metabotropic receptors

Examples of metabotropic receptors in the following topics:

• Ionotropic and Metabotropic Receptors

  • Although both ionotropic and metabotropic receptors are activated by neurotransmitters, ionotropic receptors are channel-linked while metabotropic receptors initiate a cascade of molecules via G-proteins.
  • Two types of membrane-bound receptors are activated with the binding of neurotransmitters: ligand-gated ion channels (LGICs) inotropic receptors and metabotropic G- protein coupled receptors.
  • Examples of metabotropic receptors include glutamate receptors, muscarinic acetylcholine receptors, GABAB receptors, most serotonin receptors, and receptors for norepinephrine, epinephrine, histamine, dopamine, neuropeptides, and endocannabinoids.
  • The metabotropic G protein-coupled receptors have seven hydrophobic transmembrane domains.
  • Since opening channels by metabotropic receptors involves activating a number of molecules in the intracellular mechanism, these receptors take longer to open than the inotropic receptors.

• Adrenergic Neurons and Receptors

  • Adrenergic receptors are molecules that bind catecholamines.
  • The adrenergic receptors (or adrenoceptors) are a class of metabotropic G protein-coupled receptors that are targets of the catecholamines, especially norepinephrine or noradrenaline, as seen in , and epinephrine (adrenaline) as shown in ).
  • There are two main groups of adrenergic receptors, α and β, with several subtypes. α receptors have the subtypes α1 (a Gq coupled receptor) and α2 (a Gi coupled receptor).
  • Phenylephrine is a selective agonist of the α receptor. β receptors have the subtypes β1, β2 and β3.
  • α1-adrenergic receptors are members of the G protein-coupled receptor superfamily.

• Classification of Receptors by Stimulus

  • Sensory receptors can be classified by the type of stimulus that generates a response in the receptor.
  • Sensory receptors perform countless functions in our bodies.
  • Cutaneous receptors are sensory receptors found in the dermis or epidermis.[2]
  • Encapsulated receptors consist of the remaining types of cutaneous receptors.
  • A tonic receptor is a sensory receptor that adapts slowly to a stimulus, while a phasic receptor is a sensory receptor that adapts rapidly to a stimulus.

• Blocking of Hormone Receptors

  • A receptor antagonist does not provoke a biological response upon receptor binding, but limits or dampens agonist-mediated responses.
  • A receptor antagonist is a type of receptor ligand or drug that does not provoke a biological response itself upon binding to a receptor, but blocks or dampens agonist-mediated responses.
  • Binding to the active site on the receptor regulates receptor activation directly.
  • The current accepted definition of receptor antagonist is based on the receptor occupancy model.
  • Irreversible antagonists covalently bind to the receptor target and, in general, cannot be removed; inactivating the receptor for the duration of the antagonist effects is determined by the rate of receptor turnover, the rate of synthesis of new receptors.

• Classification of Receptors by Location

  • Some sensory receptors can be classified by the physical location of the receptor.
  • Remarkably, specialized receptors have evolved to transmit sensory inputs from each of these sensory systems.
  • Receptors are sensitive to discrete stimuli and are often classified by both the systemic function and the location of the receptor.
  • Sensory receptors are found throughout our bodies, and sensory receptors that share a common location often share a common function.
  • For example, sensory receptors in the retina are almost entirely photoreceptors.

• Types of Receptors

  • There are two types of receptors: internal receptors and cell-surface receptors.
  • Internal receptors can directly influence gene expression without having to pass the signal on to other receptors or messengers.
  • There are three general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors.
  • Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme.
  • An example of this type of enzyme-linked receptor is the tyrosine kinase receptor.

• Target Cell Specificity

  • Hormones target a limited number of cells (based on the presence of a specific receptor) as they circulate in the bloodstream.
  • This androgen insensitivity occurs when the receptors on the target cells are unable to accept the hormone due to an impairment in receptor shape.
  • Target cells are capable of responding to hormones because they display receptors to which the circulating hormone can bind.
  • Finally, hormone-receptor affinity can be altered by expression of associated inhibitory or co-activating factors.
  • In some instances alterations of receptor structure due to a genetic mutation can lead to a reduction in hormone-receptor affinity as in the case of androgen insensitivity.

• Direct Gene Activation and the Second-Messenger System

  • Hormones can alter cell activity by binding with a receptor.
  • Receptors which can directly influence gene expression are termed nuclear receptors.
  • Type I nuclear receptors are located in the cytosol.
  • Type II receptors are retained in the nucleus .
  • Most hormone receptors are G protein-coupled receptors.

• Binding Initiates a Signaling Pathway

  • Ligand binding to cell-surface receptors activates the receptor's intracellular components setting off a signaling pathway or cascade.
  • Cell-surface receptors, also known as transmembrane receptors, are membrane-anchored (integral) proteins that bind to external ligand molecules.
  • There are three general categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked receptors.
  • All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.
  • Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme.

• Plasma Membrane Hormone Receptors

  • Lipid-insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors.
  • When a hormone binds to its membrane receptor, a G protein that is associated with the receptor is activated.
  • G proteins are proteins separate from receptors that are found in the cell membrane.
  • When a hormone is not bound to the receptor, the G protein is inactive and is bound to guanosine diphosphate, or GDP.
  • Describe the events that occur when a hormone binds to a plasma hormone receptor