Histochemical identification of a PVN-hindbrain feeding pathway

Unilateral coronal knife cuts through the ventrolateral pontine reticular formation produce overeating and overweight when combined with contralateral parasagittal knife cuts in the medial hypothalamus (MH). The knife cuts were in a position to sever fiber projections from the paraventricular nucleus to the hindbrain. The present study used histochemical techniques to confirm that hyperphagia-producing knife cuts transect PVN-hindbrain fiber connections. In Experiment 1, adult female rats received a unilateral coronal knife cut in the ventrolateral pontine reticular formation. Horseradish peroxidase (HRP) was applied to the knife cut region and two to three days later brains were processed for the localization of neurons labeled with HRP. HRP-labeled neurons were found in the PVN, particularly in the caudal parvocellular region. Additional HRP-labeled neurons were observed in other medial hypothalamic areas but none were found in the ventromedial nucleus. HRP-filled cells were also found in the lateral hypothalamus, central nucleus of the amygdala, and in the nucleus of the solitary tract (NST). Many of the PVN projections to the hindbrain contain oxytocin and Experiment 2 determined if hyperphagia-inducing knife cuts sever PVN oxytocinergic fibers. Adult female rats received unilateral MH cuts, unilateral pontine cuts, or a contralateral combination of both cuts. One to eight days later the brains were processed for immunocytochemistry. The MH cuts and pontine cuts were found to interrupt descending oxytocinergic fibers. Taken together, these results support the hypothesis that interruption of a direct PVN-hindbrain oxytocinergic projection is responsible for the hypothalamic hyperphagia-obesity syndrome. However, the results do not rule out the involvement of a multisynaptic pathway or additional neurochemical systems.     

Paraventricular area: Critical focus of a longitudinal neurocircuitry mediating food intake

Hyperphagia, obesity, and excessive linear growth, but not hyperdipsia, were produced by the asymmetrical combination of a parasagittal hypothalamic knife cut and a contralateral coronal knife cut. When the location of the coronal cut was varied systematically, it was found that cuts rostral to the coronal level of the paraventricular nucleus (PVN) neither produced nor prevented overeating, while cuts caudal to the PVN produced a robust hyperphagic response. Thus, the coronal level of the paraventricular nucleus was revealed as the rostral focus of a longitudinal satiety neurocircuitry.     

Effects of quinine adulterated diets on the food intake and body weight of obese and non-obese hypothalamic hyperphagic rats

Female rats given knife cuts between the medial and lateral hypothalamus overate and became obese on a high fat diet. When switched to a quinine diet the knife cut rats initially underate and lost weight, but their body weights did not fall significantly below that of controls maintained on the same diet. Knife cut rats also maintained weights at control levels when given a moderately bitter quinine diet immediately after surgery, but displayed subnormal weights when switched to a very bitter diet. Cuts lateral to the fornix produced a greater weight suppression on the quinine diet, but a smaller weight gain on a high fat diet than did cuts medial to the fornix. The results indicate that the hypothalamic knife cuts elevate the upper limit of body weight with little or no change in the lower body weight limit, and that obesity rather than hypothalamic damage per se is the major cause of the hyperphagic rat's finickiness to unpalatable quinine diets. A dual lipostatic model of the hypothalamic hyperphagia syndrome is discussed.     

Effects of gastric vs complete subdiaphragmatic vagotomy on hypothalamic hyperphagia and obesity

Complete subdiaphragmatic vagotomy totally reversed pre-existing VMH lesion induced hyperphagia and obesity independent of the magnitude of the brain lesion effects, suggesting vagal participation in the VMH syndrome. However, complete vagotomy also reduced food and water intake and body weight in brain unlesioned controls. The specificity to VMH lesion obesity of the vagotomy effect is therefore questioned. Complete subdiaphragmatic vagotomy also reversed the obesity produced by prior hypothalamic knife cuts, and prevented the development of knife cut effects in rats which received knife cuts and vagotomies concurrently, or were permitted to recover from the acute effects of vagotomy before receiving knife cuts. Section of the gastric vagus, sparing the hepatic and coeliac branches, had no effect on knife cut-induced obesity, but did reduce water/food ratios and block increases in fasting gastric acid secretion. Thus, upper gastrointestinal effects cannot alone account for the blockade of obesity seen after complete vagotomy. This implicates the coeliac and/or hepatic vagal branches as important to the expression of hypothalamic obesity.     

