FIGURE 1.

(A) Baseline input-output curves of evoked field potentials in the dorsal and the ventral hippocampus. fEPSPs evoked by relatively strong stimulation (≥200 μA), corresponding to large fiber volleys (>2 mV), were significantly larger in the dorsal than in the ventral hippocampus (horizontal lines above data, p < 0.05). Data were collected from 83 dorsal and 61 ventral slices obtained from 40 rats. (B) Photograph of a transverse hippocampal slice in which is depicted the method of electrical stimulation and recording used to study transient heterosynaptic depression (tHSD) in the CA1 hippocampal field. Two stimulation electrodes were positioned at stratum radiatum on either side of a recording electrode in a way to excite two different sets of presynaptic fibers converging to CA1 pyramidal neurons. One of the inputs was used to condition the response of a test input. The sites of conditioning and test pathways relative to recording electrode randomly alternated between slices. (C) Example traces of fEPSPs evoked by paired-pulse stimulation (PPS) of either the same pathway (upper panel, Homosynaptic PPS) or two different pathways (lower panel, Heterosynaptic PPS), in a dorsal and a ventral hippocampal slice. Responses of conditioning and conditioned (test) pathways were evoked by stimulation current of weak (1), moderate (2), and strong (3) intensity. In heterosynaptic PPS panel, superimposed to the conditioned responses are the (unconditioned) responses evoked by the first stimulus in homosynaptic PPS (traces in blue) for comparison sake. The interval between the two pulses in either stimulation configuration (i.e., homosynaptic or heterosynaptic) was 50 ms. Calibration bars: 0.5 mV and 10 ms. The independence of the two pathways was confirmed by the absence of interaction (i.e., facilitation or depression) between conditioning and conditioned responses in the heterosynaptic PPS configuration. On the contrary, homosynaptic PPS produced facilitation of conditioned responses. (D) Collective data of the ratio Conditioned fEPSP/Unconditioned fEPSP in homosynaptic (Homo) and heterosynaptic (Hetero) PPS, applied with weak, moderate, and strong conditioning stimulation (CS) current intensity. The number of slices used in each condition is indicated at the bottom of columns. Diesis (#) on top of columns indicate statistically significant difference between the dorsal and the ventral hippocampus (independent t-test, p < 0.005). Homosynaptic PPS produced synaptic facilitation. On the contrary, heterosynaptic PPS did not significantly affect test responses. (E) Examples of tHSD of a test input (Test path) produced by a high-frequency burst (50 Hz) delivered to a distinct set of adjacent fibers (Conditioning path). Upper traces represent responses of the test input before the application of the conditioning burst stimulation, while traces on the bottom represent responses to heterosynaptic stimulation at 300 ms. Calibration bars: 0.5 mV and 50 ms. In the heterosynaptic condition, unconditioned responses of the test pathway (traces in blue) are superimposed to the conditioned responses for comparison. Note that the conditioning burst stimulation produces suppression of fEPSP evoked by stimulation of the test pathway. All artifacts in trace recordings are truncated for clarity.