Ion specificity of rat chorda tympani fibers to chemical and electrical tongue stimulations

Responses of rat single chorda tympani fibers to ionic chemical stimuli and anodal tongue stimulations with ionic bathing solutions were examined. Fourteen out of 30 fibers showed the highest sensitivities to NaCl for both chemical and electrical stimulations, whereas the remaining 16 fibers showed almost no ion specificity for electrical stimulation even though most of them have the highest sensitivities to HCl for chemical stimulation. Ion specificity for electrical stimulation was significantly smaller than that for chemical stimulation.     

Amiloride inhibition of responses of rat single chorda tympani fibers to chemical and electrical tongue stimulations

Amiloride inhibition of single fiber responses of the rat chorda tympani to ionic chemical and electrical tongue stimulations was studied. Amiloride reduced responses to both chemical and electrical stimulations with NaCl or LiCl in most of the single fibers. However, the magnitude of reduction of the response by amiloride varied among the fibers and was greater for chemical than electrical stimulation with NaCl in each fiber. Thirty-two single fibers were divided into two groups, such as 18 high (HAS) and 14 low amiloride-sensitive (LAS) fibers. Percent responses (control, 100%) of the former group to chemical stimulus with NaCl after amiloride ranged from 1.1 to 42.5%, while those of the latter from 72.8 to 108.0%. In HAS fibers, amiloride also reduced responses to KCl and CaCl2, but to a smaller degree than those to NaCl and LiCl. Fifteen out of 18 HAS fibers more strongly responded to a chemical stimulus with 0.1 M NaCl than 0.01 M HCl, while the opposite was true for 13 out of 14 LAS fibers, although the threshold concentration for NaCl was rather lower in LAS fibers than in HAS fibers. These results suggest that there exist at least two different receptor mechanisms for NaCl or LiCl which are amiloride-sensitive and -insensitive, and the observed differences in relative specificities to ionic taste stimuli and sensitivities to amiloride among rat chorda tympani fibers are possibly due to a disproportional distribution of these two receptors.     

Development of chorda tympani nerve taste responses in the hamster

To determine whether changes in salt and sugar responses occur during development in the hamster, multifiber responses were recorded from the chorda tympani nerve while stimulating the anterior tongue of preweanling, early postweanling, and adult hamsters. Gustatory stimuli included 0.1 and 0.5 M solutions of NH4Cl, NaCl, LiCl, and KCl, and concentration series (0.01-1.0 M) of glucose, fructose, sucrose, maltose, lactose, and (0.0005-0.01 M) saccharin. Dramatic alterations in hamster peripheral gustatory sensitivities occurred with age, with the direction and magnitude of change dependent on the specific stimulus. Response magnitudes to 0.1 M solutions of NaCl and LiCl decreased with age compared to the NH4Cl response, whereas responses to all other salt stimuli remained constant during development. Responses to all sugars and saccharin compared to the NH4Cl response increased during development across a large concentration range; however, the age at which mature responses were achieved depended on the specific "sweet" stimulus. Whereas these findings demonstrate that the hamster peripheral gustatory system is dynamic during postnatal development, the hamster has a unique developmental pattern of salt taste development compared to other species. Specifically, the effectiveness of NaCl and LiCl decrease during development compared to NH4Cl in the hamster, but increase dramatically in the rat and sheep. Thus, the developmental patterns are opposite in direction for the hamster compared to the rat and sheep and may relate to the environmental pressures imposed upon each species.     

Postnatal development of chorda tympani axons in the rat nucleus of the solitary tract

The chorda tympani nerve (CT), one of three nerves that convey gustatory information to the nucleus of the solitary tract (NTS), displays terminal field reorganization after postnatal day 15 in the rat. Aiming to gain insight into mechanisms of this phenomenon, CT axon projection field and terminal morphology in NTS subdivisions were examined using tract tracing, light microscopy, and immunoelectron microscopy at four postnatal ages: P15, P25, P35, and adult. The CT axons that innervated NTS rostrolateral subdivision both in the adult and in P15 rats were morphologically distinct from those that innervated the rostrocentral, gustatory subdivision. In both subdivisions, CT terminals reached morphological maturity before P15. Rostrolateral, but not rostrocentral axons, went through substantial axonal branch elimination after P15. Rostrocentral CT synapses, however, redistribute onto postsynaptic targets in the following weeks. CT terminal preference for GABAergic postsynaptic targets was drastically reduced after P15. Furthermore, CT synapses became a smaller component of the total synaptic input to the rostrocentral NTS after P35. The results underlined that CT axons in rostrocentral and rostrolateral subdivisions represent two distinct populations of CT input, displaying different morphological properties and structural reorganization mechanisms during postnatal development.     

