No spatial working memory deficit in beta-amyloid-exposed rats. A longitudinal study

Amelioratory effects of testosterone treatment on cognitive performance deficits induced by soluble Aβ1-42 oligomers injected into the hippocampus

This study was undertaken to investigate the protective effects and potential mechanism of testosterone (T) on cognitive performance in adult male rats given bilateral intrahippocampal injections of beta amyloid 1-42 oligomers (Aβ1-42) combined with gonadectomy (Aβ+GDX). A series of experiments were designed to verify the optimal administration time and dose of T and to explore its potential protective mechanisms on spatial ability in Aβ+GDX rats in the Morris water maze test. Aβ1-42 was injected only once two weeks before testing, while T and the androgen receptor (AR) antagonist flutamide (F) were administered daily beginning 2 days before and throughout the 6 days of testing. The Aβ1-42 injection and GDX individually impaired cognitive performance, and the combination of these treatments was additive, leading to even greater impairment. The serum T level peaked at 48 h after administration. T doses ranging from 0.25 to 1.00 mg corresponding to serum T levels of 4.5-21.35 ng/ml improved the spatial ability. Animals administered 0.75 mg of T corresponding to the serum T level of 15.2 ng/ml had the most significantly improved behavioral performances. However, higher T doses of 1.50 and 2.00 mg resulting in serum T levels of 34.8 and 45 ng/ml, respectively, impaired the behavioral performances. F had no effect on the serum T level and spatial ability, but it blocked the activational effect of T. These findings indicate that the effect of T on behavioral performances is partly mediated through ARs.

Behavioural and cellular effects of exogenous amyloid-β peptides in rodents

A better understanding of Alzheimer's disease (AD) and the development of disease modifying therapies are some of the biggest challenges of the 21st century. One of the core features of AD are amyloid plaques composed of amyloid-beta (Aβ) peptides. The first hypothesis proposed that cognitive deficits are linked to plaque-development and transgenic mice have been generated to study this link, thereby providing a good model to develop new therapeutic approaches. Since later it was recognised that in AD patients the cognitive deficit is rather correlated to soluble amyloid levels, consequently, a new hypothesis appeared associating the earliest amyloid toxicity to these soluble species. The purpose of this review is to give a summary of behavioural and cellular data obtained after soluble Aβ peptide administration into rodents' brain, thereby showing that this model is a valid tool to investigate AD pathology when no plaques are present. Additionally, this method offers an excellent, efficient model to test compounds which could act at such early stages of the disease.

Animal model of Alzheimer’s disease: characteristics of EEG and memory

We studied the effects of aggregated amyloid β-peptide Aβ25–35 on spatial memory and the spectral-correlational characteristics of EEG of both the dorsal hippocampus and the frontal cortex both in adult and aged rats at the early stage of Aβ25–35 action. Spatial memory was characterized using a novel cognitive test. A decrease in low-frequency theta band oscillations in the dorsal hippocampus and the frontal cortex was observed. The mean coefficient of EEG cross-correlation between these structures was significantly reduced at the early stage of Aβ25–35 action both in adult and aged rats. In addition, we found that one month after Aβ25–35 injection spatial memory was impaired. These results suggest that the decrease in low-frequency theta band oscillations and the weakening of binding between the dorsal hippocampus and the frontal cortex under the action of Aβ25–35 may be an underlying cause of the typical memory breakdown associated with the Alzheimer’s disease.

Use of copper and insulin-resistance to accelerate cognitive deficits and synaptic protein loss in a rat Abeta-infusion Alzheimer's disease model

The rat amyloid-beta (Abeta) intracerebroventricular infusion can model aspects of Alzheimer's disease (AD) and has predicted efficacy of therapies such as ibuprofen and curcumin in transgenic mouse models. High density lipoprotein (HDL), a normal plasma carrier of Abeta, is used to attenuate Abeta aggregation within the pump, causing Abeta-dependent toxicity and cognitive deficits within 3 months. Our goal was to identify factors that might accelerate onset of Abeta-dependent deficits to improve efficiency and cost-effectiveness of model. We focused on: 1) optimizing HDL-Abeta preparation for maximal toxicity; 2) evaluating the role of copper, a factor typically in water that can impact oligomer stability; and 3) determining impact of insulin resistance (type II diabetes), a risk factor for AD. In vitro studies were performed to determine doses of copper and methods of Abeta-HDL preparation that maximized toxicity. These preparations when infused resulted in earlier onset of cognitive deficits within 6 weeks post-infusion. Induction of insulin resistance did not exacerbate Abeta-dependent cognitive deficits, but did exacerbate synaptic protein loss. In summary, the newly described in vivo infusion model may be useful cost-effective method for screening for new therapeutic drugs for AD.

