Module 10: Figure stimulus intensity and spine Ca2+ responses
Effect of varying stimulus intensity on Ca2+ responses in single spines or neighbouring dendrites.
If the parallel fibres are stimulated at low frequencies (10 Hz), a Ca2+ signal appears in the spine after a latency of 200 ms; it reaches a peak after a further 260 ms, but there is no response in the dendrites. The slow response is caused by the time taken for glutamate to act on the metabotropic glutamate 1 receptors (mGluR1) to generate inositol 1,4,5-trisphosphate (InsP3), which then releases Ca2+ from the internal store. At higher stimulation intensities (20 Hz), the spine response was larger, and some Ca2+ appeared in the dendrite. When stimulated at 50 Hz, glutamate activated the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors sufficiently to induce an excitatory postsynaptic current (EPSC) that depolarizes the membrane sufficiently to open voltage-operated channels (VOCs) to give an early Ca2+ spike that develops with a short delay of 7 milliseconds. This early signal due to Ca2+ entry (the grey shaded area) was then followed by a much larger signal as the slower developing InsP3-dependent release of Ca2+ began to appear. Reproduced by permission from Macmillan Publishers Ltd: Nature, Takechi, H., Eilers, J. and Konnerth, A. (1998) A new class of synaptic response involving calcium release in dendritic spines. 396:757–760. Copyright (1998); http://www.nature.com; see Takechi et al. 1998.