Quantitative analysis of synaptic contacts made between functionally identified oralis neurons and trigeminal motoneurons in cats

Modulation of reflex responses of the anterior and posterior bellies of the digastric muscle in freely moving rats

BACKGROUND

Chewing and licking are primarily activated by central pattern generator (CPG) neuronal circuits in the brainstem and when activated trigger repetitive rhythmic orofacial movements such as chewing, licking and swallowing. These CPGs are reported to modulate orofacial reflex responses in functions such as chewing.

OBJECTIVE

This study explored the modulation of reflex responses in the anterior and posterior bellies (ant-Dig and post-Dig, respectively) of the digastric muscle evoked by low-intensity trigeminal stimulation in conscious rats.

METHODS

The ant-Dig and post-Dig reflexes were evoked by using low-intensity electrical stimulation applied to either the right or left inferior alveolar nerve. Peak-to-peak amplitudes and onset latencies were measured.

RESULTS

No difference was observed between threshold and onset latency for evoking ant-Dig and post-Dig reflexes, suggesting that the latter was also evoked disynaptically. The peak-to-peak amplitude of both reflexes was significantly reduced during chewing, licking and swallowing as compared to resting period and was lowest during the jaw-closing phase of chewing and licking. Onset latency was significantly largest during the jaw-closing phase. Inhibitory level was similar between the ant-Dig and post-Dig reflex responses and between the ipsilateral and contralateral sides.

CONCLUSION

These results suggest that both the ant-Dig and post-Dig reflex responses were significantly inhibited, probably due to CPG activation during feeding behaviours to maintain coordination of jaw and hyoid movements and hence ensure smooth feeding mechanics.

Synaptic connectivity of the TRPV1-positive trigeminal afferents in the rat lateral parabrachial nucleus

Recent studies have shown a direct projection of nociceptive trigeminal afferents into the lateral parabrachial nucleus (LPBN). Information about the synaptic connectivity of these afferents may help understand how orofacial nociception is processed in the LPBN, which is known to be involved primarily in the affective aspect of pain. To address this issue, we investigated the synapses of the transient receptor potential vanilloid 1-positive (TRPV1+) trigeminal afferent terminals in the LPBN by immunostaining and serial section electron microscopy. TRPV1 + afferents arising from the ascending trigeminal tract issued axons and terminals (boutons) in the LPBN. TRPV1+ boutons formed synapses of asymmetric type with dendritic shafts and spines. Almost all (98.3%) TRPV1+ boutons formed synapses with one (82.6%) or two postsynaptic dendrites, suggesting that, at a single bouton level, the orofacial nociceptive information is predominantly transmitted to a single postsynaptic neuron with a small degree of synaptic divergence. A small fraction (14.9%) of the TRPV1+ boutons formed synapses with dendritic spines. None of the TRPV1+ boutons were involved in axoaxonic synapses. Conversely, in the trigeminal caudal nucleus (Vc), TRPV1+ boutons often formed synapses with multiple postsynaptic dendrites and were involved in axoaxonic synapses. Number of dendritic spine and total number of postsynaptic dendrites per TRPV1+ bouton were significantly fewer in the LPBN than Vc. Thus, the synaptic connectivity of the TRPV1+ boutons in the LPBN differed significantly from that in the Vc, suggesting that the TRPV1-mediated orofacial nociception is relayed to the LPBN in a distinctively different manner than in the Vc.

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.

Synaptic connectivity of urinary bladder afferents in the rat superficial dorsal horn and spinal parasympathetic nucleus

That visceral sensory afferents are functionally distinct from their somatic analogues has been known for a long time but the detailed knowledge of their synaptic connections and neurotransmitters at the first relay nucleus in the spinal cord has been limited. To provide information on these topics, we investigated the synapses and neurotransmitters of identified afferents from the urinary bladder to the superficial laminae of the rat spinal dorsal horn (DH) and the spinal parasympathetic nucleus (SPN) by tracing with horseradish peroxidase, quantitative electron microscopical analysis, and immunogold staining for GABA and glycine. In the DH, most bladder afferent boutons formed synapses with 1-2 postsynaptic dendrites, whereas in the SPN, close to a half of them formed synapses with 3-8 postsynaptic dendrites. The number of postsynaptic dendrites and dendritic spines per bladder afferent bouton, both measures of synaptic divergence and of potential for synaptic plasticity at a single bouton level, were significantly higher in the SPN than in the DH. Bladder afferent boutons frequently received inhibitory axoaxonic synapses from presynaptic endings in the DH but rarely in the SPN. The presynaptic endings were GABA- and/or glycine-immunopositive. The bouton volume, mitochondrial volume, and active zone area, all determinants of synaptic strength, of the bladder afferent boutons were positively correlated with the number of postsynaptic dendrites. These findings suggest that visceral sensory information conveyed via the urinary bladder afferents is processed differently in the DH than in the SPN, and differently from the way somatosensory information is processed in the spinal cord.

