The effects of acute tryptophan depletion on neuropsychological function, mood and movement in the healthy elderly

Techniques for Measurement of Serotonin: Implications in Neuropsychiatric Disorders and Advances in Absolute Value Recording Methods

Serotonin (5-HT) is a monoamine neurotransmitter in the peripheral, enteric, and central nervous systems (CNS). Within the CNS, serotonin is principally involved in mood regulation and reward-seeking behaviors. It is a critical regulator in CNS pathologies such as major depressive disorder, addiction, and schizophrenia. Consequently, in vivo serotonin measurements within the CNS have emerged as one of many promising approaches to investigating the pathogenesis, progression, and treatment of these and other neuropsychiatric conditions. These techniques vary in methods, ranging from analyte sampling with microdialysis to voltammetry. Provided this diversity in approach, inherent differences between techniques are inevitable. These include biosensor size, temporal/spatial resolution, and absolute value measurement capabilities, all of which must be considered to fit the prospective researcher's needs. In this review, we summarize currently available methods for the measurement of serotonin, including novel voltammetric absolute value measurement techniques. We also detail serotonin's role in various neuropsychiatric conditions, highlighting the role of phasic and tonic serotonergic neuronal firing within each where relevant. Lastly, we briefly review the present clinical application of these techniques and discuss the potential of a closed-loop monitoring and neuromodulation system utilizing deep brain stimulation (DBS).

Nutritional importance of tryptophan for improving treatment in depression and diabetes

The importance of nutrients in our diet is becoming increasingly recognized. From the viewpoint of protein synthesis and other physiologic and metabolic functions, all amino acids are important, but some of these amino acids are not synthesized endogenously. This subset, called essential amino acids, comprise dietarily indispensable nutrients. Tryptophan, an essential amino acid, is the sole precursor of neuronal as well as peripheral serotonin (5-hydroxytryptamine). Its systemic or oral administration increases serotonin synthesis because tryptophan hydroxylase, the rate-limiting enzyme of 5-hydroxytryptamine biosynthesis, is physiologically unsaturated with its substrate. Central serotonin is implicated in a number of psychiatric illnesses, including depression, and in responses to stress. Acting peripherally, serotonin affects vasoconstriction, intestinal motility, control of T cell–mediated immunity, and liver and pancreatic functions. Depression and diabetes are 2 highly prevalent diseases that often coexist. There is evidence that occurrence of depression is 2–3 times higher in people with diabetes mellitus. A comorbid condition of diabetes and depression worsens the treatment and increases risk for death. Stress, known for its causal role in depression, can also enhance risk for diabetes. Stress-induced decreases in the circulating levels of tryptophan can impair brain and pancreatic serotonin-dependent functions to precipitate these diseases. The importance of tryptophan supplementation for improving therapeutic intervention in depression and diabetes is the focus of this article. A deficiency of this essential amino acid may enhance risk for depression as well as diabetes, and can also weaken treatment efficacy of medicinal compounds for treating these diseases. Guidelines for optimal levels of circulating tryptophan can help if supplements of this amino acid can improve treatment efficacy.

Serotonin and Consciousness-A Reappraisal

The serotonergic system of the brain is a major modulator of behaviour. Here we describe a re-appraisal of its function for consciousness based on anatomical, functional and pharmacological data. For a better understanding, the current model of consciousness is expanded. Two parallel streams of conscious flow are distinguished. A flow of conscious content and an affective consciousness flow. While conscious content flow has its functional equivalent in the activity of higher cortico-cortical and cortico-thalamic networks, affective conscious flow originates in segregated deeper brain structures for single emotions. It is hypothesized that single emotional networks converge on serotonergic and other modulatory transmitter neurons in the brainstem where a bound percept of an affective conscious flow is formed. This is then dispersed to cortical and thalamic networks, where it is time locked with conscious content flow at the level of these networks. Serotonin acts in concert with other modulatory systems of the brain stem with some possible specialization on single emotions. Together, these systems signal a bound percept of affective conscious flow. Dysfunctions in the serotonergic system may not only give rise to behavioural and somatic symptoms, but also essentially affect the coupling of conscious affective flow with conscious content flow, leading to the affect-stained subjective side of mental disorders like anxiety, depression, or schizophrenia. The present model is an attempt to integrate the growing insights into serotonergic system function. However, it is acknowledged, that several key claims are still at a heuristic level that need further empirical support.

Health, pre-disease and critical transition to disease in the psycho-immune-neuroendocrine network: Are there distinct states in the progression from health to major depressive disorder?

