The role of CCK₂ receptors in energy homeostasis: insights from the CCK₂ receptor-deficient mouse

Abstract

The present study explored the contribution of type 2 cholecystokinin (CCK) receptors in energy regulation. A total of 78 CCK₂ receptor-deficient mice and 80 wild-type controls were acclimated to a 12:12 light–dark cycle at 30±1 °C. Using a computer-monitored biotelemetry system, circadian patterns of body temperature, food intake, and activity were monitored for 4 days. Body weight and water consumption were manually recorded during this period. Results indicate that CCK₂ receptor invalidation produces elevated body temperature during both the photophase and scotophase (by 0.38 and 0.12 °C, respectively), increased body weight (29.3±0.2 vs. 26.8±0.2 g) and water consumption (4.1±0.1 vs. 3.2±0.1 ml), and decreased scotophase locomotor activity (WT: 7.0±0.2 vs. KO: 6.1±0.2 counts/min). These findings suggest an important role for CCK₂ receptors in processes underlying energy regulation during basal and possibly pathological states.

Introduction

Cholecystokinin (CCK) is one of the most abundant neuropeptides of the central nervous system. It binds with equal affinity to CCK₁ (previously termed CCK_A) and CCK₂ (previously termed CCK_B/gastrin) receptors [1]. The former are primarily localized in peripheral organs and some discrete areas of the brain, whereas CCK₂ receptors are the predominant subtype of the brain where they have a ubiquitous distribution [1], [2]. Strongly concentrated in the hypothalamus [1], CCK peptide and CCK₂ receptors are strategically positioned for a pivotal role in energy regulation including thermoregulation and food intake [3].

Exogenously administered CCK has long been known to produce hypothermic and hyperthermic responses [3]. In the rat, antagonism of the CCK₁ receptor attenuated the hypothermic effect of peripherally administered CCK octapeptide (CCK-8), but failed to alter the hyperthermia induced by centrally injected CCK-8 [4]. In contrast, CCK₂ receptor antagonism reduced this hyperthermia [4]. The role of CCK receptor activation in fever genesis and maintenance is equivocal. Although CCK₁ receptor antagonism has yielded negative findings [5], [6], blockade of CCK₂ receptors, either peripherally or centrally, attenuated lipopolysaccharide-induced fever [7]. Thus, a role for the CCK₂ receptor in thermoregulation seems likely.

The satiety effect of CCK is well established [8]. Although substantial evidence exists implicating CCK₁ receptors in the mediation of satiety under various conditions [9], results concerning the CCK₂ receptor have been mixed. A delay in postprandial satiety and/or an increase in food consumption has been reported following antagonism of CCK₂ receptors [10], [11], [12]. While this led some to speculate that central CCK acts primarily at CCK₂ receptors to affect food intake, the satiety effect of CCK could not be prevented by CCK₂ receptor antagonism [13], [14], and, with the exception of one study [15], CCK₂ selective agonists have not been shown to induce satiety [16], [17]. Thus, an involvement of CCK₂ receptors in mediating satiety is questionable.

Given the role ascribed to CCK as a satiety factor, it is not surprising that the body of evidence implicating CCK in the regulation of body weight is growing. Recently, it has been suggested that peripheral CCK and central leptin act in concert to regulate body weight [18], and although some pharmacological studies indicate that CCK does not reduce body weight [19], [20], CCK₁ receptor invalidation results in obesity [21]. As yet, however, a role for CCK₂ receptors in body weight control has not emerged.

CCK receptor activation has also been implicated in the regulation of locomotor activity. Peripheral and central injection of CCK induces hypolocomotion, which is abrogated by CCK₁ antagonists. Further, CCK₁ receptor blockade reduces spontaneous motor activity in mice [22], and CCK₁ receptor-deficient mutant rats exhibit decreased levels of scotophase motor activity [23]. Although CCK₂ receptor blockade increases locomotor activity in mice [22], peripheral stimulation of CCK₂ receptors induces increased spontaneous activity through a δ-opioid receptor action [24].

Although pharmacological approaches have provided a great deal of insight into the physiologic functions mediated by CCK and its receptor subtypes, the interpretation of these studies is constrained by problems associated with metabolism, incomplete antagonism, and receptor desensitization. Furthermore, while pharmacological strategies are useful in elucidating short-term responses, they are of limited value in clarifying effects in the longer-term processes of energy regulation. Targeted gene disruption, however, represents a powerful in vivo tool for assessing the physiological significance of receptors in long-term responses.

Gene targeting has enabled the generation of mice that exhibit a disrupted CCK₂ receptor gene [25]. When backcrossed for several generations, the genetic background or wild-type (WT) for these animals is the C57BL/6J [26]. Mutants that are homozygous for the disrupted gene lack functional CCK₂ receptors. The expression of central and pancreatic CCK₁ receptor mRNA in these mice is the same as that of heterozygous littermates, and CCK₂ receptor-deficient (CCK₂RD) mice do not exhibit any obvious abnormalities in brain morphology or histopathology [25]. Although the short-term locomotor and satiety responses of these animals have been reported [27], [28], [29], as well as energy metabolism and turnover [30], an analysis of the long-term behavioral parameters of these mice is lacking. This report profiles the circadian patterns of body temperature (T_b), activity, and food and water intake, as well as the body weight of adult CCK₂RD mice compared to WTs. We report that CCK₂ receptor invalidation results in elevated photophase and scotophase T_b, increased body weight and water consumption, and decreased scotophase locomotor activity.