GABAergic signalling in health and disease Flashcards
what is GABA
gamma amino butyric acid
primary inhibitory neurotransmitter
widely distributed
what are the subtypes of gaba
2 ionotropic 1 metabotropic
GABAA and GABAC ionotropic - Cl- channel linked, GABAB changes permeability for K+
outline GABAA receptor
pentameric complex - 5 subunits
mediates most inhibitory neurotransmission in adult brain - target for many drugs and therapeutic compounds
what forces act on the movement of ions across membranes
- chemical gradients
- electrical gradients
- equilibrium
how is equilibrium for Cl- flux calculated
Nernst equation - calculates equilibirum potential 11x more chloride outside than inside
-62mV = in is more negative than outside (11x higher than inside), no net movement across membrane
what is equilibrium
reached when concentration and voltage gradients have the same energy, they oppose eachother and there is no net movement of Cl- across membrane
how does passive flux of Cl- out of the cell happen
membrane potential changes, cl- is negatively charged so repels chloride from cytoplasm and there is passive efflux of chloride, it moves out
how does passive flux oc Cl - into the cell happen
voltage gradient has less energy than concentration gradient, which dominates
cl- passively leaks into cell and is actively transported out of cell by specialised transporter molecules
what is synaptic inhibition
small hyperpolarisation is sufficient to prevent membrane from reaching threshold to generate action potentials
- membrane is slightly more negative = harder to reach threshold potential
in certain conditons GABAA can be DEPOLARISING and even sometimes EXCITATORY - when?
early development, in some phases of circadian rhythm
trauma and epilepsy
what does GABAA plasticity depend on?
chloride homeostasis
- concentration ratio maintained across cell membrane
what is EGABA
gaba reversal potential
at EGABA, gaba responses change polarity, it depends on chloride concentration
what is gaba reversal potential
membrane potential at which gaba responses change polarity from hyperpolarising to DEPOLARISING
when does EGABA shift to more depolarising levels
when there is an increase in intracellular chloride - big depolarising response maybe even action potential depending on how much chloride is in cell
what are changes in Cl in and EGABA mediated by
secondary active transporters that either take up or extrude CL-
what is the difference between primary and secondary active transporters?
primary - pump that depends on metabolic energy ATP, constantly pumps sodium out of cell and potassium in against gradient
secondary = derive energy from ionic concentration differences in sodium or potassium
what is Cl- uptake mediated by
Na+, K+, 2Cl- co-transporters (NKCCs)
Na+ independant anion exchangers (AEs)
what is Cl- extrusion mediated by
K+, Cl- co-transporters (KCCs)
Na+ dependant anion exchangers (NDAEs)
what does activity in KCC mean for GABAA
decrease in intracellular chloride activates GABAA which opens channel, chloride moves along electrochemical gradient, which is now bigger because less cl- inside cell due to extrusion, chloride moves in and hyperpolarises - normal synaptic inhibition
what does high activity in NKCC mean for gabaa
inwardly directed sodium gradient - influx driving cl- into cytoplasm, higher intracellular cl concentration activates gabaa channel, now have efflux of cl-, negative charge moving out so inside becomes more positively charged - depolarising gabaa response
what are cation chloride co-transporters
large transmembrane glycoproteins
NKCC1 is prominently expressed in CNS
KCC2 is exclusively expressed in mature neurons
what is KCC2 largely responsible for
low CL- in mature cells
when are GABAA responses depolarizing
when they elicit a cl- efflux
happens when:
chloride extruded from cell is weaker (less KCC2 activity)
or when chloride uptake by cell is stronger (more NKCC1 activity)
outline early GABAA responses
depolarising because little KCC2 and NKCC1 is prominent
as development progresses, downregulation of NKCC1 and upregulation of KCC2 = signficant reudction of intracellular chloride
activate GABAA receptor = influx of negative charges - hyperpolarisation
what does GABA influence
neuronal survival, neurite outgrowth, synapse formation
GABAA membrane depolarisation is sufficiently strong to open what
voltage gated Ca2+ channels - signficant increase in calcium - shows gabba infleucnes many important functions
calcium into cell has fundamental effects on cell function
outline excitatory GABA responses in embryonic and neonatal cortical slices
Egaba more positive than Vrest in early development -> gaba depolarises developing neocortical cells
Egaba is most positive in youngest precursor cells (VZ)
