Biomedical subjects

H J Luhmann

Publications and source records attributed to H J Luhmann.

At least 37 records · Page 2Linked to original sources

PubMed2000

Characterization of a hyperpolarization-activated inward current in Cajal-Retzius cells in rat neonatal neocortex.

Cajal-Retzius cells are among the first neurons appearing during corticogenesis and play an important role in the establishment of cortical lamination. To characterize the hyperpolarization-activated inward current (I(h)) and to investigate whether I(h) contributes to the relatively positive resting membrane potential (RMP) of these cells, we analyzed the properties of I(h) in visually identified Cajal-Retzius cells in cortical slices from neonatal rats using the whole cell patch-clamp technique. Membrane hyperpolarization to -90 mV activated a prominent inward current that was inhibited by 1 mM Cs(+) and was insensitive to 1 mM Ba(2+). The activation time constant for I(h) was strongly voltage dependent. In Na(+)-free solution, I(h) was reduced, indicating a contribution of Na(+). An analysis of the tail currents revealed a reversal potential of -45.2 mV, corresponding to a permeability coefficient (pNa(+)/pK(+)) of 0. 13. While an increase in the extracellular K(+) concentration ([K(+)](e)) enhances I(h), it was reduced by a [K(+)](e) decrease. This [K(+)](e) dependence could not be explained by an effect on the electromotive force on K(+) but suggested an additional extracellular binding site for K(+) with an apparent dissociation constant of 7.2 mM. Complete Cl(-) substitution by Br(-), I(-), or NO(3)(-) had no significant effect on I(h), whereas a complete Cl(-) substitution by the organic compounds methylsulfate, isethionate, or gluconate reduced I(h) by approximately 40%. The I(h) reduction observed in gluconate could be abolished by the addition of Cl(-). The analysis of the [Cl(-)](e) dependence of I(h) revealed a dissociation constant of 9.8 mM and a Hill-coefficient of 2.5, while the assumption of a gluconate-dependent I(h) reduction required an unreasonably high Hill-coefficient >20. An internal perfusion with the lidocaine derivative lidocaine N-ethyl bromide blocks I(h) within 1 min after establishment of the whole cell configuration. An inhibition of I(h) by 1 mM Cs(+) was without an effect on RMP, action potential amplitude, threshold, width, or afterhyperpolarization. We conclude from these results that Cajal-Retzius cells express a prominent I(h) with characteristic properties that does not contribute to the RMP.

Animals↗

PubMed1999

Effects of ionotropic glutamate receptor blockade and 5-HT1A receptor activation on spreading depression in rat neocortical slices.

The effect of the AMPA antagonist NBQX (10 microM), NMDA antagonist ketamine (100 microM) and 5-HT1A agonist 8-OH-DPAT (1, 10 and 100 microM) on the properties of a KCl-induced spreading depression (SD) was studied in parietal cortical slices of adult rats. Whereas NBQX did not significantly affect the SD, ketamine significantly (p < 0.01) reduced the amplitude of the first SD peak (12.8 +/- 4.6 mV) and blocked the second SD peak when compared with the controls (19.8 +/- 5.2 mV and 25 +/- 5 mV, respectively). Ketamine also decreased the SD duration at half maximal amplitude from 34.9 +/- 12.4 s to 22.2 +/- 12 s (p < 0.05). 8-OH-DPAT attenuated the duration of the SD from 42 +/- 15.6 s to 21.2 +/- 10.6 s (p < 0.05, 100 microM). These data indicate that not only NMDA receptor blockade, but also activation of the 5-HT1A receptor attenuates the SD and may be beneficial in the reduction of ischemic injury following focal cerebral ischemia.

Animals↗

PubMed1999

Connectivity in the somatosensory cortex of the adolescent rat: an in vitro biocytin study.

