Understanding Synapses: Electrical and Chemical Transmission

Synaptic plasticity—the ability of synapses to strengthen or weaken—is the cellular basis for learning and memory, mediated by NMDA receptors that detect coincident pre- and postsynaptic activity; long-term potentiation (LTP) involves calcium influx through NMDA receptors activating CAMKII and other kinases to increase AMPA receptor number and conductance, while long-term depression (LTD) involves similar calcium signals but produces synaptic weakening, with different phases of plasticity (induction, expression, maintenance) requiring distinct molecular mechanisms.

Synaptic plasticity is the brain's ability to strengthen or weaken neural connections in response to experience, forming the basis for learning and memory. Two key mechanisms are Long-Term Potentiation (LTP), which strengthens synapses through persistent neuronal activity to form lasting memories, and Long-Term Depression (LTD), which weakens unused or inefficient connections. These processes work together to sculpt the brain's architecture, enabling adaptation to new experiences and playing a crucial role in recovery after brain injury.

Synaptic plasticity enables neural networks to adapt through strengthening (long-term potentiation, LTP) or weakening (long-term depression, LTD). LTP occurs when glutamate, glycine, and depolarizing action potentials coincide, activating NMDA receptors and triggering insertion of additional AMPA receptors into the postsynaptic membrane. This increases the maximum depolarization current, strengthening synaptic transmission. LTD occurs when this combination is insufficient, causing AMPA receptor removal and synaptic weakening. These bidirectional mechanisms allow the brain to encode memories, learn new behaviors, and maintain efficient neural processing by strengthening useful connections while pruning less useful ones.

Long-term potentiation (LTP) and long-term depression (LTD) are bidirectional forms of synaptic plasticity that depend on the frequency and duration of neural stimulation; high-frequency stimulation (100 Hz for 1 second) causes dramatic calcium influx through NMDA receptors (increasing intracellular calcium from 100 nM to 1 μM), which activates CAMKII kinase to insert AMPA receptors and strengthen synapses, while low-frequency stimulation (10 minutes) produces sustained moderate calcium levels (400-500 nM) that activate calcineurin phosphatase to internalize AMPA receptors and weaken synapses, with these mechanisms being utilized by the brain during learning and memory formation as demonstrated by optogenetic studies showing that LTP/LTD protocols can artificially write and erase fear memories.

Drugs interact with neurotransmitters through different mechanisms. Agonists enhance neurotransmitter effects - increasing excitatory or inhibitory effects as needed. Antagonists block or reduce neurotransmitter effects. Reuptake inhibitors prevent neurotransmitter reabsorption, increasing their availability in the synapse. For example, SSRIs block serotonin reuptake, increasing serotonin levels to treat depression. Antianxiety medications like benzodiazepines act as GABA agonists, increasing inhibitory effects to reduce anxiety.

Medications can increase or decrease neurotransmitter activity through three mechanisms: (1) Agonists mimic neurotransmitters to increase their effects when there is a deficiency; (2) Antagonists bind to receptor sites on dendrites, preventing neurotransmitters from entering neurons and reducing their effects when there is an excess; (3) Reuptake inhibitors (like SSRIs) block the reabsorption of neurotransmitters back into the sending neuron, forcing more neurotransmitters to remain in the synaptic gap and be absorbed by receiving neurons. This explains how SSRIs increase serotonin availability by blocking its reuptake.

Drugs influence brain function by affecting neurotransmitter systems through three primary mechanisms: agonists bind to neurotransmitter receptors and mimic the effects of natural neurotransmitters (e.g., morphine mimics endorphins to produce euphoria); antagonists block neurotransmitter receptors and prevent the neurotransmitter's normal effects (e.g., atropine blocks acetylcholine to reduce muscle tremors); and reuptake inhibitors prevent neurotransmitters from being reabsorbed into the presynaptic neuron, thereby increasing their concentration in the synaptic cleft (e.g., cocaine blocks dopamine reuptake, maintaining elevated dopamine levels that produce feelings of happiness).

Drugs affect synaptic transmission through multiple mechanisms: neurotoxins like botulinum toxin (blocks vesicle fusion by destroying SNARE proteins, causing flaccid paralysis) and tetanus toxin (prevents inhibitory neurotransmitter release, causing spastic paralysis); receptor agonists like nicotine (stimulates nicotinic receptors on skeletal muscle) and muscarine (stimulates muscarinic receptors on cardiac muscle, decreasing heart rate); receptor antagonists like curare (blocks nicotinic receptors, causing paralysis) and atropine (blocks muscarinic receptors, used as antidote); and enzyme inhibitors like acetylcholinesterase inhibitors (increase neurotransmitter levels in the synaptic cleft) and reuptake inhibitors (increase neurotransmitter availability by blocking presynaptic reuptake).

