Abnormal behavior associated with a point mutation in the structural gene for monoamine oxidase A

Basic concepts in the study of diseases with complex genetics

Most diseases run in families--this is also true of virtually all psychiatric disorders. Twin and adoption studies have shown that most psychiatric disorders have a genetic component, yet very few genetic factors are known, as is true for most disorders with a complex genetic origin. Here I review, for nongeneticists, some of the basic terminology and concepts used when studying complex genetic diseases, with examples from psychiatric genetics. This review is intended to help in the understanding and critical evaluation of reports on genetics of psychiatric illnesses in the literature.

Benefits and pitfalls encountered in psychiatric genetic association studies

The genetic association strategy is currently being applied to a number of psychiatric phenotypes including disease vulnerability, personality variation and clinical response to psychotropic drugs. Association studies offer the prospect of identification of the specific alleles that confer significant effects on clinical phenotype. However, it should be noted that this strategy has additional advantages as well as unique drawbacks. In this paper, we review the basic methodology utilized in each step of a typical psychiatric genetic association study and discuss their potential benefits and pitfalls with particular emphasis on the selection of clinical phenotype, the identification of a candidate gene, the selection of a candidate variant, clinical data set design, and the statistical analysis of association data. With appropriate design and execution, it is hoped that the association strategy will prove to be as successful in psychiatry as it has proven to be in other branches of medicine.

Monoamine oxidase: from genes to behavior

Cloning of MAO (monoamine oxidase) A and B has demonstrated unequivocally that these enzymes are made up of different polypeptides, and our understanding of MAO structure, regulation, and function has been significantly advanced by studies using their cDNA. MAO A and B genes are located on the X-chromosome (Xp11.23) and comprise 15 exons with identical intron-exon organization, which suggests that they are derived from the same ancestral gene. MAO A and B knock-out mice exhibit distinct differences in neurotransmitter metabolism and behavior. MAO A knock-out mice have elevated brain levels of serotonin, norephinephrine, and dopamine and manifest aggressive behavior similar to human males with a deletion of MAO A. In contrast, MAO B knock-out mice do not exhibit aggression and only levels of phenylethylamine are increased. Mice lacking MAO B are resistant to the Parkinsongenic neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine. Both MAO A and B knock-out mice show increased reactivity to stress. These knock-out mice are valuable models for investigating the role of monoamines in psychoses and neurodegenerative and stress-related disorders.

The monoamine oxidase-A gene and major psychosis in Japanese subjects

BACKGROUND

Monoamine oxidase (MAO) is a critical enzyme in deamination of biogenic amines and may be involved in the pathophysiology of major psychosis, including mood disorder and schizophrenia. Recently, evidence for genetic association between the MAO-A gene and bipolar mood disorder was obtained in Caucasians.

METHODS

We investigated the polymorphisms of the MAO-A gene, which may be related to enzyme activity (T/941/G, A/1609/G), with amino-acid change (A/1609/G), in Japanese patients with bipolar disorder patients (n = 132), unipolar major depression (n = 43), or schizophrenia (n = 95), and controls (n = 169).

RESULTS

No difference in the allele frequencies or genotype distribution of the T/941/G variation was observed between any disease group and the control group. As for the A/1609/G variation, no G allele was found in the Japanese subjects.

CONCLUSIONS

No evidence for the genetic association between the MAO-A gene and major psychosis was obtained in the Japanese subjects.

Plasma membrane transporters of serotonin, dopamine, and norepinephrine mediate serotonin accumulation in atypical locations in the developing brain of monoamine oxidase A knock-outs

Genetic loss or pharmacological inhibition of monoamine oxidase A (MAOA) in mice leads to a large increase in whole-brain levels of serotonin (5-HT). Excess 5-HT in mouse neonates prevents the normal barrel-like clustering of thalamic axons in the somatosensory cortex. Projection fields of other neuron populations may develop abnormally. In the present study, we have analyzed the localization of 5-HT immunolabeling in the developing brain of MAOA knock-out mice. We show numerous atypical locations of 5-HT during embryonic and postnatal development. Catecholaminergic cells of the substantia nigra, ventral tegmental area, hypothalamus, and locus ceruleus display transient 5-HT immunoreactivity. Pharmacological treatments inhibiting specific monoamine plasma membrane transporters and genetic crosses with mice lacking the dopamine plasma membrane transporter show that the accumulation of 5-HT in these catecholaminergic cells is attributable to 5-HT uptake via the dopamine or the norepinephrine plasma membrane transporter. In the telencephalon, transient 5-HT immunolabeling is observed in neurons in the CA1 and CA3 fields of the hippocampus, the central amygdala, the indusium griseum, and the deep layers of the anterior cingulate and retrosplenial cortices. In the diencephalon, primary sensory nuclei, as well as the mediodorsal, centrolateral, oval paracentral, submedial, posterior, and lateral posterior thalamic nuclei, are transiently 5-HT immunolabeled. The cortical projections of these thalamic nuclei are also labeled. In the brainstem, neurons in the lateral superior olivary nucleus and the anteroventral cochlear nucleus are transiently 5-HT immunolabeled. None of these structures appear to express the monoamine biosynthetic enzyme L-aromatic amino acid decarboxylase. The administration of monoamine plasma membrane transporter inhibitors indicates that the 5-HT immunolabeling in these structures is attributable to an uptake of 5-HT by the 5-HT plasma membrane transporter. This points to neuron populations that form highly precise projection maps that could be affected by 5-HT during specific developmental stages.

