The effect of chronic undernourishment on the synaptic depression of cutaneous pathways in the rat spinal cord

The aim of this study was to determine the effect of chronic undernourishment on the amplitude depression of the first negative component in the cord dorsum potentials (N(1)-CDPs) caused by the conditioning stimulation of sensory cutaneous nerves in the rat spinal cord. Single electrical pulses (1Hz; 2 times threshold) applied to the sural (SU) nerve of control rats (n=14) produced CDPs with a first negative component (N(1)-CDPs) larger in amplitude (14.2±1.3%, p<0.01) than those recorded in chronically undernourished rats (n=14; 3 times threshold). The conditioning stimulation of the SP nerve (4 shocks at 300Hz, 3×T) in the control rats (n=5) evoked a long-lasting (～200ms) depression of the N(1)-CDP (60.2±7.2%). In contrast such depression was smaller in magnitude (42.5±5.7%, p<0.01) and time course (100-120ms) in undernourished rats (n=7). The systemic application of picrotoxin (PTX) reduced, but did not abolish the conditioned depression of the N(1)-CDPs and DRPs in both the control and undernourished rats. By assuming that the depression of the N(1)-CDPs is representative of presynaptic mechanisms, it is proposed that chronic undernourishment influence the activation of presynaptic neuronal pathways that regulate the transmitter release of cutaneous afferent fibers in the spinal cord and such effect could act as a compensatory mechanism that counterbalances the decreased activation of spinal neurons by the reduced afferent input in the rat.     

Phase-dependent modulation of dorsal root potentials evoked by peripheral nerve stimulation during fictive locomotion in the cat

To help elucidate the role of presynaptic mechanisms in the control of locomotor movements, the transmission of PAD pathways was investigated by recording dorsal root potentials (DRPs) evoked by electrical stimulation of cutaneous and muscle nerves of both hindlimbs at various phases of the fictive step cycle. Fictive locomotion occurred spontaneously in decorticate cats or by stimulating the mesencephalic locomotor region (MLR) as well as in low spinal cats injected with nialamide and L-DOPA. Evoked DRPs were superimposed on a fluctuating DRP accompanying the fictive locomotor rhythm (locomotor DRP) which typically consisted of two peaks of depolarization per cycle, the largest peak occurring during the flexor phase. The amplitude of evoked DRPs was substantially modulated throughout the locomotor cycle and followed a similar modulation pattern for all stimulated nerves whether ipsilateral (i-) or contralateral (co-). The amplitude of evoked DRPs decreased at the beginning of the flexor phase, dropped to a minimum later in the flexor phase and then increased during the extensor phase where it became maximum. Results were comparable in decorticate and spinal preparations and for L6 and L7 rootlets with cutaneous and muscle nerve stimulation. It is noteworthy that the modulation pattern for a given rootlet was similar for i- and co- stimulation, even though the bilateral locomotor DRPs fluctuate out-of-phase with each other, subjecting the stimulated fibres to opposite presynaptic polarization changes. This suggests that the modulation may depend more on the presynaptic mechanisms of the receiving fibres than on those of the stimulated fibres. These results demonstrate that the transmission in spinal pathways involved in primary afferent depolarization (PAD) is phasically modulated by the activity in the spinal locomotor network. It is further suggested that the presynaptic inhibition associated with PAD evoked by movement-related sensory feedback during real locomotion could be modulated in a similar way.     

Dorsal root potentials and afferent input to the spinal cord in rats with an experimental peripheral neuropathy.

