Multiple averaged records to identify Aδ-fibers in sensory nerves.

BACKGROUND: Existing methods identify only ≈10 Aδ-fibers in human sensory nerves per recording. This study examines methods to increase the detection of Aδ-fibers. NEW METHOD: Two to 20 averages of 500 replicate responses to epidermal nerve stimulation are obtained. Pairs of different averages are constructed. Each pair is analyzed with algorithms applied to amplitude and frequency to detect replication of responses to stimulation as "simultaneous similarities in two averages" (SS2AVs) at ≥99.5th percentile of control. In a pair of averages the latencies of amplitude and frequency SS2AVs for the same response to stimulation may differ by ≤0.25 ms. Therefore, Aδ-fibers are identified by the 0.25 ms moving sum of SS2AV latencies of the pairs of averages. RESULTS: Increasing averages increases pairs of different averages and detection of Aδ-fibers: from 2 to 10 Aδ-fibers with two averages (one pair) to >50 Aδ-fibers with 12-20 averages (66-190 pairs). COMPARISON WITH EXISTING METHOD(S): Existing methods identify ≤10 Aδ-fibers in 10 averages/45 pairs with the medians of amplitude and frequency algorithms applied to all 45 pairs. This study identifies Aδ-fibers (i) by applying these algorithms at the 99.5th percentile of control, (ii) to each pair of averages and (iii) by the 0.25 ms sum of algorithm identified events (SS2AVs) in all pairs. These three changes significantly increase the detection of Aδ-fibers, e.g., in 10 averages/45pairs from 10 to 45. CONCLUSIONS: Three modifications of existing methods can increase the detection of Aδ-fibers to an amount suitable (>50 with ≥12 averages) for statistical comparison of different nerves.

Median amplitude and frequency analysis of sensory nerve responses to intraepidermal stimulation.

BACKGROUND: In clinical practice, small myelinated sensory fibers conveying pain and other sensations, Aδ-fibers, cannot be examined with available nerve conduction study techniques. NEW METHOD: Equipment available in clinical neurophysiology laboratories is used to record from human sensory nerves multiple averaged responses to non-painful stimulation of intraepidermal nerves. Ten averaged responses are analyzed in all possible pair combinations with an algorithm applied to a 0.45 ms period of amplitude and frequency (power spectrum). The median of the algorithms is compared to control data to identify potentials generated as response to intraepidermal stimulation. RESULTS: Median analysis of the algorithm applied to amplitude and frequency of multiple record pairs identifies potentials with conduction velocities of Aδ-fibers. The analysis of frequency (power spectrum) adds data to the analysis of amplitude. Median analysis of multiple record pairs yields more data than analysis of one pair of alternate averages with the same algorithms. COMPARISON WITH EXISTING METHOD(S): At present, analysis of one pair of alternate average records with an algorithm is the only method to identify Aδ-fiber generated potentials. Median analysis of the same algorithm applied to the amplitude of multiple record pairs increases the number of Aδ-fiber generated potentials identified. Neither median analysis of amplitude nor frequency of multiple records pairs has ever been used for conduction studies of nerve fibers, including Aδ-fibers. CONCLUSIONS: Stimulation, recording and data analysis methods used in this study can be applied in the clinical EMG laboratory to identify the conduction velocities of Aδ-fibers in human sensory nerves.

Slowly conducting potentials in human sensory nerves.

BACKGROUND: In clinical practice, small myelinated sensory fibers, Aδ-fibers, conveying mainly pain and temperature sensations, cannot be examined with available nerve conduction study techniques. Currently, these fibers can only be examined with experimental or very specialized and not commonly available nerve conduction techniques, or only indirectly with cerebral evoked potentials. NEW METHOD: This study uses equipment and methods available in clinical neurophysiology laboratories to record from human sensory nerves ≥1000 averaged responses to focal, non-painful stimuli applied by a special electrode to epidermal nerves. The averaged responses to odd numbered stimuli are compared to the averaged responses to even numbered stimuli. An algorithm identifies potentials common in both averages. The 99th and 99.9th percentiles for this algorithm are obtained from control records without stimulation and applied to records with stimulation to identify potentials resulting from stimulation of intraepidermal nerves. RESULTS: The algorithm identifies numerous negative and positive potentials as being different from controls at the 99th and 99.9th percentile levels. The conduction velocities of the potentials range from of 1.3-29.9 m/s and are compatible with conduction velocities of Aδ-fibers. COMPARISON WITH EXISTING METHOD(S): No existing methods. CONCLUSIONS: The stimulation, recording and data analysis methods used in this study can be applied in the clinical EMG laboratory to identify Aδ-fibers in human sensory nerves.

