Mol. Cells 2021; 44(2): 63-67
Published online February 17, 2021
https://doi.org/10.14348/molcells.2021.0006
© The Korean Society for Molecular and Cellular Biology
Correspondence to : sungyonkim@snu.ac.kr
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/.
The bed nucleus of the stria terminalis (BNST)—a key part of the extended amygdala—has been implicated in the regulation of diverse behavioral states, ranging from anxiety and reward processing to feeding behavior. Among the host of distinct types of neurons within the BNST, recent investigations employing cell type- and projection-specific circuit dissection techniques (such as optogenetics, chemogenetics, deep-brain calcium imaging, and the genetic and viral methods for targeting specific types of cells) have highlighted the key roles of glutamatergic and GABAergic neurons and their axonal projections. As anticipated from their primary roles in excitatory and inhibitory neurotransmission, these studies established that the glutamatergic and GABAergic subpopulations of the BNST oppositely regulate diverse behavioral states. At the same time, these studies have also revealed unexpected functional specificity and heterogeneity within each subpopulation. In this Minireview, we introduce the body of studies that investigated the function of glutamatergic and GABAergic BNST neurons and their circuits. We also discuss unresolved questions and future directions for a more complete understanding of the cellular diversity and functional heterogeneity within the BNST.
Keywords bed nucleus of the stria terminalis, gamma-aminobutyric acid, glutamate, neural circuitry
The bed nucleus of the stria terminalis (BNST) is a basal forebrain structure situated posterior to the nucleus accumbens, anterior to the thalamus, medial to the dorsal striatum, and dorsal to the ventral pallidum and preoptic area (Alheid and Heimer, 1988). The BNST is a representative example of a highly heterogeneous and complex brain region, with 15-20 subregions harboring numerous types of neurons defined by cellular morphology (Ju et al., 1989), neurochemical makeup (Bota et al., 2012; Ju and Swanson, 1989; Kash et al., 2015; Poulin et al., 2009), electrophysiological properties (Daniel and Rainnie, 2016; Gungor and Paré, 2016), as well as their patterns of connectivity (Dong and Swanson, 2004; Dong et al., 2001; Lebow and Chen, 2016). The anatomical complexity of the BNST is well-matched with the many behavioral, autonomic, and endocrine functions it supports, which have been extensively discussed in the following excellent reviews: anxiety (Davis et al., 2010; Fox et al., 2015; Luyck et al., 2019; Robinson et al., 2019; Walker et al., 2009), stress (Daniel and Rainnie, 2016; Lebow and Chen, 2016; Radley and Johnson, 2018), aversive learning and memory (Goode and Maren, 2017; Gungor and Paré, 2016), addiction (Kash et al., 2015; Vranjkovic et al., 2017), social behavior (e.g., aggression, reproduction, and parenting) (Bayless and Shah, 2016; Flanigan and Kash, 2020), appetite control (Atasoy and Sternson, 2016; Zimmerman et al., 2017), as well as the control of heart rate and blood pressure (Crestani et al., 2013) and hormonal release (Radley and Johnson, 2018). Understanding how such diversity of functions is implemented by the heterogeneous constituents of the BNST remains to be a significant challenge.
A promising approach to tackle this problem—which turned out to be remarkably successful so far—is to investigate the anatomy and function of a specific neuron type defined by the expression of a marker gene (Huang and Zeng, 2013; Luo et al., 2018; Zeng and Sanes, 2017). This became possible with the development of neuron type-specific activity manipulation and imaging techniques, such as optogenetics and chemogenetics (Deisseroth, 2011; Roth, 2016), two-photon calcium imaging, and fiber photometry (Chen et al., 2013; Cui et al., 2013; Gunaydin et al., 2014; Svoboda and Yasuda, 2006), in conjunction with the advancement in genetic and viral methods that grant researchers reproducible access to a genetically defined subpopulation of neurons (Betley and Sternson, 2011; Huang and Zeng, 2013; Luo et al., 2018; Nectow and Nestler, 2020; Yizhar et al., 2011). Indeed, the genetic marker-based neuron type-specific investigations over the past decade yielded a striking expansion in our understanding of the structure and function of the BNST at the cellular level.
