An adrenoceptor can also be called an adrenergic receptor and is one of the proteins that span along the membranes of presynaptic and postsynaptic neurons. The adrenoceptor serves as a recognition site for the binding of the neurotransmitters norepinephrine and epinephrine. There are two major classes of adrenoceptors. They are alpha- and beta-adrenoceptors.
The protective barrier surrounding a neuron is called a cell membrane, and located within it are ion channels that allow ions to move in and out of the cell. The flow of ions across the cell membrane results in a change in potential or electrical energy within the membrane. This causes a release of a chemical substance or neurotransmitter from the presynaptic neuron into the synaptic cleft, a fluid-filled gap. When the neurotransmitter is released, it binds or attaches itself to receptor sites on the postsynaptic neuron that recognize that particular neurotransmitter. Adrenoceptors are metabotropic receptors, meaning that they do not contain ion channels.
In order for associated ion channels to open and close and for ions to flow across the cell membrane of an adrenoceptor, a series of metabolic steps must occur. This involves the stimulation of G-proteins. When neurotransmitters bind to receptors, G-proteins are activated, and secondary messengers are generated. This results in enzymes being activated and the opening and closing of associated ion channels. For this reason, an adrenoceptor is also called a G-coupled receptor.
Adrenoceptors consist of two main groups: alpha-adrenergic receptors and beta-adrenergic receptors. These two groups have been further subdivided into alpha1-adrenergic receptors, alpha2-adrenergic receptors, beta1-adrenergic receptors and beta2-adrenergic receptors. These adrenoceptors bind to the neurotransmitters norepinephrine and epinephrine.
Alpha1-adrenoceptors are situated on the membrane of postsynaptic neurons. These adrenoceptors become active when norepinephrine binds to them, resulting in an activation of secondary messengers and a release of calcium. Calmodulin, a binding protein, binds with the calcium resulting in the opening of associated ion channels. This allows ions to move across the cell membrane and the neuron to become active. Alpha2-adrenoceptors are located on the presynaptic membrane of neurons, and when activated by norepinephrine, the reduce additional release of the neurotransmitter, making the neuron inactive.
Beta1-adrenoceptors are located in the heart, and when they are stimulated by transmitters, they result in an increase in heart rate and heart contraction. Beta2-adrenoceptors are found in smooth muscles. When these adrenoceptors are activated, the smooth muscles in which they are located will relax.