Metabotropic Receptors

Why are metabotropic receptors important and describe briefly how these work?

• G-coupled and metabotropic pathways are very important in the modulation of neuronal activity

  • Signal binds to the receptor

  • Receptor activated

  • Change in function occurs

What are the two main classes of G-protein coupled receptors?

• Heterotrimeric

  • Chemical signalling molecule binds to the receptor, interacting with the heterotrimeric G-protein with 3 subunits that in response to the conformational changes of ligand binding, dissociates and alters the function of the effector protein and different intracellular signalling pathways

• Monomeric

  • are able to interact with GTPases and can affect neuronal function but not focussed on

How do G-Protein Coupled Receptors work?

Sitting in the membrane are alpha, beta and gamma subunits, lipid modified so they can associate with the membrane

• In the resting state (inactive), the molecule exists with a guanine diphosphate (GDP) bound

• When the agonist binds to the receptor, it stabilises the receptor in a conformation that binds the alpha subunit

• The alpha subunit then changes its conformation and the GDP is removed and guanine triphosphate (GTP) associates, disassociating the alpha from the beta-gamma subunit

• The alpha and the beta-gamma can then interact with effector proteins that generate a response such as intracellular signalling molecules such as cAMP, change conductance of calcium or potassium channels which affect neuronal function

• Dephosphorylation of the GTP also occurs so the alpha and beta subunits can recombine and be ready to be activated again

What is the main benefit of GPCRs?

• Massive amplification

• Low concentration of ligand and receptor can have large effects

• A receptor interacts with a single ligand, but can interact with multiple G-proteins, which each interact with an effector protein (e.g. Adenylyl cyclase) which can then have an enzymatic effect to produce MULTIPLE intracellular signalling molecules (cyclic AMP), which can interact with other molecules, having a variety of different effects

Whare are the 3 Main G-Protein Types?

• G_alpha/G_{aplha s} for stimulation (depolarisation)

• G_{alpha i} for inhibition (hyperpolarisation)

• G_{alpha q} ultimately increasing cell activity

How do the G_alpha/G_{aplha s} receptors work?

• G_alpha/G_{aplha s} for stimulation (depolarisation)

  • Interacts with and stimulates adenylyl cyclase

  • Increases cyclic AMP

  • which increases protein kinase A

  • increasing protein phosphorylation which means the cell activity will increase

How do the G_{alpha i} receptors work?

• G_{alpha i} for inhibition (hyperpolarisation)

  • Interacts with and inhibits adenylyl cyclase

  • Inhibits cyclic AMP

  • which inhibits protein kinase A

  • decreasing protein phosphorylation which means the cell activity will be decreased

How do the G_{alpha q} receptors work?

• G_{alpha q} ultimately increasing cell activity

  • Binds to GTP which leads to an interaction with the effector protein phospholipase C

  • which catalyses the reduction of phosphoinositol biphosphate, a membrane lipid protein leading to the production of:

    • inositol trisphosphate (IP3), which binds to intracellular receptors that lead to the release of calcium from the endoplasmic reticulum

    • diacylglycerol (DAG) which can interact with protein kinase C another effector which when combined with calcium can produce more effects

• This increases the protein phosphorylation and activates the calcium-binding proteins

How do intracellular signalling molecules directly interact with ion channels to alter neuronal function?

• Neurotransmitter receptors: neurotransmitter such as glutamate binds to the receptor enabling ions to flow through

• Calcium ion activated potassium channel: uses calcium as a signalling molecule to change the conductance of potassium

• Cyclic nucleotide gated channel: Cyclic AMP can also interact with an ion channel leading to changes in conductance of select ions such as sodium and potassium

What are the 8 neurotransmitter receptor classes that interact with metabotropic receptors?

• Glutamate

• GABA B

• Dopamine

• Adrenaline and Noradrenaline

• Histamine

• Serotonin

• Purines

• Muscarinic

What is the post-synaptic effect of metabotropic receptors?

Post synaptic metabotropic receptors act via trimeric G proteins which activates a pathway to alter ion channels and ultimately membrane potential

What changes other than to membrane potential can receptors have?

Metabotropic and ionotropic receptors can cause changes in intracellular calcium and other messengers that don't directly and immediately impact membrane potential, but make changes that alter neuronal function via changes in gene expression

• That change in gene expression can have a range of vast impacts

How can presynaptic actions modulate the probability of release of neurotransmitters?

• Presynaptic facilitation

• Presynaptic inhibition

• Feedback regulation

How does presynaptic facilitation modulate the probability of release of neurotransmitters?

• serotonergic neuron releases serotonin to a serotonin receptor on the presynaptic cell

• this shuts down the potassium current, which leads to further depolarisation of the nerve terminal

• making it more likely to release neurotransmitter in response to an action potential

How does presynaptic inhibition modulate the probability of release of neurotransmitters?

• GABAergic neuron interacts with a GABA B receptor

• this opens the potassium channel leading to hyperpolarisation of the terminal

• making it less likely to release neurotransmitter

  • This can also be done with the ionotropic GABA A receptor channel which leads to further hyperpolarisation by a chloride channel

How does feedback regulation modulate the probability of release of neurotransmitters?

• neuron releases noradrenaline into the post synaptic space

• noradrenaline can then also bind to a receptor on the presynaptic neuron (alpha adrenoceptor) that can in turn alter activity of calcium channels

  • Decreasing the entry of calcium will decrease the probability of release, thus inhibiting the presynaptic neuron, modulating itself

What monoamines make up the catecholamines of the aminergic systems?

dopamine, noradrenaline and adrenaline, which each have multiple monotropic receptor subtypes

How are the aminergic catecholammines synthesised?

These neurotransmitters are produced from tyrosine via an enzymatic cascade

1. dihydroxy phenylalanine is produced from tyrosine via tyrosine hydroxylase

2. Dopamine is produced by DOPA decarboxylase

3. Noradrenaline is produced by dopamine-beta-hydroxylase

4. adrenaline is produced by phenylethanolamine N-methyltransferase.

How does the production of the catecholamines occur in the presynaptic neuron?

• An action potential leads to the influx of calcium into the neuron which stimulates the production of monoamines

  • Calcium works on protein kinase which phosphorylates tyrosine, produces more dopamine to be packaged in the vesicles to be released into depolarisation of the presynaptic terminal

  • For the other monoamines, dopamine beta hydroxylase and PNMT are present within the vesicles where they convert the dopamine into noradrenaline or adrenaline

What is the transporter that puts the catecholamines into the vesicles called?

Vesicular monoamine transporter (VMAT)

How are these catecholamines cleared and recycled from the synaptic cleft?

Once released into the synaptic cleft, the amines are either taken back into astrocytes or the presynaptic terminal where they are broken down by either monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT)

• The uptake is assisted by a dopamine transporter (DAT which is a sodium dependent co-transporter) or the noradrenaline co-transporter which is more specific and can also take dopamine