Cholecystokinin release mediated by 5-HT3 receptors in rat cerebral cortex and nucleus accumbens

D2 dopamine receptor-mediated mechanisms of dopaminergic system modulation in in vivo and in vitro experimental models of migraine

The dopaminergic system is implicated in the pathophysiology of migraine. However, the underlying mechanisms remain unclear. We explored the effects and mechanisms of dopaminergic system modulation in the in vivo and in vitro rat models of migraine. Dopaminergic agonist apomorphine, D2 receptor antagonists metoclopramide and haloperidol and 5-HT3 receptor antagonist ondansetron alone and together were tested in nitroglycerin-induced migraine model, in vivo. Likewise, the combinations of drugs were also tested on basal calcitonin gene-related peptide (CGRP) release in vitro hemiskull preparations. Mechanical allodynia was tested by von Frey filaments. CGRP concentrations in trigeminovascular structures and in vitro superfusates and c-Fos levels in the brainstem were determined by enzyme-linked immunosorbent assay. Meningeal mast cells were evaluated with toluidine blue staining. Apomorphine further enhanced nitroglycerin-induced mechanical allodynia, brainstem c-fos expression, trigeminal ganglion and brainstem CGRP concentrations and meningeal mast cell degranulation, in vivo. Haloperidol completely antagonised all apomorphine-induced effects and also alleviated changes induced by nitroglycerin without apomorphine. Metoclopramide and ondansetron partially attenuated apomorphine- or nitroglycerin-induced effects. A combination of haloperidol and ondansetron decreased basal CGRP release, in vitro, whereas the other administrations were ineffective. Apomorphine-mediated dopaminergic activation exacerbated nitroglycerin-stimulated nociceptive reactions by further enhancing c-fos expression, CGRP release and mast cell degranulation in strategical structures associated with migraine pain. Metoclopramide partially attenuated the effects of apomorphine, most likely because it is also a 5-HT3 receptor antagonist. Haloperidol with pure D2 receptor antagonism feature appears to be more effective than metoclopramide in reducing migraine-related parameters in dopaminergic activation- and/or NTG-induced migraine-like conditions.

5-HT3Rs Maintain Hippocampal LTP in a CB1R-GABAA -Dependent Manner for Spatial Memory

BACKGROUND AND PURPOSE

As the only ionotropic receptor in 5-HT receptor family, 5-HT3 receptor (5-HT3R) involves in psychiatric disorders and its modulators have potential therapeutic effects for cognitive impairment in these disorders. However, it remains unclear how 5-HT3Rs shape synaptic plasticity for memory function.

EXPERIMENTAL APPROACH

Extracellular as well as whole-cell recordings were used to monitor hippocampal long-term potentiation (LTP) and synaptic transmission in hippocampal slices from 5-HT3AR knock-out or 5-HT3AR-GFP mice. Immunocytochemistry, qRT-PCR and Western blot were used to measure receptor expression. We also assessed hippocampal dependent cognition and memory using the Morris water maze (MWM) and novel object recognition.

KEY RESULTS

We found that 5-HT3R dysfunction impaired hippocampal LTP in Schaffer collateral (SC)-CA1 pathway in hippocampal slices by facilitating GABAergic inputs in pyramidal cells. This effect was dependent on 5-HT3Rs on axon-terminals. It resulted from reduced expression and function of cannabinoid receptor 1 (CB1R) co-localized with 5-HT3Rs on axon terminals, which led to diminishment of tonic inhibition of GABA release by CB1Rs. Inhibition of CB1Rs mimicked the facilitation of GABAergic transmission by 5-HT3R disruption. Consequently, mice with hippocampal 5-HT3R disruption exhibited impaired spatial memory in Morris water maze tasks.

CONCLUSION AND IMPLICATIONS

These results suggest that 5-HT3Rs are crucial in enabling hippocampal synaptic plasticity via a novel CB1R-GABA_A -dependent pathway to regulate spatial memory.

Does endogenous cholecystokinin modulate alcohol intake?

Alcohol use disorder or alcoholism is characterized by uncontrollable alcohol use and intoxication, as well as a heightened state of anxiety after alcohol withdrawal. Ethanol-associated stimuli also drive the urge to drink by means of classical conditioning. Alcoholism has been considered a dopamine (DA) dysregulation syndrome that involves the activity of the central amygdala circuitry of anxiety. Cholecystokinin (CCK) is the most abundant neuropeptide in the mammal brain, where it activates two receptors, CCK₁ and CCK₂. Genetic evidence relates CCK₁ receptors to alcoholism in humans. CCK₂ activity has been associated with the onset of human anxiety. CCK modulates DA release in the nucleus accumbens (NAc) and it is expressed in the γ-aminobutyric acid (GABA)-expressing basket interneurons in the cerebral cortex. CCK interacts with serotonin (5-HT) neurotransmission through 5-HT₃ receptors to regulate mesocorticolimbic pathways and with GABA to attenuate anxiety in the amygdala. Finally, CCK stimulates the release of orexins and oxytocin in the hypothalamus, two relevant hypothalamic neuropeptides involved in signaling satiety for ethanol and well-being respectively. Given the "dimmer-switch" function of endogenous CCK in the neurotransmission by 5-HT, DA, GABA, and glutamate in normal and pathological behaviors (Ballaz and Bourin, 2020), we hypothesize that CCK adjusts functioning of the reward and anxiety circuitries altered by ethanol. This review gathers data supporting this hypothesis, and suggests mechanisms underlying a role for endogenous CCK in alcoholism.

Ethanol induces persistent potentiation of 5-HT3 receptor-stimulated GABA release at synapses on rat hippocampal CA1 neurons

Several studies have shown that ethanol (EtOH) can enhance the activity of GABAergic synapses via presynaptic mechanisms, including in hippocampal CA1 neurons. The serotonin type 3 receptor (5-HT3-R) has been implicated in the neural actions of ethanol (EtOH) and in modulation of GABA release from presynaptic terminals. In the present study, we investigated EtOH modulation of GABA release induced by 5-HT3-R activation using the mechanically isolated neuron/bouton preparation from the rat CA1 hippocampal subregion. EtOH application before and during exposure to the selective 5-HT3 receptor agonist, m-chlorophenylbiguanide (mCPBG) potentiated the mCPBG-induced increases in the peak frequency and charge transfer of spontaneous GABAergic inhibitory postsynaptic currents. Interestingly, the potentiation was maintained even after EtOH was removed from the preparation. A protein kinase A inhibitor reduced the magnitude of EtOH potentiation. Fluorescent Ca2+ imaging showed that Ca2+ transients in the presynaptic terminals increased during EtOH exposure. These findings indicate that EtOH produces long-lasting potentiation of 5-HT3-induced GABA release by modulating calcium levels, via a process involving cAMP-mediated signaling in presynaptic terminals.

The 5-HT3A receptor is essential for fear extinction

The 5-HT3 receptor, the only ionotropic 5-HT receptor, is expressed in limbic regions, including the hippocampus, amygdala, and cortex. However, it is not known whether it has a role in fear memory processes. Analysis of 5-HT3A receptor knockout mice in fear conditioning paradigms revealed that the 5-HT3A receptor is not required for the acquisition or retention of fear memory but is essential for the extinction of contextual and tone-cued fear. Our data suggest that the 5-HT3A receptor could be a key molecule regulating fear memory processes and a potential therapeutic target for fear disorders.

