Serotonin, also called 5-hydroxytryptamine or 5-HT, is a chemical substance derived from tryptophan. Serotonin is found in mast cells, blood platelets, intestinal tissue, and especially in the brain, where it acts as a primary neurotransmitter. It is localized in distinct regions of the brain, and changes in serotonin concentrations are connected with many effects, including depression, migraines, euphoria, etc (Yeh: 1998). Ecstasy, or 3,4-Methlyenedioxymethamphetamine (MDMA), is a widely abused drug that causes a feeling of euphoria by causing a profound release of serotonin. However, frequent users of the drug have been found to enter into a clinical state of depression, and recent studies suggest that the body responds to these extraordinarily high levels of serotonin by decreasing the amount of serotonin receptors in the brain; these receptors essentially "die off." Then, when serotonin levels return to normal, the body's natural amount of serotonin level is present, but due to less receptors in the brain (which is thought to be irreversible), the users don't experience as good a feeling as they normally would. The eventual loss of more and more receptors leads to a constant state of depression (Rutty and Milroy: 1998).
There are at least 14 different types of serotonin receptors, but all belong to the family of G-protein coupled receptors or Serpentine receptors. In these types of receptors, an external ligand binds to the receptor, activating an intracellular GTP-binding protein, which regulates an enzyme that generates an intracellular second messenger (Cox and Nelson 2000).
Ecstasy, also known as "E" or "X" to the general public, is considered to be the present drug of choice among teens and young adults, and is often seen at dance "raves" or nightclubs. MDMA was first synthesized in Germany in 1914, but was forgotten about until the U.S. Army tested it for their chemical warfare program in the 1950's. Done in 1953 but not declassified until 1973, the U.S. Army tests on animals found a profound decrease in serotonin uptake sites due to a loss of serotonin neurons (Connor, et al: 1998). Current autopsy observations from those who have died from ecstasy use have proposed that MDMA acts on the pre-synaptic serotonin receptors via a calcium ion-independent carrier-mediated process. It is further thought that MDMA blocks the return of serotonin to the neuron, stopping serotonin recycling and thus amplifying the signal. This accounts for the heightened sense of awareness and increased feeling of euphoria that users encounter while on ecstasy, as serotonin serves as a natural "feel good" chemical of the body (Shankaran, Yamamoto, and Gudelsky: 2000).
However, Ecstasy also has many negative side effects which more than outweigh the short duration of euphoria achieved from taking Ecstasy. MDMA has led to many deaths from hyperthermia and heat stroke, as the high levels of serotonin released cause renal failure and severe dehydration due to increased water loss. In addition, liver damage has been observed in frequent Ecstasy users, including cases of hepatitis. Furthermore, cardiac complications, such as hypertension and arrhythmias, have resulted due to use of Ecstasy (Rutty and Milroy: 1998). Nonetheless, Ecstasy continues to be the drug of choice at the moment for teenagers and young adults, due in part to it's low price, high availability, and the relatively low incidents of deaths caused by the drug. Many users feel that Ecstasy is a relatively safe drug with no major complications. However, the major concern of physicians and others is the major side effect seen more and more often in users of the drug, that of clinical depression. Much like Diabetes Mellitus Type II is caused by a low quantity of insulin receptors, this state of depression seen in regular ecstasy users is caused not by a diminished concentration of serotonin or a halt in production, but by a loss of serotonin receptors in the central nervous system (CNS), as seen in studies done on animals following a dosage of Ecstasy. The exact mechanism resulting in the long-term depletion of serotonin axons and axon terminals remains unknown, although the evidence for potential neurodegeneration remains strong and continues to be the number one concern about those who abuse the drug (Battaglia et al: 1999).
In 1950, the neurochemical serotonin was first discovered, and in 1953 it was found to play a large function in the central nervous system. Also known as 5-hydroxytryptamine (5-HT), serotonin is produced in neurons in the brainstem raphe nuclei, and is known to belong to the most complex efferent system in the brain. 5-HT is synthesized from the amino acid l-tryptophan, which is brought to the brain by the bloodstream. Once in the brain, 5-hydroxytryptamine neurons convert the tryptophan into 5-hydroxytryptohan, (5-HTP) using the enzyme tryptophan hydroxylase, which is present only in the 5-HT-synthesizing cells. From there, the enzyme aromatic amino acid decarboxylase converts 5-HTP into 5-hydroxytrytamine (5-HT, serotonin), releasing a carbon dioxide molecule in the process (Cox and Nelson: 2000).
