Selective blockade of an excitatory synapse in rat cerebral cortex by the sigma opiate cyclazocine: an intracellular, in vitro study

Developmental neurotoxicity of ketamine in pediatric clinical use

Ketamine is widely used as an anesthetic, analgesic, and sedative in pediatric clinical practice and it is also listed as an illicit drug by most countries. Recent in vivo and in vitro animal studies have confirmed that ketamine can induce neuronal cell death in the immature brain, resulting from widespread neuronal apoptosis. These effects can disturb normal development further altering the structure and functions of the brain. Our recent studies further indicate that ketamine can alter neurogenesis from neural stem progenitor cells in the developing brain. Taken together, these findings identify a novel complication associated with ketamine use in premature infants, term newborns, and pregnant women. Recent data on the developmental neurotoxicity of ketamine are reviewed with proposed future directions for evaluating the safety of ketamine in these patient populations.

Ketamine induces failure of the oculomotor neural integrator in the cat

We studied the effect of intramuscular injection of low dose of ketamine (1 mg/kg) on the spontaneous ocular movements of the cat. Ketamine is a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptors, which is used as an anesthetic agent in human surgery. We found that ketamine administration caused a failure of gaze holding: each saccade was followed by a centripetal post-saccadic drift. This defect was selective: the dynamics of the saccades was not altered (the amplitude/maximum velocity relationship was unaffected by ketamine at the dose of 1 mg/kg). We postulated that the observed effect was due to the fact that NMDA receptors were implicated in the network of the oculomotor neural integrator that converted activity related to the saccade (pulse signal) into activity responsible for gaze holding (step signal).

Agonists, antagonists and modulators of excitatory amino acid receptors in the guinea-pig myenteric plexus

1. The receptors for glutamic acid (L-Glu) present in the guinea-pig myenteric plexus-ileal longitudinal muscle preparation have been studied by measuring the muscle contraction induced by numerous putative endogenous agonists acting at these receptors. Furthermore, the actions of different concentrations of antagonists, glycine, Mg2+ and Ca2+ on the ileal contractions induced by L-Glu have been evaluated. 2. The EC50 values of the most common putative endogenous agonists of these receptors were: L-Glu 1.9 X 10(-5) M; L-aspartate 8 X 10(-5) M; quinolinate 5 X 10(-4) M; L-homocysteate 1.4 X 10(-4) M; the dipeptide aspartyl-glutamate 8 X 10(-5) M, while N-acetyl-aspartyl-glutamate was inactive. Among the molecules used to classify excitatory amino acid receptors, N-methyl-D-aspartate (NMDA) was the most potent (EC50 5 X 10(-4) M). Kainic and quisqualic acids were almost completely inactive. 3. The responses to L-Glu were competitively antagonized by 2-amino-5-phosphonovaleric acid. They were, also, prevented by hyoscine (10(-7) M) and by tetrodotoxin (3 X 10(-7) M), suggesting that the L-Glu-induced ileal contraction was in some way dependent upon an action on the myenteric cholinergic neurones. Kynurenic acid was a non-competitive antagonist, gamma-D-glutamyl-taurine (10(-4) M) and aminophosphonobutyric acid (10(-4) M) did not modify the L-Glu-induced contractions. 4. Glycine (10(-5) M) significantly potentiated the effects of glutamate especially when the ionic composition of the superfusion medium contained concentrations of Ca2+ in the range of 0.6-1.2 mM. Strychnine 3 X 10(-5) M did not modify the actions of glycine. 5. The data presented here confirm the presence of NMDA receptors in the guinea-pig myenteric plexus, and show that these receptors, similar to those present in primary neuronal cultures may be modulated by glycine.

Phencyclidine. Physiological actions, interactions with excitatory amino acids and endogenous ligands

Phenycyclidine (PCP) produces many profound effects in the central nervous system. PCP has numerous behavioral and neurochemical effects such as inhibiting the uptake and facilitating the release of dopamine, serotonin, and norepinephrine. PCP also interacts with sigma, mu opioid, muscarinic, and nicotinic receptors. However, the psychotomimetic effects induced by PCP are believed to be mediated by specific PCP receptors, where PCP binds with greater potency than sigma compounds. Electrophysiological, behavioral, and neuro-chemical evidence strongly suggests that at least some of the many PCP actions result from antagonism of excitatory amino acid-induced responses via PCP receptors. The recent isolation and partial characterization of the alpha and beta endopsychosins and the identification of other endogenous ligands for the PCP and sigma receptors, is another promising area of research in the elucidation of the physiological role of an endogenous PCP and sigma system.

