Tsodyks, Misha, Asher Uziel, and Henry Markram. “Synchrony Generation in Recurrent Networks with Frequency-Dependent Synapses.” Journal of Neuroscience 20, no. 1 (January 1, 2000): RC50.
http://www.jneurosci.org/cgi/content/abstract/20/1/RC50

This is the original paper describing the 3-state cycle of neural transmitter for synaptic transmission. An extension to include a forth, super inactive state, is given in the paper:

Volman, Vladislav, Richard C. Gerkin, Pak-Ming Lau, Eshel Ben-Jacob, and Guo-Qiang Bi. “Calcium and Synaptic Dynamics Underlying Reverberatory Activity in Neuronal Networks.” Physical Biology 4, no. 2 (2007): 91–103.
http://www.iop.org/EJ/abstract/1478-3975/4/2/003/