Configuration of an alpha-gamma coincidence spectrometer for utilization of safeguards measurements.

An alpha-gamma coincidence spectrometry (AGCS) system was developed at the International Atomic Energy Agency's (IAEA) Safeguards Analytical Laboratory (SAL) in order to improve the detection of alpha-gamma emitting radionuclides of relevance for safeguards, such as (239)Pu and (241)Am. The AGCS design was based upon a linear gate, whose resolving time is dependent upon the amplifiers' shaping time constants (STC). A coincidence spectrum of a Pu/Am sample was acquired which showed that the AGCS system could be utilized in safeguards measurements.

Group III metabotropic glutamate receptor-mediated, chemically induced long-term depression differentially affects cell viability in the hippocampus.

In vivo, activation of group III metabotropic glutamate (mGlu) receptors leads to a reduction of basal synaptic transmission in the hippocampus, and depending on the experimental conditions in vitro, leads to neuroprotection or neurotoxicity. Here, the cellular response to cerebral application of L(+)-2-amino-4-phosphonobutanoic acid (AP4) was investigated in the CA1 region and dentate gyrus of freely moving rats. Drugs were applied via the lateral ventricle, and electrophysiological measurements were obtained via chronically implanted electrodes. AP4 produced a slowly developing depression of evoked responses in both hippocampal regions which lasted for over 4 h. Effects could be reversed by application of high frequency tetanus. Histological evaluation, 4 h or 7 d, following a single, acute AP4 injection into the lateral cerebral ventricle showed that AP4-mediated synaptic depression either amplified (CA1) or attenuated (dentate gyrus) excitotoxic neuronal death, strongly depending on the sub-region investigated. Effects were long-lasting, being still evident 7 days after AP4 application. In both hippocampal areas, the effects obtained were subtle, however, with the CA1 region being more potently affected. Interestingly, effects in the dentate gyrus comprised a slight enhancement of live cell number coupled with deterioration in cell area, suggesting that cell proliferation triggered by group III mGlu receptor activation may have masked neurotoxic effects mediated by activation of this receptor. These results show that although AP4 induces a slow-onset synaptic depression in both sub-regions, cell viability is differentially influenced by activation of group III mGlu receptors in the CA1 region and dentate gyrus.

Kindling-induced changes in plasticity of the rat amygdala and hippocampus.

Temporal lobe epilepsy (TLE) is often accompanied by interictal behavioral abnormalities, such as fear and memory impairment. To identify possible underlying substrates, we analyzed long-term synaptic plasticity in two relevant brain regions, the lateral amygdala (LA) and the CA1 region of the hippocampus, in the kindling model of epilepsy. Wistar rats were kindled through daily administration of brief electrical stimulations to the left basolateral nucleus of the amygdala. Field potential recordings were performed in slices obtained from kindled rats 48 h after the last induced seizure, and in slices from sham-implanted and nonimplanted controls. Kindling resulted in a significant impairment of long-term potentiation (LTP) in both the LA and the CA1, the magnitude of which was dependent on the number of prior stage V seizures. Saturation of CA1-LTP, assessed through repeated spaced delivery of high-frequency stimulation, occurred at lower levels in kindled compared to sham-implanted animals, consistent with the hypothesis of reduced capacity of further synaptic strengthening. Furthermore, theta pulse stimulation elicited long-term depression in the amygdala in nonimplanted and sham-implanted controls, whereas the same stimulation protocol stimulation caused LTP in kindled rats. In conclusion, kindling differentially affects the magnitude, saturation, and polarity of LTP in the CA1 and LA, respectively, most likely indicating an activity-dependent mechanism in the context of synaptic metaplasticity.

Processes and components participating in the generation of intrinsic optical signal changes in vitro.

Imaging of intrinsic optical signals has become an important tool in the neurosciences. To better understand processes underlying changes in intrinsic optical signals, we studied electrical stimulation at varying strengths in hippocampal slices of adult Wistar rats. Following serial stimulation we observed an increase in light transmittance in all tested slices. During antidromic stimulation at minimum stimulation strength the increase in light transmittance was 75 +/- 8% (P < 0.05), and during orthodromic minimum stimulation 19.6 +/- 5.6% (P < 0.001) in the stratum pyramidale of the CA1-region. During orthodromic stimulation no significant difference between submaximum, maximum and supramaximum stimulation was found, indicating saturation. In contrast, submaximum antidromic stimulation yielded 56.2 +/- 12% (P < 0.05) of maximum stimulation strength, indicating recruitment. In a further set of experiments serial stimulation was carried out under glial blockade with fluoroacetate (FAC) or blockage of mitochondrial function. Amplitude and slope of the intrinsic optical signal significantly decreased in the presence of FAC (amplitude: 36 +/- 6%, P < 0.01; slope: 37 +/- 11% as compared with baseline conditions, P < 0.05). This suggests a glial participation in signal generation. Rotenone, an inhibitor of mitochondrial complex I, yielded decreased amplitudes of the intrinsic optical signal (27 +/- 7% after 40 min, P < 0.01). Our data indicate that the intrinsic optical signal change reflects type and strength of neuronal activation and point to glia and mitochondria as important participants in signal generation.

