Is the Tonic Decerebrate Inhibition of Reflex Paths Mediated by Monoaminergic Pathways?

Motor Neurons

Motor neurons translate synaptic input from widely distributed premotor networks into patterns of action potentials that orchestrate motor unit force and motor behavior. Intercalated between the CNS and muscles, motor neurons add to and adjust the final motor command. The identity and functional properties of this facility in the path from synaptic sites to the motor axon is reviewed with emphasis on voltage sensitive ion channels and regulatory metabotropic transmitter pathways. The catalog of the intrinsic response properties, their underlying mechanisms, and regulation obtained from motoneurons in in vitro preparations is far from complete. Nevertheless, a foundation has been provided for pursuing functional significance of intrinsic response properties in motoneurons in vivo during motor behavior at levels from molecules to systems. © 2017 American Physiological Society. Compr Physiol 7:463-484, 2017.

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.

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.

Bistability in spinal motoneurons in vivo: systematic variations in rhythmic firing patterns

In the presence of the monoamines serotonin and norepinephrine, spinal motoneurons can exhibit bistable behavior, in which a brief period of excitatory input evokes prolonged self-sustained firing. A brief inhibitory input returns the cell to the quiescent state. To determine whether motoneurons differ in their capacity for bistable behavior, intracellular recordings were obtained in the decerebrate cat preparation. To enhance the likelihood of encountering bistable behavior, the noradrenergic alpha1 agonist methoxamine was applied to the ventral surface of the cord. The capacity of the cells to produce bistable behavior was assessed from the duration of self-sustained firing evoked by a brief (1.5 s) excitatory synaptic input from muscle spindle Ia afferents. About 35% (17 of 49) of the cells produced steady self-sustained firing for >3 s and were considered fully bistable. The other 32 cells ( approximately 65%) were partially bistable, with self-sustained firing lasting <3 s. Fully bistable cells tended to have lower current thresholds for spike initiation and slower axonal conduction velocities than did partially bistable cells. This suggests that fully bistable motoneurons innervate fatigue resistant muscle fibers. The frequency-current (F-I) relations of the motoneurons were characterized with slow triangular current ramps. Fully bistable cells displayed an acceleration in firing rate immediately on initiation of rhythmic firing. The F-I gain after completion of the acceleration was positive. Fully bistable cells also displayed a hysteresis in the current level for firing threshold with the ascending threshold occurring at substantially higher current level than the descending one. Additionally, these current thresholds usually were centered about zero current, so that the ascending current threshold was positive while the descending current threshold was negative. This negative offset meant that fully bistable cells could exhibit tonic firing without depolarizing injected current. Partially bistable cells exhibited very different F-I characteristics. Firing rate acceleration was just as large as in fully bistable cells but did not occur until well above the current level needed to initiate rhythmic firing. F-I gain after acceleration was negative, there was little to no hysteresis between the ascending and descending firing thresholds, and both thresholds were above the zero current level. These properties of partially bistable cells suggest their functional role is in tasks requiring relatively brief, high forces. The low thresholds of fully bistable cells mean they will be readily recruited in low force tasks like posture, where their prolonged self-sustained firing would be advantageous.

Direct evidence for the link between monoaminergic descending pathways and motor activity: II. A study with microdialysis probes implanted in the ventral horn of the spinal cord

In order to define precisely the relation between descending monoaminergic systems and the motor system, we measured in the ventral horn of spinal cord of adult rats the variations of extracellular concentrations of 5-HT, 5-HIAA, DA and MHPG. Measurements were performed during rest, endurance running on a treadmill, and a post-exercise period, with microdialysis probes implanted permanently for 45 days. We found a slight decrease in both 5-HT and 5-HIAA during locomotion with a more marked decrease during the post-exercise period compared to the mean of rest values. In contrast, the concentration of DA and MHPG increased slightly during the exercise and decreased thereafter. These results, when compared with those of a previous study, which measured monoamines in the spinal cord white matter [C. Gerin, D. Bécquet, A. Privat, Direct evidence for the link between monoaminergic descending pathways and motor activity: I. A study with microdialysis probes implanted in the ventral funiculus of the spinal cord, Brain Res. 704 (1995) 191-201], highlight the complex regulation of the release of monoamines that occurs in the ventral horn.

