Morphological and immunohistochemical characterization of interneurons within the rat trigeminal motor nucleus

Functional Connectivity Between the Trigeminal Main Sensory Nucleus and the Trigeminal Motor Nucleus

The present study shows new evidence of functional connectivity between the trigeminal main sensory (NVsnpr) and motor (NVmt) nuclei in rats and mice. NVsnpr neurons projecting to NVmt are most highly concentrated in its dorsal half. Their electrical stimulation induced multiphasic excitatory synaptic responses in trigeminal MNs and evoked calcium responses mainly in the jaw-closing region of NVmt. Induction of rhythmic bursting in NVsnpr neurons by local applications of BAPTA also elicited rhythmic firing or clustering of postsynaptic potentials in trigeminal motoneurons, further emphasizing the functional relationship between these two nuclei in terms of rhythm transmission. Biocytin injections in both nuclei and calcium-imaging in one of the two nuclei during electrical stimulation of the other revealed a specific pattern of connectivity between the two nuclei, which organization seemed to critically depend on the dorsoventral location of the stimulation site within NVsnpr with the most dorsal areas of NVsnpr projecting to the dorsolateral region of NVmt and intermediate areas projecting to ventromedial NVmt. This study confirms and develops earlier experiments by exploring the physiological nature and functional topography of the connectivity between NVsnpr and NVmt that was demonstrated in the past with neuroanatomical techniques.

Supratrigeminal Bilaterally Projecting Neurons Maintain Basal Tone and Enable Bilateral Phasic Activation of Jaw-Closing Muscles

Anatomical studies have identified brainstem neurons that project bilaterally to left and right oromotor pools, which could potentially mediate bilateral muscle coordination. We use retrograde lentiviruses combined with a split-intein-mediated split-Cre-recombinase system in mice to isolate, characterize, and manipulate a population of neurons projecting to both the left and right jaw-closing trigeminal motoneurons. We find that these bilaterally projecting premotor neurons (BPNs) reside primarily in the supratrigeminal nucleus (SupV) and the parvicellular and intermediate reticular regions dorsal to the facial motor nucleus. These BPNs also project to multiple midbrain and brainstem targets implicated in orofacial sensorimotor control, and consist of a mix of glutamatergic, GABAergic, and glycinergic neurons, which can drive both excitatory and inhibitory inputs to trigeminal motoneurons when optogenetically activated in slice. Silencing BPNs with tetanus toxin light chain (TeNT) increases bilateral masseter activation during chewing, an effect driven by the expression of TeNT in SupV BPNs. Acute unilateral optogenetic inhibition of SupV BPNs identifies a group of tonically active neurons that function to lower masseter muscle tone, whereas unilateral optogenetic activation of SupV BPNs is sufficient to induce bilateral masseter activation both during resting state and during chewing. These results provide evidence for SupV BPNs in tonically modulating jaw-closing muscle tone and in mediating bilateral jaw closing.

SIGNIFICANCE STATEMENT

We developed a method that combines retrograde lentiviruses with the split-intein-split-Cre system in mice to isolate, characterize, and manipulate neurons that project to both left and right jaw-closing motoneurons. We show that these bilaterally projecting premotor neurons (BPNs) reside primarily in the supratrigeminal nucleus and the rostral parvicellular and intermediate reticular nuclei. BPNs consist of both excitatory and inhibitory populations, and also project to multiple brainstem nuclei implicated in orofacial sensorimotor control. Manipulation of the supratrigeminal BPNs during natural jaw-closing behavior reveals a dual role for these neurons in eliciting phasic muscle activation and in maintaining basal muscle tone. The retrograde lentivirus carrying the split-intein-split-Cre system can be applied to study any neurons with bifurcating axons innervating two brain regions.

A novel fluorescent retrograde neural tracer: cholera toxin B conjugated carbon dots

The retrograde neuroanatomical tracing method is a key technique to study the complex interconnections of the nervous system. Traditional tracers have several drawbacks, including time-consuming immunohistochemical or immunofluorescent staining procedures, rapid fluorescence quenching and low fluorescence intensity. Carbon dots (CDs) have been widely used as a fluorescent bio-probe due to their ultrasmall size, excellent optical properties, chemical stability, biocompatibility and low toxicity. Herein, we develop a novel fluorescent neural tracer: cholera toxin B-carbon dot conjugates (CTB-CDs). It can be taken up and retrogradely transported by neurons in the peripheral nervous system of rats. Our results show that CTB-CDs possess high photoluminescence intensity, good optical stability, a long shelf-life and non-toxicity. Tracing with CTB-CDs is a direct and more economical way of performing retrograde labelling experiments. Therefore, CTB-CDs are reliable fluorescent retrograde tracers.

