Preparative surgery enhances the direct spinal actions of three injectable anaesthetics in the anaesthetized rat

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

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

Selective inhibition from the anterior hypothalamus of C- versus A-fibre mediated spinal nociception

Modulation of spinal nociception from the anterior hypothalamus/preoptic area (AH/POA), and consequent alterations in the pain experience may contribute to integrated responses brought into play during fear or stress and as part of the sickness response. This study was designed to compare the effects of descending control from AH/POA on A- versus C-fibre-evoked spinal nociception, since any differential control is of behavioural and clinical importance given that A-fibre and C-fibre nociceptors convey different qualities of the pain signal (first and second pain, respectively), and play different roles in the development and maintenance of chronic pain states. In anaesthetised rats, electromyographic responses were recorded to monitor thresholds of withdrawal to slow (2.5 degrees Cs(-1)) or fast (7.5 degrees Cs(-1)) rates of skin heating of the hindpaw, to preferentially activate C- or A-nociceptors, respectively. Neuronal activation by microinjection of dl-homocysteic acid at sites within a specific region of AH/POA, lateral area of the anterior hypothalamus (LAAH), significantly increased response thresholds to slow heating rates (p<0.02, n=11), but not those to fast rates of heating (p=0.48, n=10). Injection of DLH adjacent to LAAH (n=9) had no significant effect on responses to slow (n=8) or fast (n=9) rates of skin heating. The functional significance of differential descending control of spinal processing of C- and A-nociceptive inputs is discussed with respect to roles both of the LAAH in pain processing, and of C- and A-nociceptive inputs in acute and chronic pain.

A reliable method for the preferential activation of C- or A-fibre heat nociceptors

There is strong evidence that A- and C-fibre nociceptors evoke significantly different sensory experiences, are differentially sensitive to pharmacological intervention, and play different roles in pain pathology. It is therefore of considerable interest to be able to selectively activate one fibre type or the other in studies of nociceptive processing. Here, we report significant modifications to a non-invasive technique, first described by Yeomans et al. [Pain 59 (1994) 85; Pain 68 (1996) 141; Pain 68 (1996) 133], which uses different rates of skin heating to preferentially activate A- or C-nociceptors. A copper disk (diameter: 4mm) was used to transfer heat evenly across the dorsal surface of the rat hindpaw. Initial experiments established the relationship between the temperature at the skin surface and the sub-epidermal temperature. Subsequently, the vanilloid capsaicin, which sensitises unmyelinated C-mechanoheat nociceptors, was shown to decrease the thresholds of reflex responses evoked by slow rates of heating. In contrast thresholds of responses to fast rates of skin heating were unchanged, indicating that nociceptors activated by this stimulus were capsaicin-insensitive A-fibre heat nociceptors.

The effect of spinal analgesia on visceral nociceptive neurons in caudal medulla of the rat

A population of neurons resident in the caudal ventrolateral medulla are excited by noxious cutaneous and visceral stimuli from large portions of the body. These neurons act as monitors of ascending nociceptive information, and we hypothesized that they would be inhibited by spinally administered analgesics in a clinically relevant fashion. Rats were anesthetized with oxygen/ halothane. The caudal medulla was surgically exposed, and a catheter placed into the intrathecal space overlying the lower thoracic spinal cord via the surgical site. Single medullary neurons were characterized for responses to cutaneous and visceral (colorectal distension) stimuli. The effects of i.v. and intrathecally administered morphine and lidocaine were determined. The intrathecal infusion of morphine for 6 days before testing was also used as a pretreatment. Colorectal distension-evoked responses of medullary nociceptive neurons were inhibited in a dose-dependent, naloxonereversible fashion by intrathecal and i.v. morphine (50% effective dose values: 3.5 and 440 microg/kg, respectively). Intrathecal lidocaine abolished responses to colorectal distension and produced a spinal level at doses producing minimal effects when administered systemically. Prior treatment with an infusion of morphine produced tolerance to the effects of subsequent intrathecal morphine administration. These findings support the use of this preparation as a neurophysiologic model of spinal analgesia.

IMPLICATIONS

Neurons in the brainstem, isolated electrophysiologically, were used as whole body monitors of pain-related activity in the rat. As a neurophysiologic model of nociception, this preparation may prove useful for the study of regionally administered analgesics and local anesthetics.

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.