Neuronal network of panic disorder: the role of the neuropeptide cholecystokinin

Panic disorder (PD) is characterized by panic attacks, anticipatory anxiety and avoidance behavior. Its pathogenesis is complex and includes both neurobiological and psychological factors. With regard to neurobiological underpinnings, anxiety in humans seems to be mediated through a neuronal network, which involves several distinct brain regions, neuronal circuits and projections as well as neurotransmitters. A large body of evidence suggests that the neuropeptide cholecystokinin (CCK) might be an important modulator of this neuronal network. Key regions of the fear network, such as amygdala, hypothalamus, peraqueductal grey, or cortical regions seem to be connected by CCKergic pathways. CCK interacts with several anxiety-relevant neurotransmitters such as the serotonergic, GABA-ergic and noradrenergic system as well as with endocannabinoids, NPY and NPS. In humans, administration of CCK-4 reliably provokes panic attacks, which can be blocked by antipanic medication. Also, there is some support for a role of the CCK system in the genetic pathomechanism of PD with particularly strong evidence for the CCK gene itself and the CCK-2R (CCKBR) gene. Thus, it is hypothesized that genetic variants in the CCK system might contribute to the biological basis for the postulated CCK dysfunction in the fear network underlying PD. Taken together, a large body of evidence suggests a possible role for the neuropeptide CCK in PD with regard to neuroanatomical circuits, neurotransmitters and genetic factors. This review article proposes an extended hypothetical model for human PD, which integrates preclinical and clinical findings on CCK in addition to existing theories of the pathogenesis of PD.

Role of 5-HT3 Receptors in the Antidepressant Response

Serotonin (5-HT)₃ receptors are the only ligand-gated ion channel of the 5-HT receptors family. They are present both in the peripheral and central nervous system and are localized in several areas involved in mood regulation (e.g., hippocampus or prefrontal cortex). Moreover, they are involved in regulation of neurotransmitter systems implicated in the pathophysiology of major depression (e.g., dopamine or GABA). Clinical and preclinical studies have suggested that 5-HT₃ receptors may be a relevant target in the treatment of affective disorders. 5-HT₃ receptor agonists seem to counteract the effects of antidepressants in non-clinical models, whereas 5-HT₃ receptor antagonists, such as ondansetron, present antidepressant-like activities. In addition, several antidepressants, such as mirtazapine, also target 5-HT₃ receptors. In this review, we will report major advances in the research of 5-HT₃ receptor's roles in neuropsychiatric disorders, with special emphasis on mood and anxiety disorders.

A novel subpopulation of 5-HT type 3A receptor subunit immunoreactive interneurons in the rat basolateral amygdala

The amygdalar basolateral nuclear complex (BLC) has very high levels of the 5-HT type 3 receptor (5-HT(3)R). Previous studies have reported that 5-HT(3)R protein in the BLC is expressed in interneurons and that 5-HT(3)R mRNA is coexpressed with GABA and certain neuropeptides or calcium-binding proteins in these cells. However, there have been no detailed descriptions of the distribution of 5-HT(3)R+ neurons in the rat amygdala, and no quantitative studies of overlap of neurons expressing 5-HT(3)R protein with distinct interneuronal subpopulations in the BLC. The present investigation employed dual-labeling immunohistochemistry using antibodies to the 5-HT-3A receptor subunit (5-HT(3A)R) and specific interneuronal markers to address these questions. These studies revealed that there was a moderate density of nonpyramidal 5-HT(3A)R+ neurons in the BLC at all levels of the amygdala. In addition, immunostained cells were also seen in anterior portions of the cortical and medial nuclei. Although virtually all 5-HT(3A)R+ neurons in the BLC were GABA+, very few expressed neuropeptide or calcium-binding protein markers for individual subpopulations. The main interneuronal marker expressed by 5-HT(3A)R+ neurons was cholecystokinin (CCK), but only 8-16% of 5-HT(3)R+ neurons in the BLC, depending on the nucleus, were CCK+. Most of these CCK+/5-HT(3A)R+ double-labeled neurons appeared to belong to the subpopulation of large type L CCK+ interneurons. Very few 5-HT(3A)R+ neurons expressed calretinin, vasoactive intestinal peptide, or parvalbumin, and none expressed somatostatin or calbindin. Thus, the great majority of neurons expressing 5-HT(3A)R protein appear to constitute a previously unrecognized subpopulation of GABAergic interneurons in the BLC.

Cholecystokinin-2 (CCK2) receptor-mediated anxiety-like behaviors in rats

Cholecystokinin (CCK) is a neurotransmitter in the brain closely related to anxiety. Of the two CCK receptor subtypes, CCK(2) receptors are most implicated in the control of anxiety-related behavior. CCK(2) receptor activation causes anxiogenic effects while the blockade of this receptor has anxiolytic effects. This review focuses on the molecular mechanisms of CCK(2) receptors underlying anxiety-related behaviors of PVG hooded and Spraque-Dawley (SD) rats in two anxiety models (elevated plus-maze [EPM] and cat exposure test). PVG hooded rats showed prolonged freezing behavior in the cat exposure test while SD rats showed very low levels of freezing. A CCK(2) receptor antagonist (LY225910) attenuated freezing behavior in PVG hooded rats while a CCK(2) receptor agonist (CCK-4) increased freezing behavior in SD rats. In contrast, the two strains behaved similarly on the EPM. CCK-4 caused a pronounced anxiogenic effect in PVG hooded rats but only a slight effect in SD rats. CCK(2) antagonists also showed more pronounced anxiolytic effects in PVG hooded rats than in SD rats. CCK(2) receptor expression was greater in PVG hooded than in SD rats in the cortex and hippocampus. Genetic studies also demonstrated four differences in the DNA sequence of the CCK(2) receptor gene between the two rat strains.

Further Studies on the Interaction of the 5-Hydroxytryptamine3 (5-HT3) Receptor with Arylpiperazine Ligands. Development of a New 5-HT3 Receptor Ligand Showing Potent Acetylcholinesterase Inhibitory Properties

Novel arylpiperazine derivatives bearing lipophilic probes were designed, synthesized, and evaluated for their potential ability to interact with the 5-hydroxytryptamine(3) (5-HT(3)) receptor. Most of the new compounds show subnanomolar 5-HT(3) receptor affinity. Ester 6bc showing a picomolar K(i) value is one of the most potent 5-HT(3) receptor ligands so far synthesized. The structure-affinity relationship study suggests the existence of a certain degree of conformational freedom of the amino acid residues interacting with the substituents in positions 3 and 4 of the quipazine quinoline nucleus. Thus, the tacrine-related heterobivalent ligand 6o was designed in an attempt to capitalize on the evidence of such a steric tolerance. Compound 6o shows a nanomolar potency for both the 5-HT(3) receptor and the human AChE and represents the first example of a rationally designed high-affinity 5-HT(3) receptor ligand showing nanomolar AChE inhibitory activity. Finally, the computational analysis performed on compound 6o allowed the rationalization of the structure-energy determinants for AChE versus BuChE selectivity and revealed the existence of a subsite at the boundary of the 5-HT(3) receptor extracellular domain, which could represent a "peripheral" site similar to that evidenced in the AChE gorge.

