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.

Daily acute intermittent hypoxia enhances serotonergic innervation of hypoglossal motor nuclei in rats with and without cervical spinal injury.

Intermittent hypoxia elicits protocol-dependent effects on hypoglossal (XII) motor plasticity. Whereas low-dose, acute intermittent hypoxia (AIH) elicits serotonin-dependent plasticity in XII motor neurons, high-dose, chronic intermittent hypoxia (CIH) elicits neuroinflammation that undermines AIH-induced plasticity. Preconditioning with repeated AIH and mild CIH enhance AIH-induced XII motor plasticity. Since intermittent hypoxia pre-conditioning could enhance serotonin-dependent XII motor plasticity by increasing serotonergic innervation density of the XII motor nuclei, we tested the hypothesis that 3 distinct intermittent hypoxia protocols commonly studied to elicit plasticity (AIH) or simulate aspects of sleep apnea (CIH) differentially affect XII serotonergic innervation. Sleep apnea and associated CIH are common in people with cervical spinal injuries and, since repetitive AIH is emerging as a promising therapeutic strategy to improve respiratory and non-respiratory motor function after spinal injury, we also tested the hypotheses that XII serotonergic innervation is increased by repetitive AIH and/or CIH in rats with cervical C2 hemisections (C2Hx). Serotonergic innervation was assessed via immunofluorescence in male Sprague Dawley rats, with and without C2Hx (beginning 8 weeks post-injury) exposed to 28 days of: 1) normoxia; 2) daily AIH (10, 5-min 10.5% O2 episodes per day; 5-min normoxic intervals); 3) mild CIH (5-min 10.5% O2 episodes; 5-min intervals; 8 h/day); and 4) moderate CIH (2-min 10.5% O2 episodes; 2-min intervals; 8 h/day). Daily AIH, but neither CIH protocol, increased the area of serotonergic immunolabeling in the XII motor nuclei in both intact and injured rats. C2Hx per se had no effect on XII serotonergic innervation density. Thus, daily AIH may increases XII serotonergic innervation and function, enhancing the capacity for serotonin-dependent, AIH-induced plasticity in upper airway motor neurons. Such effects may preserve upper airway patency and/or swallowing ability in people with cervical spinal cord injuries and other clinical disorders that compromise breathing and airway defense.

The dorsal and the ventral side of hypoglossal motor nucleus showed different response to chronic intermittent hypoxia in rats.

PURPOSE: To study neurochemical reactions to chronic intermittent hypoxia (CIH) in the hypoglossal nucleus (HN) of rats. METHODS: Adult male Sprague-Dawley rats (n = 12) were randomly divided into two groups (the CIH and the control group). The CIH rats were housed in a hypoxic chamber with the fraction of oxygen volume alternating between 21% and 5% by providing air for 60 s and then providing nitrogen for 60 s from 8:30 am to 16:30 pm each day for 35 days. The control group was housed in a cabin with normal oxygen levels. We studied the expression of c-fos protein, 5-hydroxytryptamine (5-HT) positive terminals, and its 2A receptors in hypoglossal nuclei by immunohistochemistry. RESULTS: The expression of c-fos, 5-HT positive terminals, and accordingly 5-HT 2A receptors in the CIH group were significantly higher than that in the controls (p < 0.05). The ventral side of the HN showed a clearly higher expression of 5-HT and its 2A receptors than the dorsal side (p < 0.05). CONCLUSION: There were 2 responses of the HN to CIH. First, CIH induced a higher expression of 5-HT positive terminals and its 2A receptors, and second, this reaction was much more evident in ventral side than in the dorsal side. We postulate that these responses may serve to be a protective and compensatory mechanism for CIH.

The role of gonadal hormones in the hypoglossal discharge activity of rats exposed to chronic intermittent hypoxia.

