From Theta rhythm of hippocampal CA1 neuron activity: gating by GABAergic synaptic depolarization.
Journal of neurophysiology.
Journal of neurophysiology.
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Fig. 7. Carbachol (CCH)-induced {theta} GABAergic depolarization of hippocampal CA1 pyramidal cells enables GABAergic inputs to entrain CA1 pyramidal cells. A: {theta} rhythm stimulation evoked IPSPs before CCH application (Control). During CCH application (50 µM, 30 min; middle panel), the same pattern of stimulation entrained activity of the pyramidal cell. Action potentials were evoked at the 2nd, 3rd, and 5th pulses, with action potentials truncated. The evoked postsynaptic responses were abolished by BIC (1 µM, 30 min). B: schematic diagram of network and discharge relationship between CA1 pyramidal cells and GABAergic interneurons. Top: cholinergic inputs (synaptic or diffuse transmission) act on pyramidal cells, inducing HCO3{-} accumulation and enhanced HCO3{-} conductance through the GABAA receptor channels. The {theta} rhythmic activity of GABAergic interneurons can then directly be transmitted to the pyramidal cells, entraining their activity and altering signal processing. Bottom: peak discharge relationship of pyramidal cells (black rectangle) and interneurons (shadow oval) in {theta} rhythm in behaving and rapid eye movement (REM) sleep (based on O'Keefe and Recce 1993; Shen et al. 1997; Csicsvari et al. 1999). The arrow indicates the discharge shift of a place cell, starting from shadow rectangle, in relation to the {theta} activity as the animal travels into the place field of the place cell. GABA, GABAergic interneurons; Pyr, CA1 pyramidal cells; SCH, Schaffer collateral pathway. C: top left, top right, and bottom left are examples of traces without truncation, showing that the brief pulse of stimulation at 5.5 Hz elicited action potentials even though the 1st brief pulse of stimulation was insufficient to evoke action potential. Bottom right illustrates responses to co-stimulation of Sch at below-threshold intensity and GABAergic inputs at pre-CCH (Control) and post-CCH (CCH) periods. Inset shows the initial segment at ×3 amplification with action potential truncated. Arrowheads indicate the time when the brief pulse of stimulation was delivered.
- On the other hand, in the presence of CCH, stimulation of GABAergic inputs elicited instantly phase-locked firing of pyramidal cells (Fig. 7, A and C; n = 14).
- The postsynaptic GABAergic response to the first stimulation pulse usually did not reach action potential threshold (Fig. 7, A and C).
- The postsynaptic GABAergic responses were sensitive to BIC, indicating the involvement of the same receptor channels (Fig. 7A).
- Before the CCH administration, co-stimulation of Sch at the set intensity (50% below the threshold) and GABAergic inputs (50 µA, 50 µs) largely abolished the Sch stimulation-induced excitatory potential (by 89.5 ħ 4.3%, n = 8, P < 0.05; Fig. 7C).
- Action potentials, however, were evoked by co-stimulation of Sch at below-threshold intensity together with reversed GABAergic inputs in all cases (n = 8, P < 0.05; Fig. 7C, bottom right).
- 1999) and reconfigures the operations of hippocampal networks into patterns of activity associated with GABAergic inputs (Fig. 7B).
- Place cells thus fire in phase with progressively stronger GABAergic inputs from interneurons and at earlier phases of the cycles as the rat moves toward the center of their place field (Fig. 7B) (Csicsvari et al.
- The reversed response, although often not strong enough to reach threshold by itself, can entrain the pyramidal cells when stimulated at a frequency (Fig. 7).
- Furthermore, the reversed response can effectively enhance weak excitatory inputs to reach threshold (Fig. 7C) (Sun et al.