Chapter 30 General Principles of the Neuron Activities

Chapter 30 General Principles of the Neuron Activities

Contents l Neurotransmission l Neurotransmitter and Receptor l Synaptic Plasticity l Properties of the Synaptic Neurotransmission

Part I NEUROTRANSMISSION

I Synapse 1. Chemical synapse (Classical Synapse) – Predominates in the vertebrate nervous system 2. Non-synaptic chemical transmission 3. Electrical synapse – Via specialized gap junctions – Does occur, but rare in vertebrate NS – Astrocytes can communicate via gap junctions

1. Chemical Synapse • Terminal bouton is separated from postsynaptic cell by synaptic cleft. • Vesicles fuse with axon membrane and NT released by exocytosis. • Amount of NTs released depends upon frequency of AP.

2. Non-synaptic • chemical transmission Sympathetic Nerve • The postganglionic neurons innervate the smooth muscles. • No recognizable endplates or other postsynaptic specializations; • The multiple branches are beaded with enlargements (varicosities) that are not covered by Schwann cells and contain synaptic vesicles Fig. : Ending of postganglionic autonomic neurons on smooth muscle

Non-synaptic chemical transmission continued • In noradrenergic neurons, the varicosities are about 5 m, with up to 20, 000 varicosities per neuron • Transmitter is released at each varicosity • One neuron innervate many effector cells. Fig. : Ending of postganglionic autonomic neurons on smooth muscle

Innervation of Adrenal Medulla

3. Electrical Synapse • Impulses can be regenerated without interruption in adjacent cells. • Gap junctions: – Adjacent cells electrically coupled through a channel. – Each gap junction is composed of 12 connexin proteins. • Examples: – Smooth and cardiac muscles, brain, and glial cells.

Electrical Synapses 1. Communication takes place by flow of electric current directly from one neuron to the other 2. No synaptic cleft or vesicles cell membranes in direct contact 3. Communication not polarized- electric current can flow between cells in either direction

Electrical Synapse Chemical Synapse Purves, 2001

II The Chemical Synapse and Signal Transmission

II The Chemical Synapse and Signal Transmission l The chemical synapse – specialized junction that transfers nerve impulse information from a pre synaptic membrane to a postsynaptic membrane using neurotransmitters and enzymes

Synaptic connections • ~100, 000, 000 neurons in human brain • Each neuron contacts ~1000 cells • How many synapses?

Chemical Synapses • Neurotransmitterchemical intermediary released from one neuron and influences another • Synaptic cleft- a small gap between the sending (presynaptic) and the receiving (postsynaptic) site

Chemical Synapses • Synaptic vesiclessmall spherical or oval organelles contain chemical transmitter used in transmission • Polarizationcommunication occurs in only one direction, from sending presynaptic site to receiving postsynaptic site

1. Synaptic Transmission Model • • • Precursor transport NT synthesis Storage Release Activation Termination ~diffusion, degradation, uptake, autoreceptors

Presynaptic Axon Terminal Button Postsynaptic Membrane Dendritic Spine

(1) Precursor Transport

(2) Synthesis _ _ _ enzymes/cofactors NT

(3) Storage in vesicles

NT Terminal Button Dendritic Spine Synapse Vesicles

(4) Release Terminal Button Dendritic Spine Synapse Receptors

Terminal Button AP Dendritic Spine Synapse

Exocytosis 2+ Ca

Each vesicle contains one quanta of neurotransmitter (approximately 5000 molecules) – quanta release

(5) Activation

(6) Termination

(6. 1) Termination by. . . Diffusion

(6. 2) Termination by. . . Enzymatic degradation

(6. 3) Termination by. . . Reuptake

(6. 4) Termination by. . . Autoreceptors A

Autoreceptors • On presynaptic terminal • Binds NT same as postsynaptic receptors different receptor subtype • Decreases NT release & synthesis • Metabotropic receptors

Synaptic Transmission • AP travels down axon to bouton. • Voltage Gated Ca 2+ channels open. – Ca 2+ enters bouton down concentration gradient. – Inward diffusion triggers rapid fusion of synaptic vesicles and release of NTs. • Ca 2+ activates calmodulin, which activates protein kinase. • Protein kinase phosphorylates synapsins (突触蛋白）. – Synapsins aid in the fusion of synaptic vesicles.