Natural variation in a neuropeptide Y receptor homolog modifies social behavior and food response in C. elegans

Natural isolates of C. elegans exhibit either solitary or social feeding behavior. Solitary foragers move slowly on a bacterial lawn and disperse across it, while social foragers move rapidly on bacteria and aggregate together. A loss-of-function mutation in the npr-1 gene, which encodes a predicted G protein-coupled receptor similar to neuropeptide Y receptors, causes a solitary strain to take on social behavior. Two isoforms of NPR-1 that differ at a single residue occur in the wild. One isoform, NPR-1 215F, is found exclusively in social strains, while the other isoform, NPR-1 215V, is found exclusively in solitary strains. An NPR-1 215V transgene can induce solitary feeding behavior in a wild social strain. Thus, isoforms of a putative neuropeptide receptor generate natural variation in C. elegans feeding behavior.

What is wrong with reductionist explanations of behaviour?

Methodological reductionism has served biology well, but its problems in the study of behaviour include turning open systems into closed ones, defining the units of analysis, and interpreting correlative and causal relationships between processes studied within different biological discourses, from molecular biology to psychology The problems become more acute when methodological becomes philosophical reductionism, with its declared goal of collapsing 'higher level' explanations into 'lower level' ones. Quite apart from the vexed question of what constitutes a 'level', relevant behavioural phenomena may only be manifest at such higher levels. The reductionist programme assumes that parts have ontological and possibly historical (developmental, evolutionary) primacy over wholes, yet the nature of living systems is such that this cannot be the case. I will exemplify these problems in the context of the study of behaviour. But the worst problem arises when reductionism becomes an ideology, especially in the context of human behaviour, when it makes the claims to explain complex social phenomena (e.g. violence, alcoholism, the gender division of labour or sexual orientation) in terms of disordered molecular biology or genes. In doing so, ideological reductionism manifests a cascade of errors in method and logic: reification, arbitrary agglomeration, improper quantification, confusion of statistical artefact with biological reality, spurious localization and misplaced causality.

Control of alternative behavioral states by serotonin in Caenorhabditis elegans

Serotonin has been implicated in the regulation of a wide range of brain functions involving alternative behavioral states, including the control of mood, aggression, sex, and sleep. Here, we report that in the nematode Caenorhabditis elegans, serotonin controls a switch between two distinct, on/off states of egg-laying behavior. Through quantitative analysis of the temporal pattern of egg-laying events, we determined that egg laying can be modeled as a novel random process, in which animals fluctuate between discrete behavioral states: an active state, during which eggs are laid in clusters, and an inactive state, during which eggs are retained. Single-cell ablation experiments indicate that two pairs of motor neurons, HSNL/HSNR and VC4/VC5, can induce the active phase by releasing serotonin. These neurons also release acetylcholine, which appears to trigger individual egg-laying events within the active phase. Genetic experiments suggest that determination of the behavioral states observed for C. elegans egg laying may be mediated through protein kinase C-dependent (PKC-dependent) modulation of voltage-gated calcium channels.

Are MAO-A deficiency states in the general population and in putative high-risk populations highly uncommon?

Lack of monoamine oxidase A (MAO-A) due to either Xp chromosomal deletions or alterations in the coding sequence of the gene for this enzyme are associated with marked changes in monoamine metabolism and appear to be associated with variable cognitive deficits and behavioral changes in humans and in transgenic mice. In mice, some of the most marked behavioral changes are ameliorated by pharmacologically-induced reductions in serotonin synthesis during early development, raising the question of possible therapeutic interventions in humans with MAO deficiency states. At the present time, only one multi-generational family and a few other individuals with marked MAO-A deficiency states have been identified and studied in detail. Although MAO deficiency states associated with Xp chromosomal deletions were identified by distinct symptoms (including blindness in infancy) produced by the contiguous Norrie disease gene, the primarily behavioral phenotype of individuals with the MAO mutation is less obvious. This paper reports a sequential research design and preliminary results from screening several hundred volunteers in the general population and from putative high-risk groups for possible MAO deficiency states. These preliminary results suggest that marked MAO deficiency states are very rare.

