Synaptic transmission

Assoc. Prof. Ana-Maria Zagrean MD, PhD

Physiology & Neuroscience Division.

physiology.caroldavila@gmail.com

LECTURE 2 – NERVOUS SYSTEM PHYSIOLOGY

Charles Sherrington (1897):

The synapse, a specialized apposition between

cells, mediates the signals.

NEURONAL SYNAPSES

Santiago Ramón y Cajal (1894):

Neuron Doctrine

Neurons are distinct entities, fundamental

units of the nervous system, that are

discontinuous with each other.

• The ionic gradients across cell membranes provide a form of stored

electrochemical energy that cells can use for electrical signaling.

• The combination of resting membrane potential of −60 to −90 mV and

voltage-gated ion channels presence in the cell membrane allows excitable

cells to generate action potentials (APs).

Signal transfer between neuronal cells - premises:

• AP propagate over long distances along the surface membrane of

a single nerve axon.

• Cell-to-cell transfer of electrical signals (AP) by synaptic transmission

ELECTRICAL SYNAPSES

- provide direct electrical continuity between cells by means of gap junctions

(the membranes remain distinct, but ions and other small solutes can diffuse

through the gap junctions)

- instantaneous signal transduction, no delay – direct electrical contact

CHEMICAL SYNAPSE

- implies "contiguity, not continuity" between neurons and a chemical

neurotransmitter is released from one cell (presynaptic) and diffuses to

another (postsynaptic); synaptic cleft in between pre- and post-synaptic

segment

- 1 msec delay of signal transduction

• One AP can depolarize the postsynaptic membrane by 1 µV - there is an

attenuation of 10.000 -100.000 times of the signal’s voltage from one cell to the

next one…

Signal transfer between neuronal cells - premises:

Synaptic functions of neurons

Information → Action potentials transmission through a

succession of neurons, that can be:

(1) blocked from one neuron to the next

(2) changed from a single impulse into repetitive impulses

(3) integrated with impulses from other neurons in neuronal

networks, to cause highly intricate patterns of impulses in

successive neurons.

Types of synapses – intercellular

connections in the NS

Main types of intercellular connections:

(1) ELECTRICAL SYNAPSE – direct contact of cell membranes

(2) CHEMICAL SYNAPSE – depend on a neurotransmitter…

Other types of information transmission

(3) VOLUME TRANSMISSION – neurotransmitter diffusion in the

extracellular space, acting on extra-synaptic receptors

(4) EPHAPTIC TRANSMISSION - electric fields generated by a

specific neuron can alter the excitability of neighboring neurons

as a result of their anatomical and electrical proximity.

CHEMICAL VS ELECTRICAL SYNAPSE

Types of Synapses: Chemical and electrical - what are the differences?

Both types of synapses can coexist on the same neuron

– mixed synapses, hetero-synaptic interactions

Types of synapses

(1) ELECTRICAL SYNAPSE

- direct open fluid channels: gap junctions

- low-resistance path for electrotonic current

- directly conduct electricity from one cell to the next

- with little attenuation of the voltage signal

- with no time delay,

- but potential decays with distance

- transmit signals in either direction – bidirectional - with equal efficiency = reciprocal synapses.

- certain electrical synapses with voltage-dependent gates conduct more readily in one direction than in another = rectifying synapsis

- can also transmit metabolic signals between cells (ATP, cAMP, Ca2+, etc)

Electrical synapse : GAP junctions

Present in nearly every part of the

mammalian CNS, interconnecting:

-inhibitory neurons of the cerebral cortex

and thalamus,

-excitatory neurons of the brainstem,

-a variety of other neurons in the

hypothalamus, basal ganglia, and spinal

cord.

2 connexons of the coupled cells, with a

1.5 nm pore, open and close randomly,

with a higher probability to open when there

is an elevated level of intracellular Ca2+ or

H+ in one of the cells, or in response to

depolarization of one or both cells.

Electrical synapse : GAP junctions

Connect both neurons and glial cells

(neuronal and glial cells networks):

-typically found in dendro-dendritic sites to

synchronize the activity of neuronal

populations

-more common in embryonic nervous

system

-help the development of appropriate

synaptic connections based on

synchronous firing of neuronal populations

Types of synapses

(2) CHEMICAL SYNAPSE

- most of the synapses in the CNS

- the first neuron = presynaptic, releases the neurotransmitter that diffuses into the synaptic cleft to act on receptor proteins in the membrane of the next, postsynaptic neuron, to excite / inhibit it, or modify its sensitivity; these steps determine a 1 ms synaptic delay

> 40 neurotransmitters discovered thus far, most known are: acetylcholine, norepinephrine, epinephrine, histamine, GABA, glycine, serotonin, glutamate.

- “one-way” conduction at chemical synapses, from the presynaptic to postsynaptic neuron, allows specific transmission of signals to discrete and highly focused areas both within the CNS and PNS, to perform its numerous functions of sensation, motor control, memory etc.

CNS synapse

PNS synapse

Neuronal synapses vary widely in the size of the synaptic

contact, the identity of the neurotransmitter, the nature of the

postsynaptic receptors, the efficiency of synaptic transmission,

the mechanism used for terminating transmitter action, and the

degree and modes of synaptic plasticity.

Thus, the properties of neuronal synapses can be tuned to

achieve the diverse functions of the brain.

