Deficits in feeding behavior after intraventricular injection of 6-hydroxydopamine in rats.

Intraventricular injections of 6-hydroxydopamine produced 95 percent depletion of telencephalic norepinephrine and 62 percent depletion of striatal dopamine in rats. Treated rats maintained body weight at subnormal levels and failed to increase food intake in response to a short-term decrease in glucose utilization. After treatment with the monoamine oxidase inhibitor pargyline, 6-hydroxydopamine produced no further norepinephrine depletion but increased the dopamnine depletion to 95 percent and produced complete aphagia. These effects are comparable to events that follow bilateral electrolytic lesions of the lateral hypothalanmus.

Recovery of feeding and drinking by rats after intraventricular 6-hydroxydopamine or lateral hypothalamic lesions.

Rats given intraventricular injections of 6-hydroxydopamine after pretreatment with pargyline become aphagic and adipsic, and show severe loss of brain catecholamines. Like rats with lateral hypothalamic lesions, these animals gradually recover ingestive behaviors, although catecholamine depletions are permanent. Both groups decrease food and water intakes markedly after the administration of alpha-methyltyrosine, at doses that do not affect the ingestive behaviors of control rats. Thus, both the loss and recovery of feeding and drinking behaviors may involve central catecholamine-containing neurons.

Hyperphagia and increased growth in rats after intraventricular injection of 5,7-dihydroxytryptamine.

Juvenile male rats given intracerebroventricular injections of 5,7-dihydroxytryptamine, following treatment with desmethylimipramine, maintained body weight gains of 5 to 6 grams per day into adulthood and grew much larger than control rats. Biochemical analyses of brain tissue obtained 50 to 140 days after the injections revealed 60 to 86% depletions of telencephalic 5-hydroxytryptamine, with catecholamine levels unchanged. Hyperphagia did not develop despite comparable losses of 5-hydroxytryptamine when the pretreatment was withheld, perhaps because substantial depletions of norepinephrine occurred as well.

Compensatory increase in tyrosine hydroxylase activity in rat brain after intraventricular injections of 6-hydroxydopamine.

The neurotoxin 6-hydroxydopamine produced a permanent loss of endogenous norepinephrine and of 3H-labeled norepinephrine uptake sites in the hippocampus within 5 days. These losses were initially accompanied by parallel decreases in tyrosine hydroxylase activity and synaptosomal norepinephrine synthesis. Within 21 days, however, hippocampal tyrosine hydroxylase activity and norepinephrine synthesis rate increased three- to fivefold. These data suggest a novel form of plasticity in brain-damaged animals characterized by an increase in the capacity for transmitter biosynthesis in residual neurons.

The renin-angiotensin system and thirst: a reevaluation.

Systemic injections of renin that stimulate substantial amounts of drinking in nephrectomized rats can produce plasma renin activities that fall well above the physiological range. Furthermore, increases in plasma renin activities that occur in rats with intact kidneys during experimental hypotension appear to be too low to provide the basis for the observed elevations in water intake. These findings question the contribution of the renin-angiotensin system to thirst under normal physiological conditions.

Glucoregulatory feeding by rats after intraventricular 6-hydroxydopamine or lateral hypothalamic lesions.

Rats given intravenricular injections of 6-hydroxydopamine or bilateral electrolytic lesions of the lateral hypothalamus do not show the normal increase in food intake in response to large decreases in glucose utilization or exposure to severe cold stress. However, they will eat more during chronic glucoprivation that is less intense, or during exposure to more moderate cold stress. Thus, the feeding deficits of these lesioned rats may not reflect an inability to respond to certain qualitatively different stimuli, but rather an inability to respond to quantitatively different intensities of the same stimulus.

Water intake in hypovolemic sheep: effects of crushing the left atrial appendage.

Sheep increased their water intake in proportion to the amount of protein-free, isosmotic fluid that was removed from their blood by ultrafiltration. This behavioral response to hypovolemia was eliminated by crushing the left atrial appendage of the heart. The surgical maneuver had no effect on basal water intake or on the drinking response to a salt load. These findings suggest that left atrial stretch receptors, which influence secretion of antidiuretic hormone when stimulated, may also play an important role in mediating thirst during hypovolemia.

