Area postrema lesions block feeding induced by systemic injections of monosodium glutamate

Morphological analysis for neuronal pathway from the hindbrain ependymocytes to the hypothalamic kisspeptin neurons

Hindbrain ependymocytes are postulated to have a glucose-sensing role in regulating gonadal functions. Previous studies have suggested that malnutrition-induced suppression of gonadotropin secretion is mediated by noradrenergic inputs from the A2 region in the solitary tract nucleus to the paraventricular nucleus (PVN), and by corticotropin-releasing hormone (CRH) release in the hypothalamus. However, no morphological evidence to indicate the neural pathway from the hindbrain ependymocytes to hypothalamic kisspeptin neurons, a center for reproductive function in mammals, currently exists. The present study aimed to examine the existence of a neuronal pathway from the hindbrain ependymocytes to kisspeptin neurons in the arcuate nucleus (ARC) and anteroventral periventricular nucleus (AVPV). To determine this, wheat-germ agglutinin (WGA), a trans-synaptic tracer, was injected into the fourth ventricle (4V) in heterozygous Kiss1-tandem dimer Tomato (tdTomato) rats, where kisspeptin neurons were visualized by tdTomato fluorescence. 48 h after the WGA injection, brain sections were taken from the forebrain, midbrain and hindbrain and subjected to double immunohistochemistry for WGA and dopamine β-hydroxylase (DBH) or CRH. WGA immunoreactivities were found in vimentin-immunopositive ependymocytes of the 4V and the central canal (CC), but not in the third ventricle. The WGA immunoreactivities were detected in some tdTomato-expressing cells in the ARC and AVPV, DBH-immunopositive cells in the A1–A7 noradrenergic nuclei, and CRH-immunopositive cells in the PVN. These results suggest that the hindbrain ependymocytes have neuronal connections with the kisspeptin neurons, most probably via hindbrain noradrenergic and CRH neurons to relay low energetic signals for regulation of reproduction.

Circadian integration of sleep-wake and feeding requires NPY receptor-expressing neurons in the mediobasal hypothalamus

Sleep and feeding rhythms are highly coordinated across the circadian cycle, but the brain sites responsible for this coordination are unknown. We examined the role of neuropeptide Y (NPY) receptor-expressing neurons in the mediobasal hypothalamus (MBH) in this process by injecting the targeted toxin, NPY-saporin (NPY-SAP), into the arcuate nucleus (Arc). NPY-SAP-lesioned rats were initially hyperphagic, became obese, exhibited sustained disruption of circadian feeding patterns, and had abnormal circadian distribution of sleep-wake patterns. Total amounts of rapid eye movement sleep (REMS) and non-REMS (NREMS) were not altered by NPY-SAP lesions, but a peak amount of REMS was permanently displaced to the dark period, and circadian variation in NREMS was eliminated. The phase reversal of REMS to the dark period by the lesion suggests that REMS timing is independently linked to the function of MBH NPY receptor-expressing neurons and is not dependent on NREMS pattern, which was altered but not phase reversed by the lesion. Sleep-wake patterns were altered in controls by restricting feeding to the light period, but were not altered in NPY-SAP rats by restricting feeding to either the light or dark period, indicating that disturbed sleep-wake patterns in lesioned rats were not secondary to changes in food intake. Sleep abnormalities persisted even after hyperphagia abated during the static phase of the lesion. Results suggest that the MBH is required for the essential task of integrating sleep-wake and feeding rhythms, a function that allows animals to accommodate changeable patterns of food availability. NPY receptor-expressing neurons are key components of this integrative function.

The pedunculopontine tegmental nucleus and responding for sucrose reward

Pedunculopontine tegmental nucleus (PPTg) lesions in rodents lead to increased sucrose consumption, but the psychological deficit behind this remains uncertain. To understand better the relationship between consumption of, and motivation for, sucrose, the authors trained rats to traverse a runway for 20% or 4% sucrose solution; after 7 days, concentrations were reversed. Control rats consumed more 20% than 4% sucrose solution and promptly altered run times in response to concentration change. PPTg-lesioned rats consumed normal quantities of 4% but more 20% sucrose solution than controls and took longer to alter their runway time following the concentration change. These data suggest that lesions of the PPTg do not alter motivation per se and might be better understood as inducing a response selection deficit.

