Aversive behavior during intraoral intake in male rats

Oromotor and somatic taste reactivity during sucrose meals reveals internal state and stimulus palatability after gastric bypass in rats

After Roux-en-Y gastric bypass (RYGB), rats consume less high-energy foods and fluids, though whether this reflects a concomitant change in palatability remains unclear. By measuring behavior during intraorally delivered liquid meals across days (1 water, 8 sucrose sessions), we showed that RYGB rats (RYGB, n = 8/sex) consumed less 1.0 M sucrose than their sham surgery counterparts (SHAM, n = 8 males, n = 11 females) but displayed similarly high levels of ingestive taste reactivity responses at the start of infusions. Relative to water, both groups increased intake of sucrose, and ingestive responses were dominated by tongue protrusions rather than mouth movements. Thus, RYGB animals still found sucrose palatable despite consuming less than the SHAM group. As the intraoral infusion progressed but before meal termination, aversive behavior remained low and both RYGB and SHAM animals showed fewer ingestive responses, predominantly mouth movements as opposed to tongue protrusions. This shift in responsiveness unrelated to surgical manipulation suggests negative alliesthesia, or a decreased palatability, as rats approach satiation. Notably, only in RYGB rats, across sessions, there was a striking emergence of aversive behavior immediately after the sucrose meal. Thus, although lower intake in RYGB rats seems independent of the hedonic taste properties of sucrose, taste reactivity behavior in these animals immediately after termination of a liquid meal appears to be influenced by postoral events and reflects a state of nimiety or excessive consumption. Measurement of taste reactivity behaviors during an intraorally delivered meal represents a promising way to make inferences about internal state in nonverbal preclinical models.

Adrenergic drugs modify the level of noradrenaline in the insular cortex and alter extinction of conditioned taste aversion in rats

We compared the effect of conditioned taste aversion in rats by measuring the amount of sucrose that they drunk after conditioning, which differed according to whether rats had drunk the sucrose freely (SD: self drinking) during the conditioning session, or had been forced to drink it (IO: intra-oral administration through a chronically implanted cannula). The SD procedure delayed the extinction of conditioned taste aversion. Enhanced arousal, alertness, awareness or attention in the SD condition may have strengthened the memory of the taste. Brain noradrenergic networks are involved in such processes. We administered two noradrenergic drugs that produce opposite effects on noradrenaline release in the brain, methoxy-idazoxan, RX821002 (1mg/kg, i.p.), and guanfacine (0.12mg/kg, i.p.). We evaluated their effect (i) on the level of noradrenaline in the gustatory cortex using microdialysis, (ii) on glycaemia that is an essential factor of taste learning and (iii) on the comparative SD versus IO conditioned taste aversion protocol mentioned above. Injecting RX821001 increased the level of noradrenaline in the gustatory cortex up to two-fold of the baseline. This effect lasted 1h. The same dose of RX821002 did not elicit any alteration of glycaemia. It enhanced extinction of conditioned taste aversion in the SD group of rats. Injecting 0.12mg/kg of guanfacine produced the opposite effect. The noradrenaline level of the gustatory cortex decreased, but only down to 20% of the baseline. This decrease lasted 2h. Guanfacine increased glycaemia. Extinction of conditioned taste aversion was only marginally decreased by guanfacine in the SD group of rats. These results fit with Aston-Jones' point of view that the role of the noradrenergic coeruleo-cortical system may be to enhance arousal, alertness, awareness or attention to an event by a transient increase of cortical noradrenaline.

Intake inhibition by NPY and CCK-8: A challenge of the notion of NPY as an “Orexigen”

We tested the hypothesis that neuropeptide Y (NPY) interacts with cholecystokinin octapeptide (CCK-8) in inhibition of intake of an intraorally infused solution of sucrose, a test of consummatory ingestive behavior. Both intracerebroventricular infusion of NPY (10 microg) and intraperitoneal injection of CCK-8 (0.5 micro/kg) reduced the intake of a 1M solution of sucrose infused intraorally at a rate of 0.5 ml/min in ovariectomized female rats, but the two peptides did not interact in inhibiting intraoral intake. By contrast, NPY increased intake if the sucrose solution was ingested from a bottle, a test demanding both appetitive and consummatory ingestive responses. CCK-8 inhibited intake in this test and its inhibitory effect was increased by simultaneous treatment with NPY. The activity in the nucleus of the solitary tract (NTS), a brainstem relay mediating inhibition of intake, judged by the expression of c-fos-like immunoreactivity, was significantly increased after treatment with CCK-8 or NPY to approximately the same extent. Combined treatment with NPY and CCK-8 did not increase the c-fos-like immunoreactivity in the NTS above treatment with NPY or CCK-8 alone. These results strengthen the hypothesis that NPY, like CCK-8, is an inhibitor of consummatory ingestive behavior and suggest that this inhibition is mediated via the NTS.

