Effects of chronic intrahypothalamic infusion of insulin on food intake and diurnal meal patterning in the rat

In Experiment 1, rats were chronically infused with insulin (2.7, 27, or 270 ng/hr) or 0.9% saline into the ventromedial (VMH), medial perifornical (PF), or lateral (LH) hypothalamus. VMH infusions of insulin caused a significant, dose-dependent decrease in food intake and body weight; PF infusion of insulin was less effective, but significant; whereas LH infusions of insulin were ineffective. In Experiment 2, rats were chronically infused with insulin (0.54 ng/hr) or 0.9% saline into the VMH, paraventricular (PVN), or posterior (PN) hypothalamic nucleus. Subjects that received VMH or PN infusions of insulin failed to regain weight lost as a result of surgery even 2 weeks after infusion; subjects that received PVN infusions of insulin regained their preoperative weights faster than did controls. All of the groups that received insulin significantly increased their daytime food intake during the infusion period and decreased their night food intake slightly; 24-hr food intake remained unchanged.     

Role of intrahypothalamic insulin in circadian patterns of food intake, activity, and body temperature.

These experiments examined the extent to which chronic intrahypothalamic (IH) insulin infusions that alter circadian patterns of food intake (FI) affect the regulation of other diurnally varying behavior in the rat. One-week IH insulin infusion (1.5 microU/hr) significantly decreases rats' night FI and increases day FI but does not alter the diurnal pattern of activity. Mean daily core temperature increased slightly but significantly during insulin infusion, the daily peak of the body temperature rhythm did not shift significantly, and the daily range of body temperature increased. IH insulin infusion in rats living in constant light and thus without circadian rhythm of FI led to significant decreases in FI and body weight. These data support the conclusion that IH insulin infusion alters food intake and body weight through a specific effect on a neural system that regulates food intake and body weight, and not by altering circadian rhythms.     

Concomitant food intake and adipose tissue responses under chronic insulin infusion in rats

Body weight (BW), food intake (FI), and activity of white adipose tissue (WAT) and brown adipose tissue (BAT) were studied in adult male rats under chronic insulin infusion. Insulin was infused for 4, 7 or 10 days via implanted minipumps. Insulin-treated rats gained more BW than control rats until 7th day of infusion. At 10 days, the difference in BW decreased. The average cumulative FI was significantly higher after 4, 7 and 10 days of insulin infusion. Feed efficiency (FE) was increased in insulin-treated rats after 4 and 7 days. An increase in WAT weight was observed in insulin-treated rats together with an increased activity of lipogenic enzymes. BAT weight was augmented after 4 days of insulin infusion. This was due mainly to lipid accumulation. Specific mitochondrial guanosine diphosphate (GDP) binding was significantly decreased by 58% in insulin-treated rats after 4 days of infusion. This reduced thermogenic activity, along with the increased FI and FE were responsible for the rapid BW gain observed during the first 7 days of insulin infusion.     

Role of hepatic portal osmoreception in the control of ADH release.

Dogs were prepared for experiments by chronic implantation of catheters into the hepatic portal vein. Intraportal infusions of hypertonic saline, 5 ml/min, increased plasma antidiuretic hormones (rat bioassay) in absence of significant changes in systemic plasma osmolality. In the 5th min of 1.8, 2.7, or 3.6% saline infusion, plasma antidiuretic hormone (ADH) increased significantly from 3.2 +/- 0.7 to 14.3 +/- 3.6 (SE) microU/ml, from 2.3 +/- 0.4 to 42.0 +/- 7.6 microU/ml, and from 1.9 +/- 0.6 to 64.4 +/- 16.8 microU/ml, respectively. In control experiments plasma ADH did not change with isotonic saline infused intraportally or with 3.6% saline given into a systemic vein. The transsection of hepatic vagal afferent fibers abolished plasma ADH increase dependent on hypertonic intraportal infusion. The data support the existence of osmoreceptors within the portal vascular bed and indicate ADH involvement in the control of fluid balance by intrahepatic blood osmalality. The information on osmolality changes is neurally transmitted to the hypothalamus via hepatic vagal afferents. A demonstration of an increase in portal but not systemic blood osmolality following food intake suggests that intrahepatic osmoreception may be important in physiological regulation of water metabolism.     