The effects of chronic estradiol treatment on working memory deficits induced by combined infusion of beta-amyloid (1-42) and ibotenic acid

Estrogen limits in vitro neuron death induced by application of beta-amyloid, the cytotoxic peptide linked to Alzheimer's disease. However, the ability of estrogen to protect neurons and preserve cognitive function in vivo following exposure to beta-amyloid has not been demonstrated. Our objective was to evaluate the potential of estrogen to reduce spatial working memory deficits in female rats induced by administration of a neurotoxic form of beta-amyloid in combination with the excitotoxin, ibotenic acid. The interaction of beta-amyloid with excitotoxic factors may underlie cognitive deficits associated with Alzheimer's disease. Therefore, to create an experimental model typical of early Alzheimer's disease a low dose of ibotenic acid was administered with beta-amyloid into the dorsal hippocampus. Ovariectomized rats were implanted subcutaneously with Silastic capsules that produce physiological levels of 17beta-estradiol 10 days before bilateral intrahippocampal injections of aggregated beta-amyloid (1-42) and ibotenic acid. Capsules remained in situ throughout behavioral testing. When tested 3-10 weeks after neurotoxin treatment, females without estrogen capsules exhibited delay-dependent impairments in working memory performance on a water maze and a radial arm maze. Females treated with estrogen and combined neurotoxins displayed working memory performance comparable to unlesioned females on both tasks. Neurotoxin treatment increased immunoreactivity for glial fibrillary acidic protein but this measure was unaffected by estradiol treatment indicating that estrogen did not limit glial proliferation. Results indicate that estrogen prevented deficits in spatial working memory induced by neurotoxin treatments intended to mimic the pathology of early Alzheimer's disease.

The synthetic cannabinoid HU210 induces spatial memory deficits and suppresses hippocampal firing rate in rats

BACKGROUND AND PURPOSE

Previous work implied that the hippocampal cannabinoid system was particularly important in some forms of learning, but direct evidence for this hypothesis is scarce. We therefore assessed the effects of the synthetic cannabinoid HU210 on memory and hippocampal activity.

EXPERIMENTAL APPROACH

HU210 (100 microg kg(-1)) was administered intraperitoneally to rats under three experimental conditions. One group of animals were pre-trained in spatial working memory using a delayed-matching-to-position task and effects of HU210 were assessed in a within-subject design. In another, rats were injected before acquisition learning of a spatial reference memory task with constant platform location. Finally, a separate group of animals was implanted with electrode bundles in CA1 and CA3 and single unit responses were isolated, before and after HU210 treatment.

KEY RESULTS

HU210 treatment had no effect on working or short-term memory. Relative to its control Tween 80, deficits in acquisition of a reference memory version of the water maze were obtained, along with drug-related effects on anxiety, motor activity and spatial learning. Deficits were not reversed by the CB(1) receptor antagonists SR141716A (3 mg kg(-1)) or AM281 (1.5 mg kg(-1)). Single unit recordings from principal neurons in hippocampal CA3 and CA1 confirmed HU210-induced attenuation of the overall firing activity lowering both the number of complex spikes fired and the occurrence of bursts.

CONCLUSIONS AND IMPLICATIONS

These data provide the first direct evidence that the underlying mechanism for the spatial memory deficits induced by HU210 in rats is the accompanying abnormality in hippocampal cell firing.

Impairments of hippocampal synaptic plasticity induced by aggregated beta-amyloid (25–35) are dependent on stimulation-protocol and genetic background

The aggregation of beta-amyloid to plaques in the brain is one of the hallmarks of Alzheimer disease (AD). Numerous studies have tried to elucidate to what degree amyloid peptides play a role in the neurodegenerative developments seen in AD. While most studies report an effect of amyloid on neural activity and cognitive abilities of rodents, there have been many inconsistencies in the results. This study investigated to what degree the different genetic backgrounds affect the outcome of beta-amyloid fragment (25-35) on synaptic plasticity in vivo in the rat hippocampus. Two strains, Wistar and Lister hooded rats, were tested. In addition, the effects of a strong (600 stimuli) and a weak stimulation protocol (100 stimuli) on impairments of LTP were analysed. Furthermore, since the state of amyloid aggregation appears to play a role in the induction of toxic processes, it was tested by dual polarisation interferometry to what degree and at what speed beta-amyloid (25-35) can aggregate in vitro. It was found that 100 nmol beta-amyloid (25-35) injected icv did impair LTP in Wistar rats when using the weak but not the strong stimulation protocol (P < 0.001). One-hundred nano mole of the reverse sequence amyloid (35-25) had no effect. LTP in Lister Hooded rats was not impaired by amyloid at any stimulation protocol. The aggregation studies showed that amyloid (25-35) aggregated within hours, while amyloid (35-25) did not. These results show that the genetic background and the stimulation protocol are important variables that greatly influence the experimental outcome. The fact that amyloid (25-35) aggregated quickly and showed neurophysiological effects, while amyloid (35-25) did not aggregate and did not show any effects indicates that the state of aggregation plays an important role in the physiological effects.