Synaptic Transmission (continued) • NTs are released and diffuse across synaptic cleft. • NT (ligand) binds to specific receptor proteins in postsynaptic cell membrane. • Chemically-regulated gated ion channels open. – EPSP: depolarization. – IPSP: hyperpolarization. • Neurotransmitter inactivated to end transmission.

2 EPSP and IPSP l EPSP: l IPSP: Excitatory postsynaptic potential Inhibitory postsynaptic potential

EPSP An AP arriving in the presynaptic terminal cause the release of neurotransmitter · The molecules bind activate receptor on the postsynaptic membrane ·

EPSP Opening transmittergated ions channels ( Na+) in postsynapticmembrane · Both an electrical and a concentration gradient driving Na+ into the cell · The postsynaptic membrane will become depolarized (EPSP). ·

EPSP No threshold. • Decreases resting membrane potential. • – Closer to threshold. Graded in magnitude. • Have no refractory period. • Can summate. •

IPSP l A impulse arriving in the presynaptic terminal causes the release of neurotransmitter l The molecular bind active receptors on the postsynaptic membrane open CI- or, K+ channels l CI- influx or K+ outflux produce a hyperpolarization in the postsynaptic membrane.

• IPSPs –No threshold. –Hyperpolarize postsynaptic membrane. –Increase membrane potential. –Can summate. –No refractory period.

3 Synaptic Inhibition A • Presynaptic inhibition • Postsynaptic inhibition B

(1) Postsynaptic inhibition · Concept: · effect of inhibitory synapses on the postsynaptic membrane. · Mechanism: · IPSP · inhibitory interneuron · Types: Afferent collateral (reciprocal) inhibition) Recurrent inhibition.

Postsynaptic inhibition Reciprocal inhibition

Postsynaptic inhibition 2) Recurrent inhibition

(2) Presynaptic inhibition • Concept: the inhibition occurs at the presynaptic terminals before the signal ever reaches the synapse. • The basic structure: an axon-axon synapse (presynaptic synapse) between A and B. • Neuron A has no direct effect on neuron C, but it exert a presynaptic effect on ability of B to Influence C. • decrease the amount of neurotransmitter released from B (Presynaptic inhibition) AA B C C

Mechanisms Presynaptic inhibition • Activation of the presynaptic receptors increases CIconductance EPSP to decrease the size of the AP reaching the excitatory ending reduces Ca 2+ entry and consequently the amount of excitatory transmitter decreased.

Presynaptic Inhibition Excitatory Synapse A + • A active • B more likely to fire • Add a 3 d neuron ~ B

Presynaptic Inhibition Excitatory Synapse + A + C • Axon-axon synapse • C is excitatory ~ B

Presynaptic Inhibition Excitatory Synapse A + + B C • C active • less NT from A when active • B less likely to fire ~

4 Synaptic Facilitation: Presynaptic and Postsynaptic

(1) Presynaptic Facilitation Excitatory Synapse A + • A active • B more likely to fire ~ B

Presynaptic Facilitation Excitatory Synapse A + C + B • C active (excitatory) • more NT from A when active (Mechanism: AP of A is prolonged and Ca 2+ channels are open for a longer period. ) • B more likely to fire ~

(2) Postsynaptic facilitation: neuron that has been partially depolarized is more likely to undergo AP.