Somatotopy in the Medullary Dorsal Horn As a Basis for Orofacial Reflex Behavior

The somatotopy of the trigeminocervical complex of the rat was defined as a basis for describing circuitry for reflex behaviors directed through the facial motor nucleus. Thus, transganglionic transport of horseradish peroxidase conjugates applied to individual nerves/peripheral receptive fields showed that nerves innervating oropharyngeal structures projected most rostrally, followed by nerves innervating snout, periocular, and then periauricular receptive fields most caudally. Nerves innervating mucosae or glabrous receptive fields terminated densely in laminae I, II, and V of the trigeminocervical complex, while those innervating hairy skin terminated in laminae I-V. Projections to lamina II exhibited the most focused somatotopy when individual cases were compared. Retrograde transport of FluoroGold (FG) deposited into the facial motor nucleus resulted in labeled neurons almost solely in lamina V of the trigeminocervical complex. The distribution of these labeled neurons paralleled the somatotopy of primary afferent fibers, e.g., those labeled after FG injections into a functional group of motoneurons innervating lip musculature were found most rostrally while those labeled after injections into motoneurons innervating snout, periocular and preauricular muscles, respectively, were found at progressively more caudal levels. Anterograde transport of injections of biotinylated dextran amine into lamina V at different rostrocaudal levels of the trigeminocervical complex confirmed the notion that the somatotopy of orofacial sensory fields parallels the musculotopy of facial motor neurons. These data suggest that neurons in lamina V are important interneurons in a simple orofacial reflex circuit consisting of a sensory neuron, interneuron and motor neuron. Moreover, the somatotopy of primary afferent fibers from the head and neck confirms the "onion skin hypothesis" and suggests rostral cervical dermatomes blend seamlessly with "cranial dermatomes." The transition area between subnucleus interpolaris and subnucleus caudalis is addressed while the paratrigeminal nucleus is discussed as an interface between the somatic and visceral nervous systems.

Central connectivity of transient receptor potential melastatin 8-expressing axons in the brain stem and spinal dorsal horn

Transient receptor potential melastatin 8 (TRPM8) ion channels mediate the detection of noxious and innocuous cold and are expressed by primary sensory neurons, but little is known about the processing of the TRPM8-mediated cold information within the trigeminal sensory nuclei (TSN) and the spinal dorsal horn (DH). To address this issue, we characterized TRPM8-positive (+) neurons in the trigeminal ganglion and investigated the distribution of TRPM8+ axons and terminals, and their synaptic organization in the TSN and in the DH using light and electron microscopic immunohistochemistry in transgenic mice expressing a genetically encoded axonal tracer in TRPM8+ neurons. TRPM8 was expressed in a fraction of small myelinated primary afferent fibers (23.7%) and unmyelinated fibers (76.3%), suggesting that TRPM8-mediated cold is conveyed via C and Aδ afferents. TRPM8+ axons were observed in all TSN, but at different densities in the dorsal and ventral areas of the rostral TSN, which dominantly receive sensory afferents from intra- and peri-oral structures and from the face, respectively. While synaptic boutons arising from Aδ and non-peptidergic C afferents usually receive many axoaxonic contacts and form complex synaptic arrangements, TRPM8+ boutons arising from afferents of the same classes of fibers showed a unique synaptic connectivity; simple synapses with one or two dendrites and sparse axoaxonic contacts. These findings suggest that TRPM8-mediated cold is conveyed via a specific subset of C and Aδ afferent neurons and is processed in a unique manner and differently in the TSN and DH.

Ultrastructural analysis of low-threshold mechanoreceptive vibrissa afferent boutons in the cat trigeminal caudal nucleus

Ultrastructural parameters related to synaptic release and their correlation with synaptic connectivity were analyzed in the low-threshold mechanoreceptive vibrissa afferent boutons in laminae III and IV of the trigeminal caudal nucleus (Vc). Rapidly adapting vibrissa afferents were intra-axonally labeled, and quantitative ultrastructural analyses with serial sections were performed on the labeled boutons and their presynaptic endings (p-endings). The volume of the labeled boutons was widely distributed from small to large ones (0.8~12.3 µm³), whereas the p-endings were small and uniform in size. The volume of the labeled boutons was positively correlated with the ultrastructural parameters such as mitochondrial volume (correlation coefficient, r=0.96), active zone area (r=0.82) and apposed surface area (r=0.79). Vesicle density (r=-0.18) showed little correlation to the volume of labeled boutons, suggesting that the total vesicle number of a bouton is proportional to its volume. In addition, the bouton volume was positively correlated with the number of p-endings (r=0.52) and with the number of dendrites postsynaptic to the labeled bouton (r=0.83). These findings suggest that low-threshold mechanoreception conveyed through vibrissa afferents is processed in a bouton size-dependent manner in the Vc, which may contribute to the sensory-motor function of laminae III/IV in Vc.

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.

