Ketamine-Midazolam Anesthesia Induces Total Inhibition of Cortical Activity in the Brain of Newborn Rats.

The effects of general anesthetics ketamine and midazolam, the drugs that cause neuroapoptosis at the early stages of CNS development, on electrical activity of the somatosensory cortex in newborn rats were studied using extracellular recording of local field potentials and action potentials of cortical neurons. Combined administration of ketamine (40 mg/kg) and midazolam (9 mg/kg) induced surgical coma and almost completely suppressed early oscillatory patterns and neuronal firing. These effects persisted over 3 h after injection of the anesthetics. We concluded that general anesthesia induced by combined administration of ketamine and midazolam profoundly suppressed cortical activity in newborn rats, which can trigger neuroapoptosis in the developing brain.

[Secondary epileptogenesis in the immature brain: a role of GABA]. / Vtorichnyi épileptogenez v nezrelom mozge: rol' GAMK.

Clinical studies show that the probability of recurrent epileptiform discharges and formation of an epileptic focus (epileptogenesis) in young children is much higher than in adults. Repetitive epileptiform discharges and their potential contribution to the mechanisms of the development of the epileptic focus - an important object of clinical and scientific research. This review is based on the data from animal studies, and summarizes the current understanding of the mechanisms underlying the increased excitability of the immature brain, the formation of a secondary epileptogenic focus, and the functional changes of neurons due to deleterious effects of repetitive epileptiform discharges on the excitation and inhibition in the immature neuronal networks. The review discusses the relevance of experimental data in light of the general mechanisms of epileptogenesis in infants and identifies the gaps in current scientific knowledge, including the relationship between the data obtained in animal studies and processes underlying human acquired epilepsy.

Gamma oscillations in the somatosensory cortex of newborn rats.

Here we addressed a question of whether gamma oscillations previously described in the whisker-related barrel cortex are a universal pattern of activity in the somatosensory cortex of newborn rats. Intracortical recording of local field potentials and action potentials in neurons using multisite silicon electrodes in 2-7-day-old rats showed that mechanical stimulation of single fingers or specific areas on the plantar or back side of the foot evoked early gamma oscillations followed by spindle-burst oscillations in the corresponding regions of the somatosensory cortex. Early gamma oscillations had maximum amplitude in layer IV of the somatosensory cortex and effectively synchronized action potentials in layer IV neurons. It was concluded that early gamma oscillations evoked by activation of the topographic sensory input are a universal activity pattern of the entire somatosensory cortex of newborn rats.

[Excitatory action of GABA in ontogenesis].

GABA (gamma-aminobutyric acid) is the main inhibitory mediator in central nervous system. However, at early stages of ontogenesis, GABA has an excitatory effect on immature neurons. This review surveys modern concepts of the mechanisms of GABAergic excitation and physiological role of excitatory GABA in generation of patterns of network activity in developing brain.

[GABA--the basic mediator of excitation in the early stages of hippocampal development]. / GAMK--osnovnoi mediator vozbuzhdeniia na rannikh étapakh razvitiia gippokampa.

GABA is the principal neurotransmitter of inhibition in the adult mammalian brain. However, at early stages of development, including embryonic period and first week of postnatal life, GABA plays the role of main neurotransmitter of excitation. The paradoxical excitatory effect of GABA is due to an inversed chloride gradient and therefore a depolarizing direction of GABA-A receptor mediated responses. In addition, another type of GABAergic inhibition mediated by postsynaptic GABA-B receptors is not functional at early stage of life. In the neonatal rat hippocampus, GABA, acting via GABA-A receptors, activates voltage gated sodium and calcium channels and potentiates the activity of NMDA receptors by reducing their voltage dependent Mg2+ block. The temporal window when GABA exerts excitatory actions coincides with a particular pattern of activity of hippocampal neuronal network that is characterized by periodical giant depolarizing potentials (GDPs) reminiscent of interictal-like epileptiform discharges. Recent studies have shown that GDPs result from the synchronous discharge of GABAergic interneurons and principal glutamatergic pyramidal cells and are mediated by the synergistic excitatory actions of GABA-A and glutamate receptors. GDPs provide synchronous intracellular Ca2+ oscillations and may therefore be implicated in hebbian modulation of developing synapses and activity-dependent formation of the hippocampal network.

