The sweet taste in the calf. I. Chorda tympani proper nerve responses to taste stimulation of the tongue

Electrophysiological recordings were obtained from the chorda tympani nerve in calves during stimulation with NaCl, quinine hydrochloride, citric acid, acesulfan-K, aspartame, fructose, galactose, glucose, glycine, lactose, maltose, monellin, Na-saccharin, sucrose, thaumatin, and xylitol. In cattle the chorda tympani innervates the posterior third of the tongue as well as the anterior part. It was found that the posterior receptive field generally responded better to sweet substances than the anterior. Glycine and Na-saccharin followed by xylitol were the most effective sweet stimuli. The monosaccharides elicited larger responses than the disaccharides. Aspartame gave a weak nerve response in 5 of 13 calves. Monellin and thaumatin elicited no change in chorda tympani nerve activity and did not crossadapt with any sweetener. No effects on citric acid responses were observed after application of miraculin.     

Chorda tympani nerve modulates the rat's avoidance of calcium chloride

Calcium intake depends on orosensory factors, implying the presence of a mechanism for calcium detection in the mouth. To better understand how information about oral calcium is conveyed to the brain, we examined the effects of chorda tympani nerve transection on calcium chloride (CaCl(2)) taste preferences and thresholds in male Wistar rats. The rats were given bilateral transections of the chorda tympani nerve (CTX) or control surgery. After recovery, they received 48-h two-bottle tests with an ascending concentration series of CaCl(2). Whereas control rats avoided CaCl(2) at concentrations of 0.1mM and higher, rats with CTX were indifferent to CaCl(2) concentrations up to 10mM. Rats with CTX had significantly higher preference scores for 0.316 and 3.16 mM CaCl(2) than did control rats. The results imply that the chorda tympani nerve is required for the normal avoidance of CaCl(2) solution.     

Chorda tympani responses in two inbred strains of mice with different taste preferences.

Behavioral studies suggest that there are significant differences in the taste systems of the inbred mouse (Mus musculus) strains: C57BL/6J (B6) and DBA/2J (D2). In an attempt to understand the biological basis of the behavioral differences, we recorded whole-nerve chorda tympani responses to taste solutions and compared the results to intake of similar solutions in nondeprived mice. Stimuli included a test series composed of 0.1 M sodium chloride, 0.3 M sucrose, 10 mM sodium saccharin, 3 mM hydrochloric acid, and 3 mM quinine hydrochloride, as well as concentration series for the same substances. Neural activity of the chorda tympani that was evoked by sucrose, saccharin, or NaCl was greater in B6 than D2 mice; and neural threshold for sucrose was lower in B6 mice, but neural thresholds for HCl and quinine were lower in D2 mice. B6 mice drank more sucrose and saccharin but less quinine than D2 mice; thus, sucrose and saccharin preference were positively correlated, but NaCl and quinine aversiveness were negatively correlated with the chorda tympani results. Nonetheless, genes involved in the structuring of taste receptors and/or the chordae tympani, which transduce taste stimuli having diverse perceptual qualities, differ for the two mouse strains.     

Chorda tympani nerve transection alters linoleic acid taste discrimination by male and female rats

Taste is intimately associated with food choice, yet little is known about the role of taste in preferences for dietary fat, a major component of many foods. We measured the taste threshold for linoleic acid (LA), an essential free fatty acid found in dietary fat, before and after bilateral transections of the chorda tympani nerve (CTX) in adult male and female rats. We conditioned a taste aversion to 88 microM LA and assessed the generalization of the aversion to lower LA concentrations to determine LA discrimination thresholds. We discovered that female rats had a lower LA discrimination threshold (approximately 2.75 microM LA) than did male rats (approximately 11 microM LA). In another set of animals, we performed CTX and found that CTX elevated LA threshold to the same level (approximately 22 microM LA) in male and female rats. Finally, we evaluated licking responses to 11, 22, 44 and 88 microM LA mixed in sucrose by male rats and ovariectomized (OVX) female rats treated with estradiol benzoate or oil vehicle. All rats increased licking to increasing LA concentrations, but OVX rats responded to a lower LA concentration (22 microM) than did males (44 microM) in 10-s trials. However, estradiol did not affect this outcome. Collectively, these experiments show that male and female rats use taste to discriminate LA and that the chorda tympani nerve, which innervates taste buds on the anterior tongue, plays a role in this discrimination. Furthermore, sex differences in fat preferences may depend on differences in fatty acid taste thresholds as well as on the taste stimuli with which fat is combined.     

