Evidence that Release of Dopamine in the Brain is Involved in the Inhibitory Effect of Cholecystokinin Octapeptide on Ingestion of Intraorally Administered Sucrose in Male Rats

Peptides that regulate food intake: norepinephrine is not required for reduction of feeding induced by cholecystokinin

CCK octapeptide (CCK-8) is released by the gut in response to a meal and acts via CCK(A) receptors on vagal afferents to induce satiety. However, the central neural pathways by which peripheral CCK-8 affects feeding are poorly understood. In the present study, we tested the hypothesis that norepinephrine (NE) is necessary for satiety induced by peripheral CCK-8 by using mice lacking dopamine beta-hydroxylase (Dbh(-/-)), the enzyme responsible for synthesizing NE and epinephrine from dopamine. We found that Dbh(-/-) mice are as responsive to the satiating effects of CCK-8 as their normal littermates.

Estradiol, CCK and satiation

Estradiol has long been known to inhibit feeding in animals, but the mechanism(s) mediating its effects have not been clear. Demonstrations that estradiol's feeding effects are expressed as decreases in meal size coupled with the emerging consensus that cholecystokinin (CCK) released from the small intestines during meals is a physiological negative-feedback signal controlling meal size (i.e. satiation) suggested a new approach to the problem of the mechanisms of estradiol's inhibitory effect on feeding. Progress on this approach is reviewed here. Experimental manipulations of exogenous and endogenous CCK and estradiol have produced converging evidence that estradiol cyclically increases the activity of the CCK satiation-signaling pathway so that meal size and food intake decrease during the ovulatory or estrus phase of the ovarian cycle. This is a striking example of the modulation of the operation of a control of meal size by the physiological context in which the meal occurs. Estradiol also produces a tonic decrease in meal size, but this apparently does not involve the CCK satiation-signaling pathway. Where and how estradiol acts to increase the potency of the CCK satiating-signaling pathway are not known. Several possible sites are suggested by the observations that estradiol treatment increases feeding- and CCK-induced expression of c-Fos in ovariectomized animals in brain areas including the nucleus tractus solitarius, paraventricular nucleus of the hypothalamus, and central nucleus of the amygdala. Tests with null mutation mice indicate that estrogen receptor-alpha is necessary for estradiol's feeding effects. Finally, the possibilities that estradiol exerts important influences on normal or disordered eating in women are discussed. It is concluded that estradiol exerts a biologically significant action on CCK satiation in animals. Further research to determine whether this action of estradiol has a role in the pathogenesis, course, or treatment of disordered eating in women is indicated.

Effects of central oxytocin administration on intraoral intake of glucose in deprived and nondeprived rats

We evaluated the effects of lateral intracerebroventricular administration of oxytocin (OT) and/or a selective oxytocin-receptor antagonist (OTX), 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-OT, on ingestion of intraorally delivered 12.5% glucose in rats that were either nondeprived or deprived of food for 20 h. In deprived rats, OT delivered 30 min before an initial intake test yielded a dose-related reduction of intraoral glucose intake. The highest dose tested, 20 nmol, reduced intraoral glucose intake by 45%. The effect was short-lived, however. Intraoral intake for a second test, initiated 60 min after the termination of the first, increased as a function of OT dose so that total session intake was unaffected by OT treatment. The suppression of intraoral intake by 20 nmol OT was reversed by pretreatment (45 min before testing) with OTX. In nondeprived rats, by contrast, OT yielded no effect on first-test, second-test, or total session intakes. Significant increases in first-test and total session intakes were obtained when OTX (20 nmol) was administered alone both in deprived (32% increase in first-test intake) and nondeprived (31% increase) rats. In general, the results obtained are consistent with the suggestion that OT contributes to the control of meal size and, in particular, to the process of satiation, which is the aspect of ingestive control highlighted by the specialized intake test used in the present study.

