In vitro autoradiographic localization of amylin binding sites in rat brain

Inverse relation between amylin and glucagon secretion in healthy and diabetic human subjects

BACKGROUND

The role of amylin, which is cosecreted together with insulin by the pancreatic B-cells, in the pathogenesis of type-2 diabetes is still unclear. To elucidate a possible relation between amylin and glucagon we directly evaluated the respective prehepatic secretions following administration of a 75-g oral glucose load (OGL) in humans.

MATERIALS AND METHODS

We studied six healthy controls (C), six obese, insulin resistant subjects (O) and six patients with type 2 diabetes (D). Catheters were placed in the femoral artery and hepatic vein according to the hepatic vein catheterization technique. Splanchnic blood flow was assessed by infusion of indocyanine-green dye. The measured variables were analyzed by a general circulatory model for calculation of prehepatic secretion.

RESULTS

The total amount of released glucagon was not different between the respective groups (20.5 +/- 2.3 in C, 27.7 +/- 5.1 in O and 27.9 +/- 5.4 micro g/4 h in D). When considered as the difference from the fasting profile, however, glucagon secretion was reduced by 3.5 +/- 14% in C, 25 +/- 12% in O and increased by 36 +/- 21% in D (P = 0.051, D vs. C). Amylin secretion was increased in O (1.10 +/- 0.15) vs. C (0.63 +/- 0.05, P < 0.05) and D (0.24 +/- 0.10 nmol, P < 0.01). Following glucose administration, glucagon secretion significantly inversely correlated with secretion of amylin (r = -0.6, P < 0.01), but not with that of insulin (r =-0.23, P = 0.36).

CONCLUSIONS

The inverse correlation between amylin and glucagon secretion suggests that amylin modulates glucagon secretion following oral glucose administration. This study proves for the first time a role of endogenous amylin in the regulation of glucose homeostasis.

Neuropeptides, food intake and body weight regulation: a hypothalamic focus

Energy homeostasis is controlled by a complex neuroendocrine system consisting of peripheral signals like leptin and central signals, in particular, neuropeptides. Several neuropeptides with anorexigenic (POMC, CART, and CRH) as well as orexigenic (NPY, AgRP, and MCH) actions are involved in this complex (partly redundant) controlling system. Starvation as well as overfeeding lead to changes in expression levels of these neuropeptides, which act downstream of leptin, resulting in a physiological response. In this review the role of several anorexigenic and orexigenic (hypothalamic) neuropeptides on food intake and body weight regulation is summarized.

Amylin and glucose co-activate area postrema neurons of the rat

Glucose is an important metabolic factor controlling feeding behavior. There is evidence that physiologically relevant glucose sensors reside in the caudal hindbrain. The area postrema (AP) in particular, which has been characterized as a receptive site for the anorectic hormone amylin, may monitor blood glucose levels. To determine whether glucose and amylin co-activate the same subset of AP neurons, we performed extracellular single unit recordings from a rat AP slice preparation. In 53% of all AP neurons tested (n=32), the activity was positively correlated to the glucose concentration. Interestingly, there was a coincidental sensitivity (94%) of AP neurons to glucose and amylin, which exerted excitatory effects on these cells. We conclude that the co-sensitivity of AP neurons to glucose and amylin, both increasing in response to food intake, points to the AP as an important hindbrain center for the integration of the metabolic and hormonal control of nutrient intake.

Effects of amylin-related peptides on food intake, meal patterns, and gastric emptying in rats

We previously demonstrated that amylin inhibits food intake and gastric emptying in rats with half-maximal effective doses (ED(50)s) of 8 and 3 pmol x kg(-1) x min(-1) and maximal inhibitions of 78 and 60%, respectively. In this study of identical design, rats received intravenous infusions of salmon calcitonin (sCT), rat calcitonin (rCT), rat calcitonin gene-related peptide (rCGRP), and rat adrenomedullin (rADM) for 3 h at dark onset, and food intake was measured for 17 h or for 15 min and gastric emptying of saline was measured during the final 5 min. sCT, rCGRP, and rADM inhibited food intake with estimated ED(50)s of 0.5, 26, and 35 pmol x kg(-1) x min(-1) and maximal inhibitions of 88, 90, and 49%, respectively. rCT was not effective at doses up to 100 pmol x kg(-1) x min(-1). sCT, rCGRP, rADM, and rCT inhibited gastric emptying with ED(50)s of 1, 130, 160, and 730 pmol x kg(-1) x min(-1) and maximal inhibitions of 60, 66, 60, and 33%, respectively. These results suggest that amylin and sCT may act by a common mechanism to decrease food intake, which includes inhibition of gastric emptying.

Effects of pramlintide, an amylin analogue, on gastric emptying in type 1 and 2 diabetes mellitus

Pramlintide delays gastric emptying, possibly by a centrally mediated mechanism. Our aim was to determine whether the effects of pramlintide on gastric emptying differ in people with type 1 or type 2 diabetes who had no history of complications. Using a randomized, three-period, two-dose, crossover design, we studied the effects of 0, 30, or 60 microg t.i.d. pramlintide subcutaneously for 5 days each in six type 1 and six type 2 diabetic subjects. Gastric emptying of solids was measured by 13C-Spirulina breath test. Plasma pancreatic polypeptide (HPP) response to the test meal was also measured. Relative to placebo [t 50% 91 +/- 6 min (means +/- SEM)], pramlintide equally delayed gastric emptying following 30 or 60 microg t.i.d. (268 +/- 37 min, 329 +/- 49 min, respectively; P < 0.01]. Postprandial HPP levels were lower in response to 30 and 60 microg pramlintide compared to placebo. There were no significant differences in the effects on gastric emptying or HPP levels between type 1 and type 2 diabetic subjects. Pramlintide delays gastric emptying in diabetes unassociated with clinically detected complications. Further studies are needed in diabetic patients with impaired gastric motor function.

Unresolved challenges with insulin therapy in type 1 and type 2 diabetes: potential benefit of replacing amylin, a second beta-cell hormone

Current insulin therapy still fails to safely restore near-normoglycemia in the majority of patients. Among the barriers to achieving tight long-term glycemic control with insulin in both type 1 and type 2 diabetes are an increased risk of hypoglycemia, undesired weight gain, and a failure to normalize postprandial hyperglycemia and excessive unpredictable diurnal glucose fluctuations. Amylin is a second beta-cell hormone that is cosecreted with insulin in response to meals, and is deficient in patients with type 1 and insulin-requiring type 2 diabetes. Preclinical studies indicate that amylin acts as a neuroendocrine hormone that complements the effects of insulin in postprandial glucose regulation by suppressing postprandial glucagon secretion and slowing the rate of nutrient delivery from the stomach to the small intestine. Human amylin is not optimal for replacement therapy because of its propensity to aggregate; thus, pramlintide, a soluble, nonaggregating synthetic peptide analog of human amylin, was developed that has potency at least equal to that of human amylin. In clinical studies, subcutaneous injections of pramlintide prior to meals, in addition to insulin therapy, significantly reduced postprandial glucose excursions and lowered HbA(1c) levels in patients with type 1 and type 2 diabetes. The improvement in long-term glycemic control was associated with a significant reduction in body weight and occurred without increases in total daily insulin use or in overall severe hypoglycemia event rates. Because of this unique spectrum of clinical effects, amylin replacement with pramlintide as an adjunctive therapy to insulin is a promising approach that may fulfill some of the unmet clinical needs of insulin-using patients with type 1 and type 2 diabetes.

