3. Synaptic Plasticity: Development, LTP, LTD, and Homeostasis

Introduction to Synaptic Plasticity

The course instructor for this segment is a rotating faculty member, a common pedagogical approach in advanced neuroscience programs. This ensures that students receive instruction from faculty members who are active researchers in the specific domain being taught, providing current, in-depth, and often unpublished insights into the field. This rotation allows for a diverse set of expert perspectives on complex and evolving topics.

The previous class specifically discussed glutamate receptors and their comprehensive mechanistic details. This foundational knowledge is critical because glutamate is the primary excitatory neurotransmitter in the brain, and its receptors are central players in fast synaptic transmission, as well as in all forms of synaptic plasticity that underlie learning and memory. The discussion would have covered:

Types of Glutamate Receptors

• Ionotropic receptors: Ligand-gated ion channels, including AMPA ($\alpha$-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, NMDA (N-methyl-D-aspartate) receptors, and Kainate receptors.

• Metabotropic Glutamate Receptors (mGluRs): G-protein coupled receptors.

AMPA Receptor Mechanism

• Primarily responsible for the rapid component of excitatory postsynaptic potentials (EPSPs).

• When glutamate binds, it induces a conformational change that opens the ion channel pore, allowing a rapid influx of $Na^+$ ions (and some $K^+$ efflux), leading to depolarization of the postsynaptic membrane.

• They have fast kinetics, contributing significantly to the speed of synaptic responses.

• A typical current-voltage (I-V) trace for AMPA receptors in physiological $Mg^{2+}$ shows a nearly linear relationship, indicating non-rectifying current flow.

NMDA Receptor Mechanism

• Unique due to their dual requirement for activation: glutamate binding and postsynaptic membrane depolarization.

• At resting membrane potential ($-70mV$ to $-60mV$), the channel pore is blocked by an extracellular $Mg^{2+}$ ion.

• Depolarization, often initiated by AMPA receptor activation, expels the $Mg^{2+}$ block, allowing $Ca^{2+}$ and $Na^+$ influx (and $K^+$ efflux). This $Ca^{2+}$ influx is a crucial second messenger for initiating many forms of synaptic plasticity.

• NMDA receptors also require a co-agonist, typically glycine or D-serine.

• Their slower kinetics (longer opening duration) contribute to the prolonged component of EPSPs and are critical for coincidence detection.

• A current-voltage (I-V) trace for NMDA receptors typically shows an inward rectifying current at negative potentials (due to $Mg^{2+}$ block) and outward current at positive potentials once the block is relieved, leading to a characteristic J-shaped or N-shaped curve that illustrates their voltage dependence.

Kainate Receptors

• Similar to AMPA receptors but with distinct pharmacology and kinetics.

• They contribute to both presynaptic and postsynaptic modulation.

• #### mGluRs

  • G-protein coupled receptors. Upon glutamate binding, they activate intracellular signaling pathways (e.g., involving $Gq$, $G{i/O}$ or $G_s$ proteins) that can modulate ion channel activity, enzyme activity, or gene expression.

  • Leads to slower, longer-lasting effects on synaptic transmission.

  • Categorized into Group I ($mGluR1,5$), Group II ($mGluR2,3$), and Group III ($mGluR4,6,7,8$) based on their sequence homology, pharmacology, and signaling pathways.

A significant portion of the class has prior exposure to neuroscience courses, which implies familiarity with fundamental neurobiological concepts. For these students, some material will serve as a review, reinforcing existing knowledge and providing deeper mechanistic insights. However, all foundational concepts will be covered thoroughly and in an accessible manner for those without prior exposure, ensuring a common baseline understanding for all students.

Overview of Synapses and Plasticity

Synapses are crucial for information transmission in the brain. They are specialized junctions, typically between an axon terminal of a presynaptic neuron and the dendrite or cell body of a postsynaptic neuron. At these junctions, electrical signals in the presynaptic neuron are converted into chemical signals (neurotransmitter release), which then bind to receptors on the postsynaptic neuron, converting the chemical signal back into an electrical signal (postsynaptic potential). This intricate process allows for the serial and parallel processing of information through neural circuits, ultimately leading to all behavioral outputs (e.g., initiating a coordinated motor response like moving the head to avoid an incoming ball, processing sensory information, or forming complex thoughts and emotions).

Receptors, specific protein molecules that bind neurotransmitters and initiate a cellular response, are precisely localized at synapses. This localization is often highly organized, with specific receptors concentrated in the postsynaptic density (PSD), a dense protein scaffolding beneath the postsynaptic membrane, which ensures efficient and specific signaling. The presence and arrangement of these receptors critically determine the synapse's input-output properties.