Noradrenergic modulation of masseter muscle activity during natural rapid eye movement sleep requires glutamatergic signalling at the trigeminal motor nucleus

Noradrenergic neurotransmission in the brainstem is closely coupled to changes in muscle activity across the sleep-wake cycle, and noradrenaline is considered to be a key excitatory neuromodulator that reinforces the arousal-related stimulus on motoneurons to drive movement. However, it is unknown if α-1 noradrenoceptor activation increases motoneuron responsiveness to excitatory glutamate (AMPA) receptor-mediated inputs during natural behaviour. We studied the effects of noradrenaline on AMPA receptor-mediated motor activity at the motoneuron level in freely behaving rats, particularly during rapid eye movement (REM) sleep, a period during which both AMPA receptor-triggered muscle twitches and periods of muscle quiescence in which AMPA drive is silent are exhibited. Male rats were subjected to electromyography and electroencephalography recording to monitor sleep and waking behaviour. The implantation of a cannula into the trigeminal motor nucleus of the brainstem allowed us to perfuse noradrenergic and glutamatergic drugs by reverse microdialysis, and thus to use masseter muscle activity as an index of motoneuronal output. We found that endogenous excitation of both α-1 noradrenoceptor and AMPA receptors during waking are coupled to motor activity; however, REM sleep exhibits an absence of endogenous α-1 noradrenoceptor activity. Importantly, exogenous α-1 noradrenoceptor stimulation cannot reverse the muscle twitch suppression induced by AMPA receptor blockade and nor can it elevate muscle activity during quiet REM, a phase when endogenous AMPA receptor activity is subthreshold. We conclude that the presence of an endogenous glutamatergic drive is necessary for noradrenaline to trigger muscle activity at the level of the motoneuron in an animal behaving naturally.

Monosynaptic premotor circuit tracing reveals neural substrates for oro-motor coordination

Feeding behaviors require intricately coordinated activation among the muscles of the jaw, tongue, and face, but the neural anatomical substrates underlying such coordination remain unclear. In this study, we investigate whether the premotor circuitry of jaw and tongue motoneurons contain elements for coordination. Using a modified monosynaptic rabies virus-based transsynaptic tracing strategy, we systematically mapped premotor neurons for the jaw-closing masseter muscle and the tongue-protruding genioglossus muscle. The maps revealed that the two groups of premotor neurons are distributed in regions implicated in rhythmogenesis, descending motor control, and sensory feedback. Importantly, we discovered several premotor connection configurations that are ideally suited for coordinating bilaterally symmetric jaw movements, and for enabling co-activation of specific jaw, tongue, and facial muscles. Our findings suggest that shared premotor neurons that form specific multi-target connections with selected motoneurons are a simple and general solution to the problem of orofacial coordination.DOI: http://dx.doi.org/10.7554/eLife.02511.001.

Identification of the transmitter and receptor mechanisms responsible for REM sleep paralysis

During REM sleep the CNS is intensely active, but the skeletal motor system is paradoxically forced into a state of muscle paralysis. The mechanisms that trigger REM sleep paralysis are a matter of intense debate. Two competing theories argue that it is caused by either active inhibition or reduced excitation of somatic motoneuron activity. Here, we identify the transmitter and receptor mechanisms that function to silence skeletal muscles during REM sleep. We used behavioral, electrophysiological, receptor pharmacology and neuroanatomical approaches to determine how trigeminal motoneurons and masseter muscles are switched off during REM sleep in rats. We show that a powerful GABA and glycine drive triggers REM paralysis by switching off motoneuron activity. This drive inhibits motoneurons by targeting both metabotropic GABA(B) and ionotropic GABA(A)/glycine receptors. REM paralysis is only reversed when motoneurons are cut off from GABA(B), GABA(A) and glycine receptor-mediated inhibition. Neither metabotropic nor ionotropic receptor mechanisms alone are sufficient for generating REM paralysis. These results demonstrate that multiple receptor mechanisms trigger REM sleep paralysis. Breakdown in normal REM inhibition may underlie common sleep motor pathologies such as REM sleep behavior disorder.

