Ocular Biochem: Ocular Neurochemistry

Neurotransmission

an efficient and rapid mechanism for sending messages to and from the brain

What are the functions of neurotransmission in the eye?

-tear secretion

-miosis

-generation of IOP

-act of sight

What are the functions of neurotransmission outside of the eye?

-movement of eyes

-blinking and lid closure

What does the transmission of nervous signal through the body involve?

-transport of cations through membranes (creating an action potential in nerve)

-diffusion of neurotransmitters from one neuron to next neuron or to a muscle cell

Action Potential

-results from rapid sequential flow of Na into the cell and K out of the cell

-a cell normally has a high internal concentration of K (140 mM vs 5 mM) and a relatively high external concentration of Na (145 mM vs 10 mM)

-combination of these concentration differences creates a potential difference of 60 mV across cell membrane with the inside of the cell being negative

When does the action potential begin?

when separate gate proteins for sodium and potassium open in sequence to allow the cations to flow through the membrane

What is the potential difference established and maintained by?

constant pumping activity of Na, K ATPase and the very small leakage of both ions across the membrane

What occurs at time = 0 in the action potential?

a wave of depolarization reaches the membrane from adjacent section of nerve OR by some transduction mechanism

What activates depolarization?

-begun by small, local cation changes on both sides of the membrane

-these changes chase the initial opening of sodium channel proteins

-each opening triggers the opening of more channels

-each open channel allows 4000 Na ions to enter the cell per msec

What occurs for each mm of membrane when peak of depolarization is maximal?

-there is an influx of some 32,000 Na+ ions for the 200 msec period

-membrane potential reaches between +10 and +22 mV

-this phase represents the action potential

-opening as well as closing of the channel proteins is sensitive to the local concentration of adjacent cations

What occurs during action potential as the sodium channel proteins open?

-K+ ion channel proteins also begin to open at a significantly slower rate

-K+ ions begin to leave the cell

-this action of K+ ions then causes the Na+ ion channel proteins to close

-as the closing of Na+ ion channel proteins continues, the voltage across the membranes decreases to a value below the resting potential (at time = 0)

What occurs during action potential when all the Na ion channel proteins have closed?

-the K+ ion channel proteins also begin to close

-aka refractory period

Refractory Period

-lasts about 3 msec

-during this phase, the potential difference is lower than normal

-Na+ ion channel proteins cannot be reopened by a succeeding wave of depolarization

-limits the rate of succeeding pulses of action potentials

What is the rate of propagation of action potential?

action potentials can travel along nerves at rates between 1-100 m/sec

What are the higher rates of travel of action potentials attributed by?

-attributed to layers of myelin lipid along a nerve axon

-myelin decreases the capacitance of an axon membrane

What are the abilities of myelinated nerves?

-high potential

-very little ability to leak cations to either side of the membrane

What do areas of myelination contain?

-have regular interruptions (nodes of Ranvier) of nonmyelination

-the nerve membrane concentrates its channel proteins at these nodes

What is the process of propagation of action potentials?

-waves of depolarization leap from one node to another

-occurs by causing local flows of cations (sod and pot) on both the inside and outside of the nerve membranes

Where does depolarization take place?

at the nodes

How does myelin impart extra insulation in the nerve?

myelin is wrapped around the nerve many times to impart extra insulation

Ion Channel Proteins

-large proteins that transport ions

-their ability to transport ions is dependent upon the relative ion concentration in their vicinity of the nerve plasma membrane

What is gating (opening/closing) of the ion channel proteins controlled by?

the cations themselves

Sodium Channel Protein

-consists of 3 glycoprotein chains (α-260kD, β1=36kD and β2-33kD)

-sodium ions enter neurons through a channel (pore) in the α-chain

-one of the transmembrane segments of the α-chain (S-4) is voltage sensitive (cation sensitive) region that opens the pore

What are the steps of the release of a neurotransmitter?

-when an action potential reaches the terminal region of the axon where synaptic cleft is located, it opens a channel protein specific for calcium ions than sodium ions

-this channel is also sensitive to local cation concentration

-the inflow of calcium ions brings about the fusion of vesicles in the cytoplasm with the presynaptic membrane

-fusion of vesicles leads to release of neurotransmitters

What happens to the neurotransmitter once it is released?

