Gary J. Schwartz

Central leptin modulates behavioral and neural responsivity to CCK

The mechanisms through which leptin, the protein product of the ob gene, affects food intake remain to be determined. To assess whether the actions of leptin depend on modulation of within-meal satiety signals, we measured the effect of third ventricular leptin administration on the satiety actions of CCK. Leptin (10 micrograms) administered 1 h before 30-min access to a liquid diet had no effect on intake when administered alone, but doses of 3.5 or 10 micrograms dose dependently increased the suppression of intake produced by 1 nmol/kg CCK. Examination of patterns of c-Fos activation induced by 3.5 micrograms leptin and 1 nmol/kg CCK revealed that the combination produced significant c-Fos activation within the area postrema and the caudal and medial nucleus of the solitary tract (NST) compared with either leptin or CCK treatments alone. The leptin-CCK combination also resulted in increased c-Fos activation within the paraventricular nucleus of the hypothalamus above that produced by leptin alone. These data suggest that the actions of leptin in food intake are mediated through its ability to modulate responsivity to within-meal satiety signals.The mechanisms through which leptin, the protein product of the ob gene, affects food intake remain to be determined. To assess whether the actions of leptin depend on modulation of within-meal satiety signals, we measured the effect of third ventricular leptin administration on the satiety actions of CCK. Leptin (10 μg) administered 1 h before 30-min access to a liquid diet had no effect on intake when administered alone, but doses of 3.5 or 10 μg dose dependently increased the suppression of intake produced by 1 nmol/kg CCK. Examination of patterns of c-Fos activation induced by 3.5 μg leptin and 1 nmol/kg CCK revealed that the combination produced significant c-Fos activation within the area postrema and the caudal and medial nucleus of the solitary tract (NST) compared with either leptin or CCK treatments alone. The leptin-CCK combination also resulted in increased c-Fos activation within the paraventricular nucleus of the hypothalamus above that produced by leptin alone. These data suggest that the actions of leptin in food intake are mediated through its ability to modulate responsivity to within-meal satiety signals.

Gut vagal afferent lesions increase meal size but do not block gastric preload-induced feeding suppression

Subdiaphragmatic vagal afferent (SVA) signals arising from gut sites may provide critical feedback for the control of food intake within a meal. To evaluate the role of SVAs in both spontaneous and scheduled meals, food intake was assessed in two paradigms in male Sprague-Dawley rats. In the first study, control (Con) rats (n = 6) and rats with subdiaphragmatic vagal deafferentation (SDA) (n = 7) had 12-h nightly access to Ensure liquid diet (1 kcal/ml). SDA rats had larger and fewer meals and maintained initial rapid rates of licking, yet total numbers of licks were unaffected. In the second study, Con (n = 8) and SDA (n = 7) rats had scheduled access to 12. 5% liquid glucose after overnight food deprivation. Glucose intake was assessed after 5-ml gastric preloads of 0.9% saline or glucose, peptone, and Intralipid solutions at three concentrations (0.5, 1, and 2 kcal/ml). Glucose and peptone preloads suppressed intake similarly in Con and SDA rats, whereas Intralipid was ineffective. These results suggest that meal-related SVA signals 1) are not critical in determining preload-induced feeding suppression after deprivation, yet 2) contribute to satiety during spontaneous meals.Subdiaphragmatic vagal afferent (SVA) signals arising from gut sites may provide critical feedback for the control of food intake within a meal. To evaluate the role of SVAs in both spontaneous and scheduled meals, food intake was assessed in two paradigms in male Sprague-Dawley rats. In the first study, control (Con) rats ( n = 6) and rats with subdiaphragmatic vagal deafferentation (SDA) ( n = 7) had 12-h nightly access to Ensure liquid diet (1 kcal/ml). SDA rats had larger and fewer meals and maintained initial rapid rates of licking, yet total numbers of licks were unaffected. In the second study, Con ( n = 8) and SDA ( n = 7) rats had scheduled access to 12.5% liquid glucose after overnight food deprivation. Glucose intake was assessed after 5-ml gastric preloads of 0.9% saline or glucose, peptone, and Intralipid solutions at three concentrations (0.5, 1, and 2 kcal/ml). Glucose and peptone preloads suppressed intake similarly in Con and SDA rats, whereas Intralipid was ineffective. These results suggest that meal-related SVA signals 1) are not critical in determining preload-induced feeding suppression after deprivation, yet 2) contribute to satiety during spontaneous meals.

