MESTRADO INTEGRADO EM CIÊNCIAS FARMACÊUTICASBIOQUÍMICA II 2008-09

AULA 22Cecília M. P. Rodrigues

SumárioParte III: Organização e Funcionamento Subcelular Bases bioquímicas do funcionamento da célula nervosaSinapses e transmissão do impulsoNeurotransmissores e receptores de neurotransmissoresReciclagem de vesículas sinápticasTransdução sensorialAprendizagem e memória

Synapses and impulse transmission

• Synapses are the junctions where neurons pass signals to target cells, which may be other neurons, muscle cells, or gland cells;

• In most nerve-to-nerve signaling and all known nerve-to-muscle and nerve-to-gland signaling, the neuron releases chemical neurotransmitters at the chemical synapse that act on the target cell;

• Much rarer, but simpler in function, are electric synapses in which the action potential is transmitted directly and very rapidly from the presynaptic to the postsynaptic cell.

• Signals are transmitted across an electric synapse within a few microseconds because ions flow directly from the presynaptic cell to the postsynaptic cell through gap junctions.

• Signal transmission across a chemical synapse is delayed about 0.5 ms — the time required for secretion and diffusion of neurotransmitter and the response of the postsynaptic cell to it.

Transmission of action potentials across electric and chemical synapses

Synapses and impulse transmission

Lodish, Molecular Cell Biology

Chemical synapses

• Chemical synapses can be fast or slow, excitatory or inhibitory, and can exhibit signal amplification and computation (integrated function of all incoming signals).

• In excitatory and inhibitory synapses, the action of a neurotransmitter tends to promote or inhibit the generation of an action potential in the postsynaptic cell, by binding of the neurotransmitter to an excitatory or inhibitory receptor, respectively.

• Excitatory: acetylcholine for nicotinic receptor, glutamate for NMDA and non-NMDA receptors, serotonin for 5HT3 receptors

• Inhibitory: GABBA for A-class receptors, glycine

• In fast synapses, binding of the neurotransmitter causes an immediate conformational change in neurotransmitter receptors, which are ligand-gated ion channels.

• In slow synapses, the neurotransmitter receptors are coupled to G proteins.

• The same neurotransmitter binds many types of receptors.

• Except for acetylcholine, the neurotransmitters shown here are amino acids (glycine and glutamate) or derived from the indicated amino acids (tyrosine, tryptophan, histidine).

• The 3 transmitters synthesized from tyrosine, which contain the catechol moiety (blue highlight), are referred to as catecholamines.

Structures of several small molecules that function as neurotransmitters

Lodish, Molecular Cell Biology

(a) Acetylcholine (or nicotine) in frog skeletal muscle produces a rapid postsynaptic depolarization of about 10 mV, for 20 ms.

The nicotinic acetylcholine receptors are ligand-gated cation channels; binding of acetylcholine opens the channel, admitting both Na+ and K+.

(b) Acetylcholine (or muscarine) in frog heart muscle produces, after a lag period of about 40 ms (not visible in graph), a hyperpolarization of 2-3 mV, for several seconds.

These cells contain muscarinic acetylcholine receptors, which are coupled via a G protein to K+ channels. Activation of the receptor leads to channel opening.

Excitatory and inhibitory responses in postsynaptic cells stimulated by acetylcholine

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1. Vesicles import neurotransmitters (red circles) from the cytosol using a H+/neurotransmitter antiporter.

The low intravesicular pH, generated by a V-type ATPase in the vesicle membrane, powers neurotransmitter import.

1. The vesicles then move to the active zone near the plasma membrane.

2. Vesicles “dock” at defined membrane sites by interacting with specific proteins.

Release of neurotransmittersRecycling of synaptic vesicles

Lodish, Molecular Cell Biology

4. A rise in cytosolic Ca2+ triggers fusion of the docked vesicles and release of neurotransmitters into the synaptic cleft.

5. Synaptic-vesicle membrane proteins are then specifically recovered by endocytosis, usually in clathrin-coated vesicles. The clathrin coat is depolymerized, yielding vesicles that are the same size as synaptic vesicles.