progressively shifts to more negative values with development - determined by Cl gradient, which decreases intracellularly with development
what did a 1980s study on neonatal rats show
electrode in pyramidal cell saw spontaneous GABAergic responses (GDP = giant depolarising potentials)
P4 pyramidal cells in rats
Bicuculline (GABAAR antagonist) - membrane hyperpolarisation, reduced synaptic noise and blocked GDPs
what role do glutaminergic pyramidal cells have
pacemakers
what does depolarising GABA provide
provides tonic excitation to the network (depolarises pyramidal cells all the time, leads to influx of calcium that triggers important things in development)
what is the difference between GABAA connections and glutaminergic connections
gabaa - permissive
glutaminergic - instructive (determine the 0.3Hz rhythm)
why do the depolarising responses disappear in development
KCC2 co-transporter that pumps Cl- out of cells will be gradually upregulated
what happens when knock out KCC2 for early activity in respiratory neurons
rhythmic discharges in wild type
mice die at birth from suffocation
no rhythmic activity in KO mice
neuronal CL- intrusion is ineffective
GABAA responses remain excitatory
prevent synchronous patterned motor activity
is GABAA signalling sex-specific
yes occurs earlier in females
correlates with developmental expression profile of chloride transporters
ESTRADIOL - predominant sex hormone in females - upreguklates KCC2 mRNA
TESTOSTERONE - male, downregulates KCC2 mRNA, effect is via GABAA receptors and Ca2+ channels - CREB - important for sexual differentiation
example of developmental plasticity
what is the role of oxytocin
at birth oxytocin (hormone essential in labour) triggers transient disappearance of depolarising GABAA receptors in CA3 pyramidal cells
demonstrates important interaction between oxytocin production in mother and changes innCL- homeostaiss in foetus - protects foetal neurons during delivery
outline role of GABA in suprachiasmatic nucleus
SCN received info on light brightness and duration (day vs night) from retina
dispatches info to parts of brain and body controlling circadian rhythmicity
SCN neurons are pacemakers - most GABAERGIC
changes in GABAA activity throughout clock influence SCN output during day and night (different gating mechanism)
outline effects of GABA on SCN
opposite effects during day and night
inhibitory during day
excitatory during night
how are EGABA distributed in the SCN during night
not normally distributed, indicates functional effects of GABA are heterogenous
what is the role of day-night antagonism in GABAergic neurotransmission
provides SCN with time-dependant gating mechanism
1. dampens propagation of excitatory signals throughout biological clock during day
2. promotes it at night
outline GABAergic plasticity in patophysiological conditions
GABAA receptor -mediated responses after trauma are DEPOLARIZING (because of increase in intracellular calcium)
there is excessive release of glutamate, leading to elevated ca2+ intracellularly, excitotoxic cellular damage and cell death
what are examples of neuronal trauma
neurite transection
osmotic imbalance
excess heat
outline KCC2-mediated GABAergic plasticity during epilepsy
increasing intracellular K+ concentration can stimulate epilepsy and generate functionally excitatory GABAA response because KCC2 has high transport affinity for external potassium = leads to epileptic state
what does KCC2 having a high transport affinity for external K+ mean - for stimulating epilepsy
increase K+ out is followed by influx of CL- via KCC2 , results in positive shift in EGABA, increased gross excitability
outline activity-dependant downregulation in KCC2 expression and function in the hippocampus
epileptiform activity induced by Mg2+
decrease of KCC2 immunofluorescent staining
KCC2 downregulation mediated by BDNF acting on TrkB receptors
KCC2 downregulation has fast onset and lasts long
why are males more susceptible to seizures than females
may be linked to fact at early developmental stages GABAA signalling remains depolarising for longer in male brain areas involved in seizure network
what is the link between GABAA and autism spectrum disorder?
developmental shift from excitation to inhibition in GABAA responses is delayed in autism
this is interesting because ASD is more prevalent in males than females
what may contribute to the prevalence of autism in males
perinatal exposure to high levels of testosterone
also may be polymorphisms in GABAA receptor subunits: may also be linked to susceptibility traits for autism
-> factors controlling GABAA signalling important in pathogenesis of disease
what can we conclude about GABAA mediated responses
rapidly switch from inhibition to excitation through simple mechanism of modifying intracellular concentration of chloride via up or downregulation of cation chloride cotransporters
