μ-Opioid modulation in the rostral solitary nucleus and reticular formation alters taste reactivity: evidence for a suppressive effect on consummatory behavior

Constructing an adult orofacial premotor atlas in Allen mouse CCF

Premotor circuits in the brainstem project to pools of orofacial motoneurons to execute essential motor action such as licking, chewing, breathing, and in rodent, whisking. Previous transsynaptic tracing studies only mapped orofacial premotor circuits in neonatal mice, but the adult circuits remain unknown as a consequence of technical difficulties. Here, we developed a three-step monosynaptic transsynaptic tracing strategy to identify premotor neurons controlling vibrissa, tongue protrusion, and jaw-closing muscles in the adult mouse. We registered these different groups of premotor neurons onto the Allen mouse brain common coordinate framework (CCF) and consequently generated a combined 3D orofacial premotor atlas, revealing unique spatial organizations of distinct premotor circuits. We further uncovered premotor neurons that simultaneously innervate multiple motor nuclei and, consequently, are likely to coordinate different muscles involved in the same orofacial motor actions. Our method for tracing adult premotor circuits and registering to Allen CCF is generally applicable and should facilitate the investigations of motor controls of diverse behaviors.

Phenotyping neurons activated in the mouse brain during restoration of salt debt

Salt overconsumption contributes to hypertension, which is a major risk factor for stroke, heart and kidney disease. Characterising neuronal pathways that may control salt consumption is therefore important for developing novel approaches for reducing salt overconsumption. Here, we identify neurons within the mouse central amygdala (CeA), lateral parabrachial nucleus (LPBN), intermediate nucleus of the solitary tract (iNTS), and caudal NTS (cNTS) that are activated and display Fos immunoreactivity in mice that have consumed salt in order to restore a salt debt, relative to salt replete and salt depleted controls. Double-label immunohistochemical studies revealed that salt restoring mice had significantly greater densities of activated enkephalin neurons within the CeA and iNTS, while statistically significant changes within the LPBN and cNTS were not observed. Furthermore, within the CeA, restoration of salt debt conferred a significant increase in the density of activated calretinin neurons, while there was no change relative to control groups in the density of activated neurons that co-expressed protein kinase C delta (PKC-δ). Taken together, these studies highlight the importance of opioid systems within the CeA and iNTS in neuronal processes associated with salt restoration, and may aid the development of future pharmacological and other strategies for reducing salt overconsumption.

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

Evolutionary conserved brainstem circuits encode category, concentration and mixtures of taste

Evolutionary conserved brainstem circuits are the first relay for gustatory information in the vertebrate brain. While the brainstem circuits act as our life support system and they mediate vital taste related behaviors, the principles of gustatory computations in these circuits are poorly understood. By a combination of two-photon calcium imaging and quantitative animal behavior in juvenile zebrafish, we showed that taste categories are represented by dissimilar brainstem responses and generate different behaviors. We also showed that the concentration of sour and bitter tastes are encoded by different principles and with different levels of sensitivity. Moreover, we observed that the taste mixtures lead to synergistic and suppressive interactions. Our results suggest that these interactions in early brainstem circuits can result in non-linear computations, such as dynamic gain modulation and discrete representation of taste mixtures, which can be utilized for detecting food items at broad range of concentrations of tastes and rejecting inedible substances.

Endogenous opioids and feeding behavior: A decade of further progress (2004-2014). A Festschrift to Dr. Abba Kastin

Functional elucidation of the endogenous opioid system temporally paralleled the creation and growth of the journal, Peptides, under the leadership of its founding editor, Dr. Abba Kastin. He was prescient in publishing annual and uninterrupted reviews on Endogenous Opiates and Behavior that served as a microcosm for the journal under his stewardship. This author published a 2004 review, "Endogenous opioids and feeding behavior: a thirty-year historical perspective", summarizing research in this field between 1974 and 2003. The present review "closes the circle" by reviewing the last 10 years (2004-2014) of research examining the role of endogenous opioids and feeding behavior. The review summarizes effects upon ingestive behavior following administration of opioid receptor agonists, in opioid receptor knockout animals, following administration of general opioid receptor antagonists, following administration of selective mu, delta, kappa and ORL-1 receptor antagonists, and evaluating opioid peptide and opioid receptor changes in different food intake models.

Effects of varenicline on operant self-administration of alcohol and/or nicotine in a rat model of co-abuse

Alcohol and nicotine (in the form of tobacco) are often taken together, with increased negative health consequences. Co-use may modify intake of one or both of the drugs, or the effects of drugs used to treat nicotine or alcohol addiction. Varenicline is commonly prescribed as an aid to enhance quitting smoking. More recently it has been shown to reduce alcohol intake in humans and laboratory animals. There is little work investigating the role of co-exposure to alcohol and nicotine in the effects of varenicline. In pilot clinical studies, it has been reported that smoking enhances varenicline's effectiveness as a treatment for alcohol misuse, but this relationship has not been systematically investigated. To help resolve this, we examined if the effects of varenicline on alcohol and nicotine self-administration (SA) in rats are modified when the two drugs are taken together. Rats were trained on alcohol SA, and some were implanted with i.v. catheters for nicotine SA. Groups of animals then lever pressed for alcohol or nicotine alone, and another group lever pressed for alcohol and nicotine, using a two lever choice procedure. Varenicline did not affect alcohol SA. Varenicline reduced nicotine SA modestly. Access to both alcohol and nicotine reduced self-administration of either drug, but did not change the effects of varenicline. We found that in rats with a history of alcohol SA, varenicline reduced reinstatement of extinguished alcohol seeking induced by exposure to an alcohol prime combined with cues previously associated with alcohol.

