12 Neurotransmitter Action: G-Protein-Coupled Receptors
G-protein-coupled receptors (GPCRs), also known as metabotropic receptors, mediate slower but long-lasting effects compared to ionotropic receptors. These receptors activate G-proteins upon neurotransmitter binding, leading to the initiation of diverse intracellular signaling cascades that regulate ion channels, protein activity, and gene transcription.
Metabotropic Receptors
Like ionotropic receptors, metabotropic receptors are primarily located along the dendrites or cell body, but they can be present anywhere along the neuron if there is a synapse.
G-Proteins
G-proteins are enzymes with three subunits: alpha, beta, and gamma. In the resting state of the G-protein complex, the alpha subunit is bound to a GDP molecule. There are multiple types of alpha subunits, and each initiates different cellular cascades in the neuron.
G-Protein Coupled Receptors
When a neurotransmitter binds to a GPCR, the receptor is able to interact with an inactivated G-protein complex. The complex that binds is specific to the receptor; different metabotropic receptors for the same neurotransmitter can have different effects in the cell due to which G-protein binds. Once coupled to the receptor, the GDP molecule is exchanged for a GTP molecule, and the G-protein becomes activated.
Animation 12.1. Neurotransmitter binding to a G-protein-coupled receptor (GPCR) causes the receptor to interact with the inactive G-protein complex. This interaction causes the alpha subunit to exchange its GDP molecule for GTP, activating the G-protein complex. ‘G-Protein Binding’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.
After activation, the G-protein complex will separate into the alpha-GTP subunit and the beta-gamma subunit. Both components can alter the function of effector proteins in the cell. Effector protein functions can range from altering ion permeability across the membrane by opening ion channels to initiating second messenger cascades. Second messenger cascades can have long-term, widespread, and diverse cellular effects including activation of cellular enzymes or altering gene transcription.
Animation 12.2. After activation, the G-protein complex separates into the alpha-GTP subunit and the beta-gamma subunit. Both components diffuse along the inner membrane surface and can stimulate or inhibit effector proteins, producing diverse cellular effects. ‘G-Protein Effects’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.
Cellular Effects of G-Proteins
Open Ion Channels – Beta Gamma Subunit
Metabotropic receptors can indirectly open ion channels; this process is slower than ionotropic receptors
In certain situations, the activated beta-gamma subunit can open or close ion channels and change membrane permeability. Muscarinic acetylcholine receptors in the heart use this pathway. When acetylcholine binds to a muscarinic receptor in the heart muscle fiber, the activated beta-gamma subunit opens a type of potassium channel called G-protein-coupled inwardly-rectifying potassium (GIRK) channel, hyperpolarizing the cell. This explains why acetylcholine slows the heart rate, and why muscarinic antagonists like atropine increase it.
Animation 12.3. Beta-gamma subunit ion channel modulation. Some GPCRs directly alter membrane permeability through beta-gamma subunits. Muscarinic acetylcholine receptors in the heart release beta-gamma subunits that open GIRK potassium channels, causing potassium efflux, hyperpolarization, and reduced heart rate. ‘Beta Gamma Ion Channels’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.
Second Messenger Cascades
Metabotropic receptors can alter protein function in the cell through second messenger cascades; these cascades may lead to changes in gene transcription
In addition to direct effects like the activated beta-gamma subunit opening ion channels, G-proteins can have many indirect actions in the cell through the use of second messenger cascades. The specific second messenger pathway that is activated or suppressed by G-protein action depends on the type of alpha subunit.
For example, norepinephrine can act on either alpha- or beta-adrenergic receptors. Beta-adrenergic GPCRs couple to a stimulatory G-protein, or Gs, which initiates the cyclic AMP (cAMP) second messenger system by activating the enzyme adenylyl cyclase. Alpha 2-adrenergic receptors, however, couple to an inhibitory G-protein, or Gi, and suppress the activity of adenylyl cyclase. Alpha 1-adrenergic receptors couple to a third type of G-protein, Gq, which activates the phospholipase C pathway. One neurotransmitter can, therefore, cause a wide range of cellular effects after binding to GPCRs, unlike the single function of ion flow through the ionotropic receptors. The pathway initiated by norepinephrine will depend on the type of receptor a specific cell expresses.
Adenylyl Cyclase / cAMP Second Messenger Cascade
The cyclic AMP (cAMP) second messenger pathway is used by many GPCRs. Activation of the pathway is caused by the Gs alpha subunit and inhibition of the pathway is caused by the Gi alpha subunit. When activated, adenylyl cyclase converts ATP to cAMP in the cytoplasm. cAMP then activates another enzyme called protein kinase A (PKA) by binding to the regulatory subunits, allowing the catalytic (functional) subunits to separate and become active. Protein kinases add a phosphate molecule to proteins, a mechanism called phosphorylation. The addition of the phosphate changes the activity of the protein and how it functions in the cell.