Cholecystokinin, dopamine D2 and N-methyl-D-aspartate binding sites in the nucleus of the solitary tract of the rat: possible relationship to ingestive behavior

Receptor autoradiography was used to investigate the distribution of brainstem binding sites for cholecystokinin, dopamine and N-methyl-D-aspartate with particular reference to the nucleus of the solitary tract of the rat, an area involved in the control of ingestive behavior. Binding sites for the A and B subtypes of the cholecystokinin receptor, labeled with [(125)I]cholecystokinin octapeptide sulfate in the presence or absence of antagonists for the devazepide (A) or L-365,260 (B) receptor, were present throughout the caudal rostral extent of the nucleus of the solitary tract, the A type predominating in the commissural, medial and gelatinous part and the B type in the lateral part. In the most rostral part of the medial nucleus of the solitary tract, both A and B receptors were present. Dopamine D2 receptors, labeled with [(125)I]NCQ-298, were found in all parts of the nucleus of the solitary tract. No binding to the dopamine D1 receptor, labeled with [(125)I]SCH-23982, was found in the brainstem. N-Methyl-D-aspartate receptors, labeled with [(3)H]dizocilpine maleate, were also present in the entire caudorostral extent of the nucleus of the solitary tract. Binding to cholecystokinin A receptors was co-distributed with [(125)I]NCQ-298 and [(3)H]dizocilpine maleate binding in the caudal and rostral parts of the nucleus of the solitary tract, and binding to cholecystokinin B receptors overlapped with [(125)I]NCQ-298 and [(3)H]dizocilpine maleate binding in the rostral nucleus of the solitary tract. These results are consistent with the hypothesis that cholecystokinin, dopamine and glutamate interact in the nucleus of the solitary tract in the control of ingestive behavior.

Reward and reinforcement produced by drinking sucrose: two processes that may depend on different neurotransmitters

The capacity of sucrose drinking to produce conditioned place preference (CPP) and the effects of dopamine and opioid receptor blockade on acquisition of this preference and on sucrose consumption were evaluated. The establishment of place preference would reflect the reinforcing consequences of drinking, while the amount of sucrose solution consumed would illustrate its rewarding properties. Male Wistar rats were allowed to drink an 18% sucrose solution. No deprivation was used. Drinking produced place preference, and this was blocked with cis(z)-flupentixol, 0.5 mg/kg. Naloxone, 16 mg/kg, not only blocked place preference, but also reduced sucrose consumption. In fact, animals consumed amounts similar to those consumed by rats given plain water. Lower doses of naloxone, 1 and 4 mg/kg, also decreased sucrose consumption, but 1 mg/kg did not block place preference. As the dopamine antagonist did not affect consumption at a dose that inhibited conditioned place preference, it is suggested that dopamine is critical for the establishment of reinforcement produced by sucrose. Opioids seem to be more important for the rewarding effect because naloxone reduces drinking even at a dose that does not affect place preference.

Glutamate inhibits ingestive behaviour

Male rats treated with reserpine were motionless and ingested only a few of ten consecutive intraoral injections of a 1 M solution of sucrose. While injection of apomorphine, a dopamine agonist, stimulated locomotion and stereotyped sniffing in reserpinized rats, it did not reactivate ingestive responses. The non-competitive N-methyl-D-aspartate receptor antagonist MK801, however, stimulated locomotion as well as ingestion suggesting involvement of glutamate in the suppressive effect of resperpine on ingestive responses. A series of experiments was therefore undertaken to investigate the possible physiological role of glutamate in feeding. For this purpose, we used Grill's intraoral intake test, in which the rat is infused intraorally with a sucrose solution and the amount ingested measured. In untreated rats, MK801 dose-dependently facilitated ingestion of the sucrose solution and antagonized inhibition of ingestion by cholecystokinin octapeptide. Administration of cholecystokinin octapeptide or ingestion of sucrose increased the concentration of glutamate in the nucleus of the solitary tract, a brain stem relay transmitting sensory information from the gastrointestinal tract to the forebrain. MK801 was found to bind specifically to this brain area and block the elevation of glutamate and dopamine levels which occurred after treatment with cholecystokinin octapeptide in this neural site. Together these data suggest that dopamine and glutamate may interact within the nucleus of the solitary tract in controlling ingestive behaviour.