Compound action potentials (CAPs), dorsal root potentials (DRPs) and cord dorsum potentials evoked by stimulating the sciatic nerves have been measured in 4 control rats and in 19 rats with a constriction injury of one sciatic nerve produced by loose ligation of the nerve at mid-thigh level 5 days (n = 8) or 10 days (n = 11) before the acute experiments. The contralateral nerve was exposed but not ligated in a sham procedure. In all cases, the nerve was stimulated proximal to the lesion. At 5 days post-operative (PO) the maximal A-fibre CAPs on the nerve-injured side were not significantly different from those on the sham-operated side. At 10 days PO all animals showed a decrease in the CAP on the nerve-injured side. The mean CAP area on the nerve-injured side was 74.0% +/- 4.2 of the sham-operated side, which was significantly different (P less than 0.005). The sciatic nerves and L5 dorsal roots from 4 of the 10 day PO animals were examined histologically and showed no signs of demyelination or degeneration. The amplitude and area of the maximal DRPs were significantly smaller on the nerve-injured side than on the sham-operated side in all of the nerve-injured animals (P less than 0.01 at 5 days PO; P less than 0.05 at 10 days PO). The mean area of DRPs from the nerve-injured side was 61.7% +/- 10.1 and 46.8% +/- 7.5 of the DRPs from the sham-operated side in the 5 and 10 day PO animals, respectively. The DRPs evoked by sub-maximal afferent volleys were also measured. In all of the nerve-injured animals the CAP-DRP curve on the nerve-injured side was shifted to the right compared to that of the sham-operated side, such that a given size of CAP evoked a smaller DRP on the nerve-injured side than on the sham-operated side. We conclude that the constriction injury produces a decrease in the DRP generated by a volley in the injured nerve and that this change is independent of the decrease in the CAP seen in the injured nerve. We propose that the constriction injury affects the central mechanism responsible for generating primary afferent depolarization (PAD), and thus the pre-synaptic inhibitory control of the afferent input from the injured nerve is impaired.     

Depolarization of primary afferents evoked by cyclically modulated natural activity of proprioceptors of the hindlimb of the cat

The primary afferent depolarization (PAD) evoked during passive sinusoidal movements of a hindlimb in the ankle joint was investigated in decerebrated cats. The frequency of movements varied within 0.14-5.0 Hz, the amplitude of the joint angle with respect to the axis of the tibia changed from 90 degrees to 130 degrees. The dorsal root potential (DRP) negativity increased both during flexion and during extension of the joint. The amplitude of the evoked DRPs was about 50-100 mV. A strong negative correlation was observed between the latency and rise time of the DRP and the frequency of the joint angle changes. During flexion the latency changed from 650 ms at 0.14-0.16 Hz frequency to 100-110 ms at 2.0 Hz and higher frequencies; during extension at the same frequencies the latency changed from 300 ms to 80-85 ms. The latency and rise time became minimal at 2.0 Hz frequency and practically did not change during the further increase of the oscillation frequency. The cord dorsum potential (CDP) evoked by the cutaneous nerve stimulation was recorded in parallel with the DRP. Periodical changes of the N-component of the CDP were in the opposite phase to changes of the DRP. Mechanisms of the observed changes of the PAD and functional significance of these changes during rhythmical motor acts are discussed.     

Neurochemical and ionic mechanisms of dorsal root potentials in the spinal cord of the immature rat

Dorsal root potentials (DRP) recorded from spinal cord of 7-14-days old rats have two waves of depolarization. The fast wave of DRP is GABA-ergic in nature and the slow wave is evoked mainly by increasing of extracellular K+-ion concentration near the primary afferent terminals. The possible mechanisms of increasing extracellular K+-ion concentration evoked by dorsal root stimulation are discussed.     

Brain hypomyelination during postnatal undernourishment: a comparison of proteolipid protein synthesis versus assembly into membrane

A double isotope methodology which permits the separate analysis of (i) the synthesis of myelin proteolipid protein and (ii) its subsequent assembly into myelin membrane was applied to the case of postnatal undernourishment of rats. Results showed that brain proteolipid protein synthesis was greatly depressed, indicating that undernourishment limited the production of myelin components. This is a different result from that obtained in the mouse Quaking mutant, in which case component formation is normal whereas their assembly into myelin membrane is blocked. The results have possible implications regarding cellular development of oligodendroglia of undernourished rats.     

