Br. J. clin. Pharmac. (1976), Supplement, 42-44
THE BIOCHEMICAL EVALUATION OF PSYCHOTROPIC DRUGS A.V.P. MACKAY MRC Brain Metabolism Unit, Department of Pharmacology, 1 George Square, Edinburgh, Scotland
Perhaps the most scientifically lucid example of the use of any centrally active drug, extending from the aetiological definition of a disorder through rational pharmacological treatment to subsequent biochemical evaluation of the effects of treatment, is provided by LDOPA. The work of Hornykiewicz and his colleagues (for refs. see Hornykiewicz, 1972) elegantly demonstrated a profound deficiency in the dopamine content of the nigro-striatal pathway in the post mortem brains of patients who had suffered from Parkinson's disease. The logical therapeutic approach to the disorder was the administration of the dopamine precursor LDOPA. Biochemical evaluation of this treatment approach was possible through examination of the striatal dopamine concentration of post mortem brain material from Parkinsonian patients treated with LDOPA. Davidson, Lloyd, Dankova & Hornykiewicz (1971) have found that L-DOPA treatment indeed results in increased dopamine concentrations in caudate and putamen and that the increases are proportional to both the premortem dose of L-DOPA and to observed clinical improvement (Hornykiewicz,
1974). Very recently another interesting facet to the neurochemistry of Parkinson's disease has emerged from the work of Lloyd & Hornykiewicz (1973) who observed the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD) to be deficient in the caudate, putamen and substantia nigra of Parkinsonian brains. Brains from patients who had received L-DOPA treatment for at least 1 year before death had normal GAD activity in these areas. There is now good evidence to support the view that the amount of transmitter synthesizing enzyme present in neuronal tissue is related to the long-term electrical activity of the constituent neurones (Mackay, 1974; Thoenen, 1972). A likely interpretation of the finding of raised GAD activity following L-DOPA treatment is that chronically increased presynaptic release of dopamine onto post-synaptic GABA containing neurones could induce changes in the biosynthetic enzyme content of these post-synaptic cells. Results of lesion studies (Kim et al., 1971; Hattori et al., 1973; McGeer et al., 1974) and electrophysiological studies (Aghajanian & Bunney, 1974) in animals point to the existence of an inhibitory feedback tract whose cell bodies lie in the
corpus striatum and send axons to the substantia nigra, where the inhibitory transmitter released may be GABA. Thus the GAD loss in Parkinson's disease probably reflects a trans-synaptic 'second-order' reduction secondary to diminished dopamine receptor activity. One could imagine that a primary loss of these postulated striatonigral inhibitory neurones, with relative preservation of the nigro-striatal dopamine pathway, might produce a clinical picture very different from Parkinson's disease; there would be an undamped, overactive dopamine system. Such a situation may exist in Huntington's chorea, the pharmacological mirror-image of Parkinson's disease where all the clinically effective drugs have pharmacological actions which would be expected to impair dopaminergic transmission. The recent finding of substantial, selective loss of GAD activity in the basal ganglia of patients dying of Huntington's chorea points to significant deficit in GABA-containing neurones and is thus compatible with this view of
extra-pyramidal dysfunction (Bird, Mackay, Rayner & Iversen, 1973). Therapeutic manoeuvres aimed at potentiating GABA transmission would seem to be indicated by this finding. The biochemical evaluation of drugs used in psychiatric practice is in a sense impossible since the biochemical factors responsible for the clinical pictures remain obscure. Evaluation must be attempted within the framework imposed by the current hypotheses of the modes of action of these drugs and correlated with their clinical efficacy. The majority of psychotropic agents would be expected on pharmacological grounds to have some effect on amine transmission in the central nervous system. Drugs effective in the treatment of schizophrenia all seem capable of inducing Parkinsonian side-effects and might thus be thought of as impairing dopaminergic transmission. Many animal experiments suggest that the phenothiazines and butyrophenones are potent antagonists of dopamine receptors. As a crude measure of dopamine turnover in human brain, Chase and his colleagues (Chase, Schnur & Gordon, 1970; Chase, 1974) have measured the concentration of HVA (the major metabolite of dopamine) in the cerebrospinal fluid (CSF) of patients pre-treated with probenecid and given a stat dose of a range of neuroleptic drugs.
