Nerve Growth and Interaction in Gelfoam® Histoculture: A Nervous System Organoid

Nestin-expressing hair follicle-associated pluripotent (HAP) stem cells reside mainly in the bulge area (BA) of the hair follicle but also in the dermal papilla (DP). The BA appears to be origin of HAP stem cells. Long-term Gelfoam^® histoculture was established of whiskers isolated from transgenic mice, in which there is nestin-driven green fluorescent protein (ND-GFP). HAP stem cells trafficked from the BA toward the DP area and extensively grew out onto Gelfoam^® forming nerve-like structures. These fibers express the neuron marker β-III tubulin-positive fibers and consisted of ND-GFP-expressing cells and extended up to 500 mm from the whisker nerve stump in Gelfoam^® histoculture. The growing fibers had growth cones on their tips expressing F-actin indicating that the fibers were growing axons. HAP stem cell proliferation resulted in elongation of the follicle nerve and interaction with other nerves in 3D Gelfoam^® histoculture, including the sciatic nerve, trigeminal nerve, and trigeminal nerve ganglion.

The significance of diffusion tensor magnetic resonance imaging for patients with nasopharyngeal carcinoma and trigeminal nerve invasion

To investigate the significance of diffusion tensor imaging (DTI) for patients with nasopharyngeal carcinoma (NPC) and trigeminal nerve invasion.Fifty-two patients with NPC and unilateral infringement and 30 healthy controls were recruited for our study. Routine magnetic resonance imaging (MRI) and DTI were performed for all participants. Within-group and between-group comparisons of DTI metrics, including fractional anisotropy (FA) and the apparent diffusion coefficient (ADC) of the third (V3) branch of the bilateral trigeminal nerves of all participants, were carried out.The FA and ADC values on the affected sides of patients revealed a significant decrease and increase, respectively, when compared with those on the unaffected sides of patients and the healthy controls (P = 0.000 for all), whereas there were no significant differences in DTI metrics between both sides of healthy controls or between the unaffected sides of patients and the healthy controls (P = 0.930, 0.580, 0.095, and 0.360, respectively). The decreasing FA rate on the affected sides of patients correlated negatively with the increasing ADC rate (r = -0.675, P = 0.000).DTI can quantitatively evaluate microstructural abnormalities of the V3 branch of the trigeminal nerve in patients with NPC, which is important for the early detection of trigeminal nerve invasion to achieve a precise T classification, assess prognosis, and guide treatment.

[Participation of serotonin in the mechanisms of the formation of trigeminal motor nucleus]

The formation of trigeminal motor nucleus (TMN) was studied in the early postnatal period in 21 female Wistar rats which received the serotonin biosynthesis inhibitor para-chloro-phenylalanine at prenatal Day 16 (the period of serotoninergic system formation). It was shown that the serotonin deficit during the prenatal period in rats resulted in the changes of TMN structural organization. In the early postnatal period, the delay of neuropil development, the reduction of cell body size with the partial loss of Nissl substance in some of the neurons, the presence of degenerating neurons with the signs of hyperchromatosis in all the parts of the nucleus, especially in TMN ventromedial part, were detected. At later stages, the destruction of motoneurons became slower, though some of them had morphological abnormalities. With the increase of the postnatal age (by Day 20) the number of motor neurons decreased, apparently, as a result of the gradual intensification of cell death. Simultaneously with the motor neuron degeneration in TMN parts studied, the astrocytic gliosis was observed.

BAPTI and BAPTISM birthdating of neurons in zebrafish

This protocol describes how the photoconvertible protein Kaede can be used to determine the birthdates of neurons in live zebrafish. The methods used are birthdating analysis by photoconverted fluorescent protein tracing in vivo (BAPTI) and BAPTI combined with subpopulation markers (BAPTISM). Because Kaede can be converted from green to red fluorescence at any developmental time point, it serves as a temporal landmark for cell birth. When it is used in combination with subpopulation markers, the eventual fate of a cell can be correlated with its birthdate. We describe how we used this method to study the development of trigeminal sensory neurons and discuss how the technique can be extended to the study of other organs.

Morphometric analysis of the skull of the Sahel goat breed: basic and clinical anatomy

The work reports morphometric analysis of the skulls of the Sahel breed of goat. The calculated metric data (mean +/- SD) included the condylobasal length, 16.94 +/- 1.39 cm, while the orbital circumference was 11.30 +/- 0.48 cm. The foramen magnum height and width were 1.82 +/- 0.11 cm and 1.85 +/- 0.15 cm respectively while the foramen magnum index was 89.81 +/- 8.71. Animals above one year of age had significantly higher values for orbital length including horizontal and vertical diameters, overall skull length, basal length, and neurocranium height than animals aged one year and below. The cornual process length, maximum orbital circumference and horizontal diameter obtained in this study were higher than those reported for other Nigerian goat breeds in the literature. The data for the distances from the facial tuberosity to the infraorbital canal, from the mental foramen to the lateral extent of the alveolar root of the lower incisor, as well as from the mandibular foramen to the base of the mandible and that from the mental foramen to the caudal border of the mandible, which are important clinically in the estimation of craniofacial measurements that will aid regional anaesthesia, were however similar to those reported earlier for the Red Sokoto and West African Dwarf breeds implying that a uniform craniometric estimation for associated regional nerve blocks can be attempted for these goat breeds.

Multiple tooth-losses during development suppress age-dependent emergence of oscillatory neural activities in the oral somatosensory cortex

Tooth and tooth-related organs play important roles in not only mastication, but also sensory perception in the oral region. In general, sensory neural inputs during the developmental period are required for the maturation of functions in the sensory cortex. However, whether maturations of oral somatosensory cortex (OSC) require certain levels of sensory input from oral regions has been unclear. The present study investigated the influence of multiple tooth-losses during the developmental period on age-dependent emergence of rhythmic activities of population neurons in the OSC. Low-frequency electrical stimulation was delivered to layer IV and field potentials were recorded from layer II/III in the OSC of rat brain slices. In control rats, N-methyl-d-aspartate (NMDA) receptor-dependent oscillation at 8-10 Hz appeared during postnatal weeks 2-3. In rats with extraction of multiple teeth at 17-18 days old, oscillation did not appear even at maturity, whereas in rats with multiple teeth extracted at 37-38 days old, oscillation appearances were maintained in maturity. Thus, emergence of oscillation in the OSC was suppressed by multiple tooth-losses during postnatal 2-3 weeks. These results suggest that sufficient neural inputs from the teeth and tooth-related organs during developmental periods are essential for maturation of neural functions in the OSC.

[Expression and activity of Cdk5/p39 during rats Vc development]

PURPOSE

To evaluate the different expression of Cdk5 two activators, p35 and p39 throughout rats Vc development.

METHODS

The changes of Cdk5 activity, expression of Cdk5 and p39 in the development in spinal trigeminal subnucleus caudalis(Vc) were studied by Western blotting, immunoprecipitation and kinase assay. Statistical analysis was performed using SPSS11.0 software package.

RESULTS

Western blot showed that p39 expression was low at newborn, and highest at the 2-3 week-old rat Vc. In the adult rat Vc, expression declined to the same level as in newborn rat Vc. In contrast, the expression of Cdk5 was constant throughout the development rat Vc. Cdk5 activity in the newborn rat Vc(115.5 Kcpm),which was about 6 times higher than that in normal adult rat Vc(19.0 Kcpm). There was significant difference between each group(P<0.01).

CONCLUSIONS

Expression of p35 and p39 is differentially distributed throughout rat Vc. Cdk5/p35 and Cdk5/p39 may play different roles in distinct brain regions during different states of the rat Vc development.

Distinct roles of Hoxa2 and Krox20 in the development of rhythmic neural networks controlling inspiratory depth, respiratory frequency, and jaw opening

BACKGROUND

Little is known about the involvement of molecular determinants of segmental patterning of rhombomeres (r) in the development of rhythmic neural networks in the mouse hindbrain. Here, we compare the phenotypes of mice carrying targeted inactivations of Hoxa2, the only Hox gene expressed up to r2, and of Krox20, expressed in r3 and r5. We investigated the impact of such mutations on the neural circuits controlling jaw opening and breathing in newborn mice, compatible with Hoxa2-dependent trigeminal defects and direct regulation of Hoxa2 by Krox20 in r3.