Early and persistent abnormalities in rats with neonatally acquired borna disease virus infection

Newborn rats inoculated with Borna disease virus (BDV) develop a persistent, tolerant nervous system infection (PTI-NB), with no signs of encephalitis or Borna disease. We measured body weight, body length, taste preferences, and spontaneous locomotor activity over a 4-month period in PTI-NB and control rats. PTI-NB rats had decreased weight and length but not detectable disturbances in growth hormone and insulin-like growth factor-1 biosynthesis as compared to control rats. In single bottle taste acceptance tests, PTI-NB rats did not differ from controls and drank normal amounts of all solutions. When offered a choice of solutions in two-bottle taste preference tests, PTI-NB rats exhibited a normal preference for saccharin and a normal aversion for quinine, but an exaggerated preference for saline. At 1 and 4 months of age, PTI-NB rats were significantly more active than normal rats, although only 1-month-old PTI-NB rats had increased daytime activity. Thus, even in the absence of encephalitis, BDV infection of the PTI-NB rat is associated with a number of physiological and behavioral abnormalities.

Sub-diaphragmatic vagal afferent integration of meal-related gastrointestinal signals

We have established a method to investigate the range of mechanical, nutrient chemical and peptidergic meal-related stimuli t hat may generate vagal afferent neurophysiological signals critical to the negative feedback control of food intake in the rat. We have identified populations of fibers that respond with increased neurophysiological discharge rates to gastric loads, duodenal loads, and close celiac arterial administration of a brain-gut peptide, cholecystokinin. Load-sensitive fibers with gastric and duodenal mechanoreceptive fields are able to integrate information arising from mechanical and peptidergic stimulation, where cholecystokinin octapeptide (CCK) administration potentiates subsequent responses to distending loads, and synergizes with distending loads to produce greater excitation than either load stimulus alone or peptide stimulation alone. In addition, we have identified situations where the duodenal presence of nutrients modifies the vagal afferent activity of gastric load-sensitive fibers. Thus, our approach can mimic the temporal and spatial distribution of meal-related stimuli in the gut, and reveals the potential for nutrients in one gastrointestinal compartment to affect neutral signals arising from another gut compartment.

DEVELOPMENTAL INJURY TO THE CEREBELLUM FOLLOWING PERINATAL BORNA DISEASE VIRUS INFECTION

In rats infected as neonates, Borna disease virus (BDV) infection causes neuroanatomical, behavioral and physiological abnormalities without encephalitis. Neonatal infection with BDV provides a powerful model for studying the effects of virus replication on brain development without inflammation-induced brain damage. Here we report that neonatal BDV infection interfered with cerebellar development in the Lewis rat. Based on cerebellar cross-sectional area measurements, abnormal cerebellar growth was first noted between 7 and 14 days after infection. Reactive astrocytosis was evident by three days after infection, even without encephalitis, and even before identification of viral proteins in the cerebellum. While neonatal BDV infection caused a significant loss in granule cells, infected granule cells were not identified. BDV proteins were readily detected in the Purkinje cells. Thus, persistent BDV infection of Purkinje cells, but not granule cells, was associated with loss of granule cells during cerebellar development, in the absence of encephalitis.

Decreased responsiveness to dietary fat in Otsuka Long-Evans Tokushima fatty rats lacking CCK-A receptors.

Adult Otsuka Long-Evans Tokushima fatty (OLETF) rats lack functional cholecystokinin A (CCK-A) receptors, are diabetic, hyperphagic, and obese, and have patterns of ingestion consistent with a satiety deficit secondary to CCK insensitivity. Because dietary fat potently stimulates CCK release, we examined how dietary fat modulates feeding in adult male OLETF rats and their lean [Long-Evans Tokushima (LETO)] controls. High-fat feeding produced sustained overconsumption of high-fat diet (30% corn oil in powdered chow) over a 3-wk period in OLETF but not LETO rats. We then assessed the ability of gastric gavage (5 ml, 1-2 kcal/ml × 15 s) or duodenal preloads (1 kcal/ml, 0.44 ml/min × 10 min) of liquid carbohydrate (glucose), protein (peptone), or fat (Intralipid) to suppress subsequent 30-min 12.5% glucose intake in both strains. In OLETF rats, gastric and duodenal fat preloads were significantly less effective in suppressing subsequent intake than were equicaloric peptone or glucose. These results demonstrate that OLETF rats fail to compensate for fat calories and suggest that their hyperphagia and obesity may stem from a reduced ability to process nutrient-elicited gastrointestinal satiety signals.