THE BIOCHEMICAL EVALUATION OF PSYCHOTROPIC DRUGS
Drugs such as haloperidol and chlorpromazine caused an increased accumulation of HVA, which would be expected if the drugs were antagonizing dopamine receptors in the corpus striatum, blocking the negative feedback loop described above and evoking hyperactivity in the dopamine-releasing neurones of the substantia nigra. Comparison of the molecular structures of dopamine and a range of phenothiazines useful in the treatment of schizophrenia has revealed a close correlation between clinical efficacy and structural similarity to dopamine (Horn & Snyder, 1971). A useful in vitro dopamine receptor model has recently been developed and has allowed detailed investigation of dopamine receptor blockade. Miller & Iversen (1973) have looked at a range of neuroleptics for their ability to block a dopamine-stimulated adenyl cyclase ane have found that certain of the naturally occurring metabolites of chlorpromazine are potent dopamine receptor blockers. In particular 7-hydroxychlorpromazine, found in patients' plasma in concentrations similar to those of the parent drug, is a good dopamine antagonist. Chlorpromazine sulphoxide, another quantitatively important metabolite, is without significant pharmacological activity. Curry and his colleagues have proposed that some at least of the antischizophrenic action of chlorpromazine may be attributable to active metabolites (Sakalis et al., 1972). It might be expected that in general, patients who metabolized the drug ' to predominantly active metabolites might respond better than patients who produced inactive breakdown products. Mackay, Healey & Baker (1974) have shown that the ratio of the plasma concentration of 7-hydroxychlorpromazine to the concentration of sulphoxychlorpromazine was significantly greater in chronic schizophrenics whose global symptom control was good as compared with schizophrenics who were poorly controlled. It is conceivable then that the antischizophrenic activity of phenothiazines and butyrophenones might be attributable to interactions with dopamine receptors in the brain. Although the nigro-striatal pathway has been shown to contain dopamine in the post mortem human brain, additional dopamine pathways notably the mesolimbic system have been defined in the brains of lower mammals (Fuxe, Hokfelt & Ungerstedt, 1969) and provide theoretical sites of action for antischizophrenic drugs. Monoamine involvement in the affective disorders represents another area of hypothesis and
43
pharmacological intuition. Current hypotheses state that depression may be associated with reduced synaptic efficacy of either the catecholamines or 5-HT. Schildkraut (1965) has suggested that decreased transmitter output might be confined to certain subgroups of depressed patients. Reduced levels of the main central nervous system metabolite of noradrenaline, 3-methoxy-4-hydroxyphenyl glycol (MHPG), have been reported in urine and cerebrospinal fluid of depressed patients (for review see Schildkraut, 1974). Maas, Fawcett & Dekirmenjian (1972) investigated the effect of tricyclic antidepressant medication on the urinary excretion of MHPG in depressed patients. Those patients who responded well to medication had low pre-treatment excretion of MHPG and showed a return to normal excretion of MHPG after treatment. In contrast, those patients who failed to respond to medication had normal urinary MHPG both before and after treatment. These findings support the view that subpopulations may exist within any group of depressed patients and that the biochemical characteristics of these groups may determine their response to treatment. Ashcroft (1972) has found the concentration of the 5-HT metabolite 5-HIAA to be on average low in depressed patients. In general, the concentration of 5-HIAA in the CSF remains unchanged after treatment (Ashcroft et al., 1973). If, however, patients who have responded well to treatment with the 5-HT precursor 5-HTP are investigated, it has been found that successful treatment is associated with a rise in the cerebrospinal fluid accumulation of 5-HIAA relative to low pre-treatment levels (van Praag & Korf, 1971). Thus biochemically distinct subgroups of depressed patients may exist in whom the biochemical correlate of mood affects one monoamine system more than another. Identification of these subgroups would seem to be a useful prelude to the choice of appropriate antidepressant medication. In this brief overview, I have given a few current examples of the ways in which centrally acting drugs can be biochemically assessed. Of course evaluation of any therapeutic agent must ultimately be in terms of its clinical efficacy. Since the biochemical targets for the psychotropic drugs remain to be elucidated, evaluation at anything but the clinical level might seem to be inappropriate. However, through greater knowledge of the sites and modes of action of clinically successful drugs a clearer idea of the significant biochemical pathology of psychiatric illness might emerge.
