions in regions engaged in nociceptive processing such as the thalamus, insula, and secondary somatosensory cortex. OA produced reduced activity in the primary somatosensory cortex but was associated with greater activation in the anterior insula, dorsolateral prefrontal cortex, intraparietal sulcus, and inferior parietal lobule relative to CPM. In the brain stem, CPM consistently produced reductions in activity, while OA produced increases in activity. Conjunction analysis confirmed that CPM-related activity did not overlap with that of OA. Thus, dissociable mechanisms support inhibitory processes engaged during spatial vs temporal filtering of nociceptive information.

The effect of lowering the 5-hydroxytryptamine content of the rat spinal cord on analgesia produced by morphine

1. Injection of 5,6-dihydroxytryptamine (5,6-DHT, 50 mug) into a lateral cerebral ventricle of male rats lowered the 5-hydroxytryptamine (5-HT) content of the lumbar cord to 12.8% and reduced the analgesic effect of low doses of morphine (0.64-1.63 mg/kg), tested by exerting pressure on the foot; after doses of morphine of 1.33-1.63 mg/kg, the analgesic response was reduced or abolished in 33% of the rats, and after 0.64 mg/kg, 58% of the animals failed to respond normally.2. Two days after an I.P. injection of p-chlorophenylalanine (pCPA, 320 mg/kg), the loss of analgesic potency of morphine was more pronounced than after intraventricular 5,6-HDT. The 5-HT content was lowered to about 8% in the lumbar cord, and to 20% or less in pons and medulla.3. The experiments show that interference with the descending tryptaminergic axons innervating the cord is by itself sufficient to reduce analgesia due to morphine, but they do not exclude the possibility that other tryptaminergic neurones take part in the effect of pCPA. The contribution to the analgesic effect of morphine made by the interaction of tryptaminergic axons with the interneurones ;gating' the afferent impulses in the posterior columns is discussed.

Exposure to intermittent nociceptive stimulation under pentobarbital anesthesia disrupts spinal cord function in rats

RATIONALE

Spinal cord plasticity can be assessed in spinal rats using an instrumental learning paradigm in which subjects learn an instrumental response, hindlimb flexion, to minimize shock exposure. Prior exposure to uncontrollable intermittent stimulation blocks learning in spinal rats but has no effect if given before spinal transection, suggesting that supraspinal systems modulate nociceptive input to the spinal cord, rendering it less susceptible to the detrimental consequences of uncontrollable stimulation.

OBJECTIVE

The present study examines whether disrupting brain function with pentobarbital blocks descending inhibitory systems that normally modulate nociceptive input, making the spinal cord more sensitive to the adverse effect of uncontrollable intermittent stimulation.

MATERIALS AND METHODS

Male Sprague-Dawley rats received uncontrollable intermittent stimulation during pentobarbital anesthesia after (experiment 1) or before (experiment 2) spinal cord transection. They were then tested for instrumental learning at a later time point. Experiment 3 examined whether these manipulations affected nociceptive (thermal) thresholds.

RESULTS

Experiment 1 showed that pentobarbital had no effect on the induction of the learning deficit after spinal cord transection. Experiment 2 showed that intact rats anesthetized during uncontrollable intermittent stimulation failed to learn when later transected and tested for instrumental learning. Experiment 3 found that uncontrollable intermittent stimulation induced an antinociception in intact subjects that was blocked by pentobarbital.

CONCLUSIONS

The results suggest a surgical dose of pentobarbital (50 mg/kg) suppresses supraspinal (experiment 2) but not spinal (experiment 1) systems that modulate nociceptive input to the spinal cord by blocking the antinociception that is induced by this input (experiment 3).

Vibrotactile coding capacities of spinocervical tract neurons in the cat

The response characteristics and tactile coding capacities of individual dorsal horn neurons, in particular, those of the spinocervical tract (SCT), have been examined in the anesthetized cat. Twenty one of 38 neurons studied were confirmed SCT neurons based on antidromic activation procedures. All had tactile receptive fields on the hairy skin of the hindlimb. Most (29/38) could also be activated transynaptically by electrical stimulation of the cervical dorsal columns, suggesting that a common set of tactile primary afferent fibers may provide the input for both the dorsal column-lemniscal pathway and for parallel ascending pathways, such as the SCT. All but 3 of the 38 neurons studied displayed a pure dynamic sensitivity to controlled tactile stimuli but were unable to sustain their responsiveness throughout 1s trains of vibration at vibration frequencies exceeding 5-10 Hz. Stimulus-response relations revealed a very limited capacity of individual SCT neurons to signal, in a graded way, the intensity parameter of the vibrotactile stimulus. Furthermore, because of their inability to respond on a cycle-by-cycle pattern at vibration frequencies >5-10 Hz, these neurons were unable to provide any useful signal of vibration frequency beyond the very narrow bandwidth of approximately 5-10 Hz. Similar limitations were observed in the responsiveness of these neurons to repetitive forms of antidromic and transynaptic inputs generated by electrical stimulation of the spinal cord. In summary, the observed limitations on the vibrotactile bandwidth of SCT neurons and on the precision and fidelity of their temporal signaling, suggest that SCT neurons could serve as little more than coarse event detectors in tactile sensibility, in contrast to DCN neurons the bandwidth of vibrotactile responsiveness of which may extend beyond 400 Hz and is therefore broader by approximately 40-50 times than that of SCT neurons.

