Neurotransmitters: Chemical Messengers of Nervous System

Human brain is well encapsulated within a thick, bony skull. The chloroid plexus secretes the cerebrospinal fluid (CSF) which surrounds the brain. The fluid passes down through the four ventricles with the help of subarachnoid space and finally enter the cerebral veins through the arachnoid villi. Brain lacks lymphatic system so CSF ​​acts as a partial substitute. Dura mater is a tough, protective conductive tissue firmly attached to the skull and includes the subarachnoid space filled with the CSF, arteries and web-like connective tissue known as archanoid mater. The pia mater is a very delicate and permeable membrane composed of collagen, elastin and fibroblasts that rests on the floor of subarachnoid space and allows diffusion between CSF and the interstitial fluid of brain tissue. The pia mater is also interrupted by astrocyte processes. The dura mater, arachnoid mater and the pia mater are collectively known as meninges.

The brain and CSF are separated from each other by the blood-cerebrospinal fluid barrier and the blood-brain barrier (BBB) ​​which protects brain from undesirable blood substances. These barriers are permeable to water, oxygen, carbon dioxide, small lipid soluble molecules, electrolytes and certain essential amino acids. The barriers are formed by the combined action of endothelial cells lining the capillary walls and glial cells (astrocytes) that wrap the capillaries with fibers. The brain has a distinct chemical composition for example, structural lipid accounts for 50% dry weight of brain, a feature which is in contrast with other fatty tissues of the body that are made up of triglycerides and free fatty acids. The blood brain barrier forms a protective chemical environment through which neurotransmitters can easily participate in nerve impulse delivery.

Neurotransmitters and Their Identification

Neurotransmitters are endogenously produced chemicals that actively participate in the transmission of signals from a neuron to the target cell across a synapse. They are tightly packed inside the synaptic vesicles which remain clustered benefit the membrane on the pre-synaptic side of the synapse. Upon activation they are released into the synaptic cleft where they bind to the receptors located on the membrane of the post-synaptic side of the synapse. Release of neurotransmitters is simply an indication that action potential has produced. These chemicals are synthesized from simple precursors, chiefly the amino acids. Amino acids are easily available and only few biosynthetic steps are involved in the formation of neurotransmitters.

Ramón v Cajal discovered synaptic cleft after carefully performing histological examination of neurons. After the discovery of synaptic cleft it was suggested that some chemical messengers are involved in signal transmission. In 1921 a German pharmacologist Otto Loewi confirmed that neurons communicate with each other by releasing chemical messengers. He performed a series of experiments where vagus nerve of frog was involved. He traditionally controlled the heart rate of frog by controlling the amount of saline solution present around the vagus nerve. When the experiments were over Loewi concluded that sympathetic regulation of heart rate could be mediated through changes in the chemical concentration. He later on discovered the first neurotransmitter known as acetylcholine (Ach). However, some neurons communicate by using electrical synapses through gap junctions that allow specific ions to pass directly from one cell to the other.

Types

There are many ways through which neurotransmitters can be classified for example, they can be divided into amino acids, peptides and monoamines on the basis of their chemical composition. The amino acids that act as neurotransmitters are glutamate, aspartate, D-serine, gama-aminobutyric acid (GABA) and glycine. Monoamines and other biogenic amines include dopamine (DA), norepinephrine, epinephrine, histamine and serotonin. Other substances acting as neurotransmitters are acetylcholine (Ach), adenosine, nitric oxide and anandamide. More than 50 neuroactive peptides are known that act as neurotransmitters. Many of these peptides are released along with a small transmitter molecule. The well known example of a peptide neurotransmitter is β-endorphin which is associated with the opioid receptors of the central nervous system. Single ions such as the synaptically released zinc, some gaseous molecules like nitric oxide (NO) and carbon monoxide (CO) are also considered as neurotransmitters. Glutamate is the most prevalent neurotransmitter as it is excitatory in 90% of the synapses while GABA is inhibitory in 90% of the synapses.

