Role of serotonin in visual orientation processing

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Serotonin (5-hydroxytryptamine) is a monoamine neurotransmitter that plays a role in mood, eating, sleeping, arousal and potentially visual orientation processing. [1] [2] [3] [4] [5] To investigate its function in visual orientation, researchers have utilised MDMA, or as it is commonly referred to, Ecstasy (3,4-methylenedioxymethamphetamine). [2] [3] [4] MDMA is known to affect serotonin neurons in the brain and cause neurotoxicity. [1] [2] [3] [6] [7] [8] Serotonin has been hypothesised to be involved in visual orientation because individuals who use MDMA exhibit an increase in the magnitude of the tilt aftereffect (TAE). [2] [3] [4] [5] [9] [10] The TAE is a visual illusion where viewing lines in one direction, for an extended period of time, produces the perception of a tilt in the opposite direction to vertical lines subsequently viewed. [2] [3] [5] [11] This effect is proposed to occur due to lateral inhibition to orientation sensitive neurons in the occipital lobe. [4] [11] Lateral inhibition is where neurons that become activated to a particular orientation send inhibitory signals to their neighbouring neurons. [5] [12] The degree of orientation that each neuron becomes maximally excited to is referred to as the tuning bandwidth. [2] [3] [5] Lateral inhibition consequently plays a pivotal role in each neuron's tuning bandwidth, such that if lateral inhibition no longer occurs, a greater number of neurons will become stimulated to the same orientation. [3] This results in the activated neurons becoming adapted to the same orientation stimulus, if the stimulus is viewed for a period of time. As a consequence, if those neurons are subsequently 'shown' another stimulus that differs slightly in its orientation, those neurons are no longer able to achieve the same level of response as compared to other non-adapted neurons. [5]

Contents

Serotonin nerve pathways in the brain Serotonin nerve pathways in the brain.gif
Serotonin nerve pathways in the brain
Example of the Tilt Aftereffect TiltAfterEffect.jpg
Example of the Tilt Aftereffect

Studies have consequently utilised the TAE to assess the degree of lateral inhibition that occurs from MDMA use. The results of these studies have found evidence to support the role that serotonin plays in visual orientation. [2] [3] [4] [5] This was evidenced through individuals who solely used MDMA reporting a greater magnitude of the TAE compared to drug naive controls. [2] [3] This increased magnitude showed that serotonin plays a role in lateral inhibition by potentially having a honing effect, meaning that orientation neurons become maximally excited to their preferred orientation, and less so to others. [2] This additionally provides further evidence of the neurotoxicity of MDMA. [2] [4] This area of research, overall, has provided insights into the mechanisms of visual orientation processing and the effect that MDMA neurotoxicity has on this system. This furthers the understanding of both the role that serotonin has on the visual system and to what degree MDMA neurotoxicity affects the brain.

History and Effect of MDMA

Ecstasy is the street name that refers to the popular recreational drug that contains 3,4-methylenedioxymethamphetamine (MDMA). [13] The now frequently used drug in the rave and club scene was first synthesized by Merck, a German pharmaceutical company that was investigating the development of new medications in the early 1900s. [14] Since its development, it has undergone various phases, from controversially being used as a therapeutic aid in the 1970s, to being banned in the 1980s after the Drug Enforcement Administration concluded that it was addictive. [6] [13] [14] During and following the 1970s, however, MDMA became a popular recreational drug due to it producing feelings of euphoria, empathy, social closeness, mild hallucinations and stimulation. [2] [3] [13] [14] The popularized use of the drug amongst the general public has subsequently raised concerns as animal and human studies have shown that it has the ability to cause neurotoxicity to the brain. [2] [3] [4] [5] [6] [8] [15] [16]

MDMA is part of the amphetamine family and elicits its positive effects by altering brain serotonin, dopamine and norepinephrine neurotransmitter levels. [2] [6] [13] [14] As the drug begins to take effect, the brain becomes flooded with serotonin which can then become depleted within 3–6 hours following consumption. [14] It has also been shown that an enzyme required to synthesize serotonin becomes deactivated, therefore, inhibiting the brain's replenishment of used serotonin. [14] Due to changes that the brain undergoes during and following MDMA consumption, various consequences have been noted. These have included memory impairment, anxiety, paranoia, mood swings and depression. [13] [14] This has raised further concerns as to what extent MDMA may damage and change the brain's chemistry and what this means for its users. [4]

