Effects of spinal cord lesions on cutaneously elicited reflexes in the decerebrate cat. Tonic bulbospinal and spinobulbar inhibitory systems

Projections from the nucleus reticularis magnocellularis to the rat cervical cord using electrical stimulation and iontophoretic injection methods

The aim of this study is to clarify the fiber distribution of the nucleus reticularis magnocellularis (NRMC) and adjacent areas in the rat spinal cord. Biotinylated dextran amine was injected iontophoretically through a glass capillary into the areas, in which a single cell responded to noxious electrical stimulation of the sciatic nerve and to a pinch of the thigh skin with multiple spikes. Labeled fibers descended bilaterally through the ventral funiculi of the medulla oblongata and then through the ventral and lateral funiculi of the cervical cord with an ipsilateral predominance, and terminated in the spinal gray (laminae I-X). A single fiber sometimes ran through several laminae while bifurcating many short branches with axon varicosities and terminal buttons in one transverse section, that is, through laminae V, VII and X, through laminae V, IIl-IV and I-II, and through laminae VII to I-II. The present study showed that the wide distribution of a single fiber and a mass of fibers descending from the NRMC and adjacent areas might modulate not only somatic sensory and motor functions but also autonomic functions in the spinal cord.

Inhibitory effects from various types of dorsal column and raphe magnus stimulations on nociceptive withdrawal flexion reflexes

Most of the clinical and research reports agree about the analgesic effects of dorsal column (DC) stimulation, but there is no unanimity about the neural mechanisms involved in this stimulation. The aim of the present study was to compare the effects of segmental and rostral activation of the DCs and to investigate whether these effects are mediated through a brainstem spinal loop. Decerebrate-decerebellate cats were subjected to selective DC lesions at C(1) and C(3) spinal cervical levels and their reflex reactions to natural or electrical nociceptive stimuli were monitored either as withdrawal flexion reflexes or as motorneuronal discharges. Conditioning stimulation was performed as train of shocks (100 Hz, for 1 to 10 min or 300 Hz for 30 ms) applied on the DCs either rostral (DCr) or caudal (DCc) to the spinal lesions or on the raphe magnus (RM). Conditioning trains for 5-10 min applied on DCr inhibited the withdrawal flexion reflexes recorded as toe flexion (90% of the control). Comparisons of the effects of DCr, DCc or RM of conditioning stimuli were made on the discharges of 110 motorneurons recorded in isolated ventral root fibers. Conditioning stimulation applied to DCc produced short lived inhibition (in about 60%) or facilitation (in about 30% of the neurons) while DCr or RM conditioning produced inhibition in 90% of neurons which outlasted the duration of the conditioning trains. It was also shown that repetitive application of conditioning train on either DCr or RM resulted in longer duration of inhibition than that observed following DCc conditioning. We conclude that the stronger inhibition of motorneuronal discharges, evoked by nociceptive stimuli, is obtained by rostral activation of the DCs and that long term effects of DCst are mediated through a DC-brainstem-spinal loop.

Inflammation-induced Fos protein expression in the rat spinal cord is enhanced following dorsolateral or ventrolateral funiculus lesions

Previous studies have shown an enhanced expression of Fos protein-like immunoreactivity in the lumbar spinal cord of rats with complete spinal transection following persistent hindpaw inflammation. To further locate the spinal pathways responsible for these effects, we compared the inflammation-evoked Fos expression in rats with bilateral lesions of the dorsolateral (DLFX) or ventrolateral (VLFX) funiculus, and with rats with a sham operation. The results indicate that the number of Fos-labeled neurons was significantly increased in all laminae of the dorsal horn ipsilateral to the inflamed hindpaw and in contralateral deep dorsal horn in both DLFX and VLFX rats compared to sham-operated rats. Moreover, when comparing DLFX and VLFX rats, in the ipsilateral spinal cord, DLFX resulted in more Fos expression in the deep dorsal horn; in contrast, a larger number of Fos-labeled cells in superficial laminae was observed in VLFX rats. These results suggest that modulatory systems, which descend in both DLF and VLF pathways, mediate the enhanced net descending nociceptive inhibition after persistent inflammation, although the supraspinal sites of origin of each pathway are likely functionally diverse.