Calcium influx through presynaptic 5-HT3 receptors facilitates GABA release in the hippocampus: in vitro slice and synaptosome studies

Serotonin 5-hydroxytryptamine type 3 receptors (5HT3R) are Ca2+-permeant, non-selective cation channels that have been localized to presynaptic terminals and demonstrated to modulate neurotransmitter release. In the present study the effect of 5-HT on GABA release in the hippocampus was characterized using both electrophysiological and biochemical techniques. 5-HT elicited a burst-like, 6- to 10-fold increase in the frequency of GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) measured with whole-cell voltage-clamp recordings of CA1 neurons in hippocampal slices. When tetrodotoxin was used to block action potential propagation, the 5-HT-induced burst of IPSCs was still observed. Stimulation of hippocampal synaptosomes with 5-HT resulted in a significant increase in the amount of [3H]GABA released by hyperosmotic saline. In both preparations, the 5-HT effect was shown to be mediated by 5HT3Rs, as it was mimicked by the selective 5HT3R agonist m-chlorophenyl biguanide and blocked by the selective 5HT3R antagonist 3-tropanylindole-3-carboxylate hydrochloride. The 5HT3R-mediated increase in GABA release was blocked by 100 microM cadmium or by omitting Ca2+ in external solutions, indicating the Ca2+-dependence of the effect. The high voltage-activated Ca2+ channel blockers omega-conotoxin GVIA and omega-conotoxin MVIIC and 10 microM cadmium had no significant effect on the 5-HT3R-mediated enhancement of GABA release, indicating that Ca2+ influx through the 5-HT3R facilitates GABA release. Taken together, these data provide direct evidence that Ca2+ entry via presynaptic 5HT3Rs facilitates the release of GABA from hippocampal interneurons.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Cannabinoid CB1 receptor and serotonin 3 receptor subunit A (5-HT3A) are co-expressed in GABA neurons in the rat telencephalon

Among all described serotonin (5-HT) receptors in mammals, the type three (5-HT3) is the only ligand-gated ion channel receptor for serotonin. By using double in situ hybridization histochemistry, we found co-expression of the functional 5-HT3A subunit of the 5-HT3 receptor and the central CB1 cannabinoid receptor in neurons of the rat telencephalon. Double-labeled 5-HT3A/CB1 neurons were found in the anterior olfactory nucleus, superficial and deep layers of the cortex, hippocampal formation (hippocampus, dentate gyrus, subiculum, and entorhinal cortex) and amygdala. Analysis of the proportion of neurons co-expressing 5-HT3A and CB1 receptors in the cortex and amygdala showed that, depending on the brain region, 37-53% of all neurons expressing the 5-HT3A subunit also expressed CB1 transcripts; 16-72% of the total population of neurons expressing CB1 mRNA co-expressed the 5-HT3A subunit. By using a combination of double in situ hybridization and immunohistochemistry, we demonstrated that 5-HT3A/CB1-expressing neurons contained the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). These results imply that in distinct regions of the telencephalon, GABA neurons that react to cannabinoids may also be responsive to serotonin through 5-HT3 receptors. Cellular coexistence of 5-HT3A and CB1 transcripts in interneurons of the cortex, hippocampal formation, and amygdala suggest possible interactions between the cannabinoid and serotonergic systems at the level of GABA neurotransmission in brain areas involved in cognition, memory, and emotion.

Localization of 5-HT2A, 5-HT3, 5-HT5A and 5-HT7 receptor-like immunoreactivity in the rat cerebellum

Although serotonin (5-hydroxytryptamine, 5-HT) is known to exert a modulatory action on cerebellar function, our current knowledge of the nature of receptor subtypes mediating serotonergic activity in this part of the brain remains fragmentary. In this study, we report the presence and distribution of 5-HT3, 5-HT5A and 5-HT7 receptor-like immunoreactivity in the rat cerebellum using immunofluorescence histochemistry. 5-HT3 immunoreactivity was found in fibers sparsely distributed throughout the cerebellum. Most of them were seen in the cerebellar cortex as fine varicose 5-HT3-positive axonal processes. 5-HT5A immunoreactivity, on the other hand, was observed in neuronal somata of the cerebellar cortex and deep cerebellar nuclei. Based upon cell morphology and the use of cell-specific markers, Purkinje cells, molecular layer interneurons and Golgi cells were found to be 5-HT5A immunopositive. In addition, the use of cell-specific markers allowed us to identify previously reported large 5-HT2A-positive cells in the granular layer as being Golgi cells. Finally, 5-HT7 immunoreactivity was observed only in Purkinje cells. Corroborating previous radioligand-binding, in situ hybridization and immunohistochemical studies, our data relate serotonin receptor subtypes to specific cerebellar cell types and may consequently contribute to the elucidation of serotonergic actions in the cerebellum.

Differential subcellular localization of the 5-HT3-As receptor subunit in the rat central nervous system

Following the cloning and sequencing of the A subunit of the 5-HT3 receptor, two alternatively spliced isoforms, 5-HT3-AS and 5-HT3-AL, have been identified. In order to analyse the distribution of the receptor, a polyclonal antibody has been produced against the short form which is the most abundant in the central nervous system [Doucet et al. (2000) Neuroscience 95, 881-892]. As expected from the recognition of functional 5-HT3 receptors, immunostaining by this anti-5-HT3-R-AS antibody matched the distribution of the high-affinity 5-HT3 binding sites in the rat brain and spinal cord. 5-HT3-AS-like immunoreactivity was detected at low levels in the limbic system, particularly in the amygdala and the hippocampus, and in the frontal, piriform and entorhinal cortices. High levels of immunoreactivity were found in the brainstem, mainly in the nucleus tractus solitarius and the nucleus of the spinal tract of the trigeminal nerve, and in the dorsal horn of the spinal cord. At the ultrastructural level, immunostaining was generally found associated with axons and nerve terminals (70-80%) except in the hippocampus, where labelled dendrites were more abundant (56%). This preferential localization on nerve endings is consistent with the well-documented physiological role of 5-HT3 receptors in the control of neurotransmitter release. However, the different distribution in the hippocampus raises the question of whether differential addressing mechanisms exist for preferentially targeting 5-HT3 receptors to postsynaptic dendritic sites as compared to presynaptic nerve endings, depending on the nature of the neurons bearing these receptors.

Effects of cholecystokinin tetrapeptide (CCK(4)) and anxiolytic drugs on the electrically evoked [(3)H]5-hydroxytryptamine outflow from rat cortical slices

The outflow of [(3)H]5-hydroxytryptamine ([(3)H]5-HT) from electrically stimulated rat cortical slices was measured to ascertain the modulatory role of endogenous cholecystokinin (CCK) on the amine outflow and to test the hypothesis that different anxiolytic compounds inhibit 5-HT secretion. The [(3)H]5-HT outflow evoked at 10 Hz was increased up to +30% by CCK(4) 300-1000 nM, the effect being prevented by the CCK(B) receptor antagonist GV 150013, 3 nM. The limited sensitivity to CCK(4) seemed to depend on 5-HT auto-receptor feedback because pre-treatment with 100 nM methiothepin enhanced the [(3)H]5-HT outflow and lowered the CCK(4) threshold concentration from 300 to 30 nM. In addition, pre-treatment with 1 microM bacitracin to inhibit CCK metabolism increased [(3)H]5-HT efflux. This effect was concentration-dependently counteracted by GV150013 suggesting the presence of an endogenous CCK positive modulation. GV150013 30 nM, the 5-HT(1A) partial agonist buspirone 300 nM and the GABA(A) receptor modulator diazepam 10 nM, known to have anxiolytic properties, all significantly reduced the [(3)H] amine outflow from cortical slices by about 20%. This inhibition depended on their interaction with their respective receptors, which seemed to restrain the activity of functionally interconnected glutamatergic interneurones. In fact, APV (50 microM) and NBQX (10 microM) prevented the effect of the anxiolytic compounds. Thus, anxiolytic drugs with different receptor targets can reduce 5-HT outflow by dampening the glutamatergic signal, and in turn, the secretory process of the serotonergic nerve ending.

Potentiation of 5-HT(3) receptor functions in the hippocampal CA1 region of rats following repeated electroconvulsive shock treatments

Modulation of serotonin (5-HT)(3)-receptor function by repeated electroconvulsive shock (ECS) treatment was investigated to elucidate the mechanism underlying the effectiveness of electroconvulsive therapy (ECT) which is clinically used for drug-resistant depression. ECS (100 V, 1 s) was applied once a day for 14 days via an electrode placed on the ears of rats (ECS group). For controls, rats were handled similar to the ECS-treated group except for the stimulation. Hippocampal slices (thickness, 450 microm) were prepared 24 h after the final ECS treatment when rats were 5-6 weeks old. Intracellular recordings were made from the neurons in the CA1 pyramidal cell layer. Activation of 5-HT(3) receptors increased spontaneous postsynaptic potentials (sPSP). Increased sPSP was mainly mediated by GABA. The 5-HT(3) receptor mediated increase in sPSP was potentiated in the ECS group compared with the control group. These findings suggest that the 5-HT(3) receptor function is potentiated by repeated ECS.

