J. Bureš

Spatial navigation deficit in amnestic mild cognitive impairment

Patients with Alzheimers disease (AD) frequently have difficulties with spatial orientation in their day-to-day life. Although AD is typically preceded by amnestic mild cognitive impairment (MCI), spatial navigation has not yet been studied in MCI. Sixty-five patients were divided into five groups: probable AD (n = 21); MCI, further classified as amnestic MCI single domain (n = 11); amnestic MCI multiple domain (n = 18), or nonamnestic MCI (n = 7), and subjective memory complaints (n = 8). These patients, together with a group of healthy control subjects (n = 26), were tested by using a four-subtests task that required them to locate an invisible goal inside a circular arena. Each subtest began with an overhead view of the arena showed on a computer monitor. This was followed by a real navigation inside of the actual space, an enclosed arena 2.9 m in diameter. Depending on the subtest, the subjects could use the starting position and/or cues on the wall for navigation. The subtests thus were focused on allocentric and egocentric navigation. The AD group and amnestic MCI multiple-domain group were impaired in all subtests. The amnestic MCI single-domain group was impaired significantly in subtests focused on allocentric orientation and at the beginning of the real space egocentric subtest, suggesting impaired memory for allocentric and real space configurations. Our results suggest that spatial navigation impairment occurs early in the development of AD and can be used for monitoring of the disease progression or for evaluation of presymptomiatic AD.

Specific spatial learning deficits become severe with age in β-amyloid precursor protein transgenic mice that harbor diffuse β-amyloid deposits but do not form plaques

Memory impairment progressing to dementia is the main clinical symptom of Alzheimers disease (AD). AD is characterized histologically by the presence of β-amyloid (Aβ) plaques and neurofibrillary tangles in specific brain regions. Although Aβ derived from the Aβ precursor protein (β-APP) is believed to play a central etiological role in AD, it is not clear whether soluble and/or fibrillar forms are responsible for the memory deficit. We have generated and previously described mice expressing human wild-type β-APP751 isoform in neurons. These transgenic mice recapitulate early histopathological features of AD and form Aβ deposits but no plaques. Here we describe a specific and progressive learning and memory impairment in these animals. In the Morris water maze, a spatial memory task sensitive to hippocampal damage, one pedigree already showed significant differences in acquisition in 3-month-old mice that increased in severity with age and were expressed clearly in 6-month- and 2-year-old animals. The second transgenic pedigree displayed a milder impairment with a later age of onset. Performance deficits significantly decreased during the 6 days of training in young but not in aged transgenic animals. Both pedigrees of the transgenic mice differed from wild-type mice by less expressed increase of escape latencies after the platform position had been changed in the reversal experiment and by failure to prefer the goal quadrant in probe trials. Both pedigrees performed at wild-type level in a number of other tests (open field exploration and passive and active place avoidance). The results suggest that plaque formation is not a necessary condition for the neuronal β-APP751 transgene-induced memory impairment, which may be caused by β-APP overexpression, isoform misexpression, or elevated soluble Aβ.

β-Amyloid precursor protein transgenic mice that harbor diffuse Aβ deposits but do not form plaques show increased ischemic vulnerability: Role of inflammation

