In what 3 ways are NTs inactivated after dissociation from their postsynaptic receptors?
1. re-uptake into the pre-synaptic cell
2. uptake into glial cells
3. enzymatic degradation (such as acetylcholinesterase breaks down ACh into acetate and choline)
T or F: Electrical synapses are not delayed by diffusion of a chemical transmitter across the synaptic cleft and thus are often found where it is important to activate several neurons simultaneously.
While ____ synapses can be either excitatory or inhibitory, _____ synapses are only excitatory.
While chemical synapses can be either excitatory or inhibitory, electrical synapses are only excitatory.
What is the physical basis of an electrical synapse?
What is transmitted at these synapses?
The physical basis of an electrical synapse is the gap junction, a region of apposition of an array of transmembrane proteins (connexons) whose central pores provide continuity between the adjacent cells. In general, small ions such as K+ can pass freely through gap junctions, providing a low resistance electrical connection between the cells. Larger molecules, such as tRNA, can also pass between cells. The conductance of a gap junction can be modified. Calcium ions, pH and transjunctional voltage have been shown to alter the conductance of gap junctions in several systems. Furthermore, a gap junction may be rectifying: current may flow more easily in one direction than in the other.
What is Lambert-Eaton Myasthenic Syndrome (LEMS)?
What kind of disease is LEMS?
What clinical sign/symptom does LEMS cause and why?
What disease is often associated with LEMS?
Blocking calcium influx prevents neurotransmitter release. In the Lambert- Eaton Myasthenic Syndrome (LEMS) presynaptic voltage gated calcium channels at the neuromuscular junction are degraded via an autoimmune mechanism. The autoimmune attack is often based on molecular mimicry between presynaptic voltage gated calcium channels and certain small cell lung cancers. The degradation of the calcium channels prevents influx of calcium and causes weakness. The weakness may be the first clinical sign that leads to the detection of the lung cancer.
What are the effects of botulinum toxin? Where does it act? What is the consequence of this toxin?
What are the effects of the tetanus toxin? Where does it act? What is the consequence of this toxin?
Botulinum toxin preferentially interferes with vesicle docking and neurotransmitter (Ach) (interferes with SNARE vesicle docking proteins) release at the neuromuscular junction producing decreased muscle activity (flaccid paralysis/weakness).
Tetanus toxin cleaves synaptobrevin/VAMP blocking vesicle docking and release of inhibitory neurotransmitters (GABA/glycine) in the spinal cord and brainstem (CNS) resulting in markedly increased muscle activity (spastic paralysis), such as lockjaw and is often fatal. We are immunized against tetanus early in life to protect against this.
What are the 3 fundamental classes of NTs? (just list)
1. small molecule
2. large molecule
Where in neurons are small molecule NT's synthesized? Large molecule NT's?
What controls NT synthesis?
What is the mainstay treatment of Parkinson's disease and how does it work (generally)?
Small molecule transmitters are synthesized in the presynaptic terminal. Larger neurotransmitters (peptides) are synthesized in the cell body and transported to the terminal. The availability of precursors controls neurotransmitter synthesis. In Parkinson’s disease (PD) there is degeneration of dopamine producing neurons resulting in motor impairment. Administration of L-dopa, a dopamine precursor, increases the dopamine synthesized by the surviving neurons and is a mainstay of treatment of PD.
Name examples of small molecule NTs.
For small molecule NTs, what type of synaptic vesicles are excitatory NTs usually packaged in? Inhibitory?
In what concentration range do small molecule NTs act in?
How are small molecule NTs rid of after acting on the presynaptic receptors?
What are their molecular weights?
Small Molecule Neurotransmitters (e.g., Acetylcholine (ACh) or Dopamine (DA)).
1. They are synthesized at the site of release.
2. They are packaged in small synaptic vesicles: round vesicles usually contain excitatory neurotransmitters and flattened vesicles inhibitory transmitters
3. They act in the low micromolar range.
4. They are taken back up by the terminal from which they were released by a high affinity re-uptake pump that involves a facilitated diffusion protein (shuttle protein) OR they are catabolized by specific enzymes located in the synapse OR they can be taken up by glial cells for catabolism via a low affinity facilitated diffusion protein.
