However, given advances in pharmacology, genetics, and chemical neuroanatomy, the term "neurotransmitter" can be applied to chemicals that:
->Carry messages between neurons via influence on the postsynaptic membrane.
->Have little or no effect on membrane voltage, but have a common carrying function such as changing the structure of the synapse.
->Communicate by sending reverse-direction messages that affect the release or reuptake of transmitters.

a-amino acid

Ester of acetic acid and choline

Increase NMDA receptor activity and risk of excitotoxicity (mGluR1, mGluR5) Mainly Postsynaptic

Decrease NMDA receptor activity and risk of excitotoxicity (mGluR2, mGluR3) Mainly Presynaptic

Decrease NMDA receptor activity and risk of excitotoxicity (mGluR4, mGluR6, mGluR7, mGluR8) Mainly Presynaptic

Gi/G0 - heterotrimeric G protein subunit that inhibits the production of cAMP from ATP. Inhibits cAMP dependent pathway. These receptors are involved in presynaptic inhibition, and do not appear to affect postsynaptic membrane potential by themselves. Receptors in groups II and III reduce the activity of postsynaptic potentials, both excitatory and inhibitory, in the cortex

Gq - heterotrimeric G protein subunit that activates phospholipase C (PLC). PLC in turn hydrolyzes PIP2 to DAG and IP3. DAG acts as a second messenger that activates Protein Kinase C (PKC) and IP3 helps in phosphorylation of some proteins.
Can increase cytosolic calcium concentrations. These receptors are also associated with Na+ and K+ channels. Their action can be excitatory, increasing conductance, causing more glutamate to be released from the presynaptic cell, but they also increase inhibitory postsynaptic potentials, or IPSPs. They can also inhibit glutamate release and can modulate voltage-dependent calcium channels.

NMDA receptors
(N-methyl-D-aspartate receptor)

Each AMPAR has four sites to which an agonist (such as glutamate) can bind, one for each subunit. The binding site is believed to be formed by the N-terminal tail and the extracellular loop between transmembrane domains three and four. When an agonist binds, these two loops move towards each other, opening the pore. The channel opens when two sites are occupied, and increases its current as more binding sites are occupied.

The AMPAR's permeability to calcium and other cations, such as sodium and potassium, is governed by the GluR2 subunit. If an AMPAR lacks a GluR2 subunit, then it will be permeable to sodium, potassium, and calcium. The presence of a GluR2 subunit will almost always render the channel impermeable to calcium.

Once open, the channel may undergo rapid desensitization, stopping the current. The mechanism of desensitization is believed to be due to a small change in angle of one of the parts of the binding site, closing the pore.AMPARs open and close quickly (1ms), and are thus responsible for most of the fast excitatory synaptic transmission in the central nervous system.