Tonic descending inhibition and spinal nociceptive transmission

Thalamus: The 'promoter' of endogenous modulation of pain and potential therapeutic target in pathological pain

More recently, the thalamic mediodorsal (MD) and ventromedial (VM) nuclei have been revealed to be functioned as 'nociceptive discriminator' in discriminating noxious and innocuous peripheral afferents, and exhibits distinct different descending controls of nociception. Of particularly importance, the function of thalamic nuclei in engaging descending modulation of nociception is 'silent' or inactive during the physiological state as well as in condition exposed to insufficient noxious stimulation. Once initiation by sufficient noxious or innocuous C-afferents associated with temporal and spatial summation, the thalamic MD and VM nuclei exhibit salient, different effects: facilitation and inhibition, on noxious mechanically and heat evoked nociception, respectively. Based on series of experimental evidence, we here summarize a novel hypothesis involving thalamic MD and VM nuclei functioned as 'promoter' in initiating descending facilitation and inhibition of pain with specific spatiotemporal characteristics. We further hypothesize that clinical remedy in targeting thalamic VM nucleus by enhancing its activities in recruiting inhibition alone or decreasing thalamic MD nucleus induced facilitation may provide promising way in effectively control of pathological pain.

Effects of pruritogens and algogens on rostral ventromedial medullary ON and OFF cells

Rostroventromedial medulla (RVM) ON and OFF cells are thought to facilitate and inhibit spinal nociceptive transmission, respectively. However, it is unknown how ON and OFF cells respond to pruritic stimuli or how they contribute to descending modulation of spinal itch signaling. In pentobarbital sodium-anesthetized mice, single-unit recordings were made in RVM from ON and OFF cells identified by their respective increase or decrease in firing that occurred just before nocifensive hindlimb withdrawal elicited by paw pinch. Of RVM ON cells, 75% (21/28) were excited by intradermal histamine, 50% (10/20) by intradermal chloroquine, and 75% (27/36) by intradermal capsaicin. Most chemically responsive units also responded to a scratch stimulus applied to the injected hindpaw. Few ON cells responded to intradermal injection of vehicle (saline: 5/32; Tween 2/17) but still responded to scratching. For OFF cells, intradermal histamine and scratching inhibited 32% (6/19) with no effect of histamine in the remainder. Intradermal chloroquine inhibited 44% (4/9) and intradermal capsaicin inhibited 61% (11/18) of OFF cells. Few OFF cells were affected by vehicles (Tween: 1 inhibited, 7 unaffected; saline: 3 excited, 1 inhibited, 8 unaffected). Both ON and OFF cells that responded to one chemical usually also responded to others, whereas units unresponsive to the first-tested chemical tended not to respond to others. These results indicate that ascending pruriceptive signals activate RVM ON cells and inhibit RVM OFF cells. These effects are considered to facilitate and disinhibit spinal pain transmission, respectively. It is currently not clear if spinal itch transmission is similarly modulated. NEW & NOTEWORTHY The rostroventromedial medulla (RVM) contains ON and OFF cells that are, respectively, excited and inhibited by noxious stimuli and have descending projections that facilitate and inhibit spinal nociceptive transmission. Most RVM ON cells were excited, and OFF cells inhibited, by intradermal injection of the pruritogens histamine and chloroquine, as well as the algogen capsaicin. These results indicate that itchy stimuli activate RVM neurons that presumably give rise to descending modulation of spinal itch transmission.

Opposing effects of cervical spinal cold block on spinal itch and pain transmission

Inactivation of descending pathways enhanced responses of spinal dorsal horn neurons to noxious stimuli, but little is known regarding tonic descending modulation of spinal itch transmission. To study effects of cervical spinal cold block on responses of dorsal horn neurons to itch-evoking and pain-evoking stimuli, single-unit recordings were made from superficial dorsal horn wide dynamic range and nociceptive-specific-type neurons in pentobarbital-anesthetized mice. Intradermal histamine excited 17 units. Cold block starting 1 minute after intradermal injection of histamine caused a marked decrease in firing. The histamine-evoked response during and following cold block was significantly lower compared with control histamine-evoked responses in the absence of cold block. A similar but weaker depressant effect of cold block was observed for dorsal horn unit responses to chloroquine. Twenty-six units responded to mustard oil allyl isothiocyanate (AITC), with a further significant increase in firing during the 1-minute period of cold block beginning 1 minute after AITC application. Activity during cold block was significantly greater compared with the same time period of control responses to AITC in the absence of cold block. Ten units' responses to noxious heat were significantly enhanced during cold block, while 6 units' responses were reduced and 18 unaffected. Cold block had no effect on mechanically evoked responses. These results indicate that spinal chemonociceptive transmission is under tonic descending inhibitory modulation, while spinal pruriceptive transmission is under an opposing, tonic descending facilitatory modulation.

Changes in response properties of rostral ventromedial medulla neurons during prolonged inflammation: modulation by neurokinin-1 receptors

Activation of neurokinin-1 (NK-1) receptors in the rostral ventromedial medulla (RVM) can facilitate pain transmission in conditions such as inflammation, and thereby contribute to hyperalgesia. Since blockade of NK-1 receptors in the RVM can attenuate hyperalgesia produced by prolonged inflammation, we examined the role of NK-1 receptors in changes of response properties of RVM neurons following four days of hind paw inflammation with complete Freund's adjuvant. Recordings were made from functionally identified ON, OFF and NEUTRAL cells in the RVM. Spontaneous activity and responses evoked by a series of mechanical (10, 15, 26, 60, 100, and 180 g) and heat (34-50 °C) stimuli applied to the inflamed and non-inflamed hind paws were determined before and at 15 and 60 min after injection of the NK-1-antagonist L-733,060 or vehicle into the RVM. Prolonged inflammation did not alter the proportions of functionally-identified ON, OFF and NEUTRAL cells. ON cells exhibited enhanced responses to mechanical (60-100g) and heat (48-50 °C) stimuli applied to the inflamed paw, which were attenuated by L-733,060 but not by vehicle. Inhibitory responses of OFF cells evoked by mechanical stimuli applied to the inflamed paw were also inhibited by L-733,060, but responses evoked by stimulation of the contralateral paw were increased. Heat-evoked responses of OFF cells were not altered by L-733,060. Also, neither L-733,060 nor vehicle altered spontaneous ongoing discharge rate of RVM neurons. These data indicate that NK-1 receptors modulate excitability of ON cells which contribute to both mechanical and heat hyperalgesia, whereas NK-1 modulation of OFF cells contributes to mechanical hyperalgesia during prolonged inflammation.

Imbalance between excitatory and inhibitory amino acids at spinal level is associated with maintenance of persistent pain-related behaviors

Although the postsynaptic events responsible for development of pathological pain have been intensively studied, the relative contribution of presynaptic neurotransmitters to the whole process remains less elucidated. In the present investigation, we sought to measure temporal changes in spinal release of both excitatory amino acids (EAAs, glutamate and aspartate) and inhibitory amino acids (IAAs, glycine, ?-aminobutyric acid and taurine) in response to peripheral inflammatory pain state. The results showed that following peripheral chemical insult induced by subcutaneous bee venom (BV) injection, there was an initial, parallel increase in spinal release of both EAAs and IAAs, however, the balance between them was gradually disrupted when pain persisted longer, with EAAs remaining at higher level but IAAs at a level below the baseline. Moreover, the EAAs-IAAs imbalance at the spinal level was dependent upon the ongoing activity from the peripheral injury site. Intrathecal blockade of ionotropic (NMDA and non-NMDA) and metabotropic (mGluRI, II, III) glutamate receptors, respectively, resulted in a differential inhibition of BV-induced different types of pain (persistent nociception vs. hyperalgesia, or thermal vs. mechanical hyperalgesia), implicating that spinal antagonism of any specific glutamate receptor subtype fails to block all types of pain-related behaviors. This result provides a new line of evidence emphasizing an importance of restoration of EAAs-IAAs balance at the spinal level to prevent persistence or chronicity of pain.