Distribution of vestibulospinal contacts on the dendrites of ipsilateral splenius motoneurons: an anatomical substrate for push-pull interactions during vestibulocollic reflexes

Excitatory and inhibitory synapses may control neuronal output through a push-pull mechanism--that is, increases in excitation are coupled to simultaneous decreases in inhibition or vice versa. This pattern of activity is characteristic of excitatory and inhibitory vestibulospinal axons that mediate vestibulocollic reflexes. Previously, we showed that medial vestibulospinal tract (MVST) neurons in the rostral descending vestibular nucleus (DVN), an excitatory pathway, primarily innervate the medial dendrites of contralateral splenius motoneurons. In the present study, we tested the hypothesis that the counterparts of the push-pull mechanism, the ipsilateral inhibitory MVST synapses, are distributed on the dendritic tree such that the interactions with excitatory MVST synapses are enhanced. We combined anterograde tracing and intracellular staining in adult felines and show that most contacts (approximately 70%) between inhibitory MVST neurons in the rostral DVN and ipsilateral splenius motoneurons are also located on medial dendrites. There was a weak bias towards proximal dendrites. Using computational methods, we further show that the organization of excitatory and inhibitory MVST synapses on splenius motoneurons increases their likelihood for interaction. We found that if either excitatory or inhibitory MVST synapses were uniformly distributed throughout the dendritic tree, the proportion of inhibitory contacts in close proximity to excitatory contacts decreased. Thus, the compartmentalized distribution of excitatory and inhibitory MVST synapses on splenius motoneurons may be specifically designed to enhance their interactions during vestibulocollic reflexes. This suggests that the push-pull modulation of motoneuron output is based, in part, on the spatial arrangement of synapses on the dendritic tree.

Expression of transient receptor potential ankyrin 1 (TRPA1) in the rat trigeminal sensory afferents and spinal dorsal horn

Transient receptor potential ankyrin 1 (TRPA1), responding to noxious cold and pungent compounds, is implicated in the mediation of nociception, but little is known about the processing of the TRPA1-mediated nociceptive information within the trigeminal sensory nuclei (TSN) and the spinal dorsal horn (DH). To address this issue, we characterized the TRPA1-positive (+) neurons in the trigeminal ganglion (TG) and investigated the distribution of TRPA1(+) afferent fibers and their synaptic connectivity within the rat TSN and DH by using light and electron microscopic immunohistochemistry. In the TG, TRPA1 was expressed in unmyelinated and small myelinated axons and also occasionally in large myelinated axons. Many TRPA1(+) neurons costained for the marker for peptidergic neurons substance P (26.8%) or the marker for nonpeptidergic neurons IB4 (44.5%). In the CNS, small numbers of axons and terminals were immunopositive for TRPA1 throughout the rostral TSN, in contrast to the dense network of positive fibers and terminals in the superficial laminae of the trigeminal caudal nucleus (Vc) and DH. The TRPA1(+) terminals contained clear round vesicles, were presynaptic to one or two dendrites, and rarely participated in axoaxonic contacts, suggesting involvement in relatively simple synaptic circuitry with a small degree of synaptic divergence and little presynaptic modulation. Immunoreactivity for TRPA1 was also occasionally observed in postsynaptic dendrites. These results suggest that TRPA1-dependent orofacial and spinal nociceptive input is processed mainly in the superficial laminae of the Vc and DH in a specific manner and may be processed differently between the rostral TSN and Vc.

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.

The somatotopic organization of trigeminal premotoneurons in the cat brainstem

This study was performed to complement the results of prior intracellular recording and labeling studies by investigating the general distribution pattern of trigeminal premotoneurons in the cat brainstem using the retrograde tracing methods. The results of the present study reconfirmed the presence of premotoneurons in the trigeminal principal and oral nuclei following horseradish peroxidase injections into the jaw-opening (JO) or jaw-closing (JC) nucleus. Furthermore, we found that labeled cells from the JO nucleus and JC nucleus located in the reticular regions surrounding the trigeminal motor nucleus (Vmo; Vmo shell region) were arranged in a topographic fashion, while those in the parabrachial nucleus, supratrigeminal nucleus, lateral reticular formation caudal to the shell region and raphe nuclei were intermingled with each other. The labeling in the individual nuclei was bilateral with an ipsilateral predominance to each injection site, with the exception of the mesencephalic trigeminal nucleus, where the labeling was ipsilateral to the injection site in the JC nucleus. These results, combined with the data of the previous intracellular tracing studies, indicate that based on the presence of somatotopic organization, premotoneurons can be largely divided into two groups; those projecting to either the JO or the JC nucleus and those projecting to the two nuclei, and we offer the suggestion that roles of premotoneurons for jaw movements differ among the individual nuclei.

