Cholecystokinin turnover in brain

CCK biosynthesis and processing: recent progress and future challenges

The peptide cholecystokinin (CCK), like other peptides which pass through the regulated secretory pathway, undergoes a number of post-translational modifications during its biosynthesis including tyrosine sulfation, endoproteolytic cleavage, and trimming by carboxypeptidases. This minireview summarizes what is known about this process in endocrine cells and in the Cpe(fat)/Cpe(fat) mouse and points out what challenges remain for future research.

Modulation of apomorphine-induced stereotyped behavior by cholecystokinin

1. The goal was to verify if central or peripheral sulphated cholecystokinin octapeptide (CCK8) injections can modulate apomorphine (APO)-induced stereotyped behavior. Experiments were designed to determine the involvement of cholecystokinin receptor subtypes as well. 2. Animals which received CCK8 (0.0725, 0.145 and 14.5 nmol, icv) showed a significant (p < 0.05) decrease in APO (0.6 mg/kg, sc)-induced stereotyped behavior. 3. No other statistically significant difference was observed among groups. Since ip CCK8 (1.16 or 2.32 nmol/kg) injections did not interfere with APO-induced stereotypy, the results suggest that the CCK8 modulatory effects have a central action. 4. The results also suggest that the effects of icv CCK8 were not due to the stimulation of CCK8 receptors alone since central CCK4 (14.5 or 29.0 nmol) injections did not interfere with the expression of stereotypy.

On the origin of striatal cholecystokinin release: studies with in vivo microdialysis

In the present study, extracellular levels of the neuropeptide cholecystokinin (CCK), of the monoamine dopamine and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and of the excitatory amino acids glutamate and aspartate were simultaneously monitored by microdialysis in the neostriatum of halothane-anesthetized rats under basal and K(+)-depolarizing conditions. Extracellular CCK and dopamine levels, but not glutamate and aspartate levels, were decreased by perfusion with a Ca(2+)-free medium, under both basal and K(+)-depolarizing conditions. HPLC revealed that the majority of the CCK-like immunoreactivity in the perfusates coeluted with CCK octapeptide. Striatal extracellular CCK levels were decreased by decortication plus callosotomy, with a parallel decrease in glutamate levels. Striatal extracellular levels of dopamine, DOPAC, and HVA were significantly decreased in animals treated previously with a unilateral 6-hydroxydopamine injection into the medial forebrain bundle. In these animals, however, the effect of decortication plus callosotomy on CCK and glutamate levels was not further augmented. Thus, this study supports the hypothesis of a neuronal origin of extracellular CCK and dopamine monitored with microdialysis in the striatum of the rat, and also supports the idea of a partly contralateral origin of corticostriatal CCK and glutamate inputs.

Subcellular and developmental studies of the tyrosyl protein sulfotransferase in rat brain

1. Tyrosyl protein sulfotransferase (TPS) activity in the newborn and mature rat brain was studied using the cholecystokinin derivative terbutyloxycarbonyl-Asp-Tyr-Met-Gly-Trp-Met-Asp-PheNH2, BocCCK-8(ns), as the peptide substrate. 2. TPS activity was enriched 4 times in the microsomal and synaptic vesicular enriched fractions of rat cerebral cortex. 3. CCK-8 content, in the subcellular fractions and the peptide sulfation activity distribution was in accord with the hypothesis that tyrosyl protein sulfotransferase plays a key role in the maturation process of bioactive CCK. 4. TPS activity measured in membranes from newborn brain was 2.5 times higher than the activity observed in the mature brain membranes with a Vmax = 0.83 +/- 0.05 and 0.31 +/- 0.02 respectively. The apparent KM for the sulfate donor, 3'-phosphoadenosine 5'-phosphosulfate (PAPS), was similar, 94 +/- 4 nM and 90 +/- 6 nM and the KM for the peptide substrate, BocCCK-8(ns), was 234 +/- 16 microM and 160 +/- 12 microM in the newborn and adult brain membranes respectively. 5. TPS activity reached normal mature values within 20 days of age. 6. These data support the idea that tyrosyl protein sulfation is an important process in the secretion mechanism and in the CCK maturation.

Examination of the effects of cholecystokinin 26-33 and neuropeptide Y on responses of visual cortical neurons of the cat

