Corticofugal direct projections to primary afferent neurons in the trigeminal mesencephalic nucleus of rats

Mesencephalic trigeminal neuron dendritic structures in the macaque

It is presumed that the unusual central location of mesencephalic trigeminal neurons is a specialization that allows them to receive synaptic input. However, relatively few synaptic terminals were observed on the somata of mesencephalic trigeminal neurons in macaque monkeys via electron microscopy. This leaves the question of dendritic synaptic terminals open. Unlike the pseudounipolar neurons found in the trigeminal ganglion, some mesencephalic trigeminal neurons have been reported to be multipolar cells exhibiting a number of dendritic processes in non-primate species. To examine whether this morphological feature was also present in macaque monkeys, we retrogradely filled these cells with biotinylated dextran amine by injecting it into the trigeminal nerve entry zone. A portion of the mesencephalic trigeminal neurons exhibited short, poorly branched, dendritic processes. They also exhibited very fine, short processes believed to be somatic spines. Thus, primate trigeminal mesencephalic neurons appear to have specializations aimed at increasing the membrane surface area available for synaptic input.

The ultrastructure of macaque mesencephalic trigeminal nucleus neurons

Primary afferents originating from the mesencephalic trigeminal nucleus provide the main source of proprioceptive information guiding mastication, and thus represent an important component of this critical function. Unlike those of other primary afferents, their cell bodies lie within the central nervous system. It is believed that this unusual central location allows them to be regulated by synaptic input. In this study, we explored the ultrastructure of macaque mesencephalic trigeminal nucleus neurons to determine the presence and nature of this synaptic input in a primate. We first confirmed the location of macaque mesencephalic trigeminal neurons by retrograde labeling from the masticatory muscles. Since the labeled neurons were by far the largest cells located at the edge of the periaqueductal gray, we could undertake sampling for electron microscopy based on soma size. Ultrastructurally, mesencephalic trigeminal neurons had very large somata with euchromatic nuclei that sometimes displayed deeply indented nuclear membranes. Terminal profiles with varied vesicle characteristics and synaptic density thicknesses were found in contact with either their somatic plasma membranes or somatic spines. However, in contradistinction to other, much smaller, somata in the region, the plasma membranes of the mesencephalic trigeminal somata had only a few synaptic contacts. They did extend numerous somatic spines of various lengths into the neuropil, but most of these also lacked synaptic contact. The observed ultrastructural organization indicates that macaque trigeminal mesencephalic neurons do receive synaptic contacts, but despite their central location, they only avail themselves of very limited input.

The Cerebellar Cortex Receives Orofacial Proprioceptive Signals from the Supratrigeminal Nucleus via the Mossy Fiber Pathway in Rats

Proprioceptive sensory information from muscle spindles is essential for the regulation of motor functions. However, little is known about the motor control regions in the cerebellar cortex that receive proprioceptive signals from muscle spindles distributed throughout the body, including the orofacial muscles. Therefore, in this study, we investigated the pattern of projections in the rat cerebellar cortex derived from the supratrigeminal nucleus (Su5), which conveys orofacial proprioceptive information from jaw-closing muscle spindles (JCMSs). Injections of an anterograde tracer into the Su5 revealed that many bilateral axon terminals (rosettes) were distributed in the granular layer of the cerebellar cortex (including the simple lobule B, crus II and flocculus) in a various sized, multiple patchy pattern. We could also detect JCMS proprioceptive signals in these cerebellar cortical regions, revealing for the first time that they receive muscle proprioceptive inputs in rats. Retrograde tracer injections confirmed that the Su5 directly sends outputs to the cerebellar cortical areas. Furthermore, we injected an anterograde tracer into the external cuneate nucleus (ECu), which receives proprioceptive signals from the forelimb and neck muscle spindles, to distinguish between the Su5- and ECu-derived projections in the cerebellar cortex. The labeled terminals from the ECu were distributed predominantly in the vermis of the cerebellar cortex. Almost no overlap was seen in the terminal distributions of the Su5 and ECu projections. Our findings demonstrate that the rat cerebellar cortex receives orofacial proprioceptive input that is processed differently from the proprioceptive signals from the other regions of the body.