Effects of selective vagotomies on knife cut-induced hypothalamic obesity: Differential results on lab chow vs high-fat diets

Asymmetrical hypothalamic knife cuts were used to produce obesity in rats fed lab chow pellets. When the brain surgery was accompanied by selective section of the coeliac branch of the abomdinal vagus nerve, only 57% of the expected weight gain was observed. Additional section of the gastric branches of the vagus further reduced the knife cut effects, and complete subdiaphragmatic vagotomy suppressed body weight below control levels. Conversely, all vagotomies that spared the coeliac branch did not by themselves attenuate hypothalamic knife cut obesity. These results suggest that some function(s) under coeliac vagal control are specifically involved in mediating hypothalamic hyperphagia and obesity. When, after 30 days, the rats were switched to high-fat diet, all the knife cut rats overate and became obese (or more obese) irrespective of vagotomy status. This overeating despite vagotomy indicates that the vagus nerves must not be the exclusive mediator of hypothalamic obesity.     

Preoptic-brainstem connections and maternal behavior in rats.

This study presents evidence supporting the view that preoptic area (POA) projections through the ventral tegmental area (VTA) to lower brainstem regions are important for maternal behavior in postpartum rats. Experiment 1 demonstrated that bilateral coronal knife cuts posterior to the VTA disrupted maternal behavior, and Experiment 2 demonstrated a similar disruption when a unilateral knife cut that severed the lateral connections of the medial POA was paired with a contralateral knife cut posterior to the VTA. In a final anatomical experiment using horseradish peroxidase histochemistry, it was shown that knife cuts posterior to the VTA do sever POA efferents. However, such cuts severed other ascending and descending pathways as well, and these may also be involved in maternal behavior control.     

Role of the stria terminalis in food intake and body weight in rats

Previous studies have shown that lesions of the posterodorsal amygdala result in hyperphagia and obesity in female rats. In the present study, lesions of the stria terminalis at its most dorsal point (before it separates into dorsal and ventral components) also resulted in hyperphagia and excessive weight gains in female rats compared to female rats with sham lesions, as did coronal knife cuts anterior to the ventromedial hypothalamus. Identical lesions and knife cuts did not elevate food intake or weight gains in male rats compared to male control animals. Examination of the anterograde degeneration with the amino-cupric-silver method in the brains of two female rats with hypothalamic knife cuts revealed degenerating terminals in the capsule of the ventromedial hypothalamus and in the premammillary nuclei, two classic indicators of damage to the dorsal component of the stria terminalis. No degenerating axon terminals were observed in the paraventricular nucleus. Differences from previous studies that used male rats were attributed to a sex difference for the effects of amygdaloid and ventromedial hypothalamic lesions. It is proposed that the posterodorsal amygdala, dorsal component of the stria terminalis, and ventromedial hypothalamus are part of an inhibitory pathway in the regulation of feeding behavior.     

Noradrenergic inputs to the paraventricular hypothalamus contribute to hypothalamic-pituitary-adrenal axis and central Fos activation in rats after acute systemic endotoxin exposure

Noradrenergic (NA) neurons within the nucleus of the solitary tract (NST) and caudal ventrolateral medulla (VLM) innervate the hypothalamic paraventricular nucleus (PVN) to initiate and modulate hypothalamic-pituitary-adrenal (HPA) axis responses to interoceptive stress. Systemic endotoxin (i.e. bacterial lipopolysaccharide, LPS) activates NA neurons within the NST and VLM that project to the PVN and other brain regions that receive interoceptive signals. The present study examined whether NA neurons with axonal inputs to the PVN are necessary for LPS to activate Fos expression within the PVN and other interoceptive-related brain regions, and to increase plasma corticosterone. Male Sprague-Dawley rats received bilateral stereotaxic microinjections of DSAP (saporin toxin conjugated to an antibody against dopamine-beta-hydroxylase, DbH) into the PVN to destroy NA inputs. Control rats were microinjected with vehicle into the PVN or received no PVN injections. Two weeks later, DSAP and control rats were injected i.p. with LPS (200 microg/kg BW) or saline vehicle, and perfused with fixative 2.5-3 h later. Brain tissue sections were processed to reveal nuclear Fos protein and cytoplasmic DbH immunolabeling. DSAP lesions depleted NA terminals in the PVN and bed nucleus of the stria terminalis, reduced the number of NA cell bodies in the NST and VLM, attenuated PVN Fos activation after LPS, and attenuated LPS-induced increases in plasma corticosterone. These findings support the view that NA projections from hindbrain to hypothalamus are necessary for a full HPA axis response to systemic immune challenge.     