Biphasic modulation of spinal nociceptive transmission from the medullary raphe nuclei in the rat

The modulatory effects of electrical and chemical (glutamate) stimulation in the rostral ventromedial medulla (RVM) on spinal nociceptive transmission and a spinal nociceptive reflex were studied in rats. Electrical stimulation at a total 86 sites in the RVM in the medial raphe nuclei (n = 54) and adjacent gigantocellular areas (n = 32) produced biphasic (facilitatory and inhibitory, n = 43) or only inhibitory (n = 43) modulation of the tail-flick (TF) reflex. At these 43 biphasic sites in the RVM, facilitation of the TF reflex was produced at low intensities of stimulation (5-25 microA) and inhibition was produced at greater intensities of stimulation (50-200 microA). At 43 sites in the RVM, electrical stimulation only produced intensity-dependent inhibition of the TF reflex. Activation of cell bodies in the RVM by glutamate microinjection reproduced the biphasic modulatory effects of electrical stimulation. At biphasic sites previously characterized by electrical stimulation, glutamate at a low concentration (5 nmol) produced facilitation of the TF reflex; a greater concentration (50 nmol) only inhibited the TF reflex. In electrophysiological experiments, electrical stimulation at 62 sites in the RVM produced biphasic (n = 26), only inhibitory (n = 26), or only facilitatory (n = 10) modulation of responses of lumbar spinal dorsal horn neurons to noxious cutaneous thermal (50 degrees C) or mechanical (75.9 g) stimulation. Facilitatory effects were produced at lesser intensities of stimulation and inhibitory effects were produced at greater intensities of stimulation. The apparent latencies to stimulation-produced facilitation and inhibition, determined with the use of a cumulative sum method and bin-by-bin analysis of spinal neuron responses to noxious thermal stimulation of the skin, were 231 and 90 ms, respectively. The spinal pathways conveying descending facilitatory and inhibitory influences were found to be different. Descending facilitatory influences on the TF reflex were conveyed in ventral/ventrolateral funiculi, whereas inhibitory influences were conveyed in dorsolateral funiculi. The results indicate that descending inhibitory and facilitatory influences can be simultaneously engaged throughout the RVM, including nucleus raphe magnus, and that such influences are conveyed in different spinal funiculi.

Characterization of descending inhibition and facilitation from the nuclei reticularis gigantocellularis and gigantocellularis pars alpha in the rat

Descending influences on the spinal nociceptive tail-flick (TF) reflex produced by focal electrical stimulation and glutamate microinjection in the nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGC alpha) were examined and characterized in rats lightly anesthetized with pentobarbital. Both inhibition and facilitation of the TF reflex were produced by electrical stimulation at identical sites in the NGC/NGC alpha; glutamate microinjection only inhibited the TF reflex. The chronaxie of stimulation for inhibition of the TF reflex was 169 +/- 28 microseconds. Inhibition of the TF reflex by stimulation was produced throughout the NGC and NGC alpha; intensities of stimulation for inhibition were least in the ventral NGC and in the NGC alpha. At threshold intensities of stimulation, inhibition of the TF reflex did not outlast the period of stimulation. Facilitation of the TF reflex was produced at many of the same sites at which stimulation inhibited the TF reflex, but always at lesser intensities of stimulation (mean, 10 microA vs. 43 microA for inhibition, n = 25). Stimulation in the NGC/NGC alpha at threshold intensities for facilitation or inhibition of the TF reflex did not significantly affect blood pressure. Strength-duration characterization of electrical stimulation and microinjection of glutamate into identical sites in the NGC and NGC alpha suggest that descending inhibition of the TF reflex results from activation of cell bodies in the NGC and NGC alpha.