Inhibition of gold thioglucose lesions by intrahypothalamic saline injections

The ventromedial hypothalamus (VMH) at the level of the median eminence in CFI female mice was infused with saline unilaterally. Following surgical recovery, the mice were challenged with gold thioglucose (GTG) (800 mg/kg) intraperitoneally. The effects of saline on the GTG-induced necrosis in the VMH were assessed by light microscopy. Intrahypothalamic saline infusions yielded partial or complete inhibition of the GTG lesion on the side of the brain receiving the infusion.     

L-vasopressin inhibits oxytocin-induced increases of plasma levels of insulin conscious dogs

Oxytocin is known to increase plasma levels of insulin, glucagon and glucose in dogs. Plasma levels of vasopressin rise during stressful conditions. Since vasopressin counteracts several oxytocin-induced effects, it was decided to study how vasopressin influences the oxytocin-induced elevation of plasma levels of insulin, glucagon and glucose. Therefore oxytocin at 0.11 (which gives rise to physiological plasma concentrations) was infused i.v. for 10 min into fasted, conscious dogs either alone or in combination with 0.033 or 0.17 nmol kg-1 h-1 of L-vasopressin. Accordingly, 1.1 nmol kg-1 h-1 of oxytocin was infused alone or in combination with 0.67 or 1.7 nmol kg-1 h-1 of L-vasopressin. Repeated blood samples were drawn during and after the infusions and insulin and glucagon levels were determined by radioimmunoassay. Plasma levels of insulin increased three- and six-fold in response to 0.11 and 1.1 nmol kg-1 h-1 of oxytocin, respectively, and the elevations were inhibited by L-vasopressin. Slight (1.5-fold) increases in plasma levels of glucagon were observed following 0.11 and 1.1 nmol kg-1 h-1 of oxytocin, although the effect was significant only after the latter dose. Concomitant infusion with L-vasopressin did not markedly influence the effect caused by oxytocin. Small, insignificant increases in blood glucose levels were induced by both doses of oxytocin. These effects were not affected by concomitant infusions of L-vasopressin. The insulin levels rose before glucose levels suggesting that oxytocin stimulates the release of insulin without a previous rise in glucose levels. In conclusion, it has been shown that vasopressin, in amounts which give rise to physiological plasma concentrations, inhibits oxytocin-induced effects on insulin levels, and that oxytocin stimulates the release of insulin via a mechanism which is independent of elevations in blood glucose levels.     

Enhanced responsiveness to insulin in sheep exposed to cold

The effects of cold exposure on tissue sensitivity and responsiveness to insulin were determined by the euglycemic insulin clamp technique. Insulin was infused at rates of 0.2, 0.5, 1.0, 6.0, and 30.0 mU x kg-1 x min-1 into five adult sheep in a warm environment and after cold exposure (0 degree C) from 7 to 23 days. Cold exposure increased basal plasma glucose concentration and basal glucose irreversible loss. Glucose metabolic clearance rate (MCR) was significantly increased by cold exposure at all rates of insulin infusion, with increases ranging from 44 to 72%. The insulin concentration causing half-maximal stimulation of glucose MCR was unchanged by environment (warm, 42 microU/ml; cold, 36 microU/ml). Combined alpha + beta-adrenergic blockade did not affect the increased response to insulin during cold exposure. Endogenous (hepatic) glucose production was inhibited by insulin to a similar extent in the two environments and was less sensitive to insulin than was glucose utilization or MCR. The results suggest that cold exposure increases the responsiveness to insulin of a postreceptor event in peripheral tissues.     

