Neurochemistry Flashcards
1
Q
Blood brain barrier (BBB)
A
- Is different from the blood-CSF barrier, but both together maintain the CNS environment
- BBB is dependent on tight junctions btwn endothelial cells in (non-fenestrated) capillaries within the CNS
- Blood-CSF barrier is from tight junctions btwn ependymal epithelial cells in the choroid plexus
- Some specific areas of the brain do not have a BBB for neurosecretory products to pass into circulation (posterior pituitary)
2
Q
Other cells involved in BBB
A
- Endothelial cell tight junctions create the BBB, are surrounded by basement membrane, astrocytes, and pericytes
- Astrocytes dictate the endothelial cells to form these tight junctions (foot processes will completely surround the capillary)
- Pericytes play a role in angiogenesis and communicate to other cells (one pericyte will be directly adjacent to the capillary)
- Pericytes also are source of adult pluripotent stem cells
3
Q
Developmental aspects of BBB
A
- Tight junctions begin to form early during fetal development
- Permeability to substances is greater in developing brain
- Before 6 mos the brain is susceptible to hormones, neurotoxins, and other chemicals
4
Q
Brain uptake index (BUI)
A
- Relative uptake of a molecule as compare to deuterium water (DOD)
- DOD BUI is arbitrarily set to 100%
- Some substances (nicotine and alcohol) are over 100%, while most are under 100% (caffeine, heroine, ect)
- The ability of a substance to pass through the BBB w/o aid of a transporter is based on its hydrophobicity
- The more hydrophobic a substance, the more easily and quickly it will pass through the BBB
- Mannitol is poorly permeable and is used to dehydrate the brain via osmosis to reduce swelling
5
Q
Partition coefficient (PC)
A
- The partition btwn an oil and aqueous phase, to predict the accumulation behind the BBB
- The higher the PC (the more hydrophobic a substance), the greater the brain uptake
- Gases (NO) and volatile anesthetics can rapidly diffuse into CNS
- Glucose is taken up by facilitate transport mechanism, driven by concentration gradient but doesn’t require energy
- GLUT transporters are on endothelial cells, ependymal (?) cells, astrocytes, microglia and neurons
6
Q
General features of BBB
A
- Enz barriers are found at the luminal side of the endothelial cells
- Toxic materials can destroy the BBB: bordatella pertussis (whooping cough) toxin destroys tight junctions, LPS from GN cause neuro-inflammation and disrupts the BBB by elevating matrix metalloproteinases
- Drug abuse may lead to permanent BBB damage
- Aging and diabetic condition have profound negative effect on BBB
- BBB dysfunction is observed in AD pts who also have HTN and CVD
- Free bili in newborns is toxic as it can pass thru the BBB and be deposited in the brain
- Bili levels increase when a mother develops Abs to the fetal Hb. Bili breakdown is stimulated by light
7
Q
Formulations to penetrate BBB
A
- Abs to transferrin receptors (TR) may sneak drugs through BBB
- BBB contains many TRs that transport Fe-TF to the interstitium
- If an Ab to the TR is made and linked to a drug, that drug could be transported across the BBB via the TR
- CAMs are also upregulated in capillaries within the CNS at sites of inflammation/hypoxia
- Therefore finding proteins that bind to CAMs and coupling them w/ certain drugs can carry those drugs to specific sites of inflammation
- WBCs use these CAM markers to exit the blood and enter the CNS at times of inflammation/hypoxia
8
Q
Electrical vs chemical synapse
A
- Chemical synapse is the typical and most common synapse (using NTs)
- Electrical synapse is directly passing current from one cell to another (gap junctions)
9
Q
Steps of synaptic transmission
A
- Presynaptic neuron depolarizes down to nerve terminal, causing an influx of Ca by voltage-gated Ca channels
- Increase in [Ca] leads to fusion of synaptic vesicles w/ plasma membrane (botox interferes w/ this step), causing the NTs in those vesicles to be released into the synapse
- NTs diffuse across and bind to postsynaptic cell at their receptors, causing a change in postsynaptic membrane conductance
- This can either be excitatory (EPSP, depolarization) or inhibitory (IPSP, hyperpolarization), PSP= postsynaptic potential, in neurons
- In muscles the depolarization of the motor-end plate is called end-plate potential (EPP)
- If the depolarization is enough for Vm to reach threshold the postsynaptic cell will fire an action potential (AP)
10
Q
Synaptic delay
A
- Time btwn depolarization of presynaptic cell and initiation of postsynaptic response
- Is the time required for release of NT from the presynaptic cell in response to the presynaptic AP
11
Q
Neuromuscular junctions (NMJ) 1
A
- A chemical synapse that results in EPP in the muscle cell (leading to contraction)
- The NT acetylcholine (ACh) is released from the neuron onto the NMJ where it binds to the nicotinic ACh receptor (nAChR)
- The other AChR is the muscarinic receptor (in the heart)
