Maturation of a Central Glutamatergic Synapse

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Science  08 Nov 1996:
Vol. 274, Issue 5289, pp. 972-976
DOI: 10.1126/science.274.5289.972


  • Fig. 1.

    (A) Diagram of the whole-brain preparation showing the stimulating electrode (S) in the optic chiasm and the recording patch-pipette (R) in the optic tectal cell body region (OT). Tel, telencephalon; ON, optic nerve. (B) Plot of dendritic branchtip number versus relative distance from the caudal border of the optic tectum. (C) EPSC amplitude does not change over a range of stimulus intensities above threshold, indicating single-fiber excitation. Plot of EPSC amplitude versus trials. Stimulus intensity was changed where indicated. (D) Ensemble average from 16 cells. The stimulus duration was varied (30 to 150 μs) so that threshold responses were obtained at about 20-V stimulus, indicated as 0 V on the x axis. After the threshold was set, voltage setting was changed (x axis) to determine response sensitivity to variations in stimulus intensity.

  • Fig. 2.

    Increase in the AMPA component of the synaptic response with neuronal development. (A) Retinotectal-evoked synaptic responses recorded in rostral optic tectum have mixed AMPA and NMDA components. Upper three traces obtained at +55 mV, lower three traces obtained at −60 mV. Symbols indicate drugs (Control, no drugs; APV, 100 μM DL-APV; CNQX, 10 μM) in perfusate during recording. Horizontal black bars show positions of windows for measuring amplitudes of NMDA and AMPA currents. (B) Evoked synaptic transmission recorded from a neuron in caudal optic tectum. Traces in (A) and (B) are averages of 10 to 50 consecutive responses. (C) AMPA/NMDA ratio of single-fiber responses for neurons recorded at various positions along the RC axis. (D) Average amplitudes (mean ± SEM) of AMPA and NMDA responses and their ratio for the pooled population of rostral (RC position 0.62 to 1.0; n = 20) and caudal (RC positions 0.25 to RC 0.62; n = 21) neurons. AMPA responses in rostral neurons, 4.4 ± 0.4 pA; caudal neurons, 2.4 ± 0.4 pA. NMDA responses in rostral neurons, 1.9 ± 0.3 pA; caudal neurons, 2.2 ± 0.2 pA. Average AMPA/NMDA ratio in rostral neurons, 2.7 ± 0.3; caudal neurons, 1.3 ± 0.2. **P < 0.01, *** P < 0.001.

  • Fig. 3.

    Developmental increase in AMPA responses at single synapses revealed by spontaneous miniature synaptic events. (A) mEPSCs recorded from neurons in caudal tectum (a) and rostral tectum (b), under different recording conditions as indicated (n = 28). (a) In recordings from immature neuron, pure NMDA receptor-mediated mEPSCs can be recorded as indicated by the presence of spontaneous events in individual traces at +60 mV but not at −60 mV. (b) In mature neurons, individual traces show that mEPSCs recorded at +55 mV have fast and slow components; the latter are blocked by hyperpolarization or APV. The mEPSCs recorded at −60 mV are completely blocked by CNQX. Superimposed traces recorded at 0 mV show no synaptic responses. Each superimposed recording is composed of 20 consecutive traces. An increase in frequency (B) and amplitude (C) of AMPA mEPSC, recorded at −60 mV in the presence of Mg2+, as a function of distance along the RC axis.

  • Fig. 4.

    Decrease in the fraction of pure NMDA responses with neuronal development. (A, C, E) Evoked synaptic currents recorded at +55 mV and −60 mV from neurons located in caudal (A), middle (C), and rostral (E) optic tectum. (B, D, F) Amplitude distribution histograms of synaptic currents recorded at −60 mV (light line) and +55 mV (dark line) are shown to the right of each set of recordings (30). The peaks at 0 amplitude represent synaptic failures. Failure rates at the two potentials are similar in the neuron from rostral tectum, but are successively more disparate in neurons from middle and caudal tectum. (G) The fraction of total events that are mediated purely by NMDA receptors decreases from about 100% in caudal neurons to about 5% in rostral neurons. (H) Failure rates at hyperpolarized potentials decrease significantly from 59 ± 5% in caudal tectum to 41 ± 3% in rostral tectum. Failure rates at depolarized potentials increase from 24 ± 4% in caudal tectum to 30 ± 3% in rostral tectum. The fraction of pure NMDA responses decreases from 62 ± 5% in caudal neurons to 25 ± 4% in rostral neurons (mean ± SEM, **P < 0.01, ***P < 0.001).

  • Fig. 5.

    Selective increase in the AMPA component of evoked synaptic responses by CaMKII. (A) Sequence of 10 consecutive, individual evoked responses obtained at the indicated holding potential from neurons infected with tCaMKII-V (top) or β-Gal-V (bottom). (B) Amplitude distribution histograms of the responses at +55 mV (dark lines) and −60 mV (light lines) generated from the recordings in (A). (C) Graph of the average amplitudes of AMPA and NMDA synaptic components and their ratio in uninfected or tCaMKII-infected neurons. Amplitude of AMPA responses: uninfected controls, 2.8 ± 0.5 pA, n = 20; β-Gal, 3.0 ± 0.7 pA, n = 11; tCaMKII, 7.1 ± 0.6 pA, n = 32; NMDA responses: uninfected controls, 2.1 ± 0.2 pA, n = 20; β-Gal, 2.7 ± 0.2 pA, n = 11; tCaMKII, 2.7 ± 0.4 pA, n = 32; AMPA/NMDA ratio: 1.5 ± 0.3; β-Gal, 1.30 ± 0.4; tCaMKII, 3.9 ± 0.5; mean ± SEM, ***P < 0.001. (D) Graph of failure rate at hyperpolarized and depolarized potentials and the calculated fraction of transmission mediated by pure NMDA responses in uninfected or tCaMKII-infected neurons. Hyperpolarized failures: uninfected controls, 62 ± 5%, n = 20; β-Gal, 55 ± 4%, n = 11; tCaMKII, 37 ± 2%, n = 32; depolarized failures: uninfected controls, 33 ± 4%, n = 20; β-Gal, 24 ± 4%, n = 11; tCaMKII, 32 ± 2%, n = 32); fraction of pure NMDA responses: uninfected controls, 54 ± 8%, n = 20; β-Gal, 60 ± 6%, n = 9; tCaMKII, 13 ± 2%, n = 32; mean ± SEM, ***P < 0.001. No significant differences were observed between β-Gal and uninfected control neurons.

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