Gut Commensal E. coli Proteins Activate Host Satiety Pathways following Nutrient-Induced Bacterial Growth

The composition of gut microbiota has been associated with host metabolic phenotypes, but it is not known if gut bacteria may influence host appetite. Here we show that regular nutrient provision stabilizes exponential growth of E. coli, with the stationary phase occurring 20 min after nutrient supply accompanied by bacterial proteome changes, suggesting involvement of bacterial proteins in host satiety. Indeed, intestinal infusions of E. coli stationary phase proteins increased plasma PYY and their intraperitoneal injections suppressed acutely food intake and activated c-Fos in hypothalamic POMC neurons, while their repeated administrations reduced meal size. ClpB, a bacterial protein mimetic of α-MSH, was upregulated in the E. coli stationary phase, was detected in plasma proportional to ClpB DNA in feces, and stimulated firing rate of hypothalamic POMC neurons. Thus, these data show that bacterial proteins produced after nutrient-induced E. coli growth may signal meal termination. Furthermore, continuous exposure to E. coli proteins may influence long-term meal pattern.

Effects of the GABAB receptor agonist baclofen on primary drinking in rats

The effects of subcutaneous (s.c.) administration of the GABA(B) receptor agonist baclofen were investigated on primary drinking in rats. Baclofen (1-4 mg/kg) produced a dose-related reduction in cumulative water intake in 16 h water-deprived rats during the 120 min measurement period (Experiment 1). The suppressant effect of baclofen (2mg/kg) on water intake 16 h water-deprived rats was significantly attenuated by pretreatment with the GABA(B) receptor antagonist CGP 35348 (3-aminopropyl (diethoxymethyl)-phosphinic acid; 50mg/kg; s.c., Experiment 2.), indicating that the hypodipsic effects of the drug in thirsty rats are mediated by an action at GABA(B) receptors. Experiment 3 was undertaken to investigate the effects of baclofen on volemic drinking induced in rats pretreated with propylene glycol. S.C. administration of polyethylene glycol induces volemic drinking in rats by reducing extracellular fluid. Baclofen (2mg/kg, s.c.) significantly reduced the volemic drinking in rats pretreated with polyethylene glycol (30% w/v solution). Experiment 4 was conducted to investigate the effects of baclofen on osmotic drinking in non-deprived rats pretreated with hypertonic sodium chloride (NaCl) solution. Hypertonic NaCl will draw out intracellular fluid to stimulate osmotic drinking. Baclofen (2mg/kg; s.c.) significantly reduced osmotic drinking in rats pretreated with 1 ml hypertonic NaCl (16% w/v). The results of this study indicate that (i) the hypodipsic effect of baclofen in water-deprived rats is mediated by an action at GABA(B) receptors and (ii) baclofen suppresses both volemic and osmotic drinking.

Effects of intraperitoneal administration of the GABA B receptor agonist baclofen on food intake in rats measured under different feeding conditions

The effects of intraperitoneal (i.p.) administration of the GABA(B) receptor agonist baclofen were assessed in rats under different feeding conditions. In Experiment 1, it was observed that baclofen (1-4 mg/kg) significantly (at least, P<0.05) increased cumulative food intake in non-deprived rats during the 120 min measurement period during the early light phase of the light-dark cycle. By contrast, during the early dark phase of the light-dark cycle in non-deprived rats, the 1mg/kg doses of baclofen significantly increased cumulative feeding at 30, 60 and 120 min (at least P<0.05), the 2mg/kg dose significantly increased feeding at 30 and 60 min (at least P<0.05) and the 4 mg/kg dose had no effects on feeding. In Experiment 2, baclofen (1-4 mg/kg) was found to produce no significant effects on food intake in rats that were food-deprived for 22 h. In Experiment 3, the effects of baclofen were investigated on food intake in 16 h food-deprived rats that had received an oral preload for 2h prior to drug administration. Baclofen (1-4 mg/kg) significantly increased cumulative food consumption (at least, P<0.05) only during the first 30 min after administration in these animals. The results of this study indicate that the effects of baclofen on food intake may be related to the state of hunger or satiety of the animals and the time during the light-dark cycle when the drug is administered.

