The two basic types of neurotransmitter receptors are ionotropic and metabotropic; they use different mechanisms to transduce extracellular signals (the binding of neurotransmitters to receptors) to intracellular responses.
A brain builder would use ionotropic receptors for rapid information transfer. For slower and longer lasting functions, the builder would use metabotropic receptors.
Ionotropic receptors are also known as ligand-gated ion channels. They combine receptor and channel functions into a single protein complex. When a ligand binds to this type of receptor, the channel changes conformation such that it is more or less permeable to one or more ions. Thus, the receptor acts directly, i.e. no intermediate metabolic steps are required. Note: "tropos" means to move in response to a stimulus.
The activation of an ionotropic receptor allows ions to pass through its channel. These channels are relatively fast, the channel opens and closes rapidly, and their time constants are about 0.5 ms. AMPARs are much fasters than NMDARs.
Metabotropic receptors usually activate G-proteins, which modulate ion channels directly or indirectly through enzymes and second messengers. They do not combine receptor and channel functions into a single protein complex. Note: these receptors are called "metabotropic" because the (delayed) movement of ions through the channel requires metabolic steps.
Metabotropic glutamate (mGlu) receptors are G-protein coupled receptors (GPCRs) that have been subdivided into three groups. The groups are based on sequence similarity, pharmacology and intracellular signalling mechanisms. Group I mGlu receptors (mGlu1 and mGlu5) are coupled to PLC and intracellular calcium signalling. Group II (mGlu2 and mGlu3) and group III receptors (mGlu4, mGlu6, mGlu7 and mGlu8) are negatively coupled to adenylyl cyclase.
mGluRs plus the GABA B receptor, Ca2+ sensing receptors, pheremone receptors, and taste receptors are distinct from the adrenergic-type GPCRs.
The activation of metabotropic receptors indirectly opens nearby ion channels, e.g. a channel on the cell's outer membrane (plasma membrane) as shown in the image below.
mGluRs can also open many calcium channels on the endoplasmic reticulum (in effect, the original input signal gets amplified). After the first messenger (a neurotransmitter outside the cell) binds to the receptor on the the cell's outer membrane (PM), the second messenger is produced and delivered to the target receptors, e.g. IP3 receptors that open Ca2+ channels in the ER membrane. Channels associated with these receptors take longer to open than ionotropic receptors -- from 30 ms up to 1 second. In this case the mGluRs works slowly because the second messenger (IP3) must diffuse in the cytosol before it can bind to IP3 receptors on the ER. In cases where the second messenger is Ca2+, the process is also slow because Ca2+ is delivered by diffusion -- but Ca2+ diffuses in the cytosol less easily than IP3 due to calcium buffers (Ca2+ is inactivated when it binds to a buffer).
Metabotropic receptors not only amplify an input signal, they offer precise control over cell behaviour over a wide range of times.
Ionotropic receptors have a fast and direct effect on their micro-domains. Their channels remain open for a few milliseconds. They provide a way to deliver a sharp ion spike to a very specific location. For example: voltage dependent calcium channels can deliver a sudden [Ca2+] increase proximate to a presynaptic vesicle(s). In contrast, metabotropic receptors are much slower. They may operate on a time scale of seconds to minutes and over larger domains, e.g. elevate [Ca2+] in one or more cells.
The release of one type of neurotransmitter may activate both metabotropic receptors and ligand-gated ion channels to produce both fast and slow post synaptic potentials at the same synapse.
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