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The hormone leptin reduces food intake through actions in the peripheral and central nervous systems, including in the hindbrain nucleus of the solitary tract (NTS). The NTS receives viscerosensory information via vagal afferents, including information from the gastrointestinal tract, which is then relayed to other central nervous system (CNS) sites critical for control of food intake. Leptin receptors (lepRs) are expressed by a subpopulation of NTS neurons, and knockdown of these receptors increases both food intake and body weight. Recently, we demonstrated that leptin increases vagal activation of lepR-expressing neurons via increased NMDA receptor (NMDAR) currents, thereby potentiating vagally evoked firing. Furthermore, chemogenetic activation of these neurons was recently shown to inhibit food intake. However, the vagal inputs these neurons receive had not been characterized. Here we performed whole cell recordings in brain slices taken from lepRCre ( floxedTdTomato mice and found that lepR neurons of the NTS are directly activated by monosynaptic inputs from C-type afferents sensitive to the transient receptor potential vanilloid type 1 (TRPV1) agonist capsaicin. CCK administered onto NTS slices stimulated spontaneous glutamate release onto lepR neurons and induced action potential firing, an effect mediated by CCKR1. Interestingly, NMDAR activation contributed to the current carried by spontaneous excitatory postsynaptic currents (EPSCs) and enhanced CCK-induced firing. Peripheral CCK also increased c-fos expression in these neurons, suggesting they are activated by CCK-sensitive vagal afferents in vivo. Our results indicate that the majority of NTS lepR neurons receive direct inputs from CCK-sensitive C vagal-type afferents, with both peripheral and central CCK capable of activating these neurons and NMDARs able to potentiate these effects. [ABSTRACT FROM AUTHOR]

To investigate the role of glial cells in the regulation of glucoprivic responses in rats, a chemogenetic approach was used to activate astrocytes neighboring catecholamine (CA) neurons in the ventromedial medulla (VLM) where A1 and C1 CA cell groups overlap (A1/C1). Previous results indicate that activation of CA neurons in this region is necessary and sufficient for feeding and corticosterone release in response to glucoprivation. However, it is not known whether astrocyte neighbors of CA neurons contribute to glucoregulatory responses. Hence, we made nanoinjections of AAV5-GFAP-hM3D(Gq)-mCherry to selectively transfect astrocytes in the A1/C1 region with the excitatory designer receptor exclusively activated by designer drugs (DREADDs), hM3D (Gq). After allowing time for DREADD expression, we evaluated the rats for increased food intake and corticosterone release in response to low systemic doses of the antiglycolytic agent, 2-deoxy-D-glucose (2DG), alone and in combination with the hM3D (Gq) activator clozapine-N-oxide (CNO). We found that DREADD-transfected rats ate significantly more food when 2DG and CNO were coadministered than when either 2DG or CNO was injected alone. We also found that CNO significantly enhanced 2DG-induced FOS expression in the A1/C1 CA neurons, and that corticosterone release also was enhanced when CNO and 2DG were administered together. Importantly, CNO-induced activation of astrocytes in the absence of 2DG did not trigger food intake or corticosterone release. Our results indicate that during glucoprivation, activation of VLM astrocytes cells markedly increases the sensitivity or responsiveness of neighboring A1/C1 CA neurons to glucose deficit, suggesting a potentially important role for VLM astrocytes in glucoregulation. [ABSTRACT FROM AUTHOR]

We have previously shown that blockade of N-methyl-D-aspartate (NMDA) receptors in the caudal brain stem delays satiation and increases food intake. NMDA receptors are heterodimers made up of distinct, but different, ion channel subunits. The NR2 subunits of the NMDA receptor contain the binding site for glutamate. About half of vagal afferents express immunoreactivity for NMDA NR2B subunit and about half of the NR2B expressing afferents also express NMDA NR2C or NR2D subunits. This suggests that increased food intake may be evoked by interference with glutamate binding to NMDA channels containing the NR2B subunit. To test this, we measured deprivation-induced intake of 15% sucrose solution following fourth ventricle and intranucleus of the solitary tract (intra-NTS) injections of Conantokin G (Con G; NR2B blocker), D-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphoric acid (D-CPPene; NR2B/2A blocker), and ([+ or -])-cis-1-(phenanthren-2yl-carbonyl)piperazine-2,3-dicarboxylic acid (PPDA; NR2D/C blocker). Fourth ventricular administration of Con G (5, 20, 40, 80 ng), D-CPPene (3.0, 6.25, 12.5, 25, 50, 100 ng), and PPDA (300, 400 ng) increased sucrose intake significantly compared with control. Likewise, injections of Con G (10 ng), D-CPPene (5 ng, 10 ng), and PPDA (0.5, 1.0, 2.5, 5.0 ng) directly into the NTS significantly increased sucrose intake. These results show that hindbrain injection of competitive NMDA antagonists with selectivity or preference for the NMDA receptor NR2B or NR2C subunits increases food intake. satiation; vagal afferents; glutamate

