Homeostatic plasticity

Homeostatic presynaptic plasticity

Homeostatic presynaptic plasticity is the ability of neurons to regulate neurotransmitter release at presynaptic terminals, ensuring a steady range of brain activity. This process involves various mechanisms, such as quantal size adjustment, differential expression of presynaptic proteins, and modification of vesicle recycling. Quantal size adjustment helps maintain steady postsynaptic responses despite changes in synaptic strength. Differential expression of presynaptic proteins, such as calcium channels or synaptic vesicle proteins, can also be altered by neurons to affect neurotransmitter release rate.

Homeostatic postsynaptic plasticity

Homeostatic postsynaptic plasticity is crucial for maintaining consistent levels of synaptic activity in neurons, which are formed at specific synapses in the brain. Homeostatic processes involve changes in the expression of receptors, changes in receptor sub-unit composition, and changes to intracellular signaling pathways. For example, the NMDA receptor can change its sub-unit composition to improve sensitivity to neurotransmitters. Additionally, changes in the expression and location of neurotransmitter receptors can impact synaptic transmission when specific signaling pathways are activated. Synaptic adhesion molecules can also be influenced by homeostatic processes. Overall, homeostatic postsynaptic plasticity contributes to the stability and proper functioning of neural circuits, allowing the brain to adapt to changing conditions without compromising the overall stability of neuronal activity.^[11]

Homeostatic intrinsic plasticity

Homeostatic intrinsic plasticity refers to the ability of neurons to change their intrinsic electrical characteristics in response to changes in synaptic or network activity. This process involves alterations in the excitability or firing characteristics of individual neurons, rather than primarily adjusting synaptic strength. Intrinsic plasticity processes associated with homeostasis include ion channel expression alterations, membrane conductance modifications, action potential threshold alterations, and regulation of intrinsic excitability. Neurons can up-regulate the expression of sodium channels to maintain firing rates and increase excitability in case of a drop in synaptic activity. These changes impact the input-output link between neurons and the homeostatic control of neuronal activity.

Synaptic scaling

Synaptic scaling is a homeostatic mechanism that allows neurons to modulate the strength of all synapses to maintain stable activity levels within a specific range. This process is characterized by changes in the quantity or sensitivity of neurotransmitter receptors on the postsynaptic membrane. Neurons can reduce the number of neurotransmitter receptors in response to network activity spikes, reducing synaptic strength, or increase the density in response to network activity drops, increasing sensitivity and boosting synaptic strength. This homeostatic regulation of brain circuits supports other types of synaptic plasticity, such as long-term depression and long-term potentiation.

Homeostatic structural plasticity

Structural homeostatic plasticity is another subtype of homeostatic plasticity, and is a homeostatic mechanism that involves structurally remodeling neural circuits, such as forming, pruning, or altering synaptic size, in order to maintain stability within the network's activity.^[12] While it seems similar to synaptic scaling, it is different in the sense that synaptic scaling acts on the strength or weight of existing synapses, while homeostatic structural plasticity acts on the number of synapses. In a paper published in July 2025, it was found that synaptic scaling and structural homeostatic plasticity work hand-in-hand. In the study, mouse hippocampal tissue cultures were used to block AMPA receptors in two different doses. Results showed that a partial blockage of the AMPA receptors increased the spine density, whereas a complete blockage resulted in decreased spine density. This showed that there is not a linear relationship between neural activity and dendritic spine numbers; instead, it is biphasic (U-shaped) and dependent on the degree of inhibition. This biphasic result is related to structural homeostatic plasticity because it shows how the subtypes of homeostatic structural plasticity and synaptic scaling work together to govern the number of synapses and the strength of synapses to provide control over the homeostasis of neural circuits.^[13]

