Nested circuits mediate the decision to vocalize
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eLife assessment
Vocalizations are controlled by neural circuits connecting the amygdala and periaqueductal gray. This study presents valuable measures of the neurons that suppress vocalization in appropriate contexts using a rich variety of behavioural, imaging, optogenetic, and tracing methodologies. The evidence supporting the claims of the authors is solid, although their results only hint at the mechanisms that could underlie the hierarchical control of vocalization. The work will be of interest to neurobiologists working on motor control and vocalization.
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Abstract
Vocalizations facilitate mating and social affiliation but may also inadvertently alert predators and rivals. Consequently, the decision to vocalize depends on brain circuits that can weigh and compare these potential benefits and risks. Male mice produce ultrasonic vocalizations (USVs) during courtship to facilitate mating, and previously isolated female mice produce USVs during social encounters with novel females. Earlier we showed that a specialized set of neurons in the midbrain periaqueductal gray (PAG-USV neurons) are an obligatory gate for USV production in both male and female mice, and that both PAG-USV neurons and USVs can be switched on by their inputs from the preoptic area (POA) of the hypothalamus and switched off by their inputs from neurons on the border between the central and medial amygdala (Amg C/M-PAG neurons) (Michael et al., 2020). Here, we show that the USV-suppressing Amg C/M-PAG neurons are strongly activated by predator cues or during social contexts that suppress USV production in male and female mice. Further, we explored how vocal promoting and vocal suppressing drives are weighed in the brain to influence vocal production in male mice, where the drive and courtship function for USVs are better understood. We found that Amg C/M-PAG neurons receive monosynaptic inhibitory input from POA neurons that also project to the PAG, that these inhibitory inputs are active in USV-promoting social contexts, and that optogenetic activation of POA cell bodies that make divergent axonal projections to the amygdala and PAG is sufficient to elicit USV production in socially isolated male mice. Accordingly, Amg C/M-PAG neurons, along with POA PAG and PAG-USV neurons, form a nested hierarchical circuit in which environmental and social information converges to influence the decision to vocalize.
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Author Response
Reviewer #1 (Public Review):
This manuscript focuses on a set of neurons from the border between the central and medial amygdala (AMGc/m-PAG ) that project to neurons in the periaqueductal gray (PAG) that gate ultrasonic vocalizations (USVs). These neurons suppress vocal production and are active in contexts where vocalizations would be inappropriate (e.g. in the presence of predator cues, or aggressive encounters with conspecifics). They then further characterized these neurons, demonstrating that like in males, these neurons are GABAergic in females and in both sexes, half of these neurons express estrogen receptor alpha (Esr1). To examine the inputs into these neurons, the authors performed monosynaptically-restricted transsynaptic rabies tracing and identified numerous cortical and subcortical projections. Of …
Author Response
Reviewer #1 (Public Review):
This manuscript focuses on a set of neurons from the border between the central and medial amygdala (AMGc/m-PAG ) that project to neurons in the periaqueductal gray (PAG) that gate ultrasonic vocalizations (USVs). These neurons suppress vocal production and are active in contexts where vocalizations would be inappropriate (e.g. in the presence of predator cues, or aggressive encounters with conspecifics). They then further characterized these neurons, demonstrating that like in males, these neurons are GABAergic in females and in both sexes, half of these neurons express estrogen receptor alpha (Esr1). To examine the inputs into these neurons, the authors performed monosynaptically-restricted transsynaptic rabies tracing and identified numerous cortical and subcortical projections. Of particular interest, neurons from the preoptic area of the hypothalamus (POA) in addition to terminating on PAG-USV neurons also project to AMGc/m-PAG neurons. Imaging the terminals of these neurons revealed elevated activity during vocalization-promoting contexts and optogenetically stimulating them resulted in evoking USVs. Together, these experiments further identify and quantify a circuit incorporating external factors (e.g. predatory factors, social interactions) in the drive to produce vocalizations.
The authors are commended for use of male and female mice, demonstrating that even though they produce USVs in different social contexts, AMGc/m-PAG neurons share a function in suppressing USV production in both sexes. They do this convincingly with a variety of methodologies while incorporating appropriate controls (e.g. light-only and GFP-control in optogenetic experiments). The experiments are performed in a logical order and the data generated is elaborate.
We appreciate the reviewer’s commendations and for their appreciation of the convincing insights provided by our study. We provide detailed responses to their recommendations in the following section. We hope the reviewer finds these revisions satisfactorily address their concerns.
Reviewer #2 (Public Review):
The existence of PAG-USV-producing neurons has been recently established, alongside two independent pathways, POA->PAG, and AMG->PAG, that promote and inhibit the production of ultrasound vocalizations in female and male mice, respectively. Because vocalizations can be modulated in a variety of contexts, such as in the presence of a predator, the authors first show that the AMG->PAG pathway is activated in situations where mice stop vocalizing, such as in the presence of a predator or aggressive conspecifics, and can inhibit natural vocalizations in contexts where females vocalize (extending to their previous findings in male mice). Interestingly, AMG->PAG neurons also receive input from POA neurons that are known to promote vocalizations via their connection to PAG interneurons that inhibit PAG-USV-producing neurons. This POA->AMG and PAG pathway is inhibitory and therefore its capacity to promote vocalizations via these two parallel pathways might be achieved by its inhibition of AMG and PAG neurons that inhibit the PAG-USV producing neurons. While these results hint at possible mechanisms that could underlie the hierarchical control of vocalization, and how different external signals impinge on existing pathways to produce behavior flexibility, the study is missing important elements to draw such conclusions. Overall, the study is also missing important information on how experiments were performed.
