Motor and electrical signs of epileptiform activity induced by penicillin in the spinal cords of decapitate cats.

Convulsive activity was induced in functionally decapitate cat preparations by topical and by systemic administration of toxic amounts of penicillin. The paroxysmal movement patterns and the electrographic signs of spinal seizure activity recorded from spinal ventral and dorsal roots and from the dorsal surface of the spinal cord are described. Paroxysms of interictal myoclonic twitching as well as tonic and clonic ictal seizures reminiscent of epileptiform convulsions of intact animals were seen in the absence of descending influences from the brain. Tonic seizures consisted of flexion--extension sequences; co-contraction of antagonistic muscles was the rule. Clonic activity consisted of rhythmic discharges at 4--6/sec, In dorsal roots, electrotonically conducted paroxysmal negative potential shifts as well as antidromically conducted trains of impulses were recorded. Ictal paroxysmal waves of the cord dorsum potential consisted of either biphasic positive--negative sequences or of purely negative waves. Diphenylhydantoin effectively controlled spinal seizures in the absence of a functioning cerebellum. Diphenylthiohydantoin changed the pattern of seizures, suppressing all ictal activity and greatly enhancing the amplitude and frequency of interictal bursts. Three different barbiturates suppressed seizure activity, but diazepam was ineffective, indicating that the site of its clinical anticonvulsant action may be supraspinal. Seizure activity, once induced, continued for up to 18 h. Intravenous administration of penicillinase abolished seizures indicating that their usual persistence is caused by the presence of the drug in the tissue, not by an irreversible biochemical lesion.     

The effects of bicuculline on the isolated spinal cord of the frog

An investigation has been made of the effects of topically applied bicuculline, a reported gamma-aminobutyric acid (GABA) antagonist, on the isolated, hemisected frog spinal cord by recording ventral and dorsal root potentials and reflexes evoked by volleys to various spinal cord inputs. Bicuculline had potent excitatory effects causing depolarization, spontaneous potentials in ventral and dorsal roots, and an increased polysynaptic ventral root reflex. More importantly, the alkaloid blocked presynaptic inhibition of orthodromic reflex activity produced by preceding ventral root stimulation and primary afferent depolarization. These effects were attributed to a demonstrated antagonism of the direct depolarizing effects of GABA on dorsal root terminals by the alkaloid. These actions of bicuculline suggest that GABA may be the transmitter responsible for primary afferent depolarization and presynaptic inhibition in the amphibian.     

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.     

Reflex effects and postsynaptic membrane potential changes during epileptiform activity induced by penicillin in decapitate spinal cords.

The administration of a convulsant dose of penicillin enhanced the transmission of monosynaptic reflexes in spinal cords in which reflex transmission was feeble before the drug treatment, but it had little effect in cords where monosynaptic reflexes were powerful to begin with. Post-tetanic potentiation was not altered by penicillin. Polysynaptic reflexes were invariably enhanced by convulsant amounts of penicillin. Postsynaptic ("direct") inhibition was not affected in the seizure-free intervals in spinal cords treated with penicillin, but it seemed to be suppressed during tonic seizures. The disability of reflex inhibition during ictal discharges may be due to presynaptic depolarization of inhibitory terminals. Recurrent inhibition was partially suppressed in spinal cords treated with penicillin. Neurons in the dorsal and intermediate gray matter were sometimes excited, sometimes inhibited, and sometimes unaffected by seizure activity of their segment. Motoneurons in the ventral horns invariably participated in the interictal and ictal activity. The timing of clonic seizure sequences coincided with bursts of Renshaw cell discharges. Action potential of abnormal amplitude and configuration were frequently observed in convulsing motoneurons. Paroxysmal depolarizing shifts (PDSs) of motoneurons were similar to those observed by other investigators in neurons in experimental epileptic foci of the cerebral cortex, except that spinal PDSs were not followed by hyperpolarizing waves.     