RESULTS

We found that Hoxa2 mutants displayed an impaired oro-buccal reflex, similarly to Krox20 mutants. In contrast, while Krox20 is required for the development of the rhythm-promoting parafacial respiratory group (pFRG) modulating respiratory frequency, Hoxa2 inactivation did not affect neonatal breathing frequency. Instead, we found that Hoxa2-/- but not Krox20-/- mutation leads to the elimination of a transient control of the inspiratory amplitude normally occurring during the first hours following birth. Tracing of r2-specific progenies of Hoxa2 expressing cells indicated that the control of inspiratory activity resides in rostral pontine areas and required an intact r2-derived territory.

CONCLUSION

Thus, inspiratory shaping and respiratory frequency are under the control of distinct Hox-dependent segmental cues in the mammalian brain. Moreover, these data point to the importance of rhombomere-specific genetic control in the development of modular neural networks in the mammalian hindbrain.

Postnatal generation of neurons in the ventrobasal nucleus of the rat thalamus

Most CNS systems, including the trigeminal-somatosensory system, develop via a hierarchical order (from the periphery and up the neuraxis). We tested the hypothesis that development of the trigeminal system can proceed via a nonhierarchical mechanism (i.e., that neuronogenesis can occur postnatally). Preweanling rats were perfused, and brain sections were stained with cresyl violet or immunolabeled with NeuN (for neuronal counts), or processed for acetylcholinesterase (AChE) activity or p75 immunoreactivity [to identify boundaries of the ventrobasal nucleus (VB)]. Neuronal number decreased during the first postnatal week but increased 2.5-fold over the next 3 weeks. To determine whether this remarkable rise resulted from the generation of new neurons, preweanlings were given injections of bromodeoxyuridine (BrdU) on postnatal day 6 (P6) or P21. BrdU-positive VB cells were apparent on both days. Cumulative BrdU labeling showed that the cell cycle was 17.3 h on P6. Moreover, Ki-67, a protein elaborated throughout the cell cycle, was expressed by 25.8-29.3% of all VB cells on P6-P15, falling to 7.7% by P21. BrdU-positive VB cells coexpressed neuronal markers: NeuN, HuC/D, microtubule-associated protein 2, and a dextran placed in the somatosensory cortex. Note that postnatal neuronal generation was also evident in other thalamic nuclei (e.g., the lateral geniculate nucleus). Thus, the developing VB experiences two periods of neuronal generation. Prenatal neuronogenesis is part of hierarchical trigeminal-somatosensory development. Postnatal nonhierarchical neuronogenesis is intrathalamic and matches changes in neuromodulatory systems (exemplified by AChE activity and p75) and the arrival of corticothalamic afferents.

Developmental changes in distribution of γ-aminobutyric acid- and glycine-immunoreactive boutons on rat trigeminal motoneurons. I. Jaw-closing motoneurons

We have previously described the distribution pattern of inhibitory synapses on rat jaw-closing (JC) alpha- and gamma-motoneurons. In the present study, we investigated developmental changes in inhibitory synapses on JC motoneurons. We performed a quantitative ultrastructural analysis of putative inhibitory synaptic boutons on JC motoneuron somata by using postembedding immunogold labeling for GABA and glycine. In total, 206, 350, and 497 boutons contacting JC motoneuron somata were analyzed at postnatal days 2 (P2), 11 (P11) and 31 (P31), respectively. The size of the somata increased significantly during postnatal development. The size distribution was bimodal at P31. Mean length of the boutons and percentage of synaptic covering also increased during postnatal development, whereas bouton density did not differ significantly among the three age groups. Synaptic boutons on the somata of JC alpha-motoneurons could be classified into four types: boutons immunoreactive for 1) GABA only, 2) glycine only, 3) both GABA and glycine, and 4) neither GABA nor glycine. There was no developmental change in the proportion of putative inhibitory boutons to the total number of studied boutons. However, the glycine-only boutons increased significantly (15.1% to 27.3%), and the GABA-only boutons decreased significantly (17.7% to 2.6%) during the period from P11 to P31. Our ultrastructural data indicate that the inhibitory synaptic input to JC motoneurons is developmentally regulated and that there is a postnatal switch from GABA to glycine. The postnatal changes revealed in the present study could play an important role in the maturation of the oral motor system.

Network mechanisms of spindle-burst oscillations in the neonatal rat barrel cortex in vivo

Early in development, cortical networks generate particular patterns of activity that participate in cortical development. The dominant pattern of electrical activity in the neonatal rat neocortex in vivo is a spatially confined spindle-burst. Here, we studied network mechanisms of generation of spindle-bursts in the barrel cortex of neonatal rats using a superfused cortex preparation in vivo. Both spontaneous and sensory-evoked spindle-bursts were present in the superfused barrel cortex. Pharmacological analysis revealed that spindle-bursts are driven by glutamatergic synapses with a major contribution of AMPA/kainate receptors, but slight participation of NMDA receptors and gap junctions. Although GABAergic synapses contributed minimally to the pacing the rhythm of spindle-burst oscillations, surround GABAergic inhibition appeared to be crucial for their compartmentalization. We propose that local spindle-burst oscillations, driven by glutamatergic synapses and spatially confined by GABAergic synapses, contribute to the development of barrel cortex during the critical period of developmental plasticity.

Comparative postnatal development of spinal, trigeminal and vagal sensory root entry zones

Somatic and visceral sensory information enters the central nervous system (CNS) via root entry zones where sensory axons span an environment consisting of Schwann cells in the peripheral nervous system (PNS) and astrocytes and oligodendrocytes in the CNS. While the embryonic extension of these sensory axons into the CNS has been well-characterized, little is known about the subsequent, largely postnatal development of the glial elements of the root entry zones. Here we sought to establish a comparative developmental timecourse of the glial elements in the postnatal (P0, P3, P7, P14) and adult rat of three root entry zones: the spinal nerve dorsal root entry zone, the trigeminal root entry zone, and the vagal dorsal root entry zone. We compared entry zone development based on the expression of antigens known to be expressed in astrocytes, oligodendrocytes, oligodendrocyte precursor cells, Schwann cells, radial glial fibres and the PNS extracellular matrix. These studies revealed an unexpected distribution among glial cells of several antigens. In particular, antibodies used to label mature oligodendrocytes (RIP) transiently labelled immature Schwann cell cytoplasm, and a radial glial antigen (recognized by the 3CB2 antibody) initially decreased, and then increased in postnatal astrocytes. While all three root entry zones had reached morphological and antigenic maturity by P14, the glial elements comprising the PNS-CNS interface of cranial root entry zones (the trigeminal root entry zone and the vagal dorsal root entry zone) matured earlier than those of the spinal nerve dorsal root entry zone.

The mesencephalic trigeminal sensory nucleus is involved in acquisition of active exploratory behavior induced by changing from a diet of exclusively milk formula to food pellets in mice

Post-weaning mice fed exclusively milk display low-frequency exploratory behavior [Ishii, T., Itou, T., and Nishimura, M. (2005) Life Sci. 78, 174-179] compared to mice fed a food pellet diet. This low-frequency exploratory behavior switched to high-frequency exploration after a switch from exclusively milk formula to a food pellet diet. Acquisition of the high-frequency exploratory behavior was irreversible. Recently, we demonstrated that the mesencephalic trigeminal nucleus (Me5) is involved in the control of feeding and exploratory behavior in mice without modulating the emotional state [Ishii, T., Furuoka, H., Itou, T., Kitamura, N., and Nishimura, M. (2005) Brain Res. 1048, 80-86]. We therefore investigated whether the Me5 is involved in acquisition of high-frequency exploratory behavior induced by the switch in diet from an exclusively milk formula to food pellets. Mouse feeding and exploratory behaviors were analyzed using a food search compulsion apparatus, which was designed to distinguish between the two behaviors under standard living conditions. Immunohistochemical analysis of immediate early genes indicated that the Me5, which receives signals from oral proprioceptors, is transiently activated after the diet change. The change from low-frequency to high-frequency exploratory behavior was prevented in milk-fed mice by bilateral lesion of the Me5. These results suggest that the Me5 is activated by signals associated with mastication-induced proprioception and contributes to the acquisition of active exploratory behavior.