Chemical synaptic transmission

The most common synaptic arrangements in the CNS

The contact site and direction of

communication determine the way in

which a synapse is named:

axodendritic/ axospinous,

axosomatic, and axoaxonic

synapses.

Also, dendrodendritic, somato-

somatic, and even somato-dendritic

synapses may be found in the

mammalian brain.

In >90% of all excitatory

synapses in the CNS, the

postsynaptic site is a

dendritic spine.

-The ubiquity of spines implies that they serve prominent functions, but their small size

(usually less than 1 μm long) makes their function extremely difficult to study.

-Spines’ density and shape change → neuroplasticity.

-Spines contain proteins in high concentration: transmitter receptors, protein kinases,

structural proteins, and proteins that are involved in endocytosis and glycolysis.

B, Electron micrograph of an axospinous

synapse in the neocortex.

A, Drawings of various dendrites in the

neocortex, taken from Golgi-stained material.

The numerous protrusions are "spines."

Spiny dendrites from hippocampal pyramidal neuron.

Left: Light microscope image. Right: Reconstruction from serial electron

microscopy.

http://synapses.clm.utexas.edu/anatomy/dendrite/dendrite.stm#spines

Chemical Synapses

-structure:1. Presynaptic terminal (end-feet/axon knob)

2. Synaptic cleft (~20-30 nm, isolation by glial cells)

3. Postsynaptic membrane

-function:-unidirectional signal transmission: discrete, specific, directed towards

specific areas

-determine excitation or inhibition depending on neurotransmitters

and postsynaptic receptors

CHEMICAL SYNAPSE: microscopic aspects of vesicles with

neurotransmitters and postsynaptic receptors

Anterior motor neuron (anterior horn of the

spinal cord):

- Soma

- One single axon – extends into a peripheral

nerve

- Dendrites – projects up to 1 mm into the

surrounding areas of the cord

Physiologic Anatomy of the Synapse

-number of presynaptic terminals/neuron:

from a few up to 10,000-200,000 presynaptic terminals on the soma

of a motor neuron, from which ~80-95% lie on the dendrites

5–20% on the soma

Presynaptic terminals

(axon boutons, end-feet, or synaptic knobs)

-Presynaptic terminals contain transmitter vesicles and mitochondria

-AP at presynaptic level open voltage-gated Ca channels → [Ca2+]i

increases → transmitter release from the vesicles (exocytosis)→

chemical transmission

-vesicles with Ach store 2,000-10,000 molec./vesicle; there are

enough vesicles to transmit >10,000 APs.

-presynaptic terminals contain enzymes for synthesis of small-

molecule /nonpeptide transmitters from simple precursors;

-the neuropeptides are synthesized in the soma RER, packed in

vesicles → axonal transport→ presynaptic terminal.

Presynaptic terminals can have autoreceptors:

- receptors located presynaptically, sensitive only to the

neurotransmitters released by the neuron on which the

autoreceptor sits.

- assist a negative feedback loop in signal transduction

Chemical synapses:

Postsynaptic segment

The transmitter molecules once released bind and influence the membrane

potential of the postsynaptic membrane, determining excitation (EPSP) or

inhibition (IPSP)

The postsynaptic membrane has receptor proteins with 2 components:

• A binding component: sites for neurotransmitter

• An ionophore component that passes all the way through the

postsynaptic membrane to the interior of the postsynaptic neuron, that

can be one of 2 types:

1. an ion channel: excitatory (allows Na+ entry)

inhibitory (allows negative charge to enter: Cl-)

2. a second messenger activator – a molecule that protrudes into

the cell cytoplasm and activates one or more "second

messengers" to increase or decrease specific cellular functions.

Chemical synapse transmission - 7 steps

Step 1: Neurotransmitter molecules are packaged into membranous vesicles, and

the vesicles are concentrated and docked at the active zones / release sites of

the presynaptic terminal.

Step 2: The presynaptic membrane depolarizes, usually as the result of an AP

Step 3: The depolarization causes voltage-gated Ca2+ channels to open and

allows Ca2+ ions to flow into the terminal.

Step 4: The resulting increase in intracellular [Ca2+] triggers fusion of vesicles

with the presynaptic membrane.

The Ca2+ dependence of fusion may be conferred by synaptotagmin, a

neuron-specific protein component of the fusion apparatus. The fusion events are

incredibly fast; each individual exocytosis requires only a fraction of a msec. to be

completed.

Step 5: The transmitter is released into the extracellular space in quantized

amounts and diffuses passively across the synaptic cleft (20-30 nm thick).

Step 6: Some of the transmitter molecules bind to receptors in the

postsynaptic membrane, and the activated receptors trigger some

postsynaptic event, usually the opening of an ion channel (fast synapse) or the

activation of a G protein-coupled signal cascade (slow synapse).

Step 7: Transmitter molecules diffuse away from postsynaptic receptors and

are eventually cleared away by continued diffusion, enzymatic degradation,

or active uptake into cells.

In addition, the presynaptic machinery retrieves the membrane of the

exocytosed synaptic vesicle, perhaps by endocytosis from the cell surface.

Chemical synapse transmission occurs in 7 steps

Why are some

postsynaptic receptors

excitatory while others

are inhibitory?

Transmitter evokes

membrane depolarization

or hyperpolarization

→ Excitatory / inhibitory

transmitters

Why are some Postsynaptic Receptors excitatory while

others are inhibitory?

