Osteopontin-immunoreactivity (OPN-ir) was examined in the oro-facial tissues and trigeminal sensory nuclei (principal sensory nucleus and spinal trigeminal nucleus) to ascertain the peripheral ending and central projection of OPN-containing primary sensory neurons in the trigeminal ganglion (TG). No staining was observed using mouse monoclonal anti-OPN antibody preabsorbed with recombinant mature OPN. OPN-immunoreactive (ir) peripheral endings were classified into two types: encapsulated and unencapsulated types. Unencapsulated endings were subdivided into two types: simple and complex types. Simple endings were characterized by the thin neurite that was usually devoid of ramification. These endings were seen in the hard plate and gingiva. The complex type was characterized by the thick ramified neurite, and observed in the vibrissa, hard palate, and molar periodontal ligament. Encapsulated endings were found only in the hard palate. The trigeminal sensory nuclei contained OPN-ir cell bodies and neuropil. The neuropil was devoid of ir in laminae I and II of the medullary dorsal horn (MDH), and had various staining intensities in other regions of the trigeminal sensory nuclei. Transection of the infraorbital and inferior alveolar nerves caused an increase of OPN-ir intensity in ipsilateral TG neurons. The staining intensity of the neuropil also increased in the trigeminal sensory nuclei ipsilateral to the neurotomy excepting laminae I and II of the MDH. The present study indicates that OPN-ir primary sensory neurons in the TG innervate encapsulated and unencapsulated corpuscular endings. Such neurons probably project their central terminals to the trigeminal sensory nuclei except for the superficial laminae of the MDH.

Bone morphogenetic proteins (BMP) exert its biological functions by interacting with membrane bound receptors. However, functions of BMPs are also regulated in the extracellular space by secreted antagonistic regulators, such as chordin and noggin. Although the deep involvement of BMP signaling in the development and functions of the trigeminal nuclei has been postulated, little information is available for its expression in the trigeminal nuclei. We, thus, investigated chordin and noggin expression in the adult rat trigeminal nuclei using immunohistochemistry. Chordin and noggin were intensely expressed throughout the trigeminal nuclei. In addition, interesting differences are observed between chordin expression and noggin expression. For example, chordin prefers dendritic expression than noggin, suggesting that chordin is involved in the regulation of dendritic morphology and synaptic homeostasis. Furthermore, chordin and noggin were differentially expressed in the neuropil of the trigeminal nuclei. Since BMP signaling is known to play a pivotal role to make precise neural network, theses differences might be important to keep precise interneuronal connections by regulating local BMP signaling intensity in each region. Interestingly, we also detected chordin and noggin expression in axons of the trigeminal nerves. These data indicate that chordin and noggin play pivotal roles also in the adult trigeminal system.

Little is known about the central projection patterns of trigeminal afferent neurons expressing the vanilloid receptor TRPV1 and their coexpression of neuromodulatory peptides. To address these issues, we examined the distribution of TRPV1-positive neurons in the trigeminal ganglion (TG) and trigeminal sensory nuclei principalis (Vp), oralis (Vo), interpolaris (Vi), and caudalis (Vc) in the rat via light and electron microscopy. In addition, we studied the colocalization of TRPV1-positive neurons with substance P (SP) and calcitonin gene-related peptide (CGRP) via confocal microscopy. In TG, only small and medium-sized neurons were immunopositive for TRPV1. The staining for TRPV1 was found in axon collaterals in the dorsal parts of Vp, Vo, and Vi and in terminals and fibers throughout lamina I and the outer zone of lamina II (IIo) of Vc. With electron microscopy, TRPV1-positive fibers in the ascending and descending trigeminal tracts were found to be unmyelinated. Almost all TRPV1-positive terminals in Vc contained numerous large dense-core vesicles and formed synaptic contacts with single small dendrites. Multiple immunofluorescence revealed a high degree of colocalization of TRPV1 with SP and CGRP in TG neurons as well as in fibers and terminals confined to laminae I and IIo of Vc. These results suggest that the central projections of unmyelinated (C) afferents sensitive to noxious heat and capsaicin are organized differently between Vc and the rostral trigeminal nuclei and that Vc may play a role in the development of hyperalgesia.

