The SLC6A2 gene encodes a norepinephrine (noradrenaline) transporter, which is responsible for reuptake of norepinephrine into presynaptic nerve terminals and is a regulator of norepinephrine homeostasis. It also appears to transport the neurotransmitter dopamine in the same way, but to a lesser degree. In the CNS, norepinephrine is involved in a number of physiological responses including mood, sleep regulation, alertness and arousal, both cognitive and non-cognitive expression of behaviors, and control of the endocrine and autonomic nervous systems. Peripherally, norepinephrine is present in sympathetic nerve endings and has full control of the sympathetic nervous system. Certain antidepressant medications act to raise norepinephrine, such as the SNRIs and the tricyclic antidepressants (TCAs). In essence, these medications prevent the norepinephrine transporter from serving its purpose. Norepinephrine therefore remains in the synapse longer, allowing it to reach more normal levels. Since the norepinephrine transporter is also thought to transport dopamine to some extent, SNRIs would also raise dopamine levels in the synapse.