Ingestion in the satiated rat: role of alpha and beta receptors in mediating effects of hypothalamic adrenergic stimulation

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.

Histochemical and pharmacological analysis of noradrenergic projections to the paraventricular hypothalamus in relation to feeding stimulation

Three techniques, namely, midbrain lesions, fluorescence histochemistry, and brain cannulation, were used in combination to analyze noradrenergic projections to the paraventricular nucleus of the hypothalamus (PVN) and their function in stimulating feeding behavior. The convergence of evidence indicates that the dorsal component of the central tegmental tract (CTT), which ascends through the dorsal pons and then projects through the medial tegmental radiations (TR) into the ventral tegmentum just dorsal to the media lemniscus, contains the crucial noradrenergic axons which innervate the PVN and mediate noradrenergic stimulation of feeding behavior. The primary evidence for this conclusion is that dorsal tegmental electrolytic or 6-OHDA lesions which damaged specifically these fibers invariably caused: (1) a reduction of catecholamine varicosities within the PVN (most notably, fine and moderate-size, rounded varicosities within the parvocellular area); (2) a strong reduction or loss of the feeding response elicited by PVN injection of the presynaptically-acting drugs tranylcypromine and desipramine; and (3) a potentiation of the same response produced by injected norepinephrine. These pharmacological and neurochemical changes in the PVN were reduced in magnitude if the dorsal CTT and medial TR fibers received only partial damage, and these changes did not occur at all if the lesion fell immediately dorsal to these fibers without damaging them. Specific lesions in the ventral tegmentum, which also failed to damage the dorsal CTT and TR axons but instead damaged the ventral component of the CTT, not only failed to disrupt the action of the antidepressant agents but actually potentiated their effectiveness in the PVN. Ventromedial lesions, however, which severed the rostroventral extension of the dorsal CTT and medial TR fibers, had the same behavioral consequences as had the dorsal lesions which damaged this projection at a more dorsocaudal level. Finally, damage to other catecholamine projections had little effect on PVN function in stimulating eating.

Pharmacological characterization of perifornical hypothalamic dopamine receptors mediating feeding inhibition in the rat

Mapping studies with central drug injections in the hungry rat have identified the perifornical lateral hypothalamus as being uniquely sensitive to the feeding suppressive effects of exogenous dopamine, as well as anorexic drugs which release endogenous catecholamines. In the present study, the hypothalamic dopamine-receptive sites mediating this phenomenon were pharmacologically characterized. These sites, studied via direct drug injection into the perifornical hypothalamus of freely moving, brain-cannulated rats, were found to be most responsive to dopamine, in a dose-dependent manner, but were also activated by other catecholamine receptor stimulants, with the order of potency being dopamine greater than apomorphine = epinine greater than norepinephrine. Clinically effective neuroleptic compounds antagonized these dopamine-sensitive sites, apparently in a competitive and stereochemically specific manner. The relative potency of the neuroleptics and structurally related compounds was calculated to be haloperidol greater than fluphenazine greater than chlorpromazine greater than pimozide greater than promazine. The ineffective neuroleptic promethazine, the tricyclic antidepressants imipramine and desipramine, and the antagonists of alpha-adrenergic, beta-adrenergic, cholinergic, and serotonergic receptors, did not manifest the ability to reverse dopamine's action. These results thus reveal properties of these hypothalamic dopamine-sensitive, feeding inhibitory sites which match to a large extent the characteristics recently identified for dopamine receptors in the periphery and extrahypothalamic brain areas.