Interlimb reflex activity after spinal cord injury in man: strengthening response patterns are consistent with ongoing synaptic plasticity

OBJECTIVE

Previous reports from our laboratory have described short-latency contractions in muscles of the distal upper limb following stimulation of lower limb nerves or skin in persons with injury to the cervical spinal cord. It takes 6 or more months for interlimb reflexes (ILR) to appear following acute spinal cord injury (SCI), suggesting they might be due to new synaptic interconnections between lower limb sensory afferents and motoneurons in the cervical enlargement. In this study, we asked if once formed, the strength of these synaptic connections increased over time, a finding that would be consistent with the above hypothesis.

METHODS

We studied persons with sub-acute and/or chronic cervical SCI. ILR were elicited by brief trains of electrical pulses applied to the skin overlying the tibial nerve at the back of the knee. Responses were quantified based on their presence or absence in different upper limb muscles. We also generated peri-stimulus time histograms for single motor unit response latency, probability, and peak duration. Comparisons of these parameters were made in subjects at sub-acute versus chronic stages post-injury.

RESULTS

In persons with sub-acute SCI, the probability of seeing ILR in a given muscle of the forearm or hand was low at first, but increased substantially over the next 1-2 years. Motor unit responses at this sub-acute stage had a prolonged and variable latency, with a lower absolute response probability, compared to findings from subjects with chronic (i.e. stable) SCI.

CONCLUSIONS

Our findings demonstrate that interlimb reflex activity, once established after SCI, shows signs of strengthening synaptic contacts between afferent and efferent components, consistent with ongoing synaptic plasticity.

SIGNIFICANCE

Neurons within the adult human spinal cord caudal to a lesion site are not static, but appear to be capable of developing novel-yet highly efficacious-synaptic contacts following trauma-induced partial denervation. In this case, such contacts between ascending afferents and cervical motoneurons do not appear to provide any functional benefit to the subject. In fact their presence may limit the regenerative effort of supraspinal pathways which originally innervated these motoneurons, should effort in animal models to promote regeneration across the lesion epicenter be successfully translated to humans with chronic SCI.

The effects of strychnine, bicuculline, and ketamine on 'immersion-inhibited' dorsal horn convergent neurons in intact and spinalized rats

In both intact and spinalized rats, this study examined the effects of strychnine (a glycine antagonist), bicuculline (a GABAA antagonist), and ketamine (a non-competitive NMDA receptor antagonist) on one particular class of lumbar dorsal horn convergent neurons. This group of convergent neurons are inhibited when a rat's entire ipsilateral hindpaw is immersed in 50 degrees C water and has a strong afterdischarge as soon as the paw is removed from the water. Strychnine (2 mg/kg, iv) increased ongoing activity and blocked the 'inhibition phase' in both intact and spinalized rats demonstrating that a spinal-related glycine mechanism was involved in the inhibition. However, only in intact rats did the firing rate of the 'afterdischarge phase' increase significantly from pre-drug levels, suggesting that supraspinal sites may be involved in modulating this phase. Ketamine (15 mg/kg, iv) depressed ongoing activity and the firing rate in the afterdischarge phase of these neurons. Additionally, ketamine reversed the strychnine-induced increase in ongoing activity. Bicuculline (2 mg/kg, iv) had no effect on the activity of this cell class. As shown previously, and replicated here, these 'immersion-inhibited' neurons invariably have both inhibitory and excitatory mechano-receptive fields on the ipsilateral hindpaw. Thus, the response of this class of convergent neurons to noxious stimulation may be a function of relative inputs of glycine and EAA's, each possibly triggered by the stimulation of different receptive fields/regions on the same paw. Furthermore, when both fields are co-stimulated during noxious immersion of the entire paw, glycine has a stronger influence on activity than does the EAA's.

The effect of pentobarbital sodium on the dorsal horn of the spinal cord

The effect of intravenously administered pentobarbital sodium on the activity of single unit in Rexed lamina V of the transected feline lumbar spinal cord was studied using an extracellular microelectrode recording technique. Pentobarbital sodium 1.0 mg·kg(-1), 2.5 mg·kg(-1), and 5.0 mg·kg(-1) administered intravenously suppressed both the spontaneous and the evoked activity in Rexed lamina V cells, known to respond principally to noxious stimuli, in a dose-dependent manner. The maximum depression of cell activity occurred within 5 min after intravenous administration. The recovery of cell activity occurred within 70 min after intravenous administration of pentobarbital sodium. We conclude that pentobarbital sodium intravenously administered has a suppressive effect on single unit activity of cells in Rexed lamina V and probably has an analgesic effect. Its suppressive effect is dose-dependent.

Analgesia from electrical stimulation of the hypothalamic arcuate nucleus in pentobarbital-anesthetized rats

Inhibition of noxious heat-induced tail flick by electrical stimulation of the arcuate nucleus of the hypothalamus (ARH) was examined and characterized in pentobarbital-anesthetized rats. Systematic mapping studies revealed that inhibition of the tail flick reflex could be induced by stimulating widespread areas in the ventromedial parts of the hypothalamus, which include the paraventricular nucleus, ventromedial nucleus, dorsomedial nucleus, anterior hypothalamic area as well as the ARH areas. The ARH stimulation-produced tail flick suppression could be completely blocked by systemic naloxone (2 mg/kg) which shows the involvement of an opiate mechanism in this effect. Although the tail flick reflex in the lightly anesthetized state is of significantly shorter latency than in the unanesthetized state, thresholds of the ARH stimulation for suppressing spinal nociceptive reflexes in the lightly anesthetized state were not significantly different from the thresholds at the same ARH sites in the awake state.

