Feeding-induced changes of melanin-concentrating hormone (MCH)-like immunoreactivity in goldfish brain

Intracerebroventricular (ICV) injection of melanin-concentrating hormone (MCH) influences feeding behavior in the goldfish and exerts an anorexigenic action in goldfish brain, unlike its orexigenic action in mammals. Despite a growing body of knowledge concerning MCH function in mammals, the role of MCH in appetite has not yet been well studied in fish. The aim of the present study was to investigate the involvement of endogenous MCH in the feeding behavior of the goldfish. We examined the distribution of MCH-like immunoreactivity (MCH-LI) in the goldfish brain and the effect of feeding status upon this distribution. Neuronal cell bodies containing MCH-LI were localized specifically to four areas of the hypothalamus. Nerve fibers with MCH-LI were found mainly in the neurohypophysis, with a few in the telencephalon, mesencephalon, and diencephalon. The number of neuronal cell bodies containing MCH-LI in the dorsal area adjoining the lateral recess of the third ventricle in the posterior and inferior lobes of the hypothalamus showed a significant decrease in fasted fish compared with that in normally fed fish, although other areas showed no evident differences. We also administered an antiserum against fish MCH (anti-MCH serum) by ICV injection and examined its immunoneutralizing effect on food intake by using an automatic monitoring system. Cumulative food intake was significantly increased by ICV injection of the anti-MCH serum. These results indicate that MCH potentially functions as an anorexigenic neuropeptide in the goldfish brain, and that the further study of the evolutionary background of the MCH system and its role in appetite is warranted. This work was supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (K.M. and A.T.) and by a research grant from the Toyama Marine Biotechnology Association (K.M.).     

Ghrelin对大鼠摄食的影响及orexins信号通路的调控

目的:探究Ghrelin对大鼠摄食的影响及orexins信号通路的调控作用。方法:采用免疫组织化学染色的方法观察Ghrelin免疫阳性神经元轴突末梢与orexin神经元的突触联系以及下丘脑外侧区(LHA)内c-fos的表达。侧脑室注射抗-orexin-A IgG和抗-orexin-B IgG混合液、抗-黑色素浓集激素(MCH)IgG、NPY-1受体拮抗剂后测量大鼠摄食量,观察其对ghrelin诱导摄食的影响。结果:Ghrelin免疫阳性神经元轴突末梢与orexin神经元的突触相接触。侧脑室注射ghrelin可诱导orexin神经元内c-fos表达,但是没有引起MCH神经元内c-fos的表达。预先注射抗-NPY IgG抗体,ghrelin仍然可诱导orexin神经元内c-fos表达。侧脑室预先注射抗-orexin-A IgG和抗-orexin-B IgG抗体可减弱ghrelin促摄食作用,但是预先注射抗-MCH IgG抗体对ghrelin诱导的摄食作用没有明显影响。注射NPY受体拮抗剂可进一步加强抗-orexin-A IgG抗体和抗-orexin-B IgG抗体对ghrelin诱导摄食的抑制效应。结论:ghrelin可能与orexin系统相互作用共同参与摄食和能量平衡的调控。     

Fluctuations in central and peripheral temperatures associated with feeding behavior in rats

