The efficacy of intranasal leptin for opioid-induced respiratory depression depends on sex and obesity state

Introduction: Opioid-induced respiratory depression (OIRD) is the primary cause of death associated with opioids and individuals with obesity are particularly susceptible due to comorbid obstructive sleep apnea (OSA). Repeated exposure to opioids, as in the case of pain management, results in diminished therapeutic effect and/or the need for higher doses to maintain the same effect. With limited means to address the negative impact of repeated exposure it is critical to develop drugs that prevent deaths induced by opioids without reducing beneficial analgesia. Methods: We hypothesized that OIRD as a result of chronic opioid use can be attenuated by administration of IN leptin while also maintaining analgesia in both lean mice and mice with diet-induced obesity (DIO) of both sexes. To test this hypothesis, an opioid tolerance protocol was developed and a model of OIRD in mice chronically receiving morphine and tolerant to morphine analgesia was established. Subsequently, breathing was recorded by barometric plethysmography in four experimental groups: obese male, obese female, lean male, and lean female following acute administration of IN leptin. Respiratory data were complemented with measures of arterial blood gas. Operant behavioral assays were used to determine the impact of IN leptin on the analgesic efficacy of morphine. Results: Acute administration of IN leptin significantly attenuated OIRD in DIO male mice decreasing the apnea index by 58.9% and apnea time by 60.1%. In lean mice leptin was ineffective. Blood gas measures confirmed the effectiveness of IN leptin for preventing respiratory acidosis in DIO male mice. However, IN leptin was not effective in lean mice of both sexes and appeared to exacerbate acid-base disturbances in DIO female mice. Additionally, morphine caused a complete loss of temperature aversion which was not reduced by intranasal leptin indicating IN leptin does not decrease morphine analgesia. Discussion: IN leptin effectively treated OIRD in morphine-tolerant DIO male mice without impacting analgesia. In contrast, IN leptin had no effect in lean mice of either sex or DIO female mice. The arterial blood gas data were consistent with ventilatory findings showing that IN leptin reversed morphine-induced respiratory acidosis only in DIO male mice but not in other mouse groups. Finally, a hypercapnic sensitivity study revealed that IN leptin rescued minute ventilation under hypercapnic conditions only in DIO male mice, which suggests that differential responses to IN leptin are attributable to different leptin sensitivities depending on sex and the obesity status.

Optogenetic activation of the tongue in spontaneously breathing mice

Inadequate tongue muscle activation contributes to dysarthria, dysphagia, and obstructive sleep apnea. Thus, treatments which increase tongue muscle activity have potential clinical benefit. We hypothesized that lingual injection of an adeno-associated virus (AAV) encoding channelrhodopsin-2 (ChR2) would enable light-induced activation of tongue motor units during spontaneous breathing. An AAV serotype 9 vector (pACAGW-ChR2-Venus-AAV9, 8.29 × 1011 vg) was injected to the posterior tongue in adult C57BL/6J mice. After 12 weeks, mice were anesthetized and posterior tongue electromyographic (EMG) activity was recorded during spontaneous breathing; a light source was positioned near the injection site. Light-evoked EMG responses increased with the intensity and duration of pulses. Stimulus trains (250 ms) evoked EMG bursts that were comparable to endogenous (inspiratory) tongue muscle activation. Histology confirmed lingual myofiber transgene expression. We conclude that intralingual AAV9-ChR2 delivery enables light evoked lingual EMG activity. These proof-of-concept studies lay the groundwork for clinical application of this novel approach to lingual therapeutics.

Chemogenetic activation of hypoglossal motoneurons in a mouse model of Pompe disease

