Activity-dependent excitability changes in hippocampal CA3 cell Schaffer axons.

The membrane potential changes following action potentials in thin unmyelinated cortical axons with en passant boutons may be important for synaptic release and conduction abilities of such axons. In the lack of intra-axonal recording techniques we have used extracellular excitability testing as an indirect measure of the after-potentials. We recorded from individual CA3 soma in hippocampal slices and activated the axon with a range of stimulus intensities. When conditioning and test stimuli were given to the same site the excitability changes were partly masked by local effects of the stimulating electrode at intervals < 5 ms. Therefore, we elicited the conditioning action potential from one axonal branch and tested the excitability of another branch. We found that a single action potential reduced the axonal excitability for 15 ms followed by an increased excitability for approximately 200 ms at 24 degrees C. Using field recordings of axonal action potentials we show that raising the temperature to 34 degrees C reduced the magnitude and duration of the initial depression. However, the duration of the increased excitability was very similar (time constant 135 +/- 20 ms) at 24 and 34 degrees C, and with 2.0 and 0.5 mM Ca2+ in the bath. At stimulus rates > 1 Hz, a condition that activates a hyperpolarization-activated current (Ih) in these axons, the decay was faster than at lower stimulation rates. This effect was reduced by the Ih blocker ZD7288. These data suggest that the decay time course of the action potential-induced hyperexcitability is determined by the membrane time constant.

Unmyelinated axons in the rat hippocampus hyperpolarize and activate an H current when spike frequency exceeds 1 Hz.

The mammalian cortex is densely populated by extensively branching, thin, unmyelinated axons that form en passant synapses. Some thin axons in the peripheral nervous system hyperpolarize if action potential frequency exceeds 1-5 Hz. To test the hypothesis that cortical axons also show activity-induced hyperpolarization, we recorded extracellularly from individual CA3 pyramidal neurons while activating their axon with trains consisting of 30 electrical stimuli. Synaptic excitation was blocked by kynurenic acid. We observed a positive correlation between stimulation strength and the number of consecutive axonal stimuli that resulted in soma spikes, suggesting that the threshold increased as a function of the number of spikes. During trains without response failures there was always a cumulative increase in the soma response latency. Intermittent failures, however, decreased the latency of the subsequent response. At frequencies of > 1 Hz, the threshold and latency increases were enhanced by blocking the hyperpolarization-activated H current (Ih)by applying the specific Ih blocker ZD7288 (25 microM) or 2 mM Cs+. Under these conditions, response failures occurred after 15-25 stimuli, independent of the stimulation strength. Adding GABA receptor blockers (saclofen and bicuculline) and a blocker of metabotropic glutamate receptors did not change the activity-induced latency increase in recordings of the compound action potential. We interpret these results as an activity-induced hyperpolarization that is partly counteracted by Ih. Such a hyperpolarization may influence transmitter release and the conduction reliability of these axons.

The hippocampal lamella hypothesis revisited.

We have re-examined the hippocampal lamellar organization of the CA3-to-CA1 connection. Based on a new technique with electrophysiological quantification of Schaffer collateral density, and a review of recent literature, we conclude that the lamellar organization remains a useful concept for understanding hippocampal connectivity. Using a sheet-like hippocampal preparation, containing the whole CA1 region, we mapped the distribution of Schaffer collaterals by two procedures. First, we recorded the amplitude of the Schaffer compound action potential in various parts of CA1 after stimulation of a point in CA3. Second, we charted the CA1 positions from which we could antidromically excite individual CA3 neurones. Although the Schaffer collaterals radiated from their CA3 cells of origin within a wide, fan-shaped area, covering a large part of the septo-temporal extent of CA1, the amplitude of the compound action potential was largest in a slightly oblique, transverse band across the CA1 towards the subicular region.