Cellular electrophysiological changes in the hippocampus of freely behaving rats during local microdialysis with epileptogenic concentration of N-methyl-D-aspartate

N-methyl-D-aspartate (NMDA) receptor dysfunctions are thought to be involved in the pathophysiology of seizures of hippocampal origin. While the cellular effects of excessive NMDA receptor stimulation have been widely studied in vitro, no data are available on the sequence of cellular electrophysiological events that follow the overstimulation of hippocampal NMDA receptors in awake, behaving subjects. Therefore, the present study addressed this problem. Intrahippocampal microdialysis with 500 microM NMDA was performed in freely behaving rats, and the electrical activity of single neurons in the dialysis area were monitored. In all recorded neurons (n = 9), regardless of their type, NMDA induced a long-lasting electrical silence preceded in most cells by a brief but robust firing rate increase. During these firing rate increases, place cells lost the spatial selectivity of their discharges, and a gradual reduction in the amplitude of the action potentials was also observed. Remarkably, electroencephalographic (EEG) seizures developed exclusively after the appearance of cellular electrical silence in the recording/dialysis site. The NMDA-induced electrophysiological changes were reversible. This study demonstrates that the combined single-cell recording-intracerebral microdialysis technique can be readily used for inducing focal epileptiform events in the hippocampus and monitoring the induced cellular electrophysiological events in behaving animals.

Intrinsic optical signaling denoting neuronal damage in response to acute excitotoxic insult by domoic acid in the hippocampal slice

Using the seafood contaminant domoic acid (an AMPA/kainate receptor agonist), we demonstrate a distinct excitotoxic sequence of events leading to acute neuronal damage in the hippocampal slice as measured by (1) loss of the evoked CA1 field potential, (2) irreversible changes in light transmittance, (3) histopathology, and (4) lucifer yellow injection of single CA1 pyramidal neurons. Change in light transmittance (LT) through the submerged slice indirectly measures altered cell volume, both neuronal and glial. At 37 degrees C, a 1-min superfusion of 10 mu M domoate induced a prolonged reversible increase in LT, primarily in the dendritic regions of CA1 and dentate granule cells (GC), but not in the CA3 region. Spectral analysis (400-800 nm) revealed a wide-band transmittance increase, indicating cell swelling as a major source of the intrinsic signal. The evoked field potential recorded in the CA1 cell body region (PYR) was lost as LT peaked, but completely recovered upon return to the baseline LT level. Increasing domoate exposure to 10 min elicited a different and distinct LT sequence in CA1 and dentate regions. An initial LT increase in dendritic regions evolved in an irreversible decrease in LT. At the same time, LT irreversibly increased in cell body regions (CA1 PYR and GC) and the evoked field potential was irretrievably lost. Also, there was histological damage to cell body and dendritic regions of CA1 and granule cells. Injection of lucifer yellow into single CA1 neurons in slices displaying the irreversible LT sequence revealed extensive dendritic beading, whereas CA1 cells in control slices displayed a smoothly contoured arbor. Consistent with acute neuronal damage, the optical changes generated by domoate did not require extracellular Ca2+, and lowering the temperature protected the slice from irreversible damage to CA1 and GC regions. Although glial changes may also occur, we conclude that imaging light transmittance reveals dynamic and compartmentalized excitotoxic changes in neuronal volume. Beading of the dendritic arbor increases light scatter, thereby decreasing LT and highlighting damaged dendritic regions.

Ultrastructural localization of zinc transporter-3 (ZnT-3) to synaptic vesicle membranes within mossy fiber boutons in the hippocampus of mouse and monkey

Zinc transporter-3 (ZnT-3), a member of a growing family of mammalian zinc transporters, is expressed in regions of the brain that are rich in histochemically reactive zinc (as revealed by the Timm's stain), including entorhinal cortex, amygdala, and hippocampus. ZnT-3 protein is most abundant in the zinc-enriched mossy fibers that project from the dentate granule cells to hilar and CA3 pyramidal neurons. We show here by electron microscopy that ZnT-3 decorates the membranes of all clear, small, round synaptic vesicles (SVs) in the mossy fiber boutons of both mouse and monkey. Furthermore, up to 60-80% of these SVs contain Timm's-stainable zinc. The coincidence of ZnT-3 on the membranes of SVs that accumulate zinc, and its homology with known zinc transporters, suggest that ZnT-3 is responsible for the transport of zinc into SVs, and hence for the ability of these neurons to release zinc upon excitation.

Glutamate release correlates with brain-derived neurotrophic factor and trkB mRNA expression in the CA1 region of rat hippocampus

Synthesis of the neurotrophic factor brain-derived neurotrophic factor (BDNF) and its receptor TrkB in the hippocampus have been proposed to be influenced by endogenous glutamate. To test this hypothesis we have investigated if increases in BDNF and trkB mRNAs are associated with changes in the synaptic release of glutamate in the dorsal hippocampus in the conscious rat by combining the technique of in vivo microdialysis with in situ hybridization histochemistry. A 35% and 66% increase in extracellular levels of glutamate in the dorsal CA1 region was detected following injection into the lateral entorhinal cortex of 2.4 and 9.6 microg of the non-NMDA glutamate receptor agonist quisqualate, respectively. The increase in glutamate was attenuated by local administration of tetrodotoxin (TTX) indicating neuronal origin. Levels of BDNF and trkB mRNAs were increased in the hippocampus in a dose-dependent fashion following the stimulations. The extracellular levels of glutamate in individual animals correlated to the levels of BDNF and trkB mRNAs in the dorsal CA1 region of the hippocampus. This study provides for the first time evidence of an entorhinal cortex influenced concentration-dependent relationship between the release of endogenous glutamate in vivo and neuronal expression of mRNAs for BDNF and its receptor trkB in the hippocampus.