Extracellular recordings were made from 160 neurons in area 17 (n = 120) and area 18 (n = 40) of the visual cortex of anesthetized cats. Cells were classified according to their receptive field properties and their intracortical positions were evaluated histologically. Cholecystokinin 26-33, antagonists, (cholecystokinin 27-32, cholecystokinin 27-33 and proglumide), amino acids, neuropeptide Y and solvent vehicle (control), were administered to cells by microiontophoresis (cholecystokinin and neuropeptide Y) or by pressure (neuropeptide Y). The results of the tests with cholecystokinin 26-33 fell into four categories: enhancement (31%), suppression (24%), mixed, i.e. either biphasic responses or dose-related alterations in the direction of effect (20%), and no effect (25%). Enhancements of the visually elicited response were more prevalent in simple (43%) and unimodal/movement-sensitive (34%) cells than in complex (7%) cells. The converse was true for suppressions: 19% of simple cells, 24% of unimodal/movement-sensitive cells, and 31% of complex cells were suppressed. Thirty per cent of the unaffected cells were complex or unimodal/movement-sensitive; only 14% were simple. Cells in layers II-IV were more likely to have firing enhanced than suppressed by cholecystokinin 26-33. The converse was true for cells in layers V and VI, where 50% of responses were suppressed and only 22% were enhanced. Unaffected cells were found predominantly in layer III of areas 17, and the lower part of layer III and layer IV of area 18. Cholecystokinin 26-33 sometimes exerted delayed, response-suppressant effects; it also occasionally elevated responsiveness preferentially within the upper ranges (10-20 degrees/s) of velocity tuning curves. Cholecystokinin 26-33 altered the response-suppressant action of GABA in 11 of 19 visually sensitive cells. The peptide potentiated the visual responsiveness in half of the cells where cholecystokinin 26-33 diminished the GABA-induced suppressions (n = 8). The presumed antagonists either exerted no effect on firing or on cholecystokinin 26-33-induced effects, or had cholecystokinin 26-33-like actions themselves. There was a reversible partial antagonism of the effects of cholecystokinin 26-33 on only two of 11 cells tested. Neuropeptide Y injected by pressure or administered iontophoretically had variable and inconsistent effects on the visually evoked responses of 29 additional neurons from those described above. These effects were indistinguishable from those of the vehicle whether spontaneous activity, magnitude of the visually elicited response, spatial integrity of the RF substructure, orientation or velocity tuning was assessed.(ABSTRACT TRUNCATED AT 400 WORDS)

Procholecystokinin processing in rat cerebral cortex during development

Using a library of radioimmunoassays for essential sequences of procholecystokinin (proCCK), we have examined the post-translational processing in the rat cerebral cortex from fetal to adult state. The concentration of proCCK in the fetal cerebral cortex was 43 +/- 7 pmol/g tissue (wet weight; mean +/- S.E.M. (n = 20)). It remained constant until day 21 post partum, after which it decreased to undetectable levels. In contrast, the concentration of fully processed, bioactive CCK peptides (i.e. alpha-carboxyamidated CCK) rose from 2 +/- 1 pmol/g in the fetal cortex to 122 +/- 21 pmol/g in the adult. A particularly steep increase occurred from day 7 post partum (13 +/- 2 pmol/g) to day 21 (108 +/- 11 pmol/g). The concentration of glycine-extended intermediates rose gradually from 8 +/- 1 pmol/g in the fetal brain to 55 +/- 6 pmol/g in the adult. Gel chromatography of cortical extracts from day 7, 21 and 100 confirmed the variable processing at the C-terminal amidation site. The results show that the CCK gene is expressed as proCCK already in the fetal brain. However, the covalent modifications of proCCK follow different time courses so that only a small fraction reaches maturation until the first week post partum. We conclude that expression of transmitter-active CCK peptides in the brain is largely regulated at the post-translational rather than at the transcriptional level.

Memory effect of caerulein and its analogs in active and passive avoidance responses in the rat

The memory effects of caerulein (CER) and its analogs ([des-Gln2]-CER and [Leu5,Nle8]-CER) were compared with that of cholecystokinin octapeptide (CCK-8) using active and passive avoidance responses in rats. In the active avoidance test, single subcutaneous (SC) injection of CER and its analogs immediately after the learning trials at doses of 10 and 100 ng/kg prevented extinction of learned task for 10 days, and at a dose of 1000 ng/kg for at least 15 days, but the effect of CCK-8 was somewhat weaker. In the saline control group, the number of responses decreased after 5 days. In the passive avoidance response, electroconvulsive shock (ECS)-induced amnesia was partially prevented by CCK-8 at doses of 100 and 1000 ng/kg SC, while CER and its analogs at doses of more than 100 ng/kg totally prevented the ECS-induced amnesia. Intraperitoneal administration of scopolamine caused complete amnesia which was also partially prevented by CCK-8, while CER could totally prevent the amnesia following SC injection of 2 micrograms/kg. These results indicate that CER and its analogs are more effective than CCK-8 for preventing experimental amnesia.

Expression of cholecystokinin and enkephalin mRNA in discrete brain regions

The levels of preprocholecystokinin mRNA were measured in several regions of rat brain using RNA blot analysis. In both species, high levels of expression were observed in the thalamus, amygdala, neocortical areas and hippocampus. Intermediate levels were observed in the periaqueductal grey, hypothalamus, substantia nigra, ventral tegmental area, and olfactory bulbs; little or no mRNA was detected in the caudate nucleus, nucleus accumbens, olfactory tubercle, cerebellum or a liver control. In contrast, the caudate and olfactory tubercle expressed large amounts of preproenkephalin mRNA. Other regions, such as the periaqueductal grey and olfactory bulbs, expressed both transcripts while regions like the hippocampus contained prominent amounts of preprocholecystokinin mRNA and relatively little preproenkephalin mRNA.

