Adrenergic stimulation of the paraventricular nucleus and its effects on ingestive behavior as a function of drug dose and time of injection in the light-dark cycle

NTS Catecholamine Neurons Mediate Hypoglycemic Hunger via Medial Hypothalamic Feeding Pathways

Glucose is the essential energy source for the brain, whose deficit, triggered by energy deprivation or therapeutic agents, can be fatal. Increased appetite is the key behavioral defense against hypoglycemia; however, the central pathways involved are not well understood. Here, we describe a glucoprivic feeding pathway by tyrosine hydroxylase (TH)-expressing neurons from nucleus of solitary tract (NTS), which project densely to the hypothalamus and elicit feeding through bidirectional adrenergic modulation of agouti-related peptide (AgRP)- and proopiomelanocortin (POMC)-expressing neurons. Acute chemogenetic inhibition of arcuate nucleus (ARC)-projecting NTS^TH neurons or their target, AgRP neurons, impaired glucoprivic feeding induced by 2-Deoxy-D-glucose (2DG) injection. Neuroanatomical tracing results suggested that ARC-projecting orexigenic NTS^TH neurons are largely distinct from neighboring catecholamine neurons projecting to parabrachial nucleus (PBN) that promotes satiety. Collectively, we describe a circuit organization in which an ascending pathway from brainstem stimulates appetite through key hunger neurons in the hypothalamus in response to hypoglycemia.

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.

Hindbrain Catecholamine Neurons Activate Orexin Neurons During Systemic Glucoprivation in Male Rats

Hindbrain catecholamine neurons are required for elicitation of feeding responses to glucose deficit, but the forebrain circuitry required for these responses is incompletely understood. Here we examined interactions of catecholamine and orexin neurons in eliciting glucoprivic feeding. Orexin neurons, located in the perifornical lateral hypothalamus (PeFLH), are heavily innervated by hindbrain catecholamine neurons, stimulate food intake, and increase arousal and behavioral activation. Orexin neurons may therefore contribute importantly to appetitive responses, such as food seeking, during glucoprivation. Retrograde tracing results showed that nearly all innervation of the PeFLH from the hindbrain originated from catecholamine neurons and some raphe nuclei. Results also suggested that many catecholamine neurons project collaterally to the PeFLH and paraventricular hypothalamic nucleus. Systemic administration of the antiglycolytic agent, 2-deoxy-D-glucose, increased food intake and c-Fos expression in orexin neurons. Both responses were eliminated by a lesion of catecholamine neurons innervating orexin neurons using the retrogradely transported immunotoxin, anti-dopamine-β-hydroxylase saporin, which is specifically internalized by dopamine-β-hydroxylase-expressing catecholamine neurons. Using designer receptors exclusively activated by designer drugs in transgenic rats expressing Cre recombinase under the control of tyrosine hydroxylase promoter, catecholamine neurons in cell groups A1 and C1 of the ventrolateral medulla were activated selectively by peripheral injection of clozapine-N-oxide. Clozapine-N-oxide injection increased food intake and c-Fos expression in PeFLH orexin neurons as well as in paraventricular hypothalamic nucleus neurons. In summary, catecholamine neurons are required for the activation of orexin neurons during glucoprivation. Activation of orexin neurons may contribute to appetitive responses required for glucoprivic feeding.

Orexin-A enhances feeding in male rats by activating hindbrain catecholamine neurons

Both lateral hypothalamic orexinergic neurons and hindbrain catecholaminergic neurons contribute to control of feeding behavior. Orexin fibers and terminals are present in close proximity to hindbrain catecholaminergic neurons, and fourth ventricular (4V) orexin injections that increase food intake also increase c-Fos expression in hindbrain catecholamine neurons, suggesting that orexin neurons may stimulate feeding by activating catecholamine neurons. Here we examine that hypothesis in more detail. We found that 4V injection of orexin-A (0.5 nmol/rat) produced widespread activation of c-Fos in hindbrain catecholamine cell groups. In the A1 and C1 cell groups in the ventrolateral medulla, where most c-Fos-positive neurons were also dopamine β hydroxylase (DBH) positive, direct injections of a lower dose (67 pmol/200 nl) of orexin-A also increased food intake in intact rats. Then, with the use of the retrogradely transported immunotoxin, anti-DBH conjugated to saporin (DSAP), which targets and destroys DBH-expressing catecholamine neurons, we examined the hypothesis that catecholamine neurons are required for orexin-induced feeding. Rats given paraventricular hypothalamic injections of DSAP, or unconjugated saporin (SAP) as control, were implanted with 4V or lateral ventricular (LV) cannulas and tested for feeding in response to ventricular injection of orexin-A (0.5 nmol/rat). Both LV and 4V orexin-A stimulated feeding in SAP controls, but DSAP abolished these responses. These results reveal for the first time that catecholamine neurons are required for feeding induced by injection of orexin-A into either LV or 4V.

Extracellular Hypothalamic Monoamines Measured by In Vivo Microdialysis in a Rat Model of Dietary Fat-Induced Obesity

We tested two hypotheses about monoamine neurotransmitters in two strains of rats that differ in their sensitivity to obesity when eating a high-fat diet; 1) that the concentrations of norepinephrine and serotonin and of their metabolites differ in the extracellular fluid of the ventromedial hypothalamus of conscious, unrestrained Osborne-Mendel and S 5B/Pl rats, and 2) that these monoamines are altered differently between strains by a high-fat diet. The monoamines were measured by HPLC in dialysate collected by in vivo microdialysis in rats eating a semisynthetic low-fat diet (10% of kcal as fat) and again after either two or seven days of eating a high-fat diet (56% of kcal as fat). Norepinephrine, serotonin (5-HT), and 5-hydroxyindole-3-acetic acid (5-HIAA) were lower in Osborne-Mendel rats than in S 5B/Pl rats eating the low-fat diet. Norepinephrine and serotonin both increased in Osborne-Mendel rats with the onset of the high-fat diet so that Osborne-Mendel and S 5B/Pl rats no longer differed in these neurotransmitters. By day 7 of high-fat feeding, the concentrations of 3-methoxy-4-hydroxyphenylglycol (MHPG), 5-HIAA and the 5-HIAA/5-HT ratio rose in both strains. Ambient extracellular monoamines in the medial hypothalamus are lower in Osborne-Mendel rats than in S 5B/Pl rats and the response of these catecholamines to dietary fat was greater in Osborne-Mendel rats than in S 5B/Pl rats.