5. They have short diffusion distances due to catabolism and re-uptake.
6. Their molecular weights are in the 100s.
Name examples of large molecule NTs.
How do large molecule NTs get to axon terminals?
In what concentration range do these NTs act?
What happens to large molecule NTs after acting at the postsynaptic neuron?
What are their molecular weights?
What may large molecule NTs be co-released with? What is often required for release of large molecule NTs?
Large Molecule Neurotransmitters (neuropeptides; e.g., enkephalins (Enk), endorphins, substance P (Sub P)).
1. They are synthesized in the nucleus and packaged by the Golgi apparatus into large, dense core vesicles and transported to the nerve terminal.
2. They act in the pico- and nanomolar range.
3. They are not taken back up by the terminal, the empty vesicle is recycled back up to the cell body.
4. They are catabolized by non-specific peptidases located in extracellular space.
5. They have short diffusion distances.
6. They have molecular weights in the 1,000 -10,000.
7. They may be released at the same axon terminal, as cotransmitters, with a small molecule neurotransmitter, but often require repetitive, or higher frequency action potentials. If there is too much NT release, releasing neuropeptides may decrease excitability of postsynaptic neuron-form of regulation.
Attached pic: A. Single action potentials (1) lead to calcium influx (2) and release of small molecule neurotransmitters from small synaptic vesicles (3) in the presynaptic terminal. Repetitive action potentials (1) cause greater calcium influx (2) and co-release of both small neurotransmitter (3) and neuropeptide neurotransmitter (4).
Name examples of gas NTs.
Where are gas NTs synthesized?
How are they released? Where in the postsynaptic cell do they act?
What happens to gas NTs after acting on their targets?
Gas Neurotransmitters (e.g., nitric oxide (NO) or carbon monoxide (CO)).
1. They are synthesized locally near their release site.
2. They are not packaged in vesicles, but rather synthesized and immediately diffuse to their targets due to high lipid solubility.
3. Can work on intercellular targets instead of traditional receptors.
4. Are not catabolized per se, but rather diffuse away or are oxidized to an inactive state.
What are neuromodulators?
What kind of molecules are neuromodulators?
What are neuromodulators often co-localized with?
Where can they act?
The action of neurotransmitters is affected by Neuromodulators.
A: A catch-all category of molecules that are not responsible for direct neuron-neuron communication (as is true of the neurotransmitters), but rather modifies the action of neurotransmitters.
B: Neuromodulation can be mediated by neuropeptides and other non-peptide molecules (glucocorticoids or estrogens) that can, for example, affect a receptor protein’s affinity for a small molecule neurotransmitter.
i. Neuropeptides are often co-localized with small molecule neurotransmitters where two different vesicular pools exist.
ii. Neuropeptides can be co-localized with small molecule neurotransmitters where they are found within the same vesicular pool.
iii. They can act at a local synapse, have a more regional or field effect (paracrine effects), or act over distances (hormone).
iv. Their actions can even reverse the primary effect of the released neurotransmitter (from inhibition to excitation).
How long do the effects of NTs that act on ionotropic receptors generally last?
Neurotransmitters that act through ionotropic receptors generally produce rapid effects that are temporary and rapidly reversible. The duration of the effect of the neurotransmitter is dictated by its half-life in the synapse and terminated by being taken back up or by catabolism.
Generally, what effect do metabotropic receptors have on cells?
After ligand binding, what do metabotropic receptors do?
Generally, how long do the effects from metabotropic receptors last?
Metabotropic receptors gain their name from the fact that they act by inducing a metabolic action in the target cell. This is a G-protein coupled mechanism that leads to an increase in cAMP, a decrease in cAMP, or some other second messenger activity.
1. Metabotropic receptors may also open up ion channels and therefore possess ionotropic effects as well.
2. Other second messenger systems include cGMP, activation of the Phospholipase A2, arachidonic acid pathways, and the diacyl-glycerol (DAG)/inositol triphosphate (IP3) pathway.
