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Pathophysiology of the spinal cord studied in vitro
215
PATHOPHYSIOLOGY OF THE SPINAL COFD STUDIED IN VITRO G.G S O M J ~ and G.CZEH, Department of Physiology, Duke University Medical Center, Durham, NC 27710 U.S.A. Hemisected mouse spinal cords were maintained submerged in rapidly flowing artificial cerebrospinal fluid (ACSF)_at 22°C. The control ACSF contained 3.5 mM K+ and 1.2 mM each of Ca 2+ and Mg 2+. In an earlier study we examined the effect of raising and lowering extracellular potassium concentration ([K+]A) by changing K + levels in the bath (Cz~h, Obih and Somjen, Brain Res., in press~. In more recent experiments we varied divalent cation_concentration both in fully oxygenated and in N2-saturated ACSF. Raising [Ca2+]o enhanced the late component of segmental reflexes, of dorsal root potentials CDRP), and of dorsal root reflexes (DRR~. Spontaneous activity, synchronized in dorsal and ventral roots (DR and VR) and in focal recordings from gray matter, erupted when bath [Ca2+] was raised to 1.8 - 2.4 mM, the threshold concentration varying in different experiments. Spontaneous discharges consisted of slow wave with superimposed impulse showers, resembling "interictal" discharges induced by penicillin in spinal cord in situ (Electroenceph. Clin. Neurophysiol. 41: 237, 1976). More prolonged, "ictal" seizures were not seen. The N~DA antagonist APV reversibly reduced theA frequency and amplitude of high Ca-induced spontaneous waves. Elevated [Mg2+]^ reversibly blocked spontaneous activity, as did blCuculllne and plcrotoxln at a dose level that reduced but did not block DRPs. We conclude that the circuit generating Ca-induced spontaneous activity includes an obligatory GABA-ergic component and is facilitated by NMDA receptor activation. Jefferys & Haas (Nature, 300: 448, 1982) reported spontaneous burst activity in hippocampal tissue slices exposed to very low [Ca2+]^. This was not seen in isolated splnal cords. Conversely, in hlppocampal tlssue sllces exposed to elevated [Ca.2+ ] we did not observe spontaneous discharges, not burst-firing in response to afferent volleys, although fEPSP and orthodromic population spikes were increased. When isolated spinal cord was deprived of oxygen, evoked responses subsided rapidly in the first i0 min and more slowly thereafter. The depression of the responses was reversible after 45 min hypoxia at 22°C. When [Ca-l+] in ACSF was raised to 3.6 mM 20 min before onset of hypoxia, the pre-hypoxic evoked responses were enhanced (see above) and the initial phase of hypoxic decay was much slowed. After 30 min hypoxia the depression in the two groups (normo- and hypercalcic) became equal. .
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(Supported by grants NS17771 and NS 18670)
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PATHOPHYSIOLOGY OF THE SPINAL COFD STUDIED IN VITRO G.G S O M J ~ and G.CZEH, Department of Physiology, Duke University Medical Center, Durham, NC 27710 U.S.A. Hemisected mouse spinal cords were maintained submerged in rapidly flowing artificial cerebrospinal fluid (ACSF)_at 22°C. The control ACSF contained 3.5 mM K+ and 1.2 mM each of Ca 2+ and Mg 2+. In an earlier study we examined the effect of raising and lowering extracellular potassium concentration ([K+]A) by changing K + levels in the bath (Cz~h, Obih and Somjen, Brain Res., in press~. In more recent experiments we varied divalent cation_concentration both in fully oxygenated and in N2-saturated ACSF. Raising [Ca2+]o enhanced the late component of segmental reflexes, of dorsal root potentials CDRP), and of dorsal root reflexes (DRR~. Spontaneous activity, synchronized in dorsal and ventral roots (DR and VR) and in focal recordings from gray matter, erupted when bath [Ca2+] was raised to 1.8 - 2.4 mM, the threshold concentration varying in different experiments. Spontaneous discharges consisted of slow wave with superimposed impulse showers, resembling "interictal" discharges induced by penicillin in spinal cord in situ (Electroenceph. Clin. Neurophysiol. 41: 237, 1976). More prolonged, "ictal" seizures were not seen. The N~DA antagonist APV reversibly reduced theA frequency and amplitude of high Ca-induced spontaneous waves. Elevated [Mg2+]^ reversibly blocked spontaneous activity, as did blCuculllne and plcrotoxln at a dose level that reduced but did not block DRPs. We conclude that the circuit generating Ca-induced spontaneous activity includes an obligatory GABA-ergic component and is facilitated by NMDA receptor activation. Jefferys & Haas (Nature, 300: 448, 1982) reported spontaneous burst activity in hippocampal tissue slices exposed to very low [Ca2+]^. This was not seen in isolated splnal cords. Conversely, in hlppocampal tlssue sllces exposed to elevated [Ca.2+ ] we did not observe spontaneous discharges, not burst-firing in response to afferent volleys, although fEPSP and orthodromic population spikes were increased. When isolated spinal cord was deprived of oxygen, evoked responses subsided rapidly in the first i0 min and more slowly thereafter. The depression of the responses was reversible after 45 min hypoxia at 22°C. When [Ca-l+] in ACSF was raised to 3.6 mM 20 min before onset of hypoxia, the pre-hypoxic evoked responses were enhanced (see above) and the initial phase of hypoxic decay was much slowed. After 30 min hypoxia the depression in the two groups (normo- and hypercalcic) became equal. .
•
.
.
.
°
~2
,
(Supported by grants NS17771 and NS 18670)
•
~
°