Acute dopamine D(1) and D(2) receptor stimulation does not modulate mismatch negativity (MMN) in healthy human subjects

RATIONALE

Schizophrenia is commonly associated with an impairment in pre-attentive change detection, as represented by reduced mismatch negativity (MMN), an auditory event related potential. While the neurochemical basis of MMN has been linked to the integrity of the glutamatergic system involving N-methyl-D-aspartate (NMDA) receptors, the role of the dopaminergic system and in particular, the role of D(1) and D(2) receptors on MMN is yet to be determined.

OBJECTIVES

The aim of the present project was to investigate the acute effects of dopamine D(2) (bromocriptine) and D(1)/D(2) (pergolide) receptor stimulation on the human MMN in healthy subjects.

METHODS

Fifteen healthy male subjects participated in a double-blind, placebo-controlled, cross-over design in which each subject was tested under three acute treatment conditions separated by a 1-week wash out period; placebo, bromocriptine (2.5 mg) and pergolide (0.1 mg). The subjects were exposed to a duration-MMN paradigm with 50 ms standard tones (91%) and 100 ms deviant tones (9%).

RESULTS

The results showed that neither D(2) receptor stimulation with bromocriptine, nor simultaneous D(1) and D(2) receptor stimulation with pergolide, modulated MMN.

CONCLUSIONS

These findings suggest that acute D(1) and D(2) receptor stimulation does not modulate MMN. While the role of dopamine cannot be completely ruled out, the findings support the view that the aberrant MMN reported in schizophrenia may be linked primarily to glutamate dysfunction involving NMDA receptors.

Acute serotonin and dopamine depletion improves attentional control: findings from the stroop task

Schizophrenia is associated with impairments of attentional control on classic experimental paradigms such as the Stroop task. However, at a basic level the neurochemical mechanisms that may be responsible for such impairments are poorly understood. In this study, we sought to investigate the influence of brain monoamine function on Stroop task performance in healthy participants using the established methods of acute dietary serotonin, dopamine, and combined monoamine depletion. The study was a double-blind placebo controlled design in which 12 healthy male participants completed the Stroop task under four acute treatment conditions: (a) balanced/placebo control, (b) acute tryptophan depletion, (c) acute tyrosine/phenylalanine depletion, and (d) acute tyrosine/phenylalanine/tryptophan depletion (combined monoamine depletion). Decreased Stroop interference indicating improved attentional control was observed after both tryptophan depletion and tyrosine/phenylalanine depletion, while there was no significant change in interference after combined monoamine depletion. Findings suggest that reduced tonic dopamine or serotonin activity within specific neural circuits (such as the striatum, anterior cingulate, or prefrontal cortex) may play a critical role in attentional control, possibly by improving gating of information via reducing noise in monoaminergic systems. These findings enhance our understanding of the neurochemical basis of attentional control and the possible cause of attentional control deficits in schizophrenia.

Sex differences in the effect of acute tryptophan depletion on declarative episodic memory: A pooled analysis of nine studies

Acute tryptophan depletion (ATD) studies have shown that serotonin plays a role in learning and memory processes. In this study, we performed a pooled analysis of nine ATD studies in order to examine the nature of the memory-impairing effects of ATD and mediating factors, such as gender, age and vulnerability for disease in which disturbed serotonin was hypothesized to play a role. All studies that were used in this pooled analysis assessed declarative episodic memory using a verbal learning task paradigm. Immediate recall, delayed recall, and delayed recognition scores were examined. A total of 211 participants were included in the analysis. The analysis revealed that ATD impaired not only delayed recall, but also immediate recall. The ATD-induced impairments were larger in females than in males. Furthermore, ATD did not interact with any other serotonergic vulnerability and age. This suggests that the only factor that actually has the properties of a serotonergic vulnerability factor for declarative memory performance is female gender. The findings provide further support for a critical role of serotonin in declarative episodic memory.

Dysfunction of dorsolateral prefrontal cortex in antipsychotic-naïve schizophreniform psychosis

Reports of abnormal activation of the dorsolateral prefrontal cortex (dlPFC) are common in functional neuroimaging studies of schizophrenia, although very few have examined brain activity in patients close to the onset of illness. In this H(2)(15)O PET study, eight young male patients with first-episode schizophreniform psychosis and age-matched control subjects performed a version of the Stroop task that we have previously shown to engage the middle-frontal gyrus. At the time of testing, patients were antipsychotic-naïve and were scanned within 1 week of initial contact with our clinical program. All patients received a later diagnosis of schizophrenia 6 months after participating in the study. Whole-brain (within-group) and region-of-interest (between-group) analyses were carried out and data underwent spatial reproducibility testing. Compared with healthy subjects, patients showed significantly greater reaction-time (RT) interference but normal RT accuracy on the Stroop task. This pattern correlated with significant under-activation of the posterior left middle-frontal gyri in the patient versus control group. These findings support an emerging model of impaired cognitive control in schizophrenia and suggest that there is significant dysfunction of the dlPFC close to the onset of illness that may coincide with, or be modulated by, the transition-to-illness phase.

Evaluating brain activity in obsessive-compulsive disorder: preliminary insights from a multivariate analysis

Obsessive-compulsive disorder (OCD) has been linked to a dysfunction of brain corticostriatal networks, although functional imaging studies of OCD rarely apply network-sensitive analysis methods. In this study, we compared a univariate and a multivariate analysis of PET data in OCD patients and healthy subjects; the latter approach was considered more suitable for characterizing functional networks of brain activity. Although both methods suggested there was abnormal corticostriatal activity in OCD patients, the nature, extent and magnitude of this activity was clearly enhanced by the multivariate approach. Implications for the analysis of such studies are discussed.

Dopamine receptor stimulation does not modulate the loudness dependence of the auditory evoked potential in humans

RATIONALE

The Loudness Dependence of the Auditory Evoked Potential (LDAEP) has been suggested as a reliable measure of central serotonin function in humans; however, its specificity for the serotonin system remains a topic of debate, with possible modulation of this purported serotonin marker by other neurotransmitters, including dopamine.