Neurodegeneration of Trigeminal Mesencephalic Neurons by the Tooth Loss Triggers the Progression of Alzheimer's Disease in 3×Tg-AD Model Mice

BACKGROUND

The mesencephalic trigeminal nucleus (Vmes) is not only anatomically adjacent to the locus coeruleus (LC) but is also tightly associated with the function of the LC. The LC can be the first area in which Alzheimer's disease (AD) develops, although it is unclear how LC neuronal loss occurs.

OBJECTIVE

We investigated whether neuronal death in the Vmes can be spread to adjacent LC in female triple transgenic (3×Tg)-AD mice, how amyloid-β (Aβ) is involved in LC neuronal loss, and how this neurodegeneration affects cognitive function.

METHODS

The molars of 3×Tg-AD mice were extracted, and the mice were reared for one week to 4 months. Immunohistochemical analysis, and spatial learning/memory assessment using the Barnes maze were carried out.

RESULTS

In 4-month-old 3×Tg-AD mice, aggregated cytotoxic Aβ42 was found in granules in Vmes neurons. Neuronal death in the Vmes occurred after tooth extraction, resulting in the release of cytotoxic Aβ42 and an increase in CD86 immunoreactive microglia. Released Aβ42 damaged the LC, in turn inducing a significant reduction in hippocampal neurons in the CA1 and CA3 regions receiving projections from the LC. Based on spatial learning/memory assessment, after the tooth extraction in the 4-month-old 3×Tg-AD mice, increased latency was observed in 5-month-old 3×Tg-AD mice 1 month after tooth extraction, which is similar increase of latency observed in control 8-month-old 3×Tg-AD mice. Measures of cognitive deficits suggested an earlier shift to dementia-like behavior after tooth extraction.

CONCLUSION

These findings suggest that tooth extraction in the predementia stage can trigger the spread of neurodegeneration from the Vmes, LC, and hippocampus and accelerate the onset of dementia.

Widespread corticopetal projections from the oval paracentral nucleus of the intralaminar thalamic nuclei conveying orofacial proprioception in rats

The oval paracentral nucleus (OPC) was initially isolated from the paracentral nucleus (PC) within the intralaminar thalamic nuclei in rats. We have recently shown that the rat OPC receives proprioceptive inputs from jaw-closing muscle spindles (JCMSs). However, it remains unknown which cortical areas receive thalamic inputs from the OPC, and whether the cortical areas receiving the OPC inputs are distinct from those receiving inputs from the other intralaminar nuclei and sensory thalamic nuclei. To address this issue, we injected an anterograde tracer, biotinylated dextranamine (BDA), into the OPC, which was electrophysiologically identified by recording of proprioceptive inputs from the JCMSs. Many BDA-labeled axonal fibers and terminals from the OPC were ipsilaterally observed in the rostral and rostroventral regions of the primary somatosensory cortex (S1), the rostral region of the secondary somatosensory cortex (S2), and the most rostrocaudal levels of the granular insular cortex (GI). In contrast, a BDA injection into the caudal PC, which was located slightly rostral to the OPC, resulted in ipsilateral labeling of axonal fibers and terminals in the rostrolateral region of the medial agranular cortex and the rostromedial region of the lateral agranular cortex. Furthermore, injections of a retrograde tracer, Fluorogold, into these S1, S2, and GI regions, resulted in preferential labeling of neurons in the ipsilateral OPC among the intralaminar and sensory thalamic nuclei. These findings reveal that the rat OPC has widespread, but strong corticopetal projections, indicating that there exist divergent corticopetal pathways from the intralaminar thalamic nucleus, which process JCMS proprioceptive sensation.