We examined the pattern of temperature fluctuations in the nucleus accumbens (NAcc), temporal muscle, and skin, along with locomotion in food-deprived and nondeprived rats following the presentation of an open or closed food container and during subsequent eating or food-seeking behavior without eating. Although rats in food-deprived, quiet resting conditions had more than twofold lower spontaneous locomotion and lower temperature values than in nondeprived conditions, after presentation of a container, they consistently displayed food-seeking behavior, showing much larger and longer temperature changes. When the container was open, rats rapidly retrieved food and consumed it. Food consumption was preceded and accompanied by gradual increases in brain and muscle temperatures ( approximately 1.5 degrees C) and a weaker, delayed increase in skin temperature ( approximately 0.8 degrees C). All temperatures began to rapidly fall immediately after eating was completed, but NAcc and muscle temperatures returned to baseline after approximately 35 min. When the container was closed and rats were unable to obtain food, they continued food-seeking activity during the entire period of presentation. Similar to eating, this activity was preceded and accompanied by gradual temperature increases in the brain and muscle, which were somewhat smaller than those during eating ( approximately 1.2 degrees C), with no changes in skin temperature. In contrast to trials with eating, NAcc and muscle temperatures continued to increase for approximately 10 min after the container was removed from the cage and the rat continued food-seeking behavior, with a return to baselines after approximately 50 min. These temperature fluctuations are discussed with respect to alterations in metabolic brain activity associated with feeding behavior, depending upon deprivation state and food availability.     

Role of brain dopamine in food reward and reinforcement

The ability of food to establish and maintain response habits and conditioned preferences depends largely on the function of brain dopamine systems. While dopaminergic transmission in the nucleus accumbens appears sufficient for some forms of reward, the role of dopamine in food reward does not appear to be restricted to this region. Dopamine plays an important role in both the ability to energize feeding and to reinforce food-seeking behaviour; the role in energizing feeding is secondary to the prerequisite role in reinforcement. Dopaminergic activation is triggered by the auditory and visual as well as the tactile, olfactory, and gustatory stimuli of foods. While dopamine plays a central role in the feeding and food-seeking of normal animals, some food rewarded learning can be seen in genetically engineered dopamine-deficient mice.     

How Long do Vespula Germanica Wasps Search for a Food Source that is no Longer Available?

Vespula germanica foragers return to a food source that has not been depleted. In this work we investigate how long wasps continue searching for a food source that is no longer available. We first trained wasps to feed on a dish surrounded by four yellow cylinders, and then, during the testing phase, we removed the food, and recorded foragers’ behavior until wasps stopped visiting the array. Two groups received different treatments: one received one feeding trial and the other three. Wasps trained with three consecutive trials searched over the original array approximately three times longer than those receiving one. Furthermore, the number of hovers and landings over the array was significantly higher for wasps trained with three trials than for wasps trained with one. Finally, initial level of response (i.e. number of hovers and landings in the first visit during the testing phase) was significantly higher in the group with three trials than in the group with one. We discuss the biological significance of memory extinction in these generalist wasps, in relation to the level of uncertainty of the resources they exploit. The plasticity to extinguish differently an association between a stimulus and a food resource could be one of the various behavioral mechanisms in V. germanica wasps that had allowed the species to get successfully established in new areas of the world.     

Competitive dominance of wild and domestic Norway rats (Rattus norvegicus)

The objective was to determine differences in the relative dominance of wild and domestic Norway rats (Rattus norvegicus) when competing for food in inter-stock, round-robin pairings. Each day, after food-deprivation, wild and domestic rats were individually placed in a testing arena containing a slice of apple. On day 20 and every fourth day thereafter, eight wild-domestic pairs were allowed to compete for the apple. Domestic subjects spent more time eating than did wild rats and were more often dominant. Competition tended to wane over test days in response to defeat in earlier trials. The outcome of competition trials was not changed by increasing the extent of food deprivation together with continuous pairing. A reduction in social inhibitions accompanying the domestication process provides the best explanation for the enhanced competitive potential of the domestic rat.     

White Flank Spots Signal Feeding Dominance in Female Diamond Firetails,Stagonopleura guttata