OBJECTIVES

We examined the effect of dopaminergic modulation on the LDAEP using the D1/D2/D3 dopamine receptor agonist pergolide and the D2/D3 agonist bromocriptine.

METHODS

The study was a double-blind, placebo-controlled repeated-measures design in which healthy participants were tested under three acute treatment conditions: placebo, bromocriptine (2.5 mg), and pergolide (0.1 mg). Changes in the amplitude of the N1/P2 at intensities (60, 70, 80, 90, and 100 dB) were examined at C Z.

RESULTS

Acute stimulation of D1/D2/D3 receptors with pergolide and D2/D3 receptors with bromocriptine in comparison with placebo had no effect on the LDAEP.

CONCLUSION

These findings indicate that acute stimulation of dopamine D1, D2, and D3 receptors does not modulate the LDAEP in humans. Although the findings suggest that the LDAEP may not be modulated by acute changes in dopamine neurotransmission, further studies are needed to fully characterize its dopaminergic sensitivity.

Pindolol does not augment central serotonin function increases to citalopram in humans: an auditory evoked potential investigation

Animal studies have demonstrated that the co-administration of pindolol and selective serotonin reuptake inhibitors (SSRIs) potentiate serotonergic functioning to a greater degree than SSRIs alone. However, clinical trials of pindolol augmentation in patients with major depressive disorder have reported contradictory findings, and the central effects of this treatment regime on serotonin functioning in humans are unknown. The current double-blind placebo controlled repeated measures investigation used the loudness dependence auditory evoked potential (LDAEP) to assess central serotonin functioning in healthy participants across three acute treatment conditions: placebo, citalopram (20 mg), and pindolol (10 mg)+citalopram (20 mg). The current paper focuses on the effects of pindolol augmentation of citalopram as compared to the administration of citalopram alone. Enhancement of serotonin function with citalopram in comparison to placebo decreased the slope of the LDAEP (i.e. weaker LDAEP). However, there were no significant differences between the changes in the LDAEP induced by co-administration of pindolol and citalopram compared to citalopram. The present results indicate that, in healthy controls, pindolol augmentation of SSRIs does not potentiate central serotonin function to a greater degree than the administration of an SSRI alone. The findings may provide further support for why pindolol may not be an effective therapeutic strategy to augment serotonin function and antidepressant response.

Molecular imaging of the dopaminergic system and its association with human cognitive function

Molecular imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) has recently been used to examine dopamine (DA) function and its relationship with cognition in human subjects. This article will review PET and SPECT studies that have explored the relationship between cognitive processes and components of the DA system (pre-, intra-, and postsynaptic) in healthy and patient populations such as Parkinson's disease (PD), schizophrenia, Huntington's disease, and aging. It is demonstrated that DA activity modulates a range of frontal executive-type cognitive processes such as working memory, attentional functioning, and sequential organization, and alterations of DA within the fronto-striato-thalamic circuits might contribute to the cognitive impairments observed in PD, schizophrenia, and normal aging. Although associations between DA and cognitive measures need to be considered within the context of fronto-striato-thalamic circuitry, it is suggested that striatal (especially caudate) DA activity, particularly via D2 receptors, might be important for response inhibition, temporal organization of material, and motor performance, whereas cortical DA transmission via D1 receptors might be important for maintaining and representing on-going behavior.

Estrogen prevents 5-HT1A receptor-induced disruptions of prepulse inhibition in healthy women

The sex steroid hormone, estrogen, has been proposed to be protective against schizophrenia. This study examined the effects of estrogen treatment on modulation of prepulse inhibition (PPI) by the serotonin-1A (5-HT1A) receptor partial agonist, buspirone. PPI is a model of sensorimotor gating, which is deficient in schizophrenia and other mental illnesses. A total of 11 healthy women were tested following four acute treatment conditions: placebo, buspirone (Buspar; 5 mg), estradiol (Estrofem; 2 mg), and combined buspirone and estradiol. Electromyogram activity was measured across three interstimulus intervals (ISI): 30, 60, and 120 ms. There was no significant effect of either drug treatment on startle amplitude or habituation. At 120 ms ISI, buspirone caused a significant disruption of PPI and pretreatment with estrogen prevented this disruption. Estrogen treatment, administered in the appropriate experimental conditions, prevented PPI deficits induced by 5-HT(1A) receptor activation and may therefore also play a protective role in sensorimotor gating deficits in schizophrenia.

Muscarinic and nicotinic receptors synergistically modulate working memory and attention in humans

Functional abnormalities in muscarinic and nicotinic receptors are associated with a number of disorders including Alzheimer's disease and schizophrenia. While the contribution of muscarinic receptors in modulating cognition is well established in humans, the effects of nicotinic receptors and the interactions and possible synergistic effects between muscarinic and nicotinic receptors have not been well characterized in humans. The current study examined the effects of selective and simultaneous muscarinic and nicotinic receptor antagonism on a range of cognitive processes. The study was a double-blind, placebo-controlled, repeated measures design in which 12 healthy, young volunteers completed cognitive testing under four acute treatment conditions: placebo (P); mecamylamine (15 mg) (M); scopolamine (0.4 mg i.m.) (S); mecamylamine (15 mg)/scopolamine (0.4 mg i.m.) (MS). Muscarinic receptor antagonism with scopolamine resulted in deficits in working memory, declarative memory, sustained visual attention and psychomotor speed. Nicotinic antagonism with mecamylamine had no effect on any of the cognitive processes examined. Simultaneous antagonism of both muscarinic and nicotinic receptors with mecamylamine and scopolamine impaired all cognitive processes impaired by scopolamine and produced greater deficits than either muscarinic or nicotinic blockade alone, particularly on working memory, visual attention and psychomotor speed. These findings suggest that muscarinic and nicotinic receptors may interact functionally to have synergistic effects particularly on working memory and attention and suggests that therapeutic strategies targeting both receptor systems may be useful in improving selective cognitive processes in a number of disorders.