Monoamine oxidase A deficiency: biogenic amine metabolites in random urine samples

We have recently described an association between abnormal behaviour and monoamine oxidase A (MAO-A) deficiency in several males from a single large Dutch kindred. A characteristically abnormal excretion pattern of biogenic amine metabolites was present in 24-hour urine of affected males. Because of this strikingly abnormal metabolite pattern observed in 24 hour urine samples of MAO-A deficient males we hypothesized that it should be possible to diagnose this condition by examining random urine samples. We therefore studied multiple urine samples obtained over a two-week study period from two males with selective MAO-A deficiency. The results demonstrate that the characteristic abnormalities in the excretion of biogenic amines and their metabolites were faithfully present in every one of 12 independent samples obtained from the MAO-A deficient males over the two-week study period. We conclude that MAO-A deficiency can be reliably diagnosed by measuring the ratio of normetanephrine (NMN) to VMA (or that of NMN to MHPG) in random urine samples.

Relationship between monoamine oxidase (MAO) A specific activity and proportion of human skin fibroblasts which express the enzyme in culture

Total deficiency of monoamine oxidase A (MAO-A) in affected males of a single, human kindred appears to be associated with mild mental retardation and significant behavioral anomalies. Though total MAO-A deficiency appears to be rare, the extent and significance of individual variation in monoamine oxidase A activity in human populations is unclear. Since MAO-A activity is undetectable in blood cells, most systematic surveys of individual variation MAO-A activity have compared enzyme activity in human fibroblasts cultured from skin biopsies. Surprisingly, MAO-A activity in skin fibroblast cultures from unrelated donors ranges over 100-fold. It has been suggested that this extreme variation in fibroblast MAO-A activity between donors reflects individual, genetic variation in the regulation of MAO-A in fibroblasts. I have found from studies with immunofluorescence microscopy and flow cytometry that the proportion of MAO-A+ cells in fibroblast cultures is (a) highly variable between cultures, (b) a reproducible characteristic of each culture and (c) the primary factor responsible for variation in MAO-A specific activity in whole cell, skin fibroblast homogenates. It has been shown previously that MAO-A activity of a skin fibroblast culture is relatively constant with continued passage prior to cellular senescence. Therefore, these new data raise the possibility that MAO-A expression is confined to a functionally distinct subset of human skin fibroblasts.

Genes, free will, and criminal responsibility

Advances in human genetics have raised the possibility that genetic mechanisms can explain various aspects of human behavior. It has been suggested that such genetic explanations would tend to diminish responsibility for one's actions. In this paper I argue that the genetic approach adds little to our understanding of free will, determinism, and responsibility. Even though human beings are material systems obeying the laws of the physical and biological sciences, their behavior may still be unpredictable and essentially undetermined. Moreover, with few exceptions, behavior influenced by genes is no more deterministic than is behavior influenced by the environment. An analysis of the genetic and environmental influences and the complex interactions between them reveals a certain symmetry between genetic and environmental explanations of behavior. Consequently, any argument concerning the relevance of a genetic excuse to a criminal defense will be equally applicable to an environmental excuse.

Biopolitics: the newest synthesis?

On first impression, the disciplines of genetics and political science would appear to be unrelated. And yet, commencing more than 30 years ago, the interdisciplinary field known as Biopolitics has now taken hold. This essay traces the central thrust of the biopolitical research agenda. It describes, analyzes, and assesses how political scientists have sought to show connections between our species' genetic constitution and our species' political behavior. Important bridges between the two are the neurophysiology of the human brain and the role of evolutionary theory in charting man's adaptational political profile. The parameters of the emerging biopolitical literature raise profound policy questions, some of which are also characterized.

The renal monoamine oxidases: pathophysiology and targets for therapeutic intervention

The mitochondrial enzyme monoamine oxidases A and B are among the major metabolic agents for the degradation of the biogenic amines adrenaline, noradrenaline, dopamine and serotonin. The fact that the kidney contains a large amount of monoamine oxidase suggests that the renal effects of biogenic amines might depend in part on the activity of these enzymes.

The role of non-P450 enzymes in drug oxidation

In addition to cytochrome P450, oxidation of drugs and other xenobiotics can also be mediated by non-P450 enzymes, the most significant of which are flavin monooxygenase, monoamine oxidase, alcohol dehydrogenase, aldehyde dehydrogenase, aldehyde oxidase and xanthine oxidase. This article highlights the importance of these non-P450 enzymes in drug metabolism. A brief introduction to each of the non-P450 oxidizing enzymes is given in this review and the oxidative reactions have been illustrated with clinical examples. Drug oxidation catalyzed by enzymes such as flavin monooxygenase and monoamine oxidase may often produce the same metablolites as those generated by P450 adn thus drug interactions may be difficult to predict without a clear knowledge of the underlying enzymology. In contrast, oxidation via aldehyde oxidase and xanthine oxidase gives different metabolites to those resulting from P450 hydroxylation. Although oxidation catalyzed by non-P450 enzymes can lead to drug inactivation, oxidation may be essential for the generation of active metabolite(s). The activation of a number of prodrugs by non-P450 enzymes is thus described. It is concluded that there is still much to learn about factors affecting the non-P450 enzymes in the clinical situation.

