The morphology of neurons in trigeminal nucleus oralis projecting to the medullary dorsal horn (trigeminal nucleus caudalis): a retrograde horseradish peroxidase and Golgi study

The Trigeminal Sensory System and Orofacial Pain

The trigeminal sensory system consists of the trigeminal nerve, the trigeminal ganglion, and the trigeminal sensory nuclei (the mesencephalic nucleus, the principal nucleus, the spinal trigeminal nucleus, and several smaller nuclei). Various sensory signals carried by the trigeminal nerve from the orofacial area travel into the trigeminal sensory system, where they are processed into integrated sensory information that is relayed to higher sensory brain areas. Thus, knowledge of the trigeminal sensory system is essential for comprehending orofacial pain. This review elucidates the individual nuclei that comprise the trigeminal sensory system and their synaptic transmission. Additionally, it discusses four types of orofacial pain and their relationship to the system. Consequently, this review aims to enhance the understanding of the mechanisms underlying orofacial pain.

Neuronal Cell Types in the Spinal Trigeminal Nucleus of the Camel Brain

Neurons in the spinal trigeminal nucleus of a camel were morphologically studied by the Golgi impregnation method. The neurons were classified based on the size and shape of their cell bodies, the density of their dendritic trees, and the morphology and distribution of their appendages. At least 12 morphological types of neurons were found in the camel spinal trigeminal nucleus, including the following: stalked, islets, octopus-like, lobulated, boat-like, pyramidal, multipolar, round, oval, and elongated neurons. These neurons exhibited large numbers of various forms of appendages that arise not only from their dendrites but also from their cell bodies. Moreover, neurons with unique large dilatations especially at their dendritic branching points were also reported. The neurons reported in this study displayed an array of different sizes and shapes and featured various forms of appendages arising from cell bodies and dendrites. Such morphologically distinctive neuronal cell types might indicate an evolutionary adaptation to pain and temperature processing pathways at the level of the spinal trigeminal nucleus in camels, which traditionally live in a very harsh climatic environment and are frequently exposed to painful stimuli.

Interactions of Whisking and Touch Signals in the Rat Brainstem

Perception is an active process, requiring the integration of both proprioceptive and exteroceptive information. In the rat's vibrissal system, a classical model for active sensing, the relative contribution of the two information streams was previously studied at the peripheral, thalamic, and cortical levels. Contributions of brainstem neurons were only indirectly inferred for some trigeminal nuclei according to their thalamic projections. The current work addressed this knowledge gap by performing the first comparative study of the encoding of proprioceptive whisking and exteroceptive touch signals in the oralis (SpVo), interpolaris (SpVi), and paratrigeminal (Pa5) brainstem nuclei. We used artificial whisking in anesthetized male rats, which allows a systematic analysis of the relative contribution of the proprioceptive and exteroceptive information streams along the ascending pathways in the absence of motor or cognitive top-down modulations. We found that (1) neurons in the rostral and caudal parts of the SpVi convey whisking and touch information, respectively, as predicted by their thalamic projections; (2) neurons in the SpVo encode both whisking and (primarily) touch information; and (3) neurons of the Pa5 encode a complex combination of whisking and touch information. In particular, the Pa5 contains a relatively large fraction of neurons that are inhibited by active touch, a response observed so far only in the thalamus. Overall, our systematic characterization of afferent responses to active touch in the trigeminal brainstem approves the hypothesized functions of SpVi neurons and presents evidence that SpVo and Pa5 neurons are involved in the processing of active vibrissal touch.SIGNIFICANCE STATEMENT The present work constitutes the first comparative study of the encoding of proprioceptive (whisking) and exteroceptive (touch) information in the rat's brainstem trigeminal nuclei, the first stage of vibrissal processing in the CNS. It shows that (1) as expected, the rostral and caudal interpolaris neurons convey primarily whisking and touch information, respectively; (2) the oralis nucleus, whose function was previously unknown, encodes both whisking and (primarily) touch touch information; (3) a subtractive computation, reported at the thalamic level, already occurs at the brainstem level; and (4) a novel afferent pathway probably ascends via the paratrigeminal nucleus, encoding both proprioceptive and exteroceptive information.

Periodontitis and diabetes reshape neuronal dendritic arborization in the thalamus and nucleus oralis in the rat

Diabetes is a metabolic disorder resulting in long-term hyperglycemia that could induce oxidative stress as well as neural modifications in the central nervous system. Periodontal disease is highly comorbid with diabetes and in some cases, with exacerbated pain responses. Periodontal tissue is innervated by trigeminal afferents which extend to the nucleus oralis (NO) that sends input to the ventral posterior lateral thalamic nuclei (VPL). The present study aimed to evaluate the consequences of periodontitis, diabetes and both conditions on the dendritic morphology, spine type, and density in neurons of the NO and VPL in male and female rats. A quantitative neuromorphological analysis was performed using the Cox-Golgi staining in male and female rats in four groups: naïve control, after a periodontitis procedure, diabetic, and diabetic with periodontitis. Periodontitis decreased the total dendritic length (TDL) in the NO of the male rat but no change in the female rat and no neuronal alterations were observed in the VPL of both male and female rats. In contrast, diabetes increased the number of spines in the NO and VPL and decreased TDL in the NO in both male and female rats. We observed that periodontitis induced a dimorphic effect in the NO, whereas diabetes induced a strong neuromorphological effect regardless of sex. Moreover, while periodontitis had a limited effect on the neuronal morphology, it dramatically modified the neural consequences in the VPL and NO when comorbid with diabetes. In conclusion, these neuroplastic modifications may be relevant to understand how diabetes exacerbates the outcome of periodontitis in humans, particularly in the female population.

Suppression of the swallowing reflex by stimulation of the red nucleus

We study whether the red nucleus is involved in control of swallowing. The swallowing reflex was induced in anesthetized rats by repetitive electrical stimulation of the superior laryngeal nerve. The electromyographic activities of the mylohyoid and thyrohyoid muscles were recorded in order to identify the swallowing reflex. Repetitive electrical stimulation applied to the red nucleus reduced the number of swallows. The onset latency of the first swallow was increased during repetitive electrical stimulation applied to the magnocellular part of the red nucleus. Microinjection of monosodium glutamate into the red nucleus also reduced the number of swallows. The onset latency of the first swallow was increased after microinjection of monosodium glutamate into the magnocellular part of the red nucleus. These results imply that the red nucleus is involved in the control of swallowing.

Morphology and connections of intratrigeminal cells and axons in the macaque monkey

Trigeminal primary afferent fibers have small receptive fields and discrete submodalities, but second order trigeminal neurons often display larger receptive fields with complex, multimodal responses. Moreover, while most large caliber afferents terminate exclusively in the principal trigeminal nucleus, and pars caudalis (sVc) of the spinal trigeminal nucleus receives almost exclusively small caliber afferents, the characteristics of second order neurons do not always reflect this dichotomy. These surprising characteristics may be due to a network of intratrigeminal connections modifying primary afferent contributions. This study characterizes the distribution and morphology of intratrigeminal cells and axons in a macaque monkeys. Tracer injections centered in the principal nucleus (pV) and adjacent pars oralis retrogradely labeled neurons bilaterally in pars interpolaris (sVi), but only ipsilaterally, in sVc. Labeled axons terminated contralaterally within sVi and caudalis. Features of the intratrigeminal cells in ipsilateral sVc suggest that both nociceptive and non-nociceptive neurons project to principalis. A commissural projection to contralateral principalis was also revealed. Injections into sVc labeled cells and terminals in pV and pars oralis on both sides, indicating the presence of bilateral reciprocal connections. Labeled terminals and cells were also present bilaterally in sVi and in contralateral sVc. Interpolaris injections produced labeling patterns similar to those of sVc. Thus, the rostral and caudal poles of the macaque trigeminal complex are richly interconnected by ipsilateral ascending and descending connections providing an anatomical substrate for complex analysis of oro-facial stimuli. Sparser reciprocal crossed intratrigeminal connections may be important for conjugate reflex movements, such as the corneal blink reflex.