Excitation

• Opening of Na+ channels

Raises membrane potential closer to threshold

• Depressed conduction of Cl- or K+

Internal membrane potential becomes more positive

• Changes in the internal metabolism of the postsynaptic neuron

Excitatory Postsynaptic Potentials (EPSPs)

•Occur when:

• Na+ and/or Ca2+ channels open,

OR

• K+ and /or Cl- channels close

•Cation influx or reduced K+ efflux causes the membrane to

become depolarized

Why are some Postsynaptic Receptors

excitatory while others are inhibitory?

Inhibition

Open Cl- or K+ channels

• Membrane hyperpolarization

• Inhibit cellular metabolic functions …

Inhibitory Postsynaptic Potentials (IPSPs)

Occur when:

• Cl- channels open

OR

• K+ channels open

Cl- influx or K+ efflux ensues and the membrane becomes

hyperpolarized, i.e., membrane potential becomes lower than Vrest

→ more energy is required to achieve threshold.

Three states of a neuron. A, Resting neuron, with a normal intraneuronal potential of -65 mV.

B, Neuron in an excited state, with a less negative intraneuronal potential (-45 mV) caused by

sodium influx. C, Neuron in an inhibited state, with a more negative intraneuronal membrane

potential (-70 mV) caused by potassium ion efflux, chloride ion influx, or both.

Electrical Events During Neuronal Excitation

Postsynaptic mechanisms

There are several types of second messenger systems in the postsynaptic neuron.

One of the most prevailing types in neurons, G-protein (with , and components),

is attached to the portion of the receptor protein that protrudes to the interior of the

cell.

G-protein activation: a nerve impulse activates the activator portion of the G-prot

and separates it from the and portions, then moves free within the cell cytoplasm.

Postsynaptic mechanisms

G-proteins system

Inside the cytoplasm, the separated alpha component (G) performs one or

more of multiple functions, depending on the specific characteristic of each

type of neuron:

• Opening specific ion channels through the postsynaptic cell membrane.

These channels often stay open for a prolonged time.

• Activation of cyclic adenosine monophosphate (cAMP) or cyclic

guanosine monophosphate (cGMP) in the neuronal cell.

• Activation of one or more intracellular enzymes. In turn the enzymes can

cause any one of many specific chemical functions in the cell.

• Activation of gene transcription…

→ Prolonged and amplified response

Amplification. A, One neurotransmitter (NT) binds directly to a channel, thereby activating it.

B, One NT binds to a receptor that in turn activates 10 - 20 G-proteins. In this example, the β

subunits directly activate K+ channels. In addition, each activated α subunit activates an adenylyl

cyclase (AC) molecule, each of which produces many cAMP molecules that activate protein

kinase A (PKA). C, Each activated PKA can phosphorylate and thereby modulate many channels.

Control of transmitter activity in the synaptic cleft

In two ways:

• Reuptake by presynaptic cell

• Deactivation in the synaptic cleft

Reuptake

•Norepinephrine and glutamate are taken up into the presynaptic cell.

•Glutamate is taken through Na+-linked transport, both into neurons and

astrocytes.

Deactivation

• Acetylcholine is digested by cholinesterase

• Norepinephrine is deactivated by methylation by catechol-O methyl

transferase (COMT) and cleared into the blood circulation.

•Neuropeptides action is terminated by proteolysis or by diffusion away from

the postsynaptic membrane

Vesicle Re-uptake

Within seconds to minutes of exocytosis, the vesicle portion of the

membrane are endocytosed back to the inside of the presynaptic

terminal and pinches off to form a new vesicle.

The new vesicle still contains the appropriate transport proteins

required for synthesizing and for concentrating new transmitter

substance inside it.

– astrocytes do not fire action potentials, but are Ca2+-excitable!

– one astrocyte contacts 1000s of synapses !!!

– astrocytes ‘listen’ to neurons (all major receptors present on astrocytes)

– astrocytes release neurotransmitters (Glu, ATP, …)

– astrocytes modulate neuronal excitability and synaptic transmission

Glial presence at synaptic level

The “One-Neuron-One-Messenger” Dogma

It was previously believed that any given synaptic terminal releases one and

only one transmitter substance, which is characteristic of that kind of

synapse. This belief has been discredited.

Current Evidence:

•Multiple chemical messengers co-exist in single nerve cellsAs a rule, small transmitters are stored and released by separate sets of

neurons. The peptides, however, are usually stored and released from the same

neurons as one of the small transmitters (co-localization of neurotransmitters).

•Transmitter substances can be both excitatory and inhibitory, as the

postsynaptic response is determined by the nature of postsynaptic

receptor .