Serotonin, social status and aggression

Serotonin, social status and aggression appear to be linked in many animal species, including humans. The linkages are complex, and, for the most part, details relating the amine to the behavior remain obscure. During the past year, important advances have been made in a crustacean model system relating serotonin and aggression. The findings include the demonstration that serotonin injections will cause transient reversals in the unwillingness of subordinate animals to engage in agonistic encounters, and that at specific synaptic sites involved in activation of escape behavior, the direction of the modulation by serotonin depends on the social status of the animal.

Neuronal adaptations to changes in the social dominance status of crayfish

The effect of superfused serotonin (5-HT; 50 microns) on the synaptic responses of the lateral giant (LG) interneuron in crayfish was found to depend on the social status of the animal. In socially isolated animals. 5-HT persistently increased the response of LG to sensory nerve shock. After social isolates were paired in a small cage, they fought and determined their dominant and subordinate status. After 12 d of pairing, 5-HT reversibly inhibited the response of LG in the social subordinate and reversibly increased the response of LG in the social dominant crayfish. The effect of 5-HT changed approximately linearly from response enhancement to inhibition in the new subordinate over the 12 d of pairing. If, after 12 d pairing, the subordinate was reisolated for 8 d, the response enhancement was restored. If the subordinate, instead, was paired with another subordinate and became dominant in this new pair, the inhibitory effect of 5-HT changed to an enhancing effect over the next 12 d of pairing. If, however, two dominant crayfish were paired and one became subordinate, the enhancing effect of 5-HT persisted in the new subordinate even after 38 d pairing. These different effects of serotonin result from the action of two or more molecular receptors for serotonin. A vertebrate 5-HT, agonist had no effect on social isolates but reversibly inhibited the response of LG in both dominant and subordinate crayfish. The inhibitory effects of the agonist developed approximately linearly over the first 12 d of pairing. A vertebrate 5-HT2 agonist persistently increased the response of LG in isolate crayfish and reversibly increased the response of the cell in dominant and subordinate crayfish. Finally, although neurons that might mediate these effects of superfused 5-HT are unknown, one pair of 5-HT-immunoreactive neurons appears to contact the LG axon and initial axon segment in each abdominal ganglion in its projection caudally from the thorax.

Monoamine oxidase B (MAOB)-containing structures in MAOA-deficient transgenic mice

Monoamine oxidase (MAO)-containing structures were studied for the first time in type A MAO (MAOA)-deficient transgenic mice (Tg8) derived from C3H strain, using MAO enzyme histochemistry. In this mutant line, MAOA activity was not detected in neurons of the locus coeruleus. In contrast, in their dorsal raphe neurons, we noted an intense activity of type B MAO (MAOB). Based on pharmacological MAOA suppression experiments employing a specific inhibitor (clorgyline), we confirmed that the localization of MAOB-positive structures are not different between Tg8 mutant and normal C3H line. Many of MAOB-positive structures which have not been described previously in the rat, cat and primates were described in this study. In the forebrain, MAOB-containing neurons were discriminated in the striatum, septal nuclei, major island of Calleja, diagonal band, medial forebrain bundle, ventral pallidum and amygdaloid nucleus. Stained neurons in the thalamus and hypothalamus were much more extensively distributed in the mouse than the rat. Pontine laterodorsal tegmental neurons showed MAOB activity. The present data suggest that serotonin, a preferential substrate for MAOA, can be oxidized by MAOB in MAOA-deficient Tg8 mice.

Increased stress response and beta-phenylethylamine in MAOB-deficient mice

MAOA and MAOB are key iso-enzymes that degrade biogenic and dietary amines. MAOA preferentially oxidizes serotonin (5-hydroxytryptamine, or 5-HT) and norepinephrine (NE), whereas MAOB preferentially oxidizes beta-phenylethylamine (PEA). Both forms can oxidize dopamine (DA). A mutation in MAOA results in a clinical phenotype characterized by borderline mental retardation and impaired impulse control. X-chromosomal deletions which include MAOB were found in patients suffering from atypical Norrie's disease, which is characterized by blindness and impaired hearing. Reduced MAOB activity has been found in type-II alcoholism and in cigarette smokers. Because most alcoholics smoke, the effects of alcohol on MAOB activity remain to be determined. Here we show that targetted inactivation of MAOB in mice increases levels of PEA but not those of 5-HT, NE and DA, demonstrating a primary role for MAOB in the metabolism of PEA. PEA has been implicated in modulating mood and affect. Indeed, MAOB-deficient mice showed an increased reactivity to stress. In addition, mutant mice were resistant to the neurodegenerative effects of MPTP, a toxin that induces a condition reminiscent of Parkinson's disease.