Metaplasticity

Sometimes called the "plasticity of plasticity", metaplasticity is when a neuron or synapse's past activity changes the capacity it has for future plasticity. Instead of directly changing synaptic strength or working in the pre-synaptic or post-synaptic sense, it works on a higher level to shift the threshold at which future long-term potentiation of long-term depression can be triggered. Thus, it inhibits repeated excitability or degradation of synaptic connections through Hebbian neuroplasticity, and works as both a sub-type and mechanism of homeostatic plasticity.^[14]

Hebbian plasticity

Homeostatic synaptic plasticity is a means of maintaining the synaptic basis for learning, respiration, and locomotion, in contrast to the Hebbian plasticity associated with learning and memory. Although Hebbian forms of plasticity, such as long-term potentiation and long-term depression occur rapidly, homeostatic plasticity (which relies on protein synthesis) can take hours or days. Homeostatic plasticity is thought to balance Hebbian plasticity by modulating the activity of the synapse or the properties of ion channels.^[15]

Functional plasticity

Functional plasticity is a type of plasticity that allows the brain the ability to adapt to changes in its functions with changes in their environment. At every point in the child's life, their brain is able to balance malleable processes that represent neural plasticity and promote the change with homeostatic process in order to promote stability.^[16] This means that regardless of a child's brain development as a fetus, typical brain development or atypical brain development, can depend on their environment.

The functions and abilities of a certain part of the brain can be moved to another part of the brain when damaged. When a fetus is developing, this type of plasticity occurs rapidly establishing the brain systems.^[17]

Structural plasticity

Structural plasticity refers to morphological changes in the structure of the brain through the growth of new synaptic connections.^[18] This process is done through synaptogenesis, neuronal migration, and neurogenesis. These processes are a foundational part of fetal neuron development, but have also been found in adult brains. Neurogenesis occurs in the ventricular or subventricular zone of the brain. Neural migration is the process of neurons traveling from these zones towards their final destination in development.^[19] Synaptic remodeling in response to learning and memory lead to function consequences in the brain throughout life.^[20]

Developmental plasticity

Another broad umbrella of plasticity is developmental plasticity. It is generally defined as an organism's ability to develop in regards to the environment that it is in, allowing for one encoded genotype to result in multiple phenotypes in the developmental process. The mechanism underlying this type of plasticity is the brain and nervous system's ability to adapt to experience and the environment, even if it opposes the genetic composition of an individual. Ultimately, these phenotypic changes can influence later health outcomes and the passing on of certain traits to the next generation.^[21]

Epilepsy

In a healthy brain, neuronal excitability and synaptic strength are homeostatically regulated to maintain balance between excitation and inhibition. In epilepsy, this process can become semi-uncontrolled through damage to Narp, leading to excessive neural excitability. This can cause an event where after prolonged network inactivity and/or brain injuries, neurons may become more active to compensate for the irregularity, which can cause the person to be more prone to seizures. Homeostatic plasticity is essential for normal brain function, the overcompensation due to brain dysregulation can contribute to the epileptic activity, highlighting its complex role in both stability and dysfunction.^[27] There are ways to attempt to regulate the over-excitability, such as cortical stimulation which increases controlled activity and might decrease excessive excitability, achieving the balance homeostatic plasticity seeks. Some studies also show that electrical stimulation helps those who suffer from drug-resistant epilepsy.^[28] Traditional pharmacological approaches may be ineffective in restoring physiological balance in the neuronal network. However, therapeutic strategies targeting homeostatic plasticity mechanisms may offer a potential solution.^[26]

