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Treatment strategies for senile dementia: antagonist β-carbolines
Treatmentstrategies for senile dementia: antagonistp-carbolines Martin Sarter, Herbert H. Schneider and David N. Stephens An effective pharmacological treatment for senile dementia is not yet available and even the therapeuticpotency of muscarinicagonists has been rather disappointing up to now. Different lines of evidence and a hypothesis are presented below suggesting that ~-carbolines with antagonist or partial inverse agonist properties at the GABAbenzodiazepine receptor complex may offer a treatment for senile demenba primarily by disinhibiting the remaining cholinergic neurons of basal forebrain. Predinical behavioral and biochemical data, as well as studies with human volunteers, support the idea that such ~carbolines exert nootropic effects in general
Since Alzheimer's original work 1, no results have provided more impulse into research on senile dementia than the reports of a loss of activity of cortical cholinergic enzymes in senile dementia patients 2'3, and the corresponding anatomical evidence for cholinergic cell loss4'5. Although there is increasing criticism of the dominance of the cholinergic hypothesis, largely because of anatomical incongruities between the cortical distribution of cholinergic terminals and senile plaques6'7, the cholinergic hypothesis remains pre-eminent, at least among those attempting to develop pharmacological therapies. The two most important reasons for that may be that: (1) the degree of cortical cholinergic deafferentation seems fairly well related to the extent of the dementia3'~1°; and, (2) cholinergic drugs are well known as tools in memory research~-~3 (the antimuscarinic scopolamine is even used to test the functional integrity of the central cholinergic system in demented patients and the normal elderly~4'15).
clinically available muscarinic agonists do not seem to bind selectively to MI or M2 receptors and, therefore, by reducing acetylcholine release through an action at presynaptic receptors, may negate their own beneficial effects at the postsynaptic receptor24. Such a property would account for the failure of RS 86 to antagonize the effects of scopolamine in both human volunteers and animals2~, although evidence for some selectivity of this compound for the M1 receptor has been shown 26. According to this concept, selective muscarinic postsynaptic agonists or presynaptic antagonists would offer a more rational approach to a therapy for senile dementia. In particular, presynaptic antagonists may be useful, since enhancing acetylcholine release may provide a more physiological signal (but see Ref. 27). Stimulation of MI postsynaptic receptors, on the other hand, would disrupt the .normal patterning of physiological cholinergic transmission28. Thus, M2 antagonists might serve to amplify the signal by enhancing acetylcholine release, whereas postsynaptic agonists at M1 receptors may simply increase 'noise'. However, selective M1 and M2 ligands do not seem likely to become available in the near future. An alternative approach might be to enhance acetylcholine-turnover via an indirect disinhibitory mechanism.
Martin Sarter,Herbert H. Schneiderand DavidN. Stephensare at the Research Laboratoriesof ScheringA G, POBox 650311,D-1000 Berlin65, FRG.
GABA-benzodiazepine receptor-mediated modulation of cortical acetylcholine turnover Several pharmacological studies have demonstrated that acetylcholine release, as well as highaffinity choline uptake are under the control of GABA-benzodiazepine receptors. Direct injection of the GABAergic agonist muscimol into the cholinerMuscarinic agonists and cholinesterase inhibitors Although some positive clinical effects with mus- gic basal forebrain decreased cortical acetylcholine carinic agonists like arecoline, RS 86, and pilo- release29 as well as high-affinity choline uptake 30 . carpine, and with the cholinesterase inhibitor Systemic application of muscimol or diazepam has physostigmine h a v e been reported 16-19, no also been demonstrated to decrease acetylcholine therapeutically useful approaches have been de- turnover in the cortex 31'32. Acetylcholine turnover veloped with such compounds 2°. Bartus et a/.2° and high-affinity choline uptake in hippocampus listed four major reasons for the clinical failure of the has been reported as decreased33'34 or not available cholinergics: extremely short half life, lack influenced 32,35. Interestingly, physostigmine has been used as an of selectivity for CNS, high incidence of adverse side effects, and extremely narrow therapeutic window. antidote in accidental diazepam intoxication 36, A more general reason could be related to the which may support the idea of a strong inhibition of existence of at least two muscarinic receptor sub- acetylcholine turnover by GABA agonists and types, M1 and M2, which exert functionally oppos- benzodiazepines. Disinhibition may thus facilitate ing effects at post- and pre-synaptic sites, cholinergic neurotransmission. respectively21. Since acetylcholine shows a higher affinity to the presynaptic M2 receptor 22 , whose Anatomical evidence activation reduces acetylcholine release23, it is not The GABAergic afferent projection to the cholinsurprising that the cholinesterase inhibitor physo- ergic cell bodies in the basal forebrain may arise stigmine, for example, improves the status of from the nucleus accumbens, as suggested by patients with senile dementia only in extremely small some older anatomical studies37'38. More recently, and individually adjusted dose ranges. Similarly, the Zaborszky eta/. 39, using combinations of choline TINS, Vol. 1 I, No. I, 1988
© 1988.ElseviePubl r ications.Cambridge0378-59121881502.00
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acetyltransferase and glutamic acid decarboxylase (GAD) immunohistochemistry, as well as retrograde staining with horseradish peroxidase, demonstrated that GAD-containing terminals synapse onto cholinergic neurons (see also Ref. 40). By means of in-vitro autoradiography with the benzodiazepine [3H]lormetazepam in the rat, we found a high density of benzodiazepine binding sites in the substantia innominata compared with the parietal cortex (layers II-IV) (1080 fmol. mg -1 tissue and 890 fmol. mg -1 tissue, respectively) (see Fig. 1, and Ref. 41 for anatomical terminology). The [3H]lormetazepam binding could be displaced by diazepam (IC5o = 100 riM), the inverse agonist 13carboline FG 7142 (IC5o = 525 riM), and the antagonist 13-carboline ZK 93 426 (IC5o = 47 riM). Of particular interest is the anatomical locus of the high density of benzodiazepine binding sites. Binding to the area of the basal nucleus of Meynert was rather low, and the bulk of [3H]lormetazepam binding sites was in the substantia innominata. Bigl et al.4~have suggested that this area in the rat is the source of cholinergic neurons innervating neocortical areas, in contrast to the cells situated more medio-rostral in the globus pallidus, which primarily innervate limbic tetencephalic structures. Thus, the anatomical evidence, together with pharmacological results, suggests that the activity of basal forebrain cholinergic neurons innervating the cortex is controlled by a GABAergic input and can be modulated by GABAergic drugs as well as by benzodiazepines, acting_ at different sites of the same receptor complex ~z. If GABA and benzodiazepines can act via this mechanism to decrease cortical acetylcholine turnover, then it should be possible to employ GABA antagonists or benzodiazepine receptor inverse agonists to enhance cholinergic activity in order to exert antiarnnesic and promnesic effects.
