Inpharmacology,GABAA receptor positive allosteric modulators, also known asGABAkines orGABAA receptor potentiators,[1] arepositive allosteric modulator (PAM) molecules that increase the activity of theGABAA receptor protein in thevertebratecentral nervous system.
GABA is a major inhibitory neurotransmitter in the central nervous system. Upon binding, ittriggers the GABAAreceptor to open itschloride channel to allowchloride ions into theneuron, making the cellhyperpolarized and less likely tofire. GABAAPAMs increase the effect of GABA by making the channel open more frequently or for longer periods. However, they have no effect if GABA or anotheragonist is not present.
UnlikeGABAA receptor agonists, GABAAPAMs do not bind at the sameactive site as theγ-aminobutyric acid (GABA)neurotransmitter molecule: they affect the receptor by binding at a different site on the protein. This is calledallosteric modulation.
Inpsychopharmacology, GABAA receptor PAMs used asdrugs have mainlysedative andanxiolytic effects. Examples of GABAA PAMs includeethanol,benzodiazepines such asdiazepam (Valium) andalprazolam (Xanax),Z-drugs such aszolpidem (Ambien) and thebarbiturate drugs.
The GABAA receptors have historically been a target of drug treatment research. The earliest compounds were ionic compounds, such asbromide.
In 1903, the first psychoactive derivative ofbarbituric acid was synthesized and marketed for headaches. Within 30 years, many otherbarbiturates were developed and found use assedatives, sleep aids andgeneral anesthetics. Although barbiturates fell out of favor, they continue to serve as a short-actinganesthetic andanti-epileptic drugs.
Benzodiazepines were discovered in 1955 and largely replaced the barbiturates because of their largertherapeutic index.[2] At first benzodiazepines were considered to be safe and efficient minortranquilizers but then were criticized for theirdependence producing effects. Several efficient benzodiazepines offer choices aboutdosage form, length of action, metabolic interaction and safety.
Benzodiazepines function by binding to the benzodiazepine site on most, but not all, GABAA receptors. GABAA modulation by benzodiazepine site agonists is self-limiting. The channel conductance is not higher in the presence of benzodiazepine and GABA than the conductance with the presence of only high GABA concentrations. Additionally, in the absence of GABA the presence of benzodiazepines alone does not open the chloride channel.[3]
Certain metabolites ofprogesterone anddeoxycorticosterone are potent and selective positive allosteric modulators of the γ-aminobutyric acid type A (GABAA) receptor.[4]Hans Selye demonstrated in 1940 that certainpregnanesteroids could cause both anesthesia andsedation,[5] but 40 years later the molecular mechanism emerged to explain their depressant effect. In a rat brain slice preparation, the synthetic steroidal anestheticalphaxalone (5α-pregnan-3α-ol-11,20 dione) enhanced both stimulus-evoked inhibition and the effects ofexogenously appliedmuscimol, which is a GABAA selective agonist.[6]
The GABAA receptors are made up of subunits which form a receptor complex. Humans have 19 receptor subunits 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 thepentameric complex is put together. The most common complex that includes around 40% of the GABAA receptors is the α1β2γ2 combination. The expression of the subunits can be very different depending on brain region.[7] 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 lowchannel conductance. But if the γ2 is expressed with α1 and β2 the sensitivity is low and channel conductance is high.[8] γ2 subunit has to be present for high affinity binding of benzodiazepine. Little is known about where different complexes are located in the brain, complicating drug discovery.[7] For example, the binding site of neurosteroids in the GABAA receptor is not known[9] and barbiturates bind at a beta subunit that is distinct from the benzodiazepine binding site.
Barbiturates' precise action sites have not yet been defined. The second and thirdtransmembrane domains of the β subunit appear to be critical; binding may involve a pocket formed by β-subunitmethionine 286 as well as α-subunit methionine 236.[13]
Barbiturates were introduced ashypnotics for patients withschizophrenia. It induced a state of deep and prolonged sleep. But this was not used for long because of adverse side effects.[2]
Phenobarbital was the first truly effective drug against epilepsy. It was discovered by accident when given toepileptic patients to help them sleep. The positive side effects wereanticonvulsant properties that reduced seizure number and intensity.[2]
Pentobarbital is used as ahypnotic whenanalgesia is not required. It´s often used inCT imaging when sedation is needed. It is efficient, safe and the recovery time is short.[14] In 2013 the barbituratesphenobarbital andbutabarbital are still used as sedatives in certain cases as well as to antagonize the effects of drugs asephedrine andtheophylline. Phenobarbital is used in cases of drug withdrawal syndromes. It is used as normal and emergency treatment in some cases of epilepsy.[2]
Synaptic action of benzodiazepines: GABAA receptors located at synapses are activated when they are exposed to high concentration of GABA. Benzodiazepines enhance the receptor affinity for GABA by increasing the decay of spontaneous miniature inhibitory postsynaptic currents (mIPSC).[15][16]
Sedative actions of benzodiazepines limit their usefulness asanalgesic agents and they are therefore generally not considered to be appropriate. This limitation can be bypassed byintrathecal administration. GABAA receptors in theperiaqueductal gray are pro-nociceptive atsupraspinal sites while GABAA that are found in the spinal cord are anti-hyperalgesic. Spinal α2 and α3 containing GABAA receptors are responsible for the anti-hyperalgesic action of intrathecaldiazepam. 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 showed that the spinal α5-containing GABAA receptor has a minor role in inflammatory pain. An α2, α3 and/or α5 selective positive allosteric agonist, likeL-838,417 for example, might be useful as an analgesic drug againstinflammatory orneuropathic pain.[3] Further studies in animal neuropathic pain models have shown that stabilizing the potassium chloride cotransporter 2 (KCC2) at neuronal membranes could not only potentiate theL-838,417-inducedanalgesia but also rescue its analgesic potential at high doses, revealing a novel strategy for analgesia in pathological pain, by combined targeting of the appropriateGABAA receptor subtypes (i.e.α2,α3) and restoring Cl− homeostasis.[17]
Benzodiazepines are used as a supporting treatment in patients withschizophrenia.[3]
AGABAergic hypothesis for depression has been proposed which places the GABA system in a central role in thepathophysiology ofdepression. Clinical studies have shown thatalprazolam andadinazolam have antidepressant activities in patients with major depressive disorder. Unfortunately, it is not known 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 increasedcorticosterone 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 increasedanxiety and depression-like symptoms in conflict-based feeding tests. The fact that anxiety and depression are often linked seems to indicate that the α2 subunit might be a valid target for a GABAA antidepressant.[3]
Preclinical studies have shown that benzodiazepines can be effective in reducing the effect of strokes for up to three days after the drug has been administered.[3]
Neurosteroids can act as allosteric modulators of neurotransmitter receptors, such as theGABAA,[18][19][20][21]NMDA,[22] andsigma receptors.[23] The neurosteroidprogesterone (PROG) that activatesprogesterone receptors expressed in peripheral and central glial cells.[24][25][26][27] Additionally it has been surmised that the 3α-hydroxy ring A-reduced pregnane steroidsallopregnanolone andtetrahydrodeoxycorticosterone increase theGABA-mediated chloride currents whilepregnenolone sulfate anddehydroepiandrosterone (DHEA) sulfate on the other hand display antagonistic properties at the GABAA receptors.