3. The metabolic consequences of metabotropic receptor activation can have long-term and profound effects on the function of a neuron or glial cell.
a. Alterations in the phosphorylation status of a target cell through metabotropic actions can regulate cell function (e.g., alterations in the phosphorylation status of proteins involved with neurotransmitter release can, as a result of autoreceptor activation, be phosphorylated or dephosphorylated to influence transmitter storage). These second messengers can even bind to the receptor and decrease its response to the neurotransmitter (desensitization). Alternatively, second messengers can increase future response to neurotransmitters (sensitization).
b. Neurotransmitters can alter immediate early genes which translocate into the nucleus and subsequently enhance the synthesis of new AMPA receptors as occurs as part of long-term potentiation (LTP) following high level Glu release.
c. Regulation of trophic factors which alter the physical structure of the CNS (e.g., chronic treatment with a DA agonist can downregulate striatal production of target-derived trophic factors leading to “pruning” of presynaptic terminals; alternatively chronic administration of an antagonist increases target-derived trophic factor production leading to sprouting).
d. Production of gases such as NO and CO which, in turn, can influence the production of retrograde neurotransmitters (e.g., NO released by target neuron diffuses back into presynaptic terminal to increase future Glu release as part of long term potentiation, LTP).
What NT is the primary excitatory NT in the CNS?
What type of receptors (ionotropic, metabotropic) does glutamte bind to?
How many types of these receptors does it bind to?
What is the most "famous" receptor that glutamate binds to?
Glutamate (Glu) is the primary excitatory neurotransmitter in the CNS. Its functions are not limited to a specific behavioral function, but rather, it appears to participate in most functions. The other small molecule neurotransmitters (acetylcholine, dopamine, serotonin, norepinephrine) more than likely modulate the activity of Glu or GABA neurotransmission.
Glu acts through three types of ionotropic receptors, with emphasis on the categories NMDA receptor and non-NMDA receptors
By far the most famous is the NMDA receptor.
What kind of receptor is the NMDA receptor?
What ions does the NMDA receptor transmit?
What ions/molecules bind to this receptor and what are their roles?
By far the most famous is the NMDA receptor, which is a combined ligand and voltage gated Na+ and Ca++ channel that is involved in long-term potentiation (LTP) and excitotoxicity. The NMDA receptor is a complex protein with several binding sites in addition to that of the acceptor site for Glu. There is an obligatory Glycine binding site that must be occupied in order for Glu to activate this receptor. In addition, there is an obligatory Zn++ binding site that if occupied blocks receptor activation by Gly. Inside the receptor channel itself are two additional sites for Mg++ and phencyclidine (PCP, “angel dust”-a hallucinogen) which, if occupied, block NMDA activation. These properties confer important roles for this receptor.
What is long-term potentiation (LTP)?
What is the role of NMDA receptors in LTP?
What are the results of Ca2+ entry into cells through NMDA receptors?
NEUROPLASTICITY: LTP is a physiological process that involves use dependent enhanced synaptic responsiveness. The NMDA receptor responds to binding of its ligand Glu only when the membrane in which it sits is depolarized. It is a therefore a combined ligand-gated/voltage-gated channel. Under partial depolarization, as occurs if Glu activates another receptor (e.g. A nearby AMPA receptor, another type of Glu receptor) on the neuron, the Mg is dislodged from its binding site within the Na+ channel. If this occurs, then Ca++ also enters the cell. The calcium entry has several consequences.
(1) Calcium binds with calmodulin and the complex can activate Nitric oxide synthase (NOS). NOS converts arginine to citrulline producing NO as a gas neuromodulator. NO then can diffuse back across the synapse (retrograde neurotransmission) and via cGMP production increase the production of Glu.
(2) Calcium entry may also increase the synthesis of postsynaptic AMPA receptors.
(3) The end result is that activation of NMDA receptors leads to increased GLU release presynaptically and increased postsynaptic responsiveness due to increased AMPA receptors lasting several days to weeks. The target cell, once activated will respond in a more profound fashion to Glu signaling in the future. This is a form of synaptic plasticity that provides a neurochemical mechanism for memory.
What is excitotoxicity? What role do NMDA receptors play in excitoxicity?
The other critical feature of the NMDA receptor is that it is thought to mediate excitotoxicity. Excitotoxicity is a phenomenon whereby excessive excitation leads to neuronal death. Excessive stimulation of NMDA receptors has several consequences.
