Cell Metabolism
Previews Motivation to Eat—AgRP Neurons and Homeostatic Need Dengbao Yang,1 Tiemin Liu,1 and Kevin W. Williams1,*
1Division of Hypothalamic Research, Department of Internal Medicine, The University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75390, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cmet.2015.06.018
Activation of AgRP neurons potently induces feeding behaviors; however, whether this activity is involved in motivations of feeding behavior is unclear. A recent study in Nature (Betley et al., 2015) reports that AgRP neuron activity conditions learned behavior by transmitting a negative-valence signal: linking AgRP neurons to the preference of environmental cues associated with homeostatic need. Hunger is a complex state that includes multiple motivational processes ultimately evoking foraging and foodseeking behaviors. Agouti-related protein (AgRP) neurons in the mediobasal hypothalamus are prototypical neurons that establish a functional link between neuronal activity and feeding behavior. For example, AgRP neuronal activity and AgRP transcription is activated by energy deficit while inhibited by energy surplus (Morton et al., 2006). Also, ablation of AgRP neurons in the adult results in anorexia (Wu and Palmiter, 2011), and acute activation or inhibition of AgRP neurons results in voracious feeding or satiating behaviors, respectively (Aponte et al., 2011; Krashes et al., 2011). In a recent study, Betley and colleagues (2015) extended these former observations by performing a series of behavioral experiments combined with cell-typespecific neuronal activity manipulations. This approach allowed for the assessment of AgRP neurons in the context of learning with feeding behavior. Betley et al. utilized channel rhodopsin2 (ChR2) to acutely stimulate AgRP activity in vivo. The authors found that increased AGRP neuronal activity conditioned a negative-valence signal, resulting in the avoidance of food (flavor) or place cues associated with activation of AgRP neurons (Figure 1A). They next used pharmacologically selective effector molecule (PSEM89S) to suppress AgRP activity. In contrast to AgRP stimulation, inhibition increased preference for associated flavors and place cues (Figure 1A). Using instrumental conditioning experiments, the authors demonstrated that previously reinforced food-seeking actions progres-
sively decrease during AgRP neuron photostimulation in ad libitum mice, indicating a reduced value of nutritive food when AGRP neuronal activity remains elevated. A key advance of this study is the insight gained into how AgRP neurons behave in vivo. Due to their location, as they are intermingled with heterogeneous populations of hypothalamic neurons deep within the brain, much of our understanding of AgRP neuronal activity has been limited to ex vivo electrophysiological or post-hoc histological analyses. Betley and colleagues (2015) employed a unique intracranial gradient index (GRIN) lens with a head-mounted miniature microscope to monitor the activity of AgRP neurons genetically encoding calcium indicators in freely moving mice. Their data support a previous ex vivo slice electrophysiological analysis of AgRP neurons from fasted and fed mice (Takahashi and Cone, 2005), demonstrating that elevated AgRP neuronal activity in nutrient-deprived mice is inhibited with food consumption (Figure 1B). The current study also determined that AgRP neurons encode the receipt of nutritive foods by reducing activity with food consumption—an observation not possible with prior ex vivo methods (Figure 1B). These response properties of AgRP neurons were also influenced by homeostatic drive. In particular, the presence of food suppressed AgRP activity, while non-nutritive objects (false food) transiently reduced AgRP activity, ultimately failing to influence long-term activity (Figure 1B). Similarly, temporary exposure to food transiently decreases activity only in the presence of food, while removal of food resulted in a rebound excitatory
62 Cell Metabolism 22, July 7, 2015 ª2015 Elsevier Inc.
activity of AgRP neurons. Parallel experiments on the activity of subfornical organ (SFO) neurons demonstrated similar findings in the regulation of drinking, supporting the homeostatic-dependent nature of these behaviors. The Betley et al. (2015) study also demonstrated that a simple stimulus pattern in AgRP neurons alone gives rise to complex motivated goal-oriented behaviors. The model offered by the authors is that AgRP neurons are transmitting a negative valence signal, which influences behavior via learning. Normally, this would occur during energy deficit, coinciding with increased activity of AgRP neurons. This negative valence signal interacts with learning about the relationship of the external cues in the environment and actions the animal takes. In particular, cues that are associated with actions that don’t lead to the consumption of nutrients are devalued and are less likely to be performed. Consumption of nutrients results in the silencing of these neurons, subsequently reinforcing the relationship between external cues and actions resulting in food intake. The current study also highlights potential parallels between AgRP neuronal activation and some negative emotional aspects of weight loss. Modest (5%– 10%) weight loss improves symptoms associated with obesity. However, weight loss also paradoxically increases hunger and food-seeking behaviors, which counteract the reduced body weight and, in fact, explains why most weight loss diets fail. Defining the cellular and molecular mechanisms contributing to these behaviors is critical in advancing our understanding of weight gain and in the
Cell Metabolism
Previews A
Fasted
Fed Preference
AgRP neuron
AgRP neuron
ChR2 or eGFP
PSAML141F Avoidance
Chemogenecs
Optogenecs Food discovery or food cues
AgRP neuronal acvity
B
food “false” food Nutrient-deprived
ACKNOWLEDGMENTS
Eang This work was supported by grants to K.W.W. (NIH R01 DK100699) and T.L. (AHA 14SDG20370016).
