The Versatile GABAa Chloride Channel Receptor Complex

In today’s industrialized society we are constantly exposed to work related stresses. Consequently, anxiety and insomnia (sleeplessness) have become quite common. No wonder, we are using anxiolytics and sedatives more often; to get relief from the anxiety and insomnia respectively.

Benzodiazepines such as diazepam (Valium), chlordiazepoxide (Librium) can effectively treat anxiety and insomnia. They do so by binding with a receptor (called Benzodiazepine-GABAa-chloride ion channel complex [henceforth to be referred to simply as GABAa receptor]) in nerve cell membranes. It is known that most drugs (medicines) exert their actions by combining with receptors: macromolecular complexes present in the cell membrane or within the cytosol or the nucleus.

The GABAa receptor is a very versatile receptor complex (a hypothetical model is shown at the bottom). Its main action is to inhibit transmission along neurons in which they are present. Normally, proper functioning of the brain is ensured by a balance between the action of excitatory and inhibitory neurotransmitters [henceforth to be referred to simply as NTs]. Simply put, excitatory NTs (for example, glutamate) give a green or go signal; while inhibitory NTs (such as GABA) tell the nerve not to fire (red or stop signal). In this connection, it must be said that GABA (gamma amino butyric acid) is the most important inhibitory NT. When GABA binds with the GABA receptor ionophore complex, the receptor changes shape (conformation); and then a centrally located chloride channel, that is a part of the receptor itself, opens. Since the concentration of the chloride ions (Cl-) is much more on the outside of the cell than on the inside, Cl- now rushes in due to the increase in chloride conductance. The cell voltage goes further down and the interior of the cell becomes more negative (hyperpolarized) with respect to the outside. The cell becomes less excitable and is thus inhibited.

Apart from maintaining the much needed critical balance already mentioned, they also ensure that the brain works in a relatively noise free environment. Billions of neuronal units are always firing in the background creating a constant ‘noise’. A constant release of GABA by the brain drowns out this noise thus improving the ‘signal to noise’ ratio, making the brain’s task of finding the proverbial ‘needle in a haystack’ a lot easier.

The GABAa receptor not only binds with GABA, but it is a binding site for various other ligands. But before we discuss them, let us briefly analyze its structure first. The receptor has a pentameric structure which means that it consists of five subunits, and each subunit has four membrane-spanning (transmembrane) domains (see picture). And there are many of the polypeptide subunits to choose from a vast array consisting of alpha, beta, gamma, delta, pi, rho and so on. (In addition, there are six different forms of alpha, 4 beta and 3 gamma subunits). Thus, it’s no wonder that a great variety of GABAa receptors will be found, given the possible permutations!

This receptor heterogeneity explains actions of various pharmaceuticals on the receptor. One major form of GABAa receptor (found throughout the brain) consists of two alpha1, two beta2 and one gamma2 subunits. In this isoform, GABA ‘somewhere’ between alpha1 and beta2 subunits, and benzodiazepines bind with the BZ1 (also called omega1) pockets located between alpha1 and gamma2 subunits. Benzodiazepines act only when the receptor isoform has one of the following alpha subunits: 1, 2, 3 or 5 and the subunit should have a conserved histidine residue in the N-terminal domain. In ‘knock-in’ mice where histidine has been replaced by arginine in the alpha1 subunit (alpha1H101R; H for histidine and R for arginine in the 101st residue of alpha1 subunit) there was no sedation or amnesia (as evidenced by their unchanged ‘energy’ and memory to electric shocks). It may be mentioned at this moment that the so called ‘date rape’ pills exploit the amnestic properties of benzodiazepines. The drug plays tricks with the victims’ memories. However, the anxiolytic and muscle relaxant properties were retained in these mice.

These mice also do not respond to the hypnotic effects of zolpidem and zaleplon, non-benzodiazepines that act at GABAa receptors containing alpha1 subunits. But in mice with selective histidine arginine mutation in the alpha2 subunit of GABAa receptors, resistance to the antianxiety action of benzodiazepines has been seen. Based on these observations, it is thought that alpha1 subunit mediates sedative and amnestic effects, while alpha2 takes care of the anxiolytic and muscle relaxant ones. It also seems that we are poised to make better benzodiazepines in future (like one that works in anxiety but doesn’t wreak the patients’ memory).