OBJECTIVE: The role of gonadal hormones in chronic intermittent hypoxia (CIH)-evoked hypoglossal nerve (XII) neuroplasticity has not been thoroughly studied. The purpose of this study was to reveal the effects of gonadal hormone concentration variations on the XII discharge activity of rats exposed to CIH and the corresponding relationship with 5-hydroxytryptamine (5-HT). METHODS: This study employed five groups of female rats and six groups of male rats. Gonadal hormone levels were modified through gonadal resection and daily supplementation with gonadal hormones in rats of both sexes. Rats in the CIH groups were exposed to an additional 4 weeks of CIH once the operative incision for gonadectomy had healed. Finally, XII spontaneous discharge activities were recorded, and serum estradiol, testosterone and 5-HT concentrations were detected by ELISA. RESULTS: Among the female rats, the normal estradiol level groups expressed XII neuroplasticity, while the low estradiol level group failed to express this phenomenon. XII neuroplasticity was related to the serum estradiol concentration. In the male rats, XII neuroplasticity was successfully evoked in the normal testosterone level group but was suppressed in the low testosterone level group and aromatase inhibitor group. XII neuroplasticity was not significantly related to serum testosterone concentrations. Both estradiol and testosterone concentrations were related to 5-HT concentrations. CONCLUSIONS: This is the first study to analyze the effects of gonadal hormones on XII neuroplasticity in both female and male rats. The results suggest that the estradiol level is related to XII neuroplasticity rather than the testosterone level, and testosterone may indirectly adjust XII neuroplasticity by converting to estradiol. Estradiol and testosterone levels are related to 5-HT levels in the respective genders.

Variable colocalisation of GABAA receptor subunits and glycine receptors on neurons in the human hypoglossal nucleus.

The hypoglossal nucleus, the nucleus of the twelfth cranial nerve, is located dorsally in the midline of the medulla oblongata. The hypoglossal nucleus contains lower motor neurons which innervate the tongue muscles that control tongue movements involved in speech production, swallowing, mastication and associated respiratory movements. GABAA and glycine receptors are heteropentameric ionotropic receptors that facilitate fast-response, inhibitory neurotransmission in the mammalian brain and spinal cord. We investigated the immunohistochemical distribution of the GABAA receptor α1, α2, ß2,3 subunits and glycine receptors as well as their relationship to the vesicular GABA transporter (VGAT) in the human hypoglossal nucleus at the light and confocal laser scanning microscope levels. The results showed that all of the GABAA receptor subunits as well as glycine receptor display punctate labelling indicative of synapses on the soma and dendritic membranes of large neurons within the hypoglossal nucleus. On average, approximately 50% of glycine receptors were co localised with GABAA receptor α1 subunits. Also on average GABAA α2 and ß2,3 subunits were colocalised with approximately 30% of glycine receptor subunits. VGAT positive terminals were associated with both GABAA and glycine receptor types. Both glycinergic and GABAergic positive puncta were found adjacent to VGAT terminal-like staining. These results suggest that inhibition of human hypoglossal motor neurons occurs not only through complex interaction of separated GABAAR and glycine receptor regions, but also through synapses containing both inhibitory receptor types co-existing at the same synaptic sites.

Capsaicin causes robust reduction in glycinergic transmission to rat hypoglossal motor neurons via a TRPV1-independent mechanism.

The effect of capsaicin on glycinergic synaptic transmission to juvenile rat hypoglossal motor neurons in acute brainstem slices was evaluated in the presence of TTX. Capsaicin caused a robust decrease in miniature IPSC frequency, amplitude, and half-width, showing that this effect is independent of action potential generation. In the presence of capsazepine, a classic TRPV1 antagonist, capsaicin was still able to reduce spontaneous inhibitory postsynaptic current (IPSC) amplitude and frequency. We further investigated whether the effect of capsaicin on glycinergic transmission to hypoglossal motor neurons is pre- or postsynaptic in nature by recording pairs of evoked IPSCs. Interestingly, capsaicin also reduced evoked IPSC amplitude without affecting paired-pulse ratio, indicating a postsynaptic mechanism of action. Significant reduction was also observed in evoked IPSC half-width, rise time, and decay tau. We also show that capsaicin does not have any effect on either transient (It) or sustained (Is) potassium currents. Finally, we also show that the hyperpolarization-activated cationic current (Ih) also remains unchanged after capsaicin application. NEW & NOTEWORTHY Capsaicin reduces the amplitude of quantal and evoked glycinergic inhibitory neurotransmission to brainstem motor neurons without altering activity-dependent transmitter release. This effect of capsaicin is not due to activation of TRPV1 receptors, as it is not blocked by capsazepine, a TRPV1 receptor antagonist. Capsaicin does not alter voltage-dependent potassium current or the hyperpolarization-activated cationic current in brainstem motor neurons.

