Cholecystokinin neurotransmission in the central nervous system: Insights into its role in health and disease

Abstract

Cholecystokinin (CCK) plays a key role in various brain functions, including both health and disease states. Despite the extensive research conducted on CCK, there remain several important questions regarding its specific role in the brain. As a result, the existing body of literature on the subject is complex and sometimes conflicting. The primary objective of this review article is to provide a comprehensive overview of recent advancements in understanding the central nervous system role of CCK, with a specific emphasis on elucidating CCK's mechanisms for neuroplasticity, exploring its interactions with other neurotransmitters, and discussing its significant involvement in neurological disorders. Studies demonstrate that CCK mediates both inhibitory long‐term potentiation (iLTP) and excitatory long‐term potentiation (eLTP) in the brain. Activation of the GPR173 receptor could facilitate iLTP, while the Cholecystokinin B receptor (CCKBR) facilitates eLTP. CCK receptors' expression on different neurons regulates activity, neurotransmitter release, and plasticity, emphasizing CCK's role in modulating brain function. Furthermore, CCK plays a pivotal role in modulating emotional states, Alzheimer's disease, addiction, schizophrenia, and epileptic conditions. Targeting CCK cell types and circuits holds promise as a therapeutic strategy for alleviating these brain disorders.

The cholecystokinin and its receptors exhibit extensive expression throughout the brain, where they engage in interactions with various neurotransmitters. They play a crucial role in regulating behavioral phenotypes by modulating neuronal activities and plasticity in region‐specific or circuit‐specific manners.

1. INTRODUCTION

Neuropeptides are present in nearly all neurons within the brain. These bioactive molecules exert their effects by binding to G‐protein‐coupled receptors (GPCRs), ¹ , ² thereby modulating ion channels and modifying synaptic transmission mediated by classical neurotransmitters such as glutamate and Gamma‐aminobutyric acid (GABA) if certain signaling pathways are activated. Recent technological advancements have shed light on the pivotal role of neuropeptides in regulating the central nervous system (CNS). Among various neuropeptides, cholecystokinin (CCK) holds significant importance for several reasons. First, CCK is the most abundant neuropeptide in the brain compared to any other known neuropeptide. ³ Second, CCK exhibits a distinct distribution pattern within the brain, with the highest expression observed in the cerebral cortex and amygdala. ⁴ , ⁵ , ⁶ Third, increasing evidence implicates CCK's involvement in several neurological disorders, including depression, anxiety, post‐traumatic stress disorder (PTSD), Alzheimer's disease (AD), epilepsy, and addiction. ⁵ , ⁶ , ⁷ , ⁸ , ⁹

Despite the significant involvement of CCK in brain disorders, numerous pivotal questions regarding its role in the brain remain unanswered, leading to a frequently perplexing body of literature. One major contributing factor to this complexity is the extensive molecular heterogeneity of CCK within the brain. ³ , ¹⁰ Over the past decade, our research group has clarified the indispensable role of CCK within the CNS in both health and disease states. ⁶ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ In this literature review, we aim to enhance our understanding of the key role of CCK in brain disorders. We present a historical overview of CCK, explore the mechanisms of neuroplasticity associated with CCK, examine its interactions with other neurotransmitters, and ultimately discuss its critical involvement in various brain disorders. A comprehensive analysis of preclinical and clinical findings was provided, placing primary emphasis on preclinical rodent models.

2. BRIEF HISTORY OF CCK

In 1856, a French physiologist discovered that hydrochloric acid stimulates bile flow in the duodenum. ¹⁸ In 1903, it was found that acid and soap in the duodenum release a substance that aids in bile secretion and flow. ¹⁹ In 1928, a hormone called CCK was identified as being responsible for gallbladder emptying in dogs. ²⁰ It was later discovered that the vertebrate brain contains a small peptide called CCK, which reacts with gastrin antibodies. ²¹ Different forms of CCK have been identified, with CCK‐58 and CCK‐8 being the most prevalent. ³ , ²² , ²³ In the gastrointestinal system, CCK is synthesized and released from I‐cells located in the small intestine. CCK is primarily released in response to the presence of fatty acids and plays a crucial role in the digestion of food. It exerts its effects by inhibiting gastric emptying, thereby slowing down the movement of food from the stomach to the small intestine. Additionally, CCK stimulates the secretion of pancreatic digestive enzymes, which aids in the breakdown of nutrients in the small intestine. ²⁴ , ²⁵ A recent study demonstrated that duodenal cells that express CCK play a role in regulating the preference for sugar over artificial sweeteners in mice. ²⁶ In the brain, CCK is widely expressed in the cortex, hippocampus, amygdala, hypothalamus, and ventral tegmental area (VTA). ²⁷ Considering that CCK cells in the small intestine aid in digestion and regulate sugar intake ²⁶ and CCK cells in the brain region nucleus of the solitary tract (NTS) control appetitive behaviors, ²⁸ , ²⁹ it is possible that there exists a signaling mechanism between the CNS and the peripheral nervous system to regulate food intake. However, direct evidence is currently lacking, and further investigations are needed to explore this possibility. CCK is found to be colocalized with various neurotransmitters and neuropeptides, such as dopamine, GABA, glutamate, serotonin, and endogenous opioid peptides. ³⁰ This co‐localization suggests that CCK may interact with these molecules to modulate neuronal signaling and regulate physiological processes in the CNS. There are two types of CCK receptors in the CNS: CCKAR and CCKBR. ³¹ CCKAR is primarily expressed in the hippocampus and certain midbrain regions, while CCKBR is expressed in the neocortex, amygdala, and hippocampus to a lesser extent. ³² , ³³ Recently, our research group discovered a new, third type of CCK receptor called GPR173 (CCKCR), which has implications for the plasticity of GABAergic synapses. ¹⁵ CCK receptors can interact with various G‐protein subtypes, similar to other GPCRs. For example, CCKBR interacts with both Gq and Gi proteins, while CCKAR predominantly interacts with Gq but also with Gi and Gs proteins to modulate downstream signaling pathways. ³⁴ , ³⁵

