Cholecystokinin activates catecholaminergic neurons in the caudal medulla that innervate the paraventricular nucleus of the hypothalamus in rats

CCK-sensitive C fibers activate NTS leptin receptor-expressing neurons via NMDA receptors

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.

Hindbrain ghrelin and liver-expressed antimicrobial peptide 2, ligands for growth hormone secretagogue receptor, bidirectionally control food intake

Hindbrain growth hormone secretagogue receptor (GHSR) agonism increases food intake, yet the underlying neural mechanisms remain unclear. The functional effects of hindbrain GHSR antagonism by its endogenous antagonist liver-expressed antimicrobial peptide 2 (LEAP2) are also yet unexplored. To test the hypothesis that hindbrain GHSR agonism attenuates the food intake inhibitory effect of gastrointestinal (GI) satiation signals, ghrelin (at a feeding subthreshold dose) was administered to the fourth ventricle (4V) or directly to the nucleus tractus solitarius (NTS) before systemic delivery of the GI satiation signal cholecystokinin (CCK). Also examined, was whether hindbrain GHSR agonism attenuated CCK-induced NTS neural activation (c-Fos immunofluorescence). To investigate an alternate hypothesis that hindbrain GHSR agonism enhances feeding motivation and food seeking, intake stimulatory ghrelin doses were administered to the 4V and fixed ratio 5 (FR-5), progressive ratio (PR), and operant reinstatement paradigms for palatable food responding were evaluated. Also assessed were 4V LEAP2 delivery on food intake and body weight (BW) and on ghrelin-stimulated feeding. Both 4V and NTS ghrelin blocked the intake inhibitory effect of CCK and 4V ghrelin blocked CCK-induced NTS neural activation. Although 4V ghrelin increased low-demand FR-5 responding, it did not increase high-demand PR or reinstatement of operant responding. Fourth ventricle LEAP2 reduced chow intake and BW and blocked hindbrain ghrelin-stimulated feeding. Data support a role for hindbrain GHSR in bidirectional control of food intake through mechanisms that include interacting with the NTS neural processing of GI satiation signals but not food motivation and food seeking.

Comparative insights into the integration mechanism of neuropeptides to starvation and temperature stress

Stress is known as the process of biological responses evoked by internal or external stimuli. The ability to sense, integrate and respond to stress signals is a requisite for life. Temperature and photoperiod are very important environmental factors for animals. In addition, stress signals can also be inputted from peripheral tissue, such as starvation and inflammation. Through afferent pathways, stress signals input to the central nervous system (CNS), where various signals will integrate, and the integrated information will transmit to the peripheral effectors. As the regulators of neural activity, neuropeptides play important roles in these processes. The present review summarizes recent findings about the integration mechanism of stress signals in the CNS, emphasizing on the role of neuropeptides.

The role of nucleus of the solitary tract glucagon-like peptide-1 and prolactin-releasing peptide neurons in stress: anatomy, physiology and cellular interactions

Neuroendocrine, behavioural and autonomic responses to stressful stimuli are orchestrated by complex neural circuits. The caudal nucleus of the solitary tract (cNTS) in the dorsomedial hindbrain is uniquely positioned to integrate signals of both interoceptive and psychogenic stress. Within the cNTS, glucagon‐like peptide‐1 (GLP‐1) and prolactin‐releasing peptide (PrRP) neurons play crucial roles in organising neural responses to a broad range of stressors. In this review we discuss the anatomical and functional overlap between PrRP and GLP‐1 neurons. We outline their co‐activation in response to stressful stimuli and their importance as mediators of behavioural and physiological stress responses. Finally, we review evidence that PrRP neurons are downstream of GLP‐1 neurons and outline unexplored areas of the research field. Based on the current state‐of‐knowledge, PrRP and GLP‐1 neurons may be compelling targets in the treatment of stress‐related disorders.

Effects of age on feeding response: Focus on the rostral C1 neuron and its glucoregulatory proteins

BACKGROUND

Older people are likely to develop anorexia of aging. Rostral C1 (rC1) catecholaminergic neurons in rostral ventrolateral medulla (RVLM) are recently discovered its role in food intake control. It is well established that these neurons regulate cardiovascular function.

OBJECTIVE

This study aims to determine the effect of age on the function of rostral C1 (rC1) neurons in mediating feeding response.

METHOD

Male Sprague Dawley rats at 3-months (n = 22) and 24-months (n = 22) old were used and further divided into two subgroups; 1) treatment group with 2-deoxy-d-glucose (2DG) and 2) vehicle group. Feeding hormones such as cholecystokinin (CCK), ghrelin and leptin were analysed using enzyme-linked immunosorbent assay (ELISA). Rat brain was carefully dissected to obtain the brainstem RVLM region. Further analysis was carried out to determine the level of proteins and genes in RVLM that were associated with feeding pathway. Protein expression of tyrosine hydroxylase (TH), phosphorylated TH at Serine40 (pSer40TH), AMP-activated protein kinase (AMPK), phosphorylated AMPK (phospho AMPK) and neuropeptide Y Y5 receptor (NPY5R) were determined by western blot. Expression of TH, AMPK and NPY genes were determined by real-time PCR.

RESULTS

This study showed that blood glucose level was elevated in young and old rats following 2DG administration. Plasma CCK-8 concentration was higher in the aged rats at basal and increased with 2DG administration in young rats, but the leptin and ghrelin showed no changes. Old rats showed higher TH and lower AMPK mRNA levels. Glucoprivation decreased AMPK mRNA level in young rats and decreased TH mRNA in old rats. Aged rC1 neurons showed higher NPY5R protein level. Following glucoprivation, rC1 neurons produced distinct molecular changes across age in which, in young rats, AMPK phosphorylation level was increased and in old rats, TH phosphorylation level was increased.

CONCLUSION

These findings suggest that glucose-counterregulatory responses by rC1 neurons at least, contribute to the ability of young and old rats in coping glucoprivation. Age-induced molecular changes within rC1 neurons may attenuate the glucoprivic responses. This situation may explain the impairment of feeding response in the elderly.

High Fat Diet Attenuates Cholecystokinin-Induced cFos Activation of Prolactin-Releasing Peptide-Expressing A2 Noradrenergic Neurons in the Caudal Nucleus of the Solitary Tract

Cholecystokinin (CCK) released from the small intestine increases the activity of vagal afferents that relay satiety signals to the caudal nucleus of the solitary tract (cNTS). A caudal subset of A2 noradrenergic neurons within the cNTS that express prolactin-releasing peptide (PrRP) have been proposed to mediate CCK-induced satiety. However, the ability of exogenous CCK to activate cFos expression by PrRP neurons has only been reported in rats and mice after a very high dose (i.e., 50 μg/kg BW) that also activates the hypothalamic-pituitary-adrenal stress axis. The present study examined the ability of a much lower CCK dose (1.0 µg/kg BW, i.p) to activate PrRP-positive neurons in the rat cNTS. We further examined whether maintenance of rats on high fat diet (HFD; 45% kcal from fat) alters CCK-induced activation of PrRP neurons, since HFD blunts the ability of CCK to suppress food intake. Rats maintained on HFD for 7 weeks consumed more kcal and gained more BW compared to rats maintained on Purina chow (13.5% kcal from fat). CCK-treated rats displayed increased numbers of cFos-positive cNTS neurons compared to non-injected and saline-injected controls, with no effect of diet. In chow-fed rats, a significantly larger proportion of PrRP neurons were activated after CCK treatment compared to controls; conversely, CCK did not increase PrRP neuronal activation in HFD-fed rats. Collectively, these results indicate that a relatively low dose of exogenous CCK is sufficient to activate PrRP neurons in chow-fed rats, and that this effect is blunted in rats maintained for several weeks on HFD.

