Gold thioglucose selectively damages dorsal vagal nuclei

Orthodontic tooth movement-activated sensory neurons contribute to enhancing osteoclast activity and tooth movement through sympathetic nervous signalling

OBJECTIVES

Orthodontic tooth movement (OTM) increases sympathetic and sensory neurological markers in periodontal tissue. However, the relationship between the sympathetic and sensory nervous systems during OTM remains unclear. Therefore, the present study investigated the relationship between the sympathetic and sensory nervous systems activated by OTM using pharmacological methods.

MATERIALS AND METHODS

We compared the effects of sympathectomy and sensory nerve injury during OTM in C57BL6/J mice. Capsaicin (CAP) was used to induce sensory nerve injury. Sympathectomy was performed using 6-hydroxydopamine. To investigate the effects of a β-agonist on sensory nerve injury, isoproterenol (ISO) was administered to CAP-treated mice. Furthermore, to examine the role of the central nervous system in OTM, the ventromedial hypothalamic nucleus (VMH) was ablated using gold thioglucose.

RESULTS

Sensory nerve injury and sympathectomy both suppressed OTM and decreased the percent of the alveolar socket covered with osteoclasts (Oc.S/AS) in periodontal tissue. Sensory nerve injury inhibited increases in OTM-induced calcitonin gene-related peptide (CGRP) immunoreactivity (IR), a marker of sensory neurons, and tyrosine hydroxylase (TH) IR, a marker of sympathetic neurons, in periodontal tissue. Although sympathectomy did not decrease the number of CGRP-IR neurons in periodontal tissue, OTM-induced increases in the number of TH-IR neurons were suppressed. The ISO treatment restored sensory nerve injury-inhibited tooth movement and Oc.S/AS. Furthermore, the ablation of VMH, the centre of the sympathetic nervous system, suppressed OTM-induced increases in tooth movement and Oc.S/AS.

CONCLUSIONS

The present results suggest that OTM-activated sensory neurons contribute to enhancements in osteoclast activity and tooth movement through sympathetic nervous signalling.

Effects of gold thioglucose treatment on central corticotrophin-releasing hormone systems in mice

Systemic administration of gold thioglucose (GTG) causes a hypothalamic lesion that extends from the ventral part of the ventromedial hypothalamus (VMH) to the dorsal part of the arcuate nucleus (ARC), resulting in hyperphagia and obesity in mice. In the present study, we used in situ hybridisation histochemistry to explore the effects of GTG on the central corticotrophin-releasing hormone (CRH) system, which regulates feeding and energy homeostasis. Type 2 CRH receptor (CRHR-2) mRNA expression decreased by 40% at 8 weeks in the VMH and by 40-60% at 2 and 8 weeks in the ARC after GTG injection. By contrast, CRHR-2 mRNA expression in the hypothalamic paraventricular nucleus (PVN) and lateral septum was unchanged. Urocortin (Ucn) 3 mRNA expression in the perifornical area and medial amygdala decreased, whereas CRH mRNA expression in the PVN increased at 2 and 8 weeks after GTG injection. Ucn 1 mRNA expression in the Edingher-Westphal nucleus and Ucn 2 mRNA expression in the PVN were unchanged. Because Ucn 3 is an anorexigenic and a possible endogenous ligand for VMH CRHR-2, our results suggest that decreased Ucn 3 expression and decreased VMH CRHR-2 expression contribute, in part, to GTG-induced hyperphagia and obesity. To determine whether VMH CRHR-2 mediates the anorexigenic effects of Ucn 3, Ucn 3 was administered i.c.v. and food intake was measured 8 weeks after GTG treatment. Ucn 3 decreased cumulative food intake on days 4-7 after surgery compared to i.c.v. administration of vehicle in control mice. By contrast, the anorexigenic effects of i.c.v. Ucn 3 were abolished in GTG-treated mice. Taken together, our results indicate that the Ucn 3 pathway, which innervates the VMH, is involved in appetite regulation via CRHR-2. It remains to be determined whether CRHR-2 in the ARC has additional roles in appetite regulation by Ucn 3.