Brainstem Nuclei Associated with Mediating Apnea-Induced Respiratory Motor Plasticity.

The respiratory control system is plastic. It has a working memory and is capable of retaining how respiratory stimuli affect breathing by regulating synaptic strength between respiratory neurons. For example, repeated airway obstructions trigger a form of respiratory plasticity that strengthens inspiratory activity of hypoglossal (XII) motoneurons. This form of respiratory plasticity is known as long-term facilitation (LTF) and requires noradrenaline released onto XII motoneurons. However, the brainstem regions responsible for this form of LTF remain unidentified. Here, we used electrophysiology, neuropharmacology and immunohistochemistry in adult rats to identify the brainstem regions involved in mediating LTF. First, we show that repeated airway obstructions induce LTF of XII motoneuron activity and that inactivation of the noradrenergic system prevents LTF. Second, we show that noradrenergic cells in the locus coeruleus (LC), which project to XII motoneurons, are recruited during LTF induction. Third, we show that targeted inactivation of noradrenergic LC cells during LTF induction prevents LTF. And lastly, we show that the nucleus tractus solitarius (NTS), which has known projections to the LC, is critical for LTF because its inactivation prevents LTF. Our results suggest that both the LC and NTS are involved in mediating apnea-induced LTF, and we hypothesize that a NTS → LC → XII circuit mechanism mediates this form of respiratory motor plasticity.

Firing responses mediated via distinct nicotinic acetylcholine receptor subtypes in rat prepositus hypoglossi nuclei neurons.

We previously reported that cholinergic current responses mediated via nicotinic acetylcholine (ACh) receptors (nAChRs) in the prepositus hypoglossi nucleus (PHN), which participates in gaze control, can be classified into distinct types based on different kinetics and are mainly composed of α7- and/or non-α7-subtypes: fast (F)-, slow (S)-, and fast and slow (FS)-type currents. In this study, to clarify how each current type is related to neuronal activities, we investigated the relationship between the current types and the membrane properties and the firing responses that were induced by each current type. The proportion of the current types differed in neurons that exhibited different afterhyperpolarization (AHP) profiles and firing patterns, suggesting that PHN neurons show a preference for specific current types dependent on the membrane properties. In response to ACh, F-type neurons showed either one action potential (AP) or multiple APs with a short firing duration, and S-type neurons showed multiple APs with a long firing duration. The firing frequency of F-type neurons was significantly higher than that of S-type and FS-type neurons. An α7-subtype-specific antagonist abolished the firing responses of F-type neurons and reduced the responses of FS-type neurons but had little effect on the responses of S-type neurons, which were reduced by a non-α7-subtype-specific antagonist. These results suggest that the different properties of the current types and the distinct expression of the nAChR subtypes in PHN neurons with different membrane properties produce unique firing responses via the activation of nAChRs. NEW & NOTEWORTHY Prepositus hypoglossi nucleus (PHN) neurons show distinct nicotinic acetylcholine receptor (nAChR)-mediated current responses. The proportion of the current types differed in the neurons that exhibited different afterhyperpolarization profiles and firing patterns. The nAChR-mediated currents with different kinetics induced firing responses of the neurons that were distinct in the firing frequency and duration. These results suggest that the different properties of the current types in PHN neurons with different membrane properties produce unique firing responses via the activation of nAChRs.