The effect of non-quantal acetylcholine release on quantal miniature currents at mouse diaphragm.

1. The amplitude and exponential decay time constant of miniature endplate currents (MEPCs) were measured in mouse diaphragms treated with anti-cholinesterase under conditions known to modulate non-quantal acetylcholine (ACh) release. 2. Anti-cholinesterase prolonged MEPC decay and the extent of this initial prolongation was not influenced by non-quantal release. When non-quantal release was present, the decays of MEPCs became increasingly faster over several hours. This increased decay did not occur in the absence of non-quantal release. 3. Potentiation of the non-quantal release by zero Mg2+ and 1 x 10(-5) M choline, on the other hand, led to acceleration of MEPC shortening. 4. Increase of temperature from 15 to 26 degrees C and the presence of the desensitization-promoting drug proadifen (5 x 10(-6) M) accelerated the rate of MEPC shortening. 5. These observations are consistent with increased receptor desensitization due to non-quantal release. Repetitive binding of ACh to postsynaptic receptors which prolongs the time course of MEPC in anti-cholinesterase-treated endplates leads to progressive desensitization in the presence of non-quantal release and to the subsequent shortening of the quantal responses.

A correlation between quantal content and decay time of endplate currents in frog muscles with intact cholinesterase.

1. The relationship between quantal content and prolongation of endplate currents (EPC) was studied in the frog sartorius with intact synaptic acetylcholinesterase. 2. The prolongation of EPC was more pronounced in endplates with a higher quantal content both before and after potentiation of quantal release by 4-aminopyridine (4-AP). When the quantal content of EPC was lowered, either by high Mg2+ or repetitive stimulation, the EPC decay constant was reduced. 3. A certain critical value of about 120 quanta per nerve impulse was found, at which point the decay of EPC remained constant even through the quantal content was reduced further. 4. The reduction in both density and number of postsynaptic receptors, produced by alpha-bungarotoxin and (+)-tubocurarine led to a profound reduction in EPC decay during the progressive fall in EPC amplitude in both 4-AP-treated and -untreated endplates. Both drugs are known to produce a shortening of EPC in anti-cholinesterase (anti-ChE)-treated muscles, due to a decrease in receptor density and less frequent repetitive binding of ACh. 5. It is assumed that the prolongation of multiquantal EPC is caused by an increased ACh concentration near the receptors, which may provide the opportunity for repetitive binding even with full cholinesterase activity. The critical quantum content of about 120 might be the number of quanta at which the probability of multiple release at single active zones is increased above zero.

[Non-quantal secretion of a mediator as factor determining consequences of acetylcholinesterase inhibition]. / Nekvantovaia sekretsiia mediatora kak faktor, opredeliaiushchii posledstviia ingibirovaniia atsetilkholinésterazy.

The mechanism of shortening MEPC decay phase after initial prolongation due to acetylcholinesterase inhibition by armine and neostigmine was studied by use of two-electrode voltage-clamp at the mice diaphragm Factors which switch off non-quantal secretion of acetylcholine from the nerve (acute denervation in vitro, ouabain, high concentration of magnesium ions) only slightly reduced the prolongation of MEPC caused by AChE inhibition. So, postsynaptic potentiation of MEPC by nonquantal ACh is not significant immediately after AChE inhibition. At the same time these factors abolished the process of shortening MEPC decay phase. It is concluded, that desensitization of the postsynaptic membrane induced by nonquantal ACh is the main mechanism of the MEPC shortening and that this mechanism can compensate insufficient AChE activity.

[Effect of magnesium ions on the functional state of postsynaptic membrane by modulation of the level of non-quantal secretion of the mediator]. / Vliianie ionov magniia na funktsional'noe sostoianie postsinapticheskoi membrany cherez moduliatsiiu urovnia nekvantovoi sekretsii mediatora.

The role of non-quantal secretion (NS) of acetylcholine (ACh) in shortening of miniature end-plate currents (MEPC) after initial prolongation of MEPC due to acetylcholinesterase (AChE) inhibition was studied in the mouse diaphragm muscle under the voltage clamp conditions. Elevation of NS level by removal of magnesium ions from solution accelerated the shortening effect. After elimination of NS due to an increase of magnesium concentration up to 3 mmol/l the shortening of MEPC was absent. It was suggested that after AChE inhibition the magnesium ions in physiological range of concentrations can modulate the desensitization development on postsynaptic membranes by NS level regulation.