- Normally gNa«gK, due to no movement of Na but loss of K from leaky channels, but conductance of both ions changes to gNa=gK upon ACh binding to AChR (both ions move thru the channel)
12
Q
Neuromuscular junctions (NMJ) 2
A
- When ACh binds to nAChR, the receptor opens its channel and lets Na in and K out, making gNa=gK
- While both ions are moving in opposite directions, the overall increase in conductance of Na is greater than the increase in conductance of K so the Vm increases, closer to Ena (which is +60, as opposed to Ek which is -80)
- This change in Vm is high enough to reach threshold (when gNa=gK depolarization occurs), thus beginning an AP along the membrane of the muscle, ultimately leading to contraction
13
Q
Stopping EPPs
A
- EPPs must be transient or else tetanus would occur
- To stop EPPs, nzs called acetylcholinesterases are synthesized and present on the post junctional membrane
- These rapidly breakdown the ACh
- Some of the ACh diffuses out of the synaptic cleft
- Inhibitors of these nzs are called anticholinesterases
14
Q
nAChR structure
A
- nAChRs at the NMJ are composed of 2A,B,D,E subunits
- nAChRs in neurons are composed only of A and B subunits
- nAChR formed from different subunit combinations vary in gating kinetics, single channel conductance, sensitivity to ACh, Ca permeability, and affinity for pharmacological drugs
15
Q
Nicotine addiction
A
- Nicotine minmics the NT ACh in the brain and stimulates dopaminergic neurons in the mesolimbic reward pathways
- Desensitization occurs when the nAChRs enter a permanently closed state (at high nicotine or ACh concentrations) due to repetitive and prolonged binding
- Desensitization leads to acute tolerance and subsequent receptor upregulation (to compensate for the desensitization)
- Chronic nicotine use leads to multiple neuroadaptations that underlie nicotine withdrawal symptoms
16
Q
Neuron-neuron synapses
A
- Use many different NTs such as ACh, AAs (glutamate, glycine, aspartate, GABA), peptides (endorphins), amines (norepinephrine, DA, serotonin), lipophilic arachidonic acid (ADA) derivatives (cannabinoids)
- Each neuron can innervate many different neurons and can be innervated by many other neurons
- Synaptic transmission is terminated by breakdown of NT, reuptake of NT into glial cells or presynaptic neuron, and diffusion out of synapse
- Cocaine inhibits reuptake of DA, and prozac inhibits reuptake of serotonin (both transport mechanisms are Na dependent)
17
Q
NT receptors
A
- All are either ionotropic (binding to the receptor opens a channel in the receptor and leads to ion movement) or metabotropic (receptor is GPCR and leads to activation of other channels and/or 2nd messengers)
- EPSPs (increase in gNa, decrease in gK, or increase in both) from excitatory NTs: glutamate, aspartate
- IPSPs (increase in gK or increase in gCl) from inhibitory NTs: GABA, glycine
- EPSPs and IPSPs sum to determine the overall value of the Vm
- Each synapse will depolarize or hyperpolarize the postsynaptic cell by 1-2mV
18
Q
Summation of synaptic inputs
A
- Integration of many synaptic inputs arriving simultaneously from different presynaptic cells is spatial summation
- Integration of a series of inputs from a single synaptic input occurring in rapid succession is temporal summation
- Spatial summation inputs can cancel each other out if in opposite directions (EPSP+IPSP)
19
Q
Use-dependent changes in synaptic strength
A
- The synaptic strength is not a fixed value
- This is synaptic plasticity, which can result in potentiation (increase of a response to stimulation) or depression (decrease of a response to stimulation)
20
Q
Resting membrane potential
A
- A net outward K current is responsible for the negative resting membrane potential, Vm (which is usually about -80mV as compared to the outside of the cell)
- This outward K current is due to leaky K channels
- In contrast, there is very little movement of Na into the cell
- The NA/K ATPase is responsible for maintaining the concentration differences for both K and Na, by moving these two ions against their gradient (moves 3 Na out of the cell and 2 K into the cell)
21
Q
Equilibrium potential
A
- The value of the membrane potential that is in equilibrium w/ a concentration difference
- The equilibrium potential (E) can be calculated for an ion when it is at various concentrations in and out of the cell by the Nernst eqn
- Combining the concentration gradient and the force on the ion by the Vm is the electrochemical potential of the ion
- When these two factors cancel each other out, the ion is at E
- When Vm=E, there is no net movement of the ion in or out of the cell
- Ena= +70mV, Ek=-90mV
22
Q
Currents of ions and Ohm’s law
A
- Ohm’s law is used to calculate how much current of an ion will be generated based on the conductance, Vm, and E of an ion
- I=g(Vm-E); stating that the difference of the Vm and E for the ion times the conductance will give a value of current
- Vm-E represents the driving force for ion movement (potential difference), and g (conductance) is the same as 1/R (inverse of resistance), since conductance is the ease at which ions flow)
