Psychostimulants exert their effects by acting on a range of biological systems. One of the primary targets of psychostimulant activity is the monoamine system. Monoamines refer to the particular neurotransmitters dopamine, noradrenaline and serotonin. Dopamine and noradrenaline are sometimes also referred to as catecholamines. These neurotransmitters are involved in mediating a wide range of physiological and homeostatic functions, which vary with the part of the brain being examined.
Dopamine is a modulatory neurotransmitter. It is important in the regulation of movement, cognitive processes such as attention and working memory and motivational behaviour (Tzschentke, 2001
; Vallone, Picetti & Borrelli, 2000
). It is the primary neurotransmitter involved in reward pathways that is considered important in mediating effects of drugs of abuse (Tzschentke, 2001
). Dopamine acts on a range of dopamine receptors located in various brain regions and the periphery.
Noradrenaline (also called norephinephrine) acts on the adrenergic (or sympathetic) nervous system and is involved in mediating cardiovascular effects, arousal, concentration, attention, learning and memory (Ressler & Nemeroff, 1999
). Noradrenaline acts on adrenergic receptors in the Central Nervous System (CNS) and the periphery. There are two types of adrenergic receptors (a and b) and for each type there are a number of subtypes (Lynch, Harrison & Pearson, 1994
; Michelotti, Price & Schwinn, 2000
Serotonin is a neurotransmitter in the CNS, but is also present in platelets and the gastrointestinal mucosa. It is also known as 5-hydroxytryptamine, or 5-HT. It is involved in a variety of physiological processes, including regulation of smooth muscle function, blood pressure regulation and both peripheral and CNS neurotransmission. In the CNS it is involved in complex behaviours such as mood, appetite, sleep, cognition, perception, motor activity, temperature regulation, pain control, sexual behaviour and hormone secretion (Kema, de Vries & Muskiet, 2000
; Saxena, 1995
). Serotonin acts on serotonin (5-HT) receptors, of which there are many types and subtypes (e.g. 5-HT1A, 5-HT2C).
These neurotransmitters are synthesised within particular neurons and stored in vesicles.To exert an effect, they are released into the synapse where they are able to act on receptors. Their action at receptors is terminated either by being broken down by enzymes such as monoamine oxidase or being returned to the nerve terminal by a reuptake transporter. Psychostimulants may increase or enhance the activity of dopamine, noradrenaline or serotonin by either increasing release, blocking reuptake, inhibiting metabolism or acting directly on a receptor.