Endogenous cholecystokinin reduces food intake and increases Fos-like immunoreactivity in the dorsal vagal complex but not in the myenteric plexus by CCK1 receptor in the adult rat

We hypothesized that endogenous CCK reduces food intake by activating the dorsal vagal complex (DVC) and the myenteric neurons of the gut. To test this hypothesis, adult rats were given camostat mesilate; a nonnutrient releaser of endogenous CCK, by orogastric gavage, and Fos-like immunoreactivity (Fos-LI) was quantified in the DVC and the myenteric plexus. The results for endogenous CCK were compared with those for exogenous CCK-8. Exogenous CCK-8 reduced food intake and stimulated Fos-LI in the DVC and in myenteric neurons of the duodenum and jejunum. In comparison, endogenous CCK reduced food intake and increased DVC Fos-LI but did not increase Fos-LI in the myenteric plexus. Similar to CCK-8, devazepide, a specific CCK(1) receptor antagonist, and not L365,260, a specific CCK(2) receptor antagonist, attenuated the reduction of food intake by camostat. In addition, Fos-LI in the DVC in response to both exogenous CCK-8 and camostat administration was significantly attenuated by vagotomy, as well as by blocking CCK(1) receptors. These results demonstrate for the first time that reduction of food intake in adult rats by endogenous CCK released by a nonnutrient mechanism requires CCK(1) receptors, the vagus nerve, and activation of the DVC, but not the myenteric plexus.     

Cholecystokinin-58 and cholecystokinin-8 produce similar but not identical activations of myenteric plexus and dorsal vagal complex

The enteric nervous system (ENS: myenteric and submucosal plexuses) of the gastrointestinal tract may have a role in the reduction of food intake by cholecystokinin (CCK). Exogenous cholecystokinin-8 (CCK-8) activates the myenteric plexus and the feeding control areas of the dorsal vagal complex (DVC) of the brainstem. An increasing number of reports, however, have shown that CCK-58 is the sole or the major circulating form of CCK in rat, human and dog, and that it is qualitatively different from CCK-8 in evoking various gastrointestinal physiological responses (e.g., contraction of the gallbladder and exocrine pancreatic secretion). In the current report, we compared the abilities of exogenous CCK-58 to activate the myenteric plexus and the dorsal vagal complex with those of exogenous CCK-8 by quantifying Fos-like immunoreactivity (Fos-LI; a marker for neuronal activation). We report that CCK-58 (1, 3, and 5 nmol/kg) increased Fos-LI in the myenteric plexus (p<0.001) and in the DVC (p<0.001) compared to the saline vehicle. The highest dose of CCK-58 increased Fos-LI more than an equimolar dose of CCK-8 in the myenteric plexus and the area postrema. Thus, CCK-8 and CCK-58 produce the same qualitative pattern of activation of central and peripheral neurons, but do not provoke identical quantitative patterns at higher doses. The different patterns produced by the two peptides at higher doses, in areas open to the circulation (myenteric plexus and area postrema) may reflect endocrine actions not observed at lower doses.     

Cholecystokinin-8 activates myenteric neurons in 21- and 35-day old but not 4- and 14-day old rats

Cholecystokinin (CCK) activates the myenteric neurons of adult rats. The goal of this work is to determine the ontogeny of this activation by CCK-8 in the myenteric plexus of the duodenum (2 cm immediately following the pyloric sphincter aborally) and compare it with that of the dorsal vagal complex (DVC) - which occurs in 1-day old pups. Despite the existence of both of the CCK receptors, CCK₁ and CCK₂, in 4, 14, 21 and 35 day old rats, CCK-8 (0, 5, 10, 20 and 40 μg/kg, i.p.) increased Fos-like immunoreactivity (Fos-LI, a marker for neuronal activation) in the myenteric neurons of 21- and 35-day old rats but in the DVC of all age groups. As such, this belated activation of myenteric neurons by CCK-8 compared to the DVC may reflect a delayed role for these neurons in CCK-related functions.     