A promising way to elucidate neuronal information processing is to establish detailed structure-function relationships of identified single neurons or populations of nerve cells, especially their synaptic connectivity. This has been greatly improved by the development of acute brain slice preparations. The cellular physiology of the rodent primary somatosensory (barrel) cortex has been extensively studied. However, for a meaningful interpretation of physiological experiments the degree and pattern of connectivity has to be known for the particular preparation. Since such studies are not available for rat (P15-25) barrel cortex in vitro, we have traced the cortico-cortical and thalamo-cortical connections in 400-microm-thick slices with biocytin. In coronal slices, a wealth of axonal connections in retrograde and anterograde directions were heavily labeled, resembling the full pattern of cortico-cortical projections described in vivo. The most striking connections were vertical and horizontal connections within the primary somatosensory cortex, as well as a columnar projection to the secondary somatosensory cortex and beyond (mainly the parietal ventral area). Electron microscopic extensions of the study indicated that the full possible set of synaptic contacts with an adult-like appearance was already established in these connections. In thalamo-cortical slices, strong reciprocal connections with the ventrobasal (and to a much lesser extent also the posterior) thalamic nucleus were always observed, together with an intensive ramification of fibers in the reticular nucleus. A striatal terminal field was also consistently found. We conclude that all major intracortical and thalamo-cortical connection are richly preserved in the in vitro slice preparations of rats. Thus, these preparations are suitable for elucidation of the functional interaction of the most crucial brain structures involved in somatosensory information processing combining an in vivo-like anatomical structure with the controlled environment of an in vitro slice.

Animals↗

PubMed1999

Inhibition of collagen IV deposition promotes regeneration of injured CNS axons.

Scarring impedes axon regrowth across the lesion site and is one major extrinsic constraint to effective regeneration in the adult mammalian central nervous system. In the present study we determined whether specific biochemical or immunochemical modulation of one major component of the scar, the basal membrane (BM), would provide a means to stimulate axon regeneration in the mechanically transected postcommissural fornix of the adult rat. Basal membrane developed within the first 2 weeks after transection in spatiotemporal coincidence with the abrupt growth arrest of spontaneously regrowing axons. Local injection of anticollagen IV antibodies or alpha, alpha'-dipyridyl, an inhibitor of collagen triple helix formation and synthesis, significantly reduced lesion-induced BM deposition. This treatment allowed massive axon elongation across the lesion site. Anterograde tracing provided unequivocal evidence that regenerating axons follow their original pathway, reinnervate the appropriate target, the mammillary body, and become remyelinated with compact myelin. Presynaptic electrophysiological recordings of regenerated fibre tracts showed recovery to nearly normal conduction properties. Our results indicate that lesion-induced BM is an impediment for successful axonal regeneration and its reduction is a prerequisite and sufficient condition for regrowing axons to cross the lesion site.

2,2'-Dipyridyl↗

PubMed1999

Impairment of neocortical long-term potentiation in mice deficient of endothelial nitric oxide synthase.

The role of the possible retrograde messenger nitric oxide (NO) in the induction of long-term potentiation (LTP) was studied in supragranular layers of somatosensory cortical slices obtained from adult mice. High-frequency stimulation produced a slowly rising, long-lasting (50 min) and significant (P < 0.001) increase in the extracellular synaptic response by 23%. The induction of LTP was independent from activation of N-methyl-D-aspartate (NMDA) receptors, but prevented by bath application of NG-nitro-L-arginine methyl ester (L-NAME), indicating that one or several of the different NO synthases (NOS) produced NO within the postsynaptic neuron. No LTP could be induced in knockout mice lacking the endothelial NOS (eNOS) isoform. These data suggest that eNOS is involved in an NMDA receptor-independent form of LTP in the rodent cerebral cortex.

2-Amino-5-phosphonovalerate↗

PubMed1999

Cellular morphology and physiology of the perinatal rat cerebral cortex.

The cellular morphology and electrophysiology of the rat neocortex between embryonic day (E) 18 and postnatal day (P) 3 was studied in vitro by extracellular biocytin injections and whole-cell recordings, respectively. Most neurons were characterized by a small number of short-range dendrites and a main axon that was directed towards the white matter. Biocytin injections into the marginal zone and the cortical plate labeled far-reaching connections extending up to 2 mm in horizontal direction, indicating the existence of a dense network of long-range intrinsic projections in the neonatal cortex. Action potentials could be elicited as early as E18 and repetitive firing could first be observed at P0. Electrical stimulation of the immature cortex at various positions elicited polyphasic and long-lasting (up to 1 s) excitatory postsynaptic potentials and currents, which were significantly reduced in amplitude by a selective N-methyl-D-aspartate receptor antagonist. Our data indicate that the perinatal cortex manifests the structural and functional conditions for powerful excitatory interactions, which increase the likelihood for the generation of epileptiform activity during this developmental period.

