Aspects on the psychopharmacology of cholecystokinin

Abstract

This thesis includes studies of the effect of mild stress on brain cholecystokinin (CCK), anatomy of markers for CCK-ergic transmission in the brain and the systemic effect of CCK receptor stimulation in a clinical test for anxiety and fear. CCK is a brain-gut peptide acting as a neurotransmitter in mammalian brain via CCKA (CCK1) and CCKB (CCK2) receptors.

In the brain, CCK is widely distributed to both cortical and subcortical structures, coexisting and interacting with "classical" neurotransmitters such as dopamine (DA), glutamate and GABA. Brain CCK seems to be involved in regulation of various physiological and cognitive functions. The prefrontal cortex (PFC) is an important association area in the psychopathology of stress, anxiety disorders and schizophrenia. To date there are several models involving DA and glutamate-systems in mimicking aspects of psychosis in humans and different paradigms for studying stress, and human and animal data suggest possible CCK-ergic involvement in these conditions.

In rat PFC, mild stress such as an intraperitoneal (i.p.) saline injection leads to acute regionspecific decrease in CCK peptide levels within 20 minutes followed by increased levels at 8 hours. The decrease in CCK 20 min following i.p. saline injection was more accentuated in ketamine (glutamate NMDA receptor antagonist) pre-treated rats and the increase at 8 hours was absent in those animals. However, no changes in CCK mRNA in rat PFC up to 8 hours following i.p. saline injection were observed.

Tripeptidyl peptidase II (TPP II) is a putative CCK-degrading enzyme and the brain distribution of its mRNA was examined. The mRNA encoding TPP II is broadly distributed to regions rich in CCK in monkey brain. Distribution of the mRNA encoding the CCKA receptor in monkey brain suggests a broader localization than previously reported using receptor binding autoradiography. CCKA mRNA was localized to the cerebral cortex, ventral striatum, amygdala region, hippocampus and substantia nigra. TPP II and CCKA receptor mRNAs in Brodmann area (BA) 10 of human PFC show tendencies to decrease in subjects having had a schizophrenia diagnosis as compared to controls.

In anxiety research, CCKB receptor stimulation has been used to simulate panic attacks. In addition, gastrin/CCKB receptor stimulation is known to induce insulin/C-peptide release from the pancreas. In an effort to develop a bioassay for phenotypes relevant for anxiety sensitivity and anxiety disorders, serum levels of C-peptide were measured following CCKB receptor stimulation by pentagastrin. Pentagastrin administration in doses up to 0.2 µg/kg leads to acute transient increased serum levels of C-peptide, elevated physiological parameters such as heart rate and skin conductivity and to increases in subjective discomfort as rated on the subjective anxiety and discomfort scale (SADS) within the same time frame (2-4 minutes following i.v. pentagastrin injection). Pretest ratings on the anxiety sensitivity index (ASI) and Hamilton anxiety scale (HAS) as well as baseline levels of C-peptide correlated to ratings on SADS following pentagastrin.

In conclusion, CCK-ergic mechanisms in the PFC may be involved in the neurobiology of mild stress. Brain distribution of TPP II is in agreement with a CCK-degrading function of this enzyme. TPP II and CCKA receptors in BA 10 may be involved in the pathophysiology of schizophrenia. C-peptide measurements along with psychophysiological, psychological and personality characterization in a "pentagastrin test for anxiety and fear" may be a useful bioassay for anxiety sensitivity and anxiety disorders.