Plumage colour can be used as an honest signal to convey health and status, which has traditionally been examined in the sexual selection context of choosy females and elaborate males. We use a model avian system to study the role of plumage coloration in a social context such as inter‐ and intrasexual competition over food resources. The diamond firetail (Stagonopleura guttata) is an endemic Australian finch: females have more white flank spots than males, and white spot number was correlated with cell‐mediated immune response in females. We use two experimental designs to test the role of white flank spots for feeding dominance and dominance discrimination in a group‐living bird. The results from two‐choice trials and from single‐arena trials showed that female ornamentation was consistently important in social food contests, and males consistently responded to female spot number. Females with higher spot number fed first, in trials with males and/or females. Also, females preferred to feed next to test birds with low spot number, but males showed no preference for feeding next to birds with few or many spots. Finally, latency to feed was predicted by spot number: both males and females had longer latency to feed if test birds had more spots than the focal birds. We conclude that female, but not male, ornamentation was important for inter‐ and intrasexual food competition. This is one of the very few studies to show that the same plumage ornament can have a different function between the sexes as a signal of social status. Moreover, this study shows that white plumage can be a signal of dominance.     

Chronic loss of melanin-concentrating hormone affects motivational aspects of feeding in the rat

Current epidemic obesity levels apply great medical and financial pressure to the strenuous economy of obesity-prone cultures, and neuropeptides involved in body weight regulation are regarded as attractive targets for a possible treatment of obesity in humans. The lateral hypothalamus and the nucleus accumbens shell (AcbSh) form a hypothalamic-limbic neuropeptide feeding circuit mediated by Melanin-Concentrating Hormone (MCH). MCH promotes feeding behavior via MCH receptor-1 (MCH1R) in the AcbSh, although this relationship has not been fully characterized. Given the AcbSh mediates reinforcing properties of food, we hypothesized that MCH modulates motivational aspects of feeding.Here we show that chronic loss of the rat MCH-precursor Pmch decreased food intake predominantly via a reduction in meal size during rat development and reduced high-fat food-reinforced operant responding in adult rats. Moreover, acute AcbSh administration of Neuropeptide-GE and Neuropeptide-EI (NEI), both additional neuropeptides derived from Pmch, or chronic intracerebroventricular infusion of NEI, did not affect feeding behavior in adult pmch(+/+) or pmch(-/-) rats. However, acute administration of MCH to the AcbSh of adult pmch(-/-) rats elevated feeding behavior towards wild type levels. Finally, adult pmch(-/-) rats showed increased ex vivo electrically evoked dopamine release and increased limbic dopamine transporter levels, indicating that chronic loss of Pmch in the rat affects the limbic dopamine system.Our findings support the MCH-MCH1R system as an amplifier of consummatory behavior, confirming this system as a possible target for the treatment of obesity. We propose that MCH-mediated signaling in the AcbSh positively mediates motivational aspects of feeding behavior. Thereby it provides a crucial signal by which hypothalamic neural circuits control energy balance and guide limbic brain areas to enhance motivational or incentive-related aspects of food consumption.     

Spatial response properties of homing pigeon hippocampal neurons: correlations with goal locations,movement between goals,and environmental context in a radial-arm arena

The amniote hippocampal formation plays an evolutionarily-conserved role in the neural representation of environmental space. However, species differences in spatial ecology nurture the expectation of species differences in how hippocampal neurons represent space. To determine the spatial response properties of homing pigeon (Columba livia) HFneurons, we recorded from isolated units in birds freely navigating a radial arena in search of food present at four goal locations. Fifty of 76 neurons displayed firing rate variations that could be placed into three response categories. Location cells (n=25) displayed higher firing rates at restricted locations in the arena space, often in proximity to goal locations. Path cells (n=13) displayed higher firing rates as a pigeon moved between a subset of goal locations. Arena-off cells (n=12) were more active when a pigeon was in a baseline holding space compared to inside the arena. Overall, reliability and coherence scores of the recorded neurons were lower compared to rat place cells. The differences in the spatial response profiles of pigeon hippocampal formation neurons, when compared to rats, provide a departure point for better understanding the relationship between spatial behavior and how hippocampal formation neurons participate in the representation of space.     

