Catecholamine system involvement in age-related memory dysfunction

Possible role of locus coeruleus neuronal loss in age-related memory and attention deficits

Introduction

Aging is associated with a decline in cognitive abilities, including memory and attention. It is generally accepted that age-related histological changes such as increased neuroinflammatory glial activity and a reduction in the number of specific neuronal populations contribute to cognitive aging. Noradrenergic neurons in the locus coeruleus (LC) undergo an approximately 20 % loss during ageing both in humans and mice, but whether this change contributes to cognitive deficits is not known. To address this issue, we asked whether a similar loss of LC neurons in young animals as observed in aged animals impairs memory and attention, cognitive domains that are both influenced by the noradrenergic system and impaired in aging.

Methods

For that, we treated young healthy mice with DSP-4, a toxin that specifically kills LC noradrenergic neurons. We compared the performance of DSP-4 treated young mice with the performance of aged mice in models of attention and memory. To do this, we first determined the dose of DSP-4, which causes a similar 20 % neuronal loss as is typical in aged animals.

Results

Young mice treated with DSP-4 showed impaired attention in the presence of distractor and memory deficits in the 5-choice serial reaction time test (5-CSRTT). Old, untreated mice showed severe deficits in both the 5-CSRTT and in fear extinction tests.

Discussion

Our data now suggest that a reduction in the number of LC neurons contributes to impaired working memory and greater distractibility in attentional tasks but not to deficits in fear extinction. We hypothesize that the moderate loss of LC noradrenergic neurons during aging contributes to attention deficits and working memory impairments.

Synapsin I in Intraocular Hippocampal Transplants during Maturation and Aging: Effects of Brainstem Cografts

The role of target innervation for maintenance of synaptic proteins in the hippocampal formation during aging was investigated. Fetal CA1 tissue and brainstem tissue containing the nucleus locus coeruleus was dissected from albino rats and grafted sequentially into the anterior chamber of the eye of adult rat recipients. Synapsin protein distribution and levels were evaluated by immunohistochemistry and quantitative immunolabeling in single hippocampal grafts or brainstem-hippocampal double grafts at 6,12, or 24 mo postgrafting. The synapsin levels in 6-mo-old single hippocampal transplants were significantly lower than those in situ, and remained at these lower levels at 12 and 24 mo. On the contrary, synapsin levels were close to normal in the hippocampal portion of double grafts in the 6- and the 12-mo-group. However, in the 24-mo-old double transplants the levels had declined significantly, approaching levels seen in single hippocampal grafts. The immunoblot results were supported by morphological observations with synapsin antibodies and immunohistochemistry. The present data demonstrate that hippocampal tissue maintained near normal synapsin levels when grafted together with brainstem tissue, as compared to the lower levels seen in single hippocampal grafts. This normalization of synapsin levels was, however, not seen in the aged hippocampal-brainstem double grafts.

Monoaminergic neuropathology in Alzheimer's disease

None of the proposed mechanisms of Alzheimer's disease (AD) fully explains the distribution patterns of the neuropathological changes at the cellular and regional levels, and their clinical correlates. One aspect of this problem lies in the complex genetic, epigenetic, and environmental landscape of AD: early-onset AD is often familial with autosomal dominant inheritance, while the vast majority of AD cases are late-onset, with the ε4 variant of the gene encoding apolipoprotein E (APOE) known to confer a 5-20 fold increased risk with partial penetrance. Mechanisms by which genetic variants and environmental factors influence the development of AD pathological changes, especially neurofibrillary degeneration, are not yet known. Here we review current knowledge of the involvement of the monoaminergic systems in AD. The changes in the serotonergic, noradrenergic, dopaminergic, histaminergic, and melatonergic systems in AD are briefly described. We also summarize the possibilities for monoamine-based treatment in AD. Besides neuropathologic AD criteria that include the noradrenergic locus coeruleus (LC), special emphasis is given to the serotonergic dorsal raphe nucleus (DRN). Both of these brainstem nuclei are among the first to be affected by tau protein abnormalities in the course of sporadic AD, causing behavioral and cognitive symptoms of variable severity. The possibility that most of the tangle-bearing neurons of the LC and DRN may release amyloid β as well as soluble monomeric or oligomeric tau protein trans-synaptically by their diffuse projections to the cerebral cortex emphasizes their selective vulnerability and warrants further investigations of the monoaminergic systems in AD.

Cardiorespiratory fitness and cognitive functioning following short-term interventions in chronic stroke survivors with cognitive impairment: a pilot study

This study, a quasi-experimental, one-group pretest-post-test design, evaluated the effects on cognitive functioning and cardiorespiratory fitness of 8-week interventions (aerobic exercise alone and aerobic exercise and cognitive training combined) in patients with chronic stroke and cognitive impairment living in the community (participants: n=14, 61.93±9.90 years old, 51.50±38.22 months after stroke, n=7 per intervention group). Cognitive functions and cardiorespiratory fitness were evaluated before and after intervention, and at a 3-month follow-up visit (episodic memory: revised-Hopkins Verbal Learning Test; working memory: Brown-Peterson paradigm; attention omission and commission errors: Continuous Performance Test; cardiorespiratory fitness: peak oxygen uptake during a symptom-limited, graded exercise test performed on a semirecumbent ergometer). Friedman's two-way analysis of variance by ranks evaluated differences in score distributions related to time (for the two groups combined). Post-hoc testing was adjusted for multiple comparisons. Compared with before the intervention, there was a significant reduction in attention errors immediately following the intervention (omission errors: 14.6±21.5 vs. 8±13.9, P=0.01; commission errors: 16.4±6.3 vs. 10.9±7.2, P=0.04), and in part at follow-up (omission errors on follow-up: 3.4±4.3, P=0.03; commission errors on follow-up: 13.2±7.6, P=0.42). These results suggest that attention may improve in chronic stroke survivors with cognitive impairment following short-term training that includes an aerobic component, without a change in cardiorespiratory fitness. Randomized-controlled studies are required to confirm these findings.

Does multicomponent physical exercise with simultaneous cognitive training boost cognitive performance in older adults? A 6-month randomized controlled trial with a 1-year follow-up

Background

Cognitive impairment is a health problem that concerns almost every second elderly person. Physical and cognitive training have differential positive effects on cognition, but have been rarely applied in combination. This study evaluates synergistic effects of multicomponent physical exercise complemented with novel simultaneous cognitive training on cognition in older adults. We hypothesized that simultaneous cognitive–physical components would add training specific cognitive benefits compared to exclusively physical training.

Methods

Seniors, older than 70 years, without cognitive impairment, were randomly assigned to either: 1) virtual reality video game dancing (DANCE), 2) treadmill walking with simultaneous verbal memory training (MEMORY), or 3) treadmill walking (PHYS). Each program was complemented with strength and balance exercises. Two 1-hour training sessions per week over 6 months were applied. Cognitive performance was assessed at baseline, after 3 and 6 months, and at 1-year follow-up. Multiple regression analyses with planned comparisons were calculated.