The effects of chronic intraperitoneal administration of the GABA B receptor agonist baclofen on food intake in rats

This study was undertaken to examine the effects of repeated administration of the GABA(B) receptor agonist baclofen on food intake in male Wistar rats. In the 1st Experiment, the effects of daily administration of physiological saline and baclofen (2 mg/kg, i.p.) for 27 days were investigated on food intake and body weight in non-deprived rats (n=6 in each group). Baclofen significantly (P<0.05) increased cumulative food intake each day over the treatment period during the 60 min measurement period following administration. Tolerance did not develop to the short-term hyperphagic effect of baclofen over the course of the experiment. In addition, treatment with baclofen did not alter body weight of the animals over the 27 day treatment period when compared with the saline control rats. In the 2nd Experiment, the effects of acute and chronic administration of baclofen (2 mg/kg) were investigated on 24 h food intake in rats. The rats were injected daily for 21 days with either saline (n=6) or baclofen (n=6). Food intake was measured in 30 min time bins for 24 h on treatment Days 1, 12 and 21 following injection. The results showed that while baclofen produced short-term increases in food consumption following injection on treatment Days 1, 12 and 21, the daily (24 h) food intake of the animals was not significantly different from those of control rats. Thus, these data reveal that while chronic administration of baclofen (2 mg/kg) produces short-term increases in feeding without the development of tolerance, daily (24 h) food consumption is not affected. These findings are consistent with the observation that chronic administration of baclofen (2 mg/kg) had no effect on the body weight of these animals.

The effects of intraperitoneal administration of the GABA(B) receptor agonist baclofen on food intake in CFLP and C57BL/6 mice

The effects of the GABA(B) receptor agonist baclofen were investigated on food intake in non-deprived CFLP and C57BL/6 mice. In Experiment 1, baclofen (1-8 mg /kg) administered i.p. to CFLP mice, produced a dose-related increase in food intake. The 4 and 8 mg/kg doses produced significant increases in cumulative feeding when measure 120 min after administration (at least P < 0.05, in each case). In Experiment 2, baclofen (1-10 mg/kg), administered intraperitoneally (i.p.) to C57BL/6 mice, also produced a dose-related increase in food intake. The 4 mg/kg dose of baclofen significantly increased cumulative food intake at 60 min (P < 0.05), while the 2 and 4 mg/kg doses significantly increased cumulative food intake at 120 min (P < 0.01, in each case). The 10mg/kg dose was without effect. These data show that systemic administration of the GABA(B) agonist baclofen produces an increase in food consumption in two different strains of mice and extend previous observations made in rat to another rodent species.

The effects of 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) on food intake in non-deprived C57BL6 mice

The effects of the 5HT1A agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) were investigated on food intake in non-deprived mice. 8-OH-DPAT (50-200 mg/kg) administered subcutaneously (s.c.) 5 min prior to presentation of food, produced a dose-related increase in cumulative food intake in C57BC6 mice. The hyperphagic effect of 8-OH-DPAT (100 mg/kg, s.c.) was abolished by concurrent treatment with the 5HT1A receptor antagonist N-[2-(4-2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridyl) cyclohexanecarboxamide (WAY100635; 0.3 mg/kg, s.c.). These data show that 8-OH-DPAT produces an increase in food consumption in non-deprived mice by a 5-HT1A receptor-mediated mechanism of action.

8-Hydroxy-2-(di-N-propylamino)-tetralin (8-OH-DPAT) inhibits food intake in fasted rats by an action at 5-HT1A receptors

The effects of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) on food intake were investigated in food-deprived rats. 8-OH-DPAT (25-100 microg/kg) administered subcutaneously immediately prior to the presentation of food produced a dose-related decrease in food intake in rats that had been fasted for 22 h. The hypophagic effect of 8-OH-DPAT (50 microg/kg) was abolished by pretreatment with the selective 5-HT1A receptor antagonist n-[2-(4-2-methoxyphenyl)-1-piperazinyl]-n-(2-pyridyl) cyclohexanecarboxamide (WAY 10063; 0.3 mg/kg). The results of this study show that the acute dose-dependent depressant effect of 8-OH-DPAT on food intake in fasted rats is mediated by an action at 5-HT1A receptors.