The short term satiety peptide cholecystokinin reduces meal size and prolongs intermeal interval

Camostat mesilate (or mesylate) releases endogenous cholecystokinin (CCK) or CCK-58, the only detectable endocrine form of CCK in the rat, and reduces cumulative food intake by activating CCK₁ receptor. However, the literature lacks meal pattern analysis and an appropriate dose-response curve for this peptide. Therefore, the current study determines meal size (MS), intermeal interval (IMI) and satiety ratio (SR) by orogastric gavage of camostat (0, 12.5, 25, 50, 100, 200, 300, 400, 800 mg/kg) and compares them to those previously reported by a single dose of CCK-8 (1 nmol/kg, i.p), the most utilized form of CCK. We found that camostat (200, 300, 400 and 800 mg/kg) and CCK-8 reduced cumulative food intake and the size of the first meal, but only camostat prolonged IMI and increased SR. There was no change in the duration of the first two meals or in rated behaviors such as feeding, grooming, standing and resting in response to camostat and CCK-8, but there was more resting during the IMI in response to camostat. This study provides meal pattern analysis and an appropriate dose-response curve for camostat and CCK-8. Camostat reduces food intake by decreasing MS and prolonging IMI, whereas CCK-8 reduces food intake by reducing only meal size.     

Gastrin releasing peptides increase Fos-like immunoreactivity in the enteric nervous system and the dorsal vagal complex

We and others have shown that gastrin-releasing peptide (GRP) reduces food intake. In this study, we determined the activation of the gastrointestinal and dorsal vagal complex (DVC) neurons by various forms of GRP to determine the pathway involved in this reduction. We found the following: (1) GRP-10, -27 and -29 (2.1 nmol/kg, i.p.) increased the Fos-like immunoreactivity (Fos-LI, a marker for neuronal activation) in the myenteric neurons of the stomach and the area postrema (AP) of the DVC; (2) GRP-27 and GRP-29 increased the Fos-LI in the myenteric plexus of the duodenum; and (3) only GRP-29 increased the Fos-LI in the submucosal plexus of the duodenum. In conclusion, GRP may reduce food intake by activating the area postrema. The enteric neurons may have a potential role in this reduction through the direct activation of the AP or exerting local gut actions, such as the stimulation of gut motility or secretions.     

Cholecystokinin-8 increases Fos-like immunoreactivity in the brainstem and myenteric neurons of rats through CCK1 receptors

To examine the role of cholecystokinin1 receptor (CCK1) in the activation of brainstem and myenteric neurons by CCK, we compared the ability of exogenous CCK-8 to induce Fos-like immunoreactivity (Fos-LI) in these neurons in Otsuka Long-Evans Tokushima Fatty (OLETF) rats, lacking CCK1 receptors, and Long-Evans Tokushima Otsuka (LETO) controls. Five groups (n=4 rats per group) of OLETF rats, and five LETO control groups, were injected intraperitoneally (IP) with 5, 10, 20, and 40 microg/kg CCK-8 or saline. Forty-micrometer brainstem sections containing the area postrema, nucleus of the solitary tract, and the dorsal motor nucleus of the vagus, and myenteric neurons of the duodenum, jejunum, and ileum underwent a diaminobenzidine reaction enhanced with nickel to reveal Fos-LI. CCK-8 did not increase Fos-LI in any of the tested neurons in the OLETF rats. CCK-8 increased Fos-LI in the brainstem of the LETO rats in a dose dependent manner. In the LETO rats only 40 microg/kg CCK-8 increased Fos-LI in the myenteric plexus of the jejunum. This study demonstrates that CCK-8 activates the brainstem and myenteric neurons through the CCK1 receptor.     

