5.5: Neurotransmitters

As described previously, neurotransmitters are the substances that are released at chemical synapses, and they are the signaling molecules that allow neurons to communicate with one another. To date, scientists have identified more than 100 neurotransmitters. Here, we will describe six classical neurotransmitters, their receptors, and their actions. Additionally, three atypical neurotransmitters will be introduced.

One important note to keep in mind as you think about neurotransmitters: the effect that a neurotransmitter has on a cell depends on the receptor. In other words, a neurotransmitter molecule can either excite or inhibit a neuron depending on the composition of receptors that are present. For example, glutamate is excitatory at most synapses in the nervous system. Glutamate exerts excitation by activating ionotropic glutamate receptors, which are ligand-gated cation channels. However, at one particular synapse in the eye, glutamate activates a metabotropic glutamate receptor that causes cellular inhibition.

Glutamate

Glutamate (Glu) is the main excitatory neurotransmitter used by the nervous system. Glutamate is the same as the amino acid glutamic acid. There is more glutamate per volume of brain tissue than any other neurotransmitter. Glutamatergic neurons are identified by the presence of the vesicular glutamate transporter (vGluT).

Glutamate can activate both ionotropic and metabotropic receptors. Ionotropic glutamate receptors are all ligand-gated cation channels, which makes them excitatory since they allow Na⁺ to move into the cell. Ionotropic glutamate receptors are generally subdivided into three classes, named after exogenous chemicals that can activate the receptor. AMPA receptors are Na+ channels, but some also allow Ca²⁺ entry. NMDA receptors allow both Na⁺ and Ca²⁺ to pass across the membrane. When the cell is at rest, NMDA receptors also have a large magnesium ion in the pore that blocks ion movement through the channel. The third category of ionotropic glutamate receptors is called kainate receptors, which are similar to AMPA receptors.

The metabotropic glutamate receptors (mGluRs) signal using different G proteins. There are a total of 8 of these mGluRs, classified into three groups, called Group I, Group II, and Group III. Group I are excitatory GPCRs which signal via Gq, while Group II and Group III are inhibitory via the Gi signal transduction pathway.

One theory proposes that excess signaling by glutamate can lead to neuronal death, a phenomenon called excitotoxicity. Of the various glutamate receptors, the NMDA receptor is most strongly implicated in contributing to excitotoxicity, since uncontrolled elevated levels of calcium can be deadly for neurons. Excitotoxicity is observed in a variety of disease states ranging from neurodegenerative disorders such as Parkinson’s disease, Alzheimer’s disease, and multiple sclerosis, but also in injury such as concussion or stroke.

Figure 5.15 Glutamate is the main excitatory neurotransmitter in the nervous system, acting at different categories of receptors, three of which are shown below.

GABA + glycine

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain. According to one estimate, about 25% of neurons in the brain are GABA-ergic. Chemically speaking, GABA is remarkably similar to glutamate. In fact, GABA is synthesized from glutamate in a single step by the enzyme glutamic acid decarboxylase (GAD). GAD is often used as a biochemical marker for the presence of GABAergic neurons. Many interneurons use GABA as their chemical signaling molecule.

Figure 5.16 The inhibitory neurotransmitter GABA is synthesized from glutamate by the action of GAD.

The main action of GABA as an inhibitory neurotransmitter is to activate one of three main classes of receptors, called A, B, and C. GABA_A receptors are ligand-gated chloride channels, so activating these ion channels causes Cl^- flux, which opposes the ability for the cell to reach action potential threshold. GABA_B and GABA_C receptors are both metabotropic receptors that inhibit neuronal activity through the action of the G_i protein.

A neurotransmitter that is similar to GABA is glycine. Another small amino acid, glycine is mostly used by neurons of the spinal cord and brain stem. Glycine is also inhibitory , and acts at glycine receptors, which are ligand-gated chloride channels.

Dopamine

Dopamine (DA) is a biogenic amine derived from the amino acid tyrosine through the action of several enzymes. One in particular, tyrosine hydroxylase (TH), is the main marker that is used for identifying dopamine-producing neurons. Unlike glutamate or GABA, dopamineproducing neurons are not widely abundant in the brain. Instead, there are generally only a few patches of neurons that produce dopamine, most of which are found in the midbrain. Two areas include the ventral tegmental area and the substantia nigra.