[Nasojejunal enteral feeding tubes in critically ill patients. Successful placement without technical assistance]. / Nasojejunale Ernährungssonden bei Intensivpatienten. Erfolgreiche Platzierung ohne technische Hilfsmittel.

BACKGROUND: Critically ill patients with early enteral feeding seem to profit from post-pyloric administration. Two feeding tubes were studied that, due to their construction, are able to move into the duodenum without the necessity of technical support. The duration until successful positioning, time until total enteral feeding and possible complications were compared. PATIENTS AND METHOD: Patients with naso-gastric tubes and early enteral feeding, who had an increased reflux despite head of bed elevation and prokinetic drugs, were randomly assigned to either a Tiger tube (Cook) or a Bengmark tube (Pfrimmer Nutricia). RESULTS: A total of 28 patients from the surgical intensive care ward were included. Of the 16 Tiger tubes 14 could be successfully placed but only 2 out of the 12 Bengmark tubes. With Tiger tubes total enteral feeding was established within 6 days (median), with Bengmark tubes within 4 days. CONCLUSION: In comparison to the Bengmark tube the Tiger tube has a higher success rate in terms of positioning in intensive care patients with impaired abdominal motility.

Clinical heterogeneity of familial spastic paraplegia linked to chromosome 2p21.

The clinical features of four families with autosomal dominant spastic paraparesis (FSP) are described, along with the results of linkage analysis to markers from the regions of chromosomes 2, 14, and 15 which are known to contain spastic paraplegia genes. All families had 'pure' spastic paraparesis (FSP), but the severity of symptoms varied widely among families, and other mild neurologic signs were observed in some subjects. Although no family individually yielded a lod score >3.0, all families yielded positive lod scores with chromosome 2 markers, and a maximal lod score of 5.7 was obtained for the families combined using marker D2S352. There was no evidence of linkage to chromosome 14 or 15 in any of the families.

Ammonium ions abolish excitatory synaptic transmission between cerebellar neurons in primary dissociated tissue culture.

1. Transmitter glutamate is thought to be derived from glutamine via cleavage by glutaminase. NH+4 inhibits glutaminase. Therefore the decrease of glutamatergic excitatory synaptic transmission by NH+4 was thought to be due to the inability of glutamine to serve as precursor for glutamate. However, in cat spinal cord, NH+4 abolished excitatory synaptic transmission by a conduction block for action potentials in presynaptic terminals. The conduction block prevented inferences as to the effects of NH+4 on the availability of glutamate for synaptic transmission. This study reexamines the effects of NH+4 on glutamatergic excitatory synaptic transmission in cerebellar neurons in tissue culture. 2. Whole-cell patch voltage-clamp recordings were obtained from presumed Purkinje cells. Extracellular stimulation of presumed granule cells produced mono- and polysynaptic excitatory postsynaptic currents (EPSCs). In addition, presumed Purkinje cells showed spontaneous EPSCs that occurred independently of the addition of tetrodotoxin (TTX) or Cd2+ to the extracellular solution. 3. NH+4 (5-10 mM) abolished evoked mono- and polysynaptic EPSCs without abolishing spontaneous EPSCs and without significant effects on action currents in the Purkinje cell soma. 4. Increase of K+ in the extracellular solution to 10-12 from 5 mM abolished evoked EPSCs without abolishing spontaneous EPSCs and without significant effects on action currents in the Purkinje cell soma. 5. Mixtures of NH+4 and K+, with each ion in a concentration insufficient to affect evoked EPSCs when given alone, abolished evoked EPSCs when the sum of NH+4 and K+ exceeded 10-12 mM. 6. Increase of intracellular pH by trimethylamine had no effect on evoked and spontaneous EPSCs.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of NH4+ on reflexes in cat spinal cord.

