Developmental time course of peripheral cross‐modal sensory interaction of the trigeminal and gustatory systems

Few sensory modalities appear to engage in cross‐modal interactions within the peripheral nervous system, making the integrated relationship between the peripheral gustatory and trigeminal systems an ideal model for investigating cross‐sensory support. The present study examined taste system anatomy following unilateral transection of the trigeminal lingual nerve (LX) while leaving the gustatory chorda tympani intact. At 10, 25, or 65 days of age, rats underwent LX with outcomes assessed following various survival times. Fungiform papillae were classified by morphological feature using surface analysis. Taste bud volumes were calculated from histological sections of the anterior tongue. Differences in papillae morphology were evident by 2 days post‐transection of P10 rats and by 8 days post in P25 rats. When transected at P65, animals never exhibited statistically significant morphological changes. After LX at P10, fewer taste buds were present on the transected side following 16 and 24 days survival time and remaining taste buds were smaller than on the intact side. In P25 and P65 animals, taste bud volumes were reduced on the denervated side by 8 and 16 days postsurgery, respectively. By 50 days post‐transection, taste buds of P10 animals had not recovered in size; however, all observed changes in papillae morphology and taste buds subsided in P25 and P65 rats. Results indicate that LX impacts taste receptor cells and alters epithelial morphology of fungiform papillae, particularly during early development. These findings highlight dual roles for the lingual nerve in the maintenance of both gustatory and non‐gustatory tissues on the anterior tongue. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 626–641, 2016     

Effect of taste deafferentation on the preference for solutions of saccharose, saccharin and salt in rats

The preference of sucrose, saccharin and salt solutions to water was analyzed during 5 days in rats with bilateral section of the lingual nerve comprising the taste nerve--chorda tympani. In the process of the analysis of daily consumption and choice of solutions, different types of behavioural reactions were found: stable preference and change of preference. The number of rats preferring NaCl was greater among the animals with sectioned lingual nerve than among sham-operated control rats and rats with ligated ducts of submaxillary and sublingual salivary glands. The number of rats with the lingual nerve section preferring sucrose or saccharin solutions to water was equal to that among the sham-operated rats. At the same time the mean volume of sucrose solution taken in daily by rats with sectioned lingual nerve was twice as great as the volume of saccharin, drunk by the same animals. The role of taste in the process of choice and preference of NaCl to sweet solutions is discussed.     

The development of sucking behaviour in the newborn goat

The development and nature of the approach and sucking behaviour of newborn goats has been studied in kids born to normal mothers and to animals with one of the two mammary glands transplanted to the neck. The initial approaches of the kids appeared to be orientated primarily towards the mobile rounded forms of the dam's ventral surfaces, with random direction of movement. Sucking occurred when the mouth contacted a smooth hairless surface, whether this was a teat located in either a normal or abnormal position, or even a surgically exteriorized skin-covered vein. By the third day of life most kids had developed a preference for sucking either the transplanted or the normal teat and were quickly able to locate their preferred teat. Non-nutritive sucking was not observed at this stage, suggesting that a reward mechanism exists as an integral part of the sucking reflex in the kid.     

Morphological studies of the neuromuscular mechanism shifting from sucking to biting of mice

In order to give a neuroanatomical evidence to the mechanism of shifting from sucking to biting, we investigated in prenatal, newborn and postnatal mice whether there is a time difference in the neurogenesis of the neurons relative to sucking and biting or in the histogenesis of their peripheral effector organs by the HRP labeling technique and electron microscopy. The results obtained are as follows. (1) At birth the facial motoneurons exceed the trigeminal motoneurons in cell area and development. (2) After birth, the trigeminal motoneurons grow rapidly and outstrip the growth of the facial motoneurons at the age of 6 days. (3) Thereafter, the cell area of both neuron types continues to increase gradually. (4) The initial sign of the alpha motor end plates is found in the orbicularis oris muscle innervated by the facial nerve in 17-day-old fetuses, while that of the trigeminal nerve is delayed in the masseter muscle of 18-day-old fetuses. (5) The initial sign of the muscle spindle appears with the sensory terminals in the masseter muscle of 17-day-old fetuses and the fundamental structure of the muscle spindle is formed in 4-day-old youngs. (6) Myelination of the facial nerve begins in 3-day-old youngs, while that of the trigeminal nerve becomes apparent in 4- or 5-day-old youngs. From these bases, it is obvious that the facial nerve elements related to sucking are firstly developed at birth and that the differentiation of the trigeminal nerve elements related to biting is rapidly accelerated after birth.     