Primary afferent depolarization evoked by electroacupuncture in the lumbar cord of the cat

In precollicular decerebrate cats, experiments were performed to ascertain the presence of primary afferent depolarization at the slowly conducting fiber terminals of the sural nerve, in an attempt to substantiate our previous postulation of a possible presynaptic mechanism underlying acupuncture analgesia in the spinal cord. A well correlated temporal course has been observed to exist between the negativity of dorsal root potential, suppression of sural polysynaptic reflexes, and increased excitability of sural primary afferent terminals, under the influence of the same electroacupuncture to the left tsusanli point in the hindlimb. Furthermore, by a collision test and conduction velocity measurement, the acupuncture-evoked primary afferent depolarization thus indicated was found to occur solely at the terminals of the slowly conducting fibers of the sural nerve, fibers believed to transmit “pain” impulses. As primary afferent depolarization has powerful inhibitory actions and its existence is well demonstrated in the spinal cord and trigeminal system, we suggest that acupuncture can also utilize this well established mechanism in modulating “pain” information at the primary afferent level in the spinal cord. This spinal presynaptic inhibitory mechanism, however, is thought to be only a part of an overall process underlying the production of acupuncture analgesia.     

Presynaptic inhibition of the monosynaptic reflex produced by injections of nicotine or eserine in the spinal cat

In completely deafferented cats with spinal cord transected and paralyzed with gallamine, the close arterial injection of nicotine (5 to 33 μg) into the spinal cord circulation or the intravenous injection of eserine (2 mg/kg) produced a transient increase in excitability of the central terminals of primary afferent fibers. Continuous d-c records from dorsal roots during nicotine injections indicated that a depolarization of the terminals was probably responsible for the observed excitability increase. Mecamylamine prevented both the depolarization and the increase in excitability of primary afferent central terminals whereas atropine and gallamine did not. The monosynaptic reflex was always depressed following injections of nicotine or eserine, but the excitability of the motorpool was found to be increased by these drugs. It was concluded that the monosynaptic reflex depression following injections of nicotine or eserine had a significant presynaptic inhibitory component as a consequence of a drug-induced depolarization of the central terminals of Group Ia afferent fibers.     

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.     

Presynaptic origin of penicillin after discharges at mammalian nerve terminals.

The site of origin and mechanism underlying the generation of repetitive after-discharges produced by penicillin was studied in the isolated rat phrenic nerve-hemidiaphragm preparation. Application of low concentrations of sodium penicillin to the bathing solution initiated bursts of antidromic action potentials originating at or near the motor nerve terminals following single orthodromic stimuli to the nerve. Afterdischarges could not be elicited by direct stimulation of the muscle fibers alone, or when the nerve trunk was isolated from the neuromuscular junction and exposed to penicillin. D-Tubocurarine applied in doses sufficient to abolish postsynaptic responses did not diminish penicillin-induced after discharges. At concentrations which most reliably produced repetitive firing (5000 IU/ml; 8.5 mM), penicillin did not accelerate the frequency of spontaneous transmitter release (MEPPs), yet significantly increased the relative excitability of nerve endings to extracellular stimulation. It is concluded that penicillin acts directly and preferentially on presynaptic nerve terminals to induce repetitive afterdischarges which arise independently of postsynaptic depolarization, transmitter-mediated potassium efflux, or muscle fiber contraction. The results suggest that the convulsant effects of penicillin at a mammalian neuromuscular junction are due to non-depolarizing alterations in the intrinsic excitability of the terminal membrane which increase the probability of suprathreshold depolarizations during the recovery period of spike electrogenesis. Several models of the mechanisms which might produce hyperexcitability at presynaptic nerve terminals are discussed.     

Effect of chronic undernourishment on the cord dorsum potentials and the primary afferent depolarization evoked by cutaneous nerves in the rat spinal cord