Barbituric acid is the parent compound of barbiturate drugs although barbituric acid itself is not pharmacologically active. Barbiturates were synthesized in 1864 byAdolf von Baeyer by combiningurea andmalonic acid (Figure 5). A synthesis process was later developed and perfected by French chemist Edouard Grimaux in 1879, making possible the subsequent widespread development of barbiturate derivatives.[28] Malonic acid was later replaced bydiethyl malonate, as using theester avoids the need to deal with the acidity of thecarboxylic acid and its unreactive carboxylate (see figure 6). Barbituric acid can form a large variety of barbiturate drugs by using theKnoevenagel condensation reaction.[29]
The structure that the first benzodiazepine is based on was discovered byLeo H. Sternbach. He thought the compound had a heptoxdiazine structure (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.[30]
Neurosteroids are synthesized in thecentral nervous system (CNS) and theperipheral nervous system (PNS) from cholesterol and steroidal precursors that are imported from the peripheral sources. These sources include 3β-hydroxy-Δ5 derivatives, such aspregnenolone (PREG) anddehydroepiandrosterone (DHEA), their sulfates, and reduced metabolites such as the tetrahydro derivative ofprogesterone 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.[31][32]
Barbiturates have special uses and are organized into 4 classes: ultrashort-, short-, intermediate- and long-acting. Empirically SARs of barbiturants are based on thousands of (animal) tested compounds. They have shown that R and R' may not be H in position 5 (see figure 8). Also, position 5 confer sedative-hypnotic properties.[14] Generally alkyl branching in position 5 means less lipid solubility and less activity. Unsaturation show less activity in position 5 and alicyclic and aromatic rings show less potency. Polar substituents (-NH2, -OH, -COOH) will decrease lipid solubility but it will also eliminate activity. R'' in position 1 is usually, H but CH3 in that position yields less lipid solubility and duration. Exchanging S for O atom in position 2 produces thiobarbiturates, which are more lipid-soluble than the oxybarbiturates. In general, the more lipid-soluble the barbiturate, the more rapid its onset, the shorter its duration and the greater the degree of hypnotic activity. Barbiturates showed some hydrolytic problems in regard to formulation of liquid dosage forms. The difficulty is -OH catalyzed degration of theureide rings but that can be fixed if the pH is 6 in the formulation. S(-) form of barbiturate have shown more depressant activity while the R(+) isomers have an excitatory effect.[33]
According to research performed by Maddalena et al., usingartificial neural networks, position 7 has the most effect onreceptor affinity. When the active group in position 7 is made morelipophilic and the electronic charge is increased the receptor affinity increases. In the same study position 2´ was found to be the second-most important in affecting the affinity, but the group in that position needs to beelectrophilic to have an effect. Positions 3, 6' and 8 are of less importance.[34] Changes to 6, 8, 9 or 4´ decrease activity. If the group in position 1 is changed to N-alkyl, haloalkyl,alkynyl and small cycle or aminoalkyl the activity is increased. Position 3hydroxylation can cause rapid conjugation and decrease duration and potency, which can be clinically useful.[34]
In the mid 1980s, theneuroactive steroids 3α,5α-tetrahydroprogesterone orallopregnanolone (3α,5α-THP) and 3α,5α-tetrahydrodeoxycorticosterone (3α,5α-THDOC) were shown to modulate neuronal excitability via their interaction with GABAA receptors. The steroids 3α,5α-THP and 3α,5α-THDOC were able to enhance the GABA-elicited Cl− current.[18] In addition, these steroids might enhance the binding ofmuscimol and benzodiazepines to GABAA receptors.[35] Structure- activity studies (SAR) showed that the 3alpha-OH group is essential for the anesthetic actions of these steroids,[36] they also have an optimally-placed hydrogen bond accepting group on the β face of the steroid at the C-17 position. The four steroid rings form a rigid framework for positioning these hydrogen groups in three-dimensional space.[37] Analogues 5 and 6 (Figure 10) are weak modulators of GABAA receptor function because the flexible side chains in these analogues do not have the conformations required for high biological activity.[38]