Systemic injection of MK-801, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptor ion channels, increases meal size and delays satiation. We examined whether MK-801 increases food intake by directly interfering with actions of cholecystokinin (CCK). Prior administration of MK-801 (100 [micro]g/kg ip) reversed the inhibitory effects of CCK-8 (2 and 4 [micro]g/kg ip) on real feeding of both liquid and solid foods. MK-801 alone did not alter 30-min sham intake of 15% sucrose compared with intake after saline. Furthermore, while CCK-8 (2 or 4 [micro]g/kg ip) reduced sham intake, this reduction was not attenuated by MK-801 pretreatment. To ascertain whether MK-801 attenuation of CCK-induced reduction of real feeding was associated with attenuated inhibition of gastric emptying, we tested the effect of MK-801 pretreatment on CCK-induced inhibition of gastric emptying of 5-ml saline loads. Ten-minute gastric emptying was accelerated after MK-801 (3.9 [+ or -] 0.2 ml) compared with saline vehicle (2.72 [+ or -] 0.2 ml). CCK-8 (0.5 [micro]g/kg ip) reduced 10-min emptying to 1.36 [+ or -] 0.3 ml. Pretreatment with MK-801 did not significantly attenuate CCK-8-induced reduction of gastric emptying (0.9 [+ or -] 0.4 ml). This series of experiments demonstrates that blockade of NMDA ion channels reverses inhibition of real feeding by CCK. However, neither inhibition of sham feeding nor inhibition of gastric emptying by CCK is attenuated by MK-801. Therefore, increased food intake after NMDA receptor blockade is not caused by a direct interference with CCK-induced satiation. Rather, increased real feeding, either in the presence or absence of CCK, depends on blockade of NMDA receptor participation in other postoral feedback signals such as gastric sensation or gastric tone. sham feeding; gastric emptying; MK-801; N-methyl-D-aspartate

Patch-clamp electrophysiological methods were used on dissociated rat nodose neurons maintained in culture to determine whether responses to cholecystokinin (CCK) were associated with capsaicin-resistant (A type) or capsaicin-sensitive (C type) neurons. Nodose neurons were classified as A or C type on the basis of the characteristics of the [Na.sup.+] current, a hyperpolarization-activated current, and sensitivity to a low concentration of capsaicin to ascertain the presence of vanilloid receptor 1 that has been associated with C-type neurons in sensory ganglia. It was expected that only capsaicin-sensitive C-type neurons would respond to CCK, because most vagally mediated actions of CCK are blocked by capsaicin treatment. However, we found that subpopulations of both A- and C-type neurons responded to CCK (24 and 38%, respectively). Thus some vagally mediated actions of CCK may be mediated by capsaicin insensitive A-type neurons. nodose ganglia; [Na.sup.+] current; hyperpolarization-activated current; capsaicin