Mapping study of brain dopamine- and epinephrine-sensitive sites which cause feeding suppression in the rat

Central injections of dopamine (DA) or epinephrine (EPI) have been found to suppress feeding behavior in hungry rats. In the present study, 24 different brain areas, in 299 animals, were examined to localize the precise region of catecholamine (CA) sensitivity. Essentially all sites outside the hypothalamus, as well as in the medial portion of the hypothalamus, were relatively or totally unresponsive to DA or EPI. The area of greatest sensitivity for both agonists (where they yielded a 50--70% suppression of feeding) was found to be the perifornical region of the lateral hypothalamus, extending from the caudal aspect of the paraventricular nucleus to the caudal aspect of the ventromedial nucleus. Dorsal, lateral, or ventrolateral movement of the injection site away from the fornix and into the zona incerta or the lateral hypothalamic medial forebrain bundle area caused a dramatic reduction in the effectiveness of the CA. These findings are consistent with histochemical studies, which have shown the fornix to be surrounded by CA varicosities, and pharmacological studies, which have shown the perifornical region to be most sensitive to the anorexic effect of centrally injected amphetamine, which releases endogenous CA. It is suggested that the perifornical hypothalamus plays a role in the process of inhibiting food consumption in response to increased dopaminergic and adrenergic activity.

L-Dopa feeding suppression: effect on catecholamine neurons of the perifornical lateral hypothalamus

Injection of L-Dopa (0.8--200 nmoles) into the perifornical hypothalamus produced a dose-dependent suppression of feeding in hungry rats. This effect was positively correlated in magnitude with the same effect produced by the catecholamine agonists dopamine and epinephrine, and by the catecholamine-releasing drug amphetamine. L-Dopa's action was partially antagonized by separate injections of the dopaminergic blocker haloperidol (58% blockade) and the beta-adrenergic blocker propranolol (38% blockade). Combined injections of these two antagonists produced a 90% blockade of L-Dopa's effect. Perifornical administration of the dopa decarboxylase inhibitors Ro 4-4602 and MK-486 was also shown to reverse L-Dopas feeding suppression, at doses that enhanced the effect of injected dopamine and epinephrine. On the basis of these findings, L-Dopa appears to suppress food consumption in part through increased catecholamine synthesis, specifically within dopaminergic and adrenergic neurons of the perifornical hypothalamic region.

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.

Analysis of feeding suppression produced by perifornical hypothalamic injection of catecholamines, amphetamines and mazindol

The effects on feeding of perifornical hypothalamic injection of catecholamines, amphetamines and mazindol were examined in hungry rats. In pargyline-pretreated subjects, both dopamine and epinephrine significantly suppressed food intake, at doses as low as 31 ng for dopamine and 150 ng for epinephrine (the latter injected with an alpha-adrenoceptor blocker). This effect was reliably strengthened by inhibiting catecholamine deamination or presynaptic catecholamine uptake. Perifornical injections of amphetamine, mazindol, methamphetamine, and phenmetrazine also suppressed feeding. The magnitude of this effect in individual animals was positively correlated with the effect produced by catecholamine agonists. Moreover, this effect of mazindol was partially antagonized by perifornical injection of dopaminergic and beta-adrenoceptor blockers. The effects of amphetamine and epinephrine were abolished by these drugs, while dopamine's effect was selectively inhibited by the dopaminergic antagonist. Serotonergic antagonists produced no change. These findings lend support to the hypothesis that perifornical hypothalamic catecholamine neurons, through dopaminergic receptors and beta-adrenoceptors, are involved in inhibiting feeding behavior, as well as in mediating the anorexic action of the amphetamines and mazindol.

Adrenergic stimulation of the paraventricular nucleus and its effects on ingestive behavior as a function of drug dose and time of injection in the light-dark cycle

The paraventricular nucleus (PVN) has been identified as the most responsive site in the brain for eliciting feeding and preprandial drinking responses with local injection of norepinephrine (NE) in satiated rats. The present study examined these phenomena as a function of dose and time of injection in the light-dark cycle. The lowest effective doses for producing these ingestive responses with exogenous NE in the PVN are found to be between 5.6 and 16.9 ng for preprandial drinking and between 1.0 and 4.2 ng for feeding. Tests with NE injection into the PVN at different times in the light-dark cycle indicate that an increase in feeding effect can occur in the dark as well as in the light and at varying levels of food intake baseline and with solid and liquid food. Site of injection in the brain is believed to be a crucial factor in determining the nature of NE's effects on feeding behavior.