These new synaptic vesicles then are filled with neurotransmitters (step 1), completing the cycle, which typically takes about 60 sec.

Release of neurotransmitters and the recycling of synaptic vesicles

Removal of the neurotransmitter from the synapse is essential to ensure its repeated functioning

• Pentameric receptor (the β subunit is not shown).

• The M2 α helix (red) in each subunit is part of the lining of the ion channel. Aspartate and glutamate side chains at both ends of each M2 helix form two rings of negative charges that help exclude anions from and attract cations to the channel.

• The gate, which is opened by binding of acetylcholine, lies within the pore.

3D-structure of the nicotinic acetylcholine receptor

Lodish, Molecular Cell Biology

1. Arrival of an action potential at the terminus of a presynaptic motor neuron induces opening of voltage-gated Ca2+ channels and subsequent release of acetylcholine.

2. Acetylcholine triggers opening of the ligand-gated nicotinic receptors in the muscle plasma membrane.

3. The resulting influx of Na+ produces a localized depolarization of the membrane, leading to opening of voltage-gated Na+ channels and generation of an action potential.

4. Spreading depolarization triggers opening of voltage-gated Ca2+-release channels and release of Ca2+ from the sarcoplasmic reticulum into the cytosol.

The rise in cytosolic Ca2+ causes muscle contraction.

Acetylcholine-induced opening of gated ion channelsNeuromuscular junction

Lodish, Molecular Cell Biology

• Binding of acetylcholine by muscarinic acetylcholine receptors triggers activation of a transducing G protein by catalyzing exchange of GDP for GTP on the α subunit. The released Gβγ subunit then binds to and opens a K+ channel.

• The increase in K+ permeability hyperpolarizes the membrane, which reduces the frequency of heart muscle contraction. The activation is terminated when the GTP bound to Gα is hydrolyzed to GDP and Gα·GDP recombines with Gβγ.

Acetylcholine-induced opening of K+ channelsHeart muscle plasma membrane

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Cardiac muscarinic acetylcholine receptors activate a G protein and open K+ channels

• Serotonin secreted by an activated facilitator neuron binds to the G protein-coupled serotonin receptors, leading to activation of adenylate cyclase and an increase in cAMP in the sensory neuron.

• Phosphorylation of the voltage-gated K+ channel protein or a channel-binding protein prevents the K+ channels from opening, leading to prolonged depolarization.

• This leads to enhanced secretion of the neurotransmitter glutamate, which stimulates the motor neuron.

Action of a serotonin modulatory synapse

Lodish, Molecular Cell Biology

Serotonin receptor modulates K+ channel function via activation of adenylate cyclase

Sensory transduction systems

• The nervous system receives input from a large number of sensory receptors; • Photoreceptors in the eye, taste receptors on the tounge, odorant receptors in

the nose, touch receptors on the skin monitor various aspects of the outside environment.

• Each receptor must convert, or transduce, its sensory input into an electric signal.

• How does a sensory cell, usually a specialized epithelial cell, transduce its input into an electric signal?

• Often, the connection between a sensory receptor protein and the ion channel is indirect; the sensory receptor activates a G protein that, in turn, directly or indirectly induces the opening or closing of ion channels.

Ex: The light receptors in the rod cells in mammalian retina The olfactory receptors in the nose

• In the dark, the membrane potential of a rod cell is -30mV; rod cells constantly secrete neurotransmitters.

• A brief pulse of light causes a transient hyperpolarization of the rod cell membrane and decreases neurotransmitter release.

• Light triggered closing of sodium channels hyperpolarizes rod cells.

• How is the signal transduced into the closing of sodium channels?

Visual system

• The human retina contains 2 types of photoreceptors, rods (stimulated by weak light) and cones (involved in color vision).

• Rod cells form synapses with neurons that, in turn, synapse with others that transmit impulses to the brain.

Lodish, Molecular Cell Biology

• Absorption of light causes rapid photoisomerization of the cis-retinal to the trans isomer, forming the unstable intermediate meta-rhodopsin II, or activated opsin.

• The latter dissociates spontaneously to give opsin and all-trans-retinal, which is converted back to the cis isomer by enzymes in rod cells.