Small Molecule Co-localized Peptide(s)

Acetylcholine Enkephalin, Vasoactive intestinal polypeptide

Calcitonin gene-related peptide

Substance P, Somatostatin and enkephalin

Gonadotropin-releasing hormone

Neurotensin, Galanin

Dopamine Cholecystokinin, Enkephalin

Neurotensin

Epinephrine Enkephalin, Neuropeptide Y

Neurotensin, Substance P

GABA Cholecystokinin, Enkephalin

Somatostatin, Neuropeptide Y

Substance P, Vasoactive intestinal polypeptide

Glutamate Substance P

Glycine Neurotensin

Norepinephrine Enkephalin, Neuropeptide Y

Neurotensin, Somatostatin

Vasopressin

Serotonin Cholecystokinin, Enkephalin

Substance P and thyrotropin-releasing hormone

Thyrotropin-releasing hormone

Neuroactive peptides co-localize with small-molecule neurotransmitters

Fast Chemical Synapses

In fast chemical synapses:

-Neurotransmitter is synthesized in the presynaptic terminal

-Neurotransmitter molecules are small, eg, acetylcholine, GABA

-Storage is in small vesicles

-Vesicles are located near plasma membrane (active zones)

Slow Chemical Synapses

In slow chemical synapses:

•Transmitter molecules are large, e.g., peptides, amines

•Storage is in large, dense vesicles

•Vesicles are located further away from the terminal

•Release sites are to the side of the terminal

1

3

2

G

4

5

PO

4

Slow

G-protein signaling through

intracellular second messenger cascade

example- -adrenergic receptor

stimulates the heart by modulating

Ca2+ channels

Ca2+

NE

Intermediate

1

2

3G

2

K+

Membrane delimited signaling

through G protein

example- muscarinic AchR

inhibits the heart by activating

K+ channels

Ach

1

2

3

FastLigand-gated

ion channel

example- nicotinic AchR

stimulates by Na+ influx

Ach

Synapses

Na+

Time courses of synaptic events in the nervous system.

Different transmitter systems in the brain generate responses that vary widely in

how long they last in the postsynaptic cell, from a few milliseconds to hours and

days. Note that the time axis is logarithmic. (Data from Shepherd GM: Neurobiology,

3rd ed. New York: Oxford University Press, 1994.)

Divergence and convergence of transmitter

effects on ion channels.

A, One transmitter, norepinephrine in this case,

can activate multiple receptors, which stimulate

different G protein/second messengers, which in

turn either stimulate or depress the gating of

many types of ion channels.

IAHP stands for afterhyperpolarization current, which is

mediated by a Ca2+-activated K+ channel. Ih stands for

hyperpolarization-activated cation current.

B, Multiple transmitters bind to their specific

receptors and, by the same or different second-

messenger systems, influence the same set of

ion channels.

ACh, acetylcholine; DA, dopamine; Enk, enkephalin;

5-HT, 5-hydroxytryptamine (serotonin);

NE, norepinephrine; SS, somatostatin;

SSTR, somatostatin receptor.

Synaptic connections/networks.

The brain carries out many sensory, motor, and

cognitive functions that require fast, specific,

spatially organized neural connections and

operations (such as the detailed neural mapping

allowing you to read this sentence).

Require spatially focused networks.

Falling asleep, waking up, attention or changing mood

involve more general changes in the brain. Several

systems of neurons regulate the general excitability of

the CNS, each of these modulatory systems uses a

different neurotransmitter, and the axons of each

make widely dispersed, diffuse, almost meandering

synaptic connections to carry a simple message to

vast regions of the brain.

Require widely divergent network.

Some transmitters are used by diffusely distributed systems of neurons to

modulate the general excitability of the brain.

Modulatory systems use different neurotransmitters, in widely dispersed, diffuse,

almost meandering synaptic connections to carry a simple message to vast regions of

the brain (widely divergent networks).

The functions of the different systems are not well understood, but each appears to be

essential for certain aspects of arousal, motor control, memory, mood, motivation, and

metabolic state.

The brain has several modulatory systems with diffuse central connections:

1. Neurons of the diffuse systems arise from the central core of the brain, most of

them from the brainstem !!! (typically, several thousand)

2. Each neuron can influence many others because each one has an axon that may

contact more than 100,000 postsynaptic neurons spread widely across the brain.

3. The synapses made by some of these systems seem designed to release

transmitter molecules into the extracellular fluid so that they can diffuse to many

neurons rather than be confined to the vicinity of a single synaptic cleft – volume

transmission.

Synaptic connections/networks

The main modulatory systems of the brain are distinct anatomically and biochemically.

Separate systems use norepinephrine, serotonin (5-hydroxytryptamine [5-HT]), dopamine, ACh, or histamine as their neurotransmitter.

They all tend to involve numerous metabotropic transmitter receptors. Unlike ionotropic receptors, which are themselves channels, metabotropic receptors are coupled to enzymes such as adenylyl cyclase or phospholipase C through G proteinsFor example, the brain has 10 to 100 times more metabotropic (i.e., muscarinic) ACh receptors than ionotropic (i.e., nicotinic) ACh receptors.

Collectively, the diffuse modulatory systems may be viewed as general regulators of brain function, much like the autonomic nervous system regulates the organ systems of the body. Because their axons spread so widely within the CNS, the few modulatory neurons can have an inordinately strong influence on behavior.

Synaptic connections/networks.

The four diffusely

connected systems of

central neurons using

modulatory transmitters:

Ach, NE, Dopamine,

Serotonin

Chemical Substances That Function as Synaptic Transmitters:

Neurotransmitters and Neuromodulators

No chemical distinction between neurotransmitters and

neuromodulators.

A neurotransmitter stimulate the primary postsynaptic

response

•Is released during activity of presynaptic neuron

•Effects on postsynaptic cell and on the presynaptic cell

•Action at synapse is blocked by antagonists

Neuromodulators modify the primary synaptic response. May

act on neighboring neurons.