N-methylaspartate receptors mediate epileptiform activity evoked in some, but not all, conditions in rat neocortical slices

Using intracellular recordings from pyramidal neurons in isolated slices of rat cerebral cortex epileptiform discharges evoked (1) in the presence of gamma-aminobutyric acid antagonists, and (2) in the absence of Mg2+ were compared. Depolarization shift responses recorded in the presence of bath applied picrotoxin, or electrophoretically applied picrotoxin or bicuculline, were similar in many respects to depolarization shifts reported previously, except that they could be evoked by stimuli subthreshold for evoking discernible postsynaptic potentials in these experiments. Large depolarizations evoked by repetitive activation of an N-methylaspartate receptor mediated synapse in the absence of Mg2+, displayed several properties similar to those of depolarization shifts evoked in the presence of gamma-aminobutyric acid antagonists, i.e. similar shape, latency, inability to follow high repetition rates and a similar voltage relation, suggesting activation of the same cellular mechanism. "Slow spikes" evoked as part of the response to electrophoretically applied N-methylaspartate were augmented, i.e. they were replaced by larger, longer, more complex events, when gamma-aminobutyric acid antagonists were applied. The potentiated response, evoked in the absence of Mg2+, was dependent on the activation of an N-methylaspartate receptor mediated synapse and was blocked by N-methylaspartate antagonists. In contrast, depolarization shifts could be evoked in the presence of large doses of N-methylaspartate antagonists, when gamma-aminobutyric acid antagonists were applied. Spontaneous depolarizations similar to depolarization shifts were recorded when cells were exposed to low, tonic, electrophoretic applications of excitatory amino acids under control conditions. In addition, some potentiation of the N-methylaspartate receptor mediated excitatory postsynaptic potential was achieved in the presence of Mg2+ when cells were depolarized by 10-20 mV. Depolarization shifts evoked when bicuculline was applied electrophoretically to different parts of the dendritic field, some hundreds of microns from the soma, differed in shape, latency and time course and the depolarization shift evoked when bicuculline was applied at one site summed with the depolarization shift evoked when it was applied elsewhere. We conclude that different inputs are required to activate the responses evoked in the presence of gamma-aminobutyric acid antagonists and in the absence of Mg2+. The possibility that both involve activation of dendritic Ca2+ currents and that the magnitude of the response depends on the proportion of the dendritic field activated, is discussed.

Comparison of responses to transmitter candidates at an N-methylaspartate receptor mediated synapse, in slices of rat cerebral cortex

Intracellular recordings from pyramidal neurones in isolated slices of rat cerebral cortex allowed a comparison of postsynaptic potentials and responses to electrophoretic application of excitatory amino acids. The responses of all neurones to N-methylaspartate displayed an unusual voltage relation and were associated with an apparent increase in membrane resistance, properties that were dependent on the presence of extracellular Mg2+. Responses to N-methylaspartate could be elicited only when the electrophoretic pipette was positioned close to the cell soma and were associated with generation of slow spikes which triggered bursts of fast spikes. In contrast, in the majority of neurones, responses to glutamate, aspartate, cysteate and cysteine sulphinate demonstrated a conventional voltage relation, were associated with a decrease in membrane resistance, evoked no slow spikes, were insensitive to extracellular Mg2+ concentrations between 1 and near 0 mM and could be evoked with small currents of amino acids when the electrophoretic pipette was greater than 100 micron from the cell soma. Responses to the five amino acids were tested with the N-methylaspartate antagonist 2-amino-5-phosphonovaleric acid. In the majority of cells, this antagonist blocked responses to N-methylaspartate at doses that had only a small effect on responses to the other amino acids. In these experiments it was also possible to confirm previous reports that ketamine and cyclazocine act as selective N-methylaspartate antagonists. In 4/63 neurones, responses to glutamate and aspartate and in 1/5 neurones one component of the response to cysteate, displayed properties similar to those of responses to N-methylaspartate. In one other neurone, large applications of cysteine sulphinate or glutamate could evoke slow spikes and fast spike bursts, a firing pattern that was sensitive to 2-amino-5-phosphonovalerate. The present experiments therefore demonstrate that all four naturally occurring amino acids tested can activate N-methylaspartate receptors and indicate that any one of these could be the transmitter at the N-methylaspartate receptor-mediated synapse on cortical pyramidal neurones. However, in the majority of neurones, these putative transmitters activated other receptor types preferentially. The possibility that this may result from the greater accessibility of non-N-methylaspartate receptors to electrophoretically applied agonists, is discussed.