Excitatory and Inhibitory Neurotransmitters

Neurotransmitters can be excitatory or inhibitory but their major action is activation of one or more receptors. The effect of these chemicals on the post-synaptic side of the cell is totally dependent upon the properties of the receptors. The receptors for most of the neurotransmitters are excitatory as they activate the target cell so that action potential can be produced. On the other hand, for GABA, most of the receptors are inhibitory. However, evidences have shown that GABA acts as an excitatory neurotransmitter during early brain development. For acetylcholine the receptors are both excitatory and inhibitory. The effect of a neurotransmitter system is directly dependent upon the connections of neurons and chemical properties of the receptors. Major neurotransmitter systems are the norepinephrine, dopamine, serotonin and cholinergic systems. Drugs targeting these neurotransmitter systems affect the whole system thus, explaining the complexity of drug action. AMPT prevents the conversion of tyrosine into L-DOPA which forms dopamine. Reserpine prevents accumulation of dopamine in the vesicles. Deprenyl inhibits the activity of monoamine oxidase-B and thus, increases dopamine levels.

Precursors of Neurotransmitters

Different precursors are needed for the synthesis of different neurotransmitters. For example, L-DOPA is the precursor for dopamine synthesis that crosses the blood brain barrier and is used in the treatment of Parkinson's disease. In case of depressed patients the activity of norepinephrine is lowered, so the precursors for this neurotransmitter are administratively externally. The precursors for this neurotransmitter are L-phenylalanine and L-tyrosine. These precursors also participate in the synthesis of dopamine and epinephrine. The synthesis of these neurotransmitters also requires vitamin B6, vitamin C and S-adenosylmethionine. L-tryptophan is the precursor for serotonin synthesis and studies have indicated that its administration results in increased production of serotonin in the brain. The conversion of L-tryptophan requires vitamin C. 5-hydroxytryptophan (5-HT) also acts as a precursor for serotonin.

Degradation and Elimination

Neurotransmitters must be broken down into small molecules before they reach the post-synaptic neuron in order to participate in excitatory or inhibitory signal transduction. For example, acetylcholine (ACh) is an excitatory neurotransmitter which is broken down by acetylcholinesterase (AChE). Choline is recycled by the pre-synaptic neuron to form acetylcholine again. Other neurotransmitters like dopamine are capable to diffuse away from their synaptic junctions and are eliminated from the body via kidneys or destroyed in the liver. Each neurotransmitter has a specific degradation pathway.

Important Neurotransmitters

A number of chemicals are known to act as neurotransmitters and they will be treated here separately.

1. Acetylcholine (Ach)

Acetylcholine is a part of the peripheral nervous system and was the first neurotransmitter to be discovered. It is an excitatory neurotransmitter in contrast to the monoamines which are inhibitory. The precursors of acetylcholine are acetyl-CoA produced during the glucose metabolism and choline that are actively transported across the blood brain barrier. Production of this neurotransmitter takes place in the brain. The dietary choline comes from the phosphatidyl choline present in the membranes of plant and animal cells except bacterial cells. Acetyl-CoA and choline are independently synthesized inside the cell body of the neuron. Brain has few acetylcholine receptors but outside the brain it is the principal chemical that governs muscle activity. Body muscles may be either skeletal muscles that are under the voluntary control or smooth muscles of the autonomous nervous system lacking voluntary control. The nervous system can further be subdivided into sympathetic and parasympathetic divisions. Direct innervation of the skeletal muscles is due to acetylcholine while that of the smooth muscles is due to norepinephrine.

Acetylcholine receptors are of two types normally, a fast acting ion channel controlled receptor and a slow acting receptor that requires a G-protein (Guanine nucleotide binding protein) which stimulates second messengers to indirectly open ion channels. Direct ion channel controlling receivers respond within microseconds while indirect second messenger controlling receptors may take milliseconds in order to generate a response. The fast acting receptor is known as nicotinic as it is specifically activated by a toxin present in tobacco. The slow acting receptor is known as muscarinic as it requires a toxin muscarine and acetylcholine for its activation. Parasympathetic nerves may be either cranial or sacral. 75% of all parasympathetic nerve fibers arise from a single cranial nerve known as vagus. These fibers travel towards the ganglia and finally enter smooth muscles. The preganglionic fibers are nicotinic. The neuromuscular junction of skeletal muscles is also nicotinic while that of smooth muscles is muscarinic.