(1) Increased Ca++ influx can lead to excessive formation of NO which can combine with nitrates to form a very potent free radical capable of breaking down protein, lipids, and nucleic acids.
(2) Increased Ca++ influx can lead to increased oncotic pressure resulting in neuronal swelling due to increased diffusion of obligatory water.
(3) Increased Ca++ influx leads to activation of several protein kinases that can activate formation of proinflammatory agents including prostaglandins and proinflammatory cytokines, both of which can damage cells.
May be a factor in neuronal death in stroke, epilepsy, Huntington’s Disease, other degenerative diseases
What other receptors can glutamate bind to? (just list)
Describe the importance of the AMPA receptor in the CNS.
How does the AMPA receptor respond to Glu binding?
Describe the effects of Ca2+ transmission through AMPA receptors.
a. The AMPA receptor is the excitatory workhorse of the CNS.
b. It responds to Glu with very rapid excitatory depolarizing Na+ currents and is widely distributed throughout the CNS.
c. The ionotropic channel is impermeable to Ca++.
What is the primary inihbitory NT of the CNS?
What two receptors does GABA bind to? Which GABA receptor is the most abundant?
What is the response of the most abundant GABA receptor to GABA binding?
GABA acts through two primary receptors that are the targets of numerous pharmaceutics as well as alcohol.
The vast majority of the GABA receptors are classified as GABA-A. This pentameric protein is a pure ionophore that has several binding sites on it. The binding of GABA to the GABA-A receptor opens a Cl- channel. Opening the Cl- channel may not lead to hyperpolarization (since the equilibrium potential for Cl- is close to the resting potential), but rather a shunting of positive current when a cation channel opens.
What change to GABA-A receptors does binding of benzodiazepines induce?
What effects do benzodiazepines have? (Why is it diagnosed?)
The benzodiazepines such as Valium (diazepam; Mother’s Little Helper) are anxiolytic (reduce anxiety) GABA agonists. They bind to the benzodiazepine (BDZ) binding site on the receptor and enhance the affinity of GABA for its site. This increases the frequency at which GABA opens the channel and thereby produces global inhibition. Many benzodiazepines are anticonvulsant.
What change to GABA-A receptors does binding of barbituates induce?
What effects do barbituates have? (why is it diagnosed?)
Barbiturates can open the channel independent of the presence of GABA. When they bind to the barbiturate site, they increase the duration of time the channel is open. They can produce global depression of the CNS, but unlike the benzodiazepines, can literally shut down the CNS invoking fatal respiratory depression. The barbiturates include powerful anesthetics and anticonvulsants.
What is the other type of GABA receptor?
Where in the body is this receptor heavily concentrated?
What are agonists of this receptor used to treat?
The other major receptor subtype is the GABA-B receptor so called because the drug baclofen selectively binds to it. Although it is found in the brain, it is heavily concentrated in the spinal cord and GABA-B agonists are widely used to treat spasticity.
What are the 3 classifications of receptors?
2. pre-synaptic autoreceptors
3. pre-synaptic heteroreceptors
Where are postsynaptic receptors located?
What are autoreceptors? What is the function of autoreceptors?
1. postynaptic receptor: receptor located on the receiving side of a synapse.
2. presynaptic receptor is the receptor located ojn the pre-synaptic side of a synapse. In general, this can refer to any receptor located on the presynaptic side and at any location. However, generally we refer to two distinct types:
a. autorceptor: refers to a receptor located on the presynaptic terminal that responds to the NT released by that terminal. All small molecule NTs have autoreceptors. When too much NT is released into the synapse, the NT binds to its autoreceptor and reduces future NT release (short loop feedback inhibition). This occurs generally by acting through a second messenger system or can involve variation in ion flux. Activation of this receptor can even reduce NT synthesis. Several drug classes bind specifically to these autoreceptors to affect function.
b. heteroreceptors: refers to a receptor located on a terminal that responds to a NT released by anothe rnerve. A dopaminergic synapse is affected by ACh release from another nerve making synaptic contact on the terminal.
Receptor numbers change in response to alterations in NT availability.
Name and describe what occurs when there is too much and too little NT availability.