Serotonin, aggression, and parental psychopathology in children with attention-deficit hyperactivity disorder

OBJECTIVE

To explore the relationship between central serotonergic (5-HT) function and history of parental aggression in aggressive and nonaggressive boys with attention-deficit hyperactivity disorder (ADHD).

METHOD

History of psychiatric symptoms was assessed in the biological parents of 41 boys with ADHD. The relationship between 5-HT function in aggressive and nonaggressive probands, as assessed via the prolactin response to fenfluramine (FEN) challenge, and parental history of aggression was examined.

RESULTS

Aggressive boys with a parental history of aggressive behavior had a significantly lower prolactin response to FEN challenge than aggressive boys without a parental history of aggression. Nonaggressive boys had a prolactin response midway between those of the two aggressive subgroups, and their prolactin response did not vary as a function of parental aggression. Children subdivided on the basis of parental history of other psychiatric symptoms did not differ in their response to the FEN challenge.

CONCLUSIONS

These data indicate an association between parent aggressive behavior and lower 5-HT function in aggressive boys with ADHD but do not indicate the extent to which this association is environmentally and/or genetically transmitted. There may be different neurochemical mechanisms in familial and nonfamilial aggressive children, which have clinical implications for pharmacological interventions.

Platelet serotonin measures in adolescents with conduct disorder

Dysregulation of serotonergic function has been associated with aggression in several studies involving children, adolescents, and adults. This study investigated the relationship of platelet serotonergic measures to conduct disorder type, severity of aggression, and social skills impairment. Standardized assessments of diagnosis, aggression, impulsivity, and social skills were obtained from 43 male adolescents (ages 13-17) incarcerated at an involuntary residential treatment facility for juvenile offenders. Blood samples were collected and assayed for whole blood serotonin (5-HT) and platelet [3H]-paroxetine-labeled 5-HT-transporter binding. Whole blood 5-HT was higher in adolescents with conduct disorder, childhood type than in subjects with conduct disorder, adolescent type. Whole blood 5-HT was positively correlated with violence rating of the current offense and total offense points, and staff ratings of social skills impairment. Our findings are consistent with a relationship between 5-HT dysregulation and aggressive behavior in incarcerated adolescent boys with conduct disorder, particularly of childhood onset.

Low resting heart rate at age 3 years predisposes to aggression at age 11 years: evidence from the Mauritius Child Health Project

OBJECTIVE

Previous studies indicate that low resting heart rate is probably the best-replicated biological correlate of childhood antisocial and aggressive behavior. Nevertheless, there have been few longitudinal tests of this relationship, little control over potential confounds and mediators, and no test of its cross-cultural generalizability. This study tests the hypothesis that low resting heart rate at age 3 years predicts aggression at age 11 years.

METHOD

Resting heart rate at age 3 years was assessed in 1,795 male and female children from Mauritius. Aggressive and nonaggressive forms of antisocial behavior were assessed at age 11 years using the Child Behavior Checklist.

RESULTS

Aggressive children had lower heart rates than nonaggressive children (p < .001). Conversely, those with low heart rates were more aggressive than those with high heart rates (p < .003). There were no interactions with gender or ethnicity. Evidence was found for specificity of low heart rate to aggressive forms of antisocial behavior. Group differences in heart rate were not attributable to 11 biological, psychological, and psychiatric mediators and confounds.

CONCLUSIONS

It is concluded that low resting heart rate, a partly heritable trait reflecting fearlessness and stimulation-seeking, is an important, diagnostically specific, well-replicated, early biological marker for later aggressive behavior.

Genetic association between monoamine oxidase and manic-depressive illness: comparison of relative risk and haplotype relative risk data

There have been several conflicting reports of association of monoamine oxidase (MAO) A gene polymorphisms and bipolar affective disorder. In order to determine the possible role of the MAO region in susceptibility to affective disorders in an independent sample, we have genotyped 83 probands of bipolar affective disorder families, 56 sets of parents of bipolar probands, and 84 normal controls for intronic simple sequence repeat polymorphisms of the MAO-A and MAO-B genes. For MAO-A there were no significant differences in allele frequencies between bipolar and normal control groups for both genders. However, for MAO-B there were significant differences between groups for both genders. In contrast, allele-wise haplotype relative risk analysis for the 56 bipolar proband-parent trios found no significant differences between transmitted and non-transmitted allele frequencies for MAO-A or B. These data do not support the association of MAO-A or B with bipolar affective disorder but do demonstrate that undetected population stratification can be an important source of bias in case-control studies.

