Interaction of hypoxia and hypothermia on dorsal column conduction in adult rat spinal cord in vitro

A Novel Approach for Studying the Physiology and Pathophysiology of Myelinated and Non-Myelinated Axons in the CNS White Matter

Advances in brain connectomics set the need for detailed knowledge of functional properties of myelinated and non-myelinated (if present) axons in specific white matter pathways. The corpus callosum (CC), a major white matter structure interconnecting brain hemispheres, is extensively used for studying CNS axonal function. Unlike another widely used CNS white matter preparation, the optic nerve where all axons are myelinated, the CC contains also a large population of non-myelinated axons, making it particularly useful for studying both types of axons. Electrophysiological studies of optic nerve use suction electrodes on nerve ends to stimulate and record compound action potentials (CAPs) that adequately represent its axonal population, whereas CC studies use microelectrodes (MEs), recording from a limited area within the CC. Here we introduce a novel robust isolated "whole" CC preparation comparable to optic nerve. Unlike ME recordings where the CC CAP peaks representing myelinated and non-myelinated axons vary broadly in size, "whole" CC CAPs show stable reproducible ratios of these two main peaks, and also reveal a third peak, suggesting a distinct group of smaller caliber non-myelinated axons. We provide detailed characterization of "whole" CC CAPs and conduction velocities of myelinated and non-myelinated axons along the rostro-caudal axis of CC body and show advantages of this preparation for comparing axonal function in wild type and dysmyelinated shiverer mice, studying the effects of temperature dependence, bath-applied drugs and ischemia modeled by oxygen-glucose deprivation. Due to the isolation from gray matter, our approach allows for studying CC axonal function without possible "contamination" by reverberating signals from gray matter. Our analysis of "whole" CC CAPs revealed higher complexity of myelinated and non-myelinated axonal populations, not noticed earlier. This preparation may have a broad range of applications as a robust model for studying myelinated and non-myelinated axons of the CNS in various experimental models.

Effects of hypoxia on muscle response to tendon vibration in humans

Previous animal studies have shown that hypoxia markedly reduces the activation of muscle spindles. The present study was undertaken to determine whether a reduced oxygen supply to muscle affects the tonic vibration reflex (TVR) in humans. In resting healthy volunteers, the effects of inhalation of hypoxic gas, apnea, and total forearm ischemia produced by cuff inflation were studied on separate days. The TVR was recorded in flexor digitorum superficialis and the neuromuscular conduction time (CT) was measured from the compound muscle action potential; the latency and amplitude of the H reflex were also determined. TVR depression began during inhalation of the hypoxic gas, at the end of apnea, and during cuff inflation, and persisted during the recovery period. The H-reflex amplitude concomitantly increased or remained unchanged. Thus, hypoxia seems to directly alter muscle spindle reactivity. Such alterations of sensorimotor control may occur in patients suffering from respiratory or circulatory insufficiency and may contribute to their exercise limitation.

The role of multiple hyperbaric oxygenation in expanding therapeutic windows after acute spinal cord injury in rats

OBJECT

Hyperbaric oxygenation (HBO) therapy has been reported to improve neurological recovery after spinal cord injury (SCI). In the present study, the authors examined whether multiple HBO therapy can expand the therapeutic window after acute SCI.

METHODS

Seventy rats were randomly assigned to seven groups: sham surgery; SCI without treatment; single HBO treatment beginning at 30 minutes, 3 hours, and 6 hours after SCI; and multiple HBO treatments starting at 6 and 24 hours postinjury. Mild SCI was induced by adjusting the height of a weight drop (10 g) to 6.25 mm above the exposed spinal cord. A single HBO administration was performed at 2.82 ata for 1 hour. The multiple HBO treatment modality was performed once daily for 1 week. All rats underwent behavioral testing with the Basso-Beattie-Breshnahan locomotor rating scale twice a week. Rats were killed on Day 42 postinjury and specimens comprising the lesioned area were histopathologically examined. Those rats that received single HBO intervention beginning at 30 minutes and 3 hours and those that received multiple HBO treatment starting at 6 hours following injury made significantly greater neurological recoveries than those in the nontreatment SCI group. These rats also retained more sparing tissue than controls.