3. CCK‐MEDIATED NEUROPLASTICITY MECHANISM

Accumulating evidence revealed that memories are stored in neural networks through persistent modifications of synaptic strength, for instance via long‐term potentiation (LTP). ³⁶ It is important to note that abnormal neuroplasticity changes have been implicated in the development of various brain disorders such as depression, PTSD, AD, and addiction. ⁶ , ¹⁷ , ³⁷ , ³⁸ Given that the neuropeptides play a pivotal role in modulating neuroplasticity, and CCK, being the most abundant neuropeptide in the brain, ⁴ implies its potential significance in mediating neuroplasticity.

3.1. Inhibitory long‐term potentiation

CCK was initially identified as a biomarker for subtypes of inhibitory neurons. ³⁹ , ⁴⁰ , ⁴¹ It has been observed that CCK increases GABA release onto dorsal medial hypothalamus neurons (DMH). ⁴² The GABA neurotransmitter plays a crucial role in regulating neural excitability and plasticity, as well as various associated behaviors such as pain perception, fear responses, social interaction, and memory formation. ⁴³ , ⁴⁴ Its involvement in maintaining balance within these processes is widely acknowledged within the scientific community. Whole‐cell patch clamp recordings of basolateral amygdala (BLA) neurons have shown that CCK increases the frequency of spontaneous inhibitory postsynaptic potentials (sIPSPs) and this effect has lasted for 14 min. ⁴⁵ , ⁴⁶ This effect is blocked by the sodium channel blocker tetrodotoxin, indicating that CCK's effect is mediated by the direct enhancement of GABAergic interneurons' activities. These neural activity‐induced changes can initiate synaptic plasticity alterations through transcriptional modifications, as well as via neurotransmitter release and other signaling mechanisms. ⁴⁷

CCK release from GABAergic CCK+ neurons may trigger heterosynaptic LTP of other GABAergic interneurons, leading to an overall increase in sIPSPs. In the DMH, high‐frequency stimulation (HFS) of CCK+ neurons shifts the plasticity of GABAergic synapses from LTD to LTP, and this effect is mediated by CCKBR. ⁴⁸ CCK has the ability to induce depolarization of parvalbumin‐expressing (PV+) neurons in the hippocampus, resulting in a reduction in excitability of glutamatergic neurons and an increase in inhibitory activity. ⁴⁹ , ⁵⁰ In contrast, hippocampal CA1 region CCK+ GABAergic neurons exert significant inverse regulatory control over PV+ and pyramidal neurons. ⁵¹ , ⁵² The activity of CCK+ GABAergic neurons is closely associated with spontaneous behavior in the hippocampus, indicating that these neurons play diverse roles in various brain regions. In the prefrontal cortex, CCK+ GABAergic neurons establish a broad array of connections, with the strongest connections observed between these neurons and pyramidal neurons. ⁵³ Activation of these CCK+ GABAergic neurons predominantly regulates working memory, as opposed to other subtypes of interneurons.

Research has demonstrated that somatodendritic release of CCK from the VTA dopamine neurons triggers LTP of GABAergic synapses. ⁹ , ⁵⁴ The release of CCK is mediated by trains of optogenetic stimuli or prolonged depolarization and is dependent on T‐type calcium channels. The depolarization‐induced LTP of GABAergic synapses is also blocked by a CCKBR antagonist. CCK acting through CCKBR, also enhances the inhibition of glutamatergic neurons in the olfactory bulb. ⁵⁵ Activation of cannabinoid receptor 1 (CB1R) in the anterior piriform cortex and hippocampus led to a decrease in inhibitory synaptic transmission. ⁵⁶ , ⁵⁷ The presynaptic CB1R, known to be highly expressed on CCK+ GABAergic neurons, ⁵⁷ , ⁵⁸ , ⁵⁹ has the capability to suppress the co‐release of CCK and GABA, thereby modulating inhibitory long‐term potentiation (iLTP). Pre‐synaptically targeted interneurons (e.g., CCK), can be influenced by firing activity in the post‐synaptic neurons while dendritically targeted interneurons (e.g., somatostatin [SST]) are affected by the activity of glutamatergic neurons. ⁶⁰ Hence, the release of endocannabinoids (eCBs) from the post‐synaptic neuron can bind to presynaptic CB1R and modulate inhibitory plasticity. ⁶¹ Recent research discovered that CCK induces iLTP of GABAergic synapses specifically in the auditory cortex. ¹⁵ Importantly, this effect is mediated by a newly identified CCK receptor GPR173, rather than the CCKBR. There may be additional unidentified CCK receptors that have diverse roles in different brain functions including regulating synaptic plasticity. Since GPR173 enhances the inhibition of GABAergic synapses it has the potential to inhibit seizures in the animal model of epilepsy. ⁶² We hypothesize during HFS of CCK+ GABAergic neurons, there is a concomitant release of both GABA and CCK. Subsequently, CCK binds to the novel CCK receptor GPR173 on the postsynaptic membrane, resulting in an increase in the function of GABAAR and the induction of iLTP (Figure 1). However, it remains to be investigated whether the binding of CCK to GPR173 or the activation of GPR173 leading to increased synaptic recruitment of GABAAR is necessary for synaptic plasticity. Although CCK plays a crucial role in inhibitory plasticity, the contribution of other neurotransmitters such as GABA and neuropeptides like SST/Brain‐derived neurotrophic factor (BDNF) cannot be disregarded in this mechanism. Further investigation is warranted to comprehensively assess the involvement of GPR173 and CCKBR in regulating iLTP in other brain regions, including the VTA. Additionally, it is essential to elucidate the molecular mechanisms underlying the release of CCK from presynaptic terminals and how this release modulates the activities of other inhibitory neurotransmitters, which in turn regulate plasticity and associated behaviors.

