Unconventional neurotransmitters and retrograde signalling

Most neurotransmitters are released from a presynaptic terminal via "exocytosis". They diffuse across extracellular space and, typically, bind to postsynaptic receptors. However, not all neurotransmitters are presynaptic.

Anandamide and 2-AG are endocannabinoids. They inhibit LTP (which impairs memory, for example). Endocannabinoid production can be stimulated by a rise in [Ca2+] in postsynaptic neurons. Endocanninoid release is not well understood, but after release from a postsynaptic terminal these transmitters can bind to cannabinoid receptors such as CB1 receptors (a GPCR) on the presynaptic neuron, or other nearby cells.

Also, nitric oxide (NO) can permeate the plasma membrane of a postsynaptic neuron and diffuse into nearby cells, including the presynaptic terminal. NO acts intracellularly, so it is often considered to be "second messenger" rather than a neurotransmitter.