Adrenergic feeding: its blockade or reversal by posterior VMH lesions; and a new hypothesis

Effects of pedunculopontine tegmental nucleus lesions on responding for intravenous heroin under different schedules of reinforcement

The pedunculopontine tegmental nucleus (PPTg) is believed to play important roles in reward and learning. We examined the effect of PPTg lesions (0.5 microl of 0.1 M NMDA injected bilaterally over 10 min) on the learning of an operant response for opiate reward. In 14 adult male Long-Evans rats, bilateral lesions of the PPTg disrupted the acquisition of responding for intravenous heroin (0.1 mg/kg infused at a rate of 0.25 ml/28 sec) on a fixed ratio-1 (FR-1) schedule of reinforcement. The 12 remaining lesioned animals increased their heroin intake over the acquisition sessions but did not reach the response levels of sham-lesioned animals on the 15th and final session. The sham- and PPTg-lesioned animals that learned the FR-1 task exhibited similar patterns of responding during extinction and reacquisition sessions. When tested on a progressive ratio (PR) schedule of reinforcement, however, PPTg-lesioned animals had lower break points than sham-lesioned animals. Asymmetric lesions, which destroyed the majority of the nucleus in one hemisphere only, did not produce any behavioral deficits. Rats that were lesioned after training also did not show deficits in responding under either FR or PR schedules. These findings suggest that PPTg lesions reduce the rewarding effect of opiates but do not disrupt the ability either to learn an operant response or the response requirements of a PR schedule.

Noradrenergic receptor interactions in feeding elicited by stimulation of the para-ventricular hypothalamus

Food intake and feeding behaviour were examined after the administration of noradrenaline (NA) or clonidine to the para-ventricular nucleus of the hypothalamus, or after systemic administration of clonidine. In 20-hr deprived animals all treatments dose-dependently reduced pellet consumption; however, at a low dose (2-5 micrograms) central clonidine increased eating time and bout length. In 4-hr deprived animals all treatments increased sucrose consumption. Clonidine (peripheral and central) increased feeding time but did not alter feeding rate; NA did not alter feeding time, but did increase feeding rate; NA also increased activity and decreased resting. The effects of NA on feeding rate, activity and resting were blocked by propranolol; however, the propranolol-NA combination increased feeding time. Thus, NA and clonidine increased feeding by different mechanisms, but after propranolol pretreatment the effects of NA were similar to those of clonidine. It is concluded that clonidine enhances feeding by inhibiting satiety and that the feeding stimulant effect of NA is mediated by a complex interaction of alpha- and beta-receptors.

Modification of nocturnal spontaneous and adrenergic-induced feeding in the rat following either A5 or A7 lesions

The ingestive profiles of intact, A5 and A7 damaged animals were examined during the 2-hr nocturnal period following onset of the dark cycle. A5, A7 and intact rats consumed comparable amounts of food following initial access to food nocturnally. Sebsequent feeding declined in A5 animals below control values and failed to return to baseline at the end of the nocturnal period examined. A7 damaged rats appeared more resistant to the appetite suppressing effects of initial meal taking and consumed more food than control animals. Only A5 damaged rats were noted to be hyperdipsic during the immediate 10 postoperative days. Intracranial injection of 1-norepinephrine bitartrate (10 ug/ul) into the paraventricular nucleus (PVN) of the hypothalmus produced a reliable facilitation of feeding in A5, A7 and intact rats during the first hour of the dark cycle. A5 rats exhibited the largest increase in feeding elicited by NE administration into the PVN. This feeding response was observed in rats with A5 lesions regardless of whether testing was carried out during the initial hours of the dark cycle or during a predetermined "satiation" test. A5 lesions also effected a marked hyperglycemia while A7 lesions were ineffective in this respect. Taken together these data suggest the A5 and A7 cell groupings regulate spontaneous feeding within a rostrally coursing feeding circuitry and appear to interact with the PVN in the elicitation of noradrenergic feeding.

Feeding behavior induced by central norepinephrine injection is attenuated by discrete lesions in the hypothalamic paraventricular nucleus

Extensive brain-cannula mapping studies in the rat have demonstrated that the hypothalamic paraventricular nucleus (PVN) is the most sensitive brain site for eliciting eating behavior with central norepinephrine (NE) injection. The present experiments examined the impact of lesions aimed at the PVN on this NE-elicited eating response. In rats with NE injection cannulas aimed at the lateral ventricle, bilateral lesions of the PVN significantly attenuated, by 60 to 70%, the eating effect induced by NE, at doses ranging from 20 to 160 nmoles. PVN lesions which extended ventrally to damage tissue lying within the periventricular region were more effective in abolishing the NE response than were lesions that remained confined to the dorsal aspects of the PVN. Large lesions located just dorsal to the PVN had no impact on the NE response. This evidence supports the primary role of the PVN in mediating the eating behavior elicited by central noradrenergic activation.