Pompe disease is a lysosomal storage disease resulting from absence or deficiency of acid α-glucosidase (GAA). Tongue-related disorders including dysarthria, dysphagia, and obstructive sleep apnea are common in Pompe disease. Our purpose was to determine if designer receptors exclusively activated by designer drugs (DREADDs) could be used to stimulate tongue motor output in a mouse model of Pompe disease. An adeno-associated virus serotype 9 (AAV9) encoding an excitatory DREADD (AAV9-hSyn-hM3D(Gq)-mCherry, 2.44 × 1010 vg) was administered to the posterior tongue of 5-7-wk-old Gaa null (Gaa-/-) mice. Lingual EMG responses to intraperitoneal injection of saline or a DREADD ligand (JHU37160-dihydrochloride, J60) were assessed 12 wk later during spontaneous breathing. Saline injection produced no consistent changes in lingual EMG. Following the DREADD ligand, there were statistically significant (P < 0.05) increases in both tonic and phasic inspiratory EMG activity recorded from the posterior tongue. Brainstem histology confirmed mCherry expression in hypoglossal (XII) motoneurons in all mice, thus verifying retrograde movement of the AAV9 vector. Morphologically, Gaa-/- XII motoneurons showed histological characteristics that are typical of Pompe disease, including an enlarged soma and vacuolization. We conclude that lingual delivery of AAV9 can be used to drive functional expression of DREADD in XII motoneurons in a mouse model of Pompe disease.NEW & NOTEWORTHY In a mouse model of Pompe disease, lingual injection of adeno-associated virus (AAV) serotype 9 encoding DREADD was histologically verified to produce transgene expression in hypoglossal motoneurons. Subsequent intraperitoneal delivery of a DREADD ligand stimulated tonic and phase tongue motor output.In a mouse model of Pompe disease, lingual injection of adeno-associated virus (AAV) serotype 9 encoding DREADD was histologically verified to produce transgene expression in hypoglossal motoneurons. Subsequent intravenous delivery of a DREADD ligand stimulated tonic and phase tongue motor output.

Gene delivery to the hypoglossal motor system: preclinical studies and translational potential

Dysfunction and/or reduced activity in the tongue muscles contributes to conditions such as dysphagia, dysarthria, and sleep disordered breathing. Current treatments are often inadequate, and the tongue is a readily accessible target for therapeutic gene delivery. In this regard, gene therapy specifically targeting the tongue motor system offers two general strategies for treating lingual disorders. First, correcting tongue myofiber and/or hypoglossal (XII) motoneuron pathology in genetic neuromuscular disorders may be readily achieved by intralingual delivery of viral vectors. The retrograde movement of viral vectors such as adeno-associated virus (AAV) enables targeted distribution to XII motoneurons via intralingual viral delivery. Second, conditions with impaired or reduced tongue muscle activation can potentially be treated using viral-driven chemo- or optogenetic approaches to activate or inhibit XII motoneurons and/or tongue myofibers. Further considerations that are highly relevant to lingual gene therapy include (1) the diversity of the motoneurons which control the tongue, (2) the patterns of XII nerve branching, and (3) the complexity of tongue muscle anatomy and biomechanics. Preclinical studies show considerable promise for lingual directed gene therapy in neuromuscular disease, but the potential of such approaches is largely untapped.

Motor transfer from the corticospinal to the corticobulbar pathway

There are multiple descending neural pathways, including the corticospinal pathway (CS) and the corticobulbar pathway (CB). The corticospinal pathway has been shown to exhibit within-pathway (CS-to-CS) motor transfer. However, motor transfer across each pathway (CS-to-CB or CB-to-CS) has yet to be studied in depth. The aim of the present study was to examine the effects of cross-pathway motor transfer between the ankle (CS) and tongue (CB) after training with a ballistic goal-directed motor task. Twelve healthy participants were recruited for this two-day experimental study. Six participants performed a ballistic goal-directed task with their ankle on Day 1 (ankle dorsiflexion), then tongue on Day 2 (elevate tongue against IOPI). The other 6 participants performed the same task with their tongue on Day 1, then ankle on Day 2. Both the ankle and tongue tasks (50 trials each) required matching force and time to a visual target. Our findings indicate that participants who underwent ankle training on Day 1 exhibited decreased tongue force error on Day 2 compared with participants who completed the tongue training on Day 1, with no prior ankle training (p = 0.02) (i.e. greater accuracy). This finding suggests that cross-pathway transfer from the corticospinal pathway to the corticobulbar pathway occurred with respect to force error. In other words, training of the ankle (CS) translated to improved training performance of the tongue (CB) through a reduction in force error. However, the reverse was not true - training the tongue did not elicit improved performance of the ankle. Nonetheless, if training with the corticospinal pathway can lead to improved corticobulbar pathway functioning, incorporating multi-pathway rehabilitation techniques might be valuable for clinicians across medical disciplines.

Dysphagia Management in Acute and Sub-acute Stroke

Swallowing dysfunction is common after stroke. More than 50% of the 665 thousand stroke survivors will experience dysphagia acutely of which approximately 80 thousand will experience persistent dysphagia at 6 months. The physiologic impairments that result in post-stroke dysphagia are varied. This review focuses primarily on well-established dysphagia treatments in the context of the physiologic impairments they treat. Traditional dysphagia therapies including volume and texture modifications, strategies such as chin tuck, head tilt, head turn, effortful swallow, supraglottic swallow, super-supraglottic swallow, Mendelsohn maneuver and exercises such as the Shaker exercise and Masako (tongue hold) maneuver are discussed. Other more recent treatment interventions are discussed in the context of the evidence available.