Effect of sympathectomy and demedullation on increased myenteric and dorsal vagal complex Fos-like immunoreactivity by cholecystokinin-8

Chemical sympathectomy with daily, intraperitoneal (IP) injections of guanethidine sulfate to adult rats, attenuated myenteric, but not dorsal vagal complex (DVC) Fos-like immunoreactivity (Fos-LI) by cholecystokinin-8 (CCK). This technique destroys only 60-70% of the sympathetic neurons, and spares the hormonal source of catecholamines, the adrenal medulla. The goal of the current study is to evaluate the effect of complete sympathectomy or destroying 100% of the sympathetic neurons by injecting guanethidine to 1-day-old pups (40 mg/kg daily for 5 weeks), and surgically removing the adrenal medulla. In the DVC, demedullation and sympathectomy-demedullation increased Fos-LI by CCK in the area postrema and nucleus of the solitary tract, but sympathectomy-demedullation increased it only in the area postrema. In the myenteric plexus, sympathectomy increased this response in the duodenum, and demedullation increased it in the duodenum and jejunum. On the other hand, sympathectomy-demedullation attenuated myenteric Fos-LI in the jejunum. These results indicate that catecholamines may play an inhibitory role on the activation of the DVC neurons by CCK. In the myenteric neurons, however, catecholamines may have both inhibitory and excitatory roles depending on the level of the intestine e.g., duodenum vs. jejunum. This may also indicate that CCK activates the enteric neurons by different mechanisms or through different pathways.     

Different affinity states of CCK1 receptors on pancreatic acini and gastric smooth muscle in the rat

It has recently been shown that—after chronic cholecystokinin (CCK) treatment—an adaptation of pancreatic secretory but not gastric motor function does occur. Recent studies indicate that the CCK₁-receptor exists in two (i.e. high and low) affinity states, which could be distinguished by the CCK-analogue JMV-180. CCK occupancy of high and low affinity sites is thought to be related to the initiation of different intracellular events and consequent biological responses. Affinity states of CCK₁-receptors on pancreas and gastrointestinal (GI) smooth muscle could be different and this can offer an explanation for the different effects of CCK on pancreatic and gastric growth. We therefore studied the affinity states of CCK₁-receptors on isolated rat pancreatic acini and gastric smooth muscle preparations. When acini were incubated with increasing concentrations of CCK-8, a biphasic (i.e. stimulation followed by inhibition) effect on amylase release was observed. JMV-180 caused only stimulation of enzyme release and combined JMV-180 and CCK stimulation (at submaximal doses) resulted in an additive secretory response. CCK-8 induced contractions of pyloric, antral and fundic muscle in a concentration-dependent manner. The response was monophasic, reaching a plateau. JMV-180 had only a very weak effect on these preparations. On the contrary, it inhibited CCK-induced contractions in a competitive manner, the concentration–response curve to CCK being shifted to the right by the CCK analogue. Our data suggest that the affinity states of CCK₁-receptors on rat pancreatic and gastric tissue are different. On pancreatic acini CCK₁-receptors exist in both high- and low-affinity states whose occupation is followed by the sequence of intracellular events leading to growth. In contrast, occupation of low affinity receptors (the only ones present in the GI smooth muscle) does not lead to cell proliferation. This difference therefore explains the different adaptive response of the pancreas and the stomach to chronic CCK administration. Furthermore, different affinity states of CCK₁-receptors may mediate different functions of the digestive tract.     

Capsaicin application to central or peripheral vagal fibers attenuates CCK satiety

Capsaicin treatment destroys small primary sensory neurons including a subpopulation of vagal afferents. Intraperitoneal, fourth ventricular or perivagal application of capsaicin attenuated or abolished cholecystokinin (CCK)-induced suppression of food intake. Capsaicin applied to the thoracolumbar spinal cord or to the pyloric region of the stomach did not alter CCK-induced reductions of food intake. Intraperitoneal capsaicin treatment reduced substance P-like immunoreactivity (SPLI) in the spinal dorsal horn and parts of the dorsal hindbrain. SPLI depletion, therefore, served as a histochemical indicator of the spread of capsaicin from its site of application. Capsaicin applied directly to the vagal trunks did not reduce SPLI in the spinal cord or hindbrain. Intraventricular capsaicin reduced SPLI in the hindbrain but not in the spinal cord. These data indicate that localized capsaicin application attenuates CCK-induced suppression of food intake by impairing the function of either central or peripheral portions of vagal afferent neurons. The data also support the conclusion that intraperitoneal capsaicin attenuates CCK-induced suppression of feeding by impairing vagal sensory function.     