FIGURE 1.

CCK‐dependent iLTP. HFS but not LFS of CCK‐GABA neurons increases the release of CCK, which binds to postsynaptic GPR173. This binding leads to an overall increase in GABAergic inhibition and triggers the formation of iLTP. Additionally, CCK binding to GPR173 may enhance postsynaptic GABAar activity. It is also plausible that CB1R, NR2B, and mGluR expressed on the presynaptic terminals of GABAergic neurons may regulate the release of CCK in different pathways. CCK, cholecystokinin; HFS, high‐frequency stimulation; iLTP, inhibitory long‐term potentiation; LFS, low‐frequency stimulation.

3.2. Excitatory long‐term potentiation

While CCK was initially regarded as a biomarker for inhibitory neurons, subsequent studies have reported the presence of CCK in glutamatergic neurons. ⁸ , ¹² , ⁶³ CCK enhances excitatory long‐term potentiation (eLTP) at glutamatergic CCK synapses. For instance, the infusion of CCK in the auditory cortex of anesthetized rats enhances neuroplasticity and auditory response. ¹¹ , ¹² This effect is abolished by a CCKBR antagonist or CCK gene knockout, indicating the involvement of CCK in eLTP. Furthermore, NMDA receptor (NMDAR) activation facilitates CCK release. It has been suggested that CCK release is triggered specifically by HFS. This is consistent with the theory proposing that neuropeptide release requires HFS of neurons. ⁶⁴ CCK enhances the firing frequency of vesicular glutamate transporter 2 (vGluT2) cells and increases excitatory postsynaptic current in the rostral hypothalamic arcuate nucleus. ⁶⁵ Given that the CCK is crucial for memory formation, this effect may be attributed to CCK‐mediated eLTP formation. Indeed, HFS of the entorhinal cortex (EC) CCK+ terminals in the auditory cortex facilitate heterosynaptic eLTP. ¹³ , ⁶⁶ , ⁶⁷ If pairing visual stimulus with an electrical stimulation of the auditory cortex when these CCK+ terminals are high‐frequently activated, this leads to the formation of visual–auditory association in the auditory cortex. CCK also mediates eLTP in the BLA, ⁶ CA3‐CA1, ¹⁶ , ⁵² and motor cortex (Mo). ¹⁴ While the importance of CCK in eLTP formation is established, the precise mechanism remains unknown.

Notably, a previous study demonstrated that CCK+ glutamatergic neurons express CB1R at presynaptic terminals in the nucleus accumbens (NAc), and the absence of CB1R triggers depressive phenotypes. ⁶³ Hence, it would be intriguing to explore whether the CB1R mediates CCK release in the NAc and how the NMDAR or other receptors regulate CCK release and, consequently, eLTP. Additionally, identifying the downstream signaling molecules involved in eLTP formation is also important. Based on the available literature, we propose that presynaptic CCK release binds to postsynaptic CCKBR, triggering the activation of the Ras–Raf–MEK–MAPK pathway. This activation leads to the trafficking of AMPA receptors and ultimately induces eLTP (Figure 2). Considering the significance of metabotropic glutamate receptors (mGluRs) presynaptic inhibition, ⁶⁸ we speculate that they may also modulate CCK release either directly or through modulation of NMDAR. Additionally, mGluRs may inhibit CCK release through postsynaptic eCB release, ⁶¹ which in turn binds to presynaptic CB1R.

FIGURE 2.

CCK‐dependent eLTP. HFS of CCK+ glutamatergic neurons triggers the release of glutamate. The presence of glutamate is detected by presynaptic NMDA receptors, which then release CCK. CCK binds to CCKBR, potentially increasing AMPA activity directly or indirectly by activation of Ras–Raf–MEK–MAPK signaling pathways to facilitate eLTP. The release of CCK is inhibited by presynaptic CB1R, which is activated by the postsynaptic release of eCB mediated by mGluR5. Additionally, presynaptic mGluR could directly inhibit CCK release. CCK, cholecystokinin; eLTP, excitatory long‐term potentiation; HFS, high‐frequency stimulation.

Therefore, the novel receptor GPR173 can enhance iLTP, whereas CCKBR potentially regulates eLTP respectively. Collectively, these studies highlight the significant role of CCK in the formation of both eLTP and iLTP, emphasizing the importance of CCK in learning, memory formation, and storage. Future research should focus on examining the molecular mechanisms underlying the release of CCK from glutamatergic neurons in various circuits and brain regions, as well as the associated behaviors. Advanced techniques such as CRISPR‐Cas9 gene knockout strategies and GPCR‐based CCK sensors could be employed to investigate these phenomena.

4. INTERACTIONS OF CCK WITH OTHER NEUROTRANSMITTERS

In light of the established CCK's role in neuroplasticity, and considering the extensive distribution of CCK and its receptors throughout different brain regions where neurons expressing diverse neurotransmitters, such as dopamine and serotonin, it is reasonable to propose that CCK may exert regulatory effects on the release of these neurotransmitters and influence the plasticity of specific neural pathways (Figure 3).

FIGURE 3.

