Modeling sensitization to stimulants in humans: an [11C]raclopride/positron emission tomography study in healthy men.

CONTEXT: In animals, repeated exposure to stimulant drugs leads to an enhanced drug-induced psychomotor response and increased dopamine release. This phenomenon, known as sensitization, may confer vulnerability to drug addiction or drug-induced psychosis in humans. A similar phenomenon, referred to as endogenous sensitization, is also believed to play a role in the emergence of positive symptoms in patients with schizophrenia. OBJECTIVE: To determine whether behavioral and neurochemical sensitization occur in healthy individuals after limited exposure to amphetamine in the laboratory. DESIGN: Open-label, 1-year follow-up of repeated amphetamine administration in healthy volunteers. SETTING: Department of Psychiatry, McGill University, and McConnell Brain Imaging Center, Montreal Neurological Institute. PARTICIPANTS: Ten healthy men (mean +/- SD age, 25.8 +/- 1.8 years). INTERVENTION: Three single doses of amphetamine (dextroamphetamine sulfate, 0.3 mg/kg by mouth) were administered on days 1, 3, and 5. MAIN OUTCOME MEASURES: Using positron emission tomography and [11C]raclopride, we measured dopamine release in response to amphetamine on the first exposure (day 1) and 14 days and 1 year after the third exposure. RESULTS: The initial dose of amphetamine caused dopamine release in the ventral striatum (a reduction in [11C]raclopride binding). Consistent with a sensitization-like phenomenon, 14 and 365 days after the third dose of amphetamine there was a greater psychomotor response and increased dopamine release (a greater reduction in [11C]raclopride binding), relative to the initial dose, in the ventral striatum, progressively extending to the dorsal caudate and putamen. A high novelty-seeking personality trait and self-rating assessments indicating impulsivity predicted proneness to sensitization. CONCLUSIONS: Sensitization to stimulants can be achieved in healthy men in the laboratory. This phenomenon is associated with increased dopamine release and persists for at least 1 year.

Dopamine release in response to a psychological stress in humans and its relationship to early life maternal care: a positron emission tomography study using [11C]raclopride.

Mesolimbic dopamine is thought to play a role in the processing of rewards. However, animal studies also demonstrate dopamine release in response to aversive stressful stimuli. Also, in animal studies, disruptions of the mother-infant relationship have been shown to have long-lasting effects on the mesolimbic dopamine system and the hypothalamic-pituitary adrenal axis. We therefore investigated dopamine release in response to stress in human subjects, considering the relationship to early life parental care. We screened 120 healthy young college students for parental care in early life using a combination of telephone interviews and questionnaires. Five students from the top end and five students from the bottom end of the parental care distribution were then invited for a positron emission tomography study using [11C]raclopride and a psychosocial stress task. The psychosocial stressor caused a significant release of dopamine in the ventral striatum as indicated by a reduction in [11C]raclopride binding potential in the stress versus resting condition in subjects reporting low parental care. Moreover, the magnitude of the salivary cortisol response to stress was significantly correlated with the reduction in [11C]raclopride binding in the ventral striatum (r = 0.78), consistent with a facilitating effect of cortisol on dopamine neuron firing. These data suggest that aversive stressful events can be associated with mesolimbic dopamine release in humans, and that the method presented here may be useful to study the effects of early life events on neurobiological stress systems.

Limbic response to psychosocial stress in schizotypy: a functional magnetic resonance imaging study.

Psychological stress causes dopamine release in the striatum and is thought to play a role in susceptibility to psychotic illness. Previous work suggests that an elevated dopaminergic response to stress may index vulnerability to psychosis in certain individuals. With functional magnetic resonance imaging, we measured stress-induced changes in brain activity in healthy individuals at elevated risk of developing psychosis. Participants were 15 controls and 25 psychometric schizotypes: 12 with positive symptom schizotypy (perceptual aberrations) and 13 with negative symptom schizotypy (physical anhedonia), as determined by questionnaires (Chapman et al., 1976; Chapman and Chapman, 1978). In the scanner, participants performed the Montreal Imaging Stress Task and a matched sensory-motor control task. Measures of self-reported stress and salivary cortisol levels were taken throughout the experiment. All three groups showed significant increases in self-reported stress and significant fMRI signal change in the striatal, limbic and cortical regions. However, the Physical Anhedonia group showed greater stress-induced striatal and limbic deactivation than the other two groups. Deactivation in the striatum was significantly correlated with Physical Anhedonia score across all subjects. Our findings suggest the presence of abnormalities in striatal response to stress in negative symptom schizotypy.

An acute psychosocial stress enhances the neural response to smoking cues.

Stress plays an important role in drug addiction. It can trigger relapse in abstinent addicts, and both in the everyday world and in the laboratory, a stressor can induce drug craving. Drug cues, such as the sight of drug, can also trigger subjective craving and relapse, and this effect may be amplified by stress. Underpinning this interaction may be the fact that stress and reward-predicting drug cues act on overlapping brain regions. We exposed 15 smokers undergoing functional magnetic resonance imaging to a psychosocial stressor, the Montreal Imaging Stress Task, followed by drug cues consisting of video clips of smokers. In a separate session similar video clips were shown after a non-stress control task. We observed significantly decreased neural activity during stress in the hippocampus, amygdala, bed nucleus of the stria terminalis, hypothalamus and nucleus accumbens. Following stress there was an increased neural response to drug cues in the medial prefrontal cortex, posterior cingulate cortex, dorsomedial thalamus, medial temporal lobe, caudate nucleus, and primary and association visual areas. These regions are thought to be involved in visual attention and in assigning incentive value to cues. Stress-induced limbic deactivation predicted subsequent neural cue-reactivity. We suggest that stress increases the incentive salience of drug cues.