Effects of the novel cholecystokinin analogue Suc-Trp-N(Me)-Nle-Asp-Phe-NH2 on feeding and cortisol release in pigs

Suc-Try-N-(Me)-Nle-Asp-Phe-NH2 is a succinylated tetrapeptide derived from the C-terminal sequence of cholecystokinin octapeptide (CCK-B), which has been shown to have high agonist affinity for CCKB receptors. To test the validity of the hypothesis that implicates central CCKB receptors in the aetiology of stress-related disorders, such as anxiety and panic, we argued that activation of these receptors by a CCKB receptor agonist should (i) suppress feeding motivation in hungry animals and (ii) increase plasma concentrations of the stress hormone cortisol. The effects of systemic and central administration of Suc-Try-N-(Me)-Nle-Asp-Phe-NH2 were, therefore, investigated on operant food intake and cortisol secretion in pigs. Intravenous administration of Suc-Try-N-(Me)-Nle-Asp-Phe-NH2 (0.5-5 micrograms/kg) did not affect operant feeding in food-deprived pigs, although the highest dose (5 micrograms/kg) produced a small but significant (p < 0.05) increase in plasma cortisol levels 5-30 min after injection. Intracerebroventricular injection of Suc-Try-N-(Me)-Nle-Asp- Phe-NH2 (1-5 micrograms) had no effect on operant feeding and cortisol secretion in this species. The results obtained in this study indicate that central CCKB receptors are unlikely to be involved in stress-related behaviours in pigs.

Systemic administration of cholecystokinin (CCK) inhibits operant water intake in rats: implications for the CCK-satiety hypothesis

The demonstration that intraperitoneal administration of the sulphated octapeptide of cholecystokinin (CCK-8S) inhibits food but not water intake in rats, has led to the hypothesis that endogenous peripheral CCK acts as a food-specific satiety factor. As water-deprived rats given free access to water can satisfy their thirst fairly rapidly, it is conceivable that the apparent lack of effect of CCK on water intake reported previously may have been because the animals had satisfied their thirst before the full effects of the peptide had become apparent. To test this possibility, the effects of intraperitoneal (i.p.) administration of CCK-8S, at doses that had previously been shown to inhibit food intake, were investigated on water intake in rats trained to make operant responses for water reinforcements. Such a paradigm has the merit of slowing down the rate at which a water-deprived rat can quench its thirst, thus extending the period over which the effects of CCK-8S on water intake may be assessed. CCK-8S (2, 4 or 8 micrograms kg-1, i.p.) produced a dose-related suppression of operant water intake in 16 h water-deprived rats during the first 30 min after administration. Additional experiments indicated that, as with feeding, CCK-8S inhibits water intake by an action at peripheral CCKA receptors. These finding have important implications for the CCK-satiety hypothesis as they show that the effect of the peptide on ingestive behaviours in the rats is not specific for food intake and suggest that it is unlikely that CCK is a mediator of satiety.

Baclofen pretreatment attenuates the suppressant effect of intraperitoneal administration of cholecystokinin (CCK) on food intake in rats

The aim of this study was to investigate whether pretreatment with the GABAB receptor agonist baclofen could prevent the inhibitory effect of systemically administered cholecystokinin (CCK) on food intake in rats. Baclofen (2 mg/kg, SC) administered 60 min prior to IP injection of CCK (5 micrograms/kg) significantly attenuated the suppressant effect of the peptide on feeding in nondeprived rats (Experiment 1) and rats that had been deprived of food for 22 h (Experiment 2). Baclofen had no significant effects on food intake when administered alone. The results suggest that the inhibitory effect of exogenous peripheral CCK on food intake may be dependent on an interaction with a GABAB-receptor mediated mechanism.

