Cholecyctokinin is a neuropeptide found in the gastrointestinal system and brain. Research has shown that it has various isolated fragments that may influence several important areas of human behavior, such as nociception, satiety and anxiety. Cholecystokinin receptors located in the central nervous system (CNS) are known as CCK-B receptors, and they have high affinity for the tetrapeptide fragment CCK-4. Anxiogenic effect of CKK-4 in humans suggested that it might be involved in pathogenesis of panic disorder, and opened new avenues of research into biological aspects of anxiety.
Further research showed increased sensitivity of panic disorder patients to CCK-4 in comparison with normal volunteers. Next, substances capable of blocking CCK-B receptors (CCK-B antagonists) were synthesized and their action was evaluated. One of such antagonists, L-365,260 proved to be effective in blocking CCK-4 induced panic attacks in panic disorder sufferers. However, a pilot study failed to show the effectiveness of the same antagonist in decreasing the frequency of spontaneous panic attacks in panic disorder patients during the course of six weeks.
Though CCK-B antagonists may prove to become great potential anxiolitic agents, more research has to be done in order to understand the mechanism of CCK-4 action as a neurotransmitter and its role in naturally occurring panic attacks Ethiology of panic disorder: a brief overview Panic disorder, (PD) is a recognized psychiatric condition and is identified in DSM-III-R as a condition separate from other anxiety disorders. Its main feature is occurrence of unprovoked panic attacks, which happen at random and cannot be explained by the patients.
These attacks of fear are closely associated with an overwhelming subjective feeling of anxiety in connection with unpleasant bodily sensations, such as increased heartbeat/palpitations, hot flushes/chills, abdominal distress, nausea, sweating, trembling/shaking, etc. Along with objectively groundless emotional symptoms, e. g. fear of losing control, sense of unreality and detachment, even fear of dying they affect PD sufferers, interfering with social and professional aspects oftheir lives.
Some PD patients associate panic attacks with certain objects or situations, and therefore phobias, especially agoraphobia , are closely associated with the PD. The ethiology of PD is not clear, and most theories support either a psychological or a neurobiological view. The most developed psychological explanation is cognitive theory of PD. According to Clark’s model, the panic attack develops as a result of misinterpretation of unpleasant bodily sensations,which leads to increasing feeling of anxiety and progresses to a fully developed panic.
This misinterpretation is defined as anxiety sensitivity, and it present in PD patients. When challenged by panicogenic pharmacological agents, anxiety sensitivity causes a faster and stronger response in PD sufferers than in healthy individuals. 2 Biological theories concentrate on implicating pathological disturbances in the neurotransmitter systems, including GABA, serotonin (5HT) and noradrenaline. Recently attention was given to a less known neuropeptide cholecystokinin (CCK).
Though it was first discovered in the gastrointestinal tract (it is secreted by the small intestine and stimulates gall bladder contractions), its abundant presence in the mammalian brain indicated on its possible functions as a behavior-regulating neurotransmitter. Various electrophysiological data and animal studies linked CCK to anxiety regulation. For example, its excitatory role on pyramidal neurons of hippocampal area was first observed in rats after electrophoretic administration of CCK, and increased density of CCK-B receptors was detected in rats with low exploratory activity and with novelty-avoidance behavior.
The later, also known as novelty stress sensitivity, is often observed in panic disorder patients.. Anxiogenic properties of CCK were demonstrated in various animal models of anxiety, and results of only one of these studies suggested anxiolytic rather than anxiogenic properties of CCK. 7 The first human study which demonstrated CCK anxiogenic properties was conducted by De Montigny in 1989. The study did not include a control group and all participants were healthy volunteers. Upon injection of various doles of CCK (20-100 g) 70% of participants developed panic attack symptoms.
This discovery was confirmed a year later by Bradwejn and colleagues, who have contributed heavily to the research on the role of CCK as panicogenic agent. In 1991 they confirmed De Montignys observation with the use of a double-blind experimental design. 7 Unlike de Montigny, Bradwejns study included no healthy volunteers, but rather panic disorder patients, who were randomly subjected to injections of either 50 g of CCK or placebo. The procedure was repeated two times and results left no room for ambiguity: all participants panicked with CCK and none with placebo.