The production of 5-HT is dependent upon the concentration of tryptophan in the blood, as well as the concentration of other neutral amino acids with which tryptophan competes with for transport into the brain. Studies have shown that the rate-limiting step in 5-HT metabolism is the initial hydroxylation of tryptophan, as opposed to the subsequent decarboxylation of 5-HTP, (Naughton, Mulrooney, and Leonard: 2000).
The major concern about the wide spread use of Ecstasy is the evidence that frequent users may be losing serotonin receptors in the CNS, perhaps even irreversibly, due to the extraordinarily amounts of serotonin released while on Ecstasy. This is such a concern among physicians and neuroscientists because of the wide spread roles that serotonin receptors play in the body, and the damage that can occur due to depletion of these receptors.
Serotonin has been found to act both pre-synaptically and post-synaptically, and could be either excitatory or inhibitory in its actions. Therefore, when it was first proposed that there were more than one type of 5-HT receptor, it came as no surprise. However, scientists were still surprised as the multitude of receptor sub types that were eventually discovered. Currently, there is evidence for 16 different subtypes of 5-HT receptors, which appears to be the result of more than 750 million year of molecular evolution. The 5-HT receptors belong to the superfamily of G-protein-coupled receptors, and are found in Planaria, a relatively low life form thought to have been around (and not evolved much) for at least 750 million years. At this point, it is assumed that divergence of the 5-HT receptors began, giving rise to the 16 subtypes known to exist today (Cryan and Leonard: 2000).
At least five 5-HT1 receptor subtypes have been classified, being 5-HT1A, 5-HT1B, B-HT1D, 5-HT1E, and 5-HT1F. All belong to a class of receptors known as Serpentine receptors, which consist of seven transmembrane spanning G-protein-coupled receptors. The binding of 5-HT to the receptor causes a conformational change in the intracellular domain of the receptor, which then affects its interaction with the GTP-binding G-protein on the cytosolic side of the plasma membrane. The occupied receptor causes replacement of the GDP bound to the alpha subunit of the G protein by GTP, activating the G protein. This activated G protein (alpha subunit bound to GTP) then regulates an enzyme which generates an intracellular second messenger. If the G protein is a stimulatory G protein, it acts on the membrane bound enzyme to increase the concentration of the intracellular second messenger, while an inhibitory G protein acts to decrease the second messenger concentration (Cox and Nelson: 2000). This family of 5-HT receptor subtypes has inhibitory G proteins (Gi), and all are negatively linked to adenyl cyclase, which causes a decrease in cyclic AMP concentrations.
The 5-HT1A receptor differs from significantly from most other 5-HT receptors and even acts like an adrenergic receptor, which may explain why a high number of adrenergic agents bind to this receptor with such a high affinity (Cryan and Leonard: 2000). The adrenergic receptor family is named after the receptor type that binds acetylcholine. At resting, this type of receptor is closed. When the neurotransmitter bind to the two binding sites, the receptor is excited, and the gate opens, allowing sodium and calcium cations to freely pass through it and into the cell. After continued excitation by the neurotransmitter, the receptor becomes desensitized, and the receptor gates once again close (Cox and Nelson: 2000).
The 5-HT1B receptors are not present in primates, but are rather found in the CNS of rodents. Its analog in primates seems to be the 5-HT1D receptor. Currently, these receptors are thought to be involved in both anxiety and depression.
The 5-HT1E has been shown to show a rather low affinity for 5-HT compared to the other receptors, and its physiological function is unknown. The fifth 5-HT1 receptor subtype, 5-HT1F, also has not had its physiological significance determined (Naughton, Mulrooney, and Leonard: 2000).
5-HT2 Receptors:The 5-HT2 receptor family consists of three specific receptor subtypes: 5-HT2A, 5-HT2B, and 5-HT2C. This family of serotonin receptors are all G-protein linked molecules that are positively coupled to phosphoinoside metabolism, and are found both in the central nervous system (CNS) and in the peripheral nervous system (PNS).