β-amyloid (Aβ), derived form the β-amyloid precursor protein (APP), is important for the pathogenesis of Alzheimers disease (AD), which is characterized by progressive decline of cognitive functions, formation of Aβ plaques and neurofibrillary tangles, and loss of neurons. However, introducing a human wild-type or mutant APP gene to rodent models of AD does not result in clear neurodegeneration, suggesting that contributory factors lowering the threshold of neuronal death may be present in AD. Because brain ischemia has recently been recognized to contribute to the pathogenesis of AD, we studied the effect of focal brain ischemia in 8- and 20-month-old mice overexpressing the 751-amino acid isoform of human APP. We found that APP751 mice have higher activity of p38 mitogen-activated protein kinase (p38 MAPK) in microglia, the main immune effector cells within the brain, and increased vulnerability to brain ischemia when compared with age-matched wild-type mice. These characteristics are associated with enhanced microglial activation and inflammation but not with altered regulation of cerebral blood flow, as assessed by MRI and laser Doppler flowmetry. Suppression of inflammation with aspirin or inhibition of p38 MAPK with a selective inhibitor, SD-282, abolishes the increased neuronal vulnerability in APP751 transgenic mice. SD-282 also suppresses the expression of inducible nitric-oxide synthase and the binding activity of activator protein 1. These findings elucidate molecular mechanisms of neuronal injury in AD and suggest that antiinflammatory compounds preventing activation of p38 MAPK in microglia may reduce neuronal vulnerability in AD.

Effect of atropine on learning, extinction, retention and retrieval in rats

SummaryEEG changes induced by intraperitoneal injection of 6 mg/kg atropine sulphate were studied in freely moving rats with implanted electrodes. Sleep-like high voltage slow wave activity appeared in neocortex, hippocampus and reticular formation attaining maximum approximately 20 minutes after atropine injection. At the height of the atropine effect one-trial learning of a passive avoidance reaction was partly impaired. So was extinction of this overtrained reaction. Atropine injected 20 minutes before the retention test adversely affected retrieval of threshold conditioned reactions; this effect could not be demonstrated with slightly overtrained behavior. Atropine impaired neither the consolidation nor the storage of memory traces. The atropine induced dissociation between EEG and behavior is discussed.

Extent of the tetrodotoxin induced blockade examined by pupillary paralysis elicited by intracerebral injection of the drug

SummarySpatial extent and duration of the functional blockade elicited by intracerebral injection of tetrodotoxin (TTX) was examined in rats anesthetized with pentobarbital. Pupillary diameter was measured under dissecting microscope before and up to 24 h after injection of TTX (10 ng/l μl saline) into or 1.0, 1.5 and 2.0 mm lateral from the Edinger-Westphal nucleus. TTX administration elicited mydriasis the latency of which was directly and amplitude indirectly proportional to the target-injection distance. The maximum mydriasis attained 3.4 mm, lasted 2 h and slowly decayed over subsequent 20 h. Impulse transmission and conduction was blocked in a spherical volume of tissue about 3 mm in diameter the development of which could be approximated by diffusion from an instantaneous point source. Completeness and full reversibility make the TTX block a convenient research tool.

Systemic ketamine blocks cortical spreading depression but does not delay the onset of terminal anoxic depolarization in rats

Ketamine (KET)-induced blockade of cortical spreading depression (CSD) was examined in rats (n = 51) anesthetized with pentobarbital (50 mg/kg). CSD was elicited by intracortical injection of 1 microliter of 0.15 mol/l potassium acetate 10-40 min after i.p. injection of 6-50 mg/kg KET. KET was almost ineffective at 6 mg/kg but prevented CSD propagation at 12 mg/kg and at higher dosages. The blockade was maximal 20 min after injection. Terminal anoxic depolarization elicited by N2 breathing developed in control rats after a latency which was not significantly different from that in rats pretreated with 50 and 100 mg/kg KET. The failure of CSD blocking dosages of KET to delay the onset of terminal anoxic depolarization indicates that excitotoxic amino acids play different roles in the mechanism of the anoxia-elicited and CSD-related autoregenerative ionic shifts.