Traditional Chinese herbal formula relieves snoring by modulating activities of upper airway related nerves in aged rats.

AIM: The present study investigated whether intraperitoneal treatment with the herbal formula B210 ([B210]; a herbal composition of Gastrodia elata and Cinnamomum cassia) can reduce snoring in aged rats. Also, we studied possible neural mechanisms involved in B210 treatment and subsequent reduced snoring in rats. METHODS AND RESULT: We compared pressure and frequency of snoring, activities of phrenic nerve (PNA), activities of recurrent laryngeal nerve (RLNA) and activities of hypoglossal nerve (HNA), inspiratory time (TI) and expiratory time (TE) of PNA, and pre-inspiratory time (Pre-TI) of HNA in aged rats between sham and B210 treatment groups (30 mg/mL dissolved in DMSO). We found that aged rats that received B210 treatment had significantly reduced pressure and frequency of snoring than rats who received sham treatment. Also, we observed that aged rats that received B210 treatment had significantly increased PNA, RLNA, and HNA, extended TI and TE of PNA, and prolonged Pre-TI of HNA compared to rats that received sham treatment. In other words, B210 treatment may relieve snoring through modulating activities and breathing time of upper airway related nerves in aged rats. CONCLUSION: We suggested that the B210 might be a potential herbal formula for snoring remission.

Microglia support ATF3-positive neurons following hypoglossal nerve axotomy.

Microglia are essential in developmental processes and maintenance of neuronal homeostasis. Experimental axotomy of motor neurons results in neurodegeneration, and microglia in motor nuclei become activated and migrate towards injured neurons. However, whether these activated microglia are protective or destructive to neurons remains controversial. In the present study, we transected the hypoglossal nerve in BALB/c mice, causing activating transcription factor 3 (ATF3) and growth associated protein 43 (GAP43) induction, and partial neuronal death. Inhibition of microglial accumulation by minocycline administration impaired microglial accumulation, decreased GAP43 mRNA expression, and reduced motor neuron survival. Expression of ATF3 contributed to nerve regeneration, and increased within 6 h after axotomy, prior to microglial migration. Further, microglial contact with neuronal cell bodies was associated with neuronal ATF3 expression. Colchicine administration blocked lesion-induced ATF3 transcription in axotomized neurons and microglial accumulation. In addition, perineuronal microglia-derived ciliary neurotrophic factor (CNTF) increased, indicating that perineuronal microglia in the hypoglossal nucleus protect axotomized motor neurons by releasing trophic factors. We also observed that microglia secrete CNTF and that neurons have CNTFRα and can respond to it in vitro. CNTF promote neurite elongation and neuronal survival of primary cultured neurons. Microglia make contact through unknown neuronal signals that are possibly regulated by ATF3 in hypoglossal nucleus. Moreover, they play important roles in regenerating motor neurons and are potential new therapeutic targets for motor neuron diseases.

Phrenic and hypoglossal nerve activity during respiratory response to hypoxia in 6-OHDA unilateral model of Parkinson's disease.