Periaqueductal gray inhibition of viscerointercostal and galvanic skin reflexes

In adult cats anesthetized with urethan-chloralose, effects of descending volleys from the mesencephalic periaqueductal gray (PAG) upon the viscerointercostal and galvanic skin reflexes were studied. The viscerointercostal reflex (VIR) was evoked by electrical stimulation of the greater splanchnic nerve and was recorded from the 11th, 12th or 13th intercostal nerve. Conditioning stimuli applied to the PAG inhibited the VIR. The inhibition was particularly marked when the nucleus raphe dorsalis (NRD) or its immediately adjacent ventromedial PAG was stimulated. A train of pulses was required in order to produce a recognizable PAG/NRD inhibition of the VIR. When the PAG/NRD was stimulated at 300-500 Hz, stimulation-produced inhibition became more pronounced in parallel with increase in number of pulses in each train and levelled off at about 5 pulses. The most effective frequency of PAG/NRD stimulation was found within this frequency range. Degree of PAG/NRD stimulation-produced inhibition of the VIR was dependent upon the strength of the test stimulus applied to the splanchnic nerve; the weaker the test stimulus, the more marked the inhibition. PAG/NRD stimulation-produced inhibition of the VIR was completely eliminated by bilateral section of the dorsolateral funiculi. The same section enhanced the VIR per se. It was suggested that PAG/NRD stimulation-produced inhibition of the VIR is mediated by descending pathways in the ipsi- as well as contralateral dorsolateral funiculi, and that the VIR per se is tonically inhibited by descending impulses in these pathways. PAG/NRD stimulation inhibited the segmental polysynaptic reflex in the intercostal nerve, but had little effect on the segmental monosynaptic reflex in the same nerve. Intravenous administration of morphine suppressed the VIR. The suppression was antagonized by intravenous naloxone. In contrast, PAG/NRD stimulation-produced inhibition of the VIR was unaffected by intravenous naloxone. Electrical stimulation of the splanchnic nerve evoked the galvanic skin reflex (GSR) from the forepaw pad. The GSR was inhibited by electrical stimulation of the PAG/NRD. The PAG/NRD stimulation-produced inhibition of the GSR was completely eliminated by intravenous administration of naloxone (0.4 mg/kg).

Bilateral lesions of the dorsolateral funiculus of the cat spinal cord: effects on basal nociceptive reflexes and nociceptive suppression produced by cholinergic activation of the pontine parabrachial region

In cats, bilateral microinjections of the cholinergic agonist, carbachol (0.6 micrograms in 0.2 microliter), into an area surrounding the lateral half of the brachium conjunctivum (BC) produces a non-narcotic suppression of nociceptive responses, as assessed by flexion reflexes (tail-flick and calibrated pinch tests). Bilateral lesions of the dorsolateral funiculi (DLF) of the thoracic spina cord (T2) significantly reduced the magnitude of this nociceptive suppression. Nociceptive suppression following carbachol microinjections into sites along the dorsal aspect of BC was reduced by DLF lesions to a greater degree than nociceptive suppression following injections into sites within or ventral to BC. Relatively superficial DLF lesions produced reductions in nociceptive suppression which were equivalent to reductions induced by deeper lesions. DLF lesions, either superficial or deep, produced equivalent, reliable decreases in tail-flick test assessments of baseline nociceptive thresholds. The magnitude of decreases in baseline nociceptive thresholds produced by DLF lesions was not correlated with the magnitude of reduction of carbachol-induced suppression of nociceptive responses, indicating that DLF lesions suppress anti-nociception independent of baseline alterations. These data suggest that non-narcotic analgesia produced by cholinergic activation of cells along the dorsal aspect of BC may be predominantly mediated by fibers descending within the DLF. However, results of the retrograde horseradish peroxidase (HRP) tracing studies reported in the present investigation indicate that this pain suppression is probably mediated by polysynaptic pathways since this region dorsal to BC projects neither through DLF nor extra-DLF pathways. Retrograde HRP data show that areas ventral to and including BC projects to the cord via both DLF and extra-DLF pathways. Since DLF lesions were less effective in reducing analgesia attained from ventral compared to dorsal sites, spinal pathways other than DLF may mediate reflex suppression following carbachol microinjection into these more ventral sites. Possible cholinergic contributions to endogenous, non-opiate forms of analgesia are discussed.

Induction of spinal seizures by natural stimulation in cats

Transection of the spinal cord of the cat at a thoracic or lumbar level results, after as short a period as 12 days, in a preparation with such altered excitability that repeated natural stimulation of the dermatome just caudal to the transection site will induce, in as short a time as 3 days, seizure discharges. The trigger zone for the seizure spreads to caudal dermatomes when these caudal regions are repeatedly stimulated. The 'typical' T4-T7 seizure is a scratch reflex followed by the tonic-clonic seizure lasting for 20-30 s and ending with a scratch afterdischarge lasting for several minutes. Lower thoracic and upper lumbar seizures consist of tonic-clonic co-contractions of the muscles of the hindlegs, followed by rhythmical stepping movements lasting less than 1 min. Partial dorsal rhizotomy or local Cobalt application to the spinal cord may reduce the threshold for induction of seizure by natural stimulation and local Penicillin application to spinal cord induces seizure discharges similar to those induced by natural stimulation. Retransection of the spinal cord caudally, with elimination of the primary trigger zone, does not abolish the secondarily acquired triggers. The findings suggest that spinal circuits possess the ability to acquire new neuronal patterns of discharge and to transfer them to other more caudal segments.

