Neurochemistry+I

=**Neurochemistry I**=


 * 1. Define a neurotransmitter.**

A neurotransmitter is a substance stored in a presynaptic vesicles and released in a Ca2+ dependent manner to interact with a membrane bound receptor.


 * 2. Discuss the process of neurotransmitter release.**

(1) Movement of action potential into the presynaptic terminal. (2) Influx of Ca2+ into the terminal. //This is the primary signal for release.// (3) Fusion of synaptic vesicle with the synaptic membrane (4) Release of neurotransmitter (5) The channels that are directly involved are voltage-gated (i.e., depolarization-sensitive) Ca2+ channels at the presynaptic terminal.


 * 3. Compare the different types of neurotransmitter receptors with regard to location (pre- vs. postsynaptic), functional relationship to ion channels, neurochemical effector systems, and structure.**

Neurotransmitter receptors can be found on postsynaptic or presynaptic membranes. Receptors found on presynaptic membranes may be autoreceptors or heteroreceptors. Autoreceptors are on the presynaptic terminal that releases the neurotransmitter and serve to inhibit further release of neurotransmitter via negative feedback. Heteroreceptors are receptors for an inhibitory neurotransmitter on the presynaptic terminal of a heterologous neuron, acting to reduce neurotransmitter release via presynaptic inhibition.

Ionotrophic neurontransmission functions by binding of neurotransmitter to its receptor to open an ion channel. This is a fast-acting, short-latency response that involves a change in membrane potential: EPSP or IPSP. These receptors are ligand-gated ion channels with multiple homologous subunits. Each subunit has 3-4 transmembrane domains that together form the ion channel.

Metabotrophic transmission functions by binding of the neurotransmitter to its receptor to alter the biochemical processes in the recipient cell. Responses are of late onset but long duration. These receptors are usually single subunit receptors with 7 transmembrane domains and coupled to heterotrimeric G-proteins. G-protein couple their receptors to ion channels or to biochcemical systems. They bind and hydrolyze GTP in a cyclic manner. In the basal state, the alpha-subunit binds GDP; when the G-protein becomes activated by binding neurotransmitter, GDP is released and GTP is bound. The built-in GTPase activity of the alpha subunit converts GTP to GDP and returns the G-protein to basal state with the dissociation of neurotransmitter form its receptor. Specificity is determined by the alpha-subunit. Receptor activation may affect ion channels but only indirectly through the G-protein or intracellular secondary messengers.


 * 4. Compare the components of the major intracellular signaling pathways in neurons.**

__Neurotransmitters__ For one neurotransmitter, there can be multiple receptors types.

__Receptors__ Specificity of response is determined by the nature of the receptor that is activated.

__G-proteins__ The four most common types of G-protein receptors are //G//s, //G//i, //G//q, and //G//o. //G//s is stimulatory and increases function of adenylyl cyclase. //G//i is inhibitory and reduces function of adenylyl cyclase. //G//q or //G//p increases activity of phospholipase C. //G//o acts directly on K+ and Ca2+ channels to increase its conductance.

__Secondary Messangers__ Secondary messengers may be used to directly open receptors or altering receptor properties (sensitivity, kinetics, etc).


 * **Secondary Messenger** || **Enzyme of Synthesis** || **G-protein/Source** ||** Target** ||
 * cAMP || Adenylyl Cyclase; ATP || //G//s or //G//i || Cyclic nucleotide-gated ion channels and cyclic AMP-dependent protein kinase (e.g., PKA) ||
 * Diacylglycerol (DAG) || Phospholipase C; PIP2 || //G//q || Protein Kinase C (PKC) ||
 * Inositol Triphosphage (IP3) || Phospholipase C; PIP2 || //G//q || Receptor on endoplasmic reticulum ||
 * Ca2+ || -- || Voltage-gated Ca2+ channels, NMDA receptor channels, or endoplasmic reticulum || PKC, Calmodulin, Ca2+/Calmodulin-dependent Protein Kinase (CaMK) ||

__Protein Phosphorylation__ Secondary messengers may be used to activate a protein kinase which, in turn, phosphorylates and alters neuronal function/biochemistry.