Record here EPSP + + • Depolarization more likely to fire ~ Vm -65 mv - 70 mv AT REST Time

5 Synaptic Integration • EPSPs can summate, producing AP. – Spatial summation – Temporal summation

(1) Spatial Summation • The accumulation of neurotransmitter in the synapse due the combined activity of several presynaptic neurons entering the Area (Space) of a Convergent Synapse. • A space (spatial) dependent process.

Spatial Summation + + + • Multiple synapses vm -65 mv - 70 mv AT REST Time

(2) Temporal Summation • The accumulation of neurotransmitters in a synapse due to the rapid activity of a presynaptic neuron over a given Time period. • Occurs in a Divergent Synapse. (explain later) • Is a Time (Temporal) dependent process.

Temporal Summation + + n Vm n -65 mv - 70 mv Repeated stimulation same synapse ~ AT REST Time

(3) EPSPs & IPSPs summate • CANCEL EACH OTHER • Net stimulation – EPSPs + IPSPs = net effects ~

+ + - 70 mv - EPSP IPSP

6. Divergent and Convergent Synapse

Divergent Synapse • A junction that occurs between a presynaptic neuron and two or more postsynaptic neurons. • The stimulation of the postsynaptic neurons depends on temporal summation.

Convergent Synapse • A junction between two or more presynaptic neurons with a postsynaptic neuron • The stimulation of the postsynaptic neuron depends on the spatial summation.

Part II Neurotransmitters and receptors

1. Basic Concepts • Neurotransmitter • Endogenous signaling molecules that alter the behaviour of neurons or effector cells. • Neuroreceptor: • Proteins on the cell membrane or in the cytoplasm • Could bind with specific neurotransmitters • Alter the behavior of effector cells

A neurotransmitter must (classical definition) • • • Be synthesized and released from neurons Be found at the presynaptic terminal Have same effect on target cell when applied externally Be blocked by same drugs that block synaptic transmission Be removed in a specific way Purves, 2001

Classical Transmitters (small-molecule transmitters) • Biogenic Amines (胺) • Acetylcholine • Catecholamines • Dopamine • Norepinerphrine • Epinephrine • Serotonin (5 -HT) • Amino Acids • Glutamate • GABA ( -amino butyric acid) • Glycine Non-classical Transmitters • Neuropeptides • Neurotrophins • Gaseous messengers –Nitric oxide –Carbon Monoxide –Hydrogen sulfide

Classes of CNS Transmitters Neurotransmitter Brain % of Synapse Concentrati on s Function Primary Receptor Class Monoamines Catecholamines: DA, NE, EPI Indoleamines: serotonin (5 -HT) 2 -5 nmol/mg protein (low) Slow change in excitability (secs) GPCRs Acetylcholine (ACh) 5 -10 nmol/mg protein (low) Slow change in excitability (secs) GPCRs μmol/mg protein (high) Rapid inhibition (msecs) Ion channels μmol/mg protein (high) Rapid excitation (msecs) Ion channels Amino acids Inhibitory: GABA, glycine Excitatory: Glutamate, aspartate 15 -20 75 -80 71

Agonist l. A substance that mimics a specific neurotransmitter – able to attach to that neurotransmitter's receptor – produces the same action as the neurotransmitter. l Drugs are often designed as receptor agonists – to treat a variety of diseases and disorders l when the original chemical substance is missing or depleted.

Antagonist l Drugs bind to but do not activate neuroreceptors – blocking the actions of neurotransmitters or the neuroreceptor agonists.

• Same NT can bind to different -R • different part of NT ~ NT Receptor A Receptor B

Specificity of drugs Drug A Receptor A Drug B NT Receptor B

Synaptic vesicles • Concentrate and protect transmitter • Can be docked at active zone • Differ for classical transmitters (small, clear-core) vs. neuropeptides (large, dense-core)

Neurotransmitter Co-existence l Some neurons produce both a classical neurotransmitter and a polypeptide neurotransmitter. – contained in different synaptic vesicles – be distinguished using the electron microscope. l The neuron release either the classical or the polypeptide neurotransmitter under different conditions.

