Abstract

1. Introduction

Food consumption is necessary for survival and is affected by homeostatic and hedonic aspects of feeding. Hunger and satiety are two physiological responses that promote and reduce appetite and food intake. Signals that facilitate or impede appetite and feeding are integrated into the nucleus tractus solitarius and hypothalamus. Three hypothalamic structures play essential roles in this process. These include the ventromedial nucleus of the hypothalamus (VMN), serving to suppress appetite and food intake; the lateral hypothalamus (LH), functioning to promote appetite and food consumption; and the arcuate nucleus of the hypothalamus (ARC), operating as a switchboard to turn on or turn off the above structures to regulate appetite and food intake [1,2].

PACAP mediates food intake in these structures in distinctive manners. For example, PACAP injection in the ARC selectively decreases carbohydrate intake but not high-fat intake. The role of afferent projections from the bed nucleus of stria terminalis (BNST) to the LH in homeostatic feeding is to attenuate food intake upon PACAP injection, suggesting that PACAP may also have indirect effects on the LH’s satiety-dependent food intake (for detail, see Section 4 and Section 8).

Two groups of neurons exist in the ARC that integrate and modulate feeding behaviors. These neurons contain anorexigenic peptides, such as proopiomelanocortin (POMC) and cocaine-amphetamine-regulated transcript (CART), as well as orexigenic peptides, such as neuropeptide Y (NPY)/agouti-related peptide (AgRP). Many central and peripheral inputs can alter the activity of these neurons. For example, insulin, leptin, ghrelin, etc., secreted by peripheral tissues, can activate one or the other type of ARC neurons to increase or decrease appetite and food intake.

Changes in the levels of nutrients in blood circulation can also relay excitatory or inhibitory signals to these neurons to stimulate or suppress appetite. For example, low blood glucose increases ghrelin secretion, which increases calcium levels in NPY-containing neurons [3]. The increase in calcium facilitates exocytosis and triggers the release of NPY, promoting appetite and food intake.

Several peptides in the peripheral or central nervous system can regulate appetite and food intake. The glucagon superfamily promotes hypophagia, a conserved response throughout different species [4]. A growing body of evidence suggests that pituitary adenylyl cyclase activating polypeptide (PACAP), a peptide within this superfamily, regulates food intake. Thus, PACAP, like other analogs in the glucagon superfamily, may prove to be a potential target to develop medications to treat obesity. Likewise, the dual role of PACAP in both satiety and hedonic feeding suggests that it may prove useful in treating binge-eating disorders. In this article, we reviewed the literature (68 articles selected and included in this review) regarding the regulatory action of PACAP on food intake (Table 1).

In this review, we attempted to describe the literature on the role of endogenous PACAP and the effect of exogenous PACAP on food intake. Furthermore, we attempted to provide information regarding neuroanatomical sites where PACAP exerts its hypophagic effects in the brain. We also discussed the effect of PACAP on the hedonic and homeostatic aspects of feeding. Finally, we reviewed the role of neurotransmitters and signaling mechanisms in the hypophagic actions of PACAP.

2. PACAP, Its Receptors, and Its Physiological Roles

Pituitary adenylate cyclase-activating polypeptide (PACAP) was initially isolated from the ovine hypothalamus as an activator of adenylyl cyclase in pituitary cells [5]. PACAP is found in two forms. PACAP38 comprises 38 amino-acid residues and PACAP27, a C-terminally alpha-amidated form of PACAP; the sequence encompasses an internal cleavage-amidation site that generates a 27-residue alpha-amidated polypeptide fragment or PACAP27, which corresponds to the N-terminal 27 amino-acid portion of the PACAP38 [6].

Human PACAP27 shares 68% identity with VIP, classifying PACAP as a member of the VIP/secretin/growth hormone/glucagon superfamily. The PACAP gene contains five exons, and PACAP is encoded by exon 5. The human PACAP promoter possesses two cAMP response-like elements, which play a role in the tissue-specific factor growth hormone and six binding domains for the thyroid-specific transcription factor-1.