Distinct roles of the anterior cingulate cortex in spinal and supraspinal bee venom-induced pain behaviors

A wide variety of human and animal experiments suggest that the anterior cingulate cortex (ACC) is one of the key brain substrates subserving higher order processing of noxious information. However, no sufficient data are now available regarding the mediation by ACC of different levels of pain processing as well as its potential descending modulation of spinal nociception. Using the well-developed rat bee venom (BV) model, the present study evaluated the effect of lesions of bilateral ACC on two levels of spontaneous nociceptive behaviors (spinally-processed persistent paw flinching reflex and supraspinally-processed paw lifting/licking) and heat or mechanical hypersensitivity under the inflammatory pain state. In contrast to the sham lesion group (saline microinjection into the ACC), bilateral complete ACC chemical lesions (kainic acid microinjection into the ACC) significantly decreased the BV-induced paw lifting and licking behavior (less time spent by the animal in paw lifting/licking) but produced no influence upon spinally-processed spontaneous paw flinching reflex (no change in number of paw flinches following subcutaneous BV injection). Moreover, the bilateral ACC lesions relieved the BV-evoked primary thermal or mechanical hypersensitivity compared with the sham control group. However, incomplete lesions of bilateral ACC failed to affect the abovementioned pain-related behaviors. No effects were seen on basal pain sensitivity in either group of rats. Motor coordination, as measured by Rota-Rod treadmill test, was not impaired by bilateral ACC lesions. These results implicate that the ACC area of the brain plays differential roles in the mediation of different levels of spontaneous pain-related behaviors. The present study also provides additional evidence for the ACC-mediated descending facilitation of primary hyperalgesia (pain hypersensitivity) identified in the injured area under inflammatory pain state.

Opioid modulation of reflex versus operant responses following stress in the rat

In pre-clinical models intended to evaluate nociceptive processing, acute stress suppresses reflex responses to thermal stimulation, an effect previously described as stress-induced "analgesia." Suggestions that endogenous opioids mediate this effect are based on demonstrations that stress-induced hyporeflexia is enhanced by high dose morphine (>5 mg/kg) and is reversed by naloxone. However, reflexes and pain sensations can be modulated differentially. Therefore, in the present study direct comparisons were made of opioid agonist and antagonist actions, independently and in combination with acute restraint stress in Long Evans rats, on reflex lick-guard (L/G) and operant escape responses to nociceptive thermal stimulation (44.5 degrees C). A high dose of morphine (>8 mg/kg) was required to reduce reflex responding, but a moderate dose of morphine (1 mg/kg) significantly reduced escape responding. The same moderate dose (and also 5 mg/kg) of morphine significantly enhanced reflex responding. Naloxone (3 mg/kg) significantly enhanced escape responding but did not affect L/G responding. Restraint stress significantly suppressed L/G reflexes (hyporeflexia) but enhanced escape responses (hyperalgesia). Stress-induced hyperalgesia was significantly reduced by morphine and enhanced by naloxone. In contrast, stress-induced hyporeflexia was blocked by both naloxone and 1 mg/kg of morphine. Thus, stress-induced hyperalgesia was opposed by endogenous opioid release and by administration of morphine. Stress-induced hyporeflexia was dependent upon endogenous opioid release but was counteracted by a moderate dose of morphine. These data demonstrate a differential modulation of reflex and operant outcome measures by stress and by separate or combined opioid antagonism or administration of morphine.

Epidural Clonidine for Postoperative Pain After Total Knee Arthroplasty: A Dose–Response Study

BACKGROUND

Combinations of epidural clonidine, local anesthetics, and opioids have improved postoperative analgesia after total knee arthroplasty. In this study we sought to determine the optimal epidural bolus dose of clonidine, which provides the best analgesia and fewest side effects.

METHODS

Eighty ASA I-III patients, who underwent total knee arthroplasty were randomly assigned to one of four groups of 20 patients each. Identical epidural anesthesia procedures were used for all groups. After surgery, groups C0, C1, C2, and C4 received patient-controlled epidural analgesia (PCEA) with clonidine (0, 1.0, 2.0, or 4.0 mug/mL, respectively) and morphine (0.1 mg/mL) in 0.2% ropivacaine. The analgesia effect was estimated by PCEA consumption volume and visual analog pain scale at rest and with movement at 1, 2, 4, 12, 24, 48, and 72 h after surgery. Systolic blood pressure, heart rate, sedation, and sensory and motor blockade were also recorded for 72 h after surgery.

RESULTS

The PCEA consumption volume for groups C0, C1, C2, and C4 were 71.8 +/- 19.5 mL, 49.6 +/- 12.3 mL, 48.1 +/- 9.3 mL, and 39.4 +/- 9.0 mL, respectively. The clonidine groups experienced less postoperative pain (P = 0.002). In the C4 group, four patients had prolonged sensory blockade and one patient had both severe sedation and prolonged sensory motor blockade. No significant statistical difference in analgesic consumption (P = 0.78) and pain intensity (P = 0.66) between groups C1 and C2 were noted.

CONCLUSIONS

The optimal amount of epidural clonidine in a solution of morphine and ropivacaine for postoperative pain management is 1.0 microg/mL.

The possible role of the NO-cGMP pathway in nociception: different spinal and supraspinal action of enzyme blockers on rat dorsal horn neurones

In the literature, the pro- or antinociceptive effects of nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) are discussed controversially. Our laboratory and others have reported that in the spinal cord a local lack of NO has an excitatory action on the ongoing (background) activity of dorsal horn neurones. Here, we tested the hypothesis that this effect of NO is mediated by cGMP and that part of the controversy is due to differences in the spinal and supraspinal actions of both compounds. In anaesthetised rats, impulse activity of lumbar dorsal horn neurones was recorded, and blockers of NO- and cGMP-synthesis, as well as the phosphodiesterase 5 (PDE5) inhibitor sildenafil (which increases the cGMP level), or 8-Bromo-cGMP (a membrane permeable cGMP analogue) were administered spinally or supraspinally. Topical superfusion of the spinal cord with a blocker of the guanylyl cyclase (ODQ) to reduce the cGMP level led to an increase in background activity of nociceptive lumbar dorsal horn neurones similar to that caused by l-NAME, a blocker of the NO synthase. Spinal superfusion with sildenafil or 8-Bromo-cGMP had no excitatory effect. In contrast, injections of sildenafil or 8-Bromo-cGMP into the third cerebral ventricle caused an increased background activity in lumbar dorsal horn neurones, while l-NAME and ODQ were ineffective. The results show that at the spinal level, a lack of cGMP and NO has an excitatory action on dorsal horn neurones, whereas supraspinally an elevated level of cGMP is excitatory.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Expansion of nociceptive withdrawal reflex receptive fields in spinal cord injured humans

OBJECTIVE

In spinal cord injured (SCI) subjects, exaggerated withdrawal reflexes associated with a dominant flexor pattern irrespective of stimulation site have been reported. In the present study, withdrawal reflex receptive field (RRF) was determined in complete SCI subjects (N=9).

METHODS

Distributed electrical stimulation was applied to the sole of the foot, and reflexes in tibialis anterior, soleus, biceps femoris, and vastus lateralis muscles were recorded together with knee and ankle movement trajectories. A group of spinally intact subjects (N=10) were included as controls. With the subjects in supine position, stimulation was applied to 10 different sites on the foot sole. Based on the tibialis anterior reflex threshold for stimulation on the mid foot sole, two stimulus intensities (1.1 times the reflex threshold and 1.4 times the reflex threshold) were used for all 10 sites.