The effect of chronic undernourishment on the cord dorsum potentials (CDPs) and the dorsal root potential (DRP), closely related to primary afferent depolarization (PAD) and presynaptic inhibition in the spinal cord of the rat, was analyzed in this study. Single electrical pulses applied to the sural nerve (SU) of control (n=14) and chronically undernourished (n=16) Wistar rats produced CDPs, which are composed of four components: afferent volley (AV), two negative components (N(1) and N(2)), and one positive component (P wave) and negative DRPs recorded in a small rootlet of the L6 segment of the rat. The CDPs of the control and undernourished rats with AV components of comparable amplitude (U(AV)/C(AV)=0.96), showed N(1) components of similar amplitude (U(N1)/C(N1)=0.94), but smaller P wave (U(PW)/C(PW)=0.23). A comparable reduction in the amplitude of the DRPs was obtained in the undernourished rats (U(DRP)/C(DRP)=0.36). When normalized as a function of the body mass of the animals, the CDPs and DRPs produced in undernourished rats were of significantly smaller normalized amplitude than those evoked in the control. According to these results, it is suggested that chronic undernourishment induce a depressive effect on the mechanisms generating the P wave component in the CDP and the DRPs either by decreasing the sensory input and/or the excitability of the dorsal horn neurones involved in the generation of PAD and presynaptic inhibition in the spinal cord of the rat.     

Strychnine sensitive inhibition in the dorsal horn of mammalian spinal cord

Using an in-vitro mammalian spinal cord preparation 3 pharmacologically distinct phases of inhibition have been demonstrated in the dorsal horn. An early picrotoxin-sensitive inhibition was followed by an intermediate strychnine-sensitive phase and a late phase resistant to both strychnine and picrotoxin. The picrotoxin-sensitive inhibition was accompanied by an increase in excitability of the afferent fibres which was prolonged by strychnine, suggesting the presence of glycine mediated inhibition of GABA primary afferent depolarization.     

Epileptogenic action of tungstic acid gel on cat lumbar motoneurons

Injection of tungstic acid gel (but not pH adjusted saline) into the cat lumbar ventral horn results in spontaneous, epileptiform activity consisting of waves of repetitive, high frequency, action potentials in motoneurons surrounding the injection site. In most motoneurons the action potentials are grouped in high frequency bursts composed of action potentials triggered from the delayed depolarization of the preceding action potential. The same kind of bursting can be triggered by intracellular current pulses, indicating that altered neuronal membrane properties are associated with the bursting activity. This type of bursting differs markedly from that seen in motoneurons during penicillin or strychnine-induced spinal seizures.     

Characteristics of K- and veratridine-induced release of ATP from synaptosomes prepared from dorsal and ventral spinal cord

K+ and veratridine released 2-3 times more ATP from dorsal than from ventral spinal cord synaptosomes (P2). K+-induced release of ATP was Ca2+-dependent whereas veratridine-induced release was augmented in a Ca2+-free medium. Twenty-one to 24 days after section of the right sciatic nerve of the rat the evoked release of ATP from right dorsal synaptosomes was indistinguishable from release from left dorsal synaptosomes. Although these latter results suggest that ATP may not be a transmitter at primary afferent synapses in the spinal cord, it is possible that sciatic nerve section does not deplete the releasable pool of ATP in primary afferent terminals the releasable pool of ATP in primary afferent terminals or that ATP is also released from interneurons in the dorsal spinal cord.     

Differential decerebrate control of depolarization in the central terminals of cutaneous afferents in the sacral cord

Presynaptic depolarization of cutaneous afferents has been investigated in the sacral cord of decerebrate cats before and after spinal cord transection. In the decerebrate state the central terminals of caudal femoral cutaneous nerve are depolarized by ipsilateral volleys entering the cord via sacral and lumbar dorsal roots. A significant increase of depolarization occurring after severing the cord indicates that there is tonic decerebrate inhibition of presynaptic depolarization in terminals of caudal femoral cutaneous nerve. In contrast to this finding, presynaptic depolarization evoked in the central terminals of the pudendal nerve by ipsilateral volleys entering the cord through sacral and lumbar dorsal roots is not subjected to decerebrate inhibitory control. It is suggested that differential inhibitory control of depolarization in the central terminals of cutaneous nerves in the sacral cord is related to the intraspinal course of their fibres, to differences in the receptor types involved, and to the location of their innervation fields. In more than half of the decerebrate preparations stimulation of the central terminals of cutaneous afferents through microelectrodes evokes antidromic spikes appearing simultaneously in ipsi- and contralateral nerves. The time course of bilateral excitability changes is similar on both sides of the cord. It is assumed that presynaptic effects are transmitted to the contralateral side by collaterals of ipsilateral cutaneous afferents.     