CONCLUSIONS

The results of this study demonstrate that multiple HBO treatments can expand the therapeutic window for acute SCI to 6 hours after injury.

Protection by hypothermia of hypoxia-induced inhibition of neurogenic vasodilation in porcine cerebral arteries

Porcine cerebral arterial strips denuded of the endothelium responded to transmural electrical stimulation (5 Hz for 40 s) with a relaxation, which was abolished by tetrodotoxin and N (G)-nitro-L-arginine, a NO synthase inhibitor. Lowering the temperature of the bathing media from 37 degrees C to 33 degrees C or 25 degrees C potentiated the response to nerve stimulation, but did not affect relaxations induced by NO applied exogenously. Hypoxia suppressed the stimulation-induced relaxation at 37 degrees C, but hypothermia blunted the inhibitory effect of hypoxia in a temperature-dependent manner. It is concluded that hypothermia augments vasodilatation associated with nitroxidergic (nitrergic) nerve activation possibly by increasing the production of NO from L-arginine and, in addition, prevents impairment of NO production by hypoxia. These mechanisms likely explain how hypothermia protects nerve cells against hypoxia. Inhibitions of cyclic GMP phosphodiesterase and of superoxide production by hypoxia do not seem to participate in the action of hypothermia. Mechanisms underlying its protective action remain to be ascertained.

Multiple hyperbaric oxygenation (HBO) expands the therapeutic window in acute spinal cord injury in rats

Hyperbaric oxygenation (HBO) therapy has been reported to improve neurological recovery following spinal cord injury (SCI). In the present study, we examined whether multiple HBO expands the therapeutic window for acute SCI. Single HBO (2.8 ATA, 1 hour) treatment was used at 30 minutes, 3 hours, and 6 hours following SCI, and serial HBO treatment (once daily for 1 week) at 6 hours and 24 hours post-injury. Mild SCI was induced by adjusting the height for a weight drop insult (10 g) to 6.25 mm above the exposed spinal cord. The group of animals receiving a single HBO intervention beginning at 30 minutes and 3 hours, or serial HBO treatment starting at 6 hours following the injury had a significantly better neurological recovery than animals with SCI only. The results of this study demonstrate that multiple HBO expands the therapeutic window for acute SCI to 6 hours after injury, further that serial HBO administration is superior to single HBO therapy.

Mechanisms of signal change during intraoperative somatosensory evoked potential monitoring of the spinal cord

In scoliosis surgery, intraoperative somatosensory evoked potential (SSEP) monitoring has reduced the incidence of postoperative neurologic deficits. Many factors affect the amplitude and latency of SSEP waveforms during surgery. Somatosensory evoked potential amplitude decreases with ischemia and anoxia because of temporal dispersion of the afferent volley and conduction block in damaged axons. In conjunction with surgical manipulations, minor drops in blood pressure may result in substantial SSEP changes that reverse when perfusion pressure is increased. Irreversible anoxic injury to central nervous system white matter with loss of SSEP waveforms is dependent on calcium influx into the intracellular space. Somatosensory evoked potential monitoring may be less sensitive for detecting acute insults in the presence of preexisting white matter lesions. Increased extracellular potassium from acute baro-trauma can block axonal conduction transiently even when there is no axonal disruption. Marked temperature-related drops in SSEP amplitude may occur after exposure of the spine but before instrumentation and deformity correction. Hypothermia may increase false-negative outcomes. Short-interval double-pulse stimulation may improve the sensitivity of the SSEP in detecting early ischemic changes. For neurosurgical procedures on the spinal cord the use of SSEP monitoring in improving postoperative outcome is less well established.