Genetics of aggressive and violent behavior

This article develops the topic of the genetics of aggressive and violent behavior from three directions. Firstly, evidence from twin, family, and adoption studies will establish the case for the importance of genetically transmitted factors in the genesis of aggressivity from childhood through adulthood. Secondly, evidence from adoption studies will be presented to show that some environmental conditions interact with genetic factors in such a way as to suggest that the development of aggressivity requires that both genetic and environmental factors be present. Thirdly, additional and direct evidence of genetic factors in aggressivity is presented from the perspective of molecular genetics, where underlying biochemical mechanisms associated with aggressivity have been found to be caused by specific genes in animal models with confirmation of similar physiologic mechanisms in humans.

The neurobiology of impulsive aggression

As noted previously, it is likely that the tendency to lash out verbally or physically at others is influenced by an interaction among multiple complex biologic factors. We need to investigate how these systems interact with each other to develop a more thorough understanding of the brain's influence over aggressive behavior. We are at a very early stage in our understanding of the neurobiology of aggression. There are no simple tools for studying the complex neurophysiology of the human brain. The studies cited in this article include techniques limited in their utility. As our technologies improve, discovering a more thorough picture of the brain's influence over aggressive behavior may be possible. For example, functional neuroimaging may help to localize abnormal neurotransmitter functioning in the brains of individuals with impulsive aggressive behavior. Our technologies are beginning to reveal the differential effects of subsystems of neurotransmitter regulation. Subtypes of serotonin receptors may differentially mediate impulsive aggressive behaviors. Animal studies suggest that 5-HT 1A receptor stimulation results in a decrease in aggressive behavior. As noted previously, aggressive personality-disordered patients show a blunted prolactin response to the 5-HT1A agonist buspirone. Antagonism of 5-HT 2 receptors appears to decrease aggression, and this effect may explain the ability of newer antipsychotic agents (which, unlike older antipsychotic medications, block 5-HT 2 receptors) to produce a dramatic reduction in aggression and agitation independent of effects on psychotic symptoms. Neglecting psychosocial factors in the causes of aggressive behavior would also be naive. Although environmental factors account for much of the predisposition to aggression, there have been few systematic studies to explore the relationship between life experiences and aggression. In addition, there have been no well-designed studies of the interaction between biology and an individual's environment in the genesis of aggressive behavior. There is some evidence of an association between childhood abuse and neglect and adult antisocial personality disorder, but this relationship might be merely an artifact of the genetic relationship between parental and offspring antisocial personality disorder. As we discussed in the introduction, one of the biggest hurdles in the study of the neurobiology of aggression is the lack of a consensus on definitions. "Intermittent Explosive Disorder" is the only category in DSM-IV that directly addresses individuals with problems with aggression, but the criteria are vague and only focus on a handful of the many patients who exhibit problems with aggressive behavior. It is our hope that investigators in this field can work together toward developing more precise and encompassing diagnostic criteria to study effectively both the neurobiology and treatment of these disorders.

A key amino acid responsible for substrate selectivity of monoamine oxidase A and B

Monoamine oxidase (MAO) oxidizes biologically important amines including neurotransmitters and plays a central role in the regulation of intracellular level of these amines. Two distinct forms of MAO (MAO A and MAO B) were defined based on differences in substrate and inhibitor specificities. We earlier reported that the region between about residues 120 and 220 of rat MAO is responsible for determination of the substrate selectivity of MAO A and B (Tsugeno, Y. Hirashiki, I., Ogata, F., and Ito, A. (1995) J. Biochem. (Tokyo) 118, 974-980). To determine the essential amino acids in this region that participate in substrate recognition, a series of mutant enzymes in which amino acid residues that are conserved among various species but are different between the two forms of the enzyme were replaced with the corresponding amino acids of the counterpart and were engineered from the cDNAs of rat liver MAO A and B, and affinities for several substrates were examined. A single mutation in which Phe-208 in MAO A was substituted by the corresponding residue of Ile in MAO B was sufficient to convert the A-type substrate selectivity, and the reverse was exactly the case. Phe at this position was replaceable with Tyr for the A-type specificity and Ile was replaceable with Val and Ala for the B-type. Thus, aromatic and aliphatic residues seem to contribute to render substrate selectivity of MAO A and MAO B, respectively.

Serotonin and aggressive motivation in crustaceans: altering the decision to retreat

In crustaceans, as in most animal species, the amine serotonin has been suggested to serve important roles in aggression. Here we show that injection of serotonin into the hemolymph of subordinate, freely moving animals results in a renewed willingness of these animals to engage the dominants in further agonistic encounters. By multivariate statistical analysis, we demonstrate that this reversal results principally from a reduction in the likelihood of retreat and an increase in the duration of fighting. Serotonin infusion does not alter other aspects of fighting behavior, including which animal initiates an encounter, how quickly fighting escalates, or which animal eventually retreats. Preliminary studies suggest that serotonin uptake plays an important role in this behavioral reversal.