Whisker maps in marsupials: Nerve lesions and critical periods

In the wallaby, whisker-related patterns develop over a protracted period of postnatal maturation in the pouch. Afferents arrive simultaneously in the thalamus and cortex from postnatal day (P) 15. Whisker-related patterns are first seen in the thalamus at P50 and are well formed by P73, before cortical patterns first appear (P75) or are well developed (P85). This study used the slow developmental sequence and accessibility of the pouch young to investigate the effect of nerve lesions before afferent arrival, or at times when thalamic patterns are obvious but cortical patterns not yet formed. The left infraorbital nerve supplying the whiskers was cut at P0-93 and animals were perfused at P112-123. Sections through the thalamus (horizontal plane) and cortex (tangential) were reacted for cytochrome oxidase to visualize whisker-related patterns. Lesions of the nerve at P2-5, before innervation of the thalamus or cortex, resulted in an absence of patterns at both levels. Lesions from P66-77 also disrupted thalamic and cortical patterns, despite the fact that thalamic patterns are normally well established by P73. Lesions from P82-93 resulted in normal thalamic and cortical patterns. Thus, despite the wallaby having clearly separated times for the development of patterns at different levels of the pathway, these results suggest a single critical period for the thalamus and cortex, coincident with the maturation of the cortical pattern. Possible mechanisms underpinning this critical period could include dependence of the thalamic pattern on corticothalamic activity or peripheral signals to allow consolidation of thalamic barreloids.

TrkB signaling regulates the developmental maturation of the somatosensory cortex

In the rodent central nervous system, the region of the cortex that responds to facial whisker stimulation is anatomically segregated into discrete regions called barrels. Each barrel is made up of layer IV cortical neurons that receive input from a separate whisker via innervation from the thalamus. It has been shown that neurotrophins play important roles in the development and plasticity of thalamic axon innervation into the visual and retrosplenial cortex. We now extend those findings to the investigation of the role of neurotrophin signaling in barrel cortex formation. We show that the neurotrophin receptor TrkB is expressed in the thalamus and cortex during the time of cortical innervation. The two TrkB ligands, brain derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4), are expressed in the cortex at this time. Mice lacking TrkB demonstrate a developmental delay in the segregation of thalamic axons within barrels. In TrkB mutants, thalamic axons are abnormally uniform within layer IV of the cortex at postnatal day 4 compared to their control littermates, but show clear segregation into barrels 2 days later. This phenotype is recapitulated in BDNF mutant mice, but not in NT-4 mutant mice. These results demonstrate that BDNF is the sole TrkB ligand responsible for this phenotype. Analysis of conditional knockout mice that lack TrkB within the cortex, and not the thalamus, does not show a delay in thalamic axon segregation. These results indicate that TrkB expression in thalamic axons is important for the appropriate timing of barrel cortex development.

Metabolic barrel representations with various patterns of neonatal whisker deafferentation in rats

With various patterns of whisker deafferentation, C3 whisker stimulation produced divergently shaped metabolic barrel representations in layer IV of the primary somatosensory cortex. Whisker deafferentation results in functional and structural reorganization of the barrels in the primary somatosensory cortex. The present study examines the alteration of the metabolic barrel representations in layer IV with various configurations of selective whisker deafferentation in neonates, using [14C]2-deoxyglucose autoradiography. The deafferentation was produced by unilateral ablation of whiskers, leaving certain follicles intact. Configurations of intact follicles included: (I) row C follicles; (II) B3, C3, and D3 follicles; (III) B3, B4, C3, and C4 follicles; (IV) C2, C3, D2, and D3 follicles. The metabolic C3 barrel representations in layer IV after the deafferentations were found to have expanded only toward the barrel sites in which the corresponding whiskers were ablated, with no expansion toward the neighboring barrels. Expansion toward row D was significantly more pronounced than expansion toward row B, and expansion toward the C2 barrel was significantly more pronounced than expansion toward the C4 barrel. From these results, it can be inferred that asymmetric intrinsic structural connections are reflected in the functional metabolic barrel representation under the condition of neural plasticity in the barrel cortex following whisker deafferentation.

Jaw stretch reflexes in children

The substantial morphological transformations that occur during human development present the nervous system with a considerable challenge in terms of motor control. Variability of skilled motor performance is a hallmark of a developing system. In adults, the jaw stretch reflex contributes to the functional stability of the jaw. We have investigated the response properties of the jaw stretch reflex in two groups of young children and a group of young adults. Response latencies increased with development, and all age groups produced stimulus-magnitude-dependent increases in reflex gain and resulting biting force. Reflex gain was largest for the older children (9-10 years), yet net increases in resulting biting force were comparable across age groups. These data and earlier experiments suggest that oral sensorimotor pathways mature throughout childhood in concert with the continued acquisition of complex motor skills.

High EphA3 expressing ophthalmic trigeminal sensory axons are sensitive to ephrin-A5-Fc: Implications for lobe specific axon guidance

The ophthalmic, maxillary and mandibular axon branches of the trigeminal ganglion provide cutaneous sensory innervation to the vertebrate face. In the chick embryo, the trigeminal ganglion is bilobed, with ophthalmic axons projecting from the ophthalmic lobe, while maxillary and mandibular projections emerge from the maxillomandibular lobe. To date, target tissue specific guidance cues that discriminately guide the axon projections from the two trigeminal ganglion lobes are unknown. EphA receptor tyrosine kinases and ephrin-A ligands are excellent candidates for this process as they are known to mediate axon guidance in the developing nervous system. Accordingly, the expression of EphAs and ephrin-As was investigated at stages 13, 15, 20 of chick embryogenesis when peripheral axons from the trigeminal ganglion are pathfinding. EphA3 is expressed highly in the ophthalmic trigeminal ganglion lobe neurons in comparison to maxillomandibular trigeminal ganglion lobe neurons. Furthermore, from stages 13-20 ephrin-A2 and ephrin-A5 ligands are only localized to the mesenchyme of the first branchial arch (maxillary and mandibular processes), the target fields for maxillomandibular trigeminal ganglion axons. We found that ophthalmic and not maxillomandibular lobe axons were responsive to ephrin-A5-Fc utilizing a substratum choice assay. The implication of these results is that EphA3 forward signaling in ophthalmic sensory axons may be an important mechanism in vivo for lobe specific guidance of trigeminal ganglion ophthalmic projections.

Ontogeny of central CO2 chemoreception: chemosensitivity in the ventral medulla of developing bullfrogs

Sites of central CO2 chemosensitivity were investigated in isolated brain stems from Rana catesbeiana tadpoles and frogs. Respiratory neurograms were made from cranial nerve (CN) 7 and spinal nerve 2. Superfusion of the brain stem with hypercapnic artificial cerebrospinal fluid elicited increased fictive lung ventilation. The effect of focal perfusion of hypercapnic artificial cerebrospinal fluid on discrete areas of the ventral medulla was assessed. Sites of chemosensitivity, which are active continuously throughout development, were identified adjacent to CN 5 and CN 10 on the ventral surface of the medulla. In early- and middle-stage tadpoles and frogs, unilateral stimulation within either site was sufficient to elicit the hypercapnic response, but simultaneous stimulation within both sites was required in late-stage tadpoles. The chemosensitive sites were individually disrupted by unilateral application of 1 mg/ml protease, and the sensitivity to bath application or focal perfusion of hypercapnia was reassessed. Protease lesions at CN 10 abolished the entire hypercapnic response, but lesions at CN 5 affected only the hypercapnic response originating from the CN 5 site. Neurons within the chemosensitive sites were also destroyed by unilateral application of 1 mM kainic acid, and the sensitivity to bath or focal application of hypercapnia was reassessed. Kainic acid lesions within either site abolished the hypercapnic response. Using a vital dye, we determined that kainic acid destroyed neurons by only within 100 microm of the ventral medullary surface. Thus, regardless of developmental stage, neurons necessary for CO2 sensitivity are located in the ventral medulla adjacent to CN 5 and 10.

