Nutritional Effects on Central Serotonergic Activity: A Hypothesis on the Unconscious Self-manipulation of Mood by Food Intake and Dietary Selection

Emotionsbedingtes Essverhalten: Die Bedeutung der Emotionen

Zusammenfassung. Wirkungen von Emotionen auf das Essverhalten variieren in Abhängigkeit von Person- und Emotionsmerkmalen. Bisherige Untersuchungen waren größtenteils auf den Einfluss von Personmerkmalen gerichtet. Sie zeigten, dass ein gezügelter sowie ein emotionaler Ess-Stil zu gesteigerter Nahrungsaufnahme bei negativen Emotionen prädisponieren. Weitere Untersuchungen verweisen auf den Einfluss von Emotionsmerkmalen wie Intensität und Valenz. Der vorliegende Beitrag fasst den Forschungsstand zusammen und unterteilt die Wirkungen von Emotionen auf das Essverhalten in fünf Varianten, die sich durch definierte Person- und Emotionsmerkmale vorhersagen lassen: emotionale Steuerung der Nahrungswahl, emotionale Hemmung des Essverhaltens, emotionale Enthemmung gezügelten Essverhaltens, emotional-instrumentelles Essverhalten und emotionkongruente Modulation des Essverhaltens.

Action of CCK and 5-HT on lateral hypothalamic neurons depends on early postnatal nutrition

Wistar rats grown up during the early postnatal life (3-21 days after birth) in artificially built normal, small or large lifters developed a significantly different body weight. This difference persisted also during adulthood when they had free access to food and water. The influence of iontophoretically administered cholecystokinin (CCK8S), serotonin (5-HT) or co-ejection of both on firing of lateral hypothalamic neurons was investigated in adult, urethane anesthetized rats of the three groups. The responsiveness to CCK8S was significantly higher in large- and small-litter rats than in the normal control group. The differences were greater in males than in females. They resulted in the male large-litter group from an increase of excitatory responses, whereas in the male small-litter group the proportion of inhibitory responses was augmented. Co-administration of 5-HT generally reduced the neuronal responsiveness. Especially in the large-litter group excitatory responses were significantly reduced. It may be speculated that the availability of food in the early postnatal life influences the development of the hypothalamic regulatory network in such a way that it stabilizes the high or low food ingestion all the life. At least in males, a changed responsiveness and type of response to cholecystokinin of lateral hypothalamic neurons might be involved in this altered regulation.

Macronutrients and mental performance

There is currently intense interest in the effects of macronutrients on psychological states, mental performance, and well-being. A strong theoretical perspective has guided work on carbohydrates and their relation to brain serotoninergic function with concomitant effects on performance. The clearest and most reliable effects have been observed for the beneficial action of glucose on cognitive performance, supported by investigations of hypoglycemia, which is associated with general impairment of cognitive performance. The effects of complex carbohydrates are less distinct and change with time of day; e.g., carbohydrate at breakfast tends to improve morning performance. However, these studies are rarely decisive. Far fewer experiments have been performed on protein and fat, and it is difficult to draw any firm conclusions. Macronutrients are seldom given alone, proportions of protein and fat differ greatly between studies, and comparisons are frequently performed with no food at all. Food intake may mitigate the effects of low doses but not of high doses of alcohol on performance. Effects of macronutrients on cognitive performance may be dependent on their effects on glucose metabolism, metabolic activation, or serotonin. Other factors that modify effects include time of day, circadian rhythms, type of task, habitual diet, and vulnerability of the population.

Acute regulation and long-term modulation of presynaptic serotonin output

This contribution summarizes the present knowledge about the mechanisms involved in the regulation of presynaptic serotonin release. Under conditions of a permanently altered environmental, (psychosocial, drug-induced, nutritional etc.) input, each one of these mechanisms may be adaptively adjusted to the novel use-dependent requirements. Examples of such long-term alterations of the 5-HT-output pattern in distant projection fields of the raphe-neurons are the downregulation of cortical serotonin-transporters after long-term food restriction, the loss of serotonergic nerve terminals caused by substituted amphetamines, the serotonergic hyperinnervation of the frontal cortex seen after olfactory bulbectomy in rats, or the long lasting changes in the levels of tryptophanhydroxylase, of serotonin-transporter depression or in the density of serotonergic nerve terminals in distant projection fields caused by long-term antidepressant treatments. All these long-lasting alterations and imbalances of a previously established serotonin-output-pattern will not only affect the impact of individual regional networks on whole brain information processing but also the established synaptic connectivity in distant projection fields of the central serotonergic system.

Intrahypothalamic Serotonergic Neurons

Serotonin's role as a neuronal transmitter was established already forty years ago, and the anatomy and many of the functions of the major serotonergic systems have been carefully mapped. The intimate association of serotonergic mechanisms with central control of food intake has also been extensively studied. While the present concepts of serotonergic functions rely on the ascending, raphe nuclei-originating serotonergic pathways, there is an accumulating evidence to support that hypothalamic neurons may also exhibit many features normally attributed to serotonergic neurons only. Neurons in the hypothalamic arcuate and periventricular nuclei express tryptophan hydroxylase, the serotonin synthesizing enzyme, while they do not transport or synthesize serotonin. On the other hand, dorsomedial nucleus contains a select population of neurons that do actively accumulate serotonin, while they do not express tryptophan hydroxylase. These and some other serotonin-associated features of the hypothalamic neuronal groups are discussed. Finally the present data is projected against the prevailing concept of hypothalamic regulation of food intake.