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Fig 2. Schematic diagram of a GABAA receptor protein ((α1)2(β2)2(γ2)) which illustrates the five combined subunits that form the protein, the chloride (Cl-) ion channel pore, the two GABA active binding sites at the α1 and β2 interfaces, and the benzodiazepine (BZD) allosteric binding site at the α1 and γ2 interface.

Gamma-aminobutyric acid, otherwise known as GABA, is a major inhibitory neurotransmitter in the central nervous system ^[1] and is widely used for its pharmacological modulation of brain functions. Vast majority of the neurons in the brain have GABA receptors and of the different GABA receptor types the GABA_A are generally considered to be the most important for pharmacological modulation. A popular misconception is that many commonly used sedative and anxiolytic drugs that have an effect on the GABA receptor are agonists when many of these drugs are actually positive allosteric modulators. These positive allosteric modulators do indeed bind to the GABA receptors but they can't induce a respose from the neuron without a real GABA agonist present. Drugs from this chemical group produce a pharmacodynamic action by increasing the effects of the actual agonists when the potentiation is achieved. That is they can't function at all without the agonists. The positive allosteric modulators manage this effect by increasing the frequency with which the chlorine channels in the GABA receptors open when an agonist bind to its site. This increases the concenctration of the chlorine ions in the postsynaptic neuron hyperpolarizes the neuron and makes it less excitable and inhibits the possibility of an action-potential. ^[2]

History

The GABA_A receptors have been historically a target of several drug treatment research where the earliest compounds were ions, like for example bromide then barbiturates and finally benzodiazepines. Barbiturates were first introduced in 1904. They were a big step for treatment of psychiatric and neurological disorders of the time. They were useful in treatment of sleep disorders and were the first truly effective treatment for epilepsy. In 1950 benzodiazepines were discovered and for the biggest part replaced the barbiturates because of larger theraputical index.^[3] At first benzodiazepines were considered to be very safe and very efficient minor tranquilizers but later they were criticized for their dependence producing effects. These effects are mainly a concern for prolonged use of benzodiazepines as long acting anxiolytic but not when used as a short acting sedative or as a sleep inducing compound. In addition to the dependence producing effects prolonged use has a very clear tolerance development which has limited their use in medicine. Currently there are several efficient benzodiazepines available and so one can be selected in relation to its dosage form, length of action, metabolic interaction and safety as needed. These relatively effective compounds exist for insomnia, anxiety and sedation and in addition to those there exists a selection of α1 subunit-preferring hypnotics namely zolpidem, zopiclone, and zaleplon. ^[4] Benzodiazepines function by binding to the benzodiazepine site on most, but not all, GABA_A receptors. GABA_A modulation by benzodiazepine site agonists is self limiting and the conductance of the channel is not higher with the presence of benzodiazepine and GABA than the conductane with the presence of only high concentration of GABA. Additionally in the absence of GABA the presence of benzodiazepines alone does not open the chloride channel. ^[5]

Hans Selye demonstrated in the 1940 that certain pregnane steroids could cause both anesthesia and sedation^[6] but it wasn't until 40 years later that molecular mechanism emerged to explain the depressant effect of these steroids. In a rat brain slice preparation, the synthetic steroidal anesthetic alphaxalone (5α-pregnan-3α-ol-11,20 dione) enhanced both stimulus-evoked inhibition and the effects of exogenously applied muscimol which is a GABA_A selective agonist.^[7]

Receptor

The GABA_A receptors are made up of subunits which form a receptor complex. The receptor subunits in humans are 19 and are classified into α (1–6), β (1–3), γ (1–3), δ, ε, π, θ, and ρ (1−3). The function of the receptor is different according to how the pentameric complex is put together. The most common complex which includes around 40% of the GABA_A receptors is the α1β2γ2 combination. The expression of the subunits can be very different depending on brain region.^[8]^[9] The combination of the subunits influences how the receptor acts. For example if the α1 and β2 subunits are expressed together they have high sensitivity to GABA but low channel conductance. But if the γ2 is expressed with α1 and β2 the sensitivity is low and channel conductance is high^[10]. γ2 subunit has to be present for high affinity binding of benzodiazepine. The knowledge on where different complexes are located in the brain is still minimal. That makes it hard to develop drugs for certain regions.^[9]

Available agents

Mechanism of action

Barbiturates

Although the precise sites of action of barbiturates have not yet been defined, the second and third transmembrane domains of the β subunit appear to be critical; binding may involve a pocket formed by β-subunit methionine 286 as well as α-subunit methionine 236. In addition to effects on GABA_A receptors, barbiturates block AMPA/kainate receptors, and they inhibit glutamate release through an effect on P/Q-type high-voltage activated calcium channels. The combination of these various actions likely accounts for their diverse clinical activities.^[11]

Insomnia

Barbiturates were first introduced as hypnotics for patients with schizophrenia. It induced a state of deep and prolonged sleep. But this was not used for long because of adverse effects.^[3]

Antileptic

Phenobarbital was the first truly effective drug against epilepsy. It was discovered by accident when given to epileptic patients to help them sleep. The positive side effects were the anticonvulsant properties which reduced the number of seizures and their intensity.^[3]

