Late prolonged discharges in the motor nerves of the hindlimbs and their relationship to locomotor rhythmicity in thalamically immobilized cats]

Effects of flexor reflex afferents stimulation were investigated on high decerebrated curarized cats. Stimulation of ipsilateral flexor reflex afferents evoked late long-lasting discharges in flexor nerves. Contralateral flexor reflex afferents stimulation evoked late discharges both in extensor and flexor nerves. Transition from late discharges to rhythmic discharges was observed. Early segmental reflexes were tonically depressed in thalamic in comparison with acute spinal cats. A similar tonic depression of segmental reflexes took place in acute spinal cats after DOPA injection. Segmental reflexes were distinctly modulated during late and rhythmic discharges. On the basis of the data available possible central mechanisms of the observed changes of segmental reflexes are discussed.     

Further evidence for enhancing effects of NO on monosynaptic and polysynaptic spinal reflexes in cats

There are a number of studies on the effects of different NO donors and inhibitors on spinal cord with quite contradictory results. The aim of this study was to investigate the effects of sodium nitroprusside (SNP), a NO donor, and N(G)-nitro-L-arginine methyl ester (L-NAME), a nonselective NOS inhibitor, on monosynaptic and polysynaptic spinal reflexes in anesthetized and spinalized cats. After a dorsal laminectomy between L5 and S1, monosynaptic and polysynaptic spinal reflexes were evoked by stimulation of gastrocnemius nerves. Following control recordings, administration of L-NAME in 100, 200, 500 microM (local) and 10, 20, 50 mg/kg (i.v.) doses decreased significantly the monosynaptic and polysynaptic reflex amplitudes in a dose-dependent manner. Administration of SNP in 100, 200, 500 microM (local) and 100, 200, 500 microg/kg (i.v.) doses enhanced significantly the both reflex amplitudes in a dose-dependent manner. In another series of experiments it has been observed that the maximal decrease in reflex amplitudes caused by 500 microM local L-NAME administration in the 15th minute was reversed by locally administered SNP (500 microM). Our results support the hypothesis stating that NO may play a role in the modulation of mono- and polysynaptic spinal reflexes and the NO appears to have an enhancing role on these responses.     

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.     

Distal hind-limb reflex responses in cats with largely intact spinal reflex circuits

Reflex responses to single shocks to peripheral nerves were studied in the small muscles of the hind limb of the decerebrated and subsequently spinalized cat. Monosynaptic and high threshold polysynaptic reflexes of plantar muscles evoked by tibial nerve shocks were significantly more active in preparations that had extensive dissection of nerves and muscles. Monosynaptic reflexes were more active in the plantar flexor portion than in other parts of the interosseus. Compared with proximal limb muscles, this muscle showed features of reflex behavior that were neither consistently flexor nor extensor. In spinal animals the threshold of the tibialis anterior to tibial nerve shocks was often less than the threshold for other muscles, suggesting a function other than nociceptive flexion reflex, probably tactile influence in gait. Extensor digitorum brevis reflexes did not exhibit the flexor reflex pattern typified by those of the tibialis anterior muscle. The data further suggest activation of these reflexes by Ib interosseus muscle afferents and plantar cutaneous afferents and tonic inhibition from Ia receptors.     

Nociceptive withdrawal reflexes evoked by uniform-temperature laser heat stimulation of large skin areas in humans

Nociceptive withdrawal reflexes (NWR) were evoked by brief (200 ms) painful CO(2) laser stimulation at five intensities (1.2, 1.4, 1.6, 1.8, and 2.0 x pain threshold) applied to nine sites (2 cm(2)) separated by 1.7 cm on the dorsal side of the foot and anterior part of the lower leg of 14 healthy volunteers. The purpose of the study was to investigate the characteristics of NWRs evoked by a natural stimulation modality. The reflexes were measured as the electromyographic response from the iliopsoas (ILI), quadriceps vastus lateralis (QVL), biceps femoris (BF), tibialis anterior (TA), and soleus (SOL) muscles. Stimulus-response relationships between heat intensity and the reflex magnitude and correlation between perceived pain intensity and reflex magnitude were observed in the ILI, QVL, BF, and TA but not the SOL. No significant differences in reflex magnitude were found between the stimulation sites. NWRs were evoked more often in flexor muscles than extensor muscles, indicating a non-site-specific reflex organization. The paper presents a new method to evoke NWRs by uniform-temperature laser heat stimulation of large skin areas in humans. These heat evoked reflexes had a stimulus-response relationship.     

