Roger L. Redondo

Neuromedin S signaling has recently been identified as a modulator of both sleep and circadian rhythms within the mammalian central nervous system. However, the involvement of its preferred receptor, NMUR2, remains elusive. Following extensive profiling and refinement, we utilize R-PSOP, its only selective antagonist, to elucidate NMUR2 function in the mouse brain. We show that NMUR2 is essential for physiological sleep architecture under baseline conditions, as well as for responses to homeostatic and circadian challenges. We demonstrate that NMUR2 antagonism produces opposing effects on time spent asleep depending on the circadian phase of administration, while its effects on EEG spectral activity and locomotion persist across conditions. Combining pharmacological, neuronal-tagging, brain-clearing and chemogenetic tools, we functionally implicate a subset of PVH neurons whose activation unexpectedly promotes sleep. Together, our observations establish NMUR2 as a promising therapeutic target for the treatment of sleep and circadian neurological disorders.

Sleep disturbances and altered sensory processing are key features of neurodevelopmental and neuropsychiatric disorders (NDDs/NPDs). Clinical assessments of brain function in these conditions reveal increased connectivity between the thalamus and cortical areas, alongside changes in restorative sleep patterns that rely on thalamic function, like decreased sleep spindle density and slow-wave oscillations. To assess the role of individual thalamic nuclei in such impairments, we activated thalamic nuclei within three different thalamocortical circuits in rats using the chemogenetic receptor hM3Dq. Activation of the mediodorsal thalamic (MDT) nuclei impaired restorative sleep by reducing spindle density and slow-wave oscillations, recapitulating phenotypes associated with NDDs/NPDs. Conversely, activating ventral posterior thalamic (VPT) and ventromedial thalamic (VMT) nuclei increased wakefulness, reducing sleep spindle density and alpha/beta band power during sleep. To examine downstream network effects, we used perfusion-based pharmacological MRI; MDT activation significantly modulated neural activity in MDT and interconnected regions, including prelimbic cortex and thalamic nuclei. Finally, we tested whether pharmacologically increasing inhibition in the thalamus using the GABA_Aδ agonist gaboxadol and positive allosteric modulator DS-2, could counter phenotypes driven by thalamic hyperactivity. Gaboxadol partially normalized the balance between light and deep NREM sleep in the MDT-hM3Dq group, and in VPT- and VMT-hM3Dq animals, partially normalized resting-state oscillations but failed to rescue sleep physiology. DS-2 had minimal effects across groups. Collectively, our findings reveal that thalamic nuclei-specific activation impacts restorative sleep, and that this thalamocortical circuit model may provide valuable insights into the neural mechanisms underlying dysfunctions associated with NDDs/NPDs and neuropsychiatric conditions.

Abnormal sensory processing is core to neuropsychiatric and neurodevelopmental disorders, such as schizophrenia and autism spectrum disorders. Developing efficient therapies requires understanding the basic sensory pathways and identifying circuit abnormalities during early development. Auditory brainstem responses (ABRs) are well-established biomarkers for auditory processing on the brainstem level. Beyond their advantage of being easily applicable in clinics (given their non-invasive nature), ABRs have high reproducibility in rodents and translate well to humans (e.g. wave identity), despite species differences (e.g. wave features). We hypothesized that ABRs would reveal sensory abnormalities in neurodevelopmental models with construct validity, such as Neurexin1α knockout (Nrxn1α KO) rats during their development. In a previous study, adult Nrxn1α KO rats showed altered cortical auditory-evoked potentials and impaired prediction error to auditory stimuli (Janz in Transl Psychiat, 12:455, 2022 ). This study used ABR measurements to assess brainstem physiology during auditory processing in Nrxn1α KO rats and their wild-type littermates. Therefore, we followed the development trajectories of ABRs from the age of 3 weeks to 12 weeks longitudinally. We found that juvenile Nrxn1α KO rats (3 weeks of age) show altered ABRs, which normalized during further development. This alteration was confined to increased latency in waves II, III, and IV of the ABRs, suggesting impaired auditory processing on the level of the superior olivary complex and inferior colliculus. In conclusion, our results suggest that early but transient deficits in the processing of auditory information on the level of the brainstem are present in Nrxn1α KO rats, which may contribute to later cortical auditory processing deficits observed in adulthood. Our study emphasizes the value of ABRs as a functional readout of auditory brainstem circuit function with potential value as a translational biomarker.