Temporal summation of C-fiber afferent inputs: competition between facilitatory and inhibitory effects on C-fiber reflex in the rat

Long-lasting facilitations of spinal nociceptive reflexes resulting from temporal summation of nociceptive inputs have been described on many occasions in spinal, nonanesthetized rats. Because noxious inputs also trigger powerful descending inhibitory controls, we investigated this phenomenon in intact, halothane-anesthetized rats and compared our results with those obtained in other preparations. The effects of temporal summation of nociceptive inputs were found to be very much dependent on the type of preparation. Electromyographic responses elicited by single square-wave electrical shocks (2 ms, 0.16 Hz) applied within the territory of the sural nerve were recorded in the rat from the ipsilateral biceps femoris. The excitability of the C-fiber reflex recorded at 1.5 times the threshold (T) was tested after 20 s of electrical conditioning stimuli (2 ms, 1 Hz) within the sural nerve territory. During the conditioning procedure, the C-fiber reflex was facilitated (wind-up) in a stimulus-dependent fashion in intact, anesthetized animals during the application of the first seven conditioning stimuli; thereafter, the magnitude of the responses reached a plateau and then decreased. Such a wind-up phenomenon was seen only when the frequency of stimulation was 0.5 Hz or higher. In spinal, unanesthetized rats, the wind-up phenomenon occurred as a monotonic accelerating function that was obvious during the whole conditioning period. An intermediate picture was observed in the nonanesthetized rat whose brain was transected at the level of the obex, but the effects of conditioning were profoundly attenuated when such a preparation was anesthetized. In intact, anesthetized animals the reflex was inhibited in a stimulus-dependent manner during the postconditioning period. These effects were not dependent on the frequency of the conditioning stimulus. Such inhibitions were blocked completely by transection at the level of the obex, and in nonanesthetized rats were then replaced by a facilitation. A similar long-lasting facilitation was seen in nonanesthetized, spinal rats. It is concluded that, in intact rats, an inhibitory mechanism counteracts the long-lasting increase of excitability of the flexor reflex seen in spinal animals after high-intensity, repetitive stimulation of C-fibers. It is suggested that supraspinally mediated inhibitions also participate in long term changes in spinal cord excitability after noxious stimulation.

Neurotransmission by neurons that use serotonin, noradrenaline, glutamate, glycine, and gamma-aminobutyric acid in the normal and injured spinal cord

OBJECTIVE

The science of neurotransmission in the normal and injured spinal cord has grown. This is a review of neurotransmission using serotonin, noradrenaline, glutamate, glycine, and gamma-aminobutyric acid.

METHODS

The literature on spinal cord neurotransmission and changes that occur with trauma are reviewed.

CONCLUSION

Serotonergic and noradrenergic bulbospinal tracts influence interneurons and motor neurons via postsynaptic inhibition. Colocalization of serotonin and thyrotropin-releasing hormone occur in bulbospinal tracts, and reduction in uptake and thyrotropin-releasing hormone immunoreactivity quantitates the degree of injury in chronic spinal cord injury (SCI). Glutamate functions as an excitatory transmitter of some dorsal root afferent neurons and interneurons modulating nociceptive and motor neurons via at least five different receptors. Reactive synaptogenesis occurs after SCI, leading to an increase in the number of excitatory glutamatergic synapses below the level of SCI. gamma-Aminobutyric acid is an inhibitory transmitter of spinal interneurons that functions both pre- and postsynaptically. After SCI, a reduction occurs in the number of inhibitory synapses related to gamma-aminobutyric acid. Glycine is an inhibitory neurotransmitter that functions postsynaptically and also modulates the N-methyl-D-aspartate receptor. After SCI, a reduction in glycine adds to the loss of local inhibition below the SCI.

Direct evidence for the link between monoaminergic descending pathways and motor activity. I. A study with microdialysis probes implanted in the ventral funiculus of the spinal cord

Monoaminergic projections to the spinal cord are involved in the modulation of motor, autonomic, and sensory functions. More specifically, the increase of electrical activity of serotonergic neurons in raphe obscurus has been correlated with locomotion in treadmill-trained cats [Jacobs, B.L. and Fornal, C., Trends Neurosci., 9 (1993) 346-352]. In order to test the direct correlation between locomotion and the release of monoamines, microdialysis probes were permanently implanted for 45 days into the ventral funiculus of the spinal cord (white matter) of adult rats. Eight days after implantation, these rats were subjected to an endurant exercise on a treadmill, and dialysis sessions were organized in such a way that microdialysate samples of 15 min duration were collected during pre-, per- and post-exercise periods. Measurements of serotonin, 5-hydroxyindoleacetic acid, dopamine and 3-methoxy-4-hydroxyphenylethylglycol concentration in the extracellular space showed significant increases during locomotion when compared with both pre- and post-exercise values. Histological analysis shows that serotonergic axons were present close to the dialysis probe. These results demonstrate that the implantation of a microdialysis probe in the ventral funiculus, close to a potential target of monoaminergic projections, is a suitable technique for the collection of neuromediators released during spontaneous running.