 * **Substrate** || **Example** || **Function** ||
 * Ion Channels || K+ and Ca2+ channels || Modulate neuronal activity ||
 * Enzymes || Tyrosine hydroxylase || Neurotransmitter synthesis ||
 * Cytoskeletal proteins || Microtubule-associated proteins (MAPs) || Maintenance of neuronal structure ||
 * Transcription factors || cAMP response element binding protein (CREB)|| Gene expression ||


 * 5. List neurotransmitters with multiple receptor subtypes and their effects on neurons.**


 * **Neurotransmitter** || **Receptor Types ** || **Neurotransmission Type** || **G-Protein** || **Effect** ||
 * Dopamine || D1, D5 || Metabotropic || //G//s || increasing activity of adenylyl cyclase ||
 * || D2, D3, D4 || Metabotropic || //G//i || decreasing activity of adenylyl cyclase ||
 * Norepinephrine || Alpha 1 || Metabotropic || //G//q || increasing activity of phospholipase C ||
 * || Alpha 2 || Metabotropic || //G//i || decreasing activity of adenylyl cyclase ||
 * || Beta || Metabotropic || //G//s || increasing activity of adenylyl cyclase ||
 * Acetylcholine || Nicotinic || Ionotropic || || Increasing Na+/K+ conductance ||
 * || Muscarinic m1, m3, m5 || Metabotropic || //G//q || increasing activity of phospholipase C ||
 * || Muscarinic m2, m4 || Metabotropic || //G//i || decreasing activity of adenylyl cyclase; //G//o, some G-protein-coupling to ion channels ||
 * GABA || GABA(a) || Ionotropic || || Increases Cl- conductance ||
 * || GABA(b) || Metabotropic || //G//o || G-protein-coupled increase in K+, Ca2+ conductance ||
 * Glutamate || AMPA/Kainic Acid || Ionotropic || || Increases Na+/K+ conductance ||
 * || NMDA || Ionotropic || || Increases Ca2+, Na+, and K+ conductance; has both ligand-gated and voltage-gated properties ||
 * || Metabotropic || Metabotropic || //G//q || increases phospholipase C activity. ||


 * 6. Review the synthesis and degradation of acetylcholine.**

Acetylcholine is synthesized from acetyl CoA and choline, catalyzed by choline acetyltransferase. After vesicle release, acetylcholine is not cleared by reuptake. Instead, it is degraded by acetylcholinesterase back into acetate and choline. Choline is recycled to make new acetylcholine; acetate disappears like magic.

Some nerve gases like sarin, act by inhibiting acetylcholinesterase for long periods of time to cause death. Temporary inhibition of acetylcholinesterase can be used to treat myasthenia gravis, where an autoimmune reaction destroys ACh receptors; temporary inhibition of degradation allows ACh in the synaptic cleft longer.


 * 7. Describe the types of acetylcholine receptor.**

Nicotinic ACh receptors are ionotrophic, acting to increase Na+ and K+ conductance, resulting in fast acting EPSPs. Its agonist is nicotine and its antagonist is curare. The nicotinic ACh receptor has 5 subunits arraged to form an ion channel. ACh binds to two alpha-subunits to activate the channel.

There are five muscarinic receptors. m1, m3, and m5 are //G//q protein coupled, increasing activity of phospholipase C, while m2 and m4 are //G//i protein coupled, decreasing adenylyl cyclase activity. These receptors produce slow EPSPs or IPSPs. It is the predominant cholinergic receptor in CNS.