Small-molec., rapidly acting / neuropeptides, slowly acting

e.g., transmission of sensory signals to

the brain and of motor signals back to

the muscles.

long-term changes in number of neuronal

receptors, long-term opening/closure of certain

ion channels, possibly even long-term changes

in number or sizes of synapses.

CONVENTIONAL NEUROTRANSMITTERS (stored in synaptic vesicles [SV] and

released in quantal fashion by Ca2+ dependent exocytosis)

Small-Molecule, Rapidly Acting Transmitters

Acetylcholine

Amino Acids

Gamma-aminobutyric acid (GABA), Glycine, Glutamate, Aspartate

Purines:

ATP, Adenosine

Amines

Catecholamines: Norepinephrine, Epinephrine, Dopamine (all derived from tyrosine)

Serotonin

Histamine

Neuropeptide, Slowly Acting Transmitters or Growth Factors

UNCONVENTIONAL NEUROTRANSMITTERS (not stored in synaptic vesicles, lipid-

soluble)

Gaseous transmitters

Nitric oxide (NO), Carbon monoxide (CO)

Lipids – endogenous endocannabinods (2-arachidonoylglycerol)

Neurotransmitters: intercellular messenger molecule

Released at

classical synapses,

wiring transmission

Released both at classical synapses (wiring transmission), but also at

en passant synapses – SV varicosities along axons as they pass postsynaptic

cells (e.g. sympathetic neuron – smooth mm cell; volume transmission).

Hypothalamic-releasing hormones

Thyrotropin-releasing hormone

Luteinizing hormone-releasing hormone

Somatostatin (growth hormone inhibitory

factor)

Pituitary peptides

Adrenocorticotropic hormone (ACTH)

β-Endorphin

α-Melanocyte-stimulating hormone

Prolactin

Luteinizing hormone

Thyrotropin

Growth hormone

Vasopressin

Oxytocin

Peptides that act on gut and brain

Leucine enkephalin

Methionine enkephalin

Substance P

Gastrin

Cholecystokinin

Vasoactive intestinal polypeptide (VIP)

Nerve growth factor

Brain-derived neurotropic factor

Neurotensin

Insulin

Glucagon

From other tissues

Angiotensin II

Bradykinin

Sleep peptides

Calcitonin

Neuropeptide, Slowly Acting Transmitters

Acetylcholine (ACh)

-first neurotransmitter to be identified in 1914 by Henry Dale for its actions on

heart tissue; confirmed as a neurotransmitter by Otto Loewi who initially gave

it the name vagusstoff because it was released from the vagus nerve (1936

Nobel Prize in Physiology or Medicine).

-synthesized in the presynaptic terminal from acetyl coenzyme A and choline

in the presence of the enzyme choline acetyltransferase, and transported

into its specific vesicles.

-released into the synaptic cleft where is rapidly split to acetate and choline

by the enzyme cholinesterase from the proteoglycan reticulum.

-choline is actively transported back into the presynaptic terminal to be used

again for synthesis of new acetylcholine.

ACh - ester of acetic acid and choline: CH3COOCH2CH2N+(CH3)3

Acetylcholine secreting neurons:

(1) large pyramidal cells from the motor cortex,

(2) neurons in the basal ganglia,

(3) motor neurons innervating skeletal muscles,

(4) the preganglionic neurons of the autonomic nervous system,

(5) the postganglionic neurons of the parasympathetic nervous system

(6) some postganglionic neurons of the sympathetic nervous system.

Acetylcholine receptors:Ionotropic nicotinic receptors (neuromuscular junction)

Metabotropic muscarinic receptors (cardiac muscle)

Presynaptic Postsynaptic

Acetylcholine effects:In most instances ACh has an excitatory effect; however, it is known to

have inhibitory effects at some peripheral PS nerve endings (inhibition

of the heart by the vagus nerves).

Nicotinic

Muscarinic

EPSP

IPSPAch

Ach

Neurons containing ACh are located:

-in the basal forebrain complex, which includes the septal nuclei and nucleus basalis;

the neurons project to the hippocampus and the neocortex.

- in the pontomesencephalotegmental cholinergic complex and project to the dorsal

thalamus and part of the forebrain.

Ionotropic and metabotropic

ACh receptors.

A - nicotinic AChR: ligand-

gated channel on the

postsynaptic membrane. In a

skeletal muscle, the end result

is muscle contraction.

B - muscarinic AChR, which is

coupled to a G protein. In a

cardiac muscle, the end result

is decreased heart rate.

Norepinephrine

-Secreted by

→the terminals of many neurons whose cell bodies are located in

the hypothalamus and brain stem:

- locus ceruleus in the pons send nerve fibers to widespread areas

of the brain → control overall activity and mood (e.g., increasing the

level of wakefulness).

→most postganglionic neurons of the sympathetic nervous

system, where it excites some organs or inhibits others.

-Effects: mostly excitatory, but also inhibitory, depending on the type of

receptors:

Receptor Type Agonists* Antagonists

G

Protein Linked Enzyme

Second

Messenger

α2-Adrenergic NE ≥ Epi (clonidine) Yohimbine Gαi Adenylyl cylase ↓ [cAMP]i

β1-Adrenergic Epi > NE (dobutamine,

isoproterenol)

Metoprolol Gαs Adenylyl

cyclase

↑ [cAMP]i

β2-Adrenergic Epi > NE (terbutaline,

isoproterenol)

Butoxamine Gαs Adenylyl

cyclase

↑ [cAMP]i

β3-Adrenergic Epi > NE (isoproterenol) SR-59230A Gαs Adenylyl

cyclase

↑ [cAMP]i

Neurons containing NE are located in the locus coeruleus and innervate nearly

every part of the CNS

Dopamine

-secreted by neurons that originate in the substantia nigra.

