Drug Development Research 569-76 (1985)

Is It Possible to Design Rational Treatments For the Symptoms of Alzheimer’s Disease? Peter Davies

Departments of Pathology and Neuroscience, Albert Einstein College of Medicine, Bronx, New York

ABSTRACT

Davies, P.: Is it possible to design rational treatments for the symptoms of Alzheimer’s disease? Drug Dev. Res. 5:69-76, 1985.

This paper reviews and discusses the evidence that the primary cause of the symptoms of Alzheimer’s disease is a deficiency of acetylcholine in cerebral cortex and hippocampus. Data on other neurotransmitters in this disease are briefly reviewed. Possible means of treating acetylcholine deficiences are discussed.

Key words: Alzheimer’s disease, neurotransmitters, acetylcholine, cholinesterase inhibitors, muscarinic agonists, neuropeptides

INTRODUCTION

All of the available evidence is consistent with the presence of a major deficiency in central cholinergic transmission in patients with Alzheimer’s disease, and it seems reasonable to propose that many of the symptoms experienced by the patient are the direct result of this deficit. This paper will briefly review the data on which this proposal is based, and then discuss some areas of research which might lead to the design of appropriate therapeutic agents. Although research directed towards discovering the etiology of Alzheimer’s disease must be given a very high priority, such studies are not relevant to the design of drugs to alleviate the symptoms of this disorder; thus, this important area will not be considered further.

WHAT IS WRONG WITH THE ALZHEIMER BRAIN?

The major reason for the current level of interest in Alzheimer’s Disease is that it is the single most common cause of dementia in the developed world. Patients who have been labeled “senile, ” “organic brain syndrome, ” “organic mental syndrome, ” or simply called “de-

Received final version June 12, 1984; accepted June 14, 1984.

Address reprint requests to Peter Davies, Departments of Pathology and Neuroscience, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461.

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mented” have been proven at least half of the time to have the classical pathology of Alzheimer’s disease, irrespective of their age at death [Katzman, 1976; Plum, 19791. This pathology is evidenced by the presence of significant numbers of neuritic plaques and neurofi- brillary tangles in regions of the cerebral cortex and hippocampus and appears to be the only pathologic finding in at least 50% of demented individuals [Terry and Davies, 19801. A rising tide of interest is understandable, given that there are about 3.6 million demented individuals in the United States or about 1.8 million victims of Alzheimer’s disease [Plum, 19791. Although an understanding of the nature of the two pathologic lesions, the plaque and the tangle, is undoubtably important, it is the transmitter neurochemistry that has raised the hope that effective treatments can be designed. In late 1976 and early 1977, 3 groups in Britain reported that there were dramatic deficiencies in the activity of the enzyme choline acetyltransferase (ChAT) in the brains of Alzheimer patients [Davies and Maloney, 1976; Bowen et al, 1976; Perry et al., 1977; White et al., 19771. Numerous studies published since that time have confirmed this deficit to be both consistent and dramatic [Davies, 1979a,b; Rossor et al., 1980b; Bowen et al., 1981a; Sims et al., 1981b; Reisine et al., 1978; Yates et al., 19801. This enzyme is responsible for the synthesis of acetylcholine, a neurotransmitter with an important role in memory and cognition (see below). What was surprising then and is still remarkable today is the lack of clear evidence for deficits in other neurotransmitter systems in the brain.

Every enzyme or function we know to be associated with neurons producing acetylcho- line (cholinergic neurons) has been shown to be deficient in the Alzheimer brain. This list includes ChAT activity, acetylcholinesterase activity, high affinity choline uptake, and basal and potassium-stimulated acetylcholine production and release [Sims et al., 1981al. The choline uptake and acetylcholine release have been shown to be deficient in biopsied samples of cerebral cortex from Alzheimer patients, and some of these samples were taken from patients who were apparently still in the early stages of the disease. Findings in both biopsied and autopsied brain tissues leave no doubt that a cholinergic deficit is a dramatic and consistent feature of Alzheimer’s disease. Recently, Whitehouse, Price, and colleagues [Whitehouse et al., 19821 have presented direct evidence for the loss of a population of cholinergic neurons that supply cholinergic innervation to the cerebral cortex and hippocampus, although it is not yet clear at what stage of the disease this cell loss occurs.

