Terminal field volume of the glossopharyngeal nerve in adult rats reverts to prepruning size following microglia depletion with PLX5622

Programmed reduction of synapses is a hallmark of the developing brain, with sensory systems emerging as useful models with which to study this pruning. The central projections (terminal field) of the gustatory glossopharyngeal nerve (GL) of the rat are a prime example of developmental pruning, undergoing an approximate 66% reduction in volume from postnatal day 15 (P15) to P25. Later in adulthood, developmental GL pruning can be experimentally reversed, expanding to preweaning volumes, suggesting mature volumes may be actively maintained throughout the life span. Microglia are central nervous system glia cells that perform pruning and maintenance functions in other sensory systems, including other gustatory nerves. To determine their role in GL pruning, we depleted microglia from Sprague-Dawley rat brains from P1 to P40 using daily intraperitoneal injections of the colony-stimulating factor 1 receptor inhibitor PLX5622. This prevented GL developmental pruning, resulting in preweaning terminal field volumes and innervation patterns persisting through P40, 2 weeks after pruning is normally completed. These findings show microglia are necessary for developmental GL pruning. Ceasing PLX5622 treatments at P40 allowed microglia repopulation, and within 4 weeks the GL terminal field had reduced to control volumes, indicating that pruning can occur outside of the typical developmental period. Conversely, when microglia were depleted in adult rats, GL terminal fields expanded, reverting to sizes comparable to the neonatal rat. These data indicate that microglia are required for GL pruning and may continue to maintain the GL terminal field at a reduced size into adulthood.

Taste activity in the parabrachial region in adult rats following neonatal chorda tympani transection

The chorda tympani is a gustatory nerve that fails to regenerate if sectioned in rats 10 days of age or younger. This early denervation causes an abnormally high preference for NH₄Cl in adult rats, but the impact of neonatal chorda tympani transection on the development of the gustatory hindbrain is unclear. Here, we tested the effect of neonatal chorda tympani transection (CTX) on gustatory responses in the parabrachial nucleus (PbN). We recorded in vivo extracellular spikes in single PbN units of urethane-anesthetized adult rats following CTX at P5 (chronic CTX group) or immediately prior to recording (acute CTX group). Thus, all sampled PbN neurons received indirect input from taste nerves other than the CT. Compared to acute CTX rats, chronic CTX animals had significantly higher responses to stimulation with 0.1 and 0.5 M NH₄Cl, 0.1 and 0.5 M NaCl, and 0.01 M citric acid. Activity to 0.5 M sucrose and 0.01 M quinine stimulation was not significantly different between groups. Neurons from chronic CTX animals also had larger interstimulus correlations and significantly higher entropy, suggesting that neurons in this group were more likely to be activated by stimulation with multiple tastants. Although neural responses were higher in the PbN of chronic CTX rats compared to acute-sectioned controls, taste-evoked activity was much lower than observed in previous reports, suggesting permanent deficits in taste signaling. These findings demonstrate that the developing gustatory hindbrain exhibits high functional plasticity following early nerve injury.NEW & NOTEWORTHY Early and chronic loss of taste input from the chorda tympani is associated with abnormal taste behaviors. We found that compared to when the chorda tympani is sectioned acutely, chronic nerve loss leads to amplification of spared inputs in the gustatory pons, with higher response to salty and sour stimuli. Findings point to plasticity that may compensate for sensory loss, but permanent deficits in taste signaling also occur following early denervation.

Astrocytic response to neural injury is larger during development than in adulthood and is not predicated upon the presence of microglia

While contributions of microglia and astrocytes are regularly studied in various injury models, how these contributions differ across development remains less clear. We previously demonstrated developmental differences in microglial profiles across development in an injury model of the gustatory system. Nerves of the rat gustatory system have limited capacity to regenerate if injured during neonatal ages but show robust recovery if the injury occurs in adulthood. Using this developmentally disparate model of regenerative capacity, we quantified microglia and astrocytes in the rostral nucleus of the solitary tract (rNTS) following transection of the gustatory chorda tympani nerve (CTX) of neonatal and adult rats. We found that neonatal CTX induced an attenuated microglia response but a larger astrocyte response compared to adult CTX. To elucidate the interplay between the microglia and astrocyte responses in the CTX model, we used our novel intraperitoneal injection protocol for the colony-stimulating factor 1 receptor inhibitor PLX5622 to deplete microglia in the neonatal and adult rat brain prior to and after CTX. PLX5622 depleted microglia by 80-90% within 3 days of treatment, which increased to > 90% by 7 days. After 14 days of PLX5622 treatment, microglia were depleted by > 96% in both neonates and adults while preserving baseline astrocyte quantity. Microglia depletion eliminated the adult astrocyte response to CTX, while the neonatal astrocyte response after injury remained robust. Our results show injecting PLX5622 is a viable means to deplete microglia in neonatal and adult rats and suggest developmentally distinct mechanisms for astrogliosis following neural injury.