Role of CCK Neurocircuitry and receptors in the brain. The CCK projection is known to influence behavior phenotypes in brain regions and circuits specifically, for example, EC to BLA, HIP, AC, and Mo facilitate the fear, spatial, auditory, and motor learning, respectively. CCK projections from NTS to PVH promote positive valence, BNST to LH trigger reinforcement, PBN to BNST inhibit salt intake, median preoptic nucleus (MnPO) inhibits water intake, and BLA to NAc mediate depression. However, further investigation is needed to understand the specific connections represented by the dashed arrows from BLA to LH, VTA, LC, and EW to DR. CCK receptors, including CCKBR, CCKAR, and GPR173, are distributed across several brain regions. CCKBR is found in HIP, BLA, LH, BNST, VTA, Mo, and AC and facilitates the LTP through CCK binding. CCKAR is located in HIP, PVH, NAc, BNST, and LH. GPR173 is primarily situated in AC and BLA, mediating iLTP. These receptors have the potential to modulate the effects of CCK when present on other neurons containing other neurotransmitters within these brain regions. AC, Auditory cortex; BLA, basolateral amygdala; BNST, bed nucleus of the stria terminalis; CCK, cholecystokinin; DR, dorsal raphe; EC, entorhinal cortex; EW, Edinger Westphal; HIP, Hippocampus; LC, Locus coeruleus; LH, Lateral hypothalamus; LTP, long‐term potentiation; Mo, motor cortex; NAc, nucleus accumbens; NTS, nucleus of the solitary tract; PBN, Parabrachial nucleus; PVH, Paraventricular nucleus; VTA, ventral tegmental area.

4.1. CCK and GABA

CCK immunoreactive cells overlap with glutamic acid decarboxylase (GAD)‐positive cells, indicating their inhibitory nature, and CCK probably interacts with GABA to modulate diverse behaviors, such as memory formation, ⁶⁹ , ⁷⁰ alcohol intake, ⁷¹ appetite regulation, ⁴⁸ , ⁷² and stress‐induced emotional states. ⁷³ , ⁷⁴ The modulation of social isolation stress‐induced social avoidance behavior in mice is regulated by CB1R‐expressing CCK GABAergic neurons within the anterior cingulate cortex. ⁵⁹ While the activation of CB1R is known to inhibit the GABA release. ⁷⁵ Presynaptic GABA B receptors robustly suppress GABA release from CCK+ GABAergic neurons in the hippocampus, exhibiting a stronger effect compared to PV neurons. ⁷⁶ These findings suggest that GABA B receptors may also exert inhibitory control over CCK release from CCK+ GABAergic neurons. GABA receptors are the primary targets of benzodiazepine drugs used for treating anxiety. A previous study indicated that chronic diazepam treatment, a benzodiazepine, reduces the excitatory effect of CCK. ⁷⁷ This suggests that benzodiazepine drugs may influence stress‐induced anxiety‐like behavior through the CCK signaling pathway. However, there is currently no direct evidence to conclude whether benzodiazepines directly target CCK receptors or indirectly. Further investigation is needed to determine if benzodiazepines bind to CCK receptors, particularly the GPR173 receptor, which mediates iLTP at GABAergic CCK+ synapses. Furthermore, it would be intriguing to explore whether GABA alone can modulate the aforementioned behaviors following the knockdown of CCK, and vice versa.

4.2. CCK and glutamate

In addition to GABAergic neurons, CCK is expressed in glutamatergic neurons, suggesting their involvement in modulating glutamatergic synaptic plasticity and behavior. Indeed, the glutamate NMDAR facilitates CCK release from CCK+ glutamatergic terminals in the auditory cortex, mediating LTP and associative memory. ¹² This effect was abolished in CCK gene knockout mice. CCK‐4 infusion induces panic attacks, ⁷⁸ possibly due to the long‐term activation of CCK+ glutamatergic neurons and increased glutamate release. However, conflicting reports exist regarding glutamate levels in the anterior cingulate cortex following CCK‐4 injection. ⁷⁹ This discrepancy may arise from different experimental conditions, protocols, and improper statistical evaluation. CCK+ glutamatergic neurons influence various behaviors in different brain regions. For example, they control appetitive behavior in the NTS, ²⁸ , ²⁹ facilitate trace fear memory formation ⁸ affect depressive‐like phenotypes in the BLA, ⁶ regulate depressive‐like behavior in the NAc, ⁶³ and mediate spatial, motor, auditory, and visual associative learning in the hippocampus (HIP), Mo, and auditory cortex (AC), ¹¹ , ¹³ , ¹⁴ , ¹⁶ respectively. Targeting CCK+ glutamatergic neurons holds promise to enhance memory formation and alleviate disease states, but further research is needed to understand the advantages and disadvantages of manipulating them. For example, inhibiting CCK release or blocking CCK receptors in the BLA alleviates depression, ⁶ while decreased CCK levels or blocking CCK receptors in the AC impairs learning. ¹² Therefore, advancements in technology are necessary to selectively manipulate CCK genes in certain circuits to control specific behaviors or treat specific brain disorders. Analysis of human postmortem RNA sequences from prefrontal cortex regions revealed functional and anatomical heterogeneity among neuropeptides, with the highest expression level observed of neuropeptide CCK, which is expressed in both GABAergic and glutamatergic neurons. ⁸⁰ Mouse neocortical transcriptomic studies indicated that CCK is expressed in nearly all glutamatergic neurons of the cortex, suggesting a potential critical role for CCK in modulating cortical glutamatergic neuronal plasticity and homeostasis. ⁸¹ Future investigations are necessary to explore whether the release of CCK alone can modulate cortical neuronal activities, plasticity, and associated behaviors, particularly in scenarios involving region‐specific glutamate knockdown, circuit‐specific knockdown, or this effect is mediated by corelease of glutamate.