Results

Eighty-nine participants were randomized to the three groups initially, 71 completed the training, while 47 were available at 1-year follow-up. Advantages of the simultaneous cognitive–physical programs were found in two dimensions of executive function. “Shifting attention” showed a time×intervention interaction in favor of DANCE/MEMORY versus PHYS (F[2, 68] =1.95, trend P=0.075, r=0.17); and “working memory” showed a time×intervention interaction in favor of DANCE versus MEMORY (F[1, 136] =2.71, trend P=0.051, R²=0.006). Performance improvements in executive functions, long-term visual memory (episodic memory), and processing speed were maintained at follow-up in all groups.

Conclusion

Particular executive functions benefit from simultaneous cognitive–physical training compared to exclusively physical multicomponent training. Cognitive–physical training programs may counteract widespread cognitive impairments in the elderly.

Targeting norepinephrine in mild cognitive impairment and Alzheimer's disease

The Alzheimer's disease (AD) epidemic is a looming crisis, with an urgent need for new therapies to delay or prevent symptom onset and progression. There is growing awareness that clinical trials must target stage-appropriate pathophysiological mechanisms to effectively develop disease-modifying treatments. Advances in AD biomarker research have demonstrated changes in amyloid-beta (Aβ), brain metabolism and other pathophysiologies prior to the onset of memory loss, with some markers possibly changing one or two decades earlier. These findings suggest that amyloid-based therapies would optimally be targeted at the earliest clinically detectable stage (such as mild cognitive impairment (MCI)) or before. Postmortem data indicate that tau lesions in the locus coeruleus (LC), the primary source of subcortical norepinephrine (NE), may be the first identifiable pathology of AD, and recent data from basic research in animal models of AD indicate that loss of NE incites a neurotoxic proinflammatory condition, reduces Aβ clearance and negatively impacts cognition - recapitulating key aspects of AD. In addition, evidence linking NE deficiency to neuroinflammation in AD also exists. By promoting proinflammatory responses, suppressing anti-inflammatory responses and impairing Aβ degradation and clearance, LC degeneration and NE loss can be considered a triple threat to AD pathogenesis. Remarkably, restoration of NE reverses these effects and slows neurodegeneration in animal models, raising the possibility that treatments which increase NE transmission may have the potential to delay or reverse AD-related pathology. This review describes the evidence supporting a key role for noradrenergic-based therapies to slow or prevent progressive neurodegeneration in AD. Specifically, since MCI coincides with the onset of clinical symptoms and brain atrophy, and LC pathology is already present at this early stage of AD pathogenesis, MCI may offer a critical window of time to initiate novel noradrenergic-based therapies aimed at the secondary wave of events that lead to progressive neurodegeneration. Because of the widespread clinical use of drugs with a NE-based mechanism of action, there are immediate opportunities to repurpose existing medications. For example, NE transport inhibitors and NE-precursor therapies that are used for treatment of neurologic and psychiatric disorders have shown promise in animal models of AD, and are now prime candidates for early-phase clinical trials in humans.

A meta-regression to examine the relationship between aerobic fitness and cognitive performance

Many studies have been conducted to test the potentially beneficial effects of physical activity on cognition. The results of meta-analytic reviews of this literature suggest that there is a positive association between participation in physical activity and cognitive performance. The design of past research demonstrates the tacit assumption that changes in aerobic fitness contribute to the changes in cognitive performance. Therefore, the purpose of this meta-analysis was to use meta-regression techniques to statistically test the relationship between aerobic fitness and cognitive performance. Results indicated that there was not a significant linear or curvilinear relationship between fitness effect sizes (ESs) and cognitive ESs for studies using cross-sectional designs or posttest comparisons. However, there was a significant negative relationship between aerobic fitness and cognitive performance for pre-post comparisons. The effects for the cross-sectional and pre-post comparisons were moderated by the age group of the participants; however, the nature of this effect was not consistent for the two databases. Based on the findings of this meta-analytic review, it is concluded that the empirical literature does not support the cardiovascular fitness hypothesis. To confirm the findings of this review, future research should specifically test the dose-response relationship between aerobic fitness and cognitive performance. However, based upon the findings of this review, we also encourage future research to focus on other physiological and psychological variables that may serve to mediate the relationship between physical activity and cognitive performance.

Exercício físico e função cognitiva: uma revisão

O exercício e o treinamento físico são conhecidos por promover diversas alterações, incluindo benefícios cardiorrespiratórios, aumento da densidade mineral óssea e diminuição do risco de doenças crônico-degenerativas. Recentemente outro aspecto tem ganhando notoriedade: trata-se da melhoria na função cognitiva. Embora haja grande controvérsia, diversos estudos têm demonstrado que o exercício físico melhora e protege a função cerebral, sugerindo que pessoas fisicamente ativas apresentam menor risco de serem acometidas por desordens mentais em relação às sedentárias. Isso mostra que a participação em programas de exercícios físicos exercem benefícios nas esferas física e psicológica e que, provavelmente, indivíduos fisicamente ativos possuem um processamento cognitivo mais rápido. Embora os benefícios cognitivos do estilo de vida fisicamente ativo pareçam estar relacionados ao nível de atividade física regular, ou seja, exercício realizado durante toda a vida, sugerindo uma "reserva cognitiva", nunca é tarde para se iniciar um programa de exercícios físicos. Dessa forma, o uso do exercício físico como alternativa para melhorar a função cognitiva parece ser um objetivo a ser alcançado, principalmente em virtude da sua aplicabilidade, pois se trata de um método relativamente barato, que pode ser direcionado a grande parte da população. Assim, o objetivo da presente revisão é o de discutir os aspectos associativos entre exercício físico e função cognitiva, permitindo uma ponderação entre o seu uso enquanto alternativa e elemento coadjuvante.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

Neural Transplantation in Animal Models of Dementia

Neural transplantation provides a powerful novel technique for investigating the neurobiological basis and potential strategies for repair of a variety of neurodegenerative conditions. The present review considers applications of this technique to dementia. After a general introduction (section 1), attempts to replace damaged neural systems by transplantation are considered in the context of distinct animal models of dementia. These include grafting into aged animals (section 2), into animals with neurotransmitter-selective lesions of subcortical nuclei, in particular involving basal forebrain cholinergic systems (section 3), and into animals with non-specific lesions of neocortical and hippocampal systems (section 4). The next section considers the alternative use of grafts as a source of growth/trophic factors to inhibit degeneration and promote regeneration in the aged brain (section 5). Finally, a number of recent studies have employed transplanted tissues to model and study the neurodegenerative processes associated with ageing and Alzheimer's disease taking place within the transplant itself (section 6). It is concluded (section 7) that although neural transplantation does not offer any immediate prospect of therapeutic repair in clinical dementia, the technique does offer a powerful neurobiological tool for studying the neuropathological processes involved in both spontaneous degeneration and specific diseases of ageing. New understandings derived from neural transplantation may be expected to lead to rational development of novel strategies to inhibit the neurodegenerative process and to promote regeneration in the aged brain.

