Low-frequency stimulation induces long-term depression and slow onset long-term potentiation at perforant path-dentate gyrus synapses in vivo

Abstract

The expression of homosynaptic long-term depression (LTD) is thought to mediate a crucial role in sustaining memory function. Our in vivo investigations of LTD expression at lateral (LPP) and medial perforant path (MPP) synapses in the dentate gyrus (DG) corroborate prior demonstrations that PP-DG LTD is difficult to induce in intact animals. In freely moving animals, LTD expression occurred inconsistently among LPP-DG and MPP-DG responses. Interestingly, following acute electrode implantation in anesthetized rats, low-frequency stimulation (LFS; 900 pulses, 1 Hz) promotes slow-onset LTP at both MPP-DG and LPP-DG synapses that utilize distinct induction mechanisms. Systemic administration of the N-methyl-d-aspartate (NMDA) receptor antagonist (+/-)-cyclopiperidine-6-piperiperenzine (CPP; 10 mg/kg) 90 min before LFS selectively blocked MPP-DG but not LPP-DG slow onset LTP, suggesting MPP-DG synapses express a NMDA receptor-dependent slow onset LTP whereas LPP-DG slow onset LTP induction is NMDA receptor independent. In experiments where paired-pulse LFS (900 paired pulses, 200-ms paired-pulse interval) was used to induce LTD, paired-pulse LFS of the LPP resulted in rapid onset LTP of DG responses, whereas paired-pulse LFS of the MPP induced slow onset LTP of DG responses. Although LTD observations were very rare following acute electrode implantation in anesthetized rats, LPP-DG LTD was demonstrated in some anesthetized rats with previously implanted electrodes. Together, our data indicate in vivo PP-DG LTD expression is an inconsistent phenomenon that is primarily observed in recovered animals, suggesting perturbation of the dentate through surgery-related tissue trauma influences both LTD incidence and LTP induction at PP-DG synapses in vivo.

Keywords: LTD; dentate gyrus; in vivo; perforant path; slow onset LTP.

Fig. 1.

Isolating lateral perforant path-dentate gyrus (LPP-DG) and medial perforant path (MPP)-DG responses in vivo. A: schematic of electrode sites enabling the collection of PP-dentate gyrus responses. A recording electrode was implanted in the hilar region of the dentate gyrus and a bipolar stimulating electrode was positioned in either the lateral or medial aspect of the angular bundle to selectively stimulate LPP or MPP afferents, respectively. B: representative LPP-DG and MPP-DG responses indicate differences in peak latency, slope, and half height width of peak. In freely moving animals, LPP-DG responses (n = 4) exhibited an average peak latency of 6.1 ± 0.5 ms (means ± SE) measured from the trough of the stimulation artifact and a half height width of peak of 6.6 ± 0.9 ms, whereas MPP-DG responses (n = 6) demonstrated an average peak latency of 4.6 ± 0.5 ms and a half height width of peak of 4.6 ± 0.8 ms. Calibration: 0.5 mV, 5 ms.

Fig. 2.

Awake, freely moving rats display long-term depression (LTD) at LPP-DG synapses following low-frequency stimulation (LFS). A: LPP-DG field excitatory postsynaptic potential (fEPSP) dV/dt analysis expressed as a percent change of baseline (means ± SE). LPP LFS consisting of 900 pulses delivered at 1 Hz with stimulation intensities evoking a half-maximal LPP-DG response resulted in a sustained depression of DG fEPSPs in freely moving rats (n = 6). Calibration: 0.5 mV, 5 ms. B: normalized MPP-DG fEPSP dV/dt analysis (means ± SE) demonstrate 900 pulses, 1-Hz MPP LFS delivered at stimulation intensities evoking a half-maximal MPP-DG response generates no change in baseline DG fEPSPs in freely moving rats (n = 6). Calibration: 0.5 mV, 5 ms. C: empirical cumulative probability distributions of the alterations in dentate slopes following LPP or MPP LFS in freely moving rats. Each data point reflects the probability of individual dentate fEPSP ratios obtained from lateral or MPP LFS experiments. Dentate fEPSP ratios are derived from the average normalized slopes obtained 56–60 min post-LFS divided by the average normalized slopes collected 5 min before LFS.

Fig. 3.

LFS induces slow onset long-term potentiation (LTP) at LPP-DG and MPP-DG synapses in pentobarbital-anesthetized rats. A: comparison of normalized LPP-DG fEPSP dV/dt analysis (means ± SE) in control (n = 5) and (+/−)-cyclopiperidine-6-piperiperenzine (CPP)-treated (n = 6) pentobarbital-anesthetized rats. LPP LFS (900 pulses, 1 Hz) was delivered at stimulation intensities evoking a half-maximal LPP-DG response in both naïve rats and in animals given CPP at least 90 min before LFS. Both control and CPP-treated animals exhibit slow onset LTP following LPP LFS. B: representative LPP-DG fEPSPs recorded during baseline and 56–60 min after LPP LFS in control (top) and CPP-treated (bottom) pentobarbital-anesthetized rats. Calibration: 1 mV, 5 ms. C: assessment of normalized MPP-DG fEPSP dV/dt analysis (means ± SE) in control (n = 8) and CPP-treated (n = 6) pentobarbital-anesthetized rats. MPP LFS (900 pulses, 1 Hz) was delivered at stimulation intensities evoking a half-maximal MPP-DG response in both naïve rats and in animals given CPP at least 90 min before LFS. CPP blocked LFS-induced slow onset LTP at MPP-DG synapses. D: representative MPP-DG fEPSPs collected during baseline and 56–60 min after MPP LFS in control (top) and CPP-treated (bottom) pentobarbital-anesthetized rats. Calibration: 1 mV, 5 ms.