Amiloride inhibition of chorda tympani responses to NaCI and its temperature dependency in mice

Inhibitory effects of amiloride on salt responses of the chorda tympani nerve and its temperature dependency were compared among three inbred strains of mice (C57BL, BALB and 129). In C57BL mice, lingual treatment with amiloride significantly suppressed responses to 0.1–1.0 M NaCI at two different temperatures, 24 ± 2°C and 12 ± 2°C. The magnitude of the amiloride-inhibited component of NaCl response was slightly larger at the higher temperature. In contrast, in BALB mice, amiloride suppression of NaCl responses was observed only at the lower temperature. No such suppression was exhibited by 129 mice at either temperature levels. These results suggest that there exist at least two different amiloride-sensitive receptor components for NaCl in mice: one is more sensitive to NaCl at the higher temperature, and the other is more sensitive at the lower temperature. It is hypothesized, C57BL mice possess the former (or both) component(s), whereas BALB mice have the latter one. The 129 strain may be lacking both components.     

Recovery of two independent sweet taste systems during regeneration of the mouse chorda tympani nerve after nerve crush

In rodents, section of the taste nerve results in degeneration of the taste buds. Following regeneration of the cut taste nerve, however, the taste buds reappear. This phenomenon can be used to study the functional reformation of the peripheral neural system responsible for sweet taste. In this study we examined the recovery of sweet responses by the chorda tympani (CT) nerve after nerve crush as well as inhibition of these responses by gurmarin (Gur), a sweet response inhibitor. After about 2 weeks of CT nerve regeneration, no significant response to any taste stimuli could be observed. At 3 weeks, responses to sweet stimuli reappeared but were not significantly inhibited by Gur. At 4 weeks, Gur inhibition of sweet responses reached statistically significant levels. Thus, the Gur-sensitive (GS) component of the sweet response reappeared about 1 week later than the Gur-insensitive (GI) component. Moreover, single CT fibers responsive to sucrose could be classified into distinct GS and GI groups at 4 weeks. After 5 weeks or more, responses to sweet compounds before and after treatment with Gur became indistinguishable from responses in the intact group. During regeneration, the GS and GI components of the sucrose response could be distinguished based on their concentration-dependent responses to sucrose. These results suggest that mice have two different sweet-reception systems, distinguishable by their sensitivity to Gur (the GS and GI systems). These two sweet-reception systems may be reconstituted independently during regeneration of the mouse CT nerve.     

Impact of chorda tympani nerve injury on cell survival, axon maintenance, and morphology of the chorda tympani nerve terminal field in the nucleus of the solitary tract

Chorda tympani nerve transection (CTX) has been useful to study the relationship between nerve and taste buds in fungiform papillae. This work demonstrated that the morphological integrity of taste buds depends on their innervation. Considerable research focused on the effects of CTX on peripheral gustatory structures, but much less research has focused on the central effects. Here, we explored how CTX affects ganglion cell survival, maintenance of injured peripheral axons, and the chorda tympani nerve terminal field organization in the nucleus of the solitary tract (NTS). After CTX in adult rats, the chorda tympani nerve was labeled with biotinylated dextran amine at 3, 7, 14, 30, and 60 days post-CTX to allow visualization of the terminal field associated with peripheral processes. There was a significant and persistent reduction of the labeled chorda tympani nerve terminal field volume and density in the NTS following CTX. Compared with controls, the volume of the labeled terminal field was not altered at 3 or 7 days post-CTX; however, it was significantly reduced by 44% and by 63% at 30 and 60 days post-CTX, respectively. Changes in the density of labeled terminal field in the NTS paralleled the terminal field volume results. The dramatic decrease in labeled terminal field size post-CTX cannot be explained by a loss of geniculate ganglion neurons or degeneration of central axons. Instead, the function and/or maintenance of the peripheral axonal process appear to be affected. These new results have implications for long-term functional and behavioral alterations.     

Response of rat superior salivatory units to chorda tympani stimulation

Unit responses to chorda tympani stimulation in urethane anesthetized rats were characterized by response latency and frequency following ability. One hundred and fifty one units responded with constant latencies, mean: 16.9 msec, S.D.: 7.8 msec. Responses recorded from within the superior salivatory nucleus (SSN) had significantly longer latencies (18.8 ± 6.8, n = 94) than those from surrounding areas (13.5 ± 8.3, n = 50) (p < 0.001). This difference was due to the higher incidence of responses with latencies between 2.7 and 10 msec outside of the SSN (44%) compared to SSN responses (8%). Ability to follow high frequency stimulation ranged from 2–550 Hz, and was the same for SSN neurons and those outside the nucleus. The population of responses localized to the SSN, with latencies greater than 10 msec and with high (> 100 Hz) frequency following, had a mean latency of 20.5 msec. The estimated conduction velocity for these presumed preganglionic, parasympathetic neurons is 0.85 M/sec ± 0.25, significantly slower than that reported for similar responses in cats. The antidromic nature of these responses requires confirmation.     