RESULTS

In SCI subjects, dorsi-flexion dominated independent of stimulus site and the tibialis anterior RRF covered the entire foot sole in contrast to a well-defined tibialis anterior receptive field at the medial, distal foot sole in the spinally intact subjects. Further, the soleus RRF also covered the entire sole in the SCI subjects. The reflexes in biceps femoris and vastus lateralis muscles were small and associated with weak knee flexion at all 10 sites in the SCI subjects and in the controls.

CONCLUSIONS

The RRF of the ankle flexor and the ankle extensor muscles both covered the entire sole of the foot indicating an expansion of the RRFs following spinal cord injury. The expansion is most likely due to lack of descending inhibitory control and/or increased sensitivity of the spinal reflex loop in the SCI subjects.

SIGNIFICANCE

The study improves the understanding of spinal reflex control in spinal intact and spinal cord injured subjects.

Spinal neurons that express NK-1 receptors modulate descending controls that project through the dorsolateral funiculus

Selective ablation of spinal neurons possessing substance P receptors (NK-1 receptors) using the selective cytotoxin conjugate substance P-saporin (SP-SAP) decreases hyperalgesia and central sensitization. The mechanisms by which NK-1 expressing neurons modulate the excitability of other dorsal horn neurons are unclear. Because the majority of NK-1 expressing spinal neurons project rostrally, it is possible that they are part of a spinal-supraspinal circuitry that contributes to descending modulation of excitability of spinal nociceptive neurons. We therefore determined whether ablation of spinal neurons that possess the NK-1 receptor altered descending systems that travel via the dorsolateral funiculus (DLF). Spontaneous activity and responses of dorsal horn neurons evoked by mechanical (von Frey monofilaments) and heat (35-51 degrees C) stimuli were determined before and after transection of the DLF and were compared in rats pretreated with intrathecal application of vehicle or SP-SAP. In vehicle-treated rats, transection of the DLF caused a 233% increase in mean spontaneous activity of neurons and enhanced their responses to mechanical and heat stimuli, whereas these increases in excitation were blocked in rats pretreated with SP-SAP. Importantly, SP-SAP alone had no effect on spontaneous or evoked activity in the absence of DLF transection. These results demonstrate that spinal neurons expressing the NK-1 receptor appear to play a pivotal role in regulating descending systems that modulate activity of nociceptive dorsal horn neurons.

Neurobiology of Acupuncture: Toward CAM

It has long been accepted that acupuncture, puncturing and scraping needles at certain points on the body, can have analgesic and anesthetic effects, as well as therapeutic effects in the treatment of various diseases. This therapy, including acupuncture anesthesia, has drawn the attention of many investigators and become a research subject of international interest around the world. Numerous studies have demonstrated that the nervous system, neurotransmitters, endogenous substances and Jingluo (meridians) may respond to needling stimulation and electrical acupuncture. An abundance of information has now accumulated concerning the neurobiological mechanisms of acupuncture, in relation to both neural pathways and neurotransmitters/hormonal factors that mediate autonomic regulation, pain relief and other therapeutics. Early studies demonstrated that the analgesic effects of electroacupuncture (EA) are mediated by opioid peptides in the periaqueductal gray. Recent evidence shows that nitric oxide plays an important role in mediating the cardiovascular responses to EA stimulation through the gracile nucleus-thalamic pathway. Other substances, including serotonin, catecholamines, inorganic chemicals and amino acids such as glutamate and alpha-aminobutyric acid (GABA), are proposed to mediate certain cardiovascular and analgesic effects of acupuncture, but at present their role is poorly understood. The increased interest in acupuncture health care has led to an ever-growing number of investigators pursuing research in the processes of the sense of needling touch, transduction of needling stimulation signals, stimulation parameters and placebos. In this Review, the evidence and understanding of the neurobiological processes of acupuncture research have been summarized with an emphasis on recent developments of nitric oxide mediating acupuncture signals through the dorsal medulla-thalamic pathways.

Role of central NMDA versus non-NMDA receptor in spinal withdrawal reflex in spinal anesthetized rats under normal and hyperexcitable conditions

The present study aimed to investigate the role of central N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the spinal withdrawal reflex assessed by recording single motor unit (SMU) electromyogram (EMG) response to peripheral mechanical (pressure, pinch) stimuli and repeated electrical stimuli at 3 and 20 Hz. During normal conditions, intrathecal administration of MK-801 and CNQX apparently depressed mechanically and electrically (3 Hz) evoked EMG responses in a dose-dependent manner (10, 20 and 40 nmol in 10 microl). In contrast, the after-discharges to 20 Hz electrical stimuli were suppressed only by CNQX treatment, not by MK-801 treatment. This indicates that the central mechanisms underlying the different frequencies of electrically evoked withdrawal reflex may be different. During peripheral bee venom (BV, 0.2 mg/50 microl) induced inflammation and central sensitization, the enhanced SMU EMG responses including after-discharges to pinch stimuli and 3 Hz electrical stimuli were depressed significantly by treatments with both MK-801 and CNQX. However, the enhanced SMU activities to innocuous pressure stimuli were depressed only by treatment with CNQX. Likewise, enhanced long lasting after-discharges elicited by 20 Hz electrical stimuli were also only depressed by CNQX, indicating that different central mechanisms are involved in the persistent hyperexcitability during BV-induced inflammation. The data suggest that both central NMDA and non-NMDA receptors play important roles in the transmission of nociceptive information under normal conditions. In BV-induced inflammation, however, central non-NMDA receptors, but not NMDA receptors, play a pivotal role in the generation of persistent hyperexcitability to mechanical and electrical stimuli at different frequencies (3 Hz, 20 Hz).

Organisation of sensitisation of hind limb withdrawal reflexes from acute noxious stimuli in the rabbit

Spatial aspects of central sensitisation were investigated by studying the effects on three hind limb withdrawal reflexes of an acute noxious stimulus (20 % mustard oil) applied to a number of locations around the body in decerebrate and in anaesthetised rabbits. Reflex responses to electrical stimulation of the toes were recorded from the ankle flexor tibialis anterior (TA) and the knee flexor semitendinosus (ST), whereas responses to stimulation of the heel were recorded from the ankle extensor medial gastrocnemius (MG). In non-spinalised, decerebrated, pentobarbitone-sedated preparations, flexor reflexes were facilitated significantly from sites on the plantar surface of the ipsilateral foot but were either inhibited or unaffected by stimulation of sites away from this location. The heel-MG reflex was facilitated from the ipsilateral heel and was inhibited from a number of ipsilateral, contralateral and off-limb sites. In decerebrated, spinalised, pentobarbitone-sedated animals, mustard oil applied to any site on the ipsilateral hind limb enhanced both flexor reflexes, whereas the MG reflex was enhanced only after stimulation at the ipsilateral heel and was inhibited after stimulation of the toe tips or TA muscle. Mustard oil on the contralateral limb had no effect on any reflex. In rabbits anaesthetised with pentobarbitone and prepared with minimal surgical interference, the sensitisation fields for the heel-MG and toes-TA reflexes were very similar to those in non-spinal decerebrates whereas that for toes-ST was more like the pattern observed in spinalised animals. In no preparation was sensitisation or inhibition of reflexes related to the degree of motoneurone activity generated in direct response to the sensitising stimulus. This study provides for the first time a complete description of the sensitisation fields for reflexes to individual muscles. Descending controls had a marked effect on the area from which sensitisation of flexor reflexes could be obtained, as the sensitisation fields for the flexor reflexes evoked from the toes were larger in spinalised compared to decerebrated, non-spinalised animals. The intermediate sizes of sensitisation fields in anaesthetised animals suggests that the area of these fields can be dynamically controlled from the brain. On the other hand, the sensitisation field for the heel-MG reflex varied little between preparations and appears to be a function of spinal neurones.

