Yoshitaka Nagase

Light microscopic observations of the contacts made between two spindle afferent types and alpha-motoneurons in the cat trigeminal motor nucleus.

Previous studies indicate that cat jaw‐muscle spindle afferents can be divided into two types (type I and II) on the basis of their axonal trajectories. The present study examined the relationship between spindle afferent fibers and their target masseter α‐motoneurons in the cat by using the intracellular horseradish peroxidase (HRP) injection technique, and provided several new findings on the synaptic organization generated between the two. Five type I afferent fiber‐motoneuron pairs and nine type II afferent‐motoneuron pairs were well stained with HRP. The following conclusions were drawn: 1) A motoneuron received contacts from only one collateral of any given spindle afferent. 2) The number of contacts made between an afferent and a motoneuron ranged from one to three. 3) The contacts made by a spindle afferent were on the same dendrite or dendrites branching from the same primary dendrite. 4) The vast majority of the contacts made by an afferent on a motoneuron were distributed in the dendritic tree within 600 μm from the soma, i.e., in the proximal three fourths of the dendritic tree. The differences observed between the two afferent types were as follows. First, type II afferent terminals made contacts on more distal dendrites of the motoneurons than did type I afferent terminals. Second, the contacts made between a type I afferent and a motoneuron were clustered together, but those made between a type II afferent and a motoneuron were widely dispersed.

Organization of the descending projections from the parabrachial nucleus to the trigeminal sensory nuclear complex and spinal dorsal horn in the rat

To clarify direct descending projections from the parabrachial nucleus (PB) to the trigeminal sensory nuclear complex (TSNC) and spinal dorsal horn (SpDH), the origin and termination of descending tract cells were examined by the anterograde and retrograde transport methods. Phaseolus vulgaris leucoagglutinin (PHA‐L) and Fluorogold (FG) or dextran‐tetramethylrhodamine (Rho) were used as neuronal tracers for the anterograde and retrograde transport, respectively. The ventrolateral PB, including Kölliker‐Fuse nucleus (KF), sent axons terminating mainly in the ventrolateral parts of rostral trigeminal nuclei of the principalis (Vp), oralis (Vo), and interpolaris (Vi) as well as in the inner lamina II of the medullary (nucleus caudalis, Vc) and SpDH. Although the descending projections were bilateral with an ipsilateral dominance, TSNC received a more dominant ipsilateral projection than SpDH. The cells of origin of the descending tracts were located mainly in KF, but TSNC received fewer projections from the KF than SpDH. Namely, TSNC received a considerable projection from the medial subnucleus of PB and the ventral parts of lateral subnuclei of PB, such as the central lateral subnucleus and lateral crescent area. The other difference noted between TSNC and SpDH was that the former received projections mainly from the caudal two thirds of KF and the latter from the rostral two thirds of KF. These results demonstrate the existence of direct parabrachial projections to TSNC and SpDH that are organized in a distinct manner and suggest that both pathways are involved in the control of nociception. J. Comp. Neurol. 383:94–111, 1997.

Electron microscopic observation of synaptic connections of jaw-muscle spindle and periodontal afferent terminals in the trigeminal motor and supratrigeminal nuclei in the cat

Previous studies indicate that the trigeminal motor nucleus (Vmo) and supratrigeminal nucleus (Vsup) receive direct projections from muscle spindle (MS) and periodontal ligament (PL) afferents. The aim of the present study is to examine the ultrastructural characteristics of the two kinds of afferent in both nuclei using the intracellular horseradish peroxidase (HRP) injection technique in the cat. Our observations are based on complete or near‐complete reconstructions of 288 MS (six fibers) and 69 PL (eight fibers) afferent boutons in Vmo, and of 93 MS (four fibers) and 188 PL (four fibers) afferent boutons in Vsup. All the labeled boutons contained spherical synaptic vesicles and were presynaptic to neuronal elements, and some were postsynaptic to axon terminals containing pleomorphic, synaptic vesicles (P‐endings). In Vmo neuropil, MS afferent boutons were distributed widely from soma to distal dendrites, but PL afferent boutons predominated on distal dendrites. Most MS afferent boutons (87%) formed synaptic specialization(s) with one postsynaptic target while some (13%) contacting two or three dendritic profiles; PL afferents had a higher number of boutons (43%) contacting two or more dendritic profiles. A small but significant number of MS afferent boutons (12%) received contacts from P‐endings, but PL afferent boutons (36%) received three times as many contacts from P‐endings as MS afferents. In Vsup neuropil, most MS (72%) and PL (87%) afferent boutons formed two contacts presynaptic to one dendrite and postsynaptic to one P‐ending, and their participation in synaptic triads was much more frequent than in Vmo neuropil.

