Effects on hypothalamic and telencephalic NE and 5-HT of tegmental knife cuts that produce hyperphagia or hyperdipsia in the rat

Differential effects of NE, CLON, and 5-HT on feeding and macronutrient selection in genetically obese (ob/ob) and lean mice

The effects of central injection of norepinephrine (NE), clonidine (CLON), and 5-hydroxytryptamine (5-HT) on feeding and macronutrient selection in genetically obese (C57B1/6J, ob/ob) and lean mice (C57B1/6J, +/?) were examined. Mice were adapted to single-energy source diets of carbohydrate, protein, and fat and then injected with NE (20-80 nmol) or CLON (5-20 nmol) immediately prior to dark onset (17h00). Measurements of nutrient intake were determined 2 h postinjection. In a separate study, obese and lean mice were deprived of food for 1 h (1700-1800) and subsequently treated with 5-HT (35-140 nmol). The results of this study demonstrate that the hyperphagic effect of NE and CLON and the anorectic effect of 5-HT are dose dependent and nutrient selective. Specifically, at the onset of the nocturnal cycle, obese and lean mice exhibit a shift in diet choice resulting in an increased preference for carbohydrate and a reduction in the proportional intake of protein and fat. At this time, central injection of NE or CLON potentiates an already enhanced preference for carbohydrate; whereas injection of 5-HT suppresses carbohydrate intake (kcal) in both phenotypes without altering fat or protein intake. However, in comparison to lean mice, obese mice showed significantly augmented hyperphagic responses to NE and CLON administration but decreased inhibition of feeding after 5-HT injection. This suggests that the stimulatory effect of alpha 2-noradrenergic mechanisms controlling feeding and carbohydrate ingestion is enhanced in obese mice, while the inhibitory influence of serotonergic mechanisms is attenuated.

Hypothalamic serotonin in the control of meal patterns and macronutrient selection

Serotonin (5-HT) is believed to have an inhibitory influence over feeding behavior. The present experiments were designed to investigate the effects of hypothalamic 5-HT on spontaneously motivated feeding and appetite regulation. Freely-feeding rats were injected with 5-HT or norfenfluramine (NORFENF) directly into the paraventricular hypothalamus (PVN), and precise changes in feeding behavior were monitored by a computer. Following PVN 5-HT or NORFENF injection, animals exhibited a marked suppression in food intake, associated with a decrease in meal size, duration and eating rate, and no change in the frequency of meals consumed. This suggests that brain 5-HT may influence primarily the induction of satiety rather than the suppression of hunger. The effect of drugs presumed to affect brain 5-HT transmission on diet selection was also investigated in groups of rats injected centrally with 5-HT or NORFENF or peripherally with either fenfluramine, quipazine or cyproheptadine. In a series of 2-diet tests, rats centrally injected with 5-HT or NORFENF exhibited a selective suppression of the carbohydrate-rich diets. In animals provided with three pure macronutrient diets, protein, carbohydrate, and fat, systemic administration of serotonergic agents had its greatest impact on fat and carbohydrate ingestion, as compared to protein consumption. These findings support a role for hypothalamic 5-HT in modulating meal patterns and appetite for particular macronutrients.

The role of glucose, insulin and glucagon in the regulation of food intake and body weight