The primary (S1) and secondary (S2) somatosensory cortices project to several trigeminal sensory nuclei. One putative function of these corticofugal projections is the gating of sensory transmission through the trigeminal principal nucleus (Pr5), and some have proposed that S1 and S2 project differentially to the spinal trigeminal subnuclei, which have inhibitory circuits that could inhibit or disinhibit the output projections of Pr5. Very little, however, is known about the origin of sensorimotor corticofugal projections and their patterns of termination in the various trigeminal nuclei. We addressed this issue by injecting anterograde tracers in S1, S2 and primary motor (M1) cortices, and quantitatively characterizing the distribution of labeled terminals within the entire rostro-caudal chain of trigeminal sub-nuclei. We confirmed our anterograde tracing results by injecting retrograde tracers at various rostro-caudal levels within the trigeminal sensory nuclei to determine the position of retrogradely labeled cortical cells with respect to S1 barrel cortex. Our results demonstrate that S1 and S2 projections terminate in largely overlapping regions but show some significant differences. Whereas S1 projection terminals tend to cluster within the principal trigeminal (Pr5), caudal spinal trigeminal interpolaris (Sp5ic), and the dorsal spinal trigeminal caudalis (Sp5c), S2 projection terminals are distributed in a continuum across all trigeminal nuclei. Contrary to the view that sensory gating could be mediated by differential activation of inhibitory interconnections between the spinal trigeminal subnuclei, we observed that projections from S1 and S2 are largely overlapping in these subnuclei despite the differences noted earlier.

Trigeminal primary afferents expressing P2X(3) receptor are involved in the transmission of orofacial nociceptive information. However, little is known about their central projection pattern and ultrastructural features within the trigeminal brainstem sensory nuclei (TBSN). Here we use multiple immunofluorescence and electron microscopy to characterize the P2X(3)-immunopositive (+) neurons in the trigeminal ganglion and describe the distribution and synaptic organization of their central terminals within the rat TBSN, including nuclei principalis (Vp), oralis (Vo), interpolaris (Vi), and caudalis (Vc). In the trigeminal ganglion, P2X(3) immunoreactivity was mainly in small and medium-sized somata, but also frequently in large somata. Although most P2X(3) (+) somata costained for the nonpeptidergic marker IB4, few costained for the peptidergic marker substance P. Most P2X(3) (+) fibers in the sensory root of trigeminal ganglion (92.9%) were unmyelinated, whereas the rest were small myelinated. In the TBSN, P2X(3) immunoreactivity was dispersed in the rostral TBSN but was dense in the superficial laminae of Vc, especially in the inner lamina II. The P2X(3) (+) terminals contained numerous clear, round vesicles and sparse large, dense-core vesicles. Typically, they were presynaptic to one or two dendritic shafts and also frequently postsynaptic to axonal endings, containing pleomorphic vesicles. Such P2X(3) (+) terminals, showing glomerular shape and complex synaptic relationships, and those exhibiting axoaxonic contacts, were more frequently seen in Vp than in any other TBSN. These results suggest that orofacial nociceptive information may be transmitted via P2X(3) (+) afferents to all TBSN and that it may be processed differently in different TBSN.