Sensory perturbations using suture and sutureless repair of transected median nerve in rats

The effects of changes to cold, mechanical, and heat thresholds following median nerve transection with repair by sutures (Su) or Rose Bengal adhesion (RA) were compared to sham-operated animals. Both nerve-injured groups showed a transient, ipsilateral hyposensitivity to mechanical and heat stimuli followed by a robust and long-lasting hypersensitivity (6–7 weeks) with gradual recovery towards pre-injury levels by 90 days post-repair. Both tactile and thermal hypersensitivity were seen in the contralateral limb that was similar in onset but differed in magnitude and resolved more rapidly compared to the injured limb. Prior to injury, no animals showed any signs of aversion to cold plate temperatures of 4–16?°C. After injury, animals showed cold allodynia, lasting for 7 weeks in RA-repaired rats before recovering towards pre-injury levels, but were still present at 12 weeks in Su-repaired rats. Additionally, sensory recovery in the RA group was faster compared to the Su group in all behavioural tests. Surprisingly, sham-operated rats showed similar bilateral behavioural changes to all sensory stimuli that were comparable in onset and magnitude to the nerve-injured groups but resolved more quickly compared to nerve-injured rats. These results suggest that nerve repair using a sutureless approach produces an accelerated recovery with reduced sensorimotor disturbances compared to direct suturing. They also describe, for the first time, that unilateral forelimb nerve injury produces mirror-image-like sensory perturbations in the contralateral limb, suggesting that the contralateral side is not a true control for sensory testing. The potential mechanisms involved in this altered behaviour are discussed.     

VIP and noncholinergic vasodilatation in rabbit submandibular gland

The effect of parasympathetic nerve activation on rabbit submandibular gland (SMG) blood flow and saliva secretion were studied before and after systemic administration of atropine or hexamethonium. The parasympathetic fibers were stimulated electrically (2 and 15 Hz, 10 V, 1 msec) at the plexus around the submandibular salivary duct or at the chorda lingual nerve. In untreated animals, stimulation of parasympathetic fibers caused a frequency-dependent increase of salivary secretion and blood flow in the SMG. Atropine treatment completely abolished saliva secretion at 2 Hz and 15 Hz and the increase in SMG blood flow during stimulation at 2 Hz. Although atropine significantly reduced the vasodilatory response at 15 Hz, the highest blood flow measured under such circumstances was still about 2.5 times the prestimulation value. After hexamethonium administration no blood flow increase or saliva secretion was seen upon chorda lingual stimulation. The concentration of vasoactive intestinal polypeptide (VIP)-like immunoreactivity in the venous effluent of the SMG increased during nerve stimulation. Atropine significantly reduced, and hexamethonium abolished this VIP-output elicited by parasympathetic nerve stimulation. Local infusion of VIP, peptide histidine isoleucine (PHI) and substance P all caused atropine-resistant vasodilation but no salivation. The present data suggest that VIP and possibly PHI play a role in the atropine-resistant vasodilatation in rabbit submandibular gland elicited by parasympathetic nerve stimulation. The contribution of sensory mediators such as substance P released by stimulation of afferent nerves in the chorda lingual nerve to the salivary and vasodilatory responses seems to be of minor importance in the rabbit submandibular gland.     