We appreciate the reviewer’s efforts to evaluate our manuscript and provide constructive feedback. In the following section, we have responded to all the reviewer’s comments and concerns and provide all but one of the previously missing elements and information. We also maintain that the results and additional analysis we provide in this manuscript go beyond merely hinting at possible mechanisms, and instead provide explicit synaptic mechanisms by which vocal-promoting and vocal suppressing signals interact in the mouse’s brain.
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eLife assessment
Vocalizations are controlled by neural circuits connecting the amygdala and periaqueductal gray. This study presents valuable measures of the neurons that suppress vocalization in appropriate contexts using a rich variety of behavioural, imaging, optogenetic, and tracing methodologies. The evidence supporting the claims of the authors is solid, although their results only hint at the mechanisms that could underlie the hierarchical control of vocalization. The work will be of interest to neurobiologists working on motor control and vocalization.
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Reviewer #1 (Public Review):
This manuscript focuses on a set of neurons from the border between the central and medial amygdala (AMGc/m-PAG ) that project to neurons in the periaqueductal gray (PAG) that gate ultrasonic vocalizations (USVs). These neurons suppress vocal production and are active in contexts where vocalizations would be inappropriate (e.g. in the presence of predator cues, or aggressive encounters with conspecifics). They then further characterized these neurons, demonstrating that like in males, these neurons are GABAergic in females and in both sexes, half of these neurons express estrogen receptor alpha (Esr1). To examine the inputs into these neurons, the authors performed monosynaptically-restricted transsynaptic rabies tracing and identified numerous cortical and subcortical projections. Of particular interest, …
Reviewer #1 (Public Review):
This manuscript focuses on a set of neurons from the border between the central and medial amygdala (AMGc/m-PAG ) that project to neurons in the periaqueductal gray (PAG) that gate ultrasonic vocalizations (USVs). These neurons suppress vocal production and are active in contexts where vocalizations would be inappropriate (e.g. in the presence of predator cues, or aggressive encounters with conspecifics). They then further characterized these neurons, demonstrating that like in males, these neurons are GABAergic in females and in both sexes, half of these neurons express estrogen receptor alpha (Esr1). To examine the inputs into these neurons, the authors performed monosynaptically-restricted transsynaptic rabies tracing and identified numerous cortical and subcortical projections. Of particular interest, neurons from the preoptic area of the hypothalamus (POA) in addition to terminating on PAG-USV neurons also project to AMGc/m-PAG neurons. Imaging the terminals of these neurons revealed elevated activity during vocalization-promoting contexts and optogenetically stimulating them resulted in evoking USVs. Together, these experiments further identify and quantify a circuit incorporating external factors (e.g. predatory factors, social interactions) in the drive to produce vocalizations.
The authors are commended for use of male and female mice, demonstrating that even though they produce USVs in different social contexts, AMGc/m-PAG neurons share a function in suppressing USV production in both sexes. They do this convincingly with a variety of methodologies while incorporating appropriate controls (e.g. light-only and GFP-control in optogenetic experiments). The experiments are performed in a logical order and the data generated is elaborate.
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Reviewer #2 (Public Review):
The existence of PAG-USV-producing neurons has been recently established, alongside two independent pathways, POA->PAG, and AMG->PAG, that promote and inhibit the production of ultrasound vocalizations in female and male mice, respectively. Because vocalizations can be modulated in a variety of contexts, such as in the presence of a predator, the authors first show that the AMG->PAG pathway is activated in situations where mice stop vocalizing, such as in the presence of a predator or aggressive conspecifics, and can inhibit natural vocalizations in contexts where females vocalize (extending to their previous findings in male mice). Interestingly, AMG->PAG neurons also receive input from POA neurons that are known to promote vocalizations via their connection to PAG interneurons that inhibit …
Reviewer #2 (Public Review):
The existence of PAG-USV-producing neurons has been recently established, alongside two independent pathways, POA->PAG, and AMG->PAG, that promote and inhibit the production of ultrasound vocalizations in female and male mice, respectively. Because vocalizations can be modulated in a variety of contexts, such as in the presence of a predator, the authors first show that the AMG->PAG pathway is activated in situations where mice stop vocalizing, such as in the presence of a predator or aggressive conspecifics, and can inhibit natural vocalizations in contexts where females vocalize (extending to their previous findings in male mice). Interestingly, AMG->PAG neurons also receive input from POA neurons that are known to promote vocalizations via their connection to PAG interneurons that inhibit PAG-USV-producing neurons. This POA->AMG and PAG pathway is inhibitory and therefore its capacity to promote vocalizations via these two parallel pathways might be achieved by its inhibition of AMG and PAG neurons that inhibit the PAG-USV producing neurons. While these results hint at possible mechanisms that could underlie the hierarchical control of vocalization, and how different external signals impinge on existing pathways to produce behavior flexibility, the study is missing important elements to draw such conclusions. Overall, the study is also missing important information on how experiments were performed.
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