The brain’s intricate network relies on trillions of connections between nerve cells, or neurons. These connection points, called synapses, are where information is passed from one neuron to the next, forming the physical basis of thought, memory, and movement. To study these microscopic structures, scientists use biological markers, which are measurable substances indicating a particular biological state. A synaptic marker is a specific molecule, usually a protein, that is part of a synapse’s structure. These markers act like molecular signposts, allowing researchers to locate, count, and characterize synapses within brain tissue, investigating how the number of connections changes over time, during learning, or in the presence of disease.
Types of Synaptic Markers
Synaptic markers are categorized by their location and function within the synapse, which consists of a sending and a receiving side. The sending terminal of a neuron is the presynaptic terminal, and it releases chemical messengers. Proteins exclusive to this area are known as presynaptic markers. A common example is Synaptophysin, a protein found in the membrane of synaptic vesicles, the tiny sacs that hold and release neurotransmitters. Another is Synapsin I, which helps regulate these vesicles.
On the other side of the connection is the postsynaptic terminal, which receives chemical signals. Proteins found here serve as postsynaptic markers. A widely studied example is Postsynaptic Density-95 (PSD-95), a scaffolding protein that anchors neurotransmitter receptors in place on the receiving neuron’s membrane. By holding these receptors in a dense cluster, PSD-95 strengthens the synapse and makes communication more efficient. The presence of PSD-95 indicates an excitatory, or signal-promoting, connection.
Markers can also distinguish between the types of signals being sent. Synapses can be either excitatory, transmitting “go” signals that encourage the next neuron to fire, or inhibitory, sending “stop” signals that prevent it from firing. Scientists use markers for the transporters that load these specific neurotransmitters into vesicles. For example, vesicular glutamate transporters (VGLUTs) are used to identify excitatory synapses that use glutamate. In contrast, the vesicular GABA transporter (VGAT) is a reliable marker for inhibitory synapses that use the neurotransmitter GABA.
Methods for Detecting Synapses
To visualize and quantify synaptic markers, scientists use a technique called immunofluorescence. This method involves applying carefully designed antibodies to very thin slices of brain tissue. These antibodies are engineered to recognize and bind only to a specific synaptic marker protein, such as Synaptophysin or PSD-95. These antibodies are also tagged with fluorescent dyes that glow brightly when illuminated with a specific wavelength of light.
When researchers view the prepared brain tissue under a fluorescence microscope, the locations of the synaptic markers appear as thousands of tiny, distinct points of light, called “puncta”. To confirm the presence of a complete synapse, scientists use two different antibodies at once—one for a presynaptic marker and another for a postsynaptic marker. Each is tagged with a different colored dye. A synapse is then identified where a presynaptic punctum and a postsynaptic punctum are located right next to each other.
A complementary method to measure the overall amount of a synaptic protein in a brain region is called Western blotting. Unlike immunofluorescence, this technique does not show the precise location of individual synapses. It involves taking a sample of brain tissue, extracting all the proteins, and then using antibodies to detect the total quantity of a specific marker protein. This provides a measure of the overall synaptic protein level in that tissue sample.
More recently, advances in medical imaging have allowed scientists to study synapses in living human brains using Positron Emission Tomography (PET). This technique uses a radioactive tracer that is injected into the bloodstream and travels to the brain. Tracers have been developed, such as [11C]UCB-J, that bind specifically to a protein called Synaptic Vesicle Glycoprotein 2A (SV2A), found in the vesicles of nearly all synapses. The PET scanner detects the radioactive signal, providing a direct measure of synaptic density across different brain regions.
Interpreting Synaptic Changes
The data gathered from detection methods provide information about the state of the brain’s circuitry through the concept of synaptic density. Synaptic density refers to the number of synapses within a given volume of brain tissue. High levels of synaptic markers detected through immunofluorescence or Western blotting indicate a high synaptic density, suggesting robust neural connectivity in that area.
Changes in marker levels allow researchers to infer dynamic processes in the brain. A decrease in the quantity of synaptic markers is interpreted as synaptic loss, or neurodegeneration. This reduction signifies that connections between neurons are lost, which can impair communication within neural circuits. This process is a common feature of both normal aging and various neurological disorders.
Conversely, an increase in synaptic marker levels can indicate synaptic gain, a process related to the brain’s capacity for plasticity. Synaptic plasticity is the ability of synapses to strengthen or weaken over time, and it includes the formation of entirely new connections. This remodeling of neural circuits is the cellular basis for learning and memory. For example, studies in animals have shown that learning a new complex motor skill leads to an increase in the number of synapses in the cerebellum, the brain region involved in coordinating movement.
The ability to form, eliminate, and remodel synaptic connections is important for a healthy brain. It allows the nervous system to adapt to new experiences, store information, and fine-tune its functions. By measuring synaptic markers, scientists can gain a window into these processes, observing how the brain’s wiring responds to learning, environmental changes, or the onset of disease.
Connection to Brain Disorders
Abnormal changes in synaptic marker levels are a hallmark of many neurological and psychiatric conditions, providing insights into how these disorders disrupt brain function. In Alzheimer’s disease, the loss of synapses is an early event. This synaptic degeneration occurs long before the formation of amyloid plaques and tangles, and the degree of synapse loss correlates strongly with the severity of dementia. Studies of post-mortem brain tissue and PET imaging in living patients show reduced levels of markers like synaptophysin and SNAP-25, especially in the hippocampus, which relates to the early memory impairments that characterize the disease.
Parkinson’s disease is known as a movement disorder caused by the death of dopamine-producing neurons, but it is also a disease of synaptic dysfunction. The loss of dopamine disrupts communication in the basal ganglia, a group of brain structures that control movement. This leads to changes in both presynaptic terminals, which struggle to release the remaining dopamine, and postsynaptic terminals, which alter their sensitivity to signals. This synaptopathy, or disease of the synapse, can happen before widespread neuron death, making it a target for early therapeutic intervention.
The study of synaptic markers has also shed light on major depressive disorder (MDD). Evidence from post-mortem brain analyses reveals a reduced density of synapses and lower levels of synaptic proteins in brain regions that regulate mood and emotion, such as the prefrontal cortex and hippocampus. This deficit in synaptic connectivity is believed to contribute to the cognitive and emotional symptoms of depression. The effectiveness of certain fast-acting antidepressants like ketamine is linked to their ability to rapidly trigger synaptogenesis, the formation of new synapses, which can be tracked by measuring changes in synaptic markers.