A repertoire of rhythmic bursting produced by hypoglossal motoneurons in physiological and pathological conditions

Suckling, Feeding, and Swallowing: Behaviors, Circuits, and Targets for Neurodevelopmental Pathology

All mammals must suckle and swallow at birth, and subsequently chew and swallow solid foods, for optimal growth and health. These initially innate behaviors depend critically upon coordinated development of the mouth, tongue, pharynx, and larynx as well as the cranial nerves that control these structures. Disrupted suckling, feeding, and swallowing from birth onward-perinatal dysphagia-is often associated with several neurodevelopmental disorders that subsequently alter complex behaviors. Apparently, a broad range of neurodevelopmental pathologic mechanisms also target oropharyngeal and cranial nerve differentiation. These aberrant mechanisms, including altered patterning, progenitor specification, and neurite growth, prefigure dysphagia and may then compromise circuits for additional behavioral capacities. Thus, perinatal dysphagia may be an early indicator of disrupted genetic and developmental programs that compromise neural circuits and yield a broad range of behavioral deficits in neurodevelopmental disorders.

Inspiratory-Activated Airway Vagal Preganglionic Neurones Excited by Thyrotropin-Releasing Hormone via Multiple Mechanisms in Neonatal Rats

The airway vagal preganglionic neurons (AVPNs) providing projections to intrinsic tracheobronchial ganglia are considered to be crucial to modulation of airway resistance in physiological and pathological states. AVPNs classified into inspiratory-activated AVPNs (IA-AVPNs) and inspiratory-inhibited AVPNs (II-AVPNs) are regulated by thyrotropin-releasing hormone (TRH)-containing terminals. TRH causes a direct excitatory current and attenuates the phasic inspiratory glycinergic inputs in II-AVPNs, however, whether and how TRH influences IA-AVPNs remains unknown. In current study, TRH regulation of IA-AVPNs and its mechanisms involved were investigated. Using retrogradely fluorescent labeling method and electrophysiology techniques to identify IA-AVPNs in brainstem slices with rhythmic inspiratory hypoglossal bursts recorded by a suction electrode, the modulation of TRH was observed with patch-clamp technique. The findings demonstrate that under voltage clamp configuration, TRH (100 nM) caused a slow excitatory inward current, augmented the excitatory synaptic inputs, progressively suppressed the inhibitory synaptic inputs and elicited a distinctive electrical oscillatory pattern (OP). Such a current and an OP was independent of presynaptic inputs. Carbenoxolone (100 μM), a widely used gap junction inhibitor, fully suppressed the OP with persistence of TRH-induced excitatory slow inward current and augment of the excitatory synaptic inputs. Both tetrodotoxin (1 μM) and riluzole (20 μM) functioned to block the majority of the slow excitatory inward current and prevent the OP, respectively. Under current clamp recording, TRH caused a slowly developing depolarization and continuously progressive oscillatory firing pattern sensitive to TTX. TRH increased the firing frequency in response to injection of a square-wave current. The results suggest that TRH excited IA-AVPNs via the following multiple mechanisms: (1) TRH enhances the excitatory and depresses the inhibitory inputs; (2) TRH induces an excitatory postsynaptic slow inward current; (3) TRH evokes a distinctive OP mediated by gap junction.

Propofol Protects Rat Hypoglossal Motoneurons in an In Vitro Model of Excitotoxicity by Boosting GABAergic Inhibition and Reducing Oxidative Stress

In brainstem motor networks, hypoglossal motoneurons (HMs) play the physiological role of driving tongue contraction, an activity critical for inspiration, phonation, chewing and swallowing. HMs are an early target of neurodegenerative diseases like amyotrophic lateral sclerosis that, in its bulbar form, is manifested with initial dysphagia and dysarthria. One important pathogenetic component of this disease is the high level of extracellular glutamate due to uptake block that generates excitotoxicity. To understand the earliest phases of this condition we devised a model, the rat brainstem slice, in which block of glutamate uptake is associated with intense bursting of HMs, dysmetabolism and death. Since blocking bursting becomes a goal to prevent cell damage, the present report enquired whether boosting GABAergic inhibition could fulfill this aim and confer beneficial outcome. Propofol (0.5 µM) and midazolam (0.01 µM), two allosteric modulators of GABA_A receptors, were used at concentrations yielding analogous potentiation of GABA-mediated currents. Propofol also partly depressed NMDA receptor currents. Both drugs significantly shortened bursting episodes without changing single burst properties, their synchronicity, or their occurrence. Two hours later, propofol prevented the rise in reactive oxygen species (ROS) and, at 4 hours, it inhibited intracellular release of apoptosis-inducing factor (AIF) and prevented concomitant cell loss. Midazolam did not contrast ROS and AIF release. The present work provides experimental evidence for the neuroprotective action of a general anesthetic like propofol, which, in this case, may be achieved through a combination of boosted GABAergic inhibition and reduced ROS production.

