Glossary
Long-term depression
What is long-term depression?
Long-term depression (LTD) is a lasting reduction in the strength of communication at a synapse following particular patterns of activity. It is one form of synaptic plasticity investigated in learning and memory. Here, depression means reduced synaptic efficacy, not a depressed mood. Long-term potentiation (LTP) describes a lasting increase in synaptic strength.
How it works
LTD includes several mechanisms, including forms involving NMDA or metabotropic glutamate receptors. Its induction and expression vary across synapses. A reduction in synaptic efficacy is not necessarily elimination of the synapse, and LTD is not the sole mechanism of forgetting or updating.
One postsynaptic mechanism
In a well-studied NMDA-receptor-dependent form of LTD, the lasting reduction in AMPA-receptor-mediated transmission involves removing AMPA receptors from the receiving side of the synapse. With fewer of these receptors available, a comparable glutamate input can produce a smaller postsynaptic response. Collingridge and colleagues’ review discusses this evidence from hippocampal preparations; the explanation does not cover every form of LTD.
This separates induction from expression. A pattern such as prolonged low-frequency stimulation can initiate LTD under suitable experimental conditions. Changes in receptors can then express the weaker response after that induction period ends. The stimulation pattern is the initiating procedure, whereas the receptor change is part of how the reduced transmission is produced.
Applied example
Researchers can stimulate a neural pathway and measure its response. If the response remains weaker afterward, they can study that lasting change as LTD. Connecting it to a particular learned behavior requires a further test.
Why it matters
LTD contributes to theories of how neural circuits adjust with experience. Evidence supports roles in some forms of learning and flexible responding, rather than all unlearning.
Evidence note: Collingridge and colleagues (2010) review distinct forms of LTD and their proposed behavioral roles.