Adjunctive therapy with the amylin analogue pramlintide leads to a combined improvement in glycemic and weight control in insulin-treated subjects with type 2 diabetes

The objective of this study was to assess the effect of mealtime amylin replacement with pramlintide on long-term glycemic and weight control in subjects with type 2 diabetes. This 52-week, randomized, placebo-controlled, multicenter, double-blind, dose-ranging study in 538 insulin-treated subjects with type 2 diabetes compared the efficacy and safety of 30-, 75-, or 150-microg doses of pramlintide, a synthetic analogue of the beta-cell hormone amylin, to placebo when injected subcutaneously three times daily (TID) with major meals. Pramlintide therapy led to a mean reduction in HbA1c of 0.9% and 1.0% from baseline to week 13 in the 75- and 150-microg dose groups, which was significant compared to placebo (p = 0.0004 and p = 0.0002, respectively). In the 150-microg dose group, there was a mean reduction in HbA1c of 0.6% from baseline to week 52 (p = 0.0068 compared to placebo). The greater reduction in HbA1c with pramlintide was achieved without increases in insulin use or severe hypoglycemia, and was accompanied by a significant (p < 0.05) reduction in body weight in all dose groups compared to placebo. Three times the proportion of subjects in the 150-microg pramlintide group compared to the placebo group achieved a concomitant reduction in both HbA1c and body weight from baseline to week 52 (48% versus 16%). The most common adverse event reported with pramlintide treatment was nausea, which was mild to moderate and dissipated early in treatment. The results from this study support the safety and efficacy of pramlintide administered three times a day with major meals, in conjunction with insulin therapy, for improving long-term glycemic and weight control in subjects with type 2 diabetes.

Amylin potently activates AP neurons possibly via formation of the excitatory second messenger cGMP

Amylin is secreted with insulin from the pancreas during and after food intake. One of the most potent actions of amylin in vivo is its anorectic effect, which is directly mediated by the area postrema (AP), a circumventricular organ lacking a functional blood-brain barrier. As we recently demonstrated, amylin also stimulates water intake most likely via its excitatory action on subfornical organ (SFO) neurons. Neurons investigated under equal conditions in an in vitro slice preparation of the rat AP were 15-fold more sensitive to amylin than SFO neurons. Amylin (10(-11)-10(-8) M) excited 48% of 94 AP neurons tested; the remaining cells were insensitive. The average threshold concentration of the excitatory response was 10(-10) M and, thus, close to physiological plasma concentrations. Coapplication of the amylin receptor antagonist AC-187 reduced amylin's excitatory effect. Amylin-mediated activation of AP neurons and antagonistic action of AC-187 were confirmed in vivo by c-fos studies. Peripherally applied amylin stimulated cGMP formation in AP and SFO neurons, as shown in immunohistochemical studies. This response was independent of nitric oxide (NO) formation in the AP, while coapplication of the NO synthase inhibitors N-monomethyl-L-arginine (100 mg/kg) and nitro-L-arginine methyl ester (50 mg/kg) blocked cGMP formation in the SFO. In contrast to the SFO, where NO-dependent cGMP formation seems to represent a general inhibitory transduction pathway, cGMP acts as an excitatory second messenger in the AP, since the membrane-permeable analog 8-bromo-cGMP stimulated 65% of all neurons tested (n = 17), including seven of nine amylin-sensitive neurons (77%). The results indicate that the anorectic effect of circulating amylin is based on its excitatory action on AP neurons, with cGMP acting as a second messenger.

Amylin infusion into rat nucleus accumbens potently depresses motor activity and ingestive behavior

Amylin, a calcitonin gene-related peptide-like peptide coreleased with insulin, exerts anorexic effects on central administration. Because previous studies revealed dense amylin binding in the nucleus accumbens (Acb), we investigated the behavioral effects of amylin infusions (10, 30, and 100 ng/side) into Acb subregions. Intra-Acb shell amylin infusions decreased ambulation, rearing, feeding, and drinking in either food-deprived rats or water-deprived rats; motor activity was affected more potently than ingestive behavior. Moreover, intra-Acb shell amylin reduced motor activity in nondeprived rats tested in the absence of food or water, indicating that the expression of amylin's effects is independent of drive or proximal incentives. Intra-Acb core amylin infusions in water-deprived rats also decreased ambulation and water intake, although anterior Acb placements were associated with smaller motor effects, regardless of Acb subregion. In contrast to amylin's effects, intra-Acb shell infusions of orexin-A (50, 100, and 500 ng/side) had no effects on motor activity, feeding, or drinking. Hence the Acb may be a target for behavioral regulation by satiety-related peptides like amylin.

Amylin receptor agonists: a novel pharmacological approach in the management of insulin-treated diabetes mellitus

Amylin is a peptide hormone which is co-secreted with insulin from the pancreatic beta-cell. Type 1 diabetic individuals and some Type 2 diabetic individuals are characterised by amylin deficiency. Animal experiments have revealed several actions of amylin on intermediary metabolism, of these some have been demonstrated to be of potential physiological relevance in humans. In particular amylin appears to have important actions in controlling prandial glucose homeostasis. The peptide hormone inhibits postprandial glucagon secretion and delays gastric emptying thereby modifying postprandial hyperglycaemia in diabetic individuals which presumably adds to overall glycaemic control without a concomitant increase in the number of severe hypoglycaemic episodes. Moreover, amylin acts as a satiety agent. Amylin replacement may therefore improve glycaemic control in diabetes mellitus. However, human amylin exhibits physicochemical properties predisposing the peptide hormone to aggregate and form amyloid fibres, which makes it unsuitable for pharmacological use. A stable analogue, pramlintide, with actions and pharmacokinetic and pharmacodynamic properties similar to the native peptide has therefore been developed. The efficacy and safety of pramlintide administration to diabetic individuals have been tested in a large number of clinical trials. It is the aim of this review to describe possible (patho)physiological actions of amylin as demonstrated in animal and human models, to discuss the background for potential amylin (analogue) replacement in diabetes mellitus and to review results from clinical trials with the amylin receptor analogue pramlintide.

Cloning, characterization and central nervous system distribution of receptor activity modifying proteins in the rat

Calcitonin gene-related peptide (CGRP), adrenomedullin (ADM), amylin and calcitonin (CT) are structurally and functionally related neuropeptides. It has recently been shown that the molecular pharmacology of CGRP and ADM is determined by coexpression of one of three receptor activity-modifying proteins (RAMPs) with calcitonin receptor-like receptor (CRLR). Furthermore, RAMP proteins have also been shown to govern the pharmacology of the calcitonin receptor, which in association with RAMP1 or RAMP3, binds amylin with high affinity. In this study, we have cloned the rat RAMP family and characterized the pharmacology of rat CGRP and ADM receptors. Rat RAMP1, RAMP2 and RAMP3 shared 72%, 69% and 85% homology with their respective human homologues. As expected CRLR-RAMP1 coexpression conferred sensitivity to CGRP, whilst association of RAMP2 or RAMP3 with CRLR conferred high affinity ADM binding. Using specific oligonucleotides we have determined the expression of RAMP1, RAMP2 and RAMP3 mRNAs in the rat central nervous system by in situ hybridization. The localization of RAMP mRNAs was heterogeneous. RAMP1 mRNA was predominantly expressed in cortex, caudate putamen and olfactory tubercles; RAMP2 mRNA was most abundant in hypothalamus; and RAMP3 was restrictively expressed in thalamic nuclei. Interestingly, in specific brain areas only a single RAMP mRNA was often detected, suggesting mutual exclusivity in expression. These data allow predictions to be made of where each RAMP protein may heterodimerize with its partner G-protein-coupled receptor(s) at the cellular level and consequently advance current understanding of cellular sites of action of CGRP, ADM, amylin and CT. Furthermore, these localization data suggest that the RAMP family may associate and modify the behaviour of other, as yet unidentified neurotransmitter receptors.

