....      The mechanism of endocannabinoid synaptic transmission is believed to occur as follows: first, transmission of the excitatory neurotransmitter glutamate causes an influx of calcium ions into the post-synaptic neuron. Through a mechanism not yet fully understood, the presence of calcium post-synaptically induces the production of endocannabinoids in the post-synaptic neuron. These endocannabinoids (such as anandamide) are released into the synaptic cleft, where binding occurs at cannabinoid receptors present on pre-synaptic neurons, where they modulate neurotransmission. Thus, this form of neurotransmission is termed retrograde transmission, as the signal is carried in the opposite direction of orthodox propagation, which previously was thought to be exclusively one way.