Chorda tympani nerve transection, but not amiloride, increases the KCl taste detection threshold in rats

Water-restricted rats were trained to press one lever after KCl presentation and the other lever after distilled water. Water reinforcement was given after each correct response, and a time-out followed each incorrect response. Rats were trained and tested on KCl stimuli of varying concentrations. Threshold was defined as the KCl concentration corresponding to 1/2 the maximum asymptote of performance for each rat. The geometric mean KCl detection threshold for all rats was 0.033 M KCl. Rats that had the chorda tympani nerve (CT) bilaterally transected showed an average increase in KCl threshold of approximately 0.60 log10 units, whereas sham-operated rats showed no change. Control rats retested with 100 microM amiloride added to all KCl concentrations and water displayed no change in threshold. These results suggest that although the CT contributes significantly to the rat's sensitivity to KCl, amiloride-sensitive taste transduction pathways do not.     

Fluid intakes and chorda tympani nerve responses in vitamin B-6 deficient rats.

Rats fed a vitamin B-6 deficient diet were shown to increase their presence for NaCl solutions using two-bottle preferences tests compared to rats pair-fed a control diet with vitamin B-6 supplementation or rats fed the same control diet ad lib. Integrated whole chorda tympani nerve responses to NaCl solutions showed no difference in thresholds among the three groups of rats. This finding suggests that the change in the preference for NaCl resulting from vitamin B-6 deficiency is probably not mediated via a gustatory neural mechanism.     

Chloroform responses of the chorda tympani nerve in the rat

Electrophysiological analysis of the chorda tympani nerve response to saturated chloroform solution was performed in the rat, and the results were compared to those of the sucrose response. The chloroform response was characterized by a transient response, and it lacked an off-type response to the water rinse of the tongue. Treatment of the tongue with 0.1 M anionic detergent and 5 × 10^?4 M HgCl₂ produced the same effects on the chloroform and sucrose responses. However, the effects of some metallic ions on response to chloroform were different from those on the sucrose response. That is, 0.1 M CaCl₂ suppressed the chloroform response and the off-type response, but 0.001 M CuCl₂ suppressed only the sucrose response. Additive effect of sucrose and chloroform was rather smaller than the summation expected by doubling of the responses of sucrose and chloroform. The fibers which showed the off-type discharges also always responded to chloroform. These results suggest that sucrose and chloroform combine to the different loci within the same taste receptor macromolecule.     

Taste responses of the chorda tympani nerve in the mouse

Electrophysiological taste responses were obtained from the chorda tympani nerve of the mouse by using a technique whereby a 50 microns nichrome wire electrode was placed against the nerve where it passes through the middle ear.     

Enhanced responses of the chorda tympani nerve to sugars in the ventromedial hypothalamic obese rat

Sweet taste sensitivity in obese rats with lesions of the ventromedial hypothalamus (VMH) was studied by examining chorda tympani nerve responses to various taste stimuli including sugars. In the early progressive phase of obesity (2 wk after creating VMH lesions), there was no significant difference in the nerve responses to any taste stimulus between sham-operated and VMH-lesioned rats. In contrast, in the late phase of obesity (15-18 wk after VMH lesions), the magnitude of responses to sugars (except for fructose) was prominently greater than that in age-matched controls. High-fat diet-induced obese rats and streptozotocin-diabetic rats also showed greater chorda tympani nerve responses to sugars as was observed in VMH-lesioned obese rats, indicating that VMH lesions might not be specifically related to the enhanced gustatory neural responses to sugars. Although it has been demonstrated that the enhanced responses of the chorda tympani nerve to sugars in genetically diabetic db/db mice is largely attributable to the lack of the direct suppressive effect of leptin on the taste receptor cells, plasma leptin levels were not correlated with the changes in chorda tympani responsiveness to sugars in these models of obesity and diabetes. Accordingly, our results suggest that some chronic factors, including high blood glucose, inefficiency of insulin action, or leptin resistance may be related to the enhancement of chorda tympani nerve responses to sugars.     