Dopamine D2 receptors mediate amylin's acute satiety effect

The anorectic effect of the pancreatic peptide amylin has been established in numerous studies. Here, we investigated the influence of a pretreatment with dopamine (DA) D1- and D2-receptor antagonists on the anorectic effect of intraperitoneally injected amylin in rats fed a medium-fat (18% fat) diet. In 24-h food-deprived rats, pretreatment with the DA D2-receptor antagonist raclopride [100 μg/kg (0.2 μmol/kg) ip] significantly attenuated amylin's (5 μg/kg ip) anorectic effect, whereas raclopride alone had no effect on food intake [i.e., food intakes 1 h after injection were ( n = 12): NaCl/NaCl 7.3 ± 0.5 g; NaCl/amylin 3.9 ± 0.6; raclopride/NaCl 7.7 ± 0.7; raclopride/amylin 5.6 ± 0.7]. Pretreatment with another DA D2 receptor antagonist, sulpiride [50 mg/kg (154 μmol/kg) ip], similarly reduced amylin's satiety effect, whereas pretreatment with the DA D1-receptor antagonist SCH-23390 [10 μg/kg (0.03 μmol/kg) ip] did not influence amylin's effect. SCH-23390, however, completely blocked the anorexia induced by d-amphetamine (0.3 mg/kg ip). These results suggest that, under the present feeding conditions, the dopaminergic system mediates part of amylin's inhibitory effect on feeding in rats when administered intraperitoneally. This seems to involve DA D2 receptors but not D1 receptors.

Comparative effects of amylin and cholecystokinin on food intake and gastric emptying in rats

CCK is a physiological inhibitor of gastric emptying and food intake. The pancreatic peptide amylin exerts similar actions, yet its physiological importance is uncertain. Objectives were to compare the dose-dependent effects of intravenous infusion of amylin and CCK-8 on gastric emptying and food intake in rats, and to assess whether physiological doses of amylin are effective. Amylin and CCK-8 inhibited gastric emptying with mean effective doses (ED(50)s) of 3 and 35 pmol x kg(-1) x min(-1) and maximal inhibitions of 60 and 65%, respectively. Amylin and CCK-8 inhibited food intake with ED(50)s of 8 and 14 pmol x kg(-1) x min(-1) and maximal inhibitions of 78 and 69%, respectively. The minimal effective amylin dose for each effect was 1 pmol x kg(-1) x min(-1). Our previous work suggests that this dose increases plasma amylin by an amount comparable to that produced by a meal. These results support the hypothesis that amylin acts as a hormonal signal to the brain to inhibit gastric emptying and food intake and that amylin produces satiety in part through inhibition of gastric emptying.

Does neuropeptide Y contribute to the anorectic action of amylin?

Neuropeptide Y (NPY) is a potent feeding stimulant acting at the level of the hypothalamus. Amylin, a peptide co-released with insulin from pancreatic beta cells, inhibits feeding following peripheral or central administration. However, the mechanism by which amylin exerts its anorectic effect is controversial. This study investigated the acute effect of amylin on food intake induced by NPY, and the effect of chronic amylin administration on food intake and body weight in male Sprague Dawley rats previously implanted with intracerebroventricular (icv) cannulae. Rats received 1 nmol NPY, followed by amylin (0.05, 0.1, 0.5 nmol) or 2 microl saline. Increasing doses of amylin resulted in a dose-dependent inhibition of NPY-induced feeding by 31%, 74% and 99%, respectively (P < 0.05). To determine the chronic effects of i.c.v. amylin administration on feeding, rats received 0.5 nmol amylin or saline daily, 30 min before dark phase, over 6 days. Amylin significantly reduced food intake at 1, 4, 16 and 24 hours; after 6 days, amylin-treated rats showed a significant reduction in body weight, having lost 17.3 +/- 6.1 g, while control animals gained 7.7 +/- 5.1 g (P < 0.05). Brain NPY concentrations were not elevated, despite the reduced food intake, suggesting amylin may regulate NPY production or release. Thus, amylin potently inhibits NPY-induced feeding and attenuates normal 24 hour food intake, leading to weight loss.

Effects of long-term infusion of anorexic concentrations of islet amyloid polypeptide on neurotransmitters and neuropeptides in rat brain

Islet amyloid polypeptide (IAPP or amylin) potently reduces food intake in rats at or near physiological concentrations. Although the mechanisms of action of IAPP are not understood, the brain is a suggested site. Changes in hypothalamic and striatal neurotransmission have been reported following acute systemic administration of a pharmacological concentration of IAPP. In the current study, we evaluated the effects of chronic administration of low doses of IAPP on satiety-related neurotransmitters and neuropeptides in the hypothalamus, hippocampus, striatum, left cortex, and right cortex of the rat. Doses of 0, 5 and 25 pmol IAPP/kg-min were administered subcutaneously for 2 or 5 days. Food intake was reduced by 27 and 44% (both P<0.001) for the 5 and 25 pmol/kg-min groups, respectively, in the 2-day experiment and was decreased by 14% (P<0.01) and 24% (P<0.001), respectively, in the 5-day experiment. Body weight was significantly decreased in a dose-dependent fashion. In the 2-day experiment, norepinephrine increased in the hypothalamus in the 5 pmol IAPP/kg-min group, and neurotensin increased in the hippocampus in the 25 pmol/kg-min rats (both P<0.05). In the 5-day, 5 pmol/kg-min rats, 5-hydroxyindoleacetic acid (5-HIAA) increased in the hypothalmus and cholecystokinin (CCK) increased in the striatum (both P<0.05). In the 5-day, 25 pmol/kg-min group, neuropeptide Y (NPY) increased in the hypothalamus (P<0.01) and CCK increased in the hypothalmus and striatum (both P<0.05). The present study confirms that IAPP is a potent anorectic peptide at low doses and suggests that IAPP not only affects classical neurotransmitters in the brain but also alters concentrations of neuropeptides known to be involved in food intake.

Calcitonin and calcitonin receptors: bone and beyond

Calcitonin (CT), a 32 amino acid peptide hormone produced primarily by the thyroid, and its receptor (CTR) are well known for their ability to regulate osteoclast mediated bone resorption and enhance Ca2+ excretion by the kidney. However, recent studies now suggest that CT and CTRs may play an important role in a variety of processes as wide ranging as embryonic/foetal development and sperm function/physiology. In this review article, CT and CTR gene transcription, signal transduction and function are addressed. The effects of CT on the physiology of a variety of organ systems are discussed and the relationship between polymorphisms in the CTR gene and bone mineral density (BMD)/osteoporosis is examined. Recent studies demonstrating the ability of receptor activity modifying proteins (RAMPs) to post-translationally modify the calcitonin receptor-like receptor (CRLR) are detailed and studies employing transgenic mouse technology to determine the temporal and tissue specific transcriptional activity of the CTR gene in vivo are discussed.