The contribution of increases in extracellular potassium to primary afferent depolarization in the bullfrog spinal cord

To assess the extent to which depolarization by accumulated K+ contributes to the generation of primary afferent depolarization (PAD), the isolated bullfrog spinal cord was superfused with K+-rich Ringer solutions and the resultant dorsal root depolarizations were recorded extracellularly. Action potential blockade (with tetrodotoxin) did not reduce the K+-induced depolarization of primary afferents, indicating that the depolarization was generated locally in the region around the afferents. In this respect superfusion with K+-rich solutions adequately models the localized K+ accumulation which occurs physiologically during afferent activity. K+-induced depolarizations were decreased in the presence of 20 mM Mg2+; this effect was due to a direct decrease in the membrane response to K+ and not to blockade of K+-induced transmitter release onto primary afferents. The depolarization caused by a K+ concentration comparable to a maximum estimate of the K+ accumulating around afferent terminals following a single afferent volley was found to account for no more than about one-third of the DRP height. However, higher K+ levels, comparable to those resulting from high frequency afferent stimulation, caused large depolarizations of primary afferents, sometimes greater than the DRP amplitude. Therefore, K+-induced depolarization may contribute more significantly to PAD evoked by high frequency afferent activity.     

The effects of convulsant doses of penicillin on primary afferents, dorsal root ganglion cells, and on ‘presynaptic’ inhibition in the spinal cord

Intracellular recordings were made from neurons in dorsal root ganglia (DRG) of rats, isolated in vitro. The depolarization of DRG cells caused by the application of gamma-aminobutyric acid (GABA) diminished reversibly when penicillin (0.08–2.0 mM) was added to the bathing fluid. The decrease of the input resistance of DRG cells measured during GABA perfusion was also depressed in the presence of penicillin, but no evidence of a shift of the reversal potential of the GABA-induced depolarization was found. Nor did penicillin (up to 10 mM) cause a change in the voltage-current function, in electrical excitability, in the inclination to repetitive firing, bursting discharge, or after discharge. In decapitate cat preparation the amplitude of the negative dorsal root potential (DRP or DR V) diminished by 0–50% after the i.v. administration of 0.5–1.0 × 10⁶I.U./kg (the convulsant dose) of penicillin. Post-tetanic depression of the DRP was aggravated by penicillin. The degree of depression of the DRP bore no relationship to the promptness of the eruption, and to the intensity, of the seizure activity induced by penicillin. The rates of rise and fall of the negative DRP (DR V) were consistently slowed, the positive DRP (DR V) reduced, and the dorsal root reflex (DRR) blocked by penicillin. Inhibitory reflex effects presumed to be presynaptic were either enhanced or unchanged, never depressed by penicillin. This was seen when inhibitory function was gauged by monosynaptic reflex amplitude, and also from the inhibition of ventral root electronic excitatory postsynaptic potentials (VR EPSPs). Possible explanations of these seemingly paradoxical findings are discussed, with arguments in favor and against each.     

The effect of peripheral nerve injury on dorsal root potentials and on transmission of afferent signals into the spinal cord

The sciatic nerve adults rats was either cut and ligated or was crushed on one side. The response of the spinal cord to stimulation of the proximal part of the injured nerve was examined at various times after the lesion and compared to the effects of stimulating the intact nerve on the other side. During the first 10 days after nerve section the following measures were not affected: (i) the size of the input volley (compound action potential, CAP, measured on a dorsal root that carried sciatic nerve afferents (L5); (ii) the volley running in the dorsal columns; (iii) the dorsal root potential (DRP) evoked on neighbouring dorsal roots which do not contain sciatic afferents (L2 and L3); (iv) the post-synaptic volleys ascending in the spinal cord. However, by the fourth day after nerve section, there was a decrease of the DRP evoked on the ipsilateral L5 dorsal root by stimulation of the cut nerve. By 10 days this DRP had decreased by 50%. There was also a decrease in the DRP on the L5 root evoked by stimulation of the contralateral intact nerve. Crush lesions of the sciatic nerve did not produce DRP charge. Beginning 10–20 days after nerve cut, there was a decrease in the amplitude of the afferent CAP and of all the measures of central response to the afferent volley. We discuss the possibility that the loss of the DRP may be associated with a disinhibition which results in novel receptive fields which we observe in cord cells deafferented by the peripheral nerve section. The decrease of DRP and the appearance of novel receptive fields do not occur if the peripheral nerve is crushed rather than cut.     