How do neurons in orofacial motor cortex (MCtx) orchestrate behaviors? We show that focal activation of MCtx corticobulbar neurons evokes behaviorally relevant concurrent movements of the forelimb, jaw, nose, and vibrissae. The projections from different locations in MCtx form gradients of boutons across premotor nuclei spinal trigeminal pars oralis (SpVO) and interpolaris rostralis (SpVIr). Furthermore, retrograde viral tracing from muscles that control orofacial actions shows that these premotor nuclei segregate their outputs. In the most dramatic case, both SpVO and SpVIr are premotor to forelimb and vibrissa muscles, while only SpVO is premotor to jaw muscles. Functional confirmation of the superimposed control by MCtx was obtained through selective optogenetic activation of corticobulbar neurons on the basis of their preferential projections to SpVO versus SpVIr. We conclude that neighboring projection neurons in orofacial MCtx form parallel pathways to distinct pools of trigeminal premotor neurons that coordinate motor actions into a behavior.

We have examined the cyto- and chemoarchitecture of the trigeminal nuclei of two monotremes using Nissl staining, enzyme reactivity for cytochrome oxidase, immunoreactivity for calcium binding proteins and non-phosphorylated neurofilament (SMI-32 antibody) and lectin histochemistry (Griffonia simplicifolia isolectin B4). The principal trigeminal nucleus and the oralis and interpolaris spinal trigeminal nuclei were substantially larger in the platypus than in either the echidna or rat, but the caudalis subnucleus was similar in size in both monotremes and the rat. The numerical density of Nissl stained neurons was higher in the principal, oralis and interpolaris nuclei of the platypus relative to the echidna, but similar to that in the rat. Neuropil immunoreactivity for parvalbumin was particularly intense in the principal trigeminal, oralis and interpolaris subnuclei of the platypus, but the numerical density of parvalbumin immunoreactive neurons was not particularly high in these nuclei of the platypus. Neuropil immunoreactivity for calbindin and calretinin was relatively weak in both monotremes, although calretinin immunoreactive somata made up a large proportion of neurons in the principal, oralis and interpolaris subnuclei of the echidna. Distribution of calretinin immunoreactivity and Griffonia simplicifolia B4 isolectin reactivity suggested that the caudalis subnucleus of the echidna does not have a clearly defined gelatinosus region. Our findings indicate that the trigeminal nuclei of the echidna do not appear to be highly specialized, but that the principal, oralis and interpolaris subnuclei of the platypus trigeminal complex are highly differentiated, presumably for processing of tactile and electrosensory information from the bill.

Ligands of the mu-opioid receptor are known to inhibit nociceptive transmission in the dorsal horn, yet the cellular site(s) of action for this inhibition remain to be fully elucidated. Neurons located in lamina I of the dorsal horn are involved in distinct aspects of nociceptive transmission. Neurons projecting to the thalamus are thought to be involved in sensory-discriminative aspects of pain perception, while neurons projecting to the parabrachial nucleus are thought to be important for emotional and/or autonomic responses to noxious stimuli. The present study examined these two populations of lamina I projection neurons in the trigeminal dorsal horn to determine if the mu-opioid receptor protein (MOR1) is differentially located in these populations of neurons. Lamina I projection neurons were identified using the retrograde tracer FluoroGold (FGold). FGold was injected into either the contralateral thalamus (ventral posterolateral (VPM)/ventral posterolateral (VPL) thalamic region) or into the ipsilateral parabrachial nuclei. The distribution of MOR1 in these neurons was determined using immunocytochemistry. The distribution of MOR1-ir within these two populations of lamina I projection neurons was examined by both confocal and electron microscopy. We found that both populations of projection neurons contained MOR1. Immunogold analyses revealed the presence of MOR1-ir at membrane sites and within the cytoplasm of these neurons. Cytoplasmic receptor labeling may represent sites of synthesis, recycling or reserve populations of receptors. MOR1 was primarily found in the somata and proximal dendrites of projection neurons. In addition, these neurons rarely received synaptic input from MOR1-containing axon terminals. These results indicate that lamina I neurons in trigeminal dorsal horn that project to the thalamic and parabrachial nuclei contain MOR1 and are likely sites of action for MOR ligands that modulate sensory and/or autonomic aspects of pain transmission in the trigeminal dorsal horn.