 * 8. Discuss the chemistry and synthesis of dopamine, norepinephrine, and epinephrine.**

Catecholamines have a catechol nucleus (benzene with 2 –OH groups) and have the general formula:

Catechol – CHR’– CH2 – NHR”

Dopamine has R’, R” ``=`` H Norepinephrine has R’ ``=`` OH, R” ``=`` H Epinephrine has R’ ``=`` OH, R” ``=`` CH3

Reaction with formaldehyde gives off a green fluorescence product. Synthesis of catecholamine begins with tyrosine getting hydroxylated by tyrosine hydroxylase to form L-dihydroxyphenylalanine (L-DOPA). L-DOPA is decarboxylated by DOPA decarboxylase or L-aromatic-aminodecarboxylase to form dopamine.

Dopamine can be hydroxylated by dopamine beta-hydroxylase to form norepinephrine. Norepinephrine receives a methyl group, catalyzed by phenylethanol-amino-N-methyl-transferase to form epinephrine.

Indoleamines have an indole nucleus with the general formula:

Indole – CH2 – CH2 – N – R’R”

Seratonin has R’, R” ``=`` H

Reaction with formaldehyde gives off a yellow product. 5-HT is synthesized from tryptophan which is converted to 5-hydroxytryptophan by tryptophan-5-hydroxylase using a O2 pterin cofactor. 5-hydroxytryptophan is decarboxylated by 5-hydroxytryptophan decarboxylase using pyridoxal phosphate to 5-HT.


 * 9. Discuss the role of MAO and COMT in neurotransmitter breakdown.**

Monoamine oxidase (MAO) is an enzyme bound to the outer mitochondrial membrane and takes part in the degradation of 5-HT and catecholamines. Catecholamines are further degraded by catechol-O-methyltransferase (COMT), a cytoplasmic enzyme requiring S-adenyslmethionine and Mg2+.

MPTP is a catecholamine-like compound that is a contaminant of designer drugs. Its degradation by MAO results in toxic metabolites that cause degeneration of the substantia nigra and a severe form of Parkinson’s disease. Because of this, deprengyl, a MAO inhibitor, is useful in the treatment of Parkinson’s disease.


 * 10. Describe the major classes of pharmacological agents that interact with catecholaminergic and serotonergic systems.**

In general, excess catecholamines results in behavioral excitation and pharmacological agents which elevate catechoamines are useful in treatment of depression. Conversely, lack of catecholamines results in behavioral depression and blocking catecholamine function is useful in treating states of catecholamine excess such as schizophrenia.


 * **Drug** || **Functional Action** || **Neurochemical Action** ||
 * Neuroleptic (Haloperidol) || (-) Antipsychotic || Blockage/antagonist of catecholamine receptors ||
 * Reserpine || (-) Antipsychotic (uncommon); peripheral effects || Blockage of neurotransmitter uptake into vesicles resulting in MAO depletion of neurotransmitter ||
 * Tricyclic Antidepressant (Imipramine); Cocaine || (+) Antidepressant || Blocks reuptake from synaptic cleft, resulting in longer neurotransmitter action ||
 * MAO inhibitor || (+) Antidepressant || Inhibition of MAO, resulting in less breakdown of neurotransmitter ||
 * Amphetamine || (+) Stimulant; antidepressant || Inhibits MAO, inhibits reuptake of neurotransmitter from synaptic cleft, causes catecholamine release ||


 * 11. Compare the synthetic and degradative pathways for serotonin to those of the catecholamines.**

Both catecholamines and serotonin are synthesized from amino acids; catecholamines (DA, NE, and E) are synthesized from tyrosine while serotonin is synthesized from tryptophan. Synthesis of DA and 5-HT from their amino acid precursors both require a hydroxylation step followed by a decarboxylation. NE and E are synthesized from DA by adding another –OH to make NE and further adding a methyl group to make E.

Inactivation of catecholamines and serotonin are both mediated initially by MAO degradation. MAO metabolites of DA, NE, and E are further broken down by COMT.