1. In deeply barbiturate-anesthetized animals. NH4+ decreases spinal excitatory synaptic transmission by neuronal depolarization and subsequent block of conduction of action potentials into presynaptic terminals of low-threshold (presumably Ia-) afferents. Because barbiturates by themselves depress excitatory synaptic transmission and may have modified the effects of NH4+, this study examines the effect of NH4+ on excitatory synaptic transmission in the unanesthetized animal. 2. The effects of NH4+ on monosynaptic and polysynaptic excitatory reflexes as well as di- and polysynaptic inhibition were investigated in the spinal cord of the decerebrate and unanesthetized cat in vivo. 3. The monosynaptic excitatory reflex (MSR) elicited by muscle nerve stimulation and polysynaptic excitatory reflexes elicited by muscle (MSR-PSR) or cutaneous nerve stimulation (Cut-PSR) were recorded from the ventral roots L7 or S1. The P-wave was recorded from the cord dorsum. Di- and polysynaptic inhibition was elicited by muscle nerve stimulation and measured as decrease of the MSR. 4. Intravenous infusion of ammonium acetate (AA) decreased MSR and the monosynaptic motoneuron pool excitatory postsynaptic potential (EPSP) recorded from the ventral root (VR-EPSP). Decrease of MSR and VR-EPSP was accompanied by an increase of the intraspinal conduction time in presynaptic terminals. The maximal decrease of the MSR was preceded by a period of transient increase of the MSR and reflex discharges from previously subthreshold VR-EPSPs. 5. The effects of NH4+ on MSR and VR-EPSP are consistent with those in barbiturate-anesthetized animals and suggest that NH4+ also decreases monosynaptic excitation in unanesthetized animals by depolarization and subsequent conduction block for action potentials in presynaptic terminals. 6. Decrease of the MSR was accompanied by a decrease of the P-wave, indicating that NH4+ simultaneously decreases mono- and oligosynaptic excitatory synaptic transmission as well as presynaptic inhibition. 7. Decrease of the MSR was accompanied by increases of MSR-PSR and Cut-PSR and decreases of di- and polysynaptic postsynaptic inhibition. 8. The neuronal circuits underlying MSR-PSR and Cut-PSR include presynaptic inhibition of group I and II afferents as well as postsynaptic inhibition of motoneurons. It is suggested that increases of MSR-PSR and Cut-PSR are contributed to by decreases of pre- and postsynaptic inhibition and neuronal depolarization by NH4+. These effects increase afferent input to motoneurons, permit uncontrolled discharge of motoneurons, and initiate reflex discharges by previously subthreshold excitatory postsynaptic potentials.

Ammonium decreases excitatory synaptic transmission in cat spinal cord in vivo.

1. Glutamine is thought to be a precursor of the pool of glutamate that is used as synaptic transmitter. NH4+ inhibits glutaminase, the enzyme presumed to cleave glutamine into glutamate in synaptic terminals. Therefore a decrease by NH4+ of excitatory synaptic transmission in hippocampus was suggested to be due to the inability to utilize glutamine as a precursor for glutamate and subsequent transmitter depletion. This study reexamines the effects of NH4+ on excitatory synaptic transmission. 2. The effects of NH4+ on excitatory synaptic transmission from low-threshold afferent fibers, presumably Ia-afferent fibers, to motoneurons was investigated in the spinal cord of anesthetized cats in vivo. 3. Action potentials of low-threshold afferent fibers were recorded at the entry of the dorsal roots into the spinal cord. An extracellular electrode within a motoneuron nucleus recorded the action potential of low-threshold afferent fibers and the extracellular monosynaptic excitatory postsynaptic potential, i.e., the focal synaptic potential (FSP). This extracellular electrode also recorded the antidromic field potential (AFP) in response to ventral root stimulation. Electrodes on the ventral roots recorded the monosynaptic reflex (MSR) and the monosynaptic excitatory postsynaptic potential in motoneurons electrotonically conducted into the ventral roots (VR-EPSP). 4. Intravenous infusion of ammonium acetate (AA) reversibly decreased MSR, VR-EPSP, and FSP, i.e., decreased excitatory synaptic transmission. 5. The decrease of VR-EPSP and FSP was accompanied initially by a decrease of conduction and, eventually, a conduction block in presynaptic terminals of low-threshold afferent fibers. 6. The decreases of VR-EPSP and FSP were also accompanied by the transient appearance of a reflex discharge, triggered by VR-EPSPs of decreased amplitude, and changes of the AFP indicating increased invasion of motoneuron somata by antidromic action potentials. 7. It is suggested that NH4+ depolarizes intraspinal Ia-afferent fibers and motoneurons. This depolarization initially decreases and then blocks conduction of action potentials into the presynaptic terminals of Ia-afferent fibers. The conduction block prevents the release of excitatory transmitter and decreases excitatory synaptic transmission. 8. The suggested depolarizing action of NH4+ may be due to K+-like ionic properties of NH4+ and/or an inhibition of K+-uptake into astrocytes. 9. The conduction block in presynaptic terminals of low-threshold afferent fibers can fully explain the decrease of excitatory synaptic transmission by NH4+. Because of the conduction block in presynaptic terminals, this study does not permit a conclusion as to an inhibition by NH4+ fo the utilization of glutamine as a precursor for glutamate used as synaptic transmitter.