Involvement of cholecystokinin within craving for cocaine: role of cholecystokinin receptor ligands

In the brain, cholecystokinin (CCK) has been described to act as a central neurotransmitter or neuromodulator involved in functions such as food consumption, stress and anxiety. Recently, the CCK system has been involved in drug dependence phenomena and proposed to be correlated to a putative state of 'drug preferring' phenotype within free choice tests. CCK exerts its action in the CNS through at least two different G-protein coupled high affinity receptors, CCK1 and CCK2. Various selective CCK receptor agonists and antagonists have been synthesised. In particular, L-364,718 has been demonstrated to be a potent and selective CCK1 receptor antagonist, whereas L-365,260 is a potent and selective CCK2 receptor antagonist. More recently, GV150013 has been reported to be a highly selective CCK2 receptor antagonist. This paper reviews the putative role of the CCK system within drug dependence phenomena. In particular, it analyses the relationship between central CCK activity and the exhibition of spontaneous preference for drugs of abuse, such as cocaine or alcohol. The potential therapeutic role for CCK receptor antagonists is also discussed.

Immunolabeling of the rat central nervous system with antibodies partially selective of the short form of the 5-HT3 receptor

Polyclonal antibodies were raised against a synthetic hexadecapeptide corresponding to the portion of the second intracytoplasmic loop of the short form of the mouse 5-hydroxytryptamine-3A receptor subunit (5-HT3A-S), which differs from the long form (5-HT3A-L) by the removal of six amino acids. Antibodies were detected by enzyme-linked immunosorbent assay as soon as two months after the first injection to rabbits of the peptide coupled to keyhole limpet hemocyanin. Immunoblot detection of fusion proteins comprising glutathione-S-transferase and the second intracellular loop of 5-HT3A-S or 5-HT3A-L, and immunoprecipitation of cloned receptors showed that antibodies exhibited some selectivity for the short variant. Affinity chromatography allowed the purification of selective anti-5-HT3A-S antibodies which yielded a strong positive labeling of plasma membrane, reticulum and Golgi apparatus of COS-7 cells expressing murine 5-HT3A-S. In contrast, COS-7 cells expressing similar levels of 5-HT3A-L exhibited only a very weak labeling. Selectivity was also observed on immunoblots of cloned receptors transiently expressed in COS-7 cells, or stably expressed in CHO cells, both systems showing an immunolabeled component at 53,000-54,000 mol. wt. Immunoautoradiographic labeling of central nervous system sections showed that 5-HT3A-S-like immunoreactivity was found mostly within the nucleus of the solitary tract, the nucleus of the spinal tract of the trigeminal nerve, and the dorsal horn of the the spinal cord in the rat. After unilateral ablation of the nodose ganglion, 5-HT3A-S-like immunoreactivity decreased markedly in the ipsilateral part of the nucleus of the solitary tract, as expected of the presynaptic localization of 5-HT3 receptors. Finally, immunohistochemistry at the light and electron microscope levels revealed that 5-HT3A-S-like immunoreactivity was associated essentially with terminals and axonal profiles. All these results demonstrate that the immunolabeling exhibited by these antibodies is consistent with a specific and partially selective recognition of the short isoform of the 5-HT3A subunit. Because the pattern of immunoautoradiographic labeling matches the distribution previously established with selective radioligands, it can be inferred that these antibodies probably recognized the same fully assembled form of the 5-HT3A-S receptor subunit.

Cholecystokinin/opioid interactions

Cholecystokinin (CCK) acts as an anti-opioid peptide. The mechanisms of CCK-opioid interaction under normal and pathological conditions were examined with various techniques. Nerve injury induces upregulation of CCK mRNA and CCK2 receptors in sensory neurons. The involvement of CCK in spinal nociception in normal and axotomized rats was examined. The CCK2 receptor antagonist CI-988 did not reduce spinal hyperexcitability following repetitive C-fiber stimulation in normal or axotomized rats, suggesting that CCK is probably not released from injured primary afferents. With in vivo microdialysis intravenous (i.v.) or intrathecal (i.t.) morphine increased the extracellular level of CCK in the dorsal horn in a naloxone reversible manner. Morphine also released CCK after axotomy, but not during carrageenan-induced inflammation. In contrast, K(+)-stimulation failed to increase extracellular levels of CCK in axotomized rats, but did so in inflamed rats. Double-coloured immunofluorescence technique revealed partial co-localization between CCK-like immunoreactivity (LI) and mu-opioid receptor (MOR)-LI in superficial dorsal horn neurons. The presence of MOR in CCK containing neurons suggests a possible direct influence of opioids on CCK release in the spinal cord. Axotomy, but not inflammation, induced a moderate decrease in CCK- and MOR-LI in the dorsal horn. I.v. morphine further temporarily reduced CCK- and MOR-LIs in axotomized, but not in normal or inflamed, rats. While the effect of morphine on CCK-LI can be interpreted as the result of increased CCK release, the effect on MOR-LI may be related to changes in the microenvironment of the dorsal horn induced by nerve injury.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Calcium changes induced by presynaptic 5-hydroxytryptamine-3 serotonin receptors on isolated terminals from various regions of the rat brain

The serotonin 5-hydroxytryptamine-3 receptor is a ligand-gated ion channel that is distributed widely in the nervous system. Within the CNS, a significant portion of the 5-hydroxytryptamine-3 receptors appears to be present on presynaptic nerve terminals and, using an imaging approach, it was shown previously that presynaptic 5-hydroxytryptamine-3 receptors on individual isolated nerve terminals (synaptosomes) from rat corpus striatum display a distinctive set of properties-slow onset, little desensitization and high apparent permeability for Ca2+-when compared to those observed for 5-hydroxytryptamine-3 receptors localized at postsynaptic sites on neuronal cell bodies. To consider whether their characteristic nature is a common feature of presynaptic 5-hydroxytryptamine-3 receptors across the brain, we used confocal microscopy to measure changes in intracellular Ca2+ concentration resulting from 5-hydroxytryptamine-3 agonist-induced responses in synaptosomes from representative rat brain regions, ranging in expression of overall levels of 5-hydroxytryptamine-3 receptors from relatively low (cerebellum) to intermediate (corpus striatum and hippocampus) to high (amygdala). Application of 100 nM m-chlorophenyl biguanide, a specific 5-hydroxytryptamine-3 receptor agonist, induced changes in relative intracellular Ca2+ concentration in subsets of synaptosomes from the corpus striatum (approximately 6% of total), hippocampus (approximately 3% of total), amygdala (approximately 30% of total) and cerebellum (approximately 32% of total). In order to assure the viability of the synaptosomes that did not respond to 5-hydroxytryptamine-3 agonist stimulation, KCl (45 mM) was subsequently added to depolarize the same population of synaptosomes, and increases in intracellular Ca2+ concentration were then seen in 80-90% of the synaptosomes from all four regions. The kinetics of the intra synaptosomal Ca2+ changes produced by K+-evoked depolarization were similar in all regions, showing a rapid rise to a peak followed by an apparent plateau phase. In contrast, the changes in intracellular Ca2+ concentration evoked by m-chlorophenyl biguanide displayed substantially slower kinetics, similar to previous findings, but which varied among responding synaptosomes from one region to another. In particular, m-chlorophenyl biguanide-induced changes were notably slower in synaptosomes from the amygdala (rise time constant, tau = 25 s), when compared to responses in synaptosomes from other regions (striatum, tau = 12 s; hippocampus, tau= 9.6 s; cerebellum, tau = 7 s). To independently demonstrate the presence of 5-hydroxytryptamine-3 receptors on nerve terminals in the various regions using a molecular approach, we double-immunostained the synaptosomes for the 5-hydroxytryptamine-3 receptor and the synaptic vesicle protein synaptophysin, using, respectively, a polyclonal antibody raised against an N-terminal peptide of the 5-hydroxytryptamine-3 receptor and a monoclonal anti-synaptophysin antibody, and observed 5-hydroxytryptamine-3 receptors in varying subsets of the synaptosomes from each region, providing direct support for the results obtained in our functional experiments. These results suggest that the distinctive properties of presynaptic 5-hydroxytryptamine-3 receptors are found throughout the brain, with evident differences in the kinetics of the responses to agonist stimulation observed across the brain regions studied. As expected, the proportion of the synaptosomal population that responded on application of 5-hydroxytryptamine-3 agonist varied in preparations from one region to another; however, the presence of a relatively high proportion of presynaptic 5-hydroxytryptamine-3 receptors in the cerebellum contrasts with previous binding studies demonstrating a relatively low overall density of 5-hydroxytryptamine-3 receptors in this region. We hypothesize that presynaptic 5-hydroxytryptamine-3 receptors present on nerve terminals regulate the