Fig. 4.

LFS induces slow onset LTP at LPP-DG and MPP-DG synapses in urethane-anesthetized rats. A: evaluation of normalized LPP-DG fEPSP dV/dt analysis (means ± SE) in control (n = 5) and CPP-treated (n = 7) urethane-anesthetized rats. LPP LFS (900 pulses, 1 Hz) was delivered at stimulation intensities evoking a half-maximal LPP-DG response in both naïve rats and in animals given CPP at least 90 min before LFS. Differences in post-LFS-to-baseline LPP-DG fEPSP ratios among control and CPP-treated rats were not statistically significant. B: representative LPP-DG fEPSPs collected during baseline and 56–60 min after LPP LFS in control (top) and CPP-treated (bottom) urethane-anesthetized rats. Calibration: 1 mV, 5 ms (top); 0.5 mV, 5 ms (bottom). C: comparison of normalized MPP-DG fEPSP dV/dt analysis (means ± SE) in control (n = 5) and CPP-treated (n = 5) urethane-anesthetized rats. MPP LFS (900 pulses, 1 Hz) was delivered at stimulation intensities evoking a half-maximal MPP-DG response in both naïve rats and in animals given CPP at least 90 min before LFS. CPP blocked LFS-induced slow onset LTP at MPP-DG synapses. D: representative MPP-DG fEPSPs recorded during baseline and 56–60 min after MPP LFS in control (top) and CPP-treated (bottom) urethane-anesthetized rats. Calibration: 1 mV, 5 ms.

Fig. 5.

Slow onset LTP requires LFS of PP afferents. Normalized MPP-DG fEPSPs indicate stable baseline recordings spanned over 90 min in pentobarbital-anesthetized rats (n = 3), suggesting our initial observations of slow onset LTP were not due to a slow rising baseline drift. Subsequent LFS (900 pulses, 1 Hz) induced a significant increase in MPP-DG fEPSPs recorded 1 h after LFS in 3 out of 3 animals (P < 0.05, repeated-measures one-way ANOVA).

Fig. 6.

LPP-DG LTD was observed in some anesthetized animals with previously implanted electrodes. A: averaged LPP-DG fEPSP dV/dt analysis in pentobarbital-anesthetized rats with previously implanted electrodes (n = 6) following 900 pulses, 1-Hz LPP LFS. B: data of an individual experiment demonstrating LTD at LPP-DG synapses in a pentobarbital-anesthetized rat with previously implanted electrodes. Similar LPP-DG LTD observations occurred in 4 out of 6 animals.

Fig. 7.

LFS at low current intensities and paired-pulse LFS fail to induce LTD at LPP-DG and MPP-DG synapses in pentobarbital-anesthetized rats. A: normalized LPP-DG fEPSP dV/dt analysis (means ± SE) demonstrate no LTD following 900 pulse, 1-Hz LPP LFS delivered at stimulation intensities evoking 25% of the maximal fEPSP response in pentobarbital-anesthetized rats (n = 6). Inset: responses calibration: 0.5 mV, 5 ms. B: normalized MPP-DG fEPSP dV/dt analysis (means ± SE) reveal no change in baseline MPP-DG fEPSPs following 900 pulse, 1-Hz MPP LFS delivered at stimulation intensities producing 25% of the maximal fEPSP response in pentobarbital-anesthetized rats (n = 5). Inset: calibration: 0.5 mV, 5 ms. C: normalized LPP-DG fEPSP dV/dt analysis (means ± SE) depicting the induction of a rapid onset LTP following LPP paired-pulse LFS (900 paired pulses, 200 ms interpulse interval) in pentobarbital-anesthetized rats (n = 4). Inset: calibration: 1 mV, 5 ms. D: normalized MPP-DG fEPSP dV/dt analysis (means ± SE) demonstrate slow onset LTP following MPP paired-pulse LFS (900 paired pulses, 200-ms interpulse interval) in pentobarbital-anesthetized rats (n = 5). Inset: response calibration: 1 mV, 5 ms.

Fig. 8.

High-frequency stimulation-induced LTP occludes LFS-dependent slow onset LTP. Normalized MPP-DG fEPSP dV/dt analysis (means ± SE) depicts the expression of robust LTP following theta burst stimulation (5 trains of ten 50 ms, 400-Hz theta bursts with 20-s intertrain intervals repeated 3 times every 10 min) in pentobarbital-anesthetized rats (n = 5). Potentiated MPP-DG fEPSP exhibited no additional enhancement of fEPSP dV/dt following subsequent 900 pulse, 1-Hz MPP LFS.