-the termination of these neurons is mainly in the striatal region of the

basal ganglia.

-effects of dopamine: usually inhibition, but also excitation.

-dopamine receptors:

Receptor

Type Agonists* Antagonists G Protein Linked Enzyme

Second

Messenger

D1 Dopamine Gαs Adenylyl

cyclase

↑ [cAMP]i

D2 Dopamine Gαi Adenylyl

cyclase

↓ [cAMP]i

Neurons containing dopamine are located in the substantia nigra (and these

project to the striatum) and the ventral tegmental area of the midbrain (and

these project to the prefrontal cortex and parts of the limbic system).

Renshaw Cell and Glycine - Major inhibitory transmitter in the spinal cord

Spinal cord:

Ach

Ach

glycine

(-)

skeletal muscle:

Renshaw Interneuron

Glycine

-secreted mainly at synapses in the spinal cord.

-act as an inhibitory transmitter.

-also, influence NMDA receptor activity.

(+)

(+)

Serotonin (5-hydroxytryptamine, 5-HT)

-secreted by nuclei that originate in the median raphe of the brain

stem and project to many brain and spinal cord areas, especially to the

dorsal horns of the spinal cord and to the hypothalamus.

-acts as an inhibitor of pain pathways in the spinal cord, and an inhibitor

action in the higher regions of the nervous system;

-is believed to help control the mood of the person, perhaps even to

cause sleep.

-Serotonine receptors: G protein-coupled receptors and ligand-gated

ion channels (only 5-HT3) found in the central and peripheral nervous

system, mediating both excitatory and inhibitory neurotransmission.

Serotonine

Receptor FamilyType Mechanism Potential

5-HT1

Gi/Go-protein

coupled.↓ cAMP Inhibitory

5-HT2

Gq/G11-protein

coupled.↑ IP3 and DAG. Excitatory

5-HT3

Ligand-gated

Na+ and K+

cation channel.

Depolarizing plasma

membraneExcitatory

5-HT4

Gs-protein

coupled.↑ cAMP. Excitatory

5-HT5

Gi/Go-protein

coupled.↓ cAMP. Inhibitory

5-HT6

Gs-protein

coupled.↑ cAMP. Excitatory

5-HT7

Gs-protein

coupled.↑ cAMP. Excitatory

Neurons containing serotonin are located in two

groups of raphe nuclei and project to most of the brain.

Glutamate

-excitatory neurotransmitter

-secreted by the presynaptic terminals in many of the sensory pathways

entering the central nervous system, as well as in many areas of the cerebral

cortex.

-Glutamate receptors:

-ionotropic

NMDA rec. (N-methyl D-aspartate)

AMPA/quisqualat rec.

(AMPA = α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)

Kainic acid (KA) receptor (KA - natural marine acid in some seaweed)

-metabotropic mGluR I & II

-Involved in:

– Memory & learning

– Synapse formation

– Pathological states (with excess glutamate): epilepsy, Parkinson’s and

Alzheimer's disease, stroke, trauma and hypoxia

Glutamatergic synapse

Ionotropic glutamate receptors

-AMPA receptors normally allow Na+ to enter the cell

-NMDA receptors allow Na+ and Ca2+ inflow to enter the cell

-AMPA receptor activation normally precedes the NMDA receptor activation and

is involved in detaching of Mg2+ from the pore of the NMDA receptor channel,

allowing the NMDA receptor to open and Ca2+ to enter the cell

A, At most glutamate-mediated synapses,

the EPSP (red curve) is the sum of two

components (1) a rapid component that is

mediated by an AMPA receptor channel

(green) and (2) a slow component that is

mediated by an NMDA receptor channel

(orange); here postsynaptic Vm is

relatively negative, the contribution of

the NMDA rec. channel is very small.

B, At a relatively negative initial Vm in

the postsynaptic cell, as in A, the NMDA

receptor channel does not open. The

AMPA receptor channel, which is indep.

of postsynaptic Vm, opens. The result is a

fast depolarization. C, In this example, in

which the postsynaptic Vm is relatively

positive, the contribution of the NMDA

receptor channel is fairly large. D, At a

relatively positive initial Vm in the

postsynaptic cell, as in C, glutamate

activates both the AMPA and the NMDA

receptor channels. The recruitment of the

NMDA receptor channels is important

because unlike most AMPA receptor

channels, they allow the entry of Ca2+

and have slower kinetics.

Cooperation of glutamate-gated channels: AMPA and NMDA receptors

GABA (gamma-aminobutyric acid)

-most abundant endogenous inhibitory neurotransmitter

-secreted by nerve terminals in the spinal cord, cerebellum, basal

ganglia, hippocampus and many areas of the cortex.

-GABA receptors: found in most of the CNS neurons (60-80%)

GABA A – ionotropic ligand gated receptors (open Cl- channels);

has binding sites for benzodiazepines (diazepam), barbiturates

GABA B – metabotropic G-protein coupled receptors (open K+

channels or suppress Ca2+ channels).