Spatial distribution patterns of excitatory and inhibitory synapses in the dendritic tree differ between jaw-closing and -opening motoneurons

This paper reviews recent data on the spatial distribution of inhibitory and excitatory synapses on the dendritic tree of jaw-closing (JC) and -opening (JO) motoneurons in the cat, in which a combination of techniques employing intracellular injections of horseradish peroxidase and postembedding immunogold labelling was used. The dendritic tree is divided into three segments: primary and distal dendrites and intermediate dendrites between the two segments. The proportion of inhibitory boutons (immunoreactive for GABA and/or glycine) is slightly higher than proportion of excitatory boutons (immunoreactive for glutamate) in JC motoneurons, but this trend is reversed in JO motoneurons. In the two kinds of motoneuron, boutons immunoreactive to glycine alone are more numerous than boutons double-labelled to GABA and glycine, which, in turn, occur more frequently than boutons immunoreactive to GABA alone. In JC motoneurons, the packing density (number of boutons per 100 microm(2)) of the inhibitory boutons decreases somatofugally, but this trend is not applicable to the excitatory boutons. In contrast, the packing density of the inhibitory and excitatory boutons in JO motoneurons does not significantly differ among the three dendritic compartments, though it is slightly higher for the excitatory than the inhibitory ones on each dendritic segment. These differences have important implications for synaptic integration in JC and JO motoneurons.

Involvement of neurotrophin-4/5 in regeneration of the periodontal Ruffini endings at the early stage

Little is known about the role of neurotrophin-4/5 (NT-4/5) in the regeneration of mechanoreceptors. Therefore, the present study examined the regeneration process of Ruffini endings in the periodontal ligament in nt-4/5-deficient and wildtype mice following transection of the inferior alveolar nerve by immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker, and by computer-assisted quantitative image analysis. Furthermore, rescue experiments by a continuous administration of recombinant NT-4/5 were performed and analyzed quantitatively. At postoperative day 3 (PO 3d), almost all PGP 9.5-positive neural elements had disappeared; they began to appear in both types of animals at PO 7d. At PO 10d, almost all nerve fibers showed a beaded appearance, with fewer ramifications in both types of mice. Although the regeneration proceeded in the wildtype, a major population of the periodontal Ruffini endings continued to display smooth outlines at PO 28d in the nt-4/5 homozygous mice. The reduction ratio of neural density reached a maximum at PO 3d, decreased at PO 10d, and later showed a plateau. In a rescue experiment, an administration of NT-4/5 showed an acceleration of nerve regeneration in the homozygous mice. These findings indicate that the nt-4/5-depletion causes a delay in the regeneration of the periodontal Ruffini endings, but the delay is shortened by an exogenous administration of NT-4/5. Combined with our previous findings of bdnf-deficient mice (Harada et al. [2003] Arch Histol Cytol 66:183-194), these morphological and numerical data suggest that multiple neurotrophins such as NT-4/5 and brain-derived neurotrophic factor (BDNF) play roles in their regeneration in a stage-specific manner.

Postnatal development of noradrenergic terminals in the rat trigeminal motor nucleus: A light and electron microscopic immunocytochemical analysis

The noradrenergic (NA) innervation in the trigeminal motor nucleus (Vmot) of postnatal and adult rats was examined by light and electron microscopic immunocytochemistry using antibodies against dopamine-beta-hydroxylase or tyrosine hydroxylase. NA fibers were identified in the Vmot as early as the day of birth (postnatal day 0; P0). A continuous increase in the density of labeled fibers was observed during development up to P20, with a slight decrease at P30 and in the adult. Electron microscopic analysis of serial ultrathin sections revealed that, at P5, nearly half (46%) of the examined NA terminals made synaptic contact with other neuronal elements with membrane specializations. The percentage of examined NA varicosities engaged in synaptic contacts increased at P15 (74%), then decreased in the adult (64%). At all developmental ages, the majority of contacts made by these boutons were symmetrical, the postsynaptic elements being mainly dendrites and occasionally somata. Interestingly, some of the NA terminals made axo-axon contacts with other unidentified boutons. These results show that, although the density of NA fibers increases during postnatal development, functional NA boutons are present in the Vmot at early postnatal ages. Some of these fibers might exert their effects via nonsynaptic release of noradrenaline, the so-called volume transmission, but, in the main, they form conventional synaptic contacts with dendrites, somata, and other axonal terminals in the Vmot. These results are consistent with previous electrophysiological studies that propose an important role for the NA system in modulating mastication.

Central distribution of nociceptive intradental afferent nerve fibers in the rat

The central distribution of intradental afferent nerve fibers was investigated by combining electron microscopic observations with a selective method for inducing degeneration of the A delta- and C-type afferent fibers. Degenerating terminals were found on the proprioceptive mesencephalic trigeminal neurons and on dendrites in the neuropil of the trigeminal motor nucleus after application of capsaicin to the rat's lower incisor tooth pulp. The results give anatomical evidence of new sites of central projection of intradental A delta- and C-type fibers whereby the nociceptive information from the tooth pulp can affect jaw muscle activity.

