Vassiliki Aroniadou-Anderjaska

Glomerular synaptic responses to olfactory nerve input in rat olfactory bulb slices

In olfactory bulb slices from young rats, the field potential evoked in the glomerular layer by stimulation in the olfactory nerve layer consisted of two negative components: an early component (N1) which was blocked by bath application of the kainate/amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM), and a late, prolonged component (N2; duration > or = 350 msec) which was unaffected by CNQX, was enhanced by reduction of Mg2+ in the medium, and was blocked by the N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonovalerate (50 microM). A comparison of the glomerular field potentials before and after knife cuts that isolated the glomerular layer from the deeper layers of the olfactory bulb indicated that both N1 and N2 were produced by currents generated, for the most part, within the glomeruli. A laminar analysis of the field potential profiles evoked by olfactory nerve stimulation in standard medium, or in the presence of CNQX, showed that N1 and N2 reversed polarity in the external plexiform and mitral cell layers, suggesting that both components reflected synaptic responses in the distal, apical dendrites of mitral/tufted cells. Simultaneous field potential recordings in the glomerular layer and intracellular recordings in the mitral cell layer showed that: (i) N1 is associated with a brief, short-latency spiking activity of mitral cells, and (ii) N2 is associated with prolonged mitral cell spiking, since N2 and the late cell firing had similar time-courses, and both were blocked by bath applied DL-2-amino-5-phosphonovalerate. Application of the GABA(A) receptor antagonist bicuculline methiodide (10 microM) to standard medium selectively enhanced N2. The enhanced N2 was significantly reduced by DL-2-amino-5-phosphonovalerate. Strychnine, an antagonist of glycine receptors, had similar effects to those of bicuculline, but only at high concentrations that have been previously shown to block GABA(A) receptors; at low concentrations strychnine had no effect. The effects of all drugs tested were reversible. In the rat olfactory bulb, activation of the olfactory nerve evokes a kainate/AMPA receptor-mediated response in the distal, apical dendrites of mitral/tufted cells, followed by a slow N-methyl-D-aspartate receptor-mediated response which triggers prolonged discharge of mitral cells. GABA(A) receptor-mediated inhibition appears to suppress, preferentially, this N-methyl-D-aspartate receptor-mediated component. The presence of prolonged N-methyl-D-aspartate receptor-mediated postsynaptic activity at the primary synapses of the olfactory system may play a key role in olfactory processing by facilitating synaptic integration and plasticity.

Intrinsic inhibitory pathways in mouse barrel cortex

The organization of intrinsic connections in the mouse somatosensory cortex was studied by combining tract-tracing and immunocytochemical techniques. Injections of HRP or fluorescent beads were made into the supragranular or infragranular layers of the posteriomedial barrel subfield. Numerous cells were retrogradely labeled within a vertical column centered on each injection site. Retrogradely labeled cells were also found > 3 mm horizontal to the injection. Dual-label immunocytochemistry identified the population of inhibitory, GABAergic cells forming intrinsic horizontal connections. Double-labeled cells were found predominantly within a 200 microns radius horizontal to an injection site. These findings indicate that intrinsic inhibitory pathways in the barrel cortex do not provide a substrate for direct GABAergic interactions among barrel columns, and that inter-columnar interactions are primarily excitatory.

The GluK1 (GluR5) Kainate/α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Receptor Antagonist LY293558 Reduces Soman-Induced Seizures and Neuropathology

The possibility of mass exposure to nerve agents by a terrorist attack necessitates the availability of antidotes that can be effective against nerve agent toxicity even when administered at a relatively long latency after exposure, because medical assistance may not be immediately available. Nerve agents induce status epilepticus (SE), which can cause brain damage or death. Antagonists of kainate receptors that contain the GluK1 (formerly known as GluR5) subunit (GluK1Rs) are emerging as a new potential treatment for SE and epilepsy from animal research, whereas clinical trials to treat pain have shown that the GluK1/α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist LY293558 [(3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5-yl)ethyl]decahydroisoquinoline-3-carboxylic acid] is safe and well tolerated. Therefore, we tested whether LY293558 is effective against soman-induced seizures and neuropathology, when administered 1 h after soman exposure, in rats. LY293558 stopped seizures induced by soman and reduced the total duration of SE, monitored by electroencephalographic recordings within a 24 h-period after exposure. In addition, LY293558 prevented neuronal loss in the basolateral amygdala (BLA) and the CA1 hippocampal area on both days 1 and 7 after soman exposure and reduced neuronal degeneration in the CA1, CA3, and hilar hippocampal regions, entorhinal cortex, amygdala, and neocortex on day 1 after exposure and in the CA1, CA3, amygdala, and neocortex on day 7 after exposure. It also prevented the delayed loss of glutamic acid decarboxylase-67 immuno-stained BLA interneurons on day 7 after exposure. LY293558 is a potential new emergency treatment for nerve agent exposure that can be expected to be effective against seizures and brain damage even with late administration.