Generation of the masticatory central pattern and its modulation by sensory feedback

The basic pattern of rhythmic jaw movements produced during mastication is generated by a neuronal network located in the brainstem and referred to as the masticatory central pattern generator (CPG). This network composed of neurons mostly associated to the trigeminal system is found between the rostral borders of the trigeminal motor nucleus and facial nucleus. This review summarizes current knowledge on the anatomical organization, the development, the connectivity and the cellular properties of these trigeminal circuits in relation to mastication. Emphasis is put on a population of rhythmogenic neurons in the dorsal part of the trigeminal sensory nucleus. These neurons have intrinsic bursting capabilities, supported by a persistent Na(+) current (I(NaP)), which are enhanced when the extracellular concentration of Ca(2+) diminishes. Presented evidence suggest that the Ca(2+) dependency of this current combined with its voltage-dependency could provide a mechanism for cortical and sensory afferent inputs to the nucleus to interact with the rhythmogenic properties of its neurons to adjust and adapt the rhythmic output. Astrocytes are postulated to contribute to this process by modulating the extracellular Ca(2+) concentration and a model is proposed to explain how functional microdomains defined by the boundaries of astrocytic syncitia may form under the influence of incoming inputs.

Dopamine triggers skeletal muscle tone by activating D1-like receptors on somatic motoneurons

The dopamine system plays an integral role in motor physiology. Dopamine controls movement by modulation of higher-order motor centers (e.g., basal ganglia) but may also regulate movement by directly controlling motoneuron function. Even though dopamine cells synapse onto motoneurons, which themselves express dopamine receptors, it is unknown whether dopamine modulates skeletal muscle activity. Therefore, we aimed to determine whether changes in dopaminergic neurotransmission at a somatic motor pool affect motor outflow to skeletal muscles. We used microinjection, neuropharmacology, electrophysiology, and histology to determine whether manipulation of D(1)- and D(2)-like receptors on trigeminal motoneurons affects masseter and/or tensor palatini muscle tone in anesthetized rats. We found that apomorphine (a dopamine analog) activated trigeminal motoneurons and triggered a potent increase in both masseter and tensor palatini tone. This excitatory effect is mediated by D(1)-like receptors because specific D(1)-like receptor activation strengthened muscle tone and blockade of these receptors prevented dopamine-driven activation of motoneurons. Blockade of D(1)-like receptors alone had no detectable effect on basal masseter/tensor palatini tone, indicating the absence of a functional dopamine drive onto trigeminal motoneurons, at least during isoflurane anesthesia. Finally, we showed that D(2)-like receptors do not affect either trigeminal motoneuron function or masseter/tensor palatini muscle tone. Our results provide the first demonstration that dopamine can directly control movement by manipulating somatic motoneuron behavior and skeletal muscle tone.

The trigeminal circuits responsible for chewing

Mastication is a vital function that ensures that ingested food is broken down into pieces and prepared for digestion. This review outlines the masticatory behavior in terms of the muscle activation patterns and jaw movements and gives an overview of the organization and function of the trigeminal neuronal circuits that are known to take part in the generation and control of oro-facial motor functions. The basic pattern of rhythmic jaw movements produced during mastication is generated by a Central Pattern Generator (CPG) located in the pons and medulla. Neurons within the CPG have intrinsic properties that produce a rhythmic activity, but the output of these neurons is modified by inputs that descend from the higher centers of the brain, and by feedback from sensory receptors, in order to constantly adapt the movement to the food properties.

Effect of the Stimulation of Sensory Inputs on the Firing of Neurons of the Trigeminal Main Sensory Nucleus in the Rat

Mastication can be triggered by repetitive stimulation of the cortex or of sensory inputs, but is patterned by a brain stem central pattern generator (CPG). This CPG may include the dorsal part of the principal trigeminal sensory nucleus (NVsnpr), where neurons burst repetitively when the extracellular concentration of Ca2+ ([Ca2+]e) drops. We examined the effects of repetitive stimulation of sensory afferents of the trigeminal tract on activity of NVsnpr neurons recorded extracellularly in vitro under physiologic [Ca2+]e (1.6 mM). Spontaneously active cells had either a tonic ( n = 145) or a bursting ( n = 46) firing pattern. Afferent stimulation altered burst duration and/or burst frequency in bursting cells and firing frequency in most tonic cells. In 28% of the latter, the firing pattern switched to rhythmic bursting. This effect could be mimicked by local application of N-methyl-d-aspartate and blocked by APV but not DNQX. Detailed analysis showed that rhythm indices (RIs) of 35 tonic neurons that were negative (nonrhythmic) before stimulation became significantly rhythmic (RI ≥ 0.01) after stimulation. Mean and median bursting frequency of these units were 8.32 ± 0.72 (SE) Hz and 6.25 Hz (range, 2.5–17.5 Hz). In seven instances, two units were recorded simultaneously, and cross-correlation analysis showed that firing of six pairs was rhythmic and synchronized after stimulation. Optimal stimulation parameters for eliciting rhythmic bursting consisted in 500-ms trains of pulses delivered at 40–60 Hz. Together, our results show that repetitive stimulation of sensory afferents in vitro can elicit masticatory-like rhythmic bursting in NVsnpr neurons at physiological [Ca2+]e.