GABA increases refractoriness of adult rat dorsal column axons

We applied randomized double pulse stimulation for assessing the effects of GABA and a GABAA antagonist on compound action potentials in dorsal column axons isolated from adult rat. We stimulated the axons with double pulses at 0.2 Hz and randomly varied interpulse intervals between 3, 4, 5, 8, 10, 20, 30, 50 and 80 ms. Action potentials were measured using glass micropipettes. The first pulse was used to condition the response activated by the second test pulse. Concentrations of GABA of 1 mM, 100 microM and 10 microM did not affect action potential amplitudes or latencies activated by conditioning pulses. In the control studies, before drug administration, test pulses induced response amplitudes that were significantly decreased at 3-, 4- and 5-ms interpulse intervals. The test action potential amplitudes were 84.6 +/- 2.5%, 89.0 +/- 3.9% and 93.3 +/- 3.6% (mean +/- S.E.M.) of conditioning pulse levels, respectively. At 3-ms interpulse intervals, test response latencies were prolonged to 104.3 +/- 1.0%, but were unchanged at the other interpulse intervals. The 10 microM, 100 microM and 1 mM concentrations of GABA affected test response amplitudes. Application of 100 microM GABA reduced the amplitudes of test responses at 3-, 4-, 5- and 8-ms interpulse intervals, to 59.2 +/- 3.0%, 70.0 +/- 3.0%, 80.2 +/- 1.1% and 88.6 +/- 3.6% of the conditioning pulse amplitudes, respectively. At both 100 microM and 1 mM concentrations, GABA significantly prolonged the latencies of test responses. Treatment with 100 microM GABA prolonged the latencies of test responses at 3-, 4- and 5-ms interpulse intervals, to 119.3 +/- 3.1%, 107.3 +/- 2.8% and 105.5 +/- 2.5% of conditioning pulse latencies, respectively. The addition of 100 microM bicuculline methochloride, a GABAA antagonist, eliminated the effects of 100 microM GABA. The combined application of GABA and bicuculline (both 100 microM) did not affect amplitudes or latencies of test responses. These results suggest that GABA(A) receptor subtypes are present on the spinal dorsal column axons of adult rat, and that they modulate the excitability of the axons. The randomized double pulse methods reveal that GABA increases refractoriness of adult rat dorsal column axons.

Ion transport and membrane potential in CNS myelinated axons. II. Effects of metabolic inhibition

Compound resting membrane potential was recorded by the grease gap technique (37 degrees C) during glycolytic inhibition and chemical anoxia in myelinated axons of rat optic nerve. The average potential recorded under control conditions (no inhibitors) was -47 +/- 3 (SD) mV and was stable for 2-3 h. Zero glucose (replacement with sucrose) depolarized the nerve in a monotonic fashion to 55 +/- 10% of control after 60 min. In contrast, glycolytic inhibition with deoxyglucose (10 mM, glucose omitted) or iodoacetate (1 mM) evoked a characteristic voltage trajectory consisting of four distinct phases. A distinct early hyperpolarizing response (phase 1) was followed by a rapid depolarization (phase 2). Phase 2 was interrupted by a second late hyperpolarizing response (phase 3), which led to an abrupt reduction in the rate of potential change, causing nerves to then depolarize gradually (phase 4) to 75 +/- 9% and 55 +/- 6% of control after 60 min, in deoxyglucose and iodoacetate, respectively. Pyruvate (10 mM) completely prevented iodoacetate-induced depolarization. Effects of glycolytic inhibitors were delayed by 20-30 min, possibly due to continued, temporary oxidative phosphorylation using alternate substrates through the tricarboxylic acid cycle. Chemical anoxia (CN- 2 mM) immediately depolarized nerves, and phase 1 was never observed. However a small inflection in the voltage trajectory was typical after approximately 10 min. This was followed by a slow depolarization to 34 +/- 4% of control resting potential after 60 min of CN-. Addition of ouabain (1 mM) to CN--treated nerves caused an additional depolarization, indicating a minor glycolytic contribution to the Na+-K+-ATPase, which is fueled preferentially by ATP derived from oxidative phosphorylation. Phases 1 and 3 during iodoacetate exposure were diminished under nominally zero Ca2+ conditions and abolished with the addition of the Ca2+ chelator ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 5 mM). Tetraethylammonium chloride (20 mM) also reduced phase 1 and eliminated phase 3. The inflection observed with CN- was eliminated during exposure to zero-Ca2+/EGTA. A Ca2+-activated K+ conductance may be responsible for the observed hyperpolarizing inflections. Block of Na+ channels with tetrodotoxin (TTX; 1 microM) or replacement of Na+ with the impermeant cation choline significantly reduced depolarization during glycolytic inhibition with iodoacetate or chemical anoxia. The potential-sparing effects of TTX were less than those of choline-substituted perfusate, suggesting additional, TTX-insensitive Na+ influx pathways in metabolically compromised axons. The local anesthetics, procaine (1 mM) and QX-314 (300 microM), had similar effects to TTX. Taken together, the rate and extent of depolarization of metabolically compromised axons is dependent on external Na+. The Ca2+-dependent hyperpolarizing phases and reduction in rate of depolarization at later times may reflect intrinsic mechanisms designed to limit axonal injury during anoxia/ischemia.