Influence of peripheral and intracerebroventricular glucose and insulin infusions on peripheral and cerebrospinal fluid glucose and insulin levels

There is much evidence that glucose and insulin are related to the regulation of food intake and the maintenance of peripheral glucose homeostasis through actions of the central nervous system. However, evidence concerning the penetration of peripheral glucose and insulin into the cerebrospinal fluid (CSF) and the relationship between both peripheral and CSF glucose and insulin levels is still missing. In the reported experiments it is shown that insulin is present in the CSF (17 +/- 3.6 microU/ml CSF versus 44 +/- 6.0 microU/ml plasma) but that CSF insulin does not follow rises of peripheral insulin to 136 microU/ml plasma during 4.5 hours. On the other hand CSF glucose (55 +/- 3.1 mg/dl CSF) follows rises of peripheral glucose levels with a delay of about 30 min. Increase of CSF glucose by infusing glucose into the third brain ventricle elicits a prolonged decrease in peripheral glucose levels. Infusion into the third ventricle of insulin only does not change peripheral glucose. However, infusion of a combination of insulin and glucose in the third ventricle leads to a gradual increase in peripheral glucose and elicits a disappearance of the decrease in peripheral glucose after glucose only infusion into the CSF. During third ventricle infusion experiments no change in peripheral insulin could be observed. It will be argued that changes in CSF glucose and insulin contribute to maintenance of peripheral glucose homeostasis.     

Metering of intravenously infused nutrients in VMH lesioned rats

Rats were chronically implanted with lesioning electrodes in the ventromedial hypothalamus and with intracardiac catheters. The effects on ad lib oral food intake of a continuous intravenous infusion of glucose were studied both before and after lesioning. Before the lesion oral food intake was reduced by an average of 0.59 kcal per kcal infused. Postlesion the reduction of oral intake was correlated with the caloric value of the infusion, with a mean of 0.92 kcal per kcal. Rats with ventromedial hypothalamic lesions thus adjust their oral intakes in accordance with a systemically infused supplement. Further, the metering of systemically infused substrates actually appears to be more adequate after the lesion. This may be due to an exaggerated insulin release to systemic inputs which have bypassed the orogastric route and are thus unable to elicit preabsorptive endocrine reflexes.     

The effect of portal infusions of epinephrine on ingestion, plasma glucose and insulin in dogs

Preabsorptive satiety has been hypothesized to occur as the result of food activating oral and gastrointestinal receptors that cause the release of catecholamines in the liver. The catecholamines were then proposed to hyperpolarize hepatic glucoreceptors and produce satiety. In the present study the hepatic portal vein was chronically cannulated in six mongrel dogs. Upon recovery the dogs were infused, over three minutes, with either saline or epinephrine (0.83 and 1.5 micrograms/kg b. wt.). Infusions ended 10 minutes prior to the animals' daily one-hour feeding period. The epinephrine infusions resulted in physiological increases in plasma glucose and insulin, but did not inhibit food consumption. The animals were next prefed 20% of their normal daily food intake 30 minutes prior to infusion of epinephrine at the above noted doses. Again plasma glucose and insulin increased, but food consumption was not affected. These data show that epinephrine infusions which produce physiological changes in plasma glucose and insulin do not alter feeding behavior of mongrel dogs. These findings are in agreement with previous data that question the physiological importance of the preabsorptive catecholamine satiety hypothesis.     

Selective effects of beta-endorphin infused into the hypothalamus, preoptic area and bed nucleus of the stria terminalis on the sexual and ingestive behaviour of male rats

