Phillip S. Lasiter

Postnatal development of the rostral solitary nucleus in rat: Dendritic morphology and mitochondrial enzyme activity

Morphological and metabolic development of the gustatory zone of the rostral nucleus of the solitary tract (NST) was examined in rat. Transganglionic transport of horseradish peroxidase (HRP) was used to visualize the organization of gustatory projections to the rostral gustatory NST in rats aged postnatal day 1 (P1) to P34. Golgi impregnation studies were performed to analyze morphological development of dendrites in regions of the rostral NST that were identified as anterior tongue terminal fields. Results demonstrate that afferent fibers of the anterior tongue project to the rostral NST in rats as young as P1. The volume of NST terminal fields increased from P1 to approximately P16-P20, and was adult-like after approximately P20. Developmental increases in terminal field volume resulted from a preferential expansion in the rostrocaudal plane. Planar length of first-order dendrites associated with fusiform, multipolar, and ovoid neurons, and second-order dendrites of fusiform and ovoid neurons, increased approximately three-fold between P4 and P16-20. First-order dendritic length for all morphological types was adult-like after approximately 20-25 days of age, whereas second-order dendritic length of multipolar neurons increased significantly between P30 and P60-70. Histochemical studies confirmed that activity of the mitochondrial respiratory enzymes cytochrome c oxidase (EC 1.9.3.1), succinate dehydrogenase (EC 1.3.99.1), and NADH-dehydrogenase (EC 1.6.99.3) increased monotonically during the developmental period in which planar growth of first-order dendrites was observed. The present results, in combination with results from previous studies, indicate that morphological and metabolic development fo the NST occurs concomitantly with morphological development of taste receptors and peripheral gustatory nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal development of the parabrachial gustatory zone in rat: Dendritic morphology and mitochondrial enzyme activity

Previous studies have shown that behavioral and neurophysiological responses to tastes develop during rats postnatal life. The present experiments evaluated morphological and metabolic development of neurons in the gustatory zone of the caudal parabrachial nucleus (PBNc) of rat. Histological reconstruction studies were conducted to establish coordinate systems for PBNc gustatory zones in developing rats. Reliability of coordinate systems were evaluated in separate experiments following infusions of horseradish peroxidase in the thalamic taste area. Morphological and Golgi impregnation studies were performed to characterize neuronal and dendritic architecture in PBNc gustatory zones defined by coordinates. Conventional histochemical studies were performed for the mitochondrial respiratory enzymes cytochrome C oxidase (CO; EC 1.9.3.1) succinate dehydrogenase (SDH; EC 1.3.99.1), and NADH-dehydrogenase (NADH-DH; EC 1.6.99.3). Results show that two somatic morphologies can be statistically characterized in PBNc gustatory zones: Multipolar somatic types and fusiform somatic types. Multipolar and fusiform neurons of neonatal and adult rats project axons to the thalamic taste area, and dendrites of these neurons grow extensively between approximately 16 days after birth to approximately 35 days after birth. Activity of CO, SDH, and NADH-DH increases in the PBNc gustatory zones during the period of dendritic growth, and continues to increase slightly to approximately 45 days. These results provide the first demonstration of postnatal morphological and metabolic developmental in a central gustatory relay. Postnatal development of gustatory system therefore appears similar to that reported for other sensory systems, to the extent that morphological and metabolic development accompanies the ontogeny of taste responses.

Postnatal development of gustatory recipient zones within the nucleus of the solitary tract.