Inhibition of nociceptive responses of lumbar dorsal horn neurones by remote noxious afferent stimulation in the cat

In cats anaesthetized with nitrous oxide and sodium pentobarbital, multireceptive lumbar dorsal horn neurones excited by controlled noxious radiant heating of glabrous hind paw skin were recorded by extracellular microelectrodes. These noxious heat responses were inhibited by concomitant noxious stimulation of the ipsilateral forepaw or pinna, or repetitive electrical stimulation of the ipsilateral forelimb deep radial nerve. Similar extents of inhibition were produced by noxious peripheral stimulation and by deep radial nerve stimulation in repetitive trains at intensities sufficient to excite small myelinated fibres or unmyelinated fibres. A greater inhibitory effect was produced by continuous repetitive high-intensity stimulation of the deep radial nerve. With a constant frequency (5 Hz) of continuous deep radial nerve stimulation, graded increases in stimulation intensity revealed the threshold for inhibition in the small myelinated fibre range, and an additional increment of the inhibitory effect when unmyelinated fibres were also activated. When suprathreshold for unmyelinated fibres, the efficacy of continuous deep radial nerve stimulation increased with graded increases in stimulation frequency, with a threshold frequency for inhibition between 0.5 and 1 Hz and maximal effect at 5 Hz. Two nociceptive-specific neurones studied were also inhibited by deep radial nerve stimulation. The results indicate that 'diffuse noxious inhibitory controls' (DNIC) occur in the cat and can be activated by remote electrical or natural noxious stimulation.

Diffuse noxious inhibitory controls of lumbar spinal neurons involve a supraspinal loop in the cat

In anesthetized cats, lumbar dorsal horn neurons were excited by brief noxious radiant heating of glabrous hindpaw skin. These nociceptive responses were inhibited by concomitant repetitive electrical stimulation of the ipsilateral deep radial nerve. Noxious heat responses were linearly correlated with skin temperature during heating. The slope of this stimulus-response function was decreased, and the response threshold increased, by deep radial nerve stimulation. Microinjection of lidocaine into the medullary raphe attenuated the inhibition induced by deep radial nerve stimulation. The results indicate that in the cat, 'diffuse noxious inhibitory controls' (DNIC) involve medial medullary regions.

Inhibition of spontaneous activity of spinal dorsal horn neurons in the intact cat is naloxone-insensitive

Low-threshold neurons in the dorsal horn of the spinal cord of physiologically intact, awake, drug-free cats demonstrate minimal rates of spontaneous activity. Studies in acute animals suggest that the lack of spontaneous activity is due to descending inhibitory control mechanisms. The present study suggests that the inhibition of spontaneous activity is naloxone-insensitive. Intravenously administered naloxone in doses of up to 0.4 mg/kg failed to have any effect on the spontaneous activity of the 34 low-threshold neurons recorded from the dorsal horn of the spinal cord of physiologically intact, awake animals. Stimulus-evoked activity was also not significantly influenced by the doses of naloxone used in this study. These results confirm data previously obtained by others using acute preparations. This confirmation is important in that it demonstrates that, for this system, differences between acute and chronic preparations do not appear to alter neuronal sensitivity to specific drug manipulations. They also verify that, for this system, acute experiments accurately reflect conditions that exist in the intact animal.

Receptive field profiles and integrative properties of spinocervical tract cells in the cat

The receptive fields of sixteen spinocervical tract (s.c.t.) cells whose responses were recorded extracellularly were mapped using discrete and uniform jets of air given at equally spaced locations on the clipped fur of cats anaesthetized with chloralose. All the cells whose receptive fields were on the thigh or upper hind limb showed approximately unimodal gradients of sensitivity to stimulation within their excitatory receptive fields. The response magnitudes declined steadily as the stimuli were moved sequentially from the centres to the peripheries of the fields and abrupt edges were not found. Spatial summation from within the excitatory receptive field was studied in twelve s.c.t. cells. These cells showed a poor ability to summate the responses to two spatially separated air jets when these stimuli were applied simultaneously within their receptive fields. No significant summation was found in twenty-five out of thirty-one trials and in six of these trials (four cells) the responses were significantly reduced. Summation was found in six trials (four cells). Lack of summation or response reduction was more prevalent when the individual response levels were low (less than impulses stimulus-1). These results are discussed in relation to similar findings for cells of somatosensory relay nuclei and cortex.

Response of nucleus raphe magnus neurons to electrical stimulation of nucleus cuneiformis: role of acetylcholine

There is considerable evidence that cells in the ventral medulla which includes nucleus raphe magnus (NRM) and nucleus magnocellularis are involved in a descending pain inhibitory system. Anatomical studies indicate a strong projection from nucleus cuneiformis (NCF) to the ventral medulla and histochemical studies suggest that many NCF neurons are cholinergic. Therefore, we investigated the effect of NCF stimulation on NRM unit activity and explored the possible role of acetylcholine (ACh) in this interaction. Of 180 NRM neurons examined, 43% were excited and 14% were inhibited by NCF stimulation. The average latency to the peak excitatory response was about 14 ms with a range of 5-32 ms. There was a tendency for the response latencies to cluster around 5 and 14 ms. Inhibitory responses were between 10 and 65 ms in duration. The anatomical specificity of the effective stimulation site was assessed by determining the response of a given NRM neuron to stimulation of areas dorsal and ventral as well as within NCF. The most reliable and intense responses of NRM neurons was observed with electrode placements within NCF. The most effective NCF region for activating NRM neurons corresponded to that region of NCF that contains a large population of neurons that project directly to NRM as seen in the present histochemical studies. The involvement of ACh in the interaction between NCF and NRM was examined with iontophoretic application of ACh and its antagonists. Of NRM neurons that responded to ACh, 79% were excited, an effect which was blocked by scopolamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of feline spinal cord dorsal horn neurons following electrical stimulation of nucleus paragigantocellularis lateralis. A comparison with nucleus raphe magnus

