How amphetamine acts in minimal brain dysfunction

Intracellular methamphetamine prevents the dopamine-induced enhancement of neuronal firing

The dysregulation of the dopaminergic system is implicated in multiple neurological and neuropsychiatric disorders such as Parkinson disease and drug addiction. The primary target of psychostimulants such as amphetamine and methamphetamine is the dopamine transporter (DAT), the major regulator of extracellular dopamine levels in the brain. However, the behavioral and neurophysiological correlates of methamphetamine and amphetamine administration are unique from one another, thereby suggesting these two compounds impact dopaminergic neurotransmission differentially. We further examined the unique mechanisms by which amphetamine and methamphetamine regulate DAT function and dopamine neurotransmission; in the present study we examined the impact of extracellular and intracellular amphetamine and methamphetamine on the spontaneous firing of cultured midbrain dopaminergic neurons and isolated DAT-mediated current. In dopaminergic neurons the spontaneous firing rate was enhanced by extracellular application of amphetamine > dopamine > methamphetamine and was DAT-dependent. Amphetamine > methamphetamine similarly enhanced DAT-mediated inward current, which was sensitive to isosmotic substitution of Na(+) or Cl(-) ion. Although isosmotic substitution of extracellular Na(+) ions blocked amphetamine and methamphetamine-induced DAT-mediated inward current similarly, the removal of extracellular Cl(-) ions preferentially blocked amphetamine-induced inward current. The intracellular application of methamphetamine, but not amphetamine, prevented the dopamine-induced increase in the spontaneous firing of dopaminergic neurons and the corresponding DAT-mediated inward current. The results reveal a new mechanism for methamphetamine-induced dysregulation of dopaminergic neurons.

α-synuclein regulation of dopamine transporter

The development of effective therapeutic interventions for neurodegeneration requires a better understanding of the early events that precede neuronal loss. Recent work in various disease models has begun to emphasize the significance of presynaptic dysfunction as an early event that occurs before manifestation of neurological disorders. Dysregulation of dopamine (DA) homeostasis is implicated in neurodegenerative diseases, drug addiction, and neuropsychiatric disorders. The neuronal plasma membrane dopamine transporter (DAT) is essential for the maintenance of DA homeostasis in the brain. α-synuclein is a 140-amino acid protein that forms a stable complex with DAT and is linked to the pathogenesis of neurodegenerative disease. In this review we will examine the prevailing hypotheses for α-synuclein-regulation of DAT biology.

Adrenomedullary function during cognitive testing in attention-deficit/hyperactivity disorder

OBJECTIVE

Reported correlations between epinephrine (EPI) excretion and classroom performance, the cognition-enhancing effects of EPI infusion, increased EPI excretion with stimulants, and reports of decreased EPI excretion in attention-deficit/hyperactivity disorder (ADHD) suggest that sympathoadrenomedullary function might be altered in ADHD. This hypothesis was tested by examining sympathetic and adrenomedullary functioning during cognitive testing in boys with diagnosed ADHD.

METHOD

Urinary excretion of EPI and norepinephrine during a 3-hour cognitive test battery was assessed in 7- to 13-year-old boys. Excretion rates (nanograms per hour per square meter of body surface area) were determined in 200 individuals with ADHD (diagnosed according to DSM-IV criteria), with or without co-occurring oppositional defiant/conduct disorder or learning disorder. A non-ADHD contrast group (n = 51) with similar comorbidity was also studied.

RESULTS

Substantially lower (mean +/- SE) urinary EPI excretion was observed in the ADHD-inattentive subtype (n = 71) compared with the control group (200 +/- 22 versus 278 +/- 24 ng/hr/m2; F = 5.99, p = .015, critical alpha = .017). No diagnostic group differences were seen for norepinephrine excretion. Correlational analysis of both parent- and teacher-rated behaviors revealed that inattention factors consistently negatively predicted urinary EPI excretion.

CONCLUSIONS

The data extend findings of lower adrenomedullary activity during cognitive challenge in individuals with ADHD and suggest that the alteration is associated with inattentive behavior.