2-Naphthalenesulphanyl-L-aspartyl-2-(phenethyl) amide (2-NAP) and food intake in rats: evidence that endogenous peripheral CCK does not play a major role as a satiety factor

1. The demonstration that systemic administration of the CCKA receptor antagonist, devazepide, increases food intake in rats has provided the strongest support for the hypothesis that endogenous peripherally released cholecystokinin (CCK) acts as a satiety factor. However, interpretation of these results has been confounded by the fact that devazepide can enter the brain from the systemic circulation and may increase food intake by a central action. The present study was therefore undertaken to confirm the hypothesis that endogenous peripheral CCK is a satiety factor by investigating the effects of a novel CCKA receptor antagonist, 2-NAP, which is unlikely to cross the blood brain barrier, on food intake in rats. 2. 2-NAP (1-16 mg kg-1, i.p.) had no significant effects on the intake of a test meal in rats. 3. Pretreatment of rats with 2-NAP (2 mg kg-1, s.c.) abolished the inhibitory effects of exogenous peripheral CCK (5 micrograms kg-1, i.p.) on food intake. 4. In agreement with previous results, devazepide (50-200 micrograms kg-1, i.p.) significantly increased the intake of a test meal in rats. 5. The observations that 2-NAP, which is unlikely to penetrate the blood brain barrier, had no effect on food intake, but that 2-NAP abolished the suppressant effect of exogenous peripheral CCK, suggest that endogenously released peripheral CCK is not important as a satiety factor in rats.

Intravenous administration of cholecystokinin (CCK) stimulates prolactin and growth hormone release in the pig

Blood samples were taken from prepubertal pigs (n = 7) surgically prepared with venous catheters before, and at intervals during the 30 min period after, administration of physiological saline (vehicle) or sulphated cholecystokinin octapeptide (CCK) (0.3, 0.7 and 1.3 mu g/kg). Plasma concentrations of prolactin and growth hormone were measured by radioimmunoassay. Statistically significant dose-related increases in prolactin occurred mainly in the first 10-min period following CCK administration. The peptide also stimulated growth hormone secretion although in a less consistent manner, with the increases tending to occur over a longer time period. Together with earlier reports showing that CCK induces cortisol release in pigs, these results suggest that the peptide's effect on anterior pituitary function could be indicative of a stress response.

Intraperitoneal administration of baclofen increases consumption of both solid and liquid diets in rats

It has previously been demonstrated that systemic administration of the GABAB receptor agonist baclofen increases food intake but decreases water intake in rats. In the present study, the effects of baclofen (2-4 mg/kg i.p.) were investigated on food intake in non-deprived rats given access to either pelleted food (n = 8) or a palatable liquid diet (n = 8). Baclofen (2-4 mg/kg i.p.) significantly increased the consumption of both the pelleted and the liquid foods. The increased intake of the liquid diet (i) argues against the involvement of non-specific gnawing in the increased consumption of the solid food by baclofen, and (ii) suggests that the inhibition of water intake produced by baclofen is not due to interference with the mechanical aspects of drinking, such as licking on a drinking spout, or the swallowing of liquid.

Pretreatment with the 5-HT1A receptor agonists 8-OH-DPAT or gepirone does not attenuate the inhibitory effect of systemically administered cholecystokinin (CCK) on food intake in rats

In order to investigate whether the inhibitory effect of systemically administered cholecystokinin (CCK) on food intake is dependent on an interaction with central 5-hydroxytryptamine (5-HT) processes, we examined the effects of pretreating rats that were deprived of food for various periods of time (22 h in Experiments 1 and 3, 3 h in Experiment 2, and 1 h in Experiment 4) with the 5-HT1A receptor agonists, 8-OH-DPAT or gepirone, on the suppression of feeding induced by intraperitoneal CCK. 8-OH-DPAT (100 micrograms/kg, Experiments 1 and 2) or gepirone (2 mg/kg, Experiments 3 and 4) administered subcutaneously 60 min prior to intraperitoneal injection of CCK (6 micrograms/kg) did not attenuate the suppressant effect of CCK on feeding in any of these experiments. The 5-HT1A agonists had no significant effects on food intake on their own. As it has been established that the doses of 8-OH-DPAT and gepirone used in this study decrease 5-HT function in the central nervous system, the present results indicate that it is unlikely that the inhibitory effect of systemically administered CCK on food intake is dependent on an interaction with intact central 5-HT systems.