Oxytocin secretion in response to cholecystokinin and food: differentiation of nausea from satiety.

Administration of cholecystokinin (CCK) to rats caused a dose-dependent increase in plasma levels of the neurohypophyseal hormone oxytocin (OT). The OT secretion was comparable to that found in response to nausea-producing chemical agents that cause learned taste aversions. The effect of CCK on OT secretion was blunted after gastric vagotomy, as was the inhibition of food intake induced by CCK. Food ingestion also led to elevated plasma OT in rats, but CCK and aversive agents caused even greater OT stimulation. Thus, after administration of large doses of CCK, vagally mediated activation of central nausea pathways seems to be predominantly responsible for the subsequent decrease in food intake. Despite their dissimilar affective states, both nausea and satiety may activate a common hypothalamic oxytocinergic pathway that controls the inhibition of ingestion.

Homeostasis during hypoglycemia: central control of adrenal secretion and peripheral control of feeding.

Intravenous infusions of manose or B-hydroxybutyrate, metabolic fuels which can be oxidized by brain, abolished adrenal discharge of epinephrine in rats during insulin-induced hypoglycemia, whereas infusion of fructose, a sugar which does not cross the blood-brain barrier, did not. In contrast, increased feeding behavior during hypoglycemia was prevented both by the sugars and by B-hydroxybutyrate. Thus, while the sympathetic response during marked hypoglycemia may have been initiated by alterations in cerebal metabolism, the feeding response evidently was not, and a decrease in the utilization of glucose per se does not appear to be the critical stimulus in either case.

Compensations after lesions of central dopaminergic neurons: some clinical and basic implications.

Parkinson's disease is associated with degeneration of the dopaminergic component of the nigrostriatal pathway. However, the neurological symptoms of this disorder do not emerge until the degenerative process is almost complete. A comparable phenomenon can be observed in animal models of Parkinson's disease produced by the administration of the selective neurotoxin, 6-hydroxydopamine (6-OHDA). Studies using such models suggest that the extensive loss of dopaminergic neurons is compensated, in large part, by increased synthesis and release of dopamine (DA) from those DA neurons that remain, together with a reduced rate of DA inactivation. These findings may have important implications for the diagnosis and treatment of a variety of neurological and psychiatric diseases, as well as for our understanding of plasticity in monoaminergic systems.

Oxytocin produces natriuresis in rats at physiological plasma concentrations.

Oxytocin (OT) is known to stimulate natriuresis in rats when administered in large doses that produce high plasma levels. We examined the effects of physiological plasma OT levels on renal sodium excretion by infusing graded doses of OT sc in conscious adult male rats maintained on a sodium-deficient diet. Our results demonstrate that OT causes a dose-related increase in urinary sodium excretion during the initial day of infusion. The lowest plasma OT levels associated with increases in urinary sodium excretion (5-6 pmol/liter) were well within the range of physiological OT secretion in rats. However, this natriuretic effect was not sustained during subsequent days of maintenance on a sodium-deficient diet, suggesting that the OT-induced natriuresis was limited in part by receptor desensitization and/or a decreased exchangeable sodium pool in combination with secretion of opposing antinatriuretic factors such as aldosterone. Pretreatment with an OT receptor antagonist completely blocked the natriuresis produced by a 20 pmol/h infusion of OT, but urinary sodium excretion was not affected by a vasopressin V1 antagonist and was blocked only partially by a combined vasopressin V1 and V2 antagonist. Together with previous studies in rats demonstrating an inverse relation between pituitary OT secretion and sodium appetite, these results support the hypothesis that peripherally and centrally secreted OT act in concert in rats to produce a negative sodium balance by stimulating sodium excretion while inhibiting sodium ingestion.

Sodium excretion and renin secretion after continuous versus pulsatile infusion of oxytocin in rats.