AIMS: Parkinson's disease (PD) patients apart from motor dysfunctions exhibit respiratory disturbances. Their mechanism is still unknown and requires investigation. Our research was designed to examine the activity of phrenic (PHR) and hypoglossal (HG) nerves activity during a hypoxic respiratory response in the 6-hydroxydopamine (6-OHDA) model of PD. MAIN METHODS: Male adult Wistar rats were injected unilaterally with 6-OHDA (20µg) or the vehicle into the right medial forebrain bundle (MFB). Two weeks after the surgery the activity of the phrenic and hypoglossal nerve was registered in anesthetized, vagotomized, paralyzed, and mechanically ventilated rats under normoxic and hypoxic conditions. Lesion effectiveness was confirmed by the cylinder test, performed before the MFB injection and 14days after, before the respiratory experiment. KEY FINDINGS: 6-OHDA lesioned animals showed a significant increase in normoxic inspiratory time. Expiratory time and total time of the respiratory cycle were prolonged in PD rats after hypoxia. The amplitude of the PHR activity and its minute activity were increased in comparison to the sham group at recovery time and during 30s of hypoxia. The amplitude of the HG activity was increased in response to hypoxia in 6-OHDA lesioned animals. The degeneration of dopaminergic neurons decreased the pre-inspiratory/inspiratory ratio of the hypoglossal burst amplitude during and after hypoxia. SIGNIFICANCE: Unilateral MFB lesion changed the activity of the phrenic and hypoglossal nerves. The altered pre-inspiratory hypoglossal nerve activity indicates modifications to the central mechanisms controlling the activity of the HG nerve and may explain respiratory disorders seen in PD, i.e. apnea.

Chemogenetic stimulation of the hypoglossal neurons improves upper airway patency.

Obstructive sleep apnea (OSA) is characterized by recurrent upper airway obstruction during sleep. OSA leads to high cardiovascular morbidity and mortality. The pathogenesis of OSA has been linked to a defect in neuromuscular control of the pharynx. There is no effective pharmacotherapy for OSA. The objective of this study was to determine whether upper airway patency can be improved using chemogenetic approach by deploying designer receptors exclusively activated by designer drug (DREADD) in the hypoglossal motorneurons. DREADD (rAAV5-hSyn-hM3(Gq)-mCherry) and control virus (rAAV5-hSyn-EGFP) were stereotactically administered to the hypoglossal nucleus of C57BL/6J mice. In 6-8 weeks genioglossus EMG and dynamic MRI of the upper airway were performed before and after administration of the DREADD ligand clozapine-N-oxide (CNO) or vehicle (saline). In DREADD-treated mice, CNO activated the genioglossus muscle and markedly dilated the pharynx, whereas saline had no effect. Control virus treated mice showed no effect of CNO. Our results suggest that chemogenetic approach can be considered as a treatment option for OSA and other motorneuron disorders.

Nicotinic receptors modulate the onset of reactive oxygen species production and mitochondrial dysfunction evoked by glutamate uptake block in the rat hypoglossal nucleus.

In several neurodegenerative diseases, glutamate-mediated excitotoxicity is considered to be a major process to initiate cell degeneration. Indeed, subsequent to excessive glutamate receptor stimulation, reactive oxygen species (ROS) generation and mitochondrial dysfunction are regarded as two major gateways leading to neuron death. These processes are mimicked in an in vitro model of rat brainstem slice when excitotoxicity is induced by DL-threo-ß-benzyloxyaspartate (TBOA), a specific glutamate-uptake blocker that increases extracellular glutamate. Our recent study has demonstrated that brainstem hypoglossal motoneurons, which are very vulnerable to this damage, were neuroprotected from excitotoxicity with nicotine application through the activation of nicotinic acetylcholine receptors (nAChRs) and subsequent inhibition of ROS and mitochondrial dysfunction. The present study examined if endogenous cholinergic activity exerted any protective effect in this pathophysiological model and how ROS production (estimated with rhodamine fluorescence) and mitochondrial dysfunction (measured as methyltetrazolium reduction) were time-related during the early phase of excitotoxicity (0-4h). nAChR antagonists did not modify TBOA-evoked ROS production (that was nearly doubled over control) or mitochondrial impairment (25% decline), suggesting that intrinsic nAChR activity was insufficient to contrast excitotoxicity and needed further stimulation with nicotine to become effective. ROS production always preceded mitochondrial dysfunction by about 2h. Nicotine prevented both ROS production and mitochondrial metabolic depression with a delayed action that alluded to a complex chain of events targeting these two lesional processes. The present data indicate a relatively wide time frame during which strong nAChR activation can arrest a runaway neurotoxic process leading to cell death.