The molecular pharmacology of CGRP and related peptide receptor subtypes

Calcitonin gene-related peptides (alpha and beta isoforms), better known as CGRPalpha and CGRPbeta, were isolated twenty years ago. In fact, these were the first peptides to be characterized using a molecular cloning strategy, which is not the traditional approach of biochemical extraction and purification. Paradoxically, progress in the characterization of CGRP receptor subtypes has been extremely slow as a result of difficulties in their cloning and the lack of selective receptor subtype agonists and antagonists. However, exciting progress has been made overthe pasttwo years and is briefly reviewed here.

Adiposity signals and the control of energy homeostasis

Recent technologic innovations have enabled probing the workings of individual cells and even molecules. As a result, our knowledge of the biological controls over eating and the regulation of body adiposity is increasing at a rapid pace. We review the evidence that food intake is controlled by separate but interacting groups of molecular signals. One group, termed satiety signals, are proportional to what is being consumed and help to determine meal size. Cholecystokinin is the best known of these, and its premeal administration causes a dose-dependent reduction of meal size. In and of itself, however, cholecystokinin (and other satiety signals) has little impact on body-fat stores. The second group, termed adiposity signals, circulate in proportion to body adiposity and enter the brain, where they interact with satiety signals in the brainstem and hypothalamus. Insulin and leptin are the best known of these adiposity signals, and the administration of either into the brain causes a dose-dependent reduction of both food intake and body weight. Within the brain, parallel but opposing pathways originating in the hypothalamic arcuate nuclei integrate adiposity signals with satiety signals. Those with a net anabolic effect increase food intake and reduce energy expenditure and are represented (among many such signals) by neuropeptide Y; those with a net catabolic effect decrease food intake and energy expenditure and are represented by brain melanocortins. This complex regulatory mechanism allows individuals to adapt their feeding schedule to idiosyncratic environmental constraints, eating whenever it is desirable or possible. Body-weight regulation occurs as adiposity signals alter the efficacy of meal-generated satiety signals.

Interaction of the renal amylin and renin-angiotensin systems in animal models of diabetes and hypertension

The range of known actions of amylin are reviewed together with the proposal that an important role for amylin may be the hormonal integration of diverse physiological systems activated with feeding. Major targets for the action of amylin are found within the kidney. Components of the amylin system (AS) have been shown to influence the activity of components of the renin-angiotensin system (RAS), and vice versa, in normal, hypertensive and diabetic models. For instance, amylin injected into humans and rats elicits a rapid rise in plasma renin activity. Furthermore, in two models of hypertension (the spontaneously hypertensive rat (SHR) and the model with subtotal nephrectomy (STNx)), the density of amylin-binding sites in the renal cortex associated with the proximal tubules, was associated with elevation of blood pressure. In normotensive controls and in the STNx model, but not in the SHR model, treatment with angiotensin-converting enzyme (ACE) inhibitors reduced blood pressure and the density of amylin binding in the renal cortex. In Sprague-Dawley rats, angiotensin II (Ang II) infusion was associated with increased density of amylin-binding sites as well as elevated blood pressure. Thus, there appears to be a direct relationship between the activity of Ang II and the binding sites for amylin in the renal cortex. From these studies it has been postulated that the activation of the AS in the kidney may play a role in the genesis and/or development of hypertension in certain contexts. The transient expression of amylin mRNA has been detected perinatally, using in situ hybridization, in the subnephrogenic zone of the metanephros and is associated with proximal tubules of the developing nephron. These cells situated close to the glomeruli, represent a subset of brush border epithelial cells. Amylin immunoreactivity (IR) is also found in these cells and colocalizes with angiotensinogen IR. Thus a second important role for amylin is described in which it plays a role as a growth factor in the developing kidney and in renal regrowth in the adult kidney. In a model of IDDM (streptozotocin diabetes), amylin and angiotensinogen IR are both restricted to a subset of brush border epithelial cells close to glomeruli which, in the developing kidney, expressed amylin mRNA. Thus in this IDDM model, we hypothesize that amylin mRNA transcription which is normally downregulated in the adult, is upregulated in this subset of these brush border epithelial cells, and that it stimulates the activity of a local RAS by an intracellular mechanism, leading to the biosynthesis of Ang II. It remains to be determined that if amylin is playing a role in stimulating local Ang II production at these sites, this provides a mechanism for activation of TGF-beta, ultimately leading to interstitial fibrosis.

Pramlintide, an amylin analog, selectively delays gastric emptying: potential role of vagal inhibition

The amylin analog pramlintide delays gastric emptying in type I diabetics. The effects of multiple doses of pramlintide and the mechanism of action in non-amylin-deficient humans are unknown. We investigated the effects of pramlintide on gastrointestinal and colonic transit and on the plasma pancreatic polypeptide response to the meal in a parallel-group dose-response study with subjects randomized to placebo, or 30 or 60 microg (tid, sc) of pramlintide. Pramlintide delayed gastric emptying [half-time (t(1/2)): 112 min (SE 8.7 min), 169 min (SE 12 min), or 177 min (SE 25 min) after placebo or 30- or 60-microg pramlintide treatment, respectively; P = 0.033]. Pramlintide did not significantly affect small bowel or colonic transit. Pancreatic polypeptide concentrations in the first postprandial hour were lower with pramlintide than with placebo (P<0.01 for drug effect). An inverse correlation was observed between mean pancreatic polypeptide concentrations in the first postprandial hour and gastric emptying t(1/2) [Spearman correlation coefficient (R(s)) = 0.48; P = 0.044]. Pramlintide at 30 and 60 microg delays gastric emptying in healthy humans without affecting small bowel or colonic transit. Vagal inhibition is a potential mechanism of the effects of pramlintide on gastric emptying.

Amylin receptors mediate the anorectic action of salmon calcitonin (sCT)

The teleost salmon calcitonin (sCT), but not mammalian CT, shows similar biologic actions in the skeletal muscle as amylin and calcitonin gene-related peptide (CGRP). The peptides have also been shown to reduce food intake in rams. Because sCT, but not amylin, binds irreversibly to amylin binding sites, the aim of the present study was to compare the anorectic potency of both peptides. To determine whether sCT reduces food intake through interaction with amylin binding sites, we also tested whether appropriate antagonists (CORP 8-37, AC 187) attenuate the anorectic effect of sCT. Finally, we wanted to know whether rat calcitonin (rCT) and sCT reduce food intake to the same extent. Peptides were injected intraperitoneally at dark onset in 24 h food-deprived rats. At doses of 5 or 0.5 microg/kg, the anorectic effect of sCT was more potent and lasted much longer (e.g. 5 microg/kg: sCT > 10 h; amylin approx. 2 h) than that of amylin. Both CORP 8-37 and AC 187 (10 microg/kg) markedly reduced the anorectic action of sCT (0.5 microg/kg). In contrast to sCT, rCT (0.5 microg/kg) had no effect on food intake. It is concluded that sCT s anorectic effect is partly mediated by amylin receptors. Irreversible binding of sCT to amylin receptors may lead to a stronger and prolonged effect in comparison to amylin due to a sustained activation of the binding sites. Similar to other actions of CTs, the anorectic potency of sCT in rats was higher than that of mammalian (rat) CT. This agrees with binding profiles of amylin, sCT, and rCT at amylin binding sites as observed in in vitro studies.