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Cognitive effects of I~-carbolines acting as benzodiazepine receptor antagonists and inverse agonists. The GABA receptor exists as a domain on a protein that serves to gate chloride ions in the neuronal membrane. Associated with the GABA receptor is a binding site for benzodiazepines, which
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forebrain of the rat. The section was incubated in 0.6 nM [3H]lormetazepam (2,970 GBq/mmol), exposed to Ultrofilm, and the optical densities were transformed into radioactivity measures and standardized to [3H]microscales by the use of an ima&e-analysinE system (ASBA). (A) Pseudo-color image of the [3H]lormetazepam bindin E. The scale is in nCi per mE tissue. Among other areas, the substantia innominata shows a relatively high density of [3H]lormetazepam binding sites [see (B) for the identification of anatomical structures]. Abbreviations: A V, nucleus anteroventralis thalami; BST, nucleus stria terminalis; CP, caudate-putamen; GP; Elobus pallidus; HDB, nucleus horizontal limb of the diagonal band; LH, lateral hypothalamus; PT, nucleus paratenialis thalami; RT, nucleus reticularis thalami; 51, substantia innominata; f, fomix; ic, internal capsule; sm, stria medullaris thalami. TIN& Vol. 11, No. 1, 1988
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facilitate the effects of GABA. Other ligands for this benzodiazepine receptor reduce the effects of GABA, so that benzodiazepine receptor ligands can be used to either enhance or reduce GABAergic transmission. This bidirectionality at the receptor level is reflected in the behavioral pharmacology of the different types of benzodiazepine receptor ligands. Whereas benzodiazepines and I~-carbolines with agonist or partial agonist properties at the benzodiazepine receptor induce amnesic effects in a passive avoidance task43, the antagonist I~-carboline ZK 93 426 as well as the partial inverse agonist 13carboline FG 7142 and the inverse agonist DMCM were able to attenuate the disruptive effects of scopolamine in such a task43. However, using a spontaneous alternation test in mice, only the normal
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antagonist ZK 93 426 and not the partial inverse agonists ZK 90 886 and FG 7142 and the inverse agonists DMCM and I3-CCM antagonized the scopolamine-induced impairment of the alternation performance 44. The antagonist I~-carboline ZK 9342645 has also been demonstrated to improve performance of aged rats in a spatial delayed alternation task at a delay of 20 seconds, whereas in young animals the compound showed rather negative effects on performance 46. In a visual discrimination task analysed in accordance with signal-detection theory, ZK 93 426 not only antagonized the scopolamineinduced impairment of signal detectability, but also induced a subtle but distinct improvement of performance in senescent animals47. Recently, we tested basal forebrain-lesioned rats in an automated senile dementia
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Symbols - GABA-BZreceptor [] cholinergicreceptor El blocked GABA-BZr.
Fig. 2. Schematic summary of the hypothesis presented. In the left-hand section, the GABAerglc input to the chofiner&ic cell bodies of the basal forebrain is indicated. These cholinergic neurons may innervate54Predominantly, cortical pyramidal cells, which receive" GABAer&ic inhibition by cortical intemeurons, or which are GABAerg"ic themselves (not indicated " " the fiigure). In senile " de mentia (middle), 30-70% in of these cholinergic neurons degenerate without a major consequence for the postsynaptic muscarinic receptors. (There is, as yet, no evidence for an increased GABAergic innervation of the remaining choliner&ic neurons as sug&ested in the figure.) In the right-hand section, the possible action of the antagonist ~6-carboline ZK 93 426 is illustrated. All benzodiazepine receptors are blocked and, as the primary consequence, ACh release and high-affinity chofine uptake are enhanced (see insert). Such a disinhibitory effect at the chofinergic cell body enhances impulse conduction, and also possibly the transport of proteins and phospholipids, etc. It may be speculated that the disinhibition at the cholinergic cell bodies and at the cortical pyramidal target cells of the cholinergic projection act synergistically, resulting in cortical excitation. TINS, Vol. 11, No. 1, 1988
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radial tunnel maze; ZK 93 426 (5 mg kg-~) given during acquisition improved performance of the lesioned animals when they were required to relearn the maze (Steckler, T. and Sarter, M., unpublished observations). Venault et aL48 have demonstrated that the inverse agonist 13-CCM enhanced performance in three different paradigms, namely habituation of mice to a novel environment, memory for a simple passive avoidance task in mice, and imprinting in chicks. Both these authors and ourselves agree that the memory facilitating effects of antagonist and inverse agonist ~-carbolines seem to be restricted to acquisition 46'47. Similarly, the behavioral effects of scopolamine, which were antagonized in different models by antagonist and inverse agonist 13carbolines, are predominantly restricted to acquisition 46. The results from the signal-detection analysis47 provide perhaps the best clue as to the mechanism of these nootropic effects: enhancement of vigilance (the term is used in the sense of an improved ability to evaluate stimuli associated with the performance of a task46) may represent the primary effect of antagonist and inverse agonist I~-carbolines in improving performance in learning or memory tasks. Clinical evidence Experiments with human volunteers have suggested that there are certain similarities in the memory performance of patients with senile dementia and that of human volunteers treated with diazepam49. Moreover, the cognitive effects of benzodiazepine agonists have been suggested to be based on an impaired ability to screen relevant external stimuli 5°. According to the above usage, benzodiazepines reduce vigilance. Such an impairment has also been discussed as being responsible, at least in part, for the performance deficits in Alzheimer patients in some tasks51. If the proposed mechanism of the antiamnesic and promnesic effects of ZK 93 426 is correct, then it might be predicted that such a compound will improve the status of patients suffering from senile dementia. Data from patients are lacking, but in a double blind and placebo controlled studys2, ZK 93 426 given i.v. to normal human volunteers (mean age 25 + 6 years) induced behavioral changes experienced as stimulant and activating (self-rating, visual analogue scales) at a dose of 0.04 mg kg-lbut not 0.01 mg kg -1. Furthermore, in two tasks, a 'logical reasoning task' and a 'picture differences task', which were assumed to examine concentration and attention, respectively, an improvement in ZK 93 426-treated volunteers was observed (also at 0.04 mg kg-1). Interestingly, this dose of ZK 93 426 provoked no signs of anxiety (self ratings 52) suggesting that the 'nootropic' effects of ZK 93 426 are not likely to be due to anxiogenic properties of this compound. Duka e t al. 53 have also presented evidence that performance of normal volunteers was improved following ZK 93 426 (0.04 mg kg -1) 16
in a standard memory task based on recall of pictures. Thus the hypothesis derived from animal behavioral pharmacology seems to carry over to healthy humans at least.
