A role of GABAA receptors in hypoxia-induced conduction failure of neonatal rat spinal dorsal column axons

Glutamine synthetase protects the spinal cord against hypoxia-induced and GABA(A) receptor-activated axonal depressions

BACKGROUND

We investigated the effects of exogenous GS on hypoxia- and GABA(A) receptor-induced axonal depression in neonatal rats.

METHODS

To assess the effects of GS on spinal cord axons, CAPs were recorded. Hemicords were exposed to hypoxia by 30-minute superfusion with Ringer's solution saturated with 95% N(2) and 5% CO(2) followed by 60-minute exposure to 95% N(2) and 5% CO(2) gassing (N(2) gassing phase) and then 90 minutes of resuperfusion with oxygenated Ringer's solution (resuperfusion phase). Exogenous high GS (15 U) or low GS (1.5 U) was delivered during the N(2) gassing phase. The effects of GS on GABA(A) receptor-induced axonal depression were analyzed with oxygenated isolated dorsal columns.

RESULTS

The high GS significantly reduced the decline in the CAP amplitudes during the N(2) gassing and resuperfusion phases (P = .0185) compared to the hypoxia control. The low GS treatment showed a trend toward recovery during the N(2) gassing and resuperfusion phases, but the effect was not significant (P = .3953). In isolated dorsal columns, GS significantly reduced the CAP amplitude depression induced by GABA(A) receptor agonist.

CONCLUSIONS

Our findings suggest that GS had dose-dependent protective effects on the spinal cord against hypoxia-induced axonal depression. It may inhibit the depression of CAP amplitudes by blocking GABA(A) receptors.

Neurotransmitter receptors in the life and death of oligodendrocytes

Oligodendrocytes are crucial to the function of the mammalian brain: they increase the action potential conduction speed for a given axon diameter and thus facilitate the rapid flow of information between different brain areas. The proliferation and differentiation of developing oligodendrocytes, and their myelination of axons, are partly controlled by neurotransmitters. In addition, in models of conditions like stroke, periventricular leukomalacia leading to cerebral palsy, spinal cord injury and multiple sclerosis, oligodendrocytes are damaged by glutamate and, contrary to dogma, it has recently been discovered that this damage is mediated in part by N-methyl-D-aspartate receptors. Mutations in oligodendrocyte neurotransmitter receptors or their interacting proteins may cause defects in CNS function. Here we review the roles of neurotransmitter receptors in the normal function, and malfunction in pathological conditions, of oligodendrocytes.

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.

Anoxic and ischemic injury of myelinated axons in CNS white matter: from mechanistic concepts to therapeutics

White matter of the brain and spinal cord is susceptible to anoxia and ischemia. Irreversible injury to this tissue can have serious consequences for the overall function of the CNS through disruption of signal transmission. Myelinated axons of the CNS are critically dependent on a continuous supply of energy largely generated through oxidative phosphorylation. Anoxia and ischemia cause rapid energy depletion, failure of the Na(+)-K(+)-ATPase, and accumulation of axoplasmic Na+ through noninactivating Na+ channels, with concentrations approaching 100 mmol/L after 60 minutes of anoxia. Coupled with severe K+ depletion that results in large membrane depolarization, high [Na+]i stimulates reverse Na(+)-Ca2+ exchange and axonal Ca2+ overload. A component of Ca2+ entry occurs directly through Na+ channels. The excessive accumulation of Ca2+ in turn activates various Ca(2+)-dependent enzymes, such as calpain, phospholipases, and protein kinase C, resulting in irreversible injury. The latter enzyme may be involved in "autoprotection," triggered by release of endogenous gamma-aminobutyric acid and adenosine, by modulation of certain elements responsible for deregulation of ion homeostasis. Glycolytic block, in contrast to anoxia alone, appears to preferentially mobilize internal Ca2+ stores; as control of internal Ca2+ pools is lost, excessive release from this compartment may itself contribute to axonal damage. Reoxygenation paradoxically accelerates injury in many axons, possibly as a result of severe mitochondrial Ca2+ overload leading to a secondary failure of respiration. Although glia are relatively resistant to anoxia, oligodendrocytes and the myelin sheath may be damaged by glutamate released by reverse Na(+)-glutamate transport. Use-dependent Na+ channel blockers, particularly charged compounds such as QX-314, are highly neuroprotective in vitro, but only agents that exist partially in a neutral form, such as mexiletine and tocainide, are effective after systemic administration, because charged species cannot penetrate the blood-brain barrier easily. These concepts may also apply to other white matter disorders, such as spinal cord injury or diffuse axonal injury in brain trauma. Moreover, whereas many events are unique to white matter injury, a number of steps are common to both gray and white matter anoxia and ischemia. Optimal protection of the CNS as a whole will therefore require combination therapy aimed at unique steps in gray and white matter regions, or intervention at common points in the injury cascades.