Site-specific, inflammation-induced adaptations in withdrawal reflex pathways in the anesthetized rabbit

Transmission in spinal reflex pathways from the toes to the ankle flexor tibialis anterior (TA) and to the knee flexor semitendinosus (ST), and from the heel to the ankle extensor medial gastrocnemius (MG), has been studied during antigen-induced inflammation due to subcutaneous injection of ovalbumin at the heel or at the toes in pre-immunized, pentobarbitone-anesthetized rabbits. The ovalbumin challenge induced a mild swelling at the injection site that developed over 6 h. Inflammation at the toes facilitated both flexor reflexes evoked from the toes and inhibited MG extensor responses to stimulation at the heel, with effects usually developing within 75 min of the initial injection and persisting to the end of the 6 h post-injection recording period. When inflammation was induced at the heel, the heel-MG and toes-ST reflexes were enhanced, whereas the toes-TA reflex was inhibited. The findings support the view that long-lasting changes in reflexes evoked by noxious stimuli have evolved to enhance reflex protection of an injured site.     

Effect of partial transection of the medulla oblongata on inhibition of the mouth-opening reflex of the cat induced by stimulation of the mesencephalic central gray

Effect of sections of the spinal trigeminal tract and caudal nucleus of the spinal trigeminal tract on the high-threshold jaw-opening reflex (HJR) evoked by tooth pulp stimulation and the low-threshold jaw-opening reflex (LJR) evoked by stimulation of the A-alpha infraorbital nerve fibres as well as on the suppression of these reflexes by the central grey matter was studied in cats under chloralose-nembutal anesthesia. After the trigeminal tract section HJR has increased by 8.52%. The LJR magnitude was either invariable either negligibly reduced or increased after the section. Stimulation of the central grey matter by a short train of stimuli evoked depression of HJR which reached 100% in intact animals, while after the section--only 40-60%. Inhibition of HJR after prolonged rhythmic stimulation of the central grey matter reached 80% in intact animals, while after the section--25-30%. There were no changes in the LJR depression when the central grey matter was stimulated by short trains of stimuli after section. The LJR suppression evoked by the prolonged central grey matter stimulation reached 25-50% in intact animals, while after the section the depression increased and reached 75% (three animals) or was reduced and reached 15-20% (four animals). It is concluded that the caudal nucleus of the spinal trigeminal tract exerts a considerable effect on the HJR inhibition (but less on the LJR inhibition) after the midbrain central grey matter stimulation.     

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

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

Effects of morphine on somatocardiac sympathetic reflexes in spinalized cats

The effects of morphine on sympathetic reflexes, recorded in the inferior cardiac nerve, to myelinated A and unmyelinated C afferent stimulation were tested in 17 acutely spinalized cats. Stable sympathetic A and C reflexes of short latency (approximately 30 ms and 140 ms in the case of the ulnar nerve, respectively) could be recorded in the inferior cardiac sympathetic nerve to stimulation of somatic A and C afferents in the ulnar and upper thoracic intercostal nerves, ipsilaterally. Spinal sympathetic A reflexes, which were primarily evoked from stimulation of A delta afferent fibers, could be elicited from more segmental levels than could sympathetic C reflexes. Additionally, smaller reflexes, only from A afferent fiber activation, were identified from stimulations on the contralateral side of the body. Small doses of morphine (0.02 mg kg-1, i.v.) proved to be ineffective at altering sympathetic A and C reflexes, while somewhat larger doses (0.2 mg kg-1, i.v.) produced a clear 62% decrease in C reflexes and a 33% decrease in A reflexes, Dosages of 1 and 2 mg kg-1 severely depressed both A and C reflexes. All of the above effects of morphine administration were completely and immediately reversible by naloxone (i.v.). The results are discussed with regard to the effects of morphine on sympathetic A and C reflexes in CNS intact, anesthetized cats.     

Habituation and dehabituation of the spinal polysynaptic reflex responses: modification by naloxone and opiates and their anatomical correlates

The sustained inhibitory action of spinal endorphins could be responsible for the habituation of polysynaptic responses in the spinal cord. To test this hypothesis, acute spinalized unanesthetized cats (decerebrated and curarized) were used. Sural nerve electrical stimulation (0.2 Hz) was provided and a progressive decrease in the reflex response was found. Conversely, the field potential (lamina V) progressively increased during stimulation, reaching its maximum amplitude when ventral root response showed maximum habituation. The administration of naloxone (0.8-10.0 mg/kg) produced dehabituation or prevented habituation. The immunohistological results showed leu-enkephalin-like immunoreactive dot-like structures in close proximity to neurons of laminae VII, VIII and IX in the lumbo-sacral segment of the spinal cord. Our results suggest an involvement of opioid peptides in the habituation process.     