Concluding remarks This essay presents a hypothesis of the nootropic potential of ~-carbolines with antagonist and partial inverse agonist properties at the GABAbenzodiazepine receptor complex. This hypothesis is summarized schematically, in Fig. 2. Although the hypothesis is simplistic given the variety and extent of neuropathological processes in senile dementia, and the complexity of its neuropsychology, it provides an approach to the development of a new generation of drugs for the treatment of some symptoms of senile dementia.
Selected references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28
Alzheimer, A. (1906) AIIg. Z. Psychiatr. 64, 146-148 Bowen, D. M. (1984) Monogr. Dev. Biol. 17, 42-59 Perry, E. K., etal. (1978) Br. Med. J. 2, 1457-1459 Whitehouse, D. J., etal. (1982)Science 215, 1237-1239 Hassler, R. (1938) J. Psycho/. Neurol. (Leipzig) 48, 387-476 Mesulam, M.M., Mufson, J. E., Levey, A. I. and Wainer, B. H. (1983) J. Comp. Neurol. 214, 170-197 Collerton, D. (1986) Neuroscience 19, 1-28 Bird, T. D,, Stranakan, S., Sumi, S. M. and Raskind, M. (1983) Ann. Neurol. 14, 284-293 Wilcock, G.K., Esiri, M., Bowen, D.M. and Smith, S.T. (1982) J. Neurol. 5ci. 57, 407-417 Soininen, H. S., Jolkkonen, J. T., Reinikainen, K. J., Halonen, T. O. and Riekkinen, P. J. (1984)J. Neurol. Sci. 63, 167-172 Drachman, D.A. and Leavitt, J. (1974) Arch. Neurol. 30, 113-121 Drachman, D. A. and Leavitt, J. (1972) J. Exp. Psychol. 93, 302-308 Warburton, D.M. and Wesnes, K. (1984) Neuropsychobiology 11, 121-132 Sunderland, T., et al. (1985) Psychopharmacology 87, 247-249 Richardson, J. S., etal. (1985) Progr. Neuro-PsychopharmacoL Biol. Psychiatr. 9, 651-654 Brinkman, S. D. and Gershon, S. (1983) NeurobioL Aging4, 139-145 Peters, B. H. and Levin, H. S. (1979)Ann. Neurol. 6, 219-221 Christie, J. E., Shering, A., Ferguson, J. and Glen, A. I. M. (1981) Br. J. Psychiatr. 138, 46-50 Kurz, A., RiJster, P., Romero, 8. and Zimmer, R. (1986) Nervenarzt 57, 558-569 Bartus, R.T., Dean, R. C. and Fisher, S. K. (1986) in Treatment Development Strategies for AIzheimer's Disease (Crook, T., Bartus, R., Ferris, S. and Gershon, S., eds), pp. 421-450, Mark Powley Hoss, W. and Messer, W., Jr (1986) Biochem. Pharmacol. 35, 3895-3901 Kellar, K. J., Martino, A. M., Hall, D. P., Schwartz, R. D. and Taylor, R. L. (1985) J. Neurosci. 5, 1577-1582 Meyer, E. M. and Otero, D. H. (1985) J. NeuroscL 5, 12021207 Watson, M., Vickroy, T. W., Roeske, W. R. and Yamamura, H. I. (1985) Progr. Neuro-Psychopharmacol. Biol. Psychiatr. 9, 569-574 Azcona, A., Roth, S. and Spiegel, R. (1986) Pharmacopsychiatry 19, 323-325 Palacios, J. M., etaL (1986) Eur. J. Pharmacol. 125, 45-62 Mash, D. C. and Potter, L. T. (1986) Neuroscience 19, 551564 Crews, F.T., et al. (1986) in Treatment Development Strategies for Alzheimer's Disease (Crook, T., 8artus, R., Ferris, S. and Gershon, S., eds), pp. 385-419, Mark Powley
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29 Casamenti, F., Deffenu, G., Abbamondi, A. L. and Pepeu, G. (1986) Brain Res. Bull. 16, 689-695 30 Wenk, G. L. (1984) Neurosci. Left. 51, 99-103 31 Petkow, V., Georgier, V., Getova, D. and Petkov, V. (1983) Acta Physiol. Pharmacol. Bulg. 9, 3-13 32 Zsilla, G., Cheney, D. L and Costa, E. (1976) Arch. Pharmacol. 294, 251-255 33 Miller, J.A. and Richter, J.A. (1986) J. Neurochem. 47, 1916-1918 34 Miller, J. A. and Richter, J. A. (1985) Br. J. Pharmacol. 84, 19-25 35 Wood, P. C. (1985) Can. J. Physiol. Pharmacol. 64, 325-328 36 Di Liberti, J., O'Brien, M. L. and Turner, T. (1975) J. Pediatr. 86, 106-107 37 Nagy, J. I., Carter, D. A. and Fibiger, H. C. (1978) Brain Res. 158, 15-29 38 Mogenson, G. J., Swanson, L. W. and Wu, M. (1983) J. Neurosci. 3, 189-202 39 Zaborszky, L., Heimer, L., Eckenstein, F. and Levanth, C. (1986) J. Comp. NeuroL 250, 282-295 40 Perez de la Mora, M., etaL (1981) Neuroscience 5, 875-895 41 Bigl, V., Woolf, N. J. and Butcher, L. L. (1982) Brain Res. Bull. 8, 727-749 42 Haefely, W. (1985) Pharmacopsychiatry 18, 163-166 43 Jensen, L. H., Stephens, D. N., Sarter, M. and Petersen, E. N.
