SLC6A8 and major depressive disorder: Furthermore, because neuronal creatine is released from neurons in response to an action potential, and because creatine is subsequently transported back into the presynaptic neuron by the creatine transporter (see Jiddeke et al., 2012, for a review), creatine is considered by some to function as a neurotransmitter; it may thus be that alterations in its functioning as a neurotransmitter promote depression.