The psycho-immune-neuroendocrine (PINE) network is a regulatory network of interrelated physiological pathways that have been implicated in major depressive disorder (MDD). A model of disease progression for MDD is presented where the stable, healthy state of the PINE network (PINE physiome) undergoes progressive pathophysiological changes to an unstable but reversible pre-disease state (PINE pre-diseasome) with chronic stress. The PINE network may then undergo critical transition to a stable, possibly irreversible disease state of MDD (PINE pathome). Critical transition to disease is heralded by early warning signs which are detectible by biomarkers specific to the PINE network and may be used as a screening test for MDD. Critical transition to MDD may be different for each individual, as it is reliant on diathesis, which comprises genetic predisposition, intrauterine and developmental factors. Finally, we propose the PINE pre-disease state may form a "universal pre-disease state" for several non-communicable diseases (NCDs), and critical transition of the PINE network may lead to one of several frequently associated disease states (influenced by diathesis), supporting the existence of a common Chronic Illness Risk Network (CIRN). This may provide insight into both the puzzle of multifinality and the growing clinical challenge of multimorbidity.

Acute tryptophan depletion and Lewy body dementias

BACKGROUND

Studies using acute tryptophan depletion (ATD) to examine the effects of a rapid reduction in serotonin function have shown a reduction in global cognitive status during ATD in Alzheimer's disease (AD) and Parkinson's disease (PD). Based on the severe cholinergic loss evident in dementia with Lewy bodies (DLB) and Parkinson's disease and dementia (PDD), we predicted that a reduction of global cognitive status during ATD would be greater in these conditions than in AD.

METHODS

Patients having DLB or PDD underwent ATD in a double-blind, placebo-controlled, randomized, counterbalanced, crossover design.

RESULTS

While the study intended to test 20 patients, the protocol was poorly tolerated and terminated after six patients attempted, but only four patients - three with DLB and one with PDD - completed the protocol. The Modified Mini-Mental State Examination (3MSE) score was reduced in all three DLB patients and unchanged in the PDD and dementia patient during ATD compared with placebo.

CONCLUSIONS

This reduction in global cognitive function and the poor tolerability may fit with the hypothesis that people with dementia with Lewy bodies have sensitivity to the effects of reduced serotonin function.

Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis

The serotonergic system forms a diffuse network within the central nervous system and plays a significant role in the regulation of mood and cognition. Manipulation of tryptophan levels, acutely or chronically, by depletion or supplementation, is an experimental procedure for modifying peripheral and central serotonin levels. These studies have allowed us to establish the role of serotonin in higher order brain function in both preclinical and clinical situations and have precipitated the finding that low brain serotonin levels are associated with poor memory and depressed mood. The gut-brain axis is a bi-directional system between the brain and gastrointestinal tract, linking emotional and cognitive centres of the brain with peripheral functioning of the digestive tract. An influence of gut microbiota on behaviour is becoming increasingly evident, as is the extension to tryptophan and serotonin, producing a possibility that alterations in the gut may be important in the pathophysiology of human central nervous system disorders. In this review we will discuss the effect of manipulating tryptophan on mood and cognition, and discuss a possible influence of the gut-brain axis.

Evolution to a chronic disease niche correlates with increased sensitivity to tryptophan availability for the obligate intracellular bacterium Chlamydia pneumoniae

The chlamydiae are obligate intracellular parasites that have evolved specific interactions with their various hosts and host cell types to ensure their successful survival and consequential pathogenesis. The species Chlamydia pneumoniae is ubiquitous, with serological studies showing that most humans are infected at some stage in their lifetime. While most human infections are asymptomatic, C. pneumoniae can cause more-severe respiratory disease and pneumonia and has been linked to chronic diseases such as asthma, atherosclerosis, and even Alzheimer's disease. The widely dispersed animal-adapted C. pneumoniae strains cause an equally wide range of diseases in their hosts. It is emerging that the ability of C. pneumoniae to survive inside its target cells, including evasion of the host's immune attack mechanisms, is linked to the acquisition of key metabolites. Tryptophan and arginine are key checkpoint compounds in this host-parasite battle. Interestingly, the animal strains of C. pneumoniae have a slightly larger genome, enabling them to cope better with metabolite restrictions. It therefore appears that as the evolutionarily more ancient animal strains have evolved to infect humans, they have selectively become more "susceptible" to the levels of key metabolites, such as tryptophan. While this might initially appear to be a weakness, it allows these human C. pneumoniae strains to exquisitely sense host immune attack and respond by rapidly reverting to a persistent phase. During persistence, they reduce their metabolic levels, halting progression of their developmental cycle, waiting until the hostile external conditions have passed before they reemerge.