Abstract
Abstract: :
Purpose: Inhibitory feedback from amacrine cells (ACs) to Mb1 bipolar cells (BCs) in the goldfish retina is mediated by the activation of AMPA and NMDA type glutamate (Glu) receptors on the ACs. Here we investigated the role of NMDA receptors: their activation, magnesium (Mg) sensitivity, and temporal characteristics with respect to the reciprocal feedback. Methods: Voltage–clamped current and capacitance measurements were performed directly from the Mb1 BC terminals in goldfish retinal slices. We recorded only from terminals whose axon was cut during the slicing procedure. The pharmacology and/or temporal characteristics of the feedback were studied via evoking reciprocal inhibition from ACs by the depolarization of a single ("isolated") BC terminal. Results: Blocking the AMPA and NMDA receptors on ACs with NBQX/CNQX and AP5 completely eliminated the reciprocal feedback. Application of NBQX/CNQX alone always blocked the first, fast feedback peak with a delay of 2–3 ms, whereas AP5 alone left that intact, affecting predominantly the late, NBQX/CNQX–resistant components. With the AMPA receptors blocked, this late part had an onset delay of 8–9 ms. Under these conditions, the feedback amplitude strongly depended on the external Mg concentration: eliminating the Mg enhanced feedback, whereas elevating Mg to 2 mM completely and reversibly blocked it without markedly affecting the voltage–gated calcium current or the Glu release of the BC terminal. These findings further substantiated the NMDA receptor origin of the late component. Addition of TTX reduced the magnitude NBQX/CNQX–resistant feedback, but never blocked it, or changed the delay. Conclusions: In the majority of the BCs tested (27 out of 32), AMPA receptor block on the ACs did not eliminate the reciprocal feedback. Thus, a substantial portion of the NMDA receptors involved in the reciprocal communication at the Mb1 terminal did not require AMPA priming to operate. Although TTX–sensitive sodium currents can boost the NBQX/CNQX–resistant NMDA component of the feedback, their activation (i.e. spontaneous spiking of the ACs) did not seem to be essential for NMDA receptor activation.
Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • retinal connections, networks, circuitry • synapse