Modulation of cholecystokinin release from posterior nucleus accumbens by D-2 dopamine receptor

The effect of specific D-2 dopamine (DA) receptor agonists and antagonists on potassium (55 mM)-evoked release of cholecystokinin-like immunoreactivity (CCK-LI) was studied in tissue slices of the rat posterior nucleus accumbens (NAc). Incubating the tissue slices in 100 nM or 1 microM of LY-141865, a specific D-2 DA receptor agonist, reduced the release of CCK-LI as indicated by a significant decrease in the S2/S1 ratio. Addition of 10 microM of (-)-sulpiride, a specific D-2 DA receptor antagonist, blocked the inhibitory effect of 100 nM of LY-141865 on the release of CCK-LI. In contrast, 10 microM of the specific D-1 DA receptor antagonist SCH 23390 was unable to attenuate the decrease in release of CCK-LI caused by 100 nM of LY-141865. Furthermore, the active isomer of LY-141865, LY-171555 at 0.1 to 50 nM, also decreased the release of CCK-LI from the tissue slices, while the inactive isomer, LY-181990 at 1 nM, did not affect CCK-LI release. The inhibitory effect of LY-171555 on the release of CCK-LI was lost when its concentration was increased to 100 nM, thus revealing a biphasic effect of D-2 DA receptor stimulation on the release of CCK-LI. These results demonstrate that stimulation of D-2 DA receptor can modulate the release of CCK from in vitro slices of the rat posterior NAc.

Sulfation and desulfation of cerebral cholecystokinin

An apparently novel sulfotransferase present in microsomal and vesicular fractions from rat brain is able to transfer [35S]sulfate groups from [35S]PAPS to CCK derivatives. Its optimum pH (approximately 6), its substrate specificity, and its subcellular localization are all consistent with its function as post-translational processing enzyme. However, its presence in tissues devoid of CCK argues against this idea, or could simply mean that it is involved in the processing of other peptides besides CCK. No evidence could be obtained for extensive desulfation of endogenous CCK-8 released by depolarization of brain slices.

Sulfation of peptides and simple phenols by rat brain phenolsulfotransferase. Inhibition by dichloronitrophenol

Brain phenolsulfotransferase (PST) is involved in the sulfation of simple phenols like dopamine and of precursors of biologically active peptides like cholecystokinin octapeptide (CCK-8). Therefore, inhibition of brain PST would provide a new approach to studying the sulfation of CCK-8 and other sulfated compounds. Since 2,6-dichloro-4-nitrophenol (DCNP) produces a prolonged and selective inhibition of the sulfoconjugation of exogenous phenols by the liver, we decided to examine the applicability of DCNP to studies of sulfation of CCK-8 and other compounds by brain. DCNP was capable of completely inhibiting PST activity in rat brain homogenates incubated with p-nitrophenol, phenol or dopamine as substrates. The IC50 values for p-nitrophenol and dopamine were 12 and 14 microM respectively. The concentrations of DCNP in brain cortex and plasma were measured by high pressure liquid chromatography (HPLC) after a dose of 100 mumoles/kg, i.p. Peak concentrations of 380 microM in plasma and 25 mumoles/kg in brain were achieved 30 min after injection. Subsequently, DCNP concentrations decreased with half-lives of 8 and 6 hr in plasma and brain cortex, respectively. To establish if DCNP can inhibit CCK sulfation in vivo, rats were injected with 100 micromoles/kg, i.p., of the drug 30 min before injection of 35SO4(2-) into the cerebral cortex and were killed 4.5 hr later. DCNP caused a 55% inhibition of [35S]CCK-8-SO4 formation as measured by HPLC. No change in the content of endogenous CCK-8-SO4 was detectable, however, in the brain cortex of rats treated with DCNP for up to 4 days, indicating that the PST which remained active was capable of maintaining CCK-8 content at steady state.

Modulation of cholecystokinin concentrations in the rat hippocampus by chelation of heavy metals

Previously, we have reported that enkephalins, cholecystokinin, and heavy metals show roughly parallel distributional patterns in the hippocampus. A substantial body of evidence indicates that cholecystokinin-octapeptide (CCK-8) and enkephalins act as neurotransmitters. A CCK-8 degrading enzyme was recently detected in brain synaptosomes. Its activity depended on free thiol groups and the presence of a heavy metal. Since the heavy metal-containing neuropil is closely related to CCK-immunoreactive nerve terminals, we have investigated the effect of metal chelation on CCK components in the rat hippocampus. In vivo treatment of rats with a single dose of the chelating agent diethyldithiocarbamate caused a reversible chelation of heavy metals in the hippocampus. This effect was paralleled by a 3-fold increase in hippocampal content of CCK-8 and a smaller increase in the intermediate forms of CCK (CCK-58, CCK-39, CCK-33). Diethyldithiocarbamate also decreased the spontaneous motility and aggressiveness of the rats. These data show reversible changes of neuronal CCK processing by a drug, and hence they provide additional evidence that CCK is involved in the regulation of neuronal activities.