Excitatory and inhibitory synaptic transmission use different neurotransmitters and receptors

Whether the result of synaptic transmission will be excitatory or inhibitory depends on the type of neurotransmitter used and the ion channel receptors they interact with.

Excitatory synaptic transmission uses a neurotransmitter called L-glutamate. This is a common animo acid used throughout the body to build proteins (it is also used as a flavour enhancer - mono-sodium glutamate!!!). However, in the CNS it is the major excitatory neurotransmitter. It interacts with glutamate receptors in the post-synaptic neuron. These receptors are ion channels that are permeable to sodium ions and thus generate depolarisation waves. Inhibitory synaptic transmission uses a neurotransmitter called GABA. This interacts with GABA receptors, ion channels that are permeable to negatively charged chloride ions. Thus opening of these channels makes it harder for a neuron to generate an action potential.

In general (though not necessarily exclusively), neurons use only one neurotransmitter to 'send' information and are thus either excitatrory or inhibitory - they are termed glutamatergic or GABAergic, respectively.

In addition to L-glutamate and GABA, there are a whole range of other neurotransmitters and their receptors (acetylcholine, dopamine, serotonin and many others) that also perform vital functions in the central nervous system, some of which are excitatory and some of which are inhibtory. The functional loss of these neurotransmitters and responsive neurons are implicated some of the most devastating neurodegenerative disorders such as Alzheimer's and Parkinson's diseases while Prozac, probably the best known anti-depressant currently in use, affects serotonin-mediated neurotransmission.

Glutamate receptors come in many flavours!

Glutamate receptors come in many flavours - that is there are many different types and sub-types. There are three types of ion channel receptor, named after chemicals that specifically interact with them (AMPA, NMDA and Kainate receptors; for detailed information on the structure and function of these receptors, click here). In addition, there is another group of glutamate receptors that are not ion channels, but pass signals into the cell via other accessory protiens (G-proteins). These receptors are termed the metabotropic glutamate receptors (click here for further details).

The two categories of glutamate receptor perform very different functions in synaptic transmission. The ion channel receptors (ionotropic glutamate receptors) generate the post-synaptic depolarisation while the metabotropic glutamate receptors modulate the size and nature of that response.