Pharmacological analysis of excitatory and inhibitory synaptic transmission in horizontal brainstem slices preserving three subnuclei of spinal trigeminal nucleus

Spinal trigeminal nucleus (Vsp) consists of three subnuclei: oralis (Vo), interpolaris (Vi) and caudalis (Vc). Previous anatomical studies using antero-/retro-grade tracers have suggested that intersubnuclear ascending/descending synaptic transmissions exist between subnuclei. However, pharmacological properties of the intersubnuclear synaptic transmission have not been studied yet. Since three subnuclei are located in Vsp along rostro-caudal axis, it will be necessary to prepare horizontal brainstem slices to perform pharmacological analysis of the intersubnuclear synaptic transmission. We here show horizontal brainstem slices retaining three subnuclei, and that, using blind whole-cell recordings in the slices, synaptic transmission may be abundantly retained between subnuclei in the horizontal slices, except for the transmission from Vo to Vc. Finally, pharmacological analysis shows that excitatory and inhibitory synaptic responses, respectively, are mediated by AMPA and NMDA receptors and by GABA(A) and glycine receptors, with a differential contribution to the synaptic responses between subnuclei. We therefore conclude that horizontal brainstem slices will be a useful preparation for studies on intersubnuclear synaptic transmission, modulation and plasticity between subnuclei, as well as, further, other brainstem nuclei.

Trigeminohypothalamic and reticulohypothalamic tract neurons in the upper cervical spinal cord and caudal medulla of the rat

Sensory information that arises in orofacial organs facilitates exploratory, ingestive, and defensive behaviors that are essential to overall fitness and survival. Because the hypothalamus plays an important role in the execution of these behaviors, sensory signals conveyed by the trigeminal nerve must be available to this brain structure. Recent anatomical studies have shown that a large number of neurons in the upper cervical spinal cord and caudal medulla project directly to the hypothalamus. The goal of the present study was to identify the types of information that these neurons carry to the hypothalamus and to map the route of their ascending axonal projections. Single-unit recording and antidromic microstimulation techniques were used to identify 81 hypothalamic-projecting neurons in the caudal medulla and upper cervical (C(1)) spinal cord that exhibited trigeminal receptive fields. Of the 72 neurons whose locations were identified, 54 were in laminae I-V of the dorsal horn at the level of C(1) (n = 22) or nucleus caudalis (Vc, n = 32) and were considered trigeminohypothalamic tract (THT) neurons because these regions are within the main projection territory of trigeminal primary afferent fibers. The remaining 18 neurons were in the adjacent lateral reticular formation (LRF) and were considered reticulohypothalamic tract (RHT) neurons. The receptive fields of THT neurons were restricted to the innervation territory of the trigeminal nerve and included the tongue and lips, cornea, intracranial dura, and vibrissae. Based on their responses to mechanical stimulation of cutaneous or intraoral receptive fields, the majority of THT neurons were classified as nociceptive (38% high-threshold, HT, 42% wide-dynamic-range, WDR), but in comparison to the spinohypothalamic tract (SHT), a relatively high percentage of low-threshold (LT) neurons were also found (20%). Responses to thermal stimuli were found more commonly in WDR than in HT neurons: 75% of HT and 93% of WDR neurons responded to heat, while 16% of HT and 54% of WDR neurons responded to cold. These neurons responded primarily to noxious intensities of thermal stimulation. In contrast, all LT neurons responded to innocuous and noxious intensities of both heat and cold stimuli, a phenomenon that has not been described for other populations of mechanoreceptive LT neurons at spinal or trigeminal levels. In contrast to THT neurons, RHT neurons exhibited large and complex receptive fields, which extended over both orofacial ("trigeminal") and extracephalic ("non-trigeminal") skin areas. Their responses to stimulation of trigeminal receptive fields were greater than their responses to stimulation of non-trigeminal receptive fields, and their responses to innocuous stimuli were induced only when applied to trigeminal receptive fields. As described for SHT axons, the axons of THT and RHT neurons ascended through the contralateral brain stem to the supraoptic decussation (SOD) in the lateral hypothalamus; 57% of them then crossed the midline to reach the ipsilateral hypothalamus. Collateral projections were found in the superior colliculus, substantia nigra, red nucleus, anterior pretectal nucleus, and in the lateral, perifornical, dorsomedial, suprachiasmatic, and supraoptic hypothalamic nuclei. Additional projections (which have not been described previously for SHT neurons) were found rostral to the hypothalamus in the caudate-putamen, globus pallidus, and substantia innominata. The findings that nonnociceptive signals reach the hypothalamus primarily through the direct THT route, whereas nociceptive signals reach the hypothalamus through both the direct THT and the indirect RHT routes suggest that highly prioritized painful signals are transferred in parallel channels to ensure that this critical information reaches the hypothalamus, a brain area that regulates homeostasis and other humoral responses required for the survival of the organism.

Cells of origin of the trigeminohypothalamic tract in the rat

Recent studies have demonstrated that a large number of spinal cord neurons convey somatosensory and visceral nociceptive information directly from cervical, lumbar, and sacral spinal cord segments to the hypothalamus. Because sensory information from head and orofacial structures is processed by all subnuclei of the trigeminal brainstem nuclear complex (TBNC) we hypothesized that all of them contain neurons that project directly to the hypothalamus. In the present study, we used the retrograde tracer Fluoro-Gold to examine this hypothesis. Fluoro-Gold injections that filled most of the hypothalamus on one side labeled approximately 1,000 neurons (best case = 1,048, mean = 718 +/- 240) bilaterally (70% contralateral) within all trigeminal subnuclei and C1-2. Of these neurons, 86% were distributed caudal to the obex (22% in C2, 22% in C1, 23% in subnucleus caudalis, and 18% in the transition zone between subnuclei caudalis and interpolaris), and 14% rostral to the obex (6% in subnucleus interpolaris, 4% in subnucleus oralis, and 4% in subnucleus principalis). Caudal to the obex, most labeled neurons were found in laminae I-II and V and the paratrigeminal nucleus, and fewer neurons in laminae III-IV and X. The distribution of retrogradely labeled neurons in TBNC gray matter areas that receive monosynaptic input from trigeminal primary afferent fibers innervating extracranial orofacial structures (such as the cornea, nose, tongue, teeth, lips, vibrissae, and skin) and intracranial structures (such as the meninges and cerebral blood vessels) suggests that sensory and nociceptive information originating in these tissues could be transferred to the hypothalamus directly by this pathway.

Effects of systemic morphine on the activity of convergent neurons of spinal trigeminal nucleus oralis in the rat

The spinal trigeminal nucleus oralis has been shown to relay nociceptive inputs mainly from the oral and perioral regions. In this study, we examined the effects of intravenous administration of morphine on C-fiber-evoked activities of spinal trigeminal nucleus oralis convergent neurons in halothane-anesthetized rats. Morphine depressed the C-fiber-evoked responses of spinal trigeminal nucleus oralis convergent neurons in a dose-related (3-12 mg/kg range) and naloxone-reversible fashion. The ED50 was 6.1 mg/kg, a dose similar to that found in the spinal horn. The observed strong depressive action of morphine on noxious-evoked activities of spinal trigeminal nucleus oralis neurons is consistent with our previous statement, based on electrophysiological studies, that this region plays an important role in the transmission of trigeminal nociceptive information. The effect of morphine on the spinal trigeminal nucleus oralis neurons is discussed in relation to its possible site and mechanism of action.