Regenerative Failure Following Rat Neonatal Chorda Tympani Transection is Associated with Geniculate Ganglion Cell Loss and Terminal Field Plasticity in the Nucleus of the Solitary Tract

Neural insult during development results in recovery outcomes that vary dependent upon the system under investigation. Nerve regeneration does not occur if the rat gustatory chorda tympani nerve is sectioned (CTX) during neonatal (≤P10) development. It is unclear how chorda tympani soma and terminal fields are affected after neonatal CTX. The current study determined the impact of neonatal CTX on chorda tympani neurons and brainstem gustatory terminal fields. To assess terminal field volume in the nucleus of the solitary tract (NTS), rats received CTX at P5 or P10 followed by chorda tympani label, or glossopharyngeal (GL) and greater superficial petrosal (GSP) label as adults. In another group of animals, terminal field volumes and numbers of chorda tympani neurons in the geniculate ganglion (GG) were determined by labeling the chorda tympani with DiI at the time of CTX in neonatal (P5) and adult (P50) rats. There was a greater loss of chorda tympani neurons following P5 CTX compared to adult denervation. Chorda tympani terminal field volume was dramatically reduced 50 days after P5 or P10 CTX. Lack of nerve regeneration after neonatal CTX is not caused by ganglion cell death alone, as approximately 30% of chorda tympani neurons survived into adulthood. Although the total field volume of intact gustatory nerves was not altered, the GSP volume and GSP-GL overlap increased in the dorsal NTS after CTX at P5, but not P10, demonstrating age-dependent plasticity. Our findings indicate that the developing gustatory system is highly plastic and simultaneously vulnerable to injury.

Microglia density decreases in the rat rostral nucleus of the solitary tract across development and increases in an age-dependent manner following denervation

Microglia are critical for developmental pruning and immune response to injury, and are implicated in facilitating neural plasticity. The rodent gustatory system is highly plastic, particularly during development, and outcomes following nerve injury are more severe in developing animals. The mechanisms underlying developmental plasticity in the taste system are largely unknown, making microglia an attractive candidate. To better elucidate microglia's role in the taste system, we examined these cells in the rostral nucleus of the solitary tract (rNTS) during normal development and following transection of the chorda tympani taste nerve (CTX). Rats aged 5, 10, 25, or 50days received unilateral CTX or no surgery and were sacrificed four days later. Brain tissue was stained for Iba1 or CD68, and both the density and morphology of microglia were assessed on the intact and transected sides of the rNTS. We found that the intact rNTS of neonatal rats (9-14days) shows a high density of microglia, most of which appear reactive. By 29days of age, microglia density significantly decreased to levels not significantly different from adults and microglia morphology had matured, with most cells appearing ramified. CD68-negative microglia density increased following CTX and was most pronounced for juvenile and adult rats. Our results show that microglia density is highest during times of normal gustatory afferent pruning. Furthermore, the quantity of the microglia response is higher in the mature system than in neonates. These findings link increased microglia presence with instances of normal developmental and injury induced alterations in the rNTS.

Chronic Oral Capsaicin Exposure During Development Leads to Adult Rats with Reduced Taste Bud Volumes

INTRODUCTION

Cross-sensory interaction between gustatory and trigeminal nerves occurs in the anterior tongue. Surgical manipulations have demonstrated that the strength of this relationship varies across development. Capsaicin is a neurotoxin that affects fibers of the somatosensory lingual nerve surrounding taste buds, but not fibers of the gustatory chorda tympani nerve which synapse with taste receptor cells. Since capsaicin is commonly consumed by many species, including humans, experimental use of this neurotoxin provides a naturalistic perturbation of the lingual trigeminal system. Neonatal or adults rats consumed oral capsaicin for 40 days and we examined the cross-sensory effect on the morphology of taste buds across development.