MDMA and Visual Orientation Processing

Recent research investigating MDMA has revealed the neurotoxic effect of the drug on brain serotonin neurons. [1] [3] [6] [7] [8] Long term and potentially permanent changes to serotonergic axons have been noted in animal and primate studies where they were administered doses of MDMA similar to those taken by some human users. [2] [3] [5] [6] [8] [14] [15] [16] MDMA has subsequently been used to investigate the role that serotonin may play in visual orientation processing. [2] [3] [4] Serotonin neurons are thought to reside in the occipital lobe, which is an area of the brain responsible for visual processing of line orientation, edges, motion and stereoscopic depth perception. [2] [3] [5] Because MDMA is known to affect serotonin and that serotonin is thought to be involved in vision, individuals who take MDMA may exhibit differences in their visual orientation processing. [2] [3]

The relationship between the effect of MDMA and serotonin's role in visual orientation processing has been investigated following a prior study conducted in the 1990s by Maisini, Antonietti and Moja (1990). [2] [3] [5] Their experiment involved subjects ingesting a mixture which significantly reduced brain serotonin levels. [17] This reduction in serotonin resulted in an increase in the magnitude of the TAE in those subjects. [18] This study has since been used as the foundation for the idea that MDMA neurotoxicity, due to its effect on serotonin neurons, could influence the magnitude of the TAE in individuals who use MDMA. [2] [3] [4]

Present Research

Current findings regarding altered visual orientation processing from MDMA use comes from research by White, Brown and Edwards (2013). [2] Their study sought to extend the results found in previous research, such as Maisini et al. (1990), [18] and investigate how MDMA affects visual processing in the occipital lobe. The participants of the study were divided into three groups: Ecstasy users who were amphetamine abstinent, Ecstasy users who also used amphetamines, and drug naive control participants. [2] Ecstasy users who additionally used amphetamine were included as results from prior studies have indicated that concurrent amphetamine use may mediate the effects of MDMA on orientation neurons. [2] [3] [4]

The results of the study indicated that the amphetamine abstinent Ecstasy group showed a broader tuning bandwidth than the controls. [2] This demonstrates that MDMA use produces changes to serotonergic functioning as it disrupts lateral inhibition between orientation sensitive neurons. This disruption causes the neurons to activate to a wider range of orientations other than their preferred orientation. [2] [3] This finding, therefore, supports the idea that serotonin plays a role in sharpening the tuning bandwidths of orientation neurons. [2] Overall, the results support the idea that "MDMA-mediated serotonin depletion can lead to broader orientation tuning bandwidths" [2] p. 163. The authors do, however, go on to say that although deficits in certain tasks are present, the extent of these deficits requires further investigation. [2]

A study by Brown, Edwards, McKone and Ward (2007), [3] additionally investigated MDMA's effect on serotonin neurons. Their research also stemmed from Masini et al. (1990). [18] They were interested in serotonin's role in lateral inhibition to orientation sensitive neurons and how MDMA use may change this system and produce wider tuning bandwidths. [3] The study consisted of two groups, Ecstasy users and controls, who were shown brief displays of the TAE illusion. [3] The results of the study support the idea that serotonin damage due to MDMA use causes lateral inhibition to diminish amongst orientation sensitive neurons in the occipital lobe. [3] This was demonstrated by the Ecstasy group showing a greater increase in the magnitude of the TAE illusion compared to the controls. [3] The authors stated that perhaps "serotonin is involved in the extent to which the sensitivity of neurons is reduced during adaptation" [3] p. 445. It could be that the decrease in sensitivity of the post-adaptation orientation neurons is further diminished by decreased serotonergic functioning, which increases the magnitude of the TAE. Their research lends support to the idea that MDMA use affects lateral inhibition and that serotonin plays a role in visual orientation processing. [3]

Related Research Articles

<span class="mw-page-title-main">MDMA</span> Psychoactive drug, often called ecstasy

3,4-Methyl​enedioxy​methamphetamine (MDMA), commonly known as ecstasy, and molly, is an empathogen–entactogenic drug with stimulant and minor psychedelic properties. In studies, it has been used alongside psychotherapy in the treatment of post-traumatic stress disorder (PTSD) and social anxiety in autism spectrum disorder. The purported pharmacological effects that may be prosocial include altered sensations, increased energy, empathy, and pleasure. When taken by mouth, effects begin in 30 to 45 minutes and last three to six hours.

