Principles of Neurobiology and Neurochemistry PDF

Principles of Neurobiology and Neurochemistry Review of Endogenous Neurotransmitters Small Molecule Neurotransmitters Large Molecule Neurotransmitters Acetylcholine (ACh), Dopamine (DA), Arginine Vasopressin (AVP), Corticotropin -Amino Butyric...

Principles of Neurobiology and Neurochemistry Review of Endogenous Neurotransmitters Small Molecule Neurotransmitters Large Molecule Neurotransmitters Acetylcholine (ACh), Dopamine (DA), Arginine Vasopressin (AVP), Corticotropin -Amino Butyric acid (GABA), Glutamate (GLU), Releasing Factor (CRF), Endorphin, Enkephalin, Norepinephrine (NE), Serotonin (5HT) Gonadotropin-Releasing Hormone (GnRH), Oxytocin (OT), Substance P Non-proteinaceous (small chemical molecules) Proteinaceous – amino acid chains (peptides) Synthesized in the presynaptic terminal by Synthesized in the cell body by metabolizing enzymes (using amino acids as building blocks) larger precursor proteins Packaged into small clear synaptic vesicles in Packaged into large dense-core synaptic the presynaptic terminal vesicles in the cell body and transported to the presynaptic terminal Release from the presynaptic terminal is Release from the presynaptic terminal is stimulated by Ca++ influx into the terminal stimulated by Ca++ influx into the terminal Have high affinity for their receptors Have even higher affinity for their receptors Destroyed by metabolizing enzymes and/or Destroyed by protease enzymes in the recycled by reuptake proteins synapse (proteolysis) 1 Act as Neurotransmitters Act as Neurotransmitters or Neuromodulators Principles of Neurobiology and Neurochemistry 1. Neurotransmitters are housed in synaptic vesicles in Synaptic Function the pre-synaptic terminal 2. Arrival of the Action Potential to pre-synaptic terminal produces a Pre-synaptic Potential in the terminal 3. During the Pre-synaptic Potential Voltage-dependent Na+ channels in the pre- synaptic terminal membrane open (green) Na+ influx (+ charge) into the pre-synaptic terminal opens voltage-dependent Ca++ channels in the pre-synaptic terminal membrane (blue) 4. Ca++ enters the pre-synaptic terminal 5. Ca++ entry facilitates the fusion of synaptic vesicles with the pre-synaptic terminal membrane 6. Neurotransmitter is released into the synapse Bou nd pool gNa (Na+ influx) gCa (Ca++ influx) g Ca++ Ca++ (+) charge stimulus (Action Potential) 2 Principles of Neurobiology and Neurochemistry 1. Neurotransmitters are housed in synaptic vesicles in Synaptic Function the pre-synaptic terminal 2. Arrival of the action potential to pre-synaptic terminal produces a Pre-synaptic Potential in the terminal 3. During the Pre-synaptic Potential Voltage-dependent Na+ channels in the pre- synaptic terminal membrane open Na+ influx (+ charge) into the pre-synaptic terminal opens voltage-dependent Ca++ channels in the pre-synaptic terminal membrane 4. Ca++ enters the pre-synaptic terminal 5. Ca++ entry facilitates the fusion of synaptic vesicles with the pre-synaptic terminal membrane 10 6. Neurotransmitter is released into the synapse 7. Neurotransmitters bind (on & off) to neurotransmitter receptors on the post-synaptic cell (heteroreceptors) or the pre-synaptic cell itself (autoreceptors) 8. Neurotransmitter binding to a receptor opens ion channels and ions move across the membrane of the cell - altering the charge on the inside of the cell IPSP – inhibitory EPSP - excitatory 9. So, ion flux alters the response of the cell 3 10. Neurotransmitters have a lifecycle Principles of Neurobiology and Neurochemistry Neurotransmitter Lifecycle After release the NT undergoes: 1. Degradation in synapse by synaptic enzymes – Free enzymes in the synaptic space – Enzymes bound to post-synaptic membrane 2. Reuptake into terminal