Prostanoids synthesized by cyclo-oxygenase isoforms in rat spinal cord and their contribution to the development of neuronal hyperexcitability

1. The responses of wide dynamic range spinal dorsal horn neurones to noxious mechanical stimulation of the ankle or knee joint were tested before and after spinal administration of the non-selective cyclooxygenase (COX) inhibitors, indomethacin and meclofenamic acid. Neither of these drugs altered the responses of these neurones to noxious mechanical stimulation. 2. Wind-up of a spinal nociceptive reflex evoked by electrical stimulation of the sural nerve at C-fibre strength was dose-dependently inhibited by intravenous administration of indomethacin, a non-selective COX inhibitor, and SC58125, a selective COX-2 inhibitor. Intrathecal administration of indomethacin also reduced the wind-up of this nociceptive reflex. 3. Western blot analysis of proteins extracted from normal rat spinal cord revealed the presence of both cyclo-oxygenase (COX)-1 and COX-2 proteins. 4. Immunocytochemistry of sections of normal rat spinal cord with specific COX-1 antiserum revealed little specific COX-1-like immunoreactivity in the grey matter. With the same antiserum, intense COX-1-like immunoreactivity was observed in the cytoplasm, nuclear membrane and axonal processes of small to medium sized (< 1000 microns2) dorsal root ganglion (DRG) cell bodies. 5. Immunocytochemistry of sections of normal rat spinal cord incubated with specific COX-2 antiserum showed intense COX-2-like immunoreactivity (COX-2-li) in the superficial dorsal horn of the spinal cord (laminae I and II) and around the central canal (lamina X). COX-2-li was also observed in some neurones in deep dorsal horn and in individual motor neurones in ventral horn. COX-2-li was not observed in the cell bodies of DRG. 6. Superfusion of the lumbar spinal cord of normal rats with artificial CSF and subsequent radioimmunoassay revealed the presence of prostaglandin D2 (PGD2) < PGE2, but not PGI2 (determined by measurement of the stable metabolite, 6-keto-PGF1 alpha) or PGF2 alpha. 7. These data suggest that eicosanoids synthesized by an active COX pathway in the spinal cord of normal animals may contribute to nociceptive processing, but only when the spinal cord neurones are rendered hyperexcitable following C-fibre stimulation. Selective inhibition of one or both of the COX isoforms in normal animals may represent a novel target for spinal analgesia.

Potentiation of fentanyl suppression of the jaw-opening reflex by transcranial electrical stimulation

Stinus et al. [L. Stinus, M. Auriacombe, J. Tignol, A. Limoge, M. Le Moal, Transcranial electrical stimulation with high frequency intermittent current (Limoge's) potentiates opiate-induced analgesia: blind studies, Pain, 42 (1990) 351-363.] observed that transcranial electrical stimulation (TCES) with high-frequency intermittent current potentiated opiate-induced analgesia using the tail-flick test. In unanesthetized, chronic preparations, electrical stimulation (0.5 Hz) of the lower incisor pulp of rats elicits a short-(6 ms) and a long-latency (12-18 ms) jaw-opening reflex (JOR) without any evidence of aversive behavior [J. Azerad, F. Fuentes, I. Lendais, A. Limoge, B. Pollin, Methods for selective tooth pulp stimulation in acute and chronic preparations in rats, J. Physiol., 406 (1988) 3P.]. Fentanyl increases thresholds of both reflexes and transiently suppresses the long-latency JOR. We then decided to look at the influence of TCES on both drug-induced mean of maximal threshold variation (MMTV) and duration of JOR suppression period. These parameters have been investigated in 43 Wistar rats with or without TCES administered for 3 h before the drug injection and throughout the testing period. TCES alone has no effect. In contrast, it significantly increases the duration of the reflex suppression period (149 +/- 5% vs. control, P < 0.001) while fentanyl-increased reflex thresholds remain unchanged. The fentanyl-induced JOR suppression period returns to the control values 2 days later. When a second 3-h TCES session is delivered 2 or 4 days after the first TCES session, a similar increase of this suppression period is observed. Moreover, 2 days after a second TCES session, an increase of the duration of the fentanyl-induced JOR suppression period is systematically observed. In contrast, a 6-h TCES session never induces such effects. These results confirm a potentiating effect of TCES on opioid action and demonstrate the value of repeated TCES sessions.