Characteristics of dorsal root potentials recorded from the isolated spinal cord of the neonate rat

Summary Dorsal root potentials (DRP) and dorsal root reflexes (DRR) have been recorded from the isolated cord of the neonate rat. A single stimulus to the adjacent rostral or adjacent caudal dorsal root or dorsal columns evoked a DRP, the peak amplitude of which was reached in 110–115 msec and which decayed exponentially over most of its time course (time constant 800–850 msec). The same stimuli evoked field potentials in the dorsal horn comprising fast negative, slow negative and slow positive potentials. DRP had a lower threshold than DRR and reached a maximal amplitude at stimulus voltages sub-maximal for DRR. Increasing the intensity of stimulation shortened the latency of DRP and prolonged its time course. DRR and DRP were depressed by a prior conditioning stimulus (CS) and by the addition of Mg⁺⁺ ions to the bathing solution. A CS was more effective in producing depression of responses evoked more rostrally than more caudally.     

Furosemide depression of the dorsal root potentials and of the presynaptic inhibition of monosynaptic reflexes in the cat spinal cord

In experiments on spinal narcotized cats perfusion of the lumbosacral cord through the central canal with artificial cerebrospinal fluid containing furosemide (15-48 mmol/l) led to the reversible selective depression of negative DRP and to the depression of prolonged "presynaptic" inhibition of extensor monosynaptic reflexes produced by volleys in flexor muscle afferents of group 1.     

Epidurally recorded cervical somatosensory evoked potential in humans

Three slow wave components, P10, N13 and P18, can be seen in the cervical somatosensory evoked potential (CSEP) in response to median nerve stimulation recorded by an electrode in the epidural space at the dorsal aspect of the cervical spinal cord referenced to an electrode at the suprasternal notch. In the region of high CSEP amplitude, which extends over several cervical segments, the peak-to-peak amplitude is more than 10 microV, permitting observation of the CSEP slow waves in single, unaveraged records. The CSEP to finger nerve stimulation had a similar wave form and the same latencies (referred to the Erb's potential) as the CSEP to median nerve stimulation. The P10 activity is of presynaptic origin; it is generated in the brachial plexus, spinal roots and terminal branches of the primary sensory fibers. The N13 slow wave is of postsynaptic origin; however, the small wave on the ascending phase of this main postsynaptic component represents superimposed presynaptic activity. In bipolar epidural recordings, 3-5 fast waves are superimposed on the slow CSEP waves, which are of lower amplitude than the slow waves in unipolar recordings. The fast waves show a slight but progressive delay at the more rostral recording sites and are present even with high frequency stimulation, presumably reflecting activity in long ascending tracts. The surface recorded CSEP to median nerve stimulation is 4-7 times lower in amplitude than the CSEP in unipolar epidural recordings. The small wave on the ascending phase of N13 and the N13 peak of the unipolar epidural recordings had the same latencies as the surface N11 and N13 peaks.     

Epidurally recorded cervical spinal activity evoked by electrical and mechanical stimulation in pain patients

Spinal SEPs to electrical and mechanical stimulation of the upper limb of the non-painful side in 7 pain patients were recorded from the cervical epidural space. In response to electrical stimulation of the median nerve, the longitudinal distribution of the spinal postsynaptic negativity (N13) along the cord had a distinct level of maximal amplitude at the C5 vertebral body. When recorded at increasing distances cranial or caudal to this level, the latency of N13 was successively prolonged, in agreement with a spread-out near-field generator in the dorsal horn. Similar patterns of distribution and levels of maximal amplitude were demonstrated for the N13 wave evoked by electrical stimulation of the ulnar and thumb nerves as well as by mechanical stimulation of the thumb ball. The amplitude ratios of the N13 waves evoked by electrical stimulation of the median nerve and the thumb nerves, and by mechanical stimulation of the thumb ball were 3.9 to 1.4 to 1. The slow positive wave (P18), which has been assumed to represent recurrent presynaptic activity, had a somewhat different distribution, with a lower maximal amplitude and a less marked falling off in amplitude along the cord, as compared to the N13 component. The initial presynaptic positivity (P10) appeared with an almost constant amplitude along the cord. Tactile stimuli produced responses with considerably longer latency and duration than those obtained with electrical stimulation. There seemed to be a non-linear relationship between the amplitude of the response and the depth of skin indentation. The presented data contribute a more detailed picture of epidurally recorded spinal SEPs than previous studies. They will serve as a reference for further analysis of SEPs evoked by stimulation of the affected side in pain patients, to explore whether the painful state is associated with altered SEPs before or after therapeutic intervention.     

