Effects of neocortical ablations on eating elicited by hypothalamic stimulation.
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Rats shifted from a 32% to 4% sucrose solution consume substantially less 4% sucrose than unshifted animals that experience only the 4% solution. This negative contrast effect was found to be attenuated by lesions of the lateral aspects of the amygdala (basolateral, lateral, and basomedial nuclei) and eliminated by lesions of the medial aspects of the amygdala (corticomedial and central nuclei). The results are discussed in terms of the possible role the amygdala may play in some of the proposed determining factors mediating consummatory negative contrast (e.g., emotionality, neophobia, memory).
Four male Wistar rats, 180 days old at the start of the experiment, at 85% of their free-feeding body weight were trained to respond on a geometric cyclic-ratio schedule comprised of the following ratio values: 2, 4, 8, 16, 32 and 64, for 0.1 ml of 5% sucrose reinforcement. The response functions (response rates plotted against reinforcement rates) were linear and of negative slope over the range of ratio values from 2 to 16. IP administration of 1.0 mg/kg 5-MeODMT reduced the x- and y-intercepts of the linear portion of the response function without altering the slope relative to 2.0 ml/kg 0.85% saline IP. This was interpreted as a perceived palatability effect. IP administration of 100 mg/kg pCPA elevated the reinforcement rate intercept but also decreased the slope of the response function. This finding was interpreted as an increase in the perceived palatability of the reinforcer, coupled with a decrease in motivation at higher schedule conditions, possibly due to peripheral effects of pCPA.
The physiological stimulus for deprivation-enhanced ingestion was studied in developing rats. During an overnight deprivation period, continuous gastric infusions of isotonic saline or milk were made to 6- and 15-day-old rat pups in order to preferentially maintain hydrational or hydrational and nutritional status, respectively. Pups' ingestion was then studied in oral-infusion tests. In 6-day-old pups that received either milk or saline infusions, ingestion was depressed relative to intake in pups that were simply deprived. But in 15-day-old pups, only milk infusions reduced intake. These findings suggest that the increased ingestion stimulated by deprivation in pups less than a week of age results primarily from dehydration, and thus that nutrient-related feeding does not emerge until later in development.
The effect of treatment with the cholecystokinin antagonist L364,718 on intake of different dilutions of corn oil emulsion was tested under two levels of familiarity with the oil emulsion. No increase in intake was observed. To see if the CCK antagonist was effective under our conditions, exogenous CCK was administered under the same conditions. A complete suppression of the large reduction produced by CCK on intake was found.
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A consummatory conflict procedure that involves an abrupt reduction in magnitude of an expected reward (negative contrast) has been shown to be particularly sensitive to the effects of anxiolytic agents. As previously reported with chlordiazepoxide, another benzodiazepine (BDZ), midazolam released suppressed consummatory performance in a dose-dependent manner. This effect was not due to a general appetitie stimulatory effect of the drug. The effects of three 5-HT antagonists on negative contrast were examined to evaluate the role serotonin may play in the anxiolytic action of BDZ. Methysergide was found to be ineffective, cinanserin tended to reduce contrast at two intermediate doses, and cyproheptadine eliminated the contrast effect in a similar fashion as midazolam. The effectiveness of cyproheptadine may not be attributed to its anticholinergic or antihistaminergic actions since scopolamine and pyrilamine did not produce similar effects. The results are discussed in terms of the role serotonin may play in the anti-conflict action of BDZ, as well as possible interactional effects of GABA.
In a previous study, it was found that positive, palatability-dependent consummatory reactions in rats to intraorally infused tastes were facilitated by chlordiazepoxide (10 mg/kg). In contrast, the rats' more neutral or aversive reactions to these tastes were not facilitated by chlordiazepoxide. This suggested that chlordiazepoxide might selectively enhance the positive palatability of tastes. This effect was replicated in the present experiment, and in addition, the benzodiazepine antagonists Ro 15-1788 and CGS 8216 were found to counteract the enhancement of positive ingestive reactions produced by chlordiazepoxide. These antagonist effects generally suggest that the benzodiazepine receptor complex may be involved in making tastes more palatable after chlordiazepoxide administration.
Rats shifted from 32 to 4% sucrose consume substantially less of the 4% solution than animals that have not had prior experience with the 32% sucrose. This negative contrast effect was not substantially influenced by chlorpromazine (1, 3, and 5 mg/kg) or haloperidol (0.1, 0.5, and 1.0 mg/kg). Haloperidol decreased overall lick frequency, but this decrease occurred proportionately in shifted and unshifted rats, leaving contrast intact. The benzodiazepine flurazepam (5, 10, and 20 mg/kg), included as a positive control, reduced contrast at the two highest doses. The results suggest that neuroleptics do not disrupt consummatory contrast and that dopaminergic antagonists may not influence reward relativity.