The H-reflex in the encephalopathy due to ammonia intoxication.

Ammonia intoxication has been shown to decrease excitatory synaptic transmission in several regions of the central nervous system. To investigate the relation between an effect of ammonia on excitatory synaptic transmission and the behavioral depression in the encephalopathy due to ammonia intoxication, this study examined in the rat the effects of ammonia intoxication on the H-reflex, the behavioral and neurological signs of the encephalopathy due to ammonia intoxication, and correlated the effects on the H-reflex with the signs of encephalopathy. Ammonia intoxication abolished the H-reflex without affecting the M-response. This indicated that ammonia intoxication decreased spinal excitatory synaptic transmission without affecting neuromuscular excitatory synaptic transmission. In the encephalopathy due to ammonia intoxication, the H-reflex disappeared only during a very advanced stage of behavioral depression, i.e., coma. During early stages of behavioral depression, i.e., during a decrease of reactions to sensory stimuli, the H-reflex was not affected by ammonia intoxication. Therefore, mechanisms other than a decrease of excitatory synaptic transmission in the central nervous system may be responsible for the behavioral depression seen in early stages of the encephalopathy due to ammonia intoxication.

Synaptic transmission in ammonia intoxication.

Ammonia intoxication allegedly plays a significant role in the pathophysiology of hepatic encephalopathy. In order to understand the pathogenesis of this encephalopathy it is necessary to know the effects of ammonia on the mechanisms by which neurons communicate, i.e., excitatory and inhibitory synaptic transmissions. NH4+ decreases excitatory synaptic transmission mediated by glutamate. Possibly, this effect is related to a depletion of glutamate in presynaptic terminals. NH4+ decreases inhibitory synaptic transmission mediated by hyperpolarizing Cl(-)-dependent inhibitory postsynaptic potentials. This effect is related to the inactivation of the extrusion of Cl- from neurons by NH4+. By the very same action, NH4+ also decreases the hyperpolarizing action of Ca2+- and voltage-dependent Cl- currents. These currents may modify the efficacy of the synaptic input to neurons and increase neuronal excitability. Estimates derived from experimental observations suggest that an increase of CNS tissue NH4+ to 0.5 mumol/g is sufficient to disturb excitatory and inhibitory synaptic transmission and to initiate the encephalopathy related to acute ammonia intoxication. Chronic portasystemic shunting of blood, as in hepatic encephalopathy, significantly changes the relation between CNS NH4+ and function of synaptic transmission. A portacaval shunt increases the tissue NH4+ necessary to disturb synaptic transmission. However, after a portasystemic shunt, synaptic transmission becomes extremely sensitive to any acute increase of NH4+ in the CNS.

Porta-caval shunting changes neuronal sensitivity to ammonia.

The relations between an effect of ammonia on postsynaptic inhibition, the amount of ammonium acetate i.v. to obtain this effect, and the tissue concentrations of NH4+ and glutamine were investigated in the cerebral cortex of cats without and with portacaval shunts. Normal cats required 2.43 mmol/kg ammonium acetate to affect postsynaptic inhibition. Cerebral NH4+ and glutamine increased from 0.21 mumol/g to 0.77 mumol/g and from 2.92 mumol/g to 5.54 mumol/g, respectively. In portacaval shunted cats, postsynaptic inhibition was normal in spite of increases of NH4+ and glutamine to 1.37 mumol/g and 14.28 mumol/g, respectively. Only 0.7 mmol/kg of ammonium acetate were sufficient to affect postsynaptic inhibition. This was associated with a statistically insignificant increase of NH4+ to 1.61 mumol/g and no change of glutamine. A chronic portasystemic shunt markedly increases the tolerance of postsynaptic inhibition to NH4+. However, postsynaptic inhibition becomes very sensitive to an acute systemic ammonia load and the associated increase of tissue NH4+ in the cerebral cortex. These observations help to understand the pathogenesis of the encephalopathy precipitated in patients with portasystemic shunts by an acute systemic ammonia load such as resulting from a gastrointestinal hemorrhage.

Ammonia intoxication: effects on cerebral cortex and spinal cord.