Effects of hypothalamic knife cuts on feeding induced by paraventricular norepinephrine injections

The relationship between the fiber systems involved in the hypothalamic noradrenergic feeding response and the medial hypothalamic (MH) hyperphagia syndrome was appraised in male rats using knife cuts. Parasagittal knife cuts in the perifornical hypothalamus produced hyperphagia and excessive weight gain but failed to disrupt feeding in response to paraventricular hypothalamic injections of norepinephrine (NE). Coronal knife cuts in the posterior hypothalamus which extended from the midline to the lateral perifornical region also failed to disrupt NE feeding. These findings indicate that the output of the noradrenergic feeding system does not follow the feeding pathway implicated in the MH hyperphagia syndrome. They also suggest that the output of the noradrenergic feeding system is not directed laterally beyond the level of the fornix nor caudally into the lower brainstem over the medial forebrain bundle.

Evidence for vagal involvement in the eating elicited by adrenergic stimulation of the paraventricular nucleus

We examined the role of the vagus nerves in mediating the eating and preprandial drinking seen after injection of norepinephrine (NE) into the region of the paraventricular hypothalamic nucleus of satiated rats. Complete subdiaphragmatic vagotomy (confirmed by gastric secretion tests) abolished the NE-elicited eating response, whether the diet used was lab chow, milk, or a milk-chow misture, and attenuated, by 38%, the NE-elicited drinking response. These effects occurred independently of changes in body weight or daily food intake imposed by vagal surgery. The vagotomized rats retained the capacity to rapidly increase eating in response to food deprivation or insulin injection challenges, indicating that the effect of vagotomy on NE-induced eating was not due to some non-specific impairment. Efferent vagal blockade of intact rats with systemic injections of atropine methyl nitrate (0.4 mg/kg) prior to central NE infusions yielded similar results. Finally-selective section of the coeliac branch of the vagus produced a 49% reduction of NE, elicited eating, as compared with a 29% reduction in water intake, while selective section of the gastric plus hepatic vagal branches, leaving only the coeliac branch intact, did not significantly affect either ingestive response. Both of these selectively vagotomized groups displayed an unimpaired capacity to increase food intake in response to systemic insulin injections. These results suggest participation of efferent vagal mechanisms in the adrenergic feeding, and, to a lesser extent, drinking phenomena and are consistent with a particular role for some function under coeliac vagal control (perhaps insulin secretion) in modulating the effects of NE on feeding behavior.

Influence of diet palatability on noradrenergic feeding response in the rat

Adult male rats which displayed a reliable feeding response to intrahypothalamic injections of norepinephrine (NE) on a chow diet were subsequently tested on one of three diets: an unpalatable quinine-adulterated meal, a palatable fat-adulterated meal, or a "neutral" unadulterated meal. The quinine diet completely blocked the NE feeding response, while the fat diet produced a small and unreliable reduction in the feeding response. When food deprived all groups increased their food intake, although the fat diet group tended to overeat, and the quinine diet group tended to undereat relative to the unadulterated diet group. The failure of the palatable fat diet to potentiate the NE feeding response does not support the hypothesis that this response mimics the ventromedial hypothalamic hyperphagia syndrome. The blocking effect of the quinine diet on NE feeding is consistent with other evidence which suggests that NE mediated the eating behavior induced by glucoprivation.

The effect of bilateral ventral noradrenaline bundle lesions on lever pressing for food in rats

Food reward has been associated with activation of noradrenergic mechanisms in the brain. Using rats trained to press a lever for food reward, we have investigated the effects of 6-hydroxydopamine lesions, which severly depleted hypothalamic noradrenaline, on the willingness of the rats to press the lever for food reward. We found that performance in the food-rewarded task was significantly impaired following such lesions, and that this was especially marked when the task was made more difficult. From our results we suggest that ventral noradrenaline bundle lesions can decrease the rewarding nature of food, thus making the animals less willing to work for food reward.