-neurotransmitters diffuse across the narrow space of the synaptic cleft (20 nm or 200 A) and bind briefly to receptors on the post synaptic membrane

-activation through binding of the receptors cause a postsynaptic response (both biochemical and physiological)

-the response may consist of a depolarization or a hyperpolarization of postsynaptic neurons or a contraction of muscle tissue

What are the four chemical classes of neurotransmitters?

-acetylcholine

-catecholamines

-amino acids

-amino acid derivatives

Where is acetylcholine located?

-PNS

-ANS

-fast nicotinic response

-slow muscarinic response

Where are catecholamines located?

-norepinephrine: CNS, ANS, retina

-dopamine: CNS, retina

Where are amino acids located?

-glutamic acid: CNS, retina

-glycine: spinal cord, retina

Where are amino acid derivatives located?

-CNS

-retina

Neuromodulator

-a substance with neurotransmitter properties

-molecules that alter or fine tune the responses of neurotransmitters at the same synaptic cleft

Where is acetylcholine synthesized?

in the terminal axon bulb

What is acetylcholine synthesized from?

acetyl-CoA and choline by the enzyme choline acetyltransferase

What is acetylcholine inactivated by?

following its release from vesicle to synaptic cleft and bound to its postsynaptic receptor protein, it is inactivated by hydrolysis

Enzyme Acetylcholinesterase

-located in the synaptic cleft

-catalyzes hydrolysis of acetylcholine

What is the inactivation of acetylcholine necessary for?

to prevent excessive stimulation of the receptor proteins

Acetylcholinesterase

a very fast catalyst with a turnover number of 25,000 molecules per second

What are the inhibitors of acetylcholinesterase?

-can be inhibited by agents such as physostigmine

-this agent produces a temporary increase of acetylcholine available to stimulate receptor proteins

-physostigmine has been useful in lowering the IOP of glaucoma patients

What do receptor proteins for acetylcholine consist of?

-nicotinic receptor protein

-muscarinic receptor protein

Nicotinic Receptor Protein for Acetylcholine

-channel protein, MW 282 kD

-similar to sodium channel protein in size and function

-it is not opened by a change in cation concentration, but by the binding of a neurotransmitter

What are nicotinic receptor proteins composed of?

-5 glycoprotein subunits that together form a pore for cation transport

-nicotine upon binding to its receptor can stimulate the opening of its channel

-tubocurarine binds to and blocks the receptor and is an antagonist

Agonists

substances that imitate neurotransmitters

What are the synaptic junctions that use nicotinic receptor proteins like?

-considered fast (1 msec)

-located between nerve axons and muscles in the peripheral nervous system, or between ganglionic synapses of the autonomic nervous system

-the latter involves parasympathetic and sympathetic activity of the iris and ciliary body muscles in the eye

-pupil size and lens focus (accommodation) are controlled by the use of these receptor proteins

Muscarinic Receptor Proteins for Acetylcholine

considered slow receptors (range of sec to min) since the receptor is linked to a G protein

What can activation of the G protein linked to muscarinic receptor proteins cause?

-stimulation of potassium channel proteins

-inhibition of adenylate cyclase

-stimulation of phospholipase C

-modulation of activity of phospholipase A2

-the end effect of the enzyme activity changes is to increase or decrease K ion transport, decrease Ca ion transport, and increase general cation transport in the postsynaptic cell

-these events occur over a period of approx. 100 msec, but may be much longer

-resembles G protein receptors

-a single polypeptide passes through the plasma membrane seven times in the form of an α-helix

Where do muscarinic receptor proteins occur?

-occur in all the effector cells stimulated by parasympathetic nervous system

-only pupillary and ciliary constriction are affected to decrease light intake and focus on near objects

What is norepinephrine derived from?

amino acid tyrosine

What is needed for norepinephrine synthesis?

3 enzymes are needed for its synthesis along with molecular oxygen, 3 vitamins and copper ions

Where does complete synthesis of norepinephrine take place?

in the presynaptic region of the neuron that releases norepinephrine

What is the synthetic pathway of norepinephrine synthesis divided between?

divided between cytoplasm and the vesicles that store norepinephrine

When is norepinephrine conserved?

after binding to its receptor

What does the conservation of norepinephrine involve?