A comparison of the effects of the 5HT1A antagonists MM-77 and WAY-100635 on the mouse isolated vasa deferentia

1. Experiments were carried out to characterize the possible adrenergic properties of the 5-HT(1A) antagonists WAY 100635 and MM-77 using the mouse isolated vasa deferentia preparation. 2. When vasa deferentia were preincubated for 10 min in the presence of MM-77 (10(-8)-10(-6) m) or WAY100635 (10(-8)-7 x 10(-7) m), a concentration-dependent inhibition of the contractile response to submaximal electrical field stimulation (10 Hz, 50 V, 50 ms) was observed with pIC(50) values of 7.05 +/- 0.01 and 6.85 +/- 0.1 respectively. 3. MM-77 (10(-8)-10(-6) m) antagonized the contractile responses of the vasa deferentia to phenylephrine (PE) (10(-6)-10(-3) m) in a concentration-dependent manner. Schild plots of these data were linear and yielded a mean rhoA(2) value of 6.81 +/- 0.084. The mean slope was 1.42 +/- 0.22. 4. WAY100635 (10(-8)-10(-6) m) antagonized the contractile responses of the vasa deferentia to PE (10(-6)-10(-3) m) in a concentration-dependent manner. Schild plots of these data were linear and yielded a mean rhoA(2) value of 7.05 +/- 0.08. The mean slope was 0.97 +/- 0.1. 5. The results suggest that while WAY100635 acts as a competitive antagonist at alpha(1)-adrenoceptors, MM-77 displays non-competitive antagonist characteristics at this receptor subtype. 6. These results may have important implications for the use of these compounds as 5-HT(1A) receptor antagonists in in vivo studies.

The effects of intraperitoneal and intracerebroventricular administration of the GABAB receptor antagonist CGP 35348 on food intake in rats

In order to test the hypothesis that endogenous gamma-aminobutyric acid (GABA), acting at central GABAB receptors, plays a physiological role in the control of feeding behaviour, it was reasoned that blocking these receptors with a centrally active GABAB receptor antagonist should reduce food intake in hungry rats. In the present study, experiments were carried out to test this possibility using the GABAB receptor antagonist 3-aminopropyl-diethoxy-methyl-phosphinic acid (CGP 35348), which is water-soluble and can penetrate the blood-brain barrier from the systemic circulation. CGP 35348 (50 and 100 mg/kg, i.p.) had no effect on food intake in 22-h fasted rats, but a higher dose (i.e. 500 mg/kg., i.p.) significantly reduced cumulative food consumption. These findings are consistent with previous observations that high systemic doses of CGP 35348 are needed to block central GABAB receptors. However, to eliminate the possibility that the 500 mg/kg dose of CGP 35348 decreased food intake by a peripheral, rather than a central mode of action, further experiments were undertaken where the drug was given directly into the brain by the intracerebroventricular (i.c.v.) route. I.c.v. administration of CGP 35348 (5 and 10 microg) significantly decreased cumulative food intake food intake in rats that had been fasted for 22 h. By contrast, i.c.v. administration of CGP 35348 (10 microg) had no effect on water intake in 16-h water-deprived rats. The results indicate that CGP 35348 reduces food consumption in hungry rats by blocking central GABAB receptors in a behaviourally specific manner. These findings suggest that endogenous GABA acting at central GABAB receptors plays a physiological role in the regulation of feeding behaviour.

Differential effects of the 5-HT1A agonist gepirone on food intake in rats may depend on their motivation to eat

It has previously been suggested that the differential effects of 5-hydroxytryptamine1A (5-HT1A) receptor agonists on food intake in non-deprived and food-deprived animals may be related to their motivation to feed. The present study was therefore conducted to investigate a possible relationship between the effects of the 5HT1A agonist gepirone on food intake in rats and their motivation to eat. In the first experiment, motivation to feed was varied by depriving the rats of food for different periods of time. In the second experiment, motivation to feed was varied by presenting them with highly palatable food. The results obtained from the first experiment showed that gepirone (1-4 mg/kg) increased food intake in non-deprived rats but decreased intake in 5-h and 22-h fasted animals. The extent of the hypophagia was related to the time the animals were fasted. The results from the second experiment showed that gepirone (0.5-5 mg/kg) reduced food intake in non-deprived rats presented with a highly palatable diet. The results obtained in this study suggest that the effects of gepirone on food intake may be related to the motivation of the animals to eat.

Effects of the GABAB receptor agonists baclofen and 3-aminopropylphosphinic acid (3-APA) on food intake in rats

It has previously been demonstrated that central administration of the GABAB agonist baclofen increases food intake in non-deprived or satiated animals by action at the central GABAB receptors. It has also been shown that systemic administration of baclofen increases food intake in non-deprived rats. Although baclofen crosses the blood brain barrier from systemic circulation to enter the brain, it is conceivable that it may increase food intake by a peripheral mode of action. This possibility was examined in the present study. The effects of intraperitoneal (i.p.) administration of the GABAB receptor agonists baclofen and 3-aminopropylphosphinic acid (3-APA) were investigated on food intake in non-deprived rats. 3-APA is a potent GABAB agonist that does not penetrate the blood brain barrier. The results show that while baclofen (1-4 mg/kg) increases food intake in a dose-related manner, 3-APA (0.5-2 mg/kg) has no effects on food consumption. The results indicate that a peripheral GABAB mechanism is not involved in the hyperphagia elicited by systemic administration of baclofen and suggests that the drug increases food intake by a central mode of action.