Induction of NADPH-diaphorase/nitric oxide synthase in the brainstem trigeminal system resulting from cerebellar lesions

Recent evidence indicates that NADPH-diaphorase (NADPH-d) and nitric oxide synthase (NOS) can be induced in cerebellar afferent neurons following mechanical, thermal, or chemical damage to the cerebellar cortex (Saxon and Beitz [1994] Neuroreport 5:809-812). The present study reports on the induction of NADPH-d/NOS in neurons of the brainstem trigeminal complex (BVC). Three groups of rats were used: Group I received a unilateral glass micropipette lesion into the vermal/paravermal region of the cerebellar cortex, group II received a concurrent injection of fluoro-gold along with the pipette lesion, and in group III the cerebellar cortex on one side was aspirated. Following survival times of 7-120 days, animals were processed for NADPH-d histochemistry. All three groups showed projection-specific induction of NADPH-d in different regions of the brainstem trigeminal complex. Induced neurons were distributed throughout the ipsilateral subnucleus interpolaris, principal trigeminal nucleus, and intertrigeminal nucleus. Subnucleus oralis contained a small number of induced neurons localized to the ipsilateral dorsomedial portion of the subnucleus. Projection-specific induction was confirmed by the presence of neurons double-labeled for NADPH-d and Fluoro-Gold. Although the functional consequences of NADPH-d/NOS induction remain to be elucidated, the induction of these enzymes in precerebellar neurons suggests that nitric oxide may play a role in the neuronal response to target specific lesions.

Central projections of identified trigeminal primary afferents after molar pulp deafferentation in adult rats

It is known that removal of the tooth pulp from mandibular molar teeth in adult rats alters the mechanoreceptive field properties of many low-threshold mechanoreceptive neurons in the trigeminal brainstem nuclear complex. The present study investigates one possible way that such deafferentation-induced receptive field changes could occur: altered central projections of uninjured trigeminal low-threshold mechanoreceptive primary afferent fibers. Intra-axonal injection of horseradish peroxidase (n = 22) or neurobiotin (n = 44) into characterized fibers was performed ipsilateral to, and 10-32 days after, removal of the coronal pulp from the left mandibular molars in adult rats. Collaterals were reconstructed, quantified, and compared by means of multivariate analyses of variance to equivalent fibers stained in normal adult rats. Stained mechanosensitive fibers from experimental animals were rapidly conducting and responded to light mechanical stimulation of one vibrissa, one tooth, oral mucosa, facial hairy skin, or guard hairs. Their central projections were indistinguishable from those of control axons in all four trigeminal subnuclei. The numbers of collaterals, areas subtended by collateral arbors, numbers of boutons per collateral, and arbor circularity did not differ from those of control afferents. Collateral somatotopy was also unaffected. These data suggest that following pulpotomy, the central collaterals of uninjured trigeminal afferents display normal morphologies and maintain normal somatotopy. Changes in the morphology of low-threshold primary afferents cannot account for the changes that occur in the receptive field properties of trigeminal brainstem neurons after pulp deafferentation.

Gustatory and tactile stimulation of the posterior tongue activate overlapping but distinctive regions within the nucleus of the solitary tract

Both the gustatory and somatosensory systems provide necessary sensory input for the initiation and control of oromotor behaviors. Behavioral studies indicate that somatosensory input from the posterior tongue (PT) is important in initiating swallowing, whereas PT taste input is particularly important in gustatory rejection reflexes. However, there have been few studies of the central representation of PT gustatory or tactile responses. In the present study, electrophysiological multi-unit recording techniques were used to map the location of PT-mediated taste and tactile responses in the nucleus of the solitary tract (NST) of the rat. A stimulation technique that allows taste stimuli to be introduced directly and specifically into the papillae trenches was used to optimally activate PT taste receptors located within the circumvallate (CV) and foliate (FOL) papillae. The results demonstrated that non-PT responsive sites dominated the rostral half of the rostral division of NST (rNST), while PT-responsive sites dominated the caudal half. Some PT-responsive sites extended into the caudal NST. Both gustatory and tactile stimuli were effective at 28% of PT-responsive locations (taste-tactile sites), whereas at the remaining locations, only tactile stimulation was effective (tactile-only sites). Although these two types of PT-responsive sites exhibited some anatomical overlap, their distributions were distinctive, with taste-tactile sites restricted medially and the laterally located tactile-only sites offset caudally. On the other hand, responses arising from stimulation of the CV and FOL exhibited no anatomical organization, i.e., responses to stimulation of both papillae were coexistensive. On average, of the four tastants used (0.01 M Na saccharin, 0.3 M NaCl, 0.01 M quinine hydrochloride, 0.03 M HCl), HCl was the most effective stimulus for both the CV and FOL. The present results delimit the regions of the NST that provide a substrate for the gustatory and somatosensory limbs of PT-mediated oromotor reflexes.

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 monkey's 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.

The barrelettes--architectonic vibrissal representations in the brainstem trigeminal complex of the mouse. I. Normal structural organization

The organization of the brainstem trigeminal complex (BTC) of the mouse is described, with emphasis on the normal organization of the vibrissal representations. Thionin staining for Nissal substance was employed to reveal the cytoarchitecture. Cytochrome oxidase histochemistry was used to reveal the chemoarchitecture. Golgi impregnation methods, in combination with thionin staining, were used to examine the neuronal dendritic morphology within a defined cytoarchitectonic context. An in vitro horseradish peroxidase labelling method was used to study the distribution and morphology of primary trigeminal afferent terminals within the BTC. The BTC consists of four distinct subnuclei: principalis (nVp), oralis (nVo), interpolaris (nVi), and caudalis (nVc). The present study shows that these sub-nuclei can be distinguished from each other on the basis of several anatomical criteria, including the distribution and density of neuronal size classes, histochemical staining intensity, morphology and orientation of neuronal dendrites, and size and texture of primary afferent terminal arbors. Anatomical manifestation of vibrissal representations within the BTC can be described in nVp, nVi, and nVc, but not in nVo. Within the three subnuclei where they are found, anatomical vibrissal representations are composed to architectural subunits that form an overall pattern homeomorphic to the pattern of vibrissae on the face of the animal. Each sub-unit forms a cylindrical tube running in a rostrocaudal orientation within the BTC. These sub-units will be called barrelettes. Cytologically, each barrelette consists of cell-dense "sides," surrounding a practically cell-free "hollow." Individual sub-units are separated by narrow, cell-free "septa." Histochemically, each subunit is manifested as a discrete patch of positive-staining reaction products. Differential interference contrast optics shows that these patches correspond precisely to the barrelette hollows. Evidence is presented to show that the barrelettes are the functional units for the processing of vibrissal sensory information. Terminal arborizations of individual primary afferents seem to be confined to the hollow of single barrelettes. The majority of neurons that form the sides of a barrelette have bitufted dendritic arbors, which project predominantly into the barrelette hollow, although a minority of neurons, particularly in nVi and nVc, also extend part of their dendritic arbors into adjacent barrelette hollows. The barrelette hollows are thus the principal neuropil region in which primary afferents and their target neurons interact. Contacts are made mainly between en passant varicosities and terminal boutons on primary afferent collaterals and dendritic spines and shafts of second order neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

The spinotrigeminal pathway and its spatial relationship to the origin of trigeminospinal projections in the rat

The anterograde transport of horseradish peroxidase and tritiated amino acids was used to examine the distribution and morphology of spinal afferent fibers terminating in the rat spinal trigeminal complex. The results confirm the existence of a direct, ipsilateral projection from the spinal cord which is distributed exclusively to the deepest layers of the medullary dorsal horn narrow regions subjacent to the spinal trigeminal tract in trigeminal nucleus interpolaris, trigeminal nucleus oralis and the trigeminal main sensory nucleus. Spinal inputs also terminated in the insular trigeminal-cuneatus lateralis nucleus which is a distinct component of the interstitial system of the spinal trigeminal tract. The spinal afferent fibers which terminated in the dorsolateral parts of the spinal trigeminal complex arose from the dorsal column funiculi, while those that terminated in ventral parts of the complex arose from both the dorsal column and lateral funiculi. The tritiated amino acid experiments indicate that at least part of the spinotrigeminal pathway originates from cells located in the cervical spinal dorsal horn. The present findings also document a complex spatial relationship between the spinotrigeminal and trigeminospinal pathways which includes an extensive overlap between spinotrigeminal fibers and spinal projecting neurons in each of the lateralmost regions of the complex. This spatial overlap supports the existence of anatomical substrates which may underlie functional reciprocal loops between the spinal trigeminal complex and cervical spinal cord. Since these regions are primarily concerned with the processing of sensory information from lateral and posterior parts of the face, it follows that the spinotrigeminal pathway may be primarily concerned with the integration of head and neck functions. In addition, the spatial convergence of spinal inputs and the distribution of other trigeminal efferent neurons suggests that part of the spinotrigeminal pathway may be involved in spino-trigemino-thalamic and spino-trigemino-cerebellar pathways in parallel with other spinobulbar pathways in the medulla.