Nicotinic receptor activation contrasts pathophysiological bursting and neurodegeneration evoked by glutamate uptake block on rat hypoglossal motoneurons.

KEY POINTS: Impaired uptake of glutamate builds up the extracellular level of this excitatory transmitter to trigger rhythmic neuronal bursting and delayed cell death in the brainstem motor nucleus hypoglossus. This process is the expression of the excitotoxicity that underlies motoneuron degeneration in diseases such as amyotrophic lateral sclerosis affecting bulbar motoneurons. In a model of motoneuron excitotoxicity produced by pharmacological block of glutamate uptake in vitro, rhythmic bursting is suppressed by activation of neuronal nicotinic receptors with their conventional agonist nicotine. Emergence of bursting is facilitated by nicotinic receptor antagonists. Following excitotoxicity, nicotinic receptor activity decreases mitochondrial energy dysfunction, endoplasmic reticulum stress and production of toxic radicals. Globally, these phenomena synergize to provide motoneuron protection. Nicotinic receptors may represent a novel target to contrast pathological overactivity of brainstem motoneurons and therefore to prevent their metabolic distress and death. ABSTRACT: Excitotoxicity is thought to be one of the early processes in the onset of amyotrophic lateral sclerosis (ALS) because high levels of glutamate have been detected in the cerebrospinal fluid of such patients due to dysfunctional uptake of this transmitter that gradually damages brainstem and spinal motoneurons. To explore potential mechanisms to arrest ALS onset, we used an established in vitro model of rat brainstem slice preparation in which excitotoxicity is induced by the glutamate uptake blocker dl-threo-ß-benzyloxyaspartate (TBOA). Because certain brain neurons may be neuroprotected via activation of nicotinic acetylcholine receptors (nAChRs) by nicotine, we investigated if nicotine could arrest excitotoxic damage to highly ALS-vulnerable hypoglossal motoneurons (HMs). On 50% of patch-clamped HMs, TBOA induced intense network bursts that were inhibited by 1-10 µm nicotine, whereas nAChR antagonists facilitated burst emergence in non-burster cells. Furthermore, nicotine inhibited excitatory transmission and enhanced synaptic inhibition. Strong neuroprotection by nicotine prevented the HM loss observed after 4 h of TBOA exposure. This neuroprotective action was due to suppression of downstream effectors of neurotoxicity such as increased intracellular levels of reactive oxygen species, impaired energy metabolism and upregulated genes involved in endoplasmic reticulum (ER) stress. In addition, HMs surviving TBOA toxicity often expressed UDP-glucose glycoprotein glucosyltransferase, a key element in repair of misfolded proteins: this phenomenon was absent after nicotine application, indicative of ER stress prevention. Our results suggest nAChRs to be potential targets for inhibiting excitotoxic damage of motoneurons at an early stage of the neurodegenerative process.

Persistent GABAA/C responses to gabazine, taurine and beta-alanine in rat hypoglossal motoneurons.

In hypoglossal motoneurons, a sustained anionic current, sensitive to a blocker of ρ-containing GABA receptors, (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) and insensitive to bicuculline, was previously shown to be activated by gabazine. In order to better characterize the receptors involved, the sensitivity of this atypical response to pentobarbital (30µM), allopregnanolone (0.3µM) and midazolam (0.5µM) was first investigated. Pentobarbital potentiated the response, whereas the steroid and the benzodiazepine were ineffective. The results indicate the involvement of hybrid heteromeric receptors, including at least a GABA receptor ρ subunit and a γ subunit, accounting for the pentobarbital-sensitivity. The effects of the endogenous ß amino acids, taurine and ß-alanine, which are released under various pathological conditions and show neuroprotective properties, were then studied. In the presence of the glycine receptor blocker strychnine (1µM), both taurine (0.3-1mM) and ß-alanine (0.3mM) activated sustained anionic currents, which were partly blocked by TPMPA (100µM). Thus, both ß amino acids activated ρ-containing GABA receptors in hypoglossal motoneurons. Bicuculline (20µM) reduced responses to taurine and ß-alanine, but small sustained responses persisted in the presence of both strychnine and bicuculline. Responses to ß-alanine were slightly increased by allopregnanolone, indicating a contribution of the bicuculline- and neurosteroid-sensitive GABAA receptors underlying tonic inhibition in these motoneurons. Since sustained activation of anionic channels inhibits most mature principal neurons, the ρ-containing GABA receptors permanently activated by taurine and ß-alanine might contribute to some of their neuroprotective properties under damaging overexcitatory situations.