CCK-A receptor activation induces fos expression in myenteric neurons of rat small intestine

Cholecystokinin (CCK), a hormone secreted from endocrine cells of the small intestine, participates in the control of motility and secretion in the gastrointestinal tract, and in the control of food intake. At least some of the effects of CCK on intestinal function appear to be mediated via activation of intrinsic neurons in the myenteric plexus. However, the distribution of CCK-responsive enteric neurons within the rat small intestine is not known. Neither has the role of CCK-A receptors in the activation of rat myenteric neurons been investigated. Therefore, to determine the distribution of CCK-responsive neurons in the small intestinal myenteric plexus we utilized immunohistochemical detection of Fos, the protein product of the immediate early gene c-fos, to identify neurons that were activated by exogenous CCK. We also monitored Fos expression in the dorsal hindbrain, and examined CCK-induced Fos expression in the presence or absence of a receptor antagonist for the type-A CCK receptor. We found that CCK significantly increased Fos expression in the hindbrain and in myenteric neurons of the duodenum and jejunum, but not the ileum. Neuronal Fos responsiveness in both brain and myenteric neurons was mediated by CCK-A receptors, as CCK-induced Fos expression was eliminated in rats pretreated with a CCK-A receptor antagonist. We conclude that CCK activates small intestinal myenteric neurons, via CCK-A receptors. Activation of these intrinsic intestinal neurons may participate in reflexes and behaviors that are mediated by CCK.     

Ileal interposition attenuates the satiety responses evoked by cholecystokinin-8 and -33

One of the possible mechanisms by which the weight-reducing surgical procedure ileal interposition (II) works is by increasing circulating levels of lower gut peptides that reduce food intake, such as glucagon like peptide-1 and peptide YY. However, since this surgery involves both lower and upper gut segments, we tested the hypothesis that II alters the satiety responses evoked by the classic upper gut peptide cholecystokinin (CCK). To test this hypothesis, we determined meal size (MS), intermeal interval (IMI) and satiety ratio (SR) evoked by CCK-8 and -33 (0, 1, 3, 5 nmol/kg, i.p.) in two groups of rats, II and sham-operated. CCK-8 and -33 reduced MS more in the sham group than in the II group; CCK-33 prolonged IMI in the sham group and increased SR in both groups. Reduction of cumulative food intake by CCK-8 in II rats was blocked by devazepide, a CCK₁ receptor antagonist. In addition, as previously reported, we found that II resulted in a slight reduction in body weight compared to sham-operated rats. Based on these observations, we conclude that ileal interposition attenuates the satiety responses of CCK. Therefore, it is unlikely that this peptide plays a significant role in reduction of body weight by this surgery.     

Vagally mediated, nonparacrine effects of cholecystokinin-8s on rat pancreatic exocrine secretion