Input-output organization of jaw movement-related areas in monkey frontal cortex

The brain mechanisms underlying mastication are not fully understood. To address this issue, we analyzed the distribution patterns of cortico-striatal and cortico-brainstem axon terminals and the origin of thalamocortical and intracortical fibers by injecting anterograde/retrograde tracers into physiologically and morphologically defined jaw movement-related cortical areas. Four areas were identified in the macaque monkey: the primary and supplementary orofacial motor areas (MIoro and SMAoro) and the principal and deep parts of the cortical masticatory area (CMaAp and CMaAd), where intracortical microstimulation produced single twitch-like or rhythmic jaw movements, respectively. Tracer injections into these areas labeled terminals in the ipsilateral putamen in a topographic fashion (MIoro vs. SMAoro and CMaAp vs. CMaAd), in the lateral reticular formation and trigeminal sensory nuclei contralaterally (MIoro and CMaAp) or bilaterally (SMAoro) in a complex manner of segregation vs. overlap, and in the medial parabranchial and Kölliker-Fuse nuclei contralaterally (CMaAd). The MIoro and CMaAp received thalamic projections from the ventrolateral and ventroposterolateral nuclei, the SMAoro from the ventroanterior and ventrolateral nuclei, and the CMaAd from the ventroposteromedial nucleus. The MIoro, SMAoro, CMaAp, and CMaAd received intracortical projections from the ventral premotor cortex and primary somatosensory cortex, the ventral premotor cortex and rostral cingulate motor area, the ventral premotor cortex and area 7b, and various sensory areas. In addition, the MIoro and CMaAp received projections from the three other jaw movement-related areas. Our results suggest that the four jaw movement-related cortical areas may play important roles in the formation of distinctive masticatory patterns.

Distribution of contacts from vestibulospinal axons on the dendrites of splenius motoneurons

Current descriptions of the organization of synapses on the dendritic trees of spinal motoneurons indicate that the inputs are arranged in several patterns: some are widely distributed; some are distributed to proximal dendrites; others are distributed based on the trajectory of the dendrites. However, the principles governing the organization of synapses on spinal motoneurons remain poorly defined. Our goal was to extend the descriptions of the distribution of synapses, identified by their source, on the dendritic trees of spinal motoneurons. We combined anterograde and intracellular staining techniques in cats to determine the distribution of contacts between excitatory axons from the rostral aspect of the descending vestibular nucleus and the dendrites of motoneurons supplying a dorsal neck muscle, splenius. In five of five motoneurons, the contacts were preferentially distributed on dendrites medial to the soma. This qualitative observation was confirmed by using Monte Carlo methods. The results from this analysis showed that the distribution of contacts can be explained not by the overall distribution of the dendritic membrane area but rather by a systematic innervation of the medial regions of the dendritic trees (P < 0.02). Despite this selectivity, there was no additional bias in the distribution of contacts to proximal vs. distal dendrites. By concentrating excitatory synapses in a restricted region of the dendritic tree, the actions of vestibulospinal connections on neck motoneurons may be increased as a result of a greater probability of activating persistent inward currents on the dendrites.

The synaptic microcircuitry associated with primary afferent terminals in the interpolaris and caudalis of trigeminal sensory nuclear complex

Previous ultrastructural studies indicating a higher number of axoaxonic contacts on individual low-threshold mechanoreceptive afferents in the principalis (Vp) than in the oralis (Vo) of cat trigeminal sensory nuclear complex (TSNC) suggest that the synaptic microcircuitry associated with primary afferents manifests unique differences across the sensory nuclei of TSNC. To address this issue, we analyzed synaptic microcircuits associated with fast adapting vibrissa afferent terminals in the interpolaris (Vi) and caudalis (Vc, laminae III/IV) by using intraaxonal injections of horseradish peroxidase (HRP) in cats. Forty-two and 65 HRP-labeled boutons were analyzed in the Vi and Vc, respectively. The labeled boutons contained clear, spherical vesicles. They most frequently formed asymmetric axodendritic synapses and were commonly postsynaptic to unlabeled axon terminals containing pleomorphic vesicles (p-endings) with symmetric junctions. The examination of synaptic contacts over the entire surface of individual boutons indicated that the afferent boutons made contacts with an average of two postsynaptic targets in the Vi and Vc. In contrast, axoaxonic contacts, and labeled boutons participating in synaptic triads, where p-endings contacted both the boutons and their postsynaptic targets, were, on average, higher in the Vi than in the Vc. These results suggest that the output of sensory information conveyed through low-threshold mechanoreceptive afferents is more strongly controlled at the level of the first synapse by presynaptic and postsynaptic mechanisms in the Vi responsible for sensory discriminative functions than in the Vc for sensorimotor reflexive functions.