<span class="mw-page-title-main">Empathogen</span> Class of psychoactive drugs that produce empathic experiences

Empathogens or entactogens are a class of psychoactive drugs that induce the production of experiences of emotional communion, oneness, relatedness, emotional openness—that is, empathy or sympathy—as particularly observed and reported for experiences with 3,4-methylenedioxymethamphetamine (MDMA). This class of drug is distinguished from the classes of hallucinogen or psychedelic, and amphetamine or stimulants. Major members of this class include MDMA, MDA, MDEA, MDOH, MBDB, 5-APB, 5-MAPB, 6-APB, 6-MAPB, methylone, mephedrone, GHB, αMT, and αET, MDAI among others. Most entactogens are phenethylamines and amphetamines, although several, such as αMT and αET, are tryptamines. When referring to MDMA and its counterparts, the term MDxx is often used. Entactogens are sometimes incorrectly referred to as hallucinogens or stimulants, although many entactogens such as ecstasy exhibit psychedelic or stimulant properties as well.

<span class="mw-page-title-main">3,4-Methylenedioxyamphetamine</span> Empathogen-entactogen, psychostimulant, and psychedelic drug of the amphetamine family

3,4-Methylenedioxyamphetamine (MDA), sometimes referred to as “sass,” is an empathogen-entactogen, stimulant, and psychedelic drug of the amphetamine family that is encountered mainly as a recreational drug. In its pharmacology, MDA is a serotonin–norepinephrine–dopamine releasing agent (SNDRA). In most countries, the drug is a controlled substance and its possession and sale are illegal.

α-Ethyltryptamine Chemical compound

α-Ethyltryptamine, also known as etryptamine, is an entactogen and stimulant drug of the tryptamine family. It was originally developed and marketed as an antidepressant under the brand name Monase by Upjohn in the 1960s before being withdrawn due to toxicity.

<i>para</i>-Methoxyamphetamine Chemical compound

para-Methoxyamphetamine (PMA), also known as 4-methoxyamphetamine (4-MA), is a designer drug of the amphetamine class with serotonergic effects. Unlike other similar drugs of this family, PMA does not produce stimulant, euphoriant, or entactogen effects, and behaves more like an antidepressant in comparison, though it does have some psychedelic properties.

<span class="mw-page-title-main">Norepinephrine transporter</span> Protein-coding gene in the species Homo sapiens

The norepinephrine transporter (NET), also known as noradrenaline transporter (NAT), is a protein that in humans is encoded by the solute carrier family 6 member 2 (SLC6A2) gene.

<span class="mw-page-title-main">4-Fluoroamphetamine</span> Psychoactive research chemical

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<span class="mw-page-title-main">4-Methylthioamphetamine</span> Chemical compound

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<i>para</i>-Chloroamphetamine Chemical compound

para-Chloroamphetamine (PCA), also known as 4-chloroamphetamine (4-CA), is a substituted amphetamine and monoamine releaser similar to MDMA, but with substantially higher activity as a monoaminergic neurotoxin, thought to be due to the unrestrained release of both serotonin and dopamine by a metabolite. It is used as a neurotoxin by neurobiologists to selectively kill serotonergic neurons for research purposes, in the same way that 6-hydroxydopamine is used to kill dopaminergic neurons.

<span class="mw-page-title-main">MDAI</span> Chemical compound

MDAI (5,6-methylenedioxy-2-aminoindane) is a drug developed in the 1990s by a team led by David E. Nichols at Purdue University. It acts as a non-neurotoxic and highly selective serotonin releasing agent (SSRA) in vitro and produces entactogen effects in humans.

<span class="mw-page-title-main">Monoamine releasing agent</span> Class of compounds

A monoamine releasing agent (MRA), or simply monoamine releaser, is a drug that induces the release of a monoamine neurotransmitter from the presynaptic neuron into the synapse, leading to an increase in the extracellular concentrations of the neurotransmitter. Many drugs induce their effects in the body and/or brain via the release of monoamine neurotransmitters, e.g., trace amines, many substituted amphetamines, and related compounds.

A serenic, or antiaggressive agent, is a type of drug which reduces the capacity for irritability and aggression.

<span class="mw-page-title-main">DFMDA</span> Chemical compound

Difluoromethylenedioxyamphetamine is a substituted derivative of 3,4-methylenedioxyamphetamine (MDA), which was developed by Daniel Trachsel and coworkers, along with the corresponding fluorinated derivatives of MDMA, MDEA, BDB and MBDB, with the aim of finding a non-neurotoxic drug able to be used as a less harmful substitute for entactogenic drugs such as MDMA. Since a major route of the normal metabolism of these compounds is scission of the methylenedioxy ring, producing neurotoxic metabolites such as alpha-methyldopamine, it was hoped that the difluoromethylenedioxy bioisostere would show increased metabolic stability and less toxicity.