Hindbrain glucoregulatory mechanisms: Critical role of catecholamine neurons in the ventrolateral medulla

Glucose is the required metabolic substrate for the brain. Yet the brain stores very little glucose. Therefore, the brain continuously monitors glucose availability to detect hypoglycemia and to mobilize system-wide responses to protect and restore euglycemia. Catecholamine (CA) neurons in the hindbrain are critical elements of the brain's glucoregulatory mechanisms. They project widely throughout the brain and spinal cord, innervating sites controlling behavioral, endocrine and visceral responses. Hence, CA neurons are capable of triggering a rapid, coordinated and multifaceted response to glucose challenge. This article reviews experimental data that has begun to elucidate the importance of CA neurons for glucoregulation, the functions of specific CA subpopulations in the ventrolateral medulla, and the extended circuitry through which they engage other levels of the nervous system to accomplish their essential glucoregulatory task. Hopefully, this review also suggests the vast amount of work yet to be done in this area and the justification for engaging in that effort.

Individual Differences in Behavioral Responses to Palatable Food or to Cholecystokinin Predict Subsequent Diet-Induced Obesity

OBJECTIVE

This study investigated whether individual differences in behavioral responses to palatable food and to the satiation signal cholecystokinin (CCK) in outbred chow-maintained Sprague-Dawley rats enabled prediction of individual differences in weight gained after subsequent high-fat/high-sugar diet (HFHSD) maintenance.

METHODS

Meal size, meal number, and early dark cycle intake during initial HFHSD exposure were measured, as were early dark cycle sucrose solution and chow intake, chow meal size and meal number, the intake-suppressive effects of 0.5-µg/kg CCK injection, and CCK-induced c-Fos activation in the nucleus tractus solitarius. Subsequently, rats were maintained on an HFHSD for 5 weeks, and weight gain was determined.

RESULTS

Rats that took larger and less frequent meals on the first day of HFHSD exposure, whose early dark cycle intake (HFHSD and sucrose) was larger during initial HFHSD exposure, gained more weight after HFHSD maintenance. Rats with lesser sucrose intake suppression in response to CCK gained more weight after HFHSD maintenance and displayed reduced CCK-induced c-Fos activation in the nucleus tractus solitarius.

CONCLUSIONS

Together, these data identify individual differences in behavioral responses to palatable food and to CCK as novel predictors of diet-induced obesity.

Activation of catecholamine neurons in the ventral medulla reduces CCK-induced hypophagia and c-Fos activation in dorsal medullary catecholamine neurons

Catecholamine (CA) neurons within the A1 and C1 cell groups in the ventrolateral medulla (VLM) potently increase food intake when activated by glucose deficit. In contrast, CA neurons in the A2 cell group of the dorsomedial medulla are required for reduction of food intake by cholecystokinin (CCK), a peptide that promotes satiation. Thus dorsal and ventral medullary CA neurons are activated by divergent metabolic conditions and mediate opposing behavioral responses. Acute glucose deficit is a life-threatening condition, and increased feeding is a key response that facilitates survival of this emergency. Thus, during glucose deficit, responses to satiation signals, like CCK, must be suppressed to ensure glucorestoration. Here we test the hypothesis that activation of VLM CA neurons inhibits dorsomedial CA neurons that participate in satiation. We found that glucose deficit produced by the antiglycolytic glucose analog, 2-deoxy-d-glucose, attenuated reduction of food intake by CCK. Moreover, glucose deficit increased c-Fos expression by A1 and C1 neurons while reducing CCK-induced c-Fos expression in A2 neurons. We also selectively activated A1/C1 neurons in TH-Cre⁺ transgenic rats in which A1/C1 neurons were transfected with a Cre-dependent designer receptor exclusively activated by a designer drug (DREADD). Selective activation of A1/C1 neurons using the DREADD agonist, clozapine- N-oxide, attenuated reduction of food intake by CCK and prevented CCK-induced c-Fos expression in A2 CA neurons, even under normoglycemic conditions. Results support the hypothesis that activation of ventral CA neurons attenuates satiety by inhibiting dorsal medullary A2 CA neurons. This mechanism may ensure that satiation does not terminate feeding before restoration of normoglycemia.

Central regulation of energy metabolism by estrogens

BACKGROUND

Estrogenic actions in the brain prevent obesity. Better understanding of the underlying mechanisms may facilitate development of new obesity therapies.

SCOPE OF REVIEW

This review focuses on the critical brain regions that mediate effects of estrogens on food intake and/or energy expenditure, the molecular signals that are involved, and the functional interactions between brain estrogens and other signals modulating metabolism. Body weight regulation by estrogens in male brains will also be discussed.

MAJOR CONCLUSIONS

17β-estradiol acts in the brain to regulate energy homeostasis in both sexes. It can inhibit feeding and stimulate brown adipose tissue thermogenesis. A better understanding of the central actions of 17β-estradiol on energy balance would provide new insight for the development of therapies against obesity in both sexes.

A Predictive, Quantitative Model of Spiking Activity and Stimulus-Secretion Coupling in Oxytocin Neurons

Oxytocin neurons of the rat hypothalamus project to the posterior pituitary, where they secrete their products into the bloodstream. The pattern and quantity of that release depends on the afferent inputs to the neurons, on their intrinsic membrane properties, and on nonlinear interactions between spiking activity and exocytosis: A given number of spikes will trigger more secretion when they arrive close together. Here we present a quantitative computational model of oxytocin neurons that can replicate the results of a wide variety of published experiments. The spiking model mimics electrophysiological data of oxytocin cells responding to cholecystokinin (CCK), a peptide produced in the gut after food intake. The secretion model matches results from in vitro experiments on stimulus-secretion coupling in the posterior pituitary. We mimic the plasma clearance of oxytocin with a two-compartment model, replicating the dynamics observed experimentally after infusion and injection of oxytocin. Combining these models allows us to infer, from measurements of oxytocin in plasma, the spiking activity of the oxytocin neurons that produced that secretion. We have tested these inferences with experimental data on oxytocin secretion and spiking activity in response to intravenous injections of CCK. We show how intrinsic mechanisms of the oxytocin neurons determine this relationship: In particular, we show that the presence of an afterhyperpolarization (AHP) in oxytocin neurons dramatically reduces the variability of their spiking activity and even more markedly reduces the variability of oxytocin secretion. The AHP thus acts as a filter, protecting the final product of oxytocin cells from noisy fluctuations.

Role of the area postrema in the hypophagic effects of oleoylethanolamide

The satiety-promoting action of oleoylethanolamide (OEA) has been associated to the indirect activation of selected brain areas, such as the nucleus of the solitary tract (NST) in the brainstem and the tuberomammillary (TMN) and paraventricular (PVN) nuclei in the hypothalamus, where noradrenergic, histaminergic and oxytocinergic neurons play a necessary role. Visceral ascending fibers were hypothesized to mediate such effects. However, our previous findings demonstrated that the hypophagic action of peripherally administered OEA does not require intact vagal afferents and is associated to a strong activation of the area postrema (AP). Therefore, we hypothesized that OEA may exert its central effects through the direct activation of this circumventricular organ. To test this hypothesis, we subjected rats to the surgical ablation of the AP (APX rats) and evaluated the effects of OEA (10mgkg^-1 i.p.) on food intake, Fos expression, hypothalamic oxytocin (OXY) immunoreactivity and on the expression of dopamine beta hydroxylase (DBH) in the brainstem and hypothalamus. We found that the AP lesion completely prevented OEA's behavioral and neurochemical effects in the brainstem and the hypothalamus. Moreover OEA increased DBH expression in AP and NST neurons of SHAM rats while the effect in the NST was absent in APX rats, thus suggesting the possible involvement of noradrenergic AP neurons. These results support the hypothesis of a necessary role of the AP in mediating OEA's central effects that sustain its pro-satiety action.

