The neurochemistry of Alzheimer's disease and senile dementia

Functional consequences of brain exposure to saturated fatty acids: From energy metabolism and insulin resistance to neuronal damage

INTRODUCTION

Saturated fatty acids (FAs) are the main component of high-fat diets (HFDs), and high consumption has been associated with the development of insulin resistance, endoplasmic reticulum stress and mitochondrial dysfunction in neuronal cells. In particular, the reduction in neuronal insulin signaling seems to underlie the development of cognitive impairments and has been considered a risk factor for Alzheimer's disease (AD).

METHODS

This review summarized and critically analyzed the research that has impacted the field of saturated FA metabolism in neurons.

RESULTS

We reviewed the mechanisms for free FA transport from the systemic circulation to the brain and how they impact neuronal metabolism. Finally, we focused on the molecular and the physiopathological consequences of brain exposure to the most abundant FA in the HFD, palmitic acid (PA).

CONCLUSION

Understanding the mechanisms that lead to metabolic alterations in neurons induced by saturated FAs could help to develop several strategies for the prevention and treatment of cognitive impairment associated with insulin resistance, metabolic syndrome, or type II diabetes.

The therapeutic importance of understanding mechanisms of neuronal cell death in neurodegenerative disease

Despite major advances in our understanding of the initiating factors that trigger many neurodegenerative disorders, to date, no novel disease-modifying therapies have been shown to provide significant benefit for patients who suffer from these devastating disorders. As most neurodegenerative disorders are late-onset, slowly progressive, and appear to have long relatively asymptomatic prodromal phases, it is possible that therapies optimally targeting the triggers of these disorders may have limited benefit when treatment is initiated in the symptomatic patient. Such therapies may work in the prodromal phase, or when given prophylactically, but in the symptomatic patient there simply may be too much damage to the neuronal networks to restore functionality by reducing or even eliminating the primary stressor. As functional neuronal demise and overt neuronal death are almost certainly the key factors that mediate the functional impairment, it is clear that preventing neuronal death and dysfunction will have a huge clinical benefit. Unfortunately, we lack a detailed understanding of neuronal death pathways in almost all neurodegenerative disorders. To rationally develop new disease modifying therapies that target steps in the degenerative cascade downstream of the disease trigger will require a number of factors. First, we need to refocus our basic research efforts on identifying the precise steps in the pathological cascade that lead to neuronal death in each neurodegenerative disease and, if possible, determine the relative placement of those events within a potentially very complex cascade. Second, we will need to determine which of these steps are potentially targetable. Finally, we will need to develop novel therapies that interfere with these steps and demonstrate that such therapies alone, or in combination with therapies that target the trigger of these devastating diseases, have clinical benefit.

KTX 0101: a potential metabolic approach to cytoprotection in major surgery and neurological disorders

KTX 0101 is the sodium salt of the physiological ketone, D-beta-hydroxybutyrate (betaOHB). This neuroprotectant, which has recently successfully completed clinical Phase IA evaluation, is being developed as an intravenous infusion fluid to prevent the cognitive deficits caused by ischemic foci in the brain during cardiopulmonary bypass (CPB) surgery. KTX 0101 maintains cellular viability under conditions of physiological stress by acting as a "superfuel" for efficient ATP production in the brain and peripheral tissues. Unlike glucose, this ketone does not require phosphorylation before entering the TCA cycle, thereby sparing vital ATP stores. Although no reliable models of CPB-induced ischemia exist, KTX 0101 is powerfully cytoprotectant under the more severe ischemic conditions of global and focal cerebral ischemia, cardiac ischemia and lung hemorrhage. Neuroprotection has been demonstrated by reductions in infarct volume, edema, markers of apoptosis and functional impairment. One significant difference between KTX 0101 and other potential neuroprotectants in development is that betaOHB is a component of human metabolic physiology which exploits the body's own neuroprotective mechanisms. KTX 0101 also protects hippocampal organotypic cultures against early and delayed cell death in an in vitro model of status epilepticus, indicating that acute KTX 0101 intervention in this condition could help prevent the development of epileptiform foci, a key mechanism in the etiology of intractable epilepsy. In models of chronic neurodegenerative disorders, KTX 0101 protects neurons against damage caused by dopaminergic neurotoxins and by the fragment of beta-amyloid, Abeta(1-42), implying possible therapeutic applications for ketogenic strategies in treating Parkinson's and Alzheimer's diseases. Major obstacles to the use of KTX 0101 for long term therapy in chronic disorders, e.g., Parkinson's and Alzheimer's diseases, are the sodium loading problem and the need to administer it in relatively large amounts because of its rapid mitochondrial metabolism. These issues are being addressed by designing and synthesizing orally bioavailable multimers of betaOHB with improved pharmacokinetics.