Properties of synaptic transmission from the reticular formation dorsal to the facial nucleus to trigeminal motoneurons during early postnatal development in rats

We previously reported that electrical stimulation of the reticular formation dorsal to the facial nucleus (RdVII) elicited excitatory masseter responses at short latencies and that RdVII neurons were antidromically activated by stimulation of the trigeminal motor nucleus (MoV), suggesting that excitatory premotor neurons targeting the MoV are likely located in the RdVII. We thus examined the properties of synaptic transmission from the RdVII to jaw-closing and jaw-opening motoneurons in horizontal brainstem preparations from developing rats using voltage-sensitive dye, patch-clamp recordings and laser photostimulation. Electrical stimulation of the RdVII evoked optical responses in the MoV. Combined bath application of the non-N-methyl-d-aspartate (non-NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (APV) reduced these optical responses, and addition of the glycine receptor antagonist strychnine and the GABA(A) receptor antagonist bicuculline further reduced the remaining responses. Electrical stimulation of the RdVII evoked postsynaptic currents (PSCs) in all 19 masseter motoneurons tested in postnatal day (P)1-4 rats, and application of CNQX and the NMDA receptor antagonist (+/-)-3(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) reduced the PSC amplitudes by more than 50%. In the presence of CNQX and CPP, the GABA(A) receptor antagonist SR95531 further reduced PSC amplitude, and addition of strychnine abolished the remaining PSCs. Photostimulation of the RdVII with caged glutamate also evoked PSCs in masseter motoneurons of P3-4 rats. In P8-11 rats, electrical stimulation of the RdVII also evoked PSCs in all 14 masseter motoneurons tested, and the effects of the antagonists on the PSCs were similar to those in P1-4 rats. On the other hand, RdVII stimulation evoked PSCs in only three of 16 digastric motoneurons tested. These results suggest that both neonatal and juvenile jaw-closing motoneurons receive strong synaptic inputs from the RdVII through activation of glutamate, glycine and GABA(A) receptors, whereas inputs from the RdVII to jaw-opening motoneurons seem to be weak.

Central pattern generation involved in oral and respiratory control for feeding in the term infant

PURPOSE OF REVIEW

Drinking and eating are essential skills for survival and benefit from the coordination of several pattern generating networks and their musculoskeletal effectors to achieve safe swallows. Oralpharyngoesophageal motility develops during infancy and early childhood, and is influenced by various factors, including neuromuscular maturation, dietary and postural habits, arousal state, ongoing illnesses, congenital anomalies, and the effects of medical or surgical interventions. Gastroesophageal reflux is frequent in neonates and infants, and its role in neonatal morbidity including dysphagia, chronic lung disease, or apparent life-threatening events is not well understood. This review highlights recent studies aimed at understanding the development of oral feeding skills, and cross-system interactions among the brainstem, spinal, and cerebral networks involved in feeding.

RECENT FINDINGS

Functional linkages between suck-swallow and swallow-respiration manifest transitional forms during late gestation through the first year of life, which can be delayed or modified by sensory experience or disease processes, or both. Relevant central pattern generator (CPG) networks and their neuromuscular targets attain functional status at different rates, which ultimately influences cross-system CPG interactions. Entrainment of trigeminal primary afferents accelerates pattern genesis for the suck CPG and transition-to-oral feed in the RDS preterm infant.

SUMMARY

The genesis of within-system CPG control for rate and amplitude scaling matures differentially for suck, mastication, swallow, and respiration. Cross-system interactions among these CPGs represent targets of opportunity for new interventions, which optimize experience-dependent mechanisms to promote safe swallows among newborn and pediatric patients.