|et:;ter to the Acidic amino acid receptor nomenclature: time for change SIR:
A considerable debate, carried out largely within the pages of TINS ~-~, has been stimulated by the proposal to adopt a nomenclature for acidic amino acid receptors that avoids the use of agonist names °. Two major questions have been raised: (I) is a new nomenclature needed, and (2) what form should it take? What is wrong with the old nomenclature? There are good scientific reasons for abandoning the traditional names of N-methyI-Daspartate (NMDA), quisqualate and kainate receptors and adopting a more systematic nomenclature. The major problem is that quisqualate and kainate are n o t selective agonists for their respective receptor designations. Thus, in radioligand binding experiments, quisqualate has nanomolar affinity for three different sites those labelled by [3H]AMPA ('quisqualate' receptors), [3H]kainate ('kainate' receptors) and [3H]AP4 (representing a chloride-dependent transport site) - and micromolar affinity for NMDA receptors 6'7. Kainate has a high affinity for its own site but also possesses micromolar affinity for [3H]AMPA binding sites6,7. Non-selectivity of kainate and quisqualate is also apparent in electrophysiological experiTIN& Vol. 11, No. 1, 1988
Brain Res. Bull. (in press) 44 Sarter,M., Bodewitz,G. and Stephens,D. N. PsychopharmacoloEy (in press) 45 Jensen,L. H., etal. (1984) Psychopharmacolol&y83,249-256 46 Sarter, M. and Stephens, D. N. in Benzodiazepine Receptor Ligands: Memory and Information Processing (Hindmarch, F., Oft, H. and Roth, T., eds), Springer-Verlag(in press) 47 Stephens,D. N. and Sarter, M. in Benzodiazepine Receptor Ligands: Memory and Information Processing (Hindmarch, F., Ott, H. and Roth T., eds), Springer-Verlag(in press) 48 Venault, P., et al. (1986) Nature 321,864-866 49 Block, R. I., Voe, M. D., Stanley,B., Stanley,M. and Pomara, N. (1985) Exp. Aging Res. 2, 151-155 50 Gray,J. A. (1982) Behav. Brain Sci. 5, 469-534 51 Hart, S.A., Smith, C. M. and Swash, M. (1985) Neurobiol. Aging 6, 287-292 52 Duka, T., Stephens, D.N., Krause, W. and Dorow, R. Psychopharmacology (in press) 53 Duka, T., SchL~tt, B., Edelmann, V. and Dorow, R. in Benzodiazepine Receptor Ligands: Memory and Information Processing (Hindmarch, F., Oft, H. and Roth, T., eds),
Springer-Verlag (in press) 54 Hallanger,A. E., Wainer, B. H. and Rye, D. B. (1986) Neuroscience 19, 763-769
editor ments. Thus, biphasic depolarizing responses to kainate have been observed in hippocampal neurones which appear to reflect activation of both quisqualate and kainate receptors 8. In the rat cortical slice preparation, responses to high concentrations of quisqualate which persist in the presence of pentobarbitone (which antagonizes the 'quisqualate receptor'-mediated depolarization) can be reduced by the NMDA receptor antagonists, D-2-amino-5-phosphonovalerate and MK-8019. In addition, Sugiyama et al. ~° have provided evidence that quisqualate activates a further receptor type that is linked to inositol phospholipid metabolism. It is evident therefore, that neither quisqualate nor kainate can be used experimentally to define receptor subtypes, and that much confusion will be propagated (which is already apparent in the literature) by continuing to refer to receptors by these non-selective agonists. Indeed, it would seem unwise to base any receptor nomenclature on the activity of a single pharmacological agent (either an agonist or antagonist). A more satisfactory approach would take into account the broad pharmacological profile of the receptor, including the potencies of both agonists and antagonists. What form should a systematic nomenclature take? Both Ariens 11 and Green 12
have pointed out the need for an internationally agreed nomenclature for receptors similar to .that adopted succesfully for the systematic classification of enzymes. In the absence of this, various naming systems for receptors have evolved. The most common uses a simple letter/ number combination, e.g. H1, H2; D1, D2; NK1, NK2, NK3. This is a straightforward system in which the letter usually refers to the transmitter substance (e.g. H = histamine, D = dopamine) and the number is used to designate subtypes. Along these lines, perhaps the most natural nomenclature for acidic amino acid receptors would use the letter G (for glutamate), since this substance is the prime neurotransmitter candidate, and acts at all three receptor subtypes. However, several lines of evidence support the idea that a family of acidic amino acids, including aspartate, homocysteate and cysteine sulphinate, may serve transmitter roles in some pathways. It was for this reason that we originally suggested A1, A2 and A3 to designate Acidic amino acid receptors 6. In deference to those working in the adenosine receptor field who have adopted A1 and A2 as a nomenclature 2, we have recently TABLE I. Acidic amino acid receptor nomenclature Old nomenclature: Proposed nomenclatu re:
NMDA AA1
Quisqualate Kainate AA2
AA3
17
Since Alzheimer's original work 1, no results have provided more impulse into research on senile dementia than the reports of a loss of activity of cortical cholinergic enzymes in senile dementia patients 2'3, and the corresponding anatomical evidence for cholinergic cell loss4'5. Although there is increasing criticism of the dominance of the cholinergic hypothesis, largely because of anatomical incongruities between the cortical distribution of cholinergic terminals and senile plaques6'7, the cholinergic hypothesis remains pre-eminent, at least among those attempting to develop pharmacological therapies. The two most important reasons for that may be that: (1) the degree of cortical cholinergic deafferentation seems fairly well related to the extent of the dementia3'~1°; and, (2) cholinergic drugs are well known as tools in memory research~-~3 (the antimuscarinic scopolamine is even used to test the functional integrity of the central cholinergic system in demented patients and the normal elderly~4'15).