Pathfinding and growth termination of primary trigeminal sensory afferents in the embryonic rat hindbrain

Axons of the trigeminal ganglion convey sensory information from mechanoreceptors, thermoreceptors, and nociceptors in the face and nasal mucosa, then terminate on several groups of neurons including the principal sensory nucleus and the nuclei of the spinal trigeminal tract. To understand guidance mechanisms during the development of trigeminal sensory axons (TA) in the embryonic brain, we first investigated the growth pattern of TA in relation to organization in the hindbrain using flat whole-mount preparation from rat. We found that the primary TA from the trigeminal ganglion entered the brainstem and grew longitudinally within the hindbrain. Whereas descending axons ran just medial to the primary vestibular axons to innervate the spinal nucleus, ascending axons stayed near the entry point. In flat whole-mount culture, the TA extended both ascending and descending branches as they do in vivo. Rostral hindbrain was found to be a less permissive substrate for the TA compared to caudal hindbrain. In addition, the nonpermissive property of the ventral hindbrain substrate restricted the invasion of TA along the entire length of the hindbrain. Thus, cooperation of absolute and relative permissiveness of the substrate plays important roles in the guidance of TA to their targets.

Depolarization waves in the embryonic CNS triggered by multiple sensory inputs and spontaneous activity: optical imaging with a voltage-sensitive dye

Previously, we discovered a novel type of depolarization wave in the embryonic chick brain by using a multiple-site optical recording technique with a fast voltage-sensitive dye. This depolarization wave traveled widely over almost all the region of the CNS. This profile has raised the possibility that the depolarization wave plays some global roles in development of the CNS, rather than contributing to a specific neuronal circuit formation. To obtain more information concerning this issue, in the present study, we examined whether the depolarization wave was triggered by various types of peripheral nerve inputs. Stimulation applied to the vagus, glossopharyngeal, cochlear and trigeminal nerves evoked widely spreading depolarization waves with similar spatiotemporal distribution patterns. The developmental sequence of wave expression was parallel to the development of the excitatory postsynaptic potentials in each sensory nucleus. The depolarization wave was accompanied by a Ca(2+)-wave, suggesting that not only electrical synchrony, but also large-scale Ca(2+)-transients may affect developmental processes in the embryonic brain. Furthermore, we found that the depolarization wave also occurred spontaneously. The waveform and distribution patterns of the spontaneous optical signals were similar to those of the cranial nerve-evoked depolarization wave. These results demonstrated that the depolarization wave in the embryonic chick brain is triggered by multiple sources of external and endogenous activity. This profile supports the idea that this depolarization wave may not serve as a simple regulator of specific neuronal circuit formation, but might play more global roles in CNS development.

Transient prenatal expression of NPY-Y1 receptor in trigeminal axons innervating the mystacial vibrissae

Using immunohistochemistry in combination with confocal laser scanning microscopy, we studied the ontogeny of neuropeptide Y-Y1 receptor (Y1-R) expression in the trigeminal system of the rat. The study was limited to the nerve fibers innervating the mystacial pad and the trigeminal ganglia. In the trigeminal ganglia, Y1-R-immunoreactive (IR) neurons were first observed at E16.5. At this same stage some nerve fibers in the trigeminal ganglia also exhibited Y1-R-like immunoreactivity (LI). Strongly Y1-R-IR nerve fibers innervating the follicles of the mystacial vibrissae were first observed at E18. After double labeling, the Y1-R-LI was found to be colocalized with the neuronal marker protein gene product 9.5. At P1 only weak labeling for the Y1-R was found around the vibrissae follicles, whereas the neurons in the trigeminal ganglia were intensely labeled. The same was true for the adult rat, but at this stage no Y1-R labeling at all was observed in nerve fibers around the vibrissal follicles. These results strongly support an axonal localization of the Y1-R at this developmental stage. The transient expression of the Y1-R during prenatal mystacial pad development suggests a role for the Y1-R in the functional development of the vibrissae.

Diffusional anisotropy in cranial nerves with maturation: quantitative evaluation with diffusion MR imaging in rats

PURPOSE

To investigate the correlation between diffusional anisotropy and developmental changes in anatomy, which include myelination, in central and peripheral nerves in an animal model by using quantitative diffusion magnetic resonance (MR) imaging and electron microscopy.

MATERIALS AND METHODS

In vivo transverse and longitudinal apparent diffusion coefficients (ADCs) of the optic and trigeminal nerves in 2-10-week-old rats were measured with MR imaging. Then the animals were sacrificed at each time point, and transverse and longitudinal sections of optic and trigeminal nerves were studied with electron microscopy.

RESULTS

In the optic nerve, the ADC parallel to the neurofibers increased with development and increased contemporaneously with myelination, while the ADC perpendicular to the nerve did not change. This resulted in a significant increase in diffusional anisotropy. There were no significant changes in ADCs in either direction in the trigeminal nerve. Longitudinal sections of optic nerve showed a marked change in the orientation of each fiber. As development proceeded, the axons, which initially followed tortuous courses, assumed straighter and more parallel orientations. Trigeminal nerves displayed straight parallel courses at 2 weeks that did not change over the study period.

CONCLUSION

Changes in fiber anatomy in maturation from tortuous to straighter and more parallel orientation can account for changes in longitudinal ADC and in diffusional anisotropy.

Neurotrophin receptor (p75) in the trigeminal thalamus of the rat: development, response to injury, transient vibrissa-related patterning, and retrograde transport

We report on the transient, patterned expression of p75 in the ventrobasal (VB) thalamus, the major thalamic relay for somatosensation. We immunostained the brains of developing rats ranging in age from embryonic day (E) 14.5 to postnatal day (PD) 15 with an antibody against p75. To compare p75 expression with the developing synaptic organization within VB, we also immunolocalized the synaptic-vesicle-associated protein, synaptophysin (SYN), on alternate sections. p75-immunoreactivity (IR) was dense and uniform in the ventroposterior medial nucleus (VPM) in the late embryonic and early postnatal periods (E 16.5 to PD 3). In contrast, from PD 4-10, p75-IR in the VPM was patterned, reminiscent of cytochrome-oxidase-stained barreloids, a characteristic feature of the VB in rodents. By PD 14, p75-IR in the VPM was no longer detectable. The ventroposterior lateral nucleus (VPL), in contrast, exhibited no p75-IR. No p75-IR was detected in the ventroposterior lateral nucleus (VPL) at any developmental stage in which VPM could be distinguished from VPL. Light, but clearly patterned SYN-IR, first detectable on PD 2-3, increased in intensity in both VPL and VPM through PD 15. Sectioning the infraorbital nerve on PD 0 resulted in blurred patterns of p75- and SYN-IR within VPM in PD 7-9 rat pups. Removing large portions of the somatosensory cortex on PD 0 resulted in subsequent greatly reduced p75- and SYN-IR within VB. To specify the source of the p75-IR terminals, we stereotaxically injected into the VPM of PD 4-5 rats a monoclonal antibody to p75. One to 2 days later, IR of retrogradely transported p75 antibodies could be traced within axons and cell bodies of neurons associated with the trigeminothalamic pathway through the caudal diencephalon and mesencephalon; labelling was confined to the contralateral trigeminal principal sensory nucleus. The observed, transiently patterned p75-IR in VPM the early postpartum period suggests a role for p75 in synaptogenesis and pattern formation.

Visualization of cranial motor neurons in live transgenic zebrafish expressing green fluorescent protein under the control of the islet-1 promoter/enhancer

We generated germ line-transmitting transgenic zebrafish that express green fluorescent protein (GFP) in the cranial motor neurons. This was accomplished by fusing GFP sequences to Islet-1 promoter/enhancer sequences that were sufficient for neural-specific expression. The expression of GFP by the motor neurons in the transgenic fish enabled visualization of the cell bodies, main axons, and the peripheral branches within the muscles. GFP-labeled motor neurons could be followed at high resolution for at least up to day four, when most larval neural circuits become functional, and larvae begin to swim and capture prey. Using this line, we analyzed axonal outgrowth by the cranial motor neurons. Furthermore, by selective application of DiI to specific GFP-positive nerve branches, we showed that the two clusters of trigeminal motor neurons in rhombomeres 2 and 3 innervate different peripheral targets. This finding suggests that the trigeminal motor neurons in the two clusters adopt distinct fates. In future experiments, this transgenic line of zebrafish will allow for a genetic analysis of cranial motor neuron development.