MK-801, a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, enhances gastric emptying while increasing food intake. Although our previously reported results implicate the vagus in MK-801's effect on feeding, it is not clear whether vagal motor fibers participate in the feeding response. Control of gastric emptying is exerted, in part, by cholinergic vagal motor neurons. Therefore, we examined the ability of MK-801 to increase meal size in the presence or absence of the muscarinic receptor antagonist atropine methyl nitrate. Both central and systemic administration of MK-801 significantly increased intake of 15% sucrose. Intraperitoneal injection of atropine abolished MK-801-induced increase in sucrose intake, whereas administration into the fourth ventricle had no effect. To determine whether augmentation of cholinergic tone produces an enhancement of food intake in the absence of MK-801, we tested the ability of cisapride, a gastric prokinetic agent that promotes acetylcholine release through an action on presynaptic serotonin (5-H[T.sub.4]) receptors, to increase sucrose consumption. Cisapride (500 [micro]g/kg ip) induced a small but significant increase in 15% sucrose intake (15.5 [+ or -] 0.5 ml) compared with NaCl (13.0 [+ or -] 0.6 ml). Furthermore, when MK-801 (100 [micro]g/kg ip) was given in combination with cisapride, intake was significantly higher (19.8 [+ or -] 0.9 ml) than following either agent given alone. Pretreatment with atropine abolished the cisapride-induced increase in intake (12.1 [+ or -] 0.9 ml) as well as the increased intake induced by combining MK-801 and cisapride. These results suggest that blockade of NMDA-gated ion channels in the hindbrain increases food intake, in part, via a peripheral muscarinic cholinergic mechanism. atropine; cisapride; gastric emptying; muscarinic blockade.

In the present study, we tested the hypothesis that a single daily injection of the gut peptide CCK, together with continuous leptin infusion, would produce significantly greater loss of body weight than leptin alone. We found that a single daily intraperitoneal injection of CCK-8 (0.5 [micro]g/kg) significantly enhanced the weight-reducing effects of 0.5 [micro]g/day leptin infused continuously into the lateral ventricle of male Sprague-Dawley rats by osmotic minipump. However, CCK and leptin together did not enhance reduction of daily chow intake. Furthermore, there was no synergistic reduction of 30-min sucrose intake, although a significant main effect of both leptin and CCK was observed on sucrose intake. These results 1) confirm our previous reports of synergy between leptin and CCK on body weight, 2) demonstrate that enhancement of leptin-induced weight loss does not require bolus administration of leptin, and 3) suggest that enhanced body weight loss following leptin and CCK does not require synergistic reduction of food intake by leptin and CCK. cholecystokinin; osmotic minipump

Digestive processes adapt to provide more efficient digestion and absorption of fats in rats maintained on high-fat diets. Also, rats fed high-fat diets consume more calories and gain more weight. It is hypothesized that in addition to adaptation of digestive processes, high-fat maintenance diets may result in reduction of sensitivity to the satiating effects of fat digestion products, which inhibit food intake by activating sensory fibers in the intestine. Research indicated that such diets reduce the ability of fat to inhibit further food intake.

Experimental evidence suggests that cholecystokinin (CCK) plays a role in the regulation of body weight when central leptin concentrations are increased. This was gleaned from a study of male Sprague-Dawley rats that were intraperitoneally injected with CCK two to three hours after intracerebroventricular leptin. CCK was found to have reduced body weight and food intake over the subsequent 48 hours more than did leptin alone. CCK alone did not affect either the 48-hour food intake or body weight but dramatically reduced feeding during a 30-minute sucrose test.

Research was conducted to identify enteric neurons activated during intestinal motor activity by examining myenteric plexus of guinea pig small intestinal segments for expression of the immediate early gene product, Fos. Its immunoreactivity was observed immunohistochemically following in vitro manipulations which included distention, exposure to forskolin, electrical stimulation and peristalsis. Results suggest that Fos expression can, with some caution, be utilized in the identification of classes of enteric neurons activated by different stimuli under various conditions.

Studies of the influence of capsaicin on intraintestinal oleate-induced reduction in sham feeding reveal that intestinal infusion of capsaicin reversibly attenuates the suppression of sham feeding by oleic acid. Intraintestinal oleate acts through capsaicin-sensitive vagal sensory neurons which act as a neural substrate for mediating the oleate suppression of sham-feeding. Capsaicin temporarily incapacitates the capsaicin-sensitive substrate but does not impair cholecystokinin-induced reduction of feeding or damage to the viscerosensory neurons.

Food intake is suppressed by bombesin (BBS) activity in the caudal hindbrain in rats. BBS infusion in the fourth cerebral ventricle causes caudal hindbrain participation in suppression food intake. Lesions to destroy area postrema (AP) and medial nucleus of the solitary tract (NST) enable analysis of AP or NST role in food intake suppression. AP and NST stop fourth-ventricular BBS-induced food intake suppression, while modulating the impact of peripheral BBS on food intake. AP alone does not affect peripheral BBS impact. In central and peripheral BBS-induced suppression of food intake, NST acts as a common neural substrate.

