Long-term Depression

Long-term depression (LTD) is a process of synaptic weakening that occurs over time between pre- and postsynaptic neuronal connections. LTD works in opposition to long-term potentiation (LTP), which strengthens synapses. Together, these mechanisms underlie learning and memory by allowing the brain to weaken unused synapses while strengthening important ones, maintaining neural flexibility and efficiency.

Low calcium ion concentration in the postsynaptic neuron, resulting from infrequent presynaptic stimulation, initiates a signaling cascade that culminates in endocytosis or removal of AMPA glutamate receptors from the plasma membrane. With fewer receptors present, the postsynaptic response to sporadic stimulation becomes further weakened, reducing synaptic transmission efficiency.

LTD prevents the brain from becoming maximally strengthened at all synapses, which would plateau neural efficiency and impair learning and memory formation. By pruning weaker, unused synapses, LTD frees up resources and space that neurons can redirect toward strengthening more important synaptic connections, maintaining cognitive flexibility.

During LTD, AMPA glutamate receptors are removed from the postsynaptic membrane through endocytosis. The neuron then reuses these receptors at a later time or separates them into their constituent subunits. This receptor removal reduces the number of ion channels available for positively charged ions to enter and depolarize the membrane.

LTD was first described in the hippocampus, where it is thought to clear old memories. Subsequently, LTD has been demonstrated in many brain regions including the cerebellum, striatum, and cortex. The ubiquitous presence of LTD across these regions underscores its importance in proper brain functioning and neural plasticity.

Malfunctioning LTD mechanisms are thought to contribute to numerous neurological and cognitive disorders such as addiction, mental retardation, and Alzheimer's disease. Understanding how LTD operates and how its disruption leads to disease is essential for developing effective treatments for these conditions.

Infrequent presynaptic stimulation results in low calcium ion influx into the postsynaptic neuron. This low calcium concentration triggers removal of AMPA receptors from the membrane through endocytosis. With fewer receptors available, the postsynaptic response to the same sporadic stimulation is further weakened, progressively reducing synaptic strength and the excitatory and inhibitory effects of neurotransmitters at that synapse.