The effects of a peripherally acting cholecystokinin1 receptor antagonist on food intake in rats: implications for the cholecystokinin-satiety hypothesis

The observation that systemic administration of the peptide cholecystokinin (CCK) inhibits food intake in mammalian species has led to the hypothesis that endogenous peripheral CCK released from the small intestine during a meal acts as a satiety factor. It was predicted that if CCK does play an important role in satiety, then systemic administration of a specific CCK receptor antagonist should block the effects of the endogenous peptide released during a meal and increase food intake. The present study was undertaken to test the hypothesis by investigating the effects of the cholecystokinin(1) (CCK(1)) receptor antagonist N-alpha-3'-quinolinoyl-D-Glu-N,N-dipentylamide dicyclohexylammonium (A70104), which is unlikely to cross the blood-brain barrier, on food intake in rats. A70104 (20-200 microg/kg, i.p.) had no any significant effect on the intake of a test meal in rats under different experimental conditions. However, pretreatment of rats with A70104 (50 microg/kg, i.p.) abolished the inhibitory effects of exogenous peripheral CCK (5 microg/kg, i.p.) on food intake. The findings that A70104 had no effect on food intake when administered on its own, but abolishes the suppressant effect of exogenous peripheral CCK, suggest that endogenously released peripheral CCK does not play an important role as a satiety factor in rats.

The effects of the benzodiazepine receptor ligand 3-(methoxycarbonyl) amino-beta-carboline on food and water intake in rats

The effects of the benzodiazepine receptor ligand 3-(methoxycarbonyl) amino-beta-carboline (beta-CMC) were investigated on food intake in rats that had been fasted for 22 h, and on water intake in rats that had been deprived of water for 16 h. beta-CMC (2-8 mg/kg i.p.) produced a dose-related inhibition of food intake. Significant hyperphagia (p < 0.01) was observed with the 8 mg/kg dose. By contrast, the 8 mg/kg dose did not affect water intake in thirsty rats. The data suggests that beta-CMC has properties on food intake similar to those of a benzodiazepine receptor inverse agonist.

Effects of subcutaneous administration of the gamma-aminobutyric acid(A) receptor agonist muscimol on water intake in water-deprived rats

The effects of the gamma-aminobutyric acid(A) (GABA(A)) receptor agonist muscimol were investigated on water intake in rats that had been deprived of water for 16 h. Muscimol (0.5-2.0 mg/kg sc) produced a dose-related inhibition of water consumption in both male (n=8) and female (n=8) rats, with maximal suppression of drinking occurring during the first 30 min after administration. Doses of 1 and 2 mg/kg produced significant decreases in water intake (P<.01), while a lower dose of 0.5 mg/kg was without effect. The hypodipsic effect of muscimol (1.0 mg/kg sc) was abolished by pretreatment of the animals with the GABA(A) receptor antagonist bicuculline (1 mg/kg sc). Furthermore, muscimol (2 mg/kg sc) did not produce aversion in a two-bottle conditioned taste aversion test, indicating that the suppressant effects of muscimol on water intake are not due to drug-induced malaise. The results suggest that systemic administration of muscimol produces a behaviourally specific suppression of primary drinking in rats by a GABA(A) receptor-mediated mechanism. Moreover, this action of muscimol appears to be independent of the gender of the animals.

Effects of intracerebroventricular administration of the CCK(1) receptor antagonist devazepide on food intake in rats

The effects of intracerebroventricular administration of devazepide, a CCK(1) receptor antagonist, was investigated on food intake in rats. In the first experiment, rats (n=5) were deprived of food for 17 h and injected intracerebroventricularly with either vehicle or devazepide (1, 10, 25 or 100 ng). Five minutes after vehicle or drug administration, the animals were presented with food and intake measured for 60 min. Devazepide produced a dose-related increase in food intake. Doses of 1, 10 and 25 ng significantly increased consumption (at least P<0.01 in each case). A second experiment was subsequently undertaken to investigate whether systemic administration of the intracerebroventricular doses used in the first experiment would affect food intake. Rats (n=8) that have been deprived of food for 17 h were injected intraperitoneally with either vehicle or devazepide (3, 30, 75 or 300 ng/kg). Five minutes after vehicle or drug administration, the animals were presented with food and intake was measured for 60 min. Devazepide (3-300 ng/kg, i.p.) had no significant effects on food consumption. The results show that central administration of low doses of devazepide increase food intake in rats, while similar doses, given systemically, do not affect consumption. These findings suggest the possibility that endogenous cholecystokinin (CCK), acting at central CCK(1) receptors, may play a physiological role in the control of feeding behaviour in the rat.