Tibialis Anterior and Soleus Withdrawal Reflexes Elicited by Electrical Stimulation of the Sole of the Foot during Gait

The aim of the present study was to investigate the modulation and functional importance of nociceptive withdrawal reflexes elicited from the sole of the foot and recorded from the soleus (SOL) and tibialis anterior (TA) muscles during gait. Cutaneous electrical stimulation delivered at four locations of the sole of the foot was used to elicit the withdrawal reflex. Reflexes were recorded from eight healthy subjects during treadmill walking. The reflexes were elicited at heel‐contact, during foot‐flat, at heel‐off, and during mid‐swing. The reflexes evoked in TA were largest when the arch of the foot was stimulated, and smallest following stimulation of the heel (significant difference during stance, p ≤ 0.002). The largest soleus responses were elicited when the arch of the foot was stimulated (significant difference compared with the fifth metatarsophalangeal joint, stimulation after heel‐contact, p < 0.05). The TA reflex, expressed as a proportion of the electromyogram during unperturbed gait, was smallest during swing (p < 0.05, compared with stance) whereas the SOL reflex was maximal during swing (p < 0.05, compared with stance). The results suggest that the modulation of the reflex promotes an appropriate withdrawal while preserving balance and continuity of motion. These results may have applications in assisting gait of hemiplegics.     

Operant conditioning of somatosensory evoked potential (SEP) amplitude in rats. I. Specific changes in SEP amplitude and a naloxone-reversible somatotopically specific change in facial nociception

The aim of this experiment was to investigate possible endogenous opioid modulation of innocuous somatosensory activity. Somatosensory activity was measured by recording cortical somatosensory evoked potential (SEP) and reflex movement amplitude evoked by innocuous electrical stimulation of the spinal trigeminal tract in awake rats. Putative endogenous opioid activity was blocked using the opiate antagonist naloxone (1 mg/kg). The amplitude of midlatency SEP components (14-50 ms latency) increased following administration of naloxone and repeated stimulus presentations. The amplitude of these components decreased following administration of the opiate agonist morphine (3 mg/kg). An early cortical component (10 ms latency) habituated following the administration of saline but did not habituate following naloxone. Naloxone also enhanced habituation of the late SEP components (60-120 ms latency) and reflex movement evoked at higher stimulus intensities. Morphine decreased the amplitude of the early cortical component but had no consistent effect on the amplitude of the late SEP components.     

Differential Effects of a Distant Noxious Stimulus on Hindlimb Nociceptive Withdrawal Reflexes in the Rat

Recent studies indicate that the nociceptive withdrawal reflexes to individual muscles are evoked by separate reflex pathways. The present study examines whether nociceptive withdrawal reflexes to different muscles are subject to differential supraspinal control in rats. A distant noxious stimulus was used to activate a bulbospinal system which selectively inhibits 'multireceptive' neurons (i.e. neurons receiving excitatory tactile and nociceptive inputs) in the dorsal horn of the spinal cord. Withdrawal reflexes, recorded with electromyographic techniques in single hindlimb muscles, were evoked by standardized noxious pinch. Thirty-seven rats, anaesthetized with halothane and nitrous oxide, were used. Whereas withdrawal reflexes to the extensor digitorum longus and brevis, tibialis anterior and biceps posterior muscles were strongly inhibited, reflexes to interossei muscles were potentiated during noxious pinch of the nose. Reflexes to peronei muscles were not significantly changed. The effects on the reflexes usually had an onset latency of <0.5 s and outlasted the conditioning stimulation by up to 2 s. The monosynaptic la reflex to the deep peroneal nerve, innervating dorsiflexors of the digits and ankle, was not significantly changed during noxious pinch of the nose. Hence, the inhibitory effects on the hindlimb withdrawal reflexes induced by the conditioning stimulation were presumably exerted on reflex interneurons. It is concluded that nociceptive withdrawal reflexes to different hindlimb muscles are differentially controlled by descending pathways activated by a distant noxious stimulus. The results support our previous conclusion that there are separate nociceptive withdrawal reflex pathways to different hindlimb muscles.     

