Frequency transfer properties of a model of the photoreceptor triad synapse with ephaptic feedback from horizontal cells

Paul V. Maximov and Vadim V. Maximov

Institute for Problems of Information Transmission,
Russian Academy of Sciences, 101447 Moscow, Russia

A paper version of the poster is submitted to the 28th Goettingen Neurobiology Conference, June 7 - 10, 2001.

Retinal horizontal cells (HCs) provide an opponent signal coding for spatial and colour contrasts by regulating the synaptic transmission between receptors and bipolars (Bs) in triad synapses. According to Bysov's hypothesis on ephaptic feedback, the postsynaptic current from HCs would depolarize presynaptic release sites and increase transmitter release, thus 1) creating a positive intrasynaptic feedback for HCs and 2) regulating signal transmission from receptors to Bs. For synchronization of opponent signals this regulation must be sufficiently rapid. The models of the ephaptic feedback fairly well simulated the HC dynamics [1], including transients, but its frequency transfer properties remained unknown.

Frequency response functions (FRFs) of visual signal transmission to HCs and Bs in the model of triad synapse with an ephaptic feedback were calculated. An influence of different factors on the shape of amplitude and phase FRFs was investigated. Some factors (such as low rates of transmitter release and removal from synaptic cleft, noticeable capacity of cell membranes) inevitably restrict the rate of transmission. The other factors were demonstrated to expand the frequency range, that is essential for synchronization of opponent signals: 1) a transient component of transmitter release which accentuates high frequencies, 2) a voltage-dependent conductance of the HC non-synaptic membrane.

The model is designed as a 32-bit Windows (Windows 9x, Windows NT, Win32s) application.

Windows Application
IV-curves Three versions of the I-V curves for HC membrane used in calculations with different slopes of linear parts: a - 'passive' membrane; b - membrane with infinite slope resistance operating as a current stabilizer; c - 'active' membrane with negative resistance. The other parameters of the model correspond to rather 'slow' cells in the pike retina [1].
A case of N-shaped I-V curve (c) is of especial interest because of strong negative feedback that stabilizes transmitter concentration in the synaptic cleft and makes it independent on input signal. As a result Bs do not respond to sustained stimulation at all. Exactly such properties are necessary for triad synapses in the retina to realise valuable opponent (colour and space) interaction of visual signals.
HC frequency response functions
Amplitude and phase FRFs for HCs.		 Bipolar frequency response functions
Amplitude and phase FRFs for Bs.
1. VV Maximov & AL Byzov (1996) Vision Res., 36, 24:4077-4087

ACKNOWLEDGEMENTS

RFBR logo This work was supported by a research grant (98-04-49298) from the Russian Foundation for Basic Research.

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Last Update: 16 January, 2001