In our traditional view of the avian somatosensory system, input from the beak and head reaches the telencephalon via a disynaptic pathway, involving projections from the principal sensory nucleus (PrV) directly to nucleus basorostralis (previously called nucleus basalis), whereas input from the rest of the body follows a trisynatic pathway similar to that in mammals, involving projections from the dorsal column nuclei to the thalamus, and thence to somatosensory wulst. However, the role of the nuclei of the descending trigeminal tract (nTTD) in this scenario is unclear, partly because their ascending projections have been examined in only one species, the mallard duck. Here we examine the ascending projections of the nTTD in the zebra finch, using in vivo injections of biotinylated dextran amine and verification of projections by means of retrograde transport of the beta subunit of cholera toxin. The results show a high degree of interconnectivity within the nTTD, and that these nuclei project to PrV. We also find a projection from nTTD to the contralateral thalamic nucleus uvaeformis, a multi-sensory nucleus connected to the song system. Furthermore, our finding of a projection from nTTD to the contralateral somatosensory thalamic nucleus dorsalis intermedius ventralis anterior (DIVA) is consistent with the well-known projection in mammals from nTTD to the ventrobasal thalamus, suggesting that the ascending trigeminal pathways in birds and mammals are more similar than previously thought.

Numerous studies suggest an essential role for the intermediate (IRt) and parvocellular (PCRt) reticular formation (RF) in consummatory ingestive responses. Although the IRt and PCRt contain a large proportion of neurons with projections to the oromotor nuclei, these areas of the RF are heterogeneous with respect to neurotransmitter phenotypes. Glutamatergic, GABAergic, cholinergic, and nitrergic neurons are all found in the PCRt and IRt, but the projections of neurons with these phenotypes to the motor trigeminal (mV) and hypoglossal nucleus (mXII) has not been fully evaluated. In the present study, after small injections of Fluorogold (FG) into mV and mXII, sections were processed immunohistochemically to detect retrogradely labeled FG neurons in combination with the synthetic enzymes for nitric oxide (nitric oxide synthase) or acetylcholine (choline acetyltransferase) or in situ hybridization for the synthetic enzyme for GABA (GAD65/67) or the brainstem vesicular transporter for glutamate (VGLUT2). In three additional cases, FG injections were made into one motor nucleus and cholera toxin (subunit b) injected in the other to determine the presence of dual projection neurons. Premotor neurons to mXII (pre-mXII) were highly concentrated in the IRt. In contrast, there were nearly equal proportions of premotor-trigeminal neurons (pre-mV) in the IRt and PCRt. A high proportion of pre-oromotor neurons were positive for VGLUT2 (pre-mXII: 68%; pre-mV: 53%) but GABAergic projections were differentially distributed with a greater projection to mV (25%) compared to mXII (8%). Significant populations of cholinergic and nitrergic neurons overlapped pre-oromotor neurons, but there was sparse double-labeling (<10%). The IRt also contained a high proportion of neurons that projected to both mV and MXII. These different classes of premotor neurons in the IRt and PCRt provide a substrate for the rhythmic activation of lingual and masticatory muscles.

The sites of origin of serotoninergic afferents in the trigeminal motor (Vm), facial (VII), and hypoglossal nuclei (XII) were studied in the rat by fluorescent retrograde labeling with Fluoro-Gold, in combination with immunofluorescence histochemistry for serotonin (5-HT). The results indicated: (1) The nucleus raphe magnus, nucleus raphe pallidus, and nucleus raphe obscurus contained 5-HT neurons projecting to the Vm, VII or XII. (2) The nucleus raphe dorsalis sends 5-HT fibres to the Vm and VII, but not to the XII. (3) The gigantocellular reticular nucleus pars alpha contained 5-HT neurons projecting to the VII.