Drosophila choline acetyltransferase temperature-sensitive mutants

We used the reverse transcription-polymerase chain reaction (RT-PCR) to amplify choline acetyltransferase (ChAT) mRNA fragments from two temperature-sensitive alleles of Drosophila melanogaster, Cha(ts1) and Cha(ts2). Single base substitutions in the mutants (T1614A in Cha(ts1) and G1596A in Cha(ts2)) would result in amino acid changes for ChAT protein (Met403Lys in Ch(ts1) and Arg397His in Cha(ts2)). These base substitutions were confirmed in mRNA extracted from homozygous mutants using a Single Nucleotide Primer Extension assay (SNuPE) and are sufficient to produce thermolabile enzyme. Our results indicate that these temperature-sensitive mutants are point mutations in the structural gene for ChAT. Using a quantitative SNuPE assay we also show that similar levels of Cha(ts) and wild type transcripts are present in heterozygous flies (Cha(ts1)/+ and Cha(Ts2)/+) at both restrictive and permissive temperatures. This contrasts with RNase protection assays of ChAT mRNA in homozygous mutant animals where the levels of mutant mRNA decrease at restrictive temperature.

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.

The use of psychotropic medication in the management of problem behaviors in the patient with Alzheimer's disease

Although a variety of potential alternatives to the neuroleptics in the management of agitated behavior in Alzheimer's disease exist, more large, well-controlled studies are needed. At present, neuroleptics remain the standard treatment for this problem. It is also important for the clinician to investigate thoroughly underlying instigators of behavior change in Alzheimer's patients, such as delirium, anxiety, depression, or environmental factors, and target treatment to these disorders.

Differential effects of physostigmine and pilocarpine on the spatial memory deficits produced by two septo-hippocampal deafferentations in rats

Rats that had received two kinds of septo-hippocampal deafferentations, medial septum (MS) lesion and fimbria-fornix (FF) transection, were assayed for brain cholineacetyltransferase (ChAT) activity and spatial memory in an 8-arm radial maze task. Both lesions produced profound and long-lasting spatial memory impairments, which were characterized by a reduction in the numbers of correct arm choices and first correct choices, a reduction in the percent of correct choices and an increase in the number of errors. The degree of memory impairment was severer in FF- than in MS-lesioned rats, and paralleled that of decreases in ChAT activity in the hippocampus. MS lesion reduced ChAT activity in the hippocampus by approximately 45%, while FF lesion almost completely depleted the activity. An intraperitoneal injection of physostigmine (0.0032-0.32 mg/kg), an acetylcholinesterase (AChE) inhibitor, significantly ameliorated the spatial memory deficit induced by MS lesion, but hardly affected that by FF lesion. In contrast, intraperitoneal doses (0.032-3.2 mg/kg) of pilocarpine, a muscarinic agonist, showed a significant improvement of both types of memory deficit with bell shaped dose-response curves. The drug was more potent in the FF- than in the MS-lesioned rats. These results suggest that the septo-hippocampal cholinergic system plays a crucial role in the maintenance of spatial memory, and that the degree of septo-hippocampal deafferentation affects the efficacy of cholinergic drugs.

Propranolol for the control of disruptive behavior in senile dementia

Six patients with senile dementia, exhibiting severe, disruptive behavior were effectively treated with propranolol in doses ranging from 80 mg per day to 560 mg per day. All six patients were given a trial of propranolol after conventional therapy had failed, and in all patients, the agitated behavior significantly improved. There were no adverse side effects requiring the discontinuation of the propranolol, and in all cases, the agitated behavior was controlled without inducing general sedation. Both the nursing home staff and the families were pleased with the therapeutic effects. Propranolol represents a possible alternative way of addressing the severe problem of agitated behavior in senile dementia patients.