4.3. CCK and serotonin

The dorsal raphe (DR) region contains serotonergic neurons, while neighboring nuclei Edinger Westphal (EW) house CCK+ neurons, ⁸² suggesting a potential interaction between CCK and serotonin. In fact, CCK activates serotonergic neurons in the DR via the CCKAR. ⁸³ CCK+ neurons are also influenced by neurotransmitters linked to mood disorders, including serotonin and the cholinergic system. For example, blocking 5‐HT1a receptors reversed aversion‐related behavior induced by CCK‐8, ⁸⁴ and inhibiting 5‐HT3 receptors increased CCK release in the frontal cortex. ⁸⁵ On the other hand, CCK‐4, a CCKBR agonist, elevated serotonin levels and induced anxiety‐like behavior, which were prevented by a CCKBR antagonist. ⁸⁶ CCK‐4 can induce panic attacks, but chronic treatment with the serotonin reuptake inhibitor imipramine reduces CCK‐4‐induced panic attacks. ⁸⁷ Moreover, depleting serotonin in mice decreases CCK levels. ⁸⁸ 5‐HT1B receptors on presynaptic terminals of CCK+ GABAergic neurons inhibit GABA release, affecting granule cell activity in the HIP associated with antidepressant responses. ⁸⁹ While early studies demonstrated a close interaction between CCK and serotonin, recent research is limited. Based on previous findings and observations that CCKBR antagonists impair LTP in the BLA and mediate antidepressant effects, it is proposed that selective serotonin reuptake inhibitors (SSRIs) may exert their antidepressant effects by also blocking CCKBR. Investigating the correlation between SSRIs and CCK signaling in animal models of mood‐related disorders, particularly depression, would be important.

4.4. CCK and opioids

While the role of CCK in modulating GABAergic and glutamatergic synapses for anxiety and panic behaviors has been extensively studied, there is emerging evidence suggesting that CCK may also interact with opioids and mediate anti‐opioid functions via the mesolimbic pathways. ⁹⁰ CCKBR knockout mice exhibit increased endogenous opioid levels, ⁹¹ and CCKBR agonists block the analgesic effect of morphine while antagonists enhance it. ⁹² This implies that CCK agonists may activate nociceptive pathways by blocking the peripheral and spinal opioid system, and reducing opioid tone could potentially modulate panic behaviors and analgesic responses. Further investigation could involve increasing opioid tone using an opioid agonist to prevent CCK‐4‐induced panic attacks or exploring whether a standard opioid antagonist induces panic symptoms in patients with panic disorder. Previous studies indicate that CCK mediates both nociceptive and antinociceptive effects, influenced by factors such as dosage, receptor type, and administration route. ⁹³ , ⁹⁴ For instance, oral administration of a CCKAR antagonist enhances morphine's analgesic effect and reverses opiate tolerance in mice. ⁹⁵ Additionally, upregulation of the CCKBR facilitates somatic hyperalgesia through the MAPK pathway, ⁹⁶ and a CCKBR antagonist in the NAc blocks morphine‐induced conditioned place preference. ⁹⁷ On the other hand, activation of CCKAR restores morphine‐induced impairment of hippocampal LTP, ⁹⁸ and CCK attenuates conditioned place aversion and anxiety‐like behavior triggered by naloxone. ⁹⁹ , ¹⁰⁰ Opioid receptors are present on CCK neurons, ¹⁰¹ suggesting a reciprocal control between opioids and CCK release in the central and peripheral nervous systems. Indeed, chronic opioid administration increases CCK release and the CCKBR level in the CNS, ¹⁰² , ¹⁰³ while opioid agonists decrease CCK release in the peripheral nervous system. ¹⁰⁴ However, the detailed mechanisms underlying the interaction between CCK and opioids in pain modulation within peripheral and CNSs remain unclear and require further investigation.

4.5. CCK and dopamine

The dopamine system is known for its involvement in reward processing, appetitive behaviors, self‐stimulation, and schizophrenia. ¹⁰⁵ Recent research has revealed interactions between CCK and dopamine. For instance, CCK mediates the blocking effect of geranylgeranyacetone on morphine‐induced conditioned place preference via targeting dopaminergic neurons. ¹⁰⁶ CCK coexists with dopamine in the mesolimbic pathway and acts as a modulator of dopaminergic function in rodents. ³⁰ Consistent with this notion, studies have found that CCKAR activation potentiates dopamine‐stimulated adenylate cyclase in the medial posterior NAc and dopamine‐induced hyperlocomotion, while CCKBR activation inhibits them via targeting the rostral NAc. ¹⁰⁷ Somatodendritic release of CCK from VTA dopaminergic neurons triggers LTP of GABAergic synapses and reduces food intake and dopamine‐induced calcium activity. ⁹ Although the interaction between CCK and dopamine in the mesolimbic pathway modulates dopamine function related to drug abuse and schizophrenia, it may not directly contribute to the mechanism of CCK‐4‐induced panic attacks. The mesocortical pathway, involved in stress regulation, may be implicated in CCK‐4‐induced panic attacks. CCK‐mediated potentiation of VTA‐GABAergic signaling likely activates GABAergic input to the prefrontal cortex, potentially mediating panic behavior. ¹⁰⁸ Animal models could be used to test the hypothesis that CCK‐4 induces panic attacks through the activation of mesocortical pathways. In summary, the interaction between CCK and dopamine plays a crucial role in various behaviors, including schizophrenia, reward processing, and stress responses.

