Douglas J. Guarnieri

Endogenous leptin signaling in the caudal nucleus tractus solitarius and area postrema is required for energy balance regulation.

Medial nucleus tractus solitarius (mNTS) neurons express leptin receptors (LepRs), and intra-mNTS delivery of leptin reduces food intake and body weight. Here, the contribution of endogenous LepR signaling in mNTS neurons to energy balance control was examined. Knockdown of LepR in mNTS and area postrema (AP) neurons of rats (LepRKD) via adeno-associated virus short hairpin RNA-interference (AAV-shRNAi) resulted in significant hyperphagia for chow, high-fat, and sucrose diets, yielding increased body weight and adiposity. The chronic hyperphagia of mNTS/AP LepRKD rats is likely mediated by a reduction in leptin potentiation of gastrointestinal satiation signaling, as LepRKD rats showed decreased sensitivity to the intake-reducing effects of cholecystokinin. LepRKD rats showed increased basal AMP-kinase activity in mNTS/AP micropunches, and pharmacological data suggest that this increase provides a likely mechanism for their chronic hyperphagia. Overall these findings demonstrate that LepRs in mNTS and AP neurons are required for normal energy balance control.

MicroRNAs: A new class of gene regulators

Elucidation of the molecular basis of disease depends upon continued progress in defining the mechanisms by which genomic information is encoded and expressed. Transcription factor‐mediated regulation of mRNA is clearly a major source of regulatory control and has been well studied. The more recent discovery of small RNAs as key regulators of gene function has introduced a new level and mechanism of regulation. Mammalian genomes contain hundreds of microRNAs (miRNAs) that each can potentially downregulate many target genes. This suggests a new source for broad control over gene regulation and has inspired extensive interest in defining miRNAs and their functions. Here, the identification of miRNAs, their biogenesis, and some examples of miRNA effects on biology and disease are reviewed and discussed. Emphasis is placed on the possible role for miRNA in nervous system development, function, and disease.

Metabolic hormones, dopamine circuits, and feeding

Recent evidence has emerged demonstrating that metabolic hormones such as ghrelin and leptin can act on ventral tegmental area (VTA) midbrain dopamine neurons to influence feeding. The VTA is the origin of mesolimbic dopamine neurons that project to the nucleus accumbens (NAc) to influence behavior. While blockade of dopamine via systemic antagonists or targeted gene delete can impair food intake, local NAc dopamine manipulations have little effect on food intake. Notably, non-dopaminergic manipulations in the VTA and NAc produce more consistent effects on feeding and food choice. More recent genetic evidence supports a role for the substantia nigra-striatal dopamine pathways in food intake, while the VTA-NAc circuit is more likely involved in higher-order aspects of food acquisition, such as motivation and cue associations. This rich and complex literature should be considered in models of how peripheral hormones influence feeding behavior via action on the midbrain circuits.

Increased Ethanol Resistance and Consumption in Eps8 Knockout Mice Correlates with Altered Actin Dynamics

Dynamic modulation of the actin cytoskeleton is critical for synaptic plasticity, abnormalities of which are thought to contribute to mental illness and addiction. Here we report that mice lacking Eps8, a regulator of actin dynamics, are resistant to some acute intoxicating effects of ethanol and show increased ethanol consumption. In the brain, the N-methyl-D-aspartate (NMDA) receptor is a major target of ethanol. We show that Eps8 is localized to postsynaptic structures and is part of the NMDA receptor complex. Moreover, in Eps8 null mice, NMDA receptor currents and their sensitivity to inhibition by ethanol are abnormal. In addition, Eps8 null neurons are resistant to the actin-remodeling activities of NMDA and ethanol. We propose that proper regulation of the actin cytoskeleton is a key determinant of cellular and behavioral responses to ethanol.

Medial prefrontal D1 dopamine neurons control food intake

Although the prefrontal cortex influences motivated behavior, its role in food intake remains unclear. Here, we demonstrate a role for D1-type dopamine receptor–expressing neurons in the medial prefrontal cortex (mPFC) in the regulation of feeding. Food intake increases activity in D1 neurons of the mPFC in mice, and optogenetic photostimulation of D1 neurons increases feeding. Conversely, inhibition of D1 neurons decreases intake. Stimulation-based mapping of prefrontal D1 neuron projections implicates the medial basolateral amygdala (mBLA) as a downstream target of these afferents. mBLA neurons activated by prefrontal D1 stimulation are CaMKII positive and closely juxtaposed to prefrontal D1 axon terminals. Finally, photostimulating these axons in the mBLA is sufficient to increase feeding, recapitulating the effects of mPFC D1 stimulation. These data describe a new circuit for top-down control of food intake.