Paraventricular nucleus: a primary site mediating adrenergic stimulation of feeding and drinking

Central injection of norepinephrine (NE) has been found to elicit preprandial drinking and feeding responses in the satiated rat. In the present study, 35 different brain areas, in over 500 rats, were examined to localize the precise region of NE sensitivity. Essentially all sites outside the hypothalamus, as well as in the lateral portion of the hypothalamus, were relatively or totally unresponsive to NE. In the medial hypothalamic area, the paraventricular nucleus (PVN) was clearly distinguished as the most effective site for initiating both feeding and drinking with noradrenergic activation in the satiated animal. Sites greater than 0.5 mm rostral, caudal, dorsal, ventral or lateral to this nucleus yielded significantly smaller effects. In mildly hungry rats, NE was found to potentiate the ongoing feeding response, and anatomical analyses of this phenomenon showed the PVN to be most responsive, with a smaller but reliable potentiation occurring along the periventricular hypothalamus adjacent to the third ventricle. Norepinephrine injected into the lateral perifornical hypothalamic area actually produced a suppression of feeding in these hungry animals. These findings, together with results from other studies, converge on the medial PVN region as being a key link in the process of increased food and water consumption associated with increased noradrenergic activity.

Catecholamines and self-stimulation: evidence suggesting a reinforcing role for noradrenaline and a motivating role for dopamine

Investigation of the role of noradrenaline (NA) and dopamine (DA) in self-stimulation showed that d-amphetamine (which releases more DA than does l-amphetamine, but not more NA) was much more effective than l-amphetamine in enhancing self-stimulation of NA sites in the locus coeruleus and near-lateral hypothalamus. In DA sites in the substantia nigra and far-lateral hypothalamus the effects of the 2 isomers were confirmed to be more nearly equal. Thymoxamine HCl (10 mg/kg IP), a specific alpha-adrenergic receptor blocker, depressed self-stimulation at all sites, but significantly more severely at DA sites. Thus the drugs most effective in influencing self-stimulation at a particular site were those acting predominantly on the unstimulated system. These findings were interpreted in terms of a hypothesis that DA and NA play complementary roles in self-stimulation and that both are essential; or, more specifically, that DA pathways, implicated in other motivational activites, contribute to a state of drive or arousal necessary for self-stimulation; while response-contingent noradrenergic activity (elicited by the electrodes directly via a transsynaptic route) mediates reinforcement. Further predictions from this hypothesis were tested as follows: (1) Direct pharmacological stimulants of adrenergic alpha-receptors should disrupt self-stimulation by acting randomly on the reinforcement system and disrupting response-reward contingencies; this was confirmed by the finding that the alpha-receptor stimulant clonidine HCl (0.05 mg/kg) depressed self-stimulation at all sites tested. (2) Drect stimulants of DA receptors should enhance self-stimulation of NA sites by augmenting dopaminergic motivational activity; but in rats with DA electrodes, noncontingent stimulation of DA receptors would also impose similar noncontingent activity on the transsynaptic noradrenergic reinforcement pathways and thus depress self-stimulation; this was confirmed by the finding that apomorphine (0.3-1.0 mg/kg) was strongly stimulant for NA electrodes but strongly depressant for DA electrodes, and that the degree and direction of these effects was highly correlated with the differential effects of d- l-amphetamine (rho = .65, p less than 0.01). Neither effect of apomorphine depended on the occurrence of motor stereotypy. These results can be interpreted in terms of 2-component models for self-stimulation, with the predominant transmitter of the drive component being identified as DA and that g the reinforcing component as NA.

Cyclic AMP and central noradrenaline receptors: failure to activate diencephalic adrenergic feeding pathways

Intracranial injection of graded doses of dibutyryl 3',5'-cyclic adenosine monophosphoric acid (cAMP) at sites in the accumbens/stria terminalis nuclei of satiated rats elicited behavioural arousal and occasional convulsive episodes at higher doses, but failed to affect food consumption even in sites where injection of noradrenaline (65 nmol) consistently elicited increased feeding. Intracranial aminophylline (550 nmol) or dopamine (65 nmol) were also without effect on food consumption. This result does not support recent suggestions that cAMP serves as the second messenger in central noradrenergic motivational pathways.

Anorexia nervosa and the regulation of intake: a review

Recent work on the central mechanisms of food intake is reviewed in relation to anorexia nervosa. On the basis of the present review a number of conclusions are drawn as possible avenues for future research. A proposal is outlined for the investigation of L-dopa as a potential treatment for anorexia nervosa.