Central distribution of synaptic contacts of primary and secondary jaw muscle spindle afferents in the trigeminal motor nucleus of the cat

Little is known about the differences of the terminations of group Ia and group II afferents within the brainstem or spinal cord. The present study was performed to classify cat jaw muscle spindle afferents by the use of succinylcholine (SCh) and to examine the morphological characteristics of the physiologically classified afferents at the light and electron microscopic levels through the use of the intra‐axonal horseradish peroxidase (HRP) injection technique. The effects of SCh on stretch responses of 119 jaw muscle spindle afferents from the masseter were examined. The SCh converted the single skew distribution of the values for dynamic index (DI) into a bimodal one. Fifty‐eight and 61 afferents were classified as group Ia and group II afferents, respectively. The central projections of 17 intra‐axonally stained afferents (10 group Ia and 7 group II afferents) were examined. The spindle afferents terminated mainly in the supratrigeminal nucleus (Vsup), region h, and the dorsolateral subdivision of trigeminal motor nucleus (Vmo.dl) but differed in the pattern of projections of group Ia and group II afferents. The proportion of group Ia afferent terminals was higher in Vmo.dl but lower in Vsup than that of group II afferents. In Vmo.dl, the proportion of group Ia afferent terminals was higher in the central region but lower in the more outer regions than that of group II afferents. The ultrastructure of serially sectioned afferent boutons (63 group Ia and 72 group II boutons) also was examined. The boutons from the two groups were distributed widely from the soma to small‐diameter dendrites, but the frequency of synaptic contacts on proximal dendrites was higher in group Ia than group II afferents. The present study provides evidence that the two groups of jaw muscle spindle afferents differ in their central projection and the spatial distribution of their synaptic contacts on Vmo.dl neurons. J. Comp. Neurol. 391:50–63, 1998.

Serotonergic axonal contacts on identified cat trigeminal motoneurons and their correlation with medullary raphe nucleus stimulation.

The innervation of the trigeminal motor nucleus by serotonergic fibers with cell bodies in the raphe nuclei pallidus and obscurus suggests that activation of this pathway may alter the excitability of trigeminal motoneurons. Thus, we recorded intracellular responses from cat jaw‐closing (JC) and jaw‐opening (JO) α‐motoneurons evoked by raphe stimulation and used a combination of intracellular staining of horseradish peroxidase (HRP) and immunohistochemistry at the light and electron microscopic levels to examine the distribution of contacts made by serotonin (5‐HT)‐immunoreactive boutons on the two motoneurons types. Electrical stimulation applied to the nucleus raphe pallidus‐obscurus complex induced a monosynaptic excitatory postsynaptic potential (EPSP) in JC (masseter) α‐motoneurons and an EPSP with an action potential in JO (mylohyoid) α‐motoneurons. The EPSP rise‐times (time to peak) and half widths were significantly longer in the JC than in the JO motoneurons. The EPSPs were suppressed by systemic administration of methysergide (2 mg/kg). Six JC and seven JO α‐motoneurons were well stained with HRP. Contacts were seen between 5‐HT‐immunoreactive boutons and the motoneurons. The JC motoneurons received a significantly larger number of the contacts than did the JO motoneurons. The contacts were distributed widely in the proximal three‐fourths of the dendritic tree of JC motoneurons but were distributed on more proximal dendrites in the JO motoneurons. At the electron microscopic level, synaptic contacts made by 5‐HT‐immunoreactive boutons on motoneurons were identified. The present study demonstrated that JC motoneurons receive stronger 5‐HT innervation, and this correlates with the fact that raphe stimulation caused larger EPSPs among these neurons than among JO motoneurons. J. Comp. Neurol. 384:443–455, 1997.