Functional characteristics of the developing brain after early postnatal neural immunization in rats: a behavioral analysis

In experiments on Wistar line young rats immunized in the early postnatal period with antigenic complexes of the nervous tissue of the hippocampus or the neocortex of an adult rat, the influence was studied of the change of the immunoreactivity to antigenic complexes of these structures on the functional brain development. Experimental animals were immunized twice in the age of 5 and 12 days or daily in the first 7 days of life. Studies of the behaviour of experimental and control animals in various situations, beginning from the two-weeks age, revealed expressed distinctions in the behaviour of animals, immunized by the hippocampus antigens: reduction of the capability to change the behaviour in changed experimental conditions (two-weeks age), and also a deficit of orienting (three weeks age) and conditioned (six weeks age) activities. A raised level was found of anticerebral antibodies in 1.5-months animals immunized in the early age. Suggestion is expressed about the significance of neuroimmune interaction in the mechanisms of the functional brain development.     

Functional Recovery of Denervated Skeletal Muscle with Sensory or Mixed Nerve Protection: A Pilot Study

Functional recovery is usually poor following peripheral nerve injury when reinnervation is delayed. Early innervation by sensory nerve has been indicated to prevent atrophy of the denervated muscle. It is hypothesized that early protection with sensory axons is adequate to improve functional recovery of skeletal muscle following prolonged denervation of mixed nerve injury. In this study, four groups of rats received surgical denervation of the tibial nerve. The proximal and distal stumps of the tibial nerve were ligated in all animals except for those in the immediate repair group. The experimental groups underwent denervation with nerve protection of peroneal nerve (mixed protection) or sural nerve (sensory protection). The experimental and unprotected groups had a stage II surgery in which the trimmed proximal and distal tibial nerve stumps were sutured together. After 3 months of recovery, electrophysiological, histological and morphometric parameters were assessed. It was detected that the significant muscle atrophy and a good preserved structure of the muscle were observed in the unprotected and protective experimental groups, respectively. Significantly fewer numbers of regenerated myelinated axons were observed in the sensory-protected group. Enhanced recovery in the mixed protection group was indicated by the results of the muscle contraction force tests, regenerated myelinated fiber, and the results of the histological analysis. Our results suggest that early axons protection by mixed nerve may complement sensory axons which are required for promoting functional recovery of the denervated muscle natively innervated by mixed nerve.     

Taste placodes are primary targets of geniculate but not trigeminal sensory axons in mouse developing tongue

Tongue embryonic taste buds begin to differentiate before the onset of gustatory papilla formation in murine. In light of this previous finding, we sought to reexamine the developing sensory innervation as it extends toward the lingual epithelium between E 11.5 and 14.5. Nerve tracings with fluorescent lipophilic dyes followed by confocal microscope examination were used to study the terminal branching of chorda tympani and lingual nerves. At E11.5, we confirmed that the chorda tympani nerve provided for most of the nerve branching in the tongue swellings. At E12.5, we show that the lingual nerve contribution to the overall innervation of the lingual swellings increased to the extent that its ramifications matched those of the chorda tympani nerve. At E13.0, the chorda tympani nerve terminal arborizations appeared more complex than those of the lingual nerve. While the chorda tympani nerve terminal branching appeared close to the lingual epithelium that of the trigeminal nerve remained rather confined to the subepithelial mesenchymal tissue. At E13.5, chorda tympani nerve terminals projected specifically to an ordered set of loci on the tongue dorsum corresponding to the epithelial placodes. In contrast, the lingual nerve terminals remained subepithelial with no branches directed towards the placodes. At E14.5, chorda tympani nerve filopodia first entered the apical epithelium of the developing fungiform papilla. The results suggest that there may be no significant delay between the differentiation of embryonic taste buds and their initial innervation.     

