Adenosine is a predominantly inhibitory neurotransmitter located in the central nervous system of animals. Adenosine acts of adenosine receptors which include a number of subgroups. While evidence suggests that adenosine can have excitatory effects in some neurones, generally activation of adenosine receptors by adenosine causes a decrease in the firing rate of post-synaptic neurones and therefore a reduction in neuronal activity. Adenosine is therefore able to reduce the excitatory neuronal pathways of the brain, and this explains the current understanding that adenosine is involved in inducing sleepiness and relaxation, at least in higher mammals including humans. There are thought to be 4 subgroups of adenosine receptor, and of these the A1 and A2α receptors are involved in the regulation of alertness and sleep. These receptors have slightly different distributions within the brain. Adenosine release is under circadian control and this reflects the diurnal sleep-wake cycle apparent in humans.
Prolonged wakefulness increases brain levels of adenosine, and this then initiates the inhibition of neuronal activity. As this process proceeds, the ability to stay alert and awake diminishes until sleep is induced. Blocking the adenosine receptor is known to induce alertness as it prevent adenosine inhibiting neuronal activity. Caffeine is an adenosine antagonist, which means it fulfills this role of inhibiting the activation of the adenosine receptor and thus increases alertness. However, it is unclear as to which adenosine receptor caffeine acts upon. Caffeine may also have other effects in the central nervous system, for example it may increase the turnover and release of certain neurotransmitters including of serotonin, dopamine and noradrenaline. Caffeine is much more effective at increasing alertness in those who are not habituated to the effects of caffeine. In this regard regular caffeine drinkers may find that sleep is not affected by consumption of caffeine before sleep.
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