Interactions between cutaneous afferent inputs to a withdrawal reflex in the decerebrated rabbit and their control by descending and segmental systems

Previous studies have suggested that activation of nociceptive afferents from the heel recruits a supraspinal mechanism, which is modulated by adrenergic descending inhibition, that augments withdrawal reflexes in medial gastrocnemius (MG) motoneurones. To test this idea, we have studied the temporal evolution of reflexes evoked in MG by electrical stimulation of sural nerve A(beta)-, A(delta)- and C-fibre axons at 1 Hz, in decerebrated rabbits. Reflexes were analysed in three time bands, estimated to accord to afferent drive from A(beta)- (phase 1), A(delta)- (phase 2) and C-fibre (phase 3) inputs. Stimulation of A(delta)- and C-fibres gave significant temporal summation of all reflexes. The alpha(2)-adrenoceptor antagonist RX 821002 ((2-(2,3-dihydro-2-methoxy-1,4-benzodioxin-2-yl)-4,5-dihydro-1-H-imidazole)-HCl) (100 microg intrathecal (i.t.)) potentiated, and the alpha(2)-agonist dexmedetomidine (1-30 microg i.t.) depressed all reflexes per se, but the effects of these drugs on temporal summation were secondary to changes in baseline excitability. When C-fibres were stimulated, the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (1 mg i.t.) reduced temporal summation of phase 2 and 3 but not phase 1 reflexes. Spinalisation at L1 in the absence of drugs increased phase 2 and 3 reflexes but had no effect on phase 1, whereas spinalisation after RX 821002 resulted in decreased phase 1 responses with no significant change in later phases. Spinalisation in the presence of dizocilpine resulted in small reductions in phase 3 reflexes only. In all cases spinalisation virtually abolished temporal summation. In spinalised animals, dizocilpine selectively reduced late reflexes, and the opioid antagonist naloxone (100 microg i.t.) augmented all reflexes but gave rise to temporal subtraction of reflexes when C-fibres were stimulated.The present experiments have revealed a number of novel and important features of the sural-MG reflex pathway: (i) activity in fine afferent axons augments the reflexogenic potential of all subsequent afferent input, thereby allowing all afferent drive from the sural field to contribute to withdrawal of the heel; (ii) endogenous adrenergic control of this reflex pathway is completely non-selective; (iii) there is a non-adrenergic element of descending inhibition that is selective for the late components of MG reflex responses, and this element is directed particularly against transmission through NMDA receptors; (iv) temporal summation in this reflex is dependent on NMDA receptor-dependent and -independent mechanisms; and (v) this temporal summation is in some way dependent on the integrity of descending pathways.

Clonidine for treatment of postoperative pain: a dose-finding study

The aim of this double-blind randomized study was to evaluate the optimal intravenous dose of clonidine administrated during the peri-operative period, after lumbar hemilaminectomy for herniated disk repair. The "optimal intravenous dose" was defined as that providing minimal analgesic request, stable haemodynamic profile and a minimal sedation score during 12h after extubation. Eighty adult patients, ASA physical status I-II, undergoing lumbar hemilaminectomy for herniated disk (L(4)-L(5), L(5)-S(1)) were included in the study. All the patients were randomly assigned to one of four study groups (A, B, C, D), 20 patients each. The same standardized general anaesthesia was performed for each group. Thirty minutes before the end of surgery, group A, B and C patients received three different loading doses of intravenous clonidine (5 microg/kg, 3 microg/kg, 2 microg/kg respectively), followed by the same infusion of intravenous clonidine (0.3 microg/kg per hour). Group D patients received a bolus dose and a continuous infusion of NaCl 0.9%. In the recovery unit, postoperative pain was treated by a patient-controlled analgesia device, containing morphine. Pain relief was evaluated by the total morphine requirement during the postoperative period. Systolic blood pressure (SBP), heart rate and sedation were also noted during the first 12h postoperatively. Intravenous clonidine decreased morphine requirements in a dose-dependent manner. Group A, B, C and D patients requested 5 +/- 2, 11 +/- 3, 19 +/- 4 and 29 +/- 8 doses of morphine respectively. Clonidine also affected SBP in a dose-related manner. Group A, B and C patients had an SBP decrease respectively of 26 +/- 3%, 7 +/- 4% and 2 +/- 2% compared with basic values while, at the same time, in group D patients no SBP variation was registered. In conclusion, this study demonstrates that, when sedation and analgesic effect of clonidine is required, 3 microg/kg bolus dose followed by a continuous infusion of 0.3 microg/kg per hour has to be considered the optimal intravenous dose. The higher dose of intravenous clonidine (5 microg/kg) produced better analgesia but the degree of hypotension and sedation was more severe and longer lasting; it required ephedrine administration and careful monitoring of the patient. On the other hand, the bolus of intravenous clonidine 2 microg/kg (group C) was less effective in terms of pain relief but with similar side-effects to the 3 microg/kg dosage (group B).

On the reduction of spontaneous and glutamate-driven spinocerebellar and spinoreticular tract neuronal activity during active sleep

The present study was performed to provide evidence that dynamic neural processes underlie the reduction in dorsal spinocerebellar tract and spinoreticular tract neuron activity that occurs during active sleep. To ascertain the effect of local inhibition on the spontaneous and glutamate-evoked spike discharge of sensory tract neurons, preliminary control tests were performed during the state of quiet wakefulness, where GABA or glycine was co-administered in a sustained fashion during pulsatile release of glutamate to dorsal spinocerebellar tract (n=3) or spinoreticular tract (n=2) neurons. Co-administration of GABA or glycine also resulted in a significant marked suppression of spontaneous spike activity and glutamate-evoked responses of these cells. Extracellular recording experiments combined with juxtacellular application of glutamate were then performed on 20 antidromically identified dorsal spinocerebellar tract and spinoreticular tract neurons in the chronic intact cat as a function of sleep and wakefulness. The glutamate-evoked activity of a group of 10 sensory tract neurons (seven dorsal spinocerebellar tract, three spinoreticular tract), which exhibited a significant decrease in their spontaneous spike activity during active sleep, was examined. Glutamate-evoked activity in these cells was significantly attenuated during active sleep compared with wakefulness. In contrast, the glutamate-evoked activity of a second group of eight sensory tract neurons (four dorsal spinocerebellar tract, four spinoreticular tract), which exhibited a significant increase in their spontaneous spike activity during active sleep, was not significantly altered in a state-dependent manner. These data indicate that, during natural active sleep, a dynamic neural process is engaged onto certain dorsal spinocerebellar tract and spinoreticular tract neurons, which in turn dampens sensory throughput to higher brain centers.