There are five classes of dopamine receptors, named D1 through D5. All of them are metabotropic receptors. D1 and D5 are generally excitatory receptors, while D2, D3, and D4 are inhibitory receptors.

Since Roy Wise’s theory proposed in the 1960s, DA has been known in pop culture as the “pleasure neurotransmitter” because of its involvement in the processing of reward and motivation. For example, if we use microdialysis (a technique to measure the concentration of chemicals) in the nucleus accumbens, dopamine levels spike in response to all sorts of pleasurable or rewarding stimuli: food, water, sex, sugar, and exposure to drugs. However, we now know that dopamine is much more complex than once believed. One theory suggests that dopamine elevation serves as a “learning signal” that causes us to pay attention to salient stimuli in the environment.

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DA is also needed for normal motor control. When dopamine-producing neurons in the substantia nigra pars compacta (SNpc) degenerate, as in Parkinson’s disease, a person develops the trademark symptoms: difficulties with motor control, resulting in a resting tremor, postural instability, and bradykinesia (slowness of movement). Reversing the dopamine deficiency by introducing an exogenous source of dopamine is our current gold standard of treatment for PD.

Figure 5.18 Levels of dopamine in the NAc rise when animals lever press for delicious foods, denoted in red.

Serotonin

Serotonin (5-HT) is a neurotransmitter that is derived from the dietary amino acid tryptophan. The enzyme tryptophan hydroxylase is the first step of serotonin biosynthesis and is often used as a marker to identify serotonergic neurons. As with dopamine, there are only a few areas of the brain that synthesize serotonin, the major one being the raphe nucleus in the brain stem.
Receptors for serotonin have a wide variety of actions. We have identified seven major families of 5-HT receptors, which are designated by the number and subclasses which are designated by a letter. For example, the 5-HT2A receptor is metabotropic and excitatory via Gq signaling, while the 5-HT5 receptor is inhibitory via Gi signaling. Most of them are metabotropic receptors; only the 5-HT3 receptor is ionotropic.

Serotonin is heavily implicated in the regulation of mood and complex behavioral conditions. One of our most effective strategies for treating depression is the administration of a drug such as fluoxetine, which acts as a selectiveserotonin reuptake inhibitor (SSRI). Pharmacologically, fluoxetine increases synaptic levels of serotonin by preventing reuptake, and for some people, this has a moderate ability to reverse depression. Serotonin signaling is also a target for drugs that treat anxiety, posttraumatic stress disorder, obsessive-compulsive disorder, schizophrenia, and more.

Clinical correlation: Parkinson’s disease (PD) and L-DOPA-induced dyskinesia (LID)

Parkinson’s disease is a debilitating neurodegenerative disorder that affects as many as 1% of all people aged 60 or older. Generally, PD is lethal within 16 years. By the time a patient presents to the clinic with motor dysfunction, they have already lost almost 60-80% of dopamine-producing neurons in this area!

For decades, clinicians have been using the biochemical precursor to dopamine, L-DOPA, to treat the symptoms. However, after chronic exposure to L-DOPA, the drug becomes less effective and has a shorter duration of therapeutic action. Worse still, frequent treatment can lead patients to develop hyperkinesias, an abnormal excess of movements. This iatrogenic disorder is called L-DOPA induced dyskinesia (LID).

Biomedical engineers have developed a promising nondrug approach to treating PD called deep brain stimulation. A small stimulating device is surgically implanted into the subthalamic nucleus of the brain. When this brain area is stimulated, neural circuits are recruited which restores normal motor control.

Figure 5.19 Patients with PD have characteristic changes in gait as a result of low dopamine (top). The current best pharmacological therapy is levodopa administration, which is the biochemical precursor to dopamine (bottom).

Acetylcholine

Acetylcholine (ACh) is a small molecule that is made by the enzyme choline acetyltransferase (ChAT), which chemically bonds a molecule of acetyl-CoA with a molecule of choline. The presence of ChAT in a neuron is used as a biochemical marker for neurons that produce acetylcholine.