Role of serotonin in the paradoxical calming effect of psychostimulants on hyperactivity

The mechanism by which psychostimulants act as calming agents in humans with attention-deficit hyperactivity disorder (ADHD) or hyperkinetic disorder is currently unknown. Mice lacking the gene encoding the plasma membrane dopamine transporter (DAT) have elevated dopaminergic tone and are hyperactive. This activity was exacerbated by exposure to a novel environment. Additionally, these mice were impaired in spatial cognitive function, and they showed a decrease in locomotion in response to psychostimulants. This paradoxical calming effect of psychostimulants depended on serotonergic neurotransmission. The parallels between the DAT knockout mice and individuals with ADHD suggest that common mechanisms may underlie some of their behaviors and responses to psychostimulants.

Morphometric and neurosecretory changes in supraoptic neurons after D-amphetamine treatment

Several morphological and immunochemical characteristics of the neurosecretory neurons of the supraoptic nucleus (SON) have been studied of rats treated for 1 month with D-amphetamine sulfate (AMP) (8 mg/kg weight, daily). An increase of SON volume (11%) has been observed as a consequence of the growth of the dorsoventral axis. Neurosecretory neurons increased their nucleolar area (11.4%), their nuclear area (8.3%), and their cytoplasmatic area (18.3%). Vasopressin immunoreaction did not show any differences between treated and control animals, but oxytocin immunostaining displayed an important increase (23.7%) in the neuronal cytoplasm of the treated rats. The SON hypertrophy of the AMP-treated rats corresponded to the hypertrophy/hyperfunction of its oxytocinergic neurons, and could be considered as a new mechanism of the action of the AMP. The results are discussed in relation to the plastic features of the SON and its central (neuronal) and peripheral (hormonal) function.

Differential psychopharmacology of methylphenidate and the neuropsychology of childhood hyperactivity

The principle of rate-dependency has been proposed to explain the therapeutic effects of stimulant drugs in hyperactive children (HAC). This paper is a critical discussion of the salience of rate-dependency to childhood hyperactivity, on mathematical, theoretical and clinical levels. The results of a stimulant drug trial in 55 HAC are presented; the data are analyzed using analysis of variance to describe main drug effects, and these are compared to results derived from a traditional rate dependency analysis. The latter are found to have little salience to the actual clinical effects of stimulant drugs on a wide variety of behavioral, physiological and laboratory measures. The weakness of the rate dependency hypothesis, however, is not necessarily fatal to the idea that the state of the organism prior to drug administration influences the response profile of the drug. The heterogeneity of stimulant effects, and the relationship between stimulant effects and the predrug state of the organism, especially in electrophysiological paradigms, are clear. A hypothesis is presented to suggest that HAC may be characterized by a trait of excessive variability. Homeostatic stimulant effects in reducing response variability may be central to the therapeutic action of the drug. A neural substrate for the abnormal oscillations which characterize HAC, the correction of which is germane to therapeutic stimulant effects, is presented in terms of the regulatory functions of the frontal lobe. A neuroanatomic locus of childhood hyperactivity is proposed in terms of disorder or dysmaturation of frontal striatal systems.

Effects of methylphenidate on whirler mice: an animal model for hyperkinesis

The neurological mutant whirler mouse, one of several strains of waltzing mice, may be suitable as an animal model for testing studies relative to hyperkinesis. As in hyperkinetic children, behaviorally the mice are extremely restless, nervous, excitable, irritable and aggressive but also show symptoms of rotation behavior, head-shaking and deafness. This study demonstrated that paradoxically oral intubation of daily doses of 5.0 mg/kg of methylphenidate during a 23 week period significantly decreased circling activity in test mice. The effects on circling behavior were reversible following cessation of methylphenidate administration. After 18 weeks of cessation of the CNS stimulant, oral administration of a single dose of 2.5 mg/kg of methylphenidate caused a 37.8% increase in circling activity but the increase compared to control whirler mice was not significant. Use of this strain as an animal model may be especially beneficial in the screening of new drugs for the treatment of hyperkinesis.