Lastly, the versatility. The GABAa receptor also binds barbiturates (urea derivatives used as anesthetics, anticonvulsants, Marilyn Monroe supposedly died of its overdose) in addition to the benzos. Alcohol, alphaxolone (a steroid anesthetic), etomidate (a short acting anesthetic), propofol (diprivan, Michael Jackson supposedly used it), volatile anesthetics like halothane, anticonvulsants like gabapentin and vigabatrin, anthelminthics like ivermectin, and neurosteroids (metabolites of androgen and progesterone) exert part or all of their actions by acting through this receptor, thereby hyperpolarizing the neuron. Conversely, convulsants picrotoxin blocks the chloride channel directly, while bicuculline blocks the receptor’s GABAa binding site causing depolarization and convulsion. There's a lot more than this mere exegesis, and I hope to discuss about it furher later.

Last modified: never
Reference: Bertram G. Katzung, Basic and Clinical Pharmacology, ninth edition
Pharmacology: Rang, Dale, Ritter, Moore
Wisden, W., & Stephens, D. (1999). Pharmacology: Towards better benzodiazepines Nature, 401 (6755), 751-752 DOI: 10.1038/44482

Parkinsonism: The Shaking Palsy

Dopaminergic neurons are lost steadily with age. In Parkinsonism the loss is accelerated. It is this loss of dopaminergic neurons from substantia nigra pars compacta which causes Parkinsonism.As have been stated earlier, Parkinson’s disease is associated with features of hypoactivity such as akinesia (in hypokinesia or akinesia, there is a peculiar difficulty in starting a task, the person remains motionless with an expressionless face-> masked facies). The patient finds it very difficult to move or to carry out a job. He can still do it, but with the highest of his will power. This is achieved by the “caudate circuit”, pathway that is associated with cognitive control (of motor act). But even then the action is coarse and erratic, which adds to the patient's frustration.

Along with hypokinetic features, hyperkinetic features are also present (this is why PD also known as paralysis agitans; paralysis representing hypo- and agitans representing hyperkinetic features). They include tremor which is typically found at rest, and rigidity i.e. increased muscle tone of both flexor and extensor group of muscles. Rigidity is of lead-pipe type, meaning that you would feel uniform resistance, if you were to flex (or extend) patients arm passively, as in a malleable lead pipe. Cog wheel type rigidity is also seen, in which the limb moves in a series of catches as they are moved passively. Autonomic hyperactivity may be found in the form of sialorrhea (drooling of saliva). The person and his limbs assume a flexed posture, so that as the person moves, he is bent forward. The gait becomes festinant (festinating gait occurs as the man walks as if to catch up with his center of gravity, which now is tilted forward due to flexion). The patient gets demented (dementia) with time, and many other features develop.

It is not known what causes the destruction of dopaminergic neurons of the basal ganglia. It was accidentally discovered that heroin addicts who were receiving dopes contaminated with a substance called MPTP, developed Parkinsonian like features. MPTP got converted to MPP+, a free radical, which destroyed the neurons. Now MPTP is used to create animal models of Parkinsonism for clinical research. In some familial types of the disease, 7 genes have been identified which when mutated lead to Parkinsonian features. For example, alpha synuclein (a gene product, a protein responsible for ‘marking’ proteins for removal called ubiquitination) and Parkin (another such protein product) combine and form what is called Lewy bodies. Intracellular Lewy bodies are constant features of Parkinsonism, but the roles they subserve are yet to be identified.