Properties of nociceptive and non-nociceptive neurons in trigeminal subnucleus oralis of the rat

Recent studies have provided evidence suggesting the involvement of rostral components of the V brainstem complex such as trigeminal (V) subnucleus oralis in orofacial pain mechanisms. Since there has been no detailed investigation of the possible existence of nociceptive oralis neurons in the rat to substantiate this recent evidence, the present study was initiated to determine if neurons responsive to noxious orofacial stimuli were present in subnucleus oralis and to characterize their functional properties. In anesthetized rats, recordings were made of the extracellular activity of single neurons functionally characterized as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR) or nociceptive-specific (NS) neurons. The 342 LTM neurons responded only to light mechanical stimulation of orofacial tissues. The mechanoreceptive field of the LTM neurons included the intraoral region in 28% and was localized to the adjacent perioral area in 65%. For 95% the field was localized within one V division. Responses evoked in LTM neurons by electrical stimulation of the orofacial mechanoreceptive field revealed A fiber afferent inputs but no activity that could be attributed to C fiber afferent inputs. The 72 nociceptive neurons included 52 WDR neurons which responded to light (e.g. tactile) as well as noxious (e.g. heavy pressure; pinch) mechanical stimulation of perioral cutaneous and intraoral structures, and 20 NS neurons which responded exclusively to noxious mechanical stimuli. They also differed from the LTM neurons in that 36% of the WDR and 20% of the NS neurons had a mechanoreceptive field involving more than one V division. However, in accordance with our findings for the LTM neurons, the majority of WDR and NS neurons had a mechanoreceptive field involving the intraoral and perioral representations of the mandibular and/or maxillary divisions; those neurons having a mandibular field which especially included intraoral structures predominated in the dorsomedial zone of subnucleus oralis whereas those with a perioral mechanoreceptive field which particularly involved the maxillary division were concentrated in the ventrolateral zone of oralis. In contrast to the LTM neurons, 57% of the WDR and 67% of the NS neurons showed evidence of electrically evoked C fiber as well as A fiber afferent inputs from their mechanoreceptive field. We also noted suppression of the electrically evoked responses by heating of the tail or pinching of the paw. This effect was considered to be a reflection of diffuse noxious inhibitory controls, and was seen in NS as well as WDR neurons; most, but not all, of these neurons received A fiber as well as C fiber orofacial afferent inputs.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell structure and response properties in the trigeminal subnucleus oralis

Extra- and intracellular recording, electrical stimulation, receptive field mapping, and horseradish peroxidase injection techniques were used to study the structure of functionally identified neurons in trigeminal (V) brainstem subnucleus oralis of the rat. Of 15 heavily labeled cells located within oralis, 4 were local-circuit neurons with receptive fields restricted to either an incisor, guard hairs, one vibrissa, or deep facial tissue (nociceptors). Their morphologies were highly varied, with expansive and spiny dendritic trees and recurrent and intersubnuclear axon collaterals. Oralis local-circuit neurons therefore most closely resembled non-vibrissa-sensitive local-circuit cells in adjacent subnucleus interpolaris. Six other stained cells projected to contralateral thalamus, and two others projected to ipsilateral cerebellum. They typically had intramodality convergent receptive fields (i.e., spanning more than one receptor organ, such as multiple vibrissae or teeth) with widespread dendritic trees, and were therefore indistinguishable from similarly projecting cells in interpolaris. Two other cells projected to the ipsilateral spinal cord, as well as other V brainstem subnuclei. One of these responded to high-threshold mechanical stimulation of teeth; the other was discharged by deflection of one mystacial vibrissa. Their dendrites were very widespread and ended in spiny and bulbous appendages. Local axon collaterals were also extensive. The remaining oralis cell had two axons, one projecting to the thalamus, the other to the spinal cord. Its receptive field expressed convergence from multiple receptor organs, including vibrissae, guard hairs, and skin. Its somadendritic morphology was similar to that of oralis cells projecting only to thalamus. We conclude that, with some exceptions, the extensive dendritic trees, axon branching, convergence, and functional diversity of oralis cells approximate those described previously for functionally equivalent neurons in interpolaris (Jacquin et al., 1989a,b). Such anatomical and physiological properties are rarely seen, however, in nucleus principalis (Jacquin et al., 1988a). The structure and function of three atypical principalis cells with structural and functional characteristics typical of oralis cells are also described. It is argued that such cells are rostrally displaced oralis cells.

Structural types of marginal (lamina I) neurons projecting to the dorsal reticular nucleus of the medulla oblongata

The morphological features of lamina I neurons labelled from the medullary dorsal reticular nucleus with free or wheat germ agglutinin conjugated horseradish peroxidase and cholera toxin subunit B, were studied in the three standard anatomical planes in the rat. Orientation and way of branching of the dendritic arbors were further analysed by the method of Sholl in cells labelled with cholera toxin subunit B. Most marginal cells belong to the multipolar type (70%) of our Golgi-based classification, and a minority to the pyramidal (15%) and flattened (15%) types. Following unilateral lesions severing the greatest part of the cuneate fasciculus, a considerable decrease of the numbers of labeled cells of the three types was observed caudal and ipsilaterally to the lesion. Contralateral labelling of multipolar and pyramidal cells was less decreased, and that of flattened cells was apparently unchanged. While multipolar cells, which make up the bulk of marginal spinobulbar neurons, appear to have no other supraspinal target, pyramidal and flattened cells have been labelled from the mesencephalon and the thalamus, respectively. It is suggested that the three structural cell types subserve different aspects of the spinofugal nociceptive output.

Intersubnuclear connections within the rat trigeminal brainstem complex

Prior intracellular recording and labeling experiments have documented local-circuit and projection neurons in the spinal trigeminal (V) nucleus with axons that arborize in more rostral and caudal spinal trigeminal subnuclei and nucleus principalis. Anterograde tracing studies were therefore carried out to assess the origin, extent, distribution, and morphology of such intersubnuclear axons in the rat trigeminal brainstem nuclear complex (TBNC). Phaseolus vulgaris leucoagglutinin (PHA-L) was used as the anterograde marker because of its high sensitivity and the morphological detail provided. Injections restricted to TBNC subnucleus caudalis resulted in dense terminal labeling in each of the more rostral ipsilateral subnuclei. Subnucleus interpolaris projected ipsilaterally and heavily to magnocellular portions of subnucleus caudalis, as well as subnucleus oralis and nucleus principalis. Nucleus principalis, on the other hand, had only a sparse projection to each of the caudal ipsilateral subnuclei. Intersubnuclear axons most frequently traveled in the deep bundles within the TBNC, the V spinal tract, and the reticular formation. They gave rise to a number of circumscribed, highly branched arbors with many boutons of the terminal and en passant types. Retrograde single- or multiple-labeling experiments assessed the cells giving rise to TBNC intersubnuclear collaterals. Horseradish peroxidase (HRP) and/or fluorescent tracer injections into the thalamus, colliculus, cerebellum, nucleus principalis, and/or subnucleus caudalis revealed large numbers of neurons in subnuclei caudalis, interpolaris, and oralis projecting to the region of nucleus principalis. Cells projecting to more caudal spinal trigeminal regions were most numerous in subnuclei interpolaris and oralis. Some cells in lamina V of subnucleus caudalis and in subnuclei interpolaris and oralis projected to thalamus and/or colliculus, as well as other TBNC subnuclei. Such collateral projections were rare in nucleus principalis and more superficial laminae of subnucleus caudalis. TBNC cells labeled by cerebellar injections were not double-labeled by tracer injections into the thalamus, colliculus, or TBNC. These findings lend generality to currently available data obtained with intracellular recording and HRP labeling methods, and suggest that most intersubnuclear axons originate in TBNC local-circuit neurons, though some originate in cells that project to midbrain and/or diencephalon.