Catecholamine metabolism in the attention deficit disorder: implications for the use of amino acid precursor therapy

The use of catecholamine precursors and agonists in the attention deficit disorder and various biochemical studies have implicated deficient catecholamine metabolism in the pathogenesis of this illness. The amino acid L-tyrosine, a catecholamine precursor, is capable of augmenting central dopamine norepinephrine. Study of its use in the treatment of the attention deficit disorder as a safer alternative to stimulant therapy is thus warranted.

Effects of drugs and chemicals on the fetus and newborn (1)

Drug teratogenicity has been demonstrated experimentally for more than 30 years. After the discovery of the thalidomide-induced embryopathies, the fetal dangers of maternal drug ingestion were overemphasized. Accumulation of additional information during the past 15 years has led to a more balanced viewpoint concerning drug teratogenicity. A complex set of circumstances must prevail for a specific teratogenic effect to result. Not only the drug or environmental pollutant in question but also its dose, timing, and frequency of administration as well as the genetic and individual susceptibility of the embryo are important factors. Herein we review the currently available information on drug and environmental effects on the fetus and neonate.

Neurochemical correlates of attention deficit disorder

Evidence from both epidemiologic and pharmacologic investigations into the biology of attention deficit disorder suggests that abnormalities in neurotransmitter function exist in children with attention deficit disorder. This article reviews this evidence and discusses the limitations imposed upon such clinical investigations.

Methylphenidate decreases local glucose metabolism in the motor cortex

The local cerebral metabolism on glucose (l-CMRg) was evaluated in animals given methylphenidate (15 mg/kg) in order to investigate possible mechanisms of action of the drug. Significant increases in l-CMRg (p greater than 0.05) were found in the globus pallidus, entopeduncular nucleus, substantia nigra, subthalamic nucleus, inferior olive, red nucleus, lateral cerebellar cortex, ventral lateral nucleus of the thalamus and the midbrain reticular formation. Significant decreases (p greater than 0.05) in l-CMRg were found in the motor cortex. These results suggest possible mechanisms for methylphenidate's action in attention deficit disorders.

D-Amphetamine at low doses suppresses noradrenergic functions

This study examined the effects of d-amphetamine on the firing rate of hippocampal cells which had been shown to have an inhibitory, noradrenergic input from the nucleus locus coeruleus (LC). d-Amphetamine at low doses of 0.1 mg/kg i.v. or 0.5 mg/kg i.p. increased the firing rates of these cells. With higher doses of d-amphetamine, both increases and decreases in the firing rates of hippocampal cells were observed. These differential effects on the firing rate of hippocampal cells were statistically significant (x2 = 13.32, d.f. = 3, P less than 0.01). The increased firing rate of hippocampal cells produced by the low doses of d-amphetamine was blocked by a prior destruction of the LC indicating that the drug effect was mediated by LC neurons. d-Amphetamine also significantly attenuated the decrement in the firing rates of hippocampal cells produced by LC stimulation (P less than 0.01). These results suggest that low doses of d-amphetamine suppress rather than enhance the actions os norepinephrine.

Behavioral and neurochemical interactions of dextroamphetamine and methylphenidate in rats

The effects of dextroamphetamine and methylphenidate on locomotor activity and brain levels of norepinephrine and dopamine were compared in male Sprague-Dawley rats. Both drugs produced a dose-related increase in locomotor activity during the hour immediately following intraperitoneal administration. However, combined administration of drugs elicited only the effect of dextroamphetamine. Brain levels of norepinephrine and dopamine also increased 1 hr after dextroamphetamine dosing. Methylphenidate did not exhibit these effects and antagonized the neurochemical changes produced by dextroamphetamine. Although both drugs are considered to exert their effects by indirect activation of brain catecholamine systems, differences in their mechanism of action appear to result in a lack of additive or antagonistic effects when dextroamphetamine and methylphenidate are coadministered. These findings may have clinical significance with respect to the use of such agents in minimal brain dysfunction.

