W3 L2 (Autonomic Transmission)

Question 1

FAQ: What is the difference between alpha and beta-adrenergic receptors?

Answer 1

Alpha and beta adrenergic receptors are metabotropic (AKA G-protein coupled) receptors that are
activated when bound by norepinephrine. In the autonomic nervous system, they are used by the
sympathetic pathway when innervating targets. Alpha receptors usually cause an increase in IP3/DG or a
decrease in cAMP, while beta receptors usually cause an increase in cAMP. Alpha and beta adrenergic
receptors are NOT used by the parasympathetic pathway when innervating a target. The parasympathetic
pathways uses muscarinic receptors, which are also metabotropic (AKA G-protein coupled) receptors,
but they are activated when bound by acetylcholine.
Most text books break the classification down into alpha 1 or 2 and beta 1, 2, or 3. It is sufficient for
you to understand that adrenergic receptors (which bind norepinephrine released from sympathetic
postganglionic neurons) have two basic types (alpha and beta) which result in specific second
messengers being produced or not produced. Alpha 1 increases IP3 and DAG, alpha 2 decreases cAMP,
while beta 1, 2, or 3 increases cAMP. Similarly, muscarinic receptors (which bind acetylcholine
released from parasympathetic postganglionic neurons) also have two basic types (muscarinic 1, 3, or 5
and muscarinic 2 and 4) which again result in specific second messengers being produced or not
produced as described in lecture (muscarinic 1, 3, or 5 increases IP3 and DAG, while and muscarinic 2
and 4 decreases cAMP).