Chemogenetic stimulation of phrenic motor output and diaphragm activity

Reviewer #1 (Public Review):

Summary:

In this manuscript, the authors report that activation of excitatory DREADDs in the mid-cervical spinal cord can increase inspiratory activity in mice and rats. This is an important first step toward an ultimate goal of using this, or similar, technology to drive breathing in disorders associated with decreased respiratory motor output, such as spinal injury or neurodegenerative disease.

Strengths:

Strengths of this study include a comparison of non-specific DREADD expression in the mid-cervical spinal cord versus specific targeting to ChAT-positive neurons, and the measurement of multiple respiratory-related outcomes, including phrenic inspiratory output, diaphragm EMG activity, and ventilation. The data show convincingly that DREADDs can be used to drive phrenic inspiratory activity, which in turn increases diaphragm EMG activity and ventilation.

Weaknesses:

The main limitation is that the ligand, J60, was not given to control animals without spinal DREADD expression. Since J60 may have off-target effects (PMID: 37530882), a discussion of this limitation is warranted, particularly in light of the one rat that was reported to not have detectible mCherry expression in the mid-cervical spinal cord, yet had robust increases in diaphragm output after J60 administration.

In experiments in ChAT-Cre animals, several neuronal types will express DREADDs, including non-phrenic motor neurons and some interneurons. As such, these experiments do not specifically "target" phrenic motor neurons any more so than experiments in WT animals. Experiments in ChAT-Cre animals also do not avoid inducing "non-specific expression in the vicinity of the phrenic motor nucleus". This is not a study design flaw per se, but an overinterpretation of findings.

Reviewer #2 (Public Review):

Summary:

This study shows that when excitatory DREADD receptors are expressed in the ventral area of the cervical spinal cord containing phrenic motoneurons, the systemic administration of the DREADD ligand J60 increases diaphragm EMG activity without altering respiratory rate. The authors took a non-selective expression approach in wild-type mice, as well as a more selective Cre-dependent approach in Chat-Cre mice and Chat-Cre rats to stimulate cervical motoneurons in the spinal cord. This is a proof of principle study that supports the use of DREADD technology to stimulate the motor output to the diaphragm.

Strengths:

The strengths of the study lie in the use of both mice and rats and testing activation of diaphragmatic activity with multiple experimental approaches to show that diaphragm EMG and tidal volume are increased.

Weaknesses:

Weaknesses of the study consist in the lack of some important control experiments to consolidate the findings: a test of DREADD ligand effects in the absence of viral construct; repeated respiratory challenges within the same recording session in whole body plethysmographs that could compromise the behaving experiments; and lastly, a limited qualitative analysis of the histological data that does not allow for confirmation of expression in phrenic motoneurons.