The effects of subcutaneous administration of arginine-8-vasopressin on the electroencephalogram of conscious rats are mediated by peripheral vasopressin V1 receptors

The effects of subcutaneous (s.c.) administration of arginine-8-vasopressin (AVP) were investigated on the electroencephalogram (EEG) of conscious rats. AVP (1 mcg/kg) had no effect on the EEG. However, AVP (5 and 10 mcg/kg) produced significant changes in the spectral power of the EEG in a manner that was consistent with increase in level of arousal. Analogs of AVP which are virtually devoid of vasopressin V1 receptor agonist activity, desglycinamide-9 arginine-8 vasopressin (10 mcg/kg, s.c.) and 1-deamino-8-D arginine-8 vasopressin (10 mcg/kg, s.c.), had no effect on the EEG. Pretreatment with 1-(beta-mercapto-beta,beta-cyclopentamethylene propionic acid)2(O-methyl)tyrosine arginine-8 vasopressin (10 mcg/kg), a vasopressin V1 receptor antagonist, abolished the effects of AVP (10 mcg/kg) on the EEG. As systemically administered AVP cannot cross the blood-brain barrier, the present results indicate that the effects of s.c. AVP on the EEG of conscious rats are mediated by peripheral vasopressin V1 receptors.

Sex difference in the feeding responses of non-deprived rats to the 5-HT1A agonists 8-OH-DPAT and gepirone

The aim of this study was to determine whether there was any difference in the feeding responses of male and female rats to the 5-HT1A agonists 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) and gepirone. 8-OH-DPAT (62.5 and 500 mcg/kg, s.c.) and gepirone (1, 2 and 4 mg/kg, s.c.) significantly increased the amount of food consumed by non-deprived male rats (p < 0.01 in each case) over a 2 h period. Similar doses of 8-OH-DPAT and gepirone had no effects on food intake in non-deprived female rats. However, a higher dose of gepirone (i.e., 8 mg/kg, s.c.) significantly increased feeding (p < 0.01) in non-deprived female rats. These results thus show a clear-cut sex difference in the sensitivity of non-deprived rats to the hyperphagic effects of 5-HT1A agonists.

A70104 and food intake in pigs: implication for the CCK 'satiety' hypothesis

The aim of the present study was to determine whether CCK released from the small intestine during a meal acts as a satiety factor. The effects of the novel CCKA antagonist A70104 (50-200 micrograms kg-1, i.v.) were investigated on operant food intake in 4 h food-deprived pigs. None of the doses used had any significant effects on the amount of food consumed by the animals over the 2 h measurement period. However, pretreatment of the pigs with A70104 (100 micrograms kg-1) did block the inhibitory effect of exogenous CCK (1 microgram kg-1, i.v.) on operant feeding. These results thus question the validity of the hypothesis that endogenous peripheral CCK acts as a satiety factor.

Effects of the 5-HT1A agonist 8-OH-DPAT on food intake in food-deprived rats

The effects of 8-OH-DPAT were investigated on food intake in food-deprived rats in both non-operant and operant feeding paradigms. 8-OH-DPAT produced dose-related reductions in food intake in both paradigms. While the higher doses of 8-OH-DPAT used in this study (i.e. 125-500 micrograms kg-1) produced a number of stereotyped behaviours which may have interfered with normal feeding, these behaviours were not apparent with the lower doses (i.e. 15.625-62.5 micrograms kg-1) which also depressed food-intake. The results of this study thus suggest that the inhibitory effect of 8-OH-DPAT on feeding in food-deprived rats is not secondary to the induction of stereotypy.

The effects of intravenous administration of cholecystokinin on feeding behaviour and release of pituitary hormones in pigs are not mediated by serotonergic (5-HT3) receptors

Two experiments were carried out using the serotonergic (5-HT3) antagonist ondansetron (GR 38032F) to investigate whether the actions of peripherally-administered cholecystokinin octapeptide (CCK) in the prepubertal pig might involve 5-HT3 receptors. In Experiment 1, it was shown that ondansetron (10 or 30 mg, i.v.) did not affect the initial feeding response and did not modify the behavioural inhibition induced by a subsequent injection of CCK (1.3 microgram/kg, i.v.). In Experiment 2, it was demonstrated that the nauseogenic action of CCK, indicated by its stimulatory effect on release of vasopressin and cortisol, was not markedly altered by ondansetron (30 mg, i.v.). These results suggest that 5-HT3 receptors play a negligible part in mediating the behavioural and endocrine responses induced by bolus intravenous injections of CCK in this species.