References
AGHAJANIAN, G.K. & BUNNEY, B.S. (1974). Frontiers in catecholamine research. Proceedings of the Third International Catecholamine Symposium, Strasbourg, 1973. Oxford: Pergamon Press.
ASHCROFT, G.W. (1972). Modified amine hypothesis for the aetiology of affective illness. Lancet, ii, 573. ASHCROFT, G.W. et al. (1973). Changes on recovery in the concentrations of tryptophan and the biogenic amine
44
A.V.P. MACKAY
metabolites in the cerebrospinal fluid of patients with affective illness. Psychol. Med., 3, 319. BIRD, E.D. et al. (1973). Reduced glutamic-aciddecarboxylase activity of post-mortem brain in Huntington's chorea. Lancet, 1090. CHASE, T.N., SCHNUR, J.A. & GORDON, E.K. (1970). Cerebrospinal fluid monoamine catabolites in druginduced extrapyramidal disorders. Neuropharmacology, 9, 265. CHASE, T.. (1974). Frontiers in catecholamine research: Proceedings of the Third International Catecholamine Symposium, Strasbourg, 1973. Oxford: Pergamon Press. DAVIDSON, L. etal. (1971). Experientia, 27, 1048. FUXE, K., HOKFELT, T. & UNGERSTEDT, U. (1969). Metabolism of Amines in the Brain, ed. G. Hooper. London: Macmillan. HATTORI, T. et al. (1973). An estimation of the concentration of aminobutyric acid and glutamate decarboxylase in the inhibitory Purkinje axor terminals in the cat. Brain Res., 54, 103. HORN, A.S. & SNYDER, S.H. (1971). Proc. nat. Acad. Sci. U.SA., 68, 2325. HORNYKIEWICZ, 0. (1972). Handbook ofNeurochemistry, ed. A. Lajtha, 7,465. New York: Plenum. HORNYKIEWICZ, 0. (1974). Frontiers in Catecholamine Research: Proceedings of the Third International Catecholamine Symposium, Strasbourg, 1973. Oxford: Pergamon Press. KIM, J.S. et al. (1971). Role of y-aminobutyric acid (GABA) in extrapyramidal motor system 2. Some evidence for the existence of a type of GABA rich striat-nigral neurons. Exp. Brain Res., 14, 95. LLOYD, K. & HORNYKIEWICZ, 0. (1973). L-glutamic acid
decarboxylase in Parkinson's disease: effect of L-DOPA therapy. Nature (Lond.), 243, 521. MAAS, J.W., FAWCETT, J.A. & DEKIRMENJIAN, J. (1972).
Catecholamine metabolism, depressive illness and drug response. Arch. gen. Psychiat., 26, 252. McGEER, P.L. et al. (1974). Adv. Neur., S (in the press). MACKAY, A.V.P. (1974). The long-term regulation of tyrosine hydroxylase activity in cultured sympathetic ganglia: Role of ganglionic noradrenaline content. Br. J. Pharmac., S1, 509. MACKAY, A.V.P., HEALEY, A. & BAKER, J. (1974). In preparation. MILLER, R.J. & IVERSEN, L.L. (1973). Effect of chlorpromazine and some of its metabolites on the dopamine sensitive adenylate cyclase of rat brain striatum. J. Pharm. Pharmac., 26, 142. PRAAG, H.M. van & KORF, J. (1971). A pilot study of some kinetic aspects of the metabolism of 5hydroxytryptamine in depressive patients. Biol Psychiat., 3, 105. SAKALIS, G. et al. (1972). Physiologic and clinical effects of chlorpromazine and their relationships to plasma level. Clin. Pharm. Ther., 13, 931. SCHILDKRAUT, J.J. (1965). The catecholamine hypothesis of affective disorders: a review of supporting evidence. Am. J. Psychiat., 122, 509. SCHILDKRAUT, J.J. (1974). The current status of biological criteria for classifying the depressive disorders and predicting responses to treatment. Psychopharm. Bull., 10,(), 5. THOENEN, H. (1972). Neurotransmitters and Metabolic Regulations, ed. R.M.S. Smellie, Biochem. Soc. Symposium, 36, 3.