Cholinergic, noradrenergic, and serotonergic inhibition of fast synaptic transmission in spinal lumbar dorsal horn of rat

It is known that spinal nociceptive sensory transmission receives descending inhibitory and facilitatory modulation from supraspinal structures. Glutamate is the major fast excitatory transmitter between primary afferent fibers and spinal dorsal horn neurons. In whole-cell patch clamp recordings from dorsal horn neurons in spinal slices, we investigated synaptic mechanisms for inhibitory modulation at the lumbar level of the spinal cord. Application of the cholinergic receptor agonist carbachol produced a dose-dependent inhibition of glutamate-mediated excitatory postsynaptic currents (EPSCs) (IC(50) 13 microM). Postsynaptic injection of two different types of G-protein inhibitors, guanosine 5'-O-2-thiophosphate or guanosine 5'-O-3-thiotriphosphate, blocked the inhibition produced by carbachol. Clonidine, a selective alpha-adrenergic receptor agonist, also produced a dose-dependent inhibition of EPSCs (IC(50) 7 microM) that was reduced by postsynaptic inhibition of G-proteins. The inhibitory effect of serotonin was likewise mediated by postsynaptic G-proteins. Our results suggest that activation of postsynaptic neurotransmitter receptors plays a critical role in inhibition of glutamate mediated sensory responses by acetylcholine, norepinephrine, and serotonin. Our results support the hypothesis that descending sensory modulation may be mediated by multiple neurotransmitter receptors in the spinal cord.

Tonic adrenergic and serotonergic inhibition of a withdrawal reflex in rabbits subjected to different levels of surgical preparation

The excitability of the heel-gastrocnemius withdrawal reflex pathway has been monitored in rabbits undergoing surgical preparation for electrophysiological experimentation under Saffan anaesthesia. Reflexes were evoked by percutaneous electrodes inserted at the heel and recorded as electromyograph signals from the ipsilateral medial gastrocnemius muscle. Two levels of surgery were carried out. The "full surgical" preparation was performed under deep Saffan anaesthesia. The trachea, carotid artery, jugular vein and intrathecal space (via a small laminectomy at L1) were cannulated, the animals were decerebrated by suction, and the left hindlimb was immobilized by screw clamps applied to the tibia and the femur. The sciatic nerve and its branches were exposed by bisection of the posterior biceps muscle and the anaesthetic was withdrawn. In the "reduced surgery" preparation, procedures were carried out with a lighter level of Saffan anaesthesia and operated tissues were infiltrated with local anaesthetic. Only the cannulations were performed in these animals. The excitability of the heel-gastrocnemius reflex declined throughout the full surgical preparation, with the median threshold increasing from 0.8 to 4.2 mA (n=19) and responses to suprathreshold stimuli reducing in size. Most of this effect was reversed after surgery was complete and anaesthesia withdrawn subsequent to decerebration. There were no significant changes in reflex excitability during the reduced surgery preparation (n = 15). Animals prepared by each of these protocols were given increasing intrathecal doses of either the selective alpha2-adrenoceptor antagonist RX 821002 (0.3 to 300 microg) or the serotonin/5-hydroxytryptamine (5-HT)1A-receptor antagonist WAY-100635 (0.01 to 30 microg). Both drugs caused significant, dose-dependent increases in reflex responses, to four to six times pre-drug control in both groups of animals. There were no differences in the effects on reflexes of either drug between the preparations. Thus, surgical preparation of decerebrated rabbits for electrophysiological recording results in depression of hindlimb withdrawal reflexes, although much of this effect did not persist beyond the completion of surgery. Tonic monoaminergic inhibition of reflexes was present to the same extent in both preparations investigated and is not therefore an epiphenomenon of the way in which the animals were prepared.

Pain sensitivity in chronic psychoemotional stress in humans

Results were obtained from comparative studies of skin pain sensitivity (pain thresholds) using focused ultrasound in 51 healthy men and 101 patients with neurasthenia. Neurasthenia is a natural "model" of chronic psychoemotional stress, and patients showed a reduction in the pain threshold which was not accompanied by a reduction in the threshold of sensitivity to tactile stimulation. A reduction in the pain threshold, reflecting a weakening of central descending tonic inhibition, was probably due to a reduction in the activity of the brain's opioid system during long-term psychoemotional stress. Analysis of the relationships between the pain sensitivity threshold and pain syndromes suggests a role for changes in the nociception system in chronic psychoemotional stress, as part of the mechanism of pain formation.

Long term potentiation of single WDR neurons in spinalized rats

We report long-term potentiation (LTP) in single spinal wide dynamic range (WDR) neurons in urethane-anaesthetized spinalized rats with a complete neuromuscular blockade. Peripheral influences were excluded by a complete lidocaine block distal to the stimulation site on the sciatic nerve. As previously shown A-beta fibre evoked responses were not increased by the tetanic stimulation when there was a neuromuscular blockade during the experiment. Spinalization, excluding influences from supraspinal structures, increased all firing responses, and the LTP of C-fibre evoked responses when calculated in number of action potentials compared to intact animals and to previous studies. Furthermore, an LTP of the post discharge was observed after spinalization. An LTP of the post discharge has previously not been reported. Therefore, we conclude that LTP in the dorsal horn normally seems to be inhibited by descending pathways.

Uterine contractility and blood flow are reflexively regulated by cutaneous afferent stimulation in anesthetized rats

The effects of cutaneous mechanical afferent stimulation of various skin areas on uterine contractility and blood flow were examined in anesthetized non-pregnant rats. The contractility of the uterus was measured by the balloon method in the uterus. The uterine blood flow was measured by laser Doppler flowmetry. Noxious pinching stimulation of the perineum for 1 min induced an abrupt contraction of the uterus during stimulation. Pinching of a hindpaw or perineum and innocuous brushing of the perineum for 1 min increased uterine blood flow. Stimulation of other skin areas produced no changes in uterine contractility or blood flow. Most uterine responses were abolished by severance of the pelvic nerves, which innervated the uterus. The activity of pelvic parasympathetic efferent nerves to the uterus increased following perineal pinching. All these cutaneous stimulation-induced responses of uterine contractility, blood flow and pelvic efferent nerve activity still existed, and were even augmented, after acute spinalization. These results indicate that cutaneous mechanical sensory stimulation can regulate uterine contractility and blood flow by a segmental spinal reflex mechanism via uterine parasympathetic efferent nerves.

Descending modulation of opioid-containing nociceptive neurons in rats with peripheral inflammation and hyperalgesia

Inflammation and hyperalgesia induce a dramatic up-regulation of opioid messenger RNA and peptide levels in nociceptive neurons of the spinal dorsal horn. Descending axons modulate nociceptive transmission at the spinal level during inflammatory pain, and may play a role in the development of persistent pain. The role of descending bulbospinal pathways in opioid-containing nociceptive neurons was examined. Removal of descending inputs to the spinal cord was performed by complete spinal transection at the midthoracic level. Seven days after spinal transection, rats received a unilateral hindpaw injection of complete Freund's adjuvant, a noxious stimulus that produces inflammation and hyperalgesia. Tissues from the L4 and L5 segments of the spinal cord were removed and analysed by northern blotting and immunocytochemistry. Spinal transection resulted in a further increase in both dynorphin and enkephalin messenger RNA content following complete Freund's adjuvant injection. There was a similar distribution and number of dynorphin-immunoreactive cells in transected rats compared to rats which received sham surgery. These data suggest that increased dynorphin messenger RNA ipsilateral to inflammation, in rats without descending axons, was due to increased expression within the same cells and not to recruitment of additional dynorphin-expressing cells. This reflects a greater dynamic response of nociceptive neurons to noxious stimuli in the absence of descending modulation. Therefore, the net effect of descending afferents on spinal nociceptive circuits may be to reduce the response of opioid-containing neurons to noxious stimulation from the periphery.