Animals↗

PubMed1998

Pattern and pharmacology of propagating epileptiform activity in mouse cerebral cortex.

Multiple extracellular recording electrodes were used to study the intra- and interhemispheric spread of stimulus-evoked epileptiform responses in adult mouse neocortical slices. Bath application of 20 microM bicuculline methiodide induced epileptiform activity that propagated at approximately 0.08 m/s over several millimeters in rostro-caudal and medio-lateral direction within the ipsilateral hemisphere and across the corpus callosum to the contralateral hemisphere. A vertical incision from layer II to subcortical regions did not prevent the spread to remote cortical regions, indicating that layer I plays a major role in the lateral propagation of epileptiform activity. The intra- and interhemispheric spread was not influenced by application of an N-methyl-d-aspartate (NMDA) receptor antagonist, but blocked by an antagonist acting at the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptor. The potential role of potassium channel activation in controlling the generation or spread of epileptiform activity was tested by applying the potassium channel opener cromakalim and the serotonin type 1A (5-HT1A) receptor agonist (+/-)-8-hydroxydipropylaminotetralin (8-OH-DPAT) to the disinhibited slices. Whereas cromakalim reduced the neuronal excitability and blocked all epileptiform responses, 8-OH-DAPT did not affect the activity pattern. Our results suggest that propagating epileptiform activity in disinhibited neocortical structures is predominantly mediated by activation of AMPA receptors and controllable by activation of a voltage-dependent potassium current.

Animals↗

PubMed1998

Long-term cellular dysfunction after focal cerebral ischemia: in vitro analyses.

The long-term (< or = six months) functional consequences of permanent middle cerebral artery occlusion were studied with in vitro extra- and intracellular recording techniques in adult mouse neocortical slices. After survival times of one to three days, 28 days and six months, intracellular recordings from layers II/III pyramidal cells in the vicinity of the infarct did not reveal any statistically significant changes in the intrinsic membrane properties when compared to age-matched control animals. However, a pronounced hyperexcitability could be observed upon orthodromic synaptic stimulation in neocortical slices obtained from mice 28 days after induction of ischemia. Low-intensity electrical stimulation of the afferents elicited particularly in this group epileptiform extracellular field potential responses and intracellular excitatory postsynaptic potentials, that were longer in duration as compared to the controls. When the N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potential was pharmacologically isolated in a bathing solution containing 0.1 mM Mg2+ and 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione, the synaptic responses were longer and larger in the ischemic cortex as compared to the controls. Higher stimulus intensities evoked in normal medium a biphasic inhibitory postsynaptic potential, that contained in the 28 days post-ischemia group a prominent amino-phosphonovaleric acid-sensitive component, indicating a strong concurrent activation of a N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potential. This pronounced co-activation could only be observed in the 28 days ischemic group, and neither after one to three days or six months post-ischemia nor in the controls. The quantitative analysis of the efficiency of stimulus- evoked inhibitory postsynaptic potentials recorded in amino-phosphono-valeric acid revealed a reduction of GABA-mediated inhibition in ischemic cortex. Although this reduction in intracortical inhibition may already contribute to an augmentation of N-methyl-D-aspartate receptor-mediated excitation, our results do also indicate that the function of N-methyl-D-aspartate receptors is transiently enhanced in the ischemic cortex. This transient hyperexcitability does not only cause cellular dysfunction in the vicinity of the infarct, but may also contribute to neuronal damage due to excitotoxicity.

Animals↗

PubMed1998

Long-term changes of ionotropic glutamate and GABA receptors after unilateral permanent focal cerebral ischemia in the mouse brain.