Pharmacotherapy of Parkinsonism is far from satisfactory. We can not give the patient dopamine as such, because, it will get degraded before it reaches the brain. Moreover it can NOT reach the brain since to penetrate the lipid containing blood brain barrier, a drug has to be non polar (hydrophobic). So levodopa is used instead. It gets to the interior of the cells where it gets converted to dopamine(levodopa is thus called a prodrug). Carbidopa, a drug that prevents peripheral conversion of levodopa, is usually combined with levodopa and the action is synergistic. Drugs which prevent the degradation of dopamine by inhibiting its destroying enzymes are also used. They include the MAO-B inhibitor selegeline (deprenyl) (MAO for mono amine oxidase), COMT inhibitors tolcapone, entacapone etc (COMT for catechol O methyl transferase). Symptoms are alleviated with levodopa therapy but the effect wears off after 5-7 years and symptoms escalate. Selegeline, on the other hand, delays cell destruction. Cholinergic and dopaminergic systems in the brain strike a balance, actionwise. Thus anticholinergics such as benhexol or benztropine are also used. Dopaminergic drugs such as bromocryptine, lisuride, pergolide, cabergoline thus give some relief. (Conversely, antidopaminergic drugs like phenothiazine induce drug-induced Parkinsonism).

Therapy with antiapoptotic agents like desmethyl selegeline, antioxidant drugs, coenzyme Q10 are good therapeutic candidates. Infusion of glial cell line derived neurotrophic factors (GDNF) showed early promise. So did transplantation of fetal striatal tissue, the patients’ own carotid body or adrenal medullary tissues. DBS or deep brain stimulation, application of high frequency electric stimuli to specific brain tissues via subcutaneously located controls, also holds promise. Frank surgical means like pallidotomy (excision of globus pallidus interna) is rarely necessary.

A scientific approach and much research is direly needed to cure this neurodegenerative malady that is so prevalent among the aging population.
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Reference: hyper-links, unless specifically mentioned

Neuronal Circuitry of Basal Ganglia

Located deep within the matters of the brain are a group of structures which plan and program, initiate, control and execute motor functions in a well coordinated manner. It is also involved in the maintenance of muscle tone. The Basal Ganglia comprises of the following structures:

Caudate nucleus
Putamen
Globus Pallidus
Substantia Nigra
Subthalamic nucleus (also known as the body of Luys)

Caudate nucleus and Putamen together are called corpus striatum; while Putamen and Globus pallidus together go by the name of lenticular nucleus. So much for the anatomy part. The way these nuclei communicate with each other, and with the brain and the spinal cord, is largely unknown. A probable wiring diagram, which is presently accepted (and quite complicated), is presented here. Red lines represent excitatory fibers; green lines inhibitory fibers.

Globus Pallidus interna (GPI), the internal segment of GP, secretes a chemical called gamma amino butyric acid or GABA for short. This is an inhibitory neurotransmitter, meaning that it will NOT allow neurons it supplies, to fire. As you can see, GPI inhibits PPN (pedunculo-pontine nucleus), the motor outflow to the brainstem and spinal cord. It also inhibits thalamus, which stimulates the brain through the thalamocortical circuit. Thus we see that GPI inhibits both the brain and the spinal cord.

Now see that GPI itself gets inhibitory fibers from the striatum directly, via obviously the direct pathway. It also gets glutaminergic (also called glutamatergic) stimulatory (excitatory) fibers from the sub-thalamic nucleus (STN). But STN itself is inhibited by GABAergic fibers arising from GPE (globus pallidus externa) which again is inhibited by fibers from the striatum. (to make matters light, it reminds me of “seventh son of a seventh son”—by Iron Maiden!). Hence the net result through the second pathway (indirect pathway) is excitatory. So the direct pathway inhibits, while indirect pathway stimulates the GPI. (step1) Picture on the left.

But the things are not so easy. The guy out there called substantia nigra pars compacta (SNPC) secretes dopamine (the reward chemical). This substance is secreted in the striatum. Now dopamine combines with its receptor (a receptor is like a lover longing to unite, but not ignite, as we lovers do!!). In the direct pathway, it combines D1 receptor and stimulates. But it combines with D2 receptors on the neurons of the indirect pathway, and inhibits its outflow. Thus dopamine in effect inhibits GPI. (step2)

So what do we see now? Dopamine inhibits GPI (step1), which inhibits both the brain and the spinal cord. (step2) Thus dopamine EXCITES the brain and the spinal cord. (The final result)

The above sequence operates normally. In Parkinsonism, the dopaminergic neurons die a slow death. As a result, the brain and spinal cord is inhibited and hypokinetic symptoms appear. Excitatory features like tremor are also present in Parkinsonism. Huntington’s chorea, another disease which involves this circuit, occurs due to destruction of GABA secreting neurons, as is shown in the adjoining picture. Notice the internuclear dopaminergic and intrastriatal cholinergic fibers. Hemiballismus (flailing movements of limbs) and athetosis (slow and writhing movements) are other manifestations of diseases of the basal ganglia.