Exploring the temporal dynamics of the spatial working memory n-back task using steady state visual evoked potentials (SSVEP)

The neural networks associated with spatial working memory (SWM) are well established. However, the temporal dynamics of SWM-related brain activity are less clear. This study examined changes in temporal neurophysiology during the spatial n-back task using steady state probe topography (SSPT) to record cortical steady state visual evoked potentials (SSVEPs) at 64 scalp locations. Twenty healthy male volunteers participated in the study. The findings identified three different time periods of significance during the spatial n-back task--an early perceptual/encoding period (approximately 0-500 ms), an early delay period just following the stimulus disappearing from view (approximately 850-1400 ms), and a late period lasting the final second of the delay and anticipation of the new stimulus (approximately 2500-3500 ms). The delay period was associated with increases in frontal and occipital region amplitude, consistent with previous findings in more basic working memory tasks. The two different SSVEP components during the delay appear reflective of the additional "executive" demands associated with the n-back and may suggest variable roles for the PFC during different stages of the delay. All three n-back levels demonstrated a relative consistent electrophysiological profile, indicating that this pattern is specific to the spatial n-back task. Nevertheless, these findings supported the hypothesis that memory load modulates activity within the networks identified, consistent with previous neuroimaging studies. The current findings may offer a framework in which to further investigate the temporal aspects of SWM.

Association between amygdala hyperactivity to harsh faces and severity of social anxiety in generalized social phobia

BACKGROUND

Previous functional brain imaging studies of social anxiety have implicated amygdala hyperactivity in response to social threat, though its relationship to quantitative measures of clinical symptomatology remains unknown. The primary aim of this study was to examine the association between response to emotionally harsh faces in the amygdala, a region implicated in social and threat-related processing, and severity of social anxiety symptoms in patients with generalized social phobia (GSP).

METHODS

Ten subjects with GSP naive to psychotropic medications and without psychiatric comorbidity and ten healthy comparison subjects matched on age, gender, ethnicity, and education completed the Liebowitz Social Anxiety Scale and underwent high-field (4Tesla) functional magnetic resonance imaging while viewing blocks of emotionally salient faces.

RESULTS

Relative to happy faces, activation of the amygdala in response to harsh (angry, disgusted, fearful) faces was greater in GSP patients than in controls, and the extent of amygdala activation was positively correlated with severity of social anxiety symptoms, but not general state or trait anxiety levels.

CONCLUSIONS

Our findings suggest that amygdala activation to interpersonal threat can be specifically linked to the severity of social anxiety symptoms of individual GSP patients, and thus, may serve as a useful functional marker of disease severity.

The neuropharmacology of L-theanine(N-ethyl-L-glutamine): a possible neuroprotective and cognitive enhancing agent

L-theanine (N-ethyl-L-glutamine) or theanine is a major amino acid uniquely found in green tea. L-theanine has been historically reported as a relaxing agent, prompting scientific research on its pharmacology. Animal neurochemistry studies suggest that L-theanine increases brain serotonin, dopamine, GABA levels and has micromolar affinities for AMPA, Kainate and NMDA receptors. In addition has been shown to exert neuroprotective effects in animal models possibly through its antagonistic effects on group 1 metabotrophic glutamate receptors. Behavioural studies in animals suggest improvement in learning and memory. Overall, L-theanine displays a neuropharmacology suggestive of a possible neuroprotective and cognitive enhancing agent and warrants further investigation in animals and humans.

Whole-body biodistribution and estimation of radiation-absorbed doses of the dopamine D1 receptor radioligand 11C-NNC 112 in humans

The present study estimated radiation-absorbed doses of the dopamine D(1) receptor radioligand [(11)C]((+)-8-chloro-5-(7-benzofuranyl)-7-hydroxy-3-methyl-2,3,4,5-tetrahydro-1H-3-benzazepine) (NNC 112) in humans, based on dynamic whole-body PET in healthy subjects.

METHODS

Whole-body PET was performed on 7 subjects after injection of 710 +/- 85 MBq of (11)C-NNC 112. Fourteen frames were acquired for a total of 120 min in 7 segments of the body. Regions of interest were drawn on compressed planar images of source organs that could be identified. Radiation dose estimates were calculated from organ residence times using the OLINDA 1.0 program.

RESULTS

The organs with the highest radiation-absorbed doses were the gallbladder, liver, lungs, kidneys, and urinary bladder wall. Biexponential fitting of mean bladder activity demonstrated that 15% of activity was excreted via the urine. With a 2.4-h voiding interval, the effective dose was 5.7 microSv/MBq (21.1 mrem/mCi).

CONCLUSION

(11)C-NNC 112 displays a favorable radiation dose profile in humans and would allow multiple PET examinations per year to be performed on the same subject.

Direct evidence that acutely enhancing serotonin with the selective serotonin reuptake inhibitor citalopram modulates the loudness dependence of the auditory evoked potential (LDAEP) marker of central serotonin function

The loudness dependence of the auditory evoked potential (LDAEP) has been suggested as a reliable measure of central serotonin function in humans. The most convincing evidence for a direct relationship between serotonergic function and LDAEP to date has come from animal studies, while evidence in humans has been circumstantial and inconsistent. In the current study, we examine the direct effect of serotonergic modulation with the selective serotonin reuptake inhibitor (SSRI) citalopram on the LDAEP. The study was a double-blind placebo controlled design in which healthy participants were tested under two acute treatment conditions: placebo and citalopram (20 mg). Enhancement of serotonin function with citalopram in comparison to placebo decreased the slope of the LDAEP (i.e. weaker LDAEP). The findings provide direct evidence in humans, of a relationship between central serotonin function and the LDAEP, supporting findings previously observed in animals and clinical populations. Together the results provide further support for the validity of the LDAEP as a non-invasive in vivo measure of central serotonin function in humans.

