Changes in cholecystokinin mRNA expression after amygdala kindled seizures: an in situ hybridization study

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

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.

Cholecystokinin-Mediated Neuromodulation of Anxiety and Schizophrenia: A "Dimmer-Switch" Hypothesis

Cholecystokinin (CCK), the most abundant brain neuropeptide, is involved in relevant behavioral functions like memory, cognition, and reward through its interactions with the opioid and dopaminergic systems in the limbic system. CCK excites neurons by binding two receptors, CCK1 and CCK2, expressed at low and high levels in the brain, respectively. Historically, CCK2 receptors have been related to the induction of panic attacks in humans. Disturbances in brain CCK expression also underlie the physiopathology of schizophrenia, which is attributed to the modulation by CCK1 receptors of the dopamine flux in the basal striatum. Despite this evidence, neither CCK2 receptor antagonists ameliorate human anxiety nor CCK agonists have consistently shown neuroleptic effects in clinical trials. A neglected aspect of the function of brain CCK is its neuromodulatory role in mental disorders. Interestingly, CCK is expressed in pivotal inhibitory interneurons that sculpt cortical dynamics and the flux of nerve impulses across corticolimbic areas and the excitatory projections to mesolimbic pathways. At the basal striatum, CCK modulates the excitability of glutamate, the release of inhibitory GABA, and the discharge of dopamine. Here we focus on how CCK may reduce rather than trigger anxiety by regulating its cognitive component. Adequate levels of CCK release in the basal striatum may control the interplay between cognition and reward circuitry, which is critical in schizophrenia. Hence, it is proposed that disturbances in the excitatory/ inhibitory interplay modulated by CCK may contribute to the imbalanced interaction between corticolimbic and mesolimbic neural activity found in anxiety and schizophrenia.

Laminar Distribution of Neurochemically-Identified Interneurons and Cellular Co-expression of Molecular Markers in Epileptic Human Cortex

Inhibitory GABAergic interneurons are fundamental elements of cortical circuits and play critical roles in shaping network activity. Dysfunction of interneurons can lead to various brain disorders, including epilepsy, schizophrenia, and anxiety. Based on the electrophysiological properties, cell morphology, and molecular identity, interneurons could be classified into various subgroups. In this study, we investigated the density and laminar distribution of different interneuron types and the co-expression of molecular markers in epileptic human cortex. We found that parvalbumin (PV) and somatostatin (SST) neurons were distributed in all cortical layers except layer I, while tyrosine hydroxylase (TH) and neuropeptide Y (NPY) were abundant in the deep layers and white matter. Cholecystokinin (CCK) neurons showed a high density in layers IV and VI. Neurons with these markers constituted ~7.2% (PV), 2.6% (SST), 0.5% (TH), 0.5% (NPY), and 4.4% (CCK) of the gray-matter neuron population. Double- and triple-labeling revealed that NPY neurons were also SST-immunoreactive (97.7%), and TH neurons were more likely to express SST (34.2%) than PV (14.6%). A subpopulation of CCK neurons (28.0%) also expressed PV, but none contained SST. Together, these results revealed the density and distribution patterns of different interneuron populations and the overlap between molecular markers in epileptic human cortex.

Are there really "epileptogenic" mechanisms or only corruptions of "normal" plasticity?

Plasticity in the nervous system, whether for establishing connections and networks during development, repairing networks after injury, or modifying connections based on experience, relies primarily on highly coordinated patterns of neural activity. Rhythmic, synchronized bursting of neuronal ensembles is a fundamental component of the activity-dependent plasticity responsible for the wiring and rewiring of neural circuits in the CNS. It is therefore not surprising that the architecture of the CNS supports the generation of highly synchronized bursts of neuronal activity in non-pathological conditions, even though the activity resembles the ictal and interictal events that are the hallmark symptoms of epilepsy. To prevent such natural epileptiform events from becoming pathological, multiple layers of homeostatic control operate on cellular and network levels. Many data on plastic changes that occur in different brain structures during the processes by which the epileptogenic aggregate is constituted have been accumulated but their role in counteracting or promoting such processes is still controversial. In this chapter we will review experimental and clinical evidence on the role of neural plasticity in the development of epilepsy. We will address questions such as: is epilepsy a progressive disorder? What do we know about mechanism(s) accounting for progression? Have we reliable biomarkers of epilepsy-related plastic processes? Do seizure-associated plastic changes protect against injury and aid in recovery? As a necessary premise we will consider the value of seizure-like activity in the context of normal neural development.