4.6. CCK and norepinephrine (noradrenaline)

The locus coeruleus is known for its high norepinephrine expression, ¹⁰⁹ and lesioning it increased CCK receptor density in the frontal cortex and hippocampus of rats. ¹¹⁰ It is established that CCK‐4 induces panic attacks, and norepinephrine regulates aversive behaviors. ¹¹¹ , ¹¹² , ¹¹³ , ¹¹⁴ Therefore, it is highly plausible that CCK‐4 may induce panic attacks through increased norepinephrine signaling. Indeed, CCK‐4 treatment increases norepinephrine and epinephrine levels in healthy volunteers. ¹¹⁵ In contrast, Yohimbine, an alpha‐2 adrenergic receptor antagonist, induces panic attacks and increases plasma norepinephrine in Parkinson's disease patients. ¹¹⁶ Probably due to reduced norepinephrine inputs to the frontal cortex and hippocampus increasing CCK sensitivity. Bennett et al. demonstrated that norepinephrine affected somatostatin+ and CCK+ interneurons without influencing fast‐spiking and late‐spiking neurons. ¹¹⁷ CCK modulates norepinephrine release in the hypothalamus, ¹¹⁸ suggesting CCK's role in flight, hunger, and satiety behaviors. Indeed, CCK was found to potentiate NTS cell activity via adrenergic and glutamatergic signaling. ¹¹⁹ Overall, CCK modulates norepinephrine levels in the CNS and influences fundamental survival behaviors.

While the interaction between CCK and other neurotransmitters has been discussed, the precise mechanism remains elusive due to the majority of research being conducted in the 20th century. Recent technological advancements, such as the development of GPCRs‐based sensors, ¹²⁰ and CRISPR‐mediated cell type‐specific gene knockout, offer opportunities to gain a better understanding of the role and interaction of CCK with other neurotransmitters, as well as the regulation of synaptic plasticity and neurotransmitter release. Hence, it would be fascinating to investigate the timing of CCK and other neurotransmitter release during specific behavioral tasks, as well as determine whether CCK or other neurotransmitters are released first.

5. ROLE OF CCK IN THE CNS DISORDERS

Considering the abnormal neuroplastic changes associated with CNS disorders ⁶ , ¹⁷ , ³⁷ , ³⁸ and the established role of CCK in neuroplasticity as discussed above. In this section, we explore the role of CCK in CNS disorders, including emotional states, AD, addiction, epilepsy, and schizophrenia.

5.1. Role of CCK in emotional states (PTSD, anxiety, and depression)

Numerous studies indicate the significant role of CCK in the regulation of emotions, such as fear, ⁸ panic attacks, ⁸⁷ PTSD, ³⁸ and depression. ⁶ A clinical study showed that the CCK gene polymorphism is associated with PTSD in combat veterans. ¹²¹ Rodent studies have found the involvement of CCK in anxiety and depression, but they lacked in‐depth exploration of the underlying mechanisms. ¹²² , ¹²³ , ¹²⁴ The amygdala, a crucial region involved in emotional regulation, ¹²⁵ , ¹²⁶ exhibits a high expression of CCKBR. Our recent research demonstrates that chronic stress activates CCKBR in the BLA, and blocking CCKBR impairs LTP formation and exhibits antidepressant‐like effects. ⁶ This suggests that the impairment of LTP in the BLA is a crucial underlying mechanism behind the antidepressant effects. CCK‐expressing glutamatergic neurons of BLA project to the NAc and their activation has been observed to exhibit phenotypes associated with depression. ⁶³ In mice, the administration of ketamine, a rapid‐acting antidepressant, through infusion into the NAc has been shown to alleviate behaviors resembling PTSD. ³⁸ , ¹²⁷ This therapeutic effect of ketamine may be attributed to the modulation of CCK signaling in the NAc. Previous studies mainly focused on the role of classical neurotransmitters or neuromodulators like glutamate, GABA, serotonin, and dopamine in stress‐induced emotional states. ⁵ , ¹²⁶ , ¹²⁸ , ¹²⁹ Since CCK receptors are expressed in the VTA, DR, and NAc, ¹³⁰ the above‐established interaction between CCK, dopamine, and serotonin in these areas suggests that CCK may modulate emotional behaviors by influencing the plasticity or release of these neurotransmitters. Indeed, a study showed that CCK+ neurons modulate dopamine release and plasticity in the VTA. ⁹ The central amygdala (CeA) neurons project to the VTA ¹³¹ and DR, ¹³² and the release of CCK in the amygdala is involved in triggering emotional states and amygdala hyperactivity. ⁶ Thus, another possible mechanism could be the CeA‐GABAergic projection to the DR and VTA, inhibiting the release of serotonin and dopamine, respectively, and leading to the development of emotional states. However, future studies are required to test this hypothesize. Previous studies revealed mGluR5 implication in emotional states. ¹³³ , ¹³⁴ , ¹³⁵ While mGluR5 agonist inhibit CCK release. ¹³⁶ Hence, we hypothesize that the mGluR5 through CCK release modulate the emotional states. It is previously known that mGluR5 inhibits presynaptic release via eCB‐CB1R signaling, ¹³⁷ , ¹³⁸ and CB1R is present on presynaptic glutamatergic ¹³⁹ and CCK+ glutamatergic terminals. ⁶³ , ⁷⁴ Therefore, reduced mGluR5 levels following stress may downregulate eCB signaling and CB1R‐mediated presynaptic inhibition of CCK release, leading to emotional state change. These studies underscore the crucial role of CCK in emotional regulation and suggest interactions with various neurotransmitters and neuromodulators in depressive disorders. However, further research is needed to gain a comprehensive understanding of mechanisms in additional brain regions and neural circuits.