Electrical synapses formed by neuronal gap junctions composed of connexin36 (Cx36) are a common feature in mammalian brain circuitry, but less is known about their deployment in spinal cord. It has been reported based on connexin mRNA and/or protein detection that developing and/or mature motoneurons express a variety of connexins, including Cx26, Cx32, Cx36 and Cx43 in trigeminal motoneurons, Cx36, Cx37, Cx40, Cx43 and Cx45 in spinal motoneurons, and Cx32 in sexually dimorphic motoneurons. We re-examined the localization of these connexins during postnatal development and in adult rat and mouse using immunofluorescence labeling for each connexin. We found Cx26 in association only with leptomeninges in the trigeminal motor nucleus (Mo5), Cx32 only with oligodendrocytes and myelinated fibers among motoneurons in this nucleus and in the spinal cord, and Cx37, Cx40 and Cx45 only with blood vessels in the ventral horn of spinal cord, including those among motoneurons. By freeze-fracture replica immunolabeling, > 100 astrocyte gap junctions but no neuronal gap junctions were found based on immunogold labeling for Cx43, whereas 16 neuronal gap junctions at postnatal day (P)4, P7 and P18 were detected based on Cx36 labeling. Punctate labeling for Cx36 was localized to the somatic and dendritic surfaces of peripherin-positive motoneurons in the Mo5, motoneurons throughout the spinal cord, and sexually dimorphic motoneurons at lower lumbar levels. In studies of electrical synapses and electrical transmission between developing and between adult motoneurons, our results serve to focus attention on mediation of this transmission by gap junctions composed of Cx36.

In a previous work we found that nitric oxide (NO) and cyclicGMP (cGMP) inhibit glutamatergic synaptic transmission in trigeminal motoneurons (MnV). Here we study the actions of the NO/cGMP signaling pathway on glycinergic synaptic transmission in trigeminal and hypoglossal motoneurons (MnXII) in brain stem slices of neonatal rats. Glycinergic inhibitory postsynaptic currents (IPSCs) were recorded in MnV by stimulation of the supratrigeminal nucleus (SuV) and in MnXII by stimulation of the nucleus of Roller. The NO donor DETA/NONOate (DETA/NO) reduced the amplitude of the IPSC to 58.1±4.2% of control values in MnV. In the presence of YC-1, a modulator of guanylate cyclase that acts as a NO sensitizer, lower and otherwise ineffective concentrations of DETA/NO induced a reduction of the IPSC to 47.2±15.6%. NO effects were mimicked by 8 bromo cyclicGMP (8BrcGMP). They were accompanied by an increase in the paired pulse facilitation (PPF) and in the failure rate of evoked IPSCs. 8BrcGMP did not modify the glycinergic currents elicited by exogenous glycine. In MnXII the IPSCs were also reduced by NO donors and 8BrcGMP to 52.9±6.3% and 45.9±4% of control values, respectively. In these neurons, but not in MnV, we also observed excitatory postsynaptic actions of NO donors. We propose that the differences between the two motor pools may be due to a differential development of the nitrergic system in the two nuclei. Our data show that NO, through its second messenger cGMP, reduces inhibitory glycinergic synaptic transmission in both MnV and MnXII. For MnV, evidence in favor of presynaptic inhibition of glycine release is presented. Given our previous data together with the current results, we propose that the NO/cGMP signaling pathway participates pre- and postsynaptically in the combined regulation of MnV and MnXII activities in motor acts in which they participate.

After injecting Diamidino yellow and Fast blue respectively into the sensory trigeminal nuclei and spinal cord, we observed doubly labeled cells in the nucleus raphe magnus (NRM). Combining the fluorescent retrograde double labeling with serotonin (5-HT) immunofluorescence histochemistry, we further found that about 30% of the doubly labeled NRM neurons showed 5-HT-like immunoreactivity (5-HT-LI). Such 5-HT-LI NRM neurons may modulate nociceptive activities simultaneously in the sensory trigeminal nuclei and spinal cord by sending axon collaterals to these regions.