beta-Endorphin was infused bilaterally into the medial preoptic area-anterior hypothalamic continuum at doses of 5, 10 and 40 pmol each side. The highest dose selectively abolished mounting, intromitting and ejaculating in sexually experienced male rats paired with an oestrous female. Males infused with 40 pmol beta-endorphin still followed the female, investigated her anogenital region and other parts of her body, but made abortive attempts to mount. A dose of 5 pmol beta-endorphin had no effect, but 10 pmol proved partially effective. The same males, in other tests, were allowed to ingest a highly preferred, sweet, non-calorific solution (acesulfame-K) in the absence of a female. beta-Endorphin infusions (up to 40 pmol) into the same area of the hypothalamus had no effect on this behaviour. Control males allowed simultaneous access both to an oestrous female and to the sweet solution copulated normally but reduced their ingestive behaviour, despite there being sufficient time during tests for both to occur. beta-Endorphin (40 pmol) infused into the preoptic area-anterior hypothalamic continuum under these conditions suppressed sexual interaction, but ingestion of acesulfame-K increased to values observed when the female was absent. beta-Endorphin infused into neighbouring areas of the brain had different behavioural effects. Sexual behaviour was not inhibited, and ingestion of acesulfame-K was unaltered, when beta-endorphin was infused either into the bed nucleus of the stria terminalis or the rostral ventromedial hypothalamus. However, infusions of cholecystokinin-8 into the ventromedial hypothalamus suppressed acesulfame-K ingestion in most animals, showing that the cannulae were placed in an area regulating ingestive behaviour. The inhibition of sexual behaviour after preoptic area-anterior hypothalamic continuum infusions of beta-endorphin was prevented by either pretreating rats with 1 mg/kg naloxone intraperitoneally, or by infusing a putative delta opiate receptor blocker (0.5 pmols ICI 174864) into the preoptic area-anterior hypothalamic continuum 5 min prior to beta-endorphin treatment. ICI 174864 administered alone significantly increased mount rate and reduced the post-ejaculatory refractory period in copulating males. These experiments suggest that there is both neurochemical and neuroanatomical specificity relating beta-endorphin to sexual behaviour in the male rat.     

The influence of central infusions on drinking due to peripheral osmotic stimuli in the pigeon (Columba livia)

The drinking responses of pigeons infused simultaneously IV (0.334 ml/min) and ICV (2 microliters/min) for 15 min with various osmotic solutions were observed during, and for 60 min after, the combined infusions. Drinking in response to IV infusion of 0.5 M NaCl or 1.0 M sucrose was unaffected by simultaneous ICV infusion of 0.15 M NaCl, enhanced by ICV 0.3 M NaCl, inhibited during the infusion of water ICV and attenuated by ICV infusion of 0.9 M sucrose. Drinking in response to IV infusion of 1.0 M NaCl or 1.5 M sucrose, two solutions that would have greatly increased CSF sodium concentration, was only slightly affected by simultaneous ICV infusions of NaCl, sucrose or water. These results show that drinking following IV administration of osmotic stimuli can be affected by ICV infusions that may have further increased or decreased CSF sodium concentration thereby suggesting that CSF sodium concentration may play a "permissive role" in osmotically induced drinking.     

Localization of hypothalamic insulin receptor in neonatal chicks: evidence for insulinergic system control of feeding behavior

Feeding behavior is managed by various neuropeptides and/or neurotransmitters within the central nervous system in vertebrates. It is proposed that central insulin acts as the negative-feedback regulator of appetite via the central melanocortin system in neonatal chicks. The present study investigated the localization of insulin receptors in the chick hypothalamus using immunohistochemistry. Immunostaining revealed hypothalamic neuron expressing insulin receptors in the paraventricular nucleus, ventromedial hypothalamus, lateral hypothalamus and infundibular nucleus, the avian equivalent of the mammalian arcuate nucleus. Additionally, double-staining immunohistochemistry in the infundibular nucleus revealed the presence of insulin receptors in both α-melanocyto stimulating hormone and neuropeptide Y neurons. Immunohistological analysis indicates that the insulinergic system in the chick hypothalamus contributes to feeding behavior and this system regulates both anorexigenic and orexigenic neuropeptides. Furthermore, the mechanisms of central insulin induced-feeding behavior contributes to the regulation of the melanocortin system in the chick infundibular nucleus.     

Suppressive action of prolactin on renal response to volume expansion.

The effects of prolactin on rat renal sodium and water handling during volume expansion were studied using clearance techniques. Both control and experimental adult male Wistar rats were prehydrated with an oral water load of volume equal to 2.5% body weight (BW). At least 3 h later, a continuous intravenous infusion of ovine prolactin (NIH-P-S8), 7.1 mug/h per 100 g, was started in the experimental group. After a 1-h steady-state period, the rats were given an intravenous expansion infusion of either hypotonic saline (2.5% BW), isotonic saline (2.5% and 7.5% BW), or blood (2.5% BW). In all control hypotonic and isotonic saline-expanded animals, within 1 h the rats excreted a volume of urine equal to over 50% of the volume of saline infused. The diuretic and natriuretic responses to saline expansion of prolactin-treated rats were significantly smaller than controls. In contrast to the effects of prolactin on the renal response to saline infusions, it did not alter the natriuretic or diuretic response to blood infusion. Prolactin may be counteracting the effects of physical factors on the regulation of sodium reabsorption in the proximal tubule.     

