The somatotopic organization of trigeminal premotoneurons in the cat brainstem

Efferent and afferent connections of supratrigeminal neurons conveying orofacial muscle proprioception in rats

The supratrigeminal nucleus (Su5) is a key structure for controlling jaw movements; it receives proprioceptive sensation from jaw-closing muscle spindles (JCMSs) and sends projections to the trigeminal motor nucleus (Mo5). However, the central projections and regulation of JCMS proprioceptive sensation are not yet fully understood. Therefore, we aimed to reveal the efferent and afferent connections of the Su5 using neuronal tract tracings. Anterograde tracer injections into the Su5 revealed that the Su5 sends contralateral projections (or bilateral projections with a contralateral predominance) to the Su5, basilar pontine nuclei, pontine reticular nucleus, deep mesencephalic nucleus, superior colliculus, caudo-ventromedial edge of the ventral posteromedial thalamic nucleus, parafascicular thalamic nucleus, zona incerta, and lateral hypothalamus, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) to the intertrigeminal region, trigeminal oral subnucleus, dorsal medullary reticular formation, and hypoglossal nucleus as well as the Mo5. Retrograde tracer injections into the Su5 demonstrated that the Su5 receives bilateral projections with a contralateral predominance (or contralateral projections) from the primary and secondary somatosensory cortices, granular insular cortex, and Su5, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) from the dorsal peduncular cortex, bed nuclei of stria terminalis, central amygdaloid nucleus, lateral hypothalamus, parasubthalamic nucleus, trigeminal mesencephalic nucleus, parabrachial nucleus, juxtatrigeminal region, trigeminal oral and caudal subnuclei, and dorsal medullary reticular formation. These findings suggest that the Su5, which receives JCMS proprioception, has efferent and afferent connections with multiple brain regions that are involved in emotional and autonomic functions as well as orofacial motor functions.

Distinctive features of Phox2b-expressing neurons in the rat reticular formation dorsal to the trigeminal motor nucleus

Phox2b encodes a paired-like homeodomain-containing transcription factor essential for development of the autonomic nervous system. Phox2b-expressing (Phox2b⁺) neurons are present in the reticular formation dorsal to the trigeminal motor nucleus (RdV) as well as the nucleus of the solitary tract and parafacial respiratory group. However, the nature of Phox2b⁺ RdV neurons is still unclear. We investigated the physiological and morphological properties of Phox2b⁺ RdV neurons using postnatal day 2-7 transgenic rats expressing yellow fluorescent protein under the control of Phox2b. Almost all of Phox2b⁺ RdV neurons were glutamatergic, whereas Phox2b-negative (Phox2b^-) RdV neurons consisted of a few glutamatergic, many GABAergic, and many glycinergic neurons. The majority (48/56) of Phox2b⁺ neurons showed low-frequency firing (LF), while most of Phox2b^- neurons (35/42) exhibited high-frequency firing (HF) in response to intracellularly injected currents. All, but one, Phox2b⁺ neurons (55/56) did not fire spontaneously, whereas three-fourths of the Phox2b^- neurons (31/42) were spontaneously active. K⁺ channel and persistent Na⁺ current blockers affected the firing of LF and HF neurons. The majority of Phox2b⁺ (35/46) and half of the Phox2b^- neurons (19/40) did not respond to stimulations of the mesencephalic trigeminal nucleus, the trigeminal tract, and the principal sensory trigeminal nucleus. Biocytin labeling revealed that about half of the Phox2b⁺ (5/12) and Phox2b^- RdV neurons (5/10) send their axons to the trigeminal motor nucleus. These results suggest that Phox2b⁺ RdV neurons have distinct neurotransmitter phenotypes and firing properties from Phox2b^- RdV neurons and might play important roles in feeding-related functions including suckling and possibly mastication.