Ultrastructural observations of synaptic connections of vibrissa afferent terminals in cat principal sensory nucleus and morphometry of related synaptic elements

Previous work suggests that slowly adapting (SA) periodontal afferents have different synaptic arrangements in the principal (Vp) and oral trigeminal nuclei and that the synaptic structure associated with transmitter release may be related directly to bouton size. The present study examined the ultrastructures of SA and fast adapting (FA) vibrissa afferents and their associated unlabeled axonal endings in the cat Vp by using intra‐axonal labeling with horseradish peroxidase and a morphometric analysis.

Physiologic and morphologic properties of motoneurons and spindle afferents innervating the temporal muscle in the cat.

Little is known about physiology and morphology of motoneurons and spindle afferents innervating the temporalis and on synaptic connections made between the two. The present study was aimed at investigating the above issues at the light microscopic level by using the intracellular recording and horseradish peroxidase or biotinamide labeling techniques and by the use of succinylcholine (SCh) for the classification of spindle afferents in the cat. Temporalis motoneurons had dendritic trees that ranged from a spherical form to an egg‐shaped form. The shape deformation was more prominent for the dendritic trees made by motoneurons located closer to the nuclear border. No axon collaterals of the motoneurons were detected. On the basis of the values for the dynamic index after SCh infusion, temporalis spindle afferents were classified into two populations: presumptive groups Ia and II. The spindle afferents terminated mainly in the supratrigeminal nucleus (Vsup), region h, and the dorsolateral subdivision (Vmo.dl) of the trigeminal motor nucleus (Vmo). The proportion of group Ia afferent terminals was lower in the Vsup than that of group II afferents. In the Vmo.dl, the proportion of group Ia afferent terminals was nearly even throughout the nucleus, but that of group II afferent terminals increased in the more outlying regions. The proportion of terminal distribution in the central region of Vmo.dl was higher for group Ia than group II. The frequency of contacts (presumptive synapses) made by a single spindle afferent on a motoneuron was higher for group Ia than group II. The present study provided evidence that the central organization of spindle afferent neurons is different between groups Ia and II. J. Comp. Neurol. 406:29–50, 1999.

The Central Projections of the Monkey Tooth Pulp Afferent Neurons

Transganglionic transport of horseradish peroxidase conjugated to wheatgerm agglutinin (HRP:WGA) entrapped in hypoallergenic polyacrylamide gel was used to study the patterns of termination of primary afferents that innervate the upper and lower tooth pulps within the trigeminal sensory nuclear complex (TSNC) of the monkey. HRP:WGA injections were also made into the lower incisors and molars, in order to examine the topographic arrangement of pulpal afferent projections. HRP-labeled pulpal afferents innervating lower and upper teeth projected ipsilaterally to the rostral subnucleus dorsalis (Vpd) and caudal subnucleus ventralis (Vpv) of the nucleus principalis (Vp); the rostrodorsomedial (Vo.r) and dorsomedial (Vo.dm) subdivisions of the nucleus oralis (Vo); the dorsomedial subdivision of the nucleus interpolaris (Vi); and laminae I-II and/or V of the nucleus caudalis (Vc) at its rostralmost level. The HRP-labeled terminals from upper and lower pulpal afferents formed a rostrocaudal column from the midlevel of Vp to the rostral tip of Vc. The label in Vp and Vo was considerably dense, but the column of terminals was interrupted at the Vpd-Vpv transition. The label in Vi and Vc was much less dense compared to that in the rostral nuclei, and the column of terminals was interrupted frequently. The representation of the upper and lower teeth in TSNC was organized in a somatotopic fashion that varied from one subdivision to the next, though their terminal zones overlapped within Vpd. The upper and lower teeth were represented in Vpv, Vo.r, Vo.dm, Vi, and Vc in a ventrodorsal, dorsoventral, lateromedial, lateromedial, and lateromedial sequence, respectively. Topographic arrangement was also noticed for the projections of pulpal afferents from the lower incisors and molars: The representations of the lower incisors and molars in Vpv, Vo.r, Vo.dm, Vi, and Vc were organized in a lateromedial, dorsoventral, ventrodorsal, ventrodorsal, and lateromedial sequence, respectively. The present results indicating sparse projections from pulpal afferents in the monkeys Vc are in good correspondence with a clinical report that trigeminal tractotomy just rostral to the obex has no significant effect on dental pain perception in patients. Furthermore, the present study indicates that projection patterns of pulpal afferents--which include the termination sites, the density of terminations between nuclei, and topographic arrangement--differ among animal species.