Inputs to nucleus pontis caudalis from adjacent trigeminal areas

Recent studies suggest that the nucleus pontis caudalis (nPontc) plays a role in patterning mastication through interactions with the adjacent lateral tegmentum. In this study, we used in vitro intracellular recording and staining to describe the basic membrane properties and morphology of nPontc neurones and to further explore interactions with adjacent structures, using coronal sections of the brainstem of 78 rats, aged 9-28 days. Neurones were large, with dendrites that spread in all directions, and about 64% fired tonically even in the absence of synaptic inputs. Tonic neurones were predominant in the centre of the nucleus. Electrical stimulation of all regions of the nPontc produced mixed excitatory and inhibitory effects on interneurones of lateral tegmental nuclei. Focal inactivation of the dorsal nPontc with injections of tetrodotoxin also had mixed effects on the spontaneous firing of both interneurones and motoneurones but similar injections in the ventral nPontc produced mostly increases of firing. Sixty-five percent of nPontc neurones received synaptic inputs from the lateral tegmental areas and most of these (68%) were excitatory and mediated by glutamatergic receptors. Inhibitory postsynaptic potentials were mediated by GABA(A) or glycinergic receptors. Although most responses occurred at relatively long latencies (> 2 ms), they could follow relatively high-frequency stimulation (> 50 Hz). Excitatory and inhibitory connections between ipsi- and contralateral nPontc neurones were also documented, which could contribute to bilateral coordination of jaw movements. This study provides evidence that the nPontc exerts both tonic and phasic influences on the premotor components of the masticatory central pattern generator.

Unilateral application of an inflammatory irritant to the rat temporomandibular joint region produces bilateral modulation of the jaw-opening reflex

The aim of this study was to determine the effect of unilateral acute inflammation of craniofacial deep tissues on the ipsilateral and contralateral jaw-opening reflex (JOR). The effects of mustard oil (MO), injected into the temporomandibular joint region, were tested on the JOR recorded in the digastric muscle and evoked by low-intensity electrical stimulation of the ipsilateral and contralateral inferior alveolar nerve in anesthetized rats. The MO injection induced a long-lasting suppression of the amplitude of both ipsilaterally and contralaterally evoked JOR, although the latency and duration of the JOR were unaffected. The suppressive effect was more prominent for the contralaterally evoked JOR, and observed even when background activity in the digastric muscle was increased by the MO injection. The results indicate that changes in the JOR amplitude following MO injection do not simply reflect alterations in motoneuronal excitability, and suggest that inflammation of deep craniofacial tissues modulates low-threshold sensory transmission to the motoneurons.

The distribution of inhibitory and excitatory synapses on single, reconstructed jaw-opening motoneurons in the cat

In a previous study, we reported that the distribution of inhibitory input, in contrast to excitatory input, decreased somatofugally along dendrites of cat jaw-closing alpha-motoneurons [J Comp Neurol 414 (1999) 454]. The present study examined the distribution of GABA, glycine, and glutamate immunopositive boutons covering horseradish peroxidase-labeled cat jaw-opening motoneurons. The motoneurons were divided into four compartments: the soma, and primary, intermediate, and distal dendrites. Ninety-seven percent of the total number of studied boutons had immunoreactivity for at least one of the three amino acids. The proportion of boutons immunoreactive for GABA and/or glycine was lower than the proportion of boutons immunoreactive for glutamate. Boutons immunoreactive to glycine alone were more numerous than boutons double-labeled for GABA and glycine, which, in turn, occurred more frequently than boutons immunoreactive to GABA alone. The percentage synaptic covering (proportion of membrane covered by synaptic boutons) of the putatively excitatory (glutamate containing) and putatively inhibitory (GABA and/or glycine containing) boutons decreased somatofugally along the dendrites. Such systematic variations were not seen in the packing density (number of boutons per 100 microm(2)); the packing density showed a distinct drop between the soma and primary dendrites but did not differ significantly among the three dendritic compartments. Overall, the packing density was slightly higher for the putatively excitatory boutons than for the inhibitory ones. When taken together with previous analyses of jaw-closing alpha-motoneurons the present data on jaw-opening alpha-motoneurons indicate that the two types of neuron differ in regard to the nature of synaptic integration in the dendritic tree.

Bilateral projection of functionally characterized trigeminal oralis neurons to trigeminal motoneurons in cats

Intracellular Neurobiotin-injections were used to label functionally identified neurons in the rostro-dorsomedial part of the trigeminal oral nucleus (Vo.r) in the cat. The labeled Vo.r neurons with the mechanoreceptive field in oral tissues innervated bilaterally either jaw-opening motoneurons or jaw-closing motoneurons. This result suggests that Vo.r neurons play an important role in sensory-motor reflexes responsible for coordination of bilaterally symmetrical jaw movements.