Evidence for serotonin sensitivity of adult rat spinal axons: studies using randomized double pulse stimulation

We have recently shown both inhibitory and excitatory effects of serotonin on neonatal rat dorsal column axons. While neonatal rat dorsal column axons also respond to norepinephrine and GABA, adult rat dorsal columns are insensitive to the actions of both compounds. Therefore, we studied the effects of serotonin agonists on adult rat dorsal column axons using randomized double pulse stimuli at 0.2 Hz with random interpulse intervals of 3, 4, 5, 8, 10, 20, 30, 50 and 80 ms. The serotonin(1A) agonist, 8-hydroxy-dipropylaminotetralin-hydrobromide (8-OH-DPAT), significantly modulated test response amplitudes at 3, 4, 5 and 8 ms interpulse intervals by 29.6+/-4.0%, 17.4+/-2.1%, 9.6+/-2.3%, and 12.4+/-2.2% of conditioning pulse amplitudes, respectively. The mean latencies at 3, 4 and 5 ms interpulse intervals increased by 17.0+/-5.1%, 8.6+/-2.1%, and 5.1+/-1.4%, respectively (P<0.05). However, neither 10 microM 8-OH-DPAT nor 100 microM serotonin hydrochloride affected the compound action potentials evoked by conditioning or test pulses. In contrast, treatment with 100 microM quipazine dimaleate (a serotonin(2A) agonist) decreased the refractory period. While the response amplitudes to a 3-ms double pulse were reduced by 11.0+/-1.5% during the control period, the test response fell to only 2.4+/-1.8% of the conditioning response amplitudes after exposure to 100 microM quipazine. 8-OH-DPAT decreased the amplitude, prolonged the latency and increased the refractory periods of compound action potentials in the adult rat dorsal column, although a high concentration of the agonist (100 microM) was required for these effects. In contrast, the serotonin(2A) agonist, quipazine, decreased refractory periods. These results suggest that both serotonin(1A) and serotonin(2A) receptor subtypes are present on adult spinal dorsal column axons. Further, these receptors have opposing effects on axonal excitability, despite the fact that their sensitivities are relatively low.

A new model of acute compressive spinal cord injury in vitro

A novel in vitro method of spinal cord injury was developed to facilitate the study of cellular and molecular mechanisms underlying neural trauma. A 3-cm length of thoracic spinal cord was removed from the adult Wistar rat. A strip of dorsal column and its associated dorsal horn gray matter was excised and pinned in an in vitro recording chamber where it was constantly perfused with oxygenated Ringer's solution at either 25 degrees C or 33 degrees C. Injury was performed by compressing the dorsal column segment in vitro with a modified aneurysm clip (closing force 2.0 g) for 15 s. Microelectrode and sucrose gap recordings were generated to characterize the physiological effects of compressive injury. Longitudinal thin sections of control and injured dorsal column segments were examined by electron microscopy. At 25 degrees C, injured axons were characterized by a significant reduction in amplitude of the compound action potential (CAP) to 76.9 +/- 2.4% (P < 0.0005) and an increase in response latency to 112.5 +/- 2.5% (P <0.005). At 33 degrees C, the effects of injury on the CAP amplitude were accentuated (P< 0.0001). With the K+ channel blocker, 4-AP (1 mM), there was broadening of the CAP of injured axons and a delay in repolarization of the axonal resting membrane potential, suggesting myelin disruption with exposure of paranodal K+ channels. Ultrastructurally, injured dorsal column segments showed considerable axonal and myelin pathology including splaying of the myelin sheath and vesicular degeneration.

