Activation of CCK-B receptors elevates cytosolic Ca2+ levels in a pituitary cell line

Thermodynamic analysis of ligands at cholecystokinin CCK2 receptors in rat cerebral cortex

BACKGROUND AND PURPOSE

Several studies using radioligand binding assays, have shown that measurement of thermodynamic parameters can allow discrimination of agonists and antagonists (Weiland et al., 1979; Borea et al., 1996a). Here we investigate whether agonists and antagonists can be thermodynamically discriminated at CCK(2) receptors in rat cerebral cortex.

EXPERIMENTAL APPROACH

The pK(L) of [(3)H]-JB93182 in rat cerebral cortex membranes was determined at 4, 12, 21 and 37 degrees C in 50 mM Tris-HCl buffer (buffer B pH 6.96; containing 0.089 mM bacitracin). pK(I) values of ligands of diverse chemical structure and with differing intrinsic activity (alpha), as defined by the lumen-perfused rat and mouse stomach bioassays, were determined in buffer B at 4, 12, 21 and 37 degrees C.

KEY RESULTS

[(3)H]-JB93182 labelled a homogeneous population of receptors in rat cerebral cortex at 4, 12, 21 and 37 degrees C and the pK(L) and B(max) were not altered by incubation temperature. [(3)H]-JB93182 binding reached equilibrium after 10, 50, 90 and 220 min at 37, 21, 12 and 4 degrees C, respectively. pK(I) values for R-L-365,260, R-L-740,093, YM220, PD134,308 and JB95008 were higher at 4 degrees C than at 37 degrees C. There was no effect of temperature on pK(I) values for pentagastrin, CCK-8S, S-L-365,260, YM022, PD140,376 and JB93242.

CONCLUSIONS AND IMPLICATIONS

CCK(2) receptor agonists and antagonists at rat CCK(2) receptors cannot be discriminated by thermodynamic analysis using [(3)H]-JB93182 as the radioligand.

Role of BK potassium channels shaping action potentials and the associated [Ca(2+)](i) oscillations in GH(3) rat anterior pituitary cells

Measurements of electrical activity and intracellular Ca(2+) levels were performed in perforated-patch clamped GH(3) cells to determine the contribution of large-conductance calcium-activated K(+) (BK) channels to action potential repolarization and size of the associated Ca(2+) oscillations. By examining the dependence of action potential (AP) duration on extracellular Ca(2+) levels in the presence and the absence of the specific BK channel blocker paxilline, it is observed that plateau-like action potentials are associated to low densities of paxilline-sensitive currents. Extracellular Ca(2+) increases or paxilline additions are not able to largely modify action potential duration in cells showing a reduced expression of BK currents. Furthermore, specific blockade of these currents with paxilline systematically elongates AP duration, but only under conditions in which short APs and/or prominent BK currents recorded under voltage-clamp mode are present in the same cells. Our data indicate that in GH(3) cells, BK channels act primarily ending the action potential and suggest that by contributing to fine-tuning cellular electrical properties and hence intracellular Ca(2+) variations, BK channels may play an important role on time- and cell-dependent modulation of physiological outputs in adenohypophyseal cells.

Cholecystokinin increases cytosolic calcium in a subpopulation of cultured vagal afferent neurons

Imaging fluorescent measurements with fura 2 were used to examine cytosolic calcium signals induced by sulfated CCK octapeptide (CCK-8) in dissociated vagal afferent neurons from adult rat nodose ganglia. We found that 40% (184/465) of the neurons responded to CCK-8 with a transient increase in cytosolic calcium. The threshold concentration of CCK-8 for inducing the response varied from 0.01 to 100 nM. In most neurons (13/16) the response was eliminated by removing extracellular calcium. Depleting intracellular calcium stores with thapsigargin slightly augmented the response. Most neurons were unresponsive to nonsulfated CCK-8. The response was eliminated by the CCK-A receptor antagonist lorglumide. Low concentrations of JMV-180 had no effect; however, high concentrations of JMV-180 reduced responses to CCK-8. These results demonstrate that CCK acts at the low-affinity site of the CCK-A receptor to trigger the entry of extracellular calcium into vagal afferent neurons. Increased cytosolic calcium may participate in acute activation of vagal afferent neurons, or it may initiate long-term changes, which modulate future neuronal responses to sensory stimuli.

