Differences in opioidergic inhibition of spinal reflexes and Fos expression evoked by mechanical and chemical noxious stimuli in the decerebrated rabbit

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.

The organization of motor responses to noxious stimuli

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

Endogenous opioids support the spinal inhibitory action of an alpha 2-adrenoceptor agonist in the decerebrated, spinalised rabbit

The present study examined the possible contribution of endogenous opioids to inhibition of spinal reflexes by an alpha(2)-adrenoceptor agonist. In rabbits decerebrated and spinalised under halothane/nitrous oxide anaesthesia, the selective alpha(2)-adrenoceptor agonist dexmedetomidine (3-30 microg intrathecal) induced significant decreases in short- and long-latency reflex responses evoked in medial gastrocnemius (MG) motoneurones by stimulation of the sural nerve. After recovery from dexmedetomidine, the mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA; 100 microg intrathecal) significantly enhanced short-latency but not long-latency MG reflex responses. After beta-FNA, inhibition of all reflexes by dexmedetomidine was significantly weaker than in the control state, whereas the cardiovascular actions of dexmedetomidine were unaffected. These data confirm that activation of spinal alpha(2)-adrenoceptors depresses MG reflexes evoked by all groups of sural nerve afferent fibres, and shows that endogenous opioid tone supports the inhibitory action of alpha(2) agonists, possibly by a synergistic interaction in the spinal cord.

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.

The spatial organization of central sensitization of hind limb flexor reflexes in the decerebrated, spinalized rabbit

This study was designed to investigate the relationship between the location of a noxious stimulus and the magnitude and duration of the plastic effects induced by that stimulus in withdrawal reflexes acting about the knee and the ankle, in rabbits. Reflexes were evoked in the nerves to the anterior tibial and semitendinosus flexor muscles by electrical stimulation at the toes. Repetitive, high intensity electrical stimulation of nerve trunks (sural, medial gastrocnemius, superficial peroneal, tibial, 100 pulses, 20 V, 1 ms at 0.5 Hz) was generally found to be a poor method for inducing central sensitization in these flexor reflexes. 'Natural' noxious stimulation induced more reliable enhancement of both reflexes. Mechanical (clamp) or chemical (mustard oil) stimulation of the heel induced prolonged (median duration >30 min) increases in reflexes to both muscles. Mechanical (clamp and superficial pinch) or chemical (mustard oil) stimulation of the toes, and injection of bradykinin into the gastrocnemius muscles or into the soft tissues of the sole of the foot, also led to enhancement of both reflexes, with the median duration of potentiation between 7 and 30 min. The effects obtained from deep tissue stimulation were generally weaker than those obtained after stimulation of superficial structures. These data show that there were no major differences in the effects obtained from the heel vs the toes, or between the two reflexes. It appears that the spatial organization of the spinal mechanisms underlying central sensitization of flexor withdrawal reflexes is rather more crudely drawn than that pertaining to the reflexes themselves. Furthermore, the data indicate that in the present preparation, afferents from deep tissues are no more effective in generating central sensitization than those from superficial structures.

Opioid and GABA receptors involved in mediation and modulation of tonic and stimulus-evoked inhibition of a spinal reflex in the decerebrated and spinalized rabbit