Sedation

Pentobarbital is used as a hypnotic when analgesia is not required. It´s often used in CT imaging when sedation is needed. It´s efficient, safe and the recovery time is short.^[12] In 2013 the barbiturates Phenobarbital and butabarbital are still used as sedatives in certain cases as well as to antagonize effects of drugs as ephedrine and theophylline. Phenobarbital is used in cases of withdrawal syndromes. It´s still used as normal and emergency treatment in some cases of epilepsy.^[3]

Benzodiazepine

Synaptic action of benzodiazepines: GABA_A receptors that are located at synapses are activated when they are exposed to high concentration of GABA. Benzodiazepines enhance the receptor affinity for GABA by decreasing the decay of spontaneous miniature inhibitory postsynaptic currents (mIPSC) ^[13]^[14]

Analgesic

Sedative actions of benzodiazepines limit their usefulness as analgesic agents and they are therefore generally not considered to be one but this limitation can be bypassed by intrathecal administration. GABA_A receptors in the periaqueductal gray are pro-nocieptive at supraspinal sites while GABA_A that are found in the spinal cord are anti-hyperalgesic. Spinal α2 and α3 containing GABA_A receptors are the ones responsible for the anti-hyperalgesic action of intrathecal diazepam. This was shown when the anti-hyperalgesic action was reduced when administered in α2 and α3 mice in inflammatory pain and in neuropathic pain. Additionally studies in α5 mice have shown that the spinal α5-containing GABA_A receptor has a minor role in in inflammatory pain. An α2, α3 and/or α5 selective positive allosteric agonist, like L-838,427 for example, might be useful as an analgesic drug against inflammatory or neuropathic pain.^[5]

Schizophrenia

Benzodiazepines are used as a supporting treatment in patients with schizophrenia.^[5]

Depression

GABA ergic hypothesis for depression has been proposed. This hypothesis places the GABA system in a central role in the pathophysiology of depression and in addition to that clinical studies have shown that alprazolam and adinazolam have antidepressant activities in patients with major depressive disorder. Unfortunately we do not know which receptor subtype is responsible for the antidepressant activities. Studies in y2 knockout mice have shown that they display increased anxiety and depressive-like symptoms in despair based tests. The mice also had increased corticosterone concentration which is a symptom in major depression in humans. The y2 subunit is associated with α1-α6 subunits, which are all known α subunits, so these studies do not show which of the α subunits are related to the depressive like symptoms. Other studies with α2 knockout mice have displayed increased anxiety and depression like symptoms in conflict based feeding tests and the fact that anxiety and depression are often linked seems to indicate that the α2 subunit might be a valid target for a GABA_A antidepressant.^[5]

Stroke

Preclinical studies have have shown that benzodiazepines can be effective in reducing the effect of strokes up to three days after the drug has been administered.^[5]

Neurosteroids

Neurosteroids can act as allosteric modulators of neurotransmitter receptors, such as the GABA_A,^[15]^[16]^[17]^[18] NMDA,^[19] and sigma receptors.^[20] The neurosteroid Progesterone (PROG) which activates progesterone receptors expressed in peripheral and central glial cells.^[21]^[22]^[23]^[24] Additionally it has been surmised that the 3α-hydroxy ring A-reduced pregnane steroids allopregnanolone and tetrahydrodeoxycorticosterone increase the GABA-mediated chloride currents while pregnenolone sulfate and dehydroepiandrosterone (DHEA) sulfate on the other hand have displayed antagonistic properties at the GABA_A receptors.

Synthesis

Barbituric acid

Fig 5. Original synthesis of Barbituric acid

Fig 6. Current synthesis of barbituric acid

Barbituric acid is the parent compound of barbiturate drugs although barbituric acid itself is not pharmacologically active. Barbiturates were synthesized in 1864 by Adolf von Baeyer by combining urea and malonic acid (see figure 5). Synthetic process was later developed and perfected by the French chemist Edouard Grimaux in 1879, making possible the subsequent widespread development of barbiturate derivatives^[25]. Malonic acid has since been replaced by diethyl malonate, as using the ester avoids the problem of having to deal with the acidity of the carboxylic acid and its unreactive carboxylate (see figure 6). Barbituric acid can form a large variety of barbiturate drugs by using the Knoevenagel condensation reaction^[26].

Benzodiazepines

The structure that the first benzodiazepine is based on was discovered by Leo H. Sternbach. He thought the compound had a heptoxdiazine structure (See figure 7) but it was later determined to be a quinazoline-3-oxide. Possible drug candidates were then synthesized from that compound and screened for activity. One of these compounds was active, chlordiazepoxide. It was marketed in 1960 and became the first benzodiazepine drug.^[27]

Biosynthesis of Neurosteroids

Neurosteroids are synthesized in the central nervous system (CNS) and the peripheral nervous system (PNS) from cholesterol and steroidal precursors that are imported from the peripheral sources. These sources include 3β-hydroxy-Δ5 derivatives, such as pregnenolone (PREG) and dehydroepiandrosterone (DHEA), their sulfates, and reduced metabolites such as the tetrahydro derivative of progesterone 3α-hydroxy-5α-pregnane-20-one (3α,5α-THPROG). After the local synthesis or from metabolism of adrenal of gonadal steroids many neurosteroids accumulate in the brain. ^[28]^[29]