Quantitative analysis of the dendritic architectures of cat hypoglossal motoneurons stained intracellularly with horseradish peroxidase.

Little is known about the dendritic architecture of cat hypoglossal motoneurons. Thus, the present study was done to provide quantitative descriptions of hypoglossal motoneurons and to determine correlations between dendritic size parameters by using the intracellular horseradish peroxidase (HRP) injection technique in the cat. Twelve hypoglossal motoneurons stained with HRP were antidromically activated by stimulation applied to the medial branch of hypoglossal nerve. Eight (type I) and four (type II) of the 12 motoneurons were located in the ventral and dorsal parts of the ventromedial subnucleus of hypoglossal nucleus, respectively. The somatodendritic morphology of the two types of neurons was remarkably different, especially in the dendritic arborization pattern. The type I neurons established an egg‐shaped dendritic tree that was distributed through the nucleus to the reticular formation; the type II dendritic tree was confined within the nucleus and presented a rostrocaudally oriented, mirror‐image, fan‐shape appearance. The total dendritic area and length and the number of terminations and branch points were significantly larger for type I than for type II neurons. For the two types of neuron, there was a positive correlation between stem dendritic diameter and several dendritic size parameters. Although the slopes of the regression lines were slightly different between the two, these were not statistically significant. The present study provides evidence that hypoglossal motoneurons located in the ventromedial subnucleus could be divided into two types according to the dendritic arborization pattern and quantitative analysis of the dendritic tree and according to neuronal location and suggests that the two types of hypoglossal motoneurons can be viewed as intrinsically distinct cell types: type I and type II, which innervate extrinsic and intrinsic muscles, respectively. In addition, the morphometric analysis made it possible to estimate the size of the dendritic tree by measuring the stem dendritic diameter. J. Comp. Neurol. 405:345–358, 1999.

Nadph-diaphorase in the spinal trigeminal nucleus oralis and rostral solitary tract nucleus of rats

NADPH-diaphorase histochemical staining demonstrated a distinct neural group that might synthesize nitric oxide in the lower brainstem of rats. The NADPH-diaphorase stain revealed a Golgi-like network in the dorsomedial spinal trigeminal nucleus oralis and rostrolateral solitary tract nucleus, whereas this network was more dense in the latter nucleus. The distribution of NADPH-diaphorase-positive neurons in these areas overlapped with parts of central terminations from the chorda tympani nerve, as demonstrated with transganglionic transport of wheatgerm agglutinin conjugated horseradish peroxidase. The number of NADPH-diaphorase-positive neurons changed after chorda tympani nerve lesion relative to the contralateral side. The control value (%) was 106.0 +/- 4.9 (mean +/- S.E.M.). One hour after the nerve lesion, the value increased to 115.2 +/- 9.1 (P > 0.05). It then decreased to 83.9 +/- 5.2 two days after the lesion (P < 0.05), and remained at this reduced level for one or two weeks, 83.2 +/- 3.0 (P < 0.01) and 83.7 +/- 2.3 (P < 0.01), respectively. This statistically significant reduction recovered to control level 103.4 +/- 2.9 four weeks after the lesion. These results show that NADPH-diaphorase-positive neurons in the lower brainstem could be regulated trans-synaptically by primary afferents, possibly gustatory inputs.