Gaining insights into the internal states of the rodent brain through vocal communications

Differential neuronal functions of LNX1 and LNX2 revealed by behavioural analysis in single and double knockout mice

BACKGROUND

Ligand of NUMB protein-X 1 (LNX1) and LNX2 proteins are closely related PDZ domain-containing E3 ubiquitin ligases that interact with and potentially modulate numerous synaptic and neurodevelopmentally important proteins. While both LNX1 and LNX2 are expressed in neurons, it is noteworthy that neuronal LNX1 isoforms lack the catalytic domain responsible for ubiquitination of substrates. Thus, the shared interaction partners of LNX1 and LNX2 might be differentially regulated by these proteins, with LNX1 acting as a stabilizing scaffold while LNX2 may promote their ubiquitination and degradation. Despite the identification of many LNX interacting proteins and substrates, our understanding of the distinct in vivo functions of LNX1 and LNX2 remains very incomplete.

RESULTS

We previously reported that mice lacking both LNX1 in the central nervous system and LNX2 globally exhibit decreased anxiety-related behaviour. Here we significantly extend this work by examining anxiety-related and risk-taking behaviours in Lnx1-/- and Lnx2-/- single knockout animals for the first time and by analysing previously unexplored aspects of behaviour in both single and double knockout animals. While the absence of both LNX1 and LNX2 contributes to the decreased anxiety-related behaviour of double knockout animals in the open field and elevated plus maze tests, the elimination of LNX2 plays a more prominent role in altered behaviour in the dark-light emergence test and wire beam bridge risk-taking paradigms. By contrast, Lnx knockout mice of all genotypes were indistinguishable from wildtype animals in the marble burying, stress-induced hyperthermia and novel object recognition tests. Analysis of the ultrasonic vocalizations of pups following maternal separation revealed significant differences in call properties and vocal repertoire for Lnx1-/- and Lnx1-/-;Lnx2-/- double knockout animals. Finally, decreased body weight previously noted in double knockout animals could be attributed largely to Lnx1 gene knockout.

CONCLUSIONS

These results identify specific roles of LNX1 and LNX2 proteins in modulating distinct aspects of anxiety and risk-taking behaviour and social communication in mice. They also reveal an unexpected role for neuronally expressed LNX1 isoforms in determining body weight. These novel insights into the differential neuronal functions of LNX1 and LNX2 proteins provide a foundation for mechanistic studies of these phenomena.

Characterization of Ultrasonic Vocalization-Modulated Neurons in Rat Motor Cortex Based on Their Activity Modulation and Axonal Projection to the Periaqueductal Gray

Vocalization, a means of social communication, is prevalent among many species, including humans. Both rats and mice use ultrasonic vocalizations (USVs) in various social contexts and affective states. The motor cortex is hypothesized to be involved in precisely controlling USVs through connections with critical regions of the brain for vocalization, such as the periaqueductal gray matter (PAG). However, it is unclear how neurons in the motor cortex are modulated during USVs. Moreover, the relationship between USV modulation of neurons and anatomical connections from the motor cortex to PAG is also not clearly understood. In this study, we first characterized the activity patterns of neurons in the primary and secondary motor cortices during emission of USVs in rats using large-scale electrophysiological recordings. We also examined the axonal projection of the motor cortex to PAG using retrograde labeling and identified two clusters of PAG-projecting neurons in the anterior and posterior parts of the motor cortex. The neural activity patterns around the emission of USVs differed between the anterior and posterior regions, which were divided based on the distribution of PAG-projecting neurons in the motor cortex. Furthermore, using optogenetic tagging, we recorded the USV modulation of PAG-projecting neurons in the posterior part of the motor cortex and found that they showed predominantly sustained excitatory responses during USVs. These results contribute to our understanding of the involvement of the motor cortex in the generation of USV at the neuronal and circuit levels.

Playback of rat 22-kHz ultrasonic vocalizations as a translational assay of negative affective states: An analysis of evoked behavior and brain activity

The subjective nature of human emotions makes them uniquely challenging to investigate in preclinical models. While behavioral assays in rodents aim to evaluate affect (i.e., anxiety, hypervigilance), they often lack ethological validity. Playback of negatively valenced 22-kHz ultrasonic vocalizations (USVs) in rats shows promise as a translational tool to investigate affective processing. Much like how human facial expressions can communicate internal states, rats emit 22-kHz USVs that similarly convey negative affective states to conspecifics indicating possible threat. 22-kHz USV playback elicits avoidance and hypervigilant behaviors, and recruit brain regions comparable to those seen in human brains evoked by viewing fearful faces. Indeed, 22-kHz playback alters neural activity in brain regions associated with negative valence systems (i.e., amygdala, bed nucleus of the stria terminalis, periaqueductal gray) alongside increases in behaviors typically associated with anxiety. Here, we present evidence from the literature that supports leveraging 22-kHz USV playback in rat preclinical models to obtain clinically relevant and translational findings to identify the neural underpinnings of affective processing and neuropathological dysfunction.