-an active transport process constituting a reuptake mechanism

-this process involves a transport protein

-Na, K+ ATPase moves the neurotransmitter back into the presynaptic cytoplasm where it is taken up again into presynpatic vesicles

-a smaller amount of norepinephrine is lost from the synaptic cleft and broken down enzymatically

What are the four receptor proteins that exist for norepinephrine?

-α1

-α2

-β1

-β2

Why are there so many norepinephrine receptors?

-their anatomical location

-option for stimulation or inhibition

-need for a variety of sensitivities to neurotransmitter binding

Where are all of the receptor proteins for norepinephrine found?

on the postsynaptic membrane, except for α2 which occurs on the presynaptic membrane

What do the receptor proteins for norepinephrine contain?

-have 7 α-helices that pass through the membrane

-have a binding site in the plasma membrane portion of the protein

How do receptor proteins for norepinephrine function?

-by activating a G protein and similar to the structure of rhodopsin

-both norepinephrine and epinephrine use the same class of receptor proteins in both a hormonal and neurotransmission mode

-norepinephrine is mainly used as neurotransmitter whereas epinephrine is more a hormone

What is the function of the G protein associated with the norepinephrine receptor?

-stimulates adenyl cyclase (with β1 and β2 receptors)

OR

-inhibits it (with α2 receptors)

What do α1 receptors cause?

stimulation of muscle response using calcium ions as a second messenger instead of cyclic nucleotides

What are responses produced by catecholamines like?

are slow and similar in response produced by muscarinic receptors for acetylcholine

α2 Receptors

-located in the presynaptic membrane

-feedback function to alter neurotransmitter release

How does the autonomic nervous system make extensive use of acetylcholine or norepinephrine in the anterior segment of the eye?

-alter pupil diameter

-accommodation

-modulation of IOP

-production of tears

What is the process of transmission of light signal in the retina?

-photoreceptors send electrochemical signals to the brain by both direct (cone) and indirect (cone and rod) synaptic mechanisms

-horizontal, amacrine, and interplexiform cells modulate the signal transfer

-retinal cell-cell synapse has a complex structure similar to synaptic structures found in hair cells of the ear

What are the three important features of the cone triad synapse?

-several postsynpatic nerve processes share the synapse

-glutamate serves as neurotransmitter of postsynaptic receptors of all processes

-vesicle fusion into the presynaptic membrane is enhanced by a synaptic ribbon

What is the process of cone triad synapse?

-several neurons input from a single cone photoreceptor

-the pedicle of a cone photoreceptor contains many triad synapses such that photoreceptor may serve several bipolar cells

-it communicates with a number of adjacent photoreceptors by horizontal processes

What is the structure of the cone triad synapse?

-3 nerve synapses are present in the synaptic cleft

-there are two horizontal cell processes and one bipolar cell process

-constant release of neurotransmitter is facilitated by a synaptic ribbon apparatus

Ribeye Protein

-a synaptic ribbon protein

-forms an essential part of the ribbon structure that binds to the synaptic vesicles that hold the neurotransmitter

-transports the vesicles to the synaptic membrane at a rapid rate in order to facilitate their release

What are ribeye proteins composed of?

-4 domains with a MW of ~120kD

-2 identical A domains: essential for the formation and stabilization of ribbon structure

-2 identical B domains: responsible for binding to the presynaptic vesicles

Where does vesicle fusion with the membrane take place?

in an active zone at the bottom of the ribbon adjacent to the cleft membrane

What is the process for neurotransmitter release by the ribbon apparatus?

-calcium channels facilitate the process of vesicle fusion and neurotransmitter release

-these channels are not the typical channels found in conventional synapses

-these channels induce a partial decrease in the release of glutamate with light by slowing the ribbon fusion device

On Center Mechanism

-pathway proceeds from cone via synapse 1 to bipolar cell 1a and on to the ganglion cell by way of synapse 2

-the neurotransmitter of synapse 1 is glutamate

-in the dark, both photoreceptor types continuously release their neurotransmitters to receptors located on bipolar cells

-this is due to the continuous flow of sodium ions into photoreceptor outer segments (dark current)

What occurs when sodium ion flow is interrupted by light transduction in the on center mechanism?