Cholecystokinin (CCK) has been proposed to act in a vagally dependent manner to increase pancreatic exocrine secretion via actions exclusively at peripheral vagal afferent fibers. Recent evidence, however, suggests the CCK-8s may also affect brain stem structures directly. We used an in vivo preparation with the aims of 1) investigating whether the actions of intraduodenal casein perfusion to increase pancreatic protein secretion also involved direct actions of CCK at the level of the brain stem and, if so, 2) determining whether, in the absence of vagal afferent inputs, CCK-8s applied to the dorsal vagal complex (DVC) can also modulate pancreatic exocrine secretion (PES). Sprague-Dawley rats (250-400 g) were anesthetized and the common bile-pancreatic duct was cannulated to collect PES. Both vagal deafferentation and pretreatment with the CCK-A antagonist lorglumide on the floor of the fourth ventricle decreased the casein-induced increase in PES output. CCK-8s microinjection (450 pmol) in the DVC significantly increased PES; the increase was larger when CCK-8s was injected in the left side of the DVC. Protein secretion returned to baseline levels within 30 min. Microinjection of CCK-8s increased PES (although to a lower extent) also in rats that underwent complete vagal deafferentation. These data indicate that, as well as activating peripheral vagal afferents, CCK-8s increases pancreatic exocrine secretion via an action in the DVC. Our data suggest that the CCK-8s-induced increases in PES are due mainly to a paracrine effect of CCK; however, a relevant portion of the effects of CCK is due also to an effect of the peptide on brain stem vagal circuits.     

CCK enhances response to gastric distension by acting on capsaicin-insensitive vagal afferents

Capsaicin treatment destroys vagal afferent C fibers and markedly attenuates reduction of food intake and induction of hindbrain Fos expression by CCK. However, both anatomical and electrophysiological data indicate that some gastric vagal afferents are not destroyed by capsaicin. Because CCK enhances behavioral and electrophysiological responses to gastric distension in rats and people, we hypothesized that CCK might enhance the vagal afferent response to gastric distension via an action on capsaicin-insensitive vagal afferents. To test this hypothesis, we quantified expression of Fos-like immunoreactivity (Fos) in the dorsal vagal complex (DVC) of capsaicin-treated (Cap) and control rats (Veh), following gastric balloon distension alone and in combination with CCK injection. In Veh rats, intraperitoneal CCK significantly increased DVC Fos, especially in nucleus of the solitary tract (NTS), whereas in Cap rats, CCK did not significantly increase DVC Fos. In contrast to CCK, gastric distension did significantly increase Fos expression in the NTS of both Veh and Cap rats, although distension-induced Fos was attenuated in Cap rats. When CCK was administered during gastric distension, it significantly enhanced NTS Fos expression in response to distension in Cap rats. Furthermore, CCK's enhancement of distension-induced Fos in Cap rats was reversed by the selective CCK-A receptor antagonist lorglumide. We conclude that CCK directly activates capsaicin-sensitive C-type vagal afferents. However, in capsaicin-resistant A-type afferents, CCK's principal action may be facilitation of responses to gastric distension.     

Differential mechanism and site of action of CCK on the pancreatic secretion and growth in rats

Recent studies demonstrated that cholecystokinin (CCK) at physiological levels stimulates pancreatic enzyme secretion via a capsaicin-sensitive afferent vagal pathway. This study examined whether chemical ablation of afferent vagal fibers influences pancreatic growth and secretion in rats. Bilateral subdiaphragmatic vagal trunks were exposed, and capsaicin solution was applied. Pancreatic wet weight and pancreatic secretion and growth in response to endogenous and exogenous CCK were examined 7 days after capsaicin treatment. Perivagal application of capsaicin increased plasma CCK levels and significantly increased pancreatic wet weight compared with those in the control rats. Oral administration of CCK-1 receptor antagonist loxiglumide prevented the increase in pancreatic wet weight after capsaicin treatment. In addition, continuous intraduodenal infusion of trypsin prevented the increase in plasma CCK levels and pancreatic wet weight after capsaicin treatment. There were no significant differences in the expression levels of CCK-1 receptor mRNA and protein in the pancreas in capsaicin-treated and control rats. Intraduodenal administration of camostat or intravenous infusion of CCK-8 stimulated pancreatic secretion in control rats but not in capsaicin-treated rats. In contrast, repeated oral administrations of camostat or intraperitoneal injections of CCK-8 significantly increased pancreatic wet weight in both capsaicin-treated and control rats. Present results suggest that perivagal application of capsaicin stimulates pancreatic growth via an increase in endogenous CCK and that exogenous and endogenous CCK stimulate pancreatic growth not via vagal afferent fibers but directly in rats.     