Neural mechanisms of nociception during orthodontic treatment

Orthodontic tooth movement is accompanied by inflammatory responses in the periodontal ligament. Chemical mediators such as interleukin-1β have key roles in nociception around teeth. Such nociceptive inputs to the periodontal ligament continue for several days and potentially induce plastic changes in higher brain regions, including the cerebral cortex. This review summarizes research on orthodontic treatment-induced modulation of neural activities in the central nervous system. Furthermore, we describe our recent findings on the spatiotemporal effects of orthodontic treatment in the somatosensory and insular cortices.

Transcortical descending pathways through granular insular cortex conveying orofacial proprioception

Our motor behavior can be affected by proprioceptive information. However, little is known about which brain circuits contribute to this process. We have recently revealed that the proprioceptive information arising from jaw-closing muscle spindles (JCMSs) is conveyed to the supratrigeminal nucleus (Su5) by neurons in the trigeminal mesencephalic nucleus (Me5), then to the caudo-ventromedial edge of ventral posteromedial thalamic nucleus (VPMcvm), and finally to the dorsal part of granular insular cortex rostroventrally adjacent to the rostralmost part of secondary somatosensory cortex (dGIrvs2). Our next question is which brain areas receive the information from the dGIrvs2 for the jaw-movements. To test this issue, we injected an anterograde tracer, biotinylated dextranamine, into the dGIrvs2, and analyzed the resultant distribution profiles of the labeled axon terminals. Anterogradely labeled axons were distributed in the pontomedullary areas (including the Su5) which are known to receive JCMS proprioceptive inputs conveyed directly by the Me5 neurons and to contain premotoneurons projecting to the jaw-closing motoneurons in the trigeminal motor nucleus (Mo5). They were also found in and around the VPMcvm. In contrast, no labeled axonal terminals were detected on the cell bodies of Me5 neurons and motoneurons in the Mo5. These data suggest that jaw-movements, which are evoked by the classically defined jaw-reflex arc originating from the peripheral JCMS proprioceptive information, could also be modulated by the transcortical feedback connections from the dGIrvs2 to the VPMcvm and Su5.

Extrasynaptic homomeric glycine receptors in neurons of the rat trigeminal mesencephalic nucleus

The neurons in the trigeminal mesencephalic nucleus (Vmes) innervate jaw-closing muscle spindles and periodontal ligaments, and play a crucial role in the regulation of jaw movements. Recently, it was shown that many boutons that form synapses on them are immunopositive for glycine (Gly+), suggesting that these neurons receive glycinergic input. Information about the glycine receptors that mediate this input is needed to help understand the role of glycine in controlling Vmes neuron excitability. For this, we investigated the expression of glycine receptor subunit alpha 3 (GlyRα3) and gephyrin in neurons in Vmes and the trigeminal motor nucleus (Vmo), and the Gly+ boutons that contact them by light- and electron-microscopic immunocytochemistry and quantitative ultrastructural analysis. The somata of the Vmes neurons were immunostained for GlyRα3, but not gephyrin, indicating expression of homomeric GlyR. The immunostaining for GlyRα3 was localized away from the synapses in the Vmes neuron somata, in contrast to the Vmo neurons, where the staining for GlyRα3 and gephyrin were localized at the subsynaptic zones in somata and dendrites. Additionally, the ultrastructural determinants of synaptic strength, bouton volume, mitochondrial volume, and active zone area, were significantly smaller in Gly+ boutons on the Vmes neurons than in those on the Vmo neurons. These findings support the notion that the Vmes neurons receive glycinergic input via putative extrasynaptic homomeric glycine receptors, likely mediating a slow, tonic modulation of the Vmes neuron excitability.

