Frank Schottler

The effects of repetitive low frequency stimulation on control and “potentiated” synaptic responses in the hippocampus

Trains of low frequency stimulation (LFS) (1 sec−1 for 100 sec) were administered to the Schaffer collateral-commissural projections to the regio superior of the rat hippocampus before and after the induction of long-term potentiation (LTP). In 2 of 16 cases (12.5%) these produced a persistent depression of control responses. In 14 of 18 experiments (77.7%) however LFS significantly reduced the “potentiated” responses. The possible basis for these effects and their significance for understanding the LTP effect are discussed.

Calcium-activated proteolysis in rat neocortex induced by transient focal ischemia

Ischemia-induced elevation of intracellular calcium triggers a cascade of events which is considered to play a major role in neuronal death. One candidate to participate in this process is the calcium-sensitive protease, calpain. This protease is activated by calcium, and is capable of degrading critical cytoskeletal and regulatory proteins. In order to further elucidate the role of calpain in focal ischemic damage, the present study investigated the proteolysis of spectrin, a preferred substrate for calpain, in response to transient focal ischemia. Ischemia was induced by occluding reversibly both carotid arteries and the left middle cerebral artery for three hours in Sprague-Dawley rats. Western blotting techniques were used to identify and quantify the amounts of spectrin breakdown products (BDPs) in neocortical samples from the area destined for infarction, the peri-infarct area, and the contralateral hemisphere. Substantial increases in spectrin proteolysis were observed within the first few hours of ischemia in the areas that will undergo infarction. The increase in spectrin BDPs in these areas reached a plateau around the end of the 3 h ischemic period. In the peri-infarct zone, the levels of spectrin BDPs increased in a biphasic manner. A small to moderate increase was observed by the second hour of ischemia, followed by a larger increase between the 6th and 24th hours post-ischemia. The contralateral neocortex showed a significant increase in BDPs at 2 h after the initiation of ischemia. A smaller increase in BDPs was observed thereafter.(ABSTRACT TRUNCATED AT 250 WORDS)

Short-latency single unit processing in olfactory cortex

Single-unit recording of layer IIIII cells in olfactory (piriform) cortex was performed on awake, unrestrained rats actively engaged in learning novel odors in an olfactory discrimination task. Five of the 67 cells tested had very brief monophasic action potentials and high spontaneous firing rates (3080 Hz); it is suggested that these units were interneurons. The remainder of the neurons had broader spikes and did not discharge for prolonged periods. Thirty-nine percent of the broad spike cells responded to at least one and usually more of the odors presented to the rats during either of the first two trials on which that odor was present, but, in most cases, these responses occurred only very infrequently over the course of subsequent trials. Six percent of the broad-spike group, how ever, continued firing robustly to a single odor but not to others. From these results it appears that most cells in piriform cortex do not respond to most odors, i.e., coding is exceedingly sparse. A subgroup of the predominant broad-spike cell type does react to several odors but this response drops out with repeated exposure, perhaps because of training. However, a few members of this class (a small fraction of the total cell population) do go on responding to a particular odor, thus exhibiting a form of odor specificity. The results are discussed with regard to predictions from recently developed models of the olfactory cortex.

Distribution and initiation of seizure activity in a rat brain with subcortical band heterotopia.

Summary: Purpose: Misplaced (heterotopic) cortical neurons are a common feature of developmental epilepsies. To better understand seizure disorders associated with cortical heterotopia, the sites of aberrant discharge activity were investigated in vivo and in vitro in a seizure‐prone mutant rat (tish) exhibiting subcortical band heterotopia.