An integrative neuroanatomical perspective on some subcortical substrates of adaptive responding with emphasis on the nucleus accumbens

Neuroanatomical substrates associated in the literature with adaptive responding are discussed, with a focus on the nucleus accumbens. While it is emphasized that the accumbens exhibits multiple levels of complex organization, a fairly complete list of brief descriptions of recent studies devoted specifically to the accumbens shell and core subterritories is presented in tabular format. The distinct patterns of connectivity of the accumbens core and shell and structures related to them by connections are described. Multiple inputs, outputs and abundant reciprocity of connections within the ventral parts of the basal ganglia are emphasized and the implications for "through-put" of impulses is considered. It is noted, at least on neuroanatomical grounds, that there is ample reason to expect feed forward processing from shell and structures with which it is associated to core and structures with which it is associated. Furthermore, the potential for additional feed forward processing involving several forebrain functional anatomical systems, inlcuding the ventral striatopallidum, extended amygdala and magnocellular basal forebrain complex is considered. It is intended that from the considerations recorded here a conceptual framework will begin to emerge that is amenable to further experimental substantiation as regards how multiple basal forebrain systems and the cortices to which they are related by connections work together to fashion a unitary object--the adaptive response.

Pharmacological characterisation of amylin-related peptides activating subfornical organ neurones

Amylin, calcitonin gene-related peptide (CGRP) and calcitonin are structurally related peptides with overlapping peripheral and central actions. Amylin and calcitonin excite the majority of neurones in the subfornical organ (SFO), where high densities of so-called C-type G-protein-coupled receptors have been detected. Subcutaneous injection of these hormones stimulates drinking similar to angiotensin II (ANGII), a dipsogen acting via the SFO. We now show that in addition to amylin and rat calcitonin (rCT), CGRP and salmon calcitonin (sCT) also excite SFO neurones. In extracellular recordings of an in vitro slice preparation of the SFO, 78% of all neurones (n=31) superfused with CGRP (10(-6) M) were excited. The excitatory effect was dose-dependent and reversible with an average threshold concentration of 5x10(-7) M, which is approximately 15-fold higher than reported for amylin-induced excitations. sCT (10(-7) M), which behaves as a non-competitive agonist at amylin as well as calcitonin receptors, caused irreversible excitatory responses in 96% of all recordings (n=26). Amylin-, CRGP- and rCT-induced excitations could be blocked by the selective amylin receptor antagonist AC187 (10(-5) to 10(-6) M), whereas sCT-induced excitations were not inhibited. The receptor antagonist human CGRP(8-37) (10(-6) M) partly caused agonistic responses, but did not block CGRP-induced excitations. The pharmacological profile observed in the present work, and in a recent publication using the same preparation, indicating (1) that CGRP is a weaker agonist in the SFO than amylin, (2) that sCT excites SFO neurones, and (3) that responses are blocked by AC187 but not by CGRP(8-37), is inconsistent with activation via CGRP receptors, but is instead consistent with involvement of amylin (C3) and calcitonin (C1) receptors, which are co-localized to a high degree on the same subset of SFO-neurones. We propose that it is unlikely that blood-borne CGRP has a significant effect on neurones in the SFO.

Multiple amylin receptors arise from receptor activity-modifying protein interaction with the calcitonin receptor gene product

Receptor activity-modifying proteins (RAMPs) are single-transmembrane proteins that transport the calcitonin receptor-like receptor (CRLR) to the cell surface. RAMP 1-transported CRLR is a calcitonin gene-related peptide (CGRP) receptor. RAMP 2- or RAMP 3-transported CRLR is an adrenomedullin receptor. The role of RAMPs beyond their interaction with CRLR, a class II G protein-coupled receptor, is unclear. In this study, we have examined the role of RAMPs in generating amylin receptor phenotypes from the calcitonin (CT) receptor gene product. Cotransfection of RAMP 1 or RAMP 3 with the human CT receptor lacking the 16-amino acid insert in intracellular domain 1 (hCTRI1-) into COS-7 cells induced specific 125I-labeled rat amylin binding. RAMP 2 or vector cotransfection did not cause significant increases in specific amylin binding. Competition-binding characterization of the RAMP-induced amylin receptors revealed two distinct phenotypes. The RAMP 1-derived amylin receptor demonstrated the highest affinity for salmon CT (IC50, 3.01 +/- 1.44 x 10(-10) M), a high to moderate affinity for rat amylin (IC50, 7.86 +/- 4.49 x 10(-9) M) and human CGRPalpha (IC50, 2.09 +/- 1.63 x 10(-8) M), and a low affinity for human CT (IC50, 4.47 +/- 0.78 x 10(-7) M). In contrast, whereas affinities for amylin and the CTs were similar for the RAMP 3-derived receptor, the efficacy of human CGRPalpha was markedly reduced (IC50, 1.12 +/- 0.45 x 10(-7) M; P <.05 versus RAMP 1). Functional cyclic AMP responses in COS-7 cells cotransfected with individual RAMPs and hCTRI1- were reflective of the phenotypes seen in competition for amylin binding. Confocal microscopic localization of c-myc-tagged RAMP 1 indicated that, when transfected alone, RAMP 1 almost exclusively was located intracellularly. Cotransfection with calcitonin receptor (CTR)I1- induced cell surface expression of RAMP 1. The results of experiments cross-linking 125I-labeled amylin to RAMP 1/hCTR-transfected cells with bis succidimidyl suberate were suggestive of a cell-surface association of RAMP 1 and the receptors. Our data suggest that in the CT family of receptors, and potentially in other class II G protein-coupled receptors, the cellular phenotype is likely to be dynamic in regard to the level and combination of both the receptor and the RAMP proteins.

The amylin analog pramlintide improves glycemic control and reduces postprandial glucagon concentrations in patients with type 1 diabetes mellitus

To explore further the effects of the human amylin analog pramlintide on overall glycemic control and postprandial responses of circulating glucose, glucagon, and metabolic intermediates in type 1 diabetes mellitus, 14 male type 1 diabetic patients were examined in a double-blind, placebo-controlled, crossover study. Pramlintide (30 microg four times daily) or placebo were administered for 4 weeks, after which a daytime blood profile (8:30 AM to 4:30 PM) was performed. Serum fructosamine was decreased after pramlintide (314+/-14 micromol/L) compared with placebo (350+/-14 micromol/L, P = .008). On the profile day, the mean plasma glucose (8.3+/-0.7 v 10.2+/-0.8 mmol/L, P = .04) and postprandial concentrations (incremental areas under the curve [AUCs] from 0 to 120 minutes) were significantly decreased during pramlintide administration (P < .01 for both) despite comparable circulating insulin levels (359+/-41 v 340+/-35 pmol/L). Mean blood glycerol values were reduced (0.029+/-0.004 v 0.040+/-0.004 mmol/L, P = .01) and blood alanine levels were elevated (0.274+/-0.012 v 0.246+/-0.008 mmol/L, P = .03) after pramlintide versus placebo. Blood lactate concentrations did not differ during the two regimens. During pramlintide administration, the AUC (0 to 120 minutes) for plasma glucagon after breakfast was diminished (P = .02), and a similar trend was observed following lunch. In addition, peak plasma glucagon concentrations 60 minutes after breakfast (45.8+/-7.3 v 72.4+/-8.0 ng/L, P = .005) and lunch (47.6+/-9.0 v 60.9+/-8.2 ng/L, P = .02) were both decreased following pramlintide. These data indicate that pramlintide (30 microg four times daily) is capable of improving metabolic control in type 1 diabetics. This may relate, in part, to suppression of glucagon concentrations. Longer-term studies are required to ascertain whether these findings are sustained over time.

Effect of diabetes and aminoguanidine therapy on renal advanced glycation end-product binding

BACKGROUND

Advanced glycation end-products (AGEs) have been implicated in the pathogenesis of diabetic nephropathy, and aminoguanidine (AG) has been shown to decrease the accumulation of AGEs in the diabetic kidney.