Recurrent potentials in human peripheral sensory nerve: possible evidence of primary afferent depolarization of the spinal cord.

To study slowly conducted components of the orthodromic compound sensory action potential (CSAP), the response evoked at the lateral malleolus in the sural nerve was recorded through near-nerve needles at two to four sites along the nerve at midcalf. When 500 to 2000 responses were averaged at high gain, components with latencies of 30 to 80 ms were often recorded. In contrast to the main component and late components with latencies of less than 15 to 20 ms, the latencies of these extremely late components diminished the closer to the spinal cord that they were recorded. This suggested that the components were conducted antidromically from proximal to distal. This assumption was supported by abolishing the components by local anesthesia of the nerve proximal to the recording electrodes. These antidromic potentials therefore appear to be due to recurrent discharges in the sural nerve. Recurrent discharges were recorded from 65% of 60 subjects (18 normal subjects and 42 patients with peripheral or central nervous system disorders). The latencies of the recurrent discharges allowed conduction to and back from the spinal cord. Although the origin of these potentials remains unknown, we suggest that they are due to dorsal root reflexes within the spinal cord. In this case, the responses may be a direct expression of primary afferent depolarization (PAD) seen in presynaptic inhibition, and may be of value in further studies on the physiology and pathophysiology of presynaptic inhibition of cutaneous fibers in man.     

Phase-dependent changes in the posterior root potentials during real locomotion in rats

Dorsal root potentials (DRP) were studied during two kinds of the real locomotion (swimming and stepping) of rats. Two negative DRP waves were observed in one locomotor cycle. One wave coincided with stance on extension, while the other one--with swing phase. Dependence between the DRP amplitude and intensity of afferent input during the real locomotion and passive movements of hind limbs was established. It is concluded that wave-like changes of DRP in the real locomotion are mainly due to the influences from peripheral afferents.     

Subpially recorded cervical spinal cord evoked potentials in syringomyelia.

Intraoperative spinal cord evoked potentials (SCEPs) to median nerve stimulation were detected subpially from the dorsal surface of the cervical spinal cord in 5 patients with cervical syringomyelia and were compared to normal SCEPs obtained from the unaffected side in 6 patients during intraoperative monitoring of dorsal root entry zone lesion. Normal SCEP began with a positive deflection P9 and a complex N11/N13 with several low amplitude short potentials superimposed on the N11/N13. The complex was followed by a second negative potential N2 and a late prolonged positivity, P. In the 4 patients in whom median nerve somatosensory evoked potentials (SEPs) were present preoperatively, SCEP consisted of the N11 potential and the following low amplitude short (LAS) potentials, while the N13 wave was missing. In the fifth patient, in whom the preoperative median nerve SEP was missing, SCEPs were of much lower amplitude and shorter duration than normal. The potentials N2 and P were not recorded in any of our 5 patients. Changes in N13 wave, N2 and P potentials noted in syringomyelia were presumed to be the result of destruction of the spinal cord dorsal horn neurons caused by spinal cord central cavitation.     

Effect of ammonium acetate on depolarization processes in the central terminals of primary afferents]

Experiments on cats determined that ammonium acetate injected intravenously (2-4 mM/kg) supressed the processes of primary afferent depolarization (PAD) which are thought to be responsible for the presynaptic inhibition of spinal reflexes. The supression was transient and proceeded in paralle to depression of postsynaptic inhibition of monosynaptic reflexes. Ammonium acetate slightly decreased the amplitude of the negative postsynaptic potentials recorded form the dorsal surface of lumbar cord in response to stimulation of hind limb afferent nerves and increased polysynaptic reflex discharges in appropriate ventral roots. These findings make it unlikely that the ammonium depression of PAD is a result of impairment of interneuronal activity. A suggestion is made that ammonium depression of PAD results from diminition of the EMF for synaptic currents producing PAD.     