Structure-function relationships in rat brainstem subnucleus interpolaris: III. Local circuit neurons

Intracellular recording, electrical stimulation, receptive field mapping, and intracellular injection of horseradish peroxidase were used to assess the response properties, collateral projections, and morphology of 44 local circuit (LC) neurons in the subnucleus interpolaris (Sp Vi) of the trigeminal brainstem complex of the rat. LC neurons were defined as those with axons restricted to brainstem areas receiving trigeminal primary afferent fibers. Thus, none were antidromically activated from the thalamus, tectum, or cerebellum, and their axons could be seen terminating exclusively within the trigeminal brainstem complex or reticular formation. All neurons sampled were discharged by innocuous or noxious mechanical stimulation of a restricted portion of the face or mouth. They were classified functionally as sensitive to vibrissae (N = 22), nociceptors (N = 9), guard hairs (N = 7), hairy skin (N = 3), or periodontia (N = 3). Fifty percent of the stained neurons were vibrissa sensitive. Twenty-one of these 22 responded to deflection of only one vibrissa. The remaining functional groups also had small receptive fields. Intracellular staining revealed a consistency in vibrissa-sensitive LC morphology. Somata were small to medium in size and multipolar. Their axons had an initial transverse trajectory and gave off recurrent collaterals which arborized extensively in the region of the soma. The parent axon then bifurcated. One branch traveled rostrally to subnucleus principalis while the other branch traveled caudally to subnucleus caudalis. The branches periodically sent collaterals into regions of the trigeminal complex corresponding to the transverse position of the soma. Dendrites extended 440 +/- 140 microns rostrocaudally, forming a tree with a transverse perimeter of 459 +/- 226 microns. Distal dendrites were thin and sinuous, had few spines, and extensively arborized adjacent to the soma. They ended in multiple swellings connected by slender processes. The stereotyped morphology of vibrissa-sensitive LC neurons differed from the variable morphologies of LC neurons activated by nociceptors, guard hairs, hairy skin, or periodontia. Although no group of neurons in one of these categories displayed a distinguishing morphological characteristic, they collectively had features which distinguished them from the vibrissa-sensitive neurons. Non-vibrissa-responsive neurons generally had more expansive, but less circular, dendritic and recurrent axonal arbors; dendrites had more spines, and axons often sent endings into the reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Ulex europaeus agglutinin-I binding to dental primary afferent projections in the spinal trigeminal complex combined with double immunolabeling of substance P and GABA elements using peroxidase and colloidal gold

Ulex europaeus agglutinin I (UEA-I) is a plant lectin with an affinity for L-fucosyl residues in the chains of lactoseries oligosaccharides associated with medium- and smaller-diameter dorsal root ganglion neurons and their axonal processes. These enter Lissauer's tract and terminate within the superficial laminae of the spinal cord overlapping projections known to have a nociceptive function. This implies that the surface coatings of neuronal membranes may have a relationship with functional modalities. The present investigation further examined this concept by studying a neuronal projection with a nociceptive function to determine whether fucosyl-lactoseries residues were incorporated in its primary afferent terminals. Transganglionic transport of horseradish peroxidase (HRP) following injection into tooth pulp chambers was employed to demonstrate dental pulp terminals in the trigeminal spinal complex, while peroxidase and fluorescent tags were used concomitantly to stain for UEA-I. Double immunolabeling for substance P (SP) and gamma-aminobutyric acid (GABA) using peroxidase and colloidal gold allowed a comparison of the distribution of a known excitatory nociceptive transmitter with that of UEA-I binding in specific subnuclei. Synaptic interrelationships between UEA-I positive dental pulp primary afferent inputs and specific inhibitory terminals were also examined. SP immunoreactivity occurred in laminae I and outer lamina II (IIo) of subnucleus caudalis (Vc) and in the ventrolateral and lateral marginal region of the caudal half of subnucleus interpolaris (Vi), including the periobex area in which Vi is slightly overlapped on its lateral aspect by cellular elements of Vc. The adjacent interstitial nucleus (IN) also showed an intense immunoreactivity for this peptide antibody. UEA-I binding displayed a similar distribution pattern in both Vc and Vi, but extended into lamina IIi and the superficial part of Lamina III in Vc. Dental pulp terminals were found to have a comparable distribution; however, many extended into the dorsal portion of the caudal half of Vi and the ventromedial quadrant of rostral Vi. Electron-microscopic analysis showed that transganglionically labeled dental pulp terminals contained ovoid, complex membrane-bound vacuoles laden with transported HRP. The preterminal axon and synaptic membranes of those dental pulp terminals located in zones of Vc and Vi displaying an affinity for UEA-I were usually characterized by a patchy, electron-dense coating of the peroxidase tag. SP was demonstrated ultrastructurally with Protein-A colloidal gold (3-nm particles), whereas GABA immunoreactivity was revealed by the avidin-biotin-peroxidase method.(ABSTRACT TRUNCATED AT 400 WORDS)

Golgi and immunocytochemical analysis of neurons in trigeminal subnucleus interpolaris: correlations with cellular localization of enkephalin

Recent electrophysiological evidence shows that rostral levels of the trigeminal spinal complex are concerned with pain processing from receptive fields in the face and oral cavity. The ventrolateral quadrant of the subnucleus interpolaris contains concentrations of enkephalin, dynorphin, serotonin, substance P and GABA [Matthews M. A., Hernandez T. V. and Liles S. L. (1987) Synapse 1, 512-529; Matthews M. A., McDonald G. K. and Hernandez T. V. (1988) Somatosensory Res. 5, 205-217]. These transmitters have also been localized to the fusiform and stalked cells in Laminae I and II of the subnucleus caudalis [Basbaum A. I. and Fields H. L. (1984) A. Rev. Neurosci. 7, 309-338]. The present study compares Golgi impregnations of the subnucleus interpolaris with sections at the same levels immunoreacted against enkephalin to determine if comparable cells exist in the subnucleus interpolaris and if they occur predominantly in the ventrolateral quadrant of the subnucleus. Twelve, young adult cats were killed by perfusion, the brainstems removed and either processed for rapid Golgi impregnation or sectioned and immunoreacted for enkephalin using the avidin-biotin Vectastain method. Golgi impregnated tissue was sectioned in the coronal, transverse or sagittal plane to insure the most advantageous visualization of cells with a directional bias in their dendritic arbors. The subnucleus interpolaris contained several distinctive cell types. The predominant neuron throughout the subnucleus was the smooth pyramidal cell or multipolar cell, characterized by a large round soma (15-25 microns diameter) and a spherical dendritic arborization which allowed its identification in all planes of section. The second cell type was the fusiform cell which had a smaller ovoid soma (10-15 microns) with narrow, less ramified, dendritic arbors oriented dorsoventrally, thus giving a bipolar appearance. Fusiform cells were most concentrated along the lateral margin of the subnucleus interpolaris. Examination of sections at the same level reacted for enkephalin revealed cells with a bipolar appearance in these same locations. An additional cell population which tended to predominate in the lateral zone was the stalked cell. These displayed a rounded soma (12-20 microns) and were evident only in the transverse or sagittal plane. Two to four primary dendrites arose from the soma and extensively ramified into a dense spiny arbor directed into the body of the subnucleus interpolaris. Many examples contained enkephalin. Islet cells, characterized by a very small oval soma (6-12 microns) and dense, rostrocaudally oriented dendrites, were less common than stalked cells and were located deeper in the nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic organization of primary axons in trigeminal nucleus oralis