The rat vibrissal (whisker) system is one of the oldest and most important models for the study of active tactile sensing and sensorimotor integration. It is well established that primary sensory neurons in the trigeminal ganglion respond to deflections of one and only one whisker, and that these neurons are strongly tuned for both the speed and direction of individual whisker deflections. During active whisking behavior, however, multiple whiskers will be deflected simultaneously. Very little is known about how neurons at central levels of the trigeminal pathway integrate direction and speed information across multiple whiskers. In the present work, we investigated speed and direction coding in the trigeminal brainstem nuclei, the first stage of neural processing that exhibits multi-whisker receptive fields. Specifically, we recorded both single-unit spikes and local field potentials from fifteen sites in spinal trigeminal nucleus interpolaris and oralis while systematically varying the speed and direction of coherent whisker deflections delivered across the whisker array. For 12/15 neurons, spike rate was higher when the whisker array was stimulated from caudal to rostral rather than rostral to caudal. In addition, 10/15 neurons exhibited higher firing rates for slower stimulus speeds. Interestingly, using a simple decoding strategy for the local field potentials and spike trains, classification of speed and direction was higher for field potentials than for single unit spike trains, suggesting that the field potential is a robust reflection of population activity. Taken together, these results point to the idea that population responses in these brainstem regions in the awake animal will be strongest during behaviors that stimulate a population of whiskers with a directionally coherent motion.

Nervous systems respond with structural changes to environmental changes even in adulthood. In recent years, experience-dependent structural plasticity was shown not to be restricted to the cerebral cortex, as it also occurs at subcortical and even peripheral levels. We have previously shown that two populations of trigeminal nuclei neurons, trigeminothalamic barrelette neurons of the principal nucleus (Pr5), and intersubnuclear neurons in the caudal division of the spinal trigeminal nucleus (Sp5C) that project to Pr5 underwent morphometric and topological changes in their dendritic trees after a prolonged total or partial loss of afferent input from the vibrissae. Here we examined whether and what structural alterations could be elicited in the dendritic trees of the same cell populations in young adult rats after being exposed for 2 months to an enriched environment (EE), and how these changes evolved when animals were returned to standard housing for an additional 2 months. Neurons were retrogradely labeled with BDA delivered to, respectively, the ventral posteromedial thalamic nucleus or Pr5. Fully labeled cells were digitally reconstructed with Neurolucida and analyzed with NeuroExplorer. EE gave rise to increases in dendritic length, number of trees and branching nodes, spatial expansion of the trees, and dendritic spines, which were less pronounced in Sp5C than in Pr5 and differed between sides. In Pr5, these parameters returned, but only partially, to control values after EE withdrawal. These results underscore a ubiquity of experience-dependent changes that should not be overlooked when interpreting neuroplasticity and developing plasticity-based therapeutic strategies.

In studies of vestibulo-ocular reflex (VOR), the horizontal VOR circuit is much clearer than vertical-torsional VOR. The circuit and mechanism of gravity-related vertical-torsional VOR is probably weak. "Somatosensory vestibular interaction" is a known extra source to facilitate VOR, and cervico-ocular reflex is a representative for torsional VOR compensation. Whereas, how the cervical afferents finally reach the oculomotor system is less documented. Actually, when the head tilts, which generates cervico-ocular reflex, not only the neck muscle is activated, but also the jaw muscle is stretched by gravity dragged mandible and/or tissue-muscle connection between the mandible and clavicle. We have previously identified a projection from the jaw muscle afferent mesencephalic trigeminal nucleus (Vme) neurons to oculomotor nuclei (III/IV) and their premotor neurons in interstitial nucleus of Cajal (INC)-a well-known pre-oculomotor center manipulating vertical-torsional eye movements. We hypothesized that these projections may interact with vestibulo-ocular signals during vertical-torsional VOR, because effects of gravity on jaw muscles and bones has been reported. Thus, we injected different anterograde tracers into the Vme and medial vestibular nucleus (MVN)-the subnuclear area particularly harboring excitatory vestibulo-ocular neurons, and immunostained III/IV motoneurons. Retrograde tracer was injected into the III in the same animals after dual anterograde tracers' injections. Under confocal microscope, we observed the Vme and MVN neuronal endings simultaneously terminated onto the same III/IV motoneurons and the same INC pre-oculomotor neurons. We consider that jaw muscle proprioceptive Vme neurons projecting to the III/IV and INC would sense spindle activity if the jaw muscle is stretched by gravity dragged mandible or connection between mandible and clavicle during head rolling. Therefore, the convergent innervation of the Vme and MVN neurons onto the oculomotor and pre-oculomotor nuclei would be a neuroanatomic substrate for interaction of masticatory proprioception with the vestibulo-ocular signals upon the oculomotor system during vertical-torsional VOR.