Behavioral flexibility and adaptation (e.g., learning new skills, forming and retrieving memories, adjusting to novel environmental contexts) are fundamental to survival and higher cognitive function. These capabilities necessitate dynamic changes in how information is processed and flows through neural circuits. This adaptability fundamentally occurs by altering the strength, efficacy, and even the number and structure of individual synapses.

Synapses are dynamic and flexible: their efficacy can be robustly strengthened (potentiated) or significantly weakened (depressed) over various timescales. This dynamism manifests as both functional changes (e.g., alterations in the amount of neurotransmitter released from the presynaptic terminal, changes in the sensitivity or number of postsynaptic receptors, or modifications of their trafficking) and structural changes (e.g., remodeling of dendritic spines, changes in the physical size or shape of synaptic contacts, or even the formation of entirely new synaptic connections, or the elimination of existing ones).

Synaptic changes are critical for brain development, learning, and memory. They represent the fundamental cellular and molecular substrates through which experiences, both internal and external, can lead to enduring alterations in brain function and behavioral repertoires throughout an organism's lifespan. Hebb's Postulate ("neurons that fire together wire together") captures the essence of this activity-dependent plasticity: coordinated activity strengthens connections.

Information storage in the brain is widely believed to be largely attributed to these specific, long-lasting changes in synaptic strength and underlying structure. These modifications effectively encode new memories, skills, and knowledge within the vast and interconnected networks of neurons, creating enduring alterations in circuit function.

Synaptic plasticity is the overarching and encompassing term for the nervous system's intrinsic and profound ability to change, adapt, and readapt the strength and efficacy of its synaptic connections in response to neural activity patterns and diverse experiences encountered throughout an organism's life. It is the core mechanism enabling the brain's remarkable capacity for adaptation, learning, and memory formation, representing the dynamic interplay between genes, environment, and activity.

While rapid, short-term synaptic changes (occurring within milliseconds to seconds) do exist—these often involve transient post-translational modifications of receptor proteins (e.g., phosphorylation), temporary changes in the probability of neurotransmitter release from the presynaptic terminal (e.g., facilitation, depression), or short-term desensitization of receptors—the primary focus here is on long-term changes.

Synaptic Plasticity in the Developing Brain

The developing brain must undergo an extensive and highly orchestrated process of self-organization: this involves intelligently building functional synaptic networks from an initial state of exuberant, often somewhat diffuse or chaotic, growth. Subsequently, it must selectively refine which synaptic connections to retain, strengthen, and integrate into efficiently operating neural circuits, and which ones to prune or eliminate entirely. This process is crucial for establishing precise and functional connections necessary for sensory processing, motor control, and higher cognitive functions.

Mammalian brain development strategy unfolds in distinct, yet often overlapping, phases:

Overproduction

• Initially, the developing brain exhibits a remarkable and strategic phase of overproduction. It generates a significantly large excess of neurons and, even more strikingly, an exponentially greater number of tentative, often diffuse, synaptic contacts than are ultimately required for the fully mature and functional brain.

• This strategy creates a broad and often redundant substrate, providing a rich pool of potential connections from which precise circuits can be sculpted. For example, in the visual cortex, synapses initially form indiscriminately before activity-dependent mechanisms refine them. In some regions, like the Purkinje cells of the cerebellum, a single Purkinje cell might initially be innervated by multiple climbing fibers, which later prune down to a single, powerful connection.

Selection and Elimination (Synaptic Pruning and Apoptosis)

• Following this initial phase of exuberant growth, the system engages in a crucial competitive process to sort out favored and functionally relevant connections. This involves two main processes:

• Synaptic Pruning: The selective elimination of unwanted or non-functional synapses. This is a highly activity-dependent process where synapses that are weakly active or do not effectively contribute to circuit function are removed, while those that are strongly active and functionally integrated are strengthened. This process is often driven by competition for neurotrophic factors or by activity-dependent mechanisms that stabilize active synapses and destabilize inactive ones.

• Apoptosis (Programmed Cell Death): In many instances, even entire neurons that fail to establish appropriate or sufficiently active connections with their target cells, or those failing to receive adequate trophic support from their targets, ultimately undergo programmed cell death. For instance, in the motor neuron pools of the spinal cord or in the retinal ganglion cell layer, approximately twice as many neurons as needed are initially produced. These surplus neurons compete for limited resources (e.g., Nerve Growth Factor, NGF, or Brain-Derived Neurotrophic Factor, BDNF) secreted by their target cells. Those neurons that fail to secure sufficient trophic support, or whose synaptic connections are not sufficiently active, undergo apoptosis. This ensures that only the most robust, functionally integrated, and healthy neurons and their associated synaptic connections persist to form efficient, refined, and highly functional neural circuits.

• A hypothetical graph illustrating this developmental process might show the number of neurons or synaptic density over time: an initial rise (neurogenesis/synaptogenesis), followed by a peak, and then a decline (apoptosis/pruning) to the adult level, reflecting the selection and elimination phase. Another graph could show neuronal survival as a function of target tissue size or trophic factor availability.