This report examines the morphology and synaptic connections of small-diameter primary trigeminal axons that terminate in the border zone (BZ) and ventrolateral (VL) subdivisions of rat trigeminal nucleus oralis (Vo). Primary axons were made visible for light and electron microscopic analysis by utilizing the method of anterograde transport of horseradish peroxidase. BZ receives the terminal arborizations of two different populations of small-diameter primary axons. One of these arises from unmyelinated parent fibers and terminates in the dorsal one-half of BZ, while the other has small myelinated parent branches that arborize throughout the subdivision. Terminating within VL are the arbors of a second population of small myelinated primary axons. The endings of all three populations of primary axons lie in synaptic glomeruli. Endings in both subdivisions derived from small myelinated parent fibers lie centrally in glomeruli. Those in VL form axodendritic synapses on numerous dendritic shafts and spines, while endings in BZ glomeruli make at least one axodendritic synapse on one or two dendritic shafts. Endings of unmyelinated primary axons in BZ lie at the periphery of glomeruli where each forms a single axodendritic synapse on a central dendrite. It is at these asymmetrical axodendritic synapses that these three populations of primary axons are thought to transfer their inputs directly to the dendritic arbors of second-order BZ and VL neurons. Common to all three glomeruli is one or more small axonal endings filled with flattened synaptic vesicles that establish axoaxonic synapses on the primary ending as well as axodendritic synapses on the dendritic element(s) receiving primary input. In view of their symmetrical to intermediate synaptic contacts, these endings are thought to belong to axons derived from at least one source that can inhibit or diminish the firing rate of second-order BZ and VL neurons in response to primary input.

The spinothalamic system of the rat: structural types of retrogradely labelled neurons in the marginal zone (lamina I)

Retrogradely labelled lamina I neurons were studied after intrathalamic injections of free horseradish peroxidase mixed with dimethylsulphoxide, wheat germ agglutinin conjugated with horseradish peroxidase, and subunit B of cholera toxin. The first two tracers revealed only the perikaryal shape and the orientation of primary dendrites, while cholera toxin subunit B produced Golgi-like stainings. The morphological and morphometric analysis of the labelled marginal neurons in different planes showed them to belong to the pyramidal and the flattened types of our Golgi-based classification. These cells were located predominantly in the intermediate lateromedial portion of lamina I at all spinal levels, and it is suggested that their structural duality is matched by different functional properties. Distributions of the remaining spinothalamic cells labelled with the two horseradish peroxidase tracers were rather similar to those previously reported in the literature, including the almost exclusive occurrence of labelled cells, at lumbar levels, in the internal basilar column group. Cholera toxin subunit B labelled many more spinal cells and revealed considerable numbers of labelled cells in all cell groups at the lumbar enlargement.

Uptake and transneuronal transport of horseradish peroxidase-wheat germ agglutinin by tooth pulp primary afferent neurons

Horseradish peroxidase-wheat germ agglutinin (HRP-WGA) applied to proximal stumps of tooth pulp primary afferent neurons in rats was taken up and transported transneuronally to neurons in the ipsilateral trigeminal brainstem nuclear complex. The results of this study suggest that HRP-WGA transport may be a novel means of labeling both primary and higher order neurons that transmit tooth pulp sensory information in the rat and may be used to investigate the fine structure and synaptic contacts of central nervous system neurons that receive tooth pulp afferent input.

The organization of trigeminotectal and trigeminothalamic neurons in rodents: a double-labeling study with fluorescent dyes

Retrogradely transported fluorescent dyes (fast blue and diamidino-dihydrochloride yellow) were used to compare the distributions of trigeminofugal neurons that project to the superior colliculus and/or the thalamus in three rodent species. The objective was to determine what the projection and collateralization patterns of these trigeminofugal pathways are and whether they are similar among different species. In each anesthetized animal, one dye was injected into the superior colliculus and the other into the topographically congruent area of the thalamus. Counts of the numbers of yellow, blue, and double-labeled neurons were made throughout the trigeminal complex: principalis, pars oralis, pars interpolaris, and pars caudalis. Trigeminothalamic projections were similar in each of the rodent species studied. The densest concentration of retrogradely labeled neurons was in principalis, with substantially fewer neurons in pars interpolaris, and fewer still in pars oralis and pars caudalis. These neurons were generally small and tended to have round or fusiform somata. A common pattern was also noted among the three species for trigeminotectal neurons. Most trigeminotectal projections originated from neurons in pars interpolaris, somewhat fewer from pars oralis, and the fewest from principalis and pars caudalis. These neurons tended to be the largest in each subdivision and were often multipolar. Following paired injections of the tracers, double-labeled neurons were scattered throughout the sensory trigeminal complex and had morphologies characteristic of single-labeled trigeminotectal neurons. Although comparatively few double-labeled neurons were observed in any species, most of those seen were restricted to the ventrolateral portion of pars interpolaris, a position that corresponds to the representation of the vibrissae. These data indicate that, regardless of the rodent species, the vast majority of labeled trigeminal neurons project either to the superior colliculus or the thalamus, but not to both targets. This might be expected on the basis of the very different behavioral roles these structures play. On the other hand, a subpopulation of trigeminal neurons exists (mainly in pars interpolaris) that does project to both the superior colliculus and the thalamus, perhaps because both structures require some of the same somatosensory information to perform their behavioral functions.

The neurobiology of facial and dental pain: present knowledge, future directions

This review outlines recent research which has identified critical neural elements and mechanisms concerned with the transmission of sensory information related to oral-facial pain, and which has also revealed some of the pathways and processes by which pain transmission can be modulated. The review highlights recent advances in neurobiological research that have contributed to our understanding of pain, how acute and chronic pain conditions can develop, and how pain can be controlled therapeutically. Each section of the review also identifies gaps in knowledge that still exist as well as research approaches that might be taken to clarify even further the mechanisms underlying acute and chronic oral-facial pain. The properties of the sense organs responding to a noxious oral-facial stimulus are first considered. This section is followed by a review of the sensory pathways and mechanisms by which the sensory information is relayed in nociceptive neurones in the brainstem and then transmitted to local reflex centers and to higher brain centers involved in the various aspects of the pain experience--namely, the sensory-discriminative, affective (emotional), cognitive, and motivational dimensions of pain. Reflex and behavioral responses to noxious oral-facial stimuli are also considered. The next section provides an extensive review of how these responses and the activity of the nociceptive neurones are modulated by higher brain center influences and by stimulation of, or alterations (e.g., by trauma) to, other sensory inputs to the brain. The neurochemical processes, involved in these modulatory mechanisms are also considered, with special emphasis on the role of neuropeptides and other neurochemicals recently shown to be involved in pain transmission and its control. The final section deals with recent findings of peripheral and central neural mechanisms underlying pain from the dental pulp.

