Maintenance of Mouse Gustatory Terminal Field Organization Is Disrupted following Selective Removal of Peripheral Sodium Salt Taste Activity at Adulthood

Maternal diet during early gestation influences postnatal taste activity-dependent pruning by microglia

A key process in central sensory circuit development involves activity-dependent pruning of exuberant terminals. Here, we studied gustatory terminal field maturation in the postnatal mouse nucleus of the solitary tract (NST) during normal development and in mice where their mothers were fed a low NaCl diet for a limited period soon after conception. Pruning of terminal fields of gustatory nerves in controls involved the complement system and is likely driven by NaCl-elicited taste activity. In contrast, offspring of mothers with an early dietary manipulation failed to prune gustatory terminal fields even though peripheral taste activity developed normally. The ability to prune in these mice was rescued by activating myeloid cells postnatally, and conversely, pruning was arrested in controls with the loss of myeloid cell function. The altered pruning and myeloid cell function appear to be programmed before the peripheral gustatory system is assembled and corresponds to the embryonic period when microglia progenitors derived from the yolk sac migrate to and colonize the brain.

Rapid structural remodeling of peripheral taste neurons is independent of taste cell turnover

Taste bud cells are constantly replaced in taste buds as old cells die and new cells migrate into the bud. The perception of taste relies on new taste bud cells integrating with existing neural circuitry, yet how these new cells connect with a taste ganglion neuron is unknown. Do taste ganglion neurons remodel to accommodate taste bud cell renewal? If so, how much of the structure of taste axons is fixed and how much remodels? Here, we measured the motility and branching of individual taste arbors (the portion of the axon innervating taste buds) in mice over time with two-photon in vivo microscopy. Terminal branches of taste arbors continuously and rapidly remodel within the taste bud. This remodeling is faster than predicted by taste bud cell renewal, with terminal branches added and lost concurrently. Surprisingly, blocking entry of new taste bud cells with chemotherapeutic agents revealed that remodeling of the terminal branches on taste arbors does not rely on the renewal of taste bud cells. Although terminal branch remodeling was fast and intrinsically controlled, no new arbors were added to taste buds, and few were lost over 100 days. Taste ganglion neurons maintain a stable number of arbors that are each capable of high-speed remodeling. We propose that terminal branch plasticity permits arbors to locate new taste bud cells, while stability of arbor number supports constancy in the degree of connectivity and function for each neuron over time.

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.

Selective Removal of Sodium Salt Taste Disrupts the Maintenance of Dendritic Architecture of Gustatory Relay Neurons in the Mouse Nucleus of the Solitary Tract

Neuronal activity plays critical roles in the development of sensory circuits in the mammalian brain. Experimental procedures are now available to alter the function of specific taste transduction pathways and have been especially useful in studying how stimulus-specific taste activity influences the development of central gustatory circuits. We previously used a mouse knock-out (KO) model in which the transduction channel necessary for sodium taste is removed from taste bud cells throughout life. In these KO mice, the terminal fields that carry taste information from taste buds into the nucleus of the solitary tract (NST) fail to mature, suggesting that sodium-elicited taste activity is important for the proper development of central gustatory circuits. Here, we tested the hypothesis that the development and maintenance of the dendritic architecture of NST relay cells, the primary postsynaptic partner of gustatory nerve terminal fields, are similarly dependent on sodium-elicited taste activity. The dendritic fields of NST relay cells, from adult male and female mice in which the α-subunit of the epithelial sodium channel (αENaC) was conditionally deleted in taste bud cells throughout life, were up to 2.4× larger and more complex than that of age-matched control mice. Interestingly, these differences in dendritic architecture did not appear until after the age when terminal fields begin “pruning,” after postnatal day (P)20. Overall, our results suggest that ENaC-mediated sodium taste activity is necessary for the maintenance of dendritic fields of relay cells in the gustatory NST.