METHODS

Rats received moderate doses of oral capsaicin, with chronic treatments occurring either before or after taste system maturation. Tongue morphology was examined either 2 or 50 days after treatment cessation. Edema, which has been previously suggested as a cause of changes in capsaicin-related gustatory function, was also assessed.

RESULTS

Reductions in taste bud volume occurred 50 days, but not 2 days post-treatment for rats treated as neonates. Adult rats at either time post-treatment were unaffected. Edema was not found to occur with the 5 ppm concentration of capsaicin we used.

CONCLUSIONS

Results further elucidate the cooperative relationship between these discrete sensory systems and highlight the developmentally mediated aspect of this interaction.

IMPLICATIONS

Chronic exposure to even moderate levels of noxious stimuli during development has the ability to impact the orosensory environment, and these changes may not be evident until long after exposure has ceased.

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.

Neuronal cell death and population dynamics in the developing rat geniculate ganglion

In contrast to many neuronal systems, the pattern of developmental neuronal degeneration in the rat geniculate ganglion has remained undefined. To address this issue sectioned geniculate ganglia from embryonic day 13 to postnatal day 3 have been examined using standard histological techniques, TdT-mediated dUTP-digoxigenin nick end labeling to verify apoptotic activity, bromo-deoxyuridine incorporation to monitor neuronal precursor proliferation, and anti-beta-neurotubulin III to verify the neuronal identity of pycnotic cells. Results summed from alternate (embryonic day 13) or every third (embryonic day 14-postnatal day 3) section show that neuronal degeneration occurs as early as embryonic day 13 (6.8% of neurons counted), well before geniculate innervation of lingual taste buds at embryonic day 16. A degenerative peak occurs at embryonic day 17 (9.5%) followed by a decline (1.7% at embryonic day 18) and leveling off (0.1%-0.2% at embryonic day 22-postnatal day 3). Thus, geniculate neuronal degenerative pattern includes both innervation-associated histogenetic and morphogenetic cell death. Corresponding counts of mean neuronal numbers in the sections showed a continual rise from embryonic day 13 through embryonic day 18 (approx. 330-760) followed by a slight decline at embryonic day 19 (to approx. 630) and then a final leveling off at 800-825 by embryonic day 20. This pattern differs from many other developing neural systems which show a major population crash during initial target contact. It likely reflects different but slightly overlapping neuronal precursor proliferation and degeneration patterns in multiple geniculate neuronal subpopulations.

Chorda tympani nerve transection at different developmental ages produces differential effects on taste bud volume and papillae morphology in the rat

Chorda tympani nerve transection (CTX) results in morphological changes to fungiform papillae and associated taste buds. When transection occurs during neonatal development in the rat, the effects on fungiform taste bud and papillae structure are markedly more severe than observed following a comparable surgery in the adult rat. The present study examined the potential "sensitive period" for morphological modifications to tongue epithelium following CTX. Rats received unilateral transection at 65, 30, 25, 20, 15, 10, or 5 days of age. With each descending age at the time of transection, the effects on the structural integrity of fungiform papillae were more severe. Significant losses in total number of taste buds and filiform-like papillae were observed when transection occurred 5-30 days of age. Significant reduction in the number of taste pores was indicated at every age of transection. Another group of rats received chorda tympani transection at 10, 25, or 65 days of age to determine if the time course of taste bud degeneration differed depending on the age of the rat at the time of transection. Taste bud volumes differed significantly from intact sides of the tongue at 2, 8, and 50 days post-transection after CTX at 65 days of age. Volume measurements did not differ 2 days post-transection after CTX at 10 or 25 days of age, but were significantly reduced at the other time points. Findings demonstrate a transitional period throughout development wherein fungiform papillae are highly dependent upon the chorda tympani for maintenance of morphological integrity.

