Lectures 11 and 12 Flashcards
What was shown by Harrison and Simmonds in 1984?
Showed that GABA-A-R function was enhanced by alphaxalone (similar structure to progesterone)- suggested GABA-A-R had a binding site for neurosteroids
What situations increase the levels of neurosteroids?
In acute stress, pregnancy (increase day before), with alcohol consumption and with antidepressants
What is 5a-pregnan-3a-ol-20-one?
Naturally occurring metabolite of progesterone that is synthesised in the periphery and CNS. Steroid potently and selectively enhances the function of the GABA-A-R.
What is tonic inhibition?
Inhibitory response mediated by the activation of extra- or presisynaptic GABA-A-R through ambient concentrations of GABA. In some neurons these receptors have a distinct subunit composition, high affinity for GABA and show little/no desensitisation.
What is the phasic inhibitory response?
Transient inhibitory response that results from the activation of postsynaptic GABA-A-R by the relatively high concentration of GABA in the synapse subsequent to vesicular release.
How can the tonic response be measured?
Use whole-cell voltage clamp techniques and treat with kynuenic acid to block glutamate receptors and TTX to block sodium channels. Can then look at spontaneous release of GABA.
How do benzodiazepines and neurosteroids change the GABA-A-R?
Will prolong decay of GABA response by making the conformation more stable, enhancing neuronal inhibition by phasic transient responses (have little effect on rise time or amplitude).
How do neurons in the HPA axis that are involved in acute stress and release stress hormone CRF react to neurosteroids?
Synaptic response to phasic inhibition can be seen in presence of 10nM of 5a3a.
Spontaneous firing can be seen in PVN neurons releasing CRH and 5a3a even- as increasing amounts are administered, probability of AP release decreases.
When subject to acute stress, HP axis is activated and steroids GABA active increase and feedback to decrease the firing of these neurons- feedback mechanism.
What is shown in the early life stress model of mice?
Adult ELS mice show:
- Reduced hippocampal LTP/ cognitive performance
- Perturbed GABA signalling in the accumbens and behavioural effects to cocaine
- Impaired HPA axis
What are the mechanisms underlying the ELS mice?
Early life stress events increase CRF expression in the PVN and prevent the neurosteroid inhibition by firing increasing glutamate release; steroid now no longer stops neurons from firing and there is an increase in excitation.
What is shown in parturition with neurosteroids?
Just before mother gives birth, she has high levels of 5a3a, which stop oxytocin release, allowing it to be stored for the birth. After 24 hours, receptors stop being sensitive to 5a3a and levels fall. This is caused by phosphorylation of GABA-A-R.
How are insect GABA-A-R different to those of human?
Much less senstive to neurosteroid modulation- mutations in glutamate in the sequence make receptor neurosteroid insensitive.
What was shown in the GABA-A-R a2 Q241W mouse mutatnt?
mouse was insensitive to neurosteroid. Although normal a2 receptors function almost the same to mutant, there was an increased anxiogenic phenotype and reduced sensitivity to 5a3a.
Duration of synaptic events in nuclear succumbens were reduced.
How are neurosteroids important in development?
GABA-A-R change during development. GABA-mediated phasic inhibition reduces in duration. Thought to be due to differences in subunit composition. Changes from a2/a3 (which have slow kinetics) to a1 with fast kinetics. Shown in VB neurons this is not the case and neurosteroids are involved.
What developmental changes occur in ventrobasal neurons during development? What evidence shows this is not just due to subunit change?
Developmental changes in VB mIPSCs occur over course of 24hrs. In a1 KO mouse, P8-9 there were slow decay events and in P10-11 events sped up even without a1. At P18, no synaptic events at all, showing change involves neurosteroid as well as subunit change.