Beyond threat: amygdala reactivity across multiple expressions of facial affect

The amygdala has been consistently isolated as a key neural substrate for processing facial displays of affect. Recent evidence from human lesion and functional neuroimaging studies have begun to challenge the notion that the amygdala is reserved for signals of threat (fear/anger). We performed a 4 T fMRI study in which 20 subjects viewed a contemporary set of photographs displaying 6 different facial expressions (fearful, disgusted, angry, sad, neutral, happy) while performing a task with minimal cognitive demand. Across subjects, the left amygdala was activated by each face condition separately, and its response was not selective for any particular emotion category. These results challenge the notion that the amygdala has a specialized role in processing certain emotions and suggest that the amygdala may have a more general-purpose function in processing salient information from faces.

Combined D1/D2 receptor stimulation under conditions of dopamine depletion impairs spatial working memory performance in humans

RATIONALE

The mesocortical dopamine system is regarded as an important modulator of working memory. While it has been established that stimulation of the D1/D2 receptor in primates can improve spatial working memory performance, findings in humans are less consistent. Recent studies in humans suggest that global depletion of dopamine via tyrosine/phenylalanine depletion may impair spatial working memory performance, although these results are also inconsistent, and it has been suggested that task differences may partly underlie the inconsistent findings.

OBJECTIVES

This study had two aims: (1) to investigate the effects of acute tyrosine depletion (TPD) on a number of working memory tasks and (2) to examine whether stimulation of D1/D2 receptors under conditions of TPD can attenuate or "reverse" TPD-induced working memory impairments.

METHODS

Eighteen healthy male participants performed a spatial working memory delayed-recognition task, non-spatial working memory task and spatial n-back task on three separate occasions, after TPD, TPD and pergolide (D1/D2 agonist), and placebo.

RESULTS

TPD did not impair working memory performance on any of the tasks administered. However, stimulation of D1/D2 receptors under TPD conditions caused a subtle impairment in spatial working memory performance.

CONCLUSIONS

The finding that D1/D2 stimulation under TPD conditions impairs working memory highlights the complexity of functional effects of augmenting dopaminergic transmission within a dopamine-depleted state. The lack of TPD-related effects on a range of working memory tasks questions the reliability of TPD as a modulator of dopamine function and working memory performance in humans.

Muscarinic and nicotinic receptor modulation of object and spatial n-back working memory in humans

Working memory impairments in the n-back task in schizophrenia have been linked to sustained deficiency in mesocortical dopamine function. More recently, abnormalities in the cholinergic system have also been documented in schizophrenia, with cortical reductions in both nicotinic and muscarinic receptors. While the cholinergic hypothesis of memory is well established, the role of cholinergic receptors in modulating n-back working memory is not known. We investigated the effects of selective and simultaneous muscarinic and nicotinic antagonism on spatial and object n-back working memory performance. The study was a double-blind, placebo-controlled repeated-measures design in which 12 healthy subjects were tested under four acute treatment conditions; placebo (P), mecamylamine (M), scopolamine (S) and mecamylamine+scopolamine (MS). Muscarinic antagonism with scopolamine significantly impaired both object and spatial n-back working memory, whereas nicotinic antagonism with mecamylamine had little effect. Simultaneous antagonism of both muscarinic and nicotinic receptors produced greater impairments in both object and spatial n-back working memory performance than muscarinic or nicotinic antagonism alone. These results suggest that: (1) both muscarinic and nicotinic receptors may functionally interact to synergistically modulate n-back working memory, and (2) that n-back working memory impairments in schizophrenia may in part be due to reductions in both muscarinic and nicotinic receptors.

Pindolol augmentation of selective serotonin reuptake inhibitors: accounting for the variability of results of placebo-controlled double-blind studies in patients with major depression

Co-administration of pindolol with SSRIs in patients with depression has been suggested as a way to both hasten and augment antidepressant response. Clinical trials have examined the efficacy of this treatment regime and conflicting results have been reported. The present review briefly presents the results of placebo-controlled double-blind trials of pindolol augmentation of SSRIs in patients with major depression, and focuses on factors that may account for the variability of findings. Additionally, a profile of the subset of patients who may most benefit from pindolol augmentation is outlined. Methodological factors such as qualitative differences in definitions of antidepressant response, the timing of pindolol administration and heterogeneous clinical characteristics of patient samples may contribute to the variability in the results of clinical trials to date. Similarly, individual differences in neuropathology, neurophysiology and genotype may also account for some of the inconsistencies in the findings. Finally, the results of recent neuroimaging studies suggest that the 2.5 mg t.i.d. dose of pindolol that has been used in all but one of these investigations may be suboptimal for achieving reliable and significant occupancy of 5-HT1A autoreceptors and may explain the contradictory nature of the results of investigations of pindolol augmentation.

Functional connectivity during Stroop task performance

Using covariance-based multivariate analysis, we examined patterns of functional connectivity in rCBF on a practice-extended version of the Stroop color-word paradigm. Color-word congruent and incongruent conditions were presented in six AB trials to healthy subjects during 12 H2(15)O PET scans. Analyses identified two reproducible canonical eigenimages (CE) from the PET data, which were converted to a standard Z score scale after cross-validation resampling and correction for random subject effects. The first CE corresponded to practice-dependent changes in covarying rCBF that occurred over early task repetitions and correlated with improved behavioral performance. This included many regions previously implicated by PET and fMRI studies of this task, which we suggest may represent two "parallel" networks: (i) a cingulo-frontal system that was initially engaged in selecting and mapping a task-relevant response (color naming) when the attentional demands of the task were greatest; and (ii) a ventral visual processing stream whose concurrent decrease in activity represented the task-irrelevant inhibition of word reading. The second CE corresponded to a consistent paradigmatic effect of Stroop interference on covarying rCBF. Coactivations were located in dorsal and ventral prefrontal regions as well as frontopolar cortex. This pattern supports existing evidence that prefrontal regions are involved in maintaining attentional control over conflicting response systems. Taken together, these findings may be more in line with theoretical models that emphasize a role for practice in the emergence of Stroop phenomena. These findings may also provide some additional insight into the nature of anterior cingulate- and prefrontal cortical contributions to implementing cognitive control in the brain.

Neural substrates for voluntary suppression of negative affect: a functional magnetic resonance imaging study

BACKGROUND

Successful control of affect partly depends on the capacity to modulate negative emotional responses through the use of cognitive strategies. Although the capacity to regulate emotions is critical to mental well-being, its neural substrates remain unclear.