The effect of electrical stimulation applied to the nucleus paragigantocellularis lateralis (PGL) was assessed on the somatosensory responses of functionally identified spinal cord dorsal horn neurons in the cat. Neurons were classified as low threshold mechanoreceptive, wide dynamic range or nociceptive specific. The responses of over 95% of all neurons tested were inhibited by a conditioning stimulus to the PGL. For each cell the threshold current intensity necessary to produce inhibition from the PGL (inhibitory threshold) was determined. Analysis of the incidence of inhibition and the inhibitory thresholds showed that the PGL-induced inhibition was not selective for a particular class of neuron. Due to the many similarities between the PGL and the nucleus raphe magnus (NRM), a comparison was made between each region's potency in inhibiting the responses of spinal cord neurons. Based on an analysis of inhibitory thresholds, the PGL was found to be significantly more potent than the NRM. These results indicate the PGL to be an important site from which descending modulation of spinal cord somesthetic information emanates.

Characterization of inhibition of a spinal nociceptive reflex by stimulation medially and laterally in the midbrain and medulla in the pentobarbital-anesthetized rat

Inhibition of the spinal nociceptive tail flick (TF) reflex by electrical stimulation throughout the midbrain and medulla was examined and characterized in pentobarbital-anesthetized rats. The TF reflex in the lightly anesthetized state is of significantly shorter latency (1.63 s vs 2.36 s) and of greater amplitude than in the unanesthetized state. Systematic mapping studies revealed that inhibition of the TF reflex could be produced from widespread areas in the midbrain and medulla. Midbrain areas having the lowest thresholds for inhibition of the TF reflex were found lateral and ventrolateral to the periaqueductal gray matter, including nucleus cuneiformis, the lateral reticular formation, and extending into the central tegmental area. In the rostral medulla, the lowest thresholds for inhibition of the TF were distributed mediolaterally across the dorsal one-third of the nucleus raphe magnus and into the adjacent nucleus reticularis gigantocellularis. Thresholds for inhibition of the TF reflex were slightly higher in the ventral nucleus raphe magnus and adjacent nucleus reticularis paragigantocellularis. Chronaxies of stimulation in the midbrain and medulla were virtually the same, indicating that the same neural elements were affected by stimulation in both brainstem areas. The thresholds of stimulation for inhibition of the TF reflex in the lightly anesthetized state were not significantly different from the thresholds of stimulation at the same midbrain sites in the awake state in the same animals. These findings contribute to a growing body of literature establishing (1) the utility of the lightly pentobarbital-anesthetized rat model for investigations of antinociceptive mechanisms and (2) the presence of multiple loci and pathways in the brainstem capable of modulating spinal nociceptive processes.

Brainstem influences on transmission of somatosensory information in the spinocervicothalamic pathway

The inhibition of somatosensory responses of lateral cervical nucleus neurons resulting from stimulation of the brainstem has been investigated. Single unit extracellular recordings were obtained from neurons in the lateral cervical nucleus of chloralose-anesthetized cats. Electrical stimulation of the periaqueductal gray, nucleus raphe magnus, nucleus cuneiformis, and nuclei reticularis gigantocellularis and magnocellularis was found to be very effective in inhibiting the responses of lateral cervical nucleus neurons evoked by electrical or tactile stimulation of the skin. Additional experiments were performed to determine whether the inhibitory effects were mediated in the spinal cord dorsal horn or in the lateral cervical nucleus. These experiments which examined the effect of brainstem stimulation on the responses induced by stimulation of the dorsolateral funiculus or on the antidromic latency of activation of lateral cervical nucleus neurons from thalamus, revealed that most and possibly all the inhibition could be accounted for by an action on the spinal cord. These results are consistent with other studies showing that spinocervical tract cells in the spinal cord can be inhibited by stimulation of the same brainstem regions.

Stimulation sites in periaqueductal gray, nucleus raphe magnus and adjacent regions effective in suppressing oral-facial reflexes

Electrode penetrations were made in the mesencephalon and caudal brainstem at the levels of the periaqueductal gray matter (PAG) and nucleus raphe magnus (NRM) in chloralose-anaesthetized or decerebrate cats. In a systematic fashion, mesencephalic and brainstem loci at every 1 mm of vertical depth were electrically stimulated in a series of mediolateral or anteroposterior electrode penetrations to determine the lowest stimulation threshold at each locus for suppressing the digastric jaw-opening reflex evoked by tooth pulp or infraorbital nerve stimulation; at some loci, the threshold current required for suppressing infraorbital nerve-evoked neck reflexes was also determined. Stimulation at sites within large regions of the mesencephalon and caudal brainstem was effective in suppressing these reflexes at less than 4 X the lowest threshold for reflex suppression in each animal. However, in these regions the areas of lowest threshold in the mesencephalon generally corresponded to the ventrolateral PAG and adjacent nucleus cuneiformis and part of the lateral reticular formation (LRF) and in the caudal brainstem they corresponded to NRM and the adjacent nuclei reticularis magnocellularis (RMC) and gigantocellularis (RGC). These findings suggest that there may be mesencephalic and caudal brainstem areas in addition to PAG and NRM that are equally effective in modulating reflex activity.

