In summary, to our knowledge, the CSDS-induced CPP for HFD protocol described here is the first reported mouse model of an antidepressant-responsive, neuropsychiatric syndrome used to probe the mechanisms responsible for hedonic eating behaviors that occur upon chronic stress (32
). As mentioned, the CSDS protocol is considered a fairly representative model of prolonged psychosocial stress in humans and also produces a depression-like state, which, similar to major depressive disorder in humans, is known to be reversible only upon chronic, but not acute, administration of antidepressant agents (32
). Here, using a CPP protocol to examine complex, reward eating behaviors induced by the chronic stress/depression model, we were able to demonstrate stress-associated increases in both CPP for and intake of HFD. Thus, exposure of mice to CSDS prior to the CPP for HFD protocol models the complex changes in eating behavior and body weight gain associated with chronic stress and major depressive disorder — particularly the atypical subtype — in humans (1
). Also, this model revealed a dependence of these food-reward behaviors on intact ghrelin signaling via GHSRs.
In addition, to our knowledge, this study contains the first major use of mice with conditionally manipulated ghrelin receptor expression. More specifically, we showed that direct ghrelin signaling via GHSRs localized to catecholaminergic neurons was sufficient not only for CPP for HFD induced by ghrelin administration and for usual mood responses after chronic stress but also for stress-induced food-reward behaviors.
A schematic model, which incorporates these findings with some previous results and which describes the proposed role of ghrelin and GHSR-TH–coexpressing neurons in stress-induced depression and eating behaviors, appears in Figure and is further described as follows. We, and others, have shown that the CSDS protocol in mice, which is a model of prolonged psychosocial stress in humans, results in several persisting behavioral deficits reminiscent of depression (refs. 26
, and Figure , part i). Psychosocial stress also led to elevations in circulating levels of acyl-ghrelin, via a pathway which has not yet been elucidated (Figure , part ii). (Speculatively, this stress-induced elevation may involve stimulation of β1
-adrenergic receptors on ghrelin cells, as such a pathway has been shown to play a role in ghrelin release during a 24-hour fast and as increased sympathoadrenal tone is a known consequence of stress [51
]). Ghrelin, in turn, interacts with its receptor, GHSR, which is found distributed throughout the brain and periphery in a specific pattern (37
). The data in this report suggest that direct ghrelin interaction, specifically with GHSR-TH–coexpressing neurons (Figure , part iii), is sufficient for many of the behaviors induced in the setting of prolonged psychosocial stress. In particular, engagement of these catecholaminergic neurons by ghrelin minimized what would otherwise be worsened depression (as evidenced by the increased social avoidance in mice lacking GHSRs after CSDS; Figure , part iv), while at the same time, induced various hedonic eating behaviors and hyperphagia (Figure , part v). Of note, direct ghrelin action on catecholaminergic neurons did not appear to mediate ghrelin’s effects on glycemia, as fasting blood glucose levels were similarly and significantly lower than wild-type levels in both GHSR-null and GHSR-null/TH mice. The effects on eating behavior led to increased intake of highly palatable, calorically dense foods, which in the human experience are often termed comfort foods, and increased body weight (Figure , parti vi), which in turn can result in overweight and obesity.
Model of ghrelin’s roles in stress-induced behaviors.
Although the current study does focus in on the identity of those neurons that are sufficient for ghrelin’s actions in coordinating the response to stress (GHSR neurons that coexpress TH or, rather, are catecholaminergic), the selectivity afforded by the TH-Cre mouse did not allow us to further narrow down the site of direct ghrelin action to one particular brain nucleus. Nonetheless, we predict that GHSR-TH–coexpressing dopaminergic neurons in the VTA are very important in ghrelin’s actions in coordinating the response to stress. As mentioned, VTA dopaminergic neurons have long been known to play key roles in reward behavior of various types and mood regulation (38
), GHSRs are highly expressed in VTA dopaminergic neurons (36
), and ghrelin is known to act on these VTA neurons to influence food intake and food reward. For instance, ghrelin induces dopamine release in the NAC and increases action potential frequency in VTA dopamine neurons (36
). Direct ghrelin microinjection into the VTA increases intake of freely available food, intake of a rewarding diet (peanut butter) over regular chow, and operant lever pressing for a sucrose reward (20
). Conversely, direct VTA microinjection of a GHSR antagonist decreases food intake in response to i.p. ghrelin and also decreases operant lever pressing for a sucrose reward normally induced by an overnight fast (36
). Also, rats containing VTA lesions consume less peanut butter but equal regular chow amounts compared with that of sham-lesioned animals and spend less time exploring tubes containing peanut butter in response to i.c.v. ghrelin (20
). Additionally, selective knockdown of GHSR expression in transgenic rats expressing an antisense GHSR transcript under the control of the TH promoter decreases food intake (44
As mentioned, besides the VTA, there are other sites of presumed usual GHSR-TH coexpression, in which TH-Cre–driven GHSR expression was observed within GHSR-null/TH mice and in which ghrelin may act in the setting of stress on mood and/or eating. Most of these sites are dopaminergic, with the possible exception of scattered cells within the nucleus of the solitary tract (which, in the rat, have been shown to include neurons with adrenergic, noradrenergic, or dopaminergic signatures; ref. 49
). Direct ghrelin action on GHSR-containing dopaminergic neurons within the substantia nigra also could be mediating some of the complex food-reward behaviors we observed in the CSDS-exposed mice, as neuronal projections from the substantia nigra to the dorsal striatum are involved in establishing efficient behavioral habits aimed at obtaining rewards (54
). It would be even more speculative to propose a role for direct ghrelin action on dopaminergic (tuberoinfundibular) neurons within the ARC, since their known role has mainly been limited to regulatory effects on prolactin secretion (55
It is also intriguing to speculate whether the mechanisms herein described for stress-associated food-reward behaviors may be generalized to other reward behaviors known to occur in the setting of psychosocial stress. An association between stress and an increased risk of abusing addictive substances already is well established (56
). For example, chronic stress has been shown to be a strong predictor of addiction vulnerability (58
), and posttraumatic stress disorder is closely associated with an increased incidence of nicotine, alcohol, and other drug abuse (60
). Exposure to stress has also been shown to reinstate drug-seeking behaviors in animal models and to increase chances of relapse in addictive individuals (61
). Importantly, besides food-reward behaviors, ghrelin-signaling pathways also mediate alcohol-reward (64
), cocaine-reward (66
), and nicotine-reward behaviors (70
). Thus, ghrelin signaling, for instance, via catecholaminergic neurons, as described here, also may serve as an important link between chronic stress and drug reward.
We suspect that during evolution a link between stress, ghrelin, and eating behaviors may have instilled an important survival advantage. However, in today’s modern society, activation of these ghrelin-engaged catecholamine neurons instead may contribute substantively to the altered, complex eating behaviors and development of overweight and obesity in humans exposed to chronic psychosocial stress and in humans with major depressive disorder. Future experiments should be aimed at determining the sufficiency and requirement specifically of the VTA in these actions of ghrelin as well as further clarifying the role for glucocorticoids in ghrelin action. Future studies also should focus on determining the significance of GHSR expression in TH-containing neurons at other sites, for instance, the substantia nigra and the ARC. Furthermore, our described CSDS-CPP for HFD mouse model should prove extremely useful in future studies aimed at investigating the involvement of other hormones and neuronal signaling pathways in stress-induced eating of comfort foods and other stress-related, complex eating behaviors as well as future studies aimed at developing pharmaceutical agents to curb these potentially detrimental behaviors.