Identification of c-Fos immunoreactive brainstem neurons activated during fictive mastication in the rabbit

In the present study we used the expression of the c-Fos-like protein as a "functional marker" to map populations of brainstem neurons involved in the generation of mastication. Experiments were conducted on urethane-anesthetized and paralyzed rabbits. In five animals (experimental group), rhythmical bouts of fictive masticatory-like motoneuron activity (cumulative duration 60-130 min) were induced by electrical stimulation of the left cortical "masticatory area" and recorded from the right digastric motoneuron pool. A control group of five animals (non-masticatory) were treated in the same way as the experimental animals with regard to surgical procedures, anesthesia, paralysis, and survival time. To detect the c-Fos-like protein, the animals were perfused, and the brainstems were cryosectioned and processed immunocytochemically. In the experimental group, the number of c-Fos-like immunoreactive neurons increased significantly in several brainstem areas. In rostral and lateral areas, increments occurred bilaterally in the borderzones surrounding the trigeminal motor nucleus (Regio h); the rostrodorsomedial half of the trigeminal main sensory nucleus; subnucleus oralis-gamma of the spinal trigeminal tract; nuclei reticularis parvocellularis pars alpha and nucleus reticularis pontis caudalis (RPc) pars alpha. Further caudally-enhanced labeling occurred bilaterally in nucleus reticularis parvocellularis and nucleus reticularis gigantocellularis (Rgc) including its pars-alpha. Our results provide a detailed anatomical record of neuronal populations that are correlated with the generation of the masticatory motor behavior.

Quantitative analysis of tooth pulp afferent terminals in the rat brain stem

This study analyzed quantitatively the ultrastructural features of tooth pulp afferent terminals and their presynaptic axonal endings (p-endings) in the trigeminal principal (Vp), dorsomedial oral (Vdm), and caudal nuclei (Vc). Mitochondrial volume, active zone area, apposed surface area, and vesicle number were highly correlated with afferent bouton volume. The afferent bouton volume varied widely in Vp, compared to that in Vdm and Vc. The values of all parameters of p-endings were within a narrow range, and were smaller than those of afferent boutons. The afferent bouton volume correlated with the number of postsynaptic dendrites and p-endings. These results suggest that pulpal afferent information is regulated in a unique manner in the each trigeminal sensory nucleus.

The physiological and morphological characteristics of interneurons caudal to the trigeminal motor nucleus in rats

In this study we have characterized the membrane properties and morphology of interneurons which lie between the caudal pole of the trigeminal motor nucleus and the rostral border of the facial motor nucleus. Previous studies suggest that many of these interneurons may participate in the genesis of rhythmical jaw movements. Saggital brainstem slices were taken from rats aged 5-8 days. Interneurons lying caudal to the trigeminal motor nucleus were visualized using near-infrared differential interference contrast (DIC) microscopy, and were recorded from using patch pipettes filled with a K-gluconate- and biocytin-based solution. The 127 neurons recorded could be categorized into three subtypes on the basis of their responses to injection of depolarizing current pulses, namely tonic firing (type I), burst firing (type II) and spike-adaptive (type III) neurons. Type I interneurons had a higher input resistance and a lower rheobase than type II neurons. All three neuron subtypes showed 'sag' of the voltage response to injection of large-amplitude hyperpolarizing current pulses, and, in addition, also showed rectification of the voltage response to injection of depolarizing current pulses, with type II neurons showing significantly greater rectification than type I neurons. The axonal arborizations were reconstructed for 44 of 63 neurons labelled with tracer. Neurons of each subtype were found to issue axon collaterals terminating in the brainstem nuclei, including the parvocellular reticular nucleus (PCRt), the trigeminal motor nucleus (Vmot), the supratrigeminal nucleus or the trigeminal mesencephalic nucleus. Twenty-five of the 43 neurons issued collaterals which terminated in the Vmot and the other brainstem nuclei. When viewed under 100x magnification, the collaterals of some interneurons were seen to give off varicosities and end-terminations which passed close to the somata of unidentified neurons in the trigeminal motor nucleus and in the area close to the interneuron soma itself. This suggests that the interneurons may make synaptic contacts both on motoneurons and also on nearby interneurons. These results provide data on the membrane properties of trigeminal interneurons and evidence for their synaptic connections both with nearby interneurons and also with motoneurons. Thus, the interneurons examined could play roles in the shaping, and possibly also in the generation, of rhythmical signals to trigeminal motoneurons.

Synaptic organization of tooth pulp afferent terminals in the rat trigeminal sensory nuclei

Previous studies provide evidence that a structure/function correlation exists in the distinct zones of the trigeminal sensory nuclei. To evaluate this relationship, we examined the ultrastructure of afferent terminals from the tooth pulp in the rat trigeminal sensory nuclei: the principalis (Vp), the dorsomedial part of oral nucleus (Vdm), and the superficial layers of caudalis (Vc), by using transganglionic transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). A total of 93 labeled boutons were serially sectioned, in which some sections were incubated with gamma-aminobutyric acid (GABA) antiserum. Almost all labeled boutons formed asymmetric contact with nonprimary dendrites, in which more than half of labeled boutons in the Vc made synapses with their spines. The labeled boutons could be divided into two types on the basis of numbers of dense-cored vesicles (DCVs) in a boutons: S-type and DCV-type. Almost all labeled boutons in the Vp and Vdm were S-type, whereas two types were distributed evenly in the Vc. In contrast to DCV-type boutons, the S-type was frequently postsynaptic to unlabeled axon terminals containing a mixture of round, oval, and flattened vesicles (p-endings) and forming symmetrical synapses. Most p-endings examined were immunoreactive to GABA. The frequency of axoaxonic contacts was higher for labeled boutons in the Vp than in the Vdm and Vc. These results suggest that the three structures of trigeminal sensory nuclei serve distinct functions in nociceptive processing.