Tetrahydroaminoacridine inhibits high voltage spindle activity in aged rats after acute and chronic treatment

The present study evaluated the ability of tetrahydroaminoacridine (THA) to reverse the age-related increase in high voltage spindles (HVS). THA was injected either 15 or 90 min before EEG recordings were made. A THA dose of 3 mg/kg IP decreased the incidence of HVS, but was ineffective at doses of 0.03 and 1 mg/kg. The HVS suppressing effect of THA (3 mg/kg) declined during a 10-day treatment period. After 10 days chronic THA treatment, a challenge dose of 6 mg/kg of THA reinstated HVS suppressing effect of THA. Our results suggests that (1) THA reverses the age-related deficit of thalamo-cortical activation (2) tolerance develops to THA-induced HVS suppression (3) anti-cholinesterase activity may be important for the efficacy of THA in decreasing HVS because pilocarpine, a muscarinic agonist, also decreased HVS.

Effect of R-(-)-alpha-methylhistamine and thioperamide on in vivo release of norepinephrine in the rat hippocampus

1. The modifications of hippocampal release of norepinephrine following the administration of R-(-)-alpha-methylhistamine and thioperamide, respectively agonist and antagonist of histamine H3 receptors, were assessed in freely moving rats by microdialysis. 2. Both the systemic (2 mg/kg i.p.) and local (100 microM via the probe) administration of thioperamide caused no modifications of basal release, indicating that the histaminergic system is not tonically involved in regulating the hippocampal noradrenergic activity. 3. R-(-)-alpha-methylhistamine (1 and 100 microM) produced a slight, short-lasting and dose-dependent reduction of norepinephrine release antagonized by local perfusion (100 microM) and prevented by systemic administration of thioperamide 2 mg/kg. 4. The results seem to indicate that the modulation of norepinephrine release through presynaptic H3-receptors in the rat hippocampus plays a minor role in the memory-enhancing effects of thioperamide.

The effects of a specific alpha(2)-adrenoceptor antagonist, atipamezole, on cognitive performance and brain neurochemistry in aged Fisher 344 rats

The present experiments investigated the effects of a specific and potent alpha(2)-adrenoceptor antagonist, atipamezole, on cognitive performance and neurochemistry in aged rats. Aged control Fisher 344 rats, which had lower activities of choline acetyltransferase in the frontal cortex, were impaired in the acquisition of the linear arm maze task both in terms of repetition errors and their behavioural activity (the speed of arm visits), and they needed longer time to complete this task as compared to adult control rats. Atipamezole treatment (0.3 mg/kg) facilitated the acquisition of this task in the aged rats as they committed fewer errors and completed the task more quickly than saline-treated aged control rats. A separate experiment indicated that atipamezole enhanced the turnover of noradrenaline both in the adult and aged rats, but this effect was more pronounced in the aged rats. Furthermore, atipamezole enhanced significantly the turnover of serotonin and dopamine only in the aged rats when analysed in the whole brain samples. As alpha(2)-adrenoceptor antagonists are known to alleviate akinesia in the experimental models of Parkinson's disease, the present results could be especially relevant for the development of palliative treatment for demented Parkinsonian patients.

The hyaluronan receptor RHAMM in noradrenergic fibers contributes to axon growth capacity of locus coeruleus neurons in an intraocular transplant model

The hyaluronan receptor for hyaluronic acid-mediated motility (RHAMM) plays a role in cell migration and motility in many systems. Recent observations on the involvement of RHAMM in neurite motility in vitro suggest that it might also be important in axon outgrowth in situ. This was addressed directly by investigating both RHAMM expression in the rat CNS and the ability of anti-RHAMM reagents to interfere with tissue growth and axon outgrowth in intraocular brainstem transplants. By western blotting, anti-RHAMM antibody detected a RHAMM isoform of 75,000 mol. wt in both whole brain homogenate and synaptosome preparations, and a 65,000 mol. wt isoform in synaptosomes. Immunofluorescence of adult brain sections revealed RHAMM-like immunoreactivity in varicose fibers that were also positive for the noradrenergic marker dopamine-beta-hydroxylase. Not all noradrenergic fibers contained RHAMM, nor was RHAMM detected in other monoaminergic fiber types. Lesions of noradrenergic fiber systems with beta-halobenzylamine-N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) eliminated RHAMM-positive fibers, but noradrenergic axons that sprouted extensively after this treatment were strongly RHAMM-positive. To assess RHAMM's role in fiber outgrowth, fetal brainstem tissue containing noradrenergic neurons was grafted into the anterior chamber of the eye. Treatment of grafts with anti-RHAMM antibody caused significant inhibition of tissue growth and axon outgrowth, as did a peptide corresponding to a hyaluronan binding domain of RHAMM. These agents had no such effects on transplants containing serotonergic and dopaminergic neurons. These results suggest that RHAMM, an extracellular matrix receptor previously shown to contribute to migratory and contact behavior of cells, may also be important in the growth and/or regenerative capacity of central noradrenergic fibers originating from the locus coeruleus.

Effects of N-propargyl-1-(R)aminoindan (rasagiline) in models of motor and cognition disorders

N-propargyl-1-(R)aminoindan (rasagiline) is a new and selective irreversible MAO-B inhibitor, currently being considered as the mesylate salt for potential therapy in certain neurological disorders. It has been studied in animal models of cognition and motor dysfunction. Its ability to restore normal motor activity was determined in models of acute drug-induced dopaminergic dysfunction: Its effects in improving cognition and memory deficits was studied in adult and senescent rats that had been exposed to prolonged hypoxia, then subjected to the passive and active avoidance tests. In alpha-methyl-p-tyrosine (alpha-MpT)-induced hypokinesia (100-120 mg/kg, i.p.) pretreatment with rasagiline at 2.5 mg/kg i.p. restored motor activity to control level. But pretreatment with reserpine abolished the protective effect of rasagiline. Rasagiline at 0.5 mg/kg/day was protective against alpha-MpT also in hypoxia-lesioned rats. In haloperidol-induced catalepsy in rats (1.5 mg/kg, s.c.) or mice (4-6 mg/kg s.c.), rasagiline improved recovery of normal locomotion, gait and coordination at 0.4-2.4 mg/kg i.p. and 1.8-1.5 mg/kg i.p., respectively. In amphetamine-induced stereotypy (0.6 mg/kg s.c., rasagiline potentiated this effect at 1.5 mg/kg i.p. In hypoxia-induced impairment of memory and learning, rasagiline at 0.32-0.5 mg/kg/day per os improved performance of adult rats in passive and active avoidance, and of senescent rats in active avoidance. Selegiline was either ineffective or less effective at equivalent doses. Racemic N-propargyl-1-aminoindan (AGN-1135), besides being of lower potency, had a different dose-dependency than rasagiline in antagonizing haloperidol-induced catalepsy or alpha-MpT-induced hypokinesia. 1-(R)aminoindan ((R)AI), a metabolite of rasagiline, in relatively high doses produced effects that were distinct in certain respects from those of rasagiline.

