Characterizing the Access of Cholinergic Antagonists to Efferent Synapses in the Inner Ear

Stimulation of cholinergic efferent neurons innervating the inner ear has profound, well-characterized effects on vestibular and auditory physiology, after activating distinct ACh receptors (AChRs) on afferents and hair cells in peripheral endorgans. Efferent-mediated fast and slow excitation of vestibular afferents are mediated by α4β2*-containing nicotinic AChRs (nAChRs) and muscarinic AChRs (mAChRs), respectively. On the auditory side, efferent-mediated suppression of distortion product otoacoustic emissions (DPOAEs) is mediated by α9α10nAChRs. Previous characterization of these synaptic mechanisms utilized cholinergic drugs, that when systemically administered, also reach the CNS, which may limit their utility in probing efferent function without also considering central effects. Use of peripherally-acting cholinergic drugs with local application strategies may be useful, but this approach has remained relatively unexplored. Using multiple administration routes, we performed a combination of vestibular afferent and DPOAE recordings during efferent stimulation in mouse and turtle to determine whether charged mAChR or α9α10nAChR antagonists, with little CNS entry, can still engage efferent synaptic targets in the inner ear. The charged mAChR antagonists glycopyrrolate and methscopolamine blocked efferent-mediated slow excitation of mouse vestibular afferents following intraperitoneal, middle ear, or direct perilymphatic administration. Both mAChR antagonists were effective when delivered to the middle ear, contralateral to the side of afferent recordings, suggesting they gain vascular access after first entering the perilymphatic compartment. In contrast, charged α9α10nAChR antagonists blocked efferent-mediated suppression of DPOAEs only upon direct perilymphatic application, but failed to reach efferent synapses when systemically administered. These data show that efferent mechanisms are viable targets for further characterizing drug access in the inner ear.

Review of Pharmacotherapy for Tinnitus

Various medications are currently used in the treatment of tinnitus, including anesthetics, antiarrhythmics, anticonvulsants, antidepressants, antihistamines, antipsychotics, anxiolytics, calcium channel blockers, cholinergic antagonists, NMDA antagonists, muscle relaxants, vasodilators, and vitamins. To date, however, no medications have been specifically approved to treat tinnitus by the US Food and Drug Administration (FDA). In addition, medicines used to treat other diseases, as well as foods and other ingested materials, can result in unwanted tinnitus. These include alcohol, antineoplastic chemotherapeutic agents and heavy metals, antimetabolites, antitumor agents, antibiotics, caffeine, cocaine, marijuana, nonnarcotic analgesics and antipyretics, ototoxic antibiotics and diuretics, oral contraceptives, quinine and chloroquine, and salicylates. This review, therefore, describes the medications currently used to treat tinnitus, including their mechanisms of action, therapeutic effects, dosages, and side-effects. In addition, this review describes the medications, foods, and other ingested agents that can induce unwanted tinnitus, as well as their mechanisms of action.

The mammalian efferent vestibular system utilizes cholinergic mechanisms to excite primary vestibular afferents

Electrical stimulation of the mammalian efferent vestibular system (EVS) predominantly excites primary vestibular afferents along two distinct time scales. Although roles for acetylcholine (ACh) have been demonstrated in other vertebrates, synaptic mechanisms underlying mammalian EVS actions are not well-characterized. To determine if activation of ACh receptors account for efferent-mediated afferent excitation in mammals, we recorded afferent activity from the superior vestibular nerve of anesthetized C57BL/6 mice while stimulating EVS neurons in the brainstem, before and after administration of cholinergic antagonists. Using a normalized coefficient of variation (CV*), we broadly classified vestibular afferents as regularly- (CV* < 0.1) or irregularly-discharging (CV* > 0.1) and characterized their responses to midline or ipsilateral EVS stimulation. Afferent responses to efferent stimulation were predominantly excitatory, grew in amplitude with increasing CV*, and consisted of fast and slow components that could be identified by differences in rise time and post-stimulus duration. Both efferent-mediated excitatory components were larger in irregular afferents with ipsilateral EVS stimulation. Our pharmacological data show, for the first time in mammals, that muscarinic AChR antagonists block efferent-mediated slow excitation whereas the nicotinic AChR antagonist DHβE selectively blocks efferent-mediated fast excitation, while leaving the efferent-mediated slow component intact. These data confirm that mammalian EVS actions are predominantly cholinergic.

A Novel Bioengineered Functional Motor Unit Platform to Study Neuromuscular Interaction

Background: In many neurodegenerative and muscular disorders, and loss of innervation in sarcopenia, improper reinnervation of muscle and dysfunction of the motor unit (MU) are key pathogenic features. In vivo studies of MUs are constrained due to difficulties isolating and extracting functional MUs, so there is a need for a simplified and reproducible system of engineered in vitro MUs. Objective: to develop and characterise a functional MU model in vitro, permitting the analysis of MU development and function. Methods: an immortalised human myoblast cell line was co-cultured with rat embryo spinal cord explants in a serum-free/growth fact media. MUs developed and the morphology of their components (neuromuscular junction (NMJ), myotubes and motor neurons) were characterised using immunocytochemistry, phase contrast and confocal microscopy. The function of the MU was evaluated through live observations and videography of spontaneous myotube contractions after challenge with cholinergic antagonists and glutamatergic agonists. Results: blocking acetylcholine receptors with α-bungarotoxin resulted in complete, cessation of myotube contractions, which was reversible with tubocurarine. Furthermore, myotube activity was significantly higher with the application of L-glutamic acid. All these observations indicate the formed MU are functional. Conclusion: a functional nerve-muscle co-culture model was established that has potential for drug screening and pathophysiological studies of neuromuscular interactions.