Neurons communicate at synapses where presynaptic terminals release neurotransmitters stored in vesicles. Neurotransmitters can be excitatory (promoting action potentials) or inhibitory (preventing them). Despite ~100 billion neurons, only ~few hundred neurotransmitters exist due to brain compartmentalization—different neural networks in separate brain regions use the same neurotransmitters for different functions. Major neurotransmitters include dopamine (reward, motor control), epinephrine/norepinephrine (fight-or-flight), serotonin (sleep, appetite, mood), acetylcholine (muscle contraction), GABA (inhibitory), and glutamate (excitatory). To qualify as a neurotransmitter, substances must meet three criteria: synthesized and stored in presynaptic terminals, released by action potentials, and produce measurable effects on postsynaptic neurons. After signaling, neurotransmitters are removed via reuptake or degradation. Treatment challenges arise from brain compartmentalization—treating one region affects others, as seen in Parkinson's disease where global dopamine increase risks inducing schizophrenia-like symptoms.

Synaptopathy is pathology of synapses - the microscopic connections enabling brain cell communication. Synapses underwrite the brain's functional anatomy as a deep recurrent neural network. Neuromodulatory systems (dopamine, serotonin, acetylcholine, adrenaline, oxytocin) set synaptic gain, controlling how strongly synapses respond to inputs. Predictive coding explains how the brain makes sense of the world through generative models that generate predictions, with prediction errors weighted by precision. When synaptopathy prevents proper gain control, the brain cannot properly modulate prediction error precision, leading to Type I errors (delusions) and Type II errors (neglect syndromes). Sensory attenuation - the ability to ignore irrelevant sensory information - is critical; its failure produces Parkinson's disease, autism, and chronic pain.

Synaptopathies represent impairment at any level of synaptic function, encompassing diverse etiologies from genetic to autoimmune causes. Presynaptic dysfunction (STXBP1, SNAP25, SCN1B) affects neurotransmitter release through the SNARE complex, producing early-onset epileptic encephalopathies with characteristic movement disorders including dystonic postures, non-epileptic spasms, and distinctive gait abnormalities. Postsynaptic dysfunction (SHANK, DLG family) affects receptor scaffolding and glutamatergic transmission, causing intellectual disability, autism spectrum features, and later-onset seizures with eyelid myoclonia and sensitivity to sensory stimuli. Treatment approaches differ dramatically: presynaptic disorders often respond to sodium channel blockers while postsynaptic disorders require different therapeutic strategies. Understanding these mechanistic categories provides a framework for linking epilepsy and neurodevelopmental disorders.

Epilepsy is a cerebral cortex disorder characterized by recurrent seizures, which are sudden surges in electrical brain activity causing alterations in sensation, behavior, or consciousness; seizures are classified as partial (originating from one brain region) or generalized (involving both hemispheres), with partial seizures further divided into simple partial (preserving consciousness), complex partial (with impaired consciousness lasting 30-2 minutes), and partial with secondary generalization evolving into tonic-clonic seizures; the underlying mechanism involves excessive excitatory neurotransmitters (glutamate) and reduced inhibitory neurotransmitters (GABA), or synchronous high-frequency neuronal bursts replacing normal low-frequency discharges. Neurodegenerative diseases are age-associated progressive deterioration of neuronal structures and functions leading to cognitive disability and dementia, with Parkinson's disease resulting from degeneration of dopaminergic neurons in the substantia nigra compacta due to alpha-synucleopathy (abnormal alpha-synuclein protein aggregation forming Lewy bodies), and Alzheimer's disease resulting from deposition of extracellular amyloid plaques (beta-amyloid protein) and intracellular neurofibrillary tangles (tau protein) causing degeneration of cholinergic neurons in the hippocampus and frontal cortex.

Synapses consist of dendritic spines protruding from dendrites, forming compartments where presynaptic axons connect through transsynaptic adhesion proteins. Postsynaptic sites contain AMPA and NMDA receptors for neurotransmitter binding. Synapses undergo constant plasticity during learning, strengthening or weakening based on activity patterns. Neurological and psychiatric diseases often involve synaptic deficits: autism shows excessive synaptic connections creating overactive networks, while Alzheimer's involves early synapse loss followed by neuron degeneration. Understanding these mechanisms provides targets for treating conditions like schizophrenia, epilepsy, Parkinson's, and depression.

Beyond SAP97, other schizophrenia-associated genes including GRIN2A (NMDA receptor subunit) and Trio produce identical pathological strengthening of glutamatergic synapses in dentate granule neurons when their function is compromised. This suggests that increased glutamatergic neurotransmission in the hippocampus may represent a common mechanism underlying schizophrenia development across different genetic perturbations. Local reduction of SAP97 specifically in the dentate gyrus produces contextual memory deficits, demonstrating that this synaptic pathology is sufficient to produce schizophrenia-like behavioral alterations.