Long-term hyperexcitability was found after unilateral, permanent middle cerebral artery occlusion in exofocal neocortical areas of the adult mouse [Mittmann et al. (1998) Neuroscience 85, 15-27]. The aim of the present study was to test the hypothesis in an identical paradigm of ischemia. whether alterations in the densities of both excitatory and inhibitory amino acid receptors may underlie these pathophysiological changes. Alterations in densities of [3H]dizocilpine, [3H]D,L-amino-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, [3H]kainate and [3H]muscimol binding sites were demonstrated with quantitative in vitro receptor autoradiography. All binding sites were severely reduced in the core of the ischemic lesion. A completely different reaction was found in the exofocal, histologically inconspicuous parts of the somatosensory cortex and the more remote neocortical areas of both hemispheres. The [3H]muscimol binding sites were significantly reduced four weeks after ischemia in the motor cortex, hindlimb representation area and exofocal parts of the primary and secondary somatosensory cortices of both hemispheres. The focus of the reduction in [3H]muscimol binding sites was found in lower layer V and upper layer VI. Contrastingly, the densities of [3H]dizocilpine binding sites were found to be increased in these areas, whereas those of [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and [3H]kainate binding sites did not show significant changes. The [3H]dizocilpine binding site density increased predominantly in layers III and IV. All binding sites were also reduced in the retrogradely reacting, gliotic part of the ipsilateral ventroposterior thalamic nucleus, whereas the [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites were increased in the surround of the ipsilateral nucleus and no changes in binding sites were seen in the whole contralateral nucleus. We conclude that permanent local ischemia leads to a long-term and widespread impairment of the normal balance between binding sites of excitatory and inhibitory neurotransmitter receptors in neocortical areas far away from the focus of the post-ischemic tissue damage. The imbalance comprises an up-regulation of the [3H]dizocilpine binding sites in the ion channels of N-methyl-D-aspartate receptors and a down-regulation of [3H]muscimol binding sites of the GABA(A) receptors in the ipsi- and contralateral neocortex. These changes at the receptor level explain the previously observed hyperexcitability with the appearance of epileptiform field potentials and the long duration of excitatory postsynaptic potentials four weeks after ischemia.

Animals↗

PubMed1998

Analysing functional connectivity in brain slices by a combination of infrared video microscopy, flash photolysis of caged compounds and scanning methods.

We evaluate a novel set-up for scanning functional connectivity in brain slices from the somatosensory cortex of the rat. Upright infrared video microscopy for targeted placement of electrodes is combined with rapid photolysis of bath-applied caged neurotransmitter induced by a xenon flash lamp. Flash photolysis of caged glutamate and electrical stimulation produce comparable field potential responses and demonstrate that the viability of the submerged slices exceeds several hours. Glutamate release leads to field potential responses whose two phases are differentially affected by selective blockade of N-methyl-D-aspartate- and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type glutamate receptors with DL-2-amino-5-phosphonovaleric acid and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulphonamide, respectively. Rapid computer-controlled scanning of hundreds of distinct stimulation sites with simultaneous recordings at a fixed reference site allows construction of functional input maps from peak amplitudes and delays to peak of field potential responses. Selective laminar expansion of the functional input maps after bicuculline application demonstrates that the combination of this conveniently assembled set-up with pharmacological and physical manipulations can provide insights into the determinants of functional connectivity in brain slices.

2-Amino-5-phosphonovalerate↗

PubMed1998

Characterization of neuronal migration disorders in neocortical structures: extracellular in vitro recordings.

The majority of patients showing neuronal migration disorders in cortical structures suffer from pharmaco-resistant epilepsy. In order to study the molecular and cellular mechanisms underlying this pronounced hyperexcitability, we used an animal model of focal cortical dysplasia demonstrating structural malformations which resemble the human pathology of microgyria. Neocortical slices prepared from adult rats, which at the day of birth received a cortical freeze lesion, were analysed in vitro with an array of eight extracellular recording electrodes to investigate the pattern and pharmacology of propagating epileptiform activity in microgyric cortex. In cortical slices exhibiting neuronal migration disorders, orthodromic synaptic stimulation elicited late recurrent activity and early epileptiform responses that spread with 0.06 m/s over > or = 3.5 mm across the cortex. Application of a N-methyl-D-aspartate (NMDA) antagonist blocked the late recurrent activity, but not the propagation of the early epileptiform responses. The latter were blocked by an (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) antagonist, indicating that the spread of this activity was predominantly mediated by activation of AMPA receptors. A very similar response pattern could be observed in neocortical slices obtained from untreated age-matched control rats, when the slice was partially disinhibited by bath-application of 5 microM bicuculline methiodide. Stimulus-evoked epileptiform signals recorded in disinhibited slices propagated with 0.08 m/s across the cortex and showed the same sensitivity to ionotropic glutamate antagonists as in dysplastic cortex. Our results indicate that widespread structural and/or functional modifications of the AMPA receptor and possibly also of the gamma-amino-butyric acid type A receptor contribute to the pronounced hyperexcitability in dysplastic cortex.

