Autoreceptor
Encyclopedia
An autoreceptor is a receptor
located on presynaptic nerve cell
membrane
s and serves as a part of a feedback loop in signal transduction
. It is sensitive only to those neurotransmitter
s or hormone
s that are released by the neuron in whose membrane the autoreceptor sits.
Canonically, a presynaptic neuron releases the neurotransmitter across a synaptic cleft to be detected by the receptors on a postsynaptic neuron. Autoreceptors on the presynaptic neuron will also detect this neurotransmitter and often function to control internal cell processes, typically inhibiting further release or synthesis of the neurotransmitter. Thus, release of neurotransmitter is regulated by negative feedback. Autoreceptors are usually G protein-coupled receptor
s (rather than transmitter-gated ion channels
) and act via a second messenger.
Autoreceptors may be located anywhere on the cell body: near the terminal at the axon
, on the soma
, or on the dendrite
s.
As an example, norepinephrine
released from sympathetic
neurons may interact with alpha-2A and alpha-2C receptors to inhibit neurally released norepinephrine. Similarly, acetylcholine
released from parasympathetic neurons may interact with muscarinic-2 and muscarinic-4 receptors to inhibit neurally released acetylcholine. An atypical example is given by the β-adrenergic autoreceptor in the sympathetic peripheral nervous system
, which acts to increase transmitter release.
An example of an autoreceptor's functioning occurs in the depression of PPF (post-synaptic potential facilitation
). A feedback cell is activated by the (partially) depolarized post-synaptic neuron. The feedback cell releases a neurotransmitter to which the autoreceptor of the presynaptic neuron is receptive. The autoreceptor causes the inhibition of calcium channels (slowing calcium ion influx) and the opening of potassium channels (increasing potassium ion efflux) in the presynaptic membrane. These changes in ion concentration effectively diminish the amount of the original neurotransmitter released by the presynaptic terminal into the synaptic cleft. This causes a final depression on the activity of the postsynaptic neuron. Thus the feedback cycle is complete.
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
located on presynaptic nerve cell
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
s and serves as a part of a feedback loop in signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
. It is sensitive only to those neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
s or hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s that are released by the neuron in whose membrane the autoreceptor sits.
Canonically, a presynaptic neuron releases the neurotransmitter across a synaptic cleft to be detected by the receptors on a postsynaptic neuron. Autoreceptors on the presynaptic neuron will also detect this neurotransmitter and often function to control internal cell processes, typically inhibiting further release or synthesis of the neurotransmitter. Thus, release of neurotransmitter is regulated by negative feedback. Autoreceptors are usually G protein-coupled receptor
G protein-coupled receptor
G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
s (rather than transmitter-gated ion channels
Ligand-gated ion channel
Ligand-gated ion channels are one type of ionotropic receptor or channel-linked receptor. They are a group of transmembrane ion channels that are opened or closed in response to the binding of a chemical messenger , such as a neurotransmitter.The binding site of endogenous ligands on LGICs...
) and act via a second messenger.
Autoreceptors may be located anywhere on the cell body: near the terminal at the axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
, on the soma
Soma (biology)
The soma , or perikaryon , or cyton, is the bulbous end of a neuron, containing the cell nucleus. The word "soma" comes from the Greek σῶμα, meaning "body"; the soma of a neuron is often called the "cell body"...
, or on the dendrite
Dendrite
Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...
s.
As an example, norepinephrine
Norepinephrine
Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...
released from sympathetic
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
neurons may interact with alpha-2A and alpha-2C receptors to inhibit neurally released norepinephrine. Similarly, acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
released from parasympathetic neurons may interact with muscarinic-2 and muscarinic-4 receptors to inhibit neurally released acetylcholine. An atypical example is given by the β-adrenergic autoreceptor in the sympathetic peripheral nervous system
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
, which acts to increase transmitter release.
An example of an autoreceptor's functioning occurs in the depression of PPF (post-synaptic potential facilitation
Neural facilitation
Neural facilitation, also known as paired pulse facilitation, is a concept in neuroscience where an increase in the postsynaptic potential is evoked by a second impulse....
). A feedback cell is activated by the (partially) depolarized post-synaptic neuron. The feedback cell releases a neurotransmitter to which the autoreceptor of the presynaptic neuron is receptive. The autoreceptor causes the inhibition of calcium channels (slowing calcium ion influx) and the opening of potassium channels (increasing potassium ion efflux) in the presynaptic membrane. These changes in ion concentration effectively diminish the amount of the original neurotransmitter released by the presynaptic terminal into the synaptic cleft. This causes a final depression on the activity of the postsynaptic neuron. Thus the feedback cycle is complete.