Electrophysiological Properties of CA1 Pyramidal Neurons along the Longitudinal Axis of the Mouse Hippocampus

Abstract

Evidence for different physiological properties along the hippocampal longitudinal axis is emerging. Here, we examined the electrophysiological features of neurons at different dorso-ventral sites of the mouse CA1 hippocampal region. Cell position was defined with respect to longitudinal coordinates of each slice. We measured variations in neuronal excitability, subthreshold membrane properties and neurotransmitter responses along the longitudinal axis. We found that (i) pyramidal cells of the dorsal hippocampus (DH) were less excitable than those of the ventral hippocampus (VH). Resting Membrane Potential (RMP) was more hyperpolarized and somatic Input Resistance (Ri) was lower in DH compared to VH. (ii) The Paired-pulse ratio (PPR) of focally induced synaptic responses was systematically reduced from the DH to the VH; (iii) Long-term-potentiation was most pronounced in the DH and fell gradually in the intermediate hippocampus and in the VH; (iv) the frequency of miniature GABAergic events was higher in the VH than in the DH; (v) the PPR of evoked inhibitory post-synaptic current (IPSC) was higher in the DH than in the VH. These findings indicate an increased probability of both GABA and glutamate release and a reduced plasticity in the ventral compared to more dorsal regions of the hippocampus.

Figure 1. Hippocampal slicing procedure.

(a) Schematic diagram showing isolation and elongation of the hippocampus. Image credit: © 2015 Allen Institute for Brain Science. Allen Brain Atlas API. (b) The hippocampus is placed in a small agarose-block fixed to the cutting stage of the vibratome magnetic plate in order to prepare 350 μm thick transverse slices. (c) Schematic partition of the hippocampus along the septo-temporal axis and representative Hoechst- stained slices at different septo-temporal positions distances. White lines indicate boundaries among different zones, the black ones indicate the longitudinal position of slices used for electrophysiological recording.

Figure 2. Extracellular fields reveal excitability variation along the septo-temporal axis.

Local field potentials induced in the CA1 stratum radiatum by stimulating Schaffer collaterals. (a) Curves of normalized fEPSP slope plotted against stimulus intensity (I–O) for records from DH (n = 19/14), IH (n = 12/10) and VH (n = 12/8). (b) Current intensity for population spike threshold plotted against distance (μm) from the hippocampal dorsal pole. (c) Mean and S.E.M of the spike voltage normalized to the plateau voltage for events induced in DH (0.43 ± 0.03, n = 19/14), IH (0.32 ± 0.02, n = 12/9) and VH (0.33 ± 0.02, n = 14/10). Top: example of field events induced by increasing stimuli in one slice with an arrow showing the population spike at threshold. **Indicates p < 0.01.

Figure 3. Subthreshold responses and excitability of DH and VH neurons.

Whole cell recordings from CA1 pyramidal neurons. (a) Resting membrane potential (RMP) was significantly more depolarized for VH (white circles, average: −74.94 ± 1.03 mV, n = 15/8/8) than DH neurons (black circles, average: −79.34 ± 0.81 mV, n = 19/9/9). (b) Voltage responses from DH (upper) and VH (lower) pyramidal cells at RMP to 800 ms step current injections ranging from −50 pA to 50 pA in 10 pA increments. (c) Rin at RMP was significantly higher for VH (white bar, Rin VH = 235.44 ± 19.43 MΩ, n = 18/11/7) than DH cells (black bar, Rin DH = 167.68 ± 15.8 MΩ, n = 14/10/10). (d) Relations between injected current and firing frequency (F–I) for DH (n = 15/9/9) and VH (n = 15/7/7) pyramidal cells. Differences between F-I curves were statistically significant at 50 pA. *Indicates p < 0.05. **Indicates p < 0.01.

Figure 4. Short and long term plasticity at Schaffer collateral synapses in DH, IH and VH.

(a) PPR values for fEPSP slope plotted against distance from the dorsal pole (μm) measured from field records at different distances along the septo-temporal hippocampal axis. White circles indicate absolute values of PPR along the septo-temporal axis, and black circles show mean PPR values. The dependence of PPR from slice depth was described assuming a Boltzmann function (f = y0+a/(1+exp (-(x-x0)/b)) with the following fit parameters: dorsal pole distance for 1/2 maximal PPR (x0): 3087.53 ± 240.41 μm, y0 = 1.0838 ± 0.0536, a = 0.5468 ± 0.0950, b = −654.3618 ± 253.5688; n = 15). Note that the interpolating sigmoid function was not very far from a linear interpolating one. Above, fEPSP traces show variation in PPR for slices from DH, IH and VH zones. (b) Averaged PPR values for DH was 1.64 ± 0.03 (n = 37/21) in IH slices it was 1.31 ± 0.03 (n = 17/12) and in slices from the VH the PPR was 1.12 ± 0.03 (n = 19/11). (c) LTP of fEPSP slope from extracellular records made from the DH (n = 11/11, black circles), IH (n = 7/6, white triangle) and VH (n = 10/9, white circles). Time course of slope values from responses evoked at 0.05 Hz and normalized as detailed in the Methods. Arrows indicate time of HFS (2 trains at 100 Hz of 1 s duration with 3 s inter-train interval). Above are representative traces taken before (gray) and 35 minutes (black) after HFS for each septo-temporal region. (d) LTP of fEPSP slope measured at 35 minutes after HFS from slices of the DH, IH and VH. The mean increase was 1.48 ± 0.05 for DH (n = 10/10), 1.33 ± 0.06 for IH (n = 5/5) and 1.19 ± 0.06 for VH (n = 8/7). Data in B and D are mean ± S.E.M. *Indicates p < 0.05 and **indicates p < 0.01. All field waveforms are averaged from three traces.

Figure 5. Differences in release from inhibitory synapses in slices of DH and VH.

Whole cell records from CA1 pyramidal cells. (a) Histograms of mean mIPSC amplitude (left) and frequency (right) in DH and VH. Above: representative patch-clamp recordings of mIPSCs in CA1 pyramidal neurons in DH and VH. Bottom: cumulative probability histogram of mIPSCs amplitude (left) and IEI (right) from DH (black line) and VH (gray line) cells. Mean amplitude for mIPSCs: DH, 16.3 ± 0.7 pA (n = 10/8/8, Total number on events, 2010); VH, 16.6 ± 0.6 pA (n = 13/10/10, Total number on events, 2610). Mean frequency for mIPSCs: DH: 6.4 ± 0.8 Hz, (n = 10/8/8); VH: 9.3 ± 0.9 Hz (n = 13/10/10). Note the shift toward the left in IEI curves indicating an increase in mIPSCs frequency of VH cells. (b) Mean PPR for evoked IPSCs in CA1 pyramidal cells of DH and VH. Mean PPR: DH, 1.75 ± 0.15, n = 16/10/10; VH, 1.19 ± 0.07, n = 16/11/11). Above: evoked IPSCs showing differences in PPR at inhibitory synapses of DH and VH. Data shown as mean ± S.E.M. *Indicates p < 0.05 and **indicates p < 0.01. Traces are an average of the ten consecutive slices.