The cutaneous and mucosal surfaces in the infraorbital region around the whisker pad are innervated by the maxillary division of the afferent fibers of the trigeminal nerve, while certain ganglion cells project to the principal sensory trigeminal nucleus (Pr5). In turn, some of the neurons in the Pr5 project to the motor trigeminal nucleus (Mo5), whose neurons do not innervate the infraorbital skin. We analyzed the calmodulin (CaM) gene expression in these nuclei after dithranol-induced inflammation and subsequent treatment with corticosteroid in the infraorbital skin. CaM gene-specific mRNA populations were detected through quantitative image analysis of the distribution of CaM gene-specific riboprobes in brain stem cryostat sections of control rats and rats chronically treated with dithranol, corticosteroid or both. These nuclei displayed a differentially altered CaM gene expression in response to the treatments. While the CaM I and II mRNA contents were increased, the CaM III transcripts remained unaltered after chronic dithranol treatment in the Mo5. In the Pr5, however, the CaM mRNA contents were either unchanged (CaM I and III) or increased (CaM II). Subsequent corticosteroid treatment reversed the stimulatory effects of dithranol on the expression of all the CaM genes in the Mo5, but was without significant effects on the CaM I and II genes, or even increased the CaM III mRNA contents in the Pr5. Corticosteroid treatment alone was either ineffective or decreased the levels of CaM mRNAs in these nuclei. These data suggest that peripheral noxae of dermal origin may result in a trans-synaptically acting differential regulation of the multiple CaM genes in the brain.

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.

Trigeminal neuropathic pain (TNP) arises due to peripheral nerve injury, the mechanisms underlying which are little known. The altered gene expression profile in sensory ganglia is critical for neuropathic pain generation and maintenance. We, therefore, assessed the transcriptome of the trigeminal ganglion (TG) from mice at different periods of pain progression. Trigeminal neuropathic pain was established by partial infraorbital nerve transection (pIONT). High-throughput RNA sequencing was applied to detect the mRNA profiles of TG collected at 3 and 10 days after modeling. Injured TG displayed dramatically altered mRNA expression profiles compared to Sham. Different gene expression profiles were obtained at 3 and 10 days after pIONT. Moreover, 314 genes were significantly upregulated, and 81 were significantly downregulated at both 3 and 10 days post-pIONT. Meanwhile, enrichment analysis of these persistent differentially expressed genes (DEGs) showed that the MAPK pathway was the most significantly enriched pathway for upregulated DEGs, validated by immunostaining. In addition, TG cell populations defined by single-nuclei RNA sequencing displayed cellular localization of DEGs at a single-cell resolution. Protein-protein interaction (PPI) and sub-PPI network analyses constructed networks and identified the top 10 hub genes for DEGs at different time points. The present data provide novel information on the gene expression signatures of TG during the development and maintenance phases of TNP, and the identified hub genes and pathways may serve as potential targets for treatment.