Electrophysiological and morphological properties of rat supratrigeminal premotor neurons targeting the trigeminal motor nucleus

The electrophysiological and morphological characteristics of premotor neurons in the supratrigeminal region (SupV) targeting the trigeminal motor nucleus (MoV) were examined in neonatal rat brain stem slice preparations with Ca(2+) imaging, whole cell recordings, and intracellular biocytin labeling. First, we screened SupV neurons that showed a rapid rise in intracellular free Ca(2+) concentration ([Ca(2+)]i) after single-pulse electrical stimulation of the ipsilateral MoV. Subsequent whole cell recordings were generated from the screened SupV neurons, and their antidromic responses to MoV stimulation were confirmed. We divided the antidromically activated premotor neurons into two groups according to their discharge patterns during the steady state in response to 1-s depolarizing current pulses: those firing at a frequency higher (HF neurons, n = 19) or lower (LF neurons, n = 17) than 33 Hz. In addition, HF neurons had a narrower action potential and a larger afterhyperpolarization than LF neurons. Intracellular labeling revealed that the axons of all HF neurons (6/6) and half of the LF neurons (4/9) entered the MoV from its dorsomedial aspect, whereas the axons of the remaining LF neurons (5/9) entered the MoV from its dorsolateral aspect. Furthermore, the dendrites of three HF neurons penetrated into the principal sensory trigeminal nucleus (Vp), whereas the dendrites of all LF neurons were confined within the SupV. These results suggest that the types of SupV premotor neurons targeting the MoV with different firing properties have different dendritic and axonal morphologies, and these SupV neuron classes may play unique roles in diverse oral motor behaviors, such as suckling and mastication.

Trigeminal and telencephalic projections to jaw and other upper vocal tract premotor neurons in songbirds: sensorimotor circuitry for beak movements during singing

During singing in songbirds, the extent of beak opening, like the extent of mouth opening in human singers, is partially correlated with the fundamental frequency of the sounds emitted. Since song in songbirds is under the control of "the song system" (a collection of interconnected forebrain nuclei dedicated to the learning and production of song), it might be expected that beak movements during singing would also be controlled by this system. However, direct neural connections between the telencephalic output of the song system and beak muscle motor neurons in the brainstem are conspicuous by their absence, leaving unresolved the question of how beak movements are affected during singing. By using standard tract tracing methods, we sought to answer this question by defining beak premotor neurons and examining their afferent projections. In the caudal medulla, jaw premotor cell bodies were located adjacent to the terminal field of the output of the song system, into which many premotor neurons extended their dendrites. The premotor neurons also received a novel input from the trigeminal ganglion and an overlapping input from a lateral arcopallial component of a trigeminal sensorimotor circuit that traverses the forebrain. The ganglionic input in songbirds, which is not present in doves and pigeons that vocalize with a closed beak, may modulate the activity of beak premotor neurons in concert with the output of the song system. These inputs to jaw premotor neurons could, together, affect beak movements as a means of modulating filter properties of the upper vocal tract during singing.

Convergent Pre-motoneuronal Inputs to Single Trigeminal Motoneurons

Because pre-motor neurons targeting trigeminal motoneurons are located in various regions, including the supratrigeminal (SupV) and intertrigeminal (IntV) regions, the principal sensory trigeminal nucleus (PrV), and the region dorsal to the PrV (dRt), a single trigeminal motoneuron may receive differential convergent inputs from these regions. We thus examined the properties of synaptic inputs from these regions to masseter motoneurons (MMNs) and digastric motoneurons (DMNs) in brainstem slice preparations obtained from P1-5 neonatal rats, using whole-cell recordings and laser photolysis of caged glutamate. Photostimulation of multiple regions within the SupV, IntV, PrV, and dRt induced post-synaptic currents (PSCs) in 14 of 19 MMNs and 18 of 26 DMNs. Furthermore, the stimulation of the lateral SupV significantly induced burst PSCs in MMNs more often than low-frequency PSCs in MMNs or burst PSCs in DMNs. Similar results were obtained in the presence of the GABAA receptor antagonist SR95531 and the glycine receptor antagonist strychnine. These results suggest that both neonatal MMNs and DMNs receive convergent glutamatergic inputs from the SupV, IntV, PrV, and dRt, and that the lateral SupV sends burst inputs predominantly to the MMNs. Such convergent pre-motoneuronal inputs to trigeminal motoneurons may contribute to the proper execution of neonatal oro-motor functions.