Operant conditioning of EEG rhythms and ritalin in the treatment of hyperkinesis

Enhanced voluntary motor inhibition regularly accompanies conditioned increases in the sensorimotor rhythm (SMR), a 12--14-Hz Rolandic EEG rhythm in cats.A similar rhythm, presumably SMR, has also been identified in the human EEG. The clinical effectiveness of SMR operant conditioning has been claimed for epilepsy, insomnia, and hyperkinesis concurrent with seizure disorders. The present report attempts to follow up and replicate preliminary findings that suggested the technique's successful application to hyperkinesis uncomplicated by a history of epilepsy. SMR was defined as 12--14-Hz EEG activity in the absence of high-voltage slow-wave activity between 4 and 7 Hz. Anticipated treatment effects were indexed by systematic behavioral assessments of undirected motor activity and short attention span in the classroom. EEG and behavioral indices were monitored in four hyperkinetic children under the following six conditions: (1) No Drug, (2) Drug Only, (3) Drug and SMR Training I, (4) Drug and SMR Reversal Training, (5) Drug and SMR Training II, (6) No Drug and SMR Training. All hyperkinetic subjects were maintained on a constant drug regimen throughout the phases employing chemotherapy. Contingent increases and decreases in SMR occurred in three of four training subjects and were associated with similar changes in classroom assessments of motor inactivity. Combining medication and SMR training resulted in substantial improvements that exceeded the effects of drugs alone and were sustained with SMR training after medication was withdrawn. In contrast, these physiological and behavioral changes were absent in one highly distractible subject who failed to acquire the SMR task. Finally, pretraining levels of SMR accurately reflected both the seve-ity of original motor deficits and the susceptibility of hyperkinetic subjects to both treatments. Although the procedure clearly reduced hyperkinetic behavior, a salient, specific therapeutic factor could not be identified due to the dual EEG contingency imposed combined with associated changes in EMG. Despite these and other qualifying factors, the findings suggested the prognostic and diagnostic value of the SMR in the disorder when overactivity rather than distractibility is the predominant behavioral deficit.

Erythrosin B inhibits dopamine transport in rat caudate synaptosomes

Erythrosin B is a member of a class of fluorescein dyes that are suggested to elicit hyperkinesis when ingested by susceptible children. We found that erythrosin B inhibits dopamine uptake in rat caudate synaptosomes "uncompetitively" in the 10- to 800-micromolar range. Half maximal inhibition of uptake occurred at 45 micromolar. Uncompetitive inhibition denotes a decrease in efficacy of the dopamine membrane transport mechanism with an increase in affinity of dopamine to the carrier. Erythrosin B also decreased nonsaturable binding of dopamine to the synaptosome membrane. The inhibitory action of erythrosin B on dopamine uptake is consistent with the hypothesis that erythrosin B can act as a central excitatory agent able to induce hyperkinetic behavior.

Psychopharmacological perspectives in childhood psychoses

1. Neuroleptic drugs are still widely employed in childhood psychoses, but new experiences on a large number of cases in this field, have shown the usefullness of antidepressive drugs, either alone or associated with neuroleptics, also when depressive symptoms are not clearly evident. 2. This peculiar aspect, still noticed in pathological situations of less gravity, calls again attention to childhood depression and to the difficulty of identifying it. 3. The problem is developed and discussed as follows: a. The psychopathological features regarded from the psychodynamic point of view; b. The neurofunctional background in various ages; c. The pharmacodynamic characteristics of the drugs; d. The good results with lithium treatment in several cases allow the authors to outline some clinical pictures as well as biochemical markers as to recognize endogenous depression in childhood.

Are the effects of psychomotor stimulant drugs on hyperactive children really paradoxical?