Axonal sprouting of GABAergic interneurons in temporal lobe epilepsy

Temporal lobe epilepsy is one of the most common forms of epilepsy. Numerous contributing factors and compensatory mechanisms have been associated with temporal lobe epilepsy. One feature found in both humans and animal models is sprouting of hippocampal principal cell axons, which suggests that axonal sprouting may be a general phenomenon associated with temporal lobe epilepsy. This article highlights the evidence showing that hippocampal GABAergic interneurons also undergo axonal sprouting in temporal lobe epilepsy. The caveats and unanswered questions associated with the current data and the potential physiological consequences of reorganizations in GABAergic circuits are discussed.

CCK-8 prevents the development of kindling and regulates the GABA and NPY expression in the hippocampus of pentylenetetrazole (PTZ)-treated adult rats

Neuronal loss and irreversible brain damage often cause the worsening of symptoms and the decreased efficacy of pharmacological treatment occurring in epileptic patients and animal models of kindling. Recently we reported that the neurotransmitter/neuromodulatory peptide Cholecystokinin-8 (CCK-8) is able to induce the structural and functional neuronal recovery of chemical- and surgical-induced lesions when i.p. injected in rodents. The present study therefore, was aimed at verifying the hypothesis that treatment with a CCK-8 dose having a neuroprotective action might affect brain alterations and the development of kindling in adult rats receiving the convulsant agent pentylenetetrazole (PTZ). Compared to rats receiving Saline prior to PTZ, which manifested clonic-tonic seizures (Class 5 behavioural change scale) after three weeks of treatment, rats pre-treated with CCK-8 showed an improvement of behavioural score exhibiting myoclonus and occasionally tonic seizures (Class 3/4). This decreased susceptibility to develop convulsions was associated with the recovery of PTZ-induced reduction of ChAT levels in forebrain and GABA/GAD expression in the hippocampus. Furthermore, NPY immunoreactivity distribution and NPY mRNA levels were also increased in the hippocampus of rats receiving CCK-8 injection before each PTZ treatment. These data indicate that CCK-8 possesses the ability to prevent and/or suppress the convulsant effects of PTZ by stimulating the synthesis of neurotransmitters/peptides involved in the inhibition of hippocampal hyper-excitability. Our findings suggest that CCK-8 may have anticonvulsant and neuroprotective properties that merit further investigation.

Long-term effects of amygdala GABA receptor blockade on specific subpopulations of hippocampal interneurons

Growing evidence indicates that the amygdala modulates hippocampal functions. To test the hypothesis that this modulation may involve long-lasting effects on interneuronal networks in the hippocampus, changes in the expression of neurochemical markers specific for different interneuronal subpopulations were assessed in adult rats 96 h following acute infusion of low doses of the GABAA receptor antagonist picrotoxin into the amygdala. The numerical density (Nd) of somata showing immunoreactivity (IR) for parvalbumin (PVB) was decreased in dentate gyrus (DG) and the CA4-2 region, while that of calretinin (CR)-IR was decreased in DG and CA2. The Nd of calbindin D28k (CB)-IR somata was decreased in CA3-2. The densities of axon terminals arising from PVB-IR and cholecystokinin (CCK)-IR basket neurons were also altered, with those of CCK-IR terminals increased across all sectors, while PVB-IR terminals were decreased only in the CA region. Increases in CCK-IR terminals were paralleled by increases of terminals with IR for the 65-kD isoform of glutamate decarboxylase (GAD65). Mixed-effects statistical models, adapted specifically for these analyses, indicated that perturbations of amygdalar inputs to the hippocampus significantly alter the drive that hippocampal PVB-, CR-, and CB-IR neurons within the dentate gyrus/CA4 region exercise on CCK-IR terminals within the same region as well as in CA3-1. These results suggest that amygdalar modulation of specific neuronal subpopulations may induce lasting and far-reaching changes in the hippocampus during normal functioning, as well as in diseases involving a disruption of amygdalar activity. In particular, changes in specific interneuronal markers within selective hippocampal sectors detected in the present results are strikingly similar to those reported in this region in schizophrenia. These similarities suggest that, in this disease, a disruption of GABAergic transmission within the amygdala may play a significant role in the induction of abnormalities in the hippocampus.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Interneuron Diversity series: Interneuronal neuropeptides--endogenous regulators of neuronal excitability