Galanin and consummatory behavior: special relationship with dietary fat, alcohol and circulating lipids

Galanin (GAL) plays an integral role in consummatory behavior. In particular, hypothalamic GAL has a positive, reciprocal relationship with dietary fat and alcohol. In this relationship, GAL increases the consumption of fat or alcohol which, in turn, stimulates the expression of GAL, ultimately leading to overconsumption. Through actions in the amygdala, this relationship may become especially important in stress-induced food or drug intake. These effects of GAL in promoting overconsumption may involve various neurotransmitters, with GAL facilitating intake by stimulating norepinephrine and dopamine and reducing satiety by decreasing serotonin and acetylcholine. In addition, GAL in the hypothalamus stimulates the opioid, enkephalin, throughout the brain, which also promotes overconsumption. The relationship between GAL, fat, and alcohol may involve triglycerides, circulating lipids that are released by fat or alcohol and that correlate positively with hypothalamic GAL expression. In females, levels of endogenous GAL also fluctuate across the reproductive cycle, driven by a rise in the ovarian steroids, estrogen, and progesterone. They peak during the proestrous phase and also at puberty, simultaneous to a sharp increase in preference for fat to meet energy demands. Prenatal exposure to a high-fat diet also enhances hypothalamic expression of GAL into adulthood because of an increase in neurogenesis and proliferation of GAL-expressing neurons in this region. This organizational change may reflect the role of GAL in neuronal development, including neurite growth in adulthood, cell survival in aging, and cell stability in the disease state. By responding positively to fat and alcohol and guiding further neuronal development, GAL potentiates a long-term propensity to overconsume fat and alcohol.

Metabolic imprinting: critical impact of the perinatal environment on the regulation of energy homeostasis

Epidemiological studies in humans suggest that maternal undernutrition, obesity and diabetes during gestation and lactation can all produce obesity in offspring. Animal models have allowed us to investigate the independent consequences of altering the pre- versus post-natal environments on a variety of metabolic, physiological and neuroendocrine functions as they effect the development in the offspring of obesity, diabetes, hypertension and hyperlipidemia (the 'metabolic syndrome'). During gestation, maternal malnutrition, obesity, type 1 and type 2 diabetes and psychological, immunological and pharmacological stressors can all promote offspring obesity. Normal post-natal nutrition can reduce the adverse impact of some of these pre-natal factors but maternal high-fat diets, diabetes and increased neonatal access to food all enhance the development of obesity and the metabolic syndrome in offspring. The outcome of these perturbations of the perinatal environmental is also highly dependent upon the genetic background of the individual. Those with an obesity-prone genotype are more likely to be affected by factors such as maternal obesity and high-fat diets than are obesity-resistant individuals. Many perinatal manipulations appear to promote offspring obesity by permanently altering the development of central neural pathways, which regulate food intake, energy expenditure and storage. Given their strong neurotrophic properties, either excess or an absence of insulin and leptin during the perinatal period are likely to be effectors of these developmental changes. Because obesity is associated with an increased morbidity and mortality and because of its resistance to treatment, prevention is likely to be the best strategy for stemming the tide of the obesity epidemic. Such prevention should begin in the perinatal period with the identification and avoidance of factors which produce permanent, adverse alterations in neural pathways which control energy homeostasis.

Effects of central and systemic administration of leptin on neurotransmitter concentrations in specific areas of the hypothalamus

Leptin, a hormone produced by adipocytes, has been shown to affect a number of central functions, such as regulation of the hypothalamo-pituitary-adrenal axis, feeding, and body weight regulation. Because hypothalamic monoamines are intricately involved in the regulation of these functions, we hypothesized that leptin may produce its effects by altering the activity of these neurotransmitters. To test this hypothesis, male rats received peripheral (0, 100, or 500 μg ip), or central (0 or 5 μg icv) injections of leptin. The animals were killed 5 h later, and their brains were removed, frozen, and sectioned. Serum was collected to measure leptin and corticosterone by RIA. The paraventricular nucleus (PVN), arcuate nucleus (AN), ventromedial hypothalamus (VMH), dorsomedial dorsal nucleus (DMD), median eminence (ME), and medial preoptic area (MPA) were obtained using Palkovits' microdissection technique, and monoamine concentrations in these areas were determined using HPLC-EC. Intraperitoneal administration of leptin increased serum leptin concentrations in a dose-dependent manner ( P < 0.05). Both intraperitoneal and intracerebroventricular administration of leptin decreased serum corticosterone significantly ( P < 0.05). Norepinephrine (NE) concentration decreased significantly in the PVN, AN, and VMH after both intraperitoneal and intracerebroventricular administration of leptin ( P < 0.05). NE concentrations decreased significantly in the DMN after intracerebroventricular administration of leptin ( P < 0.05). Leptin treatment (both ip and icv) decreased dopamine concentrations significantly in the PVN. Serotonin (5-HT) concentration decreased significantly in the PVN after both intraperitoneal and intracerebroventricular injections of leptin and decreased in the VMH only with intracerebroventricular treatment of leptin. Leptin did not affect any of the monoamines in the ME and MPA. These results indicate that both central and systemic administration of leptin can affect hypothalamic monoamines in a region-specific manner, which, in turn, could mediate many of leptin's central and neuroendocrine effects.

Amphetamine, an appetite suppressant, decreases neuropeptide Y immunoreactivity in rat hypothalamic paraventriculum

Amphetamine (AMPH) is a well-known anorectic agent. The mechanism underlying the anorectic response of AMPH has been attributed to its inhibitory effect on hypothalamic neuropeptide Y (NPY), an orexigenic peptide in the brain. However, there is still lack of genomic or in situ immunohistochemical evidence to prove it. The present study was aimed to assess the molecular mechanism of AMPH anorexia by immunostaining of hypothalamic NPY protein in the area of paraventricular nucleus (PVN) and by detecting the change of hypothalamic NPY mRNA level using RT-PCR. Results revealed that an AMPH treatment might reduce the expression of NPY at both transcriptional and posttranslational levels. Comparatively, a treatment of clomipramine, a serotonin transporter inhibitor, was unable to reduce NPY mRNA level, revealing the noninvolvement of hypothalamic NPY gene in serotonin anorexia. Our results provided genomic and in situ immunohistochemical evidence to confirm the mediation of hypothalamic NPY neurons in the anorectic action of AMPH.