Previous studies have examined pre- and postsynaptic development of the first-order central gustatory relay, located in the rostral nucleus of the solitary tract (NST). This region of the NST is innervated by primary gustatory axons arising from the facial-intermediate nerve. However, a large portion of the gustatory NST is innervated by axons arising from the glossopharyngeal nerve, and although the time course for development of N.VII recipient zones has been defined development of glossopharyngeal afferent terminal fields has not been examined. Moreover, the time course for development of projection neurons located postsynaptic to gustatory afferent axons has not been examined in any portion of the NST. The objectives of the present study were to 1) define the time course for development of N.VII and N.IX terminal fields and 2) examine temporal relationships between development of afferent terminal fields and development of projection neurons located postsynaptic to gustatory afferent axons. To this end, triple fluorescent labeling procedures were used to simultaneously visualize developing axons and projection neurons. Results show that afferent terminal fields develop along the rostrocaudal axis of the NST. Axons of the N.VII terminal field are present in the rostral NST at P1 and develop to approximately P25. Axons and terminal endings of N.IX do not enter the NST until approximately P9-P10, and these terminal fields develop within the intermediate NST until approximately P45. Many NST neurons destined to project axons to the second-order central gustatory relay, located in the caudal parabrachial nucleus (PBN), do not possess axonal connections with the PBN during the first 2-3 weeks of postnatal life. As afferent terminal fields develop, these neurons establish connections with the PBN between the ages of approximately P7 and P45-P60. The delay between afferent terminal field development and development of PBN projection neurons in the N.VII terminal field is approximately 3 weeks. The delay between pre- and postsynaptic development in the N.IX terminal field is approximately 1 week. Potential relationships between pre- and postsynaptic development are discussed, in addition to relationships between anatomical development in the NST and the emergence of taste-guided behaviors.

Effects of early postnatal receptor damage on development of gustatory recipient zones within the nucleus of the solitary tract.

The temporal correspondence between neuroanatomical and neurophysiological development of peripheral and central gustatory neurons has suggested that morphological development of the first-order central gustatory relay, located in the rostral nucleus of the solitary tract (NST), may be dependent on afferent input from peripheral gustatory pathways. The objective of the present study was to determine the effects of perinatal receptor damage on development of gustatory recipient zones within the rostral and intermediate NST. Results show that damage induced to fungiform receptors of the anterior tongue at postnatal day 2 (P2) alters normal development of NST terminal fields associated with the chorda tympani nerve (CT) and greater superficial nerve (GSP), and that alterations in the CT/GSP terminal field persist in adulthood after peripheral gustatory receptors have regenerated. Damage induced to fungiform receptors at P2 does not alter the normal development of glossopharyngeal terminal fields in the intermediate NST. Receptor damage produced at P10 and P20 is without effect on normal development of the CT/GSP terminal field. Thus, fungiform receptor damage at P2 produces specific alterations in the development of NST terminal fields that receive projections from the facial-intermediate nerve, and receptor damage effects are only obtained during a critical period of postnatal development. P2 receptor damage has the overall effect of eliminating caudally directed migration of CT/GSP axons to additional projection neurons that establish connections with the second-order central gustatory relay located in the parabrachial nucleus (PBN). Behavioral studies were conducted to determine the functional consequences of early receptor damage. Results from behavioral studies show that bilateral damage to fungiform papillae at P2 alters normal adult preferences to low and intermediate concentrations of NaCl and sucrose tastes, yet aversions to citric acid and quinine HCl are not obviously affected. Therefore, anatomical alterations in the CT/GSP terminal field produced by P2 receptor damage are accompanied by specific changes in adult taste preference responses.

Effects of orochemical stimulation on postnatal development of gustatory recipient zones within the nucleus of the solitary tract

Previous receptor damage studies and artificial rearing (AR) studies in rat have demonstrated that orochemical stimulation between the postnatal ages of P2 and P14 is necessary for development of primary gustatory axons and terminal endings in the rostral nucleus of the solitary tract (NST). Objectives of the present experiment were to evaluate the qualitative nature of orochemical stimulation and amount of orochemical stimulation that is necessary to produce normal axonal and terminal development in gustatory recipient zones of the rostral and intermediate NST. To this end, ultra-pure water, 30, 150, and 500 mM NaCl, 80 and 340 mM lactose, whole rat milk, and rat milk that had been subject to extensive dialysis (12-14 kD MWCO) was delivered to independent groups of rat pups during AR procedures. Unstimulated AR animals and matched mother-reared (MR) animals were used as controls. AR animals received experimental orochemical stimulation between the ages of P4 and P10, and were thereafter returned to lactating dams until the time of weaning; MR animals received experimental orochemical stimulation during the course of normal nursing. Following maturation, anterograde fluorescent dual-labeling experiments were conducted to map the course and distribution of primary gustatory axons within the NST. Results show that experimental stimualtion with water during AR procedures is not sufficient to produce normal development of primary gustatory axons and terminal endings in the gustatory NST. Stimulation with 30, 150, and 500 mM NaCl produced normal axonal development in the NST, as did 80 and 340 mM lactose, whole rat milk, and dialyzed rat milk.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of early postnatal receptor damage on dendritic development in gustatory recipient zones of the rostral nucleus of the solitary tract

