Functional switching of GABAergic synapses by ryanodine receptor activation

Abstract

The role of the ryanodine receptor (RyR) in modifiability of synapses made by the basket interneurons onto the hippocampal CA1 pyramidal cells was examined in rats. Associating single-cell RyR activation with postsynaptic depolarization increased intracellular free Ca(2+) concentrations and reversed the basket interneuron-CA1 inhibitory postsynaptic potential into an excitatory postsynaptic potential. This synaptic transformation was accompanied by a shift of the reversal potential from that of chloride toward that of bicarbonate. This inhibitory postsynaptic potential-excitatory postsynaptic potential transformation was prevented by blocking RyR or carbonic anhydrase. Associated postsynaptic depolarization and RyR activation, therefore, changes GABAergic synapses from excitation filters to amplifier and, thereby, shapes information flow through the hippocampal network.

Figure 1

Transformation of GABAergic BAS-CA1 synapses by RyR activation. (a) Stimulation of local GABAergic neurons elicited a monophasic IPSC recorded in a hippocampal CA1 pyramidal cell under whole-cell patch voltage clamp. BIC (1 μM, 30 min) eliminates (b) whereas kynurenate (KYN; 500 μM, 20 min) does not alter (c) the evoked IPSPs. (d—f) The relationship between the evoked BAS-CA1 PSP at different membrane potentials can be described with a straight line, determined by the least-sum squares criterion, and is not altered by kynurenate.

Figure 2

Effects of cADP-ribose on [Ca²⁺]_i and depolarizing-induced [Ca²⁺]_i increases and sensitivity to RR. Under whole-cell voltage clamp, cADP-ribose increased [Ca²⁺]_i and the depolarizing-induced [Ca²⁺]_i increase (Insets) (a). The presence of RR in the internal solution eliminated cADP-ribose's effects on [Ca²⁺]_i and the enhancement of the depolarizing-induced [Ca²⁺]_i increase (Insets) (b). The zeros on the abscissas indicate the beginning of whole-cell patch formation. Depolarizing pulse was from −75 to −20 mV for 100 ms in duration. Insets (Right) Pseudocolor images of neurons about 10 min after the recording. Measurements of the ratiometric data were made in the somata near primary dendrites (white ovals; calibration bar = 50–500 nM).

Figure 3

(a–c) cADP-ribose reduces BAS-CA1 IPSP and reverses the IPSP to EPSP when associated with postsynaptic depolarization and shifts the potential response–membrane potential curve to the right. (d–f) RR prevents cADP-ribose- and cADP-ribose postsynaptic depolarization-induced changes in the BAS-CA1 responses and shifts the potential response–membrane potential curve to the left. (g) Time courses of the response to cADP-ribose postsynaptic depolarization (cADP-ribose/Ca²⁺; at arrow) as compared with those of pre-RR application (at arrowhead); each point represents the mean PSP magnitudes + SEM normalized to the average of their control values before application of agents. In the cADP-ribose/Ca²⁺ group, the same procedure (sham injection of RR at arrowhead) also was applied through the electrode containing no RR. (h) Acetazolamide (1 μM, 30 min) prevents cADP-ribose postsynaptic depolarization-induced changes in the BAS-CA1 responses (40 min after cADP-ribose postsynaptic depolarization vs. control trace).

Figure 4

Switching functions of GABAergic synapses from excitation filter to amplifier by RyR activation. (a and b) Single-pulse stimulation of BAS-CA1 (at arrowhead) and SCH [at above-threshold intensities; truncated; stimulated at arrow (a)] evokes an IPSP and action potentials, respectively. The excitatory SCH (at the same above-threshold stimulation) input is filtered out by a costimulation of BAS-CA1 (b). (c–f) Single-pulse stimulation (c) of BAS-CA1 and of SCH at below-threshold intensities evokes an IPSP and an EPSP, respectively. The excitatory SCH (at the same below-threshold stimulation) input is below threshold as evoked by costimulation (single pulse) of BAS-CA1 and SCH inputs (d) before cADP-ribose application. cADP-ribose (30 min after the application) transforms BAS-CA1 IPSP and does not change much of the SCH-CA1 EPSP, evoked by single-pulse stimulation of BAS or SCH, respectively (e). The excitatory SCH (at the same below-threshold stimulation) input is amplified by the co-BAS stimulation after the cADP-ribose-induced synaptic transformation and induces action potentials (truncated; f). Traces were from the same cell. (g) Schematic diagram of GABAergic inputs functioning as either excitatory filter (Left) or amplifier (Right). Active GABAergic inputs, either through activation of SCH as feed-forward inputs (no. 1), feedback inputs (no. 2), or of other circuits (no. 3; such as from the septum), effectively filter excitatory signals so that only very strong excitatory inputs might evoke action potentials. The GABAergic synaptic transformation results in amplifying excitatory signals so that weaker inputs can pass through the neural circuits. BAS, basket GABAergic interneurons (in gray); Pyr, CA1 pyramidal cells; SP, stratum pyramidale.