Among the exciting progress, in this Minireview, we systematically review the studies that employed this cutting-edge approach to specifically investigate glutamatergic and GABAergic neurons in the BNST and describe how each contributed to an updated view of the BNST function. We also discuss further remaining questions and ongoing attempts to overcome the limitations of this approach.
Glutamate and gamma-aminobutyric acid (GABA) are the principal excitatory and inhibitory neurotransmitters in the brain, respectively. The anterior and dorsal parts of the BNST are mostly comprised of GABAergic neurons, while the posterior and ventral parts of the BNST contain significant numbers of both glutamatergic and GABAergic neurons (Poulin et al., 2009). To target glutamatergic or GABAergic BNST subpopulations, most studies used the
Consistent with the fact that glutamatergic and GABAergic neurons can excite or inhibit the postsynaptic neurons, studies have reported the opposite roles of these neurons in the BNST in distinct behaviors (Fig. 1). For example, optogenetic stimulation of the axon terminals of
Another study that showed the opposing roles of these populations focused on their projections to the parabrachial nucleus (PB), a small pontine structure that has been implicated in a number of functions including feeding (Kim et al., 2020; Palmiter, 2018). In this study, optogenetic stimulation of the axon terminals of BNST
Notably, while the activation of both the BNST
In addition to the BNST
On the other hand, in the PAG projection study (Hao et al., 2019), the authors found that optogenetic stimulation of the axon terminals of BNST
In this Minireview, we focused on the opposing roles of glutamatergic and GABAergic neurons in the BNST and covered the literature that used genetic markers to interrogate these subpopulations. Overall, GABAergic neurons in the BNST seem to induce a positive-valence, appetitive motivational drive, whereas glutamatergic neurons in the BNST elicit a state of negative valence, aversion, and anxiety. This offers a useful dichotomous perspective to understand the role of the BNST at a simpler level.
At the same time, these studies have also clearly demonstrated the functional heterogeneity within each population. This heterogeneity was revealed by single-unit recordings (Jennings et al., 2013b) and projection-specific activity recordings and manipulations (Bhatti et al., 2020; Hao et al., 2019; Jennings et al., 2013a). Recent studies targeting subpopulations of GABAergic BNST neurons also reinforce this notion. For example, neurons in the oval nucleus of the BNST defined by the expression of the
The data from cell-type-specific interrogations provide invaluable building blocks for the construction of a detailed, realistic framework that can fully describe the inner workings of the multifunctional BNST circuitry. Undoubtedly, the field is moving forwards in this direction; such efforts are being accelerated by technical advances, including neuron type-specific activity manipulation and imaging techniques and genetic and viral methods as mentioned above, as well as the anatomical tissue processing techniques (Park et al., 2019; Seo et al., 2016; Ueda et al., 2020) and single-cell RNA-sequencing techniques (Armand et al., 2021; Tasic, 2018). In particular, the droplet-based single-cell RNA-sequencing approach has been recently applied to the BNST and revealed dozens of novel cell types (Moffitt et al., 2018; Rodriguez-Romaguera et al., 2020; Welch et al., 2019). Determining the precise functions of these subpopulations at the circuit and behavioral levels hold great promise for the comprehensive, mechanistic understanding of the diverse functions mediated by the BNST circuitry.
We are grateful to Shanti Chang and Benjamin H. Ahn for comments on the manuscript. This work was supported by National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1C1C1003350) and Creative-Pioneering Researchers Program of Seoul National University.
S.R.K. and S.Y.K. wrote the manuscript.
The authors have no potential conflicts of interest to disclose.