Scopolamine and MK801-induced working memory deficits in rats are not reversed by CBD-rich cannabis extracts

Smoking marijuana causes working and short-term memory deficits, an effect that is mediated by cannabinoid receptor (CB1) activation in the brain. While this may be due to the main psychoactive constituent Delta9-tetrahydrocannabinol (Delta9-THC), plant extracts also contain other cannabinoid and terpenoid compounds with unknown properties. Towards this end, we have recently shown that high concentrations of plant extracts rich in cannabidiol (CBD) can reverse working memory deficits induced by Delta9-THC which is a remaining contaminant of this extract [Fadda P, Robinson L, Fratta W, Pertwee RG, Riedel G. Differential effects of THC- and CBD-rich cannabis-extracts on working memory in rats. Neuropahrmacology 2004;47:1170-9]. Since this effect was dose-dependent and indicative of memory enhancing qualities of the CBD-rich extract, this prompted a wider investigation into the effects of CBD on other forms of amnesia in order to determine the mechanism of action and to reveal its potency against anticholinergic and antiglutamatergic agents. We employed a spatial delayed matching to position task in the open-field water maze. Both scopolamine (0.2 mg/kg i.p.) and dizocilpine (MK801: 0.1mg/kg i.p.) impaired working memory at delays of 30 s and 4 h. Two doses of CBD-rich extracts (5 and 10 mg/kg), which did not affect working memory when given alone, were unable to reverse these deficits when co-administered with scopolamine or MK801. These data suggest that reversal of working memory deficits by CBD-rich extracts are specific to the cannabinoid system and do not compensate for acutely induced cholinergic or glutamatergic receptor hypoactivity.

Long-term neurotoxicity of chlorpyrifos: spatial learning impairment on repeated acquisition in a water maze

Organophosphate compounds are cholinesterase inhibitors widely used in agriculture, industry, household products, and even as chemical weapons. Their major mechanism of acute toxic action is the inhibition of acetylcholinesterase, which is responsible for the degradation of the neurotransmitter acetylcholine. An organophosphorus ester-induced chronic neurotoxicity (OPICN) syndrome has been proposed. The OPICN syndrome could result from both long-term exposure to subclinical doses of OPs and after acute poisoning. Development of animal models for the cognitive decline are required and could later help to elucidate the mechanisms involved in this long-term effect on the central nervous system. Previously, we have found performance decrements in a four-trial repeated acquisition spatial task in a water maze. The present study includes two experiments to extend the long-term behavioral effects observed. Rats were injected either once or twice with chlorpyrifos (CPF) and then tested months after in a two-trial repeated acquisition task in a water maze. Our results confirm and extend the long-term behavioral effects of subcutaneous administration of CPF. The two treatments used produced performance decrements that suggest functional central nervous system alterations.

Differential effects of THC- or CBD-rich cannabis extracts on working memory in rats

Cannabinoid receptors in the brain (CB(1)) take part in modulation of learning, and are particularly important for working and short-term memory. Here, we employed a delayed-matching-to-place (DMTP) task in the open-field water maze and examined the effects of cannabis plant extracts rich in either Delta(9)-tetrahydrocannabinol (Delta(9)-THC), or rich in cannabidiol (CBD), on spatial working and short-term memory formation in rats. Delta(9)-THC-rich extracts impaired performance in the memory trial (trial 2) of the DMTP task in a dose-dependent but delay-independent manner. Deficits appeared at doses of 2 or 5 mg/kg (i.p.) at both 30 s and 4 h delays and were similar in severity compared with synthetic Delta(9)-THC. Despite considerable amounts of Delta(9)-THC present, CBD-rich extracts had no effect on spatial working/short-term memory, even at doses of up to 50 mg/kg. When given concomitantly, CBD-rich extracts did not reverse memory deficits of the additional Delta(9)-THC-rich extract. CBD-rich extracts also did not alter Delta(9)-THC-rich extract-induced catalepsy as revealed by the bar test. It appears that spatial working/short-term memory is not sensitive to CBD-rich extracts and that potentiation and antagonism of Delta(9)-THC-induced spatial memory deficits is dependent on the ratio between CBD and Delta(9)-THC.