Medial Frontal Theta Is Entrained to Rewarded Actions

Rodents lick to consume fluids. The reward value of ingested fluids is likely to be encoded by neuronal activity entrained to the lick cycle. Here, we investigated relationships between licking and reward signaling by the medial frontal cortex (MFC), a key cortical region for reward-guided learning and decision-making. Multielectrode recordings of spike activity and field potentials were made in male rats as they performed an incentive contrast licking task. Rats received access to higher- and lower-value sucrose rewards over alternating 30 s periods. They learned to lick persistently when higher-value rewards were available and to suppress licking when lower-value rewards were available. Spectral analysis of spikes and fields revealed evidence for reward value being encoded by the strength of phase-locking of a 6-12 Hz theta rhythm to the rats' lick cycle. Recordings during the initial acquisition of the task found that the strength of phase-locking to the lick cycle was strengthened with experience. A modification of the task, with a temporal gap of 2 s added between reward deliveries, found that the rhythmic signals persisted during periods of dry licking, a finding that suggests the MFC encodes either the value of the currently available reward or the vigor with which rats act to consume it. Finally, we found that reversible inactivations of the MFC in the opposite hemisphere eliminated the encoding of reward information. Together, our findings establish that a 6-12 Hz theta rhythm, generated by the rodent MFC, is synchronized to rewarded actions.SIGNIFICANCE STATEMENT The cellular and behavioral mechanisms of reward signaling by the medial frontal cortex (MFC) have not been resolved. We report evidence for a 6-12 Hz theta rhythm that is generated by the MFC and synchronized with ongoing consummatory actions. Previous studies of MFC reward signaling have inferred value coding upon temporally sustained activity during the period of reward consumption. Our findings suggest that MFC activity is temporally sustained due to the consumption of the rewarding fluids, and not necessarily the abstract properties of the rewarding fluid. Two other major findings were that the MFC reward signals persist beyond the period of fluid delivery and are generated by neurons within the MFC.

Direct projection from the lateral habenula to the trigeminal mesencephalic nucleus in rats

Trigeminal mesencephalic nucleus (Vmes) neurons are primary afferents conveying deep sensation from the masticatory muscle spindles or the periodontal mechanoreceptors, and are crucial for controlling jaw movements. Their cell bodies exist in the brain and receive descending commands from a variety of cortical and subcortical structures involved in limbic (emotional) systems. However, it remains unclear how the lateral habenula (LHb), a center of negative emotions (e.g., pain, stress and anxiety), can influence the control of jaw movements. To address this issue, we examined whether and how the LHb directly projects to the Vmes by means of neuronal tract tracing techniques in rats. After injections of a retrograde tracer Fluorogold in the rostral and caudal Vmes, a number of neurons were labeled in the lateral division of LHb (LHbl) bilaterally, whereas a few neurons were labeled in the medial division of LHb (LHbm) bilaterally. After injections of an anterograde tracer, biotinylated dextranamine (BDA) in the LHbl, a small number of labeled axons were distributed bilaterally in the rostral and caudal levels of Vmes, where some labeled axonal boutons contacted the cell body of rostral and caudal levels of Vmes neurons bilaterally. After the BDA injection into the LHbm, however, no axons were labeled bilaterally in the rostral and caudal levels of Vmes. Therefore, the present study for the first time demonstrated the direct projection from the LHbl to the Vmes and the detailed projection patterns, suggesting that jaw movements are modulated by negative emotions that are signaled by LHbl neurons.

Cholinergic and ghrelinergic receptors and KCNQ channels in the medial PFC regulate the expression of palatability