Collateral sprouting of central noradrenergic neurons during aging: histochemical and neurochemical studies in intraocular triple transplants

The sprouting capacity of aged noradrenergic neurons of the brain-stem nucleus locus coeruleus (LC) was examined using intraocular transplants of fetal tissues. Fetal hippocampal tissue (E18) and LC tissue (E15) were transplanted together as a double transplant into the anterior chamber of the eye of young adult Fischer 344 rats. The double transplants were allowed to mature for 14-18 months, after which an additional fetal hippocampal transplant was placed next to the LC graft. The triple transplants were monitored for overall growth and vascularization for an additional 2-6 months. Immunohistochemical examinations showed that both young (2-6 months old) and aged (16-24 months old) hippocampal cografts contained a plexus of thin varicose tyrosine hydroxylase (TH)-immunoreactive fibers extending throughout the grafted hippocampal tissues. However, the aged hippocampal grafts contained a denser uniform plexus of TH-positive fibers compared to the young transplants. Immunohistochemistry with synapsin antibodies demonstrated that both the young and the aged hippocampal transplants contained much higher densities of synaptic elements than the LC grafts. In vivo electrochemical measurements of potassium-evoked overflow of norepinephrine (NE) in the grafts showed that similar amounts of NE overflow were detected in both the young and the aged hippocampal grafts. HPLC-EC measurements of NE levels in the grafts revealed that there were similar amounts of NE in the young and the aged grafts, and the grafts did not contain serotonin or dopamine. In summary, the findings of the present study show that aged LC neurons are capable of undergoing collateral sprouting producing a functional NE neuronal system when introduced to an appropriate young target.

Effects of RU 52583, an alpha 2-antagonist, on memory in rats with excitotoxic damage to the septal area

The anti-amnesic action of RU 52583, an alpha 2-adrenergic receptor antagonist, was evaluated through performance of spatial tasks in a radial maze by rats with N-methyl-D-aspartic acid (NMDA) lesion of the medial septal (MS) nuclei. Memory performance of lesioned or sham-operated rats was evaluated by measuring reference memory as long-term maintenance of an acquired performance and working memory or memory for recent events. The lesion: a produced significant impairments of the animals' memory performance, b) significantly reduced the sodium-dependent high-affinity choline uptake in the hippocampal formation, and c) deeply disrupted cholinergic hippocampal theta waves. Oral administration of RU 52583 at 1 and 2 mg/kg (tested doses: 1-5 mg/kg) prior to performance of the task markedly reduced memory impairments, whereas idazoxan, another alpha 2-adrenergic receptor antagonist, had no effect at tested doses (2-5 mg/kg). Cholinergic drugs--arecoline at 0.1 and 1 mg/kg (tested doses: 0.05-1 mg/kg) and physostigmine at 0.02 and 0.1 mg/kg (tested doses: 1, 2, and 5 mg/kg)-administered intraperitoneally showed a tendency to alleviate memory deficits. The present results show that the alpha 2-adrenergic antagonist RU 52583 possesses cognition-enhancing properties in rats with damage to the septohippocampal system.

The role of brain noradrenaline in cortical activation and behavior: a study of lesions of the locus coeruleus, medial thalamus and hippocampus-neocortex and of muscarinic blockade in the rat

Local injection of 6-hydroxydopamine in the locus coeruleus resulted in a 90% depletion of noradrenaline (NA) in the cerebral cortex as assessed by high-pressure liquid chromatography. This NA depletion had no effect on scopolamine-resistant hippocampal rhythmical slow activity and only an occasional effect on scopolamine-resistant neocortical low voltage fast activity. However, NE depletion resulted in a slight deficit in a behavioral swim-to-platform test and increased the deficit produced on the test by systemic treatment with scopolamine. Large surgical lesions of the medial thalamus or hippocampal formation plus posterior neocortex greatly increased the behavioral deficit produced by scopolamine. It is concluded that ascending noradrenergic projections play only a modest and possibly indirect role in the control of electrocortical activation and that a number of different brain lesions increase the behavioral impairment produced by central muscarinic blockade.

Influence of the alpha 2 noradrenergic antagonist piperoxane on longevity in the Fischer-344 rat: a preliminary report

Piperoxane is an alpha 2-noradrenergic antagonist with demonstrated excitatory effects on neurons in the locus coeruleus, causing a corresponding increase in norepinephrine in many forebrain areas. 16 male Fischer-344 rats approximately 16 months of age were injected with 3 mg/kg of piperoxane or .09% saline. The piperoxane-treated rats lived an average of 127.1 days longer than the saline-treated rats. The results are discussed in terms of the effects of strategies designed to enhance brain levels of catecholamine and their effect on the aging process. A discussion of further research is also presented.

Changes in brain somatostatin in memory-deficient rats: comparison with cholinergic markers

To clarify the functional role of the brain somatostatinergic system in cognitive processes, changes in the performance in passive avoidance and water maze tasks and in brain somatostatin contents were comparatively investigated in young Fischer rats subjected to brain cholinergic and somatostatinergic depletion, and in aged Fischer rats. Lesioning of the nucleus basalis magnocellularis and administration of cysteamine (200 mg/kg, s.c.), a depletor of somatostatin, resulted in significant deficits in passive avoidance, but complete transection of the fimbria-fornix hardly affected the performance in the task. When cognitive performance was assessed in the Morris water maze, lesions of the nucleus basalis magnocellularis and the fimbria-fornix, and administration of cysteamine, significantly impaired the acquisition of navigatory spatial memories of rats. On the other hand, aged rats (24-27 months) showed severe impairments of memory acquisition in both tasks. Neurochemistry measurements showed that lesions of the nucleus basalis magnocellularis produced a selective reduction both in the cortical cholinergic marker choline acetyltransferase and in striatal somatostatin level, whereas lesioning of the fimbria-fornix caused a marked loss of choline acetyltransferase in the hippocampus and posterior cortex, and a significant reduction in hippocampal somatostatin. On the other hand, treatment with cysteamine significantly reduced the contents of somatostatin in all the brain regions examined, but minimally affected choline acetyltransferase activity. However, significant reduction in the striatal choline acetyltransferase activity and elevation in somatostatin content in the frontal cortex were found in aged rats compared with young rats. Taken together, these results strongly suggest that changes in the brain somatostatinergic transmission are involved in the cognitive deficits in the experimental animal models of dementia presently employed. Furthermore, the present comparative study further implies that there are differences in the relative involvement of the cholinergic and somatostatinergic systems in the performance of rats on two different tests of mnemonic function.

