N. A. Lazareva

Amyloid-β(25–35)-induced memory impairments correlate with cell loss in rat hippocampus

Amyloid beta-peptide (Abeta) plays an important role in the pathophysiology of Alzheimers disease. The relationship between amnesia induced by central administration of aggregated Abeta(25-35) and neurodegeneration in the hippocampus was investigated. One month after a single intracerebroventricular injection of Abeta(25-35) (15 nmol), male Wistar rats were tested in an eight-arm radial maze. A quantitative evaluation of cell number in hippocampal regions was carried out on H&E-stained brain sections of rats used in the behavioral study. Indices of free radical-mediated processes in the hippocampus were evaluated in additional groups of animals 1, 3, 5, and 30 days after surgery. Abeta(25-35) induced impairments of working and reference memory (RM) as well as neurodegeneration in the CA1 but not in the CA3 field of the hippocampus. A significant correlation between both reference and working memory (WM) impairments and the neuronal cell loss in the hippocampal CA1 region was demonstrated. A gradually developing oxidative stress was evident in the hippocampus of rats treated with Abeta(25-35) as indicated by the increase in 2-thiobarbituric acid (TBARS) reactive substances and superoxide generation. These data suggest the involvement of oxidative stress in Abeta(25-35)-induced neurodegeneration and a relation between memory impairment and neurodegeneration in the CA1 subfield of the hippocampus.

Single intracerebroventricular administration of amyloid-beta (25-35) peptide induces impairment in short-term rather than long-term memory in rats.

Ample experimental evidence indicates that intracerebral injection or infusion of amyloid-beta peptides (Abeta) to rodents induces learning and memory impairments as well as neurodegeneration in brain areas related to cognitive function. In the present study, we assessed the effects of a single intracerebroventricular (i.c.v.) injection of aggregated Abeta fragment (25-35) at a dose of 15nmol/rat on short-term and long-term memory in rats during the 6-month post-surgery period. The results demonstrate that Abeta(25-35)-induced memory impairments in spontaneous alternation behavior in a Y-maze at 17, 36, and 180 days after the surgery as well as in a social recognition task 110 days post-surgery. Abeta(25-35) also impaired spatial memory in an 8-arm radial maze, but did not influence performance of the step-down passive avoidance task. These results suggest that Abeta(25-35) preferably induces impairments of spatial and non-spatial short-term (working) memory rather than long-term memory in rats.

Orientation tuning and receptive field structure in cat striate neurons during local blockade of intracortical inhibition

The contribution of intracortical inhibition to orientation tuning in the cat striate cortex (area 17) was studied by responses to flashing light bars of different orientations and lengths in 68 single-units before and during microiontophoretical application of bicuculline, a GABAA antagonist, A three-fold increase in the background activity (13.3 +/- 1.3 vs 4.4 +/- 0.5 imp/s) and 4.4-fold increase in the maximal discharge frequency (264.4 +/- 22.3 vs 60.6 +/- 5.3 imp/s) was found in 96.0% of the cells studied during microiontophoresis. In most units all characteristics of orientation tuning significantly changed during application of bicuculline: i) tuning width increased in 76.3% of cells from 52.7 +/- 2.8 degrees in control to 85.2 +/- 4.6 degrees, ii) tuning selectivity diminished in 63.6% of cells by a factor of 1.5, and iii) tuning quality dropped in 68.5% of cases by a factor of 2.5. The threshold ejection current of bicuculline for widening of tuning was in 2/3 of the cells in the range from +10 to +40 nA (+31.0 +/- 4.5 nA) and the maximum effect was obtained in 3/4 of units with +30(-) + 100 nA (+67.1 +/- 6.0 nA). Unmasking of additional excitatory inputs to the studied cells due to blockade of the inputs from inhibitory interneurons in considered as the main mechanism of the described bicuculline effects.