[Effects of "non-quantal" acetylcholine on the sensitivity of the postsynaptic membrane: action of ouabain and imitation of these effects by exogenous acetylcholine]. / Vliianie "nekvantovogo" atsetilkholina na chuvstvitel'nost' postsinapticheskoi membrany: deistvie ouabaina i imitatsiia éffektov ékzogennym atsetilkholinom.

Development of postsynaptic potentiation (PSP) and desensitization (DS) caused by "non-quantal" acetylcholine after acetylcholinesterase inhibition was studied by means of ouabain, an agent known to modulate (initially increase and then decrease) the level of non-quantal secretion of ACh. Ouabain had no effect on the MEPC parameters when AChE was active. After AChE inhibition ouabain initially increased the decay time constant of MEPC (tau), i.e. caused postsynaptic potentiation (PSP). This effect of ouabain grew with time between inhibition of AChE and application of ouabain. The PSP stage was followed by shortening of MEPCs decay, due to the development of desensitization (DS), and that process was more pronounced than in control. Applied before AChE inhibition, ouabain had no effect on tau. Thus neither PSP nor DS developed under those conditions. Exogenous ACh (20 nmol/l) applied simultaneously with inhibitor of AChE partially prevented the shortening of MEPCs decay, but decreased the amplitude of MEPC. Applied after MEPCs shortening, exogenous ACh (50 nmol/l) tended to return the initial value of tau. It is concluded that nonquantal ACh produces PSP and DS on the postsynaptic membrane after inhibition of ACh and that the DS persists after cessation of nonquantal secretion for a long time.

Critical quantum content for shortening of endplate currents in the frog skeletal muscle.

The decay time of endplate currents was followed during progressive lowering of quantum content of endplate responses by reduced Ca2+. A certain critical value of about 100 quanta was found, when the decay of endplate currents remained constant even though the quantal content was reduced further.

[The early postdenervation changes in the properties of the postsynaptic membrane of amphibian muscle fiber]. / Rannie postdenervatsionnye izmeneniia svoistv postsinapticheskoi membrany myshechnogo volokna amfibii.

The amplitude-temporal characteristics of miniature end-plate currents (MEPC) were studied in denervated frog cutaneous-pectoral muscles. After the nerve cutting, the initial shortening of MEPC decay time was followed by its prolongation. In 6-8 days after the denervation, the MEPC sensitivity to membrane potential decreased, while the average open time of ACh-activated channels increased 2-fold. The autocorrelation function of ACh-induced current noise revealed two components at some of the synapses. The denervation of the frog cutaneous-pectoral muscle seems to make the postsynaptic membrane to contain two types of ACh receptors (AChR): normal synaptic AChR which gradually disappear, and new synthesized AChRs which open the channels with much longer mean time and a lesser sensitivity to membrane potential changes.

[End plate currents with a physiological level of quantal secretion and after potentiation of the mediator release by 4-aminopyridine]. / Toki kontsevoi plastinki pri fiziologicheskom urovne kvantovoi sekretsii i posle potentsiatsii osvobozhdeniia mediatora 4-aminopiridinom.

The role of postsynaptic potentiation (PSP) and asynchronous secretion of acetylcholine (ACh) in generation of multiquantal and plate currents (EPC) was studied under voltage clamp conditions in the cut sartorius muscle of frog before and after 4-aminopyridine (4-AP) treatment. If compared with miniature EPC (MEPC), multiquantal EPC had larger amplitude and more prolonged both rise-time and decay. In the presence of 4-AP the difference between the decay time constant of EPC (tau epc) and MEPC (tau mepc) became much more pronounced. Mean quantal content of EPC was 249 in control 373 and 423 in the presence of 1 X 10(-5) and 1 X 10(-4) mol/l of 4-AP, respectively. Magnesium ions (6-10 mmol/l) reduced the amplitude and tau epc of EPC without affecting its rise-time. Repetitive activity (10 Hz, 60s) resulted in the reduced amplitude and tau epc but in increased rise-time of EPC. D-tubocurarine (5 X 10(-7) mol/l) and alpha-bungarotoxin (1 X 10(-6) g/ml) diminished the difference between tau epc and tau mepc. In the presence of 4-AP all these effects were much more pronounced. It was suggested that asynchronous secretion of ACh from motor nerve terminals caused prolongation of the rise-time, reduction in the amplitude of EPC, but had little or no effect on the decay rate of EPC. The slow decay of multiquantal EPC both in the absence and in the presence of 4-AP was almost entirely due to postsynaptic interaction of ACh quanta, i. e. PSP.

