Elsevier

Neuroscience

Volume 226, 13 December 2012, Pages 101-118
Neuroscience

In vivo voltage-dependent influences on summation of synaptic potentials in neurons of the lateral nucleus of the amygdala

https://doi.org/10.1016/j.neuroscience.2012.09.012Get rights and content

Abstract

The amygdala has a fundamental role in driving affective behaviors in response to sensory cues. To accomplish this, neurons of the lateral nucleus (LAT) must integrate a large number of synaptic inputs. A wide range of factors influence synaptic integration, including membrane potential, voltage-gated ion channels and GABAergic inhibition. However, little is known about how these factors modulate integration of synaptic inputs in LAT neurons in vivo. The purpose of this study was to determine the voltage-dependent factors that modify in vivo integration of synaptic inputs in the soma of LAT neurons. In vivo intracellular recordings from anesthetized rats were used to measure post-synaptic potentials (PSPs) and clusters of PSPs across a range of membrane potentials. These studies found that the relationship between membrane potential and PSP clusters was sublinear, due to a reduction of cluster amplitude and area at depolarized membrane potentials. In combination with intracellular delivery of pharmacological agents, it was found that the voltage-dependent suppression of PSP clusters was sensitive to tetraethylammonium (TEA), but not cesium or a blocker of fast GABAergic inhibition. These findings indicate that integration of PSPs in LAT neurons in vivo is strongly modified by somatic membrane potential, likely through voltage-dependent TEA-sensitive potassium channels. Conditions that lead to a shift in membrane potential, or a modulation of the number or function of these ion channels will lead to a more uniform capacity for integration across voltages, and perhaps greatly facilitate amygdala-dependent behaviors.

Highlights

► Neurons of the LAT display clusters of post-synaptic potential (PSPs) in vivo. ► The amplitude and area of clusters depend on the membrane potential. ► The relationship to membrane potential is sublinear. ► The sublinear relationship is sensitive to TEA, but not cesium. ► Reversal of sublinearity leads to increased PSP integration and AP firing.

Introduction

Neurons of the amygdala play a critical role in the formation of fear memory and expression of learned fear. They receive a wide range of excitatory inputs that are integrated to dictate neuronal output and drive amygdala-dependent behaviors. Despite the importance of synaptic drive, very little is known about synaptic integration in amygdala neurons in vivo.

Neurons in the basolateral amygdala (BLA) display potential for non-linear integrative properties in vivo, indicated by modulation of excitability by voltage-gated channels (Lang and Pare, 1997a) and subthreshold membrane oscillations (Pare et al., 1995a, Pape et al., 1998). Furthermore, BLA neurons are subjected to strong inhibition in vivo (Li et al., 1996, Lang and Pare, 1997b). However, the impact of voltage-dependent conductances and GABAergic inhibition on integration of synaptic inputs in BLA neurons in vivo is still unknown. Previous studies in vitro have demonstrated that there may be enhancement or suppression of synaptic integration at depolarized membrane potentials. Studies in vitro indicate that depolarization of the membrane potential leads to enhanced synaptic integration via activation of a persistent sodium conductance in some circumstances (Llinas and Sugimori, 1980, Magee and Johnston, 1995, Fleidervish and Gutnick, 1996, Lipowsky et al., 1996, Margulis and Tang, 1998, Urban et al., 1998, Andreasen and Lambert, 1999, Gonzalez-Burgos and Barrionuevo, 2001, Prescott and De Koninck, 2005, Rosenkranz and Johnston, 2007, Branco and Hausser, 2011), or suppression of synaptic integration through various potassium (K+) conductances (Storm, 1988, Cash and Yuste, 1998, Cash and Yuste, 1999, Urban and Barrionuevo, 1998, Svirskis et al., 2004). Furthermore, the increased conductance state observed in vivo (or current injections in vitro that mimic in vivo conditions) either reduces (Holmes and Woody, 1989, Bernander et al., 1991, Timofeev et al., 1996, Hausser and Clark, 1997, Destexhe and Pare, 1999, Chance et al., 2002, Fellous et al., 2003, Petersen et al., 2003, Prescott and De Koninck, 2003, Leger et al., 2005, Zsiros and Hestrin, 2005) or enhances synaptic inputs, synaptic integration or excitability (Ho and Destexhe, 2000, Oviedo and Reyes, 2002, McCormick et al., 2003, Shu et al., 2003, Higgs et al., 2006, Haider et al., 2007). However, little is known about the relationship between membrane potential and synaptic integration in vivo.

BLA neurons in anesthetized animals display slow periodic synaptically driven depolarizations (Rosenkranz and Grace, 2002, Windels et al., 2010). These slow depolarizations are likely synaptic in origin, as opposed to faster intrinsic oscillations of the membrane potential (e.g. Pape et al., 1998), because their occurrence is not voltage-dependent (Rosenkranz and Grace, 2002). Moreover, BLA neuronal firing that occurs during some of these depolarizations is time locked to the EEG of cortical regions that send excitatory afferents to the BLA (Pare et al., 1995b), and they can be mimicked by stimulation of certain excitatory afferents (Windels et al., 2010). These spontaneous synaptic events provide an opportunity for examination of synaptic integration in vivo, and determination of the factors that modify synaptic integration.

The purpose of the current study is to test whether changes in somatic membrane potential influence summation of inputs in vivo, and what voltage-dependent factors contribute to this summation. In vivo intracellular recordings from neurons in the lateral nucleus of the BLA (LAT) were used to examine the impact of somatic membrane potential on PSP clusters, and determine whether fast GABAergic inhibition or K+ channels strongly influence the voltage-dependence of these clusters.

Section snippets

Experimental procedures

All experiments were performed with prior approval of the Rosalind Franklin University Institutional Animal Care and Use Committee, and complied with the NIH Guide for the Care and Use of Laboratory Animals. Adult male Sprague–Dawley rats (postnatal day 72–85, Harlan, Indianapolis, IN, USA) were pair or triple housed (standard solid bottom polycarbonate rat cages, 267 mm × 483 mm × 203 mm deep, corncob bedding from Harlan Teklad) in an environment-controlled vivarium with free access to food (pelleted

Basic aspects of spontaneous synaptic activity

In neurons recorded from the LAT of anesthetized rats, spontaneous synaptic events tend to occur in clusters. The frequency of these clusters of post-synaptic potentials (“PSP clusters”) depends on the anesthesia level, and is tightly aligned with the cortical EEG (Fig. 1A, B), similar to what has been demonstrated previously (e.g. Pare et al., 1995b). Because the amplitude and duration of these PSP clusters are sensitive to anesthesia level, all data used for recordings were obtained when the

Discussion

Integration of synaptic inputs is fundamental for neuronal processing of information. However, very little is known about what factors dominate synaptic integration in vivo. Many in vitro studies have demonstrated that a variety of factors can contribute (for review see Magee, 2000, Spruston, 2008). However, it is unknown if LAT neurons in vivo rely on these factors to a similar degree. To test this, we examined the voltage-dependence of clusters of PSPs, individual PSPs, and the relationship

Acknowledgements

The author wishes to thank Mallika Padival for histological processing of brain tissue, and Drs. Anthony West, Raymond Chitwood, and Jolee Shapiro for useful discussion. Grant support provided by the U.S. National Institutes of Health (MH084970) and the Brain Research Foundation. Funding supporters had no role in study design, data collection, analysis or interpretation, writing or in the decision to submit for publication. The author has no conflicts of interest to disclose.

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