Neuromodulation is the physiological process by which a given neuron uses one or more chemicals to regulate diverse populations of neurons. Neuromodulators typically bind to metabotropic, G-protein coupled receptors (GPCRs) to initiate a second messenger signaling cascade that induces a broad, long-lasting signal. This modulation can last for hundreds of milliseconds to several minutes. Some of the effects of neuromodulators include altering intrinsic firing activity,[1] increasing or decreasing voltage-dependent currents,[2] altering synaptic efficacy, increasing bursting activity[2] and reconfiguring synaptic connectivity.[3] Major neuromodulators in the central nervous system include: dopamine, serotonin, acetylcholine, histamine, norepinephrine, nitric oxide, and several neuropeptides. Cannabinoids can also be powerful CNS neuromodulators.[4] [5] [6] Neuromodulators can be packaged into vesicles and released by neurons, secreted as hormones and delivered through the circulatory system.[7] A neuromodulator can be conceptualized as a neurotransmitter that is not reabsorbed by the pre-synaptic neuron or broken down into a metabolite. Some neuromodulators end up spending a significant amount of time in the cerebrospinal fluid (CSF), influencing (or "modulating") the activity of several other neurons in the brain.[8]
See also: Neural pathways.
The major neurotransmitter systems are the noradrenaline (norepinephrine) system, the dopamine system, the serotonin system, and the cholinergic system. Drugs targeting the neurotransmitter of such systems affect the whole system, which explains the mode of action of many drugs.
Most other neurotransmitters, on the other hand, e.g. glutamate, GABA and glycine, are used very generally throughout the central nervous system.
| System | Origin[9] | Targets | Effects |
|---|---|---|---|
| Noradrenaline system | Locus coeruleus | Adrenergic receptors in:
|
|
| Lateral tegmental field |
| ||
| Dopamine system | Dopamine pathways:
| Dopamine receptors at pathway terminations. |
|
| Serotonin system | caudal dorsal raphe nucleus | Serotonin receptors in:
|
|
| rostral dorsal raphe nucleus | Serotonin receptors in:
| ||
| Cholinergic system | Pedunculopontine nucleus and dorsolateral tegmental nuclei (pontomesencephalotegmental complex) | (mainly) M1 receptors in: |
|
| basal optic nucleus of Meynert | (mainly) M1 receptors in: | ||
| medial septal nucleus | (mainly) M1 receptors in: |
The noradrenaline system consists of around 15,000 neurons, primarily in the locus coeruleus.[12] This is diminutive compared to the more than 100 billion neurons in the brain. As with dopaminergic neurons in the substantia nigra, neurons in the locus coeruleus tend to be melanin-pigmented. Noradrenaline is released from the neurons, and acts on adrenergic receptors. Noradrenaline is often released steadily so that it can prepare the supporting glial cells for calibrated responses. Despite containing a relatively small number of neurons, when activated, the noradrenaline system plays major roles in the brain including involvement in suppression of the neuroinflammatory response, stimulation of neuronal plasticity through LTP, regulation of glutamate uptake by astrocytes and LTD, and consolidation of memory.[13]
The dopamine or dopaminergic system consists of several pathways, originating from the ventral tegmentum or substantia nigra as examples. It acts on dopamine receptors.[14] Parkinson's disease is at least in part related to dropping out of dopaminergic cells in deep-brain nuclei, primarily the melanin-pigmented neurons in the substantia nigra but secondarily the noradrenergic neurons of the locus coeruleus. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.
The serotonin created by the brain comprises around 10% of total body serotonin. The majority (80-90%) is found in the gastrointestinal (GI) tract.[15] [16] It travels around the brain along the medial forebrain bundle and acts on serotonin receptors. In the peripheral nervous system (such as in the gut wall) serotonin regulates vascular tone.
Although changes in neurochemistry are found immediately after taking these antidepressants, symptoms may not begin to improve until several weeks after administration. Increased transmitter levels in the synapse alone does not relieve the depression or anxiety.
The cholinergic system consists of projection neurons from the pedunculopontine nucleus, laterodorsal tegmental nucleus, and basal forebrain and interneurons from the striatum and nucleus accumbens. It is not yet clear whether acetylcholine as a neuromodulator acts through volume transmission or classical synaptic transmission, as there is evidence to support both theories. Acetylcholine binds to both metabotropic muscarinic receptors (mAChR) and the ionotropic nicotinic receptors (nAChR). The cholinergic system has been found to be involved in responding to cues related to the reward pathway, enhancing signal detection and sensory attention, regulating homeostasis, mediating the stress response, and encoding the formation of memories.[23] [24]
Gamma-aminobutyric acid (GABA) has an inhibitory effect on brain and spinal cord activity. GABA is an amino acid that is the primary inhibitory neurotransmitter for the central nervous system (CNS). It reduces neuronal excitability by inhibiting nerve transmission. GABA has a multitude of different functions during development and influences the migration, proliferation, and proper morphological development of neurons. It also influences the timing of critical periods and potentially primes the earliest neuronal networks. There are two main types of GABA receptors: GABAa and GABAb. GABAa receptors inhibit neurotransmitter release and/or neuronal excitability and are a ligand-gated chloride channel. GABAb receptors are slower to react due to a GCPR that acts to inhibit neurons. GABA can be the culprit for many disorders ranging from schizophrenia to major depressive disorder because of its inhibitory characteristics being dampened.[25] [26] [27]
Neuropeptides are small proteins used for communication in the nervous system. Neuropeptides represent the most diverse class of signaling molecules. There are 90 known genes that encode human neuropeptide precursors. In invertebrates, there are ~50 known genes encoding neuropeptide precursors.[28] Most neuropeptides bind to G-protein coupled receptors, however some neuropeptides directly gate ion channels or act through kinase receptors.