TDP-43 in the hypoglossal nucleus identifies amyotrophic lateral sclerosis in behavioral variant frontotemporal dementia.

The hypoglossal nucleus was recently identified as a key brain region in which the presence of TDP-43 pathology could accurately discriminate TDP-43 proteinopathy cases with clinical amyotrophic lateral sclerosis (ALS). The objective of the present study was to assess the hypoglossal nucleus in behavioral variant frontotemporal dementia (bvFTD), and determine whether TDP-43 in this region is associated with clinical ALS. Twenty-nine cases with neuropathological FTLD-TDP and clinical bvFTD that had not been previously assessed for hypoglossal TDP-43 pathology were included in this study. Of these 29 cases, 41% (n=12) had a dual diagnosis of bvFTD-ALS at presentation, all 100% (n=12) of which demonstrated hypoglossal TDP-43 pathology. Of the 59% (n=17) cohort that presented with pure bvFTD, 35% (n=6) were identified with hypoglossal TDP-43 pathology. Review of the case files of all pure bvFTD cases revealed evidence of possible or probable ALS in 5 of the 6 hypoglossal-positive cases (83%) towards the end of disease, and this was absent from all cases without such pathology. In conclusion, the present study validates grading the presence of TDP-43 in the hypoglossal nucleus for the pathological identification of bvFTD cases with clinical ALS, and extends this to include the identification of cases with possible ALS at end-stage.

Nicotinic acetylcholine receptor-mediated responses in medial vestibular and prepositus hypoglossi nuclei neurons showing distinct neurotransmitter phenotypes.

Cholinergic transmission in both the medial vestibular nucleus (MVN) and prepositus hypoglossi nucleus (PHN) plays an important role in horizontal eye movements. We previously demonstrated that the current responses mediated via nicotinic acetylcholine receptors (nAChRs) were larger than those mediated via muscarinic acetylcholine receptors (mAChRs) in cholinergic MVN and PHN neurons that project to the cerebellum. In this study, to clarify the predominant nAChR responses and the expression patterns of nAChRs in MVN and PHN neurons that exhibit distinct neurotransmitter phenotypes, we identified cholinergic, inhibitory, and glutamatergic neurons using specific transgenic rats and investigated current responses to the application of acetylcholine (ACh) using whole cell recordings in brain stem slices. ACh application induced larger nAChR-mediated currents than mAChR-mediated currents in every neuronal phenotype. In the presence of an mAChR antagonist, we found three types of nAChR-mediated currents that exhibited different rise and decay times and designated these as fast (F)-, slow (S)-, and fast and slow (FS)-type currents. F-type currents were the predominant response in inhibitory MVN neurons, whereas S-type currents were observed in the majority of glutamatergic MVN and PHN neurons. No dominant response type was observed in cholinergic neurons. Pharmacological analyses revealed that the F-, S-, and FS-type currents were mainly mediated by α7, non-α7, and both α7 and non-α7 nAChRs, respectively. These findings suggest that cholinergic responses in the major neuronal populations of the MVN and PHN are predominantly mediated by nAChRs and that the expression of α7 and non-α7 nAChRs differ among the neuronal phenotypes.

Current injection and receptor-mediated excitation produce similar maximal firing rates in hypoglossal motoneurons.