PACAP binds to three G protein-coupled receptors: PAC1R, VPAC1, and VPAC2 [7]. The PACAP-preferring PAC1 receptors isoforms can be coupled to G_s and G_q to stimulate adenylyl cyclase and phospholipase C (PLC), respectively. In contrast, VPAC1 and VPAC2 receptors, which had an equal affinity for PACAP and VIP, are principally coupled to G_s to activate adenylyl cyclase and increase intracellular 5′-cyclic adenosine monophosphate (cAMP) levels [8].

The rise in cAMP activates protein kinase A (PKA). PLC causes the breakdown of phosphatidylinositol and the production of inositol triphosphate and diacylglycerol, leading to an increase in intracellular calcium and protein kinase C activation. These biochemical changes modulate multiple ionic currents and stimulate signaling factors. Moreover, PAC1 receptor activation led to β-arrestin-mediated receptor internalization and endosomal signaling, which led to extracellular regulated protein kinase phosphorylation [9,10].

PACAP is expressed in the central nervous system and peripheral organs and exerts multiple effects (pleiotropic). These include the control of neurotransmitter release, vasodilation, bronchodilation, activation of intestinal motility, increase in insulin and histamine secretion, immune modulation, as well as cell proliferation and differentiation [6]. A growing body of literature also suggests that PACAP and its receptors are involved in homeostatic and hedonic aspects of feeding. PACAP exerts its anorexigenic effect in different brain regions and periphery involving insulin regulation and other mechanisms. This article aims to evaluate the literature regarding the regulatory actions of PACAP on feeding.

3. PACAP and Feeding

Initial studies showed that central PACAP administration reduces feeding. Morley and colleagues found that intracerebroventricular (ICV) injection of PACAP at 2.5 (but not 1) µg reduced food intake in mice [11]. Likewise, a dose-dependent decrease in food intake was reported following ICV injection of PACAP in male Sprague–Dawley rats [12]. Subsequent studies confirmed these results showing that ICV administration of PACAP to food-deprived male CD1 mice dose-dependently reduced food consumption. Matsuda and colleagues [13,14] found that ICV injection of PACAP or VIP (11 and 22 pmol/g) reduced food consumption in goldfish, with the higher dose of PACAP and VIP inducing increases in locomotor activity as well.

In another study using Zebrafish, ICV injection of a low dose of PACAP (2 pmol/g) suppressed food intake, suggesting that PACAP acts as an anorexigenic peptide in the brain of fishes [7]. The hypophagic effect of PACAP was reduced in the presence of a PACAP receptor antagonist [15] or PACAP receptor knockout mice [16]. Additional research showed that the peptide levels were elevated in the brainstem in food-deprived rats.

Similar changes occurred in the hypothalamus and telencephalon in chickens [17], suggesting that endogenous PACAP is involved in food intake regulation. In this article, we reviewed the literature regarding the role of PACAP and its receptors in food intake. First, we discussed the role of endogenous PACAP in feeding. We next presented the findings regarding the effect of exogenous PACAP and related ligands on food intake. We finally reviewed the literature regarding the involvement of different neurotransmitters and brain regions in the hypophagic effects of PACAP.

4. Neuroanatomical Sites of PACAP Action in Regulating Food Intake

Different brain regions are implicated in the regulatory actions of PACAP on the homeostatic and hedonic aspects of food intake (Figure 1).

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Figure 1

Schematic diagram of different brain regions involved in the regulatory action of PACAP on feeding. Arcuate Nucleus (ARC); Bed Nucleus of Stria Terminalis (BNST); Central Nucleus of Amygdala (CeA); Lateral Hypothalamus (LHA); Nucleus Accumbens (NAc); Paraventricular Nucleus of the Hypothalamus (PVN): Ventromedial Nucleus of the Hypothalamus (VMN); and Ventral Tegmental Area (VTA).

The bed nucleus of stria terminalis (BNST) is important in regulating anxiety-like behavior caused by long-term threats. It also has a role in mediating stress-induced anorexia and associated reductions in weight gain [45,46]. PACAP may act in the BNST to regulate stress response during stressful events. An earlier study [47] showed that infusion of PACAP38 in the posterior BNST in male and female Sprague–Dawley rats mimics weight loss and anorectic state induced by repeated stress exposure. However, this effect was not observed following PACAP administration in the anterior BNST or ICV, which shows that PACAP exerts its action in the posterior BNST. The authors concluded that, in female rats, the site-specific action of PACAP in the BNST was inconclusive. Given the role of PACAP and the importance of BNST in the regulation of food intake and stress response, the PACAP/PAC1R system may be involved in stress-related eating disorders.