Rearing on basal or high dietary NaCl modifies chorda tympani nerve responses in rats

Two experiments were conducted to determine the influence of dietary NaCl level on the integrated responses of the chorda tympani (CT) nerve to salt stimulation alone and mixed with the sodium-channel blocker, amiloride hydrochloride. Five groups of adult male rats were reared on regular chow containing either basal 0.1%, intermediate 1.0%, or high 3.0% NaCl from conception to postnatal day (PD) 30 or from conception to adulthood. Adult rats reared from conception to adulthood on basal dietary NaCl demonstrated a reduction in the CT nerve response to NaCl due to a decrease in the amiloride sensitive transduction mechanism. However, the CT nerve responses of adult rats reared on basal dietary NaCl to PD30 and then switched to intermediate dietary NaCl were similar to those of rats reared for a lifetime on intermediate dietary NaCl. Similarly, the CT nerve responses to NaCl in animals reared on high dietary NaCl from conception to PD30 and then switched to an intermediate NaCl diet were comparable to animals reared on intermediate and basal dietary NaCl. However, we found that exposure to high dietary NaCl led to a greater amiloride inhibition of NaCl responses. Thus, there is critical association between dietary NaCl level over two different exposure periods and CT nerve responsiveness to NaCl specifically regarding the degree of amiloride inhibition.     

Chorda tympani innervation of anterior mandibular taste buds in the chicken (Gallus gallus domesticus)

While the mammalian chorda tympani innervates taste buds on the anterior two-thirds of the tongue, the chorda tympani of chickens does not enter the tongue, but rather is reported to supply the oral epithelium of the lower beak subjacent to the tongue. This study in the chicken investigated whether the integrity of taste buds in the lower beak is normally dependent upon innervation by the chorda tympani. Following unilateral ligation and removal of a large section of the chorda tympani, animals were sacrificed at 11, 14, and 21 days postoperatively. Oral tissue between the lingual frenulum and beak tip was serially examined, and the presence of each bud was recorded, noting the point at which the bud opened into the oral cavity. No buds were observed on the operated side in any of the cases, while the average bud count on the unoperated side was 33 +/- 10 (SD). On the unoperated side, taste buds were generally associated with anterior mandibular salivary gland ducts that reached surface epithelium and opened into the oral cavity. On the operated side, the cellular organization adjacent to gland ducts and in duct-free epithelium appeared as in control (i.e., bud-free) epithelium. The number of salivary gland duct openings into the oral cavity was equivalent on the operated and control sides. It is concluded that the chorda tympani of chickens innervates taste buds in the anterior lower beak epithelium and that it functions to maintain the structural integrity of these buds.     

The emergence of the chorda tympani from the tympanic cavity and its course until its entry into the lingual nerve]

Because there are a lot of contradictory opinions in the literature we investigated the intra- and extraosseous pathway of the Chorda tympani by dissecting the heads of 6 cadavers. The Chorda nerve leaves the temporal bone in the Fissura sphenopetrosa behind the Spina ossis sphenoidalis. In an angle of 24 degrees-28 degrees the Chorda nerve connects up with the lingual nerve. The extraosseous distance is about 1.3 to 3 time longer then the intraosseous one. The individual variations of the intraosseous distance are greater than those of the extraosseous one. This is behalf the individual variations in the structure of the temporal bone.     

The effects of dietary protein restriction on chorda tympani nerve taste responses and terminal field organization