Neuropeptide K suppresses feeding in the rat

We studied the effects of neuropeptide K (NPK), a 36 amino acid residue peptide of the tachykinin family, on latency to onset of feeding and cumulative 1 and 2 h food intake in three experimental paradigms. Intraperitoneal injection of NPK (1.25 and 3.14 nmol) to food-deprived rats delayed the onset of feeding and significantly decreased the cumulative food intake. Intraperitoneal injection of NPK (1.25 and 3.14 nmol) to water-deprived rats produced no effect on subsequent drinking behavior. Similarly, intraperitoneal injection of NPK (3.14 nmol) 15 min before onset of the dark phase (of the light-dark cycle) significantly delayed the occurrence of ingestive behavior and the cumulative food intake was markedly suppressed. Furthermore, administration of NPK intraperitoneally (0.5-3.14 nmol) 15 min before intraventricular (i.c.v.) injection of neuropeptide Y (NPY 0.47 nmol) to satiated rats significantly suppressed NPY-induced feeding and delayed the onset of ingestive behavior. However, when administered centrally prior to NPY injection, NPK delayed the onset of feeding response only. Collectively, these findings show that NPK can acutely and consistently suppress feeding behavior.     

Beyond skin color: emerging roles of melanin-concentrating hormone in energy homeostasis and other physiological functions

Melanin-concentrating hormone (MCH) is a cyclic peptide that mediates its effects by the activation of two G-protein-coupled seven transmembrane receptors (MCHR1 and MCHR2) in humans. In contrast to its primary role in regulating skin color in fish, MCH has evolved in mammals to regulate dynamic physiological functions, from food intake and energy expenditure to behavior and emotion. Chronic infusion or transgenic expression of MCH stimulates feeding and increases adipocity, whereas targeted deletion of MCH or its receptor (MCHR1) leads to resistance to diet-induced obesity with increased energy expenditure and thermogenesis. The involvement of MCH in energy homeostasis and in brain activity has also been validated in mice treated with non-peptide antagonists, suggesting that blockade of MCHR1 could provide a viable approach for treatment of obesity and certain neurological disorders. This review focuses on emerging roles of MCH in regulating central and peripheral mechanisms.     

Intrathecal melanin-concentrating hormone reduces sympathetic tone and blocks cardiovascular reflexes

Melanin-concentrating hormone (MCH) is a neuropeptide that acts to increase feeding behavior and decrease energy expenditure. The role of MCH in central cardiorespiratory regulation is still poorly understood. Experiments were conducted on urethane-anesthetized, vagotomized, and artificially ventilated male Sprague-Dawley rats (n = 22) to ascertain whether MCH modulates sympathetic vasomotor tone, as well as barosympathetic, chemosympathetic, and somatosympathetic reflexes at the level of the spinal cord. Intrathecal injection of 10 μl of MCH produced a dose-dependent hypotension, bradycardia, and sympathoinhibition. Peak response was observed following administration of 1 mM MCH, causing a decrease in mean arterial pressure of 39 ± 2 mmHg (P < 0.001), splanchnic sympathetic nerve activity of 78 ± 11% (P < 0.001), and heart rate of 87 ± 11 beats per minute (bpm) (P < 0.01). The two peaks of the somatosympathetic reflex were decreased by intrathecal MCH, 7 ± 3% (P < 0.01) and 31 ± 6% (P < 0.01), respectively, and the spinal component of the reflex was accentuated 96 ± 23% (P < 0.05), with respect to the baseline for MCH, compared with the two peaks and spinal component of the somatosympathetic reflex elicited following saline injection with respect to the baseline for saline. MCH decreased the sympathetic gain to 120 s of hyperoxic hypercapnea (10% CO(2) in 90% O(2)) and to 10-12 s poikilocapneic anoxia (100% N(2)) from 0.74 ± 0.14%/s to 0.23 ± 0.04%/s (P < 0.05) and 16.47 ± 3.2% to 4.35 ± 1.56% (P < 0.05), respectively. There was a 34% decrease in gain and a 62% decrease in range of the sympathetic baroreflex with intrathecal MCH. These data demonstrate that spinal MCH blunts the central regulation of sympathetic tone and adaptive sympathetic reflexes.     