Action on noradrenergic transmission of an anticholinesterase: 9-amino-1,2,3,4-tetrahydroacridine

The mechanism by which 9-amino-1,2,3,4-tetrahydroacridine (THA) inhibits beta-adrenoceptor linked cyclic AMP formation and its possible relationship with the cholinergic system were studied. In addition, the effect of THA on alpha 1-adrenoceptor coupled transduction systems was also investigated. THA was not able to influence the concentration-response curve for forskolin indicating that it is not acting on the catalytic subunit of the adenylate cyclase complex. On the other hand a cholinergic component seems to participate in the action of THA on beta-adrenoceptor stimulated adenylate cyclase activity since the blockade of muscarinic receptors with atropine (10 microM) partially prevented the reduction in cyclic AMP formation attained by THA in the hippocampus, in isoprenaline-stimulated conditions. This effect is not reproducible by another potent anticholinesterase physostigmine. Moreover, THA at concentrations up to micromolar did not affect alpha 1-adrenoceptor stimulated cyclic AMP formation or phosphoinositide hydrolysis. In conclusion, the neuropharmacological profile of THA is not to be restricted to the cholinergic system and its effectiveness in improving age-associated cognitive deterioration may involve an action on the beta-adrenoceptor coupled signal transduction system. Moreover, the action of THA on the beta-adrenergic and cholinergic systems in the brain could be relevant to the amelioration of cognitive deterioration and could lead to the development of new therapeutic strategies.

Paeoniflorin attenuates learning impairment of aged rats in operant brightness discrimination task

The effects of paeoniflorin isolated from peony were examined on an aging-induced learning deficit in an operant brightness discrimination task in Fischer 344 rats. Learning in aged (25 months) rats was significantly impaired compared with young (5 months) rats. Daily administration of paeoniflorin (0.01 mg/kg, PO) significantly attenuated the learning impairment in aged rats, whereas it did not affect the learning in young rats. Although tacrine (0.3 and 1 mg/kg, IP), a cholinesterase inhibitor, also did not affect the learning in young rats, it slightly augmented the aging-induced learning deficit in the present task. These data indicate the therapeutic potential of paeoniflorin in the treatment of senile dementia and aging-induced cognitive dysfunction.

Monoamine oxidase inhibitors, cognitive functions and neurodegenerative diseases

Recent data obtained in animals and in humans suggest that both MAO-A and MAO-B inhibitors present cognitive enhancing properties of possible interest in the treatment of cognitive disorders. In addition, the rational for using selegiline as a neuroprotector in Parkinson's disease may also be applicable in Alzheimer's disease in which a dramatic increase in the MAO-B activity has been reported. It seems then worthwhile to investigate the neuroprotective effect of MAOIs in humans and to assess, furthermore, the real therapeutical benefit of their cognitive enhancing properties.

Chronic administration of flumazenil increases life span and protects rats from age-related loss of cognitive functions: a benzodiazepine/GABAergic hypothesis of brain aging

Under barrier condition and with ad lib access to food and water, 20 Fischer-344 rats were chronically treated for 10 months with the benzodiazepine (BDZ) antagonist, flumazenil (FL; 4 mg/kg/day in drinking water acidified to pH = 3.0), beginning at the age of 13 months, while the group of 20 control age-matched rats received plain acidified water. The life span of the first 8 deceased rats treated with FL was significantly longer than that of the first 8 deceased rats in the age-matched control group. In tests for spontaneous ambulation and exploratory behavior in the Holeboard apparatus, conducted during the 3rd and the 8th month of treatment, the FL group, relative to controls, had significantly higher scores for the ambulation and exploratory behavior. In tests for unrewarded spontaneous alternation in the T maze, conducted at days 7, 39, 42, and 47 through 54 after drug withdrawal, i.e., at the age of 24-25 months, the FL-exposed group, compared to age-matched controls, showed a significantly higher percent of alternating choices, a behavior that was statistically comparable to that of the "young" 6-month-old controls. In the Radial Maze tests conducted 2 months after drug withdrawal, the FL group made significantly less "working memory" errors and "reference memory" errors, relative to the age-matched 25-month-old control group, a performance that was comparable to that of the young 7-month-old control group. In conclusion, chronic FL significantly protected rats from age-related loss of cognitive functions. It is postulated that the age-related alterations in brain function may be attributable to the negative metabolic/trophic influences of the "endogenous" benzodiazepine (BDZ) ligands and/or those ingested with food. A BDZ/GABAergic hypothesis of brain aging has been formulated which assumes that age-related and abnormally strong BDZ/GABAergic influences promote neurodegeneration by suppressing trophic functions of the aminergic and peptidergic neurons through opening of chloride channels in soma membrane and axon terminals, causing excessive hyperpolarizing and depolarizing inhibition, respectively. The review of human clinical and animal data indicates that FL has nootropic actions by enhancing vigilance cognitive and habituation processes.

Ginsenoside Rg1 inhibits the brain cAMP phosphodiesterase activity in young and aged rats

1. The in vitro effect of ginsenoside Rg1 from Panax ginseng on the low- and high-KM cyclic AMP phosphodiesterase (cAMP PDE) activity in the frontal cortex, striatum, hypothalamus and hippocampus of young (4-5-month old) and aged (22-month old) rats has been studied. 2. Administered in increasing concentrations (from 5 x 10(-5) M up to 5 x 10(-4) M), ginsenoside Rg1 exerted a pronounced inhibitory effect on the low- and high-KM enzyme activity in all brain structures studied in rats of both age groups. 3. Ginsenoside Rg1 exhibited inhibitory potency similar to that of theophylline. 4. The present results provide evidence for the CNS effects of ginsenoside Rg1 through inhibition of the intracellular level of cAMP.

Effects of chronic corticosterone ingestion on spatial memory performance and hippocampal serotonergic function

The effects of chronic ingestion of corticosterone (8 weeks via the drinking water, 400 micrograms/ml) on spatial memory performance and on monoamine levels in brain areas related to memory were investigated. Corticosterone treatment was associated with a long lasting (5 weeks post treatment) increase in 5-HT levels (44%) in the dentate gyrus of the hippocampus and decrease in 5-HT (50%) and NE (36%) levels in the frontal cortex. No effects were found in CA1, CA3 or in nucleus basalis. Performance of the rats on an 8-arm radial arm maze showed no overall effect of corticosterone treatment on trials to criterion or choice accuracy scores. However, three of the treated rats, who had consumed the most corticosterone during treatment, 12.5 +/- 0.3 mg/day, were impaired relative to all subjects. Thus, these results suggest that hippocampal serotonergic terminals show long lasting effects from corticosterone and may also be an early indicator of deleterious effects of glucocorticoids on hippocampal function. However, since only a small number of corticosterone-treated rats showed behavioral changes, future experiments are necessary to address the possibility that a higher level of corticosterone intake alters spatial memory as well as brain morphology and neurochemistry. Additional studies are also needed to determine whether such changes represent a threshold effect of the steroid or a dose-response function.