5.2. Role of CCK in AD

AD is a progressive neurodegenerative disorder characterized by cognitive decline. In AD, alterations in amyloid β‐peptide and tau protein have been observed in the EC and hippocampus, ¹⁴⁰ , ¹⁴¹ key brain regions for learning and memory. The EC, enriched with CCK‐positive neurons, is particularly affected in AD. ⁸ , ⁶⁶ , ¹⁴² A recent study has demonstrated that individuals with AD exhibit dysregulation of CCK levels along with abnormal tau peptide expression. ¹⁴³ Given that EC CCK is crucial for memory formation including spatial, fear, motor, and visual–auditory associative memory, ⁸ , ¹² , ¹³ , ¹⁴ , ¹⁶ , ⁶⁶ the decreased CCK mRNA levels in the EC with aging ¹⁴⁴ suggest its potential as an AD biomarker. A human study demonstrated that higher levels of CCK in cerebrospinal fluid are associated with better memory performance and reduced risk of AD impairment. ¹⁴⁵ In the 5XFAD mouse model of AD, there were observed deficits in LTP formation within the hippocampus and prefrontal cortex regions. ¹⁴⁶ , ¹⁴⁷ The established role of CCK in LTP formation, ⁶ , ¹² , ¹³ , ⁶⁶ a cellular mechanism associated with memory formation, further supports its role in AD. CCK+ neurons in the EC project to the hippocampus, where they mediate LTP in the CA3‐CA1 pathways. ¹⁶ Disruption of this pathway not only impairs LTP but also hampers spatial memory formation. In our recent study on 3xTg AD mice, we observed that reduced CCK levels and CCKBR expression in AD mouse models led to impaired LTP and cognition, while CCKBR agonist treatment improved memory formation, ¹⁷ extending the importance of CCK in AD treatment.

Collectively, these findings highlight CCK's role in memory formation and suggest its involvement in AD pathogenesis. Activation of EC CCK+ pathways may hold therapeutic potential for AD and decreased CCK levels could serve as a biomarker. Further research is needed to understand CCK's mechanisms in memory formation, its impact on Aβ and tau, and the development of more effective CCK agonists with improved stability, bioactivity, and half‐life holds promise for AD treatment.

5.3. Role of CCK in addiction

Addiction is closely associated with alterations in the brain's reward network. Addictive drugs enhance activity in dopaminergic neurons of the VTA, ¹⁴⁸ , ¹⁴⁹ leading to dopamine release in the NAc. The discovery of CCK co‐localization with VTA dopaminergic neurons has sparked interest in its role in addiction, ¹⁵⁰ suggesting its potential role in modulating addiction. Indeed, addictive drugs affect CCK mRNA levels in the brain's reward system. ¹⁵¹ Further studies have shown an inverse relationship between CCK levels in the VTA, dopaminergic neuron activities, and drug self‐administration. For instance, blocking or knockout CCK receptors increases baseline dopaminergic neuron activities and potentiates dopamine release in response to addictive drugs. ¹⁵² , ¹⁵³ A recent study also revealed that the CCK released by VTA dopaminergic neurons triggers iLTP, and inhibiting CCK receptors impairs iLTP. ⁹ CCK+ neurons in the bed nucleus of the stria terminalis (BNST) are activated by rewarding cues and promote reinforcing behavior. ¹⁵⁴ Cocaine‐induced neuroplasticity in the VTA involves NMDAR, ³⁷ and CCK release is mediated by NMDAR in the cortex, ¹² further suggesting CCK synergy in addictive behavior. Since CCK is expressed in the VTA ¹⁵⁰ and plays a role in LTP, ⁹ it is reasonable to conclude that CCK plays a vital role in the regulation of addictive behavior. However, the underlying molecular mechanisms remain poorly understood. Future studies are needed to explore in detail how CCK modulates dopamine levels or signaling pathways in other brain regions and its impact on addictive behavior. Additionally, it would be valuable to investigate whether CCK release in the VTA and other brain regions is also controlled by NMDAR. Based on these findings, it is plausible to consider CCK targeting as a promising avenue for addiction treatment.

5.4. Role of CCK in epilepsy

Epilepsy is a chronic disorder of the CNS that affects approximately 39 million people worldwide, with an estimated prevalence of seizures in around 1% of the population in Hong Kong. ¹⁵⁵ , ¹⁵⁶ It can be categorized into generalized epilepsy, involving widespread brain involvement and generalized seizures, and localization‐related epilepsy, characterized by abnormalities restricted to specific brain regions and partial or secondarily generalized seizures. ¹⁵⁷ Temporal lobe epilepsy (TLE), originating from the temporal lobe, is the most common form of localization‐related epilepsy.