Effect of undernutrition on developing rat cerebellum: Some electrophysiological and neuromorphological correlates

Electrophysiological studies of Purkinje cells from the developing normal and undernourished rat cerebellar cortex were correlated with the neuromorphological studies. In undernourished animals the firing pattern of Purkinje cells was simpler as compared to complex electrical activity in normal animals. The firing frequency of Purkinje cells remained reduced in all the age-matched undernourished groups. The duration of a spike was also prolonged as compared to their normal age mates. Similarly the mean amplitude was also relatively smaller in the undernourished animals when compared with the normal litter mates. Undernourished animals exhibited typical increased stimulus thresholds, prolonged N-2 peak wave latencies, reduced number of functional components, amplitude and increased duration of mossy fiber response (MFR). The mossy fiber (MF) activity exhibited a sluggish rise in its amplitude, when stimulus intensity was increased in undernourished animals. A phenomenon of fatigue was observed with a significant reduction in the amplitude and frequency of Purkinje cell unit discharge on repeated MF stimulation in the undernourished animals. However, repeated MF stimulation provided a high frequency discharge in the normal as well as undernourished adult rats. Neuromorphological studies at light-microscopic level exhibited delayed cell migration from the external granular layer in the undernourished rats. At electron-microscopic level, intracellular morphology exhibited almost similar ultrastructural details except for a few minor alterations. A free ribosomal pool, immature development of E.R., increased incidence of lysosomes and electron-dense membrane stacks were observed in the Purkinje cells at 30 days in the undernourished animals. The results of the present study indicated that undernutrition affects the maturation of Purkinje cell regional neurocircuitry.     

Chronically implanted electrodes for repeated stimulation and recording of spinal cord potentials

We have recorded and characterized the spinal cord evoked potentials (SCEPs) from the epidural space in the halothane-anesthetized rats. A group of 11 adult Wistar male rats was chronically implanted with two pairs of epidural electrodes. SCEPs were repeatedly elicited by applying electrical stimuli via bipolar U-shaped electrodes to the dorsal aspect of the spinal cord at C3-4 or Th11-12 levels, respectively. Responses were registered with the other pair of implanted electrodes, thus allowing us to monitor the descending (stimulation cervical/recording thoracic) and ascending SCEPs (stimulation thoracic/recording cervical). We studied the time-dependent changes of several SCEP parameters, among them the latency and amplitude of two major negative waves N1 and N2. During 4-weeks' survival, all major components of recordings remained stable and only minor changes in some parameters of the SCEPs were detected. We concluded that this technique enables repeated quantitative analysis of the conductivity of the spinal cord white matter in the rat. Our results indicate that SCEPs could be used in long-term experiments for monitoring progressive changes (degeneration/regeneration) in long projection tracts, primarily those occupying the dorsolateral quadrants of the spinal cord. These include projections that are of interest in spinal cord injury studies, i.e. ascending primary afferents, and important descending pathways including corticospinal, rubrospinal, reticulospinal, raphespinal and vestibulospinal tracts.     

The influence of bemegride on frog spinal cord root potentials]

The effect of the convulsant bemegride (beta-ethyl-beta-methyl-glutharimide) on the electronic potentials in the frog spinal roots was investigated in situ. Intravenous injection of the bemegride subconvulsant doses (6.8 +/- 2.7 mg/kg) depressed rapidly the electrotonic dorsal root potentials (DRP) evoked by a stimulation of the adjacent dorsal root (DR) or the ventral root (VR). They were reduced by 55--67% 3-6 min after the bemegride injection. The effect of bemegride was reversible and the DRP amplitude restored its initial value within an hour. Ventral root potentials after bemegride injection revealed only greater amplitude fluctuations. A conclusion is made that bemegride is a selective and potent depressor of the primary afferent depolarization in the frog spinal cord.