It is important to notice, that CCK induced panic attacks did not differ, in patients opinions, from naturally occurring panic attacks and fit the criteria established by the DSM III-R. The next step in CCK research was to compare the responses of normal volunteers and panic disorder sufferers. In a double-blind controlled procedure designed by Bradwejn and colleagues PD sufferers and healthy volunteers received either 25 or 50 g of CCK-4 via injection. 3 The procedure was repeated with placebo for each subject in the experimental and control groups.
The subjects had to describe their symptoms after each injection to the investigators, who recorded the onset and duration of symptoms. After the disappearance of symptoms the participants were assessed with the panic inventory consistent with DSM-III-R diagnostic criteria and were asked to describe any additional symptoms experienced during a panic attack. The occurrence of a panic attack was recorded only in case of abrupt onset and assessment results consistent with criteria for at least moderate panic attack according to DSM-III-R.
Resemblance of a natural panic attack was taken into consideration only for PD patients. Results showed that PD sufferers have enhanced sensitivity to CCK-4. When injected with 25 g of CCK-4, 91 % of them panicked in comparison with 17% from control group. The panic rate reached 100% with the 50 g dose, while only 47% of healthy volunteers panicked. Only 2 out of 23 PD patients (9%) panicked on placebo and none of the controls did. It ‘s worth mentioning that similar effects were observed in the studies of CCK-4 agonist pentagastrine, a peptide which includes aminoacid sequence identical to CCK-4.
Panic attacks occurred in 60-80% of PD patients and only in 25% of healthy participants. The double blind placebo controlled study similar to Bradwejn’s group showed higher sensitivity to pentagastrin in PD patients (van Megen et al. , 1993), though the dose effect was not observed. 7 To further investigate mechanism of CCK-induced anxiety in humans, research progressed to the employment of synthetic CCK-B antagonists. These antagonists have selective affinity for CCK-B receptors and are able to counteract the effect of CCK-4. Among the most studied antagonists are L-365,260 and CI-988.
The first one belongs to the benzodiazepine class of drugs. and the second one is a peptoid. The most often cited study of true experimental design was conducted by Dr. Bradwejn research team. 4 All participants were affected by PD. The experiment had a double-blind placebo arrangement and subjects were referred by two teaching hospitals from Canada and Netherlands. L-365, 260, a selected CCK-B antagonist of choice was given to participants orally in two different doses of 10 and 50 mg ninety minutes prior to administration of CCK-4.
The trials with different doses of the antagonist were separated by a week washout period. The 20 g dose of CCK-4 was proven to induce panic attack in 75% of the PD sufferers, as established in the earlier dose-response experiment (Bradwejn et al, 1991) and did not change for both trials. According to the experimental design an equal number of patients were supposed to receive placebo, 10 and 50 mg of antagonist. An important feature of this experimentwas that it included not only evaluation of behavioral responses or physiological responses by observation , but also of cardiovascular responses .
To record occurrence of panic attack, as in previous studies, a Panic Symptom Scale derived from DSM-III-R was used for assessment immediately following injection of CCK-4 . To measure subjective anxiety a visual analog scale was developed. The results of this study showed that L-365,260 is effective in attenuating symptoms and severity of CCK-induced panic attacks in a dose-dependent manner. The panic rate was 88% for placebo (15 out of 17 participants), 33% for 10 mg dose of antagonist (6 out of 18) and 0% for 50 mg dose.
L-365,260 showed significant difference in reducing the number of reported panic symptoms and the sum intensity of symptoms. It did not produce significant effects on the onset and duration of symptoms. Regarding individual PSS measurements, significant effects were found for both somatic and emotional symptoms, such as dyspnea, choking, chest pain/discomfort, palpitations/heart rate as well as anxiety/fear/apprehension and fear of dying. Curiously enough, fear of losing control in the Utrecht sample was less pronounced in those receiving placebo.