-causes hyperpolarization (build up of net negative charge) of the photoreceptor

-this leads to a decrease in release of glutamate by opening the Ca2+ ion channels

-resulting decrease in glutamate release causes the cation channel receptor proteins for sodium to open indirectly in the bipolar postsynaptic membrane via a cGMP channel protein

-this allows the release of bipolar cell neurotransmitters to ganglion cell receptors at synapse 2

-neurotransmitter in all bipolar cells is also glutamate and is not released unless stimulated

Off Center Mechanism

-the second direct route that occurs when light is turned off or suddenly decreased

-proceeds by synapse 1 to bipolar cell 1b

What is the process of the off center mechanism?

-when light is turned down or off, glutamate is released from the cone presynapse

-this opens sodium channels directly in the postsynaptic membrane that in turn depolarizes the bipolar cell

-this causes the release of glutamate tonically in its synapse with its ganglion cell

-this action activates the appropriate ganglion cell

Indirect Deactivation Mechanism

-involves partial inhibition of activated ganglion cells that are controlled by surround physiology

-it includes light signals received by neighboring cone photoreceptors

What is the purpose of the indirect deactivation mechanism?

-operation of VA

-ability to distinguish borders or edges of a visualized object

What are the two different situations in the indirect deactivation mechanism?

-when greater amount of light is incident on the center cone photoreceptor

-when greater amount of light is incident on the surround cone photoreceptor

What is the receptor mechanism like for cone, center, on synaptic pathway of the retina?

-photoreceptor --> bipolar (less (Glu): opening of Na+ channels via cGMP

-bipolar --> ganglion (Glu): depolarization; unknown mechanism

What is the receptor mechanism like for cone, center, off synaptic pathway of the retina?

-photoreceptor --> bipolar (Glu): closing of Na+ channels via PDE

-bipolar --> ganglion (Glu): depolarization; unknown mechanism

What is the receptor mechanism like for cone, surround, on synaptic pathway of the retina?

-photoreceptor --> horizontal (less Glu): opening of Na+ channel proteins

-horizontal --> photoreceptor (gamma-ABA): maintenance of Glu release

-photoreceptor --> bipolar (no Glu): opening of Na+ channel proteins

-bipolar --> ganglion (Glu): depolarization; unknown mechanism

What is the receptor mechanism like for rod, low light synaptic pathway of the retina?

-photoreceptor --> rod bipolar (less Glu): closing of Na+ channel proteins

-rod bipolar --> amacrine (Glu): opening of Na+ channel proteins

-amacrine --> cone bipolar (indoleamine?): opening of Na+ channel proteins

-cone bipolar --> ganglion (Glu): depolarization; unknown mechanism

What is the main function of rod?

to detect low levels of light

What are rods indirectly connected to?

ganglion cells by way of amacrine cell synapses

What does the indirect pathway of rod synapsing involve?

-rod photoreceptor --> rod bipolar cell (synapse 1), glutamate is neurotransmitter

-rod bipolar cell --> amacrine cell (synapse 2), glutamate is neurotransmitter

-amacrine cell --> cone bipolar cell (synapse 3), indoleamine as neurotransmitter

-cone bipolar cell --> ganglion cell (synapse 4), glutamate is neurotransmitter

What conditions will affect neurotransmission in the eye?

-retinitis pigmentosa

-Horner's syndrome: involves nerve lesions outside of the eye

-Parkinson's disease: known to affect visual system at the level of brain and retina

How does Parkinson's Disease affect neurotransmission in the eye?

-a correlation has been found with retinal levels of dopamine and visual disturbances in Parkinson's patients

-lose contrast sensitivity ability and their ability to read

-normal average retinal dopamine concentration, 1ng dopamine/mg of protein

-Parkinson's: 0.52 ng/dopamine/mg of protein

What is the influence of Parkinson's disease on the eye?

-some amacrine cells that help in lateral transfer of signals across the retina use dopamine

-interplexiform cell that has a similar function as amacrine cell also utilize dopamine

-dopamine receptors (D1 and D2) are widespread throughout the retinal cells

-dopamine may diffuse and cause effects beyond the synaptic cleft