METHODS

This study investigates changes in AGE binding associated with diabetes in the rat kidney using in vitro and in vivo autoradiographic techniques. Male Sprague-Dawley rats were randomized into control and diabetic groups with and without AG treatment and were sacrificed after three weeks. Frozen kidney sections (20 microm) were incubated with [125I]-AGE-RNase or [125I]-AGE-BSA. To localize the AGE binding site, in vivo autoradiography was performed by injection of 15 microCi of [125I]-AGE-BSA into the abdominal aorta of the rat.

RESULTS

Low-affinity binding sites specific for AGEs in the renal cortex (IC50 = 0.28 microm) were detected by in vitro autoradiography. There was a significant increase in [125I]-AGE binding in the diabetic kidney, which was prevented by AG treatment. Emulsion autoradiography revealed that binding was localized primarily to proximal tubules in the renal cortex. Renal AGE levels, as assessed by fluorescence or by radioimmunoassay, were increased after three weeks of diabetes. This increase was attenuated by AG therapy.

CONCLUSIONS

AGE binding sites are present within the proximal tubules of the kidney and appear to be modulated by endogenous AGE levels. It remains to be determined if these binding sites represent receptors involved in clearance of AGEs or are linked to pathogenic pathways that lead to the development of diabetic nephropathy.

Actions of amylin on subfornical organ neurons and on drinking behavior in rats

Amylin, a peptide hormone secreted by pancreatic beta-cells after food intake, contributes to metabolic control by regulating nutrient influx into the blood, whereas insulin promotes nutrient efflux and storage. We now report that amylin activates neurons in the subfornical organ (SFO), a structure in which the lack of a functional blood-brain barrier and the presence of a high density of amylin receptors may render it accessible and sensitive to circulating amylin. In an in vitro slice preparation of the rat SFO, 73% of 78 neurons were excited by superfusion with rat amylin (10(-8)-10(-7) M); the remainder were insensitive. The threshold concentration for the excitatory response of amylin was <10(-8) M and thus similar in potency to a previously reported excitatory effect of ANG II on the same neurons. The excitatory effect of amylin was completely blocked by coapplication of the selective amylin receptor antagonist AC-187 (10(-6)-10(-5) M) but was not affected by losartan (10(-5) M). Subcutaneous injections of 40 nmol of amylin significantly increased water intake in euhydrated rats, as did an equimolar dose of ANG II, which is a well-described SFO-mediated effect of circulating ANG II. These results point to the SFO as a sensory central nervous target for amylin released systemically in response to metabolic changes. Furthermore, we suggest that amylin release during food intake may stimulate prandial drinking.

Interaction of circulating hormones with the brain: the roles of the subfornical organ and the organum vasculosum of the lamina terminalis

1. Most circulating peptide hormones are excluded from much of the brain by the blood-brain barrier. However, they do have access to the circumventricular organs (CVO), which lack the blood-brain barrier. Three of the CVO, the subfornical organ (SFO), organum vasculosum of the lamina terminalis (OVLT) and area postrema, contain neurons responsive to peptides such as angiotensin II (AngII), atrial natriuretic peptide and relaxin. 2. We have studied the patterns of neuronal activation, as shown by Fos expression, in the SFO and OVLT in response to systemically infused AngII, relaxin or hypertonic saline and have found subgroups of neurons activated by the different stimuli. 3. Systemic infusion of relaxin or hypertonic saline activated neurons almost exclusively in the outer regions of the SFO and in the dorsal cap of the OVLT. Many of these neurons send axonal projections to regions of the brain subserving vasopressin secretion and thirst, such as the median preoptic, supraoptic and hypothalamic paraventricular nuclei. 4. At moderate blood concentrations, AngII only stimulates neurons in the inner core of the SFO and lateral regions of the OVLT. Higher levels of AngII in the bloodstream activate additional neurons in the outer parts of the SFO that connect to the supraoptic, paraventricular and median preoptic nuclei and these probably mediate water drinking and vasopressin secretion induced by blood-borne AngII. The efferent connections and the functions mediated by angiotensin-sensitive neurons in the inner core of the SFO and lateral part of the OVLT are unknown.

The brain as an endocrine target for Peptide hormones

Unlike circulating steroid hormones, which have a relatively unhindered passage into the central nervous system, blood-borne peptides are usually restricted by the blood-brain barrier. Some circulating peptides, such as angiotensin II, atrial natriuretic peptide and relaxin, influence central neural pathways subserving cardiovascular and body fluid homeostasis by acting on neurons in the subfornical organ, organum vasculosum of the lamina terminalis and area postrema, all of which lack a blood-brain barrier. There are some circulating peptides such as insulin and leptin that are transported from the bloodstream across cerebral blood vessel walls into sites in the hypothalamus that have appropriate neural connections to influence food intake and sympathetic control of brown fat.

Amylin: physiological roles in the kidney and a hypothesis for its role in hypertension

1. There are high-affinity binding sites for amylin in the renal cortex associated with proximal tubules. These appear to represent seven transmembrane (heptatopic) receptors that are known to form ternary complexes with G-proteins and activate second messenger systems. 2. Amylin stimulates sodium/water reabsorption from the basolateral side of the proximal tubules and plays a role in sodium homeostasis. 3. The transient expression of amylin-like mRNA has been detected perinatally, using in situ hybridization, in the subnephrogenic zone of the metanephros and is associated with proximal tubules of the developing nephron. There it is thought to play a role as a growth factor for brush border epithelial cells in the developing kidney and in renal regrowth in the adult kidney. 4. In two models of hypertension, the spontaneously hypertensive rat (SHR) and one created surgically by subtotal nephrectomy, renal amylin receptors are activated. In the SHR, activation precedes the rise in blood pressure and suggests that activation of the amylin system may be an important event in the development of hypertension.

Investigation on the mechanisms involved in the central protective effect of amylin on gastric ulcers in rats

1. The mechanisms involved in the protective effect of amylin (administered into the brain ventricle, i.c.v.) on gastric ulcers induced by the oral administration of ethanol 50% (EtOH, 2 ml/rat) or indomethacin (indomethacin, 20 mg kg(-1), at a dosing volume of 5 ml) were investigated in rats. 2. The possible involvement of endogenous nitric oxide (NO) in the beneficial effect of amylin against EtOH-induced ulcers was examined. The inhibitor of NO-synthesis, NG-nitro-L-arginine methyl ester (L-NAME, 70 mg kg(-1), s.c.) was injected 30 min before amylin (2.2 microg/rat, i.c.v.) followed by EtOH after a further 30 min. Rats were sacrificed 1 h after EtOH. L-NAME completely removed the protective effect of amylin. 3. The interaction between amylin and gastric nonprotein sulfhydryl groups was studied. The rats were treated with N-ethyl-maleimide (NEM, 25 mg kg(-1), s.c.) 30 min before amylin (2.2 microg/rat, i.c.v.) followed by EtOH 30 min after or by indomethacin 5 min after amylin. Rats were sacrificed 1 h or 6 h respectively after EtOH or indomethacin. NEM counteracted the protective effect of amylin against EtOH-induced ulcers but not against those provoked by indomethacin. 4. To determine whether amylin was able to promote ulcer healing, the peptide was injected 5 min after EtOH or 1 h after indomethacin. In the case of EtOH, the beneficial effect of amylin was lost whereas it was still effective on indomethacin-induced ulcers. 5. The results indicate that: the mechanisms involved in the antiulcer effects of amylin are different in these two types of gastric lesions probably because of the different etiopathology of various types of ulcers. Endogenous NO and nonprotein sulfhydryl groups are involved in the mucosal protective effects of amylin on EtOH and not on indomethacin-induced ulcers. Furthermore the effectiveness of amylin against indomethacin-induced lesions when administered after the ulcerogenic process has started suggests that amylin is involved not only in the protection but also in the healing mechanisms in this type of ulcer.