The influence of past experience on wasp choice related to foraging behavior

Memory has been little studied in social wasps. Vespula germanica （Fab.） （Hymenoptera： Vespidae） frequently revisits nondepleted food sources, making several trips between the resource and the nest. In this study, we analyzed this relocating behavior in order to evaluate whether this species is capable of remembering an established association after 1 h. To this end, we trained wasps to feed from a certain array. Then it was removed, setting it up again 1 h later, but this time 2 baited feeders were put in place, one at the original feeding site and the other opposite the first. We recorded the proportion of returning foragers, and their choice of feeder, after either 1 or 4 feeding trials. After 1 h, 78% of wasps trained with 4 feeding trials and 65% trained with 1, returned to the experimental area. Furthermore, during the testing phase, wasps trained with 4 feeding trials collected food from the previously learned feeder significantly more frequently than from the nonlearned one （P 〈 0.05）. In contrast, wasps that had been trained only once chose both feeders equally. Thus, memory retrieval could be observed 1 h after wasps had collected food on 4 consecutive occasions, but not after only 1. To our knowledge, this is the first study showing that V. germanica is capable of remembering an association 1 h after the last associative event, demonstrating that 1 h does not impair memory retention if4 feeding experiences have occurred.     

Chronic intracerebroventricular infusion of MCH causes obesity in mice. Melanin-concentrating hormone

Melanin-concentrating hormone (MCH) is a cyclic amino acid neuropeptide localized in the lateral hypothalamus. Although MCH is thought to be an important regulator of feeding behavior, the involvement of this peptide in body weight control has been unclear. To examine the role of MCH in the development of obesity, we assessed the effect of chronic intracerebroventricular infusion of MCH in C57BL/6J mice that were fed with regular or moderately high-fat (MHF) diets. Intracerebroventricular infusion of MCH (10 microg/day for 14 days) caused a slight but significant increase in body weight in mice maintained on the regular diet. In the MHF diet-fed mice, MCH more clearly increased the body weight accompanied by a sustained hyperphagia and significant increase in fat and liver weights. Plasma glucose, insulin, and leptin levels were also increased in the MCH-treated mice fed the MHF diet. These results suggest that chronic stimulation of the brain MCH system causes obesity in mice and imply that MCH may have a major role in energy homeostasis.     

Involvement of the opioid system in the orexigenic and hedonic effects of melanin-concentrating hormone

Melanin-concentrating hormone (MCH) exerts an orexigenic effect that resembles that of opioids, suggesting that the MCH and opioid systems could interact in controlling the food intake behavior. Three series of experiments were conducted in male Wistar rats: 1) to test the ability of the κ-, μ-, and δ-opioid receptor antagonists binaltorphimine (nor-BNI-κ), β-funaltrexamine (β-FNA-μ), and naltrindole (NTI-δ), respectively, to block the stimulating effects of MCH on food intake; 2) to verify the ability of MCH to induce a positive hedonic response to a sweet stimulus when injected into the nucleus accumbens shell (NAcSh) or right lateral ventricle (LV) of the brain; and 3) to assess the ability of nor-BNI, β-FNA, and NTI to block the effects of MCH on the hedonic response to a sweet stimulus. Nor-BNI, NTI (0, 10 and 40 nmol), and β-FNA (0, 10 and 50 nmol) were administered into the LV prior to injecting MCH (2.0 nmol). To assess the hedonic response, rats were implanted with an intraoral cannula allowing for the infusion of a sweet solution into the oral cavity. Food intake was assessed in sated rats during the first 3 h following the MCH or vehicle (i.e., artificial cerebrospinal fluid) injection. The hedonic response to a sweet stimulus was assessed by examining facial mimics, following the intraoral administration of a sucrose solution. Blockade of each of the three opioid receptors by selective antagonists prevented MCH-induced feeding. Furthermore, MCH-injections into the NAcSh and right LV resulted in enhanced hedonic responses. Finally, antagonism of the three opioid receptors blunted the LV-injected, MCH-induced, facial-liking expressions in response to an intraoral sweet stimulus. Overall, the present study provides evidence to link the MCH and opioid systems in the food intake behavior.     