- The Vm-E for Na is 5x more than it is for K, but the gK is 100x more than gNa, thus in the resting state there is Ik>Ina (leading to Vm being close to Ek)
23
Q
Chord conductance eqn (CCE)
A
- Conductance is directly proportional to the # of open ion channels
- The CCE gives the Vm, by producing a weighted average of the equilibrium potentials (using the conductance ratio for each ion as the weighting factor)
- Vm= (gK/gK+gNa)Ek + (gNa/gK+gNa)Ena
- At resting state gK is 100x more than gNa and the Vm is very close to Ek (Vm=-80mV, Ek=-90mV)
- During an AP the gNa greatly increases and drives the Vm toward Ena (depolarizing the cell)
- During inhibition the gK further increases (or gCl increases) and this drives the Vm even closer to Ek and Ecl (both are about -90mV) causing hyperpolarizaiton
24
Q
Firing an AP 1
A
- The usual threshold value of Vm for the firing of an AP (at the initial axon segment) is about -60mV
- APs are due to successive opening of voltage-gated Na channels, starting at the initial axon segment (just adjacent to axon hillock)
- Voltage-gated Na channels open upon depolarization of the membrane, leading to influx of Na (increases gNa) and further depolarization
25
Q
Firing an AP 2
A
- Axon depolarizes up to 50mV, but never reaches the Ena of 60mV because gK is never 0
- Overall current of Na dependent on # of channels, probability of channels being open, and the current of Na per channel
- The factor that is most variable is the probability of the Na channels being open, which depends on the Vm (high probability when Vm is >-50mV, low probability if <-55mV)
26
Q
Firing an AP 3
A
- A few ms after opening the Na channels spontaneously inactivate and prevent further influx of Na
- Inactivation must occur before the Na channel can be opened again
- When the Na channels inactivate, voltage-gated K channels (sensitive to depolarization) begin to open and release K from the cell
- This increases gK and thus (in combination w/ the inactivated Na channels) hyperpolarizes the cell
- K channels remain open for longer, contributing the the overshoot as the cell hyperpolarizes past resting potential until gK returns to resting state
27
Q
Threshold
A
- Threshold occurs at the point where the resting net outward current carried by K becomes a net inward current carried by Na
- This requires a lot of open Na channels per square um
- For weak stimuli, K efflux still exceeds Na influx and no AP is produced
- For a strong stimuli, Na influx will exceed K efflux, threshold will be reached, and AP will be produced
- For AP propagation there must also be enough Na channels in the membrane at the adjacent areas to allow the Na influx to exceed the K efflux (net current flow to be inward)
- Amplitude of AP is constant throughout the axon and will not be larger due to a stronger stimuli
28
Q
Propagation of APs
A
- The depolarization of an axon occurs in segments when the axon is myelinated
- This is because the myelin sheath (extensive wrappings of oligodendrocyte membrane in CNS) covers the axon in parts, and in these places there are few Na channels
- The sheath is discontinuous, and the places where there is no sheath (node of ranvier) is where the Na channels are located (in high numbers)
- Therefore the APs jump from one node to the next (saltatory conduction) as each node becomes depolarized by the activation of the previous one
29
Q
Function of myelin
A
- The myelin sheath prevents Na ions from leaving the axon cytoplasm into the ECF
- This is because it increases the resistance of passing through the member for the ions
- This high resistance leads to very little ions escaping out of the cell, retaining the ions in the cytoplasm and increasing the ionic current
- Overall, this leads to an increase in the conduction velocity of the axon
- Unmyelinated axons lose more Na ions into the ECF and therefore have a lower conduction velocity than myelinated axons
- Large diameter axons have higher conduction velocities than small diameter axons
30
Q
Graded response
A
- Responses to stimuli that are too small to generate an AP
- Instead they create membrane potential changes (local responses) that decrease exponentially with increasing distance from the site of stimulation
- The rate of decrease in membrane potential is based on lambda (length constant), which represents how much current is lost thru the membrane (thus myelin increases lambda)
- The depolarization of a stretch of cell membrane by this mechanism is electrotonic conduction
- In this the ionic influx from the initial stimulus alone is causing the depolarization, and voltage-gated channels do not contribute
31
Q
Synaptic plasticity
A
- Change in strength of a synapse btwn two cells
- Underlies the phenomena of classical conditioning where an organism associates an unconditioned pathway (salivation at smell of food) with a conditioned pathway (ringing a bell every time they are fed)
