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Heterosynaptic plasticity
Synaptic plasticity refers to a chemical synapse's ability to undergo changes in strength.〔Purves, D., Augustine, G.J., Fitzpatrick, D., Hall, W.C., LaMantia, A.S., White, L.E. (2012). Synaptic Plasticity. In Neuroscience (5th ed.) (pp. 163-182). Sunderland, Massachusetts: Sinauer Associates.〕 Synaptic plasticity is typically input-specific (i.e. homosynaptic plasticity), meaning that the activity in a particular neuron alters the efficiency of synaptic connection between that neuron and its target. However, in the case of heterosynaptic plasticity, the activity of a particular neuron leads to changes in the strength of synaptic connections between another pair of neurons. A number of distinct forms of heterosynaptic plasticity have been found in a variety of brain regions and organisms. These different forms of heterosynaptic plasticity contribute to a variety of neural processes including associative learning, the development of neural circuits, and homeostasis of synaptic input.〔Bailey, C.H., Giustetto, M., Huang, Y.Y., Hawkins, R.D., Kandel, E.R. (2000.) Is heterosynaptic modulation essential for stabilizing hebbian plasiticity and memory. Nature Reviews Neuroscience, 1:1, 11-20.〕
==Modulatory input-dependent plasticity ==
One well studied example of heterosynaptic plasticity is modulatory input-dependent plasticity. Modulatory neurons perform neuromodulation, which is the release of neuromodulators. Neuromodulators differ from classical neurotransmitters. Typically, neuromodulators do not directly generate electrical responses in target neurons. Rather, the release of neuromodulators often alters the efficacy of neurotransmission in nearby chemical synapses. Furthermore, the impact of neuromodulators is often quite long lasting in comparison to classical neurotransmitters.〔
A number of neurotransmitters can act as neuromodulators, particularly biogenic amines such as dopamine and serotonin.〔 These neuromodulators use G-protein coupled receptors which mediate slower modulatory effects and neither hyperpolarize nor depolarize cells. Due to these qualities, GPCR can initiate long-lasting changes in heterosynaptic strength.〔
The use of these neuromodulators is an example of heterosynaptic plasticity. Released by a neuron called an interneuron, neuromodulators can affect another neuron's efficiency of communication with a postsynaptic cell. Thus, because the interneuron does not specifically activate the postsynaptic neuron (strength in its synaptic plasticity is indirectly affected), this mechanism of modulatory input-dependent plasticity is heterosynaptic.〔 To better understand this process and its vast diversity, key functions of the neuromodulator serotonin in ''Aplysia californica'' and dopamine are further illustrated.
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