Ontogeny of chorda tympani nerve responses to gustatory stimuli in the rat

Electrical activity (whole-nerve responses) of the chorda tympani nerve (CT) in rats at 2–110 days of age was recorded in response to chemical (0.5 M NaCl, 0.5 M LiCl, 0.5 M NH₄Cl, 0.1 M citric acid) stimuli applied to the dorsal surface of the tongue. The CT of rats as young as two days of age was differentially responsive to the various chemical stimuli, and the responsiveness of the CT to 0.5 M NaCl and 0.5 M LiCl relative to 0.5 M NH₄Cl increased between 20 and 34 days of age and again between 35 and 90 days of age. The responsiveness of the CT to 0.1 M citric acid relative to 0.5 M NH₄Cl increased from 6–19 days of age, then decreased thereafter to adulthood. A concentration series of NH₄Cl solutions (0.1 M, 0.25 M, 0.5 M and 1.0 M) revealed that the CT of rats 6–110 days of age displayed increased response magnitudes as the concentration of NH₄Cl increased. The CT was responsive to mechanical and thermal stimulation of the tongue, and this responsiveness was essentially unchanged throughout development. These results show that the CT of rats as young as two days of age is differentially responsive to chemical stimuli applied to the tongue, and that CT responsiveness to chemical stimuli changes during development.     

Anatomy of the gustatory system in the hamster: central projections of the chorda tympani and the lingual nerve

The sensory modalities of taste and touch, for the anterior tongue, are relegated to separate cranial nerves. The lingual branch of the trigeminal nerve mediates touch: the chorda tympani branch of the facial nerve mediates taste. The chorda tympani also contains efferent axons which originate in the superior salivatory nucleus. The central projections of these two nerves have been visualized in the hamster by anterograde labelling with horseradish peroxidase (HRP). Afferent fibers of the chorda tympani distribute to all rostral-caudal levels of the solitary nucleus. They synapse heavily in the dorsal half of the nucleus at its rostral extreme; synaptic endings are sparser and located laterally in caudal regions. These taste afferents travel caudally in the solitary tract and reach different levels by a series of collateral branches which extend medially in the the solitary nucleus, where they exhibit preterminal and terminal swellings. Taste afferent axons range in diameter from 0.2 micrometer to 1.5 micrometers. The thickest axons project exclusively to the rostral and intermediate subdivisions of the solitary nucleus; the find ones may distribute predominantly to the caudal subdivision. Afferent fibers of the lingual nerve terminate heavily in the dorsal one-third of the spinal nucleus of the trigeminal nerve and also as a dense patch in the lateral solitary nucleus at the midpoint between its rostral and caudal poles. This latter projection overlaps that of the chorda tympani. Thus the two sensory nerves which subserve taste and touch from coincident peripheral fields on the tongue converge centrally on the intermediate subdivision of the solitary nucleus. Efferent neurons of the superior salivatory nucleus were labelled retrogradely following application of HRP to the chorda tympani. These cells are located ipsilaterally in the medullary reticular formation ventral to the rostral pole of the solitary nucleus; their dendrites are oriented dorsoventrally. The efferent axons course dorsally, form a genu lateral to the facial somatomotor genu, and course ventrolaterally through the spinal nucleus of the trigeminal nerve to exit the brain ventral to the entering facial afferents.     

Developmental decrease in size of peripheral receptive fields of single chorda tympani nerve fibers and relation to increasing NaCl taste sensitivity