ACh was the first neurotransmitter discovered and chemically isolated, a feat which earned two researchers the shared Nobel Prize in Physiology or Medicine in 1936. One of the two scientists, a German pharmacologist named Otto Loewi, stimulated the vagus nerve connected to an isolated frog heart, which caused the heart rate to slow down. When he put the surrounding solution on top of another heart, he observed that the second heart also slowed down, despite having no physical connection to the first heart. From this, he concluded that a chemical released by the vagus nerve is able to decrease heart rate. This chemical was first called Vagusstoff, the German word meaning Vagus substance. Today, we know it as acetylcholine.

ACh is able to act at ionotropic and metabotropic receptors, and activity at both receptor classes is essential for normal function. The ionotropic receptors of the nervous system are called nicotinic acetylcholine receptors (nAChRs) because they can be activated by nicotine in addition to acetylcholine. These ionotropic receptors are ligand-gated sodium channels and are therefore excitatory. On the other hand, the metabotropic receptors are called muscarinic acetylcholine receptors (mAChRs) since they are activated by the chemical muscarine found in some species of mushrooms. MAChRs can be coupled with either G_s or G_i, so they can be either excitatory or inhibitory.

ACh is the main neurotransmitter that the nervous system uses in order to communicate with the muscles at the neuromuscular junction (NMJ). Here, ACh is released by motor neurons, where it activates nicotinic acetylcholine receptors on muscle cells, causing them to constrict, or flex. On the other hand, muscarinic acetylcholine receptors are located in the heart, and their activation causes a decrease in heart rate (as Otto Loewi demonstrated with the isolated frog heart preparation.)

In the central nervous system, ACh is involved in a wide variety of processes, including attention and learning. One of the first theories to explain the symptoms of Alzheimer’s disease looked at a loss of ACh-producing neurons in the basal forebrain that become more severe as the disease worsens. It has since been demonstrated that Alzheimer’s disease is more complex than this early hypothesis.

Figure 5.20 An electron microscope image of the neuromuscular junction showing vesicles (T, top) forming a synapse with the muscle cell (M, bottom). Acetylcholine is the main neurotransmitter used in muscle control at the PNS.

Norepinephrine

Norepinephrine (NE) is a neurotransmitter that is synthesized from a molecule of dopamine by the enzyme dopamine betahydroxylase. Norepinephrineproducing cells are localized in the pons of the brain stem, a structure called the locus coeruleus. The locus coeruleus is very small, but these neurons send projections widely throughout the brain.

Figure 5.21 Norepinephrine in the brain is synthesized by small populations, but these cell bodies project widely across several other areas.

Outside of the brain, we think of norepinephrine as being responsible for triggering the sympathetic nervous system response of the body, the “fight-or-flight” reaction that prepares the body for physical activity in times of fear or acute stress. These norepinephrine-producing nerve cells reside in the sympathetic ganglia, a clump of nerve cells that run parallel to the spinal cord, one on each half of the body. These neurons project out towards the internal organs.

Receptors for NE are classified into two main categories, alpha or beta. There are subtypes within each category, giving us five major receptors for NE: alpha-1, alpha-2, beta1, beta-2, and beta-3. All five of these receptors are metabotropic, and some are excitatory while others are inhibitory. Our internal organs express these noradrenergic receptors. Clinically, the “beta blockers” are a class of drugs that inhibit beta-adrenergic receptors; the resulting action is a decrease in blood pressure. Conversely, some beta-agonists are used as bronchodilators for asthma.

Norepinephrine also functions in the brain to modulate behaviors including alertness and attention.

Atypical neurotransmitters

Although we generally think of neurotransmitters as neurochemicals that function as described above, there are a few atypical neurotransmitters that don’t quite fit the mold of the other chemical signals.

Neuropeptides

Neuropeptides are a class of large signaling molecules that some neurons synthesize. Neuropeptides are different from the traditional neurotransmitters because of their chemical size. Monoamines like DA, NE, or 5-HT have a molecular weight around 150- 200, while one of the smaller neuropeptides, enkephalin, has a molecular weight of 570. One of the largest, dynorphin, has a molecular weight greater than 2,000. Because of their large size, neuropeptides have to be packaged in densecore vesicles very close to the site of production near the nucleus rather than in clear vesicles right at the terminal.

