1. Defining a Receptor Capacity to bind ligands
Membrane component that induce a biological response when ligand is bound
Do not change the ligand chemically or physically move it
Necessary for a given effect of a neurotransmitter (mediates the action of the NT)

2. Allyn & Bacon 2004
Copyright © 2004 Allyn and Bacon

3. Model for receptor occupation
L + R LR
Rate of reaction is proportional to concentration of reactants
At equilibrium, forward and reverse reaction rates are equal
[L] [R] = [LR]
The equilibrium dissociation constant
kD=[L][R]
[LR]
kD is inversely related to affinity

4. Affinity and Saturation
Bmax
kD=-1/slope
Bound (pmoles)
Bound/Free kD
Bmax=x-intercept/mg protein
[Ligand]
Bound Ligand
kD: Concentration of ligand to elicit 50% maximal binding
Bmax: Receptor density in a given tissue

5. Effects of Repeated Administration
Tolerance:
A “decrease” in the effectiveness of a drug that is administered repeatedly.
Sensitization:
An “increase” in the effectiveness of a drug that is administered repeatedly.

6. Allyn & Bacon 2004
Copyright © 2004 Allyn and Bacon

7. Ionotropic Receptors
Same general structure for ionotropic receptor: 4 membrane spanning regions, disulfide bonds in the extracellular N terminal tail.

8. Ach binding sites at a-d and a-g interfaces
Ach binding sites at a-d and a-g interfaces. M2 regions bow in at the pore to constrict it until ach binding. Open duration of about 1ms.
Allyn & Bacon 2004

9. NAchR is a sodium channel
nAchR is a Na+ channel, depolarizes the cell.
Allyn & Bacon 2004

10. Roles of nAchR in CNS
nAchR are often found on presynaptic terminals where they may regulate release of other NT, like DA or GABA.

11. Glutamate Receptors Allyn & Bacon 2004
Glutamate ionotropic receptors NMDA and AMPA. Occur together in postsynaptic membrane, have different characteristics.
Allyn & Bacon 2004

12. The AMPA receptor
How the R blocks Ca permeabililty. Charge and possibly steric influences in the pore.

13. The NMDA receptor D-serine
At least 6 sites for endogenous compounds that influence channel. Glutamate, glycine/serine, polyamine (all promote receptor activation), Mg++, Zn+, H+ (all inhibit ion flux). Glutamate is by far the most potent agonist, binds NR1.

14. The NMDA receptor is necessary for LTP

15. GABA receptor
Unlike Ach and Glutamate receptors, GABAA receptors gate a Cl- channel and reduce excitability when activated. Vm is close to Ecl-, so there isn’t a big electrostatic force driving Cl- down its concentration gradient, but increased permeability to Cl- will counteract the opening of Na+ channels, making it harder to reach threshold.

16. Brain distribution of some of the GABA receptor subunits.

17. Native GABAA Receptors

18. Summary: Ionotropic Ach: nicotinic Glutamate: AMPA Glutamate: NMDA
Na+ channel
presynaptic
excitatory
Glutamate: AMPA
Excitatory
Glutamate: NMDA
Ca2+ channel
Vm dependent
GABA: GABAA
Cl- channel
inhibitory
Benzodiazepine site

19. Roles of Calcium Neurotransmitter release
Synaptic vesicle fusion
Activation of enzymes like CamK & NOS
Synaptic plasticity and gene expression
Pathological increase in Ca2+ concentration can lead to excitotoxicity and/or trigger apoptosis

20. Metabotropic Receptors
7 TM regions, single gene encodes, i3 often quite large and variable

21. Key feature is association with G proteins
Key feature is association with G proteins. GDP bound inactive state, ligand binding induces exchange for GTP. Many types of heterotrimeric g proteins. Gs, Gi, Gq, G12—mainly distinguished on basis of the alpha subunit. Many different bg subunits too.

22. GPCR directly activates an ion channel. Gi typically
GPCR directly activates an ion channel. Gi typically. Bg subunits more important, but some ai can activate GIRK directly.
a2-adrenergic, D2 dopamine, muscarinic, 5-HT1A, GABAB use this mechanism.
These receptors link to bg subunits that can also bind and reduce opening of Ca2+ channels, especially L type.
Allyn & Bacon 2004

23. Adenylyl Cyclase. 2nd messenger system #1 generated cAMP, activated PKA, phosphorylates CREB (and others) induces gene transcription. AC stimulated by Gas, inhibited by Gai (may be indirect through Gi bg subunits inhibiting a Gas).
Receptors that activate AC: D1, D5 dopamine, 5HT4, 7, b-adrenergic, and vasopressin.
Receptors that inhibit AC: D2, D3, muscarinic M2, M4, opioid, adenosine A1, 5HT1A, 5HT1B, 5HT1D.
Allyn & Bacon 2004

24. Phospholipase C: #2 second messenger system
Phospholipase C: #2 second messenger system. Often linked to Go proteins. Both a and bg subunits activate PLC to cleave membrane phospholipids. TRK receptors can also activate PLC.

25. Cleavage of PIP2 in the membrane releases IP3 (diffusible) and DAG (diacylglycerol).
IP3 binds receptor on ER, releases Ca from stores, Ca may activate CaMK to phosphorylate targets.
DAG remains in membrane and activates isoforms of PKC to phosphorylate targets.
In both cases, the second messenger enables a conformational change (via Ca) in the kinase exposing or activating the catalytic domains.
Receptors that activate PLC: muscarinic M1, M3, M5, 5HT1C, some peptides and mGluR.
Allyn & Bacon 2004

27. Dopamine
D1, D5 receptors ACTIVATE AC, stimulate cAMP production and PKA activation
Gas linked
D2, D3, D4 receptors INHIBIT AC
Gai linked
D2 receptor is an autoreceptor, inhibits presynaptic production/release of DA

28. Norepinephrine
a1: activate Gaq/11, increase PLC activity, leading to increases in DAG and IP3
a2: autoreceptor, decrease NE release
Inhibits AC, Gai coupled
Inhibits N type Ca channel
b: activate Gas to activate AC and produce cAMP

29. Serotonin

30. Glutamate mGluR1,5: increase IP3 and intracellular Ca2+
Gai/o coupled
mGluR2,3: inhibit AC, decrease cAMP
Gai coupled
mGluR4,6,7,8: inhibit AC, decrease cAMP
Probably Gai coupled
All groups inhibit VGCC

31. GABA
GABAB receptors activate K+ channels, inhibit Ca2+ channels, and/or inhibit cAMP production
Can act pre- or postsynaptically
Composed of R1 and R2 subunit heterodimers
R1 binds GABA, R2 binds the G protein

32. Tryosine Kinase receptors
Ligand binding (NGF, BDNF) leads to autophosphorylation, gains ability to phos- phorylate targets and recruit a huge array of signaling proteins.

33. Termination of activity
Desensitization: spontaneous channel closing in the presence of the ligand
GPCR: phosphorylation by GRK inactivates, may lead to internalization
GTPase inactivates a subunits
Internalization may lead to degradation or recycling. Key for regulating levels.
Phosphodiesterases break down cAMP and cGMP
Calcium is buffered or transported out
Inositol phosphates are dephosphorylated by phosphatases