Neurohypophyseal oxytocin (OT), secreted continuously under conditions of hyperosmolality, is a potent natriuretic hormone in rats. In contrast, OT secretion during lactation is pulsatile and is not accompanied by increased urinary Na+ excretion. The present experiments compared the effects of continuous and pulsatile infusion of OT on natriuresis in rats. In male rats anesthetized with Inactin, continuous infusion of OT (125 ng/kg x h) increased plasma OT to about 70 pg/ml; renal Na+ excretion increased 10-fold, and urine volume and K+ excretion also were elevated. However, when OT was administered i.v. in the same amount but in pulses given once every 5 or 10 min, to simulate the pattern of OT secretion during lactation, rats did not excrete significantly more urine, Na+, or K+ than did vehicle-treated animals. The plasma renin concentration, measured in these experiments because OT receptors are present in the macula densa, increased 2-fold when OT was infused either continuously or in pulses. These results indicate that the effects of OT administration on urinary Na+ excretion in rats varies depending on whether the infusion is pulsatile or continuous, whereas the effects of OT on renin secretion show no such difference.

Brain oxytocin receptor antagonism blunts the effects of anorexigenic treatments in rats: evidence for central oxytocin inhibition of food intake.

The inhibition of food intake in rats that results from various anorexigenic treatments is frequently associated with pituitary secretion of oxytocin (OT), but is not caused by circulating OT. We, therefore, evaluated the potential role of brain OT in mediating anorexia induced in rats by systemic administration of cholecystokinin (CCK), hypertonic saline (HS), or lithium chloride (LiCl), treatments that are known to stimulate pituitary OT secretion as well as to inhibit food intake. Food intake was analyzed in 22-h food-deprived rats pretreated with icv injections of either artificial cerebrospinal fluid (aCSF) or 9 nmol of an OT receptor antagonist, [d(CH2)5, Tyr(OMe)2,Orn8]vasotocin (OVT), which was the dose found to be most effective to antagonize the anorexia induced by CCK and HS. Pretreatment with the OT receptor antagonist icv significantly blunted the anorexigenic effect of each agent. After CCK (10 micrograms/kg, ip), food intake increased from 28 +/- 5% of basal intake after a CSF icv to 48 +/- 8% after OVT icv (P less than 0.01); after HS (2 ml 2 M NaCl, ip), food intake increased from 9 +/- 4% of basal intake after aCSF icv to 43 +/- 7% after OVT icv (P less than 0.01); and after LiCl (1.125 mmol/kg, ip), food intake increased from 55 +/- 4% of basal intake after a CSF icv to 80 +/- 9% after OVT icv (P less than 0.01). These data support the hypothesis that pituitary secretion of OT after anorexigenic treatments in rats is associated with coactivation of centrally projecting brain OT pathways, some of which are causally related to the induced inhibition of food intake.

Neurohypophyseal secretion in response to cholecystokinin but not meal-induced gastric distention in humans.

Exogenous administration of cholecystokinin octapeptide (CCK) is known to decrease food intake and slow gastric emptying in humans and animals. Recent studies have shown that CCK stimulates neurohypophyseal secretion of oxytocin (OT) in rats and arginine vasopressin (AVP) in monkeys, and that gastric distention also stimulates OT release in rats. We therefore studied AVP and OT secretion in 14 normal subjects in response to meal-induced gastric distention and administration of CCK, both separately and in combination, to assess whether these stimuli similarly activated central neurohypophyseal pathways in humans. Neither plasma AVP nor OT concentrations increased after gastric distention produced by ingestion of a large meal. However, a dose-related increase in plasma AVP, but not OT levels, occurred after CCK administration, the threshold CCK dose being 0.05 micrograms/kg body weight. The AVP secretion in response to CCK administration was significantly correlated with subjective aversive symptoms quantified by use of a numeric scale (r = 0.61, P less than 0.001). In 12 of the 14 subjects plasma AVP levels increased in association with symptoms of epigastric pressure and discomfort before the onset of overt nausea or emesis. The combination of CCK and meal-induced gastric distention did not stimulate increases in plasma AVP levels in excess of those produced by CCK administration alone. The results demonstrate that AVP secretion resulting from emetic center activation often is a graded response that can begin in association with milder degrees of visceral discomfort before symptoms of overt nausea or emesis. In addition, the stimulation of AVP secretion by CCK administration, but not by meal-induced gastric distention in association with physiological satiety, suggests that some component of the anorectic effects of exogenous CCK in man likely results from activation of brainstem emetic centers.

