Infrared-Guided Laser Stimulation as a Tool for Elucidating the Synaptic Site of Expression of Long-Term Synaptic Plasticity

Long-term potentiation is a synaptic mechanism thought to be involved in learning and memory. Long-term depression (LTD), an activity-dependent decrease in synaptic efficacy, may be an equally important mechanism that permits neural networks to store information more effectively. Two forms of LTD have been identified in the mammalian central nervous system, which are induced by the synaptic activation of N-methyl-d-aspartate (NMDA) and metabotropic glutamate (mGlu) receptors, respectively. Whereas the expression mechanisms of NMDA receptor-dependent LTD have been demonstrated to be postsynaptic, those of mGlu receptor-dependent LTD have not been clearly identified. In order to address this issue, a variety of different electrophysiological methods have been used. A very elegant way to realize this experimental approach is provided by the development of photolytic application of glutamate, which allows the temporally and spatially highly specific activation of any neuron or any part of the neuron. By means of simultaneous application of electrical and photolytic stimulation techniques, it has been demonstrated that mGlu receptor-induced LTD is compatible with a presynaptic mechanism of expression.