Post-train administration of 9-amino-1,2,3,4-tetrahydroacridine enhances passive avoidance retention and decreases beta-adrenoceptor-linked cyclic AMP formation in middle-aged rats

The possible involvement of beta-adrenoceptor system in the effectiveness of 9-amino-1,2,3,4-tetrahydroacridine (THA) to attenuate retention deficits exhibited by middle-aged rats in a one-trial passive avoidance task has been investigated. THA (2.5 mg.kg-1), injected i.p. after training, induced a significant increase in test step-through latency (STL) in middle-aged rats. Post-training injection of THA reduced basal and isoprenaline stimulated cyclic AMP accumulation in cortex and hippocampus of every group of rats. It is suggested that the effect of THA on memory processes may involve an action on beta-adrenoceptor-linked cyclic AMP accumulation.

Atipamezole, an alpha 2 antagonist, stabilizes age-related high-voltage spindle and passive avoidance defects

The present study investigates the effects of an alpha 2 antagonist, atipamezole (Ati), on the high-voltage spindle (HVS; Ati at 0.1, 1.0, and 3.0 mg/kg) activity, passive avoidance retention (PA; Ati at 3 mg/kg; injected before retention trial), and water maze (WM; Ati at 3 mg/kg; injected after daily training trials) acquisition of young and aged rats. PA retention trial performance defect of aged rats was partially alleviated by Ati at a 3-mg/kg dose. Ati at 3 mg/kg had no effect on the PA performance of young rats. Retention trial performance of nonshocked young or aged rats was not altered by a 3-mg/kg Ati dose. WM acquisition was not affected by posttraining Ati injections. Age-related increase of HVS was stabilized by Ati at 1 or 3 mg/kg. Ati at 1 and 3 mg/kg completely suppressed HVS of young rats. Ati at 0.1 mg/kg had no effect on HVS of young or aged rats. The results suggest that alpha 2-antagonist-administration-induced increase in noradrenergic activity may stabilize age-related HVS activity increase and PA performance defect.

Effects of noradrenergic DSP4 lesion on the effectiveness of pilocarpine in reversing scopolamine-induced amnesia

We studied the effectiveness of pilocarpine in reversing the scopolamine-induced water maze learning deficit (increase in escape latencies, decrease in spatial bias) in control and DSP4- (a noradrenergic neurotoxin) lesioned rats. The water maze acquisition deficit (escape latency, first spatial bias) induced by scopolamine 0.8 mg/kg was augmented by DSP4 treatment. The water maze performance deficit induced by scopolamine was reversed by pilocarpine 4 mg/kg in both DSP4-lesioned and control rats. A smaller dose of pilocarpine (1 mg/kg) did not reverse scopolamine-induced acquisition deficit in either control or DSP4-lesioned rats. Analysis of the second spatial bias test measured 2 weeks after training revealed that pilocarpine 4 mg/kg reversed scopolamine-induced retention deficit in control and DSP4-lesioned rats. Pilocarpine 1 mg/kg reversed scopolamine-induced retention performance deficit during the second spatial bias test in control but not in DSP4-lesioned rats. The present results suggest that 1) noradrenergic and cholinergic systems may interact in the regulation of spatial acquisition and retention and 2) the effectiveness of cholinergic drugs in reversing scopolamine-induced spatial retention deficit may be affected by noradrenergic lesioning.

Reduced density of NMDA receptors and increased sensitivity to dizocilpine-induced learning impairment in aged rats

About 20 min prior to training in a shock-motivated 14-unit T-maze, young (3-4 months) and aged (24-25 months) male Fischer-344 rats were given s.c. injections of either saline or dizocilpine (MK-801, 0.02 or 0.04 mg/kg), a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor. The aged rats showed a dose-dependent impairment in maze performance. Deficiencies were manifested as increases in errors, in runtime from start to goal, and in the number and duration of shocks received. In contrast, young rats exhibited no detrimental effects of dizocilpine on maze performance. Analysis of [3H]glutamate binding in these rats revealed a marked age-related decline in NMDA receptor binding in hippocampus. A significant correlation was observed between errors in the maze and hippocampal [3H]-glutamate binding, but the correlation was positive, i.e., rats that made the most errors had the highest level of NMDA receptor binding. Thus, compared to young rats, aged rats were more sensitive to the behavioral effects of NMDA receptor antagonism and they showed a hippocampal loss of [3H]glutamate in binding, which may be related to the increased sensitivity to dizocilpine. The positive correlation between poor maze performance and NMDA receptor binding suggests that the behaviors assessed involve complex interactions between NMDA receptors and other neuronal systems in the hippocampus.

DSP-4, a noradrenergic neurotoxin, produces more severe biochemical and functional deficits in aged than young rats

The present study examines the effects of noradrenergic lesions (either DSP-4 i.p. or 6-hydroxydopamine (6-OHDA) into the dorsal noradrenergic bundle on biochemical (noradrenaline (NA), dopamine (DA), serotonin (5-HT) and choline acetyltransferase (ChAT) activity) and cortical EEG (quantitative EEG (qEEG) and high-voltage spindle (HVS)) activity in young and aged rats. Near complete 6-OHDA NA lesions, but not partial DSP-4 NA lesions, increased HVS activity in young rats. DSP-4 and 6-OHDA lesions produced no significant changes in the 5-HT or DA levels or in the ChAT activity in young rats. In some of the aged rats, DSP-4 produced similar biochemical and HVS effects, as it induced in young rats. In the remainder of the aged rats, NA levels were greatly and 5-HT levels slightly decreased. DA levels and ChAT activity were unaltered in either set of aged rats. HVS activity was increased only in that group of aged rats with the greatly lowered NA content. These results suggest that: (1) some of the aged rats are more sensitive to DSP-4 treatment than young adult rats; and (2) NA depletions have to be complete to produce an increase in HVS activity in young and aged rats.

Conditional learning in aged rats: evidence of hippocampal and prefrontal cortex impairment

Groups of normal old rats and young adult rats were administered a test of conditional discrimination learning in which different visual stimuli were associated with responses to different levers. Initially, rats were tested in a zero-delay condition in which they selected their responses in the presence of the conditional stimuli. They were later tested at 5- and 15-s delays between stimulus presentation and the appearance of the levers. Old rats were impaired in learning the basic conditional discrimination, a test thought to be sensitive to frontal lobe dysfunction. Age differences increased with the length of the interval, revealing a time-dependent memory loss that was attributed to impaired hippocampal function.

Divergence of biological and chronological aging: evidence from rodent studies

Literature on aging populations of rodents supports the intuitive view that significant functional variation exists among like-aged, elderly individuals: chronological age as a solitary measure is a poor indicator of biological age. In this report, we review a variety of studies which classify aged rodents based on genetic and/or behavioral similarities, in addition to chronological age, and have provided valuable neurobiological and physiological information on age-related changes which accompany functional impairments, or the lack of them. Beyond their descriptive value for gerontological research, these findings suggest ways in which biological aging can be manipulated to promote good function in aged individuals.