Characterization of orexin A immunoreactivity in the rat area postrema

The distribution of orexin A immunoreactivity and the synaptic relationships of orexin A-positive neurons in the rat area postrema were studied using both light and electron microscopy techniques. At the light microscope level, numerous orexin A-like immunoreactive fibers were found within the area postrema. Using electron microscopy, immunoreactivity within fibers was confined primarily to the axon terminals, most of which contained dense-cored vesicles. Both axo-somatic and axo-dendritic synapses made by orexin A-like immunoreactive axon terminals were found, with these synapses being both symmetric and asymmetric in form. Orexin A-like immunoreactive axon terminals could be found presynaptic to two different immunonegative profiles including the perikarya and dendrites. Occasionally, some orexin A-like immunoreactive profiles, most likely to be dendrites, could be seen receiving synaptic inputs from immunonegative or immunopositive axon terminals. The present results suggest that the physiological function of orexin A in the area postrema depends on synaptic relationships with other immunopositive and immunonegative neurons, with the action of orexin A mediated via a self-modulation feedback mechanism.

Feeding induced by microinjections of NMDA and AMPA-kainate receptor antagonists into ventral striatal and ventral pallidal areas of the pigeon

The participation of glutamatergic circuits of the ventral basal ganglia in feeding-related regulatory mechanisms has been extensively indicated in primate and rodent species. In avian species, it has been shown that ICV injections of MK-801 or of CNQX increase food intake and reduce the latency of feeding initiation in free-feeding pigeons. In the present study, the effects of local injections of MK-801 (6 nmol), CNQX (160 nmol) or vehicle (0.2 microl) into a number of ventral striatopallidal nuclei on feeding, drinking and non-ingestive (sleep, preening) spontaneous behaviors were investigated in free-feeding pigeons (Columba livia). Intense feeding responses associated with an increased duration of feeding behavior were consistently recorded after injections of MK-801 or CNQX into the medial two-thirds of the tuberculum olfactorium (TO), the ventral aspect of lobus parolfactorium (LPOv), or the ventral pallidum (VP). In contrast, the latency of feeding initiation was unaffected by these treatments. No changes in drinking, preening or sleep responses were observed after drug injections into the TO/LPOv/VP area. These data indicate that glutamate-mediated circuits in the TO/LPOv/VP area can play an inhibitory role in feeding behavior in this species, contributing to the conclusion of a feeding bout, thus delaying satiation processes, and that these effects may be mediated by AMPA and NMDA receptors. Additionally, our data support the notion that a region functionally and anatomically comparable to the mammalian accumbens shell may be present in the TO/LPOv/VP region of the pigeon, and that the existence of a glutamatergic circuit in the ventral striatum controlling feeding-related phenomena may represent a highly conserved attribute throughout the amniote's evolution.

Lesions of the dorsal vagal complex abolish increases in meal size induced by NMDA receptor blockade

Rats increase meal size and duration after intraperitoneal injection of MK-801, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist. This effect depends upon intact vagal fibers, since the antagonist does not increase intake when visceral afferent and efferent pathways have been interrupted by bilateral subdiaphragmatic vagotomy. NMDA receptors have been demonstrated on vagal afferent fibers and on second-order neurons in the medial subnucleus of the solitary tract (NTS), the area postrema (AP), and the dorsal motor nucleus of the vagus. To determine whether neurons in these structures are crucial for NMDA receptor effects on feeding, we examined the effect of MK-801 on intake of 15% sucrose in rats with aspiration lesions of the AP and adjacent NTS. MK-801 (100 microg/kg, i.p.) significantly increased sucrose intake in these lesioned rats compared to sham-lesioned rats (32.3+/-0.1 ml versus 23.3+/-0.1 ml, P<0.001). However, when the AP/NTS aspiration lesions were combined with bilateral electrolytic destruction of the medial NTS and the DMV, lesioned rats consumed nearly the same amount of sucrose after either saline or MK-801 (25.9+/-2.4 ml versus 24.3+/-3. 0 ml; P=0.687). By contrast, sham-lesioned controls ingested significantly more sucrose following MK-801 compared to saline (19. 8+/-1.0 ml versus 13.1+/-0.8 ml, P<0.001). These results suggest that an intact caudomedial NTS and/or DMV are necessary for increases in intake induced by NMDA receptor blockade. While the AP might participate in MK-801-induced enhancement of intake, it is not essential for this effect.