for repackaging and reuse – Reuptake of the NT itself or the NT precursor (after NT degradation) into the pre-synaptic terminal by Transporter proteins 3. Diffusion and uptake into glial cells for metabolism – Uptake of the NT into a neighboring glial cell by Transporter proteins – Subsequent degradation while out of the synapse 4 Principles of Neurobiology and Neurochemistry Neurotransmitter Receptors Ionotropic Receptors – the receptor itself is an ion channel and neurotransmitter binding opens the channel Metabotropic Receptors – the receptor is NOT an ion channel but is linked to one by a 2 nd messenger signal - so neurotransmitter binding still opens a channel, but slower Ionotropic receptor Metabotropic receptor 5 Principles of Neurobiology and Neurochemistry IONOTROPIC RECEPTORS – neurotransmitter-gated (ligand-gated) receptors that form an ion-conducting pore in the center of the receptor that opens in response to the binding of a neurotransmitter. The effect of activating ionotropic receptors can be excitatory or inhibitory, according to the the ions they pass and the impact of these ions on the membrane potential. Ion concentrations & movement across the membrane Inside cell Outside cell K+ K+ Na+ Na + Cl- Cl- Ca++ Ca ++ 6 Principles of Neurobiology and Neurochemistry IONOTROPIC RECEPTORS – neurotransmitter-gated (ligand-gated) receptors that form an ion-conducting pore in the center of the receptor that opens in response to the binding of a neurotransmitter. Can have multiple binding sites for a Neurotransmitter Can also have binding sites for various “Neuromodulators” - allosteric modulators – enhance or inhibit the binding of neurotransmitters Ion Pore Neurotransmitter Neurotransmitter binding site binding site Neuromodulator binding site Neuromodulator Neuromodulator binding site binding sites 7 Principles of Neurobiology and Neurochemistry METABOTROPIC RECEPTORS – neurotransmitter receptors in the membrane that act by altering the structure of an intracellular signaling protein (called a G-protein) - Initiate the production of “2nd messengers” within the cytoplasm of the cell - 2nd messengers can activate 3rd/4th messenger signals within the cell – essentially amplifying the neurotransmitter signal These “Messengers”: - Influence ion conductance across membrane by altering ion channel opening → altering the electrical charge of neurons (excitation vs. inhibition) - Alter gene expression by increasing or decreasing transcription → altering protein production/activity in neurons So … 2nd messenger signaling can induce permanent changes in target neuron physiology, so their effects can be profound and long-lasting 8 Principles of Neurobiology and Neurochemistry METABOTROPIC RECEPTORS All metabotropic receptors use same intracellular signaling protein (called a G Protein) to initiate sequence of intracellular events leading to 2nd messenger production G Proteins ▪ Proteins located on the cytoplasmic side of the cell membrane ▪ Interact with cytoplasmic domain of the neurotransmitter receptor ▪ Become activated upon binding of the neurotransmitter to the receptor ▪ Have 3 functional subunits (alpha , beta , gamma ) Alpha () subunits are termed: Gi or Gs (sometimes Gq) – Gi inhibitory and Gs (Gq) stimulatory subunits - they are characterized as inhibitory or stimulatory by whether or not they activate the production of a 2nd messenger 9 Principles of Neurobiology and Neurochemistry METABOTROPIC RECEPTORS When receptor is at rest (no neurotransmitter bound to receptor) - The G subunit of the G protein is bound to a GDP molecule - The G subunit/GDP complex also interacts with neurotransmitter receptor and to G/G subunits X 10 Principles of Neurobiology and Neurochemistry METABOTROPIC RECEPTORS When neurotransmitter