Glucose and related pancreatic hormones play a major role in the metabolism of monogastric mammals yet their influence on hunger and/or satiety is, as yet, poorly understood. Glucose, insulin and glucagon rise during a meal and gradually decline to baseline levels shortly after a meal. A sudden drop in plasma glucose as well as insulin have been reported just prior to the onset of a meal but the functional significance of this is not yet clear. Systemic injections of glucose have no acute satiety effects but intraduodenal and intrahepatic infusions reduce food intake and free-feeding and deprived animals respectively. Treatments which decrease cellular glucose utilization directly (2-DG) or indirectly (insulin) increase food intake while exogenous glucagon (which produces hyperglycemia) decreases it. There is considerable evidence that some or all of these effects may be due to a direct central action of glucose, 2-DG, insulin, and glucagon on brain mechanisms concerned with the regulation of hunger and satiety although influences on peripheral "glucoreceptors" have been demonstrated as well. The functional significance of glucoprivic feeding is, however, questioned. The feeding response to 2-DG and related compounds is capricious, and its temporal course does not parallel the hyperglycemic reaction which presumably reflects cellular glucopenia. Moreover, numerous brain lesions which increase, decrease, or have no effect on ad lib intake and often have no effect on the response to deprivation have been shown to severely impair or abolish feeding responses to systemic injections of 2-DG that produce severe central as well as peripheral glucopenia. Feeding responses to insulin are intact after most of these lesions, suggesting that this hormone may influence food intake in a fundamentally different fashion. The mechanism of insulin action is not understood--the classic feeding response is obtained only with doses that are pharmacological when compared to normal plasma levels and there is increasing evidence that lower doses may have opposite, inhibitory effects on food intake and body weight. Relatively small doses of glucagon decrease food intake (although opposite facilitatory effects have been reported after even smaller doses) but the effect does not appear to be due to hepatic mobilization of glucose as initially assumed. Decreases in food intake after intracranial injections of very small doses suggest a direct central action.

Overeating after midbrain 6-hydroxydopamine: prevention by central injection of selective catecholamine reuptake blockers

Earlier research from this laboratory showed that 6-hydroxydopamine (6-OHDA) injected into the ventral midbrain of rats causes overeating and the gradual development of obesity in association with extensive depletion of forebrain norepinephrine. It was proposed that depletion of norepinephrine or epinephrine was the cause of the behavioral abnormality. This was questioned on the basis of experiments showing that 6-OHDA can cause non-selective lesions and also that non-selective neurotoxins can cause hyperphagia. To help resolve this question, the present experiments used 191 rats in 28 groups to test 3 doses of 6-OHDA and 3 different pretreatments with reuptake blockers, amphetamine, desmethylimipramine and benztropine. The result of midbrain 6-OHDA alone was hyperphagia that was highly correlated with the dose. Pretreatment with intraperitoneal desmethylimipramine (DMI) caused gastric distress, and systemic amphetamine caused delayed overeating, but these unwanted side effects were avoided by injecting the drugs directly into the midbrain 15 min before the 6-OHDA. DMI and amphetamine partially prevented catecholamine depletion in the forebrain and totally prevented subsequent hyperphagia and obesity. Benztropine prevented dopamine depletion, but had no effect on hyperphagia. We conclude that some of the neurons protected by local injection of amphetamine and DMI are probably norepinephrine or epinephrine fibers that ascend through the ventral midbrain and have as one of their functions the inhibition of food intake. These neurons could also play a role in the control of body weight.

The role of norepinephrine in feeding behavior

When dopamine-beta-hydroxylase is inhibited with FLA-63 (10 mg/kg) free feeding behavior is disrupted in satiated rats. While the average number of meals taken was not different from vehicle injected controls, meal size was decreased 58% in the first 9 hr after treatment with FLA-63. In starved animals, FLA-63, when given alone, produced little effect on feeding behavior, even though norepinephrine depletion was in excess of 40%. When given in combination with RO4-1284 (5 mg/kg), a vesicular reuptake inhibitor, feeding was reduced to 16% of control intake and norepinephrine was specifically depleted 99%. Feeding was reliably reinstated in animals which received FLA-63 plus RO4-1284 with either dl-threo-DOPs, a metabolic precursor to NE, or direct intrahypothalamic injections of NE. These findings suggest that the feeding inhibition observed after treatment with FLA-63 plus RO4-1284 is due to disruption of transmission in brain NE systems. A non-anorectic dosage of L110-140 (3.73 mg/kg), a specific FLA-63. Taken collectively, these findings suggest that the primary role of NE in feeding is maintenance of the consummatory response and that these effects are expressed in relation to activity in other neurochemical systems.