The participation of blood-borne cholecystokinin (CCK) in the suppression of sham feeding by intestiinal nutrients in rats was examined. Oleate, maltose, L-phenylalanine and casein were infused in rats that were not feeding. The resulting plasma CCK concentrations were measured. Results indicate that the suppression of sham feeding by intestinal nutrient infusion is not caused by the elevation of plasma CCK.

Diminished satiation in rats exposed to elevated levels of endogenous or exogenous cholecystokinin. Am J Physiol Regulatory Integrative Comp Physiol 280: R331-R337, 2001.--Rats maintained on a high-fat (HF) diet exhibit reduced sensitivity to the satiation-producing effect of exogenous CCK. Because more CCK is released in response to HF meals than low-fat (LF) meals, we hypothesized that increased circulating CCK associated with ingestion of HF diets contributes to the development of decreased CCK sensitivity. To test this hypothesis, we implanted osmotic minipumps filled with either NaCl or CCK octapeptide into the peritoneal cavity. Subsequently, we examined the effect of intraperitoneal NaCl or CCK (0.5 [micro]g/kg) injection on 30-min food intake. CCK significantly reduced 30-min food intake less in rats imp]anted with CCK-releasing minipumps compared with those with NaCl-releasing minipumps. Because dietary protein is a potent releaser of endogenous CCK, we hypothesized that rats adapted to a high-protein (HP) diet might also exhibit reduced sensitivity to exogenous CCK. Therefore, in a second experiment, we examined CCK-induced reduction of food intake in rats maintained on LF and rats maintained on HF or HP. Ingestion of LF stimulates very little endogenous CCK secretion, whereas both HF and HP markedly increase plasma CCK concentrations. Both doses of CCK reduced food intake significantly less in HF and HP rats compared with LF rats. There were no differences in 24-h food intake, body weight, or body fat composition among LF-, HF-, and HP-fed rats. These results are consistent with the hypothesis that sustained elevation of CCK either by infusion of exogenous CCK or by dietary-induced elevation of plasma CCK contributes to the development of reduced sensitivity to exogenous CCK. high-fat diet; protein diet; satiety; diet adaptation

NMDA receptor participation in control of food intake by the stomach. Am J Physiol Regulatory Integrative Comp Physiol 278: R1362-R1368, 2000.--We previously reported that MK-801 (dizocilpine), an antagonist of N-methyl-D-aspartate (NMDA)-type glutamate receptors, increased meal size and duration in rats. MK-801 did not increase sham feeding or attenuate reduction of sham feeding by intraintestinal nutrient infusions. These results suggested that the MK-801-induced increase in meal size did not depend on antagonism of postgastric satiety signals. Consequently, we hypothesized that the NMDA antagonist might increase food intake by directly antagonizing gastric mechanosensory signals or by accelerating gastric emptying, thereby reducing gastric mechanoreceptive feedback. To test this hypothesis, we recorded intake of 15% sucrose in rats implanted with pyloric cuffs that could be closed to prevent gastric emptying. Sucrose intake was increased when the pyloric cuffs were open, allowing the stomach to empty. However, intake was not increased when the pyloric cuffs were inflated, causing gastric retention of all ingested sucrose. Direct measurements of gastric emptying revealed that MK-801 accelerated the emptying of 5-ml loads of 0.9% NaCl and 15% sucrose. Furthermore, MK-801 also accelerated the rate of emptying of freely ingested sucrose regardless of the volume ingested. Taken together with our previous findings, these results indicate that blockade of NMDA receptors with MK-801 does not increase food intake by antagonizing gastric mechanosensation. Rather, it accelerates gastric emptying, and thereby may indirectly reduce gastric mechanoreceptive cues, resulting in prolongation of eating. Modulation of gastric emptying rate by NMDA receptors could play an important role in the control of meal sizes. gastric distension; pyloric cuff; satiety