The purposes of this study were to investigate: (i) the identity of the opioid peptide(s) mediating tonic and stimulus-evoked inhibition of the sural-medial gastrocnemius reflex of the decerebrated, spinalized rabbit and (ii) the modulation of these processes by endogenous GABA. The selective delta receptor antagonist naltrindole (100 nmol kg(-1) i.v.), the GABA(A) blocker bicuculline (300 nmol intrathecal, i.th.), and the GABA(B) antagonist CGP 35348 (1 micromol i.th.) increased gastrocnemius reflexes to 150-160% of pre-drug values, whereas a sub-maximal dose of naloxone (30 nmol kg(-1) i.v.) augmented reflexes to >500% of controls. Kelatorphan, an inhibitor of enkephalin metabolism (2 micromol i.th.), depressed gastrocnemius responses by 50% and potentiated the inhibitory effects of methionine enkephalin. Repetitive electrical stimulation of the superficial or common peroneal nerves inhibited reflexes for 15-20 min. This effect was significantly reduced by naltrindole and CGP 35348. It was not reduced by a low dose (30 nmol kg(-1) i.v.) of naloxone or by bicuculline. When naloxone and naltrindole were combined at 30 nmol kg(-1) each, stimulus-evoked inhibition was blocked. Given after bicuculline, naloxone at 100 nmol kg(-1) i.v. abolished peroneal-evoked inhibition, but a dose of 300 nmol kg(-1) was required to produce the same effect after CGP 35348. Kelatorphan augmented the depth and duration of inhibition evoked by peroneal nerve stimulation. These data are consistent with the involvement of enkephalin-like peptides in tonic and stimulus-evoked inhibition of the sural-gastrocnemius reflex. Tonic inhibition in rabbit spinal cord is dominated by opioids acting through mu receptors, whereas co-activation of delta, mu and GABA(B) receptors mediates stimulus-evoked inhibition. It is possible that GABA(B) receptors inhibit the release of spinal opioids while simultaneously supporting their actions at post-synaptic targets.

Adaptive changes in withdrawal reflexes after noxious stimulation at the heel and the toes in the decerebrated rabbit

In decerebrated rabbits, reflexes evoked by electrical stimulation of the toes in the ankle flexor tibialis anterior were enhanced for > 30 min after application of 20% mustard oil to the base of the toes, whereas responses of the ankle extensor medial gastrocnemius to stimulation of the heel were depressed for > 20 min by the same stimulus. Applied to the heel, mustard oil had inconsistent effects on the flexor reflex but potentiated the extensor response for approximately 1 h. Intrathecal co-administration of naloxone (25 microg) with the selective alpha(2)-adrenoceptor antagonist RX 821002 (200 microg) enhanced both reflexes to more than twice pre-drug values and reduced or abolished all effects of mustard oil. These data confirm that the location of a noxious stimulus is an important determinant of the subsequent adaptive changes in reflexes, and indicate roles for endogenous opioids and noradrenaline in these processes.

Spinal section and opioid receptor blockade induce the appearance of Fos-like immunoreactivity in the spinal cord of the decerebrated rabbit

The expression of Fos-like immunoreactivity has been studied in spinal segments L5-S1 of decerebrated, unanaesthetized, but otherwise unstimulated rabbits. The aim of the study was to establish baseline levels of Fos in such preparations, and to examine how these might change after spinalization and opioid receptor blockade. In animals with an intact spinal cord, approximately 30 Fos-positive profiles per section were found in the superficial dorsal horns (i.e. laminae I and II) of each 40-microm section, while about 20 profiles per section were found immediately adjacent to the central canal (lamina X). Fos-like immunoreactive profiles were rare elsewhere in the gray matter. When the spinal cord was sectioned at L1 (after blockade with local anaesthetic), significantly more Fos-like immunoreactivity was found in superficial and central regions of the gray matter (approximately 90 profiles per section) in animals perfused 4 h after decerebration, but not when perfusion was performed 2 or 8 h after decerebration. The opioid antagonist naloxone (0.25 mg/kg/h) had little effect on expression of Fos-like immunoreactivity in spinalized preparations, but significantly increased the numbers of Fos-positive profiles in all but the ventral areas of the spinal gray matter in non-spinalized preparations. The present data show that spinal section induces a transient increase in expression of Fos in the superficial and central parts of the spinal gray matter. It appears that spinalization induces spontaneous activity in some neurons in these regions of the cord, presumably as a result of relief of descending inhibition. The effects of naloxone indicate that endogenous opioids exert tonic inhibition over Fos-expressing spinal neurons in non-spinalized rabbits.