Interneurons are often classified according to neuropeptide content. However, it is becoming increasingly clear that neuropeptides are more than convenient neurochemical markers and can act as important modulators of neuronal activity. Recent advances in understanding neuropeptide release and physiological actions suggest that the interneuronal system of neuropeptides is crucial for maintaining appropriate brain function under normal and pathophysiological conditions. In particular, interneuronal neuropeptides appear to play roles in cognition and as endogenous anti-epileptic agents. This article describes current understanding of the conditions under which neuropeptides are released from interneurons, their specific effects on neuronal excitability and synaptic transmission, and the consequences of their loss of function.

Cholecystokinin expression after hippocampal deafferentiation: molecular evidence revealed by differential display-reverse transcription-polymerase chain reaction

The cortical information flow via the perforant path represents a major excitatory projection to the hippocampus. Lesioning this projection leads to massive degeneration and subsequently to reorganization in its termination zones as well as in primary non-affected subfields of the hippocampus. The molecular mechanisms and factors which are involved in the postlesional events are poorly defined. Using a differential display reverse transcription-polymerase chain reaction (DDRT-PCR) strategy, we located one band which occurred only in control hippocampus lanes and almost disappeared in the lanes of lesioned hippocampi. By sequencing, we identified the corresponding gene as cholecystokinin (CCK). Northern blot analysis confirmed a decreased transcription of CCK after lesion. In situ hybridization analysis was performed for localization and quantification of altered CCK transcription. We noted a significant downregulation of CCK transcription in the hippocampus (20%) and in the contralateral cortex (12%) 1-day after lesion (dal) and an increased signal in the ipsilateral cortex (10.5%). This pattern was altered, showing upregulation of CCK mRNA expression, reaching its highest level of 70% above control levels at 5 dal. In the hippocampus, the control level was reached again at 21 dal, whereas the cortex reached the control level at 10 dal. In comparison, the mRNA transcripts of the receptors CCK(A) and CCK(B) remained unchanged. Since CCK-containing neurons are involved in the modulation of pyramidal and granule cell excitability, our data indicate a time course correlation between CCK mRNA expression and postlesional axonal sprouting response in the hippocampus.

Effect of transient hippocampal inhibition on amygdaloid kindled seizures and amygdaloid kindling rate

In this study the effect of transient inhibition of the CA1 region of the dorsal hippocampus by lidocaine on amygdala kindling rate and amygdaloid kindled seizures was investigated. In experiment 1, rats were divided into four groups. In group 1, animals were implanted only with a tripolar electrode into the amygdala but in groups 2-4, two guide cannulae were also implanted into the CA1 regions of the dorsal hippocampi. Animals were stimulated daily to be kindled. In groups 3 and 4, saline or 2% lidocaine (1 microl/2 min) was also injected respectively into the hippocampus, 5 min before each stimulation. Results obtained showed that amygdala kindling rate and the number of stimulations to receive from stage 4 to stage 5 seizure were significantly increased in group 4. In experiment 2, lidocaine (1% and 2%) was infused (1 microl/2 min) into the hippocampus of amygdala kindled rats bilaterally and animals were stimulated at 5, 15 and 30 min after drug injection. Twenty four h before lidocaine injection, saline was also infused (1 microl/2 min) into the hippocampus as control. Obtained results showed that afterdischarge duration was reduced 5 min after lidocaine (1% and 2%) injection. Stage 5 seizure duration was also decreased 5 and 15 min after 2% lidocaine. Thus, it may be suggested that in amygdala kindling, activation of the hippocampal CA1 region has a role in seizure acquisition and seizure severity so that inhibition of this region results in decreasing of seizure severity and retards amygdala kindling rate.