In vivo recordings from rat geniculate ganglia: taste response properties of individual greater superficial petrosal and chorda tympani neurones

Coding of gustatory information is complex and unique among sensory systems; information is received by multiple receptor populations located throughout the oral cavity and carried to a single central relay by four separate nerves. The geniculate ganglion is the location of the somata of two of these nerves, the greater superficial petrosal (GSP) and the chorda tympani (CT). The GSP innervates taste buds on the palate and the CT innervates taste buds on the anterior tongue. To obtain requisite taste response profiles of GSP neurones, we recorded neurophysiological responses to taste stimuli of individual geniculate ganglion neurones in vivo in the rat and compared them to those from the CT. GSP neurones had a distinct pattern of responding compared to CT neurones. For example, a small subset of GSP neurones had high response frequencies to sucrose stimulation, whereas no CT neurones had high response frequencies to sucrose. In contrast, NaCl elicited high response frequencies in a small subset of CT neurones and elicited moderate response frequencies in a relatively large proportion of GSP neurones. The robust whole-nerve response to sucrose in the GSP may be attributable to relatively few, narrowly tuned neurones, whereas the response to NaCl in the GSP may relate to proportionately more, widely tuned neurones. These results demonstrate the diversity in the initial stages of sensory coding for two separate gustatory nerves involved in the ingestion or rejection of taste solutions, and may have implications for central coding of gustatory quality and concentration as well as coding of information used in controlling energy, fluid and electrolyte homeostasis.

Each sensory nerve arising from the geniculate ganglion expresses a unique fingerprint of neurotrophin and neurotrophin receptor genes

Neurons in the geniculate ganglion, like those in other sensory ganglia, are dependent on neurotrophins for survival. Most geniculate ganglion neurons innervate taste buds in two regions of the tongue and two regions of the palate; the rest are cutaneous nerves to the skin of the ear. We investigated the expression of four neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and NT-4, and five neurotrophin receptors, trkA, trkB, trkC, p75, and truncated trkB (Trn-B) in single sensory neurons of the adult rat geniculate ganglion associated with the five innervation fields. For fungiform papillae, a glass pipette containing biotinylated dextran was placed over the target papilla and the tracer was iontophoresed into the target papilla. For the other target fields, Fluoro-Gold was microinjected. After 3 days, geniculate ganglia were harvested, sectioned, and treated histochemically (for biotinylated dextran) or immunohistochemically (for Fluoro-Gold) to reveal the neurons containing the tracer. Single labeled neurons were harvested from the slides and subjected to RNA amplification and RT-PCR to reveal the neurotrophin or neurotrophin receptor genes that were expressed. Neurons projecting from the geniculate ganglion to each of the five target fields had a unique expression profile of neurotrophin and neurotrophic receptor genes. Several individual neurons expressed more than one neurotrophin receptor or more than one neurotrophin gene. Although BDNF is significantly expressed in taste buds, its primary high affinity receptor, trkB, was not prominently expressed in the neurons. The results are consistent with the interpretation that at least some, perhaps most, of the trophic influence on the sensory neurons is derived from the neuronal somata, and the trophic effect is paracrine or autocrine, rather than target derived. The BDNF in the taste bud may also act in a paracrine or autocrine manner on the trkB expressed in taste buds, as shown by others.

Injury-induced functional plasticity in the peripheral gustatory system

Combining unilateral denervation of anterior tongue taste buds with a low-sodium diet in rats results in a rapid, dramatic, and selective attenuation of neurophysiological sodium taste responses from the intact side of the tongue. The transduction pathway responsible for the attenuated response is through the epithelial sodium channel (Hill and Phillips, 1994). Current experiments extend these findings by detailing the effects of experimentally induced injury on taste responses from anterior tongue taste receptors in sodium-restricted rats. Experiments focused on functional salt taste responses from the intact chorda tympani nerve in sodium-restricted rats in which a gustatory nerve was sectioned that innervates the anterior tongue (chorda tympani), the posterior tongue (glossopharyngeal), or palatal taste receptors (greater superficial petrosal) or in which a nongustatory nerve was sectioned that also has its target in the anterior tongue (trigeminal). An additional group was studied that received thermal injury to the anteroventral tongue. Substantial and selective suppression of sodium salt responses occurred in a graded manner generally related to the distance from the target field of the injury to anterior tongue taste buds. The order of effectiveness was: chorda tympani section > trigeminal section > thermal injury = glossopharyngeal section > greater superficial petrosal section. These results support the hypothesis that local, diffusible factors liberated from immune-derived cells as a result of neural and/or epithelial damage are involved in regulating the transduction pathway responsible for sodium salt sensation, and that these factors may become evident through dietary sodium restriction.