The maximum firing rates of motoneurons (MNs), activated in response to synaptic drive, appear to be much lower than that elicited by current injection. It could be that the decrease in input resistance associated with increased synaptic activity (but not current injection) might blunt overall changes in membrane depolarization and thereby limit spike-frequency output. To test this idea, we recorded, in the same cells, maximal firing responses to current injection and to synaptic activation. We prepared 300 µm medullary slices in neonatal rats that contained hypoglossal MNs and used whole-cell patch-clamp electrophysiology to record their maximum firing rates in response to triangular-ramp current injections and to glutamate receptor-mediated excitation. Brief pressure pulses of high-concentration glutamate led to significant depolarization, high firing rates, and temporary cessation of spiking due to spike inactivation. In the same cells, we applied current clamp protocols that approximated the time course of membrane potential change associated with glutamate application and with peak current levels large enough to cause spike inactivation. Means (SD) of maximum firing rates obtained in response to glutamate application were nearly identical to those obtained in response to ramp current injection [glutamate 47.1 ± 12.0 impulses (imp)/s, current injection 47.5 ± 11.2 imp/s], even though input resistance was 40% less during glutamate application compared with current injection. Therefore, these data suggest that the reduction in input resistance associated with receptor-mediated excitation does not, by itself, limit the maximal firing rate responses in MNs.

Transcriptional regulation of mouse hypoglossal motor neuron somatotopic map formation.

Somatic motor neurons in the hypoglossal nucleus innervate tongue muscles controlling vital functions such as chewing, swallowing and respiration. Formation of functional hypoglossal nerve circuits depends on the establishment of precise hypoglossal motor neuron maps correlating with specific tongue muscle innervations. Little is known about the molecular mechanisms controlling mammalian hypoglossal motor neuron topographic map formation. Here we show that combinatorial expression of transcription factors Runx1, SCIP and FoxP1 defines separate mouse hypoglossal motor neuron groups with different topological, neurotransmitter and calcium-buffering phenotypes. Runx1 and SCIP are coexpressed in ventromedial hypoglossal motor neurons involved in control of tongue protrusion whereas FoxP1 is expressed in dorsomedial motor neurons associated with tongue retraction. Establishment of separate hypoglossal motor neuron maps depends in part on Runx1-mediated suppression of ventrolateral and dorsomedial motor neuron phenotypes and regulation of FoxP1 expression pattern. These findings suggest that combinatorial actions of Runx1, SCIP and FoxP1 are important for mouse hypoglossal nucleus somatotopic map formation.

Developmental changes in the morphology of mouse hypoglossal motor neurons.

Hypoglossal motor neurons (XII MNs) innervate tongue muscles important in breathing, suckling and vocalization. Morphological properties of 103 XII MNs were studied using Neurobiotin™ filling in transverse brainstem slices from C57/Bl6 mice (n = 34) from embryonic day (E) 17 to postnatal day (P) 28. XII MNs from areas thought to innervate different tongue muscles showed similar morphology in most, but not all, features. Morphological properties of XII MNs were established prior to birth, not differing between E17-18 and P0. MN somatic volume gradually increased for the first 2 weeks post-birth. The complexity of dendritic branching and dendrite length of XII MNs increased throughout development (E17-P28). MNs in the ventromedial XII motor nucleus, likely to innervate the genioglossus, frequently (42 %) had dendrites crossing to the contralateral side at all ages, but their number declined with postnatal development. Unexpectedly, putative dendritic spines were found in all XII MNs at all ages, and were primarily localized to XII MN somata and primary dendrites at E18-P4, increased in distal dendrites by P5-P8, and were later predominantly found in distal dendrites. Dye-coupling between XII MNs was common from E18 to P7, but declined strongly with maturation after P7. Axon collaterals were found in 20 % (6 of 28) of XII MNs with filled axons; collaterals terminated widely outside and, in one case, within the XII motor nucleus. These results reveal new morphological features of mouse XII MNs, and suggest that dendritic projection patterns, spine density and distribution, and dye-coupling patterns show specific developmental changes in mice.