Developmental nicotine exposure alters potassium currents in hypoglossal motoneurons of neonatal rat

We previously showed that nicotine exposure in utero and after birth via breast milk [developmental nicotine exposure (DNE)] is associated with many changes in the structure and function of hypoglossal motoneurons (XIIMNs), including a reduction in the size of the dendritic arbor and an increase in cell excitability. Interestingly, the elevated excitability was associated with a reduction in the expression of glutamate receptors on the cell body. Together, these observations are consistent with a homeostatic compensation aimed at restoring cell excitability. Compensation for increased cell excitability could also occur by changing potassium conductance, which plays a critical role in regulating resting potential, spike threshold, and repetitive spiking behavior. Here we test the hypothesis that the previously observed increase in the excitability of XIIMNs from DNE animals is associated with an increase in whole cell potassium currents. Potassium currents were measured in XIIMNs in brain stem slices derived from DNE and control rat pups ranging in age from 0 to 4 days by whole cell patch-clamp electrophysiology. All currents were measured after blockade of action potential-dependent synaptic transmission with tetrodotoxin. Compared with control cells, XIIMNs from DNE animals showed significantly larger transient and sustained potassium currents, but this was observed only under conditions of increased cell and network excitability, which we evoked by raising extracellular potassium from 3 to 9 mM. These observations suggest that the larger potassium currents in nicotine-exposed neurons are an important homeostatic compensation that prevents "runaway" excitability under stressful conditions, when neurons are receiving elevated excitatory synaptic input.NEW & NOTEWORTHY Developmental nicotine exposure is associated with increased cell excitability, which is often accompanied by compensatory changes aimed at normalizing excitability. Here we show that whole cell potassium currents are also increased in hypoglossal motoneurons from nicotine-exposed neonatal rats under conditions of increased cell and network excitability. This is consistent with a compensatory response aimed at preventing instability under conditions in which excitatory synaptic input is high and is compatible with the concept of homeostatic plasticity.

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-β-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 β amino acids, taurine and β-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 β-alanine (0.3mM) activated sustained anionic currents, which were partly blocked by TPMPA (100μM). Thus, both β amino acids activated ρ-containing GABA receptors in hypoglossal motoneurons. Bicuculline (20μM) reduced responses to taurine and β-alanine, but small sustained responses persisted in the presence of both strychnine and bicuculline. Responses to β-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 β-alanine might contribute to some of their neuroprotective properties under damaging overexcitatory situations.

Facing the challenge of mammalian neural microcircuits: taking a few breaths may help

Breathing in mammals is a seemingly straightforward behaviour controlled by the brain. A brainstem nucleus called the preBötzinger Complex sits at the core of the neural circuit generating respiratory rhythm. Despite the discovery of this microcircuit almost 25 years ago, the mechanisms controlling breathing remain elusive. Given the apparent simplicity and well-defined nature of regulatory breathing behaviour, the identification of much of the circuitry, and the ability to study breathing in vitro as well as in vivo, many neuroscientists and physiologists are surprised that respiratory rhythm generation is still not well understood. Our view is that conventional rhythmogenic mechanisms involving pacemakers, inhibition or bursting are problematic and that simplifying assumptions commonly made for many vertebrate neural circuits ignore consequential detail. We propose that novel emergent mechanisms govern the generation of respiratory rhythm. That a mammalian function as basic as rhythm generation arises from complex and dynamic molecular, synaptic and neuronal interactions within a diverse neural microcircuit highlights the challenges in understanding neural control of mammalian behaviours, many (considerably) more elaborate than breathing. We suggest that the neural circuit controlling breathing is inimitably tractable and may inspire general strategies for elucidating other neural microcircuits.

Intrinsic and synaptic homeostatic plasticity in motoneurons from mice with glycine receptor mutations

