Current Biology
ArticleThe conserved endocannabinoid anandamide modulates olfactory sensitivity to induce hedonic feeding in C. elegans
Introduction
It has been known for centuries that smoking or ingesting preparations of the plant Cannabis sativa stimulates appetite.1,2 Users report persistent hunger while intoxicated, even if previously satiated. This feeling of hunger is often accompanied by a specific desire for foods that are sweet or high in fat content, a phenomenon colloquially known as “the munchies.”3,4,5,6,7,8 The effects of cannabinoids on appetite result mainly from Δ9-tetrahydrobannabinol (THC), a plant-derived cannabinoid. THC acts at cannabinoid receptors in the brain, mimicking endogenous ligands called endocannabinoids, which include anandamide (N-arachidonoylethanolamine or AEA) and 2-arachidonoylglycerol (2-AG). AEA and 2-AG are the best studied signaling molecules of the mammalian endocannabinoid system, which comprises the cannabinoid receptors CB1 and CB2, metabolic enzymes for synthesis and degradation of the endocannabinoids, and ancillary proteins involved in receptor trafficking and modulation.9,10,11,12,13,14,15,16,17
Numerous studies in laboratory animals have established a strong link between endocannabinoid signaling and energy homeostasis, defined as the precise matching of caloric intake with energy expenditure.18 Food deprivation increases endocannabinoid levels in the nucleus accumbens and hypothalamus—brain regions that express CB1 receptors and contribute to appetitive regulation.19 Systemic administration of THC or endogenous cannabinoids increases feeding.20 Microinjection of cannabinoid receptor agonists or endocannabinoids directly into the nucleus accumbens also increases feeding.21,22 Thus, the endocannabinoid system can be viewed as a short-latency effector system for restoring energy homeostasis under conditions of food deprivation.18,23,24,25
To respond effectively to an energy deficit, an animal should be driven to seek food (appetitive behavior) and, once food is encountered, to maximize caloric intake (consummatory behavior). The endocannabinoid system is capable of orchestrating both aspects of this response. With respect to appetitive behavior, CB1 agonists reduce the latency to feed26,27,28,29,30,31,32 and induce animals to expend more effort to obtain a food or liquid reward,30,31,33,34 whereas CB1 antagonists have the opposite effects.26,27,28,29,30,31,32 As for consummatory behavior, rodent studies show that administration of THC or endocannabinoids not only increases consumption; it also alters food preferences in favor of palatable, calorically dense foods, such as those laden with sugars and fats. For example, THC causes rats to consume larger quantities of chocolate cake batter without affecting the consumption of concurrently available laboratory pellets.35 It also causes them to consume larger quantities of sugar water rather than plain water and of dry pellets rather than watered-down pellet mash, which is calorically dilute.36 Administration of endocannabinoids, systemically or directly into the nucleus accumbens, has similar effects, which can be blocked by administration of CB1 antagonists.22,37,38 Conversely, CB1 antagonists, when administered alone, specifically suppress consumption of sweet and fatty foods in rats39,40,41 and primates,42 indicating that basal CB1 activation can be regulated up or down to alter consumption.
There is experimental support for the hypothesis that cannabinoids amplify the pleasurable or rewarding aspects of calorically dense foods. This phenomenon has been termed hedonic amplification,21,43 whereas the food-specific increase in consumption it engenders has been termed hedonic feeding.44 Although inferences about the subjective experience of animals can be difficult to establish, cannabinoids have been shown to increase overt expressions of pleasure during feeding. In rats, for example, both THC and AEA increase the vigor of licking at spouts delivering sweet fluids.45,46 Further, the frequency of orofacial movements associated with highly palatable foods is increased or decreased by injection of THC or a CB1 antagonist, respectively, suggesting that pleasure may be increased by cannabinoid administration.47,48
The effects of cannabinoids on hedonic responses may be partially chemosensory in origin, involving both taste (gustation) and smell (olfaction). With respect to gustation, a majority of sweet-sensitive taste cells in the mouse tongue are immunoreactive to CB1, and a similar proportion shows heightened responses to saccharin, sucrose, and glucose following endocannabinoid administration.49,50 These effects are recapitulated in afferent nerves carrying gustatory signals from the tongue,49 as administration of AEA or 2-AG specifically increases chorda tympani responses to sweeteners rather than NaCl (salt), HCl (sour), quinine (bitter), or monosodium glutamate (umami). As for olfaction, CB1 receptors expressed in the olfactory bulb are required for postfasting hyperphagia in mice, and THC decreases the threshold for food-odor detection during exploratory behavior.51
The high degree of evolutionary conservation of the endocannabinoid system at the molecular level is well established.52 Although CB1 and CB2 receptors are unique to chordates, there are numerous candidates for cannabinoid receptors in most animals. Furthermore, orthologs of the enzymes involved in synthesis and degradation of endocannabinoids occur throughout the animal kingdom. This degree of molecular conservation, coupled with the universal need in organisms to regulate energy balance, suggests that the hypothesis that hedonic amplification and hedonic feeding are also widely conserved, but studies in animals other than rodents and primates appear to be lacking.