The speed with which the skeletal muscles respond clarifies that they are controlled by fast acting nicotinic receptors. The activity of acetylcholine in both nicotinic and muscarinic synapses is inhibited by acetylcholinesterase. The choline liberated after the hydrolysis of acetylcholine can be transported across the post-synaptic membrane to be used for the resynthesis of acetylcholine. Some snake venoms can block nicotinic receptors causing paralysis. Atropine is known to block muscarinic receptors. Most brain cholinergic receptors are muscarinic as they show synaptic plasticity. Major proportion of acetylcholine synthesis in brain occurs in the interpendunctural nucleus. All the inter-neurons in the striatum and the nucleus accumbens are cholinergic. The primary cholinergic input to the cerebral cortex arises from the basal nucleus of Meynert, a prominent area of ​​substantia innominata. Meynert's nucleus is also known to innervate basolateral amygdala, basal ganglia and reticular nucleus of thalamus.
If muscarinic blocking agents are administrated in normal individuals then memory loss can occur.

2. Dopamine

Dopamine, norepinephrine and serotonin are the primary monoamine neurotransmitters. Dopamine and norepinephrine are catecholamines while serotonin is an indolamine. Tyrosine is not an essential amino acid as its synthesis occurs in the liver from phenylalanine in the presence of phenylalanine hydroxylase. It can not be synthesized in the brain so must be coupled with the large neutral amino acid transporter molecules in order to enter brain. These transporter molecules also transport phenylalanine, tryptophan, methionine and branch-chained amino acids.

When tyrosine enter brain it must be converted into DOPA (Dihydroxyphenylalanine) by tyrosine hydroxylase along with oxygen, iron and Tetrahydrobiopterin (THB) that act as co-factors. DOPA is converted into dopamine by aromatic amino acid decarboxylase with pyridoxa L phosphate (PLP) co-factor. The rate of reaction fluctuates when there is vitamin B6 deficiency. Central nervous system has high proportion of dopaminergic cells than adrenergic cells. Dopamine present in the caudate nucleus of brain is responsible for maintaining post while that present in the nucleus accumbens is associated with animal's speed. Two types of primary dopamine receptors are already known as D1 (stimulatory) and D2 (inhibitory). Both these receptors require G-protein for their activity. D2 receptors are located on the dopaminergic neurons and produce negative feedback. They are also known as auto-receptors as they inhibit the release and synthesis of dopamine.

The binding of dopamine to the D1 receptors stimulates the activity of Adenylyl cyclase (AC) which converts ATP into cyclic AMP (second messenger). The cyclic AMP (cAMP) then binds with the protein kinase A (PKA). PKA participates in modulating the activity of various proteins by adding phosphate to them. Brain has four main dopaminergic tracts namely, the nigrostriatial tract, tuberoinfundibular tract, the mesolimbic tract and the mesocortical tract. Both dopamine and norepinephrine are catabolized by monoamine oxidase (MAO) and catechol-o-methyltransferase (COMT). COMT is active in synapses and uses S-adenosyl methionine (SAM) as methyl group while while MAO is active in the pre-synaptic terminal against the catecholamines. Schizophrenia is thought to occur due to the overstimulation of D2 receptors of the mesolimbic and mesocortical systems. The mesolimbic and mesocortical dopaminergic systems are believed to play active role in motivation. Cocaine is known to increase the dopaminergic activity in the mesolimbic areas of brain by inhibiting dopamine re-uptake in the ventral tegmental area and nucleus accumbens.

3. Serotonin (5-Hydroxytryptamine, 5-HT)

Serotonin was isolated from blood serum as a substance responsible for causing muscle contraction. Only 1-2% of the body's serotonin is present in brain while rest comes from platelets, mast cells etc. Synthesis of serotonin involves two steps along with tryptophan hydroxylase and co-factors naturally oxygen, iron and THB. The highest concentration of serotonin is present in the pineal gland. It is primarily methylated in the synthesis of melatonin. Melatonin is synthesized from serotonin in two steps and the whole process requires an acetyl group from acetyl Co-A and a methyl group from S-adenosyl methionine. Melatonin regulates diurnal activity, seasonal behavior and physiology of animals. In mammals the noradrenergic neurons located near the optic nerve are inhibited by light but in darkness norepinephrine stimulates pineal cells to release cyclic AMP which in turn activates N-acetyl transferase to cause acetylation of serotonin. The suprachiasmatic nucleus (SCN) of the hypothalamus is responsible for regulating the mammalian circadian clock partially in response to light. SCN receives serotonergic supply from dorsal raphe nucleus. Serotonin also reduces responsiveness of SCN to light. Sleep deprivation is responsible for increasing concentration of serotonin in SCN. Low levels of serotonin are associated with high levels of pain sensitivity, locomotion, sexual activity, aggression, depression, Obsessive Compulsive Disorder (OCD) and panic disorders.