Changes in the time course of miniature endplate currents induced by bath-applied acetylcholine.

Bath application of 0.5 and 2 microM acetylcholine (ACh) slowed the decay phase of miniature endplate currents (MEPC) recorded in isolated, voltage-clamped and prostigmine-treated frog sartorius muscle. Washout of ACh led to a decrease of the decay time constant of the MEPC to 72 +/- 5% (n = 5) and 51 +/- 3% (n = 6) of initial values, respectively, followed by very slow and incomplete recovery. MEPC amplitude changed slightly and recovered relatively fast. This discrepancy in the recovery rates is suggested to be due to a 'trapping' ability of desensitized receptors which can compete with the free receptors for ACh molecules and prevent repetitive binding. Thus the high affinity of desensitized receptors to ACh may partially compensate the absence of acetylcholinesterase activity.

[The characteristics of the action of calcium ions on miniature end-plate currents after the disruption of mediator hydrolysis]. / Osobennosti deistviia ionov kal'tsiia na miniatiurnye toki kontsevoi plastinki posle narusheniia gidroliza mediatora.

Effect of calcium on the miniature end-plate currents (MEPC) at the frog neuromuscular junction was studied by the voltage-clamp technique. Rise of the calcium concentration in the Ringer solution up to 9 mmol/l caused a decrease of the MEPC amplitude which was related to the reduction of the end-plate channel conductance. Calcium had no effect on the time course of MEPCs at the active acetylcholinesterase (AChE) but accelerated MEPC decay by 26% after AChE inhibition by neostigmin or armin. It is supposed that the shortening effect of calcium on the MEPC decay phase is based on the ability of calcium to modulate the block of ionic channels by acetylcholine or to accelerate the process of desensitization of the postsynaptic membrane.

[The development of desensitization during rhythmic activity of the synapse and in the course of generating a single signal]. / Razvitie desensitizatsii pri ritmicheskoi aktivnosti sinapsa i v khode generatsii odinochnogo signala.

The possibility of desensitization (DS) development under the rhythmic activity and during the generation of the single response to acetylcholine was studied in the frog sartorius muscle by voltage-clamp technique. It was revealed, that under conditions promoting DS (hyperpolarization, muscle warming, rise of calcium concentration as well as treatment by the DS--potentiating agent--proadifen) decrease of the postsynaptic membrane sensitivity to the acetylcholine can develop after few multiquantal end-plate currents--and when acetylcholinesterase is inhibited--during the response to a single quantum of acetylcholine.

[Role of the density of cholinoreceptors in the mechanisms slowing a drop in the postsynaptic current]. / Rol' plotnosti kholinoretseptorov v mekhanizmakh zamedleniia spada postsinapticheskogo toka.

Several factors, which prolonged the decay of the endplate current (EPC) without the loss of its exponential nature, were studied in voltage-clamp experiments on frogs. The ability of some factors to prolong EPC decay (acetylcholinesterase inhibition, ethanol) was reduced after acetylcholine receptors (AChR) density decreased upon treatment with alpha-bungarotoxin. The effect of other factors (hyperpolarization, cooling, "very fast" channel blocker dipyroxime) was independent of the AChR density. It was concluded that among mechanisms of EPC prolongation sensitive to the AChR' density, there are some connected with a decrease in the acetylcholinesterase activity and others altering the kinetics of AChR function.

[Comparison of the action of hyperpolarization and ethanol on frog end-plate currents at different cholinoceptor densities]. / Sravnenie deistviia giperpoliarizatsii i étanola na toki kontsevoi plastinki liagushki pri razlichnoi plotnosti kholinoretseptorov.

Both hyperpolarization of the postsynaptic membrane and ethanol delayed the decay rate of end-plate currents (EPC) in voltage clamp experiments of frog neuromuscular junctions. After a decrease of the acetylcholine receptors' density by alpha-bungarotoxin, the effect of ethanol, but not that of hyperpolarization, became less pronounced. It is concluded that there are differences between hyperpolarization and ethanol action on the processes controlling the EPC decay rate.