Prion infection of skeletal muscle cells and papillae in the tongue

The presence of the prion agent in skeletal muscle is thought to be due to the infection of nerve fibers located within the muscle. We report here that the pathological isoform of the prion protein, PrP(Sc), accumulates within skeletal muscle cells, in addition to axons, in the tongue of hamsters following intralingual and intracerebral inoculation of the HY strain of the transmissible mink encephalopathy agent. Localization of PrP(Sc) to the neuromuscular junction suggests that this synapse is a site for prion agent spread between motor axon terminals and muscle cells. Following intracerebral inoculation, the majority of PrP(Sc) in the tongue was found in the lamina propria, where it was associated with sensory nerve fibers in the core of the lingual papillae. PrP(Sc) staining was also identified in the stratified squamous epithelium of the lingual mucosa. These findings indicate that prion infection of skeletal muscle cells and the epithelial layer in the tongue can be established following the spread of the prion agent from nerve terminals and/or axons that innervate the tongue. Our data suggest that ingestion of meat products containing prion-infected tongue could result in human exposure to the prion agent, while sloughing of prion-infected epithelial cells at the mucosal surface of the tongue could be a mechanism for prion agent shedding and subsequent prion transmission in animals.     

Experimental stereotypy caused by disorders of GABA-ergic mechanisms in the caudate nuclei

The stereotypy was induced in rats by forming generators of pathologically intensified excitation (GPIE) on local disturbance of the inhibitory mechanisms in rostral portion of caudate nuclei, using bilateral injection of tetanus toxin. Microinjections of gamma-amino-butyric acid (GABA) into the area of the GPIE and systemic galoperidol administration inhibited the stereotypic behaviour of the animals. It is concluded that the formation of the GPIE may lie in the basis of stereotypy due to the disturbances in presynaptic link of the gamkergic system of caudate nuclei, dopaminergic neurons being an operant part of this GPIE.     

Effects of early electrical stimulation of the lateral hypothalamus on the delayed acquisition of approach and avoidance learning tasks in the rat

Three experiments indicated the effects of an early bilateral stimulation of the lateral hypothalamus on later learning behaviour of male rats. The animals were stimulated at 15 days of age and tested during the sixth week of life. Stimulated rats showed an improvement of performances in acquisition of a food-reinforced operant conditioning, but their performance was impaired in two avoidance tests, an inhibitory avoidance response and a two-way avoidance test. These results cannot be interpreted in terms of handling or early experience. An hypothesis of a modified synaptic competition favouring circuits which assure the regulation of approach behaviours is formulated.     

Early social isolation provokes electrophysiological and structural changes in cutaneous sensory nerves of adult male rats

Sensory and social deprivation from the mother and littermates during early life disturbs the development of the central nervous system, but little is known about its effect on the development of the peripheral nervous system. To assess peripheral effects of early isolation, male rat pups were reared artificially in complete social isolation (AR); reared artificially with two same‐age conspecifics (AR‐Social); or reared by their mothers and with littermates (MR). As adults, the electrophysiological properties of the sensory sural (SU) nerve were recorded. We found that the amplitude and normalized area (with respect to body weight) of the compound action potential (CAP) response provoked by single electrical pulses of graded intensity in the SU nerves of AR animals were shorter than the CAP recorded in SU nerves from MR and AR‐Social animals. The slope of the stimulus‐response curve of AR SU nerves was smaller than that of the other nerves. The histological characterization of axons in the SU nerves was made and showed that the myelin thickness of axons in AR SU nerves was significant lower (2–7µm) than that of the axons in the other nerves. Furthermore, the area and axon diameter of SU nerves of both AR and AR‐Social animals were significant lower than in MR animals. This is the first report to show that maternal and littermate deprivation by AR disturbs the development of the myelination and electrophysiological properties of axons in the SU nerve; the replacement of social cues prevents most of the effects. © 2014 Wiley Periodicals, Inc. Develop Neurobiol 74: 1184–1193, 2014     

Identification of sensory neurons supplying receptors in lingual muscles of the rat: histochemical and retrograde labeling study with horseradish peroxidase.