Central gustatory lesions. I : Preference and taste reactivity tests

Bilateral electrophysiologically guided lesions were placed in the nucleus of the solitary tract (NST), the parabrachial nucleus (PBN), and the ventral posteromedial thalamic nucleus (VPMpc) of rats, and 15-min intake and taste reactivity (TR) responses elicited by 3 concentrations each of sucrose, NaCl, HCl, and quinine (Q) HCl were subsequently measured. Compared with controls, NST lesions had no significant effects on intake, and rats with PBN lesions consumed significantly more QHCl, sucrose, NaCl, and HCl. Thalamic lesions decreased sucrose intake. Analysis of TR responses showed that the QHCl threshold for aversive responses increased after VPMpc, PBN, and NST lesions. Rats with NST or PBN lesions were unresponsive to increasing sucrose concentration. TR responses elicited by NaCl and HCl were similar across the groups.

Leptin amplifies the feeding inhibition and neural activation arising from a gastric nutrient preload.

Leptin affects food intake by reducing meal size, suggesting that it may modulate the efficacy of within-meal satiety signals. To assess whether leptin would amplify the feeding inhibitory actions of a nutrient gastric preload, we compared liquid diet food intake and patterns of c-Fos activation in response to intraventricular leptin (3.5 microg), intragastric Ensure (10 ml over 10 min), or their combination. Leptin alone did not affect Ensure intake but significantly increased the suppression of intake produced by the intragastric preload. Within the nucleus of the solitary tract (NTS), leptin alone did not stimulate c-Fos but significantly elevated the number of c-Fos positive cells in response to intragastric Ensure at medial and rostral levels. Within the paraventricular nucleus (PVN), both leptin and the gastric load stimulated c-Fos expression, but the combination resulted in significantly greater number of c-Fos positive cells. These data demonstrate that leptin modulates the feeding inhibition produced by meal-related signals and suggest that this modulation occurs at the levels of the NTS and PVN.

Integration of vagal afferent responses to duodenal loads and exogenous CCK in rats

The neurophysiological responses to 0.1 ml duodenal balloon inflation, 0.5 ml duodenal loads of normal saline, and 100 pmol close celiac arterial infusions of cholecystokinin (CCK) were obtained from 14 left cervical vagal afferent fibers in 14 rats. Duodenal, but not gastric, loads increased discharge rates in these slowly adapting fibers. CCK alone excited these fibers, and CCK pretreatment amplified subsequent duodenal load responses. Furthermore, duodenal loads generated greater responses when combined with CCK infusions. The small (< 3 mm) receptive fields of these fibers were localized to the ventral wall of the proximal duodenum, with C fiber conduction velocities (< 2 m/s). These results demonstrate for the first time rat duodenal load-sensitive vagal afferents. They can integrate signals arising from CCK and duodenal loads, and may mediate aspects of the role of CCK in the inhibition of gastric emptying and the control of food intake.

Effects of neonatal rat Borna disease virus (BDV) infection on the postnatal development of the brain monoaminergic systems.

Effects of neonatal Borna disease virus infection (BDV) on the postnatal development of brain monoaminergic systems in rats were studied. Tissue content of norepinephrine (NE), dopamine (DA) and its metabolite, 3,4-dihydroxyphenol acetic acid (DOPAC), and serotonin (5-HT) and its metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA) were assayed by means of HPLC-EC in frontal cortex, cerebellum, hippocampus, hypothalamus and striatum of neonatally BDV-infected and sham-inoculated male Lewis rats of 8, 14, 21, 60 and 90 days of age. Both NE and 5-HT concentrations were significantly affected by neonatal BDV infection. The cortical and cerebellar levels of NE and 5-HT were significantly greater in BDV-infected rats than control animals at postnatal days (PND) 60 and 90. Tissue content of NE in hippocampus was unaffected. In hippocampus, neonatally BDV-infected rats had lower 5-HT levels at PND 8 and significantly elevated levels at PND 21 and onwards. Neither striatal levels of 5-HT nor hypothalamic levels of 5-HT and NE were affected by neonatal BDV infection, suggesting that the monoamine systems in the prenatally maturing brain regions are less sensitive to effects of neonatal viral infection. 5-HIAA/5-HT ratio was not altered in BDV-infected rats indicating no changes in the 5-HT turnover in the brain regions damaged by the virus. Neither DA nor DOPAC/DA ratio was affected by neonatal BDV infection in any of the brain regions examined. The present data demonstrate significant and specific alterations in monoaminergic systems in neonatally BDV-infected rats. This pattern of changes is consistent with the previously reported behavioral abnormalities resulting from neonatal BDV infection.