Comparison of the antinociceptive effect of morphine and glutamate at coincidental sites in the periaqueductal gray and medial medulla in rats

In rats, microinjection of the opioid agonist morphine (15-45 nmol/0.5 microliter) and the excitatory amino acid monosodium glutamate (30-60 nmol/0.5 microliter) into identical brainstem sites within the mesencephalic periaqueductal gray matter (PAG; 23 sites) and the rostral ventromedial medulla (RVM; 22 sites) produced an increase of tail flick and hot plate response latencies. At both PAG and RVM sites, there was a statistically significant relationship between the effect obtained with morphine and with glutamate on the two nociceptive responses. While morphine and glutamate produced indistinguishable inhibition of tail flick and hot plate response latencies in the PAG, the tail flick inhibition following RVM morphine, but not RVM glutamate, displayed a clear plateau. One parsimonious interpretation of these data is that (1) glutamate directly increases the activity in the bulbospinal pathway, and (2) morphine inhibits an evoked or tonic suppression of a bulbospinal projection.

Course and mode of action of descending system conveying nucleus raphe magnus induced inhibition of flexion reflex in rats

In lightly anaesthetized rats, the latencies and magnitudes of heat-evoked withdrawal reflexes from the hindlimb were measured electromyographically. Low-intensity (20-50 microA) stimulation of the nucleus raphe magnus (NRM) resulted in an inhibition of the flexion reflex (commonly referred to as analgesia) as evidenced by increased latency and decreased amplitude. The effect lasted for several minutes after the stimulation was terminated. Following lesions of the dorsolateral funiculus (DLF) at the neural thoracic levels 7-8, baseline latencies were reduced and the effect of the NRM stimulation was abolished. Lesions of the DLF at lumbar level 1 resulted in unaltered baseline latencies with persistence of the inhibitory effect of NRM stimulation. The results of the present experiment show that pathways exerting a tonic inhibition of the withdrawal reflex, and mediating the effect of electrical stimulation from the NRM, descend in the DLF at the thoracic level of the spinal cord. At the lumbar level, there is a shift away from the DLF. The antinociceptive effect of 20 microA NRM stimulation was partly reduced by pretreatment with the serotonin synthesis inhibitor para-chlorophenylalanine (PCPA) or the opiate naloxone (1 mg kg-1 i.v.). In animals pretreated with PCPA, naloxone (1 mg kg-1 i.v.) completely abolished the analgesic effect of the stimulation. Thus, both serotonergic and opioid systems may be implicated in mediating the analgesia. With 50 microA stimulation the same treatment only partly attenuated the NRM-induced analgesia, indicating an additional non-serotonergic and non-opioid mechanism which requires a higher current intensity for its activation.

Is there a serotonergic tonic descending inhibition on the responses of dorsal horn convergent neurons to C-fibre inputs?

In the anaesthetized rat the intravenous injection of 5-HT antagonists, cinanserin and methysergide, induces in two-thirds of neurones studied, an increase in the responsiveness of dorsal horn convergent neurones to C-fibre stimulation. These results are in favour of the existence of serotonergic tonic descending inhibitory effects on the spinal transmission of noxious messages.