Novel potent and selective central 5-HT3 receptor ligands provided with different intrinsic efficacy. 2. Molecular basis of the intrinsic efficacy of arylpiperazine derivatives at the central 5-HT3 receptors

Novel 5-HT3 receptor ligands were designed and synthesized with the aim of obtaining deeper insight into the molecular basis of the intrinsic efficacy of arylpiperazines interacting with the central 5-HT3 receptor. The newly synthesized compounds and some previously published compounds belonging to the same class of heteroarylpiperazines were tested for their potential ability to displace [3H]granisetron from rat cortical membranes. These 5-HT3 receptor binding studies revealed subnanomolar affinity in several of the compounds under study. The most active ligands were quipazine derivatives bearing a phenyl group in the 4-position and various oxygenated alkyl side chains in the 3-position of the quinoline nucleus. Qualitative and theoretical quantitative structure-affinity relationship studies were carried out, and the interaction model for the 5-HT3 ligands related to quipazine with their receptor, proposed in part 1 of the present work, was updated to incorporate the latest data. The potential 5-HT3 agonist/antagonist activity of 12 selected compounds was assessed in vitro on the 5-HT3 receptor-dependent [14C]guanidinium uptake in NG 108-15 cells. Their intrinsic efficacy ranged from the 5-HT3 full agonist properties of compounds 7a and 8h, i to those of partial agonists 10a,d and antagonists 8b,d,e, and 9c, d,h,i. The comparison between these functional data and those relative to the previously described compounds suggested that in this class of 5-HT3 ligands the intrinsic efficacy is modulated in a rather subtle manner by the steric features of the heteroaryl moiety.

Antibodies against the extracellular domain of the 5-HT3 receptor label both native and recombinant receptors

We have developed polyclonal antibodies (pAb120) against a peptide corresponding to a region within the extracellular domain of the 5-hydroxytryptamine3 (5-HT3) receptor subunit, thus permitting, for the first time, localization of 5-HT3 receptors at the cell surface in intact (non-permeabilized) systems. The antibodies are both specific and sensitive: pAb120 recognized as little as 63 ng of protein from HEK293 cells expressing recombinant 5-HT3 receptors, whilst Western blots of recombinant 5-HT3 receptors purified from Sf9 cells revealed two bands at 48 and 54 kDa, and native 5-HT3 receptors from N1E-115 cell membranes produced a broad band at 50-54 kDa with a smaller band at 35 kDa. These bands were also labelled by antibodies against the intracellular loop of the 5-HT3 receptor. Immunofluorescent labelling revealed a ring of intense fluorescence in the plasma membrane of non-permeabilized HEK293 cells expressing recombinant 5-HT3 receptors. Studies on native 5-HT3 receptors revealed that pAb120 could recognize 5-HT3 receptors on presynaptic terminals isolated from rat striatum, and immunohistochemical studies in rat brain sections revealed labelling of cell bodies, dendrites and varicose axons in hippocampus, cortex and lateral hypothalamus; all of these areas have been reported to express 5-HT3 receptors. We conclude that pAb120 is a highly specific and sensitive antiserum that will assist in clarifying fundamental questions about 5-HT3 receptor neurobiology.

Effects of the CCK(B) antagonist CI-988 on responses to mCPP in generalized anxiety disorder

In order to evaluate the effect of the CCK(B) antagonist CI-988 on behavioral, neuroendocrine, and physiologic responses to the mixed, post-synaptic serotonin (5-HT) agonist/antagonist mCPP, 16 patients with a principal DSM-III-R diagnosis of generalized anxiety disorder (GAD) were enrolled in a study that involved two challenge tests. On one day, patients received a single oral dose of CI-988 followed 30 min later by an i.v. infusion of 0.1 mg/kg mCPP. On a second test day patients received placebo CI-988 followed 30 min later by active i.v. mCPP. The sequence of CI-988 was randomly assigned and the testing was conducted in double-blind fashion. In an initial dose-finding phase (N = 6) with a dose of CI-988 of 25 mg, there were no significant between-test differences in behavioral response to mCPP. Accordingly, the second phase of the study was conducted with a CI-988 dose of 100 mg in another of patients (N = 10). CI-988 (100 mg) was well tolerated and had no significant effects on pretest anticipatory anxiety. There was no significant blunting of the anxiety response to mCPP as a result of CI-988 administration, nor did CI-988 affect physiologic or neuroendocrine measures. Correlations between peak changes in plasma levels of CI-988 and mCPP-induced anxiety in the high-dose patient group were not significant. Overall, these findings did not provide evidence of anxiolytic effects of CI-988 in patients with GAD. The lack of effect of CI-988 on neuroendocrine and physiological measures further suggests that CI-988's pharmacological effects could be independent of 5-HT function. However, follow-up studies using higher doses of CI-988 are indicated to confirm this preliminary finding as are studies more closely evaluating the interrelationship between CCK and 5-HT function in GAD.

Ceruletide inhibits phencyclidine-induced dopamine and serotonin release in rat prefrontal cortex

Phencyclidine (PCP; 5.0 mg/kg, i.p.) produced a greater increase in extracellular dopamine (DA) levels in the prefrontal cortex than in the striatum, while PCP increased the extracellular 5-hydroxytryptamine (serotonin; 5-HT) levels in the prefrontal cortex but not the striatum, as determined by in vivo microdialysis in awake, freely moving rats. The cholecystokinin (CCK)-related decapeptide ceruletide (120 and 400 microg/kg, i.p.), administered 60 min prior to PCP, significantly attenuated the PCP-induced increase in the extracellular levels of DA and 5-HT in the prefrontal cortex, but not in the striatum. These effects were reversed by PD 135,158, a selective CCK-B receptor antagonist (0.1 mg/kg, s.c.), administered 5 min prior to ceruletide. When administered alone, ceruletide (400 microg/kg, i.p.) significantly increased basal extracellular DA levels only in the prefrontal cortex. The selective N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (0.5 mg/kg, i.p.) also increased extracellular DA levels in the prefrontal cortex, but this effect was unaffected by ceruletide pretreatment. These results suggest that ceruletide may differentially modulate basal and PCP-induced release of DA and 5-HT in the prefrontal cortex.

Effect of oral ondansetron on total cholecystokinin plasma levels following CCK-4 panic challenge procedure in healthy men

OBJECTIVE

To gain insight into whether ondansetron treatment induces changes in total cholecystokinin (CCKT) plasma levels before and after administration of the cholecystokinin tetrapeptide (CCK-4) panic challenge procedure in healthy men.

METHODS

Thirty-eight volunteers received a 50-microgram bolus of CCK-4 60 minutes after a single oral dose (acute treatment) and multiple oral doses (chronic treatment) of ondansetron or placebo.