Systemic administration of baclofen inhibits water intake in rats

1. The present experiments were carried out to investigate the effects of systemic administration of baclofen on water intake in rats. 2. Baclofen (1, 2 and 4 mg/kg, s.c.) inhibited water intake in 16 hr water-deprived rats in a dose-related manner, with maximal effects occurring during the first 30 min after administration. 3. Baclofen (0.25 and 2 mg/kg, s.c.) had no effects on water intake in non-deprived rats. 4. Baclofen (2 mg/kg) inhibited water intake elicited by i.p. injection of hypertonic NaCl in rats. 5. Baclofen (1 mg/kg) did not produce taste aversion in a taste aversion experiment. This indicates that the effects of baclofen on water intake is not due to an aversive effect of the drug.

Effects of the 5HT1A agonist, 8-OH-DPAT, on operant food intake in non-deprived rats

The aim of the present study was to determine whether the hyperphagia produced by systemic administration of 8-OH-DPAT is independent of drug-induced chewing or gnawing behaviour. The effects of 8-OH-DPAT (15.625-500 micrograms kg-1, s.c.) were therefore investigated on operant food intake in non-deprived rats. Low doses (i.e. 15.625-62.5 micrograms kg-1) increased operant food intake. Drug-induced stereotypy disrupted operant responding at the higher doses. The results thus suggest that 8-OH-DPAT-induced hyperphagia is independent of chewing or gnawing behaviour.

The effect of systemic administration of baclofen on food intake in rats

The effects of systemic administration of the GABAB agonist, baclofen was investigated on food intake in non-fasted rats. Baclofen (1.0, 2.0 and 4.0 mg/kg, s.c.) produced a dose-related increase in food intake in a free-feeding paradigm during the first 90 min after administration, with maximum increases occurring at a dose of 2 mg/kg (Experiment 1). Baclofen (0.5 and 1.0 mg/kg, s.c.) also increased food intake in the 40 min post-drug recording period in non-fasted rats, trained to make operant responses for food on a fixed-ratio schedule (Experiment 2). These results demonstrate that systemic administration of baclofen can stimulate ingestive behaviour in satiated rats and suggest a possible role for a GABAB receptor-mediated mechanism in the control of food intake.

Hormonal effects of apomorphine and cholecystokinin in pigs: modification of the response to cholecystokinin by a dopamine antagonist (metoclopramide) and a kappa opioid agonist (PD117302)

Three experiments were carried out to investigate some of the mechanisms involved in the endocrine responses of pigs to the emetic agents apomorphine and cholecystokinin. In Experiment 1, plasma levels of vasopressin and cortisol were measured in prepubertal pigs (N = 5) treated with i.v. apomorphine (25 micrograms/kg) or saline vehicle. In Experiment 2, concentrations of vasopressin and cortisol were determined in pigs given iv sulphated cholecystokinin octapeptide (1.3 micrograms/kg), metoclopramide (300 micrograms/kg), metoclopramide + cholecystokinin, and an oral dose of the kappa opioid agonist PD 117302 (20 micrograms) alone, or followed by i.v. cholecystokinin. In Experiment 3, operant feeding behaviour was quantified in pigs (N = 4) given cholecystokinin (1 microgram/kg) or cholecystokinin preceded by oral PD 117302. Following apomorphine injection in Experiment 1, there was a rapid, transient, rise in plasma vasopressin. Cholecystokinin had a similar effect on vasopressin secretion in Experiment 2 and also induced a later rise in plasma cortisol. Pre-treatment with metoclopramide appeared to reduce both of these effects of cholecystokinin, but only the decrease in cortisol was statistically significant. However, oral administration of PD 117302 abolished the effect of cholecystokinin on vasopressin release and reduced the subsequent rise in cortisol. The inhibitory effect of cholecystokinin on feeding was unaltered by PD 117302 treatment in Experiment 3. The results obtained with apomorphine and metoclopramide, together, suggest that the neuroendocrine effects of cholecystokinin in the pig may involve an action on central dopamine receptors while the effects of PD 117302 indicate that kappa opioids may modify the hormonal responses to cholecystokinin by a peripheral action.