Adipose tissue lipoprotein lipase activity in rats with obesity-inducing hypothalamic knife cuts

Three experiments examined the effects of obesity-inducing parasagittal hypothalamic knife cuts on adipose tissue lipoprotein lipase (LPL) activity in female rats. Knife cuts induced a 4-fold increase in adipose tissue LPL activity. Knife-cut rats with controlled insulin levels were hyperphagic but showed no increase in adipose tissue LPL activity or body weight gain. Prevention of the hyperphagia by food restriction also blocked the changes in LPL activity and weight gain. Finally, exogenous insulin treatment increased adipose tissue LPL activity in the absence of hyperphagia in neurologically-intact rats. We conclude that increased adipose tissue LPL activity may play a permissive role in the development of hypothalamic obesity, with the increase in enzyme activity being secondary to knife-cut-induced hyperinsulinemia.     

The rise, fall, and resurrection of the ventromedial hypothalamus in the regulation of feeding behavior and body weight

Early researchers found that lesions of the ventromedial hypothalamus (VMH) resulted in hyperphagia and obesity in a variety of species including humans, which led them to designate the VMH as the brain's "satiety center." Many researchers later dismissed a role for the VMH in feeding behavior when Gold claimed that lesions restricted to the VMH did not result in overeating and that obesity was observed only with lesions or knife cuts that extended beyond the borders of the VMH and damaged or severed the ventral noradrenergic bundle (VNAB) or paraventricular nucleus (PVN). However, anatomical studies done both before and after Gold's study did not replicate his results with lesions, and in nearly every published direct comparison of VMH lesions vs. PVN or VNAB lesions, the group with VMH lesions ate substantially more food and gained twice as much weight. Several other important differences have also been found between VMH and both PVN and VNAB lesion-induced obesity. Concerns regarding (a) motivation to work for food and (b) the effects of nonirritative lesions have also been addressed and answered in many studies. Lesion studies with weanling rats and adult pair-tube-fed rats, as well as recent studies of knockout mice deficient in the orphan nuclear receptor steroidogenic factor 1, indicate that VMH lesion-induced obesity is in large part a metabolic obesity (due to autonomic nervous system disorders) independent of hyperphagia. However, there is ample evidence that the VMH also plays a primary role in feeding behavior. Neuroimaging studies in humans have shown a marked increase in activity in the area of the VMH during feeding. The VMH has a large population of glucoresponsive neurons that dynamically respond to blood glucose levels and numerous histamine, dopamine, serotonin, and GABA neurons that respond to feeding-related stimuli. Recent studies have implicated melanocortins in the VMH regulation of feeding behavior: food intake decreases when arcuate nucleus pro-opiomelanocortin (POMC) neurons activate VMH brain-derived neurotrophic factor (BDNF) neurons. Moderate hyperphagia and obesity have also been observed in female rats with damage to the efferent projections from the posterodorsal amygdala to the VMH. Hypothalamic obesity can result from damage to either the POMC or BDNF neurons. The concept of hypothalamic feeding and satiety centers is outdated and unnecessary, and progress in understanding hypothalamic mechanisms of feeding behavior will be achieved only by appreciating the different types of neural and blood-borne information received by the various nuclei, and then attempting to determine how this information is integrated to obtain a balance between energy intake and energy output.     

Hypothalamic knife cuts alter vasopressin induced recovery of blood pressure following hemorrhage

Summary Effects of knife cuts posterior to the paraventricular nucleus (PVN) alone or to both the PVN and the supraoptic nucleus (SON) on vasopressin dependent restoration and maintenance of blood pressure following hemorrhage were tested in the rat. Conscious, unrestrained animals were hemorrhaged a volume equivalent to 1.8% of body weight from a femoral arterial catheter. Blood pressure was monitored for 30 min with no treatment, 30 min following iv injection of a specific antagonist to the pressor action of vasopressin, and 15 min during iv infusion of the competitive blocker of angiotensin II, saralasin. Restoration of blood pressure and the decrease in blood pressure with vasopressin blockade in rats with knife cuts posterior to the PVN alone were similar to that of control-operated animals. However, if knife cuts extended to the level of the SON, blood pressure was not restored, and vasopressin blockade did not result in a reduction of blood pressure. Saralasin infusion produced a similar decrease in blood pressure in all groups of animals. These data show that when knife cuts are confined to the area posterior to the PVN, vasopressin contributes to the restoration of blood pressure following hemorrhage. However, when cuts extend into the ventral hypothalamus, the contribution of vasopressin is eliminated.     