Correlation between electrical activity and intracellular Ca2+ oscillations in GH3 rat anterior pituitary cells

Simultaneous measurements of electrical activity and intracellular Ca(2+) levels were performed in perforated-patch current-clamped individual GH3 cells. Both in cells showing brief (<100 ms) and long action potentials (APs), we found a good correlation between the averaged intracellular Ca2+ concentration ([Ca2+]i) and AP frequency, but not between the mean [Ca2+]i and AP duration. Nevertheless, the magnitude of spontaneous Ca2+ oscillations was highly dependent on the size and duration of the APs. The decay of the Ca2+ transients was not slowed when the size of the oscillations was varied either spontaneously or after elongation of the AP with the K+ channel blocker tetraethyl ammonium. Furthermore, the recovery from Ca2+ loads similar to those induced by the APs was slightly retarded after treatment of the cells with intracellular store Ca2+-ATPase inhibitors. Among previous results showing that caffeine-induced [Ca2+]i increases are secondary to electrical activity enhancements in GH3 cells, these data indicate that the Ca2+ entry triggered via APs is the primary determinant of the [Ca2+]i variations, and that Ca2+-induced Ca2+ release has a minor contribution to Ca2+ oscillations recorded during spontaneous activity. They also point to modulation of electrical activity patterns as a crucial factor regulating spontaneous [Ca2+]i signalling, and hence pituitary cell functions in response to physiological secretagogues.

Modulation of rat erg1, erg2, erg3 and HERG K+ currents by thyrotropin-releasing hormone in anterior pituitary cells via the native signal cascade

The mechanism of thyrotropin-releasing hormone (TRH)-induced ether-a-go-go-related gene (erg) K+ current modulation was investigated with the perforated-patch whole-cell technique in clonal somatomammotroph GH3/B6 cells. These cells express a small endogenous erg current known to be reduced by TRH. GH3/B6 cells were injected with cDNA coding for rat erg1, erg2, erg3 and HERG K+ channels. The corresponding erg currents were isolated with the help of the specific erg channel blockers E-4031 and dofetilide and their biophysical properties were determined. TRH (1 M) was able to significantly reduce the different erg currents. The voltage dependence of activation was shifted by 15 mV (erg1), 10 mV (erg2) and 6 mV (erg3) to more positive potentials without strongly affecting erg inactivation. TRH reduced the maximal available erg current amplitude by 12% (erg1), 13% (erg2) and 39% (erg3) and accelerated the time course of erg1 and erg2 channel deactivation, whereas erg3 deactivation kinetics were not significantly altered. The effects of TRH on HERG currents did not differ from those on its rat homologue erg1. In addition, coinjection of rat MiRP1 with HERG cDNA did not influence the TRH-induced modulation of HERG channels. Rat erg1 currents recorded in the cell-attached configuration were reduced by application of TRH to the extra-patch membrane in the majority of the experiments, confirming the involvement of a diffusible second messenger. Application of the phorbol ester phorbol 12-myristate 13-acetate (PMA; 1 M) shifted the voltage dependence of erg1 activation in the depolarizing direction, but it did not reduce the maximal current amplitude. The voltage shift could not be explained by a selective effect on protein kinase C (PKC) since the PKC inhibitor bisindolylmaleimide I did not block the effects of TRH and PMA on erg1. In addition, cholecystokinin, known to activate the phosphoinositol pathway similarly to TRH, did not significantly affect the erg1 current. Various agents interfering with different known TRH-elicited cellular responses were not able to completely mimic or inhibit the TRH effects on erg1. Tested substances included modulators of the cAMP-protein kinase A pathway, arachidonic acid, inhibitors of tyrosine kinase and mitogen-activated protein kinase, sodium nitroprusside and cytochalasin D. The results demonstrate that all three members of the erg channel subfamily are modulated by TRH in GH3/B6 cells. In agreement with previous studies on the TRH-induced modulation of the endogenous erg current in prolactin-secreting anterior pituitary cells, the TRH effects on overexpressed erg1 channels are not mediated by any of the tested signalling pathways.