Central and peripheral doses of cholecystokinin that inhibit feeding in pigs also stimulate vasopressin and cortisol release

The effects on vasopressin and cortisol secretion of centrally and peripherally administered cholecystokinin octapeptide (CCK) were investigated in conscious prepubertal pigs. Injection of 1.3 micrograms CCK into the lateral cerebral ventricle resulted in a sustained increase in plasma vasopressin after a latency of 5 min but no change in cortisol concentrations. Intravenous injection of 0.7 and 1.3 micrograms/kg CCK initiated a rapid surge (within 2 min) in plasma vasopressin and a later increase in cortisol secretion. The time course of the vasopressin response to the central injection of CCK was found to be similar to the period of behavioural inhibition induced when an equivalent dose of the peptide was given by the same route in an earlier feeding experiment. An analogous situation was also observed when CCK was given peripherally and, in this case, the threshold dose at which the behavioural and endocrine responses were induced was found to be the same.

Operant food intake in pigs following intracerebroventricular (i.c.v.) administration of prolactin

1. It has previously been shown that systemic administration of prolactin stimulates food intake in a number of animal species. 2. In the present study we investigated whether central administration of prolactin would increase food intake in satiated pigs trained to make operant responses for food and water. 3. Prolactin (25-100 U) administered by intracerebroventricular (i.c.v.) injection produced no significant effects on food intake during the 60 min measurement period after injection. 4. Furthermore, none of these doses of prolactin had any effect on the daily (24 hr) consumption of food in these animals. 5. Similarly prolactin (25-100 U) i.c.v. had no significant short or long-term effects on water intake. 6. These results indicate that prolactin does not stimulate food intake in pigs by a central mode of action.

Effects of intracerebroventricular injection of dynorphin, leumorphin and alpha neo-endorphin on operant feeding in pigs

Young pigs, which are useful experimental animals for biomedical research, were prepared with lateral intracerebroventricular (ICV) cannulae and housed individually in cages fitted with operant panels, with food and water ad lib. ICV injection of 200 micrograms of dynorphin A 1-17 or 1-13 resulted in a significant meal commencing within 2-5 min. Shorter fragments of dynorphin (1-10, 1-9, 1-8) were ineffective at inducing feeding as was dynorphin B (rimorphin). In the same situation, leumorphin and alpha neo-endorphin (200 micrograms) elicited significant feeding but beta neo-endorphin did not. Dynorphin 1-17 or 1-13, administered 5 min before feeding started, increased meal size when pigs were fed after 4-h deprivation. Naloxone ICV (0.4 mg) significantly reduced food intake in pigs feeding after 4-h deprivation and its main effect was in the second half of the meal. Naloxone also abolished the effect of ICV dynorphin. It is concluded that dynorphin and related endogenous opioids are involved in the regulation of food intake in pigs.

Effect of intracerebroventricular administration of the GABAB-receptor agonist baclofen on operant feeding in satiated pigs

1. The present study investigated the effects of intracerebroventricular (i.c.v.) administration of the GABAB-receptor agonist baclofen on food and water intake in satiated pigs previously trained to make operant responses for food and water, which were available ad libitum. 2. Baclofen (25-100 nmol) i.c.v. produced a dose-related increase in food intake. Baclofen (50 nmol) increased feeding during the first 15 min after administration (P less than 0.01), while the 100 nmol dose increased feeding during the first 30 min (P less than 0.01). None of these doses of baclofen had any affect on the daily (24 h) food intake. 3. The effect of baclofen (50 nmol) on feeding was prevented by pretreating the animals with the GABAB antagonist phaclofen (500 nmol, i.c.v.). 4. Baclofen (25-100 nmol) i.c.v. had no significant effects on water intake. 5. Intravenous administration of baclofen (100 nmol) had no effect on food intake, thus eliminating the possibility that i.c.v. baclofen might have stimulated feeding by a peripheral mode of action. 6. These results show that baclofen increases food intake in satiated pigs, and that this effect is mediated by the drug acting at central GABAB-receptors.