Matson, Claire A., Dana F. Reid, Todd A. Cannon, and Robert C. Ritter. Cholecystokinin and leptin act synergistically to reduce body weight. Am J Physiol Regulatory Integrative Comp Physiol 278: R882-R890, 2000.--Leptin, the product of the obese gene, reduces food intake and body weight in rats and mice, whereas administration of the gut-peptide CCK reduces meal size but not body weight. In the current experiments, we report that repeated daily combination of intracerebroventricular leptin and intraperitoneal CCK results in significantly greater loss of body weight than does leptin alone. However, leptin plus CCK treatment does not synergistically reduce the size of individual 30-min sucrose meals during this period, and the effect of leptin-CCK combination on daily chow intake, while significant, is small compared with the robust effects on body weight loss. This synergistic effect on body weight loss depends on a peripheral action of CCK and a central action of leptin. These data suggest a previously unsuspected role for CCK in body weight regulation that may not depend entirely on reduction of feeding behavior and suggest a strategy for enhancing the effects of leptin in leptin-resistant obese individuals. satiety; obesity; meal size; food intake; adiposity

Covasa, Mihai, and Robert C. Ritter. Adaptation to high-fat diet reduces inhibition of gastric emptying by CCK and intestinal oleate. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 278: R166-R170, 2000.--Rats maintained on low-fat (LF) or high-fat (HF) diets were fitted with gastric cannulas and duodenal catheters. Intraperitoneal injection of 0.250-2.0 [micro]g/kg cholecystokinin (CCK) significantly inhibited gastric emptying of a 5-ml NaCl load in LF rats by 26.2-55.1% compared with emptying after vehicle injection. By contrast, CCK-induced inhibition of gastric emptying was significantly less in HF rats given the same CCK doses (10.0-31.7% inhibition over the same CCK dose range). A 20-min intraduodenal infusion of oleate (0.03 or 0.06 kcal/ml) also resulted in significant inhibition of gastric emptying in LF rats (24 and 89%, respectively). Oleate-induced inhibition of gastric emptying was significantly attenuated in rats maintained on the HF diet (2 and 56%, respectively). Unlike CCK injections or oleate infusion, intraduodenal maltotriose infusion inhibited gastric emptying to a similar degree in LF and HF rats (77 and 78%, respectively). These results indicate that feeding HF diets diminishes the enterogastric inhibition of gastric emptying by intestinal oleate and diminishes the ability of CCK to inhibit gastric emptying. cholecystokinin; maltotriose; dietary fat; intestinal adaptation

The influence of dietary protein source and heat processing on cholecystokinin (CCK) in cats is examined. The protein diets tested were purified soy protein vs. casein, and commercial heat-processed canned vs. a frozen uncooked canned diet. Plasma CCK was measured before and at timed intervals after a single meal. The results showed higher levels of CCK in cats fed with diets that cause taurine depletion.

Injection of the melanocortin-3/4 receptor agonist melanotan-II (MTII) into the nucleus of the solitary tract (NTS) produces rapid and sustained reduction of food intake. Melanocortin-4 receptors (MC4Rs) are expressed by vagal afferent endings in the NTS, but it is not known whether these endings participate in MTII-induced reduction of food intake. In experiments described here, we evaluated the contribution of central vagal afferent endings in MTII-induced reduction of food intake. Examination of rat hindbrain sections revealed that neuronal processes expressing immunoreactivity for the endogenous MC4R agonist -melanoctyte-stimulating hormone course parallel and wrap around anterogradely labeled vagal afferent endings in the NTS and thus are aptly positioned to activate vagal afferent MC4Rs. Furthermore, MTII and endogenous MC4R agonists increased protein kinase A (PKA)-catalyzed phosphorylation of synapsin I in vagal afferent endings, an effect known to increase synaptic strength by enhancing neurotransmitter release in other neural systems. Hindbrain injection of a PKA inhibitor, KT5720, significantly attenuated MTII-induced reduction of food intake and the increase in synapsin I phosphorylation. Finally, unilateral nodose ganglion removal, resulting in degeneration of vagal afferent endings in the ipsilateral NTS, abolished MTII-induced synapsin I phosphorylation ipsilateral to nodose ganglion removal. Moreover, reduction of food intake following MTII injection into the NTS ipsilateral to nodose ganglion removal was significantly attenuated, whereas the response to MTII was not diminished when injected into the contralateral NTS. Altogether, our results suggest that reduction of food intake following hindbrain MC4R activation is mediated by central vagal afferent endings. [ABSTRACT FROM AUTHOR]