Effects of alpha 2-drugs and pilocarpine on the high-voltage spindle activity of young and aged control and DSP4-lesioned rats

The present study investigates the effects of alpha 2-drugs and pilocarpine on the neocortical high-voltage spindle (HVS) activity in young and aged control and DSP4-lesioned rats. DSP4 partially decreased cortical and thalamic noradrenaline levels, but had no effect on HVS activity. The alpha 2-adrenoceptor agonist guanfacine (0.004, 0.02, 0.1 mg/kg) increased HVS activity in young and aged control and DSP4-lesioned rats. Guanfacine produced a significantly smaller increase in HVS activity in aged rats. A combination of pilocarpine (3 mg/kg), a muscarinic agonist, and atipamezole (1 mg/kg), an alpha 2-adrenoceptor antagonist, suppressed HVS activity more effectively than either of the drugs alone in young or aged control and DSP4-lesioned rats. The present results demonstrate that 1) the alpha 2-adrenoceptor antagonist and muscarinic agonist interact in suppressing HVSs in noradrenergically lesioned young and aged rats; 2) alpha 2-adrenoceptor agonists produce a greater increase in HVS activity in young than aged rats; and 3) partial noradrenergic lesions do not affect the HVS-modulating effects of alpha 2-adrenoceptor active drugs in young or aged rats.

Tetrahydroaminoacridine improves passive avoidance retention defects induced by aging and medial septal lesion but not by fimbria-fornix lesion

The present study examines whether tetrahydroaminoacridine (THA) can improve the deterioration in passive avoidance (PA) retention performance induced by medial septal (MS) and fimbria-fornix (FF) lesions in young rats or by aging. Retention of young MS-lesioned rats was improved by pretraining injection of THA at 3 mg/kg, but not by THA at 1 mg/kg or by either of the posttraining doses of THA (1 and 3 mg/kg). Pretraining injections of THA at 1 or 3 mg/kg had no effect on the PA retention performance of FF-lesioned rats. Age-induced PA failure was alleviated by pretraining administration of THA at 1 and 3 mg/kg. Posttraining injections of THA (1 or 3 mg/kg) had no effect on PA retention performance of aged rats. These results demonstrate that 1) THA may improve hippocampal cholinergic denervation-induced functional deficits and 2) some of the age-related PA deficits may be due to a cholinergic deficit and can be reversed with THA.

Acute effects of tetrahydroaminoacridine on beta-adrenoceptor-linked cyclic AMP accumulation in brain of young and middle-aged rats

The effects of acute treatment with 1,2,3,4-tetrahydro-9-aminoacridine (THA), a 4-aminopyridine derivative clinically effective in Alzheimer's disease, on beta-adrenoceptor-linked cyclic AMP accumulation have been investigated in cortical and hippocampal structures of young and middle-aged rats. In a first series of experiments, pretreatment with 2.5 mg/kg THA decreased basal cyclic AMP accumulation. When a phosphodiesterase inhibitor was added to the preparation, THA again decreased cyclic AMP levels in young rats, but failed to significantly modify cyclic AMP accumulation in middle-aged animals. Finally, in isoprenaline-stimulated conditions, acute treatment with tacrine was able to diminish cyclic AMP accumulation in every group of rats. It is suggested that the neurochemical action of THA in mammalian brain is more complex than earlier has been anticipated and may involve an action on beta-adrenoceptors.

Age-induced changes in single locus coeruleus brain transplants grown in oculo: an in vivo electrochemical study

Brain stem tissue from fetal Sprague-Dawley rats containing the nucleus locus coeruleus (LC) was transplanted into the anterior chamber of the eye of young adult host rats and was studied at 4-6 months (young control) or 24-28 months after grafting (old). High-speed in vivo electrochemical measurements were used to characterize the potassium-evoked synaptic overflow of norepinephrine (NE) in both young and aged LC brain grafts. The amplitudes of potassium-evoked NE overflow were attenuated in the aged grafts as compared to the young LC grafts. In addition, the rise times of potassium-evoked responses were longer in the old LC grafts than in the young transplants. In contrast, the NE content of aged LC grafts, as determined by high-performance liquid chromatography coupled with electrochemical detection (HPLC-EC), was only slightly diminished and not significantly different from the NE levels seen in young LC grafts. However, light microscopical evaluation using tyrosine-hydroxylase immunocytochemistry revealed pyknotic cell bodies and fluorescent accumulations in aged locus coeruleus transplants which were indicative of degeneration in these grafts. The present data demonstrate a significant age-related decline in the presynaptic function of NE-containing neurons in intraocular locus coeruleus transplants of Sprague-Dawley rats.

The effects of atipamezole, an alpha-2 antagonist, on the performance of young and aged rats in the delayed nonmatching to position task

The present experiments were undertaken to study whether pharmacological activation of the noradrenergic system would improve age-related deficits in short-term memory. Thus, we investigated the effects the single dose administration (0.1, 0.3, 0.9 and 2.7 mg/kg, subcutaneously) or atipamezole, a specific alpha-2 adrenoceptor antagonist, had on the performance of young and aged rats in a delayed nonmatching to position task. After substantial training, aged rats made more errors at longer delays (4-30 seconds) than did young rats, although the percent correct responses at short delays (0-2 seconds) did not differ between young and aged rats. Atipamezole (0.1-0.9 mg/kg) did not improve the performance of young and aged rats in this task. Moreover, the highest dose (2.7 mg/kg) used increased the number of omissions and increased the latency to collect food pellets, indicating disruption of the performance of rats in this task. According to the present results, alpha-2 antagonist (administered peripherally at a single dose), which increases the release of noradrenaline, did not improve age-related deficit in short-term memory in rats.

The effects of guanfacine, alpha-2 agonist, on the performance of young and aged rats in spatial navigation task

The aim of the present experiment was to study the effects of a low dose (0.001 mg/kg) of guanfacine, alpha-2 agonist, on the acquisition and retention of a water maze task measuring spatial reference memory in young and aged rats. Aged rats were impaired in the acquisition of this task. Both young and aged rats treated with guanfacine had shorter escape latencies than their saline treated counterparts. However, guanfacine treatment increased the speed of swimming in aged rats. According to the results of the probe trial, guanfacine may slightly improve the acquisition/retention of water maze task in young rats, whereas it may slightly impair the acquisition/retention of aged rats. The results suggest that a low dose of guanfacine administered peripherally may have different effects on young and aged rats in water maze performance, and a low dose of guanfacine does not improve spatial reference memory in aged rats.

The role of interactions between the cholinergic system and other neuromodulatory systems in learning and memory

Extensive evidence indicates that disruption of cholinergic function is characteristic of aging and Alzheimer's disease (AD), and experimental manipulation of the cholinergic system in laboratory animals suggests age-related cholinergic dysfunction may play an important role in cognitive deterioration associated with aging and AD. Recent research, however, suggests that cholinergic dysfunction does not provide a complete account of age-related cognitive deficits and that age-related changes in cholinergic function typically occur within the context of changes in several other neuromodulatory systems. Evidence reviewed in this paper suggests that interactions between the cholinergic system and several of these neurotransmitters and neuromodulators--including norepinephrine, dopamine, serotonin, GABA, opioid peptides, galanin, substance P, and angiotensin II--may be important in learning and memory. Thus, it is important to consider not only the independent contributions of age-related changes in neuromodulatory systems to cognitive decline, but also the contribution of interactions between these systems to the learning and memory deficits associated with aging and AD.

