Hans-Rudi Berthoud

Distribution and structure of vagal afferent intraganglionic laminar endings (IGLEs) in the rat gastrointestinal tract.

Abstract Intraganglionic laminar endings (IGLEs) are special terminal structures of vagal afferent fibers and have been demonstrated in the myenteric plexus of esophagus and stomach. In order to quantitatively map their presence and distribution over the entire gastrointestinal tract, including the small and large intestines, vagal afferents were anterogradely labeled in vivo by microinjections of the fluorescent carbocyanine dye DiI into the left or right nodose ganglion of adult male rats. In the most successfully labeled cases the highest density of IGLEs was found in the stomach, with about half to one-third of the myenteric ganglia receiving at least one IGLE. The proportion of myenteric ganglia innervated by IGLEs decreased in the small intestine; however, because of its large surface area this gut segment was estimated to contain the highest total number of IGLEs. Both the cecum and colon also contained significant numbers of IGLEs. In the stomach, this vagal afferent innervation by IGLEs was more or less lateralized, with less than 20% of labeled IGLEs found on the contralateral side with respect to the injection. The left/ventral vagus contributed a larger proportion of IGLEs to the proximal duodenum, while the right/dorsal vagus contributed a larger proportion of IGLEs to the distal duodenum and jejunum. Laser scanning confocal microscopy on select specimens revealed further structural details. The parent axon typically formed two or more branches that flanked the ganglia laterally, and in turn produced numerous highly arborizing laminar terminal branches that covered one or both flat sides of the ganglion in a dome-like fashion. The similar distribution patterns and structural details suggest a uniform function for the IGLEs throughout the gastrointestinal tract, but there is as yet no clear proof for any of the hypothesized roles as specialized mechanosensors or local effector terminals.

The lateral hypothalamus as integrator of metabolic and environmental needs: From electrical self-stimulation to opto-genetics

As one of the evolutionary oldest parts of the brain, the diencephalon evolved to harmonize changing environmental conditions with the internal state for survival of the individual and the species. The pioneering work of physiologists and psychologists around the middle of the last century clearly demonstrated that the hypothalamus is crucial for the display of motivated behaviors, culminating in the discovery of electrical self-stimulation behavior and providing the first neurological hint accounting for the concepts of reinforcement and reward. Here we review recent progress in understanding the role of the lateral hypothalamic area in the control of ingestive behavior and the regulation of energy balance. With its vast array of interoceptive and exteroceptive afferent inputs and its equally rich efferent connectivity, the lateral hypothalamic area is in an ideal position to integrate large amounts of information and orchestrate adaptive responses. Most important for energy homeostasis, it receives metabolic state information through both neural and humoral routes and can affect energy assimilation and energy expenditure through direct access to behavioral, autonomic, and endocrine effector pathways. The complex interplays of classical and peptide neurotransmitters such as orexin carrying out these integrative functions are just beginning to be understood. Exciting new techniques allowing selective stimulation or inhibition of specific neuronal phenotypes will greatly facilitate the functional mapping of both input and output pathways.

Neural Systems Controlling the Drive to Eat: Mind Versus Metabolism

With the bleak outlook that 75% of Americans will be overweight or obese in 10 years, it is essential to find efficient help very soon. Knowledge of the powerful and complex neural systems conferring the basic drive to eat is a prerequisite for designing efficient therapies. Recent studies suggest that the cross talk between brain areas involved in cognitive, emotional, and metabolic-regulatory functions may explain why energy homeostasis breaks down for many predisposed individuals in our modern environment.

Increased adiposity on normal diet, but decreased susceptibility to diet-induced obesity in μ-opioid receptor-deficient mice

The mu-opioid receptor encoded by the Oprm1 gene plays a crucial role in the mediation of food reward and drug-induced positive reinforcement, but its genetic deletion has been shown to provide food intake-independent, partial protection from diet-induced obesity. We hypothesized that mu-opioid receptor-deficient mice would show an even greater, intake-dependent, resistance to high-fat diet-induced obesity if the diet comprises a sweet component. We generated an F2 population by crossing the heterozygous offspring of homozygous female Oprm1(-/-) mice (on a mixed C57BL/6 and BALB/c genetic background) with male inbred C57BL/6 mice. Groups of genotyped wild-type (WT) and homozygous mutant (KO) males and females were fed either control chow or a high caloric palatable diet consisting of sweet, liquid chocolate-flavored Ensure together with a solid high-fat diet. Food intake, body weight, and body composition was measured over a period of 16 weeks. Unexpectedly, male, and to a lesser extent female, KO mice fed chow for the entire period showed progressively increased body weight and adiposity while eating significantly more chow. In contrast, when exposed to the sweet plus high-fat diet, male, and to a lesser extent female, KO mice gained significantly less body weight and fat mass compared to WT mice when using chow fed counterparts for reference values. Male KO mice consumed 33% less of the sweet liquid diet but increased intake of high-fat pellets, so that total calorie intake was not different from WT animals. These results demonstrate a dissociation of the role of mu-opioid receptors in the control of adiposity for different diets and sex. On a bland diet, normal receptor function appears to confer a slightly catabolic predisposition, but on a highly palatable diet, it confers an anabolic metabolic profile, favoring fat accretion. Because of the complexity of mu-opioid gene regulation and tissue distribution, more selective and targeted approaches will be necessary to fully understand the underlying mechanisms.

