Fig. 6.
Time course of dynorphin action at the mossy fiber–CA3 pyramidal cell synapse. A, Five puff applications of 5 μm dynorphin (200 msec, 200 kPa) at 1 sec intervals given to the slice surface at the recording electrode in s. lucidum produces a transient depression (n = 6;filled circles). The time course closely resembles that of tetanically induced heterosynaptic depression (HSD) from interleaved experiments (n = 6; open circles). The time of each manipulation is indicated by thearrow. B, Mossy fiber synaptic responses were depressed with 500 nm dynorphin (bar). Five puff applications of nor-BNI (1 μm, 500 msec, 200 kPa) separated by 1 sec each given to the slice surface at the stimulating electrode in the dentate gyrus (DG) had no effect on dynorphin-induced depression. The identical application given at the recording electrode in s. lucidum 10 min later (S. luc.) rapidly reversed the depression. C, Rapid application of the κ1antagonist nor-BNI or naloxone was given at time 0, as indicated by thearrow (filled circles). The application consisted of one of the following: (1) 5 puffs of 1 μm nor-BNI at 1 sec intervals (n = 2), (2) a drop of 50 μm nor-BNI directly into the recording chamber (n = 3), or (3) 1–20 μm naloxone rapidly applied in the perfusion line (n = 3). No obvious difference was seen with these different modes of application. The effect of this application on mossy fiber synaptic responses depressed by 500 nm dynorphin is shown normalized to the steady-state level to which they returned. The reversal is faster than the time course of tetanically induced heterosynaptic depression (HSD) from experiments performed concurrently (n = 12; open circles). D, Heterosynaptic depression was first induced under control conditions (open circles). After responses stabilized again, several peptidase inhibitors were added to the bath, which had no effect on baseline transmission. Heterosynaptic depression was subsequently induced again (filled circles), and no difference was seen (n = 8). The time of each tetanus is indicated by thearrow.