Intraventricular infusion of nerve growth factor as the cause of sympathetic fiber sprouting in sensory ganglia

OBJECT

The results of previous clinical trials have indicated that intraventricular infusion of nerve growth factor (NGF) in patients with Alzheimer's disease is frustrated by the appearance of weight loss and diffuse back pain. The present study tested whether NGF induces sympathetic sprouting in sensory ganglia. Such sprouting has been implicated in previous studies as a possible mechanism of sympathetically maintained pain in neuropathic animals.

METHODS

Nineteen Long-Evans rats underwent intraventricular infusion of either artificial cerebrospinal fluid (ACSF; seven animals) or NGF (12 animals). After 14 days of infusion, the sensory ganglia of the trigeminal nerve and the C-2, C-8, T-1, L-4, and L-5 dorsal roots were examined for sympathetic sprouting by using tyrosine hydroxylase immunohistochemical analysis.

CONCLUSIONS

In the animals receiving NGF, 52 of 144 ganglia showed sympathetic fiber sprouting. In the control animals receiving ACSF, only two of 72 ganglia showed minor sympathetic fiber sprouting. A preferential sprouting of sympathetic fibers was demonstrated at lower lumbar ganglia compared with the cervical and thoracic ganglia. The data presented here demonstrate that in the rat intraventricular NGF infusion caused sympathetic sprouting in dorsal root ganglia (p < 0.01). These findings may have importance both for the treatment of Alzheimer's disease and the understanding of neuropathic pain.

Aromatase in developing sensory systems of the rat brain

Sex differences in the rat brain are dependent, in part, on oestrogen exposure during specific developmental perinatal periods. The availability of oestrogen requires precursor androgen and the presence of intraneuronal aromatase. To examine sites of oestrogen formation and action in the brain, immunocytochemical and biochemical localization of aromatase in the rat brain were determined between embryonic day 14 and postnatal day 20. Aromatase-immunolabelled neuronal profiles were present in hypothalamic, cortical and limbic regions. Surprisingly, aromatase immunoreactivity was also observed in non-limbic regions of the immature brain where it was previously unsuspected. Among these regions, aromatase staining was robust in developing sensory systems, including primary afferents of the olfactory, trigeminal, vestibulocochlear, and visual systems. To determine whether this aromatase is functional in these systems, i.e. converts testosterone to estradiol, the trigeminal nerve was dissected from the hindbrain of perinatal animals and studied for enzyme activity by the tritium release method. The dpm/mg protein/h tritium release in these tissues equalled that of hypothalamic or limbic controls, indicating that these sensory areas are sites of in-situ estradiol synthesis. Our data suggests that aromatase (estradiol)-dependent mechanisms may play a role in the differentiation and maturation of sensory pathways, which, in turn, may contribute to sex differences in the activity of these systems.

Spatiotemporal distribution of dying neurons during early mouse development

Apoptosis is a critical cellular event during several stages of neuronal development. Recently, we have shown that biotinylated annexin V detects apoptosis in vivo in various cell lineages of a wide range of species by binding to phosphatidylserines that are exposed at the outer leaflet of the plasma membrane. In the present study, we tested the specificity by which annexin V binds apoptotic neurons, and subsequently investigated developmental cell death in the central and peripheral nervous system of early mouse embryos at both the cellular and histological level, and compared the phagocytic clearance of apoptotic neurons with that of apoptotic mesodermal cells. Our data indicate: (i) that biotinylated annexin V can be used as a sensitive marker that detects apoptotic neurons, including their extensions at an early stage during development; (ii) that apoptosis plays an important part during early morphogenesis of the central nervous system, and during early quantitative matching of brain-derived neurotrophic factor and neurotrophic factor 3 responsive postmitotic large clear neurons in the peripheral ganglia with their projection areas; and (iii) that apoptotic neurons are removed by a process that differs from classical phagocytosis of non-neuronal tissues.

Infraorbital nerve transection alters serotonin transporter expression in sensory pathways in early postnatal rat development

The serotonin transporter MRNA has been found throughout the trigeminal sensory system late in gestation and during early postnatal development, a period known to be critical for maturation of the sensory circuitry. The purpose of the present study was to determine whether sensory denervation in newborn rat pups would alter either the density or pattern of expression of the 5-HT transporter (5-HTT) within the trigeminal system. We combined autoradiographic localization of 5-HT transporters and in situ hybridization techniques to visualize both the transporter protein and mRNA in thalamic sensory neurons and in the somatosensory cortex following unilateral infraorbital nerve transection at postnatal day 1. For comparative purposes, similar measurements were conducted in thalamic visual neurons as well as in the visual cortex. Lesion of the infraorbital nerve decreased the [3H]citalopram labelling of 5-HT transporters in the ventral basal and ventral medial areas of the thalamus contralateral to the lesion, while labelling of 5-HT transporters was decreased in both contralateral and ipsilateral sides of the lateral genicuate (visual thalamus). Citalopram labelling of 5-HT transporters was not significantly altered in somatosensory or in cingulate cortex, however a significant decrease was observed in the visual cortex. In contrast, there were no obvious changes in the intensity of the 5-HT mRNA hybridization signal in sensory or visual thalamic areas. Given that the serotonin transporter regulates extracellular concentrations of 5-HT, the present data suggest that altered peripheral innervation and thereby altered sensory inputs to the thalamus during fetal development can potentially influence 5-HT transporter densities and thus, may influence extracellular levels of 5-HT in thalamus and cortex during a critical period of synapse formation. In turn, modulation of 5-HT transporter levels may influence extracellular concentrations of 5-HT in thalamus and cortex during a critical period of synapse formation.

Primary and secondary somatosensory projections in direct-developing plethodontid salamanders

In three species of plethodontid salamanders (Plethodon jordani, Hydromantes italicus, and Bolitoglossa subpalmata), primary and secondary somatosensory pathways were investigated by means of tract-tracing in vivo and in vitro using biocytin, horseradish peroxidase, and neurobiotin. Afferent sensory fibers of cranial nerves V, VII, and X and the brachial nerve run in the dorsal funiculus of the medulla oblongata and spinal cord. Fibers ascend to the level of, but do not enter, the cerebellum. In the caudal medulla oblongata, sensory tracts of the cranial nerves descend in a dorsal and a dorsolateral bundle and reach the level of the fourth spinal nerve. Two bundles are likewise formed by spinal afferent fibers, which descend to the level of the seventh spinal nerve. Secondary somatosensory projections ascend in contralateral ventral, contralateral lateral, and ipsilateral lateral tracts, the latter two corresponding to the spinal lemniscal tracts of Herrick. These tracts reach the cerebellum, mesencephalic, and diencephalic targets (tegmentum, torus, tectum, tuberculum posterius, pretectum, and ventral thalamus) ipsi- and contra-laterally. The projection to the tectum is confined to fiber layer 4. Fibers of the ascending tracts cross in the cerebellar and tectal commissure. Our study demonstrates that the ascending secondary somatosensory pathways of plethodontid salamanders differ remarkably from those of other amphibians.

Reutilization of trigeminal motoneurons during amphibian metamorphosis

Indirect evidence suggests that trigeminal motoneurons (Vmns) in Rana pipiens innervate distinct myofiber populations in tadpoles and adult frogs. Redeployment occurs when the larval myofibers die and are replaced during metamorphosis. To directly test this hypothesis, DiI was injected into the larval muscle and Fast Blue into the replacement myofiber population. Over 95% of the Vmns contained both tracers, providing support for the innervation of sequential targets by the same motoneurons.

Response of "naive" cutaneous and muscle afferents to potential targets in vitro

It is now well documented that motoneurons are specified to innervate particular target muscles prior to axon outgrowth. Here we investigate whether sensory neurons are similarly specified to innervate target skin or muscle, taking advantage of the avian trigeminal system where cutaneous and muscle afferents are anatomically separate. Using this system, we have previously shown that by embryonic day 10 (E10) (approximately 4-5 days after target innervation), regenerating cutaneous and muscle afferents differ in their response to various potential targets in vitro, in manners consistent with their normal innervation patterns in vivo. Thus, by E10 these two populations of sensory neurons have distinct identities as skin and muscle afferents. In contrast, we report here that the responses of younger, naive cutaneous and muscle afferents that have not yet, or only recently, innervated peripheral targets are indistinguishable, regardless of the target tissue tested. These findings suggest that at stages when innervation is being established, cutaneous and muscle afferents, unlike motoneurons, may not yet have acquired rigidly specified identities and/or the ability to recognize and respond selectively to their appropriate peripheral targets.