Mol. Cells 2021; 44(2): 63-67
Published online February 28, 2021 https://doi.org/10.14348/molcells.2021.0006
Copyright © The Korean Society for Molecular and Cellular Biology.
Seong-Rae Kim and Sung-Yon Kim*
Institute of Molecular Biology and Genetics, Department of Chemistry, Seoul National University, Seoul 08826, Korea
Correspondence to:sungyonkim@snu.ac.kr
This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/.
The bed nucleus of the stria terminalis (BNST)—a key part of the extended amygdala—has been implicated in the regulation of diverse behavioral states, ranging from anxiety and reward processing to feeding behavior. Among the host of distinct types of neurons within the BNST, recent investigations employing cell type- and projection-specific circuit dissection techniques (such as optogenetics, chemogenetics, deep-brain calcium imaging, and the genetic and viral methods for targeting specific types of cells) have highlighted the key roles of glutamatergic and GABAergic neurons and their axonal projections. As anticipated from their primary roles in excitatory and inhibitory neurotransmission, these studies established that the glutamatergic and GABAergic subpopulations of the BNST oppositely regulate diverse behavioral states. At the same time, these studies have also revealed unexpected functional specificity and heterogeneity within each subpopulation. In this Minireview, we introduce the body of studies that investigated the function of glutamatergic and GABAergic BNST neurons and their circuits. We also discuss unresolved questions and future directions for a more complete understanding of the cellular diversity and functional heterogeneity within the BNST.
Keywords: bed nucleus of the stria terminalis, gamma-aminobutyric acid, glutamate, neural circuitry
The bed nucleus of the stria terminalis (BNST) is a basal forebrain structure situated posterior to the nucleus accumbens, anterior to the thalamus, medial to the dorsal striatum, and dorsal to the ventral pallidum and preoptic area (Alheid and Heimer, 1988). The BNST is a representative example of a highly heterogeneous and complex brain region, with 15-20 subregions harboring numerous types of neurons defined by cellular morphology (Ju et al., 1989), neurochemical makeup (Bota et al., 2012; Ju and Swanson, 1989; Kash et al., 2015; Poulin et al., 2009), electrophysiological properties (Daniel and Rainnie, 2016; Gungor and Paré, 2016), as well as their patterns of connectivity (Dong and Swanson, 2004; Dong et al., 2001; Lebow and Chen, 2016). The anatomical complexity of the BNST is well-matched with the many behavioral, autonomic, and endocrine functions it supports, which have been extensively discussed in the following excellent reviews: anxiety (Davis et al., 2010; Fox et al., 2015; Luyck et al., 2019; Robinson et al., 2019; Walker et al., 2009), stress (Daniel and Rainnie, 2016; Lebow and Chen, 2016; Radley and Johnson, 2018), aversive learning and memory (Goode and Maren, 2017; Gungor and Paré, 2016), addiction (Kash et al., 2015; Vranjkovic et al., 2017), social behavior (e.g., aggression, reproduction, and parenting) (Bayless and Shah, 2016; Flanigan and Kash, 2020), appetite control (Atasoy and Sternson, 2016; Zimmerman et al., 2017), as well as the control of heart rate and blood pressure (Crestani et al., 2013) and hormonal release (Radley and Johnson, 2018). Understanding how such diversity of functions is implemented by the heterogeneous constituents of the BNST remains to be a significant challenge.
A promising approach to tackle this problem—which turned out to be remarkably successful so far—is to investigate the anatomy and function of a specific neuron type defined by the expression of a marker gene (Huang and Zeng, 2013; Luo et al., 2018; Zeng and Sanes, 2017). This became possible with the development of neuron type-specific activity manipulation and imaging techniques, such as optogenetics and chemogenetics (Deisseroth, 2011; Roth, 2016), two-photon calcium imaging, and fiber photometry (Chen et al., 2013; Cui et al., 2013; Gunaydin et al., 2014; Svoboda and Yasuda, 2006), in conjunction with the advancement in genetic and viral methods that grant researchers reproducible access to a genetically defined subpopulation of neurons (Betley and Sternson, 2011; Huang and Zeng, 2013; Luo et al., 2018; Nectow and Nestler, 2020; Yizhar et al., 2011). Indeed, the genetic marker-based neuron type-specific investigations over the past decade yielded a striking expansion in our understanding of the structure and function of the BNST at the cellular level.