SEASICKNESS - CURRENT STATE OF PREVENTION AND TREATMENT ISSUE

The paper describes the current state of development of seasickness as one of movement disease variants. The given type of pathology occurs when combining different types of ship’s motion (rolling and pitching). Circular, vertical and slow movements induce more pronounced and frequent signs of seasickness than linear, horizontal and quick ones. In the view of majority of researchers, the most likely is an intersensory conflict theory i.e. violation of coherent functioning of afferent body systems performing spatial orientation, statokinetic equilibrium and keeping balance. The leading role is played by the functional dysfunction of the vestibular analyzer. The classification of means of preventing and stopping of motion sickness is given, the mechanisms of their action, specific activity and side effects are described. It has been shown that currently the most effective drugs are M-cholinergic antagonists (scopolamine hydrobromide) and H1-histamine antagonists of the 1st generation (dimenhydrinate, diphenhydramine, cyclizine, meclizine, promethazine, etc.). Of the antipsychotics and blockers of D2 receptors, prochlorperazine and metoclopramide are recommended. It is also worth to use prokinetics (domperidone, cisapride, renzapride, etc.), tranquilizers (barbiturates, benzodiazepines), sleeping pills and local anesthetics. Particular attention is paid to combination drugs, consisting of antiemetic and psychostimulating drugs, designed to maintain working capacity under the influence of seasickness factors on the body. Non-pharmacological means of preventing seasickness and alleviating its symptoms are described. The main directions of improving the system of measures aimed at maintaining efficiency in the presence of symptoms of seasickness are determined.

Potentially Inappropriate Prescriptions for Anticholinergic Medications for Patients with Constipation

<b><i>Introduction:</i></b> Constipation is a very common functional gastrointestinal disorder in the general population and can be primary or secondary. <b><i>Objective:</i></b> The aim of this study was to estimate the anticholinergic burden of prescribed drugs in a population diagnosed with constipation in Colombia. <b><i>Methods:</i></b> This was a cross-sectional study that used a population database of 6.5 million people to identify the prescription of cholinergic antagonists and drugs for the management of constipation in outpatient services. The anticholinergic burden was evaluated using the Anticholinergic Drug Scale. Potentially inappropriate prescriptions that increased the risk of constipation were identified. <b><i>Results:</i></b> A total of 3,887 patients with constipation were identified; the identified patients had a mean age of 54.4 ± 21.9 years, and 69.4% were women. Eighty percent received at least one laxative, and the most prescribed laxative was bisacodyl (50.5%). Forty-one percent (<i>n</i> = 1,586) of all patients received drugs with cholinergic antagonist activity, in particular codeine (6.5%) and valproic acid (6.5%). Being over 30 years of age (odds ratio [OR]: 1.79; 95% confidence interval [CI]: 1.24–2.57), being treated in the cities of Manizales (OR: 2.20; 95% CI: 1.50–3.21) and Pereira (OR: 1.49; 95% CI: 1.07–2.09), and having hypothyroidism as a comorbidity (OR: 1.37; 95% CI: 1.08–1.73) were associated with a greater probability of receiving medications with an anticholinergic burden of 3 or more points. <b><i>Conclusions:</i></b> The majority of patients with constipation were women and were using laxatives to manage constipation. A large proportion of patients were prescribed at least one cholinergic antagonist drug, with an increased probability of use after 30 years of age.

Muscarinic receptors regulate auditory and prefrontal cortical communication during auditory processing

Much of our understanding about how acetylcholine modulates prefrontal cortical (PFC) networks comes from behavioral experiments that examine cortical dynamics during highly attentive states. However, much less is known about how PFC is recruited during passive sensory processing and how acetylcholine may regulate connectivity between cortical areas outside of task performance. To investigate the involvement of PFC and cholinergic neuromodulation in passive auditory processing, we performed simultaneous recordings in the auditory cortex (AC) and PFC in awake head fixed mice presented with a white noise auditory stimulus in the presence or absence of local cholinergic antagonists in AC. We found that a subset of PFC neurons were strongly driven by auditory stimuli even when the stimulus had no associative meaning, suggesting PFC monitors stimuli under passive conditions. We also found that cholinergic signaling in AC shapes the strength of auditory driven response in PFC, by modulating the intra-cortical sensory response through muscarinic interactions in AC. Taken together, these findings provide novel evidence that cholinergic mechanisms have a continuous role in cortical gating through muscarinic receptors during passive processing and expand traditional views of prefrontal cortical function and the contributions of cholinergic modulation in sensory gating.HighlightsPrefrontal cortex actively monitors non-associative stimuli under passive conditionsAcetylcholine facilitates cortical signaling even outside of attentional contextsLocal scopolamine infusion reduced intracortical signaling and impaired cortical gatingmAChR have an ongoing role in sound processing