THE BIOCHEMICAL EVALUATION OF PSYCHOTROPIC DRUGS A.V.P. MACKAY MRC Brain Metabolism Unit, Department of Pharmacology, 1 George Square, Edinburgh, Scotland
Perhaps the most scientifically lucid example of the use of any centrally active drug, extending from the aetiological definition of a disorder through rational pharmacological treatment to subsequent biochemical evaluation of the effects of treatment, is provided by LDOPA. The work of Hornykiewicz and his colleagues (for refs. see Hornykiewicz, 1972) elegantly demonstrated a profound deficiency in the dopamine content of the nigro-striatal pathway in the post mortem brains of patients who had suffered from Parkinson's disease. The logical therapeutic approach to the disorder was the administration of the dopamine precursor LDOPA. Biochemical evaluation of this treatment approach was possible through examination of the striatal dopamine concentration of post mortem brain material from Parkinsonian patients treated with LDOPA. Davidson, Lloyd, Dankova & Hornykiewicz (1971) have found that L-DOPA treatment indeed results in increased dopamine concentrations in caudate and putamen and that the increases are proportional to both the premortem dose of L-DOPA and to observed clinical improvement (Hornykiewicz,
1974). Very recently another interesting facet to the neurochemistry of Parkinson's disease has emerged from the work of Lloyd & Hornykiewicz (1973) who observed the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD) to be deficient in the caudate, putamen and substantia nigra of Parkinsonian brains. Brains from patients who had received L-DOPA treatment for at least 1 year before death had normal GAD activity in these areas. There is now good evidence to support the view that the amount of transmitter synthesizing enzyme present in neuronal tissue is related to the long-term electrical activity of the constituent neurones (Mackay, 1974; Thoenen, 1972). A likely interpretation of the finding of raised GAD activity following L-DOPA treatment is that chronically increased presynaptic release of dopamine onto post-synaptic GABA containing neurones could induce changes in the biosynthetic enzyme content of these post-synaptic cells. Results of lesion studies (Kim et al., 1971; Hattori et al., 1973; McGeer et al., 1974) and electrophysiological studies (Aghajanian & Bunney, 1974) in animals point to the existence of an inhibitory feedback tract whose cell bodies lie in the
corpus striatum and send axons to the substantia nigra, where the inhibitory transmitter released may be GABA. Thus the GAD loss in Parkinson's disease probably reflects a trans-synaptic 'second-order' reduction secondary to diminished dopamine receptor activity. One could imagine that a primary loss of these postulated striatonigral inhibitory neurones, with relative preservation of the nigro-striatal dopamine pathway, might produce a clinical picture very different from Parkinson's disease; there would be an undamped, overactive dopamine system. Such a situation may exist in Huntington's chorea, the pharmacological mirror-image of Parkinson's disease where all the clinically effective drugs have pharmacological actions which would be expected to impair dopaminergic transmission. The recent finding of substantial, selective loss of GAD activity in the basal ganglia of patients dying of Huntington's chorea points to significant deficit in GABA-containing neurones and is thus compatible with this view of
extra-pyramidal dysfunction (Bird, Mackay, Rayner & Iversen, 1973). Therapeutic manoeuvres aimed at potentiating GABA transmission would seem to be indicated by this finding. The biochemical evaluation of drugs used in psychiatric practice is in a sense impossible since the biochemical factors responsible for the clinical pictures remain obscure. Evaluation must be attempted within the framework imposed by the current hypotheses of the modes of action of these drugs and correlated with their clinical efficacy. The majority of psychotropic agents would be expected on pharmacological grounds to have some effect on amine transmission in the central nervous system. Drugs effective in the treatment of schizophrenia all seem capable of inducing Parkinsonian side-effects and might thus be thought of as impairing dopaminergic transmission. Many animal experiments suggest that the phenothiazines and butyrophenones are potent antagonists of dopamine receptors. As a crude measure of dopamine turnover in human brain, Chase and his colleagues (Chase, Schnur & Gordon, 1970; Chase, 1974) have measured the concentration of HVA (the major metabolite of dopamine) in the cerebrospinal fluid (CSF) of patients pre-treated with probenecid and given a stat dose of a range of neuroleptic drugs.