Unloading-induced bone loss was suppressed in gold-thioglucose treated mice

Loss of mechanical stress causes bone loss. However, the mechanisms underlying the unloading-induced bone loss are largely unknown. Here, we examined the effects of gold-thioglucose (GTG) treatment, which destroys ventromedial hypothalamus (VMH), on unloading-induced bone loss. Unloading reduced bone volume in control (saline-treated) mice. Treatment with GTG-reduced bone mass and in these GTG-treated mice, unloading-induced reduction in bone mass levels was not observed. Unloading reduced the levels of bone formation rate (BFR) and mineral apposition rate (MAR). GTG treatment also reduced these parameters and under this condition, unloading did not further reduce the levels of BFR and MAR. Unloading increased the levels of osteoclast number (Oc.N/BS) and osteoclast surface (Oc.S/BS). GTG treatment did not alter the basal levels of these bone resorption parameters. In contrast to control, GTG treatment suppressed unloading-induced increase in the levels of Oc.N/BS and Oc.S/BS. Unloading reduced the levels of mRNA expression of the genes encoding osteocalcin, type I collagen and Cbfa1 in bone. In contrast, GTG treatment suppressed such unloading-induced reduction of mRNA expression. Unloading also enhanced the levels of fat mass in bone marrow and mRNA expression of the genes encoding PPARgamma2, C/EBPalpha, and C/EBPbeta in bone. In GTG-treated mice, unloading did not increase fat mass and the levels of fat-related mRNA expression. These results indicated that GTG treatment suppressed unloading-induced alteration in bone loss.

Activation of the hypothalamic arcuate nucleus predicts the anorectic actions of ciliary neurotrophic factor and leptin in intact and gold thioglucose-lesioned mice

Similar to leptin, ciliary neurotrophic factor (CNTF) suppresses appetite and selectively reduces body fat in leptin-deficient ob/ob mice. To assess the relative importance of specific regions of the hypothalamus in mediating these effects, we administered a CNTF analogue (CNTFAx15) or leptin to mice made obese by administration of gold thioglucose (GTG), which destroys a well-defined portion of the medial basal hypothalamus. CNTFAx15 treatment reduced appetite and body weight in obese GTG-lesioned C57BL/6 mice, whereas leptin failed to effect similar changes regardless of whether treatment was initiated before or after the lesioned mice had become obese. Because leptin does not reduce food intake or body weight in most forms of obesity (a condition termed 'leptin resistance'), we also investigated the actions of leptin in GTG-lesioned leptin-deficient (ob/ob) mice. By contrast to C57BL/6 mice, leptin treatment reduced food intake and body weight in GTG-lesioned ob/ob mice, although the effect was attenuated. To further compare the neural substrates mediating the anorectic actions of leptin and CNTF, we determined the patterns of neurone activation induced by these proteins in the hypothalamus of intact and GTG-lesioned mice by staining for phosphorylated signal transducer and activator of transcription 3 (pSTAT3). CNTFAx15 stimulated robust pSTAT3 signalling in neurones of the medial arcuate nucleus in both intact and lesioned C57BL/6 and ob/ob mice. Leptin administration stimulated pSTAT3 signalling in only a few neurones of the medial arcuate nucleus in intact or lesioned C57BL/6 mice, but elicited a robust response in intact or lesioned ob/ob mice. By contrast to CNTFAx15, leptin treatment also resulted in prominent activation of STAT3 in several areas of the hypothalamus outside the medial arcuate nucleus. This leptin-induced pSTAT3 signal was at least as prominent in intact and GTG-lesioned C57BL/6 mice as it was in ob/ob mice, and thus was not correlated with appetite suppression or weight loss. These results indicate that the medial arcuate nucleus is a key mediator of appetite suppression and weight loss produced by CNTF and leptin, whereas GTG-vulnerable regions play a role only in leptin-induced weight loss. Other regions of hypothalamus in which pSTAT3 signal is induced by leptin may regulate energy metabolism through mechanisms other than appetite reduction.

Gold-thioglucose-induced hypothalamic lesions inhibit metabolic modulation of light-induced circadian phase shifts in mice

