Dopamine
Ambient dopamine concentrations in the brain determine our level of attention. This relationship is Π shaped: too much dopamine leads to rigid, inflexible thinking, whereas too little leads to distractibility (Cools & Robbins, 2004). Stimulants such as Ritalin and coffee increase dopamine tone in the brain. Brief bursts of dopamine indicate biologically important events, particularly unexpected rewards. They reinforce learning and repetition of behaviours that precede them.
The brain of man and monkey contains roughly 400.000 dopamine-producing neurons, located in the ventral tegmental area and substantia nigra in the midbrain (see figure). These neurons extend axons, via the medial forebrain bundle, into the frontal cortex, striatum, hippocampus and amygdala, where they release dopamine that activates D2- and, at high concentrations, D1-receptors. Dopamine release can be quantified by microdialysis probes or microelectrodes placed in target brain regions. Such studies indicate a slow, enabling and a fast, rewarding function of dopamine.
Dopamine-producing neurons continuously fire action potentials at a slow pace (2-3 Hz). These regulate the baseline concentration of dopamine in target brain regions (5-10 nM in striatum), which has an 'enabling' effect on neural activity and determines our level of attention, motivation and ability execute movements. For instance, dopamine concentrations regulate the signal-to-noise ratio in the prefrontal cortex, thus modulating the robustness of working-memory 'representations' (thoughts) (Gruber et al 2006). Stimulants such as Adderall and Ritalin enhance attention by increasing dopamine concentrations, particularly in attention deficit disorders, which involve impaired dopamine signalling. Dopamine concentrations rise by 20-100% in novel environments and in anticipation of, or on delivery of, rewards (e.g. liquid, food, drug, sex) and in response to some aversive events (Schultz 2007). About a quarter of dopamine-producing neurons also steadily increase their firing rate to about 2x baseline for several seconds in anticipation of uncertain rewards. Slow dopamine release thus seems to motivate the brain to attend to and act on biologically important events.
Many dopamine neurons fire bursts of 3-4 action potentials at 10-50 Hz in response to unexpected rewards or reward-predicting stimuli (this signal seems to constitute a reward-prediction signal but see Redgrave & Gurney, 2006). These bursts transiently raise dopamine concentrations to >100 nM, which activates D1 receptors, enabling memory consolidation and learning (Schultz 2007). Bursts may also occur spontaneously, in response to intense stimuli and, in a small number of dopamine neurons projecting to the cortex, in response to aversive events. However, most dopamine neurons respond to aversive events, including omitted rewards, by transiently reducing their firing rates. Burst activity in dopamine neurons thus seems to stimulate the brain to remember situations and learn behaviours that maximize its rewards, and to un-learn non-rewarding behaviours. Bursts of electrical stimulation to dopamine-producing neurons are extremely rewarding, can override even the desire to eat or sleep and are central to iPlant programming (Burgess et al 1991, Garner et al 1991, Hermer-Vasquez et al 2005).
YouTube: Dopamine, attention, working memory, neuronal groups and the prefrontal cortex, What is dopamine?, Dopamine and the frontal lobes
Serotonin
The neural function of serotonin is not as well understood as that of dopamine. Serotonin often counteracts the effect of dopamine but in a way that is adaptive, creative and warm. For example, serotonin depletion in the frontal cortex leads to problems re-evaluating learned patterns of behaviour (Clarke et al 2005). Impaired serotonin function is associated with mood disorders like anxiety, depression, obsessive compulsive disorder, anorexia and bulimia (Lechin et al 2006, Jans et al 2007). Many pharmacological treatments of these disorders (e.g. Prozac, Citalopram) increase baseline serotonin concentrations in the brain. Serotonin is synthesized by neurons in the raphe nuclei, which projects axons widely throughout the brain.
Noradrenaline
The structure, synthesis and function of noradrenaline is closely related to those of dopamine. See Sara 2009 for an overview.
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