The 5-HT2A receptors are found on vascular, bronchial, and urinary muscle tissue in the PNS, as well as on blood platelets, and function in vaso- and broncho-constriction as well platelet aggregation. In the CNS, they function to suppress cell firing, as well as inhibit neurotransmitter release (dopamine, acetylcholine and noradrenaline). These receptors are thought to function in numerous CNS disorders, including depression, anxiety, schizophrenia, and sleep disorders (Stone et al: 1998).
The 5-HT2B receptors are found in the stomach, liver, kidney, and intestine, and are thought to promote stomach contractions in rats, although other functional effects have not yet been demonstrated. The 5-HT2C receptors are found mostly in the CNS and are involved with initiating migraine attacks (Cryan and Leonard: 2000).
The 5-HT3 receptors are found in both the PNS and CNS are different from the other serotonin receptors in that they are non-selective Na+/K+ ion channel receptors, allowing them to alter fast synaptic transmission. 5-HT3 receptor antagonists are used to treat the nausea and vomiting induced by chemotherapy treatment in cancer patients (Cryan and Leonard: 2000).
The 5-HT4 receptors are all positively coupled to adenyl cyclase, so when serotonin binds to the receptor, GTP displaces GDP on the alpha subunit of the stimulatory G-protein, which then activates adenyl cyclase, and enzyme which catalyzes the synthesis of cyclic-AMP (cAMP). Currently, there is evidence that these 5-HT4 receptors play a vital role in memory processing (Naughton, Mulrooney, and Leonard: 2000).
This class of serotonin receptors has been found to not have a high efficiency of coupling to G-proteins, suggesting these may in fact be coupled to ion channels. These receptors are thought to play a role motor control, depression, anxiety, and even in brain development (Cryan and Leonard: 2000).
5-HT6 receptors are found primarily in the CNS, and recent evidence suggests that these play a role in many neuropsychiatric disorders. This is because numerous antidepressants (clomipramine, amitriptylamine) and antipsychotic agents (rilapine, clozapine, olanzapine) bind with a high affinity for these type of receptors, acting as antagonists. The loss of these receptors in Ecstasy users is particularly concerning, as their depression cannot be treated with the common antidepressants prescribed today (Naughton, Mulrooney, and Leonard: 2000).
The newest class of 5-HT receptors, these two subtypes (5-HT7A and 5-HT7B) are thought to be involved in both mood and learning. It has recently been found that these two receptors also have a high affinity for many antidepressants and antipsychotic agents (Naughton, Mulrooney, and Leonard: 2000).
The major effect associated with the long-term abuse of the drug Ecstasy has been the development of clinical depression in frequent users. As MDMA affects serotonin release, and since serotonin has long been known to be linked to depression, it was assumed that MDMA eventually caused a lower production in the amount of serotonin released. If this were true, then treatment with antidepressants should have fixed the problem. Most antidepressants are known as SSRI's, or selective serotonin reuptake inhibitors. This class of drugs works in that they inhibit the reuptake of serotonin back into the nerve terminal, therefore increasing the amount of available synaptic 5-HT, and thus, reversing depression (Connor: 1999).
However, in Ecstasy users, the administration of SSRI's had no effect, suggesting that the problem was not in the serotonin levels after all, as the increased serotonin levels did not provide the expected results. After studies found that even high levels of SSRI administration didn't work to decrease depression, it was then postulated that the problem wasn't in the levels of serotonin, but in the 5-HT receptors. Autopsy observations on humans who have died from complications of Ecstasy use (heart failure, heat stroke, seizures) found that their serotonin levels were normal (as measured by high performance liquid chromatography or HPLC), further suggesting that the problem was in the 5-HT receptors. However, it still wasn't known if the 5-HT receptors were merely dysfunctional, or if they had actually been completely depleted. However, evidence now exists that it is actually in the number of receptors, as recent studies in rodents has found a reduction in post-synaptic 5-HT receptors following MDMA dosage (Battaglia: 1999).
The depletion of serotonin receptors is much like Type II Diabetes Mellitus, in that the ligand is present in normal amounts, but the low concentration of receptors is what causes the problems. Therefore, antidepressants show no effect, as the increased levels of serotonin aren't any help, because the receptors aren't present to take up the ligand (Colado and Green: 2000).