binds receptor – conformational change in G subunit causes: GTP exchanges for GDP on G subunit The GTP-bound G subunit dissociates from the receptor and the G subunits * Thecontributes GTP-bound G subunit to a number of cellular functions regulating cellular activity 11 * It produces a RESPONSE within the cell Principles of Neurobiology and Neurochemistry METABOTROPIC RECEPTORS - There are an array of GTP-bound G proteins that contribute to a number of cellular functions regulating cellular activity   GTP  s i GTP GTP Stimulates production q of a second messenger Inhibits production of a second messenger GTP Stimulates or inhibits production of a second messenger 12 Principles of Neurobiology and Neurochemistry METABOTROPIC RECEPTORS SECOND MESSENGER SYSTEMS OF INTEREST 1. Cyclic Nucleotide (cAMP) Pathway cAMP is the second messenger 2. Phospholipase A2 (PLA2) & Arachodonic Acid (AA) Pathway Arachodonic acid is the second messenger 3. Diacylglycerol (DAG) – Inositol Triphosphate (IP3) Pathway DAG and IP3 are the second messengers 13 Principles of Neurobiology and Neurochemistry Cyclic Nucleotide Second Messenger Pathway GTP-bound G subunit has enzymatic activity When neurotransmitter binds the neurotransmitter receptor: - GTP is exchanged for GDP on the G subunit - The GTP-bound G subunit of the G protein dissociates from the receptor and → activates the membrane-bound enzyme adenylate cyclase (AC) - Activated adenylate cyclase cleaves ATP → producing the 2nd messenger cAMP If Gs → stimulates adenylate cyclase & cAMP production If Gi → inhibits adenylate cyclase & s/i AC cAMP production GTP ATP cAMP 14 Principles of Neurobiology and Neurochemistry Cyclic Nucleotide Second Messenger Pathway cAMP can produce activity and structural changes in neurons by: -activating cAMP dependent protein kinase (PKA) (a phosphorylation enzyme) 1. Phosphorylation can activate/inhibit the opening of ion channels 2. Activates gene expression by phosphorylating CREB (cAMP Response Element Binding Protein) Neurotransmitter Closed Phosphorylation opens Adenylyl Neurotransmitter ion channel Ion channel cyclase Receptor 1. Gs + P ATP cAMP + PKA relocates to nucleus CREB P CREB nucleus 2. CREB P See explanation of #2 Promotor 15 on next slide Phosphorylation activates gene expression Principles of Neurobiology and Neurochemistry Molecular Consequences of Metabotropic Receptor Activation 2nd messengers can also regulate gene transcription (mRNA production in the cell nucleus) → this occurs by activating 3rd/4th messengers → typically, due to phosphorylation or dephosphorylation of target proteins - Phosphorylation increases or decreases the activity of cellular proteins protein kinases – phosphorylate (add a phosphate group to proteins) phosphatases – de-phosphoryate (remove a phosphate group) e.g., - cAMP dependent protein kinase (PKA) translocates (moves) to the nucleus and phosphorylates the protein cAMP Response Element Binding protein (CREB) - CREB binds to the cAMP Response Element (CRE) on DNA (CRE is a specific DNA sequence in the regulatory promotor region of a gene) - CREB/CRE binding alters gene expression by increasing or decreasing mRNA production of that target gene so, cAMP dependent phosphorylation regulates gene expression 16 Principles of Neurobiology and Neurochemistry Phospholipase A2 (PLA2) & Arachodonic Acid (AA) Pathway When neurotransmitter binds the neurotransmitter receptor GTP-bound G subunit of the G protein dissociates from the receptor and activates the membrane bound enzyme Phospholipase A2 (PLA2) Activated PLA2 then goes onto hydrolyze Phosphoinositol (PI; membrane phospholipid) Hydrolyzing PI → releases the fatty acid 2nd messenger Arachodonic Acid (AA) q PLA2 Phosphoinositol (PI) GTP PI AA 17 Arachidonic Acid (AA) Principles of Neurobiology and Neurochemistry Phospholipase A2 (PLA2) & Arachodonic Acid (AA) Pathway Arachidonic Acid (AA) In brain → AA influences neuronal branch growth and synapse formation by acting as a retrograde messenger → AA also can alter Ca++ release from ER and ionotropic receptor function In body → AA plays a key role in cardiovascular biology, carcinogenesis, and many I inflammatory diseases (inflammation) (inflammation) Drugs influence activity of AA pathway Corticosteroids – prevent mobilization of AA from membrane - reduce production of 18 messengers that are produced from the release of AA → reduce inflammation! Principles of Neurobiology and Neurochemistry Diacylglycerol (DAG) – Inositol Triphosphate (IP3) Pathway When neurotransmitter binds the neurotransmitter receptor GTP-bound G subunit of G protein dissociates from the receptor and activates the membrane bound enzyme Phospholipase C (PLC) Activated PLC then cleaves Phosphatidylinositol (PIP2; a membrane phospholipid ) Cleaving PIP2 → produces the 2nd messengers IP3 and DAG q PLC GTP PIP2 IP3 activates the release of Ca++ from the endoplasmic reticulum (ER) of neurons Increase in Ca++ has several functions: DAG & IP3 Ca++ - Depolarizes (stimulates) the cell - Activates the enzyme Ca++/calmodulin dependent protein kinase (CaM Kinase - a phosphorylation enzyme) protein- P - CaM Kinase phosphorylation can activate gene expression - CaM Kinase phosphorylation can activate/inhibit the opening of ion channels 19 Principles of Neurobiology and Neurochemistry Diacylglycerol (DAG) – Inositol Triphosphate (IP3) Pathway When neurotransmitter binds the neurotransmitter receptor GTP-bound G subunit of G protein dissociates from the receptor and → activates the membrane bound enzyme Phospholipase C (PLC) Activated PLC then cleaves Phosphatidylinositol (PIP2; a membrane phospholipid ) Cleaving PIP2 → produces the 2nd messengers IP3 and DAG q PLC GTP PIP2 DAG activates the enzyme Protein Kinase C (PKC) (a phosphorylation enzyme) - phosphorylation can activate or inhibit the opening DAG & IP3 Ca++ of ion channels protein- P 20 Principles of Neurobiology and Neurochemistry Diacylglycerol (DAG) – Inositol Triphosphate (IP3) Pathway Ca++ 21 Principles of Neurobiology and Neurochemistry Molecular Consequences of Metabotropic Receptor Activation s/i AC PLC q GTP PIP2 GTP ATP cAMP IP3 Protein Kinase A (PKA) Ca++ PKA Calcium/Calmodulin Phosphorylation Dependent Protein Kinase PKA/CaMK P (CaMK) CREB CREB Translocate to nucleus CaMK CaMK binds CaMKII promotor element CREB- P binds CRE element in 22 in the promotor region of target genes the promotor region of target genes Principles of Neurobiology and Neurochemistry HETERORECEPTORS VS. AUTORECEPTORS Heteroreceptors - respond to neurotransmitters released by nerve endings of neighboring neurons. - Can be: Presynaptic - Axo-dendritic neuron - Axo-somatic - Axo-axonic - Axo-synaptic - Can be inhibitory or excitatory Post-synaptic Postsynaptic neuron neuron 23 Principles of Neurobiology and Neurochemistry HETERORECEPTORS VS. AUTORECEPTORS Autoreceptors - respond to neurotransmitters released by their own nerve endings. Somatodendritic autoreceptor - Can be Presynaptic neuron - Somatodendritic - Somatic - Pre-synaptic Presynaptic neuron soma or dendrite Somatodendritic Autoreceptor Presynaptic autoreceptor Presynaptic neuron terminal Presynaptic Autoreceptor Postsynaptic neuron - Usually, inhibitory (negative feedback) receptors but can also be excitatory 24 Principles of Neurobiology and Neurochemistry PSYCHOACTIVE DRUGS ACT BY ALTERING NEUROTRANSMITTER ACTION Neurotransmitter (NT) function may be stimulated (+) or inhibited (-) by drugs. So, drugs essentially seize the normal functioning of your endogenous neurochemistry 25