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.

Low-dose vigabatrin (gamma-vinyl GABA)-induced damage in the immature rat brain

The antiepileptic drug, vigabatrin, inhibits GABA transaminase, thus elevating GABA levels in the brain. In adult animal experiments, high-dose (200 mg/kg/day) chronic vigabatrin administration is associated with potentially reversible myelin vacuolation, a phenomenon not documented in humans. We hypothesized that vigabatrin might adversely affect myelination in the developing brain. Rats were given vigabatrin in doses comparable to those used clinically (15-50 mg/kg/day), from age 12 to 16 days. The rats were killed at age 19-20 days. We observed decreased myelin staining in the external capsule, axonal degeneration in white matter, evidence of glial cell death in the white matter, and reactive astrogliosis in the frontal cortex. We did not detect myelin vacuolation. These findings indicate that vigabatrin can have adverse and potentially irreversible effects on the developing rat brain. The mechanism of damage could be direct toxicity of vigabatrin or an indirect effect mediated through elevated GABA levels. Vigabatrin has been recommended as a treatment for some forms of childhood epilepsy; therefore, further studies are needed to assess the risks in children.

Independent depressive mechanisms of GABA and (+/-)-8-hydroxy-dipropylaminotetralin hydrobromide on young rat spinal axons

We compared the effect of GABA and the serotonin receptor agonist (+/-)-8-hydroxy-dipropylaminotetralin hydrobromide (8-OH-DPAT) on compound action potential amplitudes, latency, and conduction velocity in the spinal cord isolated from young (eight to 13-day-old) Long-Evans hooded rats. Supramaximally activated conducting action potentials and extracellular K+ activity were recorded with microelectrodes from the cuneatus-gracilis fasciculi and corticospinal tract. In the cuneatus-gracilis fasciculi, 8-OH-DPAT (10(-4) M) significantly reduced response amplitudes by 26.1 +/- 10.3% (mean +/- S.D., P < 0.0001, paired t-test, n = 27) and increased latencies by 20.3 +/- 7.9% (P < 0.0001). GABA (10(-4) M) reduced/amplitudes by 31.7 +/- 15.0% (P < 0.0001, n = 28) and increased latencies by 6.1 +/- 5.4% (P < 0.0001). However, neither GABA nor 8-OH-DPAT significantly altered conduction velocities, suggesting that the latency shifts are due to changes in activation time and not conduction velocity. In cortical spinal tract, 8-OH-DPAT (10(-4) M) depressed response amplitudes by 18.9 +/- 9.6% (P < 0.05, n = 5), increased latencies by 23.3 +/- 7.2% (P < 0.0001), but reduced conduction velocities by 19.9 +/- 10.2%. GABA (10(-4) M) reduced amplitudes by 16.4 +/- 7.5% (P < 0.01, n = 5), increased latencies by 5.3 +/- 2.3% (P < 0.05), and did not change conduction velocities. Bicuculline or picrotoxin blocked the GABA effects but did not affect the 8-OH-DPAT effects on both tracts. The potassium channel blocker tetraethylammonium did not alter the 8-OH-DPAT effects. The Na+/K(+)-ATPase inhibitor ouabain (10(-6) M) markedly enhanced the depressive GABA effects from 27.9 +/- 12.0% to 49.4 +/- 24.5% (P < 0.01, n = 9), but had no effect on 8-OH-DPAT-mediated effects. These results suggest that GABA and serotonin agonists depress axonal excitability through different and independent mechanisms.

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.

Control of impulse conduction in long range branches of afferents by increases and decreases of primary afferent depolarization in the rat

It has been shown previously that impulses in axons of the descending branches of myelinated afferents in rat dorsal columns may suffer a blockade of transmission along their course in the dorsal columns. This paper tests the effect of the mechanism of primary afferent depolarization on the orthodromic movement of impulses in descending dorsal column primary afferent axons originating in the L1 dorsal root. Orthodromic impulses were recorded in the L5 and 6 dorsal columns after stimulation of the L1 dorsal root. Twenty-seven out of 82 axons (33%) suffered a temporary transmission block if primary afferent depolarization had been induced by L5 stimulation before the L1 stimulus. The tendency to block peaked at 10-15 ms and persisted for up to 30-40 ms. The number of single unit orthodromic impulses originating from the L1 root and recorded during a search of the dorsal columns 15 mm caudal to L1 increased by a factor of 3.1 after the systemic administration of bicuculline (1 mg/kg). The number of single unit orthodromic impulses originating from the L1 root and recorded in axons descending in the dorsal columns 20 mm caudal to the root increased by a factor of 8.7 after the systemic administration of picrotoxin (5 mg/kg). It is concluded that the transmission of impulses in the long range caudally running axons from dorsal roots to dorsal columns may be blocked during primary afferent depolarization and that conduction may be restored by the administration of GABA antagonists.

