Calm down dear, it's only a sedative
Tranquilisers! In short, sedatives are tranquilisers. What do they do? They reduce anxiety and quieten a person down, without impairing consciousness or inducing sleep. Agents which induce sleep are called hypnotics (like sleeping pills). Though, some antidepressant sedatives do have hypnotic-like side effects (Dang and blast that non-specific drug binding!)
You may or may not know that neurons in the brain (and everywhere else) can be divided into two functional groups. Those that cause excitation and those that cause inhibition. In a massively complex structure such as the brain, a single neuron may be contacted by over one thousand other neurons. Some of those neurons will release excitatory neurotransmitter onto the neuron in question, and cause the neuron to fire an impulse and continue to drive that neural circuit it is involved in, thus mediating the behaviour it may be involved in. Other neurons will be inhibitory and will cause the neuron in question to stop firing, or make it less prone to firing if it is innervated by an excitatory neuron.
A quick diagram I drew explaining the basic ideas behind excitation and inhibition in simple neural circuits. Extrapolate this by a thousand times and you should get an idea of how things work in the brain.
To excite or inhibit the next neuron, neurons release neurotransmitter into the synapse. Neurotransmitters are chemicals released by neurons which bind to receptors on other neurons to either cause excitation (depolarisation) or inhibition (hyperpolarisation). There are many different neurotransmitters, and each neurotransmitter binds to a specific receptor (or specific receptors). Sedatives mimix inhibitory neurotransmitters - in particular, the neurotransmitter known as Gamma-amino-butyric-acid (GABA). This means that sedatives are acting like inhibitory neurotransmitters (or potentiating the action of existing inhibitory neurotransmitters) and are increasing inhibition in the brain. Now, sedative drugs are specific to particular regions of the brain. We don't just want inhibition occuring everywhere in the brain as that would be bad, and would probably more of a general anaesthetic effect as opposed to sedative - as sedatives aren't supposed to send you to sleep as their primary function.
The GABA receptor is a protein complex built up of 5 smaller protein subunits. It is embedded in the membranes of many neurons and opens to allow chloride ions into the neuron, upon stimulation by the neurotransmitter GABA. The diagram is showing where various agents physically bind to the receptor. Benzodiazepines are the sedatives I have been talking about (vallium). When benzodiazepine drugs are taken, they bind with the GABA to enhance its effect and cause increased inhibition. Propofol is a type of general anaesthetic. Ethanol also seems to bind to GABA receptors, implicating GABA action in drunk states. Neurosteroids such as progesterone also potentiate the action of GABA (and see a post on that..... here)
The amygdala is a structure in the brain responsible for mediating anxiety, fear and anger. It is a bilateral structure (there are two of them in either hemisphere of the brain). When regions of this are activated, fear, anger or anxiety levels increase. Sedatives sedate people (as the name indicates), so they reduce anxiety, fear and aggression, making a person very calm and tranquil (hence the name tranquiliser). Sedatives seem to increase inhibition in the circuits of the amygdala, and those that feed into the amygdala, thus reducing the output from this structure and decreasing anxiety, fear and aggression. These drugs are particularly useful on dangerously aggressive patients who are compromising the health of themselves and those around them, dangerous animals loose in the city, or an anxious patient who may be slightly averse to receiving a treatment which looks like it may be painful - like somebody going to the dentist or something.
It's that amygdala again...
Not all sedatives operate in this way, but the more common minor ones operate like this. Diazepam (aka Valium) can be described as a sedative, and operates via the aforementioned mechanism at GABA receptors. Anxiolytic drugs tend to have a sedative effect as they are affecting the amygdala. Well, not sure how well I explained all that, but here's to hoping some of it made an iota of sense! I guess I shouldn't get anxious over it.
It's probably also worth a mention that this post was inspired by a question from Superfoo! :D