Figure 1. Optogenetic and Chemogenetic Conditioning of Flavor and Place Cues (A) Animals avoided flavor and place cues associated with optogenetically induced AgRP neuronal activity. Conversely, animals preferred flavor and place cues associated with chemogenetic inhibition of AgRP neurons. (B) AgRP neuronal activity visualized from deep-brain calcium imaging in freely moving mice during different energy states (i.e., nutrient deprived [fasted] and eating). AgRP neuronal activity decreased in the discovery or consumption of food (blue line). Objects of non-nutritive value (false food: red line) failed to sustain decreased AgRP neuronal activity.
management and treatment of obesityrelated co-morbidities. These novel results also raise several questions. For instance, gradually reducing baseline activity of AgRP neurons in hungry mice by food consumption is in agreement with a prediction of homeostatic regulation. However, in the current study, AgRP neuronal activity is rapidly inhibited by food-related cues even prior to the consumption of nutrients. The rapid change in AgRP neuronal activity likely reflects actions independent of stimuli such as nutrients, hormones, or neurotransmitters associated with fuel status alone. Although it is unclear, these changes may be reflexive in nature and warrant further investigation. Additionally, AgRP neurons project to multiple downstream brain areas to regulate feeding
physiological need states can be learned. However, little is known about other biological implications. As suggested by the current study and supported by a recent report (Chen et al., 2015), prompt inhibition of AgRP activity provides a mechanism to rapidly inhibit foraging once food is discovered, a possible beneficial behavior to preserve energy demands associated with the pursuit of food. Thus, AgRP neuronal activity during this short window may instruct animals how to respond to food-related cues. Additionally, from nutritive to hedonic, there are multiple motivations associated with the need to eat. Understanding these stimuli may ultimately lead to effective strategies in the regulation of weight gain.
behavior (Betley et al., 2013; Gautron et al., 2015; Shah et al., 2014; Wu and Palmiter, 2011). Recent work demonstrated nuclei-specific roles for AgRP neuronal projections in the regulation of feeding (Betley et al., 2013). The current optogenetic and chemogenetic strategies have great potential to delineate the temporal and long-term roles of these neural circuits, regulating the negative valence described in the current study. In summary, Betley and colleagues (2015) propose a new role for AgRP neurons as a conditioned stimulus in learning related to feeding behavior. As a negative valence signal, AgRP neurons drive nutritive-enforced behaviors if physiological need is not reduced. Through the reduction of negative-valence signals, preference for cues associated with alleviating
REFERENCES Aponte, Y., Atasoy, D., and Sternson, S.M. (2011). Nat. Neurosci. 14, 351–355. Betley, J.N., Cao, Z.F., Ritola, K.D., and Sternson, S.M. (2013). Cell 155, 1337–1350. Betley, J.N., Xu, S., Cao, Z.F., Gong, R., Magnus, C.J., Yu, Y., and Sternson, S.M. (2015). Nature 521, 180–185. Chen, Y., Lin, Y.C., Kuo, T.W., and Knight, Z.A. (2015). Cell 160, 829–841. Gautron, L., Elmquist, J.K., and Williams, K.W. (2015). Cell 161, 133–145. Krashes, M.J., Koda, S., Ye, C., Rogan, S.C., Adams, A.C., Cusher, D.S., Maratos-Flier, E., Roth, B.L., and Lowell, B.B. (2011). J. Clin. Invest. 121, 1424–1428. Morton, G.J., Cummings, D.E., Baskin, D.G., Barsh, G.S., and Schwartz, M.W. (2006). Nature 443, 289–295. Shah, B.P., Vong, L., Olson, D.P., Koda, S., Krashes, M.J., Ye, C., Yang, Z., Fuller, P.M., Elmquist, J.K., and Lowell, B.B. (2014). Proc. Natl. Acad. Sci. USA 111, 13193–13198. Takahashi, K.A., and Cone, R.D. (2005). Endocrinology 146, 1043–1047. Wu, Q., and Palmiter, R.D. (2011). Eur. J. Pharmacol. 660, 21–27.