Autism spectrum disorder

Autism spectrum disorder is a neurodevelopment disorder that is characterized by repetitive patterns of behavior, along with difficulties in social interaction and communication. Dysregulation of homeostatic plasticity and neural imbalance can contribute to the cognitive and behavioral symptoms associated with autism. Sensory inputs and intrinsic brain activity can cause long-term changes in synaptic efficiency and eventually lead to an increase or decrease in the ratio between excitatory and inhibitory neurotransmissions is found in Autism.^[29] Alterations in the expression of synaptic proteins that regulate excitatory or inhibitory leads to dysfunction as the ratio changes. An example linked to autism is Neuroligins 1-4. Neuroligin 1 is mainly present in excitatory synapses, neuroligin 2 is more present in inhibitory synapses, and neuroligin 3 is present in both. Neuroligin 4 is present in glycinergic synapses which are major contributors to the regulation of neuronal excitability as they control fluxes of sensory information. Mutations in neuroligins 3 & 4 have been shakily linked to autism, but neuroligins 1-3 have been found to cause primary inhibitory dysfunction in autism.^[30] Pruning of developing synapses in the central nervous system regulates brain size and shape into the third decade of life. It is a necessary process to keep the brain from being too small or large. Underpruning has been associated autism and results in enlarged brain size due to overgrowth of dendrites from the lack of pruning and modification of neuron number. Motor impairment in autism has been linked specifically to an abundance of white matter in the primary motor cortex. Due to 22q11.2 gene deletion syndrome, found to be connected to autism spectrum disorder, both thicker and thinner cortical regions along with regions of increased cerebral cortex folding can be found in the brain.^[31] Environmental factors that effect normal homeostasis are heightened in autism, stopping neurons from maintaining optimum levels of activity.^[32]

Alzheimer's disease

In Alzheimer's disease, synaptic function and neuronal integrity are impaired. In a healthy brain, these mechanisms are tightly maintained by homeostatic plasticity. In the brain, there are constant changes in synaptic input. Therefore, neurons use homeostatic plasticity to keep their activity stable and prevents neurons from becoming too excited or too quiet. According to researchers, if a neuron does not receive enough activity, the synapses become stronger by adding receptors that help transmit signals.^[2] On the other hand, researchers argue that if the neuron receives too much activity the neuron will remove the receptors and weaken the synaptic connection.^[2] Researchers describe the regulation of receptors as synaptic scaling. In addition, neurons are able to adjust their intrinsic excitability and the changes help maintain the stability of neuron firing rates.^[2] Therefore, homeostatic plasticity potentially plays a significant role in neural stability. Deficits in homeostatic plasticity contribute to cognitive decline and memory impairment which are characteristic symptoms of Alzheimer's disease.

Addiction

Homeostatic plasticity plays a role in causing addiction as well. This is largely due to the maladaptive nature of homeostatic plasticity. It can occur with any sort of addictive substance, but the following example will cover drugs that are commonly abused, since they usually have a stronger effect because they cause excessive neural activation, and resulting hypoactivity when the drug wears off. When drugs are first taken, it triggers homeostatic plasticity due to the excessive neural activation and the immediate hypoactivity. Homeostatic plasticity works to counteract this disturbance by strengthening excitatory connections during withdrawal. This is an example of maladaptive homeostasis, where the nervous system tries to compensate for an inefficiency, but instead causes more damage. Here, the damage occurs because the neural networks that are responsible for craving the drug (or addictive substance) are also enhanced, and the addiction becomes stronger. The specific homeostatic maladaptations are manifested in the nucleus accumbens, and are associated with AMPA recruitment and membrane plasticity.^[33]

Depression

Homeostatic plasticity plays a role in causing depression, and can also assist in treating depression. Homeostatic plasticity plays a role in depression pathology because failed homeostatic plasticity, or even a significant lack of it, can result in an inability to stabilize neural activity, which leads to weakened synapses, resulting in forms of depression.^[34] Additionally, chronic stress can damage neurons in the prefrontal cortex, and if homeostatic plasticity is not used to reset the structure and function of these neurons, this atrophy creates room for disease.^[35] Imbalanced networks with a decrease in synaptic connectivity in areas such as the hippocampus and the prefrontal cortex but increased plasticity in areas like the amygdala can alter the balance between emotions and logical thought formation, thus resulting in depressive states.^[36] Lastly, inflammation of the microglia can cause disfunction of homeostatic plasticity, prompting depressive episodes as well.^[37]