Hypothalamic hyperphagia despite imposed diurnal or nocturnal feeding and drinking rhythms.

Rats normally do most of their eating at night. When ad lib fed rats are made hyperphagic with lesions or parasagittal hypothalamic knife cuts the increases in eating occur primarily during the day. This suggests that a disruption of circadian rhythms may mediate the overeating. However, when knife cut rats were food and water deprived all day excessive eating occurred at night. Similarly, when they were deprived all night overeating occurred during the day. Under both conditions od deprivation the food intakes and rapid weight gains of the ad lib fed knife cut group were defended. It was concluded that: (1) in hypothalamic hyperphagia either the excessive food intake or the excessive weight gain is defended when food and water are available only half of each day, and (2) disruption of nocturnal feeding and drinking rhythms is not the cause of hypothalamic hyperhagia.     

The effects of paraventricular hypothalamic lesions on maternal behavior in rats

The present study was undertaken to determine whether the disruptive effects of knife cuts which sever the lateral connections of the medial preoptic area (MPOA) on maternal behavior are mediated by interfering with the output of the paraventricular hypothalamic nucleus (PVN). Postpartum rats received one of the following: Knife cuts severing the lateral connections of the MPOA; knife cuts severing the lateral connections of the PVN; radiofrequency lesions of the PVN; sham lesions or knife cuts. Only females that received knife cuts severing the lateral connections of the MPOA showed severe deficits in maternal behavior. These results indicate that the influence of the MPOA on maternal behavior is not mediated by the output of the PVN. Since the PVN is the major source of oxytocin input to other brain regions, these results also suggest that oxytocinergic neural pathways are not critical for postpartum maternal behavior. Another important finding was that females with MPOA knife cuts that did not retrieve their young were capable of hoarding candy, suggesting that the retrieval deficit was not the result of a general oral motor deficit.     

Differential effects of hypothalamic transections on the wood gnawing behavior of rats

Bilateral parasagittal knife cuts were made in the hypothalamus of rats at lateral planes 0.5, 1.0, or 1.5 mm from the midline. Cuts through the ventromedial hypothalamus (VMH) at L-0.5 produced the hyperphagia syndrome and did not significantly depress wood gnawing. Cuts just lateral to the VMH at L-1.0 also produced the hyperphagia syndrome but depressed wood gnawing below control levels. Cuts along the medial edge of the lateral hypothalamus at L-1.5 produced a mild hypophagia-hyperphagia syndrome and completely blocked wood gnawing behavior. The findings suggested the possibility that specific hypothalamic neural pathways may control gnawing behavior.     

Effects of hypothalamic knife cuts on the ingestive responses to glucose and insulin.

Parasagittal knife cuts through the perifornical hypothalamus either medial or lateral to the fornix produced hyperphagia and obesity and altered the rat's ingestive responses to dilute glucose solutions. The lateral knife cut rats drank less dilute glucose solution under both nondeprived and food deprived conditions and displayed less of a feeding suppressive response to glucose ingestion compared to controls. The lateral cut rats were also deficient in their feeding response to insulin-induced hypoglycemia, although their altered sensitivity to glucose and insulin did not appear to be causally related. The medial knife cuts decreased the responsivity to glucose, but less so than the lateral cuts, and did not alter the ingestive response to insulin. Both the medial and lateral knife cuts did not appear to change the rat's responsivity to concentrated blucose solutions. The neuroanatomical and functional nature of the disorder responsible for these effects and its relationship to the hyper-phagia-obesity syndrome are discussed.     