Non-opiate analgesia induced by carbachol microinjection into the pontine parabrachial region of the cat

These studies investigated the effect of microinjection of the cholinergic agonist carbamylcholine (carbachol) into various sites of the dorsolateral pontine tegmentum of the cat. Carbachol microinjection into an area surrounding the lateral half of the brachium conjunctivum (parabrachial region, PBR) produced profound suppression of nociceptive responses. In the dorsal part of PBR, carbachol microinjection produced no generalized sensory, emotional or motor deficits, indicating that nociceptive transmission was primarily affected. Carbachol microinjection into the ventral part of PBR resulted in slight suppression of motor responses in addition to profound nociceptive suppression. Carbachol-produced analgesia (CPA) observed within PBR blocked supraspinally as well as spinally integrated responses normally elicited by either phasic or tonic noxious stimuli. Atropine sulfate, but not mecamylamine hydrochloride, significantly antagonized CPA, indicating that muscarinic receptors mediate this phenomenon. The opiate antagonist naloxone, systemically administered either prior to or after carbachol microinjection, did not reliably attenuate CPA. Microinjection of morphine into the sites from which CPA had previously been obtained did not produce significant effects on nociceptive responses. Thus, opiate mechanisms appear not to be necessary either for the activation of this system or for the production of the resultant analgesia. These findings indicate that the neural population examined in the present study is anatomically and pharmacologically distinct from previously identified opiate-mediated pain inhibitory systems. Results are discussed in light of other recent evidence indicating the existence of endogenous non-opiate pain inhibitory systems.

Behavioral evidence for a cholinoceptive pontine inhibitory area: descending control of spinal motor output and sensory input

Earlier studies have shown that microinjection of the cholinergic agonist carbamylcholine (carbachol) into the rostral pontine tegmentum of the cat elicits postural atonia. However, conflicting reports exist regarding other concomitant behavioral changes. The present study has demonstrated that a variety of functions supporting animals' responsiveness to external stimuli including postural somatomotor, sympathetic visceromotor and nociceptive somatosensory functions are differentially affected depending upon the injection sites. Sites associated with maximal effects on each of these functions are clustered in the dorsal pontine tegmentum, i.e. cholinoceptive pontine inhibitory area (CPIA). In a medial area of CPIA, which corresponds to an area caudal to the ventral tegmental nucleus of Gudden and ventromedial to the principal nucleus of locus coeruleus, postural somatomotor and sympathetic visceromotor functions were maximally suppressed. In a laterally adjacent area ventral to the principal nucleus of locus coeruleus, somatomotor function was predominantly suppressed. Nociceptive somatosensory function was primarily suppressed following microinjections into a more lateral area surrounding the lateral half of the brachium conjunctivum. Several lines of evidence suggest that each of these phenomena ultimately involves descending influences on the spinal motor output and/or sensory input. There was no correlation between maximal suppression of these spinal cord functions and signs of desynchronized sleep such as rapid eye movement. Carbachol microinjection into wide areas of CPIA also suppressed orienting behaviors. Taken together, these data suggest that CPIA is a system which primarily regulates animals' responsiveness to external stimuli, in part by influencing segmentally organized behaviors.

Reorganization of cerebral control of tactile placing after interrupting a spinal ascending system in cats with pyramid section

Interruption of a spinal ascending system by hemisection of the spinal cord at mid-thoracic level restores the tactile placing (TP) which is lost after bulbar pyramid section in cats. To determine the location of the ascending system in the spinal cord, partial lesions involving various dorsal, lateral, or ventral tracts were made at mid-thoracic level in cats with pyramid section. To determine the supraspinal control of the recovered TP after pyramid section and spinal cord hemisection, lesions were made in other cortical and subcortical structures essential to TP and also in cortical areas which are not normally involved in TP in cats with recovered TP. The findings demonstrate that the spinal ascending system is located in the ventral part of the lateral funiculus and that the cerebral association cortex takes over the function of motor cortex in the reorganized control of TP. A slow course of TP recovery along with no recovery of impaired distal forelimb movements resulting from pyramidotomy were also observed.