Hypothalamic implants of dilute estradiol fail to reduce feeding in ovariectomized rats

To investigate further the site where estradiol (E(2)) inhibits food intake, we tested the effects on feeding of subcutaneous and intrahypothalamic implants of 10% E(2) benzoate in cholesterol (CHOL) or CHOL alone. E(2) was implanted subcutaneously in Silastic tubes, and intrahypothalamically via bilateral 29-gauge cannulas into the paraventricular nucleus (PVN) or the medial preoptic area (MPA) of ovariectomized (OVX) Sprague-Dawley and Long-Evans rats. Three-day implant periods followed 3-day baseline periods. Rats were allowed ad libitum access to chow and tap water, and food intake and body weight were measured each day. Subcutaneous 10% E(2) implants in Sprague-Dawley rats reduced food intake 21% on Day 2 and 34% on Day 3 (P's<.01) and decreased 3-day body weight gain 11 g (P<.05). In contrast, 10% E(2) implants in the PVN of Sprague-Dawley rats did not change food intake or body weight. Implants of 10% or 20% E(2) in the MPA also failed to decrease food intake. MPA implants of 10% E(2) decreased body weight gain 8 g (P<.05), but MPA implants of 20% E(2) decreased weight gain only 4 g (P>.05). To determine whether the strain of rat affected our negative results on food intake, we implanted 10% E(2) into the PVN of Long-Evans rats. Again, PVN E(2) did not decrease food intake significantly in comparison to the pretest baseline. PVN E(2) did, however, decrease body weight gain 5 g and decreased food intake 6% compared to rats with implants of CHOL (both P<.05), but these effects appeared to be due to an increase in feeding in the CHOL group in comparison to their baseline. Finally, CHOL and E(2) implants did not impair the responsivity of the PVN because acute implants of norepinephrine (NE) into the PVN of E(2)- or CHOL-treated Long-Evans rats significantly increased food intake. Our results do not support the hypothesis that E(2)'s actions in either the PVN or the MPA are sufficient to account for its inhibitory effects on feeding.

Effects of three neurochemical stimuli on delayed feeding and energy metabolism

Infusions of norepinephrine (NE), the gamma-aminobutyric acid agonist, muscimol (MUS), or neuropeptide Y (NPY) into the paraventricular nucleus (PVN) of the hypothalamus all increase food intake. Such feeding may be due to direct activation of behavioral processes driving ingestion and/or to alterations in nutrient metabolism that feeding serves to normalize. To examine these possibilities, male Sprague-Dawley rats received PVN infusions of vehicle, 20 nmol NE, 1 nmol MUS or 100 pmol NPY at dark onset, then food intake was measured under three feeding conditions: (1) 1 and 2 h immediately after injections, (2) 1 h after a 1 h delay between injections and access to food, and (3) 1 h after a 1 h feeding delay, but with injections occurring just before presenting food. Measures of energy expenditure (EE) and respiratory quotients (RQs) in the absence of food were made over 2 h in parallel experiments. Results confirmed that NE, MUS and NPY all increased dark-onset feeding, but only NPY increased intake above control levels after a 1 h feeding delay. No neurochemically-induced changes in EE were observed, nor were there changes in RQs after NE or MUS. However, NPY reliably enhanced RQs from 30 to 120 min of testing. Our findings imply that NE and MUS initiate relatively immediate, short-term feeding that is not associated with changes in nutrient metabolism and does not summate with cues stimulated by delayed access to food. NPY initiates more protracted feeding temporally linked to enhanced carbohydrate metabolism. This may indicate that part of NPY's feeding stimulatory effects are secondary to physiological processes driving ingestion.

Histamine regulates food intake through modulating noradrenaline release in the para-ventricular nucleus

Histamine is one of the neurotransmitters suppressing appetite, but the interactions of histaminergic neurons with other neurons in satiety centers have not been clarified. Noradrenaline in the para-ventricular nucleus (PVN) has been shown to stimulate feeding. This study was designed to determine whether histamine modulates noradrenaline release via histamine H1-receptors in the PVN. Freely-fed rats were i.c.v. injected with an histamine H1-receptor antagonist, triprolidine (82 microg), and/or an alpha 2-adrenoceptor antagonist, rauwolscine (0, 20 and 40 microg), and food intake (n=8 each) over 2 h was measured. Food intake was significantly (p<0.01) increased in rats injected with triprolidine alone. The triprolidine-elicited increase in food intake was suppressed by rauwolscine at a dose of 40 microg. The noradrenaline content in perfusates collected by a microdialysis probe aimed at the PVN was significantly (p<0.05) increased by the presence of triprolidine in the perfusates. The noradrenaline concentrations in perfusates collected from the PVN were elevated after tyramine (a noradrenaline uptake inhibitor) administration, but not when both tyramine and histamine were given. In conclusion, these results suggest that histamine inhibits noradrenaline release from hypothalamic nerve terminals in the PVN, and thus suppresses feeding behavior.