Electrophysiological responses to sugars and amino acids in the nucleus of the solitary tract of type 1 taste receptor double-knockout mice

Strong evidence supports a major role for heterodimers of the type 1 taste receptor (T1R) family in the taste transduction of sugars (T1R2+T1R3) and amino acids (T1R1+T1R3), but there are also neural and behavioral data supporting T1R-independent mechanisms. Most neural evidence for alternate mechanisms comes from whole nerve recordings in mice with deletion of a single T1R family member, limiting conclusions about the functional significance and T1R independence of the remaining responses. To clarify these issues, we recorded single-unit taste responses from the nucleus of the solitary tract in T1R double-knockout (double-KO) mice lacking functional T1R1+T1R3 [KO1+3] or T1R2+T1R3 [KO2+3] receptors and their wild-type background strains [WT; C57BL/6J (B6), 129X1/SvJ (S129)]. In both double-KO strains, responses to sugars and a moderate concentration of an monosodium glutamate + amiloride + inosine 5'-monophosphate cocktail (0.1 M, i.e., umami) were profoundly depressed, whereas a panel of 0.6 M amino acids were mostly unaffected. Strikingly, in contrast to WT mice, no double-KO neurons responded selectively to sugars and umami, precluding segregation of this group of stimuli from those representing other taste qualities in a multidimensional scaling analysis. Nevertheless, residual sugar responses, mainly elicited by monosaccharides, persisted as small "sideband" responses in double-KOs. Thus other receptors may convey limited information about sugars to the central nervous system, but T1Rs appear critical for coding the distinct perceptual features of sugar and umami stimuli. The persistence of amino acid responses supports previous proposals of alternate receptors, but because these stimuli affected multiple neuron types, further investigations are necessary.NEW & NOTEWORTHY The type 1 taste receptor (T1R) family is crucial for transducing sugars and amino acids, but there is evidence for T1R-independent mechanisms. In this study, single-unit recordings from the nucleus of the solitary tract in T1R double-knockout mice lacking T1R1+T1R3 or T1R2+T1R3 receptors revealed greatly reduced umami synergism and sugar responses. Nevertheless, residual sugar responses persisted, mainly elicited by monosaccharides and evident as "sidebands" in neurons activated more vigorously by other qualities.

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.

Maintenance of Mouse Gustatory Terminal Field Organization Is Dependent on BDNF at Adulthood

The rodent peripheral gustatory system is especially plastic during early postnatal development and maintains significant anatomical plasticity into adulthood. Thus, taste information carried from the tongue to the brain is built and maintained on a background of anatomical circuits that have the capacity to change throughout the animal's lifespan. Recently, the neurotrophin brain-derived neurotrophic factor (BDNF) was shown to be required in the tongue to maintain normal levels of innervation in taste buds at adulthood, indicating that BDNF is a key molecule in the maintenance of nerve/target matching in taste buds. Here, we tested whether maintenance of the central process of these gustatory nerves at adulthood also relies on BDNF by using male and female transgenic mice with inducible CreERT2 under the control of the keratin 14 promoter or under control of the ubiquitin promoter to remove Bdnf from the tongue or from all tissues, respectively. We found that the terminal fields of gustatory nerves in the nucleus of the solitary tract were expanded when Bdnf was removed from the tongue at adulthood and with even larger and more widespread changes in mice where Bdnf was removed from all tissues. Removal of Bdnf did not affect numbers of ganglion cells that made up the nerves and did not affect peripheral, whole-nerve taste responses. We conclude that normal expression of Bdnf in gustatory structures is required to maintain normal levels of innervation at adulthood and that the central effects of Bdnf removal are opposite of those in the tongue.SIGNIFICANCE STATEMENT BDNF plays a major role in the development and maintenance of proper innervation of taste buds. However, the importance of BDNF in maintaining innervation patterns of gustatory nerves into central targets has not been assessed. Here, we tested whether Bdnf removal from the tongue or from all structures in adult mice impacts the maintenance of how taste nerves project to the first central relay. Deletion of Bdnf from the tongue and from all tissues led to a progressively greater expansion of terminal fields. This demonstrates, for the first time, that BDNF is necessary for the normal maintenance of central gustatory circuits at adulthood and further highlights a level of plasticity not seen in other sensory system subcortical circuits.