Cholecystokinin gene transcription: promoter elements, transcription factors and signaling pathways

Cholecystokinin (CCK) is a neuropeptide expressed in the small intestine and in the central and peripheral nervous system. CCK gene expression is both spatially and temporally regulated. In neurons CCK production is increased by growth factors, cyclic adenosine 3', 5'-monophosphate (cAMP), dopamine, estrogen, and injury situations, while intestinal CCK expression is mainly regulated by food intake. The function of the proximal CCK promoter has been examined by transfection of human CCK-CAT reporter constructs in cultured cells, DNase I footprinting and gel shift assays. These studies have led to the identification of regulatory elements and transcription factors important for basal and stimulated gene expression and depicted the signaling pathways involved in growth factor and cAMP induced CCK transcription. The review outlines the current knowledge of the regulation of CCK transcription and describes the role of putative transcription factors in tissue-specific CCK gene expression.

Subregional pathology of the amygdala complex and entorhinal region in surgical specimens from patients with pharmacoresistant temporal lobe epilepsy

The hippocampus, amygdala complex, and entorhinal region represent anatomically linked limbic structures of the mesiotemporal lobe. Chronic seizures and mnestic deficits in patients with pharmacoresistant mesial temporal lobe epilepsy (TLE) appear to correlate with distinct patterns of histopathological alterations in these areas. The complex anatomical organization of the amygdala and entorhinal region, however, render a detailed neuropathological evaluation of surgical specimens difficult. In this study, we present a combined cytoarchitectonical, pigmentarchitectonical, myelinarchitectonical, and immunohistochemical reconstruction of the amygdala, entorhinal region, and hippocampus from surgical TLE specimens (n = 20) in order to analyze their regional and cellular patterns of pathology. Anterior mesiotemporal lobes dissected in different spatial planes were obtained from 4 autopsy control patients and used for the characterization of neuroanatomical landmarks. Lateral, basal, and granular subnuclei of the amygdala were consistently identified in the surgical specimens. Major histopathological alterations included neuronal cell loss as revealed by extracellular lipofuscin accumulation, glial satellitosis, as well as cellular and fibrillary gliosis. The regional distribution of neuropathological changes varied considerably between different subnuclei but the lateral nucleus was more often involved than basal and granular nuclei. These amygdala nuclei appeared to be more severely affected compared to the adjacent entorhinal region. In addition, patients presenting with secondary generalized tonic-clonic seizures showed significantly more damage in mesiotemporal structures. Pathological alterations in the amygdala and entorhinal region were found to be associated with Ammon's horn sclerosis in most but not all cases. Our findings reveal the amygdala as a major target for epilepsy-associated neuronal cell damage. Significant variations in the lesional pattern among patients with chronic TLE would also be compatible with different spreading pathways of epileptogenic activity within the mesial temporal lobe.

Chemical kindling induced by pentylenetetrazol changes cholecystokinin mRNA and peptide levels in rat frontal cortex

Kindling model is useful to study the mechanism of learning and memory. Cholecystokinin (CCK) mRNA and CCK-like immunoreactivity (CCK-LI) levels in the hippocampus and frontal cortex of chemically kindled rats were determined at different time points. In the frontal cortex, chronic treatment with pentylenetetrazol (PTZ) (40 mg/kg per day for 8 days) increased CCK mRNA level at 7 days, and decreased CCK-LI level at 2 and 7 days after the last injection. However, neither CCK mRNA nor CCK-LI levels in the hippocampus changed. These results suggest that PTZ-induced kindling increases CCK mRNA expression and CCK-LI release in the frontal cortex.

Cholecystokinin increases GABA release by inhibiting a resting K+ conductance in hippocampal interneurons