Purves, 2001

Examples of Co-localized Neuropeptides and Small Molecule Neurotransmitters Peptide Small Molecule Site of Co-localization NYP NE Neurons of the locus ceruleus （蓝斑核）; sympathetic postganglionic neurons Dynorphin（强啡肽）; GABA substance P; enkephalin (脑啡肽） Striatal （纹状体）GABAergic projection neurons VIP ACh Parasympathetic postganglionic neurons CGRP ACh Spinal motor neurons Neurotension（神经降压素）; CCK DA Neurons of the substantia nigra （黑质）

Receptors determine whether • Synapse is excitatory or inhibitory – NE is excitatory at some synapses, inhibitory at others • Transmitter binding activates ion channel directly or indirectly. – Directly • ionotropic receptors • fast – Indirectly • metabotropic receptors • G-protein coupled • slow

2. Receptor Activation • Ionotropic receptor – directly controls channel – fast • Metabotropic receptor – second messenger systems – receptor indirectly controls channel ~

(1) Ionotropic Receptor Channel NT neurotransmitter

Ionotropic Receptor NT Pore

Ionotropic Receptor NT

Ionotropic Receptor NT

(2) Metabotropic Receptor • Receptor separate from channel • G proteins • 2 d messenger system – c. AMP – other types • Effects – Control channel – Alter properties of receptors – regulation of gene expression ~

(2. 1) G protein: direct control • NT is 1 st messenger • G protein binds to channel – opens or closes – relatively fast ~

G protein: direct control R G GDP

G protein: direct control R G GTP Pore

(2. 2) G protein: Protein Phosphorylation external signal: NT nt norepinephrine Receptor b adrenergic -R transducer primary effector GS adenylyl cyclase 2 d messenger c. AMP secondary effector protein kinase

G protein: Protein Phosphorylation A C R G GDP PK

G protein: Protein Phosphorylation A C R G GTP ATP c. AMP PK

G protein: Protein Phosphorylation A C R G GTP ATP P c. AMP PK Pore

3 SOME IMPORTANT TRANSMITTERS AND THE CORRESPONDING RECEPTORS

Major Neurotransmitter Receptors in the CNS Neurotransmitter Receptor Subtypes G Protein-Coupled (G) vs. Ligand-Gated Ion Channel (LG) DA D 1 D 2 D 3 D 4 D 5 G G G NE/EPI α 1 α 2 β 1 β 2 β 3 G G G 5 -HT 1 A 5 -HT 1 B 5 -HT 1 D 5 -HT 2 A 5 -HT 2 B 5 -HT 2 C 5 -HT 3 5 -HT 4 G G G LG G 95

Major Neurotransmitter Receptors in the CNS Neurotransmitter Receptor Subtypes ACh Muscarinic M 1 Muscarinic M 2 Muscarinic M 3 Muscarinic M 4 Nicotinic NMDA AMPA Kainate Metabotropic G Protein-Coupled (G) vs. Ligand-Gated Ion Channel (LG) G G LG LG G A B LG G Glutamate GABA 96

(1) Acetylcholine (ACh) as NT

Acetylcholine Synthesis choline acetyltransferase choline + acetyl Co. A ACh + Co. A

Acetylcholinesterase (ACh. E) • Enzyme that inactivates ACh. – Present on postsynaptic membrane or immediately outside the membrane. • Prevents continued stimulation.

The Life Cycle of Ach

Ach - Distribution • Peripheral N. S. • Excites somatic skeletal muscle (neuro-muscular junction) • Autonomic NS Ganglia Parasympathetic NS--- Neuroeffector junction Few sympathetic NS – Neuroeffector junction • Central N. S. - widespread Hippocampus Hypothalamus ~

Ach Receptors • ACh is excitatory or inhibitory, depending on organ involved. – Causes the opening of chemical gated ion channels. • Nicotinic ACh receptors: – in autonomic ganglia (N 1) and skeletal muscle fibers (N 2). • Muscarinic ACh receptors: – in the plasma membrane of smooth and cardiac muscle cells, – in cells of some glands.

