Homeostatic plasticity studied using in vivo hippocampal activity-blockade: synaptic scaling, intrinsic plasticity and age-dependence

Abstract

Homeostatic plasticity is thought to be important in preventing neuronal circuits from becoming hyper- or hypoactive. However, there is little information concerning homeostatic mechanisms following in vivo manipulations of activity levels. We investigated synaptic scaling and intrinsic plasticity in CA1 pyramidal cells following 2 days of activity-blockade in vivo in adult (postnatal day 30; P30) and juvenile (P15) rats. Chronic activity-blockade in vivo was achieved using the sustained release of the sodium channel blocker tetrodotoxin (TTX) from the plastic polymer Elvax 40W implanted directly above the hippocampus, followed by electrophysiological assessment in slices in vitro. Three sets of results were in general agreement with previous studies on homeostatic responses to in vitro manipulations of activity. First, Schaffer collateral stimulation-evoked field responses were enhanced after 2 days of in vivo TTX application. Second, miniature excitatory postsynaptic current (mEPSC) amplitudes were potentiated. However, the increase in mEPSC amplitudes occurred only in juveniles, and not in adults, indicating age-dependent effects. Third, intrinsic neuronal excitability increased. In contrast, three sets of results sharply differed from previous reports on homeostatic responses to in vitro manipulations of activity. First, miniature inhibitory postsynaptic current (mIPSC) amplitudes were invariably enhanced. Second, multiplicative scaling of mEPSC and mIPSC amplitudes was absent. Third, the frequencies of adult and juvenile mEPSCs and adult mIPSCs were increased, indicating presynaptic alterations. These results provide new insights into in vivo homeostatic plasticity mechanisms with relevance to memory storage, activity-dependent development and neurological diseases.

Figure 1. Hyperexcitability in the CA1 region of the hippocampus after in vivo activity-blockade.

Forty-eight hours after implantation of the control or TTX-containing Elvax polymer, extracellular field potentials were recorded from the CA1 region in acute hippocampal slices in response to low-frequency electrical stimulation of the stratum radiatum. The figure shows summary data of the population spike amplitudes from TTX-treated and control rats (for number of slices and animals, see text), with example traces in the inserts.

Figure 2. Unchanged amplitude, but increased frequency of mEPSCs in CA1 pyramidal cells from adult animals after in vivo TTX-application.

(A) Summary data (from all cells) of the mEPSC amplitudes in TTX-treated and control adult rats are shown in the form of cumulative probability plots (for numbers of events and animals, see main text); the bar graph shows the same data in the form of averages±SEM, and the insert displays representative averages of mEPSCs from single cells. (B) Summary data of the inter-event intervals of mEPSCs, with example traces.

Figure 3. Increased amplitude and frequency of mEPSCs in juvenile CA1 cells after in vivo TTX-application.

(A) Summary data of the mEPSC amplitudes; insert: representative averages from single cells. (B) Summary data of the inter-event interval of mEPSCs, with representative traces as inserts.

Figure 4. Increased amplitude and frequency of mIPSCs in adult CA1 cells after in vivo TTX-application.

(A) Summary data of the mIPSC amplitudes (for numbers of cells and animals, see main text); insert: representative averages from single cells. (B) Summary data of the inter-event interval of mIPSCs, with representative traces.

Figure 5. Increased amplitude, but no change in frequency, of mIPSCs in juvenile CA1 cells after in vivo TTX-application.

(A) Summary data of the mIPSC amplitudes; insert: representative averages from single cells. (B) Summary data of the inter-event interval of mIPSCs, with representative traces.

Figure 6. Lack of multiplicative scaling after in vivo TTX-treatment.

(A–C) Plots of miniature events, ranked by amplitude, from TTX-treated versus control cells are shown; inserts: cumulative distribution plots of control amplitudes and scaled TTX events (for scaling details see text; scaling factors: juvenile mEPSCs = 1.27; adult mIPSCs = 1.22; juvenile mIPSCs = 1.33). (A) mEPSCs from juveniles (linear fit equation; TTX = 1.27(Cont)−2.14; R² = 0.99). (B) mIPSCs from adults (linear fit equation; TTX = 1.22(Cont)+2.65; R² = 0.95). (C) mIPSCs from juveniles (linear fit equation; TTX = 1.33(Cont)−6.77; R² = 0.97). The scaled-back post-TTX event amplitudes (for details, see main text) remained significantly different from the control amplitudes, indicating lack of multiplicative scaling.

Figure 7. Increased intrinsic excitability in CA1 cells from adult and juvenile rats after in vivo TTX-application.

(A,B) Summary plots of action potential numbers as a function of current injection (from −60 mV; duration: 500ms; for number of cells and animals, see main text) from adult (A) and juvenile (B) rats, with representative traces from single cells. Asterisks indicate significant (p<0.05) difference between the control and TTX groups.