Spinal cord pathways mediating somatosensory evoked potentials

Using a CO2 laser, discrete thoracic spinal cord lesions were made in cats anesthetized with ketamine and xylazine (Rompun). Differences in cortical somatosensory evoked potentials (SEP's) produced with high-intensity stimulation (20 times the motor threshold) of each posterior tibial nerve determined for nine different combinations of unilateral spinal cord lesions. The results of these studies show that nerve fibers in the ipsilateral dorsal column, the ipsilateral dorsal spinocerebellar tract, and the contralateral ventrolateral tracts with respect to the side of leg stimulation, contribute to cortical SEP's. A lesion of the dorsal spinocerebellar tract affected only the early waves (less than 30 msec) of the SEP from leg stimulation ipsilateral to the side of the lesion, whereas a solitary lesion of the ventrolateral tract caused changes primarily in the amplitude of later waves (greater than 30 msec) of the SEP produced by contralateral leg stimulation. Lesions involving one-half of the dorsal column caused changes in the amplitude of both the early and late waves produced by stimulation ipsilateral to the side of the lesion. The effects of various combinations of lesions on the cortical SEP's were not additive, which indicates significant interaction between afferent pathways. These findings suggest that high-intensity peripheral nerve stimulation, which activates both C and A fibers, could be used intraoperatively to assess spinal cord function with more accuracy than the current practice of using a stimulus strength of twice the motor threshold. The importance of using anesthetic agents that do not depress cortical activity (which may affect the later components of the SEP) is also emphasized.

Alterations in the ipsi- and contralateral afferent inputs of dorsal horn cells produced by capsaicin treatment of one sciatic nerve in the rat

Dorsal horn cells in the lumbar spinal cord of decerebrate spinal rats were examined 7-21 days following local application of capsaicin to the sciatic nerve. Such local capsaicin treatment is known not to influence the size of the incoming A and C fibre afferent volley. The receptive field properties and primary afferent input of cells on both sides of the cord, that is ipsi and contralateral to the treated nerve, were examined. On the treated side, the percentage of cells excited by C fibres from the capsaicin treated nerve was 30% of normal and the number of cells responding to noxious heating of the cutaneous receptive field was reduced by 50%. A fibre input and low and high threshold mechanical input were normal. The receptive field size was larger in many cells innervated by the treated nerve. On the side opposite to the treated nerve, responses to noxious and non-noxious stimulation of the untreated limb were unaffected as was the input from the untreated sciatic nerve. Receptive fields were somewhat larger than normal. Effects were also observed from contralateral stimuli. Cells on both sides of the cord were found with excitatory contralateral receptive fields and excitatory responses to trains of high intensity stimulation to the contralateral sciatic nerve. In untreated animals the effect of such contralateral stimulation is inhibitory. The results show that peripheral nerve capsaicin treatment causes long lasting reduction of the C fibre input to dorsal horn cells on the treated side. However, it also results in changes in the inhibitory and excitatory receptive fields of cells on both sides of the cord.

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

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

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

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

Spinal neurones with long projections activated from the abdominal viscera of the cat

1. Recordings have been made from seventy-three neurones responding to electrical stimulation of pelvic, hypogastric or lumbar colonic nerves, in decerebrate or anaesthetized cats. Fifty-two of the units had long projections that ascended to the first cervical segment, and no units with visceral inputs were found to belong to the spino-cervical tract. Twenty-one units had long descending projections.2. Twenty percent (i.e. 11/46) responded to parasympathetic (pelvic) nerve stimulation (group 1) whilst 80% (35/46) responded to stimulation of hypogastric and/or lumbar colonic nerves (group 2). Ninety percent of group 2 neurones also responded to pelvic nerve stimulation.3. The electrical thresholds for activation of the units indicated that the largest peripheral nerve fibres responsible for the response were of the Adelta size.4. Thirty-one of the neurones had visceral mechanosensitive receptive fields; twenty-one had simple receptive fields in the bladder (seven) or in the colon (fourteen), ten units had compound receptive fields. The response of units with simple receptive fields to mechanical stimulation were either inhibitory or excitatory, and slowly adapting or rapidly adapting. Forty-two units appeared to have no visceral mechano-sensitive receptive fields in spite of showing responses to visceral nerve stimulation.5. Fifty percent of the units tested responded to innocuous somatic stimuli, mostly derived from muscle or joint receptors. Some of the units were found to respond to injections of bradykinin (10-15 mug) into a hindlimb artery.6. Group 1 had predominantly inhibitory visceral receptive fields, and somatic receptive fields in structures innervated from sacral segments of the spinal cord. Group 2 units all received inputs from visceral nerves entering the spinal cord over lumbar segments; many also received projections from sacral segmental inputs. These inputs showed an equal mixture of excitatory and inhibitory visceral receptive fields and convergence from somatic inputs arising from lumbar as well as sacral dermatomes. It seems likely that this group represents units originating in lumbar as well as sacral segments of the cord.7. The possible role of these neurones as mediators of visceral sensations and visceral reflexes is discussed.