Anthony Sclafani: Consummate scientist

In this article we review the scientific contributions of Anthony Sclafani, with specific emphasis on his early work on the neural substrate of the ventromedial hypothalamic (VMH) hyperphagia-obesity syndrome, and on the development of diet-induced obesity (DIO). Over a period of 20 years Sclafani systematically investigated the neuroanatomical basis of the VMH hyperphagia-obesity syndrome, and ultimately identified a longitudinal oxytocin-containing neural tract contributing to its expression. This tract has since been implicated in mediating the effects of at least two gastrointestinal satiety factors. Sclafani was one of the first investigators to demonstrate DIO in rats as a result of exposure to multiple palatable food items (the "supermarket diet"), and concluded that diet palatability was the primary factor responsible for DIO. Sclafani went on to investigate the potency of specific carbohydrate and fat stimuli for inducing hyperphagia, and in so doing discovered that post-ingestive nutrient effects contribute to the elevated intake of palatable food items. To further investigate this effect, he devised an intragastric infusion system which allowed the introduction of nutrients into the gut paired with the oral intake of flavored solutions, an apparatus her termed the "electronic esophagus". Sclafani coined the term "appetition" to describe the effect of intestinal nutrient sensing on post-ingestive appetite stimulation. Sclafani's productivity in the research areas he chose to investigate has been nothing short of extraordinary, and his studies are characterized by inventive hypothesizing and meticulous experimental design. His results and conclusions, to our knowledge, have never been contradicted.

Dorsomedial hypothalamic NPY affects cholecystokinin-induced satiety via modulation of brain stem catecholamine neuronal signaling

Increased neuropeptide Y (NPY) gene expression in the dorsomedial hypothalamus (DMH) has been shown to cause hyperphagia, but the pathway underlying this effect remains less clear. Hypothalamic neural systems play a key role in the control of food intake, in part, by modulating the effects of meal-related signals, such as cholecystokinin (CCK). An increase in DMH NPY gene expression decreases CCK-induced satiety. Since activation of catecholaminergic neurons within the nucleus of solitary tract (NTS) contributes to the feeding effects of CCK, we hypothesized that DMH NPY modulates NTS neural catecholaminergic signaling to affect food intake. We used an adeno-associated virus system to manipulate DMH NPY gene expression in rats to examine this pathway. Viral-mediated hrGFP anterograde tracing revealed that DMH NPY neurons project to the NTS; the projections were in close proximity to catecholaminergic neurons, and some contained NPY. Viral-mediated DMH NPY overexpression resulted in an increase in NPY content in the NTS, a decrease in NTS tyrosine hydroxylase (TH) expression, and reduced exogenous CCK-induced satiety. Knockdown of DMH NPY produced the opposite effects. Direct NPY administration into the fourth ventricle of intact rats limited CCK-induced satiety and overall TH phosphorylation. Taken together, these results demonstrate that DMH NPY descending signals affect CCK-induced satiety, at least in part, via modulation of NTS catecholaminergic neuronal signaling.

Stress-activated afferent inputs into the anterior parvicellular part of the paraventricular nucleus of the hypothalamus: Insights into urocortin 3 neuron activation

Urocortin 3 (Ucn 3) is a member of the corticotropin-releasing factor family, which plays a major role in coordinating stress responses. Ucn 3 neurons in the anterior parvicellular part of the paraventricular nucleus of the hypothalamus (PVHap) provide prominent input into the ventromedial nucleus of the hypothalamus (VMH), a well known satiety center, where Ucn 3 acts to suppress feeding and modulate blood glucose levels. In the present study, we first determined that Ucn 3 expression in the PVHap was stimulated by acute restraint stress. We then performed retrograde tracing with fluorogold (FG) combined with immunohistochemistry for Fos as a marker for neuronal activation after restraint stress to determine the stress-activated afferent inputs into the PVHap. Substantial numbers of FG/Fos double labeled cells were found in the bed nucleus of the stria terminalis, the lateral septal nucleus, the medial amygdala, and a number of nuclei in the hypothalamus including the VMH, the arcuate nucleus, the posterior nucleus, and the ventral premammillary nucleus. In the brainstem, FG/Fos positive cells were found in the periaqueductal gray, the nucleus of the solitary tract, and the ventrolateral medulla. In conclusion, the present study showed that acute stress rapidly stimulates Ucn 3 expression in the PVHap and identified specific stress-sensitive brain areas that project to the PVHap. These areas are potentially important in mediating the stress-induced activation of Ucn 3 neurons in the PVHap.

Peripheral injected cholecystokinin-8S modulates the concentration of serotonin in nerve fibers of the rat brainstem

Serotonin and cholecystokinin (CCK) play a role in the short-term inhibition of food intake. It is known that peripheral injection of CCK increases c-Fos-immunoreactivity (Fos-IR) in the nucleus of the solitary tract (NTS) in rats, and injection of the serotonin antagonist ondansetron decreases the number of c-Fos-IR cells in the NTS. This supports the idea of serotonin contributing to the effects of CCK. The aim of the present study was to elucidate whether peripherally injected CCK-8S modulates the concentration of serotonin in brain feeding-regulatory nuclei. Ad libitum fed male Sprague-Dawley rats received 5.2 and 8.7 nmol/kg CCK-8S (n=3/group) or 0.15M NaCl (n=3-5/group) injected intraperitoneally (ip). The number of c-Fos-IR neurons, and the fluorescence intensity of serotonin in nerve fibers were assessed in the paraventricular nucleus (PVN), arcuate nucleus (ARC), NTS and dorsal motor nucleus of the vagus (DMV). CCK-8S increased the number of c-Fos-ir neurons in the NTS (mean±SEM: 72±4, and 112±5 neurons/section, respectively) compared to vehicle-treated rats (7±2 neurons/section, P<0.05), but did not modulate c-Fos expression in the DMV or ARC. Additionally, CCK-8S dose-dependently increased the number of c-Fos-positive neurons in the PVN (218±15 and 128±14, respectively vs. 19±5, P<0.05). In the NTS and DMV we observed a decrease of serotonin-immunoreactivity 90 min after injection of CCK-8S (46±2 and 49±8 pixel/section, respectively) compared to vehicle (81±8 pixel/section, P<0.05). No changes of serotonin-immunoreactivity were observed in the PVN and ARC. Our results suggest that serotonin is involved in the mediation of CCK-8's effects in the brainstem.

Hindbrain oxytocin receptors contribute to the effects of circulating oxytocin on food intake in male rats

Oxytocin (OT)-elicited hypophagia has been linked to neural activity in the nucleus of the solitary tract (NTS). Because plasma OT levels increase after a meal, we hypothesized that circulating OT acts at both peripheral and hindbrain OT receptors (OTRs) to limit food intake. To initially determine whether circulating OT inhibits food intake by acting at hindbrain OTRs, we pretreated rats with an OTR antagonist administered into the fourth ventricle (4V) followed by either central or systemic OT administration. Administration of the OTR antagonist into the 4V blocked anorexia induced by either 4V or i.p. injection of OT. However, blockade of peripheral OTRs also weakened the anorectic response to ip OT. Our data suggest a predominant role for hindbrain OTRs in the hypophagic response to peripheral OT administration. To elucidate central mechanisms of OT hypophagia, we tested whether OT activates NTS catecholaminergic neurons. OT (ip) increased the number of NTS cells expressing c-Fos, of which 10%-15% were catecholaminergic. Furthermore, electrophysiological studies in mice revealed that OT stimulated 47% (8 of 17) of NTS catecholamine neurons through a presynaptic mechanism. However, OT-elicited hypophagia did not appear to require activation of α1-adrenoceptors, and blockade of glucagon-like peptide-1 receptors similarly did not attenuate anorexia induced by OT. These findings demonstrate that OT elicits satiety through both central and peripheral OTRs and that although catecholamine neurons are a downstream target of OT signaling in the NTS, the hypophagic effect is mediated independently of α1-adrenoceptor signaling.