Time course of morphological alterations of fungiform papillae and taste buds following chorda tympani transection in neonatal rats

The time course of structural changes in fungiform papillae was analyzed in rats that received unilateral chorda tympani nerve transection at 10 days of age. Morphological differences between intact and denervated sides of the tongue were first observed at 8 days postsection, with an increase in the number of fungiform papillae that did not have a pore. In addition, the first papilla with a filiform-like appearance was noted on the denervated side at 8 days postsectioning. By 11 days after surgery, the total number of papillae and the number of papillae with a pore were significantly lower on the transected side of the tongue as compared to the intact side. At 50 days postsection, there was an average of 70.5 fungiform papillae on the intact side and a mean of only 20.8 fungiform papillae the denervated side. Of those few remaining papillae on the cut side, an average of 13.5 papillae were categorized as filiform-like, while no filiform-like papillae occurred on the intact side. Significant reduction in taste bud volume was noted at 4 days posttransection and further decrements in taste bud volume were noted at 8 and 30 days postsection. Electron microscopy of the lingual branch of the trigeminal nerve from adult rats that received neonatal chorda tympani transection showed normal numbers of both myelinated and unmyelinated fibers. Thus, in addition to the well-characterized dependence of taste bud maintenance on the chorda tympani nerve, the present study shows an additional role of the chorda tympani nerve in papilla maintenance during early postnatal development.

Lack of functional and morphological susceptibility of the greater superficial petrosal nerve to developmental dietary sodium restriction

Restriction of dietary sodium during gestation has major effects on taste function and anatomy in the offspring. The chorda tympani nerve of offspring that are maintained on sodium-reduced chow throughout life (NaDep) has reduced neurophysiological responses to sodium and altered morphology of its terminal field in the nucleus of the solitary tract. There are many anatomical and physiological similarities between the chorda tympani nerve that innervates taste buds on the anterior tongue and the greater superficial petrosal nerve (GSP) that innervates taste buds on the palate. To determine if the GSP is similarly susceptible to the effects of dietary sodium restriction, the present study examined neurophysiological responses and the terminal field of the GSP in NaDep and control rats. Neurophysiological responses of the GSP to a variety of sodium and non-sodium stimuli did not differ between NaDep and control rats. Furthermore, the volume and shape of the GSP terminal field in the nucleus of the solitary tract did not differ between the groups. Therefore, despite the high degree of functional and anatomical correspondence between the chorda tympani nerve and the GSP, the GSP does not appear to be susceptible to the effects of lifelong dietary sodium restriction.

Neonatal chorda tympani transection permanently disrupts fungiform taste bud and papilla structure in the rat

The present report examined the morphology of fungiform papillae in adult rats that received bilateral chorda tympani transection at 10 days of age. Tongue tissue was examined using surface-structure analysis. Counts were made of fungiform papillae with a pore, fungiform papillae with no pore and fungiform papillae with a keratinized conical surface; a feature referred to as "filiform-like. " Neonatal chorda tympani nerve transection resulted not only in a loss of taste buds but also in a permanent loss in numbers of fungiform papillae. Compared with an average of 152 fungiform papillae in sham-operated control rats, there was an average of only 54 fungiform papillae after neonatal chorda tympani transection. Nearly 80% of these fungiform papillae in neonatal chorda tympani transected rats were filiform-like. No filiform-like papillae were noted in sham-operated rats. These results suggest that the chorda tympani nerve is necessary during an early postnatal period of development to maintain normal fungiform papillae morphology.

Taste responses in the greater superficial petrosal nerve: substantial sodium salt and amiloride sensitivities demonstrated in two rat strains

A great quantity of research has focused on neural responses of the chorda tympani nerve (CT) to taste stimuli. This report examined salt and sugar sensitivity of the greater superficial petrosal nerve (GSP) and the effect of amiloride on these neural responses. In addition to Sprague-Dawley (SD) rats that have CT responses typical of most rat strains, we included Fischer 344 (F344) rats whose CT responses to sodium chloride (NaCl) are higher than those of other strains. After a stimulation series in which water served as the rinse, a series of stimuli was presented in 100 microM amiloride. The GSP was highly responsive to NaCl, sodium acetate (NaAc), ammonium chloride, and sucrose; NaCl and NaAc responses were strongly suppressed by amiloride. Relative responses to NaCl were significantly higher in F344 than in SD rats. In summary, the GSP is highly sensitive to salt and sugar stimulation, and palatal taste receptors have a considerable degree of amiloride sensitivity.