Important evidence on the specificity of the cholinergic deficit comes from studies of muscarinic acetylcholine receptors in the Alzheimer brain: These are present in normal concentrations [Davies and Verth, 1977; Perry et al., 1981a1, indicating two things: One, that the cells receiving the acetylcholine transmission are still present in apparently normal num- bers, and two, that a potential target for therapeutic agents is not destroyed by the disease. There is still no reasonably consistent evidence that any other neuronal population (defined by specific neurotransmitter content) is affected by Alzheimer’s disease. Over the last 10 years, there have been reports of deficits in gamma-aminobutyric acid (GABA), dopamine, noradren- alin, and serotonin systems, but these do not seem to have withstood the test of time very well. In several of the more recent studies including those on biopsy tissue, the GABA system appears to be very well preserved in the majority of Alzheimer patients. Potassium-stimulated GABA release from biopsy tissue is unaffected by the presence of Alzheimer’s disease; in the same study, the release of glutamate and aspartate was also normal [Sims et al., 1981a; Bowen, 19831. Possible effects on the dopamine system remain the subject of dispute but it is clear that the majority of Alzheimer patients have intact, functioning dopaminergic neurons [Parkes et al., 1974; Yates et al., 1979; Mann et al., 1981; Bowen et al., 1981b; Bowen, 19831. A minority of cases may also have clinical or subclinical Parkinson’s disease and a resultant dopamine deficiency. The failure of the earlier studies to separate these patients from the majority probably accounts for the small and inconsistent dopamine deficits reported [Gottfries et al., 1969, Gottfries and Roos, 1973: Gottfries and Winblad, 1979; Adolfsson et al., 19781. Similar arguments may apply to studies of noradrenalin and serotonin; the brain stem nuclei which are the site of the noradrenergic and serotonergic neurons also are affected by Parkin-

Treatments for Alzheimer’s Disease 71

son’s disease (and perhaps by other poorly defined conditions) and a failure to consider this point seems to account for much of the confusion in the literature [Cross et al., 1981; Mann et al., 1981; Perry et al., 1981b; Todinson et al., 19811.

In fact, the only other neurotransmitter deficiency reported consistently is in the concen- tration of somatostatin [Davies et al., 1980; Rossor et al., 1980a; Davies and Terry, 19811. This peptide in the hypothalamus serves to inhibit growth hormone secretion but it is also present in neurons of the cerebral cortex and hippocampus. Its function in these higher brain centers is not known but there is evidence for an interaction between somatostatin and acetylcholine [Malthe-Sorenssen et al., 1978; Davies and Terry, 1981). The details of this interaction are uncertain at this time; it seem to be established that somatostatin and acetylcho- line are contained in separate neuronal populations [McKinney et al., 19821. Possible behav- ioral consequences of a somatostatin deficiency cannot be predicted at this time; we will have to await the development and use of somatostatin antagonists to conduct the necessary experi- ments. Several of the other peptide neurotransmitters (some call these peptides neuromodula- tors because their physiologic actions are rather slower and longer-lasting than that of the classical neurotransmitters [Hokfelt et al., 1980; Snyder, 19801) have been examined in the Alzheimer brain. Vasoactive intestinal peptide [Rossor et al., 1980b; Perry et al., 1981~1, cholecystokinin octapeptide [Rossor et al., 1981; Perry et al., 1981~1, neurotensin [Biggins et al., 19831, thyrotropin-releasing hormone [Biggins et al., 19831, and vasopressin [Rossor et al., 1980~1 are all present in quite normal concentrations, suggesting that the populations of neurons which use these peptides are not affected by Alzheimer’s disease.

All things considered, it seems quite clear why the so-called cholinergic hypothesis of Alzheimer’s disease is so prominent at this time. However, important support for the neuro- chemistry comes from pharmacologic studies in both animals and man.

NEGATIVE PHARMACOLOGY

Negative experiments clearly implicate a role for acetylcholine in memory and cognitive function. That is, it has been apparent for some years that drugs which interfere with cholinergic function impair memory performance and other aspects of intellectual activity [Brimblecombe, 1974; Drachman and Leavitt, 1972; Drachman and Suhakian, 1980; Davis and Berger, 19791. Drugs with this kind of effect include atropine and scopolamine, which are classical muscarinic receptor blockers. A note of caution here: We should not expect to closely model Alzheimer’s disease with anticholinergic agents. This is in part because there are, crudely speaking, three cholinergic systems in the brain: 1. The motor neurons: This system includes both the large anterior horn cells and the neurons of the cranial nerve nuclei; 2. the cholinergic interneurons of the caudate and putamen; 3. the cholinergic neurons of the ventral forebrain/septum, which project to the cerebral cortex and hippocampus.