NMDA-receptor contribution to spinal nociceptive reflexes: influence of stimulus parameters and of preparatory surgery

The contribution of NMDA receptors to nociceptive reflexes has been assessed both in awake rats and in electrophysiological tests on alpha-chloralose anaesthetized spinalized rats prepared with different degrees of surgery. Single motor unit activity was recorded in response to alternating noxious mechanical and electrical stimuli applied to one hindpaw, and the results compared with paw pressure withdrawal reflexes in awake rats. There was little contribution by NMDA receptors to nociceptive paw pinch responses either in awake rats or in rats prepared with minimal surgery, but following extensive lumbar surgery the contribution increased significantly to a level similar to that seen in the wind-up component of responses elicited by electrical stimulation. Surgery therefore has effects several segments from the sensory input that it generates. It enhances the NMDA receptor contribution in responses to some but not all types of afferent input.

Tachykininergic tone in the spinal cord of the rabbit: dependence on nociceptive input arising from invasive surgery

In the decerebrated and spinalized rabbit, reflexes evoked in the gastrocnemius medialis muscle nerve by electrical stimulation of teh sural nerve are suppressed after blockade of NK-tachykinin receptors. This observation suggests that endogenous tachykinins tonically enhance transmission between sural nerve afferents and gastrocnemius motoneurons. In the present study we have investigated some possible sources of this tachykininergic tone. Electrical stimulation of the sural nerve at 1 Hz, as used in our previous investigation, leads to increased gastrocnemius reflex responses with successive stimuli. We examined reflexes evoked by pairs of sural stimuli separated by intervals of 10-1000 ms, and found that responses to the second stimuli of such pairs were significantly enhanced at intervals from 50 to 500 ms. Treatment with the NK receptor antagonist CP-99,994 [(+)-(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine; 1 mg/kg, i.v.] reduced gastrocnemius reflex responses per se, but did not alter the facilitatory effects of pairing sural stimuli. Subsequent treatment with the glutamate N-methyl-D-aspartate receptor blocker dizocilpine (0.5 mg/kg, i.v.) further reduced reflex responses and abolished paired-pulse facilitation. In a second set of experiments, rabbits were prepared so that reflexes could be studied with minimal surgical preparation of the hindlimb. Reflex responses recorded in this way were enhanced by treatment with CP-99,994 (up to 1 mg/kg, i.v.). Subsequent administration of the opioid receptor antagonist naloxone (1-10 microg/kg, i.v.) increased reflexes, as seen previously in surgically-prepared animals. These data show that tachkininergic modulation of spinal reflexes in the rabbit results from the nociceptive input arising from surgical preparation of the leg. In contrast, tonic opioidergic inhibition of reflexes is not substantially dependent on such input.

The spinal loop dialysis catheter: characterization of use in the unanesthetized rat

To permit long-term measurement of time-dependent changes in levels of dialyzable drugs and transmitters in the spinal intrathecal (i.t.) space of the unanesthetized rat, we developed a dialysis catheter for chronic placement. This was accomplished by constructing a loop probe 9 cm in length from 0.3-mm-diameter dialysis tubing that was made impermeable except for the distal loop. This loop catheter was readily inserted though an incision in the cisternal membrane and passed to the lumbar enlargement. The ends of the catheter were then externalized on the top of the head. To permit i.t. injections, an additional i.t. catheter could also be inserted simultaneously by the same route. For dialysis, an external end of the loop catheter was connected to a syringe pump and perfused with artificial CSF (10 microliters/min) and the out flow collected. A series of studies were performed to demonstrate the characteristics and utility of this technique. (1) Stability of resting release: glutamate and glucose concentrations in spinal dialysate showed no significant changes from 3 to 10 days after implantation. (2) Spinal cord ischemia: ischemia induced by aortic occlusion or cardiac arrest evoked a time dependent increase in retrieved glutamate. (3) Spinal cord compression caused a time-dependent glutamate, aspartate and PGE2 increase. (4) Noxious afferent stimulation induced by the injection of formalin into the hindpaw resulted in a rapid and transient increase in dialysate glutamate concentration. (5) Direct activation of spinal excitatory amino acids receptors by i.t. injection of kainic acid (1 microgram) evoked a significant increase in aspartate and taurine. (6) Continuous delivery of spinal opiate (alfentanil) via dialysis resulted in a maintained, concentration dependent elevation in the thermal escape latencies in the unanesthetized rat. The loop dialysis catheter provides a robust experimental tool for studying time dependent changes in the concentration of diffusible substances in spinal CSF over an extended post-implantation interval and allows comparison of these changes with concurrently assessed behavioral indices.

