An overview of the serotonin, norepinephrine, and dopamine systems and their functions:
The serotonin system is known to play a role in mood, emotion, social disposition, sleep cycle, wakefulness, dreaming, appetite, spinal pain modulation, and endogenous opioidergic tone. The raphe nuclei (or RN) is a medium sized cluster of nuclei located in the brainstem, and is the primary source of serotonin for the brain. The RN projects serotonergic pathways up through areas of the cerebral cortex, and downward to the dorsal horn of the spinal cord. Most modern antidepressants (zoloft, paxil, prozac) enhance central serotonergic tone, through action within the raphe nuclei and its projections in other brain areas. As a general rule of thumb, 5HT levels are highest during ones wakeful hours, and lowest during sleep. Prolonged wakefulness is known to enhance serotonergic tone - the effects of sleep deprivation are similar to the effects of serotonergic antidepressants.
The norepinephrine system plays a role in wakefulness, alertness, energy, arousal, and motivation. The locus coeruleus (or LC), a nucleus located in the pons area of the brainstem, acts as a central headquarters to the norepinephrine system. The LC produces and projects NE to areas of the hindbrain, forebrain, spinal cord and elsewhere, through a system of far reaching noradrenergic projections. It plays a major role in function of the sympathetic nervous system, during states of agitation or stress. The LC, and the adrenergic pathways projecting from it, form what is known as the LC-noradrenergic system. Activation of the LC induces a state of arousal (i.e. stress response, fight or flight mode). Plenty of drugs with sedative or anxiolytic properties suppress activity of the LC (i.e. opioids, benzodiazepines, clonidine).
The dopaminergic system plays a role in psychological reward, motivation, desire, addiction, pleasure (hedonic tone), learning, and motor fine tuning. The central component of the dopaminergic system is the ventral tegmental area (or VTA). The VTA produces dopamine, and innervates the brain with dopaminergic pathways. The VTA projects dopamine to areas of the brain as a way of rewarding certain behaviors, especially those which are evolutionarily essential (such as eating and sex), but also other behaviors which must be reinforced in order to become learned by the brain. The VTA and the pathways it projects form what is known as the mesocorticolimbic system (meso refers to midbrain, while cortico refers to cerebral cortex). Dopamine and the limbic system (with the help of neuroplastic adaptation) are essential in forming complex behavioral/sensory/environmental-memory connections as well as emotional-memory connections. The VTA forms a connection with a key pleasure center known as the nucleus accumbens. Dopamine release into the nucleus accumbens is a common trademark of rewarding and addictive behaviors.