Satiation due to CCK-8 derivative infusion into VMH is related to a specific macronutrient selection

Diet self-selection from carbohydrate, protein and lipid offered simultaneously was studied in adult male rats infused for 8 days with pyroglutamyl-CCK-8 (pGlu-CCK-8) (0.8 pmol/hr), a derivative of the COOH-terminal octapeptide of cholecystokinin (CCK), or vehicle bilaterally into the ventromedial hypothalamus (VMH). Infusion with pGlu-CCK-8 markedly decreased total daily energy intake mainly due to a suppressed carbohydrate intake. Both pGlu-CCK-8 and vehicle infusions caused a nonspecific decrease in protein intake, but the vehicle infusion caused a compensatory increase in lipid intake as well as carbohydrate intake, so their total energy intake was well maintained. These results suggest that the process of infusion into the VMH elicited itself a suppressive effect on protein intake, but that pGlu-CCK-8 infusion elicited a more specific suppressive effect on carbohydrate intake and, to a lesser extent, on lipid intake. These results also support the prediction that centrally administered CCK may suppress food intake and show in addition that such a suppression occurs selectively. The similarities and the differences in the patterns of macronutrient selection produced by vehicle of pGlu-CCK-8 infusions into the VMH provide further evidence of the unique functions of VMH and CCK in the feeding process.     

Inactivation of the medial preoptic area/anterior hypothalamus by lidocaine reduces male sexual behavior and sexual incentive motivation in male rats

Permanent bilateral lesions of the medial preoptic area anterior hypothalamus (MPOA/AH) produce a drastic inhibition of male sexual behavior in all species studied to date. The present experiment was designed to evaluate if temporal inactivation of the MPOA/AH by infusions of lidocaine also inhibits sexual behavior in male rats. This would allow us to rule out the possibility that the behavioral effects observed after damage of the MPOA/AH could be associated with plastic changes induced by the lesion in other brain regions. We also evaluated sexual incentive motivation in males after the infusion of lidocaine in a test in which copulation is not possible but where males maintain approach behavior to the estrous females despite repeated testing. The percentage of animals displaying mounts, intromissions and ejaculation was significantly reduced while mount and intromission latency were prolonged after infusion of lidocaine. No changes were observed in sexual behavior after infusion of lidocaine in animals with cannulae outside the MPOA/AH suggesting that the inhibitory effects are specific to this brain region. Sexual incentive motivation was also affected by administration of lidocaine. Males consistently showed a clear preference for the sexually receptive female except when infused with lidocaine. After the infusion of the compound a significant reduction in the time spent in the incentive zone of the stimulus female was observed. These results support the hypothesis that neurons of the MPOA/AH are involved in the control of male sexual motivation.     

Temporal aspects of ventromedial hypothalamic progesterone action in the facilitation of estrous behavior in the female rat

Progesterone (P) action following estrogen priming is required normally for the facilitation of estrous behavior in female rats. Although mechanisms by which P exerts its influence on estrous responsiveness have not been elucidated, the primary site of action of P has been shown to be the ventromedial nucleus of the hypothalamus (VMN). The objective of the present series of experiments was to describe the temporal parameters of P action in the VMN in the facilitation of estrous behavior in estrogen-primed female rats. Subjects were female Long-Evans rats stereotaxically outfitted with 23-ga. guide cannulae directed towards the VMN. Crystalline P was applied directly to the brain tissue via bilateral 28-ga. insert cannulae, which could be inserted and removed easily through the guide cannulae. Animals were ovariectomized and estrogen primed with 5% estradiol Silastic capsules. They received a counterbalanced series of two experimental tests: one involving a manipulation with a P-filled implant, and another with a blank implant. In the first experiment, a significant increase in estrous responsiveness occurred only after 2 hr exposure of the VMN to P, whereas 4 hr were required for a full display of estrous behavior, including solicitation. In Experiment 2, P was lowered into the brain for either 1, 2, or 4 hr, and testing took place 4 hr after the lowering of the implant. It was found that 2 hr of P exposure was sufficient to facilitate full estrous responsiveness at 4 hr. In Experiment 3, it was revealed that the duration of estrous responsiveness was directly related to the time the P implant remained in the brain. In the fourth experiment, the time course of P retention in brain tissue, revealed by determination of 3H-progesterone levels in hypothalamus, agreed with the behavioral findings. Progesterone levels in the region of the VMN remained high while a P implant was in place, but declined rapidly after removal. A dual mechanistic hypothesis for P action in the facilitation of estrous behavior is presented.     

