Thalamic afferent activation of supragranular layers in auditory cortex in vitro: a voltage sensitive dye study.

We studied auditory thalamocortical interactions in vitro, using an auditory thalamocortical brain slice preparation. Cortical activity evoked by electrical stimulation of the medial geniculate nucleus (MGN) was investigated through field potential recordings and voltage sensitive dyes. Experiments were performed in slices obtained from adult mice (9-14 weeks). Stimulus evoked activity was detected in the granular and supragranular layers after a short latency (5-6 ms). In 9-14 weeks old mice infragranular activity was detected in 10 of 24 preparations and was found to be increased in younger mice (p 31-64). In 14 of 24 slices a prominent horizontal spread was observed, which extended into cortical areas lateral to A1. In these experiments, the shortest onset latencies and largest signal amplitudes were located in the supragranular layers of A1. In areas lateral to A1, shortest onset latencies were located in the granular layer, while largest signal amplitudes were found in the supragranular layers. Evoked cortical activity was sensitive to removal of extracellular Ca(2+) or application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). Short repetitive stimulation, resembling thalamic burst activity (three pulses at 100 Hz), resulted in an increase of signal amplitude and excited area by approximately 25%, without changing the overall spatiotemporal activity profile. Blockade of N-methyl-D-aspartate receptors by 2-amino-5-phosphonopentanoate (AP5, 50 microM) reduced amplitudes and excited area by approximately 15-30%, irrespective of stimulation frequency. Application of bicuculline (10 microM) greatly increased cortical responses to thalamic stimulation. Under these conditions, evoked activity displayed a pronounced horizontal spread in combination with a 2-3-fold increase in amplitude. In conclusion, afferent thalamic inputs primarily activate supragranular and granular layers in the auditory cortex of adult mice. This activation is predominantly mediated by non-NMDA receptors, while GABA(A) receptor-mediated inhibition limits the horizontal and vertical spread of activity.

Hyperbaric oxygen in neurosurgery.

BACKGROUND: The therapeutic use of pure oxygen, even under hyperbaric conditions, has been well established for about 50 years, whereas the discovery of oxygen occurred 250 years earlier. Many neurosurgical patients suffer from brain tissue damage, due to reduced blood flow, obstructive vessel disease, or as a result of traumatic brain injury. METHODS AND RESULTS: The application of pure oxygen in these patients is the only method of increasing the O(2) concentration in tissue with impaired blood supply and can minimize secondary impairment of brain tissue. DISCUSSION: In this brief historical overview we focus on the development and evidence of hyperbaric oxygenation in this specific field of insufficient oxygen supply to the central neural tissue. CONCLUSION: With the use of modern biological methods and new study designs, HBO has a place in evidence-based treatment of patients with neural tissue damage.

The effects of verapamil and flunarizine on epileptiform activity induced by bicuculline and low Mg2+ in neocortical tissue of epileptic and primary non-epileptic patients.

In human neocortical slices the specific L-type calcium channel blocker verapamil had been shown to be antiepileptic in the low Mg(2+)-model of epilepsy. The present investigation demonstrated: (1) verapamil exerted also an antiepileptic effect on epileptiform field potentials (EFP) induced by the GABAA-antagonist bicuculline. (2) The unspecific calcium channel modulator flunarizine, which in contrast to verapamil penetrates the blood-brain barrier, depressed EFP in the low Mg(2+)-model and in the bicuculline model. (3) There was no significant difference in the antiepileptic efficacy of verapamil and flunarizine in epileptic (epilepsy surgery) and primary non-epileptic (tumor surgery) neocortical slices.

Improvement and testing of a concentration-clamp system for oocytes of Xenopus laevis.

A system for rapid solution exchange on Xenopus laevis oocytes in the two-electrode voltage-clamp mode (Madeja et al., 1991) was improved for investigations on oocytes with removed follicular tissues. The speed of application and the time for potassium ions to reach the oocyte membrane (t50) was found to be about 40 ms in oocytes without follicular tissues and 360 ms for oocytes with follicular tissues. This rate of solution exchange is fast enough to measure the fast desensitizing component of the AMPA current response.

Influence of the organic calcium antagonist verapamil on N-methyl-D-aspartate (NMDA) induced cortical field potentials (neocortical slice, guinea pig).

In the present experiments it was tested whether the organic calcium antagonist verapamil has an influence on N-methyl-D-aspartate (NMDA) induced cortical field potentials (CFP) in neocortical slices of guinea pigs. NMDA (1 mumol/l) was applied via a micropipette by pressure pulses. Verapamil (60 mumol/l) was administered by bath application or ejected locally (100 mumol/l) and simultaneously with NMDA. Neither the systemic administration nor the local application of verapamil had an influence on NMDA induced CFP. It is concluded that the antiepileptic effect of verapamil is not mediated via the NMDA receptor.

Low magnesium induced epileptiform discharges in neocortical slices (guinea pig): increased antiepileptic efficacy of organic calcium antagonist verapamil with elevation of extracellular K+ concentration.

1. The antiepileptic effect of the organic calcium antagonist verapamil on low Mg2+ induced epileptiform discharges in the neocortex was tested. 2. The experiments were carried out on slices of the frontal neocortex (guinea pigs). Verapamil was tested at normal (4 mmol/l) and elevated (8 mmol/l) KCl levels. 3. Verapamil (40, 60 mumol/l) suppressed epileptiform activity in any case. 4. With elevated K+ concentration the suppressive effect of verapamil was significantly accelerated. 5. Epileptic activity reappeared when verapamil was omitted from the Mg(2+)-free superfusate.