Changes in ingestive behavior following interruption of a noradrenergic projection to the paraventricular nucleus: histochemical and neurochemical analyses

Histochemical investigation of the paraventricular nucleus (PVN) identified the tractus filiformis lateralis (TF) as a major noradrenergic preterminal pathway to the PVN. Bilateral knife cuts transecting the TF resulted in a discrete deafferentation of 56% of the noradrenergic input to the PVN region. TF transection also resulted in hypophagia and hypodipsia during the first four days after surgery. A marked hyperdipsia developed between postoperative Days 11 and 13 and persisted until sacrifice on Day 17. The results suggest that the TF contains a noradrenergic system which innervates the PVN and is involved in mediation of ingestive behaviors.

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.

Antigen-antibody reactions in rat brain sites induce transient changes in drinking behavior

The septum or hypothalamus of rat brain was injected through implanted cannulas with antibody against membrane antigens in the rat brain or with antibody against exogenous soluble antigens (such as ovalbumin) followed by the specific antigen. Both immunological systems produced a moderate but highly significant decrease in drinking by thirsty rats. This phenomenon is suggested as an experimental model for behavioral disorders resulting from nondegenerative, immunological processes in the brain.

Mesencephalic lesions resulting in normophagia, reduced weight and altered metabolism

The effects of bilateral lesions of the ventral noradrenergic bundle (VNA) were studied in male rats. In contrast to data reported by others, hyperphagia and obesity were not observed following VNA lesions. Indeed, except for a depression during the first three postoperative days, food intake (FI) of the VNA lesioned animals (VNAL) was normal. Interestingly, the body weight (BW) of the VNAL was significantly reduced compared to the controls, and a pair feeding study indicated that this depression of BW was not due to their FI. Computation of FI per metabolic size showed that the VNAL actually had a significantly increased FI compared to the controls. After a two day fast the VNAL lost more metabolic size than controls and upon refeeding they defended their pre-fast BW. The VNAL rats showed normal body composition and circulating glucose, insulin and prolactin. They had reduced free fatty acids, triglycerides, growth hormone and body length. The data suggest that the mesencephalon influences BW set point, some metabolites and possibly overall metabolism.

Catecholaminergic mechanisms of the lateral hypothalamus: their role in the mediation of amphetamine anorexia

The brain mechanisms mediating amphetamine's suppressive effect on feeding behavior were analyzed in rats with chronically implanted brain cannulas. Experiments in which drugs were injected directly into the anterolateral hypothalamus, the region found to be most responsive to amphetamine's action, yielded the following results. (1) Over a dose range of 6.25 nmoles (0.8 mug) to 400 nmoles (51.4 mug), hypothalamically injected D-amphetamine produced a reliable suppression of food consumption (20 percent at 6.25 nmoles, increasing to 88 percent at 200 nmoles) and was found to be approximately 3 times as potent as L-amphetamine in yielding this effect. (2) The anorexic effect of hypothalamically injected D-amphetamine was totally abolished by local administration of alpha-methyltyrosine, an inhibitor of dopamine, norepinephrine, and perhaps epinephrine synthesis, or by local administration of Fla-63, an inhibitor of only norepinephrine, and perhaps epinephrine, synthesis. (3) This effect of hypothalamic D-amphetamine was also antagonized by locally administered dopaminergic or beta-adrenergic receptor blockers but was unaffected by alpha-adrenergic, serotonergic, and cholinergic blockers. (4) Lateral hypothalamic administration of dopaminergic or beta-adrenergic receptor blockers, at quite low doses, was also effective in antagonizing the anorexia induced by peripherally administered D-amphetamine. These results strongly suggest that amphetamine, in suppressing feeding behavior, acts through the lateral hypothalamus, perhaps the anterior region, causing a release of dopamine and norepinephrine (or perhaps epinephrine) from lateral hypothalamic nerve endings and a subsequent stimulation of dopaminergic and beta-adrenergic receptors located in that region.