RDX Binds to the GABAA Receptor–Convulsant Site and Blocks GABAA Receptor–Mediated Currents in the Amygdala: A Mechanism for RDX-Induced Seizures

Background Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a high-energy, trinitrated cyclic compound that has been used worldwide since World War II as an explosive in both military and civilian applications. RDX can be released in the environment by way of waste streams generated during the manufacture, use, and disposal of RDX-containing munitions and can leach into groundwater from unexploded munitions found on training ranges. For > 60 years, it has been known that exposure to high doses of RDX causes generalized seizures, but the mechanism has remained unknown. Objective We investigated the mechanism by which RDX induces seizures. Methods and results By screening the affinity of RDX for a number of neurotransmitter receptors, we found that RDX binds exclusively to the picrotoxin convulsant site of the γ-aminobutyric acid type A (GABAA) ionophore. Whole-cell in vitro recordings in the rat basolateral amygdala (BLA) showed that RDX reduces the frequency and amplitude of spontaneous GABAA receptor–mediated inhibitory postsynaptic currents and the amplitude of GABA-evoked postsynaptic currents. In extracellular field recordings from the BLA, RDX induced prolonged, seizure-like neuronal discharges. Conclusions These results suggest that binding to the GABAA receptor convulsant site is the primary mechanism of seizure induction by RDX and that reduction of GABAergic inhibitory transmission in the amygdala is involved in the generation of RDX-induced seizures. Knowledge of the molecular site and the mechanism of RDX action with respect to seizure induction can guide therapeutic strategies, allow more accurate development of safe thresholds for exposures, and help prevent the development of new explosives or other munitions that could pose similar health risks.

LTP in the barrel cortex of adult rats

Intrinsic circuitry in the barrel cortex has been shown to be readily modifiable in response to changes in the patterns of sensory input, even in adult animals. It is believed that sensory experience modifies synaptic circuits by imposing specific patterns of repetitive presynaptic and postsynaptic coactivity, i.e. by LTP-like processes. However, the ability of intrinsic cynapses in the adult barrel cortex to express LTP has not been demonstrated. In slices from barrel cortex of adult rats, we show that input-specific LTP can be induced in the supragranular layers by tetanic stimulation of intracortical vertical and horizontal inputs.

Repeated Isoflurane Exposures Impair Long-Term Potentiation and Increase Basal GABAergic Activity in the Basolateral Amygdala

After surgery requiring general anesthesia, patients often experience emotional disturbances, but it is unclear if this is due to anesthetic exposure. In the present study, we examined whether isoflurane anesthesia produces long-term pathophysiological alterations in the basolateral amygdala (BLA), a brain region that plays a central role in emotional behavior. Ten-week-old, male rats were administered either a single, 1u2009h long isoflurane (1.5%) anesthesia or three, 1u2009h long isoflurane exposures, separated by 48u2009h. Long-term potentiation (LTP) and spontaneous GABAergic activity in the BLA were studied 1 day, 1 week, and 1 month later. Single isoflurane anesthesia had no significant effect on the magnitude of LTP. In contrast, after repeated isoflurane exposures, LTP was dramatically impaired at both 1 day and 1 week after the last exposure but was restored by 1 month after the exposures. Spontaneous GABAA receptor-mediated IPSCs were increased at 1 day and 1 week after repeated exposures but had returned to control levels by 1 month after exposure. Thus, repeated exposures to isoflurane cause a long-lasting—but not permanent—impairment of synaptic plasticity in the BLA, which could be due to increased basal GABAergic activity. These pathophysiological alterations may produce emotional disturbances and impaired fear-related learning.

Comparing the Antiseizure and Neuroprotective Efficacy of LY293558, Diazepam, Caramiphen, and LY293558-Caramiphen Combination against Soman in a Rat Model Relevant to the Pediatric Population

The currently Food and Drug Administration–approved anticonvulsant for the treatment of status epilepticus (SE) induced by nerve agents is the benzodiazepine diazepam; however, diazepam does not appear to offer neuroprotective benefits. This is of particular concern with respect to the protection of children because, in the developing brain, synaptic transmission mediated via GABAA receptors, the target of diazepam, is weak. In the present study, we exposed 21-day-old male rats to 1.2 × LD50 soman and compared the antiseizure, antilethality, and neuroprotective efficacy of diazepam (10 mg/kg), LY293558 (an AMPA/GluK1 receptor antagonist; 15 mg/kg), caramiphen (CRM, an antimuscarinic with NMDA receptor-antagonistic properties; 50 mg/kg), and LY293558 (15 mg/kg) + CRM (50 mg/kg), administered 1 hour after exposure. Diazepam, LY293558, and LY293558 + CRM, but not CRM alone, terminated SE; LY293558 + CRM treatment acted significantly faster and produced a survival rate greater than 85%. Thirty days after soman exposure, neurodegeneration in limbic regions was most severe in the CRM-treated group, minimal to severe—depending on the region—in the diazepam group, absent to moderate in the LY293558-treated group, and totally absent in the LY293558 + CRM group. Amygdala and hippocampal atrophy, a severe reduction in spontaneous inhibitory activity in the basolateral amygdala, and increased anxiety-like behavior in the open-field and acoustic startle response tests were present in the diazepam and CRM groups, whereas the LY293558 and LY293558 + CRM groups did not differ from controls. The combined administration of LY293558 and CRM, by blocking mainly AMPA, GluK1, and NMDA receptors, is a very effective anticonvulsant and neuroprotective therapy against soman in young rats.