Glutamatergic control of food intake in pigeons: effects of central injections of glutamate, NMDA, and AMPA receptor agonists and antagonists

The possible involvement of glutamatergic mechanisms in the control of food intake was studied in free-feeding and in 24-h food-deprived (FD24) pigeons for 1 h after intracerebroventricular (i.c.v.) treatment with glutamate (Glu, 0, 50, 150, 300, and 600 nmol). Glu injections dose dependently induced decreases (30-65%) in food intake (FI) and feeding duration (FD), and increases in latency to start feeding (LSF) in FD24 animals, but not in free-feeding ones. None of these treatments affected noningestive behaviors (locomotion, sleep, and preening). In FD24 pigeons, i.c.v. treatments with N-methyl-D-aspartic acid (NMDA, 0.1, 1, 4, 8, or 16 nmol) or D,L-alpha-amino-3-hydroxy-isoxazole proprionic acid (AMPA, 0.1, 1, 4, or 8 nmol) decreased FI and FD, but left LSF unchanged compared to vehicle-treated FD24 controls. Kainic acid (0.1, 0.5, and 1 nmol), or [trans-(1S,3R)-ACPD-(5NH4OH)] (ACPD, 0.1, 1, 4, 8, and 16 nmol) left unchanged the ingestive profile of FD24 pigeons. Pretreatment with the NMDA receptor antagonist MK-801 (15 nmol) and the AMPA-kainate receptor antagonist CNQX (390 nmol), 20 min before an i.c.v. injection of Glu (300 nmol) induced a partial blockade of the Glu-induced decreases in FI and FD and completely inhibited the Glu-induced increase in LSF in FD24 pigeons. I.c.v. injections of MK-801 (30 nmol) and of CNQX (780 nmol) increased FI and FD and reduced LSF in free-feeding pigeons. A lower dose of MK-801 (15 nmol) increased FI and FD, but not LSF. Conversely, a lower dose of CNQX (390 nmol) reduced LSF without changing FI or FD. These findings indicate the involvement of Glu as a chemical mediator in the regulation of food intake in the pigeon, possibly acting on multiple central mechanisms in this species through NMDA- and AMPA-sensitive Glu receptors.

Neurons containing hypocretin (orexin) project to multiple neuronal systems

The novel neuropeptides called hypocretins (orexins) have recently been identified as being localized exclusively in cell bodies in a subregion of the tuberal part of the hypothalamus. The structure of the hypocretins, their accumulation in vesicles of axon terminals, and their excitatory effect on cultured hypothalamic neurons suggest that the hypocretins function in intercellular communication. To characterize these peptides further and to help understand what physiological functions they may serve, we undertook an immunohistochemical study to examine the distribution of preprohypocretin-immunoreactive neurons and fibers in the rat brain. Preprohypocretin-positive neurons were found in the perifornical nucleus and in the dorsal and lateral hypothalamic areas. These cells were distinct from those that express melanin-concentrating hormone. Although they represent a restricted group of cells, their projections were widely distributed in the brain. We observed labeled fibers throughout the hypothalamus. The densest extrahypothalamic projection was found in the locus coeruleus. Fibers were also seen in the septal nuclei, the bed nucleus of the stria terminalis, the paraventricular and reuniens nuclei of the thalamus, the zona incerta, the subthalamic nucleus, the central gray, the substantia nigra, the raphe nuclei, the parabrachial area, the medullary reticular formation, and the nucleus of the solitary tract. Less prominent projections were found in cortical regions, central and anterior amygdaloid nuclei, and the olfactory bulb. These results suggest that hypocretins are likely to have a role in physiological functions in addition to food intake such as regulation of blood pressure, the neuroendocrine system, body temperature, and the sleep-waking cycle.