Cell Metabolism 22, July 7, 2015 ª2015 Elsevier Inc. 63
Previews Motivation to Eat—AgRP Neurons and Homeostatic Need Dengbao Yang,1 Tiemin Liu,1 and Kevin W. Williams1,*
1Division of Hypothalamic Research, Department of Internal Medicine, The University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75390, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cmet.2015.06.018
Activation of AgRP neurons potently induces feeding behaviors; however, whether this activity is involved in motivations of feeding behavior is unclear. A recent study in Nature (Betley et al., 2015) reports that AgRP neuron activity conditions learned behavior by transmitting a negative-valence signal: linking AgRP neurons to the preference of environmental cues associated with homeostatic need. Hunger is a complex state that includes multiple motivational processes ultimately evoking foraging and foodseeking behaviors. Agouti-related protein (AgRP) neurons in the mediobasal hypothalamus are prototypical neurons that establish a functional link between neuronal activity and feeding behavior. For example, AgRP neuronal activity and AgRP transcription is activated by energy deficit while inhibited by energy surplus (Morton et al., 2006). Also, ablation of AgRP neurons in the adult results in anorexia (Wu and Palmiter, 2011), and acute activation or inhibition of AgRP neurons results in voracious feeding or satiating behaviors, respectively (Aponte et al., 2011; Krashes et al., 2011). In a recent study, Betley and colleagues (2015) extended these former observations by performing a series of behavioral experiments combined with cell-typespecific neuronal activity manipulations. This approach allowed for the assessment of AgRP neurons in the context of learning with feeding behavior. Betley et al. utilized channel rhodopsin2 (ChR2) to acutely stimulate AgRP activity in vivo. The authors found that increased AGRP neuronal activity conditioned a negative-valence signal, resulting in the avoidance of food (flavor) or place cues associated with activation of AgRP neurons (Figure 1A). They next used pharmacologically selective effector molecule (PSEM89S) to suppress AgRP activity. In contrast to AgRP stimulation, inhibition increased preference for associated flavors and place cues (Figure 1A). Using instrumental conditioning experiments, the authors demonstrated that previously reinforced food-seeking actions progres-
sively decrease during AgRP neuron photostimulation in ad libitum mice, indicating a reduced value of nutritive food when AGRP neuronal activity remains elevated. A key advance of this study is the insight gained into how AgRP neurons behave in vivo. Due to their location, as they are intermingled with heterogeneous populations of hypothalamic neurons deep within the brain, much of our understanding of AgRP neuronal activity has been limited to ex vivo electrophysiological or post-hoc histological analyses. Betley and colleagues (2015) employed a unique intracranial gradient index (GRIN) lens with a head-mounted miniature microscope to monitor the activity of AgRP neurons genetically encoding calcium indicators in freely moving mice. Their data support a previous ex vivo slice electrophysiological analysis of AgRP neurons from fasted and fed mice (Takahashi and Cone, 2005), demonstrating that elevated AgRP neuronal activity in nutrient-deprived mice is inhibited with food consumption (Figure 1B). The current study also determined that AgRP neurons encode the receipt of nutritive foods by reducing activity with food consumption—an observation not possible with prior ex vivo methods (Figure 1B). These response properties of AgRP neurons were also influenced by homeostatic drive. In particular, the presence of food suppressed AgRP activity, while non-nutritive objects (false food) transiently reduced AgRP activity, ultimately failing to influence long-term activity (Figure 1B). Similarly, temporary exposure to food transiently decreases activity only in the presence of food, while removal of food resulted in a rebound excitatory
62 Cell Metabolism 22, July 7, 2015 ª2015 Elsevier Inc.
activity of AgRP neurons. Parallel experiments on the activity of subfornical organ (SFO) neurons demonstrated similar findings in the regulation of drinking, supporting the homeostatic-dependent nature of these behaviors. The Betley et al. (2015) study also demonstrated that a simple stimulus pattern in AgRP neurons alone gives rise to complex motivated goal-oriented behaviors. The model offered by the authors is that AgRP neurons are transmitting a negative valence signal, which influences behavior via learning. Normally, this would occur during energy deficit, coinciding with increased activity of AgRP neurons. This negative valence signal interacts with learning about the relationship of the external cues in the environment and actions the animal takes. In particular, cues that are associated with actions that don’t lead to the consumption of nutrients are devalued and are less likely to be performed. Consumption of nutrients results in the silencing of these neurons, subsequently reinforcing the relationship between external cues and actions resulting in food intake. The current study also highlights potential parallels between AgRP neuronal activation and some negative emotional aspects of weight loss. Modest (5%– 10%) weight loss improves symptoms associated with obesity. However, weight loss also paradoxically increases hunger and food-seeking behaviors, which counteract the reduced body weight and, in fact, explains why most weight loss diets fail. Defining the cellular and molecular mechanisms contributing to these behaviors is critical in advancing our understanding of weight gain and in the
Cell Metabolism
Previews A
Fasted
Fed Preference
AgRP neuron
AgRP neuron
ChR2 or eGFP
PSAML141F Avoidance
Chemogenecs
Optogenecs Food discovery or food cues
AgRP neuronal acvity
B
food “false” food Nutrient-deprived
ACKNOWLEDGMENTS
Eang This work was supported by grants to K.W.W. (NIH R01 DK100699) and T.L. (AHA 14SDG20370016).