Prenatal dietary sodium restriction produces profound developmental effects on rat functional taste responses and formation of neural circuits in the brainstem. Converging evidence indicates that the underlying mechanisms for these effects are related to a compromised nutritional state and not to direct stimulus-receptor interactions. We explored whether early malnourishment produces similar functional and structural effects to those seen following dietary sodium restriction by using a protein deficient, sodium replete diet. To determine if early dietary protein-restriction affects the development of the peripheral gustatory system, multi-fiber neurophysiological recordings were made from the chorda tympani nerve and anterograde track tracing of the chorda tympani nerve into the nucleus of the solitary tract (NTS) was accomplished in rats fed a protein-restricted or a control diet (6% and 20%, respectively). The dietary regimens began on embryonic day 7 and continued until rats were used for neurophysiological recordings (postnatal days (P) 35–50) or for chorda tympani terminal field labeling (P40–50). Responses to a concentration series of NaCl, sodium acetate, KCl, and to 0.50 M sucrose, 0.03 M quinine-HCl, and 0.01 N HCl revealed attenuated responses (30–60%) to sodium-specific stimuli in rats fed the 6% protein diet compared with those fed the 20% protein diet. Responses to all other stimuli were similar between groups. Terminal field volumes were nearly twofold larger in protein-restricted rats compared with controls, with the differences located primarily in the dorsal-caudal zone of the terminal field. These results are similar to the results seen previously in rats fed a sodium-restricted diet throughout pre- and postnatal development, suggesting that dietary sodium- and protein-restriction share similar mechanisms in altering gustatory development.     

Entrapment of the lingual nerve due to an ossified pterygospinous ligament.

During a routine dissection course at the University of Muenster (Germany) an unusual course of the lingual nerve was found with entrapment of the nerve between a widely ossified pterygospinous ligament and the medial pterygoid muscle. Furthermore, the nerve's mobility was restricted by a more distal anastomosis with the inferior alveolar nerve. Although incomplete or complete ossification of the pterygospinous ligament is not uncommon, the combination with a medial course of the lingual nerve has not been described before. Besides practical importance for surgeons and anesthetists, the entrapment of the lingual nerve may lead to lingual numbness and pain associated with speech impairment.     

Effect of nicotine on chorda tympani responses to salty and sour stimuli

The effect of nicotine on the benzamil (Bz)-insensitive (transient receptor potential vanilloid-1 variant cation channel, TRPV1t) and the Bz-sensitive (epithelial Na(+) channel, ENaC) salt taste receptors and sour taste was investigated by monitoring intracellular Na(+) and H(+) activity (pH(i)) in polarized fungiform taste receptor cells (TRCs) and the chorda tympani (CT) nerve responses to NaCl, KCl, and HCl. CT responses in Sprague-Dawley rats and both wildtype and TRPV1 knockout (KO) mice were recorded in the presence and absence of agonists [resiniferatoxin (RTX) and elevated temperature] and an antagonist (SB-366791) of TRPV1t, the ENaC blocker (Bz), and varying apical pH (pH(o)). At concentrations <0.015 M, nicotine enhanced and at >0.015 M, it inhibited CT responses to KCl and NaCl. Nicotine produced maximum enhancement in the Bz-insensitive NaCl CT response at pH(o) between 6 and 7. RTX and elevated temperature increased the sensitivity of the CT response to nicotine in salt-containing media, and SB-366791 inhibited these effects. TRPV1 KO mice demonstrated no Bz-insensitive CT response to NaCl and no sensitivity to nicotine, RTX, and elevated temperature. We conclude that nicotine modulates salt responses by direct interaction with TRPV1t. At pH(o) >8, the apical membrane permeability of nicotine was increased significantly, resulting in increase in TRC pH(i) and volume, activation of ENaC, and enhancement of the Bz-sensitive NaCl CT response. At pH(o) >8, nicotine also inhibited the phasic component of the HCl CT response. We conclude that the effects of nicotine on ENaC and the phasic HCl CT response arise from increases in TRC pH(i) and volume.     

Long-term recording from the chorda tympani nerve in rats

Cuff electrodes with headcap connectors were implanted around the rat chorda tympani nerve. Whole nerve recordings under anesthesia were made from these nerves every week to chemical, thermal and tactile stimuli applied to the anterior tongue. The signal/noise ratio of these recordings was similar to acute recordings from the chorda tympani nerve, and the nerves were spontaneously active. Responses to chemical as well as thermal and mechanical stimulation of the tongue were recorded as early as 2 and 3 weeks after implantation and recordings from the same nerve were made for more than 3 months. These results have demonstrated the feasibility of making long-term chronic recordings of chemosensory activity in the chorda tympani nerve. The cuff electrode has great potential to provide correlative information between neurophysiological and behavioral data.