-GABA agonists: selective for GABA A Rec - Muscimol, isoguvacine

selective for GABA B Rec - L-baclofen

-GABA antagonist: Bicucculine, gabazine

- in mature neurons GABA facilitates hyperpolarization of the cell by

gating chloride ions in the interior of the nerve cell (!)

-prevent firing of presynaptic neurotransmitters (presynaptic inhibition)

-depress CNS activity, keep the excitatory-inhibitory balance.

GABAA receptor channel

- GABA binding site

- several other sites on the outside face of the receptor, where modulator chemicals

can bind: benzodiazepines (diazepam [Valium]) and barbiturates (phenobarbital)

Alone, these drugs do very little to the channel's activity, but in the presence of

GABA, benzodiazepines increase the frequency of channel opening and

increase Cl- conductance, whereas barbiturates increase the duration of

channel opening.

→more inhibitory Cl- current, stronger IPSPs, and the behavioral

consequences of enhanced inhibition.

N.B. during development GABA A rec. is excitatory, due to increased intracellular Cl-

concentration → Cl- efflux → EPSP

? what endogenous ligands/natural chemicals exist for benzodiazepines/barbiturates

- various metabolites of the steroid hormones progesterone, corticosterone, and

testosterone, increase the lifetime or opening frequency of GABA-activated single-

channel currents, enhance inhibition, and therefore are potential natural modulators of

the GABAA receptor

- steroid hormones modulate GABAA receptor through distinct binding sites…

The GABAA receptor is also subject to modulation by the effects of phosphorylation

triggered by second-messenger signaling pathways within neurons.

GABAA receptor channel.A, a pulse of GABA released elicits a

small IPSP.

B, In the presence of a low dose of

pentobarbital, the pulse of synaptic

GABA elicits a much larger IPSP →

barbiturate enhances inhibition.

C, At the single-channel level, GABA by

itself elicits brief channel openings.

D, Barbiturate (here 50 μM pentobarbital)

does not by itself activate the GABAA

receptor channel but increases the

channel open time when GABA is

present.

E, The channel receptor is a hetero-

pentamer. It not only has a pore for Cl- but

also has separate binding sites for GABA

and several classes of channel

modulators.

The inset shows the presumed structure of one

of the 5 monomers. The M2 do-main of each of

the 5 subunits presumably lines the central

channel pore.

-short-lived gas, that diffuses a short distance to neighboring cells, where its effects

are primarily mediated by the activation of guanylyl cyclase (GC).

-first identified as endothelium-derived relaxation factor (EDRF)

-a nonconventional transmitter system that may be involved in behavior and

memory functions

- different from other small-molecule transmitters in its mechanism of formation in the

presynaptic terminal and in its actions on the postsynaptic neuron:

-is not preformed and stored in vesicles in the presynaptic terminal as are other

transmitters.

-is synthesized almost instantly as needed, and it then diffuses out of the

presynaptic terminals over a period of seconds

-diffuses into postsynaptic neurons nearby, where does not greatly alter the

membrane potential but instead changes intracellular metabolic functions that

modify neuronal excitability for seconds, minutes, or perhaps even longer.

-abnormalities of the NO system are involved in the pathophysiological processes of

adult respiratory distress syndrome, high-altitude pulmonary edema, stroke, and

other diseases →introduction of clinical treatments that modulate the NO system

(use of gaseous NO for treatment of pulmonary edema, NO generators such as

nitroglycerin for treatment of angina, and cGMP phosphodiesterase inhibitors such

as sildenafil (Viagra) for treatment of erectile dysfunction).

Nitric oxide

Neuropeptides

• widespread in both CNS and PNS

• excitatory and inhibitory

• act as hormones elsewhere in the body

-Substance P -- enhances our perception of pain

-Opioid peptides:

endorphins - released during stress, exercise

-breaks down bradykinins (pain chemicals), boosts

the immune system and slows the growth of cancer cells

-binds to mu-opioid receptors

-released by the hypothalamic neurons and by the cells

of the pituitary

enkephalins - analgesics

-breaks down bradykinins (200x stronger than morphine)

-pain-relieving effect by blocking the release of substance P

dynorphins - regulates pain and emotions

PRESYNAPTIC INHIBITION

•inhibitory fiber (usually GABAergic) connects with presynaptic excitatory

knob → presynaptic hyperpolarization →reduced transmitter release

→reduced postsynaptic potential (PSP).

• imparts the property of high selectivity - it affects only signals arriving at

that particular synapse.

• works by subtracting from all excitatory PSPs that arrive at the neuron →

nonselective inhibition.

• increases both the specificity and the complexity of the integration

that takes place at the neuronal level.

• Presynaptic inhibition occurs in many of the sensory pathways in the

nervous system

! Adjacent sensory nerve fibers often mutually inhibit one another, which

minimizes sideways spread and mixing of signals in sensory tracts.

Excitatory postsynaptic potentials (EPSP): simultaneous firing of only a few

synapses will not cause sufficient summated potential to elicit an action potential,

but that simultaneous firing of many synapses will raise the summated potential to

threshold for excitation and cause a superimposed action potential.

SUMMATION

Transmitter substance released by a single presynaptic

terminal cause an EPSP usually no greater than 0.5 to 1 mV,

instead of the 10-20 mV normally required to reach threshold

for excitation, but many presynaptic terminals are usually

stimulated at the same time → summation

Postsynaptic potentials (PSP) originating from the same or

neighboring synapses can summate.

