Elsevier

Brain Research Bulletin

Volume 98, September 2013, Pages 93-101
Brain Research Bulletin

Research report
Development of NMDA-induced theta rhythm in hippocampal formation slices

https://doi.org/10.1016/j.brainresbull.2013.07.010Get rights and content

Highlights

  • NMDA-induced theta oscillations in hippocampal slices were observed.

  • Enrichment of NMDA solution with baclofen increases the probability of inducing theta.

  • NMDA and baclofen-induced theta resembles cholinergically induced oscillations.

Abstract

During the past 20 years experimental evidence has accumulated demonstrating that the appearance of theta rhythm requires a certain level of excitation of local neuronal networks. In this study we extended our earlier in vitro observations concerning the involvement of cholinergic and GABAergic neurotransmission in hippocampal theta production. Specifically, we investigated whether the hippocampal neuronal network is capable of generating theta oscillations in the presence of N-methyl-d-aspartic acid (NMDA) in a brain slice preparation. To answer this question, the effect of different concentrations of NMDA (Experiment I) and the effect of interaction between NMDA and GABAA/B agonists and antagonists on field potentials recorded in the CA3c region of hippocampal formation (HPC) slice preparations (Experiment II) was examined. We demonstrated for the first time that apart from the epileptiform activity recorded in almost all series of Experiments I and II, only the perfusion of HPC slices with NMDA in doses of 30 and 50 μM, as well as the perfusion of HPC slices with NMDA and GABAB agonist baclofen (50 μM NMDA + 50 μM BACL), resulted in the appearance of individual theta epochs. The best synchronized theta oscillations obtained after administration of 50 μM NMDA + 50 μM BACL resembled theta activity induced by a bath perfusion of 50 μM carbachol. In light of the obtained results we conclude that besides the cholinergic and GABAergic input, NMDA glutamatergic drive is also important for the appearance of theta oscillations in HPC in vitro.

Introduction

The basic phenomena that accompany the production of all EEG patterns in the brain tissue are the cellular mechanisms of oscillations and synchrony (Bland and Colom, 1993, Lopes da Silva, 1991). Knowledge of these phenomena is important for understanding the relationship between specific EEG patterns and the activity of neuronal populations in brain structures, including hippocampal formation (HPC). This limbic structure generates a synchronized EEG activity, termed the theta rhythm. The theta rhythm is a sinusoidal, high-voltage activity (from 0.2 to 2 mV) with a frequency band ranging from 3 to 12 Hz (Bland, 1986, Bland and Oddie, 2001, Bland and Colom, 1993, Lopes da Silva, 1991). Theta waves are one of the best examples of oscillations and synchrony occurring in neuronal networks of the central nervous system (Bland, 1986, Bland and Colom, 1993). This activity occurs in the hippocampal formation during the planning and initiation of movement sequences (Bland, 1986, McNaughton et al., 2007, Oddie et al., 1997), plays a role in attention and sensorimotor integration (Ekstrom et al., 2005, Oddie et al., 1997), and affects formation of memory traces (Dusek and Eichenbaum, 1997, Huang and Kandel, 2005, Tracy et al., 2001).

Numerous studies carried out in vivo and in vitro have demonstrated that the appearance of theta rhythm requires a certain level of excitation of local neuronal networks (Bland and Colom, 1993, Bland, 2008, Cobb et al., 1995, Konopacki et al., 2006, Konopacki and Gołębiewski, 1993, Yoder and Pang, 2005). Many years of research conducted with the use of models of HPC slice preparations have allowed us to determine the specific role of the cholinergic (Kazmierska et al., 2012, Konopacki et al., 1987, Konopacki et al., 2006, Konopacki, 1998, Kowalczyk et al., 2001) and GABAergic systems (Gołębiewski et al., 1996, Konopacki and Gołębiewski, 1993, Konopacki et al., 1997) in providing the adequate level of neuronal excitation necessary for theta to appear. Our findings are supported by a large body of data from other laboratories, in which carbachol-induced theta rhythm in hippocampal slices (Bland et al., 1988, Cappaert et al., 2009, Fellous and Sejnowski, 2000, McQuiston, 2010, Natsume and Kometani, 1999) and urethanized rats (Kinney et al., 1998, Leung and Péloquin, 2010, Monmaur et al., 1997, Yoder and Pang, 2005) were observed. Interestingly, Bland (2008) proposed that cholinergic projections provide a steady tonic excitatory afferent drive for HPC theta-related cells while GABAergic projections act to reduce the overall level of inhibition by inhibiting hippocampal GABAergic interneurons (Cappaert et al., 2009, Smythe et al., 1992).