Among the exciting progress, in this Minireview, we systematically review the studies that employed this cutting-edge approach to specifically investigate glutamatergic and GABAergic neurons in the BNST and describe how each contributed to an updated view of the BNST function. We also discuss further remaining questions and ongoing attempts to overcome the limitations of this approach.
Glutamate and gamma-aminobutyric acid (GABA) are the principal excitatory and inhibitory neurotransmitters in the brain, respectively. The anterior and dorsal parts of the BNST are mostly comprised of GABAergic neurons, while the posterior and ventral parts of the BNST contain significant numbers of both glutamatergic and GABAergic neurons (Poulin et al., 2009). To target glutamatergic or GABAergic BNST subpopulations, most studies used the
Consistent with the fact that glutamatergic and GABAergic neurons can excite or inhibit the postsynaptic neurons, studies have reported the opposite roles of these neurons in the BNST in distinct behaviors (Fig. 1). For example, optogenetic stimulation of the axon terminals of
Another study that showed the opposing roles of these populations focused on their projections to the parabrachial nucleus (PB), a small pontine structure that has been implicated in a number of functions including feeding (Kim et al., 2020; Palmiter, 2018). In this study, optogenetic stimulation of the axon terminals of BNST
Notably, while the activation of both the BNST
In addition to the BNST
On the other hand, in the PAG projection study (Hao et al., 2019), the authors found that optogenetic stimulation of the axon terminals of BNST
In this Minireview, we focused on the opposing roles of glutamatergic and GABAergic neurons in the BNST and covered the literature that used genetic markers to interrogate these subpopulations. Overall, GABAergic neurons in the BNST seem to induce a positive-valence, appetitive motivational drive, whereas glutamatergic neurons in the BNST elicit a state of negative valence, aversion, and anxiety. This offers a useful dichotomous perspective to understand the role of the BNST at a simpler level.
At the same time, these studies have also clearly demonstrated the functional heterogeneity within each population. This heterogeneity was revealed by single-unit recordings (Jennings et al., 2013b) and projection-specific activity recordings and manipulations (Bhatti et al., 2020; Hao et al., 2019; Jennings et al., 2013a). Recent studies targeting subpopulations of GABAergic BNST neurons also reinforce this notion. For example, neurons in the oval nucleus of the BNST defined by the expression of the
The data from cell-type-specific interrogations provide invaluable building blocks for the construction of a detailed, realistic framework that can fully describe the inner workings of the multifunctional BNST circuitry. Undoubtedly, the field is moving forwards in this direction; such efforts are being accelerated by technical advances, including neuron type-specific activity manipulation and imaging techniques and genetic and viral methods as mentioned above, as well as the anatomical tissue processing techniques (Park et al., 2019; Seo et al., 2016; Ueda et al., 2020) and single-cell RNA-sequencing techniques (Armand et al., 2021; Tasic, 2018). In particular, the droplet-based single-cell RNA-sequencing approach has been recently applied to the BNST and revealed dozens of novel cell types (Moffitt et al., 2018; Rodriguez-Romaguera et al., 2020; Welch et al., 2019). Determining the precise functions of these subpopulations at the circuit and behavioral levels hold great promise for the comprehensive, mechanistic understanding of the diverse functions mediated by the BNST circuitry.
We are grateful to Shanti Chang and Benjamin H. Ahn for comments on the manuscript. This work was supported by National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1C1C1003350) and Creative-Pioneering Researchers Program of Seoul National University.
S.R.K. and S.Y.K. wrote the manuscript.
The authors have no potential conflicts of interest to disclose.