Dynamics of extracellular calcium activity following contusion of the rat spinal cord

The role of Ca2+ in cellular injury has received particular attention in studies of acute spinal cord trauma. In this context, the spatial and temporal distribution of extracellular Ca2+ ([Ca2+]e) may have an important bearing on the development of secondary tissue injury. We therefore studied the spatial-temporal distribution of [Ca2+]e following moderate (25 g-cm) contusive injury to the rat thoracic (T9-T11) spinal cord. Double-barreled, Ca(2+)-selective microelectrodes were used to measure the magnitude and time course of [Ca2+]e at increasing depths from the dorsal spinal cord surface. After 2 h, the tissue was frozen and later analyzed for total Ca concentration using atomic absorption spectroscopy. [Ca2+]e fell at all depths, but the decrease was maximal at 250 and 500 microns from the dorsal surface, where, at 0-10 min after injury, [Ca2+]e averaged 0.09 +/- 0.03 and 0.06 +/- 0.03 mM respectively. By 2 h postinjury, [Ca2+]e recovered to nearly 1 mM across all depths. Over this time, total tissue calcium concentration ([Ca]t) was 4.54 +/- 0.16 mumol/g in injured cords vs 2.75 +/- 0.1 mumol/g in sham-operated controls. These data place emphasis on the dorsal gray matter as a principal site of ionic derangement in acute spinal cord injury. The implications of these findings are discussed with reference to secondary injury processes.

The effects of methylprednisolone and the ganglioside GM1 on acute spinal cord injury in rats

Recent clinical trials have reported that methylprednisolone sodium succinate (MP) or the monosialic ganglioside GM1 improves neurological recovery in human spinal cord injury. Because GM1 may have additive or synergistic effects when used with MP, the authors compared MP, GM1, and MP+GM1 treatments in a graded rat spinal cord contusion model. Spinal cord injury was caused by dropping a rod weighing 10 gm from a height of 1.25, 2.5, or 5.0 cm onto the rat spinal cord at T-10, which had been exposed via laminectomy. The lesion volumes were quantified from spinal cord Na and K shifts at 24 hours after injury and the results were verified histologically in separate experiments. A single dose of MP (30 mg/kg), given 5 minutes after injury, reduced 24-hour spinal cord lesion volumes by 56% (p = 0.0052), 28% (p = 0.0065), and 13% (p > 0.05) in the three injury-severity groups, respectively, compared to similarly injured control groups treated with vehicle only. Methylprednisolone also prevented injury-induced hyponatremia and increased body weight loss in the spine-injured rats. When used alone, GM1 (10 to 30 mg/kg) had little or no effect on any measured variable compared to vehicle controls; when given concomitantly with MP, GM1 blocked the neuroprotective effects of MP. At a dose of 3 mg/kg, GM1 partially prevented MP-induced reductions in lesion volumes, while 10 to 30 mg/kg of GM1 completely blocked these effects of MP. The effects of MP on injury-induced hyponatremia and body weight loss were also blocked by GM1. Thus, GM1 antagonized both central and peripheral effects of MP in spine-injured rats. Until this interaction is clarified, the authors recommend that MP and GM1 not be used concomitantly to treat acute human spinal cord injury. Because GM1 modulates protein kinase activity, protein kinases inhibit lipocortins, and lipocortins mediate anti-inflammatory effects of glucocorticoids, it is proposed that the neuroprotective effects of MP are partially due to anti-inflammatory effects and that GM1 antagonizes the effects of MP by inhibiting lipocortin. Possible beneficial effects of GM1 reported in central nervous system injury may be related to the effects on neural recovery rather than acute injury processes.

Presynaptic modulation of spinal reflexes

Recent evidence suggests that independent sets of interneurons mediate presynaptic inhibition of primary and secondary muscle spindles and of tendon organ afferents. There is also evidence that the information which flows through different intraspinal collaterals of a single muscle spindle or tendon organ afferent fiber is selectively affected by electrical stimulation of the motor cortex. These studies suggest that presynaptic inhibition plays an important role in the selection of the sensory signals required for the execution of a specific motor task.