In an earlier study [48], PACAP injected directly into the VMN increased both core body temperature and locomotor activity for up to 7 h. When PACAP (25 pmol) was administered locally into the VMN, feeding decreased by 52%, which lasted for 3 h, while PACAP at 50 pmol reduced feeding by 80% for up to 6 h. The study also found that, when PACAP was injected into the VMN, POMC mRNA expression was greater in the ARC, without any change in NPY or AgRP mRNA activity [48]. Considering that the VMN was shown to project onto the nucleus tractus solitarius (NTS), which is responsible for regulating energy homeostasis, food intake, and locomotor activity, researchers concluded that excitation of the VMN via PACAP injections might be responsible for increased POMC signaling within the NTS [48].

When PACAP was injected into the PVN or VMN, it reduced the latency in consuming the first meal and the eating rate in Sprague–Dawley rats compared to its vehicle. When injected in the PVN, rats showed strong decreases in meal size, meal duration, and the total time spent eating. These effects were reversed by PAC1R antagonists but not VPAC antagonists. Researchers also found that PACAP administration in both the PVN and VMN increased the plasma glucose concentration but did not affect pancreatic hormone levels. Upon the injection of PACAP in the VMN, the core body temperature and spontaneous locomotor activity in rats were elevated compared to the controls. These results were reproduced using maxadilan, a PAC1R agonist, but not after VIP injections in either brain region [49].

Iemolo and colleagues found that rats infused with PACAP in the central amygdala showed decreased food intake and body weight with no significant locomotor activity changes [50]. However, intra-basolateral amygdala infusion of PACAP was without any effects on food intake and body weight or locomotor activity, highlighting the site-specificity of hypophagic action of PACAP.

In summary, PACAP acts in different brain regions to regulate food intake. It exerts its action on homeostatic feeding in the hypothalamus, whereas its action in the VTA and NAc may inhibit hedonic feeding (see below). However, further studies are needed to delineate the role of other brain regions, such as the paraventricular nucleus of the thalamus (PVT), which contains high levels of PACAP27 and has been implicated in alcohol self-administration [51,52,53].

5. The Role of Other Neurotransmitters in the Actions of PACAP on Food Intake

The regulatory effect of PACAP on food intake may be mediated via the α-melanocyte-stimulating hormones (α-MSH) system [37]. These authors showed that food deprivation increased the expression of NPY and reduced the expression of proopiomelanocortin (POMC), the precursor of the anorexigenic peptide α-MSH. Similar changes were observed in the PACAP gene expression [37], indicating that PACAP may serve as another anorexigenic peptide. Indeed, these authors showed that exogenous PACAP administration reduced food intake in food-deprived animals.

This effect of PACAP was reduced but not completely abolished by PACAP6-38, a PACAP receptor antagonist. The antagonist alone did not affect food intake. The authors concluded that PACAP acts on PAC1 receptors to reduce food intake. Although this conclusion may be plausible, considering that PACAP acts on VPAC1 and VPAC2 receptors and that PACAP6-38 has some actions at VPAC receptors [38,39], further studies are needed to characterize the receptor types involved in this process.

As stated above, anorexia induced by central injection of PACAP is mediated, in part, through activation of the α-MSH, as the POMC gene expression was increased by PACAP and reduced in PAC1 receptor knockout mice. Additionally, PACAP increased c-fos expression in POMC neurons. Furthermore, the hypophagic effect of PACAP was reduced by SHU9119, an MC3/MC4 receptor antagonist, which had no significant effect on food consumption [37]. These authors concluded that central administration of PACAP exerts its anorexic effect, at least in part, through activation of the hypothalamic melanocortin system [37].

Activation of POMC neurons suppresses food intake and appetite via the release of α-MSH, an endogenous melanocortin receptor agonist. In contrast, AgRP inhibits POMC neurons by blocking melanocortin receptors, increasing appetite and food intake [6]. PACAP was shown to interact with POMC neurons via PAC1 and VPAC2 receptors but only PAC1 receptors with AgRP neurons [6]. Nguyen and colleagues found that PACAP knockout mice had decreased AgRP mRNA expression levels and increased POMC mRNA levels after fasting and refeeding. NPY and AgRP expression was upregulated by feeding after a fasting period in both knockout and wild-type mice.

