Dopamine hypothesis of mania

Özet

Abstract

softheSpeakers/KonuşmacıleriThe discovery of dopamine and its pathwaysDopamine (DA) was first synthesized in 1910 from 3,4-dihydroxy phenyl alanine (DOPA) by Barger and Ewens at Wellcome Laboratories in London. It is a cathecholamine and in the 1940s Blaschko in Cambridge proposed that DA was a precursor in synthesis of the cat-echolamine neurotransmitters noradrenaline (norepinephrine) and adrenaline (epinephrine). In 1957 it was shown to be present in the brain with other catecholamines (1). In 1958, Carlsson discovered the function of dopamine a neurotransmitter (2), while studying the action of reserpine, isolated in 1952 from the root of Rauwolfia, used traditionally in India for the treatment of insanity. Reserpine irreversibly blocks the vesicular monoamine transporter, thus depleting neuronal serotonin, dopamine and norepinephrine. In mammals it produces loss of movement or “akinesia”, a symptom of Parkinson’s disease. The degree of akinesia induced by reserpine correlated most closely with the depletion of dopamine, and could be reversed by L-dopa. In Vienna, Hornykiewicz who had worked with Blaschko, measured dopamine in Parkinson’s disease brains, and in 1960 reported depletion of dopamine in the corpus striatum in these brains, and later postulated that striatal dopamine deficiency accounts for the motor symptoms of Parkinson’s disease (3).Carlsson demonstrated dopamine was concentrated in the basal ganglia. Using the technique of monoamine histofluorescence, dopamine pathways were mapped in the brain including the nigrostriatal path¬way, from A9 neurons in the substantia nigra to the corpus striatum (4). In 1967, Cotzias in New York developed a technique, raising the dose of DOPA gradually, up to 16 grams per day, to treat Parkinson’s disease (5). Thus the role of dopamine in movement control and Parkinson’s disease was established by 1967.Catecholamine hypothesis of maniaSchildkraut (1965) proposed a catecholamine hypothesis of affective disorders, in which depression was postulated to result from “deficiency” of “catecholamines particularly norepinephrine” at “functionally important adrenergic receptor sites in the brain”, and elation (mania) resulted from an excessive activity of catecholamines (6). He emphasised the role of norepinephrine and not dopamine.One of the supports of this hypothesis was the effect of amphetamines which release catecholamines. Low doses of amphetamines produce a state resembling mild mania but with reduced hunger and appetite and anomalous endocrine effects (7). Dopamine agonists, such as bromocriptine, used to treat prolactinomas can lead to psychotic states, including mania (8). Likewise, a manic episode may be triggered by amphetamine in predisposed individuals (9), and l-dopa can trigger mania in individuals with a bipolar predisposition (10). The tranquilising and depressant effect of reserpine in man was another piece of Schildkraut’s evidence. Furthermore alphamethyl para-tyrosine, which inhibits the rate-limiting step in catecholamine synthesis, reduces the severity of mania (11); and depleting l-dopa and lowering brain catecholamines, by ingesting a drink of branched chain amino acids, improves mania (12).Antipsychotics in mania and schizophreniaThe antimanic effect of antipsychotics became another foundation of the catecholamine hypothesis of mania, but there was considerable misunderstanding of the role of antipsychotics in mania until the era of newer antipsychotics stimulated research into their effects.Dopamine hypothesis of schizophreniaEarly antipsychotics such as chlorpromazine and haloperidol have varied pharmacological actions including antagonism of norepinephrine. However the main mechanism common to all antipsychotics was discovered by Carlsson and Lindqvist in 1963, showing that they increase the release of metabolites of dopamine and epinephrine, and speculating the mechanism to be blockade of the receptors for dopamine (13). Van Rossum in 1966 articulated the hypothesis that dopamine pathways may be overactive in schizophrenia (14). The importance of dopamine receptor blockade for antipsychotics was confirmed in 1974 by Seeman (15), showing S2 Journal of Mood Disorders Volume: 3, Supplement: 1, 2013 - www.jmood.orgmania occurred in patients with or without psychotic symptoms (25). When individual symptoms of mania were analysed, drug treatment (with olanzapine, quetiapine and presumably the other antipsychotics) improved the whole range of symptoms (including elation, flight of ideas, grandiosity, sexual interest, irritability, aggression, general appearance and insight, as well as the items most sensitive to sedation: insomnia, overactivity and pressure of speech). These findings lead to the inevitable conclusion that the drugs are not just antipsychotic, but also antimanic (25). The speed of action and size of effect of antipsychotics makes them especially useful for control of emergent (hypomanic) symptoms (25) and in the case of haloperidol for acute tranquillisation in severe mania (24). Antipsychotics in schizophrenia and maniaThe so-called “atypical” antipsychotics were screened in animal models to have fewer extrapyramidal or Parkinsonian side effects than the older drugs. The “atypicals” share no other quality than this. Meta-analyses show small but real superiority in efficacy (26), and effectiveness (27) for some new drugs (particularly clozapine) over older ones in schizophrenia. The differences are more striking when the treatment of mania is analysed. In our own experience the benzamide drug, remoxipride, did not bring about sustained improvement in mania in doses shown to be effective in schizophrenia (24). A multitreatment meta-analysis (28) confirms the impression from individual randomised controlled trials (25) that no atypical antipsychotic is superior in efficacy to haloperidol in mania and some, such as quetiapine, aripiprazole, asenapine and ziprasidone, are distinctly inferior. All produce