The non-competitive NMDA antagonist MK-801 increases food intake in rats

A role for excitatory amino acids in the control of feeding behavior has not been extensively investigated. Nevertheless, there is direct and circumstantial evidence to indicate that some circuits involved with feeding behavior include glutamatergic elements. To test the hypothesis that endogenous glutamate participates in the control of food intake, we performed experiments to determine whether MK-801, a non-competitive N-methyl-D-aspartate (NMDA) ion channel antagonist, is capable of altering intake of liquid and solid foods in hungry or satiated rats. Following a 16 h fast, intake of 15% sucrose was significantly enhanced by systemic treatment with MK-801. Water intake was not altered by the NMDA antagonist. Rats did not ingest more rat chow after MK-801, unless they had been fasted. When a more palatable food (cookies) was offered, MK-801 did increase intake. Thus MK-801 enhanced food intake only when feeding was initiated by food-deprivation or increased palatability. In conclusion, our results support the hypothesis that endogenous glutamate plays a role in the control of food intake. Blockade of NMDA receptor function by MK-801 may diminish or delay satiety signals, rather than initiate feeding behavior per se.

Patch-clamp study on membrane properties and transmitter activated currents of rabbit area postrema neurons

Using the patch-clamp technique in combination wit sliced tissue preparation the membrane properties of newborn rabbit area postrema neurons were investigated. The neurons responded upon depolarization with a fast Na+-current followed by an inactivating and non-inactivating K+-current. GABA-activated currents were investigated resulting in a large C1-(-)conductance, indicating the expression of GABAA-receptors. The expression of glutamate receptor mRNA was studied by in situ hybridization and electrophysiological measurements of these receptors by means of the patch-clamp technique. As a main result it was found that ionotropic glutamate receptors in the area postrema are composed of "flop" variants of the GluA-, GluB- and GluC-subunits.

Effects of opioid antagonists naloxone and naltrexone on neuropeptide Y-induced feeding and brown fat thermogenesis in the rat. Neural site of action

Neuropeptide Y administered intracerebroventricularly and into the paraventricular nucleus of the hypothalamus stimulates feeding and decreases brown adipose tissue thermogenesis. Although specific neuropeptide Y antagonists are not yet available, previous studies had shown that the opioid antagonist naloxone blocked neuropeptide Y-induced feeding when both drugs were injected intracerebroventricularly. We wanted to find out if naloxone injected into specific brain sites would block neuropeptide Y effects on feeding and brown fat thermogenesis. Rats were double injected in specific brain sites with neuropeptide Y and either naloxone or naltrexone (a congener of naloxone). Food intake and brown fat measures were assessed. Naloxone or naltrexone in the paraventricular nucleus weakly decreased paraventricular nucleus neuropeptide Y-induced feeding and did not affect neuropeptide Y-induced reductions in brown fat activity. Peripheral naloxone blocked intracerebroventricular neuropeptide Y-induced feeding and brown fat alterations. Fourth ventricular naloxone decreased paraventricular nucleus neuropeptide Y-induced feeding, and naltrexone given into the nucleus of the solitary tract blocked paraventricular nucleus neuropeptide Y-induced alterations in feeding and brown fat. These data indicate that neuropeptide Y in the paraventricular nucleus may act on feeding and brown fat thermogenesis through opioidergic pathways in the nucleus of the solitary tract.

The lateral hypothalamus: a primary site mediating excitatory amino acid-elicited eating