Morphological development of afferent segregation and onset of synaptic transmission in the trigeminothalamic pathway of the wallaby (Macropus eugenii)

A light and electron microscopic study has been made of the time of formation of whisker-related patterns in trigeminothalamic afferents and the onset of synapse formation between afferents and cells in the ventroposteromedial nucleus (VPM) of the marsupial mammal, the wallaby, by labelling afferents with a carbocyanine dye. A parallel in vitro study was made of the functional development of the trigeminothalamic pathway to the VPM. Evoked synaptic responses could be recorded in the VPM from the time that afferents first reached the VPM at postnatal day 15 (P15). At all stages, the excitatory response comprised both N-methyl-D-aspartate- and non-N-methyl-D-aspartate-mediated components. At P40, the response decreased markedly in duration, coinciding with the onset of synaptogenesis. This implies that transmission is occurring prior to synapse formation, probably through transmitter release from growth cones. At P50, synaptic responses became dominated by a fast, non-N-methyl-D-aspartate potential, and this coincided with the first appearance of whisker-related patterns in the VPM. A gamma-aminobutyric acid (subtype A)-mediated, inhibitory component was also present from the time of afferent arrival. These findings support the idea that functional interactions between afferents and their targets may play a role in pattern formation in the somatosensory thalamus.

A quantitative study of the progress of myelination in the rat central nervous system, using the immunohistochemical method for proteolipid protein

The temporal changes in intensity of myelination of the nervous pathways in 0 to 42-day-old Wistar rats were quantitatively analyzed by immunohistochemistry with anti-proteolipid protein and compared with that obtained by immunohistochemistry with anti-myelin basic protein. Immunohistochemistry was performed on paraffin-embedded tissue according to the standard ABC technique. Intensity of myelination was examined by an image analyzing system. We analyzed nine nervous pathways: corpus callosum, optic tract, internal capsule, spinal tract of the trigeminal nerve, inferior cerebellar peduncle, cerebellar white matter, pyramidal tract, medial longitudinal fasciculus, and cuneate fasciculus. The presence of immunoreactive fibers for proteolipid protein (PLP) in the spinal tract of the trigeminal nerve, medial longitudinal fasciculus and cuneate fasciculus was noted on postnatal day 0. Those of the corpus callosum, inferior cerebellar peduncle, cerebellar white matter, pyramidal tract and internal capsule were noted on day 7, and that of optic tract on day 14. The time required to reach the intensity of myelination of day 42 was day 14 for the cuneate fasciculus, day 21 for the spinal tract of the trigeminal nerve, inferior cerebellar peduncle and medial longitudinal fasciculus, day 28 for the optic and pyramidal tracts, day 35 for the corpus callosum and day 42 for the internal capsule and cerebellar white matter. The appearance of immunoreactive fibers for PLP was usually earlier than that for myelin basic protein (MBP) and the pattern of difference between PLP and MBP can be classified into three groups: (1) their time of appearance and progress are almost the same, as in the optic tract; (2) the appearance and progress of PLP occurs earlier than those of MBP, as in the pyramidal tract; (3) the appearance of PLP occurs earlier than that of MBP, but their progress is the same. Our findings revealed that the time of appearance and progress of myelination as measured by PLP are different among the nervous pathways, and that there is also a difference between PLP and MBP. This difference between PLP and MBP may indicate a functional difference between them.

Stimulation-induced responses of the trigeminal caudal neurons in the brainstem preparation isolated from newborn rats

The brainstem preparation with the trigeminal mandibular nerve attached was isolated from rats postnatal day 0-6 (P0-P6) to test if the potentiation could be induced in neonatal neurons in the trigeminal subnucleus caudalis by stimulation of the primary afferents. The stimulation-induced potentials in 92 neurons recorded extracellularly, and the synaptic potentials in 16 neurons recorded by the whole-cell patch clamp technique were examined. The extracellularly recorded neurons responded to stimulation (0.5 Hz) with either an increase, a decrease, or little change in spike numbers, and were classified as Type 1, Type 2, and Type 3, respectively. Type 1 neurons at P4 and older responded in a low Mg2+ solution with a progressive increase in spike number lasting for several minutes after the cessation of stimulation, i.e., short-term potentiation, STP. This potentiation was antagonized by 20 microM of (+)-MK-801 hydrogen maleate (MK-801) or 25 microM of 2-amino-5-phosphonovaleric acid. In contrast, Type 1 at P3 and younger did not exhibit STP. The age-related distinct response properties were observed between Type 1 neurons at P4-P6 and at P0-P3. The percentage of Type 1 in studied neurons increased from 24% at P0-P3 to 53% at P4-P6. In the intracellular experiment, the mean latency of excitatory postsynaptic potential (EPSP) of recorded neurons indicated that the conduction velocity of the convergent afferents was 0.37 m/s, in the range of C-fiber. Neurons were classified as Type E and Type I. Type E responded with EPSP only, or with both EPSP and inhibitory postsynaptic potentials (IPSP), while Type I responded with IPSP only. In Type E at P4 and older, a single stimulation produced a burst of spike discharges that lasted for several seconds. Stimulation at a hyperpolarized membrane potential showed that aggregated slow EPSPs lay under a burst of spike discharges, and that slow EPSPs, but not a short-latency EPSP, were completely blocked by MK-801. In contrast, Type E at P3 and younger did not evoke a burst of spikes. Morphological examination of recorded neurons showed that the formation of networks was sparse at P1 and rapidly developed up to P4. The results suggest that: (1) short-term potentiation is induced with the development of synaptic network formation in the caudal nucleus at P4 and older; (2) the summation of N-methyl-d-aspartate (NMDA)-mediated slow EPSPs build up a prolonged depolarization; and (3) the brainstem preparation is applicable for neurophysiological studies on the trigeminal pain system.

Sensitive period for lesion-induced reorganization of intracortical projections within the vibrissae representation of rat's primary somatosensory cortex

Previous experiments from this laboratory demonstrated that intracortical connections in lamina IV of the rat primary somatosensory cortex (SI) are most dense outside the patches of cytochrome oxidase (CO) staining that correspond to the mystacial vibrissae. This pattern of intracortical connections becomes apparent on postnatal day 4 (P-4), at least 2 days after the appearance of the vibrissae-related pattern of thalamocortical afferents. Transection of the infraorbital nerve (ION) on the day of birth (P-0) disrupts both the CO and intracortical projection patterns. This series of experiments was undertaken to determine whether the patterning of either thalamocortical afferents or intracortical projections defines the end of the period over which peripheral damage can alter intracortical projections in lamina IV of SI. The infraorbital nerve (ION) was transected in different cohorts of rats on P-1 through P-5, and animals were allowed to survive > or =45 days, at which time biotinylated dextran amine (BDA) injections were made into the SI. After 7 days, animals were killed, and alternate cortical sections were processed for the demonstration of BDA or CO. Transection of the ION on P-1 or P-2 altered the patterning of both CO and intracortical connections in the SI. In contrast, cutting the ION on P-3 left the pattern of CO densities in the SI intact, but significantly altered the patterning of intracortical connections. Transection of the nerve on P-5 resulted in qualitatively and quantitatively normal patterns of both CO densities and BDA-labelled intracortical projections. These results indicate that the establishment of a stable barrel pattern in layer IV of the SI is not sufficient for normal adult patterning of intracortical projections in this lamina. However, once the mature pattern of intracortical projections in layer IV is established, ION lesions can no longer alter it.