Subcortical connections of normotopic and heterotopic neurons in sensory and motor cortices of the tish mutant rat

Orthograde and retrograde tracers were used to examine subcortical connections of neurons in the neurological mutant tish rat. This animal exhibits bilateral heterotopia similar to those observed in epileptic humans with subcortical band heterotopia. Terminal varicosities were labeled in the striatum, thalamus, brainstem, and spinal cord following injections of the anterograde tracer biotinylated dextran amine (BDA) into the heterotopic cortex. The general topography of corticothalamic projections was evaluated by injecting the retrograde tracer Fluoro‐Gold (FG) into ventral thalamic nuclei. Retrograde labeling of small‐to‐medium sized neurons was observed in layer VI of topographically restricted portions of the normotopic cortex. Similar appearing cells were labeled in the neighboring portions of the underlying heterotopia; however, these neurons did not display characteristic lamination or radial orientation. Thalamocortical terminals labeled by injecting BDA into the ventroposterolateral nucleus (VPL) were observed primarily in layer IV of the medial aspect of the normotopic somatosensory cortex. In contrast, a radial column of terminals was present in the underlying heterotopia. Typical barrel labeling was found in the lateral aspect of the normotopic somatosensory cortex after injecting the ventroposteromedial nucleus (VPM), whereas more diffuse patches of labeling were observed in the underlying heterotopia.

Long-lasting physiological effects of bath applied N-methyl-D-aspartate.

The present experiments describe a long-lasting form of potentiation induced in field CA1 of rat hippocampal slices by bath application of N-methyl-D-aspartate (NMDA), in association with low magnesium concentrations, glycine and spermine. The potentiation effect consisted of a 50% increase in slope of field potentials and was stable for at least 80 min post treatment. It was not accompanied by detectable changes in antidromic responses and was completely blocked by an antagonist of NMDA receptor. The possible relationship of pharmacologically induced potentiation to long-term potentiation (LTP) is discussed.

Neuronal recovery after moderate hypoxia is improved by the calpain inhibitor MDL28170

The role of calcium-activated proteolysis in hypoxic neuronal injury was investigated using an in vitro slice model of moderate hypoxia that mimics many features of an ischemic penumbra. The calpain inhibitor, MDL28170, significantly improved the recovery of synaptic responses in hippocampal slices following prolonged, moderate hypoxia without hypoxic depolarization. This finding further implicates calpain-mediated proteolysis in the development of neuronal injury following moderate metabolic challenge such as occurs in regions of partial ischemia.

Hypoxic neuronal damage in the absence of hypoxic depolarization in rat hippocampal slices: the role of glutamate receptors

The propensity of neurons to undergo profound and precipitous depolarization is believed to contribute to their characteristic vulnerability to hypoxic injury. The length of time a neuron spends in a depolarized state following hypoxic depolarization (HD) is a critical determinant of the extent of irreversible cell damage. It is less clear, however, what the effects of moderate hypoxia are when HD does not occur. The present study examined the effects of prolonged, moderate hypoxia which does not elicit HD in rat hippocampal slices. Extracellularly-recorded population excitatory postsynaptic potentials (pEPSPs) in stratum radiatum of CA1 were eliminated 10-15 min after initiating hypoxia. Physiological damage was related to the hypoxic duration: full, intermediate, or poor recovery of pEPSP slope was observed after 30, 60, or 120 min of hypoxia, respectively. The glutamate receptor antagonists, D,L-2-amino-5-phosphonovaleric acid (APV) or 6,7-dinitroquinoxaline-2,3-dione (DNQX), enhanced the post-hypoxic recovery of synaptic responses. These findings demonstrate that profound HD is not necessary to elicit physiological damage during moderate hypoxia; moreover, the neuroprotective actions of excitatory transmitter antagonists are not limited to their capacity to delay HD. The precise characterization of cellular responses under these conditions will be of particular importance for understanding the pathophysiology of an ischemic penumbra.

Structural interactions between NOS-positive neurons and blood vessels in the hippocampus

The anatomical relationship between nitric oxide synthase (NOS)-positive neurons and blood vessels was examined in the hippocampus of the rat. NADPH-diaphorase histochemistry was used to identify NOS-positive neurons by light-microscopy. A close association of somatic, dendritic and axonal processes of NOS-positive neurons with cerebral blood vessels was observed. These findings suggest the possibility of neurovascular signaling by local NOS-containing neurons, through direct vascular innervation by terminals generating nitric oxide, and paracrine signaling from closely apposed somatic and dendritic neuronal elements.