The anorectic actions of the TGFβ cytokine MIC-1/GDF15 require an intact brainstem area postrema and nucleus of the solitary tract

Macrophage inhibitory cytokine-1 (MIC-1/GDF15) modulates food intake and body weight under physiological and pathological conditions by acting on the hypothalamus and brainstem. When overexpressed in disease, such as in advanced cancer, elevated serum MIC-1/GDF15 levels lead to an anorexia/cachexia syndrome. To gain a better understanding of its actions in the brainstem we studied MIC-1/GDF15 induced neuronal activation identified by induction of Fos protein. Intraperitoneal injection of human MIC-1/GDF15 in mice activated brainstem neurons in the area postrema (AP) and the medial (m) portion of the nucleus of the solitary tract (NTS), which did not stain with tyrosine hydroxylase (TH). To determine the importance of these brainstem nuclei in the anorexigenic effect of MIC-1/GDF15, we ablated the AP alone or the AP and the NTS. The latter combined lesion completely reversed the anorexigenic effects of MIC-1/GDF15. Altogether, this study identified neurons in the AP and/or NTS, as being critical for the regulation of food intake and body weight by MIC-1/GDF15.

Differential activation of chemically identified neurons in the caudal nucleus of the solitary tract in non-entrained rats after intake of satiating vs. non-satiating meals

Satiety signals arising from the gastrointestinal (GI) tract and related digestive organs during food ingestion and digestion are conveyed by vagal sensory afferents to the hindbrain nucleus of the solitary tract (NST). Two intermingled but chemically distinct NST neuronal populations have been implicated in meal size control: noradrenergic (NA) neurons that comprise the A2 cell group, and glucagon-like peptide-1 (GLP-1)-positive neurons. Previous results indicate that A2 neurons are activated in a meal size-dependent manner in rats that have been acclimated/entrained to a feeding schedule in order to increase meal size, whereas feeding under the same conditions does not activate GLP-1 neurons. The present study was designed to test the hypothesis that both A2 and GLP-1 neuronal populations are recruited in non-entrained rats after voluntary first-time intake of an unrestricted, satiating volume of liquid Ensure. DBH-positive A2 neurons within the caudal visceral NST were progressively recruited to express cFos in rats that consumed progressively larger volumes of Ensure. Among these DBH-positive neurons, the prolactin-releasing peptide (PrRP)-positive subset was more sensitive to feeding-induced activation than the PrRP-negative subset. Notably, significant activation of GLP-1-positive neurons occurred only in rats that consumed the largest volumes of Ensure, corresponding to nearly 5% of their BW. We interpret these results as evidence that progressive recruitment of NA neurons within the caudal NST, especially the most caudally-situated PrRP-positive subset, effectively "tracks" the magnitude of GI satiety signals and other meal-related sensory feedback. Conversely, GLP-1 neurons may only be recruited in response to the homeostatic challenge of consuming a very large, unanticipated meal.

Hindbrain noradrenergic input to the hypothalamic PVN mediates the activation of oxytocinergic neurons induced by the satiety factor oleoylethanolamide

Oleoylethanolamide (OEA) is a gut-derived endogenous lipid that stimulates vagal fibers to induce satiety. Our previous work has shown that peripherally administered OEA activates c-fos transcription in the nucleus of the solitary tract (NST) and in the paraventricular nucleus (PVN), where it enhances oxytocin (OXY) expression. The anorexigenic action of OEA is prevented by the intracerebroventricular administration of a selective OXY receptor antagonist, suggesting a necessary role of OXYergic mediation of OEA's effect. The NST is the source of direct noradrenergic afferent input to hypothalamic OXY neurons, and therefore, we hypothesized that the activation of this pathway might mediate OEA effects on PVN neurons. To test this hypothesis, we subjected rats to intra-PVN administration of the toxin saporin (DSAP) conjugated to an antibody against dopamine-β-hydroxylase (DBH) to destroy hindbrain noradrenergic neurons. In these rats we evaluated the effects of OEA (10 mg/kg, ip) on feeding behavior, on c-Fos and OXY immunoreactivity in the PVN, and on OXY immunoreactivity in the posterior pituitary gland. We found that the DSAP lesion completely prevented OEA's effects on food intake, on Fos and OXY expression in the PVN, and on OXY immunoreactivity of the posterior pituitary gland; all effects were maintained in sham-operated rats. These results support the hypothesis that noradrenergic NST-PVN projections are involved in the activation of the hypothalamic OXY system, which mediates OEA's prosatiety action.

Oxytocin projections to the nucleus of the solitary tract contribute to the increased meal-related satiety responses in primary adrenal insufficiency

Anorexia is a common clinical manifestation of primary adrenal gland failure. Adrenalectomy (ADX)-induced hypophagia is reversed by oxytocin (OT) receptor antagonist and is associated with increased activation of satiety-related responses in the nucleus of the solitary tract (NTS). This study evaluated OT projections from the paraventricular nucleus of the hypothalamus (PVN) to the NTS after ADX and the effect of pretreatment with intracerebroventricular injection of an OT receptor antagonist ([d(CH2)5,Tyr(Me)(2),Orn(8)]-vasotocin; OVT) on the activation of NTS neurons induced by feeding in adrenalectomized rats. Adrenalectomized animals showed higher OT labelling in the NTS than the sham and the ADX with corticosterone replacement (ADX + B) groups. Adrenalectomized animals exhibited co-localization of the anterograde tracer Phaseolus vulgaris leucoagglutinin and OT in axons in the NTS as well as OT fibres apposing NTS neurons activated by refeeding. After vehicle pretreatment, compared with fasting, refeeding increased the numbers of Fos- and Fos + TH-immunoreactive neurons in the NTS in sham, ADX and ADX + B groups, with a higher number of these immunolabelled neurons in adrenalectomized animals. Compared with fasting conditions, refeeding also increased the activation of NTS neurons in OVT-pretreated sham, ADX and ADX + B groups, but there was no difference among the three experimental groups. These data demonstrate that OT is upregulated in projections to the NTS following ADX and that OT receptor antagonist reverses the greater activation of NTS neurons induced by feeding after ADX. The data indicate that OT pathways to the NTS contribute to higher satiety-related responses and, thus, to reduce meal size in primary adrenal insufficiency.

