Differences between the scaling of miniature IPSCs and EPSCs recorded in the dendrites of CA1 mouse pyramidal neurons

Abstract

Anatomical studies have described inhibitory synaptic contacts on apical dendrites, and an abundant number of GABAergic synapses on the somata and proximal dendrites of CA1 pyramidal cells of the hippocampus. The number of inhibitory contacts decreases dramatically with distance from the soma, but the local electrophysiological characterization of these synapses at their site of origin in the dendrites is missing. We directly recorded dendritic GABA receptor-mediated inhibitory synaptic events in adult mouse hippocampal CA1 pyramidal neurons and compared them to excitatory synaptic currents recorded at the same sites. Miniature GABAergic events were evoked using localized application of a hyperosmotic solution to the apical dendrites in the vicinity of the dendritic whole-cell recording pipette. Glutamatergic synaptic events were blocked by kynurenic acid, leaving picrotoxin-sensitive IPSCs. We measured the amplitude and kinetic properties of mIPSCs at the soma and at three different dendritic locations. The amplitude of mIPSCs recorded at the various sites was similar along the somato-dendritic axis. The rise- and decay-times of local mIPSCs were also independent of the location of the synapses. The frequency of mIPSCs was 5 Hz at the soma, in contrast to < 0.5 Hz at dendritic sites, which could be increased to 10-20 Hz and 6-10 Hz, respectively, by our hyperosmotic stimulation protocol. Miniature glutamatergic events were evoked with the same protocol after blocking inhibitory synapses by bicucculine. The measured amplitudes increased along the somato-dendritic axis proportionally with their distance from the soma. The measured kinetic properties were independent of location. Consistent with the idea that IPSCs may have a restricted local effect in the dendrites, our data show a lack of distance-dependent scaling of miniature inhibitory synaptic events, in contrast to the scaling of excitatory events recorded at the same sites.

Figure 1. Anatomy of a mouse CA1 pyramidal neuron and schematic view of the recording protocol

Left, morphology of a typical recorded CA1 pyramidal cell filled with biocytin. Right, schematic diagram of the stimulation and recording sites in CA1 pyramidal cells using the whole-cell dendritic voltage-clamp (VC) technique, combined with local stimulation of synapses by ejecting hyperosmotic external solution. Scale bar represents 100 μm.

Figure 2. Representative recording of mEPSCs

A, representative traces of spontaneous (left, grey) and hypertonic solution-evoked (right, black) mEPSCs from proximal (∼50 μm), middle (∼120 μm) and distal (∼200 μm) apical dendrites of an adult mouse CA1 pyramidal cell. The amplitude of evoked mEPSCs increased with distance from soma. B, cumulative distribution of 50–50 miniature events of 3–3 cells at 3 different locations on apical dendrites.

Figure 4. Summary of electrophysiological properties of spontaneous and osmotically evoked mEPSCs (A and B) and mIPSCs (C and D) recorded at different dendritic distances

A and C, amplitude of evoked miniature events. B and D, frequency of spontaneous (grey columns) and evoked (black columns) events. G and I, 20–80% rise-time, H and J, 50% decay-time of evoked events. Middle panels (E and F) show single representative miniature currents recorded at different locations. *P < 0.05, **P < 0.01 and ***P < 0.005.

Figure 3. Representative recording of mIPSCs

A, representative traces of spontaneous (left, grey) and hypertonic solution-evoked (right, black) mIPSCs from proximal (∼50 μm), middle (∼120 μm) and distal (∼200 μm) apical dendrites as well as from soma of an adult mouse CA1 pyramidal cell. The amplitude of evoked mIPSCs did not change with distance from soma. Note the high spontaneous mIPSC activity at the somatic recording. B, cumulative distribution of 50–50 miniature events of 3–3 cells at soma and 3 different locations on apical dendrites.