Effect of calcitonin on the activity of ANG II-responsive neurons in the rat subfornical organ

In addition to the well-documented ability of calcitonin to lower blood calcium levels, blood-borne calcitonin may also affect neurons located outside the blood-brain barrier, e.g., in the subfornical organ (SFO), where numerous receptors for this peptide have been described. In an in vitro preparation of the rat SFO, calcitonin activated 61% of 36 neurons, only 1 neuron was inhibited, and the remainder were unresponsive. All but two of the neurons excited by 10(-7) M calcitonin were also stimulated by 10(-7) M ANG II. The threshold concentration for the excitatory effects of calcitonin was 10(-9) M and was thus similar to ANG II. Like ANG II, subcutaneous injection of calcitonin stimulated water intake, although to a lower extent. These results suggest that blood-borne calcitonin could stimulate drinking by its excitatory effect on neurons in the SFO. Calcitonin, which is released during food intake, might be involved in prandial drinking, which is presently considered an acquired behavior.

Differential permeability of the blood-brain barrier to two pancreatic peptides: insulin and amylin

Insulin and amylin are cosecreted by pancreatic B cells and have receptors within the central nervous system (CNS), where they exert multiple effects. Although these peptides are not produced in the CNS, their ability to cross the blood-brain barrier (BBB) explains their presence there. We used multiple-time regression analysis to measure, in mice, the unidirectional influx constant (Ki) of each of these peptides to compare their rates of transport with each other and in different regions of the brain. The uptake of amylin by whole brain and by the cerebellum, midbrain, frontal cortex, parietal cortex, and occipital cortex was greater than that for insulin. For amylin, the areas of highest uptake were the pons-medulla and the cerebellum, and the areas of lowest uptake were the thalamus and midbrain. For insulin, the areas of highest uptake were the pons-medulla and the hypothalamus, whereas three regions (midbrain, thalamus, and occipital cortex) did not have a measurable Ki. The peak percent of injected dose taken up by whole brain was 0.12% for amylin and 0.046% for insulin. These results show that the permeabilities of these two peptides across the BBB differed from each other and among brain regions, suggesting that differential permeability of the BBB for blood-borne peptides could provide a mechanism by which their effects on the CNS are regulated.

Characterization of binding sites for amylin, calcitonin, and CGRP in primate kidney

Analysis of receptor distributions for 125I-labeled amylin, 125I-labeled calcitonin, and 125I-labeled calcitonin gene-related peptide (CGRP) in Macaca fascicularis kidney by in vitro autoradiography revealed distinct patterns of binding for each peptide. 125I-rat amylin bound primarily to the cortex, being associated with the distal tubule, including apparent binding to the juxtaglomerular apparatus. 125I-salmon calcitonin displayed high-density binding in the cortex with low-density binding to the medulla. Emulsion autoradiography indicated that binding was associated with both distal tubule and thick ascending limb of the loop of Henle. Intense binding was also found often over juxtaglomerular apparatus. 125I-rat CGRP-alpha exhibited low- to moderate-density binding to the inner medulla/papilla with high-density binding over small-, medium-, and large-caliber arteries. Weak binding to the glomerulus was also seen, but no binding was associated with cortical tubules. Competition binding studies, performed with each of the radioligands, revealed peptide specificity profiles for CGRP and calcitonin receptors that were similar to those described in rat. However, the monkey amylin receptors differed from those in rat, exhibiting relatively higher affinity for calcitonin peptides but reduced affinity for CGRP peptides. These studies suggest potential roles for amylin, calcitonin, and CGRP in primate renal function.

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Characterization of amylin and calcitonin receptor binding in the mouse alpha-thyroid-stimulating hormone thyrotroph cell line

Recently, a high affinity amylin binding site was identified in the mouse alpha-TSH thyrotroph cell line. In this study, we have characterized binding sites for 125I-salmon calcitonin (125I-sCT), 125I-rat alpha-calcitonin gene-related peptide (125I-CGRP), and 125I-rat amylin in alpha-TSH cells. Using 125I-CGRP or 125I-rat amylin, equilibrium was rapidly reached, and binding was fully reversible. Competition binding revealed the relative potency of peptides was sCT>amylin, CGRP>>rCT, which is similar to the specificity profile of amylin receptors characterized in rat brain. Furthermore, specific binding of 125I-rat amylin and 125I-CGRP to membrane preparations was reduced by 52% and 39%, respectively, in the presence of 20 microM GTP-gamma-s, indicating a requirement of G protein coupling for high affinity binding. In contrast, 125I-sCT binding reached equilibrium more slowly, was essentially irreversible, and was unaltered by GTP-gamma-s. Competition binding studies using 125I-sCT as radioligand demonstrated only weak interaction by CGRP or amylin, consistent with other described CT receptors. Assessment of ligand-induced cAMP accumulation and intracellular calcium signaling revealed a relative specificity profile of sCT>rCT with little or no second messenger signaling stimulated by amylin or CGRP, consistent with a C1-CT receptor phenotype. RT-PCR amplification of messenger RNA indicated that the predominant isoform was the C1a CT receptor. In cross-linking studies, 125I-rat amylin and 125I-CGRP specifically labeled a major band of relative molecular mass (Mr) approximately 80K, being approximately 10 kDa higher than the major 125I-sCT binding protein. Full deglycosylation of N-linked carbohydrates with endoglycosidase F reduced the Mr of each of the labeled proteins to approximately 50K. Cross-linked amylin or CT receptors were immunoprecipitated with C-terminally directed antimouse or antirat CT receptor antibodies but were not immunoprecipitated with nonimmune sera or antihuman CT receptor antibodies. The current data demonstrate expression of two biochemically distinct receptor phenotypes in mouse alpha-TSH cells, a CT receptor phenotype and an amylin receptor phenotype that have highly similar protein backbones.

Evidence for a physiological role of central calcitonin gene-related peptide (CGRP) receptors in the control of food intake in rats

In the present study, we investigated the role of central calcitonin gene-related peptide (CGRP) and amylin receptors in mediating the anorectic effects of CGRP and amylin in rats chronically cannulated in the lateral brain ventricle. Intracerebroventricular (ICV) injection of the CGRP and amylin receptor antagonist CGRP(8-37) failed to influence the anorectic effects of peripherally injected CGRP and amylin. CGRP(8-37) alone, however, increased food intake in food deprived rats when administered 2 h before food presentation. Under the same experimental conditions, the more specific amylin receptor antagonists amylin(8-37) or AC 187 did not affect food intake. We therefore conclude, that CGRP is a physiological regulator of food intake within the central nervous system, acting at central CGRP receptors. Peripheral receptors, however, are likely to mediate the anorectic effects of peripherally administered amylin and CGRP.

