Adrenergic Drugs Quiz: Test Your Knowledge of Agonists

Study Notes

Adrenergic Agonists

Adrenergic drugs affect receptors stimulated by norepinephrine (noradrenaline) or epinephrine (adrenaline), known as adrenergic receptors or adrenoceptors.
Adrenergic drugs that activate adrenergic receptors are termed sympathomimetics, and drugs that block the activation of adrenergic receptors are termed sympatholytics.

Catecholamines

Catecholamines are compounds containing a catechol moiety (a benzene ring with two adjacent hydroxyl groups) and an amine side chain.
The most important catecholamines are:
- Noradrenaline (norepinephrine), a transmitter released by sympathetic nerve terminals.
- Adrenaline (epinephrine), a hormone secreted by the adrenal medulla.
- Dopamine, the metabolic precursor of noradrenaline and adrenaline, also a transmitter/neuromodulator in the central nervous system.
- Isoprenaline (isoproterenol), a synthetic derivative of noradrenaline, not present in the body.

Adrenergic Neurons

Adrenergic neurons release norepinephrine as the primary neurotransmitter.
These neurons are found in the central nervous system (CNS) and also in the sympathetic nervous system, where they serve as links between ganglia and the effector organs.

Neurotransmission

Neurotransmission in adrenergic neurons involves the following steps: synthesis, storage, release, and receptor binding of norepinephrine, followed by removal of the neurotransmitter from the synaptic gap.
Synthesis of norepinephrine:
- Tyrosine is transported into the adrenergic neuron and hydroxylated to dihydroxyphenylalanine (DOPA) by tyrosine hydroxylase.
- DOPA is then decarboxylated to form dopamine by aromatic L-amino acid decarboxylase.
Storage of norepinephrine in vesicles:
- Dopamine is transported into synaptic vesicles by an amine transporter system.
- Dopamine is hydroxylated to form norepinephrine by dopamine β-hydroxylase.
Release of norepinephrine:
- An action potential triggers an influx of calcium ions, causing synaptic vesicles to fuse with the cell membrane and release their contents into the synapse.
Binding to receptors:
- Norepinephrine released from the synaptic vesicles binds to postsynaptic receptors on the effector organ or to presynaptic receptors on the nerve ending.

Adrenergic Receptors

Adrenergic receptors are classified into two main families: α and β receptors.
α-Adrenoceptors:
- Show a weak response to isoproterenol, but are responsive to naturally occurring catecholamines epinephrine and norepinephrine.
- Subdivided into two subgroups, α1 and α2, based on their affinities for α agonists and blocking drugs.
α1 Receptors:
- Present on the postsynaptic membrane of the effector organs and mediate many of the classic effects, originally designated as α-adrenergic, involving constriction of smooth muscle.
- Activation of α1 receptors initiates a series of reactions through the G protein activation of phospholipase C, ultimately resulting in the generation of second messengers inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG).
α2 Receptors:
- Located primarily on sympathetic presynaptic nerve endings and control the release of norepinephrine.
- Stimulation of α2 receptors causes feedback inhibition and inhibits further release of norepinephrine from the stimulated adrenergic neuron.
β-Adrenoceptors:
- Responses differ from those of α receptors and are characterized by a strong response to isoproterenol, with less sensitivity to epinephrine and norepinephrine.
- Subdivided into three major subgroups, β1, β2, and β3, based on their affinities for adrenergic agonists and antagonists.

Description

Test your knowledge on adrenergic drugs that affect receptors stimulated by norepinephrine or epinephrine. Learn about sympathomimetics, sympatholytics, and the different ways drugs can interact with adrenergic receptors.