The effects of noxious heat on responses of spinocervical units to low intensity cutaneous stimuli

The responses of spinocervical neurons to sinusoidal hair displacements were studied during and in the absence of radiant heating of parts of the hindpaw to noxious levels (45-65 degrees C). Noxious heat usually increased background discharge and lowered the signal-to-noise ratio at low frequencies of hair displacement. At higher frequencies over 20 Hz, this ratio was slightly depressed for half of the cells, and dramatically increased for the others. Similar effects were found when the heating was off the receptive field for hair displacement, which suggests a central cause for these effects.

Pentobarbital-anesthetized and decerebrate cats reveal different neurological responses in anesthetic-induced analgesia

Cats were used to assess the significance of differences in animal preparations in the study of anesthetic-induced analgesia. Comparison was made between pentobarbital-anesthetized and decerebrate non-anesthetized cats. Bradykinin dissolved in normal saline was injected into the femoral artery as a noxious stimulus, and the neural response in the spinal cord lateral funiculus was recorded using the multi-unit activity technique. The magnitude of the neural response and the changes in spontaneous firing were compared before and after cervical cord transection at C1. Before the transection, the response was greater in anesthetized than in decerebrate cats. The cord transection potentiated the response in both preparations, but the degree of potentiation was greater in decerebrate than in anesthetized cats. These studies confirmed the presence of a descending pain inhibition system acting tonically on the nociceptive neural mechanisms in the spinal cord, and indicated the susceptibility of this system to pentobarbital. We conclude that pretreatment with pentobarbital induces pharmacologically a state of partial spinal cord transection and reduces the effects of drugs acting through supraspinal CNS structures.

Effect of peripheral nerve injury on receptive fields of cells in the cat spinal cord

When the sciatic and saphenous nerves are cut and ligated in adult cats, the immediate effect is the production of a completely anesthetic foot and a region in medial lumbar dorsal horn where almost all cells have lost their natural receptive fields (RFs). Beginning at about 1 week and maturing by 4 weeks, some 40% of cells in the medial dorsal horn gain a novel RF on proximal skin, that is, upper and lower leg, thigh, lower back, or perineum. This new RF is supplied by intact proximal nerves and not by sciatic and saphenous nerve fibers that sprouted in the periphery. During the period of switching of RFs from distal to proximal skin there was no gross atrophy of dorsal horn grey matter and no Fink-Heimer stainable degeneration of central arbors and terminals of peripherally axotomized afferents. In intact animals medial dorsal horn cells showed no sign of response to mechanical stimulation of proximal skin. RFs of some of the cells had spontaneous variations in size and sensitivity, but these were not nearly sufficient to explain the large shifts observed after chronic nerve section. Tetanic electrical stimulation of skin or peripheral nerves often caused RFs to shrink, but never to expand. Although natural stimuli of proximal skin would not excite medial dorsal horn cells in intact or acutely deafferented animals, it was found that electrical stimulation of proximal nerves did excite many of these cells, often at short latencies. In the discussion we justify our working hypothesis that the appearance of novel RFs is due to the strengthening or unmasking of normally present but ineffective afferent terminals, rather than to long-distance sprouting of new afferent arbors within the spinal cord.

Serotonergic mediation of descending inhibition from midbrain periaqueductal gray, but not reticular formation, or spinal nociceptive transmission in the cat

Electrical stimulation in the midbrain periaqueductal gray (PAG) and lateral midbrain reticular formation (LRF) strongly suppresses the responses of spinal dorsal horn neurons to noxious heating of the skin. The possible role of serotonin (5-hydroxytryptamine, 5-HT) was investigated by quantitatively comparing certain parameters of descending inhibition from PAG and LRF in normal cats [14,15] and cats whose central 5-HT levels had been reduced by pretreatment with p-chlorophenylalanine (PCPA, 300 or 500 mg/kg i.p., 72 h prior to acute experiment). Single lumbar dorsal horn neuronal responses to noxious radiant heating of glabrous footpad skin(50 degrees C, 10 sec, 1/3 min) were recorded in normal and PCPA-pretreated cats anesthetized with sodium pentobarbital and N2O. Inhibition of neuronal heat-evoked responses during midbrain stimulation (mean frequency 30 Hz, up to 800 microA current intensity) was expressed as percent of the unit's control response in the absence of midbrain stimulation. Inhibition by PAG stimulation of units from cats pretreated with 300 mg/kg PCPA (mean inhibition at 450 microA to 60% of control in 12 units) was not detectably different from that in control (non-pretreated) cats. However, inhibition by PAG stimulation was significantly weaker in units from cats pretreated with 500 mg/kg PCPA (mean to 83.4% of control in 9 units). In the latter group, mean current threshold for inhibition was higher, and slope of current-intensity plots lower, than in the control and 300 mg/kg PCPA pretreatment groups. In contrast, mean inhibition by LRF stimulation was enhanced in the 300 and 500 mg/kg PCPA treatment groups in a dose-related manner. In normal (non-pretreated) cats, systemic administration of the putative 5-HT antagonist methysergide (0.07--1 mg/kg) reduced or abolished inhibition by PAG stimulation in each of 8 units. Low doses of methysergide had little or no effect on inhibition produced by LRF stimulation in 6 units. The results suggest pharmacologically distinct mechanisms of inhibition produced by stimulation in PAG and LRF.