Brain, appetite and obesity.

Food intake and energy expenditure are controlled by complex, redundant, and distributed neural systems that reflect the fundamental biological importance of adequate nutrient supply and energy balance. Much progress has been made in identifying the various hormonal and neural mechanisms by which the brain informs itself about availability of ingested and stored nutrients and, in turn, generates behavioral, autonomic, and endocrine output. While hypothalamus and caudal brainstem play crucial roles in this homeostatic function, areas in the cortex and limbic system are important for processing information regarding prior experience with food, reward, and emotion, as well as social and environmental context. Most vertebrates can store a considerable amount of energy as fat for later use, and this ability has now become one of the major health risks for many human populations. The predisposition to develop obesity can theoretically result from any pathological malfunction or lack of adaptation to changing environments of this highly complex system.

Conditioned taste aversion produced by inhibitors of fatty acid oxidation in rats

The aversive effects of mercaptoacetate (MA) and methyl palmoxirate (MP) were examined in the present experiment. We used a conditioned taste aversion (CTA) paradigm for Sprague-Dawley rats maintained on a low- or high-fat diet, and determined that MA and MP both produce profound aversions to a novel saccharin solution. Because it is known that the stimulation of food intake brought about by MA administration is blocked by destruction of vagal afferents. we repeated the CTA experiment in control and capsaicin-treated rats. Results show that although the capsaicin-treated rats did not increase food intake after MA administration, the CTA produced by MA remained. Therefore, the neural pathways for the aversive and orexigenic effects of MA are distinct. We conclude that MA and MP are aversive, and that the aversive signal generated by MA does not involve vagal afferents or other fibers damaged by capsaicin.

Food reward: Orexin-signaling in ventral tegmental area contributes to high-fat intake induced by accumbens opioid stimulation.

The nucleus accumbens in the ventral striatum is considered a pivotal structure in food reward and addictive behavior. Injection of the mu-opioid agonist DAMGO into this structure powerfully stimulates intake of preferred palatable foods such as high fat, but the downstream neural mechanisms are not well understood. We have previously shown that accumbens-hypothalamic projections and orexin-signaling are important since ICV administration of the selective orexin receptor antagonist SB334768 blocks about 70% of the feeding response to accumbens DAMGO. Here we examined the effects of focal injections of the orexin-blocker into brain sites receiving dense orexin-innervation in order to identify the site of action of orexin-signaling. SB334768 injected bilaterally into the ventral tegmental area of rats, significantly attenuated accumbens DAMGO-induced high-fat intake by about 50%. Focal injections into other areas receiving heavy orexin projections, including the arcuate nucleus, paraventricular nucleus of the hypothalamus, and paraventricular nucleus of the thalamus did not significantly change DAMGO-induced high-fat intake. We conclude that orexin-signaling in the ventral tegmental area is crucial for the expression of palatable food consumption elicited by opioid-stimulation of the nucleus accumbens. This pathway is likely to activate the mesolimbic dopamine system eliciting the “wanting” aspect of food reward. Involvement of additional sites of orexin-signaling such as the lateral hypothalamus, locus coeruleus, and or nucleus of the solitary tract in this model for reward-driven appetite remains to be tested. Supported by NIDDK 47348.

Melanocortinergic modulation of food intake in the medulla: Evidence for presynaptic MC4-receptors on vagal afferents.

The dorsal vagal complex in the caudal medulla is considered an important integrative area in the distributed neural network controlling food intake with melanocortin receptor-4 (MC4R) signaling playing a crucial role. Here, we used patch-clamp whole cell recording from NTS neurons to investigate the effects of MC4R ligands that have previously been shown to powerfully modulate meal structure and total food intake. Thirty-nine percent (25/64) of NTS neurons responded to perfusion with MTII or alpha-MSH, 28% with an increase and 11% with a decrease in the frequency of spontaneous EPSPs, without affecting their amplitude. MTII-induced effects in the frequency of EPSPs were unaffected by TTX, but all effects were abolished by the MC3/4R antagonist SCHU9119. To test whether the mediating MC4-receptors are located on vagal afferent terminals, we also recorded NTS neurons from rats that had undergone vagal afferent rhizotomies 4 days before slice preparation. Only 1 out of 10 neurons from such rats responded with an increase in the frequency of spontaneous EPSCs following MTII. Together with earlier observations, these results suggest that alpha-MSH released from hypothalamic and local POMC neurons acts on presynaptic MC4-receptors on vagal afferent terminals to predominantly increase glutamate transmission to NTS neurons, but that in a minority of cases the effect can also be inhibitory. Such a mechanism is consistent with the notion that descending melanocortinergic projections from leptin-sensitive areas in the basomedial hypothalamus can suppress food intake by changing the capacity of vagal hindbrain mechanisms of satiation. Supported by NIDDK 47348.