Figure 1. Optogenetic and Chemogenetic Conditioning of Flavor and Place Cues (A) Animals avoided flavor and place cues associated with optogenetically induced AgRP neuronal activity. Conversely, animals preferred flavor and place cues associated with chemogenetic inhibition of AgRP neurons. (B) AgRP neuronal activity visualized from deep-brain calcium imaging in freely moving mice during different energy states (i.e., nutrient deprived [fasted] and eating). AgRP neuronal activity decreased in the discovery or consumption of food (blue line). Objects of non-nutritive value (false food: red line) failed to sustain decreased AgRP neuronal activity.
management and treatment of obesityrelated co-morbidities. These novel results also raise several questions. For instance, gradually reducing baseline activity of AgRP neurons in hungry mice by food consumption is in agreement with a prediction of homeostatic regulation. However, in the current study, AgRP neuronal activity is rapidly inhibited by food-related cues even prior to the consumption of nutrients. The rapid change in AgRP neuronal activity likely reflects actions independent of stimuli such as nutrients, hormones, or neurotransmitters associated with fuel status alone. Although it is unclear, these changes may be reflexive in nature and warrant further investigation. Additionally, AgRP neurons project to multiple downstream brain areas to regulate feeding
physiological need states can be learned. However, little is known about other biological implications. As suggested by the current study and supported by a recent report (Chen et al., 2015), prompt inhibition of AgRP activity provides a mechanism to rapidly inhibit foraging once food is discovered, a possible beneficial behavior to preserve energy demands associated with the pursuit of food. Thus, AgRP neuronal activity during this short window may instruct animals how to respond to food-related cues. Additionally, from nutritive to hedonic, there are multiple motivations associated with the need to eat. Understanding these stimuli may ultimately lead to effective strategies in the regulation of weight gain.
behavior (Betley et al., 2013; Gautron et al., 2015; Shah et al., 2014; Wu and Palmiter, 2011). Recent work demonstrated nuclei-specific roles for AgRP neuronal projections in the regulation of feeding (Betley et al., 2013). The current optogenetic and chemogenetic strategies have great potential to delineate the temporal and long-term roles of these neural circuits, regulating the negative valence described in the current study. In summary, Betley and colleagues (2015) propose a new role for AgRP neurons as a conditioned stimulus in learning related to feeding behavior. As a negative valence signal, AgRP neurons drive nutritive-enforced behaviors if physiological need is not reduced. Through the reduction of negative-valence signals, preference for cues associated with alleviating
REFERENCES Aponte, Y., Atasoy, D., and Sternson, S.M. (2011). Nat. Neurosci. 14, 351–355. Betley, J.N., Cao, Z.F., Ritola, K.D., and Sternson, S.M. (2013). Cell 155, 1337–1350. Betley, J.N., Xu, S., Cao, Z.F., Gong, R., Magnus, C.J., Yu, Y., and Sternson, S.M. (2015). Nature 521, 180–185. Chen, Y., Lin, Y.C., Kuo, T.W., and Knight, Z.A. (2015). Cell 160, 829–841. Gautron, L., Elmquist, J.K., and Williams, K.W. (2015). Cell 161, 133–145. Krashes, M.J., Koda, S., Ye, C., Rogan, S.C., Adams, A.C., Cusher, D.S., Maratos-Flier, E., Roth, B.L., and Lowell, B.B. (2011). J. Clin. Invest. 121, 1424–1428. Morton, G.J., Cummings, D.E., Baskin, D.G., Barsh, G.S., and Schwartz, M.W. (2006). Nature 443, 289–295. Shah, B.P., Vong, L., Olson, D.P., Koda, S., Krashes, M.J., Ye, C., Yang, Z., Fuller, P.M., Elmquist, J.K., and Lowell, B.B. (2014). Proc. Natl. Acad. Sci. USA 111, 13193–13198. Takahashi, K.A., and Cone, R.D. (2005). Endocrinology 146, 1043–1047. Wu, Q., and Palmiter, R.D. (2011). Eur. J. Pharmacol. 660, 21–27.
Cell Metabolism 22, July 7, 2015 ª2015 Elsevier Inc. 63