Reduction in bioelectric GABA and NMDA responses in organotypic neocortical explants by chronic elevation of potassium.

Long-term cultures of organotypic neonatal rat neocortex slices were maintained in a serum-free medium supplemented with 25 mM potassium. Such cultures continue to be bioelectrically silent upon return to a 5 mM potassium medium. The absence of responses to pressure ejected gamma-aminobutyric acid (GABA) and N-methyl-D-aspartate (NMDA) from neurons in treated explants suggests that one consequence of chronic depolarization is a reduction in the density of postsynaptic transmitter receptors. [3H]Muscimol binding to neocortical membrane preparations shows a large reduction in the binding of this agonist to GABAA receptors. These data show that the quantitative expression of at least one neurotransmitter receptor, the GABAA receptor, relies on voltage-dependent activity in developing neocortical neurons in vitro.

Augmentation of glutamate responses by GABA in the rat's motorcortex in vivo.

The influence of the inhibitory transmitter gamma-aminobutyric acid (GABA) on cortical field potential changes (CFPs) elicited by the excitatory transmitter glutamate and its subreceptor agonists N-methyl-D-aspartate (NMDA) and quisqualate was tested in the motorcortex of anesthetized and artificially ventilated rats. Drugs were applied through a 3-barrelled micropipette by ionophoresis or pressure ejection. Glutamate and its agonists evoked negative and GABA positive CFPs. Applications of GABA before (up to 300 s) the ejection of glutamate, NMDA or quisqualate increased the amplitude of the negative CFP induced by the excitatory transmitters. This augmentation was more pronounced with NMDA and quisqualate than with glutamate. It could be mimicked by the GABAA-agonist muscimol but not by the GABAB-agonist baclofen. It is suggested that the enhancement of glutamate responses by GABA may be mediated by an intracellular common pathway.

Motor cortical epileptic foci in vivo: actions of a calcium channel blocker on paroxysmal neuronal depolarizations.

Focal epileptiform activity was induced by local application of penicillin to the surface of the rat motor cortex. Neurons located within the epileptic focus displayed typical paroxysmal depolarization shifts (PDS). The participation of membrane calcium currents in the generation of PDS was examined by injecting the quaternized calcium entry blocker D890 into single neurons by iontophoresis or by pressure pulses. After intracellular injections of D890, PDS were depressed in amplitude by up to 55%. In a few cases the depression of PDS following intracellular application of D890 was preceded by a transient increase. Similar increases of PDS amplitude were obtained by injections of the calcium chelator EGTA. Control experiments in preparations without epileptic activity revealed that excitatory potentials elicited by thalamic stimulation and Cl(-)-dependent inhibitory postsynaptic potentials evoked by epicortical stimulation were not affected by intracellular D890. In these experiments successful intracellular drug application was verified by monitoring the transient shift of the Cl(-)-equilibrium potential induced by injection of KCl together with D890. It is concluded that in the penicillin-induced epileptic focus of the motor cortex Ca2+ inward currents participate in the generation of neuronal PDS.

Anticonvulsant effects of calcium channel blockers in partial and generalized model epilepsies.

The calcium channel blockers D890 and verapamil reduced neuronal calcium currents in single snail neurons with intra- and extracellular applications, respectively. Epileptic discharges of single neurons in the rat's motor cortex (in vivo) were depressed in amplitude by intracellular injection of D890. Focal seizure discharges and generalized tonic-clonic seizure activity in the cerebral cortex of the rat were reduced by intracerebroventricular perfusion of verapamil. In non-epileptic rats verapamil failed to exert a depressive effect on somatosensory evoked potentials and on the waves of the spontaneous EEG.

Paroxysmal neuronal depolarizations in the rat motorcortex in vivo: intracellular injection of the calcium agonist BAY K 8644.

Epileptic activity was elicited in the rat's motor cortex by local application of penicillin. At the neuronal level it consisted of typical paroxysmal depolarization shifts. The calcium agonist BAY K 8644 was injected into neurons showing such a discharge pattern. The application of this drug increased amplitude and after depolarization of paroxysmal neuronal depolarizations.

Focal interictal epileptiform discharges (FIED) in the epicortical EEG and their relations to spinal field potentials in the rat.

To study the relations between epileptiform potentials in the surface EEG and motor phenomena, focal interictal epileptiform discharges (FIEDs) were induced by the combined application of penicillin and penicillinase to the cortical surface of the rat. Spinal field potentials (SFPs) served as an indicator of descending activity. The following results were obtained. (1) The focus induced by the technique generated epileptiform activity within a very restricted cortical region. (2) FIEDs were accompanied by synchronized SFPs in the cervical and lumbar cord if the focus was located in the corresponding cortical motor area. With FIEDs in non-motor areas SFPs failed to occur. A unilateral focus led to SFPs on both sides of the spinal cord. (3) The transverse distribution of field potentials within the spinal cord revealed that the early negative peak of the SFP reached its maximum in the dorsal horn. (4) After the application of the drugs the development of FIEDs preceded that of SFPs by ca. 5 min. When the drug effect ceased SFPs remained often fully developed up to 10 min after the FIEDs had disappeared. (5) The temporal relationship between the cortical and spinal events showed individual and interindividual variations. (6) Anodal and cathodal electrical polarization of the cortical surface by currents sufficient to alter the shape, amplitude and polarity of the FIEDs failed to change the SFPs. The present investigations demonstrate that the cortical output from a restricted focus does not correspond to a definite epicortical field potential.