Taste responses in the greater superficial petrosal nerve: substantial sodium salt and amiloride sensitivities demonstrated in two rat strains

A great quantity of research has focused on neural responses of the chorda tympani nerve (CT) to taste stimuli. This report examined salt and sugar sensitivity of the greater superficial petrosal nerve (GSP) and the effect of amiloride on these neural responses. In addition to Sprague-Dawley (SD) rats that have CT responses typical of most rat strains, we included Fischer 344 (F344) rats whose CT responses to sodium chloride (NaCl) are higher than those of other strains. After a stimulation series in which water served as the rinse, a series of stimuli was presented in 100 microM amiloride. The GSP was highly responsive to NaCl, sodium acetate (NaAc), ammonium chloride, and sucrose; NaCl and NaAc responses were strongly suppressed by amiloride. Relative responses to NaCl were significantly higher in F344 than in SD rats. In summary, the GSP is highly sensitive to salt and sugar stimulation, and palatal taste receptors have a considerable degree of amiloride sensitivity.

Sodium appetite in the sham-drinking rat after chorda tympani nerve transection

Sodium depletion in the neurologically intact rat (Intact) produces a prompt and robust intake of NaCl. After chorda tympani nerve transection (CTX), there is a longer latency and a reduced intake of NaCl. The CTX rat depends on remaining gustatory and postingestive information to direct NaCl intake. In the present study, the effect of the removal of the postingestive signals of ingested NaCl (by means of a chronic gastric fistula) on the NaCl intakes and licking patterns of Intact and CTX rats was studied. When the gastric fistula was open (Sham), ingested NaCl did not pass beyond the stomach, thus negative postingestive stimulation was absent. After overnight sodium depletion, when postingestive stimulation was present (i.e., gastric fistula closed; Real), the CTX group drank significantly less 0.3 M NaCl than the Intact group over the 2-h test [11.7 +/- 1.6 (CTX) vs. 15.3 +/- 2.8 (Intact) ml]. In contrast, when postingestive signals were absent (i.e., Sham) the Intact group ingested 52.5 +/- 4.4 ml, whereas the CTX group had ingested only 12.4 +/- 3.1 ml of 0.3 M NaCl. Lickometer data analysis revealed that even during the first minute of the test the CTX/Real group generated significantly fewer licks than any of the other groups. Thus, although the CTX group was sensitive to inhibitory postingestive signals in the early portion of the appetite test, the absence of these signals did not release the robust and sustained intake of NaCl characteristic of the Intact group. These results suggest that information provided by the chorda tympani nerve is critically important to the strong motivational properties of NaCl after sodium depletion.

Retention of conditioned taste aversion to NaCl after chorda tympani transection in Fischer 344 and Wistar rats

Fischer 344 (F344) rats fail to prefer sodium chloride (NaCl) solution to water, and this behavior is dramatically altered by bilateral transection of the chorda tympani nerve (CTX). Tests of retention and generalization of a conditioned taste aversion (CTA) to 0.15 M NaCl were used to assess alterations in salt taste perception after CTX. A CTA was established to 0.15 M NaCl in groups of F344 and Wistar rats after two pairings with LiCl (IP, 2% body weight; 0.15 M). After conditioning, animals received bilateral CTX or sham operations. Approximately 2 weeks after surgery animals were tested for retention of the CTA. Aversion to 0.15 M NaCl was evident in CTX and SHAM rats that had been conditioned prior to surgery, with no apparent difference in magnitude as a function of surgical condition. Thus, although CTX profoundly alters F344 rats' hedonic response to NaCl, it does not alter perceptual characteristics so markedly that NaCl is no longer recognized as the stimulus presented during aversion conditioning. Rather these studies suggest that in both the F344 and Wistar strains the chorda tympani nerve is not necessary for retention of a presurgical CTA to NaCl. These studies, therefore, do not provide evidence of changes in NaCl perceptual "quality" as a consequence of CTX either in F344 rats, where such changes were indicated by the preference data, or in the Wistar rat, where they were not.

