Assessment of central dopaminergic function using plasma-free homovanillic acid after debrisoquin administration

Monoamine oxidase and catecholamine metabolism

The enzyme which has come to be known as monoamine oxidase was discovered in liver over 60 years ago as tyramine oxidase (Hare, 1928). Almost 10 years later, Blaschko et al. (1957a,b) established that epinephrine, norepinephrine and dopamine were also substrates for this enzyme. Zeller (1938) distinguished monoamine oxidase as different from several other amine oxidases, such as diamine oxidase. Although it was generally assumed that catecholamines were metabolized by MAO, this was not established until isotopically labelled epinephrine and an MAO inhibitor became available. Schayer (1951) found that after administration of N-methyl-14C-epinephrine, only about 50% of the radioactivity appeared in the urine, whereas when the 14C label was incorporated into the beta-position on the side chain, almost all of the radioactivity could be recovered. One year later, Zeller et al. (1952) discovered that isonicotinic acid hydrazide (iproniazid) inhibited MAO. When animals pretreated with the MAO inhibitor were administered N-methyl-14C-epinephrine, almost all of the radioactivity was recovered (Schayer et al., 1955), indicating that the enzyme was responsible for the metabolism of about half of the administered catecholamine. Schayer et al. (1952, 1953) had found that five urinary metabolite products of beta-labelled-14C-norepinephrine could be separated by paper chromatography, but the chemical structures of these compounds were not known. Armstrong et al. (1957) showed that 3-methoxy-4-hydroxymandelic acid (vanillyl mandelic acid, VMA) was the major metabolite of norepinephrine and Shaw et al. (1957) demonstrated that large amounts of homovanillic acid (HVA) were excreted in urine after administration of 3,4-dihydroxy-phenylalanine (DOPA). These observations led Axelrod to examine the possibility that O-methylation might precede deamination and to his discovery of catechol-O-methyl transferase (Axelrod, 1957, 1959). At that time it became apparent that there were two possible routes for metabolism of norepinephrine to VMA--either deamination followed by O-methylation or O-methylation and subsequent deamination. The relative roles of these two pathways in terminating the physiological actions of catecholamines then became a focus of attention. Biochemical methods were used to access directly the relative importance of the two metabolic pathways. Physiological methods, based on the effects of drugs which alter metabolism of the catecholamine, were used to examine the role of MAO and COMT in terminating the actions of administered or endogenously released catecholamines.

Plasma homovanillic acid in neuroleptic responsive and nonresponsive schizophrenics

Changes in plasma homovanillic acid (HVA) were investigated in neuroleptic responsive and nonresponsive schizophrenics in order to delineate parameters of dopamine regulation, which may underlie differences in neuroleptic responsivity. Nineteen schizophrenics were treated with haloperidol for 6 weeks. HVA was sampled at baseline, 24 hr after initial neuroleptic dose, and after 6 weeks of treatment. Subjects were pretreated with debrisoquin in order to reduce the peripheral production of HVA. The responders had an initial rise in HVA at 24 hr after first neuroleptic dose, followed by a decline back to baseline over the 6 weeks of treatment. The nonresponders' HVA failed to rise at 24 hr after first neuroleptic dose. At 6 weeks of treatment their HVA had fallen to significantly below baseline. Thus, a rise in HVA 24 hr after the first dose of neuroleptic predicted treatment response; a fall in HVA at 6 weeks to below pretreatment values was associated with neuroleptic nonresponse.

Acute effects of neuroleptics on unmedicated schizophrenic patients and controls

Acute administration of haloperidol (0.2 mg/kg) produced many more side effects in normal controls than in unmedicated schizophrenic patients. Prior to the neuroleptic challenge, both groups were on the peripheral monoamine oxidase inhibitor, debrisoquin, for at least 1 week, in order to enhance the relative contribution of CNS catecholamine metabolites to those measured in both plasma and urine. The patient group had higher plasma levels of methoxyhydroxyphenylglycol (MHPG) and homovanillic acid (HVA) and higher urinary MHPG output than controls, but there were no effects of haloperidol challenge, compared to placebo challenge. In both groups there were significant declines in plasma HVA levels from 8:30 AM to 12 NOON. These declines were unaffected by the haloperidol challenge. Explanations for the marked differences in behavioral effects of haloperidol on patients and controls include the possibility that dopamine receptor numbers were increased in the brains of the schizophrenic patients.

Measurement of plasma homovanillic acid concentrations in schizophrenic patients

1. Several lines of evidence suggest that abnormalities of central dopaminergic transmission may be involved in the expression of some schizophrenic symptoms. However, elucidation of the role of dopamine (DA) in schizophrenia has eluded investigative efforts partially because no accurate and easily repeatable measure of brain DA activity exists. 2. The development of a technique to measure homovanillic acid in plasma has offered the possibility of performing serial measurements of this major DA metabolite. 3. Assuming that plasma homovanillic acid (PHVA) concentrations is an index of brain DA activity, measurement of PHVA can play a role in elucidating the DA abnormality in schizophrenia. 4. Results to date suggest that plasma homovanillic acid concentrations are lower in chronic schizophrenic patients compared to normal controls, and that PHVA values correlate with schizophrenic symptom severity. 5. In addition, PHVA levels were shown to initially rise and subsequently decline during chronic neuroleptic administration in treatment responsive but not in treatment refractory schizophrenic patients.