Inactivating one hippocampus impairs avoidance of a stable room-defined place during dissociation of arena cues from room cues by rotation of the arena

Unilateral intrahippocampal injections of tetrodotoxin were used to temporarily inactivate one hippocampus during specific phases of training in an active allothetic place avoidance task. The rat was required to use landmarks in the room to avoid a room-defined sector of a slowly rotating circular arena. The continuous rotation dissociated room cues from arena cues and moved the arena surface through a part of the room in which foot-shock was delivered. The rat had to move away from the shock zone to prevent being transported there by the rotation. Unilateral hippocampal inactivations profoundly impaired acquisition and retrieval of the allothetic place avoidance. Posttraining unilateral hippocampal inactivation also impaired performance in subsequent sessions. This allothetic place avoidance task seems more sensitive to hippocampal disruption than the standard water maze task because the same unilateral hippocampal inactivation does not impair performance of the variable-start, fixed hidden goal task after procedural training. The results suggest that the hippocampus not only encodes allothetic relationships amongst landmarks, it also organizes perceived allothetic stimuli into systems of mutually stable coordinates. The latter function apparently requires greater hippocampal integrity.

Radial maze as a tool for assessing the effect of drugs on the working memory of rats

The effect of physostigmine (0.2 mg/kg), scopolamine (0.1 mg/kg), d,l-amphetamine (1 mg/kg), apomorphine (0.05 mg/kg), and piracetam (100 mg/kg) on working memory was examined in 12 rats that were highly overtrained in the radial maze. In experiment 1, drugs administered 10 min before the trial did not worsen performance of rats in the 12-arm maze. In experiment 2, insertion of a 5-min delay between the sixth and seventh choices increased the number of errors over choices 7–12. Performance was unaffected by pretreatment with physostigmine or apomorphine, but was significantly impaired by scopolamine, amphetamine, and piracetam. In experiment 3, performed in a 24-arm maze, the number of errors and trial duration increased, but performance was not decreased by amphetamine or piracetam. It is concluded that the uninterrupted radial maze task is relatively resistant to pharmacological disruption, but that scopolamine, amphetamine, and piracetam enhance the effect of stimuli interfering with the storage of spatial information over delays.

Ketamine blockade of cortical spreading depression in rats

The failure to elicit cortical spreading depression (SD) under ketamine anesthesia has been examined in 25 rats. SD was elicited by microinjection of K+ acetate (1 microliter, 0.15 mol/l) into the cerebral cortex and monitored by recording the accompanying slow-potential waves. I.p. injection of ketamine HCl (50 mg/kg) elicited after 5-10 min blockade of SD propagation lasting 30-40 min. SD penetration into a cortical area superfused with 10(-4) and 10(-3) mol/l ketamine was partly or completely blocked, respectively. Systemic ketamine doses eliciting SD blockade only slightly reduced spontaneous activity of cortical units recorded with carbon fiber microelectrodes and did not increase but rather decreased the rate of K+ removal from a KCl pool (30 microliters, 40 mmol/l) contacting a 12.5 mm2 area of exposed cortical surface. The results indicate that the ketamine-induced SD blockade is due neither to epileptic activity nor to enhanced active transport of ions but rather to interference with chemically gated ionic channels and/or to stabilization of postsynaptic membranes.

Differential involvement of gustatory insular cortex and amygdala in the acquisition and retrieval of conditioned taste aversion in rats

Lesion studies of the role of the gustatory insular cortex (GC) and amygdala (Am) in conditioned taste aversion (CTA) are confounded by the irreversibility of the intervention. Functional ablation methods allow more specific influencing of different phases of CTA acquisition and retrieval. Bilateral tetrodotoxin (TTX) blockade of GC (10 ng) or Am (3 ng) before or after saccharin drinking in rats with chronically implanted intracerebral cannulae showed that GC is indispensable for the initial processing of the taste stimulus but not for the association of the gustatory trace with the symptoms of LiCl poisoning. Gustatory signals can by-pass the blocked Am, the inactivation of which, however, impairs the gustatory trace-poisoning association. TTX injection into both GC and Am impairs CTA retrieval more than isolated blockade of either of these structures. It is argued that GC and Am implement processing of gustatory and visceral signals, respectively, but that formation and consolidation of the CTA engram proceeds outside forebrain, probably at the level of the brainstem.