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

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

Influence of spinalization on spinal withdrawal reflex responses varies depending on the submodality of the test stimulus and the experimental pathophysiological condition in the rat

The influence of midthoracic spinalization on thermally and mechanically induced spinal withdrawal reflex responses was studied in the rat. There were three experimental groups of rats: healthy controls, rats with a spinal nerve ligation-induced unilateral neuropathy, and rats with a carrageenan-induced inflammation of one hindpaw. Tail flick response was induced by radiant heat. Hindlimb withdrawal was induced by radiant heat, ice water, and innocuous or noxious mechanical stimulation of the paw. Prior to spinalization, spinal nerve ligated and carrageenan-treated animals had a marked unilateral allodynia and hyperalgesia. Spinalization tended to induce a facilitation of noxious heat-evoked reflexes. This spinalization-induced facilitation was stronger on tail than hindlimb withdrawal. Spinalization-induced skin temperature change did not explain the facilitation of noxious heat-evoked reflexes. In contrast, spinal withdrawal responses induced by noxious cold or mechanical stimulation were significantly suppressed following spinalization. The spinalization-induced facilitatory effects as well as inhibitory ones on spinal reflexes were enhanced in inflamed/neuropathic animals. The results indicate that the tonic descending control of spinal nocifensive responses varies depending on the submodality of the test stimulus, the segmental level of the reflex (tail vs. hindlimb), and on the pathophysiological condition.

Spinal 5-HT-receptors and tonic modulation of transmission through a withdrawal reflex pathway in the decerebrated rabbit

1. In decerebrated, non-spinalized rabbits, intrathecal administration of either of the selective 5-HT1A-receptor antagonists (S)WAY-100135 or WAY-100635 resulted in dose-dependent enhancement of the reflex responses of gastrocnemius motoneurones evoked by electrical stimulation of all myelinated afferents of the sural nerve. The approximate ED50 for WAY-100635 was 0.9 nmol and that for (S)WAY-100135 13 nmol. Intrathecal doses of the antagonists which caused maximal facilitation of reflexes in non-spinalized rabbits had no effect in spinalized preparations. 2. In non-spinalized animals, intravenous administration of (S)WAY-100135 was significantly less effective in enhancing reflexes than when it was given by the intrathecal route. 3. When given intrathecally, the selective 5-HT 2A/2C-receptor antagonist, ICI 170,809, produced a bellshaped dose-effect curve, augmenting reflexes at low doses (< or = 44 nmol), but reducing them at higher doses (982 nmol). Idazoxan, the selective alpha 2-adrenoceptor antagonist, was less effective in enhancing reflex responses when given intrathecally after ICI 170,809 compared to when it was given alone. Intravenous ICI 170,809 resulted only in enhancement of reflexes and the facilitatory effects of subsequent intrathecal administration of idazoxan were not compromised. 4. The selective 5-HT3-receptor blocker ondansetron faciliated gastrocnemius medialis reflex responses in a dose-related manner when given by either intrathecal or intravenous routes. This drug was slightly more potent when given i.v. and it did not alter the efficacy of subsequent intrathecal administration of idazoxan. 5. None of the antagonists had any consistent effects on arterial blood pressure or heart rate. 6. These data are consistent with the idea that, in the decrebrated rabbit, 5-HT released from descending axons has multiple roles in controlling transmission through the sural-gastrocnemius medialis reflex pathway. Thus, it appears 5-HT tonically inhibits transmission between sural nerve afferents and gastrocnemius motoneurones by an action at spinal 5-HT1A-receptors. Spinal 5-HT2A/2C-receptors may mediate a weak inhibition of transmission in the spinal cord, but more convincing evidence was obtained for their involvement in descending facilitatory tone. Further, some of the facilitatory consequences of spinal alpha 2-adrenoceptor blockade may be mediated through 5-HT2 type receptors. Spinal 5-HT3 receptors do not appear to have a major role in tonic modulation of the sural-gastrocnemius medialis reflex.

[Physiology of nociception]

Nociception is related to the mechanisms elicited by stimuli threatening the integrity of the organism. At the peripheral level, unmyelinated C fibres (C polymodal nociceptores) or fine myelinated A delta fibres are excited by noxious stimulation, directly or indirectly by inflammatory processes. Nociceptive afferent fibres terminate in the superficial laminae of the dorsal horn of the spinal cord where informations are integrated and controlled. These first synapses are modulated by excitatory amino acids (glutamate and aspartate) and many peptides (substance P, CGRP, CCK, endogenous opiods). The majority of ascending pathways involved in nociception are located in the ventrolateral controlateral quadrant of the cord (spinorelicular and spinothalamic tracts). Many supraspinal sites are activated following nociceptive stimuli, with relays in the reticular formation of the brain stem (including the subnucleus reticularis dorsalis), the ponto-mesencephalic regions (periaqueducal gray matter and parabrachial area) and thalamic sites. Amygdala and hypothamic targets could be involved in motivational reactions and neuroendocrine adaptations to a noxious event. The cingular, insular and somatosensory cortices also receive nociceptive informations. Nociceptive signals are modulated at all levels of their transmission; the more extensively studied controls are located at the spinal level. Segmental controls are inhibitory effects produced by non-noxious mechanical stimuli. Spinal signals can also be inhibited following activation of bulbopinal descending inhibitor pathways and release of serotonin, norepinephrine and, indirectly, endogenous opiods. Inhibitory controls triggered by noxious stimuli could facilitate the extraction of the nociceptive tone of informations having priority over other stimuli.

Potentiation of morphine-induced antinociception in acute spinal rats by the NMDA antagonist dextrorphan

Neurophysiologic and behavioral evidence indicates that excitatory amino acid (EAA) antagonists may provide a new class of selective analgesics for opiate resistant, neuropathic pain syndromes. Therapeutic applications have been limited because of unacceptable side effects of most EAA blockers. However, dextrorphan, a metabolite of the antitussive drug, dextromethorphan, is a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) subtype of EAA receptor with few side effects and a moderate analgesic effect in animals with peripheral neuropathy. It may therefore, have clinical benefit, either alone or in combination with opiates, for neuropathic pain. In this study a subeffective dose of dextrorphan (15 mg/kg) was combined with several doses of morphine (1.5, 3.0 and 6.0 mg/kg) and assessed in an animal model of central injury, the tail flick response of the acute spinal rat. At doses which were individually ineffective, the combination of dextrorphan and morphine (15 and 1.5 mg/kg, respectively) produced a significant antinociceptive response. The same dose of dextrorphan also increased the antinociceptive response to 3.0 and 6.0 mg/kg of morphine. Coadministration of low doses of an NMDA antagonist and an opiate, might have clinical benefit for the relief of pain with reduced risk of undesirable side effects.

Selective antinociceptive effect of excitatory amino acid antagonists in intact and acute spinal rats

Results of neurophysiologic and behavioral studies suggest that excitatory amino acid (EAA) antagonists may provide a new class of analgesic agents, which might be selective for neuropathic pain states that are resistant to opiate treatment. Most of these paradigms involve animal models of peripheral injury. The present study evaluated the antinociceptive effect of spinally [intrathecally (IT)] administered EAA antagonists after central injury, produced by spinal transection. Intrathecal injection of the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione produced dose-dependent antinociception on the thermal tail withdrawal [tail-flick (TF)] reflex test in Intact rats, which was significantly potentiated after spinal transection. In contrast, IT injection of the NMDA antagonist, 2-amino-5-phosphonopentanoic acid (AP5) did not affect the TF in intact rats, but significantly blocked this response in spinal rats. However, some of the spinal rats did not recover the reflex, suggesting a possible toxic action of AP5.

Review article: the hypersensitive gut--peripheral kappa agonists as a new pharmacological approach

Hypersensitivity to pain is a common component of functional bowel disorders. Hyperalgesia may be induced by various stimuli which produce a cocktail of inflammatory mediators that decrease the pain threshold. Drugs able to block these peripheral events within the gut may offer a new pharmacological approach for treating functional bowel disorders. Kappa opioids have been shown to inhibit somatic pain through a peripheral mechanism of action, acting directly on receptors located on peripheral sensory endings. They can block both the nociceptive messages as well as the release of sensory peptides. This paper reviews the effects of opioid agonists on gut visceral pain and motility anomalies induced by visceral pain. Kappa opioids have strong effects on all models tested, with a peripheral mechanism of action allowing the design of drugs acting only in the periphery and having no central nervous system side-effects. This contrasts with mu agonists which are centrally active on pain and worsen the subsequent transit and motility anomalies.