Depending on the synapse(s) of origin, there are spatial or

temporal summation.

Spatial Summation (in space)

• Postsynaptic potentials (PSPs)

originate from different synapses

• PSPs spread easily because of the low

resistance of soma membrane

• If an EPSP, the membrane potential is

elevated in neighboring area. Facilitation

occurs. Thus a smaller voltage change is

required to reach threshold in the

neighboring area.

• If an IPSP, the reverse occurs.

• EPSPs can summate with IPSPs and

diminish the size of the summated

potential.

The effect of summing simultaneous

postsynaptic potentials by activating

multiple terminals on widely spaced

areas of the neuronal membrane is

called spatial summation.

Temporal Summation (in time)

Postsynaptic potentials decay slowly.

Because of the slow decay of a prior

PSP, a second PSP may arrive at the

same synapse before the initial one

has decayed.

This allows PSPs to summate.

Successive discharges from a

single presynaptic terminal

occurring rapidly enough, can add to

one another = “temporal summation“.

Temporal Summation (in time)

Summation is dependent on impulse

frequency, i.e., is frequency modulated.

The higher the frequency, the greater

the summation and the greater the

potential.

EPSPs combine to cause an AP.

IPSPs combine to cause greater

hyperpolarization, making an AP less

likely.

The effects of spatial and temporal

summation are additive.

Spatial versus temporal summation of excitatory postsynaptic potentials (EPSPs).

Some Special Characteristics of Synaptic Transmission

Fatigue of Synaptic Transmission: When excitatory synapses are repetitively stimulated at a rapid rate, the number of

discharges by the postsynaptic neuron is at first very great, but the firing rate

becomes progressively less in succeeding millisec. or seconds.

Fatigue is an exceedingly important characteristic of synaptic function because

when areas of the nervous system become overexcited, fatigue causes them to

lose this excess excitability after a while.

Fatigue is probably the most important means by which the excess excitability of

the brain during an epileptic seizure is finally subdued so that the seizure ceases -

the development of fatigue is a protective mechanism against excess

neuronal activity.

The mechanism of fatigue - exhaustion or partial exhaustion of the stores of transmitter substance in the

presynaptic terminals: excitatory terminals store enough excitatory transmitter to

cause about 10,000 APs in only a few seconds to a few minutes of rapid

stimulation

- progressive inactivation of many of the postsynaptic membrane receptors

- slow development of abnormal concentrations of ions inside the postsynaptic

neuronal cell.

Some Special Characteristics of Synaptic Transmission

Effect of Acidosis or Alkalosis on Synaptic Transmission.

-most neurons are highly responsive to changes in pH of the surrounding

interstitial fluids.

-normally, alkalosis greatly increases neuronal excitability: a rise in

arterial blood pH from the 7.4 norm to 7.8 to 8.0 often causes cerebral

epileptic seizures because of increased excitability of some or all of the

cerebral neurons - asking a person who is predisposed to epileptic

seizures to overbreathe→ blows off carbon dioxide and therefore

elevates the pH of the blood momentarily, but even this short time can

often precipitate an epileptic attack.

- acidosis greatly depresses neuronal activity; a fall in pH from 7.4 to

below 7.0 usually causes a comatose state. For instance, in very severe

diabetic or uremic acidosis, coma virtually always develops.

Some Special Characteristics of Synaptic Transmission

Effect of Hypoxia on Synaptic Transmission.

Neuronal excitability - highly dependent on an adequate supply of oxygen. Cessation

of oxygen for only a few seconds can cause complete inexcitability of some neurons

(when the brain’s blood flow is temporarily interrupted → within 3-7 sec. the person

becomes unconscious).

Effect of Drugs on Synaptic Transmission.

-caffeine, theophylline, and theobromine, which are found in coffee, tea, and

cocoa, respectively, all increase neuronal excitability, presumably by reducing the

threshold for excitation of neurons.

-strychnine is one of the best known of all agents that increase excitability of

neurons, by inhibiting the action of some normally inhibitory transmitter substances,

especially the inhibitory effect of glycine in the spinal cord → the effects of the

excitatory transmitters become overwhelming, and the neurons become so excited

that they go into rapidly repetitive discharge, resulting in severe tonic muscle

spasms.

-most anesthetics increase the neuronal membrane threshold for excitation and

thereby decrease synaptic transmission at many points in the nervous system.

Because many of the anesthetics are especially lipid soluble, it has been reasoned

that some of them might change the physical characteristics of the neuronal

membranes, making them less responsive to excitatory agents.

Synaptic Delay

(1) discharge of the transmitter substance by the presynaptic terminal,

(2) diffusion of the transmitter to the postsynaptic neuronal membrane,

(3) action of the transmitter on the membrane receptor,

(4) action of the receptor to increase the membrane permeability, and

(5) inward diffusion of sodium to raise the excitatory postsynaptic

potential to a high enough level to elicit an action potential.

The minimal period of time required for all these events to take

place, even when large numbers of excitatory synapses are

stimulated simultaneously, is about 0.5 millisec. = synaptic delay.

By measuring the minimal delay time between an input cascade of impulses

into a pool of neurons and the consequent output, one can estimate the

number of series neurons in the circuit.

Boron 3rd Edition – Synaptic transmission

• Chapter 8 (p. 204-227)

• Chapter 13 (p. 307-328)