Distribution and neurochemical phenotypes of caudal medullary neurons activated to express cFos following peripheral administration of cholecystokinin.

Immunocytochemical localization of the protein product of the proto-oncogene c-fos allows anatomical identification of physiologically activated neurons. The present study examined the subnuclear distribution of cFos protein in the rat caudal medulla following peripheral administration of cholecystokinin octapeptide, which reduces feeding and gastric motility by a vagally mediated mechanism. To begin phenotypic characterization of neurons activated to express cFos following cholecystokinin treatment, double-labeling techniques were used to identify vagal motor neurons and neurons immunoreactive for tyrosine hydroxylase, neuropeptide Y, and neurotensin. Activated cells were most prevalent in the subnucleus medialis of the nucleus of the solitary tract, less prevalent in the subnucleus commissuralis, and virtually absent in the subnuclei centralis and gelatinosus. Many activated cells occupied the caudal area postrema; some of these were catecholaminergic. In contrast, activated cells were sparse within the medial rostral area postrema. Other activated cells occupied the dorso- and ventrolateral medulla and the midline raphe nuclei. Retrograde labeling of vagal motor neurons confirmed that very few were activated. Those that were activated occupied the caudal dorsal motor nucleus. In the dorsomedial medulla, 51% of catecholaminergic neurons and 39% of neurons positive for neuropeptide Y were activated, but no neurotensin-positive neurons were activated. In the ventrolateral medulla, 25% of catecholaminergic neurons and 27% of neuropeptide Y-positive neurons were activated. By characterizing the subnuclear distribution and chemical phenotypes of neurons activated by exogenous cholecystokinin, these data contribute to elucidation of the neural circuits mediating the behavioral, physiological, and neuroendocrine effects produced by this peptide.

Cholecystokinin activates catecholaminergic neurons in the caudal medulla that innervate the paraventricular nucleus of the hypothalamus in rats.

Stimulation of gastric vagal afferents by systemic administration of cholecystokinin octapeptide (CCK) inhibits gastric motility, reduces food intake, and stimulates pituitary secretion of oxytocin and adrenocorticotropic hormone in rats. To characterize further the central neural circuits responsible for these effects, the present study used triple-labeling immunocytochemical methods to determine whether or not exogenous CCK activates cFos expression in catecholaminergic neurons in the caudal medulla that project to the paraventricular nucleus of the hypothalamus (PVN). To identify these neurons, the retrograde tracer fluorogold (FG) was iontophoresed into the PVN of anesthetized rats under stereotaxic guidance. After 2 weeks, rats were injected with CCK (100 micrograms/kg, i.p.) and then anesthetized and killed 1 hour later by perfusion fixation. Medullary sections were processed for triple immunocytochemical localization of cFos, retrogradely transported FG, and tyrosine hydroxylase (TH). In rats with FG injections centered in the PVN (n = 10), approximately 70% of the FG-labeled neurons in the caudal nucleus of the solitary tract (NST) and ventrolateral medulla (VLM) expressed cFos. Of these activated PVN-projecting neurons, approximately 78% in the NST and 89% in the VLM were catecholaminergic (TH positive). These results indicate that PVN-projecting catecholaminergic neurons within the caudal medulla are activated by peripheral administration of CCK, further implicating these ascending catecholaminergic pathways in the neuroendocrine, physiological, and behavioral effects produced by gastric vagal stimulation.

Paradoxical kinesia in parkinsonism is not caused by dopamine release. Studies in an animal model.