Descending influences on receptive fields and activity of single units recorded in laminae 1,2 and 3 of cat spinal cord

Units (108) were isolated in laminae 1,2 and 3 in segments L7-Sl of decerebrate cat spinal cord. For each unit, the size and nature of its receptive field (RF) was delineated. Then the dorsolateral funiculus (DLF) was stimulated for 1 sec with 10 or 50 Hz, 0.1 msec square waves and the response characteristics of the unit were again examined. Of the 108 units, 55 were excited or facilitated, 6 were inhibited (all in lamina 3) and 47 were unaffected. While some of the excited units responded only during the stimulus train, the majority showed prolonged excitation or facilitation lasting over one minute. The excited units were predominantly those responding to pressure or to brush, touch and pressure. Of the pressure units, 73% were excited or facilitated in contrast to only 29% of the brush/touch units. Most of the excited units showed expansion of their RFs. While many units of this type show ongoing variations of excitability and RF size, the evoked responses reported were sufficiently time-locked to the stimulus for it to be apparent that they were caused by the DLF stimulation. The unit's responses still occurred when the DLF was stimulated caudal to a complete cord transection so that the effects did not pass through the brain stem. The major effect of descending systems or of DLF stimulation previously reported on the large cells of laminae 1,4 and 5 has been inhibition. Here we report that a major descending influence on many units of laminae 1,2 and 3 is excitatory. Therefore it is suggested that a population of small interneurons in the superficial laminae could contribute to the descending inhibition of large dorsal horn neurons.

Noradrenalin, serotonin, and the dorsal horn

There is anatomical, pharmacological, and physiological evidence that descending systems from the brain stem using noradrenalin and serotonin (5-hydroxytryptamine, 5-HT) participate in the control of segmental sensory processing. Study of these systems is described in this paper. L-dopa was given intravenously to cats to cause the release of noradrenalin and 5-HT. The resultant effects on the responses of dorsal horn cells to innocuous and noxious thermal stimulation of skin were determined. Using the catecholamine cell neurotoxin, 6-hydroxydopamine, the dopamine beta-hydroxylase inhibitor, fusaric acid, and the tryptophan hydroxylase inhibitor, parachlorophenylalanine, the available central stores of noradrenalin or 5-HT were altered, thus allowing separation of the effects of noradrenalin and 5-HT release on dorsal horn cells. The results indicate that noradrenalin facilitates the responses of dorsal horn cells to innocuous cutaneous stimuli, and has an inhibitory effect on the responses to noxious stimuli. Serotonin inhibits the responses of dorsal horn cells that respond to noxious stimuli, to both innocuous and noxious stimuli. The results are discussed in light of current developments concerning aminergic control of segmental sensory processing.

Spinal evoked potential in the monkey

Computer-averaged evoked potential responses (EPs) to stimulation of the sciatic nerve and cervical spinal cord were recorded from the dura and skin over the causda equina and spinal cord in seven monkeys, three with chronic spinal cord lesions. Sciatic EPs consited of predominantly negative triphasic propagated potentials recorded at all spinal levels and greatest in amplitude over the cauda equina and caudal spinal cord. The conduction velocity of this EP was faster over the cauda equina and rostral spinal cord than over caudal cord segments. Triphasic potentials were succeeded by small negative potentials over the cauda equina and larger negative potentials over the lumbar enlargement. Sciatic EPs over the upper lumbar and thoracic cord were more sensitive to asphyxia than the initial triphasic potentials recorded over cauda equina and caudal cord but resisted changes from increasing the rate of stimulation up to 100 per second. Propagated thoracic EPs were preceded by nonpropagated potentials. The longer latency negative potentials occurring locally over the cauda equina and lower lumbar enlargement were abolished at levels of asphyxia and were attenuated at rates of stimulation that did not affect the preceding triphasic potentials. Following complete spinal cord transection, nonpropagated sciatic EPs were recorded in leads rostral to the section. In preparations with chronic partial cord hemisection involving dorsal and lateral quadrants, ipsilateral sciatic EPs had increased latency, reduced amplitude, and poor definition in the vicinity of and rostral to the lesion. Direct cervical cord stimulation elicited caudally propagated potentials which were followed by large, broad potentials over the number enlargement.

Excitability changes in lumbosacral spinothalamic neurons produced by non-noxious mechanical stimuli and by graded electrical stimuli applied to the face

The changes in the excitability of lumbosacral spinothalamic neurons produced by activating afferents in the trigeminal nerve using electrical or mechanical stimuli was investigated in cats anesthetized with alpha-chloralose. In most spinothalamic neurons, weak electrical stimuli or step indentations of the skin of the face produced an increase followed by decrease in the excitability of these cells. In experiments in which the effect of activating specific groups of trigeminal afferent fibers on these excitability changes was evaluated, the suppression could be produced by activating only the fastest conducting cutaneous afferent fibers. Step indentations of the facial skin affected the excitability of spinothalamic neurons in a manner similar to electrical stimuli. The duration of the suppression phase appeared to be largely independent of the duration of the step indentation of the facial skin. It was concluded that the descending system mediating the suppression phase is activated largely by cutaneous afferents from rapidly adapting receptors. The effects of subtotal spinal cord lesions on the excitation and suppression phases produced by facial stimulation indicate that the pathways mediating the supppression descend bilaterally in the dorsal part of the lateral fasciculus. The excitation phase appears to be mediated largely by pathways in the dorsal part of the ipsilateral lateral fasciculus.