It is only the third group of cholinergic neurons that are attacked by Alzheimer’s disease, but the use of anticholinergic drugs results in a blockade of virtually all cholinergic synapses. This much more widespread and diffuse action may produce a somewhat different pattern of functional deficits than the more specific cholinergic deficit found in the Alzheimer brain. It should also be noted that the loss of cholinergic innervation to the cortex and hippocampus results in the denervation of both muscarinic and nicotinic receptors, while the drugs com- monly used experimentally tend to be more specific.

Other agents which can disrupt cholinergic transmission include cholinesterase inhibitors and, indeed, many of the so-called nerve gases or chemical warfare weapons are members of this class of compounds [Karczmar, 1967, 19761. It might seem illogical that increasing the amount of acetylcholine by preventing its degradation would be detrimental to function, but too much acetylcholine can be as bad as too little. What is important to keep in mind in attempts to design “ procholinergic” agents is that levels of acetylcholine at synapses (or indeed any extracellular site) are kept very low by the action of cholinesterases and that cholinergic

72 Davies

transmission is a pulse phenomenon. Analagous to morse code, the sense of the transmission can be lost if the essential temporal sequence of the transmission degenerates into a continuous stream of signal.

ATTEMPTS AT POSITIVE PHARMACOLOGY

While there are numerous agents to produce decreased or disrupted cholinergic trans- mission, there are few that can reliably enhance central cholinergic transmission in people. One of these is physostigmine, which, of course, belongs to the cholinesterase inhibitor family. It has been used with some success in the treatment of Alzheimer’s disease [Thal et al., 19831, and this will be discussed by K . Davis in this issue. However, it is obvious that virtually all postsynaptically active cholinomimetic agents (muscarinic receptor agonists or cholinesterase inhibitors) will have an inverted U dose response curve. If this strategy to treat the cholinergic deficiency of Alzheimer’s disease is to be adopted, there are several points to be kept in mind. Probably the major concern will be to attempt to make these agents as specific to brain receptors as possible. Because of the widespread distribution of muscarinic receptors in the periphery, at least some of the decrements in performance produced by high doses will result from peripheral actions. Whether or not it is possible to design muscarinic agonists specific to central nervous system receptors is not known: To date, no clear differences between central and peripheral sites have been identified. There are clearly two types of muscarinic receptor which can be distinguished by agonist binding and by some of the newer antagonists [Aronstam et al. 1977, 1979; Birdsall et al., 1978; Hulme et al., 19781. However, both types are found at central and peripheral sites.

Similar considerations may apply to the use of cholinesterase inhibitors: There is little information to suggest a systematic difference between central and peripheral acetylcholines- terase. It might prove possible to exploit differences in central versus peripheral drug metabo- lism, as might have been unintentionally done with physostigmine in Alzheimer patients. Although the half-life of this drug in plasma is measured in minutes [Groff et al., 19771, it appears to be much more slowly degraded in the central nervous system, at least as judged by the duration of its therapeutic effects [Thal et al., 19831. The overall aim in using post- synaptically active compounds would seem to be to provide an elevated baseline of cholinergic transmission: The following example will serve to illustrate this principle.

Imagine that in the normal brain, each neuron is contacted by 200 acetylcholine-releasing terminals, and that 120 of these have to release the transmitter for the neuron to fire and pass along the message. In the Alzeheimer brain, there are only 100 terminals remaining per cell and 100 empty receptor sites. If we can provide sufficient agonist to occupy 50 receptor sites, we can restore some function to the system because our occupation of the 50 and the release of acetylcholine from the 100 remaining terminals can allow normal transmission to take place. This type of baseline raising should be possible without disruption of the normal temporal sequence of transmission, because the amount of drug we supply is not enough to result in aberrant firing: Information is transmitted only when cholinergic neurons want it to be. Whether or not this scheme can be translated into the complex clinical situation is far from clear.

