Synaptic depression at type B to A photoreceptor connections in Hermissenda

Abstract

We quantified synaptic depression at the type B to A photoreceptor connections of Hermissenda. Type B cell action potentials were evoked at interstimulus intervals (ISIs) of 0.3, 3 or 30 s and the resulting inhibitory postsynaptic potentials (IPSPs) were recorded in a type A cell. A progressive decline in IPSP amplitude occurred at all three ISIs. Synaptic depression was greater at shorter ISIs, as was the level of recovery 2 min after the stimulus series. The profound level of synaptic depression observed (79.0+/-3.2% at the 0.3 s ISI) implies that synaptic depression is an important control process in the neuronal circuitry that drives phototactic behavior.

Post-light Potentiation at Type B to A Photoreceptor Connections in Hermissenda

“…The duration of manually delivered light stimuli was not always precisely 3 s. IPSP amplitudes were depressed relative to their own prestimulation baseline. By contrast, the decrease observed after depolarization-elicited type B cell activity was not significantly different from that observed in control preparations ( p Ͼ .05), suggesting that the modest decreases in these two groups probably resulted from homosynaptic depression arising from the testing procedure (Schultz et al, 1998).…”

“…For instance, synaptic facilitation that occurs at the inhibitory type B to A cell connections after either behavioral training (Frysztak & Crow, 1994) or in vitro US-alone manipulations (Clark & Schuman, 1992;Schultz & Clark, 1997a;Schuman & Clark, 1994) may also contribute to learned suppression of phototaxis (Crow & Alkon, 1980;Farley & Alkon, 1982;Frysztak & Crow, 1994;West et al, 1982). Although type B to A cell synaptic transmission is depressed during the actual activation of the CS pathway (Schultz, Lee, & Clark, 1998), a light-evoked potentiation of synaptic strength at these connections expressed after the light presentations could play a mechanistic role in CS-elicited nonassociative learning. Hence, we investigated whether the long (ϳ30-s) or short (ϳ3-s) light stimuli that have been used during behavioral training would yield post-light potentiation (PLP) at the type B to A cell connections.…”