Sensitivity to cross-like figures in the cat striate neurons

The responses and orientation tuning in 48 out of 62 (77.4%) neurons of the cat striate cortex (area 17) significantly, but with different sign, changed at stimulation by specific cross-like figure flashing in receptive field as compared with single light bar of preferred orientation. Neurons of the first group (19 units from 62, 30.6%) were found to increase the responses by 3.3 times if stimulated by a certain cross-like figure, specific for each cell configuration and orientation. Under the same conditions, neurons of the second group (29 or 46.8% revealed a three-fold decrease of responses and all tuning characteristics worsened. Among them 8% of total number of cells showed bimodal or double orientation tuning when stimulated by some configurations of crosses due to an angle specific inhibition. Dependence of the revealed effects on excitatory convergence from neurons with different orientation tuning, on inhibitory influences from end-stop and side-zones of receptive field, as well as possible functional implication of the first group neurons for an angle and line-crossing detection are discussed.

Effects of tumor necrosis factor-alpha central administration on hippocampal damage in rat induced by amyloid beta-peptide (25-35).

Male Wistar rats received unilateral intrahippocampal injections of 3 nmol (3.18 μg) aggregated Aβ(25–35), intracerebroventricular bilateral injections of 0.5 μg human recombinant TNFα or both (Aβ(25–35) + TNFα‐treated animals). Seven days after the surgery brain sections were stained with cresyl violet (Nissl), for fragmented DNA (TUNEL), glial fibrillar acidic protein (GFAP) and isolectin B4‐reactive microglia. In addition, caspase‐3 activity in brain regions was measured fluorometrically. The morphology of the hippocampus after the injection of Aβ(25–35) or both Aβ(25–35) and TNFα (but not TNFα alone) showed cell loss in the CA1 pyramidal cell layer. The extension of neuronal degeneration measured in the CA1 field was significantly larger in Aβ(25–35)‐treated groups compared to the contralateral hemisphere of both vehicle‐treated controls and animals injected with TNFα alone. TNFα augmented the Aβ(25–35)‐induced damage, significantly increasing the extension of degenerating area. Administration of Aβ(25–35) caused reactive gliosis in the ipsilateral hemisphere as demonstrated by upregulation of GFAP expression and the presence of hypertrophic astrocytes in the hippocampus. This effect was much more prominent in the hippocampi of rats treated with Aβ(25–35) + TNFα but absent after administration of TNFα alone. In both Aβ(25–35)‐treated groups, the damaged area of the hippocampal CA1 field and lateral band of dentate gyrus displayed many darkly stained round isolectin B4‐positive phagocyte‐like microglial cells. Sparse TUNEL‐positive nuclei were found in the hippocampi of rats treated with Aβ(25–35) alone or together with TNFα, but not in the control brain sections or in brain sections of TNFα‐injected animals. The activity of caspase‐3 increased significantly in the ipsilateral hippocampus after the injection of Aβ(25–35). Surprisingly, administration of TNFα into the cerebral ventricles prevented this Aβ(25–35)‐induced increase in hippocampal caspase‐3 activity. The results are discussed from the perspective of dual (neuroprotective and neurodestructive) roles of TNF in the brain.

Selective and invariant sensitivity to crosses and corners in cat striate neurons

Many neurons (56/174, or 32.2%) studied in the cat striate cortex (area 17) increased significantly (by 3.3 times on average) their responses under stimulation by cruciform or corner figures of specific or non-specific shape and orientation flashing in receptive field as compared with single light bar of preferred orientation. Most of these neurons (71.4%) were found to be highly selective to both the shape (the angle between the figures lines) and orientation of these figures. In the neuronal selection studied we have also found all possible types of invariance of the cross and corner tuning to orientation and/or shape of these figures. We found neurons with selectivity to the form of the figures and invariance to their orientation and, on the contrary, units invariant to shape but selective to orientation. Some cells were found invariant to both the form and orientation of the cruciform or corner figure but highly sensitive to appearance of any such figure in the receptive field. Two main hypotheses about the mechanisms of selective sensitivity to crosses and angles can be considered. They are as follows: an excitatory convergence of two units with different preferred orientations, and intracortical inhibitory interactions. The cells with double orientation tuning for a single bar are found relatively rarely (about 20%), thus making the first suggestion the most unlikely. This circumstance is of special importance since it provides evidence against the hierarchic formation of the higher-order cortical units from a set of lower-order cells that is still under discussion. The units with high sensitivity to cross or corner seem to be ideally suitable for their selection, rather than to serve as classical orientation detectors only.