• Strengthening of Synapses in the Developing Brain: Glutamatergic Silent Synapses and NMDAR Signaling

  • A unique and critical mechanism for synapse strengthening during development involves glutamatergic silent synapses.

  • What are Silent Synapses? These are postsynaptic sites that initially possess only NMDA receptors and lack functional AMPA receptors. Despite presynaptic glutamate release, at resting membrane potential, no current flows through the NMDAR channel due to the $Mg^{2+}$ block. Thus, the synapse is "silent" to typical presynaptic activity, meaning it does not produce a significant EPSP.

  • Key Role of NMDAR Signaling: The activation of NMDARs in silent synapses is crucial for their maturation. When a silent synapse is depolarized (e.g., coincidentally with other active synapses on the same dendrite) and glutamate is released, the $Mg^{2+}$ block is relieved, allowing $Ca^{2+}$ influx through the NMDARs.

  • Unsilencing of Synapses: This $Ca^{2+}$ signal acts as a powerful intracellular messenger, triggering a cascade of events. It activates various protein kinases (e.g., CaMKII) which lead to the phosphorylation of existing AMPA receptors and, more importantly, the trafficking and synaptic insertion of new AMPA receptors into the postsynaptic membrane. Once AMPA receptors are inserted and become functional, the synapse is no longer silent and can mediate fast excitatory transmission, contributing significantly to the developing circuit. This process effectively converts an "unresponsive" synapse into a functional, strong one. This phenomenon is a fundamental mechanism for establishing and strengthening excitatory connections during brain development, often preceding AMPA receptor insertion.

Synapse Activity in the Adult Brain: Long-Term Potentiation (LTP) and Long-Term Depression (LTD)

In the adult brain, synaptic plasticity continues to be a fundamental mechanism for learning and memory, primarily mediated by long-lasting changes in synaptic strength known as Long-Term Potentiation (LTP) and Long-Term Depression (LTD).

Long-Term Potentiation (LTP)

• Definition: LTP is a persistent, activity-dependent strengthening of synaptic transmission. It represents a cellular model for how memories might be stored—by making connections between neurons more effective.

• Induction Mechanisms: LTP is typically induced by brief, high-frequency stimulation (e.g., tetanic stimulation at $100 ext{ Hz}$ for 1 second) of presynaptic axons or by temporally coincident pre- and postsynaptic activity (e.g., at $\theta$-burst frequencies). These patterns lead to significant postsynaptic depolarization.

• Key Role of NMDAR Activation: In many forms of LTP (especially NMDAR-dependent LTP), the strong depolarization during induction relieves the $Mg^{2+}$ block of postsynaptic NMDARs. This allows a substantial influx of $Ca^{2+}$ into the postsynaptic spine.

• Intracellular Signaling: This large $Ca^{2+}$ transient acts as a second messenger, primarily activating calcium-dependent protein kinases, such as Calmodulin-dependent Kinase II (CaMKII) and Protein Kinase C (PKC). These kinases phosphorylate existing AMPA receptors (increasing their conductance) and, crucially, promote the insertion of new AMPA receptors into the postsynaptic membrane from intracellular pools.

• Expression Mechanisms: The increased number of functional AMPA receptors, along with their enhanced conductance, leads to a larger excitatory postsynaptic current (EPSC) and EPSP in response to subsequent presynaptic firing. This is often referred to as a postsynaptic expression of LTP. There can also be presynaptic contributions to some forms of LTP, such as increased neurotransmitter release probability.

• Typical Graph Description: An experimental trace illustrating LTP would show the amplitude or slope of evoked EPSPs (or EPSCs) recorded in the postsynaptic neuron. After a baseline period, a strong potentiating stimulus is applied (e.g., an arrow indicating tetanus). The synaptic response amplitude then shows a sustained increase, often $1.5$ to $3$ times the baseline, persisting for tens of minutes to hours or even longer, demonstrating the long-lasting enhancement of synaptic efficacy.

Long-Term Depression (LTD)

• Definition: LTD is a persistent, activity-dependent weakening of synaptic transmission. It is thought to be crucial for clearing old memories, selective circuit refinement, and preventing runaway excitation induced by LTP.

• Induction Mechanisms: LTD is typically induced by prolonged low-frequency stimulation (e.g., $-1 ext{ Hz}$ for $10-15 ext{ minutes}$) or specific patterns of activity (e.g., presynaptic activity followed by mild postsynaptic depolarization).

• Key Role of NMDAR Activation and $Ca^{2+}$ Dynamics: While NMDARs are also involved in many forms of LTD, the nature of the $Ca^{2+}$ influx is different compared to LTP. LTD typically results from a smaller, slower, or more prolonged increase in postsynaptic $Ca^{2+}$ concentration.