Sensory innervation of lingual musculature was studied in young adult Wistar rats using retrograde labeling by horseradish peroxidase (HRP) and combined silver impregnation and acetylcholinesterase (AchE) methods. Intra-lingual injection of HRP resulted in labeling of neuronal somata in the trigeminal, superior vagal, and second cervical spinal (C2) ganglia. When HRP was directly applied to the proximal stump of severed hypoglossal nerve, labeling occurred only in the cervical and superior vagal ganglia. Morphometric analysis revealed that the labeled neurons were of the small-sized category in all ganglia. However, in the trigeminal and C2 ganglia, labeling occurred also among the medium-sized neurons. Combined silver and AchE preparations from lingual muscles revealed the absence of typical muscle spindles. Instead, there were free and spiral nerve terminals in the interstitium, and epilemmal knob-like or bouton-like endings surrounding non-encapsulated muscle fibers. These terminals showed AchE -ve reaction in contrast to the motor ones. Few ganglionic cells were scattered along the hypoglossal nerve with uniform AchE +ve reaction in their perikarya. This indicates that medium-sized neurons in the trigeminal and C2 ganglia, and probably sensory neurons along the hypoglossal nerve mediate lingual muscle sensibility perceived by atypical sensory terminals.     

The effect of forelimb deafferentation in early postnatal ontogeny on the development of synaptic transmission in the hippocampus]

Hippocampal slices from 15-20-day-old Wistar rats were used to study the development of some features of synaptic transmission in hippocampus and the influence of partial limitation of the sensory inflow in the early ontogeny of this transmission. The dynamics of population spike changes was observed in the CA1 hippocampal field in response to stimulation of Schaffer collaterals. The early ontogenetic limitation of the sensory inflow was accomplished by cutting n. medianus on the 13th day. Between the 15th and 20th days, the dynamics of the population spike amplitude increase in the control and experimental animals was similar, however, the response amplitude of the control rats remained higher than in the experimental animals throughout the whole period of observation. It is suggested that the partial limitation of sensory inflow from a forelimb at the early stages of the ontogeny alters the formation of synaptic transmission in hippocampus.     

Organization of motor neurons innervating the proboscis musculature in Drosophila melanogaster meigen (diptera : Drosophilidae)

The present study addresses the question as to how the motor neurons involved in feeding in Drosophila melanogaster Meigen (Diptera : Drosophilidae) are organized. The motor neurons have been visualized both by Golgi-silver impregnation and by intramuscular injection of horseradish peroxidase, and analyzed in light of the existing information on taste sensory system and the feeding behaviour. The motor neurons have been broadly classified into the following types: labial nerve motor neurons, pharyngeal nerve motor neurons, and accessory pharyngeal nerve motor neurons, depending on the nerve through which their axons exit. The arborization of all the motor neurons is confined to the suboesophageal ganglion (SOG). All of them have predominantly ipsilateral and some contralateral arborizations. Their dendrites predominantly occupy the ventral region of the neuropil of the SOG and partially overlap the taste sensory projections, thereby providing an opportunity for interaction with the taste sensory input. The pharyngeal motor neurons arborize more extensively in the ventral tritocerebram, anteroventral. and mid-ventral neuropil, whereas the dendritic fields of labial motor neurons are confined to the mid-ventral neuropil. There is a functional segregation in motor neuron organization: cibarial muscles involved in sucking are innervated by pharyngeal motor neurons, while the proboscis muscles involved in positioning, of the proboscis are innervated by labial motor neurons. We have also observed projections of the stomodaeal nerve in the tritocerebrum.     