Acetylcholine Neurotransmission • “Nicotinic” subtype Receptor: – Membrane Channel for Na+ and K+ – Opens on ligand binding – Depolarization of target (neuron, muscle) – Stimulated by Nicotine, etc. – Blocked by Curare, etc. – Motor endplate (somatic) (N 2), – all autonomic ganglia, hormone producing cells of adrenal medulla (N 1)

Ligand-Operated ACh Channels N Receptor

Acetylcholine Neurotransmission • “Muscarinic” subtype Receptor: M 1 – Use of signal transduction system • Phospholipase C, IP 3, DAG, cytosolic Ca++ – Effect on target: cell specific (heart , smooth muscle intestine ) – Blocked by Atropine, etc. – All parasympathetic target organs – Some sympathetic targets (sweat glands, skeletal muscle blood vessels - dilation)

G Protein-Operated ACh Channel

Acetylcholine Neurotransmission • “Muscarinic” subtype: M 2 – Use of signal transduction system • via G-proteins, opens K+ channels, decrease in c. AMP levels – Effect on target: cell specific – CNS – Blocked by Atropine, etc.

Cholinergic Agonists • Direct – Muscarine – Nicotine • Indirect – ACh. E Inhibitors ~

Cholinergic Antagonists • Direct Nicotinic - Curare Muscarinic - Atropine

(2) Monoamines as NT

Monoamines • Catecholamines – Dopamine - DA Norepinephrine - NE Epinephrine - E • Indolamines （ 吲哚胺）Serotonin - 5 -HT

Norepinephrine (NE) as NT • NT in both PNS and CNS. • PNS: – Smooth muscles, cardiac muscle and glands. • Increase in blood pressure, constriction of arteries. • CNS: – General behavior.

Adrenergic Neurotransmission • 1 Receptor – Stimulated by NE, E, – blood vessels of skin, mucosa, abdominal viscera, kidneys, salivary glands – vasoconstriction, sphincter constriction, pupil dilation

Epi 1 G protein PLC IP 3 Ca+2

Adrenergic Neurotransmission • 2 Receptor – stimulated by, NE, E, …. . – Membrane of adrenergic axon terminals (presynaptic receptors), platelets – inhibition of NE release (autoreceptor) – promotes blood clotting, pancreas decreased insulin secretion

Adrenergic Neurotransmission • 1 receptor – stimulated by E, …. – Mainly heart muscle cells – increased heart rate and strength

Mechanism of Action ( receptor)

Adrenergic Neurotransmission • 2 receptor – stimulated by E. . – Lungs, most other sympathetic innervate organs, blood vessels serving the heart (coronary vessels) – dilation of bronchioles & blood vessels (coronary vessels), relaxation of smooth muscle in GI tract and pregnant uterus

Adrenergic Neurotransmission • 3 receptor – stimulated by E, …. – Adipose tissue, – stimulation of lipolysis

(3) Amino Acids as NT • Excitatory Amino Acid (EAA) • Glutamate acid and aspartate acid • Inhibitory AA • gamma-amino-butyric acid (GABA) and glycine:

Glutamate l Neurotransmitter at 75 -80% of CNS synapses l Synthesized within the brain from – Glucose (via KREBS cycle/α-ketoglutarate，a-酮 戊二酸) – Glutamine （谷氨酸盐） (from glial cells) l Actions terminated by uptake through excitatory amino acid transporters (EAATs) in neurons and astrocytes

124

Glutamate Receptor Subtypes

Ionotropic Glutamate Receptors

NMDA receptor as a coincidence detector : requirement for membrane depolarization 127