Overnight food deprivation markedly attenuates hindbrain noradrenergic, glucagon-like peptide-1, and hypothalamic neural responses to exogenous cholecystokinin in male rats

Systemic administration of sulfated cholecystokinin-8 (CCK) activates neurons within the hindbrain nucleus of the solitary tract (NTS) that project directly to the paraventricular nucleus of the hypothalamus (PVN), and these projections underlie the ability of exogenous CCK to activate the hypothalamic-pituitary-adrenal (HPA) stress axis. CCK inhibits food intake, increases NTS neuronal cFos expression, and activates the HPA axis in a dose-dependent manner. While the hypophagic effects of exogenous CCK are attenuated in food-deprived rats, CCK dose-response relationships for NTS and hypothalamic activation in fed and fasted rats are unknown. Within the NTS, noradrenergic A2 and glucagon-like peptide-1 (GLP-1) neurons express cFos after high doses of CCK, and both neuronal populations project directly to the medial parvocellular (mp)PVN. We hypothesized that increasing and correlated proportions of A2, GLP-1, and mpPVN neurons would express cFos in rats after increasing doses of CCK, and that food deprivation would attenuate both hindbrain and hypothalamic neural activation. To test these hypotheses, ad libitum-fed (ad lib) and overnight food-deprived (DEP) rats were anesthetized and perfused with fixative 90min after i.p. injection of 1.0ml saline vehicle containing CCK at doses of 0, 3, or 10μg/kg BW. Additional ad lib and DEP rats served as non-handled (NH) controls. Brain tissue sections were processed for dual immunocytochemical localization of cFos and dopamine-β-hydroxylase to identify A2 neurons, or cFos and GLP-1. Compared to negligible A2 cFos activation in NH control rats, i.p. vehicle and CCK dose-dependently increased A2 activation, and this was significantly attenuated by DEP. DEP also attenuated mpPVN cFos expression across all treatment groups, and A2 activation was strongly correlated with mpPVN activation in both ad lib and DEP rats. In ad lib rats, large and similar numbers of GLP-1 neurons expressed cFos across all i.p. treatment groups, regardless of CCK dose. Surprisingly, DEP nearly abolished baseline GLP-1 cFos expression in NH controls, and also in rats after i.p. injection of vehicle or CCK. We conclude that CCK-induced hypothalamic cFos activation is strongly associated with A2 activation, whereas the relationship between mpPVN and GLP-1 activation is less clear. Furthermore, activation of A2, GLP-1, and mpPVN neurons is significantly modulated by feeding status, suggesting a mechanism through which food intake and metabolic state might impact hypothalamic neuroendocrine responses to homeostatic challenge.

The interaction of amylin with other hormones in the control of eating

Twenty years of research established amylin as an important control of energy homeostasis. Amylin controls nutrient and energy fluxes by reducing energy intake, by modulating nutrient utilization via an inhibition of postprandial glucagon secretion and by increasing energy disposal via a prevention of compensatory decreases of energy expenditure in weight reduced individuals. Like many other gastrointestinal hormones, amylin is secreted in response to meals and it reduces eating by promoting meal-ending satiation. Not surprisingly, amylin interacts with many of these hormones to control eating. These interactions seem to occur at different levels because amylin seems to mediate the eating inhibitory effect of some of these gastrointestinal hormones, and the combination of some of these hormones seems to lead to a stronger reduction in eating than single hormones alone. Amylin's effect on eating is thought to be mediated by a stimulation of specific amylin receptors in the area postrema. Secondary brain sites that were defined to mediate amylin action - and hence potential additional sites of interaction with other hormones - include the nucleus of the solitary tract, the lateral parabrachial nucleus, the lateral hypothalamic area and other hypothalamic nuclei. The focus of this review is to summarize the current knowledge of amylin interactions in the control of eating. In most cases, these interactions have only been studied at a descriptive rather than a mechanistic level and despite the clear knowledge on primary sites of amylin action, the interaction sites between amylin and other hormones are often unknown.

Satiation and stress-induced hypophagia: examining the role of hindbrain neurons expressing prolactin-releasing Peptide or glucagon-like Peptide 1

Neural circuits distributed within the brainstem, hypothalamus, and limbic forebrain interact to control food intake and energy balance under normal day-to-day conditions, and in response to stressful conditions under which homeostasis is threatened. Experimental studies using rats and mice have generated a voluminous literature regarding the functional organization of circuits that inhibit food intake in response to satiety signals, and in response to stress. Although the central neural bases of satiation and stress-induced hypophagia often are studied and discussed as if they were distinct, we propose that both behavioral states are generated, at least in part, by recruitment of two separate but intermingled groups of caudal hindbrain neurons. One group comprises a subpopulation of noradrenergic (NA) neurons within the caudal nucleus of the solitary tract (cNST; A2 cell group) that is immunopositive for prolactin-releasing peptide (PrRP). The second group comprises non-adrenergic neurons within the cNST and nearby reticular formation that synthesize glucagon-like peptide 1 (GLP-1). Axonal projections from PrRP and GLP-1 neurons target distributed brainstem and forebrain regions that shape behavioral, autonomic, and endocrine responses to actual or anticipated homeostatic challenge, including the challenge of food intake. Evidence reviewed in this article supports the view that hindbrain PrRP and GLP-1 neurons contribute importantly to satiation and stress-induced hypophagia by modulating the activity of caudal brainstem circuits that control food intake. Hindbrain PrRP and GLP-1 neurons also engage hypothalamic and limbic forebrain networks that drive parallel behavioral and endocrine functions related to food intake and homeostatic challenge, and modulate conditioned and motivational aspects of food intake.

Brainstem origins of glutamatergic innervation of the rat hypothalamic paraventricular nucleus

Multiple lines of evidence document a role for glutamatergic input to the hypothalamic paraventricular nucleus (PVH) in stress-induced activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. However, the neuroanatomical origins of the glutamatergic input have yet to be definitively determined. We have previously shown that vesicular glutamate transporter 2 (VGLUT2) is the predominant VGLUT isoform expressed in the basal forebrain and brainstem, including PVH-projecting regions, and that the PVH is preferentially innervated by VGLUT2-immunoreactive terminals/boutons. The present study employed a dual-labeling approach, combining immunolabeling for a retrograde tract tracer, Fluoro-Gold (FG), with in situ hybridization for VGLUT2 mRNA, to map the brainstem and caudal forebrain distribution of glutamatergic PVH-projecting neurons. The present report presents evidence for substantial dual labeling in the periaqueductal gray, caudal portions of the zona incerta and subparafascicular nucleus, and the lateral parabrachial nucleus. The current data also suggest that relatively few PVH-projecting neurons in ascending raphe nuclei, nucleus of the solitary tract, or ventrolateral medulla are VGLUT2 positive. The data reveal multiple brainstem origins of glutamatergic input to PVH that are positioned to play a role in transducing a diverse range of stressful stimuli.

Cervical stimulation activates A1 and locus coeruleus neurons that project to the paraventricular nucleus of the hypothalamus

In female rats, stimulation of the uterine cervix during mating induces two daily surges of prolactin. Inhibition of hypothalamic dopamine release and stimulation of oxytocin neurons in the paraventricular nucleus (PVN) are required for prolactin secretion. We aim to better understand how stimulation of the uterine cervix is translated into two daily prolactin surges. We hypothesize that noradrenergic neurons in the A1, A2, and locus coeruleus (LC) are responsible for conveying the peripheral stimulus to the PVN. In order to determine whether projections from these neurons to the PVN are activated by cervical stimulation (CS), we injected a retrograde tracer, Fluoro-Gold (FG), into the PVN of ovariectomized rats. Fourteen days after injection, animals were submitted to artificial CS or handling and perfused with a fixative solution. Brains were removed and sectioned from the A1, A2, and LC for c-Fos, tyrosine hydroxylase (TH), and FG triple-labeling using immunohistochemistry. CS increased the percentage of TH/FG+ double-labeled neurons expressing c-Fos in the A1 and LC. CS also increased the percentage of TH+ neurons expressing c-Fos within the A1 and A2, independent of their projections to the PVN. Our data reinforce the significant contributions of the A1 and A2 to carry sensory information during mating, and provide evidence of a functional pathway in which CS activates A1 and LC neurons projecting to the PVN, which is potentially involved in the translation of CS into two daily prolactin surges.