Animals↗

PubMed1998

Characterization of neuronal migration disorders in neocortical structures: quantitative receptor autoradiography of ionotropic glutamate, GABA(A) and GABA(B) receptors.

Epileptiform activity was previously described [Luhmann et al. (1998) Eur. J. Neurosci., 10, 3085-3094] in the neocortex of the adult rat following freeze lesioning of the newborn neocortex. After a survival time of 3 months, a small area of dysplastic cortex surrounded by histologically normal (exofocal) neocortex was observed. The dysplastic cortex is characterized by the formation of a small sulcus and a three- to four-layered architecture. Two questions are addressed here: (i) is the hyperexcitability associated with changes in binding to major excitatory and inhibitory transmitter receptors in the dysplastic cortex?; and (ii) do such changes also occur in the exofocal cortex? Alterations in binding to glutamatergic N-methyl-D-aspartate (NMDA), (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), kainate and GABA(A) and GABA(B) (gamma-aminobutyric acid) receptors are demonstrated with quantitative in vitro receptor autoradiography by using the ligands [3H]MK-801, [3H]AMPA, [3H]kainate, [3H]muscimol and [3H]baclofen, respectively. In the dysplastic cortex, the binding to NMDA, AMPA and kainate receptors is significantly increased, whereas the binding to GABA(A) and GABA(B) receptors is reduced. Exofocal areas of the lesioned hemisphere show an imbalance between excitatory and inhibitory receptor binding with an up-regulation of the binding to AMPA and kainate, and a down-regulation to GABA(A) receptors. The binding to GABA(B) and NMDA receptors is not significantly changed in the exofocal areas. The imbalance between excitatory and inhibitory receptors may cause the hyperexcitability, as previously found in the identical experimental model, and may also induce epileptiform activity in the human cortex with migration disorders.

Animals↗

PubMed1998

Involvement of GABA(B) receptors in convulsant-induced epileptiform activity in rat neocortex in vitro.

The role of gamma-aminobutyric acid B (GABA(B)) receptors in the generation and maintenance of bicuculline-induced epileptiform activity in rat neocortical slices was studied using electrophysiological methods. A block of GABA(B) receptors in the presence of functional GABA(A) receptor-mediated inhibition was not sufficient to induce epileptiform activity. In the presence of the GABA(A) receptor antagonist bicuculline (10 microM) and at suprathreshold stimulation, the GABA(B) receptor antagonist CGP 35348 (10-300 microM) significantly potentiated epileptiform activity. With stimulation at threshold intensity, low concentrations of CGP 35348 (10-30 microM) potentiated bicuculline-induced activity, whereas higher concentrations (100-300 microM) invariably led to a reversible suppression of stimulus-evoked epileptiform discharges. CGP 35348 also enhanced picrotoxin-induced epileptiform activity, but at higher concentrations it was considerably less effective in suppressing such epileptiform discharges. The GABA uptake inhibitor nipecotic acid partially mimicked the actions of CGP 35348: with stimulation at threshold intensity, it reversibly suppressed bicuculline-induced epileptiform field potentials, but it did not influence epileptiform activity induced by picrotoxin. We conclude that a postsynaptic blockade of GABA(B) receptors induces an amplification of epileptiform activity in neocortical slices disinhibited by GABA(A) receptor antagonists. An additional blockade of presynaptic GABA(B) receptors, especially under conditions of weak stimulation of the neurons, reduces the inhibitory auto-feedback control of GABA release, leading to a displacement of competitive antagonists from the postsynaptic GABA(A) receptor and hence, to a suppression of epileptiform activity induced by competitive GABA(A) receptor antagonists.

Animals↗

PubMed1998

Characterization of neuronal migration disorders in neocortical structures. II. Intracellular in vitro recordings.