Lateral hypothalamic (LH) injections of the excitatory neurotransmitter glutamate, or its excitatory amino acid (EAA) agonists, kainic acid (KA), D,L-alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid (AMPA), or N-methyl-D-aspartic acid (NMDA), can rapidly elicit an intense feeding response in satiated rats. To determine whether the LH is the actual locus of this effect, we compared these compounds' ability to stimulate feeding when injected into the LH, versus when injected into sites bracketing this region. Food intake in groups of adult male rats was measured 1 h after injection of glutamate (30-900 nmol), KA (0.1-1.0 nmol), AMPA (0.33-3.3 nmol), NMDA (0.33-33.3 nmol) or vehicle, through chronically implanted guide cannulas, into one of seven brain sites. These sites were: the LH, the anterior and posterior tips of the LH, the thalamus immediately dorsal to the LH, the amygdala just lateral to the LH, or the paraventricular and perifornical areas medial to the LH. The results show that across doses and agonists the eating-stimulatory effects were largest with injections into the LH. In the LH, glutamate between 300 and 900 nmol elicited a dose-dependent eating response of up to 5 g within 1 h (P < 0.01). Each of the other agonists at doses of 3.3 nmol or less elicited eating responses of at least 10 g with injections into this site. Injections into the other brain sites produced either no eating, or occasionally smaller and less consistent eating responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Lateral hypothalamic injections of glutamate, kainic acid, D,L-alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid or N-methyl-D-aspartic acid rapidly elicit intense transient eating in rats

A convergence of evidence suggests that stimulation of lateral hypothalamic (LH) neurons can elicit eating, but the neurotransmitters that mediate this effect are unknown. To determine whether glutamate might be involved, it was injected through chronic guide cannulas directly into the LH of satiated adult male rats and consequent food intake was measured. Glutamate produced a dose-dependent eating response (mean intakes of 3.7 g at 300 nmol and 5.2 g at 900 nmol) only within the first hour after injection. As a first step in determining the receptor types mediating this response, agonists for specific excitatory amino acid (EAA) receptors were similarly tested. Kainic acid (KA), D,L-alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid (AMPA) or N-methyl-D-aspartic acid (NMDA) injected into the LH each elicited eating in a dose-dependent fashion beginning at 0.33 to 1.0 nmol. At maximally effective doses (1.0-33 nmol), each agonist elicited food intakes of approximately nine grams within 1 h. Finally, analysis of meal and behavioral patterns produced by LH injection of glutamate (600 nmol) and KA (1.0 nmol) revealed that the elicited eating usually began 2-3 min postinjection and consisted of a single normal to large size meal. There were no other behavioral effects during this initial postinjection period and no effects on other oral behaviors, like drinking or gnawing, at any time. Collectively, these findings suggest that glutamate may act through several subtypes of its receptors on some LH neurons to elicit eating.

Central effects of monosodium glutamate on feeding behavior in adult Long-Evans rats

Monosodium glutamate (MSG) is known as a neurotoxic molecule when injected neonatally in rats, where it produces a marked decrease in food intake and an increase in adipose tissue mass. But, in adult rats subcutaneous injections of MSG produce a small, dose-dependent increase in food intake. It is not known if this action is centrally or systemically mediated. Therefore, the feeding pattern of adult rats injected intracerebroventricularly with MSG was measured. Seven days after installation of a cannula in the right lateral ventricle, rats were injected either with artificial cerebrospinal fluid or twice with 3 mg/brain MSG within a 3-day interval. The feeding pattern was recorded via a complete computerized system during 24 h. Feeding behavior was significantly modified by MSG treatments. These effects were observed immediately after drug injections, that is, upon the first meal, as well as during the 24 h that followed. For the first meal, modifications in meal size (+285%; p = 0.0001), meal duration (x10; p = 0.0005), postmeal interval (x4; p = 0.0005), and the satiety ratio (-50%; p = 0.01) were observed. During the 24-h postinjection period, modifications in meal number (-3; p = 0.0007), total amount of food eaten (+21%,; p = 0.007), time spent eating (+40%; p = 0.007), meal duration (+53%; p = 0.005), and meal size (+44%; p = 0.01) were noted. When the two MSG injections were compared, differences were also noted. For the first meal, postmeal interval (-50%; p < 0.005) and satiety ratio (-50%; p < 0.005) were decreased after the second injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of male sexual behavior by serotonin application in the medial preoptic area

The study investigated the possible involvement of serotonin in the medial preoptic area in the regulation of sexual behavior of male rats. Injection of serotonin in the medial preoptic area resulted in an inhibition, whereas cyproheptadine (a serotonin antagonist) produced a slight facilitation, of male sexual behavior.