Control of energy homeostasis by amylin

Amylin is an important control of nutrient fluxes because it reduces energy intake, modulates nutrient utilization by inhibiting postprandial glucagon secretion, and increases energy disposal by preventing compensatory decreases of energy expenditure in weight-reduced individuals. The best investigated function of amylin which is cosecreted with insulin is to reduce eating by promoting meal-ending satiation. This effect is thought to be mediated by a stimulation of specific amylin receptors in the area postrema. Secondary brain sites to mediate amylin action include the nucleus of the solitary tract and the lateral parabrachial nucleus, which convey the neural signal to the lateral hypothalamic area and other hypothalamic nuclei. Amylin may also signal adiposity because plasma levels of amylin are increased in adiposity and because higher amylin concentrations in the brain result in reduced body weight gain and adiposity, while amylin receptor antagonists increase body adiposity. The central mechanisms involved in amylin's effect on energy expenditure are much less known. A series of recent experiments in animals and humans indicate that amylin is a promising option for anti-obesity therapy especially in combination with other hormones. The most extensive dataset is available for the combination therapy of amylin and leptin. Ongoing research focuses on the mechanisms of these interactions.

CCK stimulation of GLP-1 neurons involves α1-adrenoceptor-mediated increase in glutamatergic synaptic inputs

OBJECTIVE

Glucagon-like peptide 1 (GLP-1) is involved in the central regulation of food intake. It is produced within the brain by preproglucagon (PPG) neurons, which are located primarily within the brain stem. These neurons project widely throughout the brain, including to the appetite centers in the hypothalamus, and are believed to convey signals related to satiety. Previous work demonstrated that they are directly activated by leptin and electrical activity of the afferent vagus. Another satiety hormone, cholecystokinin (CCK), has also been linked to activation of brain stem neurons, suggesting that it might act partially via centrally projecting neurons from the nucleus tractus solitarius (NTS). The aim of this study was to investigate the neuronal circuitry linking CCK to the population of NTS-PPG neurons.

RESEARCH DESIGN AND METHODS

Transgenic mice expressing yellow fluorescent protein (Venus) under the control of the PPG promoter were used to identify PPG neurons in vitro and to record their electrical and pharmacological profile.

RESULTS

PPG neurons in the NTS were excited by CCK and epinephrine, but not by the melanocortin receptor agonist melanotan II. Both CCK and epinephrine acted to increase glutamatergic transmission to the PPG neurons, and this involved activation of α(1)-adrenergic receptors. Inhibition of adrenergic signaling abolished the excitatory action of CCK.

CONCLUSIONS

CCK activates NTS-PPG cells by a circuit involving adrenergic and glutamatergic neurons. NTS-PPG neurons integrate a variety of peripheral signals that indicate both long-term energy balance and short-term nutritional and digestional status to produce an output signal to feeding and autonomic circuits.

Early life experience shapes the functional organization of stress-responsive visceral circuits

Emotions are closely tied to changes in autonomic (i.e., visceral motor) function, and interoceptive sensory feedback from body to brain exerts powerful modulatory control over motivation, affect, and stress responsiveness. This manuscript reviews evidence that early life experience can shape the structure and function of central visceral circuits that underlie behavioral and physiological responses to emotive and stressful events. The review begins with a general discussion of descending autonomic and ascending visceral sensory pathways within the brain, and then summarizes what is known about the postnatal development of these central visceral circuits in rats. Evidence is then presented to support the view that early life experience, particularly maternal care, can modify the developmental assembly and structure of these circuits in a way that impacts later stress responsiveness and emotional behavior. The review concludes by presenting a working hypothesis that endogenous cholecystokinin signaling and subsequent recruitment of gastric vagal sensory inputs to the caudal brainstem may be an important mechanism by which maternal care influences visceral circuit development in rat pups. Early life experience may contribute to meaningful individual differences in emotionality and stress responsiveness by shaping the postnatal developmental trajectory of central visceral circuits.

Ghrelin inhibits visceral afferent activation of catecholamine neurons in the solitary tract nucleus

Brainstem A2/C2 catecholamine (CA) neurons in the solitary tract nucleus (NTS) are thought to play an important role in the control of food intake and other homeostatic functions. We have previously demonstrated that these neurons, which send extensive projections to brain regions involved in the regulation of appetite, are strongly and directly activated by solitary tract (ST) visceral afferents. Ghrelin, a potent orexigenic peptide released from the stomach, is proposed to act in part through modulating NTS CA neurons but the underlying cellular mechanisms are unknown. Here, we identified CA neurons using transgenic mice that express enhanced green fluorescent protein driven by the tyrosine hydroxylase promoter (TH-EGFP). We then determined how ghrelin modulates TH-EGFP neurons using patch-clamp techniques in a horizontal brain slice preparation. Ghrelin inhibited the frequency of spontaneous glutamate inputs (spontaneous EPSCs) onto TH-EGFP neurons, including cholecystokinin-sensitive neurons, an effect blocked by the GHSR1 antagonist, d-Lys-3-GHRP-6. This resulted in a decrease in the basal firing rate of NTS TH-EGFP neurons, an effect blocked by the glutamate antagonist NBQX. Ghrelin also dose-dependently inhibited the amplitude of ST afferent evoked EPSCs (ST-EPSCs) in TH-EGFP NTS neurons, decreasing the success rate for ST-evoked action potentials. In addition, ghrelin decreased the frequency of mini-EPSCs suggesting its actions are presynaptic to reduce glutamate release. Last, inhibition by ghrelin of the ST-EPSCs was significantly increased by an 18 h fast. These results demonstrate a potential mechanism by which ghrelin inhibits NTS TH neurons through a pathway whose responsiveness is increased during fasting.

Sulfated cholecystokinin-8 activates phospho-mTOR immunoreactive neurons of the paraventricular nucleus in rats

The serin/threonin-kinase, mammalian target of rapamycin (mTOR) was detected in the arcuate nucleus (ARC) and paraventricular nucleus of the hypothalamus (PVN) and suggested to play a role in the integration of satiety signals. Since cholecystokinin (CCK) plays a role in the short-term inhibition of food intake and induces c-Fos in PVN neurons, the aim was to determine whether intraperitoneally injected CCK-8S affects the neuronal activity in cells immunoreactive for phospho-mTOR in the PVN. Ad libitum fed male Sprague-Dawley rats received 6 or 10 μg/kg CCK-8S or 0.15M NaCl ip (n=4/group). The number of c-Fos-immunoreactive (ir) neurons was assessed in the PVN, ARC and in the nucleus of the solitary tract (NTS). CCK-8S increased the number of c-Fos-ir neurons in the PVN (6 μg: 103 ± 13 vs. 10 μg: 165 ± 14 neurons/section; p<0.05) compared to vehicle treated rats (4 ± 1, p<0.05), but not in the ARC. CCK-8S also dose-dependently increased the number of c-Fos neurons in the NTS. Staining for phospho-mTOR and c-Fos in the PVN showed a dose-dependent increase of activated phospho-mTOR neurons (17 ± 3 vs. 38 ± 2 neurons/section; p<0.05), while no activated phospho-mTOR neurons were observed in the vehicle group. Triple staining in the PVN showed activation of phospho-mTOR neurons co-localized with oxytocin, corresponding to 9.8 ± 3.6% and 19.5 ± 3.3% of oxytocin neurons respectively. Our observations indicate that peripheral CCK-8S activates phospho-mTOR neurons in the PVN and suggest that phospho-mTOR plays a role in the mediation of CCK-8S's anorexigenic effects.