Immunohistochemical distribution of CD44 and some of its isoforms during human taste bud development

 Taste buds are accumulations of elongated bipolar cells situated on lingual papillae. The factors that determine the sites where a taste bud may develop are largely obscure, although it is known that the early invasion of nerve fibers plays one of the key roles in taste bud development and maturation. The conditions under which taste bud primordium cells develop are influenced by the interaction between epithelial cells and extracellular matrix molecules of the mesenchyma, such as hyaluronan. Thus, we investigated immunohistochemically the distribution pattern of the receptor for hyaluronan, CD44s, and its epithelial variant isoforms CD44v6 and CD44v9, in taste buds of human embryonic, fetal, perinatal, and adult tongues. Furthermore, we wanted to determine the temporal and spatial relationships of CD44 to sensory innervation of taste bud primordia. In early gestational stages (weeks 7–9), CD44 and its isoforms are expressed on membranes of apical perigemmal (marginal) cells covering taste bud primordia. It seems that CD44 serves as a marker for marginal cells (perigemmal cells) in early developmental stages. The expression of CD44 follows rather than precedes the invasion of sensory nerve fibers and the development of taste bud primordia (weeks 7–8). In new-born and adult taste bud cells, only the standard molecule, CD44s, is expressed; the variant isoforms, CD44v6 and CD44v9, occur only in the adjacent epithelium. From these results it is likely that marginal cells are of the utmost importance for the development and maturation of taste buds. We presume that CD44 is involved in local binding, reuptake, and degradation of hyaluronan in the early stages of taste bud formation. CD44 probably does not induce the transformation of epithelial cells into taste bud primordial cells. What is more, CD44 may change its function in the course of developmental events. Accepted: 13 January 1998     

Acute lung injury augments hypoxic ventilatory response in the absence of systemic hypoxemia.

The objective of the present study was to examine the impact of early stages of lung injury on ventilatory control by hypoxia and hypercapnia. Lung injury was induced with intratracheal instillation of bleomycin (BM; 1 unit) in adult, male Sprague-Dawley rats. Control animals underwent sham surgery with saline instillation. Five days after the injections, lung injury was present in BM-treated animals as evidenced by increased neutrophils and protein levels in bronchoalveolar lavage fluid, as well as by changes in lung histology and computed tomography images. There was no evidence of pulmonary fibrosis, as indicated by lung collagen content. Basal core body temperature, arterial Po(2), and arterial Pco(2) were comparable between both groups of animals. Ventilatory responses to hypoxia (12% O(2)) and hypercapnia (7% CO(2)) were measured by whole body plethysmography in unanesthetized animals. Baseline respiratory rate and the hypoxic ventilatory response were significantly higher in BM-injected compared with control animals (P = 0.003), whereas hypercapnic ventilatory response was not statistically different. In anesthetized, spontaneously breathing animals, response to brief hyperoxia (Dejours' test, an index of peripheral chemoreceptor sensitivity) and neural hypoxic ventilatory response were augmented in BM-exposed relative to control animals, as measured by diaphragmatic electromyelograms. The enhanced hypoxic sensitivity persisted following bilateral vagotomy, but was abolished by bilateral carotid sinus nerve transection. These data demonstrate that afferent sensory input from the carotid body contributes to a selective enhancement of hypoxic ventilatory drive in early lung injury in the absence of pulmonary fibrosis and arterial hypoxemia.     

Interactions of somatosensory evoked potentials: simultaneous stimulation of two nerves

To analyse the mechanism by which sensory inputs are integrated, interactions of somatosensory evoked potentials (SEPs) in response to simultaneous stimulation of two nerves were examined in 12 healthy subjects. Right, left and bilateral median nerves were stimulated in random order so that a precise comparison could be made among the SEPs. The arithmetical sum of the independent right and left median nerve SEPs was almost equal within 40 msec of stimulus onset to that evoked by the simultaneous stimulation of bilateral median nerves. However, a difference emerged after 40 msec. The greatest difference was recorded after 100 msec. Sensory information from right and left median nerves may interact in the late phase of sensory processing. Left median, left ulnar, and both nerves together were stimulated. The sum of the SEPs of left median and ulnar nerves was not equal to that evoked by the simultaneous stimulation of the two nerves even at early latencies. Differences between them were first recorded at 14–18 msec and became greater after 30–40 msec. It is suggested that the neural interactions between impulses in the median and ulnar nerves begin below the thalamic level.