Effects of morphine on visceral nociception evoked by colorectal distension in rats: comparative examinations of electrophysiological and behavioral responses

The purpose of this study was to compare the effects of intravenously administered morphine on electrophysiological and behavioral responses to colorectal distension (CRD) and to examine the influence of noxious stimuli applied to another part of the body (a laminectomy) on the visceromotor response to CRD. The effects of morphine (0.1-6.4 mg·kg^-1) were examined in rate anesthetized with pentobarbital. Electrophysiological (n=16) and behavioral experiments (n=47) were done. Electrophysiological experiments were conducted to examine the effects of morphine on the responses of visceral dorsal horn neurons to CRD; behavioral studies were conducted to compare the effects of morphine with and without a laminectomy (intact group:n=24; laminectomy group:n=23). Morphine suppressed the evoked activities of the visceral dorsal horn neurons in a dose-dependent manner. Similar suppression of the behavioral visceromotor response was observed. Visceromotor thresholds were significantly lower in the intact group than in the laminectomy group during the control study. When morphine was administered, the visceromotor thresholds in both groups increased to a similar level. Behavioral and neurophysiological responses to CRD were suppressed in a similar fashion by morphine. Although laminectomy affected the threshold values of CRD for visceromotor responses, the laminectomy per se plays an insignificant role when adequate morphine is administered.

Endogenous modulation of excitatory amino acid responsiveness by tachykinin NK1 and NK2 receptors in the rat spinal cord

1. The effects of selective tachykinin (neurokinin, NK) NK1 and NK2 receptor antagonists have been examined on spinal neurones in alpha-chloralose anaesthetized, spinalized rats. They were tested for effects on responses both to excitatory amino acids (EAA) and to noxious heat stimuli. They were also tested for their ability to reverse the actions of selective NK agonists. 2. The NK1-selective antagonists GR82334 (peptide) and CP-99,994 (non-peptide), when applied by microiontophoresis, both reduced responses to kainate > AMPA > NMDA. Intravenous CP-99,994 (3 mg kg-1) also reduced responses to kainate but had inconsistent effects on nociceptive responses. 3. GR82334, applied microiontophoretically, reduced the enhancement by the selective NK1 agonist, GR73632 of both responses to EAAs and background activity. Systemic CP-99,994 (< or = 10 mg kg-1) failed to reverse the effects of GR73632. 4. The selective peptide NK2 antagonist, GR103537, had no consistent effects on responses to EAAs when applied by iontophoresis. In contrast, the non-peptide NK2 antagonist, GR159897, administered systemically (0.5-2 mg kg-1, i.v.) enhanced responses to kainate (but not NMDA); responses to noxious heat were enhanced only weakly. 5. Iontophoretically-administered GR103537 attenuated the effects of the NK2 agonist GR64349, which selectively reduced responses to kainate compared to those to NMDA. Systemically administered GR159897 (< or = 2 mg kg-1, i.v.) caused little antagonism of the effects of GR64349. 6. The data indicate that under these conditions the non-peptide antagonists are not reliable at reversing the actions of selective NK agonists. 7. These results suggest that there is a tonic release of endogenous tachykinins that can modulate glutamatergic neurotransmission in the spinal cord. They provide further support for the hypothesis that release of endogenous NKs acting on NK1 and NK2 receptors can promote NMDA receptor mediated glutamatergic transmission.

The dominant class of somatosensory neurone recorded in the spinal dorsal horn of awake sheep has wide dynamic range properties

In order to investigate the properties of dorsal horn neurones in the absence of the distorting influences of anaesthesia, preparative surgery, prior training or excessive restraint, recordings have been made in sheep chronically prepared for single-cell recording. Within the limitations of sampling error of dorsal horn neurones with cutaneous receptive fields, the cell type most frequently encountered had wide dynamic range (WDR; convergent; multireceptive) properties; these accounted for 59% of the 46 neurones that were examined in detail. High-threshold mechanoreceptive (nocispecific) and low-threshold mechanoreceptive neurones formed 11% and 30% of the sample, respectively. These and other data indicate that under normal physiological conditions in the awake state, many spinal neurones do indeed have WDR properties, implying that these cells have an important function in nociceptive processing.