Quantitation and identification of two cholecystokinin peptides, CCK-4 and CCK-8s, in rat brain by HPLC and fast atom bombardment mass spectrometry

A sensitive HPLC method has been described for quantitation of two cholecystokinin (CCK) peptides in discrete rat brain regions. Separation and quantitation was performed by the reversed-phase HPLC combined with electrochemical detection. Analytical recoveries of the tetrapeptide (CCK-4) and octapeptide-sulphate (CCK-8s) were 96% and 94%, respectively. The between assay coefficient of variation (CV) was less than 3% for both peptides. The within-assay CV was 4% and 6% for CCK-4 and CCK-8s and the detection limit was 2 and 10 pmol/mL, respectively. For identification of structures, the peptides were fractionated by semi-preparative HPLC using a novel SMART system for micropurification. The fractions were analysed by fast atom bombardment mass spectrometry (FAB MS) which confirmed the presence of both CCK-4 and CCK-8s in the rat brain tissue.

A comparison between the effect of cholecystokinin octapeptide and apomorphine on ingestion of intraorally administered sucrose in male rats

The involvement of dopamine receptors in the inhibitory effect of Cholecystokinin octapeptide on ingestive behaviour was investigated. Male rats were infused intraorally with a 1 M solution of sucrose and the amount ingested after treatment with the dopamine receptor agonist apomorphine was compared with that after treatment with Cholecystokinin octapeptide. The test allows a distinction between the consummatory aspects of ingestive behaviour, i.e. responses used to ingest food, from the appetitive aspects, i.e. responses used to obtain food, because it ignores the latter aspects. Comparisons were also made between the effects of apomorphine and Cholecystokinin octapeptide on pellet intake, a test in which the rat has to display appetitive ingestive behaviour. Injection of apomorphine (400 μg) increased the concentration of plasma apomorphine within 0.3 min and the concentration of dopamine in the cerebrospinal fluid within 1 min of injection and induced behavioural stereotypes within 10 min in food-deprived male rats. Plasma apomorphine and cerebrospinal fluid dopamine levels had decreased by 30 min and the behavioural stereotypies had decreased by 40 min after the injection. Injection of apomorphine also inhibited the consumption of food pellets and the ingestion of sucrose. Inhibition of pellet and sucrose ingestion paralleled the effect of apomorphine on Stereotypie behaviour. Thus, injection of a dopamine receptor agonist is followed by alterations in plasma levels of the agonist, cerebrospinal fluid dopamine levels and in Stereotypie and ingestive behaviour which occur in parallel, in an inverted U-shaped manner and with a temporal delay between each event. These results show a close correlation between dopamine receptor stimulation and inhibition of ingestive behaviour. However, reversal of the inhibitory effect of apomorphine on ingestive behaviour required pretreatment with a lower dose of a dopamine receptor antagonist (cis-flupentixol) (0.1 mg) than reversal of Stereotypie behaviour (0.8 mg). The effect of dopamine receptor stimulation on consummatory ingestive behaviour is thus relatively weak and not secondary to the induction of Stereotypic behaviour. Treatment with a high dose of cis-flupentixol (0.8 mg) caused a prolonged period of immobility but had no effect on the ingestion of sucrose. Dopamine receptor blockade, therefore, interferes with appetitive, but not consummatory ingestive behaviour. Injection of Cholecystokinin octapeptide (5 μg) suppressed pellet and sucrose intake in a manner comparable to that of apomorphine, but induced no behavioural stereotypes and caused a gradual, rather than inverted U-shaped, increase in the concentration of dopamine in the cerebrospinal fluid that did not correlate with the effect on ingestive behaviour. Furthermore, while the inhibitory effect of apomorphine on the ingestion of sucrose was reversed by pretreatment with a low dose of cis-flupentixol (0.1 mg), the inhibitory effect of Cholecystokinin octapeptide was only partially reversed by cis-flupentixol and a higher dose (0.8 mg) was required. Blockade of cholecystokinin-A receptors, by treatment with L-364,718, but not cholecystokinin-B receptors, by treatment with L-365,260, blocked the inhibitory effect of Cholecystokinin octapeptide and, by itself, L-364,718 increased the amount of ingested sucrose. The inhibitory effect of Cholecystokinin octapeptide on consummatory ingestive behaviour, which is mediated by cholecystokinin-A receptors, is likely to involve mechanisms in addition to dopaminergic ones.