While MDMA appears to effect all the receptor subtypes and cause an eventual marked reduction, only a few of these have been implicated as causing depression. The major receptors involved in depression are the 5-HT1A receptor, 5-HT1B/1D receptors, two of the 5-HT2 receptors (A and C), the 5-HT3 receptor, and the 5-HT6 receptor (Cryan and Leonard: 2000).
Although not enough studies have been done to prove this conclusively, nor has the drug been around the mainstream long enough to examine older users for long-term consequences, neuro-scientists propose that this depletion in serotonin receptors is irreversible. At present time, the majority of antidepressants are SSRI's, and thus they are ineffective at treating Ecstasy users who experience depression, because an increase in serotonin is futile without the appropriate receptors to bind to the serotonin.
While clinical depression is currently making headlines as the number one effect caused by the depletion of 5-HT receptors in the brain, neuro-scientists speculate that many more effects will soon be seen, due to the wide range of functions that the different 5-HT receptors are involved in, including memory, anxiety, and various psychosis. Regardless of these concerns, the use of Ecstasy is at an all time high, and the consequences of the "rave" scene continued to be diagnosed by physicians at an alarming rate.
Battaglia, G., Yeh, S.Y., O'Hearn, E., and Molliver, M.E. "3,4-Methylenedioxymethamphetamine and MDA Destroy Serotonin Terminals in Rat Brain: Quantification of Neurodegeneration by Measurement of [3H] Paroxetine-Labelled Serotonin Uptake Sites." Journal of Pharmacological Experience and Therapy, 242 (1999). pp. 911-916.
Colado, M.I., and Green, A.R. "The Spin Trap Reagent alpha-Phenyl-N-tert-Butyl Nitrone Prevents 'Ecstasy'-Induced Neurodegeneration of 5-Hydroxytryptamine Neurons." The European Journal of Pharmacology, 280 (2000). pp. 343-346.
Connor, T.J., McNamara, M.G., Kelly, J.P., and Leonard, B.E. "3,4-Methylenedioxymethamphetamine (MDMA; Ecstasy) Administration Produces Dose-Dependent Neurochemical, Endocrine and Immune Changes in the Rat." Human Psychopharmacological Clinical Experience, 14 (1999). pp. 95-104.
Cox, Michael. M. and Nelson, David L. Lehninger Principles of Biochemistry, Third Edition. Worth Publishers: 2000, pp. 426, 443-451, 644-648, 844-845.
Cryan, J.F., and Leonard, B.E. "5-HT1A and Beyond: The Role of Serotonin and its Receptors in Depression and the Antidepressant Response." Human Psychopharmacological Clinical Experience, 15 (2000). pp. 113-135.
Naughton, M., Mulrooney, J.B., and Leonard, B.E. "A Review of the Role of Serotonin Receptors in Psychiatric Disorders." Human Psychopharmacological Clinical Experience, 15 (2000). pp. 397-415.
Rutty, G.N., and Milroy, C.M. "The Pathology of the Ring-Substituted Amphetamine Analogue 3,4-Methylenedixymethylamphetamine (MDMA, 'Ecstasy')." Journal of Pathology, Volume 181 (1998). pp. 255-256.
Shankaran, M., Yamamoto, B.K., and Gudelsky, G.A. "Ascorbic Acid Prevents 3,4-Methylenedioxymethamphetamine (MDMA) Induced Hydroxyl Radical Formation and the Behavioral and Neurochemical Consequences of the Depletion of Brain 5-HT." The Journal of Neurochemistry, Aug. 23, 2000. pp. 55-64.
Stone, D.M., Merchant, K.M., Hanson, G.R., and Gibb, J.W. "Immediate and Long-Term Effects of 3,4-Methylenedioxymethamphetamine on Serotonin Pathways in Brain of Rat. Journal of Neuropharmacology, 26 (1998). pp 1677-1683.
Yeh, S.Y. "N-tert-Butyl-alpha-Phenylnitrone Protects Against 3,4- Methylenedioxymethamphetamine-Induced Depletion of Serotonin in Rats." The Journal of Pharmacological and Biochemical Behavior, May 1998. pp.169-177.
Copyright © 2001 Bobby Huggins and Koni Stone
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