The cholecystokininB receptor is coupled to two effector pathways through pertussis toxin-sensitive and -insensitive G proteins

Previous binding studies have suggested the existence of two affinity states for type B cholecystokinin receptors (CCK(B)R), which could correspond to different coupling states of the receptor to G proteins. To test this hypothesis, we have further investigated signal transduction pathways coupled to rat CCK(B)R stably transfected in Chinese hamster ovary cells. We show that CCK(B)R are coupled to two distinct transduction pathways involving two different G proteins, a pertussis toxin-insensitive/phospholipase C pathway leading to the production of inositol phosphate and arachidonic acid, and a pertussis toxin-sensitive/phospholipase A2 pathway leading to the release of arachidonic acid. We further demonstrate that the relative degree of activation of each effector pathway by different specific CCK(B)R agonists is the same, and that a specific CCK(B)R antagonist, RB213, can differentially antagonize the two signal transduction pathways elicited by these agonists. Taken all together, these data could be explained by the recently proposed theory assuming that the receptor can exist in a three-state model in which two active conformations corresponding to the complex formed by the receptor with two different G proteins coexist. According to this model, agonists or antagonists could recognize preferentially either conformation of the activated receptor, leading to variable behavior in a system containing a single receptor type.

Proliferative effects of cholecystokinin in GH3 pituitary cells mediated by CCK2 receptors and potentiated by insulin

1. Proliferative effects of CCK peptides have been examined in rat anterior pituitary GH3 cells, which express CCK2 receptors. 2. CCK-8s, gastrin(1-17) and its glycine-extended precursor G(1-17)-Gly, previously reported to cause proliferation via putative novel sites on AR4-2J and Swiss 3T3 cells, elicited significant dose dependent increases of similar magnitude in [3H]thymidine incorporation over 3 days in serum-free medium of 39 +/- 10% (P < 0.01, n = 20), 37 +/- 8% (P < 0.01, n = 27) and 41 +/- 6% (P < 0.01, n = 36) respectively. 3. CCK-8s and gastrin potentially stimulated mitogenesis (EC50 values 0.12 nM and 3.0 nM respectively), whilst G-Gly displayed similar efficacy but markedly lower potency. L-365,260 consistently blocked each peptide. The CCK2 receptor affinity of G-Gly in GH3 cells was 1.09 microM (1.01;1.17, n = 6) and 5.53 microM (3.71;5.99, n = 4) in guinea-pig cortex. 4. 1 microM G-Gly weakly stimulated Ca2+ increase, eliciting a 104 +/- 21% increase over basal Ca2+ levels, and was blocked by 1 microM L-365,260 whilst CCK-8s (100 nM) produced a much larger Ca2+ response (331 +/- 14%). 5. Insulin dose dependently enhanced proliferative effects of CCK-8s with a maximal leftwards shift of the CCK-8s curve at 100 ng ml(-1) (17 nM) (EC50 decreased 500 fold, from 0.1 nM to 0.2 pM; P < 0.0001). 10 microg ml(-1) insulin was supramaximal reducing the EC50 to 5 pM (P = 0.027) whilst 1 ng ml(-1) insulin was ineffective. Insulin weakly displaced [125I]BHCCK binding to GH3 CCK2 receptors (IC50 3.6 microM). 6. Results are consistent with mediation of G-Gly effects via CCK2 receptors in GH3 cells and reinforce the role of CCK2 receptors in control of cell growth. Effects of insulin in enhancing CCK proliferative potency may suggest that CCK2 and insulin receptors converge on common intracellular targets and indicates that mitogenic stimuli are influenced by the combination of extracellular factors present.

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.

