Anxiolytic and hypnotic drugs
- Used to treat anxiety and insomnia
- Anxiety: an emotional state characterized by excessive feelings of worry fear and nervousness
- Insomnia: the inability to initiate maintain or achieve restful sleep
Content:
1. Role of GABA in the central nervous system
2. Limbic system
3. Anxiolytic and hypnotic drugs
a. Benzodiazepines
b. Barbiturates
c. Non-benzodiazepine hypnotics
2. Limbic system
3. Anxiolytic and hypnotic drugs
a. Benzodiazepines
b. Barbiturates
c. Non-benzodiazepine hypnotics
1. Role of GABA in the central nervous system
Gamma-aminobutyric acid (GABA)
- a major inhibitory neurotransmitter
- when GABA reaches and binds to specific receptor located on a neuron, it reduces the activity level of that neuron and makes it less likely to fire its action potential
- recognized by both ligand-gated and G-protein linked GABA receptors
- ligand-gated GABA receptor (GABA-A): main target of many anxiolytic and hypnotic agents
- GABA-A receptor is made up of five subunit proteins arranged to form a central pore
- 2 alpha, 2 beta, 1 gamma or 1 delta
- alpha-1: sleep
- alpha-2 or alpha-3: anxiety
Activation
When two GABA molecules bind to the GABA-A receptor,
- conformational change is induced
- opening of the central pore
- negatively charged chloride ions to rush into the cell
- increases the negative charge inside the postsynaptic cell
- more difficult for an excitatory inputs to cause an action potential
2. Limbic system
- GABA-A receptors are present in high concentration in the limbic system
- amygdala which contains many neurons that use the neurotransmitter GABA
- control feelings of fear and anxiety
- stress excitatory neurons in the amygdala fire rapidly sending excitatory signals to other areas of the brain, leads to feeling of panic or fear
- the inhibitory interneurons in the amygdala regulate these emotions by releasing GABA and its binding to postsynaptic receptors such as GABA-A inhibits the excitatory signals, thus has a calming tranquilizing effect on our emotions and prevents us from becoming overwhelmed in stressful situations
3. Anxiolytic and hypnotic drugs
can be divide into 3 groups: benzodiazepines, barbiturates and non-benzodiazepine hypnotics
a. Benzodiazepines
- a benzene ring fused to a diazepine ring
- Binding site: alpha and gamma subunit on the GABA-A receptor
- benzodiazepines do not bind to the GABA recognition site located at the junction of the alpha and beta subunits 👉 they don't induce opening of the channel
- decreased anxiety and greater feeling of calm
- binding of a benzodiazepine to its receptor site
- increases the affinity of GABA for the GABA binding site
- increased frequency of the channel opening
- increased influx of chloride ions at the postsynaptic cell
- ultimately makes it difficult for any excitatory inputs to cause an action potential
- can also produce mild muscle relaxation
- their action on GABA receptors in the spinal cord cerebellum and brain stem
Drug:
- Alprazolam
- Chlordiazepoxide
- Clonazepam
- Diazepam
- Lorazepam
- Temazepam
Side-effects of benzodiazepines
- Drowsiness
- Dizziness
- Decreased alertness
- Decreased concentration
- Impairment of motor coordination
- all due to sedating and muscle relaxing action
b. Barbiturates
- Barbiturates bind to GABA-A receptor at the alpha or beta subunit
- enhance the effect of GABA at the GABA-A receptor
- instead of increasing the ion channel opening frequency, they increase the ion channel opening duration
- (barbiturates hold the ion channel open continuously for long periods)
- can decrease the activity of excitatory neurotransmitters such as acetylcholine and glutamate
- At higher doses, they can also mimic GABA and directly activate GABA receptors, thus leading to profound central nervous system depression
- Hence, its has lower therapeutic index than benzodiazepines
Drug:
- Pentobarbital
- Phenobarbital
- Secobarbital
Side effects of barbiturates
- Dizziness
- Lightheadedness
- Sedation
- Memory and attention impairments
- coma or death due to respiratory depression
- because of their narrow therapeutic index
c. Non-benzodiazepine hypnotics
- selectively bind to the alpha-1 subunit of the GABA-A receptor thus triggering chloride channel opening with resulting membrane hyperpolarization
- because alpha-1 containing GABA-A receptors are highly expressed in wake promoting regions of the brain
- results in sleepiness
Drug:
- Zolpidem
- Zaleplon
- Eszopiclone
Side-effects of non-benzodiazepine hypnotics
- cognitive impairments including memory loss daytime sedation
- impairment of motor function
4. Reference
https://youtu.be/4ZHudeMho8g
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