Dorsolateral connections of the medial preoptic area and maternal behavior in rats

The lateral connections of the medial preoptic area (MPOA) are essential for maternal behavior in rats. The purpose of this study was to more exactly specify the nature of this pathway. Experiment 1 found that knife cuts that severed the dorsolateral connections of the MPOA were as effective as complete cuts in disrupting maternal behavior, whereas knife cuts that severed the ventrolateral MPOA connections were ineffective. These results suggest that MPOA efferents and afferents critical for maternal behavior leave or enter the MPOA dorsolaterally. Experiment 2 located possible sources of critical afferent input. Lactating rats received MPOA lateral cuts with a horseradish peroxidase (HRP)-coated wire knife. Full lateral cuts and dorsolateral cuts disrupted maternal behavior and labeled more cells with HRP in the nucleus of the solitary tract and the locus coeruleus than did ventrolateral cuts, which did not disrupt maternal behavior.     

Hypothalamic hypoactivity prevented but not reversed by subdiaphragmatic vagotomy

Parasagittal hypothalamic knife cuts produce a syndrome which includes obesity and hypoactivity. When the subdiaphragmatic vagi were cut at the same time as the brain cuts were made, both the obesity and the hypoactivity were prevented. This suggested that both the obesity and the hypoactivity are mediated by vagal hyperactivity. In contrast, however, when the subdiaphragmatic vagotomy was done 40–70 days subsequent to the brain cuts, the obesity was reversed, but the hypoactivity persisted. Thus, some aspect of the interoperative period irreversibly prevents the ability of vagotomy to reinstate normal activity levels in the rat.     

Paraventricular nucleus efferents mediating photoperiodism in male golden hamsters

Knife cuts were placed around the paraventricular nucleus of the hypothalamus (PVN) in an effort to identify the pathways mediating gonadal regression in male golden hamsters housed in a short photoperiod. Horizontal cuts dorsal to the PVN only affected photoperiodic responsiveness if they actually damaged the PVN. Coronal cuts caudal to the PVN had no effect on testes regression. Parasagittal cuts adjacent to the medial PVN had little or no effect on gonadal regression induced by short photoperiod. We conclude that efferents of the PVN arising dorsally or medially are not necessary for photoperiodic control of testes function, whereas projections from or through the lateral PVN region descending towards the spinal cord are important.     

Differential effects of cuts through the posterior hypothalamus on food intake and body weight in male and female rats

Large knife cuts in the coronal plane through the posterior hypothalamus which transected the caudal connections of the medial hypothalamus without producing significant direct damage to cellular components of the region resulted in a very marked increase in food intake and rate of weight gain in female rats, but had little or no effect in male rats of the same strain. Water intake was equally elevated in both sexes. A detailed analysis of the effects of smaller cuts in the same region indicated that the pathways that are responsible for the effectiveness of the larger cuts do not course through the region immediately behind the ventromedial nuclei (VMN) but, instead, through the perifornical region lateral and posterior to the VMN. In the female, hyperphagia is not seen after cuts behind the VMH or lateral to the fornix but all cuts that invade the perifornical region to a significant extent result in overeating and obesity. In the male, a very mild hyperphagia (but no obesity) is seen after cuts that are restricted to this perifornical region but larger cuts that involve even minor aspects of the lateral hypothaalmus (LH) are entirely ineffective. Analysis of the effects of cuts that involve the medial, perifornical, and lateral hypothalamus to varying extent suggested that the disinhibitory effects of perifornical transections may be counteracted by relatively strong inhibitory effects due to damage to fibers in the lateral hypothalamus in the male, where the influence of the perifornical system appears to be weak, but not in the female, where the influence of the perifornical system appears to be very strong. The marked sex differences which characterize the effects of all cuts on body weight indicate that the perifornical system may be concerned specifically with metabolic functions that contribute to the development and/or maintenance of the organism's setpoint for body weight.     

Effects of knife-cut lesions of the medial forebrain bundle in self-stimulating rats

Unilateral frontal-plane knife-cut lesions were made in the anterior medial forebrain bundle ipsilateral to a lateral hypothalamic self-stimulation electrode. Behavioral effects of the knife cut on self-stimulation reward and operant performance capacity were measured via the reward summation function method. Knife cuts placed at the level of the anterior commissure were ineffective in altering reward or motor/performance capacity, whereas knife cuts just posterior in the caudal lateral preoptic area degraded reward and sometimes impaired motor/performance capacity. In a second experiment, knife cuts placed posterior to the ventral tegmental area were ineffective unless they intruded on the ventral tegmental area itself. Several small knife cuts placed just anterior to the ventral tegmental were effective in reducing self-stimulation reward. The results are discussed in terms of the anatomical substrate of lateral hypothalamic self-stimulation reward and as a first step in a larger mapping study.