Neonatal chorda tympani transection alters adult preference for ammonium chloride in the rat

The immature gustatory system of the neonatal rat is characterized by sensitivity to disruption by early interventions such as receptor or nerve damage. The present studies examined the effect of chorda tympani transection (neoCTX) of neonates on adult preference for salt and nonsalt stimuli. NeoCTX at 10 days of age led to a striking change in adult rats' preference for NH4Cl solutions but little change in preference for other solutions, including NaCl and KCl. Permanent anatomical effects of neoCTX included failure of the nerve to regenerate and a loss of all fungiform taste buds. Preference for NH4Cl was not due to an inability to discriminate it from NaCl. Following taste aversion conditioning to NaCl, neoCTX rats clearly distinguished between NaCl and NH4Cl. The effects on NH4Cl preference reflect a sensitive period during development because adult rats receiving similar surgery did not show any change in NH4Cl preference.

The CS-US interval and taste aversion learning: a brief look

Temporal parameters of taste aversion learning are known to differ markedly from other learning paradigms in that acquisition occurs despite lengthy delays between exposure to conditioned (CS) and unconditioned stimulus (US). Far less consideration has been paid to very brief CS-US intervals and the possibility that this learning may also be distinguished by an ineffectiveness of close temporal contiguity between CS and US. The effectiveness of a very brief CS-US interval (10 s) was compared with that of 2 lengthier intervals (15 and 30 min). Temporal control of CS delivery (0.15% saccharin solution) into the oral cavity and US delivery (7.5 mg/kg apomorphine hydrochloride) into circulation involved infusion pumps and indwelling catheters. Using a 1-trial learning paradigm, CS-US delays of 15 and 30 min led to significant aversions whereas the 10-s CS-US interval did not, suggesting that close temporal contiguity between CS and US is neither necessary nor sufficient for conditioned taste aversion acquisition.

Amiloride sensitivity in the neonatal rat

Amiloride-sensitive sodium channels in taste buds appear to play a key role in the response to NaCl stimulation, at least in adult rats. The researchers examined whether neonatal rats, which display an exaggerated preference for hypertonic NaCl solutions, lack functional amiloride-sensitive sodium channels. NaCl intake was significantly reduced by amiloride pretreatment, but water and ammonium chloride, NH4Cl, were unaffected. The researchers assessed whether the early appearance of amiloride sensitivity was mediated by effects on chorda tympani (CT) activity by sectioning the CT before testing. CT transection reduced intake of NaCl solutions and eliminated evidence of amiloride sensitivity. Amiloride sensitivity was also assessed by recording of whole-nerve CT activity at 8-11 days of age; the response to NaCl stimulation was significantly suppressed by amiloride. These data indicate that amiloride-sensitive sodium channels develop earlier than previously believed.

Amiloride sensitivity in the neonatal rat

Amiloride-sensitive sodium channels in taste buds appear to play a key role in the response to NaCl stimulation, at least in adult rats. The researchers examined whether neonatal rats, which display an exaggerated preference for hypertonic NaCl solutions, lack functional amiloride-sensitive sodium channels. NaCl intake was significantly reduced by amiloride pretreatment, but water and ammonium chloride, NH4Cl, were unaffected. The researchers assessed whether the early appearance of amiloride sensitivity was mediated by effects on chorda tympani (CT) activity by sectioning the CT before testing. CT transection reduced intake of NaCl solutions and eliminated evidence of amiloride sensitivity. Amiloride sensitivity was also assessed by recording of whole-nerve CT activity at 8-11 days of age; the response to NaCl stimulation was significantly suppressed by amiloride. These data indicate that amiloride-sensitive sodium channels develop earlier than previously believed.