4. Glycine

Glycine is the simplest amino acid composed of an amino and a carboxyl group. The role of glycine as a neurotransmitter is very simple. When released into a synapse it binds to the receptors making the membrane permeable to chloride ions. Thus, it is inhibitory in action and can be easily deactivated in the synapse. It is found only in vertebrates and is primarily present in the ventral spinal cord.

5. Aspartic acid (Aspartate)

Aspartate is also present in the ventral spinal cord just like glycine. It also participates in the opening of the ion channels and is soon inactivated by reabsorption into pre-synaptic membrane. It is an excitatory neurotransmitter as it increases depolarization in the post-synaptic membrane.

6. Glutamic acid (Glutamate)

Glutamate is the most common excitatory neurotransmitter present in brain and increases the flow of positive ions by opening ion channels. Its stimulation is terminated by the membrane transport system used for reabsorption of the aspartate and glutamate across the pre-synaptic membrane. NMDA-glutamate receptor is the most complicated receptor. It is the only receptor which is regulated by a ligand and voltage. It has five binding sites for glutamate, glycine, magnesium, zinc and a site that binds a hallucinogenic substance, phencyclidine. NMDA receptors are more densely located in the cerebral cortex, amygdala and basal ganglia. Glutamate is not able to cross the blood brain barrier.

7. Gamma amino butyric acid (GABA)

GABA is the major inhibitory neurotransmitter accounting for 30-40% of all synapses. It is present in high concentration in the substantia nigra and globus pallidus nuclei of basal ganglia, hypothalamus, periaqueductal gray matter and hippocampus. The concentration of GABA in brain is 200-1000 times greater than that of monoamines or acetylcholine. It is somewhat unique as it is rapidly inactivated during its transport into the glial cells. Both GABA and glutamate are synthesized in the brain from alpha-keto glutarate, a molecule produced during Kreb's cycle. Like glycine GABA receptor are coupled with the chloride ion channels.

8. Norepinephrine

Norepinephrine and acetylcholine are the neurotransmitters of the peripheral nervous system. It is synthesized from dopamine in the presence of dopamine beta-hydroxlase along with cofactors namely, oxygen, copper and vitamin C. Dopamine synthesis occurs in the cytoplasm while synthesis of norepinephrine takes place inside the neurotransmitter storage vesicles. Cells utilizing norepinephrine for making epinephrine use S-adenosyl methionine as a methyl group donor. The levels of epinephrine are low as compared to that of norepinephrine. The major proportion of norepinephrine is present in the locus ceruleus of pons while rest is found in neocortex, hippocampus, and cerebellum. Most of the dopaminergic innervations of hypothalamus are derived from lateral tegmental nuclei. It plays a major role in awakenness-arousal cycle.

9. Peptides

Peptides are the most common neurotransmitters located in the hypothalamus. Their complex structure is responsible for their high receptor specificity. Their synthesis takes place in the ribosomes and they are rapidly inactivated at the synapses by hydrolysis. They are more potent than other neurotransmitters as their very small amounts are sufficient enough to produce a response. Opioid peptides combine endorphins, enkephalins and dynorphins. Opiates and enkephalins cause inhibitory of the neuron firing at the locus ceruleus. The concentration of the opioid receptors is very high in the sensory, limbic and hypothalamic regions of brain. Their concentration is also high in the amygdala and preiaqueductal gray area. Cholecystokinin (CCK) is known to participate in satiety. Injection of small doses of CCK in the paraventricular area can inducing feeding. It is also known to modulate dopamine release. Low doses of the peptide vasopressin are known to enhance learning process in laboratory animals.

10. Endorphins

Solomon Snyder and Candace Pert of Johns Hopkins discovered endorphin in 1973. It resembles opioids in structure and function. It is inhibitory and is involved in pain reduction and pleasure. Opioid drugs work by attaching to the receptor sites of endorphin. It also causes hibernation in bear and other animals.