Animation 12.4. Adenylyl cyclase/cAMP pathway. Gs-coupled GPCRs activate adenylyl cyclase, which converts ATP to cAMP. Multiple cAMP molecules activate PKA by releasing its catalytic subunits. Active PKA phosphorylates diverse cellular proteins, altering their function. This cascade amplifies signals and produces widespread cellular effects. ‘Adenylyl Cyclase Pathway’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.
The end effects of this pathway will depend on which proteins are targeted. For example, cAMP can gate ion channels and PKA can phosphorylate ion channels altering permeability and membrane potential. Phosphorylation can open the channel, or it may modulate the activity of the channel, making the channel easier to open or remain open longer.
Animation 12.5. The cAMP pathway can alter membrane permeability through two mechanisms: cAMP directly gates certain ion channels, opening their pores upon binding, and PKA phosphorylates ion channels, altering their function. This demonstrates how GPCR signaling can control membrane excitability without direct receptor-channel coupling. ‘Second Messenger Ion Channel Action’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.
In addition to altering ion channel function, PKA can phosphorylate other proteins important for neuron function, such as proteins involved with neurotransmitter synthesis and release. One other critical target of PKA phosphorylation is the transcription factor CREB (cAMP response element binding-protein). Transcription factors bind to DNA in the nucleus and change the rate of gene transcription. Phosphorylation by PKA can cause CREB to initiate transcription of genes, creating new proteins for the neuron. Depending on which genes are transcribed, the effects on the neuron can be long-lasting.
Overall, neurotransmitters working through GPCRs and second messenger cascades like the adenylyl cyclase pathway can cause a diverse range of cellular effects: from opening ion channels, to changing protein activity via phosphorylation, to altering the proteins synthesized in the neuron.
Animation 12.6. PKA target diversity. PKA phosphorylates multiple protein targets: cytoplasmic proteins controlling neurotransmitter synthesis, packaging, and release (rapid effects), and nuclear CREB transcription factor (long-term effects through gene transcription). This allows GPCRs to produce both immediate functional changes and lasting alterations in neuronal properties. ‘PKA Targets’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.
Phospholipase C / IP3 / DAG Second Messenger Cascade
The Gq alpha subunit initiates a separate signaling pathway in the cell by activating phospholipase C. Phospholipase C targets PIP2 (phosphatidylinositol 4,5-bisphosphate), which is a phospholipid present in the plasma membrane of the cell. PIP2 is split into two cellular molecules: IP3 (inositol 1,4,5-trisphosphate) and DAG (diacylglycerol). DAG remains in the membrane and interacts with protein kinase C (PKC). IP3 moves to the endoplasmic reticulum where it opens calcium channels and allows calcium to flow into the cytosol.
Calcium is also a second messenger in the cell. One important effect is the binding of calcium to calmodulin protein. This complex can then activate another kinase, the calcium/calmodulin-dependent protein kinase (CaMK). Both PKC and CaMK can phosphorylate diverse cellular and nuclear proteins, similar to the targets of PKA.
Animation 12.7. The Gq alpha subunit activates phospholipase C, which cleaves the membrane phospholipid PIP2 into DAG and IP3. DAG remains in the membrane and activates PKC. IP3 moves to the endoplasmic reticulum, opening calcium channels and releasing calcium into the cytoplasm. Calcium acts as a second messenger, binding calmodulin to activate CaMK, which phosphorylates additional protein targets alongside PKC. ‘IP3-DAG Pathway’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.
Signal Amplification
One characteristic of GPCR activation is the signal amplification that takes place. One receptor is able to activate more than one G-protein complex. The effector protein activated by the G-protein can create many second messengers, and the activated protein kinases can each phosphorylate multiple cellular proteins. This means that one neurotransmitter can have a significant effect on cellular function.
Signal Termination
Eventually, the cascade initiated by binding of the neurotransmitter to the GPCR needs to end. The alpha subunit of the G-protein is able to convert the bound GTP back to GDP after a short period of time, inactivating the G-protein. The alpha subunit will then interact with a beta-gamma subunit and stay in the resting state until activated by another GPCR. Enzymes in the cell called protein phosphatases find and remove the phosphate groups added to cellular proteins by the protein kinases. And finally, other cellular mechanisms exist to remove calcium from the cytoplasm and degrade other second messengers.