A quantitative light microscopic analysis and ultrastructural description of cholecystokinin-like immunoreactivity in the spinal trigeminal nucleus of the rat

The spinal trigeminal nucleus is involved in orofacial sensory transmission. Cholecystokinin octapeptide has been identified in axons in this nucleus and appears to play a role in the transmission of orofacial sensation from the trigeminal ganglia to the spinal trigeminal nucleus. Although cholecystokinin has been reported in axonal processes within the spinal trigeminal nucleus at the light microscopic level, nothing is known about the synaptic relationships of these cholecystokinin axons. The goals of this study were to quantitatively determine the volume fraction of cholecystokinin-like immunoreactive cell bodies and fibers in the three subnuclei of the spinal trigeminal nucleus, to provide the first ultrastructural description of cholecystokinin-like immunoreactive processes within these subnuclei and to analyse the synaptic relationships of cholecystokinin-like immunoreactive processes within the spinal trigeminal nucleus neuropil. Cholecystokinin-like immunoreactivity was localized by the peroxidase-antiperoxidase method or the peroxidase labeled, avidin-biotin technique and quantified at the light microscopic level by point counting. Immunoreactive fibers were present in all three subnuclei, but the greatest volume fraction of immunoreactive axons was obtained in laminae I and II of the nucleus caudalis. No immunoreactive cell bodies were evident in any of the subnuclei. The majority of immunoreactive profiles in all three subnuclei were identified ultrastructurally as axon terminals that contained both small and medium sized agranular vesicles and infrequently, large dense core vesicles. These immunoreactive terminals were usually found in close contact with non-immunoreactive dendrites with which they were observed to form asymmetric synapses. Immunoreactive terminals were occasionally observed to contact the cell bodies of large non-immunoreactive neurons on the border of laminae I and II in the nucleus caudalis. These results indicate that cholecystokinin-like immunoreactive processes are present throughout the spinal trigeminal nucleus, and in nucleus caudalis show a distribution similar to that reported for the spinal cord dorsal horn. Immunoreactive axons make synaptic contact with both the dendrites and perikarya of spinal trigeminal nucleus neurons. No axoaxonic synapses were observed. These findings suggest that cholecystokinin plays an important role in spinal trigeminal nucleus function. The possible colocalization of cholecystokinin and substance P in the spinal trigeminal nucleus, and the possible role of cholecystokinin in attenuating the action of opioids in the spinal trigeminal nucleus are also discussed.

Immunocytochemistry of enkephalin and serotonin distribution in restricted zones of the rostral trigeminal spinal subnuclei: comparisons with subnucleus caudalis

The spinal trigeminal subnucleus caudalis processes nociceptive input from the head. However, physiological and behavioral studies in monkeys and humans indicate that painful stimuli from the central face and oral cavity also project through trigeminal nuclei rostral to the spinal subnucleus caudalis. Both enkephalin (ENK) and serotonin (5-HT) are present in rostral trigeminal nuclei and these regions receive inputs from the raphe complex. Thus, it appears that elements of pain-modulating circuitry proposed by Basbaum and Fields (Annu. Rev. Neurosci., 7:309-338, 1984) for the spinal and medullary dorsal horn may also exist in this region. In order to begin an exploration of this circuitry, the present study combines the techniques of retrograde transport of HRP from the ventral posteromedial thalamic nucleus (VPM) of the cat's thalamus to label trigeminothalamic relay cells. Secondarily, immunocytochemical techniques are employed to define the distribution patterns of ENK and 5-HT cells and terminals in relationship to both labeled and nonlabeled neurons in each of the subnuclei of the spinal trigeminal nucleus. Trigeminothalamic relay cells were observed in laminae I and II, the magnocellular region, and the interstitial nucleus (IN) of subnucleus caudalis (Vc). ENK was found in axodendritic and axosomatic terminals, together with a population of small fusiform neurons in all these same areas except the magnocellular region. ENK axosomatic contacts innervated approximately 30% of labeled relay cells, chiefly in lamina I and the IN, or small unlabeled neurons in the same area. Serotonin activity occurred principally in lamina I and the IN and was confined almost exclusively to axodendritic terminals. Examination of subnucleus interpolaris (Vi) revealed relay cells distributed throughout the length of the nucleus and increasing in numbers at rostral levels. A rostral extension of the IN was found just ventrolateral to the main body of Vi and contained numerous labeled cells. The distribution of ENK activity was restricted to the ventral part of Vi and the IN and occurred in axodendritic and axosomatic terminals. These latter elements innervated 30-40% of labeled relay cells in Vi, particularly those located in the IN. Cells containing ENK generally resembled the fusiform cells found in Vc and were distributed in ventral Vi and the IN. Some ENK cells were larger, displayed several dendrites, and occurred only in the ventral Vi. Serotonin within Vi and Vc was confined principally to axodendritic terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of glutamate in trigeminothalamic projection neurons: a combined retrograde transport-immunohistochemical study

Trigeminothalamic projection neurons are important components of the pathways for conscious perception of pain, temperature, and tactile sensation from the orofacial region. The neurotransmitters utilized by trigeminal neurons projecting to the thalamus are unknown. By use of a monoclonal antibody specific for fixative-modified glutamate and a polyclonal antiserum against glutaminase, we recently identified neurons in the trigeminal sensory complex that contain glutamate-like immunoreactivity (Glu-LI) and glutaminase-like immunoreactivity. In the present study, we utilized combined retrograde transport-immunohistochemical techniques to localize putative glutamatergic trigeminothalamic neurons. Following injection of the retrograde tracer, wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP), into the ventroposterior medial thalamus (VPM), the number of neuronal profiles that were double-labeled with WGA:HRP and Glu-LI was greatest in principal sensory nucleus (Pr5), followed by subnuclei interpolaris (Sp5I) and caudalis (Sp5C). The average percentages of projection neurons double-labeled with Glu-LI were approximately 60-70% in Pr5 and Sp5I and 40% in Sp5C. The majority of double-labeled profiles in Sp5C were located in the magnocellular layer, as opposed to the marginal and substantia gelatinosa layers. A large injection site that spread into the intralaminar thalamic nuclei and nucleus submedius--areas implicated in the processing of nociceptive information--resulted in an increase in the ratio of single-labeled to double-labeled projection profiles in Sp5C. These results suggest that glutamate may be the neurotransmitter for a majority of trigeminothalamic projection neurons located in Sp5I and Pr5. However, on the basis of anatomical association, glutamate does not appear to be the major transmitter for neurons in Sp5C that forward nociceptive information to the thalamus.

Morphology and synaptic connections of unmyelinated primary axons in the border zone of rat trigeminal nucleus oralis

This anterograde horseradish peroxidase study examines the morphology and synaptic connections of a population of primary axons which terminate in the dorsal one-half of the border zone (BZ) of rat trigeminal nucleus oralis. Unmyelinated parent fibers in the spinal V tract enter BZ directly and each terminate by continuing as a sparsely branched, long caudally directed strand containing several axonal endings. Primary endings lie in glomeruli where each forms an asymmetrical synapse on a central dendrite. Other glomerular components include two types of non-primary endings. One contains flattened synaptic vesicles, and forms a symmetrical synapse on either the primary ending or the central dendrite, while the other contains pleomorphic synaptic vesicles and establishes a symmetrical synapse on the central dendrite.

Morphology and synaptic connections of myelinated primary axons in the ventrolateral region of rat trigeminal nucleus oralis

Neurons in the ventrolateral (VL) subdivision of rat trigeminal nucleus oralis (Vo) have most of their dendritic arbors confined within this region. This study examines the morphology and synaptic connections of a population of myelinated primary trigeminal axons that arborize within VL and are in a position to provide input directly to VL neurons. Primary axons were visualized for light and electron microscopic analysis by injecting 30% horseradish peroxidase (HRP) in 2% dimethylsulfoxide (DMSO) into the sensory root of the trigeminal nerve and allowing 24-36 hours for the anterograde transport of HRP into the terminal axonal arbors. This population is characterized by its cone-shaped terminal arbors, which generate many axonal endings (2-8 micron in diameter) along unmyelinated terminal strands. These arbors arise from collaterals emanating from thinly myelinated (2-5 micron in diameter) parent branches descending in the spinal V tract, which, on the basis of their size, are considered to be small myelinated (A sigma) primary trigeminal axons. HRP-labeled P endings belonging to this population of primary axons are scalloped, filled with spherical to ovoid (40-70 nm in diameter) synaptic vesicles, and lie centrally in glomeruli where they make asymmetrical axodendritic synapses on dendritic shafts and spine heads. It is at these synapses that this population of primary trigeminal axons is probably transferring its input directly to the dendritic arbors of VL neurons. The dendritic shafts and spine heads also receive symmetrical to intermediate axodendritic synapses from endings containing flattened (70 X 29 nm) synaptic vesicles. These terminals also establish axo-axonic synapses on the P ending. Other synaptic components found less often in the glomeruli include small terminals containing oval (14-23 nm) synaptic vesicles that establish symmetrical to intermediate synapses on the P ending, boutons containing pleomorphic (35-80 nm) synaptic vesicles that form symmetrical to intermediate synapses on the P ending as well as on dendritic shafts, and small peripheral endings containing round (20-40 nm) synaptic vesicles that establish asymmetrical synapses on dendritic shafts.