Gustatory deafferentation and desalivation: effects on NaCl preference of Fischer 344 rats

The chorda tympani nerve (CT) appears to be particularly responsive to NaCl stimulation of the tongue. However, in most strains of rat, bilateral transection of the CT (CTX) results in little alteration of salt preference. The Fischer 344 (F344) strain of rat is unusual in its lack of preference for any concentration of salt. We recently reported a dramatic change from aversion to preference for salt in F344 rats after CTX. The present studies further explored this alteration in salt preference of F344 rats with an expanded range of NaCl concentrations. Additionally, the specificity of the CT in mediating F344 salt aversion was examined by testing NaCl preference after glossopharyngeal nerve (GL) transections, combined CT and GL deafferentation, and partial desalivation. Comparison with Wistar rats revealed that salt preference of F344 CTX rats followed a typical preference/aversion pattern across a range of NaCl concentrations. Studies of GL section, either alone or combined with CT sectioning, indicated that F344 rats' aversion was dependent on the integrity of the CT and not the GL. Removal of sublingual and submaxillary salivary glands did not lead to preference changes similar to those after CTX, indicating that the effects of CTX are not secondary to desalivation. Overall, the findings point to signals from the CT as playing a primary role in the NaCl aversion displayed by the intact F344 rat.

Sodium appetite after transection of the chorda tympani nerve in Wistar and Fischer 344 rats

Acute sodium depletion in rats leads to dramatic increases in intake of hypertonic NaCl solutions, a behavior known as sodium appetite. The importance of signals conveyed by the chorda tympani (CT) nerve to the expression of sodium appetite is unclear. We examined the effects of bilateral CT transection on the short- and long-term response to sodium depletion in Wistar and Fischer 344 (F344) rat strains, because Wistar rats normally display a NaCl preference in the absence of need whereas F344 rats avoid NaCl. In both strains, sodium appetite after CT transection and treatment with the diuretic furosemide was delayed and blunted or eliminated. The results suggest that signals conveyed by the CT nerve are important in the expression of a sodium appetite. Effects on F344 rats are particularly interesting because CT transection surgery appears to have opposite effects on NaCl intake depending on whether F344 rats are sodium replete or deplete.

Sodium appetite after transection of the chorda tympani nerve in Wistar and Fischer 344 rats

Acute sodium depletion in rats leads to dramatic increases in intake of hypertonic NaCl solutions, a behavior known as sodium appetite. The importance of signals conveyed by the chorda tympani (CT) nerve to the expression of sodium appetite is unclear. We examined the effects of bilateral CT transection on the short- and long-term response to sodium depletion in Wistar and Fischer 344 (F344) rat strains, because Wistar rats normally display a NaCl preference in the absence of need whereas F344 rats avoid NaCl. In both strains, sodium appetite after CT transection and treatment with the diuretic furosemide was delayed and blunted or eliminated. The results suggest that signals conveyed by the CT nerve are important in the expression of a sodium appetite. Effects on F344 rats are particularly interesting because CT transection surgery appears to have opposite effects on NaCl intake depending on whether F344 rats are sodium replete or deplete.

Reversal of the sodium chloride aversion of Fischer 344 rats by chorda tympani nerve transection

Fischer 344 (F344) rats are atypical in their lack of preference for any concentration of NaCl solution over water. It was hypothesized that abnormal signals mediated by the chorda tympani nerve (CT) could be causally involved in NaCl avoidance by F344 rats. This study assessed whether CT transection would normalize the salt preference of F344 rats. Preference for NaCl solutions (0.6%, 0.8%, and 1.0%) versus water was assessed using two-bottle preference tests. At all concentrations tested, CT-transected animals preferred NaCl solutions to water. This preference differed dramatically from the avoidance of these solutions by controls. These findings are striking, particularly because CT transections have generally failed to significantly affect NaCl preference in other rat strains. The results are consistent with the hypothesis that in F344 rats the avoidance of the taste of NaCl stems from input mediated by the CT.

Reversal of the sodium chloride aversion of Fischer 344 rats by chorda tympani nerve transection

Fischer 344 (F344) rats are atypical in their lack of preference for any concentration of NaCl solution over water. It was hypothesized that abnormal signals mediated by the chorda tympani nerve (CT) could be causally involved in NaCl avoidance by F344 rats. This study assessed whether CT transection would normalize the salt preference of F344 rats. Preference for NaCl solutions (0.6%, 0.8%, and 1.0%) versus water was assessed using two-bottle preference tests. At all concentrations tested, CT-transected animals preferred NaCl solutions to water. This preference differed dramatically from the avoidance of these solutions by controls. These findings are striking, particularly because CT transections have generally failed to significantly affect NaCl preference in other rat strains. The results are consistent with the hypothesis that in F344 rats the avoidance of the taste of NaCl stems from input mediated by the CT.