THE BIOCHEMICAL EVALUATION OF PSYCHOTROPIC DRUGS
Drugs such as haloperidol and chlorpromazine caused an increased accumulation of HVA, which would be expected if the drugs were antagonizing dopamine receptors in the corpus striatum, blocking the negative feedback loop described above and evoking hyperactivity in the dopamine-releasing neurones of the substantia nigra. Comparison of the molecular structures of dopamine and a range of phenothiazines useful in the treatment of schizophrenia has revealed a close correlation between clinical efficacy and structural similarity to dopamine (Horn & Snyder, 1971). A useful in vitro dopamine receptor model has recently been developed and has allowed detailed investigation of dopamine receptor blockade. Miller & Iversen (1973) have looked at a range of neuroleptics for their ability to block a dopamine-stimulated adenyl cyclase ane have found that certain of the naturally occurring metabolites of chlorpromazine are potent dopamine receptor blockers. In particular 7-hydroxychlorpromazine, found in patients' plasma in concentrations similar to those of the parent drug, is a good dopamine antagonist. Chlorpromazine sulphoxide, another quantitatively important metabolite, is without significant pharmacological activity. Curry and his colleagues have proposed that some at least of the antischizophrenic action of chlorpromazine may be attributable to active metabolites (Sakalis et al., 1972). It might be expected that in general, patients who metabolized the drug ' to predominantly active metabolites might respond better than patients who produced inactive breakdown products. Mackay, Healey & Baker (1974) have shown that the ratio of the plasma concentration of 7-hydroxychlorpromazine to the concentration of sulphoxychlorpromazine was significantly greater in chronic schizophrenics whose global symptom control was good as compared with schizophrenics who were poorly controlled. It is conceivable then that the antischizophrenic activity of phenothiazines and butyrophenones might be attributable to interactions with dopamine receptors in the brain. Although the nigro-striatal pathway has been shown to contain dopamine in the post mortem human brain, additional dopamine pathways notably the mesolimbic system have been defined in the brains of lower mammals (Fuxe, Hokfelt & Ungerstedt, 1969) and provide theoretical sites of action for antischizophrenic drugs. Monoamine involvement in the affective disorders represents another area of hypothesis and
43
pharmacological intuition. Current hypotheses state that depression may be associated with reduced synaptic efficacy of either the catecholamines or 5-HT. Schildkraut (1965) has suggested that decreased transmitter output might be confined to certain subgroups of depressed patients. Reduced levels of the main central nervous system metabolite of noradrenaline, 3-methoxy-4-hydroxyphenyl glycol (MHPG), have been reported in urine and cerebrospinal fluid of depressed patients (for review see Schildkraut, 1974). Maas, Fawcett & Dekirmenjian (1972) investigated the effect of tricyclic antidepressant medication on the urinary excretion of MHPG in depressed patients. Those patients who responded well to medication had low pre-treatment excretion of MHPG and showed a return to normal excretion of MHPG after treatment. In contrast, those patients who failed to respond to medication had normal urinary MHPG both before and after treatment. These findings support the view that subpopulations may exist within any group of depressed patients and that the biochemical characteristics of these groups may determine their response to treatment. Ashcroft (1972) has found the concentration of the 5-HT metabolite 5-HIAA to be on average low in depressed patients. In general, the concentration of 5-HIAA in the CSF remains unchanged after treatment (Ashcroft et al., 1973). If, however, patients who have responded well to treatment with the 5-HT precursor 5-HTP are investigated, it has been found that successful treatment is associated with a rise in the cerebrospinal fluid accumulation of 5-HIAA relative to low pre-treatment levels (van Praag & Korf, 1971). Thus biochemically distinct subgroups of depressed patients may exist in whom the biochemical correlate of mood affects one monoamine system more than another. Identification of these subgroups would seem to be a useful prelude to the choice of appropriate antidepressant medication. In this brief overview, I have given a few current examples of the ways in which centrally acting drugs can be biochemically assessed. Of course evaluation of any therapeutic agent must ultimately be in terms of its clinical efficacy. Since the biochemical targets for the psychotropic drugs remain to be elucidated, evaluation at anything but the clinical level might seem to be inappropriate. However, through greater knowledge of the sites and modes of action of clinically successful drugs a clearer idea of the significant biochemical pathology of psychiatric illness might emerge.