Aberrant neuronal circuit dynamics are at the core of complex neuropsychiatric disorders, such as schizophrenia (SZ). Clinical assessment of the integrity of neuronal circuits in SZ has consistently described aberrant resting-state gamma oscillatory activity, decreased auditory-evoked gamma responses, and abnormal mismatch responses. We hypothesized that corticothalamic circuit manipulation could recapitulate SZ circuit phenotypes in rodent models. In this study, we optogenetically inhibited the mediodorsal thalamus-to-prefrontal cortex (MDT-to-PFC) or the PFC-to-MDT projection in rats and assessed circuit function through electrophysiological readouts. We found that MDT–PFC perturbation could not recapitulate SZ-linked phenotypes such as broadband gamma disruption, altered evoked oscillatory activity, and diminished mismatch negativity responses. Therefore, the induced functional impairment of the MDT–PFC pathways cannot account for the oscillatory abnormalities described in SZ.

Sensory processing in the auditory brainstem can be studied with auditory brainstem responses (ABRs) across species. There is, however, a limited understanding of ABRs as tools to assess the effect of pharmacological interventions. Therefore, we set out to understand how pharmacological agents that target key transmitter systems of the auditory brainstem circuitry affect ABRs in rats. Given previous studies, demonstrating that Nrxn1α KO Sprague Dawley rats show substantial auditory processing deficits and altered sensitivity to GABAergic modulators, we used both Nrxn1α KO and wild-type littermates in our study. First, we probed how different commonly used anesthetics (isoflurane, ketamine/xylazine, medetomidine) affect ABRs. In the next step, we assessed the effects of different pharmacological compounds (diazepam, gaboxadol, retigabine, nicotine, baclofen, and bitopertin) either under isoflurane or medetomidine anesthesia. We found that under our experimental conditions, ABRs are largely unaffected by diverse pharmacological modulation. Significant modulation was observed with (i) nicotine, affecting the late ABRs components at 90 dB stimulus intensity under isoflurane anesthesia in both genotypes and (ii) retigabine, showing a slight decrease in late ABRs deflections at 80 dB stimulus intensity, mainly in isoflurane anesthetized Nrxn1α KO rats. Our study suggests that ABRs in anesthetized rats are resistant to a wide range of pharmacological modulators, which has important implications for the applicability of ABRs to study auditory brainstem physiology.

Sensory processing in the auditory brainstem can be studied with auditory brainstem responses (ABRs) across species. Although ABRs have been widely utilized to evaluate abnormalities in auditory brainstem physiology, there is limited understanding if ABRs can be useful tool to assess the effect of pharmacological interventions. Therefore, we set out to understand how pharmacological agents that target key transmitter systems of the auditory brainstem circuitry affect ABR physiology in rats. Given previous studies, demonstrating that Nrxn1α KO Sprague Dawley rats show substantial auditory processing deficits and altered sensitivity to GABAergic modulators, we used both Nrxn1α KO and wildtype littermates in our study. First, we probed how different commonly used anesthetics (isoflurane, ketamine/xylazine, medetomidine) affect ABRs waveforms. In the next step, we assessed the effects of different pharmacological compounds (diazepam, gaboxadol, retigabine, nicotine, baclofen and bitopertin) either under isoflurane or medetomidine anesthesia. We found that under our experimental conditions, ABRs are largely unaffected by diverse pharmacological modulation. Significant modulation was observed with i.) nicotine, affecting the late ABR components at 90 dB stimulus intensity under isoflurane anesthesia in both genotypes, and ii.) retigabine, showing a slight decrease in late ABRs deflections at 80 dB stimulus intensity, mainly in isoflurane-anesthetized Nrxn1α KO rats. Our study suggest that ABRs in anesthetized rats are resistant to a wide range of pharmacological modulators, which has important implications for the applicability of ABRs to study auditory brainstem physiology.