<span class="mw-page-title-main">UWA-101</span> Chemical compound

UWA-101 is a phenethylamine derivative researched as a potential treatment for Parkinson's disease. Its chemical structure is very similar to that of the illegal drug MDMA, the only difference being the replacement of the α-methyl group with an α-cyclopropyl group. MDMA has been found in animal studies and reported in unauthorised human self-experiments to be effective in the short-term relief of side-effects of Parkinson's disease therapy, most notably levodopa-induced dyskinesia. However the illegal status of MDMA and concerns about its potential for recreational use, neurotoxicity and potentially dangerous side effects mean that it is unlikely to be investigated for medical use in this application, and so alternative analogues were investigated.

<i>para</i>-Chloromethamphetamine Chemical compound

para-Chloromethamphetamine is a stimulant that is the N-methyl derivative and prodrug of the neurotoxic drug para-chloroamphetamine (4-CA). It has been found to decrease serotonin in rats. Further investigation into the long-term effects of chloroamphetamines discovered that administration of 4-CMA caused a prolonged reduction in the levels of serotonin and the activity of tryptophan hydroxylase in the brain one month after injection of a single dose of the drug.

Una D. McCann is a board certified psychiatrist and researcher at Johns Hopkins School of Medicine in the Department of Psychiatry. She is also the Director of the Anxiety Disorders Program, and Co-Director of the Center for Interdisciplinary Sleep Medicine and Research, and Associate Program Director at the Johns Hopkins Bayview Medical Center. McCann is considered to be an expert in anxiety and stress disorders and her primary areas research revolves around amphetamine-induced monoamine neurotoxicity and neurobiology of anxiety disorders.

<span class="mw-page-title-main">Monoamine neurotoxin</span> Compounds that damage or destroy monoaminergic neurons

A monoamine neurotoxin, or monoaminergic neurotoxin, is a drug that selectively damages or destroys monoaminergic neurons. Monoaminergic neurons are neurons that signal via stimulation by monoamine neurotransmitters including serotonin, dopamine, and norepinephrine.

<span class="mw-page-title-main">ODMA (drug)</span> MDMA analogue

ODMA is a bioisosteric analogue of 3,4-methylenedioxy-N-methylamphetamine (MDMA) which was developed in an attempt to create an improved MDMA alternative for potential clinical use. It is the analogue of MDMA in which the 1,3-benzodioxole ring has been replaced with a 2,1,3-benzoxadiazole ring. TDMA and SeDMA are closely related analogues. ODMA, TDMA, and SeDMA are releasing agents of serotonin, norepinephrine, and dopamine similarly to MDMA. However, they are less potent and efficacious in activating the serotonin 5-HT2A, 5-HT2B, and 5-HT2C receptors than MDMA and show differing and potentially improved metabolic and pharmacokinetic properties in comparison. ODMA, TDMA, and SeDMA were first described in the scientific literature in June 2024.

<span class="mw-page-title-main">TDMA (drug)</span> MDMA analogue

TDMA is a bioisosteric analogue of 3,4-methylenedioxy-N-methylamphetamine (MDMA) which was developed in an attempt to create an improved MDMA alternative for potential clinical use. It is the analogue of MDMA in which the 1,3-benzodioxole ring has been replaced with a 2,1,3-benzothiadiazole ring. ODMA and SeDMA are closely related analogues. ODMA, TDMA, and SeDMA are releasing agents of serotonin, norepinephrine, and dopamine similarly to MDMA. However, they are less potent and efficacious in activating the serotonin 5-HT2A, 5-HT2B, and 5-HT2C receptors than MDMA and show differing and potentially improved metabolic and pharmacokinetic properties in comparison. ODMA, TDMA, and SeDMA were first described in the scientific literature in June 2024.

<span class="mw-page-title-main">SeDMA</span> MDMA analogue

SeDMA is a bioisosteric analogue of 3,4-methylenedioxy-N-methylamphetamine (MDMA) which was developed in an attempt to create an improved MDMA alternative for potential clinical use. It is the analogue of MDMA in which the 1,3-benzodioxole ring has been replaced with a 2,1,3-benzoselenadiazole ring. ODMA and TDMA are closely related analogues. ODMA, TDMA, and SeDMA are releasing agents of serotonin, norepinephrine, and dopamine similarly to MDMA. However, they are less potent and efficacious in activating the serotonin 5-HT2A, 5-HT2B, and 5-HT2C receptors than MDMA and show differing and potentially improved metabolic and pharmacokinetic properties in comparison. ODMA, TDMA, and SeDMA were first described in the scientific literature in June 2024.

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