Increase of extensor tonus of forelimbs in chronic cats with bilateral serial hemisections of the spinal cord at different levels

An increase of extensor tonus of the forelimbs was observed in chronic cats with serial double hemisections of the spinal cord, first at a lower thoracic level followed by a contralateral hemisection at the midthoracic cord at intervals of 5-18 weeks. A similar increase in forelimb extensor tonus was observed in chronic cats with serial double hemisections, first at the high cervical cord followed by a contralateral hemisection of the mid-thoracic cord at intervals of 3-7 weeks. The results suggest that the augmentation of extensor tonus was brought about by release of bulbar centers from ascending inhibitory mechanisms from the lower spinal cord.

Tonic descending influences on cat spinal cord dorsal horn neurons

The extent and nature of tonic supraspinal influences was determined on cat spinal cord dorsal horn neurons that received both noxious (radiant heat) and nonnoxious (hair movement) inputs or only a nonnoxious input. The former cells receive a tonic inhibition that descends in the dorsolateral funiculi and which is selective for the noxious input. The latter neurons are under a tonic facilitation.

Motoneuron membrane changes associated with spinal shock and the Schiff-Sherrington phenomenon

Following transection of the thoracic spinal cord, in a decerebrate cat, the forelimbs exhibit an enhanced extensor stretch reflex while the ipsilateral flexion reflex is more difficult to elicit (Schiff-Sherrington Phenomenon). The hindlimbs on the other hand have an increased threshold for the extensor stretch reflex while flexion reflexes are hyperactive (spinal shock). This investigation was designed to examine the synaptic events mediating the Schiff-Sherrington Phenomenon (SSP) and spinal shock; and to characterize any differences in the response of flexor and extensors alpha-motoneurons. This was accomplished by measuring membrane potential and input resistance of identified forelimb and hindlimb alpha-motoneurons before, during, and after coldblock of the low thoracic cord. During post-brachial spinal cord coldblock, forelimb extensor motoneurons depolarized while flexor motoneurons hyperpolarized. Both flexor and extensor motoneurons in the lumbar cord hyperpolarized. These observations account at least in part for the behavioral manifestation of increased extensor tone seen during the SSP and for the reflex depression seen during spinal shock. The membrane potential changes observed in this study were mediated through a direct effect on alpha-motoneurons since these animals were flaxedilized and gamma loop activity was probably negligible. The hyperpolarization of hindlimb motoneurons was apparently due to the removal of descending facilitation, while the depolarization seen in forelimb extensor motoneurons appeared to be due to a release of facilitation. The mechanism of the hyperpolarization observed in some forelimb motoneurons was unclear.

Descending influences on receptive fields and activity of single units recorded in laminae 1,2 and 3 of cat spinal cord

Units (108) were isolated in laminae 1,2 and 3 in segments L7-Sl of decerebrate cat spinal cord. For each unit, the size and nature of its receptive field (RF) was delineated. Then the dorsolateral funiculus (DLF) was stimulated for 1 sec with 10 or 50 Hz, 0.1 msec square waves and the response characteristics of the unit were again examined. Of the 108 units, 55 were excited or facilitated, 6 were inhibited (all in lamina 3) and 47 were unaffected. While some of the excited units responded only during the stimulus train, the majority showed prolonged excitation or facilitation lasting over one minute. The excited units were predominantly those responding to pressure or to brush, touch and pressure. Of the pressure units, 73% were excited or facilitated in contrast to only 29% of the brush/touch units. Most of the excited units showed expansion of their RFs. While many units of this type show ongoing variations of excitability and RF size, the evoked responses reported were sufficiently time-locked to the stimulus for it to be apparent that they were caused by the DLF stimulation. The unit's responses still occurred when the DLF was stimulated caudal to a complete cord transection so that the effects did not pass through the brain stem. The major effect of descending systems or of DLF stimulation previously reported on the large cells of laminae 1,4 and 5 has been inhibition. Here we report that a major descending influence on many units of laminae 1,2 and 3 is excitatory. Therefore it is suggested that a population of small interneurons in the superficial laminae could contribute to the descending inhibition of large dorsal horn neurons.