Action of cholecystokinin and serotonin on lateral hypothalamic neurons of rats

Identification of Cholecystokinin by Genome-Wide Profiling as Potential Mediator of Serotonin-Dependent Behavioral Effects of Maternal Separation in the Amygdala

Converging evidence suggests a role of serotonin (5-hydroxytryptamine, 5-HT) and tryptophan hydroxylase 2 (TPH2), the rate-limiting enzyme of 5-HT synthesis in the brain, in modulating long-term, neurobiological effects of early-life adversity. Here, we aimed at further elucidating the molecular mechanisms underlying this interaction, and its consequences for socio-emotional behaviors, with a focus on anxiety and social interaction. In this study, adult, male Tph2 null mutant (Tph2 -/-) and heterozygous (Tph2 +/-) mice, and their wildtype littermates (Tph2 +/+) were exposed to neonatal, maternal separation (MS) and screened for behavioral changes, followed by genome-wide RNA expression and DNA methylation profiling. In Tph2 -/- mice, brain 5-HT deficiency profoundly affected socio-emotional behaviors, i.e., decreased avoidance of the aversive open arms in the elevated plus-maze (EPM) as well as decreased prosocial and increased rule breaking behavior in the resident-intruder test when compared to their wildtype littermates. Tph2 +/- mice showed an ambiguous profile with context-dependent, behavioral responses. In the EPM they showed similar avoidance of the open arm but decreased prosocial and increased rule breaking behavior in the resident-intruder test when compared to their wildtype littermates. Notably, MS effects on behavior were subtle and depended on the Tph2 genotype, in particular increasing the observed avoidance of EPM open arms in wildtype and Tph2 +/- mice when compared to their Tph2 -/- littermates. On the genomic level, the interaction of Tph2 genotype with MS differentially affected the expression of numerous genes, of which a subset showed an overlap with DNA methylation profiles at corresponding loci. Remarkably, changes in methylation nearby and expression of the gene encoding cholecystokinin, which were inversely correlated to each other, were associated with variations in anxiety-related phenotypes. In conclusion, next to various behavioral alterations, we identified gene expression and DNA methylation profiles to be associated with TPH2 inactivation and its interaction with MS, suggesting a gene-by-environment interaction-dependent, modulatory function of brain 5-HT availability.

Overeating after restraint stress in cholecystokinin-a receptor-deficient mice

In mammals, including humans, a brain-gut hormone, cholecystokinin (CCK) mediates the satiety effect via CCK-A receptor (R). We generated CCK-AR gene-deficient (-/-) mice and found that the daily food intake, energy expenditure, and gastric emptying of a liquid meal did not change compared with those of wild-type mice. Because CCK-AR(-/-) mice show anxiolytic status, we examined the effects of restraint stress. Seven hours of restraint stress was found to significantly decrease both body weight and food intake during the subsequent 3 days in all tested animals. On the fourth day after restraint stress, the CCK-AR(-/-) mice showed a significantly higher level of daily food intake than prior to stress, and food intake recovered to prestress levels in the wild-type mice. Since peripheral CCK-AR has been known to mediate gastric emptying, both gastric emptying and gastric acid secretion were determined to examine the mechanism of overeating in CCK-AR(-/-) mice. Neither gastric emptying nor gastric acid secretion differed between CCK-AR(-/-) and wild-type mice on the fourth day after stress. In contrast, however, the contents of dopamine and its metabolites in the cerebral cortex of CCK-AR(-/-) mice were increased by stress, but were rather decreased in wild-type mice. Changes in 5-hydroxytryptamine (5-HT) and its metabolite 5HIAA did not differ between the genotypes. In conclusion, CCK-AR(-/-) mice showed overeating after restraint stress, and dopaminergic hyperfunction in the brain of these mice was observed. The present evidence suggests that the CCK-AR function, possibly via altering the dopaminergic function, might be involved in overeating after stress.