The circadian clock located in the suprachiasmatic nuclei is entrained by the 24-h variation in light intensity. The clock's responses to light can, however, be reduced when glucose availability is decreased. We tested the hypothesis that the ventromedial hypothalamus, a key area in the integration of metabolic and hormonal signals, mediates the metabolic modulation of circadian responses to light by injecting C57BL/6J mice with gold-thioglucose (0.6 g/kg) which damages glucose-receptive neurons, primarily located in the ventromedial hypothalamus. Light pulses applied during the mid-subjective night induce phase delays in the circadian rhythm of locomotor activity in mice kept in constant darkness. As previously observed, light-induced phase delays were significantly attenuated in fed mice pre-treated with 500 mg/kg i.p. 2-deoxy-D-glucose and in hypoglycemic mice fasted for 30 h, pre-treated with 5 IU/kg s.c. insulin or saline, compared to control mice fed ad libitum. In contrast, similar metabolic challenges in mice with gold-thioglucose-induced hypothalamic lesions did not significantly affect light-induced phase delays compared to mice treated with gold-thioglucose and fed ad libitum. These results indicate that destruction of gold-thioglucose-sensitive neurons in the ventromedial hypothalamus prevent metabolic regulation of circadian responses to light during shortage of glucose availability. Therefore, the ventromedial hypothalamus may be a central site coordinating the metabolic modulation of light-induced phase shifts of the circadian clock.

Lesions of glucose-responsive neurons impair synchronizing effects of calorie restriction in mice

Calorie restriction can induce phase-advances of daily rhythms in rodents exposed to light-dark cycles. To test whether glucose-responsive neurons are involved in the synchronizing effects of calorie restriction, C57BL/6J mice were injected with gold-thioglucose (GTG; 0.6 g/kg) which damages glucose-responsive neurons, primarily located in the ventromedial hypothalamus. From the day of injection, GTG-treated and control mice received a hypocaloric diet (66% of ad libitum food intake) 2 h after lights on. When mice were transferred to constant darkness after 4 weeks and fed ad libitum, the onset of circadian rhythm of locomotor activity was phase-advanced by 1 h in control but not in GTG-treated mice. Therefore, glucose-responsive neurons in the ventromedial hypothalamus may play a role in the synchronizing effects of calorie restriction on circadian rhythmicity.

Ventromedial hypothalamic lesions produced by gold thioglucose do not impair induction of NPY mRNA in the arcuate nucleus by fasting

There is increasing evidence that neuropeptide Y (NPY) plays an important role in the regulation of food intake. Neuropeptide Y mRNA in the arcuate nucleus increases after fasting and it has been proposed that this increase in NPY activity occurs as a result of the decreased circulating levels of both insulin and glucose associated with a fast. Glucose-responsive neurons in the ventromedial nucleus (VMN) of the hypothalamus alter their activity in response to changes in circulating glucose levels and these neurons have been proposed to be involved in the regulation of feeding behavior and metabolism. However, it is not known if these glucose-responsive neurons are involved in the response of NPY mRNA in the arcuate nucleus to fasting. To address this relationship, mice were injected with either saline or gold thioglucose (GTG), which appears to act on glucose-responsive neurons, and killed 6 weeks later after a 72 h fast or under ad lib fed conditions. In situ hybridization histochemistry for NPY mRNA was performed on hypothalamic sections containing the arcuate nucleus. The number of labelled cells was counted and the density of autoradiographic silver grains overlying the cells was also quantified (i.e. pixels per cell). Fasting resulted in increased levels of total NPY mRNA (number of labelled cells multiplied by the pixels per cell) in the arcuate nucleus of both control and GTG-treated mice. In addition, the relative fasting-induced increase (i.e. the fasted to fed ratio) in number of cells detected, number of pixels per cell, and total NPY mRNA was similar in both the control and GTG-treated mice. These data suggest that GTG-sensitive VMN neurons play little role in the induction of NPY mRNA by fasting in the arcuate nucleus.

Morphology of identified preganglionic neurons in the dorsal motor nucleus of the vagus

