Robert O. Pihl

Tryptophan Depletion, Executive Functions, and Disinhibition in Aggressive, Adolescent Males

Low serotonin has been associated with aggressive behavior and impulsivity. Executive functions (cognitive abilities involved in the initiation/maintenance of goal attainment) have also been related to aggression. We tested whether dietary depletion of tryptophan, the amino acid precursor of serotonin, would increase disinhibition (impulsivity) in aggressive male adolescents. Cognitive-neuropsychological variables predictive of disinhibition were explored. Stable aggressive and nonaggressive adolescent men received balanced and tryptophan-depleted amino acid mixtures separately (counterbalanced, double-blind). Commission errors on a go/no-go learning task (i.e., failures to inhibit responding to stimuli associated with punishment/nonreward) measured disinhibition. Aggressive adolescent males made more commission errors as compared to nonaggressives. Lower executive functioning was significantly related to commission errors over and above conventional memory abilities. Tryptophan depletion had no effect on commission errors in the aggressive adolescents, possibly because of a ceiling effect.

Prediction of Alexithymic Characteristics from Physiological, Personality, and Subjective Measures

Noting concerns for a comprehensive conceptualization of alexithymic characteristics, the present study examines the potential utility of considering these characteristics as manifestations of deficits in cognitive schemata. Research guided by this conceptualization has identified physiological, subjective, and personality features of alexithymic characteristics. It is reasoned that if this conceptualization has merit, it should be possible to predict the presence of alexithymic characteristics from these features. Results of the present study indicate that a combination of physiological, subjective, and personality variables significantly predicts the presence of alexithymic characteristics as measured by the Schalling-Sifneos Personality Scale. These results are discussed in terms of their implications for a more comprehensive description of alexithymia and the value of the cognitive schema conceptualization.

Tryptophan Depletion and Behavioral Disinhibition in Men at Risk for Alcoholism and Antisocial Behavior

The present studies tested the hypothesis that experimentally altering central nervous system serotonin (5-HT) synthesis through tryptophan depletion (T-) would increase behavioral disinhibition in susceptible individuals. The oral administration of a mixture of amino acids devoid of tryptophan (trp), the amino acid precursor of 5-HT, is a safe and effective method of lowering brain 5-HT synthesis, and presumably 5-HT function. Previous clinical studies have reported a negative relationship between brain serotonergic functioning and impulsivity. Impulsive fire-setters, particularly those with a family history of alcoholism, as well as impulsive violent offenders, have lower levels of cerebrospinal fluid (CSF) 5-hydroxyindoleacetic acid (5-HIAA), a metabolite of 5-HT, compared to nonimpulsive violent offenders and healthy controls (Linnoila, De Jong & Virkkunen, 1989; Virkkunen et al., 1994). Two groups, selected on the basis of an increased risk for alcoholism or antisocial behavior, respectively, were tested: young men with multigenerational family histories of alcoholism, and adolescent males with past histories of aggressive, disruptive behavior.

Biochemical Factors in Aggression and Violence

Various neurochemical factors have been strongly implicated in the regulation of violence as well as prosocial behavior. This article will review the neurochemical findings to date, focusing on hormonal and neurotransmitter influences on aggressive and violent as well as prosocial and affiliative behavior in humans.