Hindbrain noradrenergic A2 neurons: diverse roles in autonomic, endocrine, cognitive, and behavioral functions

Central noradrenergic (NA) signaling is broadly implicated in behavioral and physiological processes related to attention, arousal, motivation, learning and memory, and homeostasis. This review focuses on the A2 cell group of NA neurons, located within the hindbrain dorsal vagal complex (DVC). The intra-DVC location of A2 neurons supports their role in vagal sensory-motor reflex arcs and visceral motor outflow. A2 neurons also are reciprocally connected with multiple brain stem, hypothalamic, and limbic forebrain regions. The extra-DVC connections of A2 neurons provide a route through which emotional and cognitive events can modulate visceral motor outflow and also a route through which interoceptive feedback from the body can impact hypothalamic functions as well as emotional and cognitive processing. This review considers some of the hallmark anatomical and chemical features of A2 neurons, followed by presentation of evidence supporting a role for A2 neurons in modulating food intake, affective behavior, behavioral and physiological stress responses, emotional learning, and drug dependence. Increased knowledge about the organization and function of the A2 cell group and the neural circuits in which A2 neurons participate should contribute to a better understanding of how the brain orchestrates adaptive responses to the various threats and opportunities of life and should further reveal the central underpinnings of stress-related physiological and emotional dysregulation.

Central effects of estradiol in the regulation of food intake, body weight, and adiposity

In recent years, obesity and its associated health disorders and costs have increased. Accumulation of adipose tissue, or fat, in the intra-abdominal adipose depot is associated with an increased risk of developing cardiovascular problems, type-2 diabetes mellitus, certain cancers, and other disorders like the metabolic syndrome. Males and females differ in terms of how and where their body fat is stored, in their hormonal secretions, and in their neural responses to signals regulating weight and body fat distribution. Men and post-menopausal women accumulate more fat in their intra-abdominal depots than pre-menopausal women, resulting in a greater risk of developing complications associated with obesity. The goal of this review is to discuss the current literature on sexual dimorphisms in body weight regulation, adipose tissue accrual and deposition.

Responses to cholecystokinin in the ventromedial nucleus of the rat hypothalamus in vivo

The peptide cholecystokinin (CCK) is a short-term satiety signal released from the gastrointestinal tract during food intake. From the periphery, CCK signalling travels via the vagus nerve to reach the brainstem from which it is relayed higher into the brain. The hypothalamus is a key integrator of appetite-related stimuli and the ventromedial nucleus of the hypothalamus (VMN) is thought to have an important role in the regulation of satiety. We investigated the effect of intravenous injections of CCK on the spontaneous firing activity of single VMN neurons in urethane-anaesthetised rats in vivo. We found that the predominant effect of CCK on the electrical activity in the VMN is inhibitory. We analysed the responses to CCK according to electrophysiologically distinct subpopulations of VMN neurons and found that four of these VMN subpopulations were inhibited by CCK, while five were not significantly affected. Finally, CCK-induced inhibitory response in VMN neurons was not altered by pre-administration of intravenous leptin.

Oxytocin as feeding inhibitor: maintaining homeostasis in consummatory behavior

Initial studies showed that the anorexigenic peptide oxytocin (OT) regulates gastric motility, responds to stomach distention and to elevated osmolality, and blocks consumption of toxic foods. Most recently, it has been proposed to act as a mediator of general and carbohydrate-specific satiety and regulator of body weight. In the current review, we discuss the function of OT as a homeostatic inhibitor of consumption, capable of mitigating multiple aspects of ingestive behavior and energy metabolism.

Peripherally injected CCK-8S activates CART positive neurons of the paraventricular nucleus in rats

Cholecystokinin (CCK) plays a role in the short-term inhibition of food intake. Cocaine- and amphetamine-regulated transcript (CART) peptide has been observed in neurons of the paraventricular nucleus (PVN). It has been reported that intracerebroventricular injection of CART peptide inhibits food intake in rodents. The aim of the study was to determine whether intraperitoneally (ip) injected CCK-8S affects neuronal activity of PVN-CART neurons. Ad libitum fed male Sprague-Dawley rats received 6 or 10 microg/kg CCK-8S or 0.15M NaCl ip (n=4/group). The number of c-Fos-immunoreactive neurons was determined in the PVN, arcuate nucleus (ARC), and the nucleus of the solitary tract (NTS). CCK-8S dose-dependently increased the number of c-Fos-immunoreactive neurons in the PVN (mean+/-SEM: 102+/-6 vs. 150+/-5 neurons/section, p<0.05) and compared to vehicle treated rats (18+/-7, p<0.05 vs. 6 and 10 microg/kg CCK-8S). CCK-8S at both doses induced an increase in the number of c-Fos-immunoreactive neurons in the NTS (65+/-13, p<0.05, and 182+/-16, p<0.05). No effect on the number of c-Fos neurons was observed in the ARC. Immunostaining for CART and c-Fos revealed a dose-dependent increase of activated CART neurons (19+/-3 vs. 29+/-7; p<0.05), only few activated CART neuron were observed in the vehicle group (1+/-0). The present observation shows that CCK-8S injected ip induces an increase in neuronal activity in PVN-CART neurons and suggests that CART neurons in the PVN may play a role in the mediation of peripheral CCK-8S's anorexigenic effects.

Developing small molecule nonpeptidergic drugs for the treatment of anxiety disorders: is the challenge still ahead?

Neuropeptide systems have been considered a major opportunity for the development of novel treatment approaches for anxiety disorders based on preclinical evidence and neurochemical alterations seen in anxiety disorders. This excitement was further facilitated by the fact that drugs acting at these systems, such as CRF1 antagonists, NK1 antagonists, NK3 antagonists or CCK2 antagonists, may have unique properties not seen with drugs affecting more classical mechanisms involved in anxiety. Consequently, there have been major efforts to develop such small-molecule, nonpeptide receptor ligands. A number of these molecules have been tested in the clinic, either in trials where levels of anxiety served as a secondary measure, or in a few studies with patients suffering from anxiety disorders. But unfortunately, and despite all the efforts of the field as a whole, we still lack convincing clinical proof-of-concept for any of the neuropeptidergic approaches in patients. It must, therefore, be concluded that neuropeptide targets remain a promising approach for the development of the next generation drugs to treat anxiety disorders, but that they continue to be high-risk targets for drug development.

A potential gastrointestinal link between enhanced postnatal maternal care and reduced anxiety-like behavior in adolescent rats

Early life experience impacts emotional development in the infant. In rat pups, repeated, brief (i.e., 15 min) maternal separation (MS15) during the first 1-2 postnatal weeks has been shown to increase active maternal care and to reduce later anxiety-like behavior in the offspring. We hypothesized that the anxiolytic effect of MS15 is partly due to increased intestinal release of cholecystokinin (CCK) in rat pups as a result of increased maternal contact. We predicted that rats with a history of MS15 would display less anxiety in the elevated plus maze (EPMZ) and novelty-suppressed feeding (NSF) tests, as compared with nonseparated (NS) controls, and that the anxiolytic effect of MS15 would be attenuated in rats in which daily MS15 was accompanied by systemic administration of a CCK-1 receptor antagonist (i.e., devazepide). Treatment groups included NS control litters, litters exposed to MS15 from postnatal days (P)1-10, inclusive, and litters exposed to MS15 with concurrent subcutaneous injection of devazepide or vehicle. Litters were undisturbed after P10 and were weaned on P21. Subsets of adolescent males from each litter were tested in the EPMZ on P40-41, while others were tested for NSF on P50-52. As predicted, rats with a developmental history of MS15 displayed reduced anxiety-like behavior in the EPMZ and NSF tests. The anxiolytic effect of MS15 was preserved in vehicle-treated rats, but was reversed in devazepide-treated rats. These results support the view that endogenous CCK-1 receptor signaling in infants is a potential pathway through which maternal-pup interactions regulate the development and functional organization of emotional circuits that control anxiety-like behavior in the offspring.