Brainstem activates paroxysmal discharge in human generalized epilepsy

In nine patients with generalized epilepsy of convulsive seizures, the excitability change of the brainstem was evaluated over the course of the interictal paroxysmal discharge (poly spike-and-wave complex, poly SWC). The evaluation was carried out by a sequential analysis of brainstem auditory evoked potentials (BAEPs) before and during one sequence of poly SWC. The characteristics of BAEPs, i.e. far-field evoked potentials, allowed the evaluation of the excitability change in the brainstem, which was not influenced by the cortical activity. The excitability in the ventral brainstem, measured with the parameters of wave-III, showed a biphasic fluctuation (deceleration--acceleration) before the onset of poly SWC (minima at -0.7+/-0.4 s). On the other hand, the excitability in the dorsal brainstem, measured with the parameters of wave-V, showed no significant difference over the course of poly SWC. The results suggest that the biphasic excitability change in the ventral brainstem is conveyed to the cortex through the ascending activating system. The excitability acceleration preceded by deceleration in the ventral brainstem probably synchronizes the cortical activity profoundly enough to produce poly SWC through the activation of intralaminar thalamic neurons.     

The role of 5-HT, GABA and opioid peptides in presynaptic inhibition of tooth pulp input from the medial brainstem

In decerebrate or chloralose-anaesthetized cats electrical stimulation in the spinal trigeminal nucleus evoked antidromic responses in the mandibular canine tooth pulp. Conditioning stimulation in nucleus raphe magnus (NRM) and in the adjacent contralateral medullary reticular formation, nucleus reticularis gigantocellularis (NRGC) and nucleus reticularis magnocellularis (NRMC), produced a decrease in the threshold for the antidromic responses in a proportion of the tooth pulp inputs. This was interpreted as being due to depolarization of the tooth pulp afferent terminals, reflecting presynaptic inhibition. The primary afferent depolarization (PAD) of tooth pulp afferent terminals by NRM stimulation could be selectively blocked by bicuculline applied intravenously or by iontophoresis in the terminal region. Intravenous naloxone, cinanserin and methysergide had no effect on the PAD evoked from NRM, NRGC or NRMC. Thus NRM appears to exert presynaptic inhibitory control of A delta tooth pulp input to the spinal trigeminal nucleus via GABA-containing neurones.     

The possibility of different effects of primary afferent depolarization on spinal reflexes]

Intensive depolarization of central primary afferent terminals evoked by strong stimulation of afferent nerves or dorsal root produces recurrent discharges which may be recorded as antidromic dorsal root reflexes. It is shown that the discharges are simultaneously propagating in the dorso-ventral direction and thus produce facilitation of spinal reflexes. The obtained results allow suggesting the existence of two types of influences of the primary afferent depolarization on the reflex transmission to the spinal cord.     

Voltage clamp study of cat spinal motoneurons during strychnine-induced seizures

Cat spinal motoneurons were examined by the technique of somatic voltage clamp during strychnine-induced spinal seizures. No clear alteration of voltage-dependent ionic currents was required in order for typical strychnine-induced paroxysmal depolarization shifts (PDSs) to develop in contrast to results previously obtained during penicillin-induced spinal seizures. Voltage clamp of evoked and spontaneous PDSs indicates these are generated by a synchronized mixture of excitatory and inhibitory synaptic currents with excitation predominating.     

Polarization changes in the terminals of identified primary afferent fibers at the gracile nucleus

Several cutaneous fiber types project directly to the gracile nucleus of the cat. Under barbiturate anesthesia, terminal excitability changes on single fibers of identified type were produced by electrical and natural stimulation of the ipsilateral hind limb but not by stimulation of other limbs. The time course of primary afferent depolarization evoked by electrical stimulation of the sciatic nerve was similar for all fiber types. Light brushing of hairs on the hind limb was effective in producing primary afferent depolarization on all fiber types. Rubbing the skin did not produce significantly greater excitability changes. Comparison of periphery-to-nucleus conduction time and onset time of primary afferent depolarization for individual fibers showed no clear relationship between these two parameters, although some fibers with long conduction times also had a late onset of primary afferent depolarization. The data suggest that under barbiturate anesthesia, ipsilateral peripheral inputs activate a single interneuron pool which directs primary afferent depolarization onto fibers of all types reaching the gracile nucleus directly.     