However, AgRP expression was higher in wild-type compared to knockout mice. POMC expression was downregulated after fasting and refeeding, with the extent of the downregulation increased in knockout compared to wild-type mice. An ICV injection of PACAP6-38 decreased body weight, food consumption, and AgRP expression after fasting and refeeding in wild-type mice but with no changes in POMC expression [6]. These results suggest that PACAP may act as a regulator of anorexigenic and orexigenic peptides in the hypothalamus. Considering that the authors measured the expression of these peptides, further studies are needed to determine if endogenous or exogenous PACAP would regulate the level of these peptides and especially the release of these peptides.

PACAP was reported to decrease NPY-stimulated food and water intake in male Sprague–Dawley rats [54]. An earlier study found that PACAP (2.5 µg, ICV) decreased food consumption and increased locomotor activity in fasted mice [11]. When mice were administered NPY, food intake increased. Furthermore, a combination of PACAP and NPY induced a decrease in food consumption and increased locomotor activity, suggesting that PACAP and NPY exert opposing effects on food intake in the brain. Despite PACAP-induced suppression of NPY-mediated food intake in rats, it did not alter the level of NPY in the hypothalamus [54]. However, further studies using lower doses of PACAP are needed to differentiate whether the effect of PACAP is due to an action of the peptide on food intake or the peptide selectively alters the NPY-stimulated food intake.

Nakata and colleagues (2004) showed that food intake was reduced in PACAP-deficient mice compared to their wild-type controls. They also examined the underlying mechanism of this regulatory action of PACAP and discovered that NPY mRNA was reduced in PACAP-deficient mice. They also showed that PACAP increased intracellular calcium ([Ca²⁺]i) in NPY-containing neurons of the ARC [16].

Ca²⁺ is a regulator of neuronal activities; it was shown that orexigenic substances of physiological relevance, such as low glucose, ghrelin, and orexin, induce increases in [Ca²⁺]i in NPY-containing neurons [16]. Given that NPY neurons are affected by glucose and other chemicals, these authors also studied if consumption of a high-carbohydrate diet would be changed in these mice. Their results revealed that consumption of a high-carbohydrate but not high-fat diet was reduced in PACAP knockout than wild-type control mice [16]. Together, these findings suggest that endogenous PACAP influences carbohydrate intake via the NPY-releasing neurons in the ARC.

Per Shioda et al. [55], PACAP can increase or decrease the activity of glutamate ionotropic and metabotropic receptors [56] by activating Src protein tyrosine kinases, suggesting that PACAP regulates caloric intake by activating or inhibiting satiety circuits, like in the VMN; and appetite signals, in the NAc; suppressing homeostatic and hedonic feeding drives, which can be helpful towards developing a treatment for obesity and overeating.

AP5, an NMDA receptor antagonist injected into the VMN of male Sprague–Dawley rats at doses ranging from 10 to 1 nmol, did not seem to alter the feeding behaviors but blocked the effects of PACAP injected into the VMN on food intake when administered before PACAP injections [57]. Appetite and food intake were also decreased by inhibiting ghrelin and stimulating GLP1 and leptin [15]. Per Adams et al. [58], PACAP’s role is to maintain the energy balance. PACAP-null mice were not obese or hyperphagic compared to wild-type mice regardless of the temperature or diet. PACAP-null mice were initially leaner than wild-type mice if maintained on a regular chow diet and at 21 °C.

Researchers proposed that the decrease in body mass could not be attributed to hypophagia, increased metabolism, or motor activity but rather sensitivity to cold temperatures in these mice. The reduced weight was due to decreased fat mass, leptin, and insulin levels. Researchers found that PACAP-null mice compared with wild-type mice had an improved response to insulin tolerance but normal glucose tolerance. Furthermore, adult PACAP-null mice were sensitive to cold at 4 °C, as they lost temperature faster than wild-type mice, suggesting that the thyroid axis is affected because of an observed reduction in thyrotropin-releasing hormone (TRH) mRNA. The study concluded that the reduced body mass of PACAP-null mice could be reversed with a thermoneutral environment or a high-fat diet.