The rostral gustatory zone of the nucleus of the solitary tract (NST) exhibits extensive anatomical development during the first 3 weeks of postnatal life, and this development requires the presence of intact gustatory receptors during a critical period. We have previously shown that unilateral damage induced to fungiform papillae of the anterior tongue at postnatal day 2 (P2) alters normal migration and ramification of chorda tympani (CT) axons in the rostral NST. In addition to alterations of axonal development, P2 receptor damage decreases the intraneuronal distance between neurons that project axons to the second-order central gustatory relay, located in the caudal parabrachial nucleus (PBN). This observation suggested that P2 receptor damage may alter both axonal development and dendritic development in the rostral gustatory NST. The present study evaluated potential changes in dendritic development of PBN projection neurons following either P2 or P10 receptor damage. Morphological studies were first conducted to quantitatively define somatic characteristics of neurons that project axons to the PBN. Independent experiments used fluorescent labeling combined with subsequent Golgi-impregnation to study dendritic architecture of identified PBN projection neurons. Results confirmed that P2 receptor damage alters dendritic development of PBN projection neurons located in CT terminal fields. Anterior tongue receptor damage at P2 (1) reduces planar length of first- and second-order dendritic branches, (2) reduces the mean number of second-order branches per neuron, and (3) reduces the density of spine processes on second-order dendritic branches. A critical period exists for these effects, similar to that reported for axonal development, insofar as P2 receptor damage alters dendritic development of PBN projection neurons, whereas P10 receptor damage does not. Dendrites of identified PBN projection neurons located in regions of the NST that receive primary afferent axons from the glossopharyngeal nerve are not affected by anterior tongue damage at P2. These results show that early postnatal receptor damage influences both pre- and postsynaptic development in the rostral gustatory NST. These anatomical changes are undoubtedly related to alterations in taste-guided behaviors that are observed following P2 receptor damage.

Elevated NADH-dehydrogenase activity characterizes the rostral gustatory zone of the solitary nucleus in rat

The regional distribution of NADH-dehydrogenase (NADH-DH; EC 1.6.99.3) activity was examined in the nucleus of the solitary tract (NST) in the rat, with specific emphasis given to the rostral gustatory zone. Activity of NADH-DH in the rostral gustatory zone was compared to intermediate and caudal divisions of the NST which receive projections from the glossopharyngeal and vagus nerves. Animals received injections of horseradish peroxidase (HRP) in the anterior tongue, or applications of HRP crystals to the cut chorda tympani nerve to visualize chorda tympani terminal fields in the rostral NST. Tissue was subsequently reacted for NADH-dehydrogenase and transported HRP using sequential histochemical procedures. Results show that relative activity of NADH-DH is highest in areas of the NST that receive projections from the chorda tympani nerve. Lateral divisions of the NST, which receive projections from the lingual-trigeminal nerve, show moderate enzymatic activity, but such activity is quantitatively lower than that observed in the chorda tympani terminal field. Intermediate and caudal portions of the NST show the lowest NADH-DH activity of all NST regions examined. These results confirm that histochemistry for NADH-dehydrogenase can serve as an endogenous marker for chorda tympani terminal fields in the rostral NST.

