GABA-A diversity Flashcards
What is the role of GABA?
Main inhibitory neurotransmitter in the adult CNS and is responsible for mediating rapid inhibition via GABA-ARs
How do GABA-A ion channels act?
Increase the membrane permeability to chloride and bicarbonate ions, usually resulting in a net inward flow of negative charge and an IPSP
GABA-ARs belong to a family of pentameric LGICs. How are these organised?
Multi-subunit proteins surrounding a central pore
How many genes are there for GABA-AR subunits?
19: a1-6, b1-3, gamma1-3, delta, epsilon, theta, pi and rho1-3; co-assemble to form heteromeric receptor complexes
What does a typical GABA-AR consist of?
Two a subunits, two beta subunits and a variable fifth subunit that depends on function and relative location to the synaptic cleft
What determines the functional properties and localisation of the receptor?
Specific subunit composition, molecular heterogeneity confers a large degree of variability in the response
What is the structure of an individual subunit?
Long N-terminal hydrophilic domain, comprising of a putative cys-loop, and four TM alpha-helices with a large intracellular loop connecting TM3 and 4. Ends with a short, extracellular C-terminus and the TM2 forms the lining of the pore.
What influences the sensitivity of different GABA-AR subtypes to GABA (EC50)? (ref)
Type of alpha subunit present: radioligand binding assays and electrophys. techniques found a segment of 4 amino acids in the N-terminal domain regulating sensitivity of axb3g2S receptors (Boehme et al., 2004)
What is the rank order of affinities?
a6 > a1 > a2 > a4 > a5»_space; a3
What does the magnitude of the IPSP depend on?
Duration of channel opening, time spent in open conformation, length of bursts of channel openings as well as number of receptors and associated input resistance
What did single-channel recordings of three receptor isoforms (a1b3, a1b3d, a1b3g2L) demonstrate? (+ref)
While these are similar in the presence/absence of a delta subunit, this produced a 5-fold increase in the duration of channel opening (Fisher and Macdonald, 1997) + increased open probability
What else is the alpha subunit critical for?
Determining the rise time of the current evoked, with activation rates in rank order a2 > a1 > a3
What is the effect of the gamma subunit on activation rate? (ref)
Gamma, but not delta, accelerates activation (Haas and Macdonald, 1999)
How is phasic inhibition defined? (ref)
Rapid, coordinated opening of a small cluster of postsynaptic channels, resulting from their exposure to a highly concentrated, transient pulse of GABA (Farrant and Nusser, 2005)
What is phasic inhibition dependent on?
Propagation of an AP, associated calcium ion influx that drives exocytosis, resulting in the synchronous activation of postsynaptic channels
Why are these receptors required to respond rapidly (phasic)?
As a result of the rapid clearance of GABA from the synaptic cleft by various uptake mechanisms (e.g. GAT1 and GAT3)
Where does the gamma2 subunit co-localise? (ref)
In postsynaptic densities alongside a1, a2, a3 and b2/3 subunits in synapses of the cerebellum, globus pallidus, hippocampus and neocortex (three predominant receptor types that mediate phasic inhibition (Farrant and Nusser, 2005)
What does the presence of the gamma2L subunit confer?
Ability to respond almost instantaneously, and in a synchronous manner, to transient concentrations of transmitter
What else is the gamma2L subunit implicated in?
Receptor trafficking, allowing the quick and efficient regulation of postsynaptic receptor pools and positioning of the clusters
What does the large extent of desensitisation serve? (ref)
To prevent the generation of abnormally large and sustained IPSCs (Nusser et al., 1995)
What does phasic inhibition play an important role in?
The generation of rhythmic activity and network oscillations
What have gamma oscillations been implicated in?
Memory processing, exploratory behaviour and consciousness - regulated by GABAergic interneurons
What do GABAergic interneurons in the hippocampus do (ref)?
Innervate hundreds of principal neurons and phase-lock the oscillatory signals of PCs to synchronise their activity, generating gamma frequency network oscillations (Merker, 2016)
What does synchrony allow for?
Transduction of signals that can be detected by downstream coincidence detectors, requires fast activation rate (conferred in part by gamma2L)
What would the absence of phasic inhibitory mechanisms lead to?
Runaway excitation in the cortex, possibly seizure-like activity at the network level and saturation of PC firing rate (Moore et al., 2010)
How is tonic inhibition transduced? (ref)
Through high-affinity extra/perisynaptic receptors that are exposed to low concentrations of GABA spilling over (Lee and Maguire, 2014)
What do ambient concentrations of GABA do?
Activate a low-amplitude tonic current that serves to regulate neuronal excitability
What is the specific subunit of these (tonic) receptors? (ref)
Incorporate the delta subunit in the dentate gyrus, cerebellar granule cells and thalamic neurons or the a5 subunit in CA1 and CA3 pyramidal cells (Lee and Maguire, 2014)
What does deletion of the a5 subunit lead to? (ref)
Eliminates tonic conductance in the hippocampus, as seen in a5-deficient mice that presented with an improved capacity for spatial learning and memory (Collinson et al., 2002)
What is the role of the delta subunit?
Subcellular localisation of GABA-ARs in non-synaptic regions, increase sensitivity to GABA as well as number of openings per burst
What does the persistent activation of high-affinity GABA receptors serve at the network level?
Reduce the magnitude and duration of the EPSPs, narrowing the temporal resolution over which signal integration can occur, thereby decreasing the probability of AP generation
What does the resulting shunting inhibition lead to? (ref)
A reduction in input resistance and produces a dextral shift in the input-output curve, without any changes in gain (Torres-Torrelo et al., 2014)
What is the role of tonic inhibition in the dentate gyrus? (ref)
Maintains the integrity of the dentate gate, which regulates neuronal excitability within the hippocampus by filtering synchronous excitatory activity in the EC (Coulter and Carlson, 2007)
What are properties of granule cells in the DG?
Low firing rates and relatively negative resting membrane potentials, due to persistent activation of delta-subunit containing receptors
What underlies differences in GABAergic conductance in the thalamus?
Sensory and motor inputs are relayed between cortical and subcortical areas, differences are attributed to subunit composition
What do thalamocortical neurons in the lateral geniculate nucleus express? (ref)
delta-subunit containing receptors that mediate tonic conductance; on the contrary, neurons in the nucleus reticularis display no evidence of tonic inhibition as they fail to express a5 or delta subunits (Cope et al., 2005)