In chicks, PACAP and CRH both inhibited food intake and induced behavioral changes after central injection, suggesting that the anorexigenic effects may be regulated by CRH pathways [59]. In the goldfish, studies on the effect of CRH on feeding behavior after central injection showed that it acts as a hypothalamic anorexigenic peptide [60]. PACAP was shown to regulate the stress response and affective aspect of pain via hypothalamic and extra-hypothalamic structures. For example, PACAP infusions into the central CeA caused pain hypersensitivity and increased anxiety in an elevated plus-maze [61]. Similarly, PACAP infusions in the BNST heightened acoustic startle [20], increased peripheral corticosterone release [62], caused an anorexic phenotype [47], and reinstated cocaine-seeking behavior [63].

The anorexigenic effects of PACAP and VIP in male layer chicks were mediated by CRH, as this action of the peptides was reduced by astressin, a CRH receptor antagonist [64]. However, the CRH antagonist had a differential effect on the anorexigenic effect of PACAP and VIP. Astressin significantly reversed the anorexigenic effects of PACAP at 90 min with lesser effect at earlier time points. In contrast, it significantly reversed the hypophagic effect of VIP throughout the 90-min test period, suggesting CRH is differentially involved in the hypophagic effect of VIP and PACAP. Both peptides also increased corticosterone secretion via the CRH system. However, VIP-induced corticosterone secretion was only partially attenuated by astressin [64].

The endogenous PACAP/PAC1 receptor system may also be involved in the regulatory action of leptin [32]. Animals injected with leptin into the VMN ate less than controls for 3 h after injection, and this effect was blocked by PACAP6–38. Administration of PACAP6–38 alone in the VMN had no effects on food intake or body weight. Leptin administration also increased body temperature 20 h after injection. This response was also reversed by PACAP6-38, injected before leptin in the VMN. These results suggest leptin may exert some of its action via the PAC1R activation in the VMN.

Researchers found that PACAP mRNA was co-localized with brain-derived neurotrophic factor (BDNF) mRNA and that PAC1R mRNA was co-expressed with BDNF and the leptin receptor [32]. When PACAP was administered in the VMN, researchers observed STAT3 phosphorylation and increased BDNF and SOCS3 mRNA levels, indicators of leptin receptor activation. These effects of PACAP and leptin were blocked by PACAP6-38, suggesting that PACAP is downstream of leptin and mediating its action on food intake. We propose that there may be a positive feedback loop, where central PACAP release causes leptin release from the periphery, and leptin, in turn, activates the SF-1/PACAP expressing neuron in the VMN resulting in excitation of POMC neurons [26,65].

Bath application of PACAP increased neuronal activity, although administration of leptin had a bidirectional effect in which cells that generally fired less often showed an increase in firing rate. In contrast, cells that fired more often showed a decrease in firing rate after leptin administration. Administration of PACAP6–38 attenuated the effects of leptin on firing and entirely blocked the stimulatory effects of PACAP on neuronal activation.

In summary, these findings suggest that PACAP exerts its anorectic effects via different peptide systems, including α-MSH, NPY, AgRP, leptin, and CRH. However, the involvement of different peptides has been shown in different species. Thus, although the anorectic effect of PACAP is conserved in several species, further studies are needed to delineate the role of orexigenic and anorexigenic peptides in the action of PACAP in the brain and the periphery. It is important to assess if melanocortin receptor antagonists would alter the action of PACAP in different species to examine if the role of α-MSH is also conserved in different species. Importantly, most of the studies have measured changes in the expression of orexigenic and anorexigenic peptides. It is better to measure the release of these peptides in response to PACAP administration.

6. The Role of PACAP Receptor(s) in the Regulatory Actions of PACAP on Feeding

PACAP activates PAC1 receptors with higher affinity than VIP. On the other hand, PACAP and VIP exhibit similar affinity toward VPAC1 and VPAC2 receptors. The PACAP and PAC1 system is thought to play a key role in feeding and energy homeostasis in the hypothalamus [66]. Still, few studies assessed the role of extrahypothalamic PACAP in these processes. PACAP was shown to increase POMC and MC4R mRNA levels in the ARC. The melanocortin system was reported to mediate the central effects of PACAP on thermogenic sympathetic and digestive parasympathetic outflow [67]. Based on this finding, it is speculated that PAC1 receptors are found presynaptically on POMC neuronal terminals to increase the synthesis of α-MSH.