Selective enhancement of emotional, but not motor, learning in monoamine oxidase A-deficient mice

Mice deficient in monoamine oxidase A (MAOA), an enzyme that metabolizes monoamines such as norepinephrine and serotonin, have elevated norepinephrine and serotonin levels in the frontal cortex, hippocampus, and cerebellum, compared with normal wild-type mice. Since monoamines in these areas are critically involved in a variety of behaviors, we examined learning and memory (using emotional and motor tasks) in MAOA mutant mice. The MAOA-deficient mice exhibited significantly enhanced classical fear conditioning (freezing to both tone and contextual stimuli) and step-down inhibitory avoidance learning. In contrast, eyeblink conditioning was normal in these mutant mice. The female MAOA-deficient mice also displayed normal species-typical maternal behaviors (nesting, nursing, and pup retrieval). These results suggest that chronic elevations of monoamines, due to a deletion of the gene encoding MAOA, lead to selective alterations in emotional behavior.

Assessment, management, and prognosis of canine dominance-related aggression

Aggression directed toward owners is a common complaint, and one that causes a great deal of emotional conflict. Assessment and treatment of this disturbing behavior problem must address owner safety as well as realistic expectations for improvement. Relatively mild aggression may be treated with a combination of prevention of injury, increased structure in the home, and safe control of the dog, including obedience training to reward the dog for deference to the owner. Disproportionately severe or unpredictable aggression is less likely to respond to treatment. Mounting evidence exists that aggressiveness is genetically and neurobiologically driven. Research in other species, and early research in the dog, suggest that aggression may be reduced by drug therapy to modify brain neurochemistry. Such treatment is not a cure, however, and should be paired with a lifelong, systematic program of safety and control in the home.

Predominant expression of monoamine oxidase B isoform in rabbit renal proximal tubule: regulation by I2 imidazoline ligands in intact cells

Previous studies have shown that a subpopulation of the catecholamine-degrading enzymes monoamine oxidase (MAO) A and B holds a previously unknown regulatory site, the I2-imidazoline binding site (I2BS). In the present work, we characterized the isoforms of monoamine oxidases expressed in the rabbit renal proximal tubule, defined their relationship with I2BS, and investigated the ability of I2BS ligands to inhibit enzyme activity in intact cells. Two findings indicate that MAO-B is the predominant isoform expressed in the renal proximal tubule cells: 1) Western blot performed with an anti-MAO-A/MAO-B polyclonal antiserum revealed a single 55-kDa band corresponding to MAO-B; 2) enzyme assays showed an elevated MAO-B activity ([14C]beta-phenylethylamine oxidation: Vmax = 1.31 +/- 0.41 nmol/min/mg protein), whereas MAO-A activity was only detectable ([14C]5-HT oxidation: Vmax = 80.3 +/- 19 pmol/min/mg protein). Photoaffinity labeling with the I2BS ligand [125I]2-(3-azido-4-iodophenoxy)-methylimidazoline revealed a single 55-kDa band, which indicates that MAO-B of the renal proximal tubule cells holds the I2 imidazoline binding site. [3H]Idazoxan binding studies and enzyme assays showed that, in intact cells, I2BS ligands bind to and inhibit MAO-B. Indeed, the increase in the accessibility of intracellular compartment by cell permeabilization did not enhance [3H]idazoxan binding, which indicates that, in intact cells, intracellular I2BS are fully occupied by imidazoline ligands. In addition, enzyme assays showed that incubation of proximal tubule cells with imidazoline ligands leads to a complete, dose-dependent inhibition of MAO activity. These data show the predominant expression of MAO-B in rabbit renal proximal tubule and its regulation by imidazoline ligands in intact cells.

Sequence, splice site and population frequency distribution analyses of the polymorphic human tryptophan hydroxylase intron 7

A human tryptophan hydroxylase intron seven polymorphism previously associated with low CSF 5-HIAA and suicidal behavior was sequenced and characterized for its potential role in TPH pre-mRNA splicing. Two polymorphic sites were identified: A218C and A779C. The 779A allelic frequency in various populations ranged from 0.43 to 0.61 and was in strong linkage disequilibrium with the A218C site. A218C provides a site for restriction fragment length polymorphism analysis. TPH mRNA was reverse-transcribed and sequenced. No aberrant splice products from the 779A or 779G TPH genes were detected nor were any other polymorphic nucleotides found.