Melanin concentrating hormone induces hippocampal acetylcholine release via the medial septum in rats

Among various actions of melanin concentrating hormone (MCH), its memory function has been focused in animal studies. Although MCH neurons project to various areas in the brain, one main target site of MCH is hippocampal formation for memory consolidation. Recent immunohistochemical study shows that MCH neurons directly project to the hippocampal formation and may indirectly affect the hippocampus through the medial septum nucleus (MS). It has been reported that sleep is necessary for memory and that hippocampal acetylcholine (ACh) release is indispensable for memory consolidation. However, there is no report how MCH actually influences the hippocampal ACh effluxes in accordance with the sleep–wake cycle changes. Thus, we investigated the modulatory function of intracerebroventricular (icv) injection of MCH on the sleep–wake cycle and ACh release using microdialysis techniques. Icv injection of MCH significantly increased the rapid eye movement (REM) and non-REM episode time and the hippocampal, not cortical, ACh effluxes. There was a significant correlation between REM episode time and hippocampal ACh effluxes, but not between REM episode time and cortical ACh effluxes. Microinjection of MCH into the MS increased the hippocampal ACh effluxes with no influence on the REM episode time. It appears that the effect sites of icv MCH for prolongation of REM episode time may be other neuronal areas than the cholinergic neurons in the MS. We conclude that MCH actually increases the hippocampal ACh release at least in part through the MS in rats.     

Alpha-melanocyte-stimulating hormone mediates melanin-concentrating hormone-induced anorexigenic action in goldfish

In goldfish, intracerebroventricular (ICV) administration of melanin-concentrating hormone (MCH) inhibits feeding behavior, and fasting decreases hypothalamic MCH-like immunoreactivity. However, while MCH acts as an anorexigenic factor in goldfish, in rodents MCH has an orexigenic effect. Therefore, we examined the involvement of two anorexigenic neuropeptides, alpha-melanocyte-stimulating hormone (alpha-MSH) and corticotropin-releasing hormone (CRH), in the anorexigenic action of MCH in goldfish, using an alpha-MSH receptor antagonist, HS024, and a CRH receptor antagonist, alpha-helical CRH((9-41)). ICV injection of HS024, but not alpha-helical CRH((9-41)), suppressed MCH-induced anorexigenic action for a 60-min observation period. We then examined, using a real-time PCR method, whether MCH affects the levels of mRNAs encoding various orexigenic neuropeptides, including neuropeptide Y (NPY), orexin, ghrelin and Agouti-related peptide (AgRP), in the goldfish diencephalon. ICV administration of MCH at a dose sufficient to inhibit food consumption decreased the expression of mRNAs for NPY and ghrelin, but not for orexin and AgRP. These results indicate that the anorexigenic action of MCH in the goldfish brain is mediated by the alpha-MSH signaling pathway and is accompanied by inhibition of NPY and ghrelin synthesis.     