• Intracellular Signaling: This specific $Ca^{2+}$ signal primarily activates calcium-dependent protein phosphatases, such as Calcineurin (protein phosphatase 2B, PP2B) and Protein Phosphatase 1 (PP1). These phosphatases dephosphorylate AMPA receptors and trigger their internalization (removal from the postsynaptic membrane) via endocytosis.

• Expression Mechanisms: The reduction in the number of functional AMPA receptors on the postsynaptic membrane leads to a decreased excitatory postsynaptic current and EPSP in response to subsequent presynaptic firing, thereby weakening the synapse. Presynaptic forms of LTD, involving reduced neurotransmitter release, also exist (e.g., endocannabinoid-mediated LTD).

• Typical Graph Description: An experimental trace illustrating LTD would show the amplitude or slope of evoked EPSPs (or EPSCs). After a baseline period, the depression-inducing stimulus is applied. The synaptic response amplitude then shows a sustained decrease, typically reducing to 50 ext{%} to 80 ext{%} of the baseline, persisting for tens of minutes to hours, reflecting the long-lasting weakening of synaptic efficacy.

• #### Multiple Mechanisms of LTP and LTD

  • It is crucial to understand that LTP and LTD are not monolithic phenomena. They are umbrella terms encompassing a diverse array of molecular and cellular mechanisms.

  • Variability: The specific molecular pathways involved can vary significantly depending on the brain region (e.g., hippocampus vs. cerebellum vs. cortex), the specific type of synapse (e.g., mossy fiber LTP is often NMDAR-independent and presynaptic), the developmental stage, and the precise induction protocol used.

  • Examples of diverse forms: Besides NMDAR-dependent LTP/LTD, other forms include mGluR-dependent LTD (often involving endocannabinoid signaling), $\beta$-adrenergic receptor-mediated LTP, and changes in intrinsic excitability. This complexity allows for sophisticated and context-dependent changes in neural circuits.

Homeostatic Plasticity Mechanisms

While Hebbian forms of plasticity like LTP and LTD are essential for encoding specific information, unchecked Hebbian plasticity can lead to network instability, where neurons become either saturated (always firing maximally) or silent (never firing). To counteract this, the brain employs homeostatic plasticity mechanisms.

Need for Homeostatic Mechanisms

• Maintaining Network Stability: Homeostatic mechanisms are crucial for maintaining the overall stability and excitability of neural circuits. Without them, specific strengthening (LTP) could lead to runaway excitation and epileptiform activity, while widespread weakening (LTD) could lead to network silencing, both of which would impair brain function.

• Regulating Firing Rates: They act to keep neuronal firing rates within a physiological, functional range. This ensures that neurons remain responsive to new inputs and can continue to encode information effectively.

• Preserving Information Storage: By preventing saturation or silencing, homeostatic plasticity ensures that the dynamic range for further Hebbian plasticity is preserved, allowing the network to continue learning and storing new information without losing old information or becoming unstable.

• These mechanisms operate on longer timescales (hours to days) than Hebbian plasticity and often involve global adjustments rather than synapse-specific changes.

Homeostasis Up- or Down-Regulates Synapses and Intrinsic Excitability

• Homeostatic plasticity mechanisms adjust neuronal properties in response to chronic changes in activity to restore a set point of activity.

• Synaptic Scaling: This is a prominent homeostatic mechanism where a neuron globally adjusts the strength of all its excitatory synapses proportionally to compensate for widespread changes in its activity.

  • For example, if a neuron experiences prolonged periods of activity blockade (e.g., due to pharmacological manipulation or sensory deprivation), it will globally upregulate the number and/or conductance of AMPA receptors at all its excitatory synapses. This makes the neuron more sensitive to future inputs, thereby helping to restore its baseline firing rate. Conversely, prolonged periods of elevated activity can lead to a global downregulation of synaptic strength.

  • This mechanism allows a neuron to maintain its average activity level while preserving the relative differences in strength between its synapses, which is critical for retaining stored information (e.g., the "memory trace").

• Intrinsic Plasticity: In addition to synaptic scaling, neurons can also adjust their intrinsic excitability. This involves changing the expression or properties of voltage-gated ion channels (e.g., $Na^+$ channels, $K^+$ channels) in the neuronal membrane.

  • For instance, if a neuron is chronically deprived of synaptic input, it might increase the density of $Na^+$ channels or decrease the density of $K^+$ channels, making it more excitable and increasing its likelihood of firing an action potential in response to a given input.

  • Conversely, prolonged hyperexcitability can lead to adjustments that reduce intrinsic excitability, such as increasing $K^+$ channel expression.

• Together, synaptic scaling and intrinsic plasticity work in concert to ensure that neuronal networks remain stable and functional over long periods, capable of both encoding new information through Hebbian plasticity and maintaining overall operational efficacy.