The medial prefrontal cortex (mPFC) is a key brain region for the control of consummatory behavior. Neuronal activity in this area is modulated when rats initiate consummatory licking and reversible inactivations eliminate reward contrast effects and reduce a measure of palatability, the duration of licking bouts. Together, these data suggest the hypothesis that rhythmic neuronal activity in the mPFC is crucial for the control of consummatory behavior. The muscarinic cholinergic system is known to regulate membrane excitability and control low-frequency rhythmic activity in the mPFC. Muscarinic receptors (mAChRs) act through KCNQ (Kv7) potassium channels, which have recently been linked to the orexigenic peptide ghrelin. To understand if drugs that act on KCNQ channels within the mPFC have effects on consummatory behavior, we made infusions of several muscarinic drugs (scopolamine, oxotremorine, physostigmine), the KCNQ channel blocker XE-991, and ghrelin into the mPFC and evaluated their effects on consummatory behavior. A consistent finding across all drugs was an effect on the duration of licking bouts when animals consume solutions with a relatively high concentration of sucrose. The muscarinic antagonist scopolamine reduced bout durations, both systemically and intra-cortically. By contrast, the muscarinic agonist oxotremorine, the cholinesterase inhibitor physostigmine, the KCNQ channel blocker XE-991, and ghrelin all increased the durations of licking bouts when infused into the mPFC. Our findings suggest that cholinergic and ghrelinergic signaling in the mPFC, acting through KCNQ channels, regulates the expression of palatability.

Orthodontic Force Facilitates Cortical Responses to Periodontal Stimulation

Somatosensory information derived from the periodontal ligaments plays a critical role in identifying the strength and direction of occlusal force. The orthodontic force needed to move a tooth often causes uncomfortable sensations, including nociception around the tooth, and disturbs somatosensory information processing. However, it has mostly remained unknown whether orthodontic treatment modulates higher brain functions, especially cerebrocortical activity. To address this issue, we first elucidated the cortical region involved in sensory processing from the periodontal ligaments and then examined how experimental tooth movement (ETM) changes neural activity in these cortical regions. We performed in vivo optical imaging to identify the cortical responses evoked by electrical stimulation of the maxillary and mandibular incisor and the first molar periodontal ligaments in the rat. In naïve rats, electrical stimulation of the mandibular periodontal ligaments initially evoked neural excitation in the rostroventral part of the primary somatosensory cortex (S1), the ventrocaudal part of the secondary somatosensory cortex (S2), and the insular oral region (IOR), whereas maxillary periodontal ligaments elicited excitation only in S2/IOR rostrodorsally adjacent to the mandibular periodontal ligament-responding region. In contrast, maximum responses to mandibular and maxillary periodontal stimulation were observed in S1 and S2/IOR, and the 2 responses nearly overlapped. One day after ETM (maxillary molar movement by Waldo's method), the maximum response to stimulation of the maxillary molar periodontal ligament induced larger and broader excitation in S2/IOR, although the initial responses were not affected. Taken together with the histologic findings of IL-1β expression and macrophage infiltration in the periodontal ligament of the ETM models, inflammation induced by ETM may play a role in the facilitation of S2/IOR activity. From the clinical viewpoints, the larger amplitude of cortical excitation may induce higher sensitivity to pain responding to nonnoxious stimuli, and enlargement of the responding area may reflect radiating pain.

Projections from the dorsal peduncular cortex to the trigeminal subnucleus caudalis (medullary dorsal horn) and other lower brainstem areas in rats

This study has revealed direct projections from the dorsal peduncular cortex (DP) in the medial prefrontal cortex (mPfC) to the trigeminal brainstem sensory nuclear complex and other lower brainstem areas in rats. We first examined the distribution of mPfC neurons projecting directly to the medullary dorsal horn (trigeminal subnucleus caudalis [Vc]) and trigeminal subnucleus oralis (Vo) which are known to receive direct projections from the lateral prefrontal cortex (insular cortex). After injections of the retrograde tracer Fluorogold (FG) into the rostro-dorsomedial part of laminae I/II of Vc (rdm-I/II-Vc), many neurons were labeled bilaterally (with an ipsilateral predominance) in the rostrocaudal middle level of DP (mid-DP) and not in other mPfC areas. After FG injections into the lateral and caudal parts of laminae I/II of Vc, or the Vo, no neurons were labeled in the mPfC. We then examined projections from the mid-DP by using the anterograde tracer biotinylated dextranamine (BDA). After BDA injections into the mid-DP, many axons and terminals were labeled bilaterally (with an ipsilateral predominance) in the rdm-I/II-Vc, periaqueductal gray and solitary tract nucleus, and ipsilaterally in the parabrachial nucleus and trigeminal mesencephalic nucleus. In addition, the connections of the mid-DP with the insular cortex were examined. Many BDA-labeled axons and terminals from the mid-DP were also found ipsilaterally in the caudalmost level of the granular and dysgranular insular cortex (GI/DI). After BDA injections into the caudalmost GI/DI, many axons and terminals were labeled ipsilaterally in the mid-DP. The projections from the mid-DP to the rdm-I/II-Vc and other brainstem nuclei suggest that mid-DP neurons may regulate intraoral and perioral sensory processing (including nociceptive processing) of rdm-I/II-Vc neurons directly or indirectly through the brainstem nuclei. The reciprocal connections between the mid-DP and caudalmost GI/DI suggest that this regulation may involve mid-DP interactions with the caudalmost GI/DI neurons.

