Glutamatergic Transmission Flashcards
What are the subunits of AMPA receptors, their conformation and characteristics?
GluA1-4 with flip flop isoforms and RNA editing; flop isoform predominates in adults (faster desensitisation). Each subunit is composed of NTD, TM1-4 and CTD. They are arranged in a tetrameric fashion, forming either homomeric or heteromeric receptors. GluA2 is RNA edited to from Q to R rendering it Calcium impermeable - important to synapse plasticity and function. They also associate with auxilliary subunit proteins such as TARPs (stargazin - PSD95) which can modulate receptor function via interactions with certain receptor motifs, or with intracellular proteins to link cell signalling to receptor function.
What is the function of AMPA receptors?
Fast excitatory synaptic transmission (initial fast component),
Trigger for NMDA receptor unblock,
Undergo change in number, type or phosphorylation state to exert plasticity.
What are some common agonists and antagonists, and modulators, of AMPA receptors?
Agonist include: AMPA, glutamate
Antagonists: CNQX, kynurenic acid (A2)
Intracellular modulator is the polyamine spermine which blocks the receptor internally at depolarised membrane potentials (neuroprotective?), giving the current a characteristic inwardly rectifying shape (not A2 however).
What are some characteristics of AMPA receptor mediated currents?
The current is an EPSC which is inwardly rectifying when mediated by calcium permeable AMPA receptors. It has a rapid rise time (<1ms) and time course of 1-3ms. This can vary subject to the type of subunits constituting the receptor. Calcium permeable receptors have a high single channel conductance
What is the role of AMPA receptors in plasticity?
AMPA receptors are subject to modulatiom by a variety of intracellular proteins, often triggered by Calcium dependent mechanisms, which can alter the number, subunit conformation, or phosphorylation state of the receptor. The presence of calcium permeable receptors at a synapse confers a large flux of calcium when activated thereby giving a stronger response and triggering of intracellular signals, these may be upregulated in LTP. Conversely, GluA2 containing receptors may be endocytosed via clathrin coated pits, removed from the cytoskeleton and allowed to diffuse laterally to extrasynaptic sites. In times of low frequency stimulation, these A2 receptors may be upregulated to decrease the calcium influx, leading to LTD.
Where are AMPA receptors implicated in disease?
Post-ischemia there seems to be a downregulation of GluA2 subunits, leading to greater Calcium influx and excitotoxicity.
In motor neuron disease, there is an editing deficiency of calcium permeable GluA2 subunits, causing disease activity.
During acidosis, insertion of GluA2 subunits is triggered - a possible neuroprotective mechanism.
What are the mechanisms of AMPA receptor plasticity?
Glutamate application increases the rate of AMPA receptor diffusion to the synapse, including from extrasynaptic sites (pool of receptors ready?), and decreases immobile receptors. Blocking of NSF by botox reduces LTP, showing importance of vesicle fusion. Kinases are implicated in LTP ie CaMKII, PKC, PKA, MAPK. Phosphatases are implicated in LTD ie PPI, PP2B.
What is the function of NMDA receptors
They function in normal synaptic transmissiom to a small extent, but more so at depolarised membrane potentials where they act to trigger Calcium dependent second messenger systems which can lead to long term changes such as synaptic plasticity; they therefore convert patterns of synaptic activity into physiological changes. They carry the slower second component of glutamate mediated EPSC, owing to slow unbinding of glutamate from the agonist binding clamshell domain.
What is the structure, stoichiometry and characteristics of NMDA receptors?
They are tetrameric receptors of GluN1(1a-4b), 2(A-D) and 3(A-B) subunits with NTD, 4TMs and CTD. They are arranged in a heterotetrameric fashion of GluN1 and 2 and require glycine binding as a coagonist (GluN1 or 3). They are usually diheteromers but can be triheteromers. They are blocked by extracellular magnesium at resting potentials which is increasingly releived at depolarised potentials, allowing cation influx. They have high calcium capacitance. Much of the characteristics of the receptor is determined by the type of GluN2 subunit present.
What is the EPSC of NMDA receptors like?
It shows a characteristic āJā shape on an I-V plot, due to the inherent voltage dependent Mg ion pore block. The time course is slow (>100ms) and the rise time is ~10ms. The conductance levels vary depending on the GluN2 subunit with A and B at 50ps and C and D at 22/36 and 16/35 respectively. Deactivation time decreases from A to D, as done Mg affinity (A,B - 15um; C,D - 80um)
What are some modulators of NMDA receptor function?
Endogenous modulators include: Zinc at 2A subunits, polyamines at 2B subunits and protons at 2B/2D at physiological pH (all extracellular).
Exogenous modulators include: Ifenprodil at 2B, DQP-1105 at 2D, AP-5 at all.
What impact do different subunits have on the NMDA receptor?
GluN1-2 is widely distributed.
GluN1-1,4 have complementary expression.
Embryonic rat have 2B and 2D only.
2A rises strongly after birth to widely.
2B maintains then becomes restricted.
2D drops markedly. 2C is cerebellum (weak Mg = function at low freq = faster plasticity) and olfactory bulb.
3A peaks postnatal then declines.
3B slowly increases into adulthood.
2B,2D,3A in development = role in synapse maturation/synaptogenesis?
2A,2B in adult = synapse function/plasticity?
At the synapse = 2A or 2A/2B
Peri/extrasynaptic = 2B (nb. not final - also may be 2C & 2D at both sites)
LTD = 2B make larger contribution
LTP = 2A make larger contribution
What are the molecular mechanisms of NMDA receptor mediated plasticity?
The GluN2 subunit has a large CTD rich in domains able to interact with intracellular proteins or residues able to be phosphorylated. It also has a high calcium flux which triggers such proteins that are calcium dependent. This region is the least conserved between subunits so confers a high level of subunit specificity.
Retention of the 2B subunit at the synapse depends on PDZ binding MAGUKs whereas 2A remains without, indicating that high frequency and this calcium influx increases MAGUK activity inserting more 2B receptors (longer time course = more calcium flux ability). There is also evidence that CaMKII preferentially binds to the 2B CTD having further implications for plasticity.
It is likely that the synapse contains 2A for high calcium flux and 2A/2Bs for signalling.
There is also evidence of neuregulin supressing src kinase which in turn suppresses LTP.
What are diseases associated with NMDA receptor?
Schizophrenia, ischemia-repurfusion injury, PD, pathological pain.
What are the types of memory and where is it likely located?
Declarative (explicit - facts,events) and Non-declarative (implicit - conditioning, procedures, emotions, skeletal tone).
Mainly studied in the hippocampus in CA3-CA1-dentate gyrus.
Patient HM had his hippocampus removed as the focus of epilepsy and could not retain new information.