Age-related changes of the effects of a group of nootropic drugs on the content of rat brain biogenic monoamines

1. The changes in the levels of brain biogenic monoamines (BMAs) after chronic (7 days) treatment with piracetam, aniracetam and structural analogues of aniracetam (p-H, p-F, p-Cl, p-P and m-D) were studied in young and old rats. 2. An age-related significant decrease in the BMA content was established in old rats. 3. Most of the investigated compounds increased the level of one or other BMA in one or other of the brain structures studied. This elevation was predominantly established in old rats. 4. The present results and those from previous behaviour studies show that elevation of one or more of the BMA levels in one or more brain regions plays a beneficial role in the realization of their effects on the processes of learning and memory.

Effects of cholinergic-rich neural grafts on radial maze performance of rats after excitotoxic lesions of the forebrain cholinergic projection system--I. Amelioration of cognitive deficits by transplants into cortex and hippocampus but not into basal forebrain

After ibotenate (10.0 mg/ml) lesions to the nucleus basalis and medial septal regions, at the source of the cortical and hippocampal branches of the forebrain cholinergic projection system, rats displayed long-lasting stable impairment in reference and working memory in both spatial (place) and associative (cue) radial maze tasks. Cell suspension transplants of cholinergic-rich fetal basal forebrain tissue dissected at embryonic day 15 substantially improved all aspects of radial maze performance to a comparable degree whether sited in cortex, hippocampus, or both regions of the host brain. No additive effects were obtained with grafts in both terminal regions, but total graft volume, assessed stereologically, showed a significant negative correlation with error scores. Rats with behaviourally effective grafts, like controls, were disrupted in the place task when tested in dim light which obscured extra-maze spatial cues. Lesioned rats were not affected by change in lighting. Grafts of cholinergic-poor fetal hippocampal tissue did not improve radial maze performance; neither did grafts of cholinergic-rich tissue placed within the host basal forebrain lesion sites. In rats with cholinergic-rich terminal grafts, cortical and hippocampal choline acetyltransferase activity was restored to control level, commensurate with site of transplant, whereas it was significantly reduced in lesioned animals and those with functionally ineffective grafts. The indiscriminate error pattern and insensitivity to changes in lighting shown by lesioned rats suggested that lesioning primarily disrupted attention rather than short- or long-term spatial or associative memory processes. Since rats with cholinergic-rich grafts showed both reduced errors and recovery of stimulus control, the data indicated that grafts affected information processing, rather than changes in motor or motivational processes. Changes in choline acetyltransferase activity and the behavioural efficacy of cholinergic-rich grafts are consistent with the involvement of acetylcholine in the behavioural deficits and recovery displayed by lesioned and grafted groups, but do not rule out contributions from other factors. The equipotency of grafts within each terminal region suggests also that there may be a considerable degree of functional cooperation between the two branches of the forebrain cholinergic projection system. Functional recovery may involve local, nonspecific synaptic or paracrine mechanisms within the target regions, since grafts were efficacious only when placed in the terminal areas, but not when sited homotopically in the basal forebrain, indicating that they did not achieve any functionally significant structural repair to the host brain at that site.

Spatial memory deficits in aged rats: contributions of monoaminergic systems

Age-dependent changes in monoaminergic systems and their relationship to senescent memory decline were investigated in 4- and 25-26-month-old, female, Fischer 344 rats. Spatial memory performance was tested on an 8-arm radial maze, and levels of norepinephrine (NE), dopamine (DA) and metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid, serotonin (5-HT) and metabolite 5-hydroxyindoleacetic acid were measured in brain areas which contribute to memory function--basal forebrain cholinergic nuclei, subfields of the hippocampus, frontal and entorhinal cortex--and in monoaminergic cell body areas. The performance of aged subjects was significantly impaired as compared to young subjects, and alterations of 20-60% in monoamine and metabolite levels were measured in specific brain areas of aged rats. Decreased NE levels were found in basal forebrain nuclei and cortical areas but not in hippocampal subfields of aged rats. Changes in the 5-HT system were present in hippocampal, cortical and basal forebrain sites. Changes in the DA system were the most pervasive with aged rats showing decreased DA and/or metabolites in several basal forebrain nuclei, cortical areas, and the hippocampus. Aged rats showed 50% decreases of monoamines in locus coeruleus and substantia nigra and 30% decreases in the dorsal raphe nucleus. Some but not all of the changes correlated with memory performance. The present results in rats support evidence that age-dependent changes in monoaminergic function in discrete brain sites contribute to senescent memory decline and suggest that monoaminergic-cholinergic interactions within basal forebrain nuclei may be important in this decline.

Memory following cholinergic (NBM) and noradrenergic (DNAB) lesions made singly or in combination: potentiation of disruption by scopolamine

Groups of rats were trained on either delayed matching or nonmatching to position tasks, then divided into four subgroups and given the following bilateral lesions: (a) SHAM [vehicle injection into the nucleus basalis magnocellularis (NBM) and dorsal noradrenergic bundle (DNAB)], (b) DNAB (6-hydroxydopamine lesion of the DNAB, vehicle into the NBM), (c) NBM (quisqualic acid lesion of the NBM, vehicle into the DNAB) and (d) DUAL (neurotoxin lesions of both DNAB and NBM). Following postoperative recovery, the DUAL lesion subjects were slightly impaired, but by the seventh day of testing all groups were performing at similar levels. This strongly suggests that quisqualate lesions of the NBM are not sufficient to produce severe and lasting mnemonic disorders resembling those seen in Alzheimer's disease (AD). These data also indicate that the noradrenergic system may not be of critical importance with respect to cognition. It was reasoned that an additional anticholinergic treatment might exacerbate an underlying deficiency. All groups were injected, peripherally, with the cholinergic antagonist scopolamine (0-0.5 mg/kg). This drug dose-dependently disrupted performance in all groups. Moreover, the highest dose had a marked effect in the DUAL group, impairing performance even when no mnemonic burden was present (at zero delay). The results suggest that cholinergic NBM and noradrenergic DNAB lesions produce only transient mnemonic deficiencies. A combination of the two can be disruptive, but longer term task (or reference) memory is the primary process affected, and only under certain conditions. The implication of these findings to research concerning animal models relating to Alzheimer's disease is discussed.

The correlation of passive avoidance deficit in aged rat with the loss of nucleus basalis choline acetyltransferase-positive neurons

The present study investigates the effects of ageing on the number of choline acetyltransferase (ChAT)-positive neurons in the nucleus basalis (NB) and the correlation between the number of ChAT-positive neurons and passive avoidance (PA) retention in young (3-month-old) and aged (26-month-old) rats. The results indicate that the number of ChAT-positive neurons is decreased in aged rats and that the degree of loss of NB neurons is related to the degree of PA retention deficit in aged rats.