The improved "attention" exhibited by hyperactive children treated with amphetamine-like compounds is postulated to be related to a normal action of these drugs in producing stereotyped behavior. Such activity can be conceptualised as an increased "focusing" of attention, which would be expected to aid performance in tasks involving sustained concentration of attention, but impair performance on tasks involving reversals in cognitive strategy. These behavioural actions of the drugs can be linked to the functioning of central dopaminergic mechanisms.

Effects of imipramine and methylphenidate on perceptual-motor performance of hyperactive children

Perceptual-motor effects of imipramine and methylphenidate were evaluated in a double-blind study of 47 hyperactive children. No effects were found for imipramine, although methylphenidate improved performance on several tests. Improvement due to methylphenidate was not related to base-line scores. A discriminant function was computed to compare baseline perceptual-motor scores of the hyperactive and 41 normal children. Only half of the hyperactive children were clearly discriminated from normal children by the discriminant function. The digit-span test, which was not sensitive to methylphenidate, effectively discriminated hyperactive from normal children.

Selective brain dopamine depletion in developing rats: an experimental model of minimal brain dysfunction

Administration of 6-hydroxydopamine to neonatal rats produces a rapid and profound depletion of brain dopamine. Total activity of treated animals is significantly greater than that of controls between 12 and 22 days of age, but then declines, an activity pattern similar to that seen in affected children. This suggests a functional deficiency of brain dopamine in the pathogenesis of minimal brain dysfunction.

[The influence of an early brain damage on psychic diseases in adulthood. A study documented by the AMP-system (author's transl)]

The data of 1926 psychiatric in-patients documented by the AMP-system were evaluated with respect to the anamnestic item "early brain damage". There were 82 cases (4.26%). For each patient with an early brain damage a control-case matched for age, sex and diagnosis was selected. Patients with mental handicaps were examined seperately. For methodical reasons the demonstrated frequencies are an underestimation, and subtle differences between index- and controlgroups are not expected to be visible, but those differences which could be found, will be real. The highest frequency for "early brain damage" was found in the diagnostic group which also contained the psychic disturbances in epilepsy. Among neuroses and schizophrenia the item was found with the average-frequency of the total psychiatric population. It was under-represented among affective psychoses and involutional diseases. An early brain damage predisposed to an earlier age of first manifestation, and the psychic disease showed a more chronic course. Hypotheses concerning the uneven distribution over the diagnostic spectrum were discussed with particular emphasis on the fact that hypoxic perinatal disturbances do not lead to diffuse and unspecific consequences, but predominantly to a sensory impairment and possibly to a deficit in the aminergic system. Early brain damage was also discussed as one mode of the transmission of psychic diseases.

Involvement of brain monoamines in the stimulant and paradoxical inhibitory effects of methylphenidate

The significance of central noradrenergic, dopaminergic and serotonergic neural systems for the locomotor stimulant effects of methylphenidate was investigated in the rat. In order to study the role of brain catecholamines, rats were pretreated with reserpine (2.5 mg/kg) followed 24 hrs later by treatment with alpha-methyltyrosine (25 mg/kg) or U-14,624 (75 mg/kg), a dopamine-beta-hydroxylase inhibitor. In these experiments, methylphenidate stimulated motor activity was antagonized by alpha-methyltyrosine and enhanced after treatment with U-14,624, suggesting that release of newly synthesized dopamine is important to a locomotor stimulant action of methylphenidate. Evidence implicating brain serotonin in the actions of methylphenidate was obtained in rats pretreated with pargyline or p-chlorophenylalanine (PCPA). Administration of pargyline 1 hr prior to methylphenidate was found to reduce the locomotor activity induced by methylphenidate and this was antagonized by pretreatment with low doses of PCPA. Higher doses of PCPA caused a significant elevation of methylphenidate induced activity which could be reduced by 5-hydroxytryptophan. Destruction of serotonergic neurons with 5,7-dihydroxytryptamine also potentiated methylphenidate induced locomotion. These latter findings suggest that serotonergic fibers have an inhibitory function in brain. These results are discussed in relation to the possible mechanism by which methylphenidate may act in hyperkinesis.