Last modified: March 27, 2009
Reference: 1.William F Ganong, Review of medical physiology, 22nd ed., pp 213-217
2. Textbook of medical physiology, 11th ed., Guyton & Hall 707-712

Dendrites and Memories

In my previous post 'Spiny Dendrites', I discussed about dendrites' role in learning and memory as well as its role in evoking consciousness. Exactly how these dendrites learn and more importantly memorize, has not been clearly understood, but they are being discovered at a rapid pace.

On the dendritic cell membrane, nerve endings from various other neurons converge at the synapse, the gap between those neuronal processes and the neurone on which it is converging upon. The synapses are generally chemically coupled (by neurotransmitters), but they may also be electrically coupled (as in the lateral vestibular nucleus) or both electrical and chemical coupling may be found (conjoint). Chemical coupling is the most common. In this mode of signal transmission, whenever a presynaptic nerve gets an electrical signal (in the form of an action potential), it releases a neurotransmitter (the chemical coupler) into the synaptic cleft. The post-synaptic nerve (the nerve it verges upon, downstream) senses these chemicals (acetylcholine, adrenaline, glutamic acid, GABA etc) by virtue of 'receptors', located mostly in the 'post synaptic densities' (thickened portions of post-synaptic dendritic/neuronal cell membranes, adjacent to that part of the presynaptic membranes releasing the neurotransmitters, i.e. the synaptic vesicles).

Recently, it has been discovered that PSD-95 (post synaptic density-95), a key protein constituent (of postsynaptic densities), on which other proteins are assembled, needed to be phosphorylated, for it to act correctly (as a scaffold, for other proteins to act). Researchers suggested that this (phosphorylation) could help improve cognitive function in schizophrenia, autism and depression. I, personally, think that they will be of use as nootropics too.

In another development in neurobiology, it was shown that memories were burned into the neurons (as you burn 'memories' into a CD using Nero or other programs), and these basic units of memories had to be constantly recycled so that the memories remained there. These memory units, in the form of 'receptors' (of the neurotransmitters; which were located in the postsynaptic neuronal membranes), were in constant motion within the 'fluid' cell membranes. This 'recycling' might explain how the dendritic spines maintain their characteristic identity despite constant molecular turnover. This research might help us fight neurological disorders, such as Alzheimer's disease, schizophrenia, or learning disorders like autism.

Microtubules: The Cytoskeletal Rail-Road

The cells have a rich cytoskeletal network, consisting of microfilaments, microtubules, intermediate filaments, and proteins that hold them to the interior of the cell. These cytoskeletal structures subserve various functions such as giving the cells its shape and allows small organelles inside the cells to go to other places in the cells. For example, in a nerve cell, the synaptic vesicles, which contain neurotransmitters, are synthesized in the cell bodies. These neurotransmitter filled vesicles are then transported by 'molecular motors' form the cell body to the synaptic knobs, situated at the other end of the neuron.

Not only do they (microtubules) serve as a 'rail line' on which molecular motors move (walk), they also act as 'traction wires' which pull chromosomes apart during cell division (mitosis). Thus in cancer, when cell division is excessive, drugs which interfere with microtubule formation (polymerization) can be employed effectively to curb this cell replication gone haywire. Vinca alkaloids (obtained from the Vinca rosea plant) such as vincristine (used in Hodgkin's Lymphoma, small cell lung cancer etc.), vinblastine (used in ALL or acute lymphoblastic leukemia in children), vinorelbine (used in NSCLC or non small cell carcinoma of the lung); and taxanes such as taxol (paclitaxel, used for the treatment of cancers of the breast and ovary) extracted from the western yew tree, are molecules which stabilize microtubules and thus arrest cell division.