Modulation of morphine and alcohol motor stimulant effects by cannabinoid receptors ligands

Previous studies evidenced in rats the suppression by cannabinoids of motor stimulant effects of various drugs of abuse. Here we investigated, in mice, the effects of an acute or a chronic administration with the cannabinoid agonist HU 210 on the motor stimulant effects of either morphine or alcohol. HU 210 (12.5-200 microg/kg), when acutely administered, antagonized the stimulant effects of morphine (7.5 mg/kg) or alcohol (1 or 1.5 g/kg). A tolerance to this antagonistic interaction with morphine and alcohol occurred after a 7-day or a 14-day HU 210 treatment, leading to the reappearance of morphine- and alcohol-induced stimulation. The CB1 receptor antagonist rimonabant (10 mg/kg) enhanced the stimulant effect induced by low doses of morphine (5 or 7.5 mg/kg). Rimonabant (3 or 10 mg/kg) altered the locomotor effect of alcohol in a biphasic manner. It enhanced the stimulant effect of low doses of alcohol (1 or 1.5 g/kg) while decreasing the locomotor activity of mice treated with a high dose (3 g/kg) of alcohol. Furthermore, rimonabant (3 and 10 mg/kg) enhanced the duration of alcohol-induced loss of righting reflex (4 g/kg), suggesting a dual implication of cannabinoidergic pathways in acute effects of alcohol.     

Presynaptic dopaminergic inhibition of the spinal reflex in rats.

Dopaminergic influence on spinal monosynaptic transmission was examined in rats. Monosynaptic mass reflex (MMR) was recorded from the ventral root L6 following supramaximal stimulation (0.2 Hz; 0.1 ms) to the ipsilateral dorsal root L6 in spinalized rat under pentobarbitone sodium (40 mg/kg, i.p.) anaesthesia. MMR was inhibited by intravenous administration of the dopaminergic agonist, apomorphine (50-200 ug/kg) in a dose-dependent manner. The attenuatory effect of apomorphine (200 ug/kg i.v.) on the reflex could be reversed by the dopaminergic antagonist haloperidol (0.5 mg/kg, i.v.). Under tetanic stimulation (200 Hz; 15s), the pretetanic relative inhibition induced by apomorphine (200 ug/kg, i.v.) was increased only for a short period immediately after the cessation of tetanic stimulation. The results indicate existence of presynaptic dopamine receptors on the afferent terminals converging on the motoneurone which may functionally modulate the spinal motor output.     

Noxious stimulation of the toes evokes long-lasting, naloxone-reversible suppression of the sural-gastrocnemius reflex in the rabbit

Repetitive stimulation of the small myelinated and non-myelinated afferents of the common peroneal (c.p.) nerve evokes a long-lasting (20-25 min), naloxone-reversible inhibition of the sural-gastrocnemius reflex in the decerebrated and spinalized rabbit. Altering the number and frequency of stimuli applied to the c.p. nerve showed that this inhibition was dependent on temporal summation of afferent input from that nerve, and that the optimum frequency for producing the effect was between 2 and 10 Hz. Application of natural conditioning stimuli in and around the receptive field of the c.p. nerve showed that noxious, but not innocuous, mechanical and thermal stimuli could evoke long-lasting inhibition of the sural-gastrocnemius reflex. Thermal stimuli produced a biphasic change in the excitability of the reflex with facilitation followed by inhibition. The opioid antagonist naloxone (250 micrograms.kg-1) blocked all suppression resulting from these natural noxious stimuli. Chemical stimulation of the skin with mustard oil did not evoke naloxone-reversible inhibition of the reflex. These results indicate that intensely noxious stimuli can promote the release of opioid peptides in the spinal cord, and that one of the functions of these peptides may be to regulate the level of excitability in withdrawal reflex pathways.     

Reduction in parasympathetic reflex vasodilatation following stereotaxic ear-bar insertion: importance of reduced afferent input

As in our previous report, when cats were fitted with stereotaxic ear-bars 'type A' animals (26 out of 41) still exhibited a parasympathetic reflex lip blood flow (LBF) increase in response to lingual nerve stimulation, while in 'type B' animals (the remaining 15) it was greatly reduced or abolished. We compared (in both magnitude and in their sensitivity to hexamethonium, 10 mg/kg, i.v.) the LBF responses evoked by electrical stimulation of various sites within the reflex arc (lingual nerve, trigeminal ganglion, spinal trigeminal nucleus (Vsp)) in type A and type B animals to examine where the suppressive effect of ear-bar insertion might be exerted (using artificially ventilated, cervically vago-sympathectomized cats deeply anesthetized with alpha-chloralose and urethane). After ear-bar insertion: (a) in type A animals, stimulation of both lingual nerve and Vsp evoked a similar, hexamethonium-sensitive LBF increase; (b) in type B animals (in which lingual-nerve stimulation evoked no LBF increase), Vsp stimulation evoked a hexamethonium-sensitive LBF increase; (c) in both type A and type B animals, trigeminal ganglion stimulation consistently elicited an LBF increase (abolished by hexamethonium in type A, but reduced by only 50% in type B). These results suggest (i) that abolition of the lingual nerve-induced parasympathetic reflex vasodilatation by ear-bar insertion is due to reduced afferent traffic (in peripheral trigeminal or facial nerves) rather than to a damaged efferent output, and (ii) this effect in type B animals seems somehow to allow an antidromic (hexamethonium-insensitive) vasodilatation to occur on trigeminal ganglion stimulation.     