References
AGHAJANIAN, G.K. & BUNNEY, B.S. (1974). Frontiers in catecholamine research. Proceedings of the Third International Catecholamine Symposium, Strasbourg, 1973. Oxford: Pergamon Press.
ASHCROFT, G.W. (1972). Modified amine hypothesis for the aetiology of affective illness. Lancet, ii, 573. ASHCROFT, G.W. et al. (1973). Changes on recovery in the concentrations of tryptophan and the biogenic amine
44
A.V.P. MACKAY
metabolites in the cerebrospinal fluid of patients with affective illness. Psychol. Med., 3, 319. BIRD, E.D. et al. (1973). Reduced glutamic-aciddecarboxylase activity of post-mortem brain in Huntington's chorea. Lancet, 1090. CHASE, T.N., SCHNUR, J.A. & GORDON, E.K. (1970). Cerebrospinal fluid monoamine catabolites in druginduced extrapyramidal disorders. Neuropharmacology, 9, 265. CHASE, T.. (1974). Frontiers in catecholamine research: Proceedings of the Third International Catecholamine Symposium, Strasbourg, 1973. Oxford: Pergamon Press. DAVIDSON, L. etal. (1971). Experientia, 27, 1048. FUXE, K., HOKFELT, T. & UNGERSTEDT, U. (1969). Metabolism of Amines in the Brain, ed. G. Hooper. London: Macmillan. HATTORI, T. et al. (1973). An estimation of the concentration of aminobutyric acid and glutamate decarboxylase in the inhibitory Purkinje axor terminals in the cat. Brain Res., 54, 103. HORN, A.S. & SNYDER, S.H. (1971). Proc. nat. Acad. Sci. U.SA., 68, 2325. HORNYKIEWICZ, 0. (1972). Handbook ofNeurochemistry, ed. A. Lajtha, 7,465. New York: Plenum. HORNYKIEWICZ, 0. (1974). Frontiers in Catecholamine Research: Proceedings of the Third International Catecholamine Symposium, Strasbourg, 1973. Oxford: Pergamon Press. KIM, J.S. et al. (1971). Role of y-aminobutyric acid (GABA) in extrapyramidal motor system 2. Some evidence for the existence of a type of GABA rich striat-nigral neurons. Exp. Brain Res., 14, 95. LLOYD, K. & HORNYKIEWICZ, 0. (1973). L-glutamic acid
decarboxylase in Parkinson's disease: effect of L-DOPA therapy. Nature (Lond.), 243, 521. MAAS, J.W., FAWCETT, J.A. & DEKIRMENJIAN, J. (1972).
Catecholamine metabolism, depressive illness and drug response. Arch. gen. Psychiat., 26, 252. McGEER, P.L. et al. (1974). Adv. Neur., S (in the press). MACKAY, A.V.P. (1974). The long-term regulation of tyrosine hydroxylase activity in cultured sympathetic ganglia: Role of ganglionic noradrenaline content. Br. J. Pharmac., S1, 509. MACKAY, A.V.P., HEALEY, A. & BAKER, J. (1974). In preparation. MILLER, R.J. & IVERSEN, L.L. (1973). Effect of chlorpromazine and some of its metabolites on the dopamine sensitive adenylate cyclase of rat brain striatum. J. Pharm. Pharmac., 26, 142. PRAAG, H.M. van & KORF, J. (1971). A pilot study of some kinetic aspects of the metabolism of 5hydroxytryptamine in depressive patients. Biol Psychiat., 3, 105. SAKALIS, G. et al. (1972). Physiologic and clinical effects of chlorpromazine and their relationships to plasma level. Clin. Pharm. Ther., 13, 931. SCHILDKRAUT, J.J. (1965). The catecholamine hypothesis of affective disorders: a review of supporting evidence. Am. J. Psychiat., 122, 509. SCHILDKRAUT, J.J. (1974). The current status of biological criteria for classifying the depressive disorders and predicting responses to treatment. Psychopharm. Bull., 10,(), 5. THOENEN, H. (1972). Neurotransmitters and Metabolic Regulations, ed. R.M.S. Smellie, Biochem. Soc. Symposium, 36, 3.
