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)

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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

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

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.

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

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

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

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

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

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

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.

The NMDA receptor is necessary for LTP

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.

Brain distribution of some of the GABA receptor subunits.

Native GABAA Receptors

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

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

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

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.

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

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

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.

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

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

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

Serotonin

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

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

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

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