This study tests the hypothesis that the hedonic effects of cannabinoids are conserved in the nematode C. elegans. This organism diverged from the lineage leading to mammals more than 500 million years ago.53 Nevertheless, C. elegans has a fully elaborated endocannabinoid signaling system including54: (1) functionally validated endocannabinoid receptors NPR-19, which most closely resembles the mammalian CB1 receptor, and OCTR-1, and putative receptors encoded by npr-32, osm-9, and trp-455,56,57; (2) the endocannabinoids AEA and 2-AG, which it shares with mammals45,58,59,60; (3) orthologs of the mammalian endocannabinoid synthesis enzymes N-acetylphosphatidylethanolamine-hydrolysing phospholipase D (NAPE-PLD) and diacylglycerol lipase (DAGL)61; and (4) orthologs of the endocannabinoid degradative enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL, orthologous to Y97E10AL.2 in C. elegans).55 Endocannabinoid signaling in C. elegans is currently known to contribute to six main phenotypes: (1) axon navigation during regeneration,56,62 (2) lifespan regulation related to dietary restriction,61,63 (3) progression through developmental stages,61,64 (4) suppression of nociceptive withdrawal responses,55 (5) inhibition of feeding rate,55 and (6) inhibition of locomotion.55,57
The feeding ecology of C. elegans supports the possibility of hedonic feeding in this organism. C. elegans feeds on bacteria in decaying plant matter.65 It finds bacteria through chemotaxis guided by a combination of gustatory and olfactory cues.66,67 Bacteria are ingested through the worm's pharynx, a rhythmically active muscular pump that constitutes the animal’s throat. Although C. elegans is an omnivorous bacterivore, different species of bacteria have a characteristic nutritional quality as a food source, which is defined by the growth rate of individual worms feeding on that species.68 Hatchlings are naive to food quality but, in a matter of hours, begin to exhibit a preference for nutritionally superior species (i.e., superior food) over nutritionally inferior species (inferior food).69
Here we show that exposure of C. elegans to the endocannabinoid AEA biases both consummatory and appetitive responses toward superior food. With respect to consummatory behavior, animals exposed to AEA increase their feeding rate on superior food and decrease their feeding rate on inferior food. As for appetitive behavior, the fraction of worms approaching and dwelling on patches of superior food increases, whereas the fraction approaching and dwelling on inferior food decreases. Taken together, the consummatory and appetitive manifestations of cannabinoid exposure in C. elegans imply increased consumption of superior food characteristic of hedonic feeding on calorically dense foods by mammals. We also find that AEA’s effects require the NPR-19 cannabinoid receptor. Further, AEA's effects persist when the npr-19 gene is replaced by the human CB1 receptor-gene CNR1, indicating a high degree of conservation between the nematode and mammalian endocannabinoid systems. At the neuronal level, we find that under the influence of AEA, AWC, an olfactory neuron required for chemotaxis to food, becomes more sensitive to superior food and less sensitive to inferior food. Together, our findings indicate that the hedonic effects of endocannabinoids may be conserved in C. elegans.
Section snippets
AEA exposure increases consumption of superior foods
In mammals, cannabinoids can selectively increase consumption of foods that are nutritionally superior in the sense that they are calorically dense.35,36 We asked whether cannabinoids can selectively increase consumption of nutritionally superior foods in C. elegans, in which nutritional quality is defined in terms of the growth rate of individual worms.68 C. elegans swallows bacteria by rhythmically contracting its pharynx; each contraction is called a pump. To quantify consumption, we
Discussion
In mammals, administration of THC or endocannabinoids induces hedonic feeding. This study provides two converging lines of evidence supporting the hypothesis that cannabinoids induce hedonic feeding in C. elegans. First, in the five bacteria strains for which food quality has previously been characterized,68 AEA reciprocally altered food consumption, causing worms to feed at higher and lower rates on superior food and inferior food, respectively (Figure 1C), with no effect on a food of
Key resources table
REAGENT or RESOURCE SOURCE IDENTIFIER Bacterial strains OP50 C. elegans Genetic Center (CGC) RRID:WB-STRAIN:WBStrain00041969 DA1877 CGC RRID:WB-STRAIN:WBStrain00040995 DA1885 CGC RRID:WB-STRAIN:WBStrain00040997 DA837 CGC RRID:WB-STRAIN:WBStrain00040994 HB101 CGC RRID:WB-STRAIN:WBStrain00041075 DA1881 (S13) Raizen lab
https://doi.org/10.1242/jeb.00433N/A C. elegans strains (genotype) N2, Bristol
Wild typeCGC RRID:WB-STRAIN:WBStrain00000001 FK311 ceh-36(ks86) CGC RRID:WB-STRAIN:WBStrain00007515 RB1668 npr-19(ok2068) CGC
Acknowledgments
We thank Richard Komuniecki for the npr-19(ok2068) and rescue strains and David Raizen for providing the DA1881 (B. cereus) bacterial strain. The unc-13, unc-31, ceh-36, cho-1, and eat-4 worm strains were provided by the CGC, which is funded by the NIH Office of Research Infrastructure Programs (P40 OD010440). We thank Oliver Hobert, Jonathan Millet, and Jon Pierce for discussion and Leon Avery and Matthew Smear for comments on the manuscript. We thank Chris Doe for use of a Zeiss LSM800
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