Amyloid-β (25-35) increases activity of neuronal NO-synthase in rat brain

Nitric oxide (NO) is a free radical with multiple functions in the nervous system. NO plays an important role in the mechanisms of neurodegenerative diseases including Alzheimers disease. The main source of NO in the brain is an enzymatic activity of nitric oxide synthase (NOS). The aim of the present study was to analyze the expression and activity of both neuronal (nNOS) and inducible (iNOS) isoenzymes in the cerebral cortex and hippocampus of rats after intracerebroventricular administration of amyloid-beta (A beta) peptide fragment A beta(25-35). NADPHd histochemistry as well as immunohistochemistry were also used to investigate nNOS and iNOS expression in rat brain. The data presented here show that A beta(25-35) did not influence levels of nNOS or iNOS mRNA or protein expression in both structures studied. A beta(25-35) activated nNOS in the cerebral cortex and hippocampus without effect on iNOS activity. A beta(25-35) decreased the number of NADPHd-expressing neurons in the neocortex, but it did not significantly influence the number NADPHd-positive cells in the hippocampus. The peptide had no effect on the number of nNOS containing cells. We hypothesize that increased synthesis of NO induced by A beta(25-35) is related to qualitative alterations of nNOS molecule, but not to changes in NOS protein expression.

Studies of the effects of central administration of beta-amyloid peptide (25-35): pathomorphological changes in the Hippocampus and impairment of spatial memory.

The possible link between amnesia induced by central administration of β-amyloid (25–35) (Aβ(25–35)) and neurodegenerative changes in the hippocampus was studied. Male Wistar rats received single intracerebroventricular injections of Aβ(25–35) at a dose of 15 nmoles and one month later were trained in an eight-arm radial maze. Training was followed by histological assessment of the state of the hippocampus on brain sections stained with hematoxylin and eosin. Aβ(25–35) induced impairments in long-term (reference) and working memory on testing in the maze. There was a moderate reduction in the number of neurons in hippocampal field CA1; there was no change in the number of cells in field CA3. The numbers of errors made by the animals on testing in the maze were found to correlate negatively with the numbers of nerve cells in hippocampal field CA1. Thus, this is the first demonstration that impairments of learning and memory induced by single doses of Aβ(25–35) are specifically associated with neurodegenerative changes in hippocampal field CA1 in rats.

Double orientation tuning in the cat visual cortex units

Orientation tuning of 271 neurons of the cat visual cortex (area 17) was studied with a light bar flashing in the receptive field. Under different conditions, 27-57% of units were found to have double-orientation tuning: they demonstrated the main preferred orientation and an additional preferred orientation. The statistical reliability and reproducibility of additional preferred orientation were shown. The quality of orientation tuning in the second maximum did not differ statistically from the first one. The angle between preferred orientation and additional preferred orientation was either 90 degrees (29% of cases) or an acute one (60.1 +/- 3.1 degrees, 71% of cases). The ratio of discharge frequency in responses to additional preferred orientation and preferred orientation was equal to 0.74 +/- 0.05. Neurons with double-orientation tuning clearly preferred 67 degrees and 157 degrees, while monomodal units preferred 0 degrees and 90 degrees. Probability of the double-tuning increased under bar lengths of near 3 degrees and near 10 degrees and with increase of stimulus/background contrast. At the same time some neurons displayed double-orientation tuning only with relatively low stimulus/background contrast. The proportion of units with double-orientation tuning was lowered by about 1.5-times under general Nembutal narcotization as compared with local anesthesia of the animal. In about one-third of units simultaneous stimulation by two flashing lines crossing in the receptive field center under an angle specific for the cell, evoked a response from 1.5 to four times larger than to the preferred orientation.(ABSTRACT TRUNCATED AT 250 WORDS)

Administration of aggregated beta-amyloid peptide (25-35) induces changes in long-term potentiation in the hippocampus in vivo.

Intracerebroventricular administration of aggregated β-amyloid protein fragment (25–35) (7.5 nmol/ventricle) was followed one month later by significant changes in the dynamics of long-term potentiation in the hippocampus in vivo, expressed as powerful and stable increases in the amplitude of evoked potentials. This phenomenon may be associated with oxidative stress in the hippocampus, which has previously been demonstrated in this model, and, thus, with disturbances in ion homeostasis.