As stated above, ICV PACAP6-38 injection did not affect food intake in mice deprived of food for 18 h but attenuated the inhibitory effect of PACAP on feeding during the 60-min observation period [37]. It was also found that the PACAP antagonist, PACAP6-38, attenuated the inhibitory effects of PACAP on feeding, a response that was not mimicked by VIP, indicating that PACAP has an anorexigenic action through the PAC1 receptor.

Another study [16] concluded that PACAP increased depolarization of NPY neurons of the ARC via PAC1 and VPAC2R, resulting in reduced carbohydrate intake by promoting glucose output [43], which may prevent hypoglycemia and lack of energy in PACAP deficient mice. These findings suggest that PACAP has dual insulinotropic and glucagonotropic effects of regulating energy and glucose homeostasis. Given that PACAP could act not only on PAC1 but also VPAC1 and VPAC2 receptors and that PACAP6-38 could activate VPAC2 receptors [68], further studies are needed to characterize the receptor type involved in this process.

7. Signaling Involved in the Modulatory Actions of PACAP on Food Intake

According to a study by Dore and colleagues, PACAP released after stress exposure induces CRH expression via the activation of PAC1 receptors in the PVN and increases intracellular cAMP/protein kinase A signaling [66]. Therefore, CRH can be a target and mediator of PACAP’s actions. However, the weight loss and anorectic effects of PACAP seem to be independent of CRH. Researchers found that the anorexigenic effects of PACAP were not blocked by the CRH receptor antagonist D-Phe-CRF(12–41) but attenuated by SHU9119, a melanocortin receptor (MCR3/MCR4) antagonist, and by a tyrosine kinase B (TrKB) inhibitor k-252. These observations suggest that the anorexigenic effects of PACAP may be mediated by the melanocortin and TrkB/BNDF pathways [50].

Tyrosine phosphorylation of the N-methyl-D-aspartate receptors (NMDA) receptor by PACAP is associated with increased Src family kinase activity [69,70]. In particular, PACAP induces tyrosine phosphorylation of the GluN2B subunit in the NMDA receptor through Src kinase activation, which plays an important role in food intake regulation. This view was based on the finding that VMN PACAP injections produced hypophagia and GluN2B tyrosine phosphorylation [57]. The state of phosphorylation of NMDA by Src kinases is also essential in this regard, as protein phosphatase-1 (PP1), which inhibits the tyrosine phosphorylation of the NMDA receptor, reduced the hypophagic effect of PACAP.

PACAP signaling may also regulate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, as PAC1 receptor-dependent potentiation of AMPA receptors at low PACAP concentrations and VPAC2R depression of AMPA receptors at high concentrations of PACAP in the hippocampus [71]. The PACAP-PAC1 receptor signaling increases due to the AMPA receptor activity in the amygdala.

Further works are needed in this area to delineate the role of other signaling mechanisms in the hypophagic effects of PACAP. Particularly, PACAP can activate protein kinase A and protein kinase C. Thus, the role of downstream phosphorylated products of these enzymes needs to be assessed in the hypophagic effects of PACAP.

8. The Action of PACAP on Homeostatic and Hedonic Aspects of Feeding

Homeostatic food intake is driven by hunger, whereas hedonic food intake is driven by palatability. The lateral hypothalamic area (LHA), containing receptors for various neurohormones and neurotransmitters including GABA, TRH, orexin, and hypocretin, proves to be an area in which the homeostatic and hedonic pathways interact. The homeostatic food intake is mediated by ARC and PVH neurons, whereas hedonic food intake is mediated by the mesolimbic dopaminergic (DAergic) neurons.

Researchers found that dopaminergic signaling within the basal ganglia attenuates homeostatically driven food intake in the LHA. Researchers found that activation of AgRP neurons in the LHA, which is thought to co-release GABA, and inhibitory projections from the BNST to the LHA prove to drive hedonic food intake. The NAc may also play a role in hedonic food intake, as it was shown to have efferent and afferent projections to the LHA, ARC, and VTA.