RESULTS

Results showed no difference in CCKT plasma levels of CCKT elimination rate constant between the ondansetron and the placebo groups after either acute or chronic treatment.

CONCLUSION

Results from this study suggest that total CCK plasma levels are not influenced by either acute or chronic treatment with ondansetron. However, the effect of ondansetron on the different CCK component fractions still needs exploration.

Evidence for functional native NMDA receptors activated by glycine or D-serine alone in the absence of glutamatergic coagonist

In this study we have examined the effects of N-methyl-D-aspartate (NMDA) receptor activation on the release of cholecystokinin and somatostatin from rat neocortical nerve endings. The release of cholecystokinin-like immunoreactivity (CCK-LI) and of somatostatin-like immunoreactivity (SRIF-LI) elicited by 12 mM K+ from superfused synaptosomes, but not the spontaneous release, was increased by NMDA in a concentration-dependent manner. The effects of NMDA could be prevented by antagonists selective for the glutamate recognition site, the receptor channel and the glycine site of the NMDA receptor. In the absence of NMDA, glycine increased on its own and in a concentration-dependent manner the depolarization-evoked release of both CCK-LI and SRIF-LI. This effect of glycine was strychnine-insensitive and could be mimicked by D-serine, a stereoselective agonist at the NMDA receptor glycine site. Antagonists selective for the glycine site or for the NMDA receptor channel prevented the effects of glycine/D-serine; these effects were, however, insensitive to blockade of the glutamate recognition site of the NMDA receptor, suggesting that glutamate released from synaptosomes or present as contaminant was not involved. The neuropeptide release elicited by D-serine was strongly inhibited by ifenprodil (0.3 microM) and by Zn2+ ions (50 nM), selective ligands at the NR2B and NR2A subunits of NMDA receptors, respectively. It is concluded that nerve terminals of CCK- and SRIF-releasing neurons possess non-conventional NMDA receptors whose channels can be operated by glycine or D-serine without apparent activation of the glutamatergic coagonist site. These receptors may display the triple subunit combination NR1/NR2A/NR2B.

Novel potent and selective central 5-HT3 receptor ligands provided with different intrinsic efficacy. 1. Mapping the central 5-HT3 receptor binding site by arylpiperazine derivatives

Synthesis and pharmacological evaluation of a series of condensed quinoline and pyridine derivatives bearing a N-methylpiperazine moiety attached to the 2-position of the quinoline or pyridine nucleus are described. 5-HT receptor binding studies revealed subnanomolar affinity for the 5-HT3 receptor subtype in some of the compounds under study. The most active compound (5b) displayed a Ki value about 1 order of magnitude higher than that of quipazine along with a higher selectivity. The potential 5-HT3 agonist/antagonist activity of four selected compounds was assessed in vitro on 5-HT3 receptor-dependent [14C]guanidinium uptake in NG 108-15 cells. Compound 5j acted as a 5-HT3 agonist in this assay with an EC50 value close to that reported for quipazine, while 5b was a partial agonist with an EC50 value of about 0.25 nM, and compound 5c possessed antagonist properties with an IC50 value (approximately 8 nM) in the same range as those of previously characterized 5-HT3 receptor antagonists. Qualitative and quantitative structure-affinity relationship studies carried out by making use of theoretical molecular descriptors allowed to elucidate the role of the main pharmacophoric components and to develop a model for the interaction of the 5-HT3 ligands related to quipazine with their receptor.

Neurobiology of panic disorder

Various provocative agents, including sodium lactate, carbon dioxide (CO2), caffeine, yohimbine, serotoninergic agents, and cholecystokinin (CCK), have been utilized as panicogenics in studies on healthy volunteers as well as in panic disorder patients. An overview of the utilization of these agents to study the neurobiology of panic disorder is presented. The possible roles of several neurotransmitters and neuromodulators in the etiology of panic disorder and in the actions of drugs used in its treatment are also discussed.

Spontaneous preference for ethanol in naive rats is influenced by cholecystokinin A receptor antagonism

Naive adult male Wistar rats free to choose between water or 10% ethanol (v/v) spontaneously became water-preferring (WP) rats, as they drank mainly water (approximately 35 ml per day), or alcohol-drinking (ED) rats, as they also drank a significant amount of ethanol (approximately 14 ml per day). The selective CCKA receptor antagonist L-364,718 at doses selective for the CCKA receptor (5 micrograms/kg, IP) halved the consumption of alcohol of the ED rats without modifying their total liquid in-take. In contrast, the CCKB antagonists L-365,260 or GV150013 were without effect when used at doses selective for the CCKB receptor. These data indicate that the CCK system could be involved in the modulation of alcohol intake. In particular, they suggest that CCKA receptors could play a role in the ethanol preference.

The 5-HT3 receptor is present in different subpopulations of GABAergic neurons in the rat telencephalon

The type 3 serotonin receptor (5-HT3R) is a ligand-gated ion channel whose presence in the CNS has been established by radioligand binding, in situ hybridization, and immunohistochemical analysis. To analyze further the role of the 5-HT3R in the CNS, we used in situ hybridization and immunocytochemistry to determine that 5-HT3R-expressing neurons are mainly GABA-containing cells in the rat telencephalon. We determined that 5-HT3R/GABA-containing neurons do not exhibit somatostatin immunoreactivity but often contain cholecystokinin (CCK) immunoreactivity. 5-HT3R-expressing cells with CCK immunoreactivity were observed in the neocortex, olfactory cortex, hippocampus, and amygdala. The 5-HT3R/CCK interneurons represent between 35 and 66% of the total population of CCK-containing cells in the neocortex. Further characterization of the 5-HT3R/GABAergic neurons was based on their calcium-binding protein immunoreactivity and showed that these neurons lack parvalbumin (PV) and represent a subpopulation of calbindin (CB)-containing interneurons that were preferentially present in the CA1-CA3 subfield of the hippocampus. Although some 5-HT3R/GABAergic neurons with calretinin (CR) were found in the neocortex, olfactory cortex, hippocampus, and amygdala, these neurons were more often present in the agranular insular and piriform cortices. We conclude that the neuronal expression of the 5-HT3R is selective within the GABA neuron population in the rat telencephalon. These 5-HT3R-expressing interneurons might contain CCK, CB, and CR. We suggest that serotonin through the 5-HT3R may regulate GABA and CCK neurotransmission in the telencephalon.

The 5-HT3 receptor is present in different subpopulations of GABAergic neurons in the rat telencephalon

The type 3 serotonin receptor (5-HT3R) is a ligand-gated ion channel whose presence in the CNS has been established by radioligand binding, in situ hybridization, and immunohistochemical analysis. To analyze further the role of the 5-HT3R in the CNS, we used in situ hybridization and immunocytochemistry to determine that 5-HT3R-expressing neurons are mainly GABA-containing cells in the rat telencephalon. We determined that 5-HT3R/GABA-containing neurons do not exhibit somatostatin immunoreactivity but often contain cholecystokinin (CCK) immunoreactivity. 5-HT3R-expressing cells with CCK immunoreactivity were observed in the neocortex, olfactory cortex, hippocampus, and amygdala. The 5-HT3R/CCK interneurons represent between 35 and 66% of the total population of CCK-containing cells in the neocortex. Further characterization of the 5-HT3R/GABAergic neurons was based on their calcium-binding protein immunoreactivity and showed that these neurons lack parvalbumin (PV) and represent a subpopulation of calbindin (CB)-containing interneurons that were preferentially present in the CA1-CA3 subfield of the hippocampus. Although some 5-HT3R/GABAergic neurons with calretinin (CR) were found in the neocortex, olfactory cortex, hippocampus, and amygdala, these neurons were more often present in the agranular insular and piriform cortices. We conclude that the neuronal expression of the 5-HT3R is selective within the GABA neuron population in the rat telencephalon. These 5-HT3R-expressing interneurons might contain CCK, CB, and CR. We suggest that serotonin through the 5-HT3R may regulate GABA and CCK neurotransmission in the telencephalon.

