EGR4 is critical for cell-fate determination and phenotypic maintenance of geniculate ganglion neurons underlying sweet and umami taste.

The sense of taste starts with activation of receptor cells in taste buds by chemical stimuli which then communicate this signal via innervating oral sensory neurons to the CNS. The cell bodies of oral sensory neurons reside in the geniculate ganglion (GG) and nodose/petrosal/jugular ganglion. The geniculate ganglion contains two main neuronal populations: BRN3A+ somatosensory neurons that innervate the pinna and PHOX2B+ sensory neurons that innervate the oral cavity. While much is known about the different taste bud cell subtypes, considerably less is known about the molecular identities of PHOX2B+ sensory subpopulations. In the GG, as many as 12 different subpopulations have been predicted from electrophysiological studies, while transcriptional identities exist for only 3 to 6. Importantly, the cell fate pathways that diversify PHOX2B+ oral sensory neurons into these subpopulations are unknown. The transcription factor EGR4 was identified as being highly expressed in GG neurons. EGR4 deletion causes GG oral sensory neurons to lose their expression of PHOX2B and other oral sensory genes and up-regulate BRN3A. This is followed by a loss of chemosensory innervation of taste buds, a loss of type II taste cells responsive to bitter, sweet, and umami stimuli, and a concomitant increase in type I glial-like taste bud cells. These deficits culminate in a loss of nerve responses to sweet and umami taste qualities. Taken together, we identify a critical role of EGR4 in cell fate specification and maintenance of subpopulations of GG neurons, which in turn maintain the appropriate sweet and umami taste receptor cells.

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.

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.

Contributory role of sex differences in the variations of gustatory function.

Remarkable variability between males and females occurs for an array of taste-guided behaviors in both rodents and humans. Sex differences have been noted for taste preference, detection thresholds, and reactivity to taste stimuli. Manipulating sex hormones during early postnatal development or altering the amount of circulating estrogen in adulthood can dramatically alter the pattern of these behaviors. Receptors for sex hormones appear to be prominent in several nuclei associated with central gustatory pathways, indicating that steroid hormones may modulate central taste processing. Electrophysiological recordings from the rat brainstem suggest that taste-elicited activity to sweet stimuli is organized by hormones during early development, and activity during bitter stimulation is altered by circulating ovarian hormones. Sex differences in gustatory function appear to emerge at the level of the taste bud. Among ovariectomized rats, estradiol treatment decreases activity in the chorda tympani nerve during NaCl stimulation. Although there is no evidence that chorda tympani responses to NaCl differ between intact male and female rats, glossopharyngeal nerve responses are lower in intact females for both NaCl and sodium acetate. Responses in the glossopharyngeal nerve to citric acid stimulation are also higher in female rats relative to males. These findings suggest that, in addition to differential central modulation of taste input based on sex, taste information from the periphery varies between males and females. Although the extent of sex differences in taste processing and the underlying causal mechanisms require further clarification, it is clear that studying one sex alone provides an incomplete picture of gustatory function. © 2016 Wiley Periodicals, Inc.

Long-term alterations in peripheral taste responses to NaCl in adult rats following neonatal chorda tympani transection.

The peripheral taste system of the adult rodent is highly resilient against damage, with morphological, behavioral, and functional recovery evident after regeneration of a transected nerve. If chorda tympani transection (CTX) occurs at early postnatal ages however, the nerve fails to regenerate and effects on tongue morphology and behavior are more severe and longer-lasting compared to adult denervation. To examine whether neonatal CTX induces functional changes in intact nerves, whole-nerve electrophysiology was performed on the glossopharyngeal (GL) and chorda tympani (CT) nerves of adult rats that received CTX at P10. Attenuation of NaCl-elicited GL responses were observed in CTX rats 2 months after surgery, with bilateral denervation causing the largest decreases in responses. When assessed 1 year after neonatal CTX, amiloride-sensitive responses to NaCl in the contralateral CT increased while amiloride-insensitive responses decreased. Responses to other tastants were consistent with control animals. This is the first evidence of long-term functional changes to the peripheral taste system after injury in rats fed a normal diet. This study further characterizes the developing peripheral taste system as highly susceptible to change following neural injury.