The differential effects of intravenously administered 8-OH-DPAT on operant food intake in satiated and food-deprived pigs are mediated by central 5-HT(1A) receptors

It has previously been shown that the intravenous administration of the 5-HT(1A) receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), increases food intake in satiated pigs and decreases food intake in fasted pigs. The present experiments were conducted to investigate the effects of central administration of the 5-HT(1A) receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-N-2-pyridinyl-cyclohexane carbox-amide maleate (WAY 100635), on the stimulant and depressant effects of 8-OH-DPAT on operant food intake in satiated and hungry pigs. In Experiment 1, 8-OH-DPAT (25 microg/kg) produced an increase in operant feeding during the first 30 min following intravenous administration to satiated pigs. The 8-OH-DPAT-induced hyperphagia was completely abolished by pretreatment with WAY 100635 (0.3 mg) administered by intracerebroventricular injection. In Experiment 2, 8-OH-DPAT (25 microg/kg) administered intravenously 15 min prior to the onset of feeding in pigs that had been fasted for 22.5 h produced a decrease in operant food intake, which was most apparent during the first 30 min of the feeding period. The hypophagic effect was completely abolished by pretreatment with WAY 100635 (0.3 mg icv) administered 30 min before the start of the feeding period. In both experiments, WAY 100635 (0.3 mg icv) did not have any significant effects on feeding. The results of the present study extend previous results in the pig and show that both the hyperphagic and the hypophagic effects of 8-OH-DPAT in satiated and fasted pigs, respectively, are mediated by central 5-HT(1A) receptors.

Ipsapirone suppresses food intake in food-deprived rats by an action at 5-HT(1A) receptors

The effects of the 5-HT(1A) receptor agonist ipsapirone (2.5-10 mg/kg, s.c.) on food intake were investigated in rats that had been fasted for 22 h. Ipsapirone administered immediately prior to presentation of food produced a dose-related decrease in food intake. The effect of ipsapirone (5 mg/kg) was reversed by pretreatment with the 5-HT(1A) receptor antagonist N-[2[4-)2-methoxyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexaneca rboxamide trihydrochloride (WAY100635) (0.3 mg/kg, s.c.). The results of this study show that ipsapirone has acute dose-dependent depressant effects on food intake in fasted rats, mediated by 5-HT(1A) receptors.

Effects of the 5-HT1A receptor agonist 8-OH-DPAT on operant food intake in food-deprived pigs

The effects of the 5-HT1A agonist 8-hydroxy-2 (di-n-propylamino)tetralin (8-OH-DPAT) were investigated on operant food intake in food-deprived pigs. In Experiment 1, 8-OH-DPAT (5-20 microg/kg) administered intravenously (i.v.) 15 min prior to the occurrence of feeding produced a dose-related decrease in operant food intake in pigs that had been fasted overnight. The effects were mainly apparent during the first 30 min after the start of the feeding period. In Experiment 2, 8-OH-DPAT (25 and 50 microg/kg, i.v.) administered 60 min prior to the occurrence of feeding in pigs that were fasted overnight also produced significant decreases in food intake. The effects were mainly apparent during the first 30-40 min after the start of the feeding period. In Experiment 3, 8-OH-DPAT (20 microg/kg, i.v.) significantly increased operant feeding in satiated pigs during the first 30 min after administration. These results show that 8-OH-DPAT has complex effects on feeding behaviour in pigs, increasing operant food intake in satiated pigs, while producing a reduction in food intake in food-deprived animals.

Intraperitoneal, but not subcutaneous, administration of the sulphated cholecystokinin octapeptide (CCK-8S) inhibits operant and nonoperant food intake in rats: implications for the CCK-satiety hypothesis

The effects of administering CCK-8S by the subcutaneous (s.c.) and intraperitoneal (i.p.) routes were investigated on both operant and nonoperant food intake in rats that had been fasted for 22 h. Intraperitoneal administration of CCK-8S (5 micrograms/kg) significantly reduced both operant and nonoperant food intake. In contrast, CCK-8S (5-50 micrograms/kg) administered s.c. had no effects on food intake in both types of behavioral paradigms. The results show that the peripheral route of administration of CCK-8S is an important factor to be taken into consideration when investigating its effects on food intake. The results are discussed in terms of the hypothesis that CCK released from the small intestine during a meal acts in a paracrine fashion to produce satiety.