Inhibitory synaptic inputs to hypoglossal motoneurons (HMs) are important for modulating excitability in brainstem circuits. Here we ask whether reduced inhibition, as occurs in three murine mutants with distinct naturally occurring mutations in the glycine receptor (GlyR), leads to intrinsic and/or synaptic homeostatic plasticity. Whole cell recordings were obtained from HMs in transverse brainstem slices from wild-type (wt), spasmodic (spd), spastic (spa), and oscillator (ot) mice (C57Bl/6, approximately postnatal day 21). Passive and action potential (AP) properties in spd and ot HMs were similar to wt. In contrast, spa HMs had lower input resistances, more depolarized resting membrane potentials, higher rheobase currents, smaller AP amplitudes, and slower afterhyperpolarization current decay times. The excitability of HMs, assessed by "gain" in injected current/firing-frequency plots, was similar in all strains whereas the incidence of rebound spiking was increased in spd. The difference between recruitment and derecruitment current (i.e., ΔI) for AP discharge during ramp current injection was more negative in spa and ot. GABAA miniature inhibitory postsynaptic current (mIPSC) amplitude was increased in spa and ot but not spd, suggesting diminished glycinergic drive leads to compensatory adjustments in the other major fast inhibitory synaptic transmitter system in these mutants. Overall, our data suggest long-term reduction in glycinergic drive to HMs results in changes in intrinsic and synaptic properties that are consistent with homeostatic plasticity in spa and ot but not in spd. We propose such plasticity is an attempt to stabilize HM output, which succeeds in spa but fails in ot.

Effects of medullary lesions on conditional pacemaker activity of neonatal rat hypoglossal motoneurons in vitro

N-methyl-D-aspartate (NMDA) has been demonstrated to induce rhythmic activity in various neurons, including hypoglossal motoneurons (XIIms) and converts them to conditional pacemakers. Using whole-cell patch clamp recording in a slice preparation from neonatal rats, we confirmed that some XIIms act as conditional pacemakers, with TTX-insensitivity and a burst period that is voltage-dependent during NMDA application. Other XIIms in this study only fired tonically with NMDA application. Effects of medullary structures on conditional pacemaker XIIms were assessed using lesioned preparations. As a result, NMDA-induced rhythm (NIR) in the XIIm was observed with ventral lesions (excluding inspiratory neurons) and with dorsal lesions (excluding the swallowing center located in the nucleus of the solitary tract). The NIR was also observed with lateral lesions, but with a significantly decreased burst period. These data suggest that NMDA receptor activation selects a subset of XIIms and changes them to pacemakers whose properties can be altered by their excitability. The data also demonstrate that structures fundamental to the NIR are located within the area near the XII nucleus, indicating that the NIR is distinct from inspiratory and swallowing activities. The lateral medulla is considered to be a source of modulation of the excitability of XIIms.

Postnatal developmental profile of neurons and glia in motor nuclei of the brainstem and spinal cord, and its comparison with organotypic slice cultures

In vitro preparations of the neonatal rat spinal cord or brainstem are useful to investigate the organization of motor networks and their dysfunction in neurological disease models. Long-term spinal cord organotypic cultures can extend our understanding of such pathophysiological processes over longer times. It is, however, surprising that detailed descriptions of the type (and number) of neurons and glia in such preparations are currently unavailable to evaluate cell-selectivity of experimental damage. The focus of the present immunohistochemical study is the novel characterization of the cell population in the lumbar locomotor region of the rat spinal cord and in the brainstem motor nucleus hypoglossus at 0-4 postnatal days, and its comparison with spinal organotypic cultures at 2-22 days in vitro. In the nucleus hypoglossus, neurons were 40% of all cells and 80% of these were motoneurons. Astrocytes (35% of total cells) were the main glial cells, while microglia was <10%. In the spinal gray matter, the highest neuronal density was in the dorsal horn (>80%) and the lowest in the ventral horn (≤57%) with inverse astroglia numbers and few microglia. The number of neurons (including motoneurons) and astrocytes was stable after birth. Like in the spinal cord, motoneurons in organotypic spinal culture were <10% of ventral horn cells, with neurons <40%, and the rest made up by glia. The present report indicates a comparable degree of neuronal and glial maturation in brainstem and spinal motor nuclei, and that this condition is also observed in 3-week-old organotypic cultures.

Respiratory motoneurons and pathological conditions: lessons from hypoglossal motoneurons challenged by excitotoxic or oxidative stress

Hypoglossal motoneurons (HMs) are respiration-related brainstem neurons that command rhythmic contraction of the tongue muscles in concert with the respiratory drive. In experimental conditions, HMs can exhibit a range of rhythmic patterns that may subserve different motor outputs and functions. Neurodegenerative diseases like amyotrophic lateral sclerosis (ALS; Lou-Gehrig disease) often damage HMs with distressing symptoms like dysarthria, dysphagia and breathing difficulty related to degeneration of respiratory motoneurons. While the cause of ALS remains unclear, early diagnosis remains an important goal for potential treatment because fully blown clinical symptoms appear with degeneration of about 30% motoneurons. Using a simple in vitro model of the rat brainstem to study the consequences of excitotoxicity or oxidative stress (believed to occur during the onset of ALS) on HMs, it is possible to observe distinct electrophysiological effects associated with HM experimental pathology. In fact, excitotoxicity caused by glutamate uptake block triggers sustained bursting and enhanced synaptic transmission, whereas oxidative stress generates slow depolarization, augmented repeated firing, and decreased synaptic transmission. In either case, only a subpopulation of HMs shows abnormal functional changes. Although these two insults induce separate functional signatures, the consequences on HMs after a few hours are similar and are preceded by activation of the stress transcription factor ATF-3. The deleterious action of excitotoxicity is inhibited by early administration of riluzole, a drug currently employed for the symptomatic treatment of ALS, demonstrating that this in vitro model can be useful for testing potential neuroprotective agents.