Acute or chronic effects of cannabinoids on spontaneous or pharmacologically induced yawning in rats

Yawning is a reflex or event that is not fully understood. It is controlled by many neurotransmitters and neuropeptides and can be induced pharmacologically by cholinergic or dopaminergic agonists. Amongst their many actions, cannabinoids acting on cannabinoid (CB(1) or CB(2)) receptors can alter cholinergic and/or dopaminergic activity. This study examined the effects of Delta(8)-tetrahydrocannabinol (Delta(8)-THC) administered acutely (2.5 mg/kg intraperitoneally [ip], 15 min before test) or chronically (5 mg/kg for 30 days followed by 24 h or 7 days of discontinuation) on yawning induced by pilocarpine, a cholinergic agonist (0, 1, 2, 4 or 8 mg/kg ip), or apomorphine, a dopaminergic agonist (0, 20, 40 or 80 microg/kg subcutaneously [sc]). Acute effects of different doses of Delta(9)-tetrahydrocannabinol (Delta(9)-THC: 0, 0.5, 1.25 or 2.5 mg/kg ip) on yawning induced by pilocarpine (2 mg/kg ip) or apomorphine (40 microg/kg sc) were also investigated. Both pilocarpine and apomorphine produced yawning in a dose-related manner. Acute administration of Delta(8)-THC and Delta(9)-THC significantly reduced yawning induced by both pilocarpine and apomorphine. Chronic administration of Delta(8)-THC did not change yawning induced by either agonist 24 h or 7 days after discontinuation of Delta(8)-THC. However, a high frequency of spontaneous yawning was observed 7 days after Delta(8)-THC discontinuation. These results suggest that cannabinoid agonists inhibited yawning induced by cholinergic or dopaminergic agonists. In addition, the increased frequency of spontaneous yawning following cessation of chronic administration of a cannabinoid agonist may be of importance as a withdrawal sign for these drugs.

Effect of oral sucrose on ingestive behavior in the near-term ovine fetus

BACKGROUND

Dipsogen-mediated ingestion matures acutely in late gestation because the preterm fetus may demonstrate absent responses to putative dipsogens. Although central appetite-mediated ingestive behavior is functional near term, it is unknown whether peripheral mechanisms for stimulation of appetite also are functional. In the adult, sweet taste stimulates and potentiates ingestive behavior. We sought to determine whether oropharyngeal sucrose exposure stimulates ingestive behavior in the near-term ovine fetus.

STUDY DESIGN

Time-dated pregnant ewes with near-term singleton fetuses (n = 6) were chronically prepared with fetal vascular and sublingual catheters and esophageal electromyogram electrodes and studied at 129 +/- 1 days of gestation. After an initial 2-hour baseline period, successive solutions of distilled water and 2.5%, 10%, and 40% sucrose were infused sublingually (0.25 mL/min), each for 2 hours. Maternal and fetal arterial blood samples were drawn at timed intervals.

RESULTS

During the basal period, fetal swallowing averaged 0.9 +/- 0.1 swallows per minute. Swallowing did not change in response to distilled water (0.9 +/- 0.2 swallows per minute) but significantly increased after sublingual infusion of 2.5% sucrose (1.3 +/- 0.1 swallows per minute), 10% sucrose (1.8 +/- 0.1 swallows per minute), and 40% sucrose (1.3 +/- 0.1 swallows per minute, P =.001). There were no significant changes in other fetal or maternal parameters.

CONCLUSIONS

The stimulation of fetal swallowing in response to sublingual sucrose infusion suggests that taste-mediated ingestive behavior is functional in the near-term fetus and that both central and systemic appetite mechanisms are intact near term. Fetal swallowing increased in response to an increase in sucrose concentration to peak at 10% and then decreased with further rises in concentration, possibly mediated by aversive fetal reaction to a high-intensity sucrose concentration.

Intake inhibition by NPY: role of appetitive ingestive behavior and aversion

Intraventricular infusion of neuropeptide Y (NPY) decreases the amount female rats ingest during intraoral infusion (consummatory behavior) of a 1-M solution of sucrose at a rate of 0.5 ml/min and simultaneously increases the number of times the rats visit a bottle filled with sucrose (appetitive behavior). In this study, we investigated if the suppression of consummatory behavior was dependent upon the increase of appetitive behavior. The shift from consummatory to appetitive ingestive behavior was attenuated by adding 3-mM quinine HCl (QHCl) to the sucrose solution in the bottle. However, the intraoral intake of the sucrose solution was still decreased in NPY-treated rats. NPY did not modify taste reactivity as measured by aversive responses during continuous intraoral infusion of sucrose or ingestive and aversive responses to brief intraoral infusion of sucrose (0, 0.3 or 1 M) or QHCl (0, 0.3 or 3 mM). NPY stimulated visits to a bottle and intake from the bottle and inhibited sexual behavior in male rats but had no effect on the sexual behavior in the absence of a bottle. The visits and the intake were suppressed, but sexual behavior was not activated by adding QHCl (3 mM) to the solution in the bottle. Obstructing appetitive ingestive behavior, therefore, does not indiscriminately facilitate consummatory behavior. Male rats showed aversive or ingestive behavior and sexual behavior simultaneously during intraoral infusion of QHCl or condensed milk. It is suggested that NPY decreases intraoral intake and increases appetitive ingestive behavior via partially separable mechanisms that are independent of taste aversion.