Food for thought: a critique on the hypothesis that endogenous cholecystokinin acts as a physiological satiety factor

This review evaluates the various lines of evidence supporting the hypothesis that cholecystokinin (CCK) released from the small intestine during feeding plays a physiological satiety. Issues considered include, the effects of systemic injection of CCK on consummatory and operant feeding, the role of the vagus nerve, the effects of CCKB receptor antagonists, and the neuroendocrine responses to exogenous CCK. A critical appraisal of this research indicates that while it is clearly demonstratable that exogenous peripheral CCK can alter food intake by acting on CCKA receptors, the mechanism involved may be more closely related to the induction if aversion and nausea, rather than satiety. With regard to peripheral endogenous CCK, the available evidence also does not seem to support a role for the hormone in satiety. In particular, it is doubtful whether plasma concentrations of CCK following a meal are sufficiently high to inhibit feeding. Moreover, CCKA receptor antagonist which do not cross the blood brain barrier fail to increase meal size, as would be expected if peripheral CCK was an effective satiety factor. In addition, the recent literature concerned with the possibility that CCK may have a direct action within the brain in the control of food intake has been reviewed. These studies show that CCK administered intracerebroventicularly, or by micoinjection into discrete brain regions, also inhibits feeding via a CCKA receptor mechanism. However, the physiological relevance of these findings have yet to be determined.

A study on the involvement of GABAB receptor ligands in stress-induced antinociception in male mice

Swimming at 21 degrees C for 3 min induced antinociceptive effects in male mice with all animals showing a significant increase in response time to the hot plate test measured 10 min after swimming. This antinociceptive activity was still evident at 20 min after swimming. Prior administration of the GABAB receptor agonist, baclofen, potentiated the swimming-induced antinociception. For the group receiving the higher dose of baclofen (2.0 mg/kg, s.c.) the potentiation was still evident 40 min postswimming. However, prior administration of CGP-35348, a GABAB receptor antagonist, had no effect on the antinociceptive activity observed after swimming. Restraint for 1 h also induced significant antinociceptive activity in male mice. This restraint-induced antinociceptive activity was enhanced by prior administration of baclofen and was completely abolished by the administration of CGP-35348 before restraint. The present findings suggest that GABAB receptors may play a role in stress-induced antinociception and different GABAB receptor subtypes may be involved depending on the nature of the stress.

Effects of buspirone on operant and nonoperant food intake in food deprived rats

The effects of the clinically available 5-HT1A agonist, buspirone, were investigated on food intake in food-deprived rats in both nonoperant and operant feeding paradigms. Buspirone (0.5-2.0 mg/kg), administered subcutaneously (s.c.) 15 min prior to presentation of food, produced a dose-related inhibition of food intake in the nonoperant feeding paradigm. The main inhibitory effects of buspirone were apparent during the first 30 min after food presentation. Likewise, in the operant feeding paradigm, buspirone (0.25-1.0 mg/kg, s.c.) produced a dose-related suppression of food intake in the first 30 min after administration. None of the doses of buspirone produced stereotyped or abnormal behavioral changes in the animals. The results of the study, therefore, suggest that the inhibitory effects of buspirone on food intake in food-deprived rats are unlikely to be due to nonspecific disruptions of behavior. Furthermore, the present findings may have important clinical implications with regard to possible ingestible side effects that use of this drug may have in the treatment of psychiatric disorders.

The effects of central administration of the CCK-B receptor agonist pentagastrin on feeding and cortisol release in sheep

In order to test the validity of the hypothesis that implicates central cholecystokinin-B (CCK-B) receptors in the etiology of anxiety and panic disorders, we argued that activation of these receptors by the CCK-B receptor agonist pentagastrin should (i) suppress feeding in hungry animals, and (ii) increase plasma concentrations of the stress hormone cortisol. We found that intracerebroventricular administration of pentagastrin (4 and 10 micrograms) did not inhibit feeding nor increase cortisol secretion in sheep (n = 6). Moreover, the peptide did not produce behavioral responses in the animals. We conclude that central CCK-B receptors are unlikely to be involved in behaviors associated with anxiety or panic in this species.

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)