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Background A pseudotyped modified rabies virus lacking the rabies glycoprotein (G-protein), which is crucial for transsynaptic spread, can be used for monosynaptic retrograde tracing. By coupling the pseudotyped virus with transgene expression of the G-protein and the avian leukosis and sarcoma virus subgroup A receptor (TVA), which is necessary for cell entry of the virus, researchers can investigate specific neuronal populations. Responder mouse lines, like the R phi GT mouse line, carry the genes encoding the G-protein and TVA under Cre-dependent expression. These mouse lines are valuable tools because they reduce the number of viral injections needed compared to when using helper viruses. Since R phi GT mice do not express Cre themselves, introducing the pseudotyped rabies virus into their brain should not result in viral cell entry or spread. Results We present a straightforward flowchart for adequate controls in tracing experiments, which we employed to demonstrate Cre-independent expression of TVA in R phi GT mice. Conclusions Our observations revealed TVA leakage, indicating that R phi GT mice should be used with caution for transgene expression of TVA. Inaccurate tracing outcomes may occur if TVA is expressed in the absence of Cre since background leakage leads to nonspecific cell entry. Moreover, conducting appropriate control experiments can identify the source of potential caveats in virus-based neuronal tracing experiments.

In the spinal cord, sensory-motor circuits controlling motor activity are situated in the dorso-ventral interface. The neurons identified by the expression of the transcription factor Doublesex and mab-3 related transcription factor 3 (Dmrt3) have previously been associated with the coordination of locomotion in horses (Equus caballus, Linnaeus, 1758), mice (Mus musculus, Linnaeus, 1758), and zebrafish (Danio rerio, F. Hamilton, 1822). Based on earlier studies, we hypothesized that, in mice, these neurons may be positioned to receive sensory and central inputs to relay processed commands to motor neurons. Thus, we investigated the presynaptic inputs to spinal Dmrt3 neurons using monosynaptic retrograde replication-deficient rabies tracing. The analysis showed that lumbar Dmrt3 neurons receive inputs from intrasegmental neurons, and intersegmental neurons from the cervical, thoracic, and sacral segments. Some of these neurons belong to the excitatory V2a interneurons and to plausible Renshaw cells, defined by the expression of Chx10 and calbindin, respectively. We also found that proprioceptive primary sensory neurons of type Ia2, Ia3, and Ib, defined by the expression of calbindin, calretinin, and Brn3c, respectively, provide presynaptic inputs to spinal Dmrt3 neurons. In addition, we demonstrated that Dmrt3 neurons receive inputs from brain areas involved in motor regulation, including the red nucleus, primary sensory-motor cortex, and pontine nuclei. In conclusion, adult spinal Dmrt3 neurons receive inputs from motor-related brain areas as well as proprioceptive primary sensory neurons and have been shown to connect directly to motor neurons. Dmrt3 neurons are thus positioned to provide sensory-motor control and their connectivity is suggestive of the classical reflex pathways present in the spinal cord.

Nicotinic acetylcholine receptors (nAChRs) play crucial roles in various human disorders, with the alpha 7, alpha 4, alpha 6, and alpha 3-containing nAChR subtypes extensively studied in relation to conditions such as Alzheimer's disease, Parkinson's disease, nicotine dependence, mood disorders, and stress disorders. In contrast, the alpha 2-nAChR subunit has received less attention due to its more restricted expression and the scarcity of specific agonists and antagonists for studying its function. Nevertheless, recent research has shed light on the unique expression pattern of the Chrna2 gene, which encodes the alpha 2-nAChR subunit, and its involvement in distinct populations of inhibitory interneurons. This review highlights the structure, pharmacology, localization, function, and disease associations of alpha 2-containing nAChRs and points to the unique expression pattern of the Chrna2 gene and its role in different inhibitory interneuron populations. These populations, including the oriens lacunosum moleculare (OLM) cells in the hippocampus, Martinotti cells in the neocortex, and Renshaw cells in the spinal cord, share common features and contribute to recurrent inhibitory microcircuits. Thus, the alpha 2-nAChR subunit's unique expression pattern in specific interneuron populations and its role in recurrent inhibitory microcircuits highlight its importance in various physiological processes. Further research is necessary to uncover the comprehensive functionality of alpha 2-containing nAChRs, delineate their specific contributions to neuronal circuits, and investigate their potential as therapeutic targets for related disorders.

Identifying the spinal circuits controlling locomotion is critical for unravelling the mechanisms controlling the production of gaits. Development of the circuits governing left-right coordination relies on axon guidance molecules such as ephrins and netrins. To date, no other class of proteins have been shown to play a role during this process. Here, we have analyzed hop mice, which walk with a characteristic hopping gait using their hindlimbs in synchrony. Fictive locomotion experiments suggest that a local defect in the ventral spinal cord contributes to the aberrant locomotor phenotype. Hop mutant spinal cords had severe morphologic defects, including the absence of the ventral midline and a poorly defined border between white and gray matter. The hop mice represent the first model where, exclusively found in the lumbar domain, the left and right components of the central pattern generators (CPGs) are fused with a synchronous hindlimb gait as a functional consequence. These defects were associated with abnormal developmental processes, including a misplaced notochord and reduced induction of ventral progenitor domains. Whereas the underlying mutation in hop mice has been suggested to lie within the Ttc26 gene, other genes in close vicinity have been associated with gait defects. Mouse embryos carrying a CRISPR replicated point mutation within Ttc26 displayed an identical morphologic phenotype. Thus, our data suggest that the assembly of the lumbar CPG network is dependent on fully functional TTC26 protein.