Neuronal migration disorders (NMD) are involved in a variety of different developmental disturbances and in therapy-resistant epilepsy. The cellular mechanisms underlying the pronounced hyperexcitability in dysplastic cortex are not well understood and demand further clinical and experimental analyses. We used a focal freeze-lesion model in cerebral cortex of newborn rats to study the functional consequences of NMD. Intracellular recordings from supragranular regular spiking cells in cortical slices from adult sham-operated rats revealed normal passive and active intrinsic membrane properties and normal stimulus-evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively). Regular spiking neurons recorded in rat dysplastic cortex showed on average a significantly smaller action potential amplitude, a slower spike rise, and a less steep primary frequency-current relationship. Stimulus-elicited EPSPs in NMD-affected cortex consisted of multiphasic burst discharges, which coincided with extracellular field potentials and lasted 150-800 ms. These epileptiform responses could be recorded at membrane potentials between -50 and -110 mV and were blocked by -2-amino-5-phosphonovaleric acid (APV), indicating the involvement of N-methyl--aspartate (NMDA) receptors. Isolated NMDA-mediated and APV-sensitive EPSPs could be recorded at membrane potentials negative to -70 mV, suggesting that NMDA receptors are activated at relatively negative membrane potentials. In comparison with the controls, polysynaptic IPSPs mediated by the gamma-aminobutyric acid (GABA) type A and B receptor were either absent or reduced in peak conductance in microgyric cortex by 27% (P < 0.05) and 17%, respectively. However, monosynaptic IPSPs recorded in the presence of ionotropic glutamate receptor antagonists revealed a similar efficacy in NMD and control cortex, indicating that GABAergic neurons in microgyric cortex get a weaker excitatory input. Our data indicate that the expression of epileptiform activity in NMD-affected cortex rather results from an imbalance between excitatory and inhibitory synaptic transmission than from alterations in the intrinsic membrane properties. This imbalance is caused by an increase in NMDA-receptor-mediated excitation in pyramidal neurons and a concurrent decrease of glutamatergic input onto inhibitory interneurons.

2-Amino-5-phosphonovalerate↗

PubMed1997

Morphology, electrophysiology and pathophysiology of supragranular neurons in rat primary somatosensory cortex.

Intracellularly biocytin-labelled neurons in layers II/III of adult rat primary somatosensory cortex were analysed for their morphological and electrophysiological properties and studied for their response pattern to transient hypoxia under in vitro conditions. The largest dendritic region is formed by the basal dendrites, which constitute an average area of 0.06 mm2 and which can receive synaptic inputs over horizontal distances of more than 300 microns. The dendritic territories formed by the oblique dendrites situated on the apical trunk and by the apical tuft are much smaller. The spine density is highest on the apical trunk, suggesting that large numbers of excitatory synapses are present in this region of the cell. All neurons revealed intrinsic membrane properties of typical regular spiking cells and received an excitatory and a strong biphasic inhibitory input. Whereas a significant correlation could be detected between the cell's input resistance and soma area, no correlation existed between the cell's total dendritic length and input resistance or membrane time constant/input resistance. Neurons responded to transient hypoxia either with an anoxic hyperpolarization with an apparent reversal potential of -82.4 mV, or with a gradual anoxic depolarization which reversed at -56 mV. Oxygen deprivation caused a significant reduction in the extent of axonal collaterals, whereas dendritic proportions and spine density were unaffected. The present study indicates that the dendritic tree is well preserved under in vitro conditions, whereas axonal connections are diminished by oxygen deprivation. Our results further suggest that certain structural properties correlate with the cellular physiology, but that the cell's morphology does not determine its responsiveness to hypoxia.

Animals↗

PubMed1997

Hypoxia-induced dysfunction in developing rat neocortex.