clinically available muscarinic agonists do not seem to bind selectively to MI or M2 receptors and, therefore, by reducing acetylcholine release through an action at presynaptic receptors, may negate their own beneficial effects at the postsynaptic receptor24. Such a property would account for the failure of RS 86 to antagonize the effects of scopolamine in both human volunteers and animals2~, although evidence for some selectivity of this compound for the M1 receptor has been shown 26. According to this concept, selective muscarinic postsynaptic agonists or presynaptic antagonists would offer a more rational approach to a therapy for senile dementia. In particular, presynaptic antagonists may be useful, since enhancing acetylcholine release may provide a more physiological signal (but see Ref. 27). Stimulation of MI postsynaptic receptors, on the other hand, would disrupt the .normal patterning of physiological cholinergic transmission28. Thus, M2 antagonists might serve to amplify the signal by enhancing acetylcholine release, whereas postsynaptic agonists at M1 receptors may simply increase 'noise'. However, selective M1 and M2 ligands do not seem likely to become available in the near future. An alternative approach might be to enhance acetylcholine-turnover via an indirect disinhibitory mechanism.
Martin Sarter,Herbert H. Schneiderand DavidN. Stephensare at the Research Laboratoriesof ScheringA G, POBox 650311,D-1000 Berlin65, FRG.
GABA-benzodiazepine receptor-mediated modulation of cortical acetylcholine turnover Several pharmacological studies have demonstrated that acetylcholine release, as well as highaffinity choline uptake are under the control of GABA-benzodiazepine receptors. Direct injection of the GABAergic agonist muscimol into the cholinerMuscarinic agonists and cholinesterase inhibitors Although some positive clinical effects with mus- gic basal forebrain decreased cortical acetylcholine carinic agonists like arecoline, RS 86, and pilo- release29 as well as high-affinity choline uptake 30 . carpine, and with the cholinesterase inhibitor Systemic application of muscimol or diazepam has physostigmine h a v e been reported 16-19, no also been demonstrated to decrease acetylcholine therapeutically useful approaches have been de- turnover in the cortex 31'32. Acetylcholine turnover veloped with such compounds 2°. Bartus et a/.2° and high-affinity choline uptake in hippocampus listed four major reasons for the clinical failure of the has been reported as decreased33'34 or not available cholinergics: extremely short half life, lack influenced 32,35. Interestingly, physostigmine has been used as an of selectivity for CNS, high incidence of adverse side effects, and extremely narrow therapeutic window. antidote in accidental diazepam intoxication 36, A more general reason could be related to the which may support the idea of a strong inhibition of existence of at least two muscarinic receptor sub- acetylcholine turnover by GABA agonists and types, M1 and M2, which exert functionally oppos- benzodiazepines. Disinhibition may thus facilitate ing effects at post- and pre-synaptic sites, cholinergic neurotransmission. respectively21. Since acetylcholine shows a higher affinity to the presynaptic M2 receptor 22 , whose Anatomical evidence activation reduces acetylcholine release23, it is not The GABAergic afferent projection to the cholinsurprising that the cholinesterase inhibitor physo- ergic cell bodies in the basal forebrain may arise stigmine, for example, improves the status of from the nucleus accumbens, as suggested by patients with senile dementia only in extremely small some older anatomical studies37'38. More recently, and individually adjusted dose ranges. Similarly, the Zaborszky eta/. 39, using combinations of choline TINS, Vol. 1 I, No. I, 1988
© 1988.ElseviePubl r ications.Cambridge0378-59121881502.00
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acetyltransferase and glutamic acid decarboxylase (GAD) immunohistochemistry, as well as retrograde staining with horseradish peroxidase, demonstrated that GAD-containing terminals synapse onto cholinergic neurons (see also Ref. 40). By means of in-vitro autoradiography with the benzodiazepine [3H]lormetazepam in the rat, we found a high density of benzodiazepine binding sites in the substantia innominata compared with the parietal cortex (layers II-IV) (1080 fmol. mg -1 tissue and 890 fmol. mg -1 tissue, respectively) (see Fig. 1, and Ref. 41 for anatomical terminology). The [3H]lormetazepam binding could be displaced by diazepam (IC5o = 100 riM), the inverse agonist 13carboline FG 7142 (IC5o = 525 riM), and the antagonist 13-carboline ZK 93 426 (IC5o = 47 riM). Of particular interest is the anatomical locus of the high density of benzodiazepine binding sites. Binding to the area of the basal nucleus of Meynert was rather low, and the bulk of [3H]lormetazepam binding sites was in the substantia innominata. Bigl et al.4~have suggested that this area in the rat is the source of cholinergic neurons innervating neocortical areas, in contrast to the cells situated more medio-rostral in the globus pallidus, which primarily innervate limbic tetencephalic structures. Thus, the anatomical evidence, together with pharmacological results, suggests that the activity of basal forebrain cholinergic neurons innervating the cortex is controlled by a GABAergic input and can be modulated by GABAergic drugs as well as by benzodiazepines, acting_ at different sites of the same receptor complex ~z. If GABA and benzodiazepines can act via this mechanism to decrease cortical acetylcholine turnover, then it should be possible to employ GABA antagonists or benzodiazepine receptor inverse agonists to enhance cholinergic activity in order to exert antiarnnesic and promnesic effects.
