Article Figures & Data
Figures
- Fig. 3.
The AHP is preferentially located in the apical dendritic tree. A, Experimental paradigm. EPSPs were evoked with (asterisk) or without a preceding depolarizing current injection to activate the AHP. B, Current-clamp recordings were obtained from the soma, and EPSPs were evoked either in s. radiatum (radiatum) (a) or in s. oriens (oriens) (b). Fast IPSPs were blocked with picrotoxin (100 μm). Superimposed, baselined EPSPs are shown on the right with (asterisk) and without a concurrent AHP. The amplitude of the EPSP evoked inradiatum was preferentially shunted in comparison to the EPSP evoked in oriens. Note that the faster decay of the EPSPs with an AHP reflects the faster membrane time constant of the whole cell when the AHP is active. C, Control experiment showing that when the AHP is blocked by 10 μmisoprenaline (Iso, right), the shunting of the apical EPSP is absent. Asterisks identify the EPSPs obtained after the AHP-inducing stimulus.
- Fig. 4.
Blockade of the AHP converts STP into LTP.A, Graph of normalized field EPSP slope against time from experiments (n = 5 slices) where a weak tetanus (W; 50 Hz, 0.5 sec) was applied first by itself and then after blockade of the AHP with isoprenaline. A decremental potentiation (STP) is elicited by the first weak tetanus. When the AHP is blocked by isoprenaline (Iso; 0.2 μm), the same weak tetanus now elicits LTP. Sample field EPSPs are shown below for the times indicated. B, Graph of normalized field EPSP slope against time from experiments (n = 5 slices) where a weak tetanus (W; 50 Hz, 0.5 sec) was applied first by itself and then in the presence of isoprenaline plus propranolol (Iso + Propranolol) to block β-adrenergic receptors. Both times, the weak tetanus elicited STP. A subsequent strong tetanus (S; 100 Hz, 1 sec) elicited LTP, indicating that the slices were capable of potentiation. Sample field EPSPs are shown below for the times indicated.
- Fig. 1.
Current generating the AHP has a more distal distribution than current underlying the somatic GABAergic IPSC.A, Schematic diagram illustrating the experimental setup. The cell was voltage-clamped at the soma at a holding potential of −50 mV. Somatic and dendritic inhibitory postsynaptic currents (IPSCs) were independently stimulated in s. pyramidale and s. radiatum, respectively. Excitatory postsynaptic currents were blocked with CNQX (10 μm) and d-APV (50 μm). B, a andc show superimposed traces with and without a voltage step to the IPSC reversal potential (−65 mV). The stimulus protocol is shown in the inset; arrow indicates synaptic stimulation. The relaxations after the voltage step are shown expanded in b and d and have been fitted with a single exponential for comparison. As expected for an electrotonically distant input, the dendritic IPSC (a, b) relaxed at a slower rate (τ = 10 msec) than the somatic IPSC (c,d; τ = 3.7 msec) recorded in the same cell. C, The AHP current in the same neuron, with and without a voltage step to −65 mV (a). The current relaxation is shown expanded inb and has been fitted with a single exponential. The AHP current relaxed with a time course (τ = 8.3 msec) slower than that of the somatic IPSC, indicating that it must have a large contribution from extra-somatic channels. Calibration: 50 pA, 20 msec (B,a and c); 50 pA, 200 msec (Ca).
- Fig. 2.
Electrotonic locations of the AHP and somatic GABAA IPSC channels, estimated using an equivalent cable model of a CA1 pyramidal neuron. A, Somatic IPSC (left) and AHP current (right) recorded in the same cell, when interrupted by a step from the holding potential (−50 mV) to −65 mV. B, The same currents shown expanded, with predictions from the model superimposed (dashed lines). Labels a–c correspond to different channel distributions in the model shown in C. Trace a (soma) best fits the IPSC switchoff, and trace b (proximal apical dendrite) best fits the AHP. C, Schematic diagram showing the equivalent cylinder model used to fit the data. The dendritic trees (apical at right) are shown to scale, using a different scale for the lengths and diameters, but the soma (circle) is not to scale. Filled regions represent the membrane surface that was given a constant conductance density: a, soma only; b, initial 166 μm of the apical dendrite;c, all of the apical dendrite. Adding the same conductance density to the soma made little difference to the predictions of modelsb and c, because the soma has a much smaller surface area than the dendrites. The arrow in bis 75 μm from the soma; injecting the AHP conductance at this point gave an identical switchoff to the distributed conductance inb.