The efficacy of the fluorescent conjugates of cholera toxin subunit B for multiple retrograde tract tracing in the central nervous system

Cholera toxin subunit B (CTB) is a sensitive neuroanatomical tracer that generally transports retrogradely in the nervous system, and has been used extensively in brightfield microscopy. Recently, Alexa Fluor (AF) conjugates of CTB have been made available, which now allows multiple tracing with CTB. In this study, we examined the efficacy of these new AF-CTB conjugates when injected into the brain, and compared the results to our previous experiences using fluorescent 3k dextran amines. To test this, we injected AF 488 and AF 594 CTB into the anterior cingulate cortex and the medial agranular cortex in the rat, and examined the retrograde transport to the lateral posterior nucleus of the thalamus. We found that CTB was very viscous but yet very sensitive: small injection sites revealed very intense and detailed retrograde labeling. Anterograde transport was seen only when tissue at the injection site was damaged. These findings suggest that AF-CTB is a very reliable and sensitive retrograde tracer, and should be the first choice retrograde tracer for experiments examining multiple pathways within the same brain.

Multiple neuroanatomical tract-tracing using fluorescent Alexa Fluor conjugates of cholera toxin subunit B in rats

Cholera toxin subunit B (CTB) is a highly sensitive retrograde neuroanatomical tracer. With the new availability of fluorescent Alexa Fluor (AF) conjugates of CTB, multiple neuroanatomical connections can be reliably studied and compared in the same animal. Here we provide a protocol that describes the use of AF-CTB for studying connections in the central nervous system of rats. The viscous properties of CTB allow small and discreet injection sites yet still show robust retrograde labeling. Furthermore, the AF conjugates are extremely bright and photostable, compared with other conventional fluorescent tracers. This protocol can also be adapted for use with other neuroanatomical tracers. Including a 7-d survival period, this protocol takes approximately 11 to 12 d to complete in its entirety.

Morphine increases acetylcholine release in the trigeminal nuclear complex

STUDY OBJECTIVES

The trigeminal nuclear complex (V) contains cholinergic neurons and includes the principal sensory trigeminal nucleus (PSTN) which receives sensory input from the face and jaw, and the trigeminal motor nucleus (MoV) which innervates the muscles of mastication. Pain associated with pathologies of V is often managed with opioids but no studies have characterized the effect of opioids on acetylcholine (ACh) release in PSTN and MoV. Opioids can increase or decrease ACh release in brainstem nuclei. Therefore, the present experiments tested the 2-tailed hypothesis that microdialysis delivery of opioids to the PSTN and MoV significantly alters ACh release.

DESIGN

Using a within-subjects design and isoflurane-anesthetized Wistar rats (n=53), ACh release in PSTN during microdialysis with Ringer's solution (control) was compared to ACh release during dialysis delivery of the sodium channel blocker tetrodotoxin, muscarinic agonist bethanechol, opioid agonist morphine, mu opioid agonist DAMGO, antagonists for mu (naloxone) and kappa (nor-binaltorphimine; nor-BNI) opioid receptors, and GABAA antagonist bicuculline.

MEASUREMENTS AND RESULTS

Tetrodotoxin decreased ACh, confirming action potential-dependent ACh release. Bethanechol and morphine caused a concentration-dependent increase in PSTN ACh release. The morphine-induced increase in ACh release was blocked by nor-BNI but not by naloxone. Bicuculline delivered to the PSTN also increased ACh release. ACh release in the MoV was increased by morphine, and this increase was not blocked by naloxone or nor-BNI.

CONCLUSIONS

These data comprise the first direct measures of ACh release in PSTN and MoV and suggest synaptic disinhibition as one possible mechanism by which morphine increases ACh release in the trigeminal nuclei.

GABA distribution in lamprey is phylogenetically conserved

The localization of gamma-aminobutyric acid (GABA) has been well described in most classes of vertebrates but not in adult lampreys. The question if the GABA distribution is similar throughout the vertebrate subphylum is therefore still to be addressed. We here investigate two lamprey species, the sea lamprey, Petromyzon marinus, and the river lamprey, Lampetra fluviatilis, and compare the GABA pattern with that of other vertebrates. The present immunohistochemical study provides an anatomical basis for the general distribution and precise localization of GABAergic neurons in the adult lamprey forebrain and brainstem. GABA-immunoreactive cells were organized in a virtually identical manner in the two species. They were found throughout the brain, with the following regions being of particular interest: the granular cell layer of the olfactory bulb, the nucleus of the anterior commissure, the septum, the lateral and medial pallia, the striatum, the nucleus of the postoptic commissure, the thalamus, the hypothalamus, and pretectal areas, the optic tectum, the torus semicircularis, the mesencephalic tegmentum, restricted regions of the rhombencephalic tegmentum, the octavolateral area, and the dorsal column nucleus. The GABA distribution found in cyclostomes is very similar to that of other classes of vertebrates, including mammals. Since the lamprey diverged from the main vertebrate line around 450 million years ago, this implies that already at that time the basic vertebrate plan for the GABA innervation in different parts of the brain had been developed.