Microtubules are composed of smaller subunits made of proteins, alpha tubulin and beta tubulin. Alpha and beta tubulins together form a heterodimer (hetero since alpha and beta tubulins are different molecules). 13 such hetero dimers join to form a ring and many such rings are joined in a stacked manner to form this hollow structure called microtubules. Microtubules are dynamic structures with one end forming by polymerization (+end) and the other end destroying by depolymerization (-end), as this 10 second video clip illustrates.

To add Spice: Don't "Bend it like Beckham"

In my previous article, On her majesty's service..., I wrote about how the sperms 'sniffed' and chased their way to the ovum till it nailed her down. But for the sperms to reach it's destination, it has to be deposited in the right place. Thats where this organ comes into functioning. Its name is penis. It is a very interesting organ. Just like the states of matter (solid, liquid, gas and plasma), it can be soft, rubbery, and small or it can be hard and long. It not only injects (ejaculates) sperms, but also it acts as a conduit for urine to flow out of our system. Hence it has been christened as the genitourinary organ.

When mosquitoes bite us, their proboscis get hard, for a soft one can not penetrate our skin. Likewise, it needs a hard (erect) penis to enter the vulva. You would say, its nothing great; the muscles there would do this job. Here, we have no skeletal muscles, only smooth muscles are there. When we are stimulated erotically, say by seeing an erogenic picture/video or in our vivid REM sleep dreams, the brain sends the corresponding electrical impulses through the nerves (nervi erigentes). These nerves secrete (neurotransmitter), neither nor-adrenaline, nor acetyl choline and hence are classified into a separate group called the NANC fibers. They secrete a chemical substance called nitric oxide (NO). (Nitrogen has five valence states and thus forms 5 different compounds with oxygen; nitrous oxide, N2O, is another compound that is used in medicine, as an anesthetic). This gas, previously called EDRF or the endothelium-derived relaxing factor, relaxes non-vascular smooth muscles in the corpora cavernosa of the penis, so that more blood can rush through it. The penis gets tough, as a result. Nitrous oxide stimulates/activates an enzyme called guanylyl cyclase to form cyclic GMP (guanosine mono phosphate). It is cGMP which brings about erection. Well, this (cGMP) is finally broken down by an enzyme, called phosphodiesterase and the erection ends. Sildenafil (the popular drug viagra), tadalafil etc inhibit phosphodiesterase, making the staff stay erected longer. It can act only if the nerve supply is intact. It also causes visual disturbances (impaired blue-green discrimination) as phosphodiesterase also plays a role in vision. Viagra can add vigor to the diabetics suffering erectile dysfunction. Bear in mind, erection for too long a duration can be hazardous, a condition known as priaprism. The penis may slough out! Viagra, can also be used to treat spasmodic arterial diseases such as Raynauds phenomenon (blanching, bluish and cold fingers;- in a nutshell), refractory pulmonary hypertension (raised blood pressure within the lungs resulting from vasospasm) and other diseases. However, since it dilates arterioles, there is a chance of hypotension, due to diminution in the resistance in the vascular bed, especially in persons taking concomitant drugs, such as nitroglycerin (used for the treatment of cardiac pain or angina) and antihypertensives (blood pressure lowering drugs). A few cases of myocardial infarction or heart attack have been reported. Prostaglandins such as alprostadil; phentolamine and apomorphine can help us add spice to our lives.

I would do injustice, if I did not mention how the testicles are kept in an air-conditioned chamber called scrotum, in this context. There is a heat radiator system that operate in the scrotum which keeps the temperature 2 degree Celsius (centigrade) lower than the core body temperature. It is effected by the position of the testicles (which is not in direct contact/apposed with the body) and the radiator fins just mentioned (the pampiniform venous plexus). This is necessary for effective spermatogenesis to occur. Persons who wear tight pants, works in hot environment (furnace workers, vehicle drivers exposed to engine heat) have impaired spermatogenesis and the chance of infertility (NOT impotence) is great in them.

Lastly, about the nomenclature of this post. 'Bend it like Beckham' is a movie about the legendary football player David Beckham and Victoria (of Spice Girls fame) is her wife. How Beckham makes the ball bend in 'mid air' can be explained by Magnus effect, which I have discussed in my previous post. I hope I have explained my stand.