Brain function depends on complex circuit interactions between excitatory and inhibitory neurons embedded in local and long-range networks. Systemic GABAA-receptor (GABAAR) or NMDA-receptor (NMDAR) modulation alters the excitatory-inhibitory balance (EIB), measurable with electroencephalography (EEG). However, EEG signatures are complex in localization and spectral composition. We developed and applied analytical tools to investigate the effects of two EIB modulators, MK801 (NMDAR antagonist) and diazepam (GABAAR modulator), on periodic and aperiodic EEG features in freely-moving male Sprague Dawley rats. We investigated how, across three brain regions, EEG features are correlated with EIB modulation. We found that the periodic component was composed of seven frequency bands that presented region-dependent and compound-dependent changes. The aperiodic component was also different between compounds and brain regions. Importantly, the parametrization into periodic and aperiodic components unveiled correlations between quantitative EEG and plasma concentrations of pharmacological compounds. MK-801 exposures were positively correlated with the slope of the aperiodic component. Concerning the periodic component, MK-801 exposures correlated negatively with the peak frequency of low-γ oscillations but positively with those of high-γ and high-frequency oscillations (HFOs). As for the power, θ and low-γ oscillations correlated negatively with MK-801, whereas mid-γ correlated positively. Diazepam correlated negatively with the knee of the aperiodic component, positively to β and negatively to low-γ oscillatory power, and positively to the modal frequency of θ, low-γ, mid-γ, and high-γ. In conclusion, correlations between exposures and pharmacodynamic effects can be better-understood thanks to the parametrization of EEG into periodic and aperiodic components. Such parametrization could be key in functional biomarker discovery.

Cognitive deficits and impaired sensory processing are hallmarks of several neurodevelopmental and neuropsychiatric disorders. N‐methyl‐d‐aspartate receptor (NMDAR) hypofunction contributes to these deficits by disrupting the excitation‐to‐inhibition balance in neuronal networks. Although preclinical data suggest that the activation of gamma‐Aminobutyric acid B receptors (GABA_BR) may restore excitation‐to‐inhibition balance and rescues some behavioral deficits, GABA_BR agonists have failed to meet their clinical study endpoints, suggesting more complex interactions at play. Here, we studied the effects of Baclofen (a GABA_BR agonist) and MK‐801 (a non‐competitive NMDAR antagonist) on the neurophysiology of limbic‐auditory circuits in freely‐moving rats. The pharmacological effects were assessed using resting‐state EEG, auditory‐evoked oscillation, and mismatch negativity paradigms. MK‐801 elevated resting‐state oscillatory power, mainly in the gamma and higher frequency ranges, and impaired auditory‐evoked responses. Baclofen partially normalized resting‐state oscillations but failed to rescue auditory‐evoked oscillatory abnormalities. Coherence analysis indicated that NMDAR hypofunction alters the functional coupling of limbic and thalamocortical circuits in several frequency bands. Baclofen normalized only a fraction of MK‐801‐induced abnormalities (e.g., theta coherence between frontal cortex and amygdala) while reducing delta‐theta and augmenting gamma coherence in thalamocortical circuits. Finally, we report that Baclofen intensified the MK‐801‐induced deficits in auditory mismatch responses. In summary, while Baclofen partially normalizes MK‐801‐induced gamma abnormalities, it either fails to rescue or exacerbates deficits in other phenotypes like functional coupling and auditory processing. We hope that the presented complex interactions between pharmacologically induced NMDAR hypofunction and GABABR agonism inspire a new understanding of the therapeutic potential around GABAergic modulation. image