CCK-B receptor-mediated stimulation of polyphosphoinositide turnover in GH3 pituitary cells in response to cholecystokinin and pentagastrin

CCK-B receptor-mediated polyphosphoinositide (PPI) turnover in GH3 pituitary cells has been examined and comparisons are made with Ca2+ mobilisation and receptor binding data, previously described. Sulphated cholecystokinin octapeptide (CCK-8s) and the CCK-B-selective agonist pentagastrin dose-dependently stimulated PPI turnover in GH3 cells with similar maximal increases of 3.0 fold and 3.3 fold, respectively, in production of [3H]inositol phosphates over control. Responses, measured over 30min periods in the presence of 10mM LiCl, were generally maximal for both agonists at 100nM. Consistent with their [125]Bolton Hunter CCK-8s (BHCCK) binding affinities and with effects on Ca2+ mobilisation, CCK-8s was slightly more potent than pentagastrin in stimulating PPI turnover (EC50s 1.3nM and 3.9nM respectively). Both peptides showed higher potency in the PPI assay than in Ca2+ studies. 100nM pentagastrin-induced PPI turnover was dose-dependently inhibited by the CCK-B receptor-selective antagonist L-365,260 (IC50 470nM) whilst the CCK-A receptor antagonist, devazepide, only produced weak partial inhibition (18% at 10,000nM). Antagonists alone were observed to depress control activity in PPI turnover but not in Ca2+ mobilisation assays. The selectivity of L-365,260 compared to devazepide was similar in binding studies to that for both 100nM pentagastrin-induced functional responses. Schild analysis of antagonism of PPI turnover by L-365,260 yielded a line with slope close to unity (1.07) and a pKB of 8.27+/-0.05.

Effects of guanyl nucleotides on CCKB receptor binding in brain tissue and continuous cell lines: a comparative study

The effects of non-hydrolyzable guanyl nucleotide analogue GTP-gamma S on CCKB receptor binding in human and guinea-pig cortex, Jurkat T-cells, rat pituitary GH3 cells, rat glioma C6 cells and human small cell lung cancer NCI-H69 cells were investigated by using [3H]CCK-8S saturation and competition binding studies. GTP-gamma S caused inhibition of specific [3H]CCK-8S binding in a concentration dependent manner with a plateau at 10-25 microM. 25 microM GTP-gamma S resulted in a small but significant increase in Kd and IC50 values with amount very similar in all CCKB receptor models tested. However, the maximal number of specific [3H]CCK-8S binding sites (Bmax) was unaffected. Results suggest that CCKB receptors are G-protein coupled in a similar way to human and guinea-pig cortex, Jurkat cells, GH3 cells, C6 cells and NCI-H69 cells.

Binding of cholecystokinin, bombesin and muscarine to neurons and astrocytes in explant cultures of rat central nervous system: autoradiographic and immunohistochemical studies

The cellular localization of binding sites for the gastrointestinal peptides [3H]cholecystokinin and [125I]bombesin as well as the cholecystokininB-antagonist [3H]L-365,260 was investigated in explant cultures of rat cortex, cerebellum, brainstem and spinal cord using autoradiographic techniques. Many neurons in cortical, brainstem and spinal cord cultures revealed intense labelling of the radioligands whereas cerebellar neurons showed only little binding. In addition to neurons, binding sites for these peptides were also observed on astrocytes. Labelling of glial cells in cerebellar cultures was usually weaker than in the other CNS areas studied, suggesting a certain specialization of astrocytes in various brain regions. By means of combined immunohistochemical and autoradiographic techniques it was demonstrated that many neurons and astrocytes which expressed binding sites for [3H]cholecystokinin, [3H]L-365,260 and [125I]bombesin were also immunostained by the monoclonal muscarinic receptor antibody M 35 providing evidence for a co-localization of peptidergic and cholinergic receptors on the membrane of these cells. Our autoradiographic findings suggesting the presence of receptors for cholecystokinin and bombesin on astrocytes are supported by electrophysiological studies demonstrating that both peptides induce a hyperpolarization of glial cells.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.