How the brainstem controls orofacial behaviors comprised of rhythmic actions

Mammals perform a multitude of well-coordinated orofacial behaviors such as breathing, sniffing, chewing, licking, swallowing, vocalizing, and in rodents, whisking. The coordination of these actions must occur without fault to prevent fatal blockages of the airway. Deciphering the neuronal circuitry that controls even a single action requires understanding the integration of sensory feedback and executive commands. A far greater challenge is to understand the coordination of multiple actions. Here, we focus on brainstem circuits that drive rhythmic orofacial actions. We discuss three neural computational mechanisms that may enable circuits for different actions to operate without interfering with each other. We conclude with proposed experimental programs for delineating the neural control principles that have evolved to coordinate orofacial behaviors.

Salivary peptide tyrosine-tyrosine 3-36 modulates ingestive behavior without inducing taste aversion

Hormone peptide tyrosine-tyrosine (PYY) is secreted into circulation from the gut L-endocrine cells in response to food intake, thus inducing satiation during interaction with its preferred receptor, Y2R. Clinical applications of systemically administered PYY for the purpose of reducing body weight were compromised as a result of the common side effect of visceral sickness. We describe here a novel approach of elevating PYY in saliva in mice, which, although reliably inducing strong anorexic responses, does not cause aversive reactions. The augmentation of salivary PYY activated forebrain areas known to mediate feeding, hunger, and satiation while minimally affecting brainstem chemoreceptor zones triggering nausea. By comparing neuronal pathways activated by systemic versus salivary PYY, we identified a metabolic circuit associated with Y2R-positive cells in the oral cavity and extending through brainstem nuclei into hypothalamic satiety centers. The discovery of this alternative circuit that regulates ingestive behavior without inducing taste aversion may open the possibility of a therapeutic application of PYY for the treatment of obesity via direct oral application.

The μ-opioid receptor agonist DAMGO presynaptically suppresses solitary tract-evoked input to neurons in the rostral solitary nucleus

Taste processing in the rostral nucleus of the solitary tract (rNST) is subject to modulatory influences including opioid peptides. Behavioral pharmacological studies suggest an influence of μ-opioid receptors in rNST, but the underlying mechanism is unknown. To determine the cellular site of action, we tested the effects of the μ-opioid receptor agonist DAMGO in vitro. Whole cell patch-clamp recordings were made in brain stem slices from GAD67-GFP knockin mice expressing enhanced green fluorescent protein (EGFP) under the control of the endogenous promoter for GAD67, a synthetic enzyme for GABA. Neuron counts showed that ∼36% of rNST neurons express GABA. We recorded monosynaptic solitary tract (ST)-evoked currents (jitter ≤ 300 μs) in both GAD67-EGFP-positive (GAD67+) and GAD67-EGFP-negative (GAD67-) neurons with equal frequency (25/31; 22/28), but the inputs to the GAD67+ neurons had significantly smaller paired-pulse ratios compared with GAD67- neurons. DAMGO (0.3 μM) significantly suppressed ST-evoked currents in both cell types (mean suppression = 46 ± 3.3% SE), significantly increased the paired-pulse ratio of these currents, and reduced the frequency of spontaneous miniature excitatory postsynaptic currents but did not diminish their amplitude, indicating a presynaptic site of action. Under inhibitory amino acid receptor blockade, DAMGO was significantly more suppressive in GAD67+ neurons (59% reduction) compared with GAD67- neurons (35% reduction), while the reverse was true in normal artificial cerebrospinal fluid (GAD67+: 35% reduction; GAD67-: 57% reduction). These findings suggest that DAMGO suppresses activity in rNST neurons predominantly via a presynaptic mechanism, and that this effect may interact significantly with tonic or evoked inhibitory activity.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Activation of NPY receptors suppresses excitatory synaptic transmission in a taste-feeding network in the lower brain stem

Consummatory responses to taste stimuli are modulated by visceral signals processed in the caudal nucleus of the solitary tract (cNST) and ventrolateral medulla. On the basis of decerebrate preparations, this modulation can occur through local brain stem pathways. Among the large number of neuropeptides and neuromodulators implicated in these visceral pathways is neuropeptide Y (NPY), which is oftentimes colocalized in catecholaminergic neurons themselves implicated in glucoprivic-induced feeding and satiety. In addition to the cNST and ventrolateral medulla, noradrenergic and NPY receptors are found in circumscribed regions of the medullary reticular formation rich in preoromotor neurons. To test the hypothesis that NPY may act as a neuromodulator on preoromotor neurons, we recorded the effects of bath application of NPY and specific Y1 and Y2 agonists on currents elicited from electrical stimulation of the rostral (taste) NST in prehypoglossal neurons in a brain stem slice preparation. A high proportion of NST-driven responses were suppressed by NPY, as well as Y1 and Y2 agonists. On the basis of paired pulse ratios and changes in membrane resistance, we concluded that Y1 receptors influence these neurons both presynaptically and postsynaptically and that Y2 receptors have a presynaptic locus. To test the hypothesis that NPY may act in concert with norepinephrine (NE), we examined neurons showing suppressed responses in the presence of a Y2 agonist and demonstrated a greater degree of suppression to a Y2 agonist/NE cocktail. These suppressive effects on preoromotoneurons may reflect a satiety pathway originating from A2 neurons in the caudal brain stem.