The effect of clonidine on intra-operative requirements of fentanyl during combined epidural/general anaesthesia

The study evaluates the analgesic effects of epidural clonidine in patients undergoing abdominal hysterectomy under combined epidural/general anaesthesia. Forty ASA 1-2 patients were divided into two groups who received epidurally either clonidine 300 micrograms (group 1) or placebo (group 2). Anaesthesia was maintained with oxygen/nitrous oxide, a midazolam infusion, vecuronium, and boluses of fentanyl 100 micrograms administered as needed to maintain cardiovascular stability. The mean (SD) intraoperative fentanyl requirements were 2.05 (0.18) and 3.66 (0.3) micrograms.kg-1.h-1 for groups 1 and 2 respectively (p < 0.001). Patients in Group 1 had a lower heart rate after tracheal intubation and surgical incision (p < 0.02). In the recovery room, pain intensity was lower in group 1 (p < 0.003) and the mean (SD) time until analgesia request was increased from 48.5 (8.4) min in group 2 to 235.7 (33.2) min in group 1 (p < 0.001). Our results demonstrate that epidural clonidine produces decreased fentanyl requirements, improved cardiovascular stability, reduced pain intensity and effective postoperative analgesia in the recovery room.

Intrathecal excitatory amino acid (EAA) agonists increase tail flick latencies (TFLs) of spinal rats

The facilitation of spinal nociceptive reflexes that occurs after spinal transection reveals the existence of descending, supraspinally mediated inhibition. Substantial evidence indicates that the excitatory amino acids (EAAs) are involved in these spinal circuits. Therefore, it was hypothesized that reflex facilitation in the spinal animal might be due to the removal of inhibitory input normally exerted on the spinal action of EAAs. If so, the facilitatory decrease in reflex latency, observed in the spinal preparation, might be potentiated by intrathecal (IT) administration of EAA agonists. This was tested by comparing the effect of IT injections of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) on the thermally elicited tail flick (TF) response of Intact and acute spinal rats. In intact rats, a low (0.25 nM) dose of NMDA produced a hyperalgesic decrease in latency, relative to saline, whereas higher doses produced an overall increase in latency. A large dose (0.5 microM) produced overt signs of toxicity (crippling, self-mutilation, and loss of the reflex). Only the highest (1.0 nM) dose of AMPA affected the response, resulting in a significant increase. After spinal transection, the hyperalgesic reaction to 0.25 nM of NMDA was absent, and latencies were significantly increased by 1.0 nM. The toxic reaction to 0.5 microM appeared to be potentiated. Tail flick responses to AMPA were also significantly increased in spinal rats. Contrary to the prediction, reflex latencies were significantly increased by these drugs after spinal transection. It was suggested that, although the spinal action of EAAs appears to be supraspinally modulated, this influence may be facilitatory rather than inhibitory.

Evidence of a role for N-methyl-D-aspartate (NMDA) receptors in the facilitation of tail withdrawal after spinal transection

Peripheral injury produces a characteristic excitation of spinal cord dorsal horn cells (wind-up) which is associated with a facilitation of spinal nociceptive reflexes (hyperalgesia). These phenomena are believed to be mediated by a trauma-induced increase in the release of excitatory amino acids (EAAs). A similar increase in the activity of dorsal horn neurons and spinal reflexes occurs after spinal transection. Therefore, the present studies examined the possibility that EAAs, acting through the NMDA receptor, might also be involved in behavioral hyperalgesia produced by central injury. The first experiment assessed the effect of pretreatment with the NMDA antagonist, ketamine, on the facilitated tail flick (TF) response of spinally transected rats. Separate groups of animals were spinalized under isoflurane anesthesia alone, intramuscular ketamine anesthesia alone, or a combination of isoflurane and intrathecal ketamine. The TF was examined 24 h later, before and 30 min after an intrathecal injection of morphine. In the second experiment, the effect of intraperitoneal or intrathecal ketamine on the TF was assessed to separate groups of rats that underwent spinal transection or sham surgery under isoflurane anesthesia. Pretreatment with either systemic or intrathecal ketamine did not alter TF facilitation or morphine-induced antinociception in spinal rats. However, both systemic and intrathecal ketamine significantly increased TF latencies in spinal, relative to intact rats. These results indicate that ketamine did not prevent the development of spinal reflex facilitation, but it selectively reduced this reaction once it was established in spinal rats. The data support an involvement of EAAs in reflex facilitation produced by spinal transection.

The spinal antinociceptive activity of the alpha 2-adrenoceptor agonist, xylazine in sheep

1. The intrathecal administration of xylazine (100 micrograms), via a chronic indwelling, cervical intrathecal catheter, produced a marked elevation of the mechanical nociceptive thresholds in the sheep. This antinociceptive effect was abolished by the prior intrathecal administration of the alpha 2-adrenoceptor antagonist, idazoxan. 2. The intrathecal administration of the selective alpha 2-antagonists, idazoxan (100 micrograms) and RX811059 (33 micrograms), significantly attenuated the antinociceptive activity of intravenous xylazine, with a 60-65% reduction in the area under the antinociceptive curve. The intrathecal administration of the antagonists alone had no significant effect on nociceptive thresholds. 3. Examination of the distribution of tritiated idazoxan (25 microCi in 100 microliters) indicated that the site of action of the drug was limited to the cervical spinal cord after intrathecal administration. 4. These studies demonstrate that a significant proportion of the antinociceptive effect of systemically administered xylazine is mediated by spinal alpha 2-adrenoceptors.

Role of cervical neurons in propriospinal inhibition of thoracic dorsal horn neurons

We previously reported that electrical or glutamate stimulation of the cervical spinal cord elicits a 40-60% decrease in renal sympathetic nerve activity (RSA) in the anesthetized rats. This sympatho-inhibition was possible, however, only after transection of the spinal cord at C1 or GABAergic inhibition of neurons in the rostral ventrolateral medulla. We postulated that cervical neurons inhibit RSA by inhibiting the activity of spinal interneurons that are antecedent to sympathetic preganglionic neurons (SPNs), and that these interneurons may be, in turn, excited by afferent signals. In this study, we tested the hypothesis that cervical neurons can inhibit visceroceptive thoracic spinal neurons. We recorded the spontaneous and evoked activity of 45 dorsal horn neurons responsive to splanchnic stimulation before, during, and after chemical or electrical stimulation of the cervical spinal cord in chloralose-anesthetized spinal rats. Cervical spinal stimulation that inhibited RSA also inhibited the spontaneous and/or evoked activity of 44 dorsal horn neurons. In addition to inhibiting splanchnic-evoked neuronal responses, cervical stimulation also inhibited responses, in the same neurons, evoked by noxious heat or light brushing of receptive dermatomes. We concluded that cervical neurons participate in propriospinal inhibition of afferent transmission and that this inhibitory system may be involved in controlling the access of afferent information to SPNs.