Cardiovascular response to CCK-4 was recorded only in the Montreal site, and there was significant decrease in maximum heart rate following CCK-4 injection for both 50 and 10 mg of the antagonist in comparison with placebo. The visual analog scale demonstrated that 50 mg of L-365,260 significantly lowered anxiety scores after administration of CCK4. The authors believe that their study demonstrated that CCK-B receptors are “an important site of action of exogenous CCK-4 in humans”4, though it is not clear due to the lack of knowledge about CCK permeability of blood-brain barrier whether these receptors are the primary source of action.
The study with another CCK-4 antagonist, CI-988, was conducted by the same group of researchers a year later ( Bradwejn et al. , 1995). 5 Unlike the experiment with L-365,260, this study included only healthy volunteers with no history of panic disorder. The participants were injected with either 50 g of CCK-4 or placebo. A higher dose of CCK-4 than in the L-365,260 experiment was chosen for the healthy subjects, and the antagonist was given orally 30 min before CCK-4 injection. The evaluation of CI-988 performances was judged with the help of the PSS scale employed in the previous studies and cardiovascular response monitoring.
The results of the experiment showed that CI-988 had weaker anxiolytic properties than L-365,260. The number of symptoms was not significantly reduced, though a sum intensity of symptoms and panic attack rate decreased significantly. The subjective experience of anxiety/fear/apprehension was attenuated significantly in comparison with all other symptoms, and researchers ascribe this specific effect to a decrease in panic attack rate. The scientist view ability of CI-988 to reduce anxiety levels as an indicator of CCK-4 involvement in ethiology of panic attacks.
They contribute moderate efficacy of this antagonist to its poor availability in the systemic circulation, rather than to unsuitability of healthy volunteers for CCK-B antagonists studies. In support, they refer to the corroborating L-365,260 studies with healthy subjects and CI-988 studies with PD patients None of these studies was designed to define the role of CCK-4 in the development of naturally occurring panic attacks. If cholecystokinin is involved in ethiology of naturally occuring panic attacks, then treatment with CCK-B antagonists should have clinical effect on patients with panic disorder and modulate their level of anxiety.
Clinical studies with CCK-4 antagonists To test this hypothesis a pilot study was undertaken by Mark S. Kramer and colleagues (MS Kramer et al. , 1995). 6 In a placebo-controlled, double -blind study the PD patients were given either L-365,260 or placebo for six weeks. Eighty three patients out of total eighty eight from six cities in the US completed the study. Patients did not receive any psychotropic medication during the study. The experiment lasted a total of eignt weeks, including the first and last week of the single-blind placebo treatment. Unlike in Dr.
Bradwejns studies, the effectiveness of L-365,260 was evaluated with the help of Physician’s Global Improvement Scale (PGIS) and Hamilton Anxiety and Depression Scales. Participants were taking 120 mg of L-365,260 daily in the form of capsules, of the maximum size and frequency acceptable to them. The researchers anticipated at least 40% difference between placebo group and antagonist group. These expectations were based on the study of Ballenger et al9, which tested efficacy of the benzodiazepin class drug alprazolam and also used the PGIS as evaluation tool.
Though L-365,260 had a higher response rate than placebo (13% difference), it was not enough to assign this antagonist clinically significant anxiolytic properties. However, researchers noticed that there were some numerical trends in favor of the drug upon the completion of the study. L-365,260 reduced panic attack frequency in comparison with the mean baseline, and demonstrated improvement of Work, Social and Family Disability Scales scores. The total anxiety and panic attack intensity were not affected by the drug.
In discussion of the experiment the authors mention that the baseline panic attack frequency and disability scores were significantly higher in the antagonist than in placebo group. The authors suggest several explanations to the unexpectedly poor response to L-365,260. Like Bradwejns group, they mention unclear locus of action of this CCK-B antagonists, and therefore possibility of inadequate distribution of L-365,260 to the appropriate regions. Secondly the measured 1-hour plasma levels might have been insufficient for blockage of endogenous panic attacks, though they proved to be adequate for blockage of CCK-4 induced panic attacks.