The differential effects of 5-hydroxytryptamine, noradrenaline and raphe stimulation on nociceptive and non-nociceptive dorsal horn interneurones in the cat

The effects of 5-hydroxytryptamine (5-HT), noradrenaline (NA) and stimulation of the inferior central nucleus of the raphe (RN) were examined on nociceptive and non-nociceptive spinal neurones in anaesthetized cats. 5-HT reduced excitation evoked by noxious stimulation, but increased spontaneous firing and firing evoked by DL-homocysteic acid (DLH) on both nociceptive and non-nociceptive cells. NA reduced spontaneous activity, DLH-evoked excitation and excitation evoked by a noxious stimulus on nociceptive neurones, but had little action on non-nociceptive units. RN inhibited spontaneous, stimulus-evoked and DLH-evoked firing of nociceptive cells and caused briefer inhibitions of non-nociceptive cells. Excitatory effects were also observed. Strychnine antagonized short-duration inhibitions from RN of non-nociceptive cells responding to hair movement, but failed to antagonize any of the other effects of RN. No antagonism of the inhibitory effect of RN was observed with phenoxybenzamine, phentolamine, sotalol, bicuculline or methysergide. However, methysergide antagonized some excitatory effects of 5-HT and RN, but also produced non-specific actions on some cells. It was concluded that, although glycine may mediate some of the brief duration inhibitions evoked by RN, the longer duration inhibitions were unlikely to have been mediated by either glycine or GABA. 5-HT may be a mediator of raphe-spinal actions but may have presynaptic inhibitory actions coupled with postsynaptic excitatory effects. NA could mediate some descending inhibition of nociceptive neurones.

The effects of interaction between large and small diameter fiber systems on the somatosensory evoked potential

The effect of interaction between large and small diameter fiber systems on the somatosensory evoked potential (SEP) was studied in anesthetized cats. Activation of large diameter fibers of the peroneal or radial nerves eliminates the late components of the SEP produced by stimulation of all fibers in the contralateral median or radial nerves. The inhibitory effects of a selective conditioning stimulus to the large diameter fibers of the peroneal nerve on the radial nerve evoked SEP was eliminated by bilateral transection of the dorsal column and spino-cervical tracts. However, interaction could still be obtained following transection when both large and small diameter fibers in the peroneal nerve were stimulated. The results of this study support the hypothesis that a correlation exists between activity in different fiber groups in afferent nerves, their conduction pathways through the cord, and the components of the cortical evoked potential.

Effects of electrical stimulation of periaqueductal gray matter on evoked potentials recorded in the primary somesthetic cortical areas of the rat

The influence of focal electrical stimulation of PGM on primary cortical evoked potentials elicted by nociceptive peripheral stimulation was studied in anaesthetized and conscious rats. This analgesic electrical stimulation produces an abolition of cortical responses during its application and a significant decrease of the positive and negative amplitude waves after interruption of PGM electrical stimulation. Since these effects were observed in animals under barbiturate anesthesia and in conscious rats, they are interpreted as a supraspinal action on the primary pain pathway.

Nucleus raphe magnus inhibition of spinal cord dorsal horn neurons

In decerebrate cats, electrical stimulation of nucleus raphe magnus (NRM) of the medulla produced marked inhibition of spinal neurons in lumbosacral dorsal horn. Only neurons with high threshold inputs were inhibited. These cells were located in lamina I and in or near laminae V and VI. The duration of inhibition produced was related to the stimulus train length. An ipsilateral lesion of the dorsolateral funiculus at L1 markedly reduced the inhibition of neurons caudal to the lesion. Although NRM stimulation was the most effective, inhibition from more lateral sites could be obtained at higher stimulus intensities. NRM induced inhibition is probably mediated by a direct projection via the dorsolateral funiculus to spinal dorsal horn laminae I, II, V and VI. The results are discussed in relation to proposed mechanisms underlying the analgesia produced by NRM stimulation.

An analysis of response properties of spinal cord dorsal horn neurones to nonnoxious and noxious stimuli in the spinal rat

Electrophysiological properties of neurones in the spinal cord dorsal horn were studied in decerebrated, immobilized spinal rats. Extracellular recordings were performed at the thoraco-lumbar junction level. Each track was systematically located by extracellular injection of pontamine sky blue. According to their responses to mechanical peripheral stimuli, cells were classified in four classes: Class 1 cells: Cells activated only by nonnoxious stimuli. They were divided into - 1A: hair movement and/or touch and 1B: hair movement and/or touch and pressure or pressure only. Class 2 cells: Cells driven by both nonnoxious and noxious stimuli, divided into - 2A: hair movement and/or touch, pressure, pinch and/or pin-prick, and 2B: pressure, pinch and/or pin-prick. Class 3 cells: Cells only activated by noxious stimuli (pinch and/or pin-prick). Class 4 cells: Cells responding to joint movement or pressure on deep tissues. Peripheral transcutaneous or sural nerve stimulation clearly showed that class 1 cells were activated only by A fiber input while 68% of classes 2 and 3 cells received A and C input. Histological examination indicated that cells driven only by noxious input were located either in the deepest part or in the marginal zone (lamina I) of the dorsal horn. Nevertheless, some lamina I cells were also driven by both nonnoxious and noxious stimuli. In addition, there is a great deal of overlap between class 1 and class 2 cells. This fact was confirmed by considering the wide distribution in the dorsal horn of cells receiving A and C input. However, spinal organization of the different classes of cells consists of a preferential distribution rather than a strict lamination. This study indicates that properties of dorsal horn inter-neurones in the rat have a high degree of similarity with those previously described in other species (cat and monkey).