Physiology and pathophysiology of the serotonergic system and its implications on mental and physical performance. Part II.

As shown in the first part of this review, well equilibrated neurotransmission in which 5-HT plays a dominant role is important for proper neuromodulation and adjustment of neuronal network elements. Adequate 5-HT system function supports regulation of intercommunicative neuronal transmission in the brain, which optimizes behavioral neuromodulation during and after different forms of exertions, thereby preventing transient dysregulation. Impairment of neuromodulation and neuronal network in the brain with transient dysfunctions or permanent substantial deficits at manifestation of various types of depression results from prevalent impairment of 5-HT neurotransmission and its central interaction with other neurotransmitter systems. Exercise-induced increase of free tryptophan (TRP) in blood occurs due to liberation from albumin, which is caused by adrenergically induced lipolysis of free fatty acids and results in higher free TRP uptake into the brain. Consecutively enhanced serotonin (5-HT) biosynthesis does not per se initiate mood impairment or central fatigue. It is suggested that in overtrained athletes central fatigue, mental deficiency and behavioral alterations with depressive mood are probably not primarily caused by metabolic and neuromuscular alterations. The primary trigger of these transient behavioural alterations might instead be initiated by a central exhaustive exercise stress which elicits impairment of complex neuromodulation, also afflicting the interaction of central neurotransmitters or hypothalamic neuropeptides and releasing factors. In a consecutive correction of the variation, the implication of the serotonergic system on the central neuromodular disturbance might improve or prevent the progressive course both in transient and in permanent mental disorders. However, an unsuccessful attempt to improve the depressive symptomatology leads mostly to an overproportional exaggeration of the behavioral changes.