Assessment of axonal dysfunction in an in vitro model of acute compressive injury to adult rat spinal cord axons

An in vitro model of spinal cord injury was developed to study the pathophysiology of posttraumatic axonal dysfunction. A 25 mm length of thoracic spinal cord was removed from the adult male rat (n = 27). A dorsal column segment was isolated and pinned in a recording chamber and superfused with oxygenated (95% O2/5% CO2) Ringer. The cord was stimulated with a bipolar electrode, while two point responses were recorded extracellularly. Injury was accomplished by compression with a modified aneurysm clip which applied a 2 g force for 15 s. With injury the compound action potential (CAP) amplitude decreased to 53.7 +/- 5.4% (P < 0.001), while the latency increased to 115.6 +/- 3.1% (P < 0.0025) of control values. The absolute refractory period increased with injury from 1.7 +/- 0.1 ms to 2.1 +/- 0.1 ms (P < 0.05). The infusion of 5 mM 4-aminopyridine (4-AP), a blocker of voltage-sensitive 'fast' K channels confined to internodal regions, resulted in broadening of the CAP of injured axons to 114.9 +/- 3.1% of control (P < 0.05). Ultrastructural analysis of the injured dorsal column segments revealed marked axonal and myelin pathology, including considerable myelin disruption. In conclusion, we have developed and characterized an in vitro model of mammalian spinal cord injury which simulates many of the features of in vivo trauma. Injured axons display characteristic changes in physiological function including a shift in refractory period and high frequency conduction failure. The ultrastructural data and response of injured axons to 4-AP suggest that myelin disruption with exposure of 'fast' K+ channels contributes to posttraumatic axonal dysfunction.

Excitatory and inhibitory effects of serotonin on spinal axons

We studied the effects of serotonin on compound action potentials in dorsal columns isolated from young (nine to 13 days old) rats. Conducting action potentials were activated by submaximal (50%) and supramaximal constant current electrical stimuli and recorded with glass micropipettes. At 10 microM and 100 microM concentrations, serotonin significantly increased mean action potential amplitudes by 9.6 +/- 6.5% (+/- S.D., P < 0.05) and 16.6 +/- 12.2% (+/- S.D., P < 0.005), respectively. Likewise, 10 microM and 100 microM of quipazine (a serotonin2A agonist) increased the amplitudes by 9.6 +/- 2.5% (+/- S.D., P < 0.0005) and 37.7 +/- 8.7% (+/- S.D., P < 0.0005), respectively. In contrast, 10 microM and 100 microM concentrations of 8-hydroxy-dipropylaminotetralin-hydrobromide (a serotonin 1A agonist) reduced axonal excitability by -9.4 +/- 5.5% (+/- S.D., P < 0.05) and -32.9 +/- 10.6% (+/- S.D., P < 0.0005), respectively. At 50 microM concentration, mianserin (a serotonin2A and serotonin2C antagonist) eliminated the excitatory effects of 100 microM quipazine dimaleate. The combination of 50 microM mianserin and 100 microM serotonin reduced action potential amplitudes by -5.6 +/- 4.9% (+/- S.D., P < 0.05). These results suggest that serotonin1A and serotonin2A receptor subtypes are present on spinal dorsal column axons. These two receptor subtypes have opposing effects on axonal excitability. The ratios and sensitivities of these two axonal receptor subtypes may modulate axonal excitability in rat dorsal column axons and have important implications for both development and injury of axons.