Forebrain projections to brainstem nuclei involved in the control of mandibular movements in rats

Mandibular movements occur through the triggering of trigeminal motoneurons. Aberrant movements by orofacial muscles are characteristic of orofacial motor disorders, such as nocturnal bruxism (clenching or grinding of the dentition during sleep). Previous studies have suggested that autonomic changes occur during bruxism episodes. Although it is known that emotional responses increase jaw movement, the brain pathways linking forebrain limbic nuclei and the trigeminal motor nucleus remain unclear. Here we show that neurons in the lateral hypothalamic area, in the central nucleus of the amygdala, and in the parasubthalamic nucleus, project to the trigeminal motor nucleus or to reticular regions around the motor nucleus (Regio h) and in the mesencephalic trigeminal nucleus. We observed orexin co-expression in neurons projecting from the lateral hypothalamic area to the trigeminal motor nucleus. In the central nucleus of the amygdala, neurons projecting to the trigeminal motor nucleus are innervated by corticotrophin-releasing factor immunoreactive fibers. We also observed that the mesencephalic trigeminal nucleus receives dense innervation from orexin and corticotrophin-releasing factor immunoreactive fibers. Therefore, forebrain nuclei related to autonomic control and stress responses might influence the activity of trigeminal motor neurons and consequently play a role in the physiopathology of nocturnal bruxism.

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.

Heterogeneous activity level of jaw-closing and -opening muscles and its association with arousal levels during sleep in the guinea pig

Exaggerated jaw motor activities during sleep are associated with muscle symptoms in the jaw-closing rather than the jaw-opening muscles. The intrinsic activity of antagonistic jaw muscles during sleep remains unknown. This study aims to assess the balance of muscle activity between masseter (MA) and digastric (DG) muscles during sleep in guinea pigs. Electroencephalogram (EEG), electroocculogram, and electromyograms (EMGs) of dorsal neck, MA, and DG muscles were recorded with video during sleep-wake cycles. These variables were quantified for each 10-s epoch. The magnitude of muscle activity during sleep in relation to mean EMG activity of total wakefulness was up to three times higher for MA muscle than for DG muscle for nonrapid eye movement (NREM) and rapid-eye-movement (REM) sleep. Although the activity level of the two jaw muscles fluctuated during sleep, the ratio of activity level for each epoch was not proportional. Epochs with a high activity level for each muscle were associated with a decrease in δEEG power and/or an increase in heart rate in NREM sleep. However, this association with heart rate and activity levels was not observed in REM sleep. These results suggest that in guinea pigs, the magnitude of muscle activity for antagonistic jaw muscles is heterogeneously modulated during sleep, characterized by a high activity level in the jaw-closing muscle. Fluctuations in the activity are influenced by transient arousal levels in NREM sleep but, in REM sleep, the distinct controls may contribute to the fluctuation. The above intrinsic characteristics could underlie the exaggeration of jaw motor activities during sleep (e.g., sleep bruxism).

Central amygdaloid axon terminals are in contact with retrorubral field neurons that project to the parvicellular reticular formation of the medulla oblongata in the rat

The retrorubral field (RRF) contains numerous dopaminergic neurons and projects to the parvicellular reticular formation (RFp) of the medullary and pontomedullary brainstem, where many premotor neurons project to the orofacial motor nuclei. To know how the amygdala affects the RRF-RFp pathway in the rat, we first examined the synaptic organization between the central amygdaloid nucleus (CeA) fibers and the RFp-projecting RRF neurons by using combined anterograde and retrograde tracing techniques. After ipsilateral injections of biotinylated dextran amine (BDA) into the CeA and Fluoro-gold (FG) into the RFp, the prominent overlapping distribution of BDA-labeled axon terminals and FG-labeled neurons was found in the lateral part of the RRF ipsilateral to the injection sites, where the BDA-labeled axon terminals made symmetrical synapses with somata and dendrites of the FG-labeled neurons. Using a combination of retrograde tracing and immunohistochemistry for tyrosine hydroxylase (TH), we secondly demonstrated that the RFp-projecting RRF neurons were immunonegative for TH. Using a combination of anterograde tracing and immunohistochemistry for glutamic acid decarboxylase (GAD), we finally revealed that the CeA axon terminals in the RRF were immunoreactive for GAD. The present results suggest that GABAergic CeA neurons may exert inhibitory influences on non-dopaminergic RRF neurons that project to the RFp in the control of orofacial movements closely related to emotional behavior.