During development in rats, sheep, and humans, the taste system acquires increasing responsiveness to NaCl, compared with a variety of other salts and chemicals. To better understand the neural basis of changes in salt taste responses, we studied receptive field size and response properties of single chorda tympani nerve fibers in fetal, perinatal, and postnatal sheep. Individual fungiform papillae were stimulated electrically with 5 microA anodal current to determine the location and number of papillae in receptive fields. Response characteristics of NH4Cl, NaCl, and KCl were determined for the entire field. Receptive fields were dissected for later histological reconstruction and taste bud identification. Median receptive field size decreased during development. Field sizes in lambs were smaller than those in younger animals. This decrease was accompanied by an increase in the NaCl/NH4Cl response ratio of single fibers and an increase in the proportion of fibers and associated fields that responded with higher frequency to NaCl, compared with NH4Cl. In addition, for fibers across all age groups, receptive field size correlated negatively with the NaCl/NH4Cl response ratio; that is, fields most responsive to NaCl had fewer papillae than those most responsive to NH4Cl. For all fibers, receptive field size correlated with response frequencies to NH4Cl and KCl but not NaCl. For NaCl-best fibers, receptive field size correlated with the response frequencies to all 3 salts. There was no relation between number of taste buds in a single fungiform papilla and the response frequency elicited during electrical stimulation of the papilla.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ultrastructure of primary afferent terminals and synapses in the rat nucleus of the solitary tract: comparison among the greater superficial petrosal, chorda tympani, and glossopharyngeal nerves

The greater superficial petrosal (GSP), chorda tympani (CT), and glossopharyngeal (IX) nerves terminate in overlapping patterns in the brainstem in the rat nucleus of the solitary tract (NTS). There is one region, in particular, that receives overlapping inputs from all three nerves and is especially plastic during normal and experimentally altered development. To provide the requisite data necessary ultimately to delineate the circuitry in this region, we characterized the morphology of the synaptic inputs provided by the GSP, CT, and IX nerves through transmission electron microscopy. Although all three nerves had features characteristic of excitatory nerve terminals, ultrastructural analysis revealed dimorphic morphologies differentiating IX terminals from GSP and CT terminals. IX terminals had a larger area than GSP and CT terminals, and more synapses were associated with IX terminals compared with GSP and CT terminals. Additionally, IX terminals formed synapses most often with spines, as opposed to GSP and CT terminals, which formed synapses more often with dendrites. IX terminals also exhibited morphological features often associated with synaptic plasticity more often than was seen for GSP and CT terminals. These normative data form the basis for future studies of developmentally and environmentally induced plasticity in the rodent brainstem.     

Calcium sensitivity of fast- and slow-twitch human muscle fibers

Muscle fibers from the lateral gastrocnemius muscles of five normal adult males were chemically skinned (sarcolemma disrupted) and each fiber was divided into two parts for histochemical determination of fiber type and calcium-induced tension measurements. The calcium concentration associated with half-maximal tension was 2.5 microM for fast-twitch and 1.3-1.4 microM for slow-twitch fibers. Fast-oxidative-glycolytic (type 2A) and fast-glycolytic (type 2B) fibers had similar calcium sensitivities. Maximum tensions (kg/cm2) were 2.24 for fast-twitch and 2.19 for slow-twitch fibers.     

Nucleus of the solitary tract in the C57BL/6J mouse: Subnuclear parcellation,chorda tympani nerve projections,and brainstem connections

The nucleus of the solitary tract (NST) processes gustatory and related somatosensory information rostrally and general viscerosensory information caudally. To compare its connections with those of other rodents, this study in the C57BL/6J mouse provides a subnuclear cytoarchitectonic parcellation (Nissl stain) of the NST into rostral, intermediate, and caudal divisions. Subnuclei are further characterized by NADPH staining and P2X₂ immunoreactivity (IR). Cholera toxin subunit B (CTb) labeling revealed those NST subnuclei receiving chorda tympani nerve (CT) afferents, those connecting with the parabrachial nucleus (PBN) and reticular formation (RF), and those interconnecting NST subnuclei. CT terminals are densest in the rostral central (RC) and medial (M) subnuclei; less dense in the rostral lateral (RL) subnucleus; and sparse in the ventral (V), ventral lateral (VL), and central lateral (CL) subnuclei. CTb injection into the PBN retrogradely labels cells in the aforementioned subnuclei; RC and M providing the largest source of PBN projection neurons. Pontine efferent axons terminate mainly in V and rostral medial (RM) subnuclei. CTb injection into the medullary RF labels cells and axonal endings predominantly in V at rostral and intermediate NST levels. Small CTb injections within the NST label extensive projections from the rostral division to caudal subnuclei. Projections from the caudal division primarily interconnect subnuclei confined to the caudal division of the NST; they also connect with the area postrema. P2X₂‐IR identifies probable vagal nerve terminals in the central (Ce) subnucleus in the intermediate/caudal NST. Ce also shows intense NADPH staining and does not project to the PBN. J. Comp. Neurol. 522:1565–1596, 2014. © 2013 Wiley Periodicals, Inc.     