Effects of intracerebroventricular injection of muscimol or GABA on operant feeding in pigs

Young pigs were prepared with lateral intracerebroventricular (ICV) cannulae. They were housed individually in cages fitted with operant panels and could obtain food and water ad lib. The GABA-A receptor agonist muscimol (25-200 nmol) ICV produced an increase in food intake in which the dose-response relation was most obvious 30-60 min after dosing. The 25-nmol dose had no effect on feeding. However, muscimol (50 nmol) caused a significant increase in feeding (p less than 0.01) during the first 30 min after injection, while the 100- and 200-nmol doses increased food intake (p less than 0.01) during the first 60 min. The effect of muscimol (100 nmol) on food intake was completely abolished by the simultaneous administration of the GABA-A receptor antagonist bicuculline (100 nmol). GABA (40-1600 nmol) ICV also produced a dose-related increase in food intake (p less than 0.01) in the 15 min after injection. Only doses of 800 nmol and above were effective. The effects of GABA (1600 nmol) were completely abolished by the simultaneous administration of bicuculline (50 nmol). Neither muscimol nor GABA influenced food intake for the 24-hr time period or water intake during any time period. The results indicate that stimulation of central GABA-A receptors induces operant feeding in the satiated pig.

The effect of intracerebroventricular administration of baclofen on food intake in rats

In the present study the effects of intracerebroventricular (ICV) administration of baclofen (0.1-5.0 nmol) was investigated on food intake in non-fasted rats. Baclofen (1.0, 2.5 and 5.0 nmol) increased food consumption in a dose-related manner during the first 15-30 min after administration. The effects of baclofen (5.0 nmol) on feeding were prevented by pretreating the rats with the specific GABA-B receptor antagonist phaclofen (40 nmol, ICV). These results suggest that baclofen increases food intake in rats by an action at central GABA-B receptors.

Effects of the CCK receptor antagonist MK-329 on food intake in pigs

The cholecystokinin (CCK) receptor antagonist MK-329 (previously L364,718) was administered intravenously (IV) (17.5-140 micrograms/kg) to pigs trained to make operant responses for food reinforcements after 4 hr of food deprivation. MK-329 produced a dose-related increase in food intake during the 2-hr test period, with maximum increases occurring at a dose of 70 micrograms/kg. CCK (1 micrograms/kg IV) produced a short-term reduction in feeding and this effect was completely abolished by pretreating the animals with MK-329 (70 micrograms/kg). The present results lend support to the hypothesis that endogenous CCK is involved in satiety.

Anterior and posterior pituitary hormone release induced in sheep by cholecystokinin

The effect of an intravenous bolus injection of cholecystokinin octapeptide (CCK-8; 0.85 micrograms/kg) on the release of cortisol, prolactin, vasopressin, and oxytocin was studied in sheep (n = 10). Concentrations of these hormones were measured in blood samples taken before (-10, 0 min) and after (5, 10, 20 min) administration of a saline vehicle or vehicle + CCK. Following CCK treatment, levels of cortisol were raised after 10 and 20 min, prolactin and vasopressin concentrations were increased after 5 min, and oxytocin secretion was unaffected.

Can vasopressin alone act as an unconditioned stimulus to produce passive avoidance behaviour in rats in a typical memory experiment?

The claim that vasopressin improves memory has been largely based on results obtained from shock avoidance experiments. In the majority of these studies, "memory" was defined operationally as the hesitation of a rat to enter the darkened compartment of a box in which it had once received an electric foot-shock. A single post-trial injection of arginine vasopressin (AVP) enhances such passive avoidance behaviour. In view of the recent demonstration that AVP has aversive effects, it was argued that vasopressin alone (without giving the rats foot-shock, prior to the peptide) might be a sufficient inhibitory stimulus to produce passive avoidance behaviour in a typical memory experiment. This hypothesis was tested in the present study. The results of these experiments indicate that a behaviourally active dose of AVP (10 micrograms/kg; s.c.) was a sufficient stimulus to produce passive avoidance behaviour in the rats. A small dose of AVP (1.25 micrograms/kg; s.c.) was without effect. However, AVP (10 microgram/kg) was only effective with repeated administration (Experiment 1). This result is in contrast with the post-trial effect of the peptide on inhibitory avoidance behaviour, which is obtained with just one injection in the normal single trial step-through experiment. However, it was found that if the rats were injected with AVP (10 micrograms/kg) and placed in the dark compartment of the apparatus for 20 min, thereby ensuring that the animals made the explicit connection between the aversive effects of the peptide and the dark environment, they displayed avoidance behaviour after a single trial (Experiment 2).(ABSTRACT TRUNCATED AT 250 WORDS)