Heterosynaptic plasticity is a form of synaptic plasticity, in which changes in synaptic strength are induced by activity at neighboring synapses or by modulatory inputs, rather than by activity at the synapse itself. Synaptic plasticity more broadly refers to activity-dependent changes in the strength of connections between neurons and is widely associated with learning and memory. In contrast, to homosynaptic (Hebbian) plasticity, which is input-specific, heterosynaptic plasticity involves changes driven by external or modulatory signals and can result in either synaptic potentiation or depression. Heterosynaptic mechanisms have been implicated in processes such as synaptic homeostasis, neural circuit development, and associative learning. These mechanisms enable neural systems to integrate multiple sources of activity while maintaining stability and flexibility in network function.
Modulatory input-dependent plasticity (Heterosynaptic Plasticity) Exemplified
Modulatory input-dependent plasticty is a form of heterosynaptic plasticity mediated by neuromodulation, in which modulatory neurons release neuromodulators that alter synaptic function. Unlike classical neurotransmitters, neuromodulators typically do not directly produce fast electrical responses in target neurons. Instead, they modify the efficacy of neurotransmission at nearby synapses, often producing longer-lasting effects. These modulatory influences can result in either synaptic potentiation or depression and are often regulated by metaplasticity, in which prior synaptic activity alters the capacity for future plastic changes. Several neurotransmitters can function as neuromodulators, particularly biogenic amines such as dopamine and serotonin. Their effects are commonly mediated through G-protein Coupled Receptors (GPCRs), although ionotropic mechanisms may also occur depending on the neuronal context. Activation of these pathways can influence synaptic transmission by altering factors such as neurotransmitter release probability or postsynaptic responsiveness. The use of these neuromodulators is an example of heterosynaptic plasticity, as these signals originate from neurons that are not directly involved in the active synapse. Neuromodulators, often released by interneurons, can influence communication efficiency between presynaptic and postsynaptic neurons indirectly by altering neurotransmitter release probability. This process does not directly trigger postsynaptic activation but instead modifies synaptic strength through modulatory input. Examples include serotonergic modulation in Aplysia californica and dopaminergic signaling in other neural systems.
Aplysia californica
In Aplysia californica, modulatory interneurons release serotonin, which can induce synaptic plasticity in motor neurons. Aplysia californica is a commonly used model organism for studying synaptic plasticity and neural signaling due to its relatively simple nervous system, which allows individual neural circuits to be identified and studied. Work by Eric Kandel and colleagues, using Aplysia contributed to the understanding that learning and memory are associated with long-term synaptic strength changes in the brain. Early studies in Aplysia demonstrated both heterosynaptic plasticity and neuromodulation through behavioral paradigms such as habituation and sensitization. Habituation, which is associated with synaptic depression, and sensitization, which is associated with synaptic facilitation, were observed by measuring the gill withdrawal reflex in response to tactile stimulation. Light touch to the siphon activates sensory neurons, while repeated stimulation leads to a reduced response over time. Mechanical stimulation of the siphon (see figure), activates sensory neurons, producing excitatory postsynaptic potentials (EPSP) in motor neurons that drive the gill withdrawal reflex. With repeated stimulation, habituation occurs, leading to reduced synaptic efficacy at the sensory-motor neuron synapse and decreased EPSPs, resulting in a diminished behavioral response. This process involves depression at the glutamatergic sensory-motor synapse and reflects reduced synaptic transmission underlying habituation. Conversely, noxious stimulation, such as stimulation of the tail, produces sensitization of the gill contraction response. This stimulus activates modulatory interneurons that release serotonin onto siphon neurons, enhancing neurotransmitter release and increasing EPSPs in motor neurons. When noxious stimulation is paired with light touch to the siphon, the resulting facilaition can persist beyond the initial stimulus. Compared to other forms of short-term synaptic plasticity, these neuromodulatory effects can last for longer periods, and repeated pairing can lead to more persistent changes in synaptic strength. These findings provide evidence for heterosynaptic strengthening between sensory and motor neurons in Aplysia neural circuits.
… excerpt ends here. Continue reading the full article.