Timecourse of development of the wallaby trigeminal pathway: III. Thalamocortical and corticothalamic projections

The development of trigeminal projections between the thalamus and cortex has been investigated in the marsupial mammal, the wallaby, by using a carbocyanine dye, horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), Neurobiotin, and biocytin as pathway tracers. The appearance of whisker-related patterns in the cortex in relation to their appearance in the brainstem and thalamus was examined, as was the presence or absence of a waiting period for thalamocortical afferents and the identity of the first cortical cells to project to the thalamus. Thalamic afferents first reached the cortex at postnatal day (P) 15 and were distributed up to the deep edge of the compact cell zone in the superficial cortical plate throughout development, in both dye and WGA-HRP labelled material, with no evidence of a waiting period. The initial corticothalamic projection, detected by retrograde transport of WGA-HRP from the thalamus, occurred at P60 from layer 5 cells. This was confirmed by labelling of corticothalamic axons after cortical injections of Neurobiotin and biocytin. Scattered, labelled cells seen before P60 after dye labelling from the thalamus presumably resulted from transcellular labelling via thalamic afferents. Clustering of afferents in layer 4 and cell bodies and their dendrites in layers 5 and 6 first occurred simultaneously at P76. There is a sequential onset of pattern formation, first in brainstem, then in thalamus, and finally in cortex, with a long delay between afferent arrival and pattern formation at each level. Independent regulation at each level, likely depending on target maturation, is suggested.

Functional connectivity in the rodent trigeminal pathway grown in vitro

In explant cocultures of the rat trigeminal pathway, embryonic trigeminal ganglion cells grow their axons into peripheral cutaneous and central nervous system targets (R.S. Erzurumlu, S. Jhaveri, Target influences on the morphology of trigeminal axons, Exp. Neurol, 135 (1995) 1-16; R.S. Erzurumlu, S. Jhaveri, H. Takahashi, R.D.G. McKay, Target-derived influences on axon growth modes in explant cocultures of trigeminal neurons, Proc. Natl. Acad. Sci. USA 90 (1993) 7235-7239). In heterochronic cocultures, composed of embryonic trigeminal ganglion, embryonic whisker pad and postnatal brainstem slice, trigeminal axons develop arbors and terminal boutons in the brainstem trigeminal nuclei. To determine whether these terminal arbors establish functional connections with the brainstem neurons, we examined the electrophysiological properties of brainstem neurons and their responsiveness to trigeminal ganglion stimulation. Intracellular recordings were done in vitro on cells of the trigeminal subnucleus interpolaris (SPI) in trigeminal pathway cocultures (E15 whisker pad, E15 trigeminal ganglion, and postnatal day (PND) 0-2 brainstem slice) or in the SPI of acutely prepared brainstem slices. Electrophysiological properties of SPI cells in both preparations were virtually identical. The voltage responses of SPI neurons to intracellular current injection were highly linear suggesting they lacked a number of voltage-dependent conductances. Depolarizing current injection produced trains of action potentials with a frequency that varied with stimulus intensity. In explant cocultures, electrical activation of the trigeminal ganglion evoked EPSPs, and EPSPs coupled with IPSPs in SPI cells. Bicuculline blockade of IPSP activity resulted in long lasting EPSPs whose duration increased with membrane depolarization. These results show that brainstem trigeminal neurons can retain their functional properties in culture and establish functional connections with primary sensory afferents.

Changes in Fos-like immunoreactivity evoked by maturation of the sneeze reflex triggered by nasal air puff stimulation in kittens

The sneeze reflex is a valuable tool for exploring the maturation of the respiratory control in the newborn as it alters both inspiratory and expiratory activities. Air puff stimulation of the superior nasal meatus innervated by ethmoidal afferents consistently evokes sneeze in adult cats. Such stimulation evokes only a reinforcement of expiratory activities in newborn kittens. This study demonstrates that the pattern of Fos-like immunoreactivity evoked by nasal stimulation changes during functional maturation of sneeze. Nasal stimulation evoked immunoreactivity (i) in the trigeminal sensory complex, at the levels where nasal afferents project, (ii) in the reticular formation, (iii) in the solitary complex and (iv) in the parabrachial area of mature kittens. The evoked immunoreactivity was the same in newborn kittens as in mature kittens in the projection areas of the nasal primary afferents. Fos response was less than half that in mature kittens in the reticular formation and absent in the solitary complex or the parabrachial area. Sneeze can be elicited from the time when evoked immunoreactivity in the solitary complex and the parabrachial area is above control levels. These data provide evidence that the maturation of sneeze is dependent on the development of central relays allowing peripheral inputs to be integrated by neurons engaged in respiratory control.

Timecourse of development of the wallaby trigeminal pathway. II. Brainstem to thalamus and the emergence of cellular aggregations

This paper is the second in a series which makes use of the protracted postnatal maturation of the wallaby to study the development of the trigeminal sensory system. Previous work has established similarities in the organisation of the trigeminal sensory system in the wallaby and in rodents. This study describes the structure and development of the ventroposteromedial nucleus in the wallaby in relationship to the arrival of afferents from the trigeminal nuclei, the formation of neuronal aggregations and naturally occurring cell death. Enzyme histochemistry, Nissl and myelin stains were used. Pathway development was followed using carbocyanine dyes. In the adult wallaby the nucleus demonstrates evidence of a parcellated organisation. Cells are arranged in dorsoventrally aligned bands resembling fingers. In the horizontal plane, these appear as circular clusters which are encircled by fine myelinated bundles. The clusters of cells are believed to correspond to the mystacial vibrissae. The first afferents from the principal trigeminal nucleus arrive between 10 and 15 days postnatal. This is more than two weeks prior to the time at which the borders of the nucleus can be discerned cytoarchitecturally. The first hints of segmentation are visible around day 50, and discrete aggregations form over the ensuing 3-4 weeks. Coincident with the aggregation of the neurons is an increase in their level of reactivity for acetylcholinesterase. A high level of acetylcholinesterase reactivity is maintained for at least 4 months, but has disappeared in adult animals. The peak of cell death occurs subsequent to the appearance of aggregations in the thalamus, but coincident with the appearance of vibrissae related patches in the cortex at day 85 (Waite et al. [1991] Dev. Brain Res. 58:35-41). The timing of the appearance of the neuronal aggregations supports the hypothesis that pattern formation occurs sequentially at successive levels of the pathway, and suggests the importance of target maturation in pattern formation.

The composition of trigeminal nerve branches in normal adult chickens and after debeaking at different ages

The long term effects of amputation of the tip of the beak were studied in adult hens that were debeaked on the day of hatching, at the age of 8 d and at 6 wk, by EM analysis of fibre spectra of the medial branch of the ophthalmic nerve and of the intramandibular nerve. Three categories of fibre were distinguished for further analysis, i.e. unmyelinated axons, small myelinated fibres and large myelinated fibres. In normal birds the ophthalmic nerve contains relatively more large fibres than the intramandibular nerve. Amputation consistently results in a reduction of the number of large fibres and a substantial increase in the number of small myelinated fibres. The proportion of unmyelinated axons is rather variable, but is not affected by beak trimming. Age at debeaking has no effect. The observations are inconclusive concerning the possibility of heightened nociception.

Structure-function relationships in rat brainstem subnucleus interpolaris. XI. Effects of chronic whisker trimming from birth

Whisker trimming from birth reduces activity and alters receptive fields (RFs) in the barrel cortex and thalamus. To assess whether or not this reflects deprivation effects on trigeminal (V) first- and second-order neurons, 59 primary afferents and 343 cells in V brainstem subnucleus interpolaris (SpVi) were studied in rats whose whiskers were trimmed daily for 6-9 weeks from birth. Deprivation did not effect brainstem somatotopy or primary afferent RFs. However, many SpVi cells had abnormal RFs and higher-order inputs, resembling the changes caused by infraorbital nerve injury. For example, in controls, only 3% of whisker-sensitive local circuit neurons responded to more than one whisker, whereas 35% of the deprived and 41% of the infraorbital nerve cut samples had multiwhisker. RFs. Deprived rats also had higher than normal incidences of cells with split or absent RFs, RFs spanning more than one V division, intermodality convergence, and directional or high-velocity sensitivity. Because these changes mimic those caused by nerve section, deprivation may underlie some nerve injury effects on V brainstem RF size and character. Insofar as cytochrome oxidase, anterograde labeling, and unit recordings revealed normal topography in deprived primary afferents and SpVi cells, RF changes in SpVi cells may reflect altered SpVi circuitry. To test this hypothesis, we assessed the morphology of 32 similarly deprived V primary afferents. In SpVi, deprived fibers had normal numbers of collaterals with normal shapes, transverse arbor areas, and topography. However, the total number of boutons per collateral was significantly reduced. Thus, deprivation effects on V higher-order RFs reflect quantitative changes in V afferent terminals.