Neocortical slices from young [postnatal day (P) 5-8], juvenile (P14-18), and adult (>P28) rats were exposed to long periods of hypoxia. Field potential (FP) responses to orthodromic synaptic stimulation, the extracellular DC potential, and the extracellular Ca2+ concentration ([Ca2+]o] were measured simultaneously in layers II/III of primary somatosensory cortex. Hypoxia caused a 42 and 55% decrease in the FP response in juvenile and adult cortex, respectively. FP responses recorded in slices from young animals were significantly more resistant to oxygen deprivation as compared with the juvenile (P < 0.01) and adult age group (P < 0.001) and declined by only 3% in amplitude. In adult cortex, hypoxia elicited, after 7 +/- 4.5 min (mean +/- SD), a sudden anoxic depolarization (AD) with an amplitude of 14 +/- 6 mV and a duration of 0.89 +/- 0.28 min at half-maximal amplitude. Although the AD onset latency was significantly longer in P5-8 (12.5 +/- 4.9 min, P < 0.001) and P14-18 (8.7 +/- 3.2 min, P < 0.002) cortex, the amplitude and duration of the AD was larger in young (45.7 +/- 7.6 mV, 2.19 +/- 0.71 min, both P < 0.001) and juvenile animals (29.9 +/- 9.1 mV, P < 0.001, 0.96 +/- 0.26 min, P > 0.05) when compared with the adults. The hypoxia-induced [Ca2+]o decrease was significantly (P < 0.002) larger in young cortex (1,115 +/- 50 microM) as compared with the adult (926 +/- 107 microM). Prolongation of hypoxia after AD onset for >5 min elicited in young and juvenile cortex a long-lasting AD with an amplitude of 40.5 mV associated with a decrease in [Ca2+]o by >1 mM. On reoxygenation, only slices from these age groups showed spontaneous repetitive spreading depression in 3 out of 26 cases. In adults, the same protocol caused a significantly (P < 0.05) smaller and shorter AD and never a spreading depression. However, recovery in synaptic transmission after this long-term hypoxia was better in young and juvenile cortex, indicating a prolonged or even irreversible deficiency in synaptic function in mature animals. Application of ketamine caused a 49% reduction in the initial amplitude of the AD in juvenile cortex but did not significantly affect the AD in slices from adult animals. These data indicate that the young and juvenile cortex tolerates much longer periods of oxygen deprivation as compared with the adult, but that a sufficiently long hypoxia causes severe pathophysiological activity in the immature cortex. This enhanced sensitivity of the immature cortex is at least partially mediated by activation of N-methyl-D-aspartate receptors.

Aging↗

PubMed1996

Repetitive spreading depression causes selective suppression of GABAergic function.

Cortical spreading depression (SD) represents a pathophysiological signal that has been associated with the induction of migraine and ischaemic brain damage. The properties of repetitive SDs and their effects on excitatory and inhibitory synaptic transmission were analysed in neocortical slices obtained from adult rats. The SD showed only small variations in amplitude, duration and integral when elicited four times at intervals of 30 min. Extracellularly recorded paired pulse inhibition was, however, significantly reduced by approximately 10% with each SD episode. Since excitatory synaptic transmission was unaffected, our data indicate that repetitive SD causes a selective reduction of intracortical inhibition.

Animals↗

PubMed1996

Ischemia and lesion induced imbalances in cortical function.

Cortical structures are often critically affected by ischemic and traumatic lesions which may cause transient or permanent functional disturbances. These disorders consist of changes in the membrane properties of single cells and alterations in synaptic network interactions within and between cortical areas including large-scale reorganizations in the representation of the peripheral input. Prominent functional modifications consisting of massive membrane depolarizations, suppression of intracortical inhibitory synaptic mechanisms and enhancement of excitatory synaptic transmission can be observed within a few minutes following the onset of cortical hypoxia or ischemia and probably represent the trigger signals for the induction of neuronal hyperexcitability, irreversible cellular dysfunction and cell death. Pharmacological manipulation of these early events may therefore be the most effective approach to control ischemia and lesion induced disturbances and to attenuate long-term neurological deficits. The complexity of secondary structural and functional alterations in cortical and subcortical structures demands an early and powerful intervention before neuronal damage expands to intact regions. The unsatisfactory clinical experience with calcium and N-methyl-D-aspartate antagonists suggests that this result might be achieved with compounds that show a broad spectrum of actions at different ligand-activated receptors, voltage-dependent channels and that also act at the vascular system. Whether the same therapy strategies developed for the treatment of ischemic injury in the adult brain may be applied for the immature cortex is questionable, since young cortical networks with a high degree of synaptic plasticity reveal a different response pattern to hypoxic and ischemic insults. Age-dependent molecular biological, morphological and physiological parameters contribute to an enhanced susceptibility of the immature brain to these noxae during early ontogenesis and have to be investigated in more detail for the development of adequate clinical therapy.

Adult↗