GABA, anxiety, insomnia

What is anxiety?

• Extension of flight or fight response

• When threatened, autonomic reflexes, defensive behaviour and negative emotions activated

• In anxiety this occurs without the stimulus

• GAD, phobias, PTSD, COD

What are the problems with early stage experiments of anti-anxiety drugs?

• Animal models

• Can only infer what an animal is feeling through behavioural responses

What is anxiety?

• Extension of flight or fight response

• When threatened, autonomic reflexes, defensive behaviour and negative emotions activated

• In anxiety this occurs without the stimulus

• GAD, phobias, PTSD, COD

What are the problems with early stage experiments of anti-anxiety drugs?

• Animal models

• Can only infer what an animal is feeling through behavioural responses

Describe the action of diazepam

• Positive allosteric activator of GABA-A

• Diazepam alone does nothing, but add low dose GABA and gives stronger response on patch clamp EP than same dose GABA alone

• High dose saturating GABA response is not increased, therefore diazepam is increasing affinity for GABA

• More inhibition of excitatory signals that could be triggering autonomic F/F fe

Discuss the use of benzodiazepines as anti-anxieties

• Can cause sedation and lack of motor coordination (as increased doses are used after these purposes, wide range of subunits bound)

• Great for fast effect (within 1h, in a crisis e.g. hospital)

• Dependence, tolerance and addiction risk, so use limited to about 1 month

• Withdrawal can cause rebound anxiety or withdrawal-induced seizures

Describe how benzodiazapines can be anti-anxiety - name one

• Can only bind alpha1-3, alpha5 and gamma2 subunit subtypes, but it is alpha2 binding with anti anxiety effects

• Increase GABAergic inhibition to calm autonomic F/F

• Alprazolam (Xanax)

Describe fluoxetine as an anti-anxiety

• SSRI - Bock SERT

• Initally decreases 5HT (bind Gi inhibitory 5HT receptors in preSN)

• Then these desensitise, so 5HT can be released

• Less reuptake so increases [5HT] to decrease anxiety

Describe Buspirone as an anti-anxiety

• 5-HT1AR partial agonist

• These are inhibitory Gi autoreceptors that decrease 5HT release

• Constant buspirone binding desensitises them, can release 5HT again but now no autoinhibition

Discuss use of drugs that target seratonin as anti-anxieties, and name two

• Buspirone, fluoxetine

• Slower onset than BZD

• Onset action can initially increase anxiety, missing doses can cause reflex anxiety, dependence means need to be titrated off

Describe the use of propranolol as an anti-anxiiety

• Non selective beta adrR antagonist (beta blocker)

• Inhibits the physical symptoms of anxiety e.g. beta1 tachycardia or beta2 muscle tremor

• These drive further feelings of anxiety, beta blocking can be sufficient

• Asthma no

Summerise the different targets of anti-anxieties, giving a drug for each

• GABA-A agonism: benzodiazapines e.g. alprazolam

• Increase 5-HT: SSRIs e.g. fluoxetine and inhibitory autoreceptor partial agonist buspirone

• Physical symptoms: propranolol

Is GABA excitatory or inhibitory?

• Inhibitory (main inhibitory neurotransmitter)

Describe the different types of GABA receptor

• GABA-A - ionotropic (LGIC), expressed post- and extra-synaptically so that GABA can be a neurotransmitter and neuromodulator

• GABA-B - metabotropic (GPCR)

Describe the synthesis of GABA and what this means?

• Glutamate —> GABA via glutamic acid decarboylase (GAD) int eh pre-SN

• Only formed in neurones expressing GAD, whereas all neurones contain glutamate

How is GABA packaged in vesicles?

• VIAAT antiporter of GABA for H+, could potentially be co-transp with Cl-

Describe GABA inactivation

• Reuptake into preSN via GAT1/2 symporter with 2Na+,Cl-

• Uptake into glia via GAT2/3 symporter with 2Na+, Cl-

Compare glutamate and gaba synthesis, storage and release

• Glutamate in all neurones, GABA only in those that express GAD

• Glutamate into vesicles by VGLUT1-3, GABA by VIAAT antiporter

• Both reputake into preSN or uptake into glia for termination, but glutamate uses GlnT whereas GABA is GAT1/2 on preSN, GAT2/3 for glia

What is the subunit structure of the GABA-AR?

• 2alpha1, 2beta2, gamma2

• Arranged in alpha-beta-alpha-beta-gamma pattern around central pore

What does GABA-A influx upon ligand binding? What does this do?

• Cl-

• Hyperpolarisation (terminatesdepolarisation / makes threshold value harder to reach)

What is a difficulty of GABA-A drug design?

• Pentameric, 19 possible subunits plus splice variants

Where does GABA bind on GABA-A?

• Alpha/beta site

What drugs can bind the alpha/beta site of GABA-A and therefore what are they? What does the drug do?

• Competitive antagonists with GABA

• Bicuculline

• Reduces GABAergic inhibition, increasing excitation, convulsant

What drugs can bind the alpha/gamma site of GABA-A and therefore what are they? What does the drug do?

• Positive allosteric modulators to increase GABAergic inhibition, decreasing excitation

• Benzodiazapemes e.g. diazepam for anxiety

• Z-drugs e.g. zoldipem for isomnia

Where do anaesthetics act on the GABA-A? Name one

• Site that is not alpha/beta or alpha/gamma

• Positive allosteric

• Propofol

What is insomnia?

• Difficulty falling and/or staying asleep

• Daytime sleepiness, irritability, low energy

• Can be caused by anxiety, stress, depression

Describe zoldipem as a hypnotic

• Binds the same site as benzos: alpha/gamma of GABA-A, unsure as to whether widepsread effect is needed or if there is a “sleep centre” included

• Shorter half life (2h vs 20h of diazepam)

• Increase inhibitory signaling can calm brain, decreasing time to fall asleep and increasing time asleep

• Tolerance/dependence fast, but slower than benzos, 2 week course

Describe promethazine as a hypnotic

• H1 receptor antagonist

• Histamine agonism drives wakefulness

Describe amitryptiline as a hypnotic

• Tricyclic antidepressent, but also blocks H1 receptor - useful if insomnia is being caused by depression

• Histamine agonism drives wakefulness

Discuss the different targets for insomnia and a drug for each

• GABA-A increase inhibition: zoldipem

• Decrease histamine agonism: promethazine, amitriptyline

Compare anti-anxieties and anti-insomnias

• Both can target GABA-A: benzos vs zoldipem, PK as due to shorter halflife, so tolerance slower

• Antidepressents used: in anxiety, SSRIs to increase seratonin, in insomnia, tricyclic due to H1 offsite action and if insomnia is being caused by depression

• H1 antagonism e.g. promethazine in insomnia

• Beta blockers e.g. propranolol in anxiety