Feedback control dynamics for glucose controlled insulin infusion system.

Miles Laboratories has developed a Glucose Controlled Insulin Infusion System (GCIIS) designated by the Trademark (BIOSTATOR) as a tool to investigate the physiologic control parameters of carbohydrate metabolism and regulatory deficiencies in diabetes. It consists, in principle, of a rapid on-line glucose analyzer, a computer/controller for the calculation and control of insulin or dextrose infusion, and a multichannel infusion system. A silent printer records, on a minute-by-minute basis, the glucose value measured, the insulin and/or dextrose infusion rates, and the cumulative total of the insulin infused. The on-line glucose analyzer employs an electrochemical sensor with immobilized glucose oxidase and measures the hydrogen peroxide produced. Its linearity extends beyond 700 mg/dl glucose; it permits a rapid two-point calibration of the sensor and the overall calibration of the on-line analyzer without disconnecting the catheter from the patient. The system's response time, including blood sample transport from the patient, is less than 90 seconds with a blood loss of approximately 50 ml per 24 h. The insulin and dextrose infusions are governed by control algorithms; various mathematical models have been developed and employed toward improved feedback control dynamics. The rapid glucose analyzer eliminates the need for the calculation of a "predicted" glucose value and permits, instead, the use of a derivative function for dynamic control. A multichannel infusion system combines the principles of a peristaltic pump with the advantages of a precision pump performance and individual computer control for each of the pump channels.     

Oxytocin infusions increase plasma levels of insulin and VIP but not of gastrin in conscious dogs

In the present study, a radioimmunoassay for oxytocin determinations is presented. In addition, we investigated whether the elevation of insulin, VIP and gastrin levels demonstrated to occur in response to suckling in lactating dogs may be induced by released oxytocin. Therefore, oxytocin was infused i.v. into conscious dogs in amounts calculated to give rise to plasma levels observed during physiological circumstances. Plasma levels of oxytocin, insulin, VIP (vasoactive intestinal polypeptide) and gastrin were measured by radioimmunoassay. When oxytocin was infused at a rate of 0.22 and 2.2 nmol kg-1 h-1, plasma oxytocin levels rose to 176 +/- 25 fmol ml-1 and to 1490 +/- 400 fmol ml-1, respectively, 10 min after the infusions were started. Plasma insulin levels rose in response to oxytocin administered at a rate of 0.22 and 2.2 nmol kg-1 h-1. A peak was recorded within 5 min of oxytocin infusion, that is, before maximal oxytocin levels were recorded, and basal levels were reached within about 20 min. The VIP levels rose slightly following infusion of oxytocin at 0.22 nmol kg-1 h-1, but a clear-cut response that lasted for 60 min was observed following infusion of oxytocin at the highest dose. In contrast, gastrin levels were not influenced by the oxytocin infusions. Suckling in dogs is followed by rapidly occurring short-lasting elevations of oxytocin levels in plasma which amount to 50-100 fmol ml-1. Since insulin and VIP were released by oxytocin when administered in amounts that give rise to plasma levels close to those levels, it is suggested that the secretion of insulin and VIP that occurs in response to suckling in lactating dogs may in part be caused by previously released oxytocin.(ABSTRACT TRUNCATED AT 250 WORDS)