Hypothalamic-brainstem circuits controlling eating

It is now axiomatic that neurons in the hypothalamic arcuate nucleus have a primary role in responding to changes in circulating levels of leptin and transmitting signals to downstream circuits that influence eating and energy expenditure. Signals generated from the gastrointestinal tract during meals reach the brainstem, via the vagus nerve and other routes, and impinge on neural circuits that influence the timing and size of meals and amount of food consumed. One of the mechanisms by which leptin exerts its anorexic effects is by increasing the effectiveness of intestinal signals that cause satiation during a meal. It is clear that the effects of gut satiation signals such as CCK can be amplified by leptin acting in the CNS, and in the arcuate nucleus in particular. The present article describes the state of our knowledge about specific neural circuits between the hypothalamus and brainstem that play a role in the interaction of leptin and meal-control signals to control food intake.

Immune challenge and satiety-related activation of both distinct and overlapping neuronal populations in the brainstem indicate parallel pathways for viscerosensory signaling

Caudal brainstem viscerosensory nuclei convey information about the body's internal state to forebrain regions implicated in feeding behavior and responses to immune challenge, and may modulate ingestive behavior following immune activation. Illness-induced appetite loss might be attributed to accentuated "satiety" pathways, activation of a distinct "danger channel" separate from satiety pathways, or both. To evaluate neural substrates that could mediate the effects of illness on ingestive behavior, we analyzed the pattern and phenotypes of medullary neurons responsive to consumption of a preferred food, sweetened milk, and to intraperitoneal lipopolysaccharide challenge that reduced sweetened milk intake. Brainstem sections were stained for c-Fos, dopamine beta-hydroxylase, phenylethanolamine-N-methyltransferase, and glucagon-like peptide-1 (GLP-1) immunoreactivity. Sweetened milk intake activated many neurons throughout the nucleus of the solitary tract (NTS), including A2 noradrenergic neurons in the caudal half of the NTS. LPS challenge activated a similar population of neurons in the NTS, in addition to rostral C2 adrenergic and mid-level A2 noradrenergic neurons in the NTS, many C1 and A1 neurons in the ventrolateral medulla, and in GLP-1 neurons in the dorsal medullary reticular nucleus. Increased numbers of activated GLP-1 neurons in the NTS were only associated with sweetened milk ingestion. Evidence for parallel processing was reflected in the parabrachial nucleus, where sweetened milk intake resulted in activation of the inner external lateral, ventrolateral and central medial portions, whereas LPS challenge induced c-Fos expression in the outer external lateral portions. Thus, signals generated in response to potentially dangerous physiological conditions seem to be propagated via specific populations of catecholaminergic neurons in the NTS and VLM, and likely include a pathway through the external lateral PBN. The data indicate that immune challenge engages multiple ascending neural pathways including both a distinct catecholaminergic "danger" pathway, and a possibly multimodal pathway derived from the NTS.

Phenotype of neurons in the nucleus of the solitary tract that express CCK-induced activation of the ERK signaling pathway

The satiating potency of CCK has been well characterized, including its mediation by capsaicin-sensitive vagal primary afferents. We have previously shown that peripherally administered CCK activates the MAPK-signaling cascade in a population of nucleus of the solitary tract (NTS) neurons and that preventing ERK1/2 phosphorylation partly attenuates CCK's satiating potency. The aim of this study was to identify the neurochemical phenotypes of the NTS neurons that exhibit CCK-induced activation of ERK1/2. Using confocal microscopy, we demonstrate that intraperitoneal CCK administration increases the number of neurons that express phosphorylated ERK1/2 (pERK1/2) in the medial and commissural subnuclei of the NTS and that CCK-induced expression of ERK1/2 is increased in tyrosine hydroxylase-immunoreactive neurons. Using Western blot analysis, we show that the robust increase in tyrosine hydroxylase phosphorylation obtained with intraperitoneal CCK is significantly attenuated in rats pretreated with the ERK-pathway blocker U0126 injected into the 4th ventricle. In addition, CCK injections increased pERK1/2 expression in POMC neurons in the NTS. In contrast, only the rare GAD67, neuronal nitric oxide synthase, and leptin-responsive neuron exhibited CCK-induced pERK immunoreactivity. We conclude that activation of POMC-immunoreactive neurons and tyrosine hydroxylase activity via the ERK-signaling pathway in the NTS likely contributes to CCK's satiating effects.

Hypophagia induced by glucocorticoid deficiency is associated with an increased activation of satiety-related responses

Glucocorticoids have major effects on food intake, demonstrated by the decrease of food intake following adrenalectomy. Satiety signals are relayed to the nucleus of the solitary tract (NTS), which has reciprocal projections with the arcuate nucleus (ARC) and paraventricular nucleus (PVN) of the hypothalamus. We evaluated the effects of glucocorticoids on the activation of hypothalamic and NTS neurons induced by food intake in rats subjected to adrenalectomy (ADX) or sham surgery 7 days before the experiments. One-half of ADX animals received corticosterone (ADX+B) in the drinking water (B: 25 mg/l). Fos/tyrosine hydroxylase (TH), Fos/corticotrophin-releasing factor (CRF) and Fos immunoreactivity were assessed in the NTS, PVN, and ARC, respectively. Food intake and body weight were reduced in the ADX group compared with sham and ADX+B groups. Fos and Fos/TH in the NTS, Fos, and Fos/CRF immunoreactive neurons in the PVN and Fos in the ARC were increased after refeeding, with higher number in the ADX group, compared with sham and ADX+B groups. CCK administration showed no hypophagic effect on ADX group despite a similar increase of Fos/TH immunoreactive neurons in the NTS compared with sham and ADX+B groups, suggesting that CCK alone cannot further increase the anorexigenic effect induced by glucocorticoid deficiency. The present data indicate that glucocorticoid withdrawal reduced food intake, which was associated with higher activation of ARC, CRF neurons of the PVN, and catecholaminergic neurons of the NTS. In the absence of glucocorticoids, satiety signals elicited during a meal lead to an augmented activation of brain stem and hypothalamic pathways.

Central control of body weight and appetite

CONTEXT

Energy balance is critical for survival and health, and control of food intake is an integral part of this process. This report reviews hormonal signals that influence food intake and their clinical applications.

EVIDENCE ACQUISITION

A relatively novel insight is that satiation signals that control meal size and adiposity signals that signify the amount of body fat are distinct and interact in the hypothalamus and elsewhere to control energy homeostasis. This review focuses upon recent literature addressing the integration of satiation and adiposity signals and therapeutic implications for treatment of obesity.

EVIDENCE SYNTHESIS

During meals, signals such as cholecystokinin arise primarily from the GI tract to cause satiation and meal termination; signals secreted in proportion to body fat such as insulin and leptin interact with satiation signals and provide effective regulation by dictating meal size to amounts that are appropriate for body fatness, or stored energy. Although satiation and adiposity signals are myriad and redundant and reduce food intake, there are few known orexigenic signals; thus, initiation of meals is not subject to the degree of homeostatic regulation that cessation of eating is. There are now drugs available that act through receptors for satiation factors and which cause weight loss, demonstrating that this system is amenable to manipulation for therapeutic goals.

CONCLUSIONS

Although progress on effective medical therapies for obesity has been relatively slow in coming, advances in understanding the central regulation of food intake may ultimately be turned into useful treatment options.