METHODS

We used functional magnetic resonance imaging to ascertain brain regions involved in the voluntary regulation of emotion and whether dynamic changes in negative emotional experience can modulate their activation. Fourteen healthy subjects were scanned while they either maintained the negative affect evoked by highly arousing and aversive pictures (e.g., experience naturally) or suppressed their affect using cognitive reappraisal. In addition to a condition-based analysis, online subjective ratings of intensity of negative affect were used as covariates of brain activity.

RESULTS

Inhibition of negative affect was associated with activation of dorsal anterior cingulate, dorsal medial prefrontal, and lateral prefrontal cortices, and attenuation of brain activity within limbic regions (e.g., nucleus accumbens/extended amygdala). Furthermore, activity within dorsal anterior cingulate was inversely related to intensity of negative affect, whereas activation of the amygdala was positively covaried with increasing negative affect.

CONCLUSIONS

These findings highlight a functional dissociation of corticolimbic brain responses, involving enhanced activation of prefrontal cortex and attenuation of limbic areas, during volitional suppression of negative emotion.

Neural correlates of internally-generated disgust via autobiographical recall: a functional magnetic resonance imaging investigation

Converging lines of evidence suggest the involvement of the insula and basal ganglia in the processing of disgust, an important primary emotion that guides the avoidance of potential physical contamination and disease. Prior human lesion and functional brain imaging studies have employed exteroceptive sensory stimuli such as facial expressions of disgust, and disgust-eliciting pictures. Thus, the neural substrates underlying the internal experience of disgust remain unknown. The present fMRI study examined the neural correlates of self-induced disgust aided by the recall and re-experience of personally salient life events. Subjects were scanned while they recalled and re-experienced either a recent situation that evoked intense disgust or a time-matched, equally vivid neutral/non-emotional event. Relative to the emotionally neutral condition, self-induced disgust was associated with activation of the insula, hippocampus, anterior and posterior cingulate cortex, basal ganglia, thalamus, and primary visual cortex. These findings suggest that areas previously associated with the perception of disgust (e.g., insula, basal ganglia) are also involved interoceptive experience of disgust.

The acute effects of L-theanine in comparison with alprazolam on anticipatory anxiety in humans

L-Theanine (delta-glutamylethylamide) is one of the predominant amino acids ordinarily found in green tea, and historically has been used as a relaxing agent. The current study examined the acute effects of L-theanine in comparison with a standard benzodiazepine anxiolytic, alprazolam and placebo on behavioural measures of anxiety in healthy human subjects using the model of anticipatory anxiety (AA). Sixteen healthy volunteers received alprazolam (1 mg), L-theanine (200 mg) or placebo in a double-blind placebo-controlled repeated measures design. The acute effects of alprazolam and L-theanine were assessed under a relaxed and experimentally induced anxiety condition. Subjective self-reports of anxiety including BAI, VAMS, STAI state anxiety, were obtained during both task conditions at pre- and post-drug administrations. The results showed some evidence for relaxing effects of L-theanine during the baseline condition on the tranquil-troubled subscale of the VAMS. Alprazolam did not exert any anxiolytic effects in comparison with the placebo on any of the measures during the relaxed state. Neither L-theanine nor alprazalam had any significant anxiolytic effects during the experimentally induced anxiety state. The findings suggest that while L-theanine may have some relaxing effects under resting conditions, neither L-theanine not alprazolam demonstrate any acute anxiolytic effects under conditions of increased anxiety in the AA model.

Evidence for synergistic modulation of early information processing by nicotinic and muscarinic receptors in humans

Impairments in early information processing are a hallmark feature of diverse neuropsychiatric disorders including schizophrenia and Alzheimer's disease (AD). Several lines of evidence implicate a dysfunction of the cholinergic system in these disorders, particularly in AD where there is known degeneration in major cholinergic pathways. Inspection time (IT), a measure of early visual information processing speed, has been shown to be sensitive to cholinergic manipulation. The current study employed the IT task to (1) examine the independent roles of nicotinic and muscarinic receptors in modulating information processing and (2) investigate the interaction of nicotinic and muscarinic receptor systems in modulating information processing. Twelve healthy participants completed a randomized, double-blind, placebo-controlled study under four drug conditions; (1) placebo, (2) mecamylamine (15 mg; oral), (3) scopolamine (0.4 mg, s.c.), (4) mecamylamine (15 mg) + scopolamine (0.4 mg). IT measures were examined at baseline and 2.5 h post drug administration. Selective blockade of nicotinic receptors with mecamylamine did not significantly impair IT, whereas selective blockade of muscarinic receptors with scopolamine produced a significant but small impairment in IT. Combined blockade of both receptor types with scopolamine and mecamylamine produced a large impairment in IT performance. The results indicate that both nicotinic and muscarinic receptors are involved in modulating IT, and that the two systems may function synergistically to modulate early visual information processing. These findings suggest that functional abnormalities in both nicotinic and muscarinic systems may underlie deficits in early visual information processing seen in disorders such as Alzheimer's disease and schizophrenia.

Augmentation of serotonin enhances pleasant and suppresses unpleasant cortical electrophysiological responses to visual emotional stimuli in humans