Neural sites and pathways regulating food intake in birds: a comparative analysis to mammalian systems

The paper reviews hypotheses explaining the regulation of food intake in mammals that have addressed specific anatomical structures in the brain. An hypothesis, poikilostasis, is introduced to describe multiple, homeostatic states whereby the regulation of metabolism and feeding occur in birds. Examples are given for both wild and domestic avian species, illustrating dynamic shifts in homeostasis responsible for the changes in body weights that are seen during the course of an annual cycle or by a particular strain of bird. The following neural structures are reviewed as each has been shown to affect food intake in birds or in mammals: ventromedial hypothalamic nucleus (n.), lateral hypothalamic area, paraventricular hypothalamic n., n. tractus solitarius and area postrema, amygdala, parabrachial n., arcuate n. and bed n. of the stria terminalis. Two neural pathways are described which have been proposed to regulate feeding. The trigeminal sensorimotor pathway is the most complete neural pathway characterized for this behavior and encompasses the mechanics of pecking, grasping and mandibulating food particles from the tip of the bill to the back of the buccal cavity. A second pathway, the visceral forebrain system (VFS), affects feeding by regulating metabolism and the balance of the autonomic nervous system. Wild, migratory birds are shown to exhibit marked changes in body weight which are hypothesized to occur due to shifts in balance between the sympathetic and parasympathetic nervous systems. Domestic avian species, selected for a rapid growth rate, are shown to display a dominance of the parasympathetic nervous system. The VFS is the neural system proposed to effect poikilostasis by altering the steady state of the autonomic nervous system in aves and perhaps is applicable to other classes of vertebrates as well.

Activation of fos in mouse amygdala by local infusion of norepinephrine or atipamezole

Norepinephrine (NE) is known to activate a number of immediate-early genes (IEGs) in the brain which may be involved in prolonged changes in neuronal function. To investigate the function of these genes it would be useful to have a model system in which they are induced in specific populations of cells in specific brain regions without systemic drug administration which can affect multiple sites. In the present paper we have shown that local infusions of NE or of the alpha2-adrenoceptor antagonist, atipamezole, in the mouse amygdala produces localized expression of fos. The expression of fos was blocked by a cocktail of an alpha1-(prazosin) and beta1-adrenoceptor (betaxolol) blocker but not by a selective 5-HT1A blocker (WAY100135). Prazosin and betaxolol did not have a nonspecific reducing action on fos expression. It is concluded that localized expression of fos after NE infusion in the mouse amygdala represents a model system for further studies of the role of IEG expression in central noradrenergic function.

GRF-induced feeding: evidence for protein selectivity and opiate involvement

Growth hormone-releasing factor (GRF) is a hypothalamic peptide named for its ability to induce release of growth hormone from the anterior pituitary. GRF also acts as a neurotransmitter in the suprachiasmatic nucleus/medial preoptic area (SCN/MPOA) to stimulate food intake. The purpose of this series of experiments was to explore the nature of GRF-induced feeding, with a particular emphasis on macronutrient selectivity, and to examine the role of opiate activity in the paraventricular nucleus of the hypothalamus (PVN). Chow intake stimulated by GRF microinjection (1 pmol/0.5 microliters) into the SCN/MPOA was blocked by injection of methyl-naltrexone (3 micrograms/0.5 microliters) into the PVN. In animals habituated to macronutrient diets (Teklad, WI), GRF preferentially stimulated intake of protein at 2 and 4 h postinjection, whereas it had no effect on carbohydrate intake. Further, this effect was blocked by injection of naloxone (40 nmol/0.5 microliters) into the PVN. Microinjection of morphine (0, 1, 10, and 17 micrograms/0.5 microliter) into the PVN also specifically stimulated protein intake at 2 and 4 h postinjection. These results suggest that feeding derived from GRF actions in the SCN/MPOA is macronutrient selective, and is dependent on PVN opiate activity for expression.

Food and water intake responses of the domestic fowl to norepinephrine infusion at circumscribed neural sites

The effect on food and water intake of injection of norepinephrine into circumscribed brain sites of the domestic fowl was investigated. Injection of norepinephrine into sites throughout the preoptic area caused reliable increases in food intake. Food intake was also increased by injection of norepinephrine in the ventromedial nucleus, paraventricular nucleus, and medial septal sites. Food intake was decreased by injections near the lateral septal organ and the anterior portion of both the nucleus reticularis superior, pars dorsalis, and the tractus occipitomesencephalicus. Within the preoptic area, water intake was increased at basolateral sites but was inconsistently affected at more medial sites. No consistent trends were noted at sites examined outside the preoptic-hypothalamic area.

Activation of the satiety center by auricular acupuncture point stimulation

Stimulation of the rat inner auricular regions that correspond to the human pylorus, lung, trachea, stomach, esophagus, endocrine, and heart acupuncture points evoked potentials in the hypothalamic ventromedial nucleus (HVM), the satiety center. Needle implantation into any of these points reduced the body weight to its initial 290 g after the rat had gained about 410 g in 20 days, and significantly reduced initial 450-g body weights (p less than 0.01, Student's t test) in 14 days. Stimulation of other acupuncture points did not evoke HVM potentials and did not reduce body weight. After the HVM was lesioned, body weight increased and acupuncture point needling had no effect on body weight. Needling of the auricular acupuncture points evoked no potentials in the lateral hypothalamus (LHA), the feeding center, and had almost no influence on weight reduction induced by LHA lesion.

The pharmaco-ontogeny of the paraventricular alpha 2-noradrenergic receptor system mediating norepinephrine-induced feeding in the rat

The present study was undertaken to assess the functional ontogeny of alpha 2-noradrenergic receptors in the hypothalamic paraventricular nucleus (PVN) that mediate noradrenergic stimulation of feeding in the rat. Rat pups, ranging in age from 2 to 15 days, were removed from their mothers and implanted with a brain cannula directed unilaterally at the PVN or third ventricle. On the following day, each pup was implanted with an intra-oral cannula for oral infusion of milk or water that could be swallowed or rejected. Following a 1-h period of satiation, each pup received an intracerebral injection of saline, or a single dose of norepinephrine (NE, 0.01-100.0 nmol) or the alpha 2-noradrenergic receptor agonist clonidine (0.01-1.0 nmol). Milk or water intake was then assessed following a 1-h period of infusion. Injection of NE into the PVN and third ventricle significantly enhanced milk intake at 2 days of age. NE was 10-fold more potent in the PVN than in the ventricle. While paraventricular injections of NE stimulated milk and water intake equally at 2 days of age, NE produced a greater stimulation of milk than water intake at 15 days of age. Like NE, clonidine significantly enhanced milk intake at 2 days of age following injection into the PVN. Collectively, these findings suggest that alpha 2-noradrenergic receptors, mediating noradrenergic stimulation of feeding, are functionally mature very early in the postnatal development of the rat. Moreover, consistent with evidence in the adult rat, these findings indicate that alpha 2-noradrenergic receptors relevant to feeding are located in the vicinity of the PVN.