Effect of salivation on neural taste responses in freely moving rats: analyses of salivary secretion and taste responses of the chorda tympani nerve

The outer surface of the mammalian taste receptor cell is usually covered with saliva, which may affect the initial process of gustation. To ascertain the interaction between salivation and gustation, salivary secretion from the submandibular and parotid glands and taste responses of the chorda tympani nerve were analyzed in the rat, during grooming, eating, and licking of the four standard taste stimuli (sucrose, NaCl, HCl, and quinine hydrochloride). Regions of the tongue surface bathed by saliva secreted from the each gland were examined, and it was found that: (1) Rats frequently groomed, and the anterior part of the tongue, innervated by the chorda tympani nerve, was usually covered with a mixture of submandibular saliva and substances on the body surface. (2) Licking of acceptable sucrose and NaCl solutions elicited initial phasic and long-lasting tonic taste responses, and did not evoked saliva enough to wash away the stimuli from the oral cavity. Licking of rejectable quinine evoked only a small phasic taste response and was followed by taste rejection behavior, accompanied by maximum salivation which could wash out the stimuli. (3) When taste responses were compared under awake and anesthetized (the tongue adapted to water) condition, sucrose response was larger, while responses to other taste stimuli were smaller under the awake condition. Rise time of the phasic NaCl response was longer under the awake condition. These taste response alterations may reflect the effects of prolonged adaptation of the tongue to the mixture of submandibular saliva and body surface substances, and flow rate of licked taste stimuli on the tongue surface.     

Hypoglossal motor nerve activity elicited by taste and thermal stimuli applied to the tongue in rats

Single unit activity of hypoglossal motor nerve fibers which innervate the tongue muscles was recorded in lightly anesthetized non-decerebrate and acute decerebrate rats. The pattern of responses to taste and thermal stimuli applied to the tongue surface was classified into 4 types. The type 1 response is characterized by short-lasting rhythmic burst discharges, the type 2 consists of both the rhythmic burst and tonic discharges, the type 3 is long-lasting tonic discharges and the type 4 shows short-lasting burst or short-lasting tonic discharges. In non-decerebrate rats, most of the fibers (93%) showed no or a few spontaneous firings. Sucrose and NaCl were the most effective stimulants, and 70–80% of the fibers showed the type 1 response to these stimuli. Calculating the correlations between response patterns of the fiber to a pair of the stimuli, sucrose and NaCl, and HCl and quinine produced a similar response profile, respectively. In decerebrate rats, however, about 21% of fibers showed a highly regular spontaneous firing (about 30 Hz). Rhythmic burst responses (types 1 and 2) were not induced, and thermal (especially cold) stimulation elicited much better responses than the taste stimuli. HCl and quinine, but not sucrose and NaCl, produced a similar response profile. These characteristic properties of the response in acute decerebrate rats may in part be attributed to inactivation of a ‘rhythmic center’ in the brain stem.     

Repetitive anodal transcranial direct current stimulation improves neurological recovery by preserving the neuroplasticity in an asphyxial rat model of cardiac arrest

BackgroundNon-shockable rhythms present an increasing proportion of out-of-hospital cardiac arrest (CA) patients, but are associated with poor prognosis and received limited therapeutic effect of targeted temperature management (TTM). Previous study showed repetitive anodal transcranial direct current stimulation (tDCS) improved neurological outcomes in animals with ventricular fibrillation. Here, we examine the effectiveness of tDCS on neurological recovery and the potential mechanisms in a rat model of asphyxial CA.MethodCardiopulmonary resuscitation was initiated after 5 min of untreated asphyxial CA. Animals were randomized to three experimental groups immediately after successful resuscitation (n = 12/group, 6 males): no-treatment control (NTC) group, TTM group, and tDCS group. Post resuscitation hemodynamics, quantitative electroencephalogram (EEG), neurological deficit score, and 96-h survival were evaluated. Brain tissues of additional animals undergoing same experimental procedure was harvested for enzyme-linked immunoassay-based quantification assays of neuroplasticity-related biomarkers and compared with the sham-operated rats (n = 6/group).ResultsWe observed that after resuscitation tDCS-treated animals exhibited significantly higher mean arterial pressure and left ventricular ejection fraction than NTC group and showed greatly improved EEG characteristics including weighted-permutation entropy and gamma band power, and neurologic deficit scores and 96-h survival rates compared to NTC and TTM groups. Furthermore, neuroplastic biomarkers including microtubule-associated protein 2, growth-associated protein 43, postsynaptic density protein 95 and synaptophysin, were significantly higher in tDCS group when compared with NTC and TTM groups.ConclusionIn this rat model of asphyxial CA, repetitive anodal tDCS commenced after resuscitation improved neurological recovery, and it may exert a neuroprotective effect by preserving the neuroplasticity.