Homeostatic plasticity can be utilized in treatment practices for clinical depression through multiple mechanisms. Primarily, drugs like ketamine that act as an NMDA receptor antagonist have been shown to have rapid antidepressant effects by engaging homeostatic mechanisms such as rapid synaptic upscaling through pathways like the mTORC1 signaling pathway.^[38] This ketamine-induced signaling can also increase synaptogenesis (creation of synapses) in the pre-frontal cortex, which can compensate for stress-related damage done to homeostatic plasticity mechanisms and neurons. There is also medicine and prescriptions that work to therapeutically establish new and healthy homeostatic set points in neural networks that had previously lost synaptic function.^[39]

Neurodegenerative diseases

Several neurological disorders are affected by homeostatic plasticity. Dysregulation of homeostatic plasticity can cause an excitatory or inhibitory network activity. Parkinson's disease, Huntington's disease, and ALS are all examples of disorders where dysregulation of neuronal networks contributes to the pathophysiology of the disorders.^[40] For instance, synaptic impairments occur early in ALS, so the early chances may serve as a method to counteract neuron dysfunction. Although the nervous system does attempt to adapt using synaptic plasticity, the attempts to maintain neural function are most times temporary and may eventually become harmful. For example, researchers believe that the spinal cord attempts to adapt by increasing the size of synaptic contacts, such as glutamatergic synapses to preserve motor function before the disease becomes detrimental.^[41] However, the increased excitatory synaptic activity becomes too excessive and leads to excitotoxicity. Thus, the neurodegenerative disease progresses further.

Other neurodegenerative diseases that are caused by damage to genes or molecules required for homeostatic synaptic signaling include migraines, ataxia, ADHD, and (damage to Ca2+ channels); intellectual disabilities (damage to FMRP); tuberous sclerosis complex (damage to mTOR/elF4E); myasthenia gravis (damage to acetylcholine receptors); Tourette's syndrome (damage to neurexin and neuroligin); and Warburg Micro and Martsolf Syndrome (damage to Rab3GAP).^[42]

Genetic disorders

Another way that neurodegenerative diseases can be caused by a maladaptive mechanism is when homeostatic plasticity fails to adapt the nervous system to genetic defects that can arise during development. A clear example of this is Angelman syndrome^[43], which is a neurodevelopmental disorder that results from mutations on chromosome 15. The specific mutation, which is a loss the UBE3A gene, impairs homeostatic plasticity by damaging Arc/Arg3.1. As a result, improper synaptic scaling leads to hyper-excitability within neural circuits, causing intellectual disability, speech impairment, gait ataxia, and other symptoms.^[44] Another genetic disorder of a similar accord is Rett syndrome, which is caused by a mutation in the MECP2 gene.^[45] This mutation is responsible for helping neurons adjust to activity changes, thereby maintaining homeostatic plasticity. With the loss of function mutation, homeostatic plasticity is impaired, and patients suffer symptoms of loss of mobility in the hands, slowed brain growth, problems with walking, and more.^[46] Fragile X syndrome is also a neurodegenerative disorder that is caused by a impairment to synaptic scaling and homeostatic plasticity through the inactivation of the FMR1 gene that is responsible for retinoic acid signaling through FMRP. Studies have shown that repairing the mutation has restored the signaling and opened doors to potential treatment opportunities.^[47]

Schizophrenia

Schizophrenia is characterized by disruptions in thought processes, perceptions, and emotions. Alterations in synaptic strength and connectivity due to dysregulation in homeostatic mechanisms lead to the symptoms observed in schizophrenic patients, specifically in problems with dysbindin. This dysregulation contribute to the cognitive deficits and delusions observed in schizophrenia.^[49] In fact, a study was conducted to prove that disturbance in the homeostasis of Purkinje cells is an example of how developmental plasticity homeostasis is crucial in preventing mental illnesses like Schizophrenia. Purkinje cells are neurons located in the cerebellum and are involved in cognitive functions.^[50] Moreover, research has shown that Purkinje cells keep the brain's activity stable after traumatic experiences, and unregulated responses to trauma disrupt normal brain activity, which leads to mental illnesses.^[51]