The study shows that at the particular dose of 120 mg/day L-365,260 did not prove to be clinically useful in relieving panic attacks, but the outcome of the higher doses could have been different. Also, the through levels of the drug were not measured in the study. It is worth mentioning that Westenberg and den Boer in their review article on panic disorder treatments noticed that in Balenger study that Kramer and co-authors used as the guideline, alprazepam proved to be clinically effective in much higher doses than it was used for General Anxiety disorder9.
It is also not known whether CCK-4 is the main component of endogenous panic attack, to what stage of its development it contributes and how it interacts with other neurotransmitter. Eight years has passed since De Montegny proved CCK-4 to possess anxiogenic qualities in humans. Its panicogenic effect is replicated in the studies described in this review, and now CCK-4 is a proven panic-inducing agent, along with sodium lactate, carbon dioxide and yohimbine. It satisfies four out of seven criteria for an ideal panicogenic agent, and therefore can serve an extremely useful tool in investigation of panic disorder.
Besides being of great help to researchers, the potential usefulness of its antagonists in treatment of PD patients is even more exiting . As James L. Abelson writes in his review article a “non-sedating, non-addicting anxiolytics have been a kind of pharmacological holy grail . “1 The most common adverse effect, as reported by Dr. Kramer group, was orthostatic dizziness (21%), followed by muscle pain, nausea and headaches (12%). 6 Among currently used anxiolitic agents, as described in Dr. Westenbergs article9, Tricyclic Antidepressants, such as cloripramine and imipramine cause jitterness syndrome, i. e. acerbation of symptoms in the first week of treatment, a major cause of dropout, as well as anticholinergic side effects. Benzodiazepines often induce daytime sedation.
Serotonin reuptake inhibitor fluvoxamine is one of the few clinically successive agents without major side effect. Furthermore, research data suggest that the mechanism of its action can shed light on CCK-4 role in panic disorder. In discussion of his ground breaking study on the effect of CCK-4 on panic disorder patients Bradwejn at al emphasized importance of investigation of possible blockage of CCK-4 induced attacks by known antipanic medications.
The evidence of CCK interaction with other neurotransmitter systems was first established in animal studies, which showed that anxiogenic action of CCK can be antagonized by benzodiazepines and 5-HT3 receptor antagonists. The Bradwejns group tested imipramine effects on CCK-4 challenge, and the in most recent study another research group concentrated its efforts on fluvoxamine(van Megen et al. , 1997). 8 The authors hypothesis was based on circumstantial evidence of interaction between 5-HT and CCK-containing neuronal pathways in earlier animal studies.
Twenty six patients were given fluvoxamine or placebo for eight weeks in a double blind manner starting with 50 mg and increasing the dose to 150 mg daily by the end of second week. The single blind challenge with 50 g of CCK-4 was accomplished on first and last day of the experiment. Hamilton Anxiety Scale , the Symptoms checklist 90 and the Fear Questionnaire were used for assessment a week before the first CCK-4 challenges and in the course of the study. The frequency of panic attacks was measured daily using the Utrecht Panic Inventory.
The results were strongly in favor of fluvoxamine-treated patients. The panic rate decreased from 76 to 29% overall and 75 to 13% among responders. Fluvoxamine also decreased the frequency of naturally occurring panic attacks and general anxiety, though statistical significance was reported only for the last condition. To broaden the clinical research spectrum, the studies evaluating the role of CCK-antagonists on panic attacks induced by other agents, as well as studies with medications which are not able to affect panic attacks caused by other agents need to be conducted.
On the other hand, the fundamental studies of CCK-receptor distribution in the peripheral and central nervous system, and further research of CCK interaction with other neurotransmitters should help to improve design of clinical studies. As one of the review articles suggests “whether or not we are ever able to clinically manipulate CCK receptors to human benefit, they are unquestionably of great importance to human CNS functioning and are of great interest to psychiatry”.