Seizure spread through the life cycle: optical imaging in combined brain slices from immature, adult, and senile rats in vitro.

The semiology of epileptic seizures changes during the lifetime. Hence, it can be assumed that age-related changes in brain plasticity influence the patterns of seizure onset, spread and propagation velocity. We employed the 4-aminopyridine model of epilepsy to study seizure-like events in vitro. Combined entorhinal cortex-hippocampus brain slices from juvenile (10-13 days), adult (2-3 months), and senile (24-27 months) rats were examined using electrophysiological recordings and imaging of intrinsic optical signals. In the juvenile group, seizure onset was multifocal in all slice regions including the hippocampus. Onset in adult animals was confined to the entorhinal cortex and to neocortical regions. In slices from senile animals, there was a preponderance of seizure onsets in the neocortex. Spread patterns were highly variable in the juvenile group and became gradually more monomorph with increasing age. Propagation velocities were highest in the adult group, with maximum values of 1.51 +/- 0.68 mm/s. In the juvenile group, they amounted to 0.97 +/- 0.39 mm/s, and to 1.18 +/- 0.42 mm/s in senile slices. The results of this study indicate that age-related changes in brain plasticity profoundly affect spread patterns, which may contribute to the clinically observed changes in seizure semiology during early childhood, adulthood and senescence.

Reduction of high-frequency network oscillations (ripples) and pathological network discharges in hippocampal slices from connexin 36-deficient mice.

Recent evidence suggests that electrotonic coupling is an important mechanism for neuronal synchronisation in the mammalian cortex and hippocampus. Various types of network oscillations have been shown to depend on, or be sharpened by, gap junctions between inhibitory interneurones or excitatory projection cells. Here we made use of a targeted disruption of the gene coding for Cx36, a recently discovered neuronal gap junction subunit, to analyse its role in hippocampal network behaviour. Mice lacking Cx36 are viable and lack obvious morphological or behavioural abnormalities. Stimulation of afferent and efferent fibre pathways in hippocampal slices revealed a largely normal function of the synaptic circuitry, including tetanically evoked network oscillations. Spontaneous sharp waves and ripple (approximately 200 Hz) oscillations, however, occurred less frequently in slices from Cx36 -/- mice, and ripples were slightly slower than in littermate controls. Moreover, epileptiform discharges elicited by 4-aminopyridine were attenuated in slices from Cx36 -/- mice. Our findings indicate that Cx36 plays a role in the generation of certain forms of network synchronisation in the hippocampus, namely sharp wave-ripple complexes and hypersynchronous epileptiform discharges.

Intrinsic optical imaging reveals regionally different manifestation of spreading depression in hippocampal and entorhinal structures in vitro.

The spatiotemporal features of spreading depression (SD) were analyzed in vitro by using combined hippocampal-entorhinal cortex slices. SDs were induced by microinjection of 1 M KCl in the stratum radiatum of the CA1 region of the hippocampus. Measurements of extracellular field potentials, extracellular space (ECS) volume changes and intrinsic optical signal changes were combined to study SD features in different regions of the slice. Each SD was associated with a pronounced shrinkage of the extracellular space (ECS) volume and a decrease in light transmittance. The beginning of the optical signal change occurred simultaneously with the electrographic onset as measured with extracellular microelectrodes but outlasted the dc shift for tens of seconds. The amplitude of the intrinsic optical signal change displayed marked regional variations with greatest changes of 12% in cortical regions. The signal amplitudes were considerably lower in hippocampal regions. The analysis of spread patterns revealed two types of waves: fully propagated waves spreading from CA1 all the way to the temporal neocortex and abortive waves that ceased earlier. The spread velocities displayed pronounced regional differences with highest velocities of 5.4 +/- 0.3 mm/min in the area CA3 of the hippocampal formation and lowest velocities of 2.7 +/- 0.1 mm/min in cortical regions.