Up-regulation describes an increase in receptor number when neurotransmitter availability is reduced. Physiologically this is referred to as Denervation Hypersensitivity. This commonly occurs when the motor neurons in a peripheral nerve are injured. Postsynaptic Ach receptors are upregulated at the neuromuscular junction, contributing to a partial recovery of function. This can also occur pharmacologically when a direct-acting receptor antagonist is administered and is referred to as Pharmacological Denervation Hypersensitivity.
Down-regulation of receptor number occurs when neurotransmitter availability is increased. This is the traditional mechanism through which tolerance (decreased responsiveness to a drug) occurs. Thus, chronic stimulation of a neurotransmitter receptor eventually leads to reduced numbers of receptor which reduces the response of the synaptic target.
Receptor affinity for its neurotransmitter does not appear to change in response to alterations in neurotransmitter levels. Rather, it is a pure change in number.
The changes in receptor number should be viewed as part of an overall homeostatic system designed to maintain neurotransmitter tone around a given set point. Thus, they are compensatory alterations designed to keep neurotransmitter activity static.
Small molecule NTs are generally taken back up into the presynaptic termina (or by glia).
How specific are reuptake sites for their NTs?
How is reuptake able to be regulated in the long and short term?
What are the affects of amphetamines, methylphenidate (Ritalin), and cocaine on NE and/or DA (dopamine) reuptake and release?
Small molecule neurotransmitters are generally taken back up into the presynaptic terminal (or by glia).
1. This process is 85-95% efficient.
2. Uptake sites are reasonably specific for a given neurotransmitter, but some overlap does occur (e.g., 5HT can be taken into a DA terminal).
3. Neurotransmitter re-uptake is via facilitated diffusion, secondarily operated by Na+ gradients. ???
a. Re-uptake is regulatable in the short term (coupling efficiency regulated through phosphorylation) and the long term (new protein synthesis)
b. Re-uptake pumps are saturable and reversible and thus provide a mechanism for Ca++-independent release.
(1) Amphetamine and methylphenidate cause release of DA and NE by acting as ligands for the presynaptic DA Transporter (DAT). Methylphenidate (Ritalin) acts as a dopamine and norepinephrine reuptake inhibitor, resulting in a prolongation of dopamine receptor effects and is effective in treating attention deficit hyperactivity disorder (ADHD). It improves concentration and attention to academic work.
(2) Cocaine also binds to an allosteric regulatory site on the DAT, but is not a ligand for the DAT. As such, it blocks re-uptake, but does not induce release of DA. The longer DA remains in the synaptic cleft the greater impact it has on the postsynaptic cell.
After re-uptake of an NT into its presynaptic terminal, what are its potential fates?
Once Neurotransmitters are taken up into the terminal (the so-called mobile pool), they have several fates.
1. The vast majority of the neurotransmitter is taken up by a transporter on the synaptic vesicles.
a. This transporter in biogenic amine terminals is referred to as the vesicular monoamine transporter or VMAT.
b. In cholinergic terminals it is called VAChT.
c. Reserpine depletes stores of catecholamines and serotonin by blocking their reuptake into the synaptic vesicle by the VMAT. Reserpine has antihypertensive and antipsychotic effects (reducing agitation in schizophrenia).
2. Neurotransmitters can bind to the rate limiting enzyme in the synthetic pathway (end-product inhibition) to reduce synthesis.
3. Neurotransmitters can bind to re-uptake transporter sites that are on the mobile pool side of the membrane at that time in which case they are transported back out.
4. The neurotransmitter can be catabolized by scavenger enzymes, generally located in mitochondria (e.g., Mono amine oxidase (MAO)).
What are monoamine oxidase inhibitors (MAOIs) used for the treatment of? What do they do?
Monoamine oxidase inhibitors (MAOIs) are a class of powerful antidepressant drugs prescribed for the treatment of depression. They prevent the breakdown of the monoamine neurotransmitters (dopamine, norepinephrine, serotonin).
Describe spatial and temporal summation.
Spatial summation is the additive effect produced by many EPSPs or IPSPs that have been generated at many different synapses on the same postsynaptic neuron at the same time.
Temporal summation of EPSPs or IPSPs is the additive effect produced by many EPSPs or IPSPs that have been generated at the same synapse by a series of high-frequency action potentials on the presynaptic neuron.