Lack of diet-induced thermogenesis following lesions of paraventricular nucleus in rats

The effects of electrolytic lesions in the hypothalamus paraventricular nucleus were studied in adult male and female Sprague-Dawley rats, fed different diets, consisting of either palatable human food plus chow (cafeteria diet) or chow alone. The results showed that both cafeteria diet and lesions induced an increase in energy intake and weight gain in rats of both sexes. Oxygen consumption rate and colonic temperature were significantly decreased by lesions, while cafeteria diet increased the same parameters only in intact animals. The lesion decreased weight, protein and DNA, and temperature of brown adipose tissue, while cafeteria diet increased the values considered in brown adipose tissue of sham-injured rats, but not in lesioned animals. The response to norepinephrine administration was significantly greater in intact rats and those fed cafeteria diet. The results suggest that the larger body weight gain observed in lesioned rats, particularly evident in rats fed cafeteria diet, is partly due to the disappearance of diet-induced thermogenesis that depends on the reduced mass and functional activity of brown adipose tissue.

Inhibitory effect of norepinephrine on the single-unit activity of caudally projecting paraventricular neurons

The role of norepinephrine (NE) in controlling the single-unit activity of paraventricular (PVN) neurons projecting to or passing through the caudal ventrolateral medulla (CVLM) was investigated in adult male rats anesthetized with urethane. Of 72 PVN neurons studied, 19 were antidromically activated by CVLM stimulation (Group I) and 48 were antidromically activated by posterior pituitary (PP) stimulation (Group II). The remaining 5 neurons were antidromically driven by both CVLM and PP stimulation (Group III). In 14 of the 19 Group I neurons and in all the 5 Group III neurons, iontophoretically applied NE was demonstrated to be inhibitory to the single-unit activity. No excitatory effect of NE was observed. In contrast, both excitatory and inhibitory actions of NE were observed in the Group II neurons. Of 37 Group II neurons tested, 28 were excited and 7 were inhibited by NE. The inhibitory effect of NE in Group I and Group III neurons was selectively blocked by the alpha antagonist, phentolamine, that was coiontophoresed with NE, but not by the beta antagonist, timolol (n = 9). The unit activity of Group I neurons that were inhibited by NE was not altered by an increase in arterial blood pressure (n = 3), whereas the unit activity of one NE-insensitive Group I neuron was decreased by an increase in blood pressure. Taken together, the results suggest that NE plays an alpha-adrenoreceptor-mediated inhibitory role in controlling the single-unit activity of caudally projecting PVN neurons. These neurons include a subpopulation of PVN neurons that project caudally as well as to the PP. The possible function associated with the NE-sensitive, caudally projecting PVN neurons may be other than the regulation of blood pressure.

Noradrenergic receptor interactions in feeding elicited by stimulation of the para-ventricular hypothalamus

Food intake and feeding behaviour were examined after the administration of noradrenaline (NA) or clonidine to the para-ventricular nucleus of the hypothalamus, or after systemic administration of clonidine. In 20-hr deprived animals all treatments dose-dependently reduced pellet consumption; however, at a low dose (2-5 micrograms) central clonidine increased eating time and bout length. In 4-hr deprived animals all treatments increased sucrose consumption. Clonidine (peripheral and central) increased feeding time but did not alter feeding rate; NA did not alter feeding time, but did increase feeding rate; NA also increased activity and decreased resting. The effects of NA on feeding rate, activity and resting were blocked by propranolol; however, the propranolol-NA combination increased feeding time. Thus, NA and clonidine increased feeding by different mechanisms, but after propranolol pretreatment the effects of NA were similar to those of clonidine. It is concluded that clonidine enhances feeding by inhibiting satiety and that the feeding stimulant effect of NA is mediated by a complex interaction of alpha- and beta-receptors.

Neuropeptide Y, epinephrine and norepinephrine in the paraventricular nucleus: stimulation of feeding and the release of corticosterone, vasopressin and glucose

The paraventricular nucleus (PVN) is known to have an important function in mediating a variety of behavioral and endocrine responses. In the present study, the responsiveness of the PVN to the effects of the coexisting neurotransmitters, neuropeptide Y (NPY), epinephrine (EPI) and norepinephrine (NE), was examined. Albino rats were each chronically implanted with a swivel brain-cannula that permits chemicals to be infused without disturbing the animals' ongoing behavior. When infused into the PVN, each of these neurotransmitters elicited a reliable feeding response during the first hour after injection. The response to EPI was significantly stronger than that of NE and NPY, while the latency to eat after injection was considerably longer for NPY as compared to the catecholamines. In tests with food absent, each of these substances also increased blood levels of corticosterone (EPI greater than NE = NPY) and vasopressin (NPY greater than EPI greater than NE) and revealed a significant positive correlation between circulating levels of these two hormones. In addition, EPI and NE, in contrast to NPY, caused a simultaneous rise in blood glucose, producing levels that were positively correlated with the hormones. No relationship, however, was detected between these endocrine changes and the rats' feeding-stimulatory responses. Together with other evidence, these results suggest that adrenergic as well as noradrenergic innervation to the PVN has a key role in the behavioral and endocrine systems of this nucleus and, moreover, that NPY generally mimics the effects of these catecholamines in the PVN.