The serotonergic system is one of the major systems targeted in the pharmacological treatment of a wide range of mood disorders including depression; however, little is known about the neurophysiological mechanisms underlying the effects of serotonin (5-HT) on affective phenomena including emotional behaviours, mood and emotional processing. The aim of the current study was to investigate how 5-HT acutely modulates steady-state visually evoked potentials (SSVEP), heart rate (HR) and verbal ratings associated with the viewing of differently valent emotional images. In a randomised double-blind, placebo-controlled design, 17 healthy subjects were tested under two acute treatment conditions: placebo and citalopram (20 mg) (a selective serotonin re-uptake inhibitor, or SSRI). Participants were tested 2 h post treatment whilst viewing 75 images (categorised as pleasant, neutral or unpleasant). Results indicate that under placebo treatment, processing of unpleasant valence [unpleasant (-) neutral images] was associated with decreases in SSVEP amplitude and latency in frontal and occipital cortices, whereas processing of pleasant valence [pleasant (-) neutral images] was associated with amplitude decreases and latency increases within frontal and left temporoparietal cortices. Decreases in both amplitude and latency are both interpreted as surrogate measures of cortical activation or excitation. Citalopram relative to placebo attenuated the electrophysiological activation to unpleasant valence within frontal and occipital cortices, but potentiated electrophysiological activation (amplitude only) to pleasant valence within parietooccipital cortices. Citalopram relative to placebo also suppressed differences in heart rate associated with the viewing of pleasant and unpleasant images, but did not alter subject's subjective responses to emotional images. Results suggest that responsiveness to pleasant and unpleasant stimuli following neurochemical modulation may vary across different response systems (i.e. self-report, HR and SSVEP). Electrophysiological findings suggest that acute serotonergic augmentation with citalopram modulates cortical processing of emotionally valent stimuli such that response to pleasant valence is potentiated and response to unpleasant valence is suppressed. The findings suggest a possible neurophysiological mechanism underlying antidepressant drug action on emotion.

Simultaneous depletion of tryptophan, tyrosine and phenylalanine as an experimental method to probe brain monoamine function in humans

Brain monoamines are important regulators of affective and cognitive processes and are involved in the aetiology of a number of psychiatric disorders. While methods to probe serotonin and catecholamine function are established, limited methods are available to probe monoamine function as a whole in humans. In the current study, we examined if simultaneous depletion of monoamine precursors can be used as a possible probe of monoamine function. Ten healthy subjects were tested under two treatment conditions; balanced control (B) condition and combined monoamine depletion (CMD) condition. Monoamine precursor depletion was associated with significant reductions in plasma-free tryptophan (46%), tyrosine (74%) and phenylalanine (78%). Greater reductions were achieved for ratios of each precursor to other large neutral amino acids (LNAA); tryptophan/LNAA (86%), tyrosine/LNAA (94%) and phenylalanine/LNAA (94%). Findings suggest that simultaneous depletion of monoamine precursors can achieve significant plasma monoamine depletion in the range expected to affect brain monoamine function.

Gender differences in the cortical electrophysiological processing of visual emotional stimuli

The processing of visual emotional stimuli has been investigated previously; however, gender differences in the processing of emotional stimuli remain to be clarified. The aim of the current study was to use steady-state probe topography (SSPT) to examine steady-state visually evoked potentials (SSVEPs) during the processing of pleasant and unpleasant images relative to neutral images, and to determine whether this processing differs between males and females. Thirty participants (15 males and 15 females) viewed 75 images low on the arousal dimension (categorised as pleasant, neutral or unpleasant) selected from the International Affective Picture System (IAPS), whilst a 13-Hz sinusoidal white visual flicker was superimposed over the visual field and brain electrical activity was recorded from 64 electrode sites. Results suggest that pleasant and unpleasant images relative to neutral images are associated with reductions in frontal latency and occipital amplitude. In addition, electrophysiological gender differences were observed despite there being no differences found between males and females on subjective mood or behavioural ratings of presented images (valence and arousal dimensions). The main gender difference reported in the current study related to the processing of unpleasant images (relative to neutral images) which is associated with widespread frontal latency reductions (predominantly right sided) in females but not in males. Our results suggest that gender differences do exist in the processing of visual emotional stimuli, and illustrate the importance of taking these differences into account during investigations of emotional processing. Finally, these gender differences may have implications for the pathophysiology of mood disorders such as depression.

Effects of a combined extract of Ginkgo biloba and Bacopa monniera on cognitive function in healthy humans

Extracts of Ginkgo biloba and Bacopa monniera have been shown to produce positive effects on cognitive function in healthy subjects. While the exact mechanisms are not known, it has been suggested that antioxidant properties and cholinergic modulation may play a role. In the current study the sub-chronic (2 weeks) and chronic (4 weeks) effects of an extract containing Ginkgo biloba (120 mg) and Bacopa monniera (300 mg) (Blackmores Ginkgo Brahmi) on cognitive function were examined. The study was a randomized, double-blind, placebo-controlled, independent group design in which 85 healthy subjects were allocated to one of two treatment conditions (placebo or combined Ginkgo biloba and Bacopa monniera extract). Testing was conducted at baseline and 2 and 4 weeks post treatment. The results showed that the combined extract relative to placebo did not demonstrate any significant effects on tests investigating a range of cognitive processes including attention, short-term and working memory, verbal learning, memory consolidation, executive processes, planning and problem solving, information processing speed, motor responsiveness and decision making. These findings suggest that at least within the current treatment duration and doses, an extract containing Ginkgo biloba and Bacopa monniera had no cognitive enhancing effects in healthy subjects.

Selective effects of acute serotonin and catecholamine depletion on memory in healthy women

There is converging evidence that brain serotonin and dopamine may selectively modulate learning and memory in humans. However, this has not been directly demonstrated. In the current study, we used the method of amino acid precursor depletion to explore the effects of low serotonin and catecholamine function on memory in healthy female volunteers. Participants completed three experimental sessions: (i) tryptophan depletion (TD to lower 5-HT); (ii) tyrosine and phenylalanine depletion (TPD to lower catecholamines); and (iii) a balanced control condition (Bal). All testing was conducted in a double-blind, placebo-controlled, crossover design. Cognitive and mood assessments were performed at baseline and 5 h after ingesting the amino acid mixture. Consistent with previous studies, TD impaired declarative memory consolidation on a structured word-learning task, while TPD, acting to lower brain dopamine availability, impaired spatial working memory. No secondary deficits were observed on measures of attention, short-term memory or subjective mood state. These findings suggest that low brain serotonin versus dopamine selectively impairs memory performance in humans. This may shed light on the role of these neurotransmitters in disorders that are characterized by significant memory impairment.