Epinephrine- and norepinephrine-evoked potential changes of frog primary afferent terminals: Pharmacological characterization of α and β components

The effects of superfused epinephrine (E) and norepinephrine (NE) on the membrane potential of primary afferent fibers of the isolated frog spinal cord were studied by sucrose gap recordings from the dorsal root. In all preparations both E and NE, applied in concentrations ranging from 0.1 microM to 1.0 mM, produced a hyperpolarization of afferent terminals. In many instances this was followed by a slow depolarization and, in a small number of cords, a small depolarization preceded the increase in membrane potential. E- and NE-induced hyperpolarizations were blocked by the selective alpha 2-antagonists yohimbine and piperoxan, but not by the selective alpha 1-antagonists prazosin and corynanthine or by the beta-blockers propranolol and sotalol. The alpha 2-agonists clonidine, alpha-methylnorepinephrine and guanabenz also hyperpolarized terminals, causing a change in potential that was reduced by yohimbine and piperoxan. Taken together, these results suggest that alpha 2-receptors mediate the hyperpolarizing effects of E and NE. The beta-agonist isoproterenol evoked a slow depolarization similar to that produced by E and NE. The isoproterenol-depolarization was antagonized by propranolol. Sometimes, application of E and NE after superfusion with yohimbine produced only a depolarization of the dorsal root and this depolarization was sensitive to propranolol. It would appear therefore that the late depolarization seen after the application of E and NE is produced by activation of beta-receptors. In contrast, the alpha 1-agonist phenylephrine elicited a short latency, short duration depolarization similar to those seen preceding approximately 10% of the E- and NE-hyperpolarizations. Such short-latency depolarizations were blocked by prazosin and corynanthine. The major component of the response to both E and NE is indirectly mediated through a synaptic process: application of Mn2+, Mg2+, procaine or tetrodotoxin in concentrations sufficient to block synaptic transmission substantially reduced, but never eliminated, the actions of the catecholamines. Interneurons are probably involved because mephenesin, which reduces interneuronal transmission, significantly decreased the E and NE effects. Furthermore, interneurons which secrete excitatory amino acids and/or GABA may mediate the indirect effects of the catecholamines on afferent terminals because (-)baclofen and D.L-alpha-aminoadipate decrease, and picrotoxin and bicuculline increase, the dorsal root (DR) effects of E and NE.(ABSTRACT TRUNCATED AT 400 WORDS)     

Optical imaging of spreading depolarization waves triggered by spinal nerve stimulation in the chick embryo: possible mechanisms for large-scale coactivation of the central nervous system

Using a multiple-site optical recording technique with a voltage-sensitive dye, we found that widely spreading depolarization waves were evoked by dorsal root stimulation in embryonic chick spinal cords. Spatiotemporal maps of the depolarization waves showed that the signals were mainly distributed in the ventral half of the slice, with the highest activity in the ventrolateral area. The propagation velocity of the waves was estimated to be in the order of mm/s. Depolarization waves were evoked in the ventral root-cut preparation, but not in the dorsal root-cut preparation, suggesting that the wave was triggered by synaptic inputs from the primary afferents, and that activation of the motoneurons was not essential for wave generation. In intact spinal cord-brain preparations, the depolarization wave propagated rostrally and caudally for a distance of several spinal segments in normal Ringer's solution. In a Mg(2+)-free solution, the amplitude and extent of the signals were markedly enhanced, and the depolarization wave triggered in the cervical spinal cord propagated to the brainstem and the cerebellum. The depolarization wave demonstrated here had many similarities with the vagus nerve-evoked depolarization wave reported previously. The results suggest that functional cell-to-cell communication systems mediated by the depolarization wave are widely generated in the embryonic central nervous system, and could play a role in large-scale coactivation of the neurons in the spinal cord and brain.