RhodopsinRhodopsin, the photoreceptor in rod cells, is formed from

11-cis-retinal and opsin, a transmembrane protein

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• In dark-adapted rod cells, a high level of cGMP acts to keep nucleotide-gated nonselective cation channels open and the membrane depolarized compared with the resting potential of other cell types.

• Light absorption leads to activation of opsin (O*) and conversion of inactive transducin (Gt) with bound GDP to the active state with bound GTP accompanied by dissociation of Gβγ (not shown).

cGMP is a key transducing molecule

Coupling of light absorption by rhodopsin to activation of cGMP phosphodiesterase in rod cells

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• The free Gtα·GTP thus generated then activates cGMP phosphodiesterase (PDE) by binding to and dissociating its two inhibitory subunits; as a result, the released catalytic α and β subunits of activated PDE (PDE*) can convert cGMP to GMP.

• The resultant decrease in cGMP causes dissociation of cGMP from the nucleotide-gated channels in the plasma membrane; the channels then close and the membrane becomes transiently hyperpolarized.

cGMP is a key transducing molecule

Coupling of light absorption by rhodopsin to activation of cGMP phosphodiesterase in rod cells

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• Light-activated opsin (opsin*), but not dark-adapted rhodopsin, is a substrate for rhodopsin kinase.

• The extent of opsin* phosphorylation is directly proportional to the ambient light level, and the ability of an opsin* molecule to catalyze activation of transducin is inversely proportional to the number of sites phosphorylated.

• Thus the higher the ambient light level, the larger the increase in light level needed to activate the same number of transducin molecules. At very high light levels, arrestin binds to the completely phosphorylated opsin, forming a complex that cannot activate transducin at all.

Role of opsin phosphorylation in adaptation of rod cells to changes in ambient light levels

Lodish, Molecular Cell Biology

Individual cone cells express one of the three cone opsins. The spectra were determined by measuring in a microspectrophotometer the light absorbed by individual cone cells obtained from cadavers.

The absorption spectra of the three human opsins responsible for color vision

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Memory and neurotransmitters

• Learning is a process by which we modify our behavior as a result of experience or as a result of aquisition of information about the environment.

• Memory is the process by which this information is stored and retrieved.

• Depending upon how long it persits, memory can be short term (minutes to hours) and long term (days to years).

• Memory results from changes in the structure or function of particular synapses.

• Long term memory involves the formation or elimination of specific synapses in the brain and the synthesis of new mRNAs and proteins.

• In contrast, short term memory is very rapid and involves changes in the release and function of neurotransmitters at particular synapses

Memory and neurotransmitters

• The hyppocampus is associated with many types of short-term memory.

• Long-term potentiation is a type of short-term memory.

• In long-term potentiation, continual stimulation of a postsynaptic neuron makes it more responsive to subsequent stimulation by presynaptic neurons.

• Two types of glutamate receptors in the postsynaptic neuron combine to generate long-term potentiation, NMDA and non-NMDA receptors.

Glutamate receptors in long-term potentiation

NMDA, N-methyl-D aspartate(nonnatural amino acid)

• The ion channel in the NMDA receptor (green) is normally blocked by Mg2+, and thus the glutamate released by firing of presynaptic neurons leads, at first, to opening of only the non-NMDA glutamate receptors (pink).

• The resultant influx of Na+ partially depolarizes the membrane.

Two types of glutamate receptors in long-term potentiation

Lodish, Molecular Cell Biology

• If many presynaptic neurons (here 2 are shown) fire in synchrony, the membrane of the postsynaptic cell becomes sufficiently depolarized so that the Mg2+ blocking the NMDA receptors are removed; then both the NMDA and the non-NMDA glutamate receptors open in response to glutamate.

• Ca2+ as well as Na+ enter through the open NMDA receptors, causing an enhanced response in the postsynaptic cells.

• The synapse “learns” to have an enhanced response to the electric signals in the presynaptic cells.

Two types of glutamate receptors in long-term potentiation

Lodish, Molecular Cell Biology

Bom estudo!!!

cmprodrigues@ff.ul.pt