Simultaneous depletion of serotonin and catecholamines impairs sustained attention in healthy female subjects without affecting learning and memory

Monoamine neurotransmitters, serotonin, noradrenaline and dopamine modulate many important cognitive processes such as attention, learning and memory. While the selective effects of serotonin and catecholamine depletion on such processes have been investigated, the effects of simultaneous depletion of these monoamines on cognition remain unclear. This is of particular interest given that multiple neurotransmitter abnormalities have been implicated in many psychiatric disorders. The aim of the current study was to examine the effects of lowered brain monoamine function on cognitive performance, using the technique of amino acid precursor depletion. The study was a double-blind, placebo-controlled design in which 20 healthy female subjects were tested under a combined monoamine depletion condition (CMD) and a balanced control condition (B). Cognitive testing was conducted at baseline and 5 h post-depletion. The CMD condition relative to the B condition resulted in deficits in digit vigilance (accuracy and reaction time), a measure of sustained attention. There were no effects on measures of learning and memory or psychomotor function. These findings suggest that simultaneously depleting the availability of brain serotonin and catecholamines in healthy female subjects selectively impairs sustained attention, without affecting other cognitive domains.

Selective effects of simultaneous monoamine depletion on mood and emotional responsiveness

Monoamines play a significant role in the regulation of mood and emotion. While the selective effects of serotonin and catecholamine depletion on mood have been described, the effects of simultaneous monoamine depletion on subjective measures of mood and emotional responsiveness are yet to be examined. The current study aimed to investigate the effects of simultaneous monoamine depletion on mood and emotional responsiveness in healthy participants. Twenty female participants completed a randomized, double-blind, placebo-controlled study, under a balanced control condition (B), and a combined monoamine depletion condition (CMD; via tryptophan, tyrosine and phenylalanine depletion). Mood ratings Visual Analogue Mood Scale (VAMS) and Profile Of Mood States (POMS) and measures of emotional responsiveness (Emotional Stroop and International Affective Picture System) were examined at baseline and 5 h post-depletion. Following CMD, participants rated themselves as feeling sadder, more antagonistic, and mentally slower on three VAMS subscales. There were no significant mood changes found on the POMS or measures of emotional responsiveness. These findings suggest that simultaneous depletion of all monoamines may have selective effects on mood. The findings provide evidence that the simultaneous monoamine depletion technique may be a useful experimental method to probe central monoamine function in humans.

Acute augmentation of serotonin suppresses cardiovascular responses to emotional valence

A key component in visceral reactivity to emotional states is heart rate (HR), however little is known about how HR response to emotional stimuli is modulated by neurochemicals. The present study investigated the way in which acute enhancement of serotonin (5-HT) function with a selective serotonin reuptake inhibitor (SSRI) modulates the HR associated with differently valent images (pleasant, neutral, unpleasant). Sixteen healthy participants viewed 75 images whilst HR was recorded. Participants were tested under two single-dose treatment conditions: placebo and citalopram (20 mg). Our findings suggest: (1) HR is able to differentiate differently valent images during placebo treatment and, (2) administration of citalopram suppresses the differences in HR between differently valent images. These results suggest that 5-HT may modulate the cardiovascular HR response to visual emotional stimuli and indicate that 5-HT may have a protective effect on the cardiovascular responses to emotional stimuli.

Cortical neurophysiology of anticipatory anxiety: an investigation utilizing steady state probe topography (SSPT)

The precise role of the cortex in human anxiety is not well characterised. Previous imaging research among healthy controls has reported alterations in regional cerebral blood flow (rCBF) within the prefrontal and temporal cortices during periods of anxious anticipation; however, the temporal dynamics of this activity has yet to be examined in detail. The present study examined cortical Steady State Probe Topography (SSPT) changes associated with anticipatory anxiety (AA), allowing examination of the temporal continuity and the excitatory or inhibitory nature of AA activations. We recorded Steady State Visually Evoked Potentials (SSVEPs) at 64 scalp locations, skin conductance, and self reported anxiety among 26 right-handed males while relaxed and during the anticipation of an electric shock. Relative to the baseline condition, the AA condition was associated with significantly higher levels of self-reported anxiety and increased phasic skin conductance levels. Across the seven second imaging window, AA was associated with increased SSVEP latency within medial anterior frontal, left dorsolateral prefrontal and bilateral temporal regions. In contrast, increased SSVEP amplitude and decreased SSVEP latency were observed within occipital regions. The observed SSVEP latency increases within frontal and temporal cortical regions are suggestive of increased localised inhibitory processes within regions reciprocally connected to subcortical limbic structures. Occipital SSVEP latency decreases are suggestive of increased excitatory activity. SSVEP amplitude increases within occipital regions may be associated with an attentional shift from external to internal environment. The current findings provide further support for the involvement of frontal, anterior temporal, and occipital cortical regions during anticipatory anxiety, and suggest that both excitatory and inhibitory processes are associated with AA alterations.

Drug-induced modification of the system properties associated with spontaneous human electroencephalographic activity

The benzodiazepine (BZ) class of minor tranquilizers are important modulators of the gamma-amino butyric acid (GABA(A))/BZ receptor complex that are well known to affect the spectral properties of spontaneous electroencephalographic activity. While it is experimentally well established that the BZs reduce total alpha band (8-13 Hz) power and increase total beta band (13-30 Hz) power, it is unclear what the physiological basis for this effect is. Based on a detailed theory of cortical electrorhythmogenesis it is conjectured that such an effect is explicable in terms of the modulation of GABAergic neurotransmission within locally connected populations of excitatory and inhibitory cortical neurons. Motivated by this theory, fixed order autoregressive moving average (ARMA) models were fitted to spontaneous eyes-closed electroencephalograms recorded from subjects before and approximately 2 h after the oral administration of a single 1 mg dose of the BZ alprazolam. Subsequent pole-zero analysis revealed that BZs significantly transform the dominant system pole such that its frequency and damping increase. Comparisons of ARMA derived power spectra with fast Fourier transform derived spectra indicate an enhanced ability to identify benzodiazepine induced electroencephalographic changes. This experimental result is in accord with the theoretical predictions implying that alprazolam enhances inhibition acting on inhibitory neurons more than inhibition acting on excitatory neurons. Further such a result is consistent with reported cortical neuronal distributions of the various GABA(A) receptor pharmacological subtypes. Therefore physiologically specified fixed order ARMA modeling is expected to become an important tool for the systematic investigation and modeling of a wide range of cortically acting compounds.