The effect of an acute systemic ammonia intoxication on the metabolic states of the cerebral cortex and the spinal cord of the same animal was studied in the cat. The intravenous infusion of ammonium acetate (2 and 4 mmol/kg body weight/30 min) increased the gross levels of tissue NH4+, glutamine, glutamine/glutamate ratio, lactate, and the lactate/pyruvate ratio in the cerebral cortex and the spinal cord. Pyruvate increased, but significantly only in the spinal cord; aspartate decreased, but significantly only in the cerebral cortex. The infusion of ammonium acetate did not significantly change the levels of phosphocreatine, ATP, ADP, AMP, total adenine nucleotides, adenylate energy charge, glucose, glutamate, alpha-ketoglutarate, and malate in either tissue. The changes of NH4+, glutamine, and lactate levels as well as glutamine/glutamate and lactate/pyruvate ratios in the spinal cord correlated significantly with the corresponding changes of these metabolites in the cerebral cortex. Thus, cerebral cortex and spinal cord show certain specific and comparable metabolic changes in response to a systemic ammonia intoxication. The effect of ammonia intoxication on the increases of glutamine and lactate levels is discussed.

Pathophysiology of ammonia intoxication.

Ammonia intoxication affects postsynaptic inhibition and disturbs inhibitory neuronal interactions. This study investigated whether or not the effect of ammonia on postsynaptic inhibition was associated with a change of the EEG, i.e., a change in the function of the central nervous system such as in an encephalopathy. We showed that the effect of ammonia on postsynaptic inhibition was associated with a marked change of the EEG, and that this change was not due to an effect of ammonia on the brain stem reticular activating system. In addition, it was shown that in the central nervous system a NH+4 concentration of about 1 mumol/g affected postsynaptic inhibition. Because ammonia simultaneously affected postsynaptic inhibition and the EEG at a NH+4 tissue concentration comparable to that observed in encephalopathy, it is proposed that a dysfunction of postsynaptic inhibition caused the encephalopathy due to ammonia intoxication by simultaneously disturbing inhibitory neuronal interactions in many regions of the central nervous system.

Ammonia, postsynaptic inhibition and CNS-energy state.

Ammonia intoxication decreases the hyperpolarizing action of postsynaptic inhibition. This study examines the metabolic state of the spinal cord during this effect of ammonia intoxication on spinal motoneurons. ATP, ADP, AMP, the adenylate energy charge, glucose, PCr, pyruvate, alpha-ketoglutarate and glutamate were unchanged during the effect of ammonia on the hyperpolarizing action of postsynaptic inhibition. NH4+, glutamine and lactate were increased. Ammonia intoxication affected postsynaptic inhibition without changes of the resting membrane potential, the neuron input resistance, the action potential and EPSPs. The encephalopathy caused by ammonia intoxication is known to occur without an alteration of the tissue energy state. The effect of ammonia intoxication on postsynaptic inhibition can be considered as a cause of the encephalopathy because postsynaptic inhibition is altered without a change of the tissue energy state, the resting membrane potential, the whole neuron resistance, the action potential and EPSPs.

Ammonia intoxication and hyperpolarizing postsynaptic inhibition.

Systemic ammonia intoxication abolished the hyperpolarizing action of postsynaptic inhibition in the CNS at tissue concentrations of NH+4 which are sufficient to produce the earliest signs of encephalopathy. Therefore, the action of NH+4 on hyperpolarizing postsynaptic inhibition has to be considered as the cause, or as a contributing cause, of the encephalopathy due to systemic ammonia intoxication.

Ammonia and methionine sulfoximine intoxication.

Intoxication with ammonium acetate abolished the suppression of action potential generation by cortical postsynaptic inhibition, i.e. produced 'disinhibition', due to the inactivation of neuronal Cl- extrusion. With the occurrence of disinhibition cerebral ammonia increased to 445% of normal; glutamine increased to 170%. Methionine sulfoximine (MSO), an inhibitor of glutamine synthetase, produced disinhibition about 3 h after administration; at this time cerebral ammonia was increased to 290% of normal, glutamine was unchanged. Intoxication with MSO for less than 3 h significantly decreased the amount of ammonium acetate needed to produce disinhibition at cerebral ammonia concentrations ot 340-430% of normal. MSO produces an endogenous ammonia intoxication which: (i) decreases the amount of exogenous ammonia required to affect cortical postsynaptic inhibitions; and (ii) eventually becomes sufficiently severe to disturb cortical inhibitory neuronal interactions by itself.