Figure 5.22 Enkephalin, one of the smaller neuropeptides, is very large compared to other neurotransmitters. Enkephalin is an agonist for both δ and μ opioid receptors.

Neuropeptides such as enkephalin and dynorphin are agonists at a class of receptors called the opioid receptors. These opioid receptors fall into four main types. The three classical opioid receptors are named using Greek letters: δ (delta), μ (mu), and κ (kappa), and the fourth class is the nociceptin receptor. All of these receptors are inhibitory metabotropic receptors which signal using the \(\mathrm{G_{\alpha i}}\) protein.

These receptors are expressed in several brain areas, but expression is particularly heavy in the periaqueductal gray, a midbrain area that functions to inhibit the sensation of pain. Drugs that activate the opioid receptors, like morphine, oxycontin, or fentanyl, are the most effective clinical treatments that we know of for acute pain. Unfortunately, these same drugs also represent a tremendous health risk, as opioid drugs can be lethal in overdose and have a high risk of substance use disorder.

Endocannabinoids (eCBs)

The eCBs are a class of lipidbased neurotransmitters. They are unusual neurochemicals in a few ways. Instead of sending information from the axon of one neuron to the dendrite of the next neuron (anterograde signaling), eCBs allow the postsynaptic dendritic component to communicate with the presynaptic axon terminal. Since they communicate information in the “opposite” direction of classic neurotransmitter signaling, eCBs are called retrograde signaling molecules. Secondly, eCBs are not packaged into vesicles and released by fusion processes. Instead, eCBs are synthesized de novo, meaning they get created right when they needed and used at that moment. The two most well-characterized eCBs in humans are called 2-AG and AEA.

Figure 5.23 ECBs are synthesized from the postsynaptic cell membrane and signal through presynaptic cannabinoid receptors

ECBs activate one of two receptors, CB1 and CB2. Both of them are inhibitory metabotropic receptors that couple with Galphai. Generally, CB1 receptors are found in the nervous system, while the CB2 receptors are found elsewhere in the body, such as in the immune system.

The eCB system is widely used by various systems in the body. It is estimated that eCB receptors are the most abundant GPCRs in the whole body.

These substances were named because they are endogenous chemicals that are functionally similar to compounds found in plants of the genus Cannabis. One reason cannabis is used is because of its ability to interact with our eCB receptors.

Nitric oxide

The nervous system is capable of signaling via the gas nitric oxide (NO). This gasotransmitter is not stored in vesicles but rather is synthesized as it is needed. NO is formed when the amino acid arginine is degraded by the enzyme NO synthase (NOS).

Because NO is a gas, it easily permeates across cell membranes. Therefore, the receptors for NO do not need to be transmembrane proteins expressed on the cell surface. Instead, the receptor for NO is an intracellular receptor called soluble guanylate cyclase (sGC). SGC works through a signaling pathway that is different from other metabotropic receptors so far described. SGC is linked with the signaling molecule cyclic GMP (cGMP), which activates protein kinase G. PKG can either be excitatory or inhibitory, depending on the intracellular components.

Chapter summary

Neurons communicate with one another in a variety of ways. Anatomically, neurons are separated by a small extracellular gap called the synapse. This synapse may directly connect the intracellular cytoplasm, as in an electrical synapse. Alternatively, the gap may be much larger, and chemicals that get released and diffuse across the synapse in order to signal to the following neuron. These chemicals, the neurotransmitters, are stored in vesicles, tiny spheres that are located in the presynaptic axon terminal. The release of these chemicals is very closely regulated, and neurons have several mechanisms that regulate vesicular fusion.

Following release, the neurotransmitters diffuse across the synapse and can bind to the active site on transmembrane proteins called receptors. Upon binding a molecule of neurotransmitter, these receptors physically change shape, resulting in ion flow across the membrane (ionotropic receptors) or a change in the activity of intracellular signaling molecules (metabotropic receptors). Binding of neurotransmitter changes the excitability of neurons.

We have so far identified more than 100 neurotransmitters. Many of them are small molecules that are packaged in vesicles, which then diffuse from the presynaptic neuron to the postsynaptic neuron, such as acetylcholine, glutamate, or GABA. However, there are some atypical neurotransmitters such as neuropeptides, endocannabinoids, and nitric oxide that have different methods of communication.