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Cognitive effects of I~-carbolines acting as benzodiazepine receptor antagonists and inverse agonists. The GABA receptor exists as a domain on a protein that serves to gate chloride ions in the neuronal membrane. Associated with the GABA receptor is a binding site for benzodiazepines, which
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forebrain of the rat. The section was incubated in 0.6 nM [3H]lormetazepam (2,970 GBq/mmol), exposed to Ultrofilm, and the optical densities were transformed into radioactivity measures and standardized to [3H]microscales by the use of an ima&e-analysinE system (ASBA). (A) Pseudo-color image of the [3H]lormetazepam bindin E. The scale is in nCi per mE tissue. Among other areas, the substantia innominata shows a relatively high density of [3H]lormetazepam binding sites [see (B) for the identification of anatomical structures]. Abbreviations: A V, nucleus anteroventralis thalami; BST, nucleus stria terminalis; CP, caudate-putamen; GP; Elobus pallidus; HDB, nucleus horizontal limb of the diagonal band; LH, lateral hypothalamus; PT, nucleus paratenialis thalami; RT, nucleus reticularis thalami; 51, substantia innominata; f, fomix; ic, internal capsule; sm, stria medullaris thalami. TIN& Vol. 11, No. 1, 1988
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facilitate the effects of GABA. Other ligands for this benzodiazepine receptor reduce the effects of GABA, so that benzodiazepine receptor ligands can be used to either enhance or reduce GABAergic transmission. This bidirectionality at the receptor level is reflected in the behavioral pharmacology of the different types of benzodiazepine receptor ligands. Whereas benzodiazepines and I~-carbolines with agonist or partial agonist properties at the benzodiazepine receptor induce amnesic effects in a passive avoidance task43, the antagonist I~-carboline ZK 93 426 as well as the partial inverse agonist 13carboline FG 7142 and the inverse agonist DMCM were able to attenuate the disruptive effects of scopolamine in such a task43. However, using a spontaneous alternation test in mice, only the normal
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antagonist ZK 93 426 and not the partial inverse agonists ZK 90 886 and FG 7142 and the inverse agonists DMCM and I3-CCM antagonized the scopolamine-induced impairment of the alternation performance 44. The antagonist I~-carboline ZK 9342645 has also been demonstrated to improve performance of aged rats in a spatial delayed alternation task at a delay of 20 seconds, whereas in young animals the compound showed rather negative effects on performance 46. In a visual discrimination task analysed in accordance with signal-detection theory, ZK 93 426 not only antagonized the scopolamineinduced impairment of signal detectability, but also induced a subtle but distinct improvement of performance in senescent animals47. Recently, we tested basal forebrain-lesioned rats in an automated senile dementia
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Fig. 2. Schematic summary of the hypothesis presented. In the left-hand section, the GABAerglc input to the chofiner&ic cell bodies of the basal forebrain is indicated. These cholinergic neurons may innervate54Predominantly, cortical pyramidal cells, which receive" GABAer&ic inhibition by cortical intemeurons, or which are GABAerg"ic themselves (not indicated " " the fiigure). In senile " de mentia (middle), 30-70% in of these cholinergic neurons degenerate without a major consequence for the postsynaptic muscarinic receptors. (There is, as yet, no evidence for an increased GABAergic innervation of the remaining choliner&ic neurons as sug&ested in the figure.) In the right-hand section, the possible action of the antagonist ~6-carboline ZK 93 426 is illustrated. All benzodiazepine receptors are blocked and, as the primary consequence, ACh release and high-affinity chofine uptake are enhanced (see insert). Such a disinhibitory effect at the chofinergic cell body enhances impulse conduction, and also possibly the transport of proteins and phospholipids, etc. It may be speculated that the disinhibition at the cholinergic cell bodies and at the cortical pyramidal target cells of the cholinergic projection act synergistically, resulting in cortical excitation. TINS, Vol. 11, No. 1, 1988
15
radial tunnel maze; ZK 93 426 (5 mg kg-~) given during acquisition improved performance of the lesioned animals when they were required to relearn the maze (Steckler, T. and Sarter, M., unpublished observations). Venault et aL48 have demonstrated that the inverse agonist 13-CCM enhanced performance in three different paradigms, namely habituation of mice to a novel environment, memory for a simple passive avoidance task in mice, and imprinting in chicks. Both these authors and ourselves agree that the memory facilitating effects of antagonist and inverse agonist ~-carbolines seem to be restricted to acquisition 46'47. Similarly, the behavioral effects of scopolamine, which were antagonized in different models by antagonist and inverse agonist 13carbolines, are predominantly restricted to acquisition 46. The results from the signal-detection analysis47 provide perhaps the best clue as to the mechanism of these nootropic effects: enhancement of vigilance (the term is used in the sense of an improved ability to evaluate stimuli associated with the performance of a task46) may represent the primary effect of antagonist and inverse agonist I~-carbolines in improving performance in learning or memory tasks. Clinical evidence Experiments with human volunteers have suggested that there are certain similarities in the memory performance of patients with senile dementia and that of human volunteers treated with diazepam49. Moreover, the cognitive effects of benzodiazepine agonists have been suggested to be based on an impaired ability to screen relevant external stimuli 5°. According to the above usage, benzodiazepines reduce vigilance. Such an impairment has also been discussed as being responsible, at least in part, for the performance deficits in Alzheimer patients in some tasks51. If the proposed mechanism of the antiamnesic and promnesic effects of ZK 93 426 is correct, then it might be predicted that such a compound will improve the status of patients suffering from senile dementia. Data from patients are lacking, but in a double blind and placebo controlled studys2, ZK 93 426 given i.v. to normal human volunteers (mean age 25 + 6 years) induced behavioral changes experienced as stimulant and activating (self-rating, visual analogue scales) at a dose of 0.04 mg kg-lbut not 0.01 mg kg -1. Furthermore, in two tasks, a 'logical reasoning task' and a 'picture differences task', which were assumed to examine concentration and attention, respectively, an improvement in ZK 93 426-treated volunteers was observed (also at 0.04 mg kg-1). Interestingly, this dose of ZK 93 426 provoked no signs of anxiety (self ratings 52) suggesting that the 'nootropic' effects of ZK 93 426 are not likely to be due to anxiogenic properties of this compound. Duka e t al. 53 have also presented evidence that performance of normal volunteers was improved following ZK 93 426 (0.04 mg kg -1) 16
in a standard memory task based on recall of pictures. Thus the hypothesis derived from animal behavioral pharmacology seems to carry over to healthy humans at least.
