Fig. 2.

Contributions of retinogeniculate short-term plasticity. (A) Representative traces of AMPAR and NMDAR mediated currents recorded before eye opening (left) and in a mature mouse (right) in response to the stimulation of the optic tract. Whole-cell voltage clamp recordings were performed with bicuculline to block GABA_A−receptor mediated currents. At −70 mV holding potential, AMPARs mediate the fast activating and decaying current. AMPAR and NMDAR currents both contribute to the EPSCs recorded at +40 mV with AMPARs contributing to the rapid rise and the NMDAR currents contributing to the slow decay of the EPSC. The average amplitude of AMPAR currents increases over development. (B) 5-CT-mediated activation of serotonin receptors alters retinogeniculate short-term plasticity. Experiments were performed in retinogeniculate slices from mature mice. Top and bottom traces overlay pairs of retinogeniculate EPSCs evoked with varying ISI before (top) and after (bottom) the application of 5-CT to active 5HT-1 receptors expressed in presynaptic retinogeniculate boutons. Application of 5-CT reduces the amplitude of the first EPSC and relieves short-term depression, increasing the amplitude of the second EPSC preferentially at short interstimulus interval. (C) Physiologically relevant stimulation frequencies preferentially diminish the contribution of AMPARs to relay neuron firing. Current clamp recordings of action potential firing in response to trains of optic tract stimulation in the presence of AMPAR (NBQX) or NMDAR (CPP) antagonists. Holding potential −50 mV. Blockade of AMPARs alters the latency to first spike but only minimally reduces the overall number of spikes. In contrast, blockade of NMDARs abolished EPSC summation toward action potential firing; only the first stimulus evokes an action potential, reflecting the contribution of AMPARs that rapidly desensitize after the first pulse. Therefore, NMDAR currents can sustain action potential generation without AMPAR contribution. Adapted from (A) Chen and Regehr (2000), B) Liu and Chen (2008) and (C) Augustinaite and Heggelund (2007). All figures reprinted with permission.