Cholecystokinin (CCK) is found co-localized with the inhibitory neurotransmitter GABA in interneurons of the hippocampus. Also, CCK receptors are found in abundance in this brain region. The possibility that CCK alters interneuron activity was examined using whole-cell current- and voltage-clamp recordings from visualized interneurons in the stratum radiatum of area CA1 in rat hippocampal slices. The effect of CCK on GABA-mediated IPSCs was also determined in pyramidal neurons. The sulfated octapeptide CCK-8S increased action potential frequency or generated inward currents in the majority of interneurons. These effects of CCK persisted in the presence of tetrodotoxin and cadmium, suggesting that they were direct. Current-voltage plots revealed that CCK-8S inhibited a conductance that was linear across command potentials and reversed near the equilibrium potential for K+ ions. The K+ channel blocker tetraethylammonium (10 mM) generated inward currents similar to those initiated by CCK, and it occluded the effect of the peptide. BaCl2 (1 mM) and 4-aminopyridine (2 mM) did not alter the effect of CCK. The CCKB receptor antagonist PD-135,158 completely blocked the inward currents generated by CCK-8S. CCK also resulted in an increase in spontaneous action potential-dependent IPSC frequency, but no changes in action potential-independent miniature IPSCs or evoked IPSCs in pyramidal neurons. These results provide evidence that CCK can depolarize hippocampal interneurons through the inhibition of a resting K+ conductance, leading to increased tonic inhibition of pyramidal neurons. This action of CCK may contribute to its anticonvulsant properties, as observed in limbic seizure models.

Lipofectin-facilitated transfer of cholecystokinin gene corrects behavioral abnormalities of rats with audiogenic seizures

To evaluate the potential for lipofectin-mediated central nervous system gene transfer, the plasmid coding for cholecystokinin was administered intracerebroventricularly to rats, which have congenital audiogenic seizures and high responses to peripheral electric stimulation-induced analgesia. Previous studies had shown that low brain cholecystokinin levels may be the neurochemical variable of rat's audiogenic seizure and high responses to the analgesia because cholecystokinin is an anticonvulsant and anti-opioid neuropeptide. Gene transfer of cholecystokinin corrected the increased susceptibility to audiogenic seizures and the high responses to analgesia for about one week. Similar administration of plasmid expressing beta-galactosidase indicated that the vector mainly transfected ependymal cells lining the ventricle and pia mater cells. The increased cholecystokinin messenger RNA and immunoreactivity in the hippocampus following stereotactic intrahippocampal administration of cholecystokinin plasmid was also demonstrated with in situ hybridization and immunohistochemistry techniques. These results suggest that lipofectin-mediated gene transfer will be useful for studies of brain function, the modification of behavior and gene therapy for central nervous system disorders.

Amygdala-kindled seizures increase the expression of corticotropin-releasing factor (CRF) and CRF-binding protein in GABAergic interneurons of the dentate hilus

Kindling, a model of temporal lobe epilepsy, induces a number of neuropeptides including corticotropin-releasing factor (CRF). CRF itself can produce limbic seizures which resemble kindling in some aspects. However, tolerance to the convulsant effects of CRF develops rapidly. Hypothetically, this could be explained should seizures also induce the CRF-binding protein (CRF-BP), which has been postulated to restrict the actions of CRF. Therefore, in the present study, we used in situ hybridization to examine the effects of amygdala-kindled seizures on the mRNA levels of CRF and CRF-BP. Kindled seizures markedly elevated CRF and CRF-BP in the dentate gyrus of rats. CRF and CRF-BP were induced almost exclusively in GABAergic interneurons of the dentate hilus. The CRF and CRF-BP interneurons also expressed neuropeptide Y but not cholecystokinin. CRF appeared to have an excitatory role in the dentate gyrus as it decreased the afterhyperpolarization of dentate granule neurons. These results suggest that CRF may contribute to the development of amygdala kindling. However, the compensatory induction of CRF-BP may serve to limit the excitatory effects of CRF in the dentate gyrus.

Effects of chronic lithium and electroconvulsive stimuli on cholecystokinin mRNA expression in the rat brain

This study compares the effect of lithium (Li+) and electroconvulsive stimuli (ECS), two treatments commonly used in the treatment of affective disorders, on CCK mRNA expression in the rat brain. Two groups of rats receiving either 4 week Li+ or vehicle food supplementation and two groups receiving 6 ECS or 6 sham ECS during 2 weeks were studied. A significant decrease in CCK mRNA levels was seen in the caudate putamen both after Li+ as compared to vehicle and ECS as compared to sham ECS, 27 and 25%, respectively. A small (10%), yet significant, decrease was also seen in the inner entorhinal cortex after Li+. The results indicate that both Li+ and ECS inhibit CCK synthesis in the caudate putamen and are consistent with other findings of presumed decreased dopaminergic action in this part of the brain following these treatments.