Medial hypothalamic serotonin: effects on deprivation and norepinephrine-induced eating

Evidence to date suggests an inhibitory role for serotonin (5-HT) in the regulation of feeding behavior. In the present study, hypothalamic 5-HT was investigated for its anorexic potency under different feeding conditions. In fasted rats, 5-HT (1.1-4.4 micrograms) injected into the hypothalamic paraventricular nucleus (PVN), produced a reliable dose-dependent reduction in food consumption. Under satiated conditions, this inhibitory effect was significantly larger and apparent at lower doses (down to 0.1 micrograms), in animals induced to eat by PVN injection of norepinephrine (NE). Tests with receptor antagonists, injected into the PVN immediately prior to 5-HT, revealed a dose-dependent blockade of 5-HT's action by the serotonergic blockers, metergoline, methysergide and cinanserin. While the effect of 5-HT was somewhat attenuated by administration of certain beta-adrenergic and phenothiazine-type dopamine receptor antagonists, 5-HT was totally resistant to the actions of more selective dopamine blockers and of the cholinergic and histaminergic antagonists, atropine and dexbrompheniramine. PVN injection of various serotonergic compounds, known to inhibit feeding when peripherally administered, also suppressed NE-induced feeding in a dose-related manner (fluoxetine = dl-norfenfluramine greater than quipazine greater than chlorimipramine greater than dl-fenfluramine). Further tests with PVN administration of the dextro isomer and metabolite of fenfluramine showed a considerably stronger inhibitory effect with d-norfenfluramine as compared to dexfenfluramine, and a particular effectiveness of peripherally injected dexfenfluramine in NE-injected rats, at doses at least 10-fold higher than centrally effective doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrahypothalamic noradrenaline injection in the rat enhances operant licking but not lever pressing for milk reward

Injection of noradrenaline into the hypothalamus of the rat produces poor performance of lever pressing for liquid food reinforcement at moderate ratio values, even though such injection facilitates intake of liquid food available without an operant requirement. The present work shows that a tube-licking operant response was more facilitated and less suppressed by noradrenaline than a lever-pressing response. Noradrenaline produced rear-limb ataxia which might have affected lever pressing more than licking, but results of manipulating the height of the tube suggested that this was unlikely. It is concluded that the operant performance deficit is more evident with lever pressing than with a response naturally involved in ingestion.

Destruction of noradrenergic innervation to the paraventricular nucleus: deficits in food intake, macronutrient selection, and compensatory eating after food deprivation

Norepinephrine (NE) injected into the paraventricular nucleus (PVN) has a stimulatory effect on feeding behavior and is found to selectively enhance preference for carbohydrate in the rat. The present experiments were conducted to assess the impact of chronic depletion of NE within the PVN on food intake and appetite regulation. The catecholamine (CA) neurotoxin, 6-hydroxydopamine (6-OHDA), when administered into the PVN, produced a significant depletion of PVN NE in association with a variety of behavioral changes. The immediate consequence of the neurotoxin lesion was a dramatic increase in 24-hr food intake, attributed predominantly to a preferential increase in carbohydrate and fat consumption. The long-term effects related to CA depletion were a deficit in daily food consumption, particularly of carbohydrate (-42%). Although animals with diminished PVN NE maintained a normal diurnal feeding pattern, they failed to exhibit the increased ingestion of an energy-rich carbohydrate diet which rats normally show during the dark period of the diurnal cycle. Rats injected with 6-OHDA directly into the PVN exhibited a normal response to glucoprivic challenge, but demonstrated a deficit in their ability to produce compensatory feeding, particularly of carbohydrate and fat, in response to food deprivation. These findings suggest a specific function for PVN noradrenergic mechanisms in normal energy repletion when body energy stores are reduced.

Efferent projections from the paraventricular nucleus mediating alpha 2-noradrenergic feeding

Feeding behavior elicited by central injection of the alpha-noradrenergic agonists, norepinephrine (NE) and clonidine (CLON), are believed to be mediated via postsynaptic alpha 2-type receptors located in the paraventricular nucleus (PVN). To map the course taken by essential efferent (descending) fibers of this PVN system for noradrenergically-stimulated feeding, the impact of diencephalic and lower brainstem coronal knife cuts, on the responses elicited by PVN-injected NE and CLON, was assessed. Rats that sustained damage in the periventricular gray area of the caudal thalamus and midbrain exhibited significant losses in feeding elicited by PVN injections of these drugs. In the case of animals with midbrain periventricular gray knife cuts, a significant increase in daily food intake was also observed, and this increase was positively correlated in magnitude with the attenuation of NE-induced feeding. This decrease in sensitivity to alpha 2-noradrenergic stimulation occurred with discrete periventricular knife cuts extending only 0.5 mm lateral to midline. In contrast, large ventral or lateral coronal knife cuts throughout the dorsal and ventral midbrain tegmentum left intact NE- and CLON-induced feeding. These findings provide evidence for localization of anatomical substrates which underlie PVN alpha 2-noradrenergic feeding. The efferent fibers of this system appear to exit from the PVN in a dorsomedial direction and course through the thalamic periventricular area. As this projection descends into the midbrain, it remains quite medial, maintaining this position throughout the midbrain central gray substance. At the level of the pons, just rostral to the locus coeruleus, this fiber projection appears to course ventrolaterally into the dorsolateral pontine tegmentum and possibly continue towards the dorsal vagal complex of the dorsomedial medulla.

Norepinephrine, clonidine, and tricyclic antidepressants selectively stimulate carbohydrate ingestion through noradrenergic system of the paraventricular nucleus

Using a self-selection feeding procedure, the present experiments examined the impact of central and peripheral injection of the alpha-adrenergic agonist clonidine (CLON) and the tricyclic antidepressant drugs amitriptyline (AMIT) and chlorimipramine (CIMIP) on nutrient selection in the adult male rat. In tests with mixed diets or with separate sources of the 3 macronutrients (carbohydrate, protein, and fat) simultaneously available, the following results were obtained: Peripheral and paraventricular nucleus (PVN) injection of CLON stimulated total food intake and preferentially increased ingestion of carbohydrate. Little or no change in protein or fat intake was observed. This pattern of response is similar to that observed with norepinephrine. PVN injection of AMIT and peripheral injection of CIMIP also selectively enhanced carbohydrate intake. These drug effects on carbohydrate selection occurred under a variety of conditions, including with mixed diets and pure dietary nutrients; under ad lib and restricted feeding conditions; in short (1 hr) as well as long (6 hr) test intervals; and in the absence or presence of a change in total calorie intake. Based on this and other evidence, it is proposed that noradrenergic neurons innervating the PVN in the rat play a role in regulating carbohydrate selection, and that this neurochemical system mediates the stimulating action of CLON and antidepressants on carbohydrate ingestion.