Central cholinergic systems and the P3 evoked potential

A cognitive evoked potential, the P3, is commonly altered in dementia states but little is known of the specific neuronal generators that are the source of this potential. Event-related evoked potentials and neuropsychological testing were obtained in six normal subjects during neuropharmacological manipulation of the central cholinergic system. Scopolamine (an anticholinergic) impaired recent memory, prolonged P3 latency and decreased P3 amplitude. These abnormalities were partially reversed by physostigmine (an anticholinesterase). The results imply that the cholinergic system is involved in the generation of the P3 potential.

Central catecholamines, cognitive impairment, and affective state in elderly schizophrenics and controls

Central catecholamine concentrations were determined in autopsy samples from older schizophrenic and control subjects for both the hypothalamus and the nucleus accumbens. The results of these analyses and demographic variables were regressed on antemortem measures of cognitive function and mood state. In the hypothalamus, there are significant direct relationships of homovanillic acid (HVA) and 3-methoxy-4-hydroxyphenylglycol (MHPG) with depressed mood, as measured by an adaptation of the Hamilton Rating Scale for depression. In the nucleus accumbens, dopamine (DA) and MHPG had significant inverse relationships with antemortem cognitive function, as measured by an adaptation of the Mini Mental State Exam. Results in this sample indicate that after controlling for age, the catecholamine concentrations accounted for approximately 50% of the variance in the antemortem measures of mood or cognition, depending on the loci measured.

Specific toxic effects of ethylcholine nitrogen mustard on cholinergic neurons of the nucleus basalis of Meynert

The putative cholinergic neurotoxin, ethylcholine aziridinium ion (AF64A), was injected unilaterally into the nucleus basalis of Meynert (nbM) in order to determine whether it would produce specific damage to the cholinergic cell bodies of this nucleus. Injections of small amounts of AF64A (0.01 nmol in 1 microliter) or of its vehicle had little effect on the appearance of the nbM or on the levels of choline acetyltransferase (ChAT) in the cortex. Injections of larger amounts of AF64A (0.02 and 0.05 nmol in 1 microliter and 0.02 nmol in 10 microliters) produced a loss of diffuse acetylcholinesterase staining in the nbM and a loss of large positively staining neurons. Furthermore, these injections produced a significant reduction of ChAT activity in the central portion of the cortex. However, non-cholinergic neurons in the area of the nbM were not affected by these AF64A injections. In addition, cortical uptake of monoamines was not affected by these lesions. Further increases in the amount of AF64A injected (0.1 nmol in 1 microliter and 0.035 nmol in 10 microliters) caused damage at the site of the injection which was not limited to the cholinergic elements of the nbM. These results suggest that AF64A can be used to produce specific lesions of cholinergic neurons, and therefore may be useful in developing animal models of human disorders involving cholinergic hypofunction, such as senile dementia of the Alzheimer type. However, there is a narrow dose range for producing these specific effects.

Neural transplantation: a review of recent developments and potential applications to the aged brain

Mammalian neural transplantation has recently been recognized to be a valuable technique for studying normal development and regeneration in the central nervous system. In addition, the ability of grafted neurons to reinnervate damaged regions of the host brain and to ameliorate some neuroendocrine deficits, cognitive disorders and motoric dysfunctions in young adult rodents has suggested that transplantation therapy may be effective in treating human neurodegenerative diseases and neurotransmitter deficiencies related to aging. It is of particular interest that initial studies of neuron transplants in aged rodents indicate that cholinergic, dopaminergic and noradrenergic neurons all integrate to some extent with the aged brain, and that the product of this graft-host interaction is improved behavioral performance of aged subjects. The present paper critically reviews the present domain of neural transplantation, its application to studies on the properties of the aged mammalian brain and discusses the possible therapeutic use of transplants in ameliorating transmitter-specific abnormalities associated with Parkinson's disease and Alzheimer's disease.