Riluzole is a potent drug to protect neonatal rat hypoglossal motoneurons in vitro from excitotoxicity due to glutamate uptake block

Excitotoxic damage to motoneurons is thought to be an important contribution to the pathogenesis of amyotrophic lateral sclerosis (ALS), a slowly developing degeneration of motoneurons that, in most cases of sporadic occurrence, is associated with impaired glial glutamate uptake. Riluzole is the only drug licensed for symptomatic ALS treatment and is proposed to delay disease progression. As riluzole is administered only after full ALS manifestation, it is unclear if its early use might actually prevent motoneuron damage. We explored this issue by using, as a simple in vitro model, hypoglossal motoneurons (a primary target of ALS) of the neonatal rat brainstem slice preparation exposed to excitotoxic stress due to glutamate uptake block by DL-threo-β-benzyloxyaspartate (TBOA). TBOA evoked sustained network bursting, early (1 h) enhancement of the S100B immunostaining of gray matter astrocytes, and activated the motoneuronal stress ATF-3 transcription factor; 4 h later, loss (30%) of motoneuron staining ensued and pyknosis appeared. Riluzole (5 μM; applied 15 min after TBOA) inhibited bursting, decreased the frequency of spontaneous glutamatergic events, reversed changes in S100B immunostaining and prevented late loss of motoneuron staining. These results show that excitotoxicity induced by glutamate uptake block developed slowly, and was sensed by glia and motoneurons with delayed cell death. Our data provide novel evidence for the neuroprotective action of riluzole on motoneurons and glia when applied early after an excitotoxic stimulus.

Chapter 11--novel mechanism for hyperreflexia and spasticity

We established that hyperreflexia is delayed after spinal transection in the adult rat and that passive exercise could normalize low frequency-dependent depression of the H-reflex. We were also able to show that such passive exercise will normalize hyperreflexia in patients with spinal cord injury (SCI). Recent results demonstrate that spinal transection results in changes in the neuronal gap junction protein connexin 36 below the level of the lesion. Moreover, a drug known to increase electrical coupling was found to normalize hyperreflexia in the absence of passive exercise, suggesting that changes in electrical coupling may be involved in hyperreflexia. We also present results showing that a measure of spasticity, the stretch reflex, is rendered abnormal by transection and normalized by the same drug. These data suggest that electrical coupling may be dysregulated in SCI, leading to some of the symptoms observed. A novel therapy for hyperreflexia and spasticity may require modulation of electrical coupling.

Cerebellar cortical output encodes temporal aspects of rhythmic licking movements and is necessary for normal licking frequency

Rodents consume water by performing stereotypic, rhythmic licking movements that are believed to be controlled by brainstem pattern-generating circuits. Previous work has shown that synchronized population activity of inferior olive neurons was phase-locked to the licking rhythm in rats, suggesting a cerebellar involvement in temporal aspects of licking behavior. However, what role the cerebellum has in licking behavior and whether licking is represented in the high-frequency simple spike output of Purkinje cells remains unknown. We recorded Purkinje cell simple and complex spike activity in awake mice during licking, and determined the behavioral consequences of loss of cerebellar function. Mouse cerebellar cortex contained a multifaceted representation of licking behavior encoded in the simple spike activities of Purkinje cells distributed across Crus I, Crus II and lobus simplex of the right cerebellar hemisphere. Lick-related Purkinje cell simple spike activity was modulated rhythmically, phase-locked to the lick rhythm, or non-rhythmically. A subpopulation of lick-related Purkinje cells differentially represented lick interval duration in their simple spike activity. Surgical removal of the cerebellum or temporary pharmacological inactivation of the cerebellar nuclei significantly slowed the licking frequency. Fluid licking was also less efficient in mice with impaired cerebellar function, indicated by a significant decline in the volume per lick fluid intake. The gross licking movement appeared unaffected. Our results suggest a cerebellar role in modulating the frequency of the central pattern-generating circuits controlling fluid licking and in the fine coordination of licking, while contributing little to the coordination of the gross licking movement.