Norepinephrine-facilitated eating: reduction in saccharin preference and conditioned flavor preferences with increase in quinine aversion

Paraventricular (PVN) hypothalamic norepinephrine (NE) injections which facilitated feeding were nonetheless found to reduce both the unconditioned preference for saccharin and starch-conditioned preferences for almond odor and lemon taste, as well as enhancing aversion to quinine. These results add to the evidence that PVN NE elicits eating by attenuating a satiety signal.

Hypophysectomy disturbs the noradrenergic feeding system of the paraventricular nucleus

Injection of norepinephrine (NE) into the hypothalamic paraventricular nucleus (PVN) of satiated rats is known to stimulate eating behavior. In addition, drinking behavior is potentiated just prior to the onset of eating, followed by a strong inhibition of water intake. To understand the relationship between these PVN noradrenergic phenomena and endocrine processes associated with the PVN, chronically hypophysectomized animals were tested for their behavioral responsiveness to PVN NE injection. Pituitary ablation was found to abolish the NE-elicited eating response and the NE drinking suppressive effect. However, hypophysectomy had no impact on the NE-elicited preprandial drinking response, nor did it affect drinking produced by carbachol, angiotensin, and histamine, or the feeding and drinking responses induced by insulin. These results demonstrate that hypophysectomy disturbs PVN noradrenergic mechanisms in a behaviorally and pharmacologically specific specific manner.

Peptides and feeding

This report reviews the hypothesis that peptides play a role in appetite modulation, stressing that the available evidence is predominantly pharmacological and thus caution needs to be taken in assigning physiological significance at this time. Two peptide systems have been postulated--a peripheral satiety system, typified by the gastrointestinal hormone cholecystokinin and a central feeding system driven by the opioid peptides and neuropeptide Y. This review also discusses the putative role of peptides in the anorexia of aging, drinking elicited by feeding and as mediators of the autonomic effects seen in association with ventromedial hypothalamic lesions.

Chronic norepinephrine injection into the hypothalamic paraventricular nucleus produces hyperphagia and increased body weight in the rat

A single injection of norepinephrine (NE) into the paraventricular nucleus (PVN) is known to elicit a feeding response in the satiated rat. Through repeated NE injections, the present study set out to determine whether chronic noradrenergic stimulation of the PVN is effective in producing changes in total daily food intake, as well as in body weight gain. The results indicate that repeated injections of NE (20 nmoles/injection given 4 times/day) cause a stimulation of eating with each injection and consequently produce a significant increase in total daily food intake. This stimulatory effect on feeding behavior occurs under food-restricted conditions, where food is available only at times (in the daytime) when NE is injected, and also under food-satiated conditions were food is available essentially ad lib. This hyperphagia results in a gradual increase in body weight which develops over the course of a 5-day sequence of repeated NE injections. There is some evidence to suggest that the overeating produced by NE throughout the day may be attributed specifically to an increase in meal size rather than to a change in meal frequency. This evidence suggests that medial hypothalamic NE, particularly within the PVN, may play a role in long-term feeding behavior and body weight regulation.

Chronic infusion of norepinephrine and clonidine into the hypothalamic paraventricular nucleus

Previous experiments have shown that acute injection of NE and CLON into the PVN initiates a short-term feeding response in satiated rats. This study examined, in brain-cannulated rats, the impact of remote, chronic injections of NE, CLON, or saline on daily food intake and body weight gain. Over a period of 14 days, NE was infused into the PVN, either continuously at a rate of 12 nm/microliter/hr, or discretely at a rate of 6 nm/microliter/sec. In addition, the alpha 2-adrenergic agonist CLON was infused into the PVN discretely at a rate of 3 nm/0.5 microliter/30 sec. Relative to saline infusion, chronic (continuous or discrete) stimulation of the PVN with either of these drugs was effective in potentiating daily food intake by 12-19% and in increasing body weight gain, from approximately 1.5 g/day to 3.3 g/day. This evidence indicates that medial hypothalamic NE, especially within the PVN, is sufficiently robust to alter long-term feeding patterns and body weight regulation.

Modification of nocturnal spontaneous and adrenergic-induced feeding in the rat following either A5 or A7 lesions

The ingestive profiles of intact, A5 and A7 damaged animals were examined during the 2-hr nocturnal period following onset of the dark cycle. A5, A7 and intact rats consumed comparable amounts of food following initial access to food nocturnally. Sebsequent feeding declined in A5 animals below control values and failed to return to baseline at the end of the nocturnal period examined. A7 damaged rats appeared more resistant to the appetite suppressing effects of initial meal taking and consumed more food than control animals. Only A5 damaged rats were noted to be hyperdipsic during the immediate 10 postoperative days. Intracranial injection of 1-norepinephrine bitartrate (10 ug/ul) into the paraventricular nucleus (PVN) of the hypothalmus produced a reliable facilitation of feeding in A5, A7 and intact rats during the first hour of the dark cycle. A5 rats exhibited the largest increase in feeding elicited by NE administration into the PVN. This feeding response was observed in rats with A5 lesions regardless of whether testing was carried out during the initial hours of the dark cycle or during a predetermined "satiation" test. A5 lesions also effected a marked hyperglycemia while A7 lesions were ineffective in this respect. Taken together these data suggest the A5 and A7 cell groupings regulate spontaneous feeding within a rostrally coursing feeding circuitry and appear to interact with the PVN in the elicitation of noradrenergic feeding.

Feeding behavior induced by central norepinephrine injection is attenuated by discrete lesions in the hypothalamic paraventricular nucleus

Extensive brain-cannula mapping studies in the rat have demonstrated that the hypothalamic paraventricular nucleus (PVN) is the most sensitive brain site for eliciting eating behavior with central norepinephrine (NE) injection. The present experiments examined the impact of lesions aimed at the PVN on this NE-elicited eating response. In rats with NE injection cannulas aimed at the lateral ventricle, bilateral lesions of the PVN significantly attenuated, by 60 to 70%, the eating effect induced by NE, at doses ranging from 20 to 160 nmoles. PVN lesions which extended ventrally to damage tissue lying within the periventricular region were more effective in abolishing the NE response than were lesions that remained confined to the dorsal aspects of the PVN. Large lesions located just dorsal to the PVN had no impact on the NE response. This evidence supports the primary role of the PVN in mediating the eating behavior elicited by central noradrenergic activation.