High fat maintenance diet attenuates hindbrain neuronal response to CCK

Rats maintained on a high fat diet reduce their food intake less in response to exogenous cholecystokinin (CCK) than rats maintained on a low fat diet. In addition, inhibition of gastric emptying by CCK is markedly attenuated in rats maintained on a high fat diet. Both inhibition of food intake and gastric emptying by CCK are mediated by sensory fibers in the vagus nerve. These fibers terminate on dorsal hindbrain neurons of the nucleus of the solitary tract and area postrema. To determine whether diet-induced changes in the control of feeding and gastric emptying are accompanied by altered vagal sensory responsiveness, we examined dorsal hindbrain expression of Fos-like immunoreactivity (Fos-li) following intraperitoneal CCK injection of rats maintained on high fat or low fat diets. Following CCK, there were numerous Fos-li nuclei in the area postrema and in the commissural and medial subnuclei of the nucleus of the solitary tract of rats maintained on a low fat diet. However, Fos-li was absent or rare in the brains of rats maintained on a high fat diet. These data suggest that the vagal sensory response to exogenous CCK is reduced in rats maintained on a high fat diet. Our results also are consistent with our previous findings that CCK-induced reduction of food intake and gastric emptying are both attenuated in rats maintained on a high fat diet. In addition our results support the hypothesis that attenuation of CCK-induced inhibition of food intake and gastric emptying may be due to diet-induced diminution of vagal CCK responsiveness.     

Rapakinin, Arg-Ile-Tyr, derived from rapeseed napin, shows anti-opioid activity via the prostaglandin IP receptor followed by the cholecystokinin CCK(2) receptor in mice

Rapakinin, Arg-Ile-Tyr, is a vasorelaxing, anti-hypertensive and anorexigenic peptide derived from rapeseed napin. In this study, we found that rapakinin intracerebroventricularly administered to mice inhibited the analgesic effect of morphine, evaluated by the tail-pinch test. The anti-opioid activity of rapakinin was blocked by LY225910, an antagonist of the cholecystokinin (CCK) CCK₂ receptor, but not by lorglumide, an antagonist of the CCK₁ receptor. The anti-opioid activity of rapakinin was also blocked by CAY10441, an antagonist of the prostaglandin (PG) IP receptor. These results suggest that the anti-opioid activity of rapakinin is mediated by the CCK₂ and IP receptors. The anti-opioid activity induced by ciprostene, an IP receptor agonist, was blocked by LY225910, while that of CCK-8 was not blocked by CAY10441. Thus, it is demonstrated that the CCK-CCK₂ system was activated downstream of the PGI₂-IP receptor system. Taken together, rapakinin shows anti-opioid activity via the activation of the PGI₂-IP receptor system followed by the CCK-CCK₂ receptor system.     

Degradation of cholecystokinin octapeptide,related fragments and analogs by human and rat plasma in vitro

The rate of degradation of cholecystokinin octapeptide, related fragments and analogs by human and rat plasma was investigated, using high pressure liquid chromatography for the separation and identification of the degradation products.CCK tetrapeptide showed a half-life of 13 min in human plasma while its cleavage in rat plasma occurred at a very high rate (half-life of less than 1 min).The kinetics of disappearance of both sulphated and unsulphated CCK-8 indicated more than a single rate of degradation; the faster degrading system showed a half-life of 18 min for unsulphated CCK-8 and of 50 min for the sulphated peptide in human plasma as compared respectively with 5 and 17 min in rat plasma. The cleavage of CCK-8 was inhibited by bestatin and puromycin, suggesting that aminopeptidases play a major role in the breakdown of this peptide.CCK-9 analogs were rapidly converted into their corresponding octapeptides (half-life of 2.7 min in human plasma). This conversion was inhibited by puromycin and bestatin, suggesting the participation of aminopeptidase(s) probably specific for basic amino acids.CCK decapeptide exhibited a greater stability than the nonapeptides (half-life of 30 and 45 min in human and rat plasma respectively) and also gave rise to CCK-8 as degradation product. This cleavage was not affected by aminopeptidase inhibitors but was decreased by aprotinin (Trasylol®), suggesting that trypsin-like and/or kallikrein-like enzyme(s) were involved in the plasma metabolism of this peptide.     