Conclusion
GPCRs are versatile receptors that allow neurotransmitters to influence a broad array of cellular processes through G-protein activation and second messenger pathways. Their slower yet amplified effects make them essential for prolonged and complex signaling in the nervous system.
Key Takeaways
- G-protein-coupled receptors (GPCRs) activate intracellular signaling cascades through G-proteins, allowing diverse and long-lasting effects on cellular function.
- G-proteins consist of alpha, beta, and gamma subunits, with the alpha subunit initiating different signaling pathways depending on its subtype (Gs, Gi, Gq).
- Second messenger systems, such as the cAMP pathway or phospholipase C pathway, regulate ion channel permeability, protein activity, and gene transcription.
- Signal amplification by GPCRs allows a single neurotransmitter to produce widespread cellular effects.
- Cellular mechanisms like GTP hydrolysis and protein phosphatases terminate GPCR signaling.
Important Terminology
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An enzyme activated by the Gs alpha subunit that converts ATP to cAMP in the cytoplasm. Part of the cAMP second messenger pathway.
The subunit of the G-protein complex that binds GDP or GTP. Different types of alpha subunits (Gs, Gi, Gq) initiate different cellular cascades.
The portion of the G-protein complex formed by the beta and gamma subunits. After G-protein activation, the beta-gamma subunit separates from the alpha subunit and can alter effector protein function.
Cyclic adenosine monophosphate, a second messenger synthesized from ATP by adenylyl cyclase. cAMP activates protein kinase A.
A protein that binds calcium and activates calcium/calmodulin-dependent protein kinase (CaMK).
Calcium/calmodulin-dependent protein kinase, an enzyme activated by the calcium-calmodulin complex that phosphorylates cellular proteins.
cAMP response element binding-protein, a transcription factor phosphorylated by PKA that binds to DNA and initiates gene transcription.
Diacylglycerol, a second messenger produced by phospholipase C cleavage of PIP2. DAG remains in the membrane and activates protein kinase C.
A protein whose function is altered by activated G-protein subunits. Effector proteins include ion channels and enzymes that initiate second messenger cascades.
An enzyme with three subunits (alpha, beta, gamma) that couples to metabotropic receptors. When activated, G-proteins separate into alpha-GTP and beta-gamma subunits that alter effector protein function.
A neurotransmitter receptor that activates G-proteins when a neurotransmitter binds. GPCRs initiate slower but longer-lasting effects compared to ionotropic receptors. Also called a metabotropic receptor.
Guanosine diphosphate, a molecule bound to the alpha subunit of an inactive G-protein complex.
A type of G-protein alpha subunit that inhibits adenylyl cyclase, decreasing cAMP production.
G-protein-coupled inwardly-rectifying potassium channel, a type of potassium channel opened by activated beta-gamma subunits.
A type of G-protein alpha subunit that activates phospholipase C, initiating the IP3/DAG second messenger pathway.
A type of G-protein alpha subunit that activates adenylyl cyclase, increasing cAMP production.
Guanosine triphosphate, a molecule that replaces GDP on the alpha subunit to activate the G-protein complex.
Inositol 1,4,5-trisphosphate, a second messenger produced by phospholipase C cleavage of PIP2. IP3 opens calcium channels in the endoplasmic reticulum.
A neurotransmitter receptor that activates G-proteins when a neurotransmitter binds. Also called a G-protein coupled receptor.
A G-protein coupled receptor activated by acetylcholine. In the heart, muscarinic receptors open GIRK channels and slow heart rate.
An enzyme activated by the Gq alpha subunit that cleaves PIP2 into IP3 and DAG.
The addition of a phosphate group to a protein by a protein kinase. Phosphorylation changes protein activity and function.
Phosphatidylinositol 4,5-bisphosphate, a phospholipid in the plasma membrane that is cleaved by phospholipase C into IP3 and DAG.
Protein kinase A, an enzyme activated by cAMP that phosphorylates cellular proteins.
Protein kinase C, an enzyme activated by DAG that phosphorylates cellular proteins.
An enzyme that adds phosphate groups to proteins through phosphorylation.
An enzyme that removes phosphate groups from proteins, terminating the effects of phosphorylation.
A molecule that transmits signals from receptors to target proteins inside the cell. Second messengers include cAMP, IP3, DAG, and calcium.
A series of intracellular signaling events initiated by G-protein activation. Second messenger cascades can alter ion channel function, protein activity, and gene transcription.
The process by which one receptor activates multiple G-proteins, each G-protein activates multiple effector proteins, and each effector protein produces multiple second messengers. Signal amplification allows one neurotransmitter to have widespread cellular effects.
A protein that binds to DNA in the nucleus and changes the rate of gene transcription. CREB is a transcription factor activated by PKA phosphorylation.
