Post on 19-Jan-2016
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Ionotropic and Metabotropic Receptors
Recall the 2 Kinds of Synapses?
Electrical• 2 neurons linked together
by gap junctions• Function in nervous system:
- rapid communication- bidirectional communication- excitation/inhibition at the same synapse
• Some between neurons and glia cells
Chemical• Signal transduction• Excitatory• Inhibitory• Slower communication• Unidirectional
communication
Recall where chemical synapses are found?
Recall the Chemical Synapse?
Communication Across a Synapse
1. Action Potential
2. Voltage-gated Ca channels open
3. Ca triggers exocytosis
4. Nt diffuses and binds to receptor
5. Response in cell
Response is terminated by removing nt from synaptic cleft
6. Degradation
7. Reuptake
8. Diffusion
Signal Transduction at Synapses
• Rate of the response is due to the mechanism by which the signal is received and transferred at the plasma membrane.
• Fast responses at ionotropic receptors (channel-linked).
• Slow responses at metabotropic receptors (G-protein-linked).
Ionotropic Receptors
• The receptor is a ligand-gated ion channel.• Ligand binding directly opens ion channel.• Fast action, short latency between nt binding
and response.• Response is brief.
Ionotropic Receptors• 5 subunits form the pore through
the membrane.• Binding of ligand opens the pore.• Ions flow into or out of the cell.• Produces EPSP or IPSP
(depending on the ion channel).• Rapid desensitization (loss of
activity) if continuously exposed to nt.
• Limits postsynaptic responding when presynaptic neurons are highly active for a period of time.
Ionotropic ReceptorsSensitization
Ion Flow
Time, ms, in exposure to neurotransmitter
High
Low
Ionotropic Receptors
• Can have multiple binding sites for various neuromodulators.
• Can enhance or inhibit binding of endogenous ligands.
• Are good targets for drugs.
Fast Responses at Ionotropic Receptors
Metabotropic Receptors
• Most common type of receptor.
• Coupled to G protein.• No direct control of ion
channels.• Second messengers.
Metabotropic Receptors
• Single subunit with 7 transmembrane spanning domains.
• Highly conserved across the “receptor superfamily”.
• Ligand binds in cleft on external face.• Ligand binding activates G protein• G protein activate various effectors.• Sometimes the effectors are the ion channels.
1) The ß-adrenergic receptor is a 7-transmembrane spanning protein. A negatively charged Asp residue on the 3rd transmembrane region (TM3), along with other charged, polar residues, allows a positively charged norepinephrine (NE) molecule to bind to the hydrophobic core of the receptor.
(Click to see animation; click again for next step)
2) Binding of NE causes the third intracellular loop (i3) of the receptor to change conformation and bind to the GDP-bound αs subunit of the Gs protein.
(Click to see animation; click again for next step)
3) Binding of i3 to the αs subunit of the Gs protein results in a conformation change in αs, causing GDP to dissociate and GTP to bind.
(Click to see animation; click again for next step)
4) The GTP-bound αs subunit dissociates from the β subunit and from the βAR receptor and binds to adenyl cyclase (AC). (Meanwhile, norepinephrine may dissociate from the receptor, but the αs subunit can remain active for many seconds after this dissociation.)
(Click to see animation; click again for next step)
5) Activated adenyl cyclase produces many molecules of cAMP from ATP.
(Click to see animation; click again for next step)
6) After hydrolysis of GTP to GDP, the αs subunit returns to its original conformation, dissociates from AC (which then becomes inactive), and reforms the trimeric Gs protein complex.
(Click to see animation; click again for next slide)
GDPGTP
TM1
TM5
TM4TM3
TM2Asp -
NE +
N
C
β-adrenergic receptor
TM7 TM6
Gs
protein
GDP
αsAC
i3 loop GTPα s
TM1
TM5
TM4TM3
TM2
N
TM7 TM6
Asp -
βγ
ATP
cAMP
GDPαs
ACβγ
cAMPcAMP
cAMP
Cytoplasm
Extracellular space
Slow Responses at Metabotropic Receptors: Direct G-Protein Coupling
Slow Responses at Metabotropic Receptors: Second Messenger Coupling
Postsynaptic Potential
• Change in membrane potential in response to neurotransmitter binding to receptor.
• Can be excitatory or inhibitory:- Excitatory: likely to elicit action potential:
Deporalization-Inhibitory: less likely to elicit action
potential: HypoerpolarizationMembrane Stabilization
Excitatory Synapses• Depolarize postsynaptic cell
-Brings membrane potential closer to Threshold by opening or closing ion channels.
• Channels affected are:- Open Na channels- Close K channels- Open channels that are equally permeable to Na and K
Causes depolarization because of the stronger force of Na to flow into the cell
• Depolarization = EPSP (excitatory postsynaptic potential)
Fast EPSPs
Slow EPSPs
EPSPs are Graded Potentials
• Higher freq of APs (presynaptic)
• More neurotransmitter released (presynaptic)
• More neurotransmitter binds to receptors to open (or close) channels
• Greater increase (or decrease) ion permeability
• Greater (or lesser) ion flux
• Greater depolarization
Inhibitory Synapses
• Neurotransmitter binds to receptor.• Channels for either K or Cl open hyperpolarizes
the cell.• If K channels open, then…
K moves out IPSP (inhibitory postsynaptic potential)
• If Cl channels open, then either… Cl moves in IPSP Cl stabilizes membrane potential.
