6-cyano-7-nitroquinoxaline-2-3-dione and bicuculline-methochloride

Excitation of neurons by an externally induced electric field is a long standing question that has recently attracted attention due to its relevance in novel clinical intervention systems for the brain. Here we use patterned quasi one-dimensional neuronal cultures from rat hippocampus, exploiting the alignment of axons along the linear patterned culture to separate the contribution of dendrites to the excitation of the neuron from that of axons. Network disconnection by channel blockers, along with rotation of the electric field direction, allows the derivation of strength-duration (SD) curves that characterize the statistical ensemble of a population of cells. SD curves with the electric field aligned either parallel or perpendicular to the axons yield the chronaxie and rheobase of axons and dendrites respectively, and these differ considerably. Dendritic chronaxie is measured to be about 1 ms, while that of axons is on the order of 0.1 ms. Axons are thus more excitable at short time scales, but at longer time scales dendrites are more easily excited. We complement these studies with experiments on fully connected cultures. An explanation for the chronaxie of dendrites is found in the numerical simulations of passive, realistically structured dendritic trees under external stimulation. The much shorter chronaxie of axons is not captured in the passive model and may be related to active processes. The lower rheobase of dendrites at longer durations can improve brain stimulation protocols, since in the brain dendrites are less specifically oriented than axonal bundles, and the requirement for precise directional stimulation may be circumvented by using longer duration fields.

Topics: 2-Amino-5-phosphonovalerate; 4-Aminopyridine; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Animals; Axons; Bicuculline; Calcium; Chronaxy; Dendrites; Embryo, Mammalian; Excitatory Amino Acid Antagonists; GABA Antagonists; Hippocampus; Potassium Channel Blockers; Primary Cell Culture; Rats; Rats, Wistar; Receptors, AMPA; Receptors, GABA; Receptors, N-Methyl-D-Aspartate

We examined the effects of beta-pompilidotoxin (beta-PMTX), a neurotoxin derived from wasp venom, on synaptic transmission in the mammalian central nervous system (CNS). Using hippocampal slice preparations of rodents, we made both extracellular and intracellular recordings from the CA1 pyramidal neurons in response to stimulation of the Schaffer collateral/commissural fibers. Application of 5-10 microM beta-PMTX enhanced excitatory postsynaptic potentials (EPSPs) but suppressed the fast component of the inhibitory postsynaptic potentials (IPSPs). In the presence of 10 microM bicuculline, beta-PMTX potentiated EPSPs that were composed of both non-NMDA and NMDA receptor-mediated potentials. Potentiation of EPSPs was originated by repetitive firings of the presynaptic axons, causing summation of EPSPs. In the presence of 10 microM CNQX and 50 microM APV, beta-PMTX suppressed GABA(A) receptor-mediated fast IPSPs but retained GABA(B) receptor-mediated slow IPSPs. Our results suggest that beta-PMTX facilitates excitatory synaptic transmission by a presynaptic mechanism and that it causes overexcitation followed by block of the activity of some population of interneurons which regulate the activity of GABA(A) receptors.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Animals; Bicuculline; Dendrites; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; GABA Antagonists; Gerbillinae; Hippocampus; Insect Proteins; Interneurons; Neurons; Neurotoxins; Phosphinic Acids; Propanolamines; Pyramidal Cells; Rats; Rats, Wistar; Receptors, GABA-A; Receptors, N-Methyl-D-Aspartate; Sodium Channels; Synaptic Transmission; Wasp Venoms

Agrin is an extracellular matrix protein involved in the formation of the postsynaptic apparatus of the neuromuscular junction. In addition to spinal motor neurons, agrin is expressed by many other neuronal populations throughout the nervous system. Agrin's role outside of the neuromuscular junction, however, is poorly understood. Here we use the polymerase chain reaction to examine expression and alternative splicing of agrin in mouse somatosensory cortex during early postnatal development in vivo and in dissociated cell culture. Peak levels of agrin gene expression in developing cortex coincide with ingrowth of thalamic afferent fibres and formation of thalamocortical and intracortical synapses. Analysis of alternatively spliced agrin messenger RNA variants shows that greater than 95% of all agrin in developing and adult somatosensory cortex originates in neurons, including isoforms that have little or no activity in acetylcholine receptor aggregation assays. The levels of expression of "active" and "inactive" isoforms, however, are regulated during development. A similar pattern of agrin gene expression is also observed during a period when new synapses are being formed between somatosensory neurons growing in dissociated cell culture. Changes in agrin gene expression, observed both in vivo and in vitro, are consistent with a role for agrin in synapse formation in the central nervous system.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Aging; Agrin; Alternative Splicing; Animals; Bicuculline; Cells, Cultured; Cellular Senescence; DNA Primers; Gene Expression Regulation, Developmental; Genetic Variation; Mice; Mice, Inbred ICR; Neurons; Patch-Clamp Techniques; Polymerase Chain Reaction; Receptors, Cholinergic; RNA, Messenger; Somatosensory Cortex; Synapses; Transcription, Genetic

The morphological and functional consequences of epileptic activity were investigated by applying the convulsants bicuculline and/or picrotoxin to mature rat hippocampal slice cultures. After 3 days, some cells in all hippocampal subfields showed signs of degeneration, including swollen somata, vacuolation, and dendritic deformities, whereas others displayed only a massive reduction in the number of their dendritic spines. Intracellular recordings from CA3 pyramidal cells revealed a decrease in the amplitude of evoked excitatory synaptic potentials. gamma-Aminobutyric acid-releasing interneurons and inhibitory synaptic potentials were unaffected. Seven days after withdrawal of convulsants, remaining cells possessed a normal number of dendritic spines, thus demonstrating a considerable capacity for recovery. The pathological changes induced by convulsants are similar to those found in the hippocampi of human epileptics, suggesting that they are a consequence, rather than a cause, of epilepsy.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Animals, Newborn; Bicuculline; Dendrites; Electric Stimulation; Epilepsy; Evoked Potentials; Hippocampus; Membrane Potentials; Organ Culture Techniques; Organophosphorus Compounds; Pyramidal Tracts; Quinoxalines; Rats; Synapses