Microinjection of exogenous somatostatin in the dorsal vagal complex inhibits pancreatic secretion via somatostatin receptor-2 in rats

Previous studies have suggested that somatostatin inhibits pancreatic secretion at a central vagal site, and the dorsal vagal complex (DVC) is involved in central feedback inhibition of the exocrine pancreas. The aim of this study was to investigate the effect of exogenous somatostatin in the DVC on pancreatic secretion and the somatostatin receptor subtype(s) responsible for the effect. The effects of somatostatin microinjected into the DVC on pancreatic secretion stimulated by cholecystokinin octapeptide (CCK-8) or 2-deoxy-d-glucose (2-DG) were examined in anesthetized rats. To investigate the somatostatin inhibitory action site, a somatostatin receptor antagonist [SRA; cyclo(7-aminoheptanoyl-Phe-d-Trp-Lys-Thr)] was microinjected into the DVC before intravenous infusion of somatostatin and CCK-8/2-DG. The effects of injection of a somatostatin receptor-2 agonist (seglitide) and combined injection of somatostatin and a somatostatin receptor-2 antagonist (CYN 154806) in the DVC on the pancreatic secretion were also investigated. Somatostatin injected into the DVC significantly inhibited pancreatic secretion evoked by CCK-8 or 2-DG in a dose-dependent manner. SRA injected into the DVC completely reversed the inhibitory effect of intravenous administration of somatostatin. Seglitide injected into the DVC also inhibited CCK-8/2-DG-induced pancreatic protein secretion. However, combined injection of somatostatin and CYN 154806 did not affect the CCK-8/2-DG-induced pancreatic secretion. Somatostatin in the DVC inhibits pancreatic secretion via somatostatin receptor-2, and the DVC is the action site of somatostatin for its inhibitory effect.     

Cholecystokinin activates specific enteric neurons in the rat small intestine

Cholecystokinin (CCK) is a peptide hormone released from the I-cells of the upper small intestine. CCK evokes a variety of physiological responses, such as stimulation of pancreatic secretion, reduction of food intake and inhibition of gastric emptying. Previously, we reported that CCK activates enteric neurons in the rat. However the specific subpopulations of enteric neurons activated by CCK have not been identified. In the work reported here, we utilized immunohistochemical detection of nuclear Fos, a marker for neuronal activation, and selected phenotypic markers to identify some of the neuronal subpopulations activated by CCK. The phenotypic markers that we examined were: nitric oxide synthase (NOS), neurokinin-1 receptor (NK-1R), calbindin (Cal), Calretinin (Calr), and neurofilament-M (NF-M). We found that in the myenteric plexus of the rat duodenum and jejunum, CCK activated NOS immunoreactive neurons. In the submucosal plexus of duodenum and jejunum, CCK activated Cal, Calr and NF-M immunoreactive neurons. CCK failed to activate NK-1R immunoreactive neurons in either plexus. Our results indicate that CCK activates distinct enteric neurons in the rat upper small intestine. Furthermore the fact that NOS immunoreactive neurons were activated suggests that CCK modulates the activity of inhibitory motor neurons in the myenteric plexus. Expression of Fos immunoreactivity in Calr and Cal immunoreactive neurons is consistent with a role for CCK in modulation of intrinsic sensory and/or secretomotor neuronal activity in the submucosal plexus.