Integration of posture and locomotion in acute decerebrate cats and in awake, freely moving cats

For the past 10 years, our group has been engaged in the study of posture and locomotion in decerebrate cats and in freely moving awake cats. Our initial objective was to analyse the neuronal mechanisms of locomotion from a viewpoint of "postural control". Therefore, in this review, I have focussed my attention on two major control aspects of the brain stem; one related to the interaction of posture and locomotion; and the other to initiation of locomotion. It is apparent that elucidation of the second aspect is feasible. In Fig. 15, I have summarized all the neuronal structures that have been functionally identified as being actively involved in the regulation of posture and locomotion. Obviously, contribution of the cerebello-cerebral pathways and the basal ganglia to both the postural and to the locomotor control cannot be elucidated in decerebrate preparations (Fig. 15A). These contributions can, to a certain degree, be elucidated in intact awake cats (Fig. 15B). Although it is difficult to directly compare the results obtained in the decerebrate cats with those obtained in intact cats, it has been encouraging that the selective activation of certain neuronal structures within the brain stem allowed us to evoke comparable postural and locomotor changes in both groups of preparations. It can be expected that the knowledge obtained from studies of the cerebello-cerebral pathways and of the basal ganglia, together with those that can be further obtained from studies of the brain stem, should result in the elucidation of the two major control aspects of the brain stem described above. In this sense, both the models of the decerebrate locomotor preparation and the freely moving, awake cat seem to provide an unique opportunity to study the nature and the sources of command signals that set the postural and the locomotor synergies into a single functional synergy, and even to approach elucidation of the intriguing question as to how and where volitional control signals for initiating and/or halting locomotion are organized. More than 70 years have passed since the pioneer studies by Sherrington (1906) and by Graham Brown (1911, 1914) on postural control and on locomotion, as exemplified by "decerebrate reflex standing, and stepping" and the "central rhythmic generator" relating to locomotion. Based on these discoveries, Shik, Severin and Orlovsky (1966) have made a splendid breakthrough in a wide area of investigation relating to locomotion.(ABSTRACT TRUNCATED AT 400 WORDS)

Funicular trajectories of brainstem neurons projecting to the lumbar spinal cord in the monkey (Macaca fascicularis): a retrograde labeling study

Brainstem nuclei projecting to the lumbar spinal cord in the monkey were identified by using horseradish peroxidase and the fluorescent dye granular blue. These retrogradely transported tracers were used in fluid and/or gel forms to determine the funicular trajectories of the brainstem-spinal projections. The major descending components of the dorsal funiculus arose from the n. gracilis, n. cuneatus, and the n. of the solitary tract. Major components of the dorsolateral funiculus (DLF) came from the raphe complex, medullary and pontine reticular formation, locus coeruleus, Edinger-Westphal n., and red n. Other nuclei giving rise to minor contributions to the DLF included n. gracilis, n. cuneatus, n. of the solitary tract, medial and spinal vestibular n., subcoeruleus, periaqueductal gray, interstitial n. of Cajal, n. of Darkschewitsch, and the anteromedian n. The major components of ventral cord paths (ventrolateral and ventral funiculi) arose from the raphe complex, the medullary and pontine reticular formation, lateral and spinal vestibular n., and the coerulean complex. Minor contributions to the ventral paths descended from the dorsal motor n. of X, n. of the solitary tract, medial vestibular n., paralemniscal reticular formation, dorsal parabrachial n., n. cuneiformis, periaqueductal gray, Kölliker-Fuse n., and red n. The possible functional implications of the funicular distribution of these descending pathways are discussed from the perspective of descending inhibition and pain modulation.

Motoneurons innervating the cremaster muscle of the rat are characteristically densely innervated by serotoninergic fibers as revealed by combined immunohistochemistry and retrograde fluorescence DAPI-labelling

The lumbar spinal cord of the rat was studied by combined retrograde fluorescent labelling with 4',6-diamidino-2-phenylindole-2HCl (DAPI) and immunoperoxidase procedure using serotonin antiserum. A peculiar small neuronal group endowed more densely than other anterior horn neurons with serotonin-like immunoreactive fibers was recognized in the anterior column of lumbar segments L1-L2. At the same time, this small nucleus was shown to contain the motoneurons innervating the cremaster muscle by means of retrograde labelling with DAPI. It is tentatively suggested that the bulbospinal descending serotonin system is particularly intimately connected with the function of the cremaster muscle.

The location of brainstem neurones tonically inhibiting dorsal horn neurones of the cat

In an investigation of the origin of tonic descending inhibition of dorsal horn neurones by impulses in unmyelinated primary afferents, brainstem regions were electrolytically lesioned. With each neurone studied, tonic descending inhibition was measured before and after brainstem lesions by cooling a segment of spinal cord cephalic to the recording site. Such inhibition was not reduced by lesions of areas which, when stimulated, produce analgesia. These included the periaqueductal grey and the raphé areas of the midbrain and pons-medulla. Tonic descending inhibition was reduced by bilateral lesions of the ventrolateral caudal medulla in the region of the lateral reticular nuclei. Lateral reticular areas may have a functional role in the control of pain.