Morphology and synaptic connections of small myelinated primary trigeminal axons arborizing among neurons in the border zone of rat trigeminal nucleus oralis

The anterograde transport of horseradish peroxidase (HRP) was used to examine the morphology and synaptic connections of a morphologically distinct group of small-diameter primary trigeminal axons that arborize throughout the border zone (BZ) of rat trigeminal nucleus oralis. Thinly myelinated parent branches (0.75-1.5 micron in diameter) descending in the spinal V tract (SVT) were seen to issue medially directed collaterals that entered BZ, where they branched and eventually terminated by giving rise to thin terminal strands characterized by several relatively widely spaced axonal endings. Based on the size and morphology of the parent branches in SVT, in the root entry zone, and in the sensory root of the trigeminal nerve, these primary axonal (P) endings are considered to be derived from small-diameter myelinated primary trigeminal axons (SDMA). The P endings measured 1-2 micron in diameter and contained numerous agranular spherical (40-60 nm) synaptic vesicles. In the BZ neuropil, most P endings lay in glomeruli, where each formed at least one asymmetrical axodendritic synapse on a dendritic shaft. It is at these synapses that this group of primary axons is thought to transfer its input directly to the dendritic arbors of BZ neurons. A small (0.5-1.5 micron) axonal (F) ending filled with flattened synaptic vesicles (29 X 60 nm) was observed to form at least one symmetrical to intermediate axoaxonic synapse on the P ending, as well as at least one axodendritic synapse on the same dendritic shaft receiving the primary input. Some F endings only contacted dendritic shafts. In view of their symmetrical to intermediate synaptic contacts, F endings are thought to belong to axons derived from at least one source that can inhibit or diminish the firing rate of BZ neurons in response to SDMA input. This would be accomplished either postsynaptically through the axodendritic synapses on the dendritic shafts, and/or presynaptically through the axoaxonic synapses on the P endings.

Morphological features of identified trigeminocerebellar projection neurons in the border zone of rat trigeminal nucleus oralis

The retrograde transport of horseradish peroxidase (HRP) was used to assess the overall morphology of neurons in the dorsal half of the border zone (BZ) of rat trigeminal nucleus oralis (Vo) that project to ipsilateral orofacial portions of four major tactile areas (crura I and II, the paramedian lobule, and the uvula) of the cerebellar cortex. We wished to answer two important questions: Does this group of cells consist of one or more morphologically distinct types, each projecting to different cerebellar tactile areas? Does the overall morphology or morphologies of these BZ neurons resemble one or more of the five types of identified trigeminocerebellar neurons in the dorsomedial (DM) subdivision of Vo, or do these trigeminocerebellar cells represent an additional morphologically distinct type or types restricted to BZ? The morphology of BZ neurons innervating the orofacial portions of all four cerebellar tactile areas was similar and did not resemble that of any of the five types of DM cells. They were characterized by a pyramidal- to fusiform-shaped cell body measuring 10-13 X 20-25 microns, which emitted three or four primary dendrites; one was directed dorsally, while the others took a more ventral trajectory. The primary dendrites generated a dendritic arbor arranged as a flattened disk oriented parallel to the spinal V tract. The dendritic field was largely restricted to BZ; it measured up to 150 microns in width, and spanned up to 450 microns dorsoventrally and rostrocaudally. An axon arose from the dorsal aspect of the cell body and gave rise to a single short collateral within 10 microns of its origin. This collateral remained unbranched and generated several boutons within BZ, while the parent axon, without branching further, traveled dorsolaterally toward the inferior cerebellar peduncle. Frequently, a second axon arose ventrally from the soma, and after a short unbranched course entered a deep axon bundle, where it assumed a descending trajectory. The intranuclear portion of this second axon was characterized by several boutons en passant.

The dorsomedial portion of trigeminal nucleus oralis (Vo) in the rat: cytology and projections to the cerebellum

Electrophysiological studies have described four major tactile areas in the rat cerebellar cortex. These areas are in crus I, crus II, the paramedian lobule (PML), and the uvula, and a major portion of each is related to the ipsilateral orofacial region. This study demonstrates that neurons in trigeminal nucleus oralis (Vo) that project to the orofacial portions of these four major tactile areas are localized in the dorsomedial (DM) subdivision of the nucleus. The distribution, light-microscopic morphology, and relative densities of trigeminocerebellar neurons within DM, retrogradely labeled with horseradish peroxidase (HRP) following injections into each of the four major tactile areas, were analyzed and compared as well as correlated with the myelo- and cytoarchitecture of DM observed in Nissl sections, 1-micron sections, and Golgi material. On the basis of myelo- and cytoarchitectonic as well as trigeminocerebellar connectional criteria, three portions of DM were identified: caudal DM (CDM), middle DM (MDM), and rostral DM (RDM). The greatest portion of DM is made up of MDM (1.3 mm long), which can be further subdivided into dorsal (MDMd) and ventral (MDMv) zones. CDM forms the caudal 800 microns of DM, while RDM makes up the rostral 280 microns of the subdivision. Longitudinally running deep axon bundles permeate CDM, MDMv, and RDM, but are conspicuously absent from MDMd. The majority of neurons found throughout CDM, MDMv, and RDM have medium-sized (15- to 30-microns) somata and can be divided into two types on the basis of their somatodendritic morphology. CDM, MDMv, and RDM also contain a small neuronal cell type (5- to 15-microns cell body) that is encountered less frequently than either one of the two types of medium-sized cells. A fourth type of neuron with a large (25- to 50-microns) fusiform- to pyramidal-shaped cell body is the least frequently observed neuronal cell type and is located principally in CDM and MDMv. MDMd contains a fifth type of neuron characterized by a small (5- to 15-microns) oval soma. Data from the retrograde HRP experiments show that all five of these neuronal cell types in their respective portions of DM project to one or more of the orofacial portions of the four major tactile areas of the cerebellar cortex. Many medium-sized neurons of both types in CDM, MDMv, and RDM project to crus I, crus II, and/or PML.(ABSTRACT TRUNCATED AT 250 WORDS)

Axonal endings terminating on dendrites of identified large trigeminospinal projection neurons in rat trigeminal nucleus oralis

The retrograde horseradish peroxidase technique was used to: (1) identify and assess the overall morphology of large neurons in the ventrolateral portion (VL) of rat trigeminal nucleus oralis projecting to cervical, thoracic and lumbosacral levels of the spinal cord; and (2) characterize the synaptic endings terminating on their dendrites. The morphology of large VL neurons projecting to all spinal levels is similar. They have 25-50 microns pyramidal-shaped somata which emit 3-6 primary dendrites. These primary dendrites give rise to spherical to elliptical-shaped dendritic arbors measuring up to 700 microns in diameter. Labeled axons enter either a deep axon bundle or the medial portion of the spinal V tract. Dendrites of labeled neurons are contacted by axonal endings of 3 types. The most numerous endings are filled with clear, spherical synaptic vesicles and usually form single asymmetrical contacts along the entire length of dendritic shafts. Synapsing less frequently on dendritic shafts are endings containing pleomorphic synaptic vesicles and forming single symmetrical synaptic contacts. The least frequently encountered synaptic terminal contains flattened synaptic vesicles and makes a single symmetrical synaptic contact with a dendritic shaft.