The non-antiemetic uses of serotonin 5-HT3 receptor antagonists. Clinical pharmacology and therapeutic applications

The discovery of multiple subtypes of the serotonin 5-HT receptor has generated enormous interest over the past few years. Possibly the most exciting, in terms of psychiatric clinical practice, appeared to be the 5-HT3 receptor. Early animal studies suggested that the 5-HT3 receptor antagonists, in addition to their well recognised antiemetic use, might be clinically useful in a number of areas. These included anxiety disorders, psychotic disorders, drug and alcohol abuse disorders, depressive disorders, cognitive disorders, the treatment of pain and the treatment of irritable bowel syndrome. With the exception of antiemetic actions, this review examines these potential therapeutic areas carefully, paying particular attention not only to the animal literature, but to the clinical studies which have resulted from these initial findings. Unfortunately, studies in many of these therapeutic areas have not lived up to their initial promise. Indeed, no clinical studies have yet clearly demonstrated the usefulness of 5-HT3 receptor antagonists in the treatment of CNS disorders. Nonetheless, in view of the absence of published results from double-blind, placebo-controlled studies in many of these therapeutic areas, further research would be useful in confirming the effectiveness, or otherwise, of this group of compounds.

The role of the 'central' cholecystokinin-B receptor in panic disorder

Research investigating the neurobiological underpinnings of anxiety disorder have mainly been focused on dysfunction of the GABA, noradrenergic and serotonergic (5-HT) neuronal systems. Just recently, in both animal and human studies, evidence has been found for a possible role of the cholecystokinin (CCK) neuronal system in the pathogenesis of anxiety disorder. Behaviorally, animal studies revealed anxiogenic-like properties for the 'central' CCKB receptor agonists, while CCKB receptor antagonists displayed intrinsic anxiolytic properties. Similarly, in man, CCKB receptor agonists, like pentagastrin and CCK4, were found to be able to elicited panic attacks in both panic disorder (PD) patients and healthy volunteers. These effects appear due to stimulation of the CCKB receptor. In addition, clinically effective panicolytic agents reduce the sensitivity to CCK4 in PD patients. Taken together, these findings may suggest a role for CCK in the neurobiology of PD. On the other hand, there is circumstantial evidence for involvement of several other neuronal systems, such as the serotonergic, noradrenergic and GABA-ergic system, in the regulation of anxiety. Interestingly, evidence has been found for an interaction between CCK and 5-HT and that this interaction plays a role in the mediation of anxiety. This presentation will critically discuss the evidence for the role of the CCKB receptor in anxiety and in addition, will focus on the putative evidence that the role of CCK in anxiety is mediated by its interaction with the serotonergic neuronal system.

Cholecystokinin in anxiety

Cholecystokinin (CCK) plays an important role in both the alimentary tract and the central nervous system (CNS). At present it seems to be the most abundant neuropeptide in the CNS. This paper reviews the CCK neuronal system and its interactions with gamma-aminobutyric acid (GABA) and serotonin (5-hydroxytryptamine; 5-HT). In addition, its putative role in anxiety will be discussed on the basis of animal data and studies in healthy volunteers and panic disorder patients. According to these investigations, the CCK4 challenge test fulfills most criteria for an ideal panicogenic agent and evidence has been found that CCKB receptor antagonists might possess anxiolytic properties in man.

Methylenedioxymethamphetamine-induced inhibition of neuronal firing in the nucleus accumbens is mediated by both serotonin and dopamine

Methylenedioxymethamphetamine (MDMA) is a mood-altering, legally restricted drug that has been reported to inhibit glutamate-evoked firing of cells in the nucleus accumbens. This study used extracellular recording combined with microiontophoresis to examine whether the inhibitory effect of MDMA on neuronal firing in the nucleus accumbens is mediated by serotonin and/or dopamine. Serotonin and serotonin agonists with relative selectivity for the receptor subtypes 5-HT1A, 5-HT1B, 5-HT2A/2C and 5-HT3 all significantly (P < 0.01) inhibited glutamate-evoked firing of cells in the nucleus accumbens compared to the effects of an acidic saline control solution (30-60 nA, 60 s ejection currents for all). The current (dose)-dependent inhibition produced by the serotonin agonists did not differ significantly from the inhibition produced by MDMA except for the 5-HT1A agonist 8-hydroxy-(2-di-n-propylamino) tetralin, which inhibited glutamate-evoked firing significantly more than MDMA or any of the other serotonin agonists. At the highest ejection current tested (60 nA, 60 s), glutamate-evoked firing was inhibited by MDMA in 94% of tested cells, by serotonin in 80% of tested cells and by the serotonin receptor subtype agonists in 95-100% of the tested cells. In addition to being mimicked by serotonin and serotonin agonists, MDMA-induced inhibition of glutamate-evoked firing in the nucleus accumbens was partially blocked by the serotonin antagonists ketanserin (100% of tested cells), methysergide (80% of tested cells), methiothepin (100% of tested cells) and WAY100135 (100% of tested cells). Furthermore, application of the serotonin uptake blocker fluoxetine, which prevents MDMA-induced serotonin release, also significantly attenuated MDMA-induced inhibition of glutamate-evoked firing in all of the cells that were tested. These observations suggest that MDMA-induced inhibition of nucleus accumbens cell firing is at least partially mediated by serotonin. Depletion of dopamine by pretreatment with the neurotoxin 6-hydroxydopamine and the synthesis inhibitor alpha-methyl-p-tyrosine blocked the inhibition of glutamate-evoked firing produced by MDMA applied with low ejection currents (30-40 nA, 60 s). However, this dopamine depletion had no effect on inhibition of glutamate-evoked firing produced by serotonin ejected with low or high currents (20-60 nA, 60 s). These results suggest that both dopamine release and an intermediate step of MDMA-induced serotonin release are necessary for the inhibitory effects of MDMA on neuronal excitability in the nucleus accumbens. The dopamine- and serotonin-mediated inhibitory effects of MDMA on glutamate-evoked firing of nucleus accumbens cells may play a role in the mood-altering properties of this increasingly popular drug.

The type 3 serotonin receptor is expressed in a subpopulation of GABAergic neurons in the rat neocortex and hippocampus

We used in situ hybridization and immunocytochemistry to investigate the presence of GABA in neurons that express the type 3 serotonin receptor (5-HT3R). Quantitative analysis indicated that more than 90% of 5-HT3R expressing cells are GABAergic in the neocortex and hippocampus. The co-existence of 5-HT3R and GABA in cortical and hippocampal neurons indicates that serotonin, via 5-HT3R, can affect GABA release and suggests the participation of 5-HT3R in the inhibitory regulation of forebrain neurons.

c-fos expression in specific rat brain nuclei after intestinal anaphylaxis: involvement of 5-HT3 receptors and vagal afferent fibers

The c-fos immediate-early gene is acutely induced in brain after various stimuli from the digestive tract. 5-HT3 receptors and vagal afferents have been found involved in intestinal motor disturbances induced by intestinal anaphylaxis. Our aim was to determine whether intestinal anaphylaxis activates brain structures, using c-fos expression, and to evaluate the modulation of c-fos induction by 5-HT3 receptors and vagal afferents. The effects of antigen challenge on intestinal motility were evaluated in ovalbumin-sensitized Hooded Lister rats chronically fitted with NiCr electrodes in the jejunal wall. Intestinal motility was assessed in conscious rats pretreated or not by perivagal capsaicin or a 5-HT3 antagonist (ondansetron). In sensitized rats, ovalbumin disrupted for 62.4 +/- 9.5 min the jejunal migrating motor complexes (MMC) and an important c-fos expression was detected in the nucleus tractus solitarius (NTS), lateral parabrachial nucleus (LPB) and paraventricular nucleus of the hypothalamus (PVN). Intraperitoneal administration of ondansetron or perivagal capsaicin treatment significantly reduced the duration of MMC disruption and attenuated markedly c-fos staining in the 3 brain sites. In contrast, intracerebroventricular administration of ondansetron significantly reduced jejunal motor alterations but did not diminish the c-fos expression, suggesting a role of central 5-HT3 receptors in the efferent control of the intestinal disturbances. Blockade of both c-fos expression and MMC disruption by systemic ondansetron and by perivagal capsaicin indicates that some brainstem nuclei are involved in digestive disturbances after intestinal anaphylaxis, and reflects an involvement of peripheral 5-HT3 receptors on vagal afferents. The reduction of c-fos staining in NTS as well as in LPB and PVN after perivagal capsaicin suggests that the NTS is the primary relay in the activation of the central nervous system during intestinal allergic challenge.