The modulatory effects of dopamine D1 and D2 receptor function on object working memory in humans

The role of dopamine D1 and D2 receptors in modulating object working memory in humans is poorly understood. The current study was designed to investigate the effects of D1 and D2 receptor modulation on object working memory. Twelve healthy subjects underwent testing under three conditions (0.05 mg pergolide (D1/D2 receptor agonist), 2.5 mg bromocriptine (D2 receptor agonist) and placebo] in a double-blind, placebo-controlled repeated measures design. Subjects performed the object working memory N-back task pre-drug and 1.5 h and 3 h post-drug administration. Neither pergolide nor bromocriptine had an effect on object working memory performance. These findings suggest that object working memory may not be modulated by D1 and D2 receptors in humans.

Steady-state visually evoked potential topography during processing of emotional valence in healthy subjects

The International Affective Picture System (IAPS) is increasingly used in brain imaging studies to examine emotional processes. This task allows valence and arousal content to be systematically investigated; however, previous studies have generally failed to select images that vary in one dimension as well as hold constant the variability on the other dimension. In addition, no studies have investigated the temporal structure associated with the conscious, ongoing processing of emotional stimuli following systematic selection of IAPS images. The aim of the present study was therefore to use steady-state probe topography (SSPT) to examine the steady-state visually evoked potentials (SSVEPs) associated with the processing of pleasant and unpleasant images low in arousal content. Seventy-five IAPS images, categorized as unpleasant, neutral, or pleasant, were presented to 16 healthy subjects while brain activity was recorded from 64 scalp sites. Analysis subtracted the activity associated with the presentation of neutral images from the activity associated with the presentation of pleasant as well as unpleasant images. Results demonstrate that both pleasant and unpleasant valence is associated with transient, widespread, and bilateral frontal SSVEP latency reductions. Unpleasant images were also associated with a transient bilateral anterior frontal amplitude decrease. Latency reductions are interpreted as increases in neural information processing speed, while amplitude reductions are interpreted in the current paper as analogous to an event-related desynchronisation commonly associated with the alpha bandwidth. These key findings support previous literature in terms of there being substantial overlap in frontal neural circuitry when the brain processes pleasant and unpleasant valence relative to neutral valence.

Brain monoamines and early visual information-processing speed

Visual inspection time (IT) is a measure of information-processing speed, which correlates reliably with psychometric intelligence. Pharmacological research into IT indicates that manipulation of the cholinergic system modulates performance on the IT task, however the contribution of other neurotransmitters to this modality remains unclear. This study was designed to examine the effects of low brain serotonin and catecholamine availability on IT using the established method of amino-acid precursor depletion. Female participants (n=13) completed three experimental sessions; tryptophan depletion (TD); tyrosine/phenylalanine depletion (TPD); and a balanced control condition (B) in a randomized, double-blind cross-over design. IT assessments were performed at baseline and approx. 5 h post-mixture administration. IT scores were unaffected by either of the treatment conditions. These findings suggest that monoamines, whilst implicated in various forms of cognition are not central to IT, which measures the efficiency of perceptual intake and information-processing speed.

Test-retest reliability of the emotional stroop task: examining the paradox of measurement change

The Emotional Stroop (ES) task (I. H. Gotlib & C. D. McCann, 1984) has been proposed as an experimental measure to assess the processing of emotion or the bias in attention of emotion-laden information. However, study results have not been consistent. To further examine its reliability for empirical research, the authors of this study administered the ES task to 33 participants on 2 separate occasions separated by 1 week. Results indicated that retest reliabilities for reaction times (RTs) derived from the 3 separate emotion conditions (manic, neutral, and depressive) across the 1 week interval were very high. However, consistent with previous research, the reliabilities were very low for the interference indices (manic and depressive). These low reliabilities reflect the very high intercorrelation between the RTs derived from the 3 conditions. The authors concluded that a better indicator of the reliability for this task is the individual RTs from each emotion condition.

Effects of serotonin and catecholamine depletion on interleukin-6 activation and mood in human volunteers

There is increasing evidence that depression and related neurotic illnesses are associated with alterations in immune function that may contribute to their pathogenesis. For example, clinical and experimental studies have shown that abnormal HPA-axis activation and monoamine neurotransmission may be related to an increased release of proinflammatory cytokines from stimulated lymphocytes in the periphery and brain. In the present investigation, the effects of tryptophan depletion (TD) on unstimulated plasma interleukin-6 (IL-6) concentrations were investigated in order to determine whether acute changes in serotonin (5-HT) neurotransmission would induce a proinflammatory response in healthy individuals. The effects of TD were compared with the analogous procedure of tyrosine depletion (TPD), which reduces catecholamine metabolism in humans. Thirteen female participants completed three experimental sessions: TD, TPD and a balanced-control condition (B). Mood-ratings and blood sampling were performed at baseline and 5 h after the administration of the mixtures. Analyses revealed that TD and TPD markedly reduced tryptophan and tyrosine/phenylalanine levels, respectively. No changes in plasma IL-6 production or ratings of lowered mood were observed, however, subjects did report feeling more fatigued after TD. These findings indicate that a transient disruption in global monoamine function does not stimulate a proinflammatory response of IL-6 in normal volunteers.

A review of the status of Western herbal medicine in Australia

Western herbal medicine is the most widely used form of herbal medicine in Australia although Ayurvedic and Chinese herbal medicines are becoming better known. The agricultural production and manufacture of locally grown herbs is, with some exceptions, relatively underdeveloped, as is the research and development of indigenous flora. However, the use of herbal medicine is increasingly becoming mainstream with retail sales of herbal products in Australia estimated to be 200 million dollars. Concurrent with the increase in popularity of herbal medicine with health consumers have come advances in herbal medicine education and regulation. Although small by European standards, research into herbal medicines is increasing, mainly through industry and University-based initiatives.