Role of WDR neurons in a hind limb noxious heat evoked flexion withdrawal reflex

Behavioral experiments and neurophysiological experiments, the two major types of preclinical studies which have paved the way for the development of spinal analgesia were compared under identical conditions utilizing the same animals. The results demonstrate that the activation of the wide dynamic range (WDR) neurons preceded the behavioral withdrawal reflexes, and that the activation of the WDR neurons occurred at lower stimulus temperature than that for the withdrawal reflex. The results suggest that the neuronal activation began before the behavioral reflex but also that the firing frequency of the WDR neurons at the time of the withdrawal reflex could not distinguish between non-noxious and noxious stimuli. Further study is needed to elucidate the neuronal mechanisms of the activation of the behavioral reflex.

Stimulus intensity and the comparative efficacy of mu- and kappa-opioid agonists on nociceptive spinal reflexes in the rat

The influence of stimulus intensity was tested on the relative spinal efficacy of intravenously administered mu- (fentanyl) and kappa-opioid (U-50,488) agonists. Spinal reflexes were generated by different intensities of noxious electrical stimuli in alpha-chloralose anaesthetized, spinalized rats. Both drugs became less effective as the intensity of C-fibre generated responses was increased, but U-50,488 retained the ability to reduce responses to the same degree as fentanyl. The effects were naloxone reversible. The results indicate that kappa-opioid receptor activation has similar potential for spinal analgesia as does activation of mu-opioid receptors.

Evidence for central summation of C and A delta nociceptive activity in man

Using two different stimulators, we have induced activity in A delta and C afferents in order to investigate a possible summation of nociceptive activity from these two fiber types. We used nociceptive electrical stimulation to evoke activity in A delta fibers. High-intensity light from a xenon lamp, focused into a liquid light guide which was positioned on a spot painted black under the sole of the foot, resulted in a characteristic delayed burning sensation, indicating selective C-fiber activation. By varying the delay between radiant heat and electrical stimuli (0-3000 msec), sensations evoked by these stimuli were brought to coincide. When we elicited the electrical stimulation during on-going burning pain, corresponding to a delay of approximately 1 sec between the stimulations, we found a significantly higher nociceptive withdrawal reflex in tibialis anterior (P < 0.01) and a higher overall pain rating (P < 0.05). The existence of a summation mechanism at the spinal cord is the most likely explanation for our findings. Furthermore, the results demonstrate that the nociceptive reflex may be modulated by on-going C-fiber activity.

The role of nitric oxide in spinal nociceptive reflexes in rats with neurogenic and non-neurogenic peripheral inflammation

This in vivo electrophysiological study concerns the role of nitric oxide (NO) in mechanical and thermal spinal nociceptive reflexes in alpha-chloralose anaesthetized rats. The effects of the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME; 5-40 mg/kg i.v.) on reflexes were compared both in normal rats and in those with peripheral inflammation induced neurogenically (mustard oil) and non-neurogenically (carrageenan). Methoxamine (0.1 mg/kg i.v.) was used to mimic the marked hypertension caused by L-NAME. Thermal nociceptive reflexes were equally reduced by methoxamine and L-NAME in both normal and inflamed rats, implying that NO has no role in mediating thermal reflexes. However, L- (but not D-) NAME dose dependently and significantly inhibited mechanical reflexes in both carrageenan inflamed (to 37 +/- 12% control) and mustard oil inflamed rats (to 75 +/- 8% control). Moreover, these reductions were greater than those by methoxamine. In contrast, L-NAME did not reduce mechanical reflexes in rats with no inflammation or in spinalized rats with inflammation. The inhibition of mechanical reflexes with L-NAME in carrageenan inflamed rats was reversed and prevented by pre- or post-treatment with L- (but not D-) arginine (50-200 mg/kg i.v.). These data imply a supraspinal role for NO in mediating mechanical (but not thermal) nociceptive reflexes only in those rats with peripheral inflammation.

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.

Functional evidence for multiple receptor activation by kappa-ligands in the inhibition of spinal nociceptive reflexes in the rat

1. The evidence for kappa-receptor heterogeneity is equivocal. We have now investigated this question by comparing the effects of five putatively selective kappa-agonists. The parameters examined were: the relative potencies in depressing hindlimb flexor muscle reflexes to noxious pinch stimuli in both spinalized and sham-spinalized rats; the reversibility of these effects by naloxone; and the effects on blood pressure. 2. Two types of drug effect was discriminated. One drug group, represented by U-50,488, U-69,593 and PD-117,302, had a potency ratio between sham and spinalized rats approximately 10 fold lower than the other group, which comprised GR103545 and CI-977. 3. Under sham-spinalized conditions, CI-977 and GR103545 at high doses caused only sub-maximal reductions of spinal reflexes. U-50,488 was still active when superimposed on these high doses of GR103545. 4. Naloxone reversed all effects, but different doses were required between compounds, with GR103545 taking some 20 times higher doses of naloxone to cause reversal than did U-50,488. 5. The effects on mean arterial pressure were opposite between groups. 6. The results imply that more than one type of naloxone-sensitive non-mu opioid receptor must be involved in mediating these complex actions of ligands that have been claimed to be selective for kappa-receptors.