Rats become akinetic after large dopamine-depleting brain lesions, yet they show an activation-induced restoration of motor function. In this study, rats were given intraventricular injections of the neurotoxin 6-hydroxydopamine to permanently reduce the dopamine content of the corpus striatum by 98%. Although the rats were akinetic in their home cages, they swam effectively when placed in deep water and escaped from a shallow floating ice bath. These behaviors were not abolished by pretreating the animals with the dopamine antagonists haloperidol and SCH-23390. In contrast, haloperidol completely blocked the brain-damaged animals' behavioral responses to amphetamine. These results suggest that the paradoxical kinesia of dopamine-depleted rats is not a consequence of dopamine release from residual dopaminergic fibers.

Neurochemical compensation after nigrostriatal bundle injury in an animal model of preclinical parkinsonism.

Parkinson's disease usually involves a lengthy preclinical period during which few neurological symptoms are observed despite extensive damage to the dopaminergic nigrostriatal bundle. Injury to this projection in the rat also fails to produce major neurological dysfunctions. In our studies, damage to the nigrostriatal bundle of the rat, resulting in the loss of up to 95% of the dopaminergic terminals in striatum, was accompanied by apparent increases in the synthesis and release of dopamine (DA) from those dopaminergic terminals that remained. More specifically, both the activity of the rate-limiting biosynthetic enzyme, tyrosine hydroxylase, and the content of the principal DA metabolite, dihydroxyphenylacetic acid, were increased in striatum relative to DA levels. The increases were exponentially related to DA loss.

Compound complementarities in the study of motivated behavior.

In his classic article, Stellar (1954) proposed that diverse motivated behaviors reflected the activity of excitatory and inhibitory centers in the hypothalamus. His specific and testable ideas provided the theoretical focus for a great deal of fruitful research on the biological bases of behavior for 2 decades. Subsequently, new findings and technical developments again changed the perspective and experimental approaches in behavioral neuroscience. The authors suggest that the modern emphasis on the anatomy and chemical function of neuronal systems has come at the expense of understanding the subcomponents of behavior and the hierarchical levels of integration involved in transforming reflexes into operant acts. Increased attention in the future to the infrastructure of the behaviors being elucidated, when combined with reductionistic studies of neurons, will fulfill the potential contribution to behavioral neuroscience that is implicit in Stellar's article.

Central c-Fos expression in neonatal and adult rats after subcutaneous injection of hypertonic saline.

Centrally-mediated responses to plasma hyperosmolality include compensatory drinking and pituitary secretion of vasopressin and oxytocin in both adult and neonatal rats. However, the anorexia that is produced by plasma hyperosmolality in adult rats is not evident in neonates, perhaps due to functional immaturity of osmoresponsive hindbrain circuits. To examine this possibility, the present study compared treatment-induced brain expression of the immediate-early gene product c-Fos as a marker of neural activation in adult and two-day-old rats after subcutaneous injection of 2 M NaCl (0.1 ml/10 g body weight). This treatment produced marked hypernatremia in adult and two-day-old rats without altering plasma volume. Several brain regions (including components of the lamina terminalis, the paraventricular and supraoptic nuclei of the hypothalamus, and the area postrema) were activated to express c-Fos similarly in adult and two-day-old rats after 2 M NaCl injection, consistent with previous reports implicating a subset of these regions in osmotically-stimulated drinking and neurohypophyseal secretion. In contrast, other areas of the brain that were activated to express c-Fos in adult rats after 2 M NaCl injection were not activated in neonates: these areas included the central nucleus of the amygdala, the parabrachial nucleus and catecholamine cell groups within the caudal medulla. This study demonstrates that certain brain regions that are osmoresponsive in adult rats (as defined by induced c-Fos expression) are not osmoresponsive in two-day-old rats. When considered in the context of known differences between the osmoregulatory capacities of adult and neonatal rats, our results are consistent with the idea that osmoresponsive forebrain centres are primarily involved in osmotically-stimulated compensatory drinking and neurohypophyseal secretion, whereas osmoresponsive regions of the hindbrain are important for concomitant inhibition of feeding and gastric emptying.