Fast Inhibitory Synapses Involving
K Channels
IPSPs are Grade Potentials
• Higher freq of APs (presynaptic)
• More neurotransmitter released (presynaptic)
• More neurotransmitter binds to receptors to open (or close) channels
• Greater increase (or decrease) ion permeability
• Greater (or lesser) ion flux
• Greater depolarization
Neural Integration
• Divergence/convergence• Summation• The summing of input from various synapses
at the axon hillock of the postsynaptic neuron to determine whether the neuron will generate action potentials
Divergence
Convergence
Convergence of Input as a Factor in Summation
Temporal Summation from the same Synapse
Spatial Summation from Different Synapses
Neurotransmitters
• Acetylcholine• Biogenic Amines• Amino Acid Neurotransmitters• Neuropeptides• Autonomic Nervous Sysntem
Acetylcholine
• Found in the CNS and PNS• Most abundant neurotransmitter in PNS.• Synthesis
- Acetyl CoA + choline acetylcholine +CoA- Synthesized in cytoplasm of axon terminal- Biosynthetic enzyme: choline acetyltransferase (CAT)
• Breakdown- Acetylcholine acetate + choline- Degradation occurs in synaptic cleft- Degradative enzyme: acetylcholinesterase (AchE)
Cholinergic Synapse
Cholinergic Receptors
• Nicotinic- Ionotropic- Found mostly in the skeletal muscle- Some found in the CNS
• Muscarinic- Metabotropic- Found mostly in the CNS
Actions at Nicotinic Cholinergic Receptors
Actions at Muscarinic Cholinergic Receptors
Biogenic Amines• Derived from amino acids• Catecholamines – derived from tyrosine
- Dopamine- Norepinephrine (noradrenaline)- Epinephrine (adrenaline)
• Norepineprine and epinephrine bind adrenergic receptors- Alpha and beta adrenergic receptors- Slow responses at all adrenergic receptors
• Adrenergic receptors are G-protein-coupled• Generally linked to second messengers
Dopamine
• Category: biogenic amine• Postsynaptic effect: Excitatory or inhibitory
Fig. 6.11
Dopamine Receptors
• Large diversity of metabotropic dopamine receptors (at least 6).
• Bound by many antipsychotic drugs
Kandel, 2000
Norepinephrine
• Category: biogenic amine• Formed from dopamine• also in PNS– sympathetic NS
Norepinephrine Receptors
• Effect depends on receptor bound– α-receptors
α1- vs. α2-receptors (see next slide)
– ß-receptors
Silverthorn 2004
Receptors can be Located Presynaptically too – This will determine their effect
Presynaptic GABAB receptor actions
Isaacson, J Neuophysiolgy, 1998
Epinephrine• Category: biogenic amine• synthesized from norepinephrine• Effect depends on receptor bound– α-receptors– ß-receptors
Histamine• Category: biogenic amine• Postsynaptic effect: Excitatory
Fig. 6-12
Histamine effects
• Receptors are all G-protein coupled• In brain, affects arousal and attention• In periphery affects inflamation, vasodilation.• Why do some cold medicines make you
sleepy? (good exam question).
Serotonin (5-HT)Category: Biogenic amines• Postsynaptic effect: Excitatory
Serotonin effects
• Involved in sleep/wakefulness cycle• Most receptors are metabotropic, but one
group are ionotropic.• Why does turkey make you sleepy?• SSRI and depression
Amino Acid Neurotransmitters
• Amino acid neurotransmitters at excitatory Synapses: glutamate
• Amino acid neurotransmitters at inhibitory Synapses: GABA (gamma-amino butyric acid)
Glutamate• Category: small-molecule• Glutaminergic neurons• Postsynaptic effect:
depends• Very important in CNS• Synthesized from
glutamine from glia
Fig. 6.6
Glutamate Receptors
• Ionotropic– NMDA• late EPSP• Glycine & Mg2+ dependent
– AMPA• early EPSP
– kainate• early EPSP
• Metabotropic
Kandel 2000
GABA (γ-aminobutyric acid)
• Category: small-molecule• GABAergic neurons• Postsynaptic effect:
Inhibitory• Made from glucose
Fig. 6.8
GABA Receptors
• GABAA – Ionotropic– gates Cl- channel
• GABAB – Metabotropic– gates K+ channel
Fig. 6.9
Neuropeptides
• Short chains of amino acids• E.G., endogenous opiates
- endorphins – found in the brain, morphine-like
- Vasopressin – Anjtidiuretic hormone (ADH) – found in the posterior pituitary
Autonomic Nervous System (ANS)
• Both branches of the ANS innervate most effector organs
• Primary function – regulate organs to maintain homeostasis
• Parasympathetic and sympathetic activities tend to oppose each other- Parasympathetic Nervous system – rest- Sympathetic nervous system – fight or flight
response
Autonomic Pathways
Neurotransmitters and their Receptors in the ANS