Drosophila melanogaster, A genetic model system for alcohol research

In its natural environment, which consists of fermenting plant materials, the fruit fly Drosophila melanogaster encounters high levels of ethanol. Flies are well equipped to deal with the toxic effects of ethanol; they use it as an energy source and for lipid biosynthesis. The primary ethanol-metabolizing pathway in flies involves the enzymes alcohol dehydrogenase (ADH) and acetaldehyde dehydrogenase (ALDH); their role in adaptation to ethanol-rich environments has been studied extensively. The similarity between Drosophila and mammals is not restricted to the manner in which they metabolize ethanol; behaviors elicited by ethanol exposure are also remarkably similar in these organisms. Flies show signs of acute intoxication, which range from locomotor stimulation at low doses to complete sedation at higher doses, they develop tolerance upon intermittent ethanol exposure, and they appear to like ethanol, showing preference for ethanol-containing media. Molecular genetic analysis of ethanol-induced behaviors in Drosophila, while still in its early stages, has already revealed some surprising parallels with mammals. The availability of powerful tools for genetic manipulation in Drosophila, together with the high degree of conservation at the genomic level, make Drosophila a promising model organism to study the mechanism by which ethanol regulates behavior and the mechanisms underlying the organisms adaptation to long-term ethanol exposure.

Orexin mediates morphine place preference, but not morphine-induced hyperactivity or sensitization

Orexin (or hypocretin) has been implicated in mediating drug addiction and reward. Here, we investigated orexins contribution to morphine-induced behavioral sensitization and place preference. Orexin-/- (OKO) mice and littermate wild-type (WT) controls (n=56) and C57BL/6J mice (n=67) were tested for chronic morphine-induced locomotor sensitization or for conditioned place preference (CPP) for a morphine- or a cocaine-paired environment. C57BL/6J mice received the orexin receptor 1 (Ox1r) antagonist, SB-334867, prior to test sessions. OKO mice did not significantly differ from WT controls in locomotor activity following acute- or chronic-morphine treatments. Similarly, mice treated with the Ox1r antagonist did not differ from vehicle controls in locomotor activity following acute- or chronic-morphine treatments. In contrast, while OKO mice did not differ from WT controls in preference for a morphine-paired environment, the Ox1r antagonist significantly attenuated place preference for a morphine-, but not a cocaine-paired, environment. These data suggest that orexin action is not required for locomotor responses to acute and chronic morphine, but Ox1r signaling can influence morphine-seeking in WT animals.

Orexin Signaling Via the Orexin 1 Receptor Mediates Operant Responding for Food Reinforcement

BACKGROUND Orexin (hypocretin) signaling is implicated in drug addiction and reward, but its role in feeding and food-motivated behavior remains unclear. METHODS We investigated orexins contribution to food-reinforced instrumental responding using an orexin 1 receptor (Ox1r) antagonist, orexin -/- (OKO) and littermate wildtype (WT) mice, and RNAi-mediated knockdown of orexin. C57BL/6J (n = 76) and OKO (n = 39) mice were trained to nose poke for food under a variable ratio schedule of reinforcement. After responding stabilized, a progressive ratio schedule was initiated to evaluate motivation to obtain food reinforcement. RESULTS Blockade of Ox1r in C57BL/6J mice impaired performance under both the variable ratio and progressive ratio schedules of reinforcement, indicating impaired motivational processes. In contrast, OKO mice initially demonstrated a delay in acquisition but eventually achieved levels of responding similar to those observed in WT animals. Moreover, OKO mice did not differ from WT mice under a progressive ratio schedule, indicating delayed learning processes but no motivational impairments. Considering the differences between pharmacologic blockade of Ox1r and the OKO mice, animals with RNAi mediated knockdown of orexin were then generated and analyzed to eliminate possible developmental effects of missing orexin. Orexin gene knockdown in the lateral hypothalamus in C57BL/6J mice resulted in blunted performance under both the variable ratio and progressive ratio schedules, resembling data obtained following Ox1r antagonism. CONCLUSIONS The behavior seen in OKO mice likely reflects developmental compensation often seen in mutant animals. These data suggest that activation of the Ox1r is a necessary component of food-reinforced responding, motivation, or both in normal mice.

Neural mechanisms underlying obesity and drug addiction

Increasing rates of obesity have alarmed health officials and prompted much public dialogue. While the factors leading to obesity are numerous, an inability to control intake of freely available food is central to the problem. In order to understand this, we need to better define the mechanisms by which the brain regulates food intake, and why it is often difficult to control consumption. From this point of view, it seems valuable to consider the commonalities between food intake and drug abuse. While research in the two fields has historically emphasized different neural substrates, recent data have increased interest in better defining elements that may underlie both drug addiction and obesity. Here we discuss some of these shared elements with an emphasis on emerging areas of research that better define common mechanisms leading to overconsumption.

arouser Reveals a Role for Synapse Number in the Regulation of Ethanol Sensitivity

A reduced sensitivity to the sedating effects of alcohol is a characteristic associated with alcohol use disorders (AUDs). A genetic screen for ethanol sedation mutants in Drosophila identified arouser (aru), which functions in developing neurons to reduce ethanol sensitivity. Genetic evidence suggests that aru regulates ethanol sensitivity through its activation by Egfr/Erk signaling and its inhibition by PI3K/Akt signaling. The aru mutant also has an increased number of synaptic terminals in the larva and adult fly. Both the increased ethanol sensitivity and synapse number of the aru mutant are restored upon adult social isolation, suggesting a causal relationship between synapse number and ethanol sensitivity. We thus show that a developmental abnormality affecting synapse number and ethanol sensitivity is not permanent and can be reversed by manipulating the environment of the adult fly.