The effects of amphetamine or caffeine on the response to glucoprivation in rats with rostral zona incerta lesions

Lesions in the rostral zona incerta (ZI) of male albino rats severely impaired feeding responses to 2-deoxy-D-glucose (2DG) and drinking responses to hypertonic saline during the first month after surgery. There was evidence of recovery 6 months after surgery but the magnitude of the improvement was small and severe impairments persisted in most subjects. A small but significant deficit in the feeding response to insulin persisted unabated after the 6-month recovery period. Caffeine or amphetamine pretreatment, but not apomorphine, increased ad lib feeding as well as the response to low doses of 2DG in rats with ZI lesions as well as in controls. The increased feeding response to 2DG after caffeine or amphetamine was larger than the sum of the effects of 2DG alone plus the effect of caffeine or amphetamine alone.

Mesencephalic lesions in female rats resulting in normophagia and reduced body weight

The effects of bilateral mescencephalic lesions in the area of the ventral noradrenergic bundle (VNA) were studied in immature and mature female rats. The food intake consumption of the immature lesioned rats did not differ from sham operated controls whether fed a chow or high fat diet. However, after the surgery the body weights of the two groups began to diverge with the immature lesioned rats obtaining a significantly lower body weight. By the end of the study the immature lesioned rats were also significantly shorter than the controls. Both groups showed normal body composition throughout the measurement period. Upon refeeding after a one day fast the immature female rats defended their lower than normal pre-fast body weight. These data are in good agreement with our earlier findings using immature male rats. The mature lesioned animals also showed normal food consumption when fed a chow type diet. However the lesioned animals did show a brief hyperphagia when placed on a high fat diet. Unlike the immature lesioned rats the body weight of these lesioned animals did not differ from their sham operated controls. It is suggested that lesions in the area of the VNA may result in hyperphagia or, in no effects on food intake or body weight as reported by others or normophagia with reduced body weight. Exact lesion placement may be responsible for these divergent findings.

Aphagia and adipsia after preferential destruction of nerve cell bodies in hypothalamus

Microinjections of the excitatory neurotoxin kainic acid into the lateral hypothalamus of rats produced a period aphagia and adipsia. Kainate-treated rats displayed transient motor effects during the first hours after the injection but did not show the persisting sensory-motor and arousal disturbances typically observed in animals with electrolytic lesions in this part of the hypothalamus. Histological examination revealed a significant reduction in the number of nerve cell bodies in the lateral hypothalamus. Silver-stained material indicated no evidence of damage to fiber systems passing through the affected region. Assays of dopamine in hypothalamus, striatum, and telencephalon did not indicate significant differences between experimental and control animals. These results are in agreement with recent reports of the anatomical and biochemical effects of intracerebral kainic acid injections and suggest that the observed effect on feeding behavior is related to the destruction of neurons in the lateral hypothalamus.

Some biochemical effects of zona incerta lesions that interfere with the regulation of water intake

The concentration of NE, DA and 5-HT in forebrain, striatum, and hypothalamus was measured after zona incerta (ZI) lesions that have been shown to result in general hypodipsia; adipsia during periods of food deprivation; impaired or abolished drinking in response to osmotic challenges (but not polyethylene glycol); impaired drinking after systemic isoproterenol or central angiotensin; and impaired or abolished feeding in response to 2-deoxy-D-glucose. The lesions produced a significant (40-50%) depletion of forebrain NE but a correlational analysis of the behavioral and biochemical effects of the lesions failed to indicate a causal relationship. The lesions did not reliably affect (a) forebrain DA or 5-HT; (b) striatal DA or 5-HT; (c) hypothalamic DA, NE or 5-HT. The results of these experiments indicate that significant impairments in ingestive behavior can be demonstrated in animals with diencephalic lesions that do not result in striatal (or forebrain) DA depletions. This confirms previous behavioral analyses showing that ZI lesions which interfere with ingestive behavior do not produce the debilitating sensory or motor dysfunctions typical of the rat with lateral hypothalamic lesions.