Concluding remarks This essay presents a hypothesis of the nootropic potential of ~-carbolines with antagonist and partial inverse agonist properties at the GABAbenzodiazepine receptor complex. This hypothesis is summarized schematically, in Fig. 2. Although the hypothesis is simplistic given the variety and extent of neuropathological processes in senile dementia, and the complexity of its neuropsychology, it provides an approach to the development of a new generation of drugs for the treatment of some symptoms of senile dementia.
Selected references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28
Alzheimer, A. (1906) AIIg. Z. Psychiatr. 64, 146-148 Bowen, D. M. (1984) Monogr. Dev. Biol. 17, 42-59 Perry, E. K., etal. (1978) Br. Med. J. 2, 1457-1459 Whitehouse, D. J., etal. (1982)Science 215, 1237-1239 Hassler, R. (1938) J. Psycho/. Neurol. (Leipzig) 48, 387-476 Mesulam, M.M., Mufson, J. E., Levey, A. I. and Wainer, B. H. (1983) J. Comp. Neurol. 214, 170-197 Collerton, D. (1986) Neuroscience 19, 1-28 Bird, T. D,, Stranakan, S., Sumi, S. M. and Raskind, M. (1983) Ann. Neurol. 14, 284-293 Wilcock, G.K., Esiri, M., Bowen, D.M. and Smith, S.T. (1982) J. Neurol. 5ci. 57, 407-417 Soininen, H. S., Jolkkonen, J. T., Reinikainen, K. J., Halonen, T. O. and Riekkinen, P. J. (1984)J. Neurol. Sci. 63, 167-172 Drachman, D.A. and Leavitt, J. (1974) Arch. Neurol. 30, 113-121 Drachman, D. A. and Leavitt, J. (1972) J. Exp. Psychol. 93, 302-308 Warburton, D.M. and Wesnes, K. (1984) Neuropsychobiology 11, 121-132 Sunderland, T., et al. (1985) Psychopharmacology 87, 247-249 Richardson, J. S., etal. (1985) Progr. Neuro-PsychopharmacoL Biol. Psychiatr. 9, 651-654 Brinkman, S. D. and Gershon, S. (1983) NeurobioL Aging4, 139-145 Peters, B. H. and Levin, H. S. (1979)Ann. Neurol. 6, 219-221 Christie, J. E., Shering, A., Ferguson, J. and Glen, A. I. M. (1981) Br. J. Psychiatr. 138, 46-50 Kurz, A., RiJster, P., Romero, 8. and Zimmer, R. (1986) Nervenarzt 57, 558-569 Bartus, R.T., Dean, R. C. and Fisher, S. K. (1986) in Treatment Development Strategies for AIzheimer's Disease (Crook, T., Bartus, R., Ferris, S. and Gershon, S., eds), pp. 421-450, Mark Powley Hoss, W. and Messer, W., Jr (1986) Biochem. Pharmacol. 35, 3895-3901 Kellar, K. J., Martino, A. M., Hall, D. P., Schwartz, R. D. and Taylor, R. L. (1985) J. Neurosci. 5, 1577-1582 Meyer, E. M. and Otero, D. H. (1985) J. NeuroscL 5, 12021207 Watson, M., Vickroy, T. W., Roeske, W. R. and Yamamura, H. I. (1985) Progr. Neuro-Psychopharmacol. Biol. Psychiatr. 9, 569-574 Azcona, A., Roth, S. and Spiegel, R. (1986) Pharmacopsychiatry 19, 323-325 Palacios, J. M., etaL (1986) Eur. J. Pharmacol. 125, 45-62 Mash, D. C. and Potter, L. T. (1986) Neuroscience 19, 551564 Crews, F.T., et al. (1986) in Treatment Development Strategies for Alzheimer's Disease (Crook, T., 8artus, R., Ferris, S. and Gershon, S., eds), pp. 385-419, Mark Powley
TINS, Vol. 11, No. 1, 1988
29 Casamenti, F., Deffenu, G., Abbamondi, A. L. and Pepeu, G. (1986) Brain Res. Bull. 16, 689-695 30 Wenk, G. L. (1984) Neurosci. Left. 51, 99-103 31 Petkow, V., Georgier, V., Getova, D. and Petkov, V. (1983) Acta Physiol. Pharmacol. Bulg. 9, 3-13 32 Zsilla, G., Cheney, D. L and Costa, E. (1976) Arch. Pharmacol. 294, 251-255 33 Miller, J.A. and Richter, J.A. (1986) J. Neurochem. 47, 1916-1918 34 Miller, J. A. and Richter, J. A. (1985) Br. J. Pharmacol. 84, 19-25 35 Wood, P. C. (1985) Can. J. Physiol. Pharmacol. 64, 325-328 36 Di Liberti, J., O'Brien, M. L. and Turner, T. (1975) J. Pediatr. 86, 106-107 37 Nagy, J. I., Carter, D. A. and Fibiger, H. C. (1978) Brain Res. 158, 15-29 38 Mogenson, G. J., Swanson, L. W. and Wu, M. (1983) J. Neurosci. 3, 189-202 39 Zaborszky, L., Heimer, L., Eckenstein, F. and Levanth, C. (1986) J. Comp. NeuroL 250, 282-295 40 Perez de la Mora, M., etaL (1981) Neuroscience 5, 875-895 41 Bigl, V., Woolf, N. J. and Butcher, L. L. (1982) Brain Res. Bull. 8, 727-749 42 Haefely, W. (1985) Pharmacopsychiatry 18, 163-166 43 Jensen, L. H., Stephens, D. N., Sarter, M. and Petersen, E. N.
