Caffeine is found in several plants in nature that are consumed by humans, including tea leaves; coffee beans; guarana and mate, which are plants native to South America; cola nuts and cocoa beans. Caffeine has been shown to act as a pesticide in the plants in which it occurs, helping ward off damaging insects. In humans the chemical has a mild stimulant effect, making it a popular substance and a subject of considerable research.
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Multiple Points of Action
Caffeine has its stimulant effects by competing with the molecule adenosine, a by-product of energy production. Accumulation of adenosine within cells signals that energy has been used up, putting the brain and body in a low energy state. A study published in the 2010 "Journal of Alzheimer's Disease" described the stimulant effects of caffeine as having a variety of effects within a network of nerve pathways that adenosine influences, and that caffeine competes with adenosine both before and after its points of action within the central nervous system. For example, caffeine affects the release of dopamine, an important neurotransmitter associated with motivation and alertness, by inducing glutamate, an excitatory neurotransmitter that activates dopamine. Caffeine also influences the dopamine system at points after the production of dopamine.
Differences to Other Stimulants
A study published in the 2004 "Cell and Molecular Life Sciences" focused on the effects of caffeine on adenosine receptors in a portion of the brain known as the striatum -- an area involved in stimulation and control of motor activity. The researchers were interested in the striatum as the main area upon which caffeine has its affect. The study also compared the effects of caffeine on the striatum with those of two other notable stimulants, cocaine and amphetamine, and noted a distinct difference in their mechanisms of action. While all three activate the neurotransmitter dopamine via the striatum, caffeine acts indirectly on the striatal system, and cocaine and amphetamine have a direct mode of action. Additionally, the study cites an association between caffeine and decreased risk of Parkinson's disease via a neuroprotective effect of caffeine on neurons involved in transmission of dopamine, the main neurotransmitter affected by the striatum. .
Manipulates Calcium Release
One of the important effects caffeine has on neurotransmission is to stimulate cells to release calcium, according to a study reported in the 2008 "PLoS One." Calcium, in turn, acts as a signal that enables nerves to release the neurotransmitters that they use to relay messages to other cells. Caffeine specifically has the ability to activate the release of glutamate. The researchers also noted the ability of caffeine to selectively block calcium release from neurons, under certain conditions.