Prolonged potentiation of transmission through a withdrawal reflex pathway after noxious stimulation of the heel in the rabbit

The sural-gastrocnemius reflex of the spinalized rabbit was potentiated to an average of 3-6 times control levels after the application of noxious mechanical, thermal or chemical stimuli to the skin of the heel. Facilitation of the reflex was maximal within 1 min of the noxious stimulus, and in many cases persisted for more than 1 h. Prolonged increases in the excitability of the sural-gastrocnemius reflex were not seen after innocuous mechanical or thermal stimulation of the heel. Repetitive electrical stimulation of the sural nerve (100 shocks given at 0.5 Hz) caused persistent facilitation of the reflex when small myelinated A delta fibres or non-myelinated C-fibres were recruited by the conditioning stimulus. Such protracted increases in the excitability of the sural-gastrocnemius pathway would enhance the protective functions of this reflex. The mechanisms described here have probably evolved to provide a high level of reflex protection to the heel after tissue damage has occurred at that site.

Prolonged inhibition of a spinal reflex after intense stimulation of distant peripheral nerves in the decerebrated rabbit

1. In decerebrated rabbits, repetitive stimulation of the high-threshold afferents of the left common peroneal (CP) nerve evokes prolonged depression of the sural-gastrocnemius medialis (GM) reflex recorded in the same limb. This inhibition is antagonized by co-administration of the opioid antagonist naloxone with the alpha 2-adrenoceptor antagonist idazoxan. The present study was designed to investigate whether such inhibition could be elicited from the contralateral hindlimb or the forelimbs. 2. The sural-GM reflex of decerebrated rabbits was depressed for more than 15 min after stimulation of either ipsilateral or contralateral common peroneal (CP) or median nerves with 500 pulses of 20 V, 1 ms given at 5 Hz. The order of efficacy for generating this inhibition was ipsilateral CP greater than contralateral CP greater than or equal to ipsilateral median = contralateral median. In three of thirty-nine rabbits, stimulation of the median nerves caused facilitation of the sural-GM reflex. 3. Idazoxan (1-2 mg/kg I.V.) did not significantly alter the depressant effect of ipsilateral CP stimulation but reduced that evoked by either median nerve and almost abolished the inhibition evoked from the contralateral CP nerve. 4. Naloxone (0.25 mg/kg I.V.) reduced the effects of ipsilateral CP stimulation, did not alter the inhibition evoked from contralateral CP, and had equivocal actions on the responses to median nerve stimulation. 5. When given together, the two antagonists almost abolished the effects of stimulating the median nerves and the contralateral CP nerve, and markedly reduced the inhibition evoked from the ipsilateral CP nerve. 6. These data show that prolonged inhibition of the sural-GM reflex can be evoked by stimulation of nerves in all four limbs and that in each case the inhibition can be blocked or reduced by co-administration of antagonists to opioid and alpha 2-adrenergic receptors. Such persistent inhibition of reflexes may serve to inhibit withdrawal reflexes in situations where interruptions to normal movement would be disadvantageous.

Properties and ionic basis of the action potentials in the periaqueductal grey neurones of the guinea-pig

1. Action potentials of neurones of the ventral part of the guinea-pig periaqueductal grey (PAG) were studied by intracellular recording in a mesencephalic slice preparation maintained in vitro. 2. Fast spikes spontaneously fired last 2.8 +/- 0.6 ms (mean +/- S.D.) and have an amplitude of 72.3 +/- 5.3 mV (n = 28). The neurones could be antidromically activated from the neighbouring white matter and these spikes show an initial segment component that triggers the soma-dendritic spike. These two components were dissociated by hyperpolarization. Action potentials are Na+ dependent and a Ca2+ conductance is responsible for the hump on the falling phase. Hyperpolarization makes the hump disappear and a faster rate of rise and fall are seen. Accommodation of the firing threshold is observed in response to depolarizing ramps, which is eliminated with hyperpolarization. 3. High-threshold Ca2+ spikes are evoked in either Na(+)-free solution or in the presence of tetrodotoxin (TTX). These presumed dendritic action potentials display a fast repolarization and a large after-hyperpolarization (AHP) that prevent repetitive firing. This AHP is mainly generated by Ca(2+)-dependent K+ conductances. 4. The repolarization of fast action potentials depends on the activation of K+ conductances as well as a Na+ inactivation process. A fast-activated tetraethyl-ammonium (TEA)-sensitive K+ conductance, that could be Ca2+ dependent, and a K+ conductance blocked by apamin seem to be involved in the repolarization. 5. Each fast action potential is followed by a pronounced AHP with two components, an initial fast and a slow decaying phase. Membrane hyperpolarization around -60 mV eliminated the first component and the AHP acquired a plateau-like shape. At -90 mV the AHP was nullified. The slow phase was Ca2+ dependent and an apamin-sensitive K+ conductance is involved in its generation. This conductance may be active during the early part of the AHP, but a fast-activated TEA-sensitive K+ conductance and other voltage-dependent K+ conductances might also be present. A Ca2+ conductance is hypothesized to account for the fast depolarizing change after the AHP peak. 6. A delayed return to the baseline is observed after hyperpolarizing pulses. It is generated by the activation of a transient voltage-dependent K+ conductance that is inactive at resting membrane potential (RMP, around -50 mV). This transient hyperpolarization is abolished by Ca2+ channel blockers and insensitive to high external concentrations of 4-aminopyridine, TEA and Cs+.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of intracerebroventricular injection of naloxone, phentolamine and methysergide on the transmission of nociceptive signals in rat dorsal horn neurons with convergent cutaneous-deep input

In anaesthetized rats, recordings were made from nociceptive dorsal horn neurons with convergent input from the skin and deep somatic tissues. The results of a previous study have shown that in these neurons the input from deep nociceptors is subjected to a much stronger tonic descending inhibition than is the input from cutaneous nociceptors. The aim of the present study was to find out whether at supraspinal levels opioidergic, adrenergic, or serotoninergic transmitters are involved in this quite specific inhibition of deep nociception. Injections of naloxone, phentolamine, and methysergide into the third ventricle showed that only naloxone is capable of abolishing the tonic inhibition of the deep nociceptive input to spinal neurons. The input from cutaneous nociceptors to the same cells was largely unaffected by naloxone. Thus the effects of intracerebroventricular injection of naloxone resembled those obtained with a spinal cold block in a previous study; with the exception that the increase in background activity--which is prominent during cold block--was missing after the injection of naloxone. The present results demonstrate that the tonic descending inhibition of the deep nociception operates with opioidergic synapses at the supraspinal level. In contrast, supraspinal adrenergic and serotoninergic mechanisms do not appear to contribute to the tonic inhibition. The data confirm and extend previous results which suggested that a particular portion of the descending antinociceptive system may act mainly on the input from deep nociceptors. Pharmacologically, this particular portion seems to be opioidergic in nature.

Activation by high intensity peripheral nerve stimulation of adrenergic and opioidergic inhibition of a spinal reflex in the decerebrated rabbit

The short-latency sural to gastrocnemius reflex in the decerebrated rabbit was depressed for 20-30 min following high intensity conditioning stimulation of the common peroneal nerve. This effect was observed in animals with or without spinal section, but was greater in non-spinalized preparations. Graded conditioning stimuli showed that it was necessary to activate fine myelinated common peroneal axons to inhibit the reflex. In spinalized rabbits, maximal inhibition was achieved with conditioning stimulation of fine myelinated axons and was completely reversed by the opioid antagonist naloxone. In non-spinalized rabbits, maximal inhibition was only obtained with conditioning stimuli which activated non-myelinated axons. In these preparations the effects of common peroneal nerve stimuli were only blocked by co-administration of naloxone with the alpha 2-adrenoceptor antagonist idazoxan. Thus high intensity peripheral nerve stimuli activated a segmental opioidergic and a supraspinal adrenergic suppression of the sural-gastrocnemius withdrawal reflex. Such long-lasting suppression of reflex excitability may contribute to recovery from intensely noxious stimuli.