Effects of monoamine oxidase inhibitors on levels of catechols and homovanillic acid in striatum and plasma

Levels of homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC) and dihydroxyphenylglycol (DHPG) in plasma and the striatium were measured after inhibition of monoamine oxidase type A (MAO-A) by clorgyline (4 mg/kg i.p.), MAO-B by (-)deprenyl (1 mg/kg i.p.), both MAO-A and MAO-B by nialamide (75 mg/kg i.p.) or peripheral neuronal MAO by debrisoquin (40 mg/kg i.p.). Levels of HVA in plasma decreased by about 60% after single doses of nialamide or clorgyline, by about 80% after repeated doses of nialamide, by about 40% after a single dose of debrisoquin and by about 50% after repeated doses of debrisoquin. The administration of clorgyline, nialamide or debrisoquin significantly decreased concentrations of DOPAC and DHPG in plasma, whereas (-)deprenyl did not affect levels of DHPG or HVA. None of the MAO inhibitors produced more than about 80% depression of levels of any of the deaminated metabolites. The results suggest that most of the HVA in plasma is derived from deamination of DA by MAO-A in peripheral neurons; that DOPAC in plasma is derived from cells outside the central nervous system; that DHPG in plasma is derived virtually exclusively from the metabolism of norepinephrine in sympathetic nerve endings and that residual levels of HVA after treatment with debrisoquin provide an improved but limited indication of central dopaminergic activity.

A new approach to biochemical evaluation of brain dopamine metabolism

1. Dopaminergic neurotransmission in brain is receiving increased attention because of its known involvement in Parkinson's disease and new methods for the treatment of this disorder and because of hypotheses relating several psychiatric disorders to abnormalities in brain dopaminergic systems. 2. Chemical assessment of brain dopamine metabolism has been attempted by measuring levels of its major metabolite, homovanillic acid (HVA), in cerebrospinal fluid, plasma, or urine. Because HVA is derived in part from dopamine formed in noradrenergic neurons, plasma levels and urinary excretion rates of HVA do not adequately reflect solely metabolism of brain dopamine. 3. Using debrisoquin, the peripheral contributions of HVA to plasma or urinary HVA can be diminished, but the extent of residual HVA formation in noradrenergic neurons is unknown. By measuring the levels of methoxy-hydroxyphenylglycol (MHPG) in plasma or of urinary norepinephrine metabolites (total MHPG in monkeys; the sum of total MHPG and vanillyl mandelic acid (VMA) in humans) along with HVA, it is possible to estimate the degree of impairment by debrisoquin of HVA formation from noradrenergic neuronal dopamine and thereby better assess brain dopamine metabolism. 4. This method was applied to a monkey before and after destruction of the nigrostriatal pathway by the administration of MPTP.

Reproducibility of the measurement of plasma noradrenergic and dopaminergic metabolites in normal subjects

The object of this study was to determine the reproducibility of the measurement of plasma catecholamine metabolites in normal control subjects and to assess the influence of factors such as time of day, diet, activity, blood pressure, and mood on the variance of these measures. Plasma free homovanillic acid (HVA), 3-methoxy-4-hydroxyphenylglycol (MHPG), and vanillylmandelic acid (VMA) were simultaneously measured by high performance liquid chromatography with electrochemical detection. Samples were collected from 15 doctors and nurses at 8 a.m. and at noon on 2 separate days. After the fasting 8 a.m. sample, the subjects ate a regular hospital breakfast. Activity in their usual tasks on an inpatient psychiatric unit was monitored electronically by an activity meter. Levels of each metabolite were not significantly different between days at the respective assay times and were highly correlated for individuals. MHPG showed a significant increase from morning to noon, while HVA showed a significant decrease. Activity, dietary intake of tyrosine and tyramine, blood pressure, pulse, scores on the Profile of Mood States, age, and sex were not related to plasma levels. The results demonstrate that measures of dopamine and norepinephrine metabolites have significant reliability in normal subjects in a setting used for research studies with psychiatric patients.

Attention deficit disorder: current perspectives

This review focuses on selected issues in the rapidly proliferating scientific literature on attention deficit disorder (ADD). It encompasses a brief overview of ADD, including a review of the historical trends, causation, and a description of the clinical characteristics. Critical issues fundamental to understanding of ADD are addressed next. Evidence is presented to suggest that there are several groups of children with ADD who are currently under-identified and therefore, underserved (girls with ADD; ADD without hyperactivity). The concept is introduced of uncomplicated ADD and ADD-Plus (ADD-P), a term used to designate ADD associated with other complicating features such as conduct disorder. Specific methodologic issues and their relationship to the under-identification of these groups of children follows. This discussion encompasses the reasons for diagnostic imprecision and inconsistency, emphasizing the problem of selective referral patterns and how it results in an inaccurate portrait of ADD. We then present new approaches to the definition and diagnosis of ADD based on empiric studies designed to circumvent some of these problems. We conclude with a summary of the implications of these findings, and recommendations for public policy in ADD, particularly the need to recognize that ADD may occur even in the most intellectually gifted individuals and even in students in select colleges. Often these children with the highest potential are penalized most, not because of conceptual limitations or because they do not understand, but because educators often fail to recognize the symptom complex. Many of these children are succeeding through intelligence and great effort, but many more could succeed with proper identification.