Susceptibility for homeostatic plasticity is down-regulated in parallel with maturation of the rat hippocampal synaptic circuitry

Abstract

Homeostatic regulation, i.e. the ability of neurons and neuronal networks to adjust their output in response to chronic alterations in electrical activity is a prerequisite for the pronounced functional plasticity in the developing brain. Cellular mechanisms of homeostatic plasticity have mainly been studied in cultured preparations. To understand the developmental time frame and properties of homeostatic plasticity under more physiological conditions, we have here compared the effects of activity deprivation on synaptic transmission in acutely isolated and cultured hippocampal slices at different stages of development. We find that transmission at both glutamatergic and GABAergic synapses is strongly and rapidly (15 h) regulated in the opposite directions in response to inactivity during narrow, separated time windows early in development. Following this critical period of synaptic development, induction of the homeostatic response requires longer periods (40 h) of inactivity. At glutamatergic synapses, activity blockade led to an increase in the amplitude and frequency of mEPSCs, and the threshold for induction of this response was increased during development. In contrast, homeostatic regulation at GABAergic synapses was expressed in a qualitatively distinct manner at different developmental stages. Immature neurons responded rapidly to inactivity by regulating mIPSC frequency, while longer activity blockade led to a decrease in the mIPSC amplitude independent of the neuronal maturation. The susceptibility of immature networks to homeostatic regulation may serve as a safety mechanism against rapid runaway destability during the time of intense remodelling of the synaptic circuitry.

Figure 1. Developmentally regulated effect of activity blockade on mEPSCs in CA3 pyramidal neurons in acute slices

A, 15–17 h TTX treatment leads to an increase in mEPSC amplitude and frequency at P4. Aa, example recordings of mEPSCs from CA3 pyramidal cell in control and TTX-incubated slice at P4. Ab, examples of single and averaged (10) events on an expanded scale. Ac, cumulative distributions and average values of the mEPSC interval in acute (n = 7), control (n = 11) and TTX-incubated (n = 6) slices. Ad, cumulative distributions and average values of the mEPSC amplitude from the same recordings. B, 15–17 h TTX treatment has no effect on mEPSCs at P8. Cumulative distributions and average values on the mEPSC interval (a) and amplitude (b) from acute (n = 12), control-incubated (n = 11) and TTX-incubated (n = 12) slices. C, time course of the effect at P4. Pooled data on the effect of 2–5 h (n = 10 for both groups), 7–10 h (n = 6) and 15–17 h TTX treatment on mEPSCs at P4. *P < 0.05, **P < 0.01 and ***P < 0.001.

Figure 2. Developmentally regulated effect of 15–17 h TTX treatment on mIPSCs frequency

A, 15–17 h TTX treatment leads to decrease in mIPSC frequency only at immature neurons at P0. Aa, example recordings of mIPSCs from CA3 pyramidal neurons (C_m < 30 pF) from control and TTX-treated slices, cut at P0 and incubated for 15–17 h. Ab, single and averaged + events on an expanded time scale.Ac, cumulative distributions of the inter-event interval of mIPSCs at acute (P1, n = 6), control (n = 5) and TTX-incubated (n = 4) slices, from neurons with capacitance < 30 pF. Inset: plot of the mIPSC intervals as a function of the cell membrane capacitance (C_m) from acute, control and TTX- incubated slices (n = 4–9 for each group). *P < 0.02. Ad, equivalent data as in Ac, for the effect of TTX incubation on the amplitude of mIPSCs. B, 15–17 h TTX treatment had no effect on mIPSCs at P4. Ba, cumulative distributions of the inter-event intervals of mIPSCs from acute (n = 10), control (n = 11) and TTX-incubated (n = 18) slices at P4. Inset: average mIPSC intervals from the same data. Bb, equivalent data for the effect of TTX incubation on the amplitude of mIPSCs.

Figure 3. Developmentally regulated effect of activity blockade on mEPSCs in organotypic cultures

A, effect of 15 h and 48 h TTX treatment on mEPSCs at 7–8 DIV. Aa, examples of mEPSC recordings from CA3 pyramidal neurons from control and TTX-treated cultures at 8 DIV. Ab, cumulative distributions of the mEPSC intervals in control (n = 6) and TTX-treated cultures (TTX15, n = 5; TTX48, n = 5) from one batch of slices at 8 DIV. Inset: pooled data from recordings at 7–8 DIV, showing the average inter-event interval of mEPSCs at the three groups normalized to the control level (control, n = 16; TTX15, n = 12; TTX48, n = 11). Ac, analysis of the amplitude of mEPSCs from the same neurons as in b. B, equivalent data for cultures at 14–16 DIV. Example traces (a) and cumulative distributions (b and c) from one batch of slices at 16 DIV (control, n = 6; TTX15 n = 5; TTX48 n = 6). Insets show the pooled data from all recordings at 14–16 DIV, normalized to the control level (control, n = 19; TTX15 n = 10; TTX48 n = 18).

Figure 4. Effect of 37–42 h TTX treatment on mIPSCs amplitude at P0

A, example recordings of mIPSCs from CA3 pyramidal neurons (C_m < 30 pF) from control and TTX-treated slices, cut at P0. B, single and averaged (10) events on an expanded time scale. C, cumulative distributions of the inter-event interval of mIPSCs at control (n = 8) and TTX-incubated (n = 5) slices, from P0 neurons with capacitance < 30 pF. Inset: plot of the mIPSC intervals as a function of the cell membrane capacitance (C_m) from control and TTX-incubated slices (n = 4–9 for each group). *P < 0.05 and ***P < 0.001. D, equivalent data as in C, for the effect of TTX incubation on the mIPSC amplitude.