Melanin-concentrating hormone producing neurons: Activities and modulations

Regulation of energy homeostasis in animals involves adaptation of energy intake to its loss, through a perfect regulation of feeding behavior and energy storage/expenditure. Factors from the periphery modulate brain activity in order to adjust food intake as needed. Particularly, “first order” neurons from arcuate nucleus are able to detect modifications in homeostatic parameters and to transmit information to “second order” neurons, partly located in the lateral hypothalamic area. These “second order” neurons have widespread projections throughout the brain and their proper activation leads them to a coordinated response associated to an adapted behavior. Among these neurons, melanin-concentrating hormone (MCH) expressing neurons play an integrative role of the various factors arising from periphery, first order neurons and extra-hypothalamic arousal systems neurons and modulate regulation of feeding, drinking and seeking behaviors. As regulation of MCH release is correlated to regulation of MCH neuronal activity, we focused this review on the electrophysiological properties of MCH neurons from the lateral hypothalamic area. We first reviewed the knowledge on the endogenous electrical properties of MCH neurons identified according to various criteria which are described. Then, we dealt with the modulations of the electrical activity of MCH neurons by different factors such as glucose, glutamate and GABA, peptides and hormones regulating feeding and transmitters of extra-hypothalamic arousal systems. Finally, we described the current knowledge on the modulation of MCH neuronal activity by cytokines and chemokines. Because of such regulation, MCH neurons are some of the best candidate to account for infection-induced anorexia, but also obesity.     

Foraging arena size and structural complexity affect the dynamics of food distribution in ant colonies

Food acquisition by ant colonies is a complex process that starts with acquiring food at the source (i.e., foraging) and culminates with food exchange in or around the nest (i.e., feeding). While ant foraging behavior is relatively well understood, the process of food distribution has received little attention, largely because of the lack of methodology that allows for accurate monitoring of food flow. In this study, we used the odorous house ant, Tapinoma sessile (Say) to investigate the effect of foraging arena size and structural complexity on the rate and the extent of spread of liquid carbohydrate food (sucrose solution) throughout a colony. To track the movement of food, we used protein marking and double-antibody sandwich enzyme-linked immunosorbent assay, DAS-ELISA. Variation in arena size, in conjunction with different colony sizes, allowed us to test the effect of different worker densities on food distribution. Results demonstrate that both arena size and colony size have a significant effect on the spread of the food and the number of workers receiving food decreased as arena size and colony size increased. When colony size was kept constant and arena size increased, the percentage of workers testing positive for the marker decreased, most likely because of fewer trophallactic interactions resulting from lower worker density. When arena size was kept constant and colony size increased, the percentage of workers testing positive decreased. Nonrandom (clustered) worker dispersion and a limited supply of food may have contributed to this result. Overall, results suggest that food distribution is more complete is smaller colonies regardless of the size of the foraging arena and that colony size, rather than worker density, is the primary factor affecting food distribution. The structural complexity of foraging arenas ranged from simple, two-dimensional space (empty arenas) to complex, three-dimensional space (arenas filled with mulch). The structural complexity of foraging arenas had a significant effect on food distribution and the presence of substrate significantly inhibited the spread of food. Structural complexity of foraging arenas and the resulting worker activity patterns might exert considerable influence on socioecological processes in ants and should be considered in laboratory assays.     

Melanin concentrating hormone increase hippocampal synaptic transmission in the rat

A retrograde facilitation has been demonstrated in the one trial step-down inhibitory avoidance of melanin-concentrating hormone (MCH), when it was infused into rat hippocampal formation. Considering the high density of specific binding sites for the MCH peptide on the hippocampus and the participation of this structure on learning and memory processes we have studied the effects of MCH on the hippocampal synaptic transmission. For this purpose, slices of rat hippocampus were perfused with different concentration of MCH. The main result of the present study was a long-lasting potentiation on the hippocampal evoked response on dentate gyrus induced by MCH (4-11 microM) at 30, 60 and 120 min with a maximum effect at 120 min. Previous perfusion of DL - 2- amino - 5 phosphonovaleric acid (APV, 20 microM) was unable to impair the increased hippocampal evoked response induced by MCH 4 microM. On the other hand, the channel blocker Dizocilpine (MK-801, 10 microM) completely impaired the increased hippocampal synaptic plasticity induced by MCH perfusion. We postulate the increased hippocampal synaptic efficacy induced by MCH as one of the mechanisms underlying the retrograde facilitation on the inhibitory avoidance paradigm, observed after MCH hippocampal microinjection. We cannot rule out other MCH neurochemical mechanism and other areas of the brain involved in the MCH effects.