Endorphinergic and alpha-noradrenergic systems in the paraventricular nucleus: effects on eating behavior

Brain cannulated rats were injected with the opioid peptide beta-endorphin (beta-EP) directly into the hypothalamic paraventricular nucleus (PVN) where norepinephrine (NE) is most effective in stimulating eating behavior. Beta-Endorphin (1.0 nmole) reliably increased food intake in satiated animals, and this response was blocked by local administration of the selective opiate antagonist naloxone. The eating induced by beta-EP was positively correlated in magnitude with the NE response and, like NE, was antagonized by PVN injection of the alpha-noradrenergic blocker phentolamine. Naloxone had no effect on NE-induced eating, and the dopaminergic blocker fluphenazine failed to alter either beta-EP or NE eating. When injected simultaneously, at maximally effective doses, beta-EP and NE produced an eating response which was significantly larger than either of the responses elicited separately by beta-EP or NE and was essentially equal to the sum of these two responses. The evidence obtained in this study suggests that beta-EP and NE stimulate food ingestion through their action on PVN opiate and alpha-noradrenergic receptors, respectively, and that beta-EP's action is closely related to, and in part may be dependent upon, the PVN alpha-noradrenergic system for feeding control.

Histochemical and pharmacological analysis of noradrenergic projections to the paraventricular hypothalamus in relation to feeding stimulation

Three techniques, namely, midbrain lesions, fluorescence histochemistry, and brain cannulation, were used in combination to analyze noradrenergic projections to the paraventricular nucleus of the hypothalamus (PVN) and their function in stimulating feeding behavior. The convergence of evidence indicates that the dorsal component of the central tegmental tract (CTT), which ascends through the dorsal pons and then projects through the medial tegmental radiations (TR) into the ventral tegmentum just dorsal to the media lemniscus, contains the crucial noradrenergic axons which innervate the PVN and mediate noradrenergic stimulation of feeding behavior. The primary evidence for this conclusion is that dorsal tegmental electrolytic or 6-OHDA lesions which damaged specifically these fibers invariably caused: (1) a reduction of catecholamine varicosities within the PVN (most notably, fine and moderate-size, rounded varicosities within the parvocellular area); (2) a strong reduction or loss of the feeding response elicited by PVN injection of the presynaptically-acting drugs tranylcypromine and desipramine; and (3) a potentiation of the same response produced by injected norepinephrine. These pharmacological and neurochemical changes in the PVN were reduced in magnitude if the dorsal CTT and medial TR fibers received only partial damage, and these changes did not occur at all if the lesion fell immediately dorsal to these fibers without damaging them. Specific lesions in the ventral tegmentum, which also failed to damage the dorsal CTT and TR axons but instead damaged the ventral component of the CTT, not only failed to disrupt the action of the antidepressant agents but actually potentiated their effectiveness in the PVN. Ventromedial lesions, however, which severed the rostroventral extension of the dorsal CTT and medial TR fibers, had the same behavioral consequences as had the dorsal lesions which damaged this projection at a more dorsocaudal level. Finally, damage to other catecholamine projections had little effect on PVN function in stimulating eating.

Analysis of feeding suppression produced by perifornical hypothalamic injection of catecholamines, amphetamines and mazindol

The effects on feeding of perifornical hypothalamic injection of catecholamines, amphetamines and mazindol were examined in hungry rats. In pargyline-pretreated subjects, both dopamine and epinephrine significantly suppressed food intake, at doses as low as 31 ng for dopamine and 150 ng for epinephrine (the latter injected with an alpha-adrenoceptor blocker). This effect was reliably strengthened by inhibiting catecholamine deamination or presynaptic catecholamine uptake. Perifornical injections of amphetamine, mazindol, methamphetamine, and phenmetrazine also suppressed feeding. The magnitude of this effect in individual animals was positively correlated with the effect produced by catecholamine agonists. Moreover, this effect of mazindol was partially antagonized by perifornical injection of dopaminergic and beta-adrenoceptor blockers. The effects of amphetamine and epinephrine were abolished by these drugs, while dopamine's effect was selectively inhibited by the dopaminergic antagonist. Serotonergic antagonists produced no change. These findings lend support to the hypothesis that perifornical hypothalamic catecholamine neurons, through dopaminergic receptors and beta-adrenoceptors, are involved in inhibiting feeding behavior, as well as in mediating the anorexic action of the amphetamines and mazindol.

14C-dopamine microinjected into the brain-stem of the rat: dispersion kinetics, site content and functional dose

A morphological analysis was undertaken of both the dispersion characteristics and tissue content of dopamine (DA) microinjected acutely into the brain-stem of the anesthetized rat. 14C-DA, with a specific activity of 56-62 mCi/mMol, was infused unilaterally into the pars compacta of the substantia nigra in one of four test volumes: 0.5, 1.0, 4.0 or 8.0 microliters. The concentration of the 14C-DA solution was 1.0 microCi/microliter, equivalent to 3.01 micrograms/microliters, which was delivered at an injection rate of 1.0 microliter per 45 sec. At an interval of either one min or 15 min following the microinjection, the rat's brain was removed rapidly from its calvarium, flash frozen and then cut in the coronal plane on a freezing microtome in 500 micron slabs. After each of the respective serial slabs was mounted on glass, the Eik Nes-Brizzee trochar technique for the discrete removal of tissue samples was used to obtain 0.5 mm dia. cylindrical plugs of meso-diencephalic tissue at distances from the site of injection ranging from 0.5 to 2.5 mm, center to center. Each sample plug was subsequently solubilized and 14C-DA activity quantitated by liquid scintillation spectrometry. The results show that regardless of volume, the spatial patterning of the microinjected solution assumes a tear-drop or pear shape, not a sphere. Further, as the volume of the injection is increased from 0.5 to 8.0 microliters, the magnitude of the dispersion of 14C-DA is enhanced throughout the surrounding parenchyma, but not in a linear fashion.(ABSTRACT TRUNCATED AT 250 WORDS)