Central neurotransmitter systems in the morphine suppression of jaw-opening reflex in rabbits: the cholinergic system.

The possible participation of the central cholinergic system in morphine suppression of the jaw-opening reflex was investigated in rabbits lightly anesthetized with pentobarbital sodium (30 mg/kg, i.v.). The averaged amplitude of EMG signals from the digastric muscle, evoked by intrapulpal stimulation, was used as the pain index. Drugs were administered directly into the cerebral circulation via a carotid artery, cannulated centrifugally from the heart. Injection of a subliminal dose of morphine (3 mg/kg) produced no appreciable effect on the jaw-opening reflex, which was, however, significantly depressed after a subsequent injection of physostigmine (0.1 mg/kg). Such physostigmine-potentiated analgesia still persisted after atropine administration (1 or 5 mg/kg). An optimal dose of morphine (6 mg/kg) was found to elicit drastic inhibition of the evoked digastric activity, which was again not reversed by atropine (1 or 5 mg/kg). We concluded that the cholinergic system is involved in the morphine suppression of the jaw-opening reflex, and suggest that shifting of the balance between central neurotransmitter systems may be a key to the precipitation of the analgesic process induced by the opiate.     

The NSAID dexketoprofen trometamol is as potent as μ-opioids in the depression of wind-up and spinal cord nociceptive reflexes in normal rats

The aim of this study was to examine the potency of the antinociceptive effects of the non-steroidal antiinflammatory drug (NSAID), Dexketoprofen Trometamol (the active enantiomer of ketoprofen) on spinal cord nociceptive reflexes. These effects were compared with those of the μ-opioid receptor agonist fentanyl in normal animals. The experiments were performed in male Wistar rats anaesthetised with alpha-chloralose. The nociceptive reflexes were recorded as single motor units in peripheral muscles, activated by mechanical and electrical stimulation. Both dexketoprofen and fentanyl inhibited responses evoked by mechanical and electrical stimulation with doses in the same nanomolar range (dexketoprofen ID50s: 100 and 762 nmol kg⁻¹ and fentanyl: 40 and 51 nmol kg⁻¹, respectively). Dexketoprofen and fentanyl also significantly inhibited wind-up. Since fentanyl has been shown to be some 1000 times more potent than morphine in this type of experiments, we conclude that dexketoprofen has central analgesic actions in normal animals and depresses nociceptive responses with a potency similar to that of μ-opioid agonists.     

Central neurotransmitter systems in the morphine suppression of jaw-opening reflex in rabbits: The dopaminergic system

The possible participation of the central dopaminergic system in the suppression by morphine of the jaw-opening reflex was investigated in rabbits lightly anesthetized with pentobarbital sodium (30 mg/kg, i.v.). The averaged amplitude of EMG signals from the digastric muscle, evoked by intrapulpal stimulation, was used as the pain index. Drugs were administered either directly into the cerebral circulation via a cannulated carotid artery or intraperitoneally. Pretreatment of the animals with haloperidol (0.5 mg/kg) or pimozide (0.5 mg/kg) essentially eliminated the inhibitory action of an optimal dose of morphine (6 mg/kg) on the jaw-opening reflex. Such antagonistic action of pimozide (and to a lesser extent, of haloperidol) on the opiate effect could be reversed by physostigmine (0.1 mg/kg). Apomorphine (0.5 mg/kg) pretreatment sufficiently potentiated the effect of a suboptimal dose of morphine (3 mg/kg), resulting in significant suppression of the evoked digastric activities. We concluded that the dopaminergic system is involved synergistically in morphine suppression of the jaw-opening reflex; the present study also reinforced a previous proposal that shifting of the balance between central neurotransmitter systems may be a key to the precipitation of the analgesic process induced by the opiate.