Quantitative analysis of the dendritic architectures of single jaw-closing and jaw-opening motoneurons in cats

Little is known about the dendritic architectures of trigeminal motoneurons innervating antagonistic muscles. Thus, the aim of the present study was to provide a quantitative description of jaw-closing (JC) and jaw-opening (JO) alpha motoneurons and to determine geometrical similarities and differences of the dendritic tree between the two. Seven JC alpha motoneurons and four JO alpha motoneurons were intracellularly labeled with horseradish peroxidase (HRP) in the cat and quantitatively analyzed with a computer-assisted three-dimensional system. The dendritic tree of JC alpha motoneurons was confined within the JC motor nucleus, despite locations of the cell body. In contrast, JO alpha motoneurons generated extensive extranuclear dendrites in the reticular formation. The branching pattern of proximal dendritic segments was simpler in the JC than in the JO alpha motoneurons. Despite these differences, the mean values of dendritic parameters examined per neuron were not different between the two kinds of alpha motoneurons, and the stem dendrite diameter was positively correlated with several dendritic parameters in a linear manner. The present study provides new evidence that underlying design principles of the geometry of the dendritic tree are not concerned with the differences in configuration and branching pattern of the dendritic tree of trigeminal alpha motoneurons innervating antagonistic muscles. In addition, we estimated the number of excitatory and inhibitory synapses covering dendrites of single JC alpha motoneurons.

Convergence of selected inputs from sensory afferents to trigeminal premotor neurons with possible projections to masseter motoneurons in the rabbit

Peripheral input convergence on trigeminal premotor neurons in the vicinity of trigeminal motor nucleus has been investigated. Thirty neurons were identified by their antidromic responses to microstimulation of the masseteric subnucleus of trigeminal motor nucleus (NVmot-mass). Peripheral receptive fields were found in the buccal mucosae, periodontal ligaments, palate, tongue and vibrissae for 16 neurons located in the intertrigeminal area (NVint), supratrigeminal area (NVs), main sensory trigeminal nucleus (NVsnpr) and subnucleus gamma of the oral nucleus of the spinal trigeminal tract (NVspo-gamma). Eleven neurons in the NVint, NVs and NVspo-gamma responded to passive jaw opening: nine neurons were activated and two were inhibited. None of the neurons responded to both the orofacial mechanical stimulation and passive jaw opening. Forty-six percent of neurons (13 out of 28 tested) received inputs from the inferior alveolar nerve (IAN) and 53% of neurons (8 out of 15 tested) received inputs from the infraorbital nerve (ION). Out of 15 neurons tested for inputs from the IAN and ION, 7 neurons in the NVsnpr and NVspo-gamma received input from both. Sixteen percent of neurons (4 out of 25) received inputs from the masseteric nerve (MassN). None of the neurons with inputs from IAN and/or ION also received inputs from the MassN. We suggest that trigeminal premotor interneurons with projections to the NVmot-mass fall into two broad categories, those with inputs from the IAN and/or ION and those with inputs from the MassN, possibly muscle spindle afferents, and no neuron receiving inputs from both.

Quantitative ultrastructural analysis of glycine- and gamma-aminobutyric acid-immunoreactive terminals on trigeminal alpha- and gamma-motoneuron somata in the rat

Detailed knowledge of the inhibitory input to trigeminal motoneurons is needed to understand better the central mechanisms of jaw movements. Here a quantitative analysis of terminals contacting somata of jaw-closing (JC) and jaw-opening (JO) alpha-motoneurons, and of JC gamma-motoneurons, was performed by use of serial sectioning and postembedding immunogold cytochemistry. For each type of motoneuron, the synaptic boutons were classified into four groups, i.e., immunonegative boutons or boutons immunoreactive to glycine only, to gamma-aminobutyric acid (GABA) only, or to both glycine and GABA. The density of immunolabeled boutons was much higher for the alpha- than for the gamma-motoneurons. In the alpha-motoneuron populations, the immunolabeled boutons were subdivided into one large group of boutons containing glycine-like immunoreactivity only, one group of intermediate size harboring both glycine- and GABA-like immunoreactivity, and a small group of boutons containing GABA-like immunoreactivity only. The percentage of immunolabeled boutons was higher for JC than JO alpha-motoneurons, the most pronounced difference being observed for glycine-like immunoreactivity. In contrast, on the somatic membrane of gamma-motoneurons, the three types of immunoreactive bouton occurred at similar frequencies. These results indicate that trigeminal motoneurons are strongly and differentially controlled by premotoneurons containing glycine and/or GABA and suggest that these neurons play an important role for the generation of masticatory patterns.