CCK, a neuropeptide associated with the inhibitory system, has been studied in the context of epilepsy. Research has demonstrated alterations in CCK expression and release in epilepsy. For instance, studies conducted on patients with TLE have observed heightened CCK mRNA expression in the cortical samples, while the actual CCK content has been found to be reduced in the temporal cortex. ¹⁵⁸ These alterations may be attributed to the persistent release of CCK in the brains of TLE patients due to the abnormal firing of CCK neurons within the temporal cortex. Moreover, rat models of epilepsy induced by kainic acid or kindling have also shown increased CCK mRNA levels in the brain. ¹⁵⁹ , ¹⁶⁰ , ¹⁶¹ In contrast, the anticonvulsive effects of CCK8, a specific form of CCK, have been investigated in various seizure models, demonstrating delayed onset and improved seizure control. ¹⁶² , ¹⁶³ This effect may be attributed to the mediation of CCK‐8 binding to GPR173, which enhances the inhibition of GABAergic neurons, potentially leading to an overall inhibition of cortical region. In the hippocampus, the depolarization of PV+ cells by CCK occurs via the activation of CCKBR receptors located on PV+ cells. This process is mediated through a pertussis‐toxin‐sensitive pathway, ultimately resulting in a decrease in the excitability of excitatory neurons, ⁴⁹ , ⁵⁰ suggesting another potential mechanism for the anticonvulsive effect of CCK. However, the large molecular weight of CCK8 and its limited penetration through the blood–brain barrier raises questions about its efficacy. Under normal physiological conditions, neurons in the deep layers of the EC exhibit lower burst propensity compared to superficial layers. ¹⁶⁴ However, in epilepsy, synchronized and high‐frequency neuronal firing occurs. Recent findings suggest that CCK+ neurons in the EC, with extensive cortical projections, release CCK during high‐frequency stimulation, playing a crucial role in cortical neuroplasticity. ⁸ , ¹² , ¹³ , ¹⁴ , ¹⁶ , ⁶⁶ In light of these findings, we speculated that seizures, characterized by high‐frequency firing and CCK release in the cortex, may enhance CCK concentration, strengthening excitatory connections and facilitating seizure propagation. In epilepsy patients with elevated CCK levels, the peptide could activate CCKBR and further strengthen excitatory connections in the cortex. Consequently, administering CCKBR antagonists to epilepsy models could potentially suppress this process and alleviate epilepsy. These studies highlight the significant involvement of CCK signaling in seizure induction, suggesting overactivation of CCK signaling as a potential mechanism in epilepsy. However, further research is needed to fully comprehend the detailed mechanisms underlying CCK‐mediated seizure propagation and develop novel drug targets for the treatment of epilepsy, particularly in patients with treatment‐resistant epilepsy.

5.5. Role of CCK in schizophrenia

Schizophrenia is a severe disease that leads to psychosis, characterized by a range of symptoms. Positive symptoms of schizophrenia include hallucinations, disorganized speech, and abnormal movements, while negative symptoms include social withdrawal, anhedonia (inability to experience pleasure), and a lack of emotions. ¹⁶⁵ Researchers have observed structural and functional changes in specific brain regions, such as the prefrontal cortex and mesostriatal brain regions, among individuals with schizophrenia. ¹⁶⁶ , ¹⁶⁷ The dopamine system has been extensively studied for its role in psychosis, which is a key symptom of schizophrenia. ¹⁰⁵ Notably, dopaminergic input to limbic regions is known to be involved in positive psychotic symptoms, while input to the frontal cortex regulates negative psychotic symptoms. ¹⁶⁸ Furthermore, CCK, a neuropeptide that coexists with dopamine, has been implicated in the development of schizophrenia. ⁹ CCK GABAergic neurons, which are a major population of inhibitory neurons in the prefrontal cortex, play a role in regulating working memory retrieval. ⁵³ The expression of CB1R in CCK neurons further supports the involvement of CCK in schizophrenia, as the overuse of cannabinoids has been shown to increase the risk of schizophrenia. ¹⁶⁹ Clinical studies have reported impaired CCK‐GABAergic signaling in the prefrontal cortex of schizophrenia patients. ¹⁷⁰ , ¹⁷¹ Additionally, a specific single‐nucleotide polymorphism of the CCKAR gene, characterized by a higher frequency of the C allele, has been found in individuals with schizophrenia. ¹⁷² CCKAR gene polymorphism has also been associated with auditory hallucinations and language lateralization. ¹⁷³ , ¹⁷⁴ Moreover, CCK gene polymorphism has been linked to an increased risk of smoking behavior. ¹⁷⁵ Interestingly, patients with Parkinson's disease who experience hallucinations have also shown an increased prevalence of CCK T and CCKAR C allele polymorphisms. ¹⁷⁶ Taken together, these findings suggest that targeting CCK signaling holds potential for the treatment of schizophrenia and psychosis.

6. CONCLUDED REMARKS AND PERSPECTIVE

Over the past decade, studies have explored the physiological roles of CCK and its association with neurological disorders, leading to further investigations. Understanding the molecular and cellular mechanisms underlying CCK's adaptive responses is crucial for comprehending nervous system development, plasticity, and function. CCK exhibits heterogeneity in its responses based on factors such as brain region, neural circuit, and neuron subtype.

CCK plays a critical role in both eLTP and iLTP formation, ⁹ , ¹² , ¹³ , ⁶⁶ which contributes to memory formation in neuronal networks. However, our understanding of CCK release mechanisms in different brain regions is limited, presenting an opportunity to investigate the molecular machinery involved in CCK‐mediated synaptic plasticity. Impaired CCK plasticity may contribute to various neurological and neuropsychiatric diseases, highlighting the need for a mechanistic understanding of CCK for developing targeted therapeutic interventions. For instance, CCK‐dopamine signaling in addiction and emotional states underscores the importance of such knowledge, although specific developmental stages and human midbrain diversity of CCK require further exploration.

Recent technological advancements, such as CCK GPCR‐dependent sensors, ¹²⁰ have enabled the detection of CCK release during behavioral tasks, overcoming challenges posed by its rapid degradation. These advancements create new opportunities to explore CCK's role in behavior modulation and develop potential targets for severe CNS disorders. Future studies utilizing these sensitive tools will provide novel insights into CCK's role in the CNS in both healthy and diseased states.

AUTHOR CONTRIBUTIONS

The manuscript was written and reviewed by Muhammad Asim and Xi Chen. Huajie Wang contributed to figure preparation and manuscript revision. Abdul Waris and Gao Qianqian authored the section on the role of CCK in schizophrenia and also participated in the manuscript revision. All authors reviewed the final draft and provided their approval.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

Asim M, Wang H, Waris A, Qianqian G, Chen X. Cholecystokinin neurotransmission in the central nervous system: Insights into its role in health and disease. BioFactors. 2024;50(6):1060–1075. 10.1002/biof.2081