A previous study [32] examined the effect of PACAP on the hedonic and homeostatic aspects of feeding. These authors developed a binge-eating paradigm where animals consumed a large amount of food within a short time. Using this model of binge eating, these authors studied the neuroanatomical site of action of PACAP in regulating homeostatic and hedonic drives of food intake. It was found that injection of PACAP into the VMN decreased hunger-mediated food consumption, while injection of the peptide into the NAc did not influence this response. In contrast, local injection of PACAP into the NAc decreased the intake of high-carbohydrate and high-fat foods. This study suggests that PACAP within the VMN reduces the homeostatic aspect of feeding, while injection within the NAc suppresses the hedonic aspect of eating.

Researchers examined whether PACAP excites POMC neurons via PAC1 receptor mediation and TRPC channel activation, a potential mechanism through which PACAP could regulate the homeostatic aspects of food intake. In recordings from POMC neurons, PACAP induced depolarization and increased firing in the voltage clamp. These postsynaptic actions were abolished by PAC1 receptor antagonists, suggesting that PAC1R is involved in these actions of PACAP. They also showed that this process involved TRPC channels, phospholipase C, phosphatidylinositol-3-kinase, and protein kinase C.

The stimulatory effect of PACAP on POMC neurons is potentiated by estradiol and attenuated under diet-induced obesity/insulin resistance [65]. PACAP normally inhibits food intake via a negative feedback loop. However, it has been proposed that, in an addictive state, PACAP acts through a positive feedback mechanism to stimulate food intake and promote drug and food addiction cycles (for a review, see [27]). The limbic system may play a functional role in the actions of PACAP on food intake since PACAP is stimulated by chronic stress in the BNST and PVN.

Chronic stress can lead to anxious and depressive-like moods that trigger the hedonic mechanism of food intake and promote addictive cycles [27]. However, more recent work from Wagner’s research team showed that PACAP nerve terminals were optogenetically stimulated in the VMN, and Channel Rhodopsin-2 (Chr2) expression was found in the cell bodies of PACAP neurons localized in the VMN and terminals of PACAP neurons in the VTA [72]. Optogenetic stimulation and patch-clamp recordings of this PACAP pathway showed that PACAP neurons in the VMN inhibit A10 dopamine neurons within the VTA.

Consistent with this, they showed wild-type male mice fed on an intermittent HFD showed increased anticipatory locomotion, caloric intake, heat production, meal size, bout duration, and O2 consumption. PACAP injected into the VTA (30 pmol; 0.2 L) reduced these binge-eating behaviors in males. Although binge-eating caused increased anticipatory locomotion, caloric intake, and O2 consumption in ovariectomized female mice, PACAP failed to alter these effects in these mice.

The intake of regular chow was not altered in mice of either sex. Together, these results suggest that the effect of PACAP on hedonic feeding may be reduced in obese/diabetic and female subjects. However, further studies are needed to recapitulate these results and provide a mechanistic explanation for these changes in the action of PACAP in these subjects.

9. Concluding Remarks

A growing body of evidence suggests that PACAP regulates food intake in several species, including fishes, chicks, and rodents. This action of PACAP is primarily via the PACAP-preferring (PAC1) receptor, as the action of PACAP is absent in mice lacking PAC1 receptors or in rodents treated with the PAC1 receptor antagonist PACAP6-38. The anorexic effect of PACAP involves different brain regions with a selective action of the peptide on the homeostatic aspect of food intake into the VMN in the hypothalamus and NAc as the neuroanatomical site mediating the effect of PACAP on the hedonic aspect of food intake.

However, further research is needed to define the role of other brain regions, such as the VTA, PVT, and NTS, in the regulatory action of PACAP on different aspects of food intake. Additionally, the underlying mechanism of the effect of PACAP in suppressing food intake, particularly its actions on affecting the expression and release of orexigenic and anorexigenic peptides in the hypothalamus, needs to be determined. PACAP appears to be a potential target to develop medications to alter energy and glucose metabolism and possibly binge eating. However, developing medications with selective actions on energy homeostasis without affecting the stress response and cardiovascular and other systems requires further research.

Acknowledgments

Author Contributions

Conceptualization, K.L.; methodology, A.S., K.S. and S.M.A.; investigation, A.S., K.S., S.M.A. and K.L.; writing—original draft preparation, A.S. and K.S.; writing—review and editing, K.L. and K.S.; supervision, K.L.; project administration, K.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

References