The effects of amylin on motivated behavior in rats

The effects of administration of amylin into the lateral brain ventricle on active and passive avoidance behavior and open field activity were studied in rats. Amylin increased the latency of passive avoidance behavior in a dose-dependent manner between 250 ng and 1 microgram, but 50 and 100 ng and 2 micrograms were ineffective when given immediately after passive avoidance conditioning. As concerns active avoidance behavior, the extinction was facilitated in a dose-dependent manner. The locomotion in an open field was inhibited at 3 h at 500 ng and 1 microgram doses and at 6 h at 250 and 500 ng and 1 microgram doses. However, the action was still present 24 h after administration of a large dose (1 microgram). The rearing activity was increased 3 and 6 h after administration of 1 microgram. Smaller doses (50, 100, 250, and 500 ng) were ineffective. Twenty-four hours following administration, none of the doses used exhibited any effect. The grooming activity was increased 3, 6, and 24 h after administration of 1 microgram amylin. Other doses were ineffective. The results suggest that amylin influences the behavioral reaction in these paradigms mainly by acting on locomotion, and not by modifying learning and memory processes.

Different pharmacological characteristics in L6 and C2C12 muscle cells and intact rat skeletal muscle for amylin, CGRP and calcitonin

1. We compared the ability of rat amylin, rat calcitonin gene-related peptide (CGRP) and rat and salmon calcitonins to elevate cyclic AMP levels and to inhibit [U-14C]-glucose incorporation into glycogen in insulin-stimulated intact rat soleus muscle and in two cell lines derived from rodent skeletal muscle, L6 and C2C12. 2. In intact soleus muscle, both amylin (EC50S of 0.7-6.1 nM) and salmon calcitonin (EC50S of 0.5-1.4 nM) were more potent than CGRP (EC50S of 5.6-15.8 nM) and were much more potent than rat calcitonin (EC50S of 50-137 nM) at stimulating cyclic AMP production, activating glycogen phosphorylase and inhibiting insulin-stimulated [14C]-glycogen formation. 3. In contrast, in both L6 and C2C12 cells, CGRP (EC50S of 0.042-0.12 nM) stimulated cyclic AMP formation and inhibited insulin-stimulated [U-14C]-glucose incorporation into glycogen approximately 1000 times more potently than amylin (EC50S 34-240 nM), while salmon calcitonin was without measurable effect. 4. There was a correlation between elevation of cyclic AMP and inhibition of insulin-stimulated [U-14C]-glucose incorporation into glycogen evoked by these peptides in both intact muscle (r2 = 0.69, P < 0.0004) and muscle cell lines (r2 = 0.96, P < 0.0001). 5. In conclusion, the effects of amylin, CGRP, and calcitonin on soleus muscle glycogen metabolism appear to be mediated by adenylyl cyclase-coupled receptors which show a pharmacological profile similar to high affinity amylin binding sites that have been previously reported in rat brain. In contrast, the effects of amylin and CGRP in L6 and C2C12 rodent muscle cell lines appear to be mediated by adenylyl cyclase-coupled receptors that behave like CGRP receptors.

In vitro autoradiographic localization of the calcitonin receptor isoforms, C1a and C1b, in rat brain

In this study the distribution of the calcitonin receptor isoforms, C1a and C1b, were mapped in rat brain using in vitro autoradiography and manipulation of their different pharmacological specificities. While salmon calcitonin binds to both receptors with high affinity, only the C1a receptor interacts with human calcitonin. Thus, the distribution of C1a specific binding sites was mapped using [125I]human calcitonin. The C1b receptors were mapped using [125I]salmon calcitonin in the presence of unlabelled human calcitonin and rat amylin, displacing binding of [125I]salmon calcitonin to C1a and C3 (amylin) sites, respectively. The distribution of C1a and C1b receptors was found to predominantly overlap. Brain regions displaying C1a, but little or no C1b, binding sites included the nucleus of the solitary tract, area postrema and the intermediate lobe of the pituitary. Although there were no nuclei expressing exclusively C1b receptors, parts of the mesencephalic and pontine reticular formation, and the thalamic paraventricular nucleus were enriched in C1b receptors relative to the density of C1a receptors in other brain regions. These data indicate that the relative expression of the two receptor isoforms, although predominately parallel, is not uniform in the rat brain.

Permeability of the blood-brain barrier to amylin

Amylin is co-secreted with insulin from the pancreas of patients with non-insulin dependent diabetes mellitus, and its deposition may contribute to the central nervous system (CNS) manifestations of this disease. Amylin, but not its mRNA, is found in brain, suggesting that CNS amylin is derived from the circulation. This would require amylin to cross the blood-brain barrier (BBB). We used multiple-time regression analysis to determine the unidirectional influx constant (Ki) of blood-borne, radioactively labeled amylin (I-Amy) into the brain of mice. The Ki was 8.99(10(-4)) ml/g-min and was not inhibited with doses up to 100 micrograms/kg, but it was inhibited by aluminum (Al). About 0.11 to 0.13 percent of the injected dose of I-Amy entered each gram of brain. Radioactivity recovered from brain and analyzed by HPLC showed that the majority of radioactivity taken up by the brain represented intact I-Amy. Capillary depletion confirmed that blood-borne I-Amy completely crossed the BBB to enter the parenchymal/interstitial fluid space of the cerebral cortex. Taken together, these results show that blood-borne amylin has access to brain tissue and may be involved in some of the CNS manifestations of diabetes mellitus.

Amylin stimulates plasma renin concentration in humans

Although insulin resistance and hypertension are commonly associated, the underlying cause for this association remains unknown. Plasma concentrations of the recently described hormone amylin, which is cosecreted with insulin by the pancreatic beta cell, are reported to be elevated in various states of insulin resistance, including hypertension and obesity. Preliminary studies by our group have suggested that there are amylin binding sites in the kidney. In nine healthy humans an infusion of human amylin that resulted in steady state plasma amylin levels in the subnanomolar range led to significant increases in plasma renin and aldosterone concentrations. These changes occurred in the absence of significant changes in plasma electrolytes, catecholamines, vasopressin, total renin, or osmolality. Diastolic pressure at 30 minutes and plasma glucose at 60 minutes rose modestly. Since amylin has both metabolic and renal actions, this peptide may be an important link between hypertension, insulin resistance, and the renin-angiotensin system.

Comparative distribution of receptors for amylin and the related peptides calcitonin gene related peptide and calcitonin in rat and monkey brain

The distribution of amylin receptors (125I-labelled rat amylin) in brains of rat and monkey were mapped and compared with the distribution of receptors for calcitonin (CT) (125I-labelled salmon CT) and calcitonin gene related peptide (CGRP) (rat, 125I-labelled rat CGRP alpha; monkey, 125I-labelled human CGRP alpha. In rat, amylin receptors were discretely distributed with the highest receptor densities found in mid-caudal accumbens nucleus, parts of the bed nucleus of the stria terminalis, amygdala, and hypothalamus. Moderate to high densities of binding also occurred in the area postrema, subfornical organ, vascular organ of the lamina terminalis, locus ceruleus, dorsal raphe, and caudal solitary tract nucleus. In monkey, the distribution of amylin binding sites was similar, although the highest densities of receptors were in the hypothalamus, with relatively fewer sites present in the accumbens nucleus. In rat, the distribution of amylin receptors formed a subset of the receptor distributions for 125I-labelled salmon CT and 125I-labelled rat CGRP alpha. In contrast, in monkey, although the amylin receptors again formed a subset of the binding sites identified with 125I-labelled salmon CT, there was very little overlap with the pattern of CGRP receptor distribution. This suggests that the specificity profile of amylin receptors in primates differs from that of amylin receptors in the rat, with CGRP alpha having relatively lower affinity for the primate amylin receptors.