Effect of DLF lesions at different spinal levels on morphine induced analgesia

The effect of morphine on tail flick latency was tested in 3 groups of rats. It is shown that in normal rats and in rats with a bilateral DLF lesion at Th10 or lower spinal root segments, morphine increased the tail flick latency. After a bilateral DLF lesion at Th9 or higher segments, morphine did not change the tail flick latency.

The somatotopic organization of the nucleus raphe magnus and surrounding brain stem structures as revealed by HRP slow-release gels

Horseradish peroxidase (HRP) in slow-release gel was unilaterally implanted in the dorsal lateral funiculus (DLF) of either cervical, midthoracic, lumbar or sacral spinal cord levels of adult male rats. Complete mappings and cell counts were made of HRP-filled cells from the level of the rostral trapezoid body caudal to the inferior olive. Following cervical or midthoracic placements, labelled cells often formed a continuous band surrounding the medial, dorsal and lateral aspects of the ipsilateral pyramid and extending into the ventral region medial to the facial nucleus and lateral to the pyramid. It appears that this population of HRP-filled cells in the nucleus raphe magnus (NRM) and reticularis magnocellularis (Rmc) corresponds to the serotonergic cell group B3 of Dahlström and Fuxe. In lumbar and sacral cases, there was a noticeable difference both in the pattern and number of labelled cells, with fewer labelled cells and greater tendency for a 'gap' to occur in the population of cells midway over the pyramid. Based on the patterns of labelling and a review of the literature, its appears that the actual functional unit involved in the descending control of nociception may be the combined population of Rmc and NRM.

The origin of descending pathways in the dorsolateral funiculus of the spinal cord of the cat and rat: further studies on the anatomy of pain modulation

There is considerable evidence that the dorsolateral funiculus (DLF) of the spinal cord contains descending pathways critical for both opiate and brainstem stimulation-produced analgesia. To obtain a comprehensive map of brainstem neurons projecting to the spinal cord via the DLF, large injections of horseradish peroxidase (HRP) were made into the lumbosacral spinal cord of cat and rat. These injections were made caudal to midthoracic lesions which spared only a single DLF or ventral quadrant (VQ); thus only those neurons whose axons descended in the spared funiculus would be labelled. Cells with descending axons in the VQ were concentrated in the medullary nucleus raphe pallidus and obscurus, nucleus retroambiguus and in various subregions of the reticular formation including the nucleus reticularis ventralis, gigantocellularis, magnocellularis, pontis caudalis and pontis oralis. Significant numbers of neurons were also found in medial and lateral vestibular nuclei and in several presumed catecholamine-containing neurons of the dorsolateral pons. In the rat, but not in the cat, considerable numbers of cells are present in the mesencephalic reticular formation just lateral to the periaqueductal gray. In both species, some cells were found in the paraventricular nucleus of the hypothalamus. Brainstem cells projecting in the DLF were concentrated in the nucleus raphe magnus and in the adjacent nucleus reticularis magnocellularis, ipsilateral to the spared funiculus. Significant numbers of cells were found in the dorsolateral pons, differing somewhat in their distribution from those projecting in the VQ. DLF-projecting cells were also present in the ipsilateral Edinger-Westphal nucleus and periaqueductal grey contralateral red nucleus of the midbrain and in the ipsilateral hypothalamus. Smaller projections from other sites are described. These results are discussed in terms of the differential contribution of several brainstem neuronal groups, including the serotonergic nucleus, raphe magnus, the ventromedial reticular formation of the medulla, and various catecholamine-containing neurons of the dorsolateral pontine tegmentum to the analgesia produced by opiates and electrical brain stimulation.

The effect of morphine and some other narcotic analgesics on brain tryptophan concentrations

An acute dose of morphine increased brain tryptophan in mice. This effect was not prevented by naloxone nor was it produced by other narcotic analgesics. Dextrorphan, but not levorphanol, had a similar effect to morphine. A large dose of tryptophan had no effect on the antinociceptive action of morphine in mice. Morphine increased brain tryptophan in rats. This effect was prevented by naloxone. A large dose of tryptophan antagonised the antinociceptive action of morphine in the rat.