Neuromodulators may alter the output of a physiological system by acting on the associated inputs (for instance, central pattern generators). However, modeling work suggests that this alone is insufficient,[31] because the neuromuscular transformation from neural input to muscular output may be tuned for particular ranges of input. Stern et al. (2007) suggest that neuromodulators must act not only on the input system but must change the transformation itself to produce the proper contractions of muscles as output.
Neurotransmitter systems are systems of neurons in the brain expressing certain types of neurotransmitters, and thus form distinct systems. Activation of the system causes effects in large volumes of the brain, called volume transmission.[32] Volume transmission is the diffusion of neurotransmitters through the brain extracellular fluid released at points that may be remote from the target cells with the resulting activation of extra-synaptic receptors, and with a longer time course than for transmission at a single synapse.[33] Such prolonged transmitter action is called tonic transmission, in contrast to the phasic transmission that occurs rapidly at single synapses.[34] [35]
Tonic Transmission
There are three main components of tonic transmission: Continued release, sustained release, and baseline regulation. In the context of neuromodulation, continuous release is responsible for releasing neurotransmitters/neuromodulators at a constant low level from glial cells and tonic active neurons. Sustained Influence provides long-term stability to the entire process, and baseline regulation ensures that the neurons are in a continued state of readiness to respond to any signals. Acetylcholine, noradrenaline, dopamine, norepinephrine, and serotonin are some of the main components in tonic transmission to mediate arousal and attention. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3429030/
Phasic Transmission
There are three main components of phasic transmission: burst release, transient effects, and stimulus-driven effects. As the name suggests, burst release is in charge of releasing neurotransmitters/neuromodulators in intense, acute bursts. Transient effects create acute momentary adjustments in neural activity. Lastly, as the name suggests, stimulus-driven effects react to sensory input, external stressors, and reward stimuli, which involve dopamine, norepinephrine, and serotonin. https://www.nature.com/articles/s41380-024-02608-3
There are two main categories for neuromodulation therapy: chemical and electrical.
Electrical Neuromodulator Therapies
Electrical neuromodulation has three subcategories: deep brain, spinal cord, and transcranial, each aiming to treat specific conditions. Deep brain stimulation involves electrodes being surgically implanted into specific sections of the brain that are commonly responsible for movement and motor control deficiencies and disorders like Parkinson's and tremors. Spinal cord stimulation works by being placed near the spinal cord to send electrical signals through the body to treat various forms of chronic pain like lower back pain and CRPS. This form of neuromodulator treatment is considered one of the more high-risk treatments because of its manipulation near the spinal cord. Transcranial magnetic stimulation is slightly different in that it utilizes a magnetic field to generate electrical currents throughout the brain. This treatment is widely used to remedy various mental health conditions like depression, obsessive-compulsive disorder, and other mood disorders.https://www.sciencedirect.com/topics/immunology-and-microbiology/neuromodulation#:~:text=spasticity%2C%20and%20others.-,Neuroprostheses%20such%20as%20cochlear%20implants%20and%20sacral%20root%20stimulators%20are,epidural%20or%20intrathecal%20delivery%20systems.https://uab.grlcontent.com/dynpain/page/ch2pg6
Neuromodulation is often used as a treatment mechanism for moderate to severe migraines by way of nerve stimulation. These treatments work by utilizing the basic ascending pathways. There are three main modes. It works by connecting a device to the body that sends electrical pulses directly to the affected site (Transcutaneous Electrical Nerve Stimulation), directly to the brain (Transcranial Magnetic Stimulation), or by holding a device close to the neck that works to block pain signals modulation from the PNS to the CNS. https://www.barrowneuro.org/treatment/neuromodulation-for-headache/#:~:text=Recent%20studies%20show%20that%20TMS,lasting%20changes%20in%20brain%20activity. and sends two of the most notable modes of that treatment, which are electrical and magnetic stimulation. Electrical nerve stimulation and some of the characterizations include transcranial alternating stimulation and transcranial direct current stimulation. The other is magnetic stimulation, which includes single pulse and repetitive transcranial stimulation.
Chemical Neuromodular Therapies
Chemical neuromodulation mostly consists of collaborating natural and artificial chemical substances to treat various conditions. It uses both invasive and non-invasive modes of treatment, including pumps, injections, and oral medications. This mode of treatment can be used to manage immune responses like inflammation, mood, and motor disorders. https://neuro.stonybrookmedicine.edu/neuromodulation/disorderstreated#:~:text=Neuromodulation%20works%20by%20delivering%20gentle%20electrical%20impulses,Brain%20Stimulation%20(DBS)%20Spinal%20Cord%20Stimulation%20(SCS)