To determine the degree of variation of neuronal morphology both within and between the subnuclei of the dorsal motor nucleus of the vagus (dmnX), structural features of the preganglionic neurons of each of the five primary subnuclei in the rat dmnX were characterized quantitatively. Each of the columnar subnuclei was separately labeled by application of the retrograde tracer fast blue to its corresponding subdiaphragmatic vagal branch. Fixed brain slices of 100 microns thickness were then prepared in coronal, sagittal, and horizontal orientations. Next, randomly selected fast blue labeled neurons (n = 1,256) were injected with Lucifer yellow, drawn with camera lucida, and digitized. For each cell, three features of the perikaryon and twelve of the dendritic tree were measured. Dorsal motor nucleus neurons with up to eight primary dendrites, 30 dendritic segments, and seventh order dendritic branches were observed. Throughout the dmnX, the dendrites of preganglionic neurons were preferentially oriented in the horizontal plane. Consistent with an organizing role for the columnar subnuclei, most dendrites remained within their column of origin. However, between 5 and 30% of the neurons in each of the columns projected dendrites into adjacent dmnX subnuclei or other brainstem nuclei, including the nucleus of the solitary tract (NTS). The cyto- and dendroarchitectural analyses revealed systematic gradations in morphology, although they did not support the idea that the dmnX was composed of multiple distinct preganglionic types. The most parsimonious interpretation of the data is that dmnX motorneurons are variants of a single prototype, with dendrites varying widely in length and degree of ramification. The extent of an individual preganglionic neuron's dendritic field was predicted by three factors: the cell's rostrocaudal position within the dmnX, its location within a transverse plane (i.e., its coronal position within or ectopic to the dmnX), and its subnucleus of origin. Neurons at rostral and midlongitudinal levels of each column had more extensive dendritic arbors than those at caudal levels. Ectopic neurons had more extensive dendritic fields than similar cells in the corresponding columns; in fact, of all vagal preganglionic neurons, ectopics had the most extensive dendritic fields. Somata and dendrites of celiac column neurons were more extensive than those of hepatic and gastric column cells. These differential regional distributions of vagal preganglionics suggest that their structure and function are correlated.

Unique alterations of neuropeptide content in median eminence, amygdala, and dorsal vagal complex of 3- and 6-week-old diabetes mutant mice

The nature of the genetic defects which define the obese (ob) and diabetes (db) loci in mice remain unknown, but both produce similar syndromes when maintained in the same strain of mice. There is some evidence suggesting a lesion in the central nervous system (CNS) in db/db mice, while ob/ob mice appear to have a primary lesion outside the CNS. In a search for further evidence of a unique central lesion in db/db mice, we have examined neuropeptide content in selected, microdissected brain areas in both of these mutants and lean controls. In order to rule out possible interactions of the db mutation with the genetic background, diabetes mice of both C57BL/KsJ and C57BL/6J strains were studied. When concentrations of nine neuropeptide immunoreactivities were examined in up to seven microdissected areas of the brain, C57BL/6J ob/ob mice showed only one reproducible alteration, a lower content of beta-endorphin-like immunoreactivity (LI) in the preoptic area at both 3 and 6 weeks of age as compared with lean littermates. In contrast, db/db mice of both C57BL/6J and C57BL/KsJ strains exhibited alterations in a total of four peptides in three brain areas: lower concentration of somatostatin-LI in median eminence, higher Met-enkephalin-LI in dorsal vagal complex of the medulla oblongata, higher substance P-LI and lower vasoactive intestinal polypeptide (VIP)-LI in amygdala. The concentrations of the peptides studied in medial basal hypothalamus, lateral hypothalamus, substantia nigra, and preoptic area were not reproducibly altered in db/db mice. These data provide preliminary evidence for unique brain abnormalities in db/db mice in specific areas that are involved in processing of neural signals that can affect the islets of Langerhans, gonadotrophin secretory patterns, and many other visceral functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Brown adipose tissue of mice with GTG-induced obesity: altered circadian control

The effect of feeding a "cafeteria" diet and of feeding a restricted amount of chow on brown adipose tissue (BAT) of lean and gold thioglucose (GTG)-obese mice was studied at various times of the day and night. Objectives were to find out 1) whether our previous finding of diet-induced growth of BAT of the GTG-obese mouse without thermogenic activation could be explained by a transient stimulation at a time of day not studied and 2) whether lack of stimulation of BAT thyroxine 5'-deiodinase (TD) by diet seen previously in lean mice and rats could be explained by a transient increase at times of day not studied. A transient activation of BAT thermogenesis, indicated by an increase in mitochondrial GDP binding, occurs immediately after cafeteria food is presented to the GTG-obese mouse, but the effect of diet is absent at other times. This transient stimulation of BAT in the GTG-obese mouse may be sufficient to produce the tissue growth observed. A circadian rhythm in GDP binding occurred in both lean and obese mice, whether they were eating chow or the cafeteria diet. Restricted feeding suppressed BAT mitochondrial GDP binding in lean mice but did not suppress any further the low level in GTG-obese mice. A circadian rhythm in TD activity in BAT also occurred in lean and obese mice, but no effect of cafeteria diet or of restricted feeding on this enzyme was detected at any time of day, except for a brief increase in obese mice at 0500.(ABSTRACT TRUNCATED AT 250 WORDS)