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.

Somatotopic direct projections from orofacial areas of primary somatosensory cortex to pons and medulla, especially to trigeminal sensory nuclear complex, in rats

The primary somatosensory cortex (S1) projects to the thalamus and brainstem somatosensory nuclei and modulates somatosensory information ascending to the S1 itself. However, the projections from the S1 to the brainstem second-order somatosensory neuron pools have not been fully studied. To address this in rats, we first revealed the somatotopic representation of orofacial areas in the S1 by recording cortical surface potentials evoked by stimulation of the lingual, mental, infraorbital, and frontal nerves. We then examined the morphology of descending projections from the electrophysiologically defined orofacial S1 areas to the pons and medulla after injections of an anterograde tracer, biotinylated dextranamine (BDA), into the orofacial S1 areas. BDA-labeled axon terminals were seen mostly in the trigeminal sensory nuclear complex (TSNC) and had a strong contralateral predominance. They also showed a somatotopic arrangement in dorsoventral and superficial-deep directions within almost all rostrocaudal TSNC levels, and in a rostrocaudal direction within the trigeminal caudal subnucleus. In the principal nucleus (Vp) or oral subnucleus (Vo) of TSNC, the BDA-labeled axon terminals showed a somatotopic arrangement closely matched to that of the electrophysiologically defined projection sites of orofacial primary afferents; these projection sites were marked by injections of a retrograde tracer, Fluorogold (FG), into the Vp or Vo. The FG injections labeled a large number of S1 neurons, with a strong contralateral predominance, in a somatotopic manner, which corresponded to that presented in the electrophysiologically defined orofacial S1 areas. The present results suggest that the orofacial S1 projections to somatotopically matched regions of trigeminal second-order somatosensory neuron pools may allow the orofacial S1 to accurately modulate orofacial somatosensory transmission to higher brain centers including the orofacial S1 itself.

The expression and localization of Prune2 mRNA in the central nervous system

A family of Bcl-2/adenovirus E1B 19kDa-interacting proteins (BNIPs) plays critical roles in several cellular processes such as cellular transformation, apoptosis, neuronal differentiation, and synaptic function, which are mediated by the BNIP2 and Cdc42GAP homology (BCH) domain. Prune homolog 2 (Drosophila) (PRUNE2) and its isoforms -C9orf65, BCH motif-containing molecule at the carboxyl terminal region 1 (BMCC1), and BNIP2 Extra Long (BNIPXL) - have been shown to be a susceptibility gene for Alzheimer's disease, a biomarker for leiomyosarcomas, a proapoptotic protein in neuronal cells, and an antagonist of cellular transformation, respectively. However, precise localization of PRUNE2 in the brain remains unclear. Here, we identified the distribution of Prune2 mRNA in the adult mouse brain. Prune2 mRNA is predominantly expressed in the neurons of the cranial nerve motor nuclei and the motor neurons of the spinal cord. The expression in the dorsal root ganglia (DRG) is consistent with the previously described reports. In addition, we observed the expression in another sensory neuron in the mesencephalic trigeminal nucleus. These results suggest that Prune2 may be functional in these restricted brain regions.