FUTURE DIRECTIONS AND SPECULATIONS

In my opinion, the line of research with the potential for development would be the design of agents to enhance the ability of cholinergic neurons to produce acetylcholine. This was the idea behind the extensive trials that have been conducted with choline or lecithin. Sadly, these compounds do not provide relief of the symptoms of Alzheimer’s disease, at least when given for short periods of time (up to 3 months) [Boyd et al., 1977; Etienne et al., 19781. The rationale for the use of choline and lecithin was that choline availability was a rate-limiting factor for acetylcholine synthesis. This may be true in some circumstances [Cohen and

Treatments for Alzheimer’s Disease 73

Wurtman, 19761, but it does not appear to be the case very often [Brunello et al., 19821. What is the rate-limiting step for acetylcholine synthesis? We really are not clear on this point, although there are some things we do know.

1. Even under conditions of maximum activity, no more than about 25% of the ChAT present in nerve terminals is being used to make acetylcholine [Barker, 1976; Haubrich, 19761. This observation implies (at least to those of us who like to dream) that the Alzheimer patient retains enough ChAt activity to support normal rates of acetylcholine synthesis until very late in the course of the disease [Bowen, 19831.

2. The availability of excess choline does not allow maximum rates of acetylcholine synthesis: This was most dramatically demonstrated by Sims et al. [1981a], who studied acetylcholine synthesis rates using 2-mM choline. This concentration is about two orders of magnitude greater than that present in brain and at least three times the Km of ChAT for choline.

3. It is entirely possible that acetylcholine synthesis is limited by the availability of acetyl coenzyme A (coA), the acetyl group donor [Jope, 1979; Jope and Jenden, 19801.

This latter point is intriguing but also somewhat confusing. The concentration of acetyl coA in nerve terminals is between 5 and 20 pmol/L. This is in the same range as choline concentrations and close to the K, for ChAT. Thus, it is reasonable to assume that there is enough acetyl coA to saturate the ChAT. However, most of the acetyl coA is within mitochron- dria and is rapidly turning over to provide entry of glucose carbons into the Krebs cycle. How much acetyl coA is present in the cytoplasm, where acetylcholine synthesis is taking place, is not known. Since only a few percent at most of the glucose carbons end up in acetylcholine, it might be reasonable to predict that the majority of the acetyl coA present is cycling through the much more rapid energy-producing pathway. There is currently no hard information on the concentration of acetyl coA in the cytoplasm of cholinergic nerve terminals and this point would seem to be worth investigating further. What is clear is that acetylcholine synthesis is very sensitive to decreases in oxidative metabolism. Deprivation of oxygen or glucose or inhibition of pyruvate dehydrogenase [Gibson et al., 19831 produces much larger decreases in acetylcholine production than in energy metabolism, clearly indicating that even quite small decreases in the availability of acetyl coA can have serious consequences for rates of synthesis of acetylcholine.

How does acetyl coA move out of mitochrondria into the cytoplasm? This point has been debated for years and, as yet, the mechanism remains obscure [Szutowicz and Lysiak, 1980; Jope, 19791. There is some evidence that the process is calcium dependent, but we do not have any real idea of how the necessary transport process might work. There is real potential here because, under normal conditions, only a very small percentage of the acetyl coA generated is used for acetylcholine synthesis: Doubling or tripling the amount directed towards the transmitter synthesis is unlikely to have any significant consequence for energy production.

One final approach to the presynaptic cholinergic terminal is worth considering. The work of Gibson and colleagues [Gibson, 1983; Peterson and Gibson, 19831 has suggested that the use of agents which promote the entry of calcium into the terminals enhances acetylcholine release. Work with 4-aminopyridine and 3,4-diaminopyridine suggests that the enhanced release also facilitates acetylcholine synthesis: Release and synthesis appear to be quite tightly coupled under most conditions. The 3,4-compound appears to be quite potent in reversing some of the behavioral consequences of reduced acetylcholine release, at least in aged mice [Peterson and Gibson, 19831.

CONCLUSIONS

If it is accepted that the principle neurochemical deficit present in the Alzheimer brain is a cholinergic deficiency, then the road to developing successful treatments is clear. The required agent would enhance central cholinergic transmission without significant peripheral

74 Davies

side effects and with some specificity for cortical and hippocampal cholinergic pathways. Amongst the almost infinite number of chemicals that can be produced, is there such an agent‘? I don’t know, but there a re huge rewards awaiting the discoverer of this compound.

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