Development, critical period plasticity, and adult reorganizations of mammalian somatosensory systems

This review covers recent progress in three major areas of investigation in somatosensory systems: development, developmental plasticity and functional reorganization. Important findings relate to the development of periphery-related patterning in thalamic afferents to somatosensory cortex, the controversial role of neural activity in the development and plasticity of periphery-related afferent patterning in the brainstem and cortex, experience-dependent reorganizations in adult somatosensory cortex, and the locus of these changes.

Postnatal development of the anterior ethmoidal nerve in cats: unmyelinated and myelinated nerve fiber analysis

This is the first quantitative electron microscopic study of anterior ethmoidal nerve in adult and newborn cats. The adult nerve comprises about 1,000 myelinated fibers including A delta (65%) and A beta (35%) fibers and 6,000 unmyelinated fibers. At birth, only 27% of the adult myelinated fibers complement is already present. The immaturity of the nerve is discussed in relation to that of the sneeze reflex.

Development of the mesencephalic nucleus of the trigeminal nerve in chick embryos: target innervation, neurotrophin receptors, and cell death

The goal of this study was to determine whether processes of neurons in the mesencephalic nucleus of the trigeminal nerve (Mes V) of chick embryos arrive in their peripheral target prior to the period of developmental cell death, and to determine whether neurons with early target contact survive to a greater extent than neurons with processes that reach their peripheral target later. The arrival of Mes V nerve fibers in the masticatory muscles was determined by injecting the fluorescent tracer DiI, and the position of labeled and unlabeled neurons was mapped in subdivisions of the Mes V nucleus. Developmental changes in the numerical configuration of Mes V subdivisions were studied in DiI-labeled as well as Nissl-stained material. The expression of low-affinity (p75) neurotrophin receptors was investigated throughout development of the Mes V nucleus with in situ hybridization to assess whether and how levels of expression of this trophic receptor may relate to target innervation and cell death. The extent of cell death was evaluated by counting pyknotic nuclei. Processes of Mes V neurons invade their peripheral target between 5 and 7 days of incubation (E5-7). At E7-12, between 800 and 1,400 labeled Mes V neurons were distributed throughout the two main subdivisions of the Mes V nucleus, the tectal commissure and the optic tectum. Only few Mes V neurons were labeled in the posterior commissure or outside the brain. Cell counts in Nissl-stained material from E7-13 revealed that the numbers of Mes V neurons in the optic tectum decreased to about 40-60%, and in the tectal commissure to 20-25%, whereas Mes V neurons in the posterior commissure disappeared almost entirely. Few Mes V neurons remained in the leptomeninges at E8-10, but a considerable number was found outside the midbrain at E11, indicating ongoing migration of some Mes V neurons. Neurotrophin receptors were differentially expressed in the Mes V nucleus: Before and after the period of cell death, 90-100% of Mes V neurons expressed neurotrophin receptors, whereas during, and immediately preceding the period of developmental cell death (E9-E13), merely 70% of Mes V neurons expressed this receptor. These findings are consistent with the hypothesis that early target contact may provide an advantage for the survival of Mes V neurons and that competition for trophic factors may occur in the peripheral target of this nucleus prior to the period of cell death.

Motor components of the trigeminal nerve and organization of the mandibular arch muscles in vertebrates. Phylogenetically conservative patterns and their ontogenetic basis

The peripheral branching pattern of the mandibular ramus of the Vth cranial nerve and the organization of the trigeminal motor column are highly conserved in craniate phylogeny regardless of the vast modifications in attachments and structure of the mandibular arch musculature. Proximal, intermediate and distal series of mandibular nerve branches supply three major muscle groups and are in register with three neuronal populations of the trigeminal motor column. The adult branching pattern is established in response to the differentiation of mandibular muscles and is important in determining the organization of the motor nucleus of V. The innervation of the muscles of the mandibular segment of the head, and the location of motoneurons reflect their segmental origins and are reliable criteria for homologizing mandibular muscles among the craniates.

Redeployment of trigeminal motor axons during metamorphosis

As a consequence of the degeneration and replacement of the jaw muscle fibers in the leopard frog, Rana pipiens, trigeminal motoneurons innervate different targets before and after metamorphosis. This investigation examined the morphological correlates of the reassignment of trigeminal motoneurons during the initial phases of myofiber turnover. Specifically, silver-cholinesterase histochemistry and electron microscopy were used to 1) identify the fate of motor axons within the neuromuscular junctions (NMJs) applied to degenerating larval myofibers and 2) to determine the origin(s) of the motor axons that innervate the postmetamorphic muscle fibers of the jaw. The results demonstrate that the NMJs are retained on larval myofibers throughout their degeneration and are readily identifiable on the residual larval basal laminae that remain after involution of the sarcoplasm. Light and electron microscopic observations provide evidence that both pre- and post-synaptic elements are present on the degenerating fibers. Furthermore, morphometric analyses indicate that the preponderance (86%) of motor axons supplying adult muscle fibers originates from the larval NMJs. This condition suggests that metamorphic redeployment of trigeminal motoneurons occurs through the resumption of growth at the axon terminal supplying larval muscle rather than through the proximal collateralization of these axons and resorption of larval terminals.

Prenatal ontogeny of substance P-like immunoreactivity in the trigeminal spinal caudal subnucleus of the human fetus--an immunocytochemical study

Using an immunocytochemical method, the prenatal ontogeny of substance P-like immunoreactivity (SP-LI) was demonstrated in the trigeminal spinal caudal subnucleus (TSCS) of human fetus, fetal age (menstruation age) 11.5-40 weeks. The time of initial appearance of SP-LI in the human brainstem TSCS was between the fetal age 11.5 weeks and 16 weeks. While the fetus grew, the density of SPLI fibers and terminals in the human fetus brainstem TSCS increased constantly from fetal age 16 weeks to 40 weeks. These findings indicated that substance P (SP) might play an important role in the human trigeminal nerve system development and in its functional establishment during the prenatal period.

Cortical local circuit axons do not mature after early deafferentation

The processes underlying development, refinement, and retention of the intracortical connections critical for the function of the mammalian brain are unknown. Horseradish peroxidase-labeled fibers in mouse somatosensory barrel cortex, which is patterned like the whiskers on the contralateral face from which it receives inputs, were evaluated by automated image analysis. The sensory nerve to the whiskers was sectioned on postnatal day 7, after the whisker map is set. The deprived barrel cortices, examined in adults, showed drastically diminished intracortical projections relative to normal controls, although the map of the whiskers in the cortex was unchanged. This demonstrates anatomically that the normal pattern of intracortical connections, like the normal sensory map, is dependent upon the sensory periphery four synapses away.

Neuronal differentiation and maturation in the mouse trigeminal sensory system, in vivo and in vitro

We have isolated and characterized four monoclonal antibodies (mAbs B33, E1.9, B30, and B10) that recognize mouse trigeminal sensory neurons at specific times during development. These antibodies permit the study of neuronal differentiation, axon outgrowth, and neuronal maturation in the trigeminal sensory system. With B33, we can follow migrating neural crest and placode cells into the anlagen of the trigeminal ganglion. E1.9 immunoreactivity marks neuronal differentiation and appears in the central nervous system at embryonic day 8.5 (E8.5) and in the peripheral nervous system at E9, E1.9 and B30 show the axonal outgrowth of trigeminal sensory neurons and reveal the pioneering of the peripheral tracts by an early population of ganglionic neurons. At this stage, in the central nervous system, mesencephalic trigeminal neurons are also E1.9 and B30 positive as they migrate to their final location in the rostral metencephalon. B30 and B10 allow us to follow the maturation of these neurons. Also, in about 1% of the embryos, we identified mispositioned or misrouted trigeminal neurons. Furthermore, these biochemical markers facilitate the study of neuronal development in vitro. We find that, based on morphological and biochemical criteria, the maturation of trigeminal neurons in culture is target independent.