NMDA receptor uses glycine as a co -agonist 128

NMDA receptor channel is blocked by phencyclidine ( 苯环已哌啶， PCP) 129

Glutamate non-NMDA receptors Na+ channels open depolarization removes blockade Ca 2+ channels open (Mg 2+ blockade) Ca 2+ enters when Mg 2+ is removed postsynaptic effects (learning) reinstates blockade Ca 2+dependent K+ channels open repolarization

a. Non-NMDA Na+ channels open, Na+ enters and depolarizes membrane b. Mg 2+ blockade of NMDA Ca 2+channels removed by membrane depolarization; Ca 2+enters c. Ca 2+ dependent K+ channels open; membrane repolarized d. Mg 2+ blockade reinstated a b c d

Gamma Aminobutyric acid (GABA) l inhibitory neurotransmitter of CNS and retina. l formed by decarboxylation of glutamate. l three types of GABA receptors – GABAA & B receptors are widely distributed in CNS. – GABAC are found in retina only 132

GABA Synthesis COOH Glutamic acid decarboxylase (GAD) NH 2 – CH 2 - COOH Glutamate NH 2 – CH 2 - COOH GABA 133

134

Subunit composition of ionotropic GABAA receptors Five subunits, each with four transmembrane domains (like n. ACh. R) l Most have two alpha (α), two beta (β), one gamma (γ) subunit l α 2 β 2 γ 1 is predominant in mammalian brain but there are different combinations in specific brain regions l 135

Modified from n. ACh. R, G and G 2011 136

137

The metabotropic GABAB receptor l l l GPCRs Largely presynaptic, inhibit transmitter release Most important role is in the spinal cord 138

(4) Polypeptides as NT • CCK: – Promote satiety following meals. • Substance P: – Major NT in sensations of pain.

(5) Monoxide Gas: NO and CO • Nitric Oxide (NO) – Exerts its effects by stimulation of c. GMP. – Involved in memory and learning. – Smooth muscle relaxation. • Carbon monoxide (CO): – Stimulate production of c. GMP within neurons – Promotes odor adaptation in olfactory neurons – May be involved in neuroendocrine regulation in hypothalamus. • Hydrogen Sulfide (H 2 S)

Part III SYNAPTIC PLASTICITY

Synaptic Plasticity l Concept – the ability of the synapse to change in strength in response to either use or disuse of transmission over synaptic pathways l Classification – Short term Plasticity (paired-pulse facilitation, short- term potentiation, synaptic depression) – Long-term Plasticity l l Long-term potentiation (LTP) Long-term depression (LTD)

Presynaptic vs. Postsynaptic I. The size of synaptic potentials can be modulated: • by regulating the amount of transmitter released at the synapse • by regulating the size of the current generated by postsynaptic receptors. II. Short term modulation (msecs - minutes) • Mechanism: are almost always presynaptic. • Paired-pulse facilitation (~10 to 100 msecs) • Synaptic depression (50 msecs to mins) • Post-tetanic potentiation (mins) III. Long-term plasticity • Mechanisms: complex and usually both pre- and postsynaptic • LTP (30 minutes to years) • LTD (30 minutes to years)

Facilitation and Depression

Facilitation of Transmitter Release Cause: increased mean number of quanta of transmitter released by the presynaptic terminal, probably by increasing the probability of release and perhaps increasing the number of release sites.

Depression of Transmitter Release caused by depletion of vesicles from the presynaptic terminal during the conditioning train, and reduced release efficacy.

Part IV GENERAL PRINCIPLES OF THE REFLEX (SYNAPTIC TRANSMISSION)

Special Characteristics of Synaptic Transmission l l l l One way conduction Delay, 0. 5 ms every synapse Summation and occlusion Change of frequency Afterdischarge and feedback Localization and generalization Fatigue Affected by – acidosis and alkalosis – Hypoxia – Drug

Occlusion

Afterdischarge