Recent evidence indicates that beside the cholinergic and GABA-ergic transmission, hippocampal formation may also receive the glutamatergic input directly from the median raphe nuclei (MRn) (Crooks et al., 2012), or indirectly through the complex of the medial septum and diagonal band of Broca (MS/DBB) (Huh et al., 2010, Leung and Shen, 2004). The latter authors concluded that both the N-methyl-d-aspartic acid (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in the HPC are involved in programming the amplitude of hippocampal theta, while on the level of the septum the amplitude of theta is probably controlled only by NMDA receptors. An NMDA receptor is a type of ionotropic glutamate receptor, named after the specific agonist – N-methyl-d-aspartate, and consists of NR1 subunits combined with one or more NR2 (A-D) subunits, forming channels permeable to sodium and calcium ions. At rest, the pore of this receptor is blocked by magnesium ions, which are removed after membrane depolarization (Lasoń et al., 2011). In addition to the recognized role of these receptors in controlling the mechanisms of neuroplasticity such as LTP (Davis et al., 1992, Harris et al., 1984, Jin and Feig, 2010), it is commonly believed that the activation of NMDA receptors plays a significant role in epileptogenesis (Bradford, 1995, Jones, 1988, Makarska-Białek et al., 2005, Moschovos et al., 2008, Wojtal et al., 2006), excitotoxicity (Cheng et al., 1999, Tian et al., 2012) and attention processing (Deco and Thiele, 2009). Recent studies have also reported the existence of NMDA-evoked membrane voltage oscillations in spinal locomotory neurons (Hochman et al., 1994, Schmidt et al., 1998), gamma oscillations (Carlén et al., 2012, Lazarewicz et al., 2010, Phillips et al., 2012), high frequency oscillations (Papatheodoropoulos, 2007) and sharp wave-ripples (Papatheodoropoulos, 2010) in hippocampal slices as well as frequency shifts in human EEG (Tsuda et al., 2007).

The research concerning the neurochemical substrates of hippocampal theta, has sparked an interest in the role of NMDA receptors in the development of this particular EEG pattern. First reports concerning the influence of NMDA receptors on hippocampal EEG come from the late 1980s. Those early studies demonstrated that intraventricular injections of NMDA receptors’ antagonist APV blocked the hippocampal theta in vivo (Leung and Desborough, 1988), while the synthetic aspartate analogue of NMDA triggered a rhythmic firing burst of CA1 pyramidal neurons in rat hippocampal slices (Peet et al., 1987). Interestingly, recent reports by Bland et al. (2007) have provided evidence of the generation of a new, pharmacologically distinct type of theta rhythm after intrahippocampal administration of N-methyl-d-aspartate in urethanized rats, suggesting the existence of a new independent pathway responsible for the synchronization of hippocampal EEG. These results are in agreement with other in vivo findings, which reveal that ablation of NMDA receptors on parvalbumin-positive hippocampal interneurons causes altered theta oscillations in freely behaving mice (Korotkova et al., 2010). Similarly, research by Pitkänen et al. (1995) has shown that a noncompetitive NMDA receptor antagonist MK-801 decreased both the power and the frequency of type 2 hippocampal theta in freely moving rats. In addition, microiontophoresis of NMDA to the region of CA1 pyramidal HPC cells induced rhythmic action potential (AP) bursts at a frequency of ≈6 Hz (Bonansco et al., 2002), an activity reminiscent of the “intracellular theta rhythm” (Nuñez et al., 1987, Nuñez et al., 1991, Ylinen et al., 1995).

In light of the above discussion, the question arises whether the HPC neuronal network is capable of generating theta oscillations in the presence of NMDA in a brain slice preparation. To address this issue, electrophysiological experiments were conducted on HPC slice preparations in the presence of different concentrations of NMDA. Portions of this research have been presented in an abstract form (Kazmierska et al., 2011).

Section snippets

Animals and HPC slice preparation procedure

The experiments were performed on 237 hippocampal formation (HPC) slices obtained from 38 male Wistar rats (100–150 g). All the experiments described below were monitored by a Local Ethical Commission (permission no. 24/ŁB 547/2011; in accordance with the European Communities Council Directive of 24 November 1986). Before the experimental procedure animals were housed in groups on a controlled light/dark cycle (the light on: 7:00–19:00) and had free access to standard food and tap water. Each

Experiment I: NMDA-induced field potentials in HPC slice preparations

In Experiment I the effect of different concentrations of NMDA on the HPC slice preparations’ field potentials was studied. The NMDA-induced field potentials were analyzed both qualitatively and quantitatively. When perfused with NMDA at a concentration of 1 μM, the hippocampal formation slice preparations (n = 23 slices) responded with EEG “silence” (i.e. the field activity did not differ from control recordings in ACSF) (Fig. 1, Fig. 2). After application of 3–80 μM of NMDA two different patterns

Discussion

The present study was aimed at investigating whether the hippocampal neuronal network, when isolated from ascending theta-synchronizing pathways, is capable of generating theta oscillations in the presence of NMDA. To answer this question the NMDA-induced field potentials (Experiment I) and the effect of interaction between NMDA and GABAA/B agonists and antagonists on field potentials recorded in the CA3c region of HPC slice preparations (Experiment II) were examined.

We demonstrated that apart

Conflict of interest

The authors declare that they have no competing financial interests.

Acknowledgments

The authors wish to thank Jacek Grebowski, MSc, for helpful comments and assistance in the data analysis. This study was financially supported by the National Science Centre Grant No. 2011/01/N/NZ4/01722.

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