Murine strain differences in taste responsivity and organization of the rostral nucleus of the solitary tract

Taste responsivity and organization of fungiform papillae, geniculate ganglion neurons and gustatory recipient zones of the nucleus of the solitary tract (NST) were examined in C57BL/6NCrlBR (C57) mice, BALB/c6NCrlBR (BALB/c) mice and CB6F1/CrlBr (CB6) mice, an F1 hybrid cross between BALB/c and C57 mice. Results from behavioral studies confirm that C57 and CB6 mice exhibit higher preferences to sucrose and lower preferences to NaCl, as compared to BALB/c mice. No strain differences were confirmed for aversion responses to citric acid or quinine HCl taste stimuli. Anatomical analyses show that the number and organization of fungiform papillae do not reliably differ between C57, BALB/c, and CB6 mice, nor do volumes of glossopharyngeal terminal fields in the NST. However, strain-specific differences exist in the number of neurons contained in the geniculate ganglion, volume of chorda tympani (CT) terminal fields in the rostral NST, and number of NST neurons contained in CT terminal fields. BALB/c and CB6 mice possess a greater number of geniculate ganglion neurons and larger CT terminal fields, as compared to C57 mice. However, strain differences in the number of geniculate ganglion neurons and terminal field volume are not obviously correlated with strain differences in gustatory responsivity. The only reliable relationship confirmed between taste responsivity and neuroanatomical organization of the rostral NST relates to the absolute number of neurons contained in CT terminal fields, and corresponding neuronal density within CT terminal fields. Chorda tympani terminal fields of C57 and CB6 mice contain an average of 379 neurons, whereas CT terminal fields of BALB/c mice contain an average of 531 neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in geniculate ganglion proteins following fungiform receptor damage

Previous anatomical studies in rat have shown that damage produced to fungiform receptors of the anterior tongue at postnatal age 2 (P2) alters the growth and ramification of primary gustatory axons in the rostral nucleus of the solitary tract (NST). Studies employing artificial rearing (AR) procedures, which functionally deprive rat pups of orochemical stimulation during critical periods of postnatal life, produce similar alterations in the development of primary gustatory axons in the NST. Therefore, orochemical stimulation during rats early postnatal life is necessary for normal development of primary gustatory axons in the rostral NST. One hypothesis concerning receptor-damage effects and AR effects is that receptor damage during critical periods of development may alter the regulation (i.e. transcription/translation) and/or distribution (i.e. transport) of proteins in geniculate ganglion neurons, thereby affecting growth of primary gustatory axons in the rostral NST. Specific aims of the present experiments were to comprehensively examine electrophoretic profiles of geniculate ganglion proteins following P2 receptor damage and late (> P40) receptor damage. Results show that concentrations of particular geniculate ganglion proteins are differentially altered following P2 receptor damage and late receptor damage, and that early receptor damage and late receptor damage produces distinct effects on the electrophoretic profiles of particular classes of proteins. Between the ages of P7-P38, P2 receptor damage lowers ganglion concentration of an acidic membrane glycoprotein designated as A1, with an apparent M(r) of 64-67 kDa and a pI of 4.8-5.2 P2 receptor damage also lowers ganglion concentrations of GAP-43. P2 receptor damage produces transient decreases in ganglion concentrations of NF-160, NF-200, and 8 additional acidic proteins. Three of these proteins may correspond to peripheral nerve sheath proteins analyzed in previous studies of the sciatic nerve, and one of these proteins may correspond to a 24 kDa growth-associated protein characterized in regenerating optic nerve. The time-course for changes observed in ganglion proteins following P2 damage was consistent with that observed for normal anatomical development of primary gustatory axons in both the lingual epithelium and NST. Receptor damage produced at P40 and later yielded different patterns of changes in geniculate ganglion proteins. Late receptor damage produced a transient increase in ganglion concentrations of NF-160, NF-200, GAP-43 and four additional acidic proteins within the 29-57 kDa M(r) range. Late receptor damage also produced a transient decrease in the concentrations of protein A1 and a 30 kDa protein that was not affected by P2 damage. Therefore, proteins that were preferentially affected by P2 damage may be involved in the regulation of initial axonal growth within the lingual epithelium and NST, as opposed to the structural repair or maintenance of extant axons. Relationships between normal anatomical development in peripheral and central components of primary gustatory axons are discussed in relation to availability of particular cytoskeletal and growth-associated proteins.