Organization of inhibitory feed-forward synapses from the dorsal to the ventral cochlear nucleus in the cat: a quantitative analysis of endings by vesicle morphology.

The main ascending, excitatory pathway from the cochlea undergoes synaptic interruption in the dorsal and ventral cochlear nuclei. The dorsal cochlear nucleus also forms a feed-forward circuit, which receives cochlear input and projects to the ventral cochlear nucleus by a tuberculo-ventral tract. This circuit may provide an inhibitory fringe (side bands) surrounding the center bands of the main ascending pathway. Biotinylated dextran injections into the dorsal cochlear nucleus anterogradely labeled the tuberculo-ventral tract and its endings in the anteroventral cochlear nucleus but also retrogradely filled cochlear nerve fibers and their terminals in the same regions. To distinguish tuberculo-ventral from cochlear nerve terminals, we used electron microscopy of the immunolabeled endings. Images were digitized and filter-enhanced, and the sizes and shapes of synaptic vesicles were used to construct quantitative profiles of the terminal types. The cochlear nerve endbulbs mapped to the same iso-frequency band of the injection site (main band). Flanking the main band were smaller labeled endings. About 45% of labeled terminals were pleomorphic and equally represented in the main band and side bands. Therefore, if there is an inhibitory fringe in the main projection pathway, it was not selective for tuberculo-ventral tract endings. Surprisingly, an excitatory category of round vesicles of intermediate size was labeled in the main band but not in the side bands. These intermediate endings may balance the feed-forward inhibition from the tuberculo-ventral tract. The quantitative method devised for classification of ending types by their vesicle profiles should be a generally useful tool for analysis.

Repeated contact with subtoxic soman leads to synaptic vulnerability in hippocampus.

Soman, an anticholinesterase and dangerous nerve agent, produces convulsions, memory impairment, and cell loss in the brain, especially in the hippocampus. Soman-induced accumulation of acetylcholine initiates mechanisms responsible for the development of incapacitating seizures. The prolonged epileptiform nature of these seizures causes the release of another excitatory neurotransmitter, glutamate, which has been linked to the toxic action of the nerve agent. Here, we tested whether subtoxic soman exposures influence the brain's sensitivity to glutamate-based excitotoxicity. Over a 1-week period, hippocampal slice cultures were exposed daily to a transient level of soman that produced no evidence of synaptic deterioration. After the subtoxic soman treatments, however, the tissue became vulnerable to a brief episode of glutamate receptor overstimulation that normally resulted in little or no excitotoxic damage. In those slice cultures treated with subtoxic soman, a decline in synaptic markers as well as an increase in spectrin breakdown occurred 24 hr after the mild excitotoxic event. Exposure to high soman concentrations alone produced similar synaptic degeneration, but without evident cell death, suggesting that synaptic decline is an early neurotoxicological response to the nerve agent. Interestingly, enhanced excitotoxic sensitivity caused the brain tissue to become susceptible to disparate insults initiated before or after the soman contact. These findings indicate that seemingly innocuous soman exposures leave the hippocampus sensitive to the types of insults implicated in traumatic brain injury and stroke. They also warn that asymptomatic contact with soman may lead to progressive synaptopathogenesis and that early indicators of soman exposure are critical to prevent potential brain injury.

Positive modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors elicits neuroprotection after trimethyltin exposure in hippocampus.

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamatergic receptors have been linked to survival signaling, especially when the receptors are allosterically modulated by members of the Ampakine family. While increased glutamatergic communication through AMPA receptors has been shown to protect against toxic conditions that target hippocampal subfield CA1, protection in other subfields has not been shown. Accordingly, positive modulation of AMPA receptors by Ampakine compounds CX727 and CX516 was tested for effects on trimethyltin (TMT) neurotoxicity in rat hippocampal slice cultures. TMT was applied for 4 h followed by a rapid washout and antagonistic quenching of AMPA and N-methyl-D-aspartate (NMDA) receptors. After a 24-h period, the TMT-exposed slices exhibited increased levels of calpain-mediated spectrin breakdown as well as synaptic deterioration. TMT selectively targeted CA3 pyramidal neurons and dentate gyrus (DG) granule cells as evidenced by degeneration and neuronal loss. The cytoskeletal and synaptic damage was reduced when Ampakine modulation was initiated during the postinsult period. Furthermore, the extent of protection was comparable to that produced by the NMDA receptor antagonist AP5. The above results were substantiated by histological experiments, revealing that Ampakine treatment prevented TMT-induced cell loss in CA3 and DG. These results indicate that AMPA receptor signals are part of cellular repair responses following exposure to an environmental toxin.

Survival signaling and selective neuroprotection through glutamatergic transmission.

In the brain, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors mediate glutamatergic neurotransmission and, when intensely activated, can induce excitotoxic cell death. In addition to their ionotropic properties, however, AMPA receptors have been functionally coupled to a variety of signal transduction events involving Src-family kinases, G-proteins, and the mitogen-activated protein kinase (MAPK). In the present study, we tested whether AMPA receptors are linked to appropriate signaling events in order to prevent neuronal injury and/or enhance recovery. AMPA stimulation in hippocampal slice cultures caused the selective activation of MAPK through the upstream activator MAPK kinase (MEK). Inhibition of either component of the AMPA receptor--MAPK pathway potentiated cellular damage due to serum deprivation, suggesting that this pathway facilitates compensatory signals in response to injury. Correspondingly, positive modulation of AMPA receptors with the Ampakine 1-(quinoxalin-6-ylcarbonyl)piperidine (CX516) enhanced MAPK activation and reduced the extent of synaptic and neuronal degeneration resulting from excitotoxic episodes. CX516 was neuroprotective when infused into slices either before or after the insult. The Ampakine derivative also elicited neuroprotection in an in vivo model of excitotoxicity as evidenced by reduction in lesion size and preservation of two different types of neurons. Interestingly, the AMPA receptor--MAPK pathway selectively protects against excitotoxicity since enhancing the pathway did not protect against the nonexcitotoxic, slow pathology initiated by lysosomal dysfunction. The results indicate that glutamatergic communication is important for cellular maintenance and that AMPA receptors activate survival signals to counterpoise their own excitotoxic potential.