Investigating Long-term Synaptic Plasticity in Interlamellar Hippocampus CA1 by Electrophysiological Field Recording

Summary

We used recording and stimulation electrodes in longitudinal hippocampal brain slices and longitudinally positioned recording and stimulation electrodes in the dorsal hippocampus in vivo to evoke extracellular postsynaptic potentials and demonstrate long-term synaptic plasticity along the longitudinal interlamellar CA1.

Abstract

The study of synaptic plasticity in the hippocampus has focused on the use of the CA3-CA1 lamellar network. Less attention has been given to the longitudinal interlamellar CA1-CA1 network. Recently however, an associational connection between CA1-CA1 pyramidal neurons has been shown. Therefore, there is the need to investigate whether the longitudinal interlamellar CA1-CA1 network of the hippocampus supports synaptic plasticity.

We designed a protocol to investigate the presence or absence of long-term synaptic plasticity in the interlamellar hippocampal CA1 network using electrophysiological field recordings both in vivo and in vitro. For in vivo extracellular field recordings, the recording and stimulation electrodes were placed in a septal-temporal axis of the dorsal hippocampus at a longitudinal angle, to evoke field excitatory postsynaptic potentials. For in vitro extracellular field recordings, hippocampal longitudinal slices were cut parallel to the septal-temporal plane. Recording and stimulation electrodes were placed in the stratum oriens (S.O) and the stratum radiatum (S.R) of the hippocampus along the longitudinal axis. This enabled us to investigate the directional and layer specificity of evoked excitatory postsynaptic potentials. Already established protocols were used to induce long-term potentiation (LTP) and long-term depression (LTD) both in vivo and in vitro. Our results demonstrated that the longitudinal interlamellar CA1 network supports N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) with no directional or layer specificity. The interlamellar network, however, in contrast to the transverse lamellar network, did not present with any significant long-term depression (LTD).

Introduction

The hippocampus has been widely used in cognitive studies^1,2,3. The hippocampal lamellar network in the transverse axis forms the tri-synaptic circuitry that is made up of the dentate gyrus, CA3, and CA1 regions. The lamellar network is considered to be a parallel and independent unit^4,5. This lamellar viewpoint has influenced the use of transverse orientation and transverse slices for both in vivo and in vitro electrophysiological studies of the hippocampus. In light of emerging research, the lamellar hypothesis is b....

Protocol

All animals were treated in accordance with the guidelines and regulations from the Animal Care and Use of Laboratory of National Institute of Health. All methods described here have been approved by the Institutional Animal Care and Use Committee (IACUC) of City University of Hong Kong and Incheon National University.

1. In vivo field recording

1. Animal preparation
   1. Inject urethane (0.06 g per 25 g weight) intraperitoneally to anesthetize mouse. Supplement with intramuscular injection of atropine (0.05 mg/kg). Keep the mouse in a dark quiet spot until full anesthesia takes effect.
      CAUTION: Urethane has carcinog....

Results

We explored long-term synaptic plasticity of longitudinal CA1 pyramidal neurons of the hippocampus using extracellular field recordings both in vivo and in vitro. LTP and LTD are facets of long-term synaptic plasticity that have been demonstrated in the transverse axis of the hippocampus to be unidirectional.

We showed here that using longitudinal hippocampal brain slices, there is LTP in the CA1 longitudinal axis of the hippocampus. We prepared longitudinal slices of the hippocampus along the.......

Discussion

The protocol demonstrates the method to induce long-term synaptic plasticity in vivo as well as from brain slices in the longitudinal CA1-CA1 axis of the hippocampus in vitro. The steps outlined give enough details for an experimenter to investigate LTP and LTD in a longitudinal hippocampal CA1-CA1 connection. Practice is needed to hone the skills required to successfully record field excitatory potentials.

In addition to needing practice, there are several critical steps that are essential to.......

Materials

Name	Company	Catalog Number	Comments
Atropine Sulphate salt monohydrate, ≥97% (TLC), crystalline	Sigma-Aldrich	5908-99-6	Stored in Dessicator
Axon Digidata 1550B
Calcium chloride	Sigma-Aldrich	10035-04-8
Clampex 10.7
D-(+)-Glucose ≥ 99.5% (GC)	Sigma-Aldrich	50-99-7
Eyegel	Dechra
Isoflurane	RWD Life Sciences	R510-22
Magnesium chloride hexahydrate, BioXtra, ≥99.0%	Sigma-Aldrich	7791-18-6
Matrix electrodes, Tungsten	FHC	18305
Multiclamp 700B Amplifier
Potassium chloride, BioXtra, ≥99.0%	Sigma-Aldrich	7447-40-7
Potassium phosphate monobasic anhydrous ≥99%	Sigma-Aldrich	7778-77-0	Stored in Dessicator
Pump	Longer precision pump Co., Ltd	T-S113&JY10-14
Silicone oil	Sigma-Aldrich	63148-62-9
Sodium Bicarbonate, BioXtra, 99.5-100.5%	Sigma-Aldrich	144-55-8
Sodium Chloride, BioXtra, ≥99.5% (AT)	Sigma-Aldrich	7647-14-5
Sodium phosphate monobasic, powder	Sigma-Aldrich	7558-80-7
Sucrose, ≥ 99.5% (GC)	Sigma-Aldrich	57-50-1
Temperature controller	Warner Instruments	TC-324C
Tungsten microelectrodes	FHC	20843
Urethane, ≥99%	Sigma-Aldrich	51-79-6
Vibratome	Leica	VT-1200S
Water bath	Grant Instruments	SAP12