Analysis of a functional catechol-O-methyltransferase gene polymorphism in schizophrenia: evidence for association with aggressive and antisocial behavior

We have recently characterized a functional polymorphism in the catechol-O-methyltransferase (COMT) gene that is responsible for substantial variability in COMT enzymatic activity found in humans. A common low-activity variant of the enzyme contains a methionine residue at amino acid 158 of membrane-bound COMT whereas the common high activity variant has a valine at this site. Considering the role of COMT in dopamine metabolism and the involvement of dopaminergic pathways in the pathogenesis of schizophrenia and violence, we screened 37 patients with schizophrenia to determine whether or not a behavioral association with the COMT polymorphism exists. Patients were assessed for dangerousness on the basis of a history of violent and threatening behavior, crime, cocaine and alcohol abuse, and other antisocial behaviors. We found that schizophrenic patients who were homozygous for the low activity allele were judged by their psychiatrists to be at higher risk for aggressive and dangerous behavior than those who were homozygous for the high activity allele (Kruskal-Wallis statistic = 10.43; P = 0.003).

Screening the monoamine oxidase B gene in 100 male patients with schizophrenia: a cluster of polymorphisms in African-Americans but lack of functionally significant sequence changes

The monoamine oxidase B (MAO-B) gene was examined in 100 alleles derived from 80 Caucasian, 10 African-American, 5 Asian, and 5 Native American male patients with schizophrenia to identify sequence changes that might be associated with the disease. Approximately 235 kb of genomic sequence, primarily in coding regions, were screened by dideoxy fingerprinting, a modification of single-strand conformational polymorphism (SSCP) analysis that detects virtually 100% of sequence changes [Sarkar et al. (1992): Genomics 13:441-443; Liu and Sommer (1994): PCR Methods Appl 4:97-108]. No sequence changes of likely functional significance were identified, suggesting that mutations affecting the structure of the MAO-B protein are uncommon in the general population and are unlikely to contribute significantly to the genetic predisposition to schizophrenia. Eight polymorphisms were identified in African-Americans and Native Americans, but none were identified among Caucasians. Of the eight observed polymorphisms, a set of five transitions and one microdeletion was identified within approximately 17 kb of genomic sequence in the same 3 African-American individuals, while the remaining 7 African-Americans had a sequence identical to that in Caucasians. The presence of two such haplotypes, without intermediates, is compatible with the hypothesis that germline mutations can occur in clusters, as also suggested by other recent findings.

Human hair follicles as a peripheral source of tyrosine hydroxylase and aromatic L-amino acid decarboxylase mRNA

Total RNA from human hair follicles was reverse transcribed and amplified using primers specific for aromatic L-amino acid decarboxylase (AADC) and tyrosine hydroxylase (TH). Agarose electrophoresis of the amplified products showed reverse transcription polymerase chain reaction (RT-PCR) products of the expected size for both TH and AADC. Sequencing showed that the amplified products matched the known sequences of TH and AADC. This study identifies hair follicles as a convenient, uninvasive, source of the mRNA for TH and AADC. Analysis of these mRNA's may be useful in the diagnosis and investigation of conditions resulting from qualitative changes in the genes that code for these enzymes.

Relationship between imidazoline/guanidinium receptive sites and monoamine oxidase A and B

Imidazoline binding sites or imidazoline/guanidinium receptive sites (IGRS) recognize bioactive endogenous substances and a variety of pharmacologically active compounds containing imidazoline or guanidinium moieties. The family of imidazoline binding proteins consists of multiple membrane-associated proteins that differ in their tissue/subcellular localization, M(r) and ligand recognition properties. Two of the imidazoline binding proteins are identical to the mitochondrial enzyme monoamine oxidase (MAO) A and B isoforms, which contain imidazoline binding domains distinct from the enzyme active site. The relationship between the imidazoline binding proteins and monoamine oxidases was further characterized in the present report using a covalent probe (2-[3-azido-4[125I]iodophenoxy] methyl imidazoline, [125I]-AZIPI) to label the imidazoline binding proteins in different species and following transient expression of MAO- A and -B in COS 7 cells. Species homologues of MAO-A and -B in rat and human differ in their apparent molecular weight by approximately 2000 Da. In rat and human liver [125I]-AZIPI identified peptides with apparent molecular weights similar to those of the species homologues of MAO. Peptides of M(r) approximately 63,000 (MAO-A) and approximately 59,000 (MAO-B) were also photolabeled in membranes prepared from COS-7 cells transfected with human cDNA clones encoding MAO-A or -B. Additional experiments indicate that the imidazoline binding domains on MAO-A and -B exhibit different ligand recognition properties. The covalent labeling of human liver MAO-B was more sensitive than that of placenta MAO-A to inhibition by the imidazoline 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224). These data indicate that the A and B isoforms of MAO possess imidazoline binding domains that differ in their ligand recognition properties. Allosteric regulation of the activity of MAO via the imidazoline binding domains may be of significance in various disease states associated with elevated enzyme expression or in which the enzyme is a therapeutic target.