Pre- and post-synaptic effects of the 5-HT3 agonist 2-methyl-5-HT on the 5-HT system in the rat brain

Microiontophoretic applications of 5-HT and of the 5-HT3 agonist 2-methyl-5-HT produced a current-dependent suppression of firing activity of both hippocampal (CA1 and CA3) and cortical neurons in anesthetized rats. Concomitant microiontophoretic applications of the 5-HT3 antagonists BRL 46470A and S-zacopride, as well as their intravenous injection, did not antagonize the inhibitory effect of 5-HT and 2-methyl-5-HT. In contrast, the 5-HT1A antagonist BMY 7378, applied by microiontophoresis or administered intravenously, significantly reduced the inhibitory action of 5-HT and 2-methyl-5-HT. The firing activity of dorsal raphe 5-HT neurons was also reduced by 5-HT, 2-methyl-5-HT and the 5-HT1A agonist 8-OH-DPAT applied by microiontophoresis. While BRL 46470A (0.1 and 1 mg/kg, i.v.) did not antagonize the inhibitory effect of the three 5-HT agonists on 5-HT neuronal firing activity, only that of 8-OH-DPAT was attenuated by the 5-HT1A antagonist (+) WAY 100135. R-zacopride significantly reduced the duration of suppression of firing activity of CA3 pyramidal neurons induced by the electrical stimulation of the ascending 5-HT pathway, and this reducing effect was prevented by the three 5-HT3/5-HT4 antagonists renzapride, S-zacopride and tropisetron, but not by BRL 46470A. Finally, in in vitro superfusion experiments, both BRL 46470A and S-zacopride antagonized the enhancing action of 2-methyl-5-HT on the electrically-evoked release of [3H]-5-HT in both rat frontal cortex and hippocampus slices. These findings suggest that, in vivo, the suppressant effect of 2-methyl-5-HT on the firing activity of dorsal hippocampus pyramidal, somatosensory cortical, and dorsal raphe 5-HT neurons is not mediated by 5-HT3 receptors, but rather by 5-HT1A receptors. The attenuating effect of R-zacopride on the effectiveness of the stimulation of the ascending 5-HT pathway is not mediated by 5-HT3 receptors. In contrast, in vitro, the enhancing action of 2-methyl-5-HT on the electrically-evoked release of [3H]5-HT in both frontal cortex and hippocampus slices is mediated by 5-HT3 receptors.

Evidence for a high-affinity uptake system for cholecystokinin octapeptide (CCK8) in rat cortical synaptosomes

Given the high resistance of the cholecystokinin octapeptide (CCK8) to in vivo peptidase degradation, the possible existence of a reuptake system for this peptide was investigated. Efficient accumulation of intact, tritiated propionyl CCK8 ([3H]pCCK8) was observed following its incubation with rat cortical synaptosomes but not with cerebellar synaptosomes, where no cholecystokinin immunoreactivity was found. This uptake process appeared to be dependent on temperature, duration of incubation, concentration of radioligand, the presence of glucose and the integrity of the synaptosomes. A Lineweaver-Burk analysis indicated that the putative uptake process is characterized by a single Km value of 10.7 nM and a Vmax of 8.5 fmol/min/mg of protein. Carbonyl cyanide-m-chlorophenyl hydrazone, an uncoupler of oxidative phosphorylation, blocked accumulation of [3H]pCCK8, whereas ouabain did not. The uptake was found to be highly specific since, among all the cholecystokinin analogues tested, only CCK8 and, to a lesser extent, CCK7, were able to inhibit [3H]pCCK8 uptake. The rate of [3H]pCCK8 uptake was not affected by CCK4, CCK5, D-Trp CCK8, BC 264, a potent and radioactivity was observed using [3H]pBC 264, a result which is not in favour of a cholecystokinin receptor-induced internalization mechanism. The potent and selective uptake mechanism characterized in this study could participate, in conjunction with extra and intracellular degradation of CCK8 by peptidases, in the interruption of cholecystokinin-conveyed messages in the brain.

Transient and long-lasting actions of 5-HT on rat dentate gyrus neurones in vitro

1. The actions of 5-hydroxytryptamine (5-HT) on rat dentate gyrus neurones were measured with conventional intracellular recording techniques in brain slices maintained in vitro at 32 degrees C. 2. Bath application of 5-HT (0.3-100 microM) hyperpolarized the membrane potential and reduced the input resistance; these effects persisted in tetrodotoxin (1 microM) and were abolished by MDL 73,005EF, a 5-HT1A receptor antagonist. 3. Local application of 5-HT via a pressure pipette also elicited a hyperpolarization and a reduction in resistance, and evoked a transient 'burst' of spontaneous inhibitory postsynaptic potentials (IPSPs) which were blocked by tetrodotoxin or bicuculline. 4. The 'burst' of IPSPs was subject to desensitization. It was completely abolished in the presence of the 5-HT3 receptor antagonist dolasetron. 5. In some cells, a longer lasting increase in spontaneous IPSP frequency was observed during application of 5-HT; this effect was blocked by the 5-HT2 receptor antagonist MDL 100,907. 6. 5-HT (30 microM) shortened the decay time constants of the glutamatergic and GABAergic evoked EPSPs and IPSPs without changing their amplitudes. 7. It is concluded that 5-HT hyperpolarizes granule cells via postsynaptic 5-HT1A receptors and increases spontaneous GABA release from inhibitory interneurones via the activation of 5-HT3 receptors and/or 5-HT2 receptors.

The role of 5-HT3 receptors in the anti-ulcer effect of calcitonin

1. The aim of the present study was to determine the role of 5-HT3 receptors of the gastroprotective effect of salmon calcitonin (sCT) and sCT-induced changes in gastric, hepatic, brain and brainstem glutathione (GSH) and lipid-peroxidation (LP) levels in rats subjected to cold-immobilization stress. 2. Stress exposure resulted in ulcer formation and a decrease in GSH levels of the liver, brain and brainstem and an increase in gastric and hepatic LP (P < 0.05). 3. sCT prevented stress-induced gastric ulcer development (P < 0.01) and reversed the decrease in hepatic and brain GSH levels (P < 0.05). 4. In the present study, a 5-HT3 receptor antagonist, ICS 205,930 was used. Interestingly, the effect of the blocker on GSH and LP levels of the tissues studied was similar to those of sCT. 5. ICS 205,930 dose dependently reversed the anti-ulcer effect of sCT although it did not antagonize the effect of sCT on GSH and LP levels, but it seemed to show an additive interaction for brain and brainstem GSH and gastric LP levels with sCT.

Expression of the cholecystokinin gene in rat hippocampal interneurons is independent of extrinsic input

In the present study we have used slice cultures of hippocampus and in situ hybridization techniques in order to study the ability of hippocampal neurons to synthesize CCK mRNA in the absence of extrinsic afferents. Our results show that very similar types of hippocampal neurons express CCK mRNA in culture as in situ. We conclude that the expression of the CCK gene in hippocampal neurons is not dependent on extrinsic afferent input.