Cholecystokinin combined with serotonin in the hypothalamus limits accumbens dopamine release while increasing acetylcholine: a possible satiation mechanism

Serotonin (5-HT) or cholecystokinin (CCK) injected in the hypothalamic paraventricular nucleus (PVN) inhibits feeding, but the mechanism is unknown. Prior research suggests that dopamine (DA) input to the nucleus accumbens (NAc) motivates behavior, and a component of that motivation circuit includes hypothalamic feeding systems. Acetylcholine (ACh) in the NAc, on the other hand, may act in part to inhibit feeding and generate satiety. If so, 5-HT and/or CCK in the PVN should lower extracellular DA or release ACh in the NAc. Rats were prepared with microdialysis probes in the NAc and injectors in the PVN. Serotonin (7.75 microg) or CCK-8 (0.12 microg) injected in the PVN significantly decreased ipsilateral accumbens DA (63 and 73% of baseline, respectively, without effect on ACh). However, 5-HT plus CCK injected in combination decreased DA to 72% (P<0.001) and simultaneously increased extracellular ACh to 128% of baseline (P<0.001). In later tests with the same doses and the same animals, unilateral PVN injections of 5-HT, CCK, or both combined, significantly inhibited food intake in the early dark period. The results suggest that 5-HT in the PVN acts as a neural modulator that primes a hypothalamic satiation system to respond to CCK when the gastrointestinal tract contains food to be digested. The synergistic action of 5-HT plus phasic CCK may then activate a circuit that simultaneously limits DA and releases ACh in the accumbens as part of the satiation process.

Distribution of serotonin 5-hydroxytriptamine 1B (5-HT(1B)) receptors in the normal rat hypothalamus

This is the study first attempting to evaluate distribution of neurons expressing serotonin 5-hydroxytriptamine 1B (5-HT(1B)) receptors in hypothalamus by using immunocytochemistry. The 5-HT(1B)-immunoreactive neurons were widely distributed in hypothalamus. Accumulations of 5-HT(1B) neurons occurred in magnocellular nuclei, supraoptic nucleus, paraventricular nucleus (dorsolateral part) and accessory perifornical, circular and retrochiasmatic nuclei. Magnocellular neurons manifested an intense immunostaining suggesting a high level of 5-HT(1B) receptors. Large and middle-sized neurons with different 5-HT(1B) staining patterns were scattered throughout lateral hypothalamus, periventricular nucleus and lateral preoptic area. Immunofluorescent double-labeling revealed a great overlapping of the distribution 5-HT(1B) neurons and dense network of neuropeptide Y-immunoreactive fibers in paraventricular, supraoptic and arcuate nuclei. The potential functional significance of 5-HT(1B) receptors in the 5-HT control of endocrine functions and feeding are discussed.

Action of CCK and 5-HT on lateral hypothalamic neurons depends on early postnatal nutrition

Wistar rats grown up during the early postnatal life (3-21 days after birth) in artificially built normal, small or large lifters developed a significantly different body weight. This difference persisted also during adulthood when they had free access to food and water. The influence of iontophoretically administered cholecystokinin (CCK8S), serotonin (5-HT) or co-ejection of both on firing of lateral hypothalamic neurons was investigated in adult, urethane anesthetized rats of the three groups. The responsiveness to CCK8S was significantly higher in large- and small-litter rats than in the normal control group. The differences were greater in males than in females. They resulted in the male large-litter group from an increase of excitatory responses, whereas in the male small-litter group the proportion of inhibitory responses was augmented. Co-administration of 5-HT generally reduced the neuronal responsiveness. Especially in the large-litter group excitatory responses were significantly reduced. It may be speculated that the availability of food in the early postnatal life influences the development of the hypothalamic regulatory network in such a way that it stabilizes the high or low food ingestion all the life. At least in males, a changed responsiveness and type of response to cholecystokinin of lateral hypothalamic neurons might be involved in this altered regulation.

Does increased endogenous CCK interact with serotonin to reduce food intake in rats?

The present study was aimed to test the hypothesis that increased endogenous CCK may interact with the anorectic serotonergic agent dl-fenfluramine to reduce food intake in rats. Previous studies, using selective CCK receptor antagonists, could demonstrate CCK-dependent 5-HT-induced anorexia. In the present approach, we used protease inhibitors to increase levels of endogenous CCK instead of blocking CCK receptors by antagonists. The protease inhibitors we used were soybean trypsin inhibitor (STI) and camostate. We hypothesized that combining the anorectic serotonergic drug dl-fenfluramine with either STI or camostate should result in an enhanced hypophagic effect when compared to single drug treatment. All feeding experiments were performed in non-deprived rats during night time feeding. Given alone, STI (500 mg/kg, po), camostate (200 mg/kg po) and also fenfluramine (1-9 mg/kg ip) reduced significantly food intake, with a more pronounced effect following fenfluramine. However, the experiments do not provide evidence for any additive or synergistic action between camostate or STI and the anorectic serotonergic drug dl-fenfluramine on food intake.