|et:;ter to the Acidic amino acid receptor nomenclature: time for change SIR:
A considerable debate, carried out largely within the pages of TINS ~-~, has been stimulated by the proposal to adopt a nomenclature for acidic amino acid receptors that avoids the use of agonist names °. Two major questions have been raised: (I) is a new nomenclature needed, and (2) what form should it take? What is wrong with the old nomenclature? There are good scientific reasons for abandoning the traditional names of N-methyI-Daspartate (NMDA), quisqualate and kainate receptors and adopting a more systematic nomenclature. The major problem is that quisqualate and kainate are n o t selective agonists for their respective receptor designations. Thus, in radioligand binding experiments, quisqualate has nanomolar affinity for three different sites those labelled by [3H]AMPA ('quisqualate' receptors), [3H]kainate ('kainate' receptors) and [3H]AP4 (representing a chloride-dependent transport site) - and micromolar affinity for NMDA receptors 6'7. Kainate has a high affinity for its own site but also possesses micromolar affinity for [3H]AMPA binding sites6,7. Non-selectivity of kainate and quisqualate is also apparent in electrophysiological experiTIN& Vol. 11, No. 1, 1988
Brain Res. Bull. (in press) 44 Sarter,M., Bodewitz,G. and Stephens,D. N. PsychopharmacoloEy (in press) 45 Jensen,L. H., etal. (1984) Psychopharmacolol&y83,249-256 46 Sarter, M. and Stephens, D. N. in Benzodiazepine Receptor Ligands: Memory and Information Processing (Hindmarch, F., Oft, H. and Roth, T., eds), Springer-Verlag(in press) 47 Stephens,D. N. and Sarter, M. in Benzodiazepine Receptor Ligands: Memory and Information Processing (Hindmarch, F., Ott, H. and Roth T., eds), Springer-Verlag(in press) 48 Venault, P., et al. (1986) Nature 321,864-866 49 Block, R. I., Voe, M. D., Stanley,B., Stanley,M. and Pomara, N. (1985) Exp. Aging Res. 2, 151-155 50 Gray,J. A. (1982) Behav. Brain Sci. 5, 469-534 51 Hart, S.A., Smith, C. M. and Swash, M. (1985) Neurobiol. Aging 6, 287-292 52 Duka, T., Stephens, D.N., Krause, W. and Dorow, R. Psychopharmacology (in press) 53 Duka, T., SchL~tt, B., Edelmann, V. and Dorow, R. in Benzodiazepine Receptor Ligands: Memory and Information Processing (Hindmarch, F., Oft, H. and Roth, T., eds),
Springer-Verlag (in press) 54 Hallanger,A. E., Wainer, B. H. and Rye, D. B. (1986) Neuroscience 19, 763-769
editor ments. Thus, biphasic depolarizing responses to kainate have been observed in hippocampal neurones which appear to reflect activation of both quisqualate and kainate receptors 8. In the rat cortical slice preparation, responses to high concentrations of quisqualate which persist in the presence of pentobarbitone (which antagonizes the 'quisqualate receptor'-mediated depolarization) can be reduced by the NMDA receptor antagonists, D-2-amino-5-phosphonovalerate and MK-8019. In addition, Sugiyama et al. ~° have provided evidence that quisqualate activates a further receptor type that is linked to inositol phospholipid metabolism. It is evident therefore, that neither quisqualate nor kainate can be used experimentally to define receptor subtypes, and that much confusion will be propagated (which is already apparent in the literature) by continuing to refer to receptors by these non-selective agonists. Indeed, it would seem unwise to base any receptor nomenclature on the activity of a single pharmacological agent (either an agonist or antagonist). A more satisfactory approach would take into account the broad pharmacological profile of the receptor, including the potencies of both agonists and antagonists. What form should a systematic nomenclature take? Both Ariens 11 and Green 12
have pointed out the need for an internationally agreed nomenclature for receptors similar to .that adopted succesfully for the systematic classification of enzymes. In the absence of this, various naming systems for receptors have evolved. The most common uses a simple letter/ number combination, e.g. H1, H2; D1, D2; NK1, NK2, NK3. This is a straightforward system in which the letter usually refers to the transmitter substance (e.g. H = histamine, D = dopamine) and the number is used to designate subtypes. Along these lines, perhaps the most natural nomenclature for acidic amino acid receptors would use the letter G (for glutamate), since this substance is the prime neurotransmitter candidate, and acts at all three receptor subtypes. However, several lines of evidence support the idea that a family of acidic amino acids, including aspartate, homocysteate and cysteine sulphinate, may serve transmitter roles in some pathways. It was for this reason that we originally suggested A1, A2 and A3 to designate Acidic amino acid receptors 6. In deference to those working in the adenosine receptor field who have adopted A1 and A2 as a nomenclature 2, we have recently TABLE I. Acidic amino acid receptor nomenclature Old nomenclature: Proposed nomenclatu re:
NMDA AA1
Quisqualate Kainate AA2
AA3
17