Regulatory deficits after surgical transections of three components of the MFB: correlation with regional amine depletions

Parasagittal knife cuts along the lateral border of the diencephalon (PS), coronal cuts across the lateral (LMFB) or medial (MMFB) components of the medial forebrain bundle reproduce most of the persisting deficits in responding to glucoprivic and hydrational challenges that characterize rats with lateral hypothalamic lesions or intracranial injections of 6-hydroxydopamine (60HDA). Each of these cuts produced a differnet pattern of regulatory deficits, suggesting that individual components of the LH syndrome may be mediated by different neural substrates. This interpretation is supported by the results of our correlational analysis of the relationships between specific behavioral and biochemical effects of our cuts. For example, feeding responses to insulin were reliably correlated with striatal DA concentrations but feeding responses to 2-deoxy-d-glucose (2DG) were not. Water intake during periods of food deprivation was reliably correlated with striatal DA but water intake after an experimental osmotic challenge was not. Only one of the common persisting deficits (impaired feeding response to peripheral injections of insulin) was positively correlated with the duration of aphagia and adipsia.

Deficits in food and water intake after knife cuts that deplete striatal DA or hypothalamic NE in rats

Knife cuts ventral or medial to the striatum were used to interrupt some of the principal connections of this structure. All of the cuts depleted striatal dopamine and produced aphagia and adipsia but there was no indication that the two classes of effects were always correlated. Cuts medial to the striatum produced the most severe DA depletions, persistent aphagia and adipsia, and the full complement of deficits in responding to glucoprivic and hydrational challenges that characterize rats that have recovered from lateral hypothalamic lesions. Cuts ventral to posterior portions of the striatum produced comparable periods of aphagia and adipsia (but few of the persisting impairments in responsiveness to regulatory challenges) even though their effect on striatal DA was relatively small (the average depletion was 51% compared to 89% for rats with cuts medial to the striatum). A second group of rats with cuts below more anterior aspects of the striatum sustained severe DA depletions (70%) but only very brief periods of aphagia and adipsia and only slight deficits in responding to osmotic challenges. The effects of the DA depleting cuts were compared with the behavioral consequences of coronal cuts in the midbrain tegmentum which selectively depleted hypothalamic norepinephrine. These cuts did not produce reliable effects on either food or water intake but abolished the normal feeding response to 2-deoxy-d-glucose without affecting the response to insulin. A correlational analysis of the biochemical and behavioral results of our cuts indicated a significant positive relationship between drinking in response to cellular thirst stimuli and hypothalamic NE as well as striatal DA. The postoperative body weights of our experimental animals were positively correlated with striatal dopamine and negatively related to hypothalamic norepinephrine.

A correlational analysis of the effects of surgical transections of three components of the MFB on ingestive behavior and hypothalamic, striatal, and telencephalic amine concentrations

A retractable wire knife was used to transect medial or lateral components of the MFB or its lateral projections to the striatum and amygdaloid complex. All cuts produced significant depletions of NE, DA, and 5-HT from telencephalon and striatum but little or no effect on hypothalamic NE or 5-HT. Two of our cuts resulted in aphagia and adipsia, the third in hyperphagia and obesity. A detailed correlational analysis of the magnitude and direction of the behavioral and biochemical consequences of our cuts indicated that the ingestive behavior of all of our experimental animals (including animals which had been aphagic and adipsic after surgery as well as animals which were hyperphagic and obese) was positively correlated with the concentration of DA in striatum and telencephalon and negatively correlated with telencephalic 5-HT. Less consistent evidence for facilitatory noradrenergic influences on food intake was also obtained. Our results suggest that the regulation of food intake may be the result of an interaction between telencephalic serotonergic mechanisms and dopaminergic pathways which exert opposite effects on ingestive behavior.