- This generates the association of food and the bell and leads to salivation at the sound of the bell
32
Q
Long-term potentiation (LTP)
A
- High frequency stimulation (100 Hz for 900 pulses) results in enhanced synaptic strength of the postsynaptic cell
- LTP has 2 properties: input specificity and associativity
- Input specificity: only the input that has undergone high-frequency stimulation expresses LTP
- Associativity: a weak input cannot be potentiated by itself. If it is coupled w/ a strong input it can be potentiated together w/ the strong input
33
Q
LTP induction mechanisms 1
A
- Strong synaptic stimulation (100 Hz, a strong input) can stimulate LTP on its own
- A weak stimulation (10 Hz, a weak input) will induce LTD (long term depression) instead, unless it is spatially near a strong input from another neuron
- Ca influx is important for signaling biochemical changes that lead to enhanced synaptic responses
- Blocking [Ca] rises by adding chelators or channel antagonists prevents induction of LTP
- Duration and amplitude of [Ca] rise in the cell determines whether LTP or LTD is produced
34
Q
LTP induction mechanisms 2
A
- NMDA receptors are required for LTP (blocking them prevents LTP generation
- These receptors have 2 requirements that must be met to be functional: glutamate binding and cell depolarization
- An excitatory synapse usually contains NMDA and AMPA receptors (both bind glutamate)
- The glutamate must bind to NMDA to allow for proper conformational change, and bind to AMPA to induce EPSP (depolarization)
- The depolarization kicks out the Mg that remains in the NMDA channel, fully activating the channel which allows both Ca and Na to enter the cell (confers associativity of LTP)
- The AMPA channel activation and depolarization is the strong input necessary to induce NMDA activation and thus LTP
35
Q
LTP expression mechanisms
A
- In principle both pre and postsynaptic changes can lead to LTP, but mostly postsynaptic changes are observed
- Presynaptic changes: increase in quantity of NTs releases
- Postsynaptic changes: increase in number and function of AMPA receptors
- Phosphorylating AMPA receptors leads to an increase in receptor conductance, thus LTP/LTD can be controlled by phosphorylation of AMPA receptors
36
Q
Mechanisms for induction of AMPA receptors
A
- Brief and large Ca influx from NMDA receptors leads to activation of protein kinases (one is CAMK2)
- CAMK2 phosphorylates AMPA receptors and increases their conductance for Na
- CAMK2 also phosphorylates other proteins and leads to an increase in the number of AMPA receptors expressed on the synaptic membrane
- Overall these two factors leads to easier induction of LTP
- Prolonged and moderate Ca influx leads to activation of protein phosphatases, which leads to reduced AMPA receptor conductance and expression
37
Q
Silent synapses
A
- Silent synapses are synapses btwn two cells that do not communicate effectively with each other
- This is because the post synaptic cell region expresses NMDA only, and thus cannot fully be depolarized
- These synapses must be converted into functional synapses by other neurons causing depolarization at other locations on the postsynaptic cell
- This depolarization allows for the activation of the NMDA receptors on the post synaptic membrane
- In turn this converts the first neuronal synapse into a functional synapse, since the activation of the NMDA channels allows for Ca influx and up regulation of AMPA receptors
- This process is called synapse maturation, and s crucial for normal brain development
38
Q
Metaplasticity
A
- The change in synaptic plasticity
- LTP neuronal connections would generate a positive feedback loop (leading to saturation of the postsynaptic cell) if there weren’t limitations on LTP
- Thus the more LTP a synapse has the harder it is for that synapse to increase its LTP
- Likewise, the LTP’d synapse will more easily become LTD if the strong inputs cease (future LTD easier)
- The same is true for LTD: LTD’d synapses will be more difficult to become more depressed (future LTD harder), and less difficult to become potentiated (future LTP easier)
39
Q
Firing rate homeostasis
A
- Chronically reducing a cell’s firing rate increases its ability to fire spikes (slow firing neurons want to fire faster)
- Chronically increase a cell’s firing rate reduces it’s ability to fire (fast firing neurons burn out)
- These regulations are achieved through modifying synaptic drive to bring firing rate back into a targeted functional range
40
Q
Synaptic scaling
A
- Chronically changing the cell’s firing rate (at least 2 days) scales the strength of the synapses on the postsynaptic cell multiplicatively (after plasticity occurs)
- The relative strength of each synapse is maintained (thus retains the memory of what neuron is activating it- the memory trace), but the overall firing rate is kept the same
- The response to each input (synaptic drive) is modified to maintain firing rate homeostasis, and to preserve the relative strength of each synapse