A sensitive test for studying the effects of opioids on a C-fibre reflex elicited by a wide range of stimulus intensities in the rat

A C-fibre reflex elicited by electrical stimulation within the receptive field of the sural nerve was recorded from the ipsilateral biceps femoris muscle in anaesthetized rats. Recruitment curves were built by varying the stimulus intensity from 0 to 50 mA and temporal evolution was studied by using a constant level of stimulation. At a constant level of stimulation, intrathecal administration of morphine resulted in a depressive effect on the C-fibre reflex in the 0.18-0.75 microgram range (ED50 = 0.2 microgram). Study of the recruitment curves showed that, in the 0.18-0.375 microgram range, morphine had little effect on the threshold, but induced significant decreases in the slopes. At doses above 0.75 microgram, morphine modified both the threshold and the slope of the recruitment curves. Systemic naloxone totally reversed these effects. It is concluded that intrathecal morphine not only produces a shift in the encoding functions of the spinal cord but also reduces the gain of these functions. It is suggested that this method is reliable for the pharmacological study of the spinal transmission of nociceptive signals.

A technique for recording from spinal neurones in awake sheep

A technique is described for implanting a chamber on 1 or 2 vertebrae of the spinal column of the sheep. This chamber protrudes permanently through the dorsal skin of the back and is covered by a light bandage. Between recording sessions the chamber houses an inner cap sealing the hole that gives access to the cord. During recording sessions this cap is removed and a miniature manipulator inserted instead. This manipulator can accept a motor drive that holds a glass-coated tungsten microelectrode. The drive has a hole through which an arthroscope tube can be passed so that insertion of the electrode can be performed under visual control. Extracellular recordings have been made of single spinal neurones for up to 4 h in animals that are drug-free, untrained and only lightly restrained. Recording sessions can be repeated on a daily basis for several weeks until the dura and/or arachnoid becomes too thickened to permit electrode penetrations. Animals remain healthy and their behaviour remains normal throughout this time.

The effects of sham and full spinalization on the systemic potency of mu- and kappa-opioids on spinal nociceptive reflexes in rats

1. Flexor withdrawal reflexes to noxious mechanical pinch stimuli were recorded as single motor unit activity in alpha-chloralose anaesthetized rats, by means of tungsten bipolar electrodes inserted percutaneously into hindlimb flexor muscles. The relative spinal and supraspinal contributions to mu- and kappa-opioid agonists in inhibiting these spinal reflexes, together with possible potency changes elicited by surgical trauma, were examined by comparing their relative potencies in spinally unoperated, sham spinalized and spinalized rats. 2. The noxious stimuli, which were of comparable intensity in the three groups, elicited similar mean firing rates of the motor units in all groups. This indicates that the excitability levels in the reflex pathway were not greatly affected by either sham or actual spinalization. 3. The mu-agonists morphine and fentanyl, and the kappa-agonist U-50,488H, inhibited the reflexes in a dose-dependent manner, when administered intravenously in a log2 cumulative dose regime. 4. The surgery of sham spinalization had little effect on the potency of morphine and fentanyl, whereas it doubled the potency of U-50,488. 5. Spinalization did not affect the potency of morphine. In contrast it decreased the potency of fentanyl 2-4 fold and that of U-50,488 approximately 6 fold. 6. The effects of all agonists were antagonized by naloxone. Dose-dependence studies indicating that antagonism of U-50,488H required about 5 times the dose of naloxone that antagonized morphine. 7. The data suggest that surgical trauma to the spinal column and/or dura mater triggers supraspinal mechanisms that significantly enhance the potency of kappa- but not mu-agonists. 8. It is concluded that most of the effects of systemic morphine on spinal reflexes are mediated, under all three conditions tested, by direct effects in the spinal cord. In contrast, the inhibition of reflexes by U-50,488H is mediated at both spinal and supraspinal levels, the latter being enhanced in the presence of surgical trauma. The differences between morphine and fentanyl remain unexplained.