Pasireotide Long-Acting Release Treatment for Diabetic Cats with Underlying Hypersomatotropism.

BACKGROUND: Long-term medical management of hypersomatotropism (HS) in cats has proved unrewarding. Pasireotide, a novel somatostatin analogue, decreases serum insulin-like growth factor 1 (IGF-1) and improves insulin sensitivity in cats with HS when administered as a short-acting preparation. OBJECTIVES: Assess once-monthly administration of long-acting pasireotide (pasireotide LAR) for treatment of cats with HS. ANIMALS: Fourteen cats with HS, diagnosed based on diabetes mellitus, pituitary enlargement, and serum IGF-1 > 1000 ng/mL. METHODS: Uncontrolled, prospective cohort study. Cats received pasireotide LAR (6-8 mg/kg SC) once monthly for 6 months. Fructosamine and IGF-1 concentrations, and 12-hour blood glucose curves (BGCs) were assessed at baseline and then monthly. Product of fructosamine concentration and insulin dose was calculated as an indicator of insulin resistance (Insulin Resistance Index). Linear mixed-effects modeling assessed for significant change in fructosamine, IGF-1, mean blood glucose (MBG) of BGCs, insulin dose (U/kg) and Insulin Resistance Index. RESULTS: Eight cats completed the trial. Three cats entered diabetic remission. Median IGF-1 (baseline: 1962 ng/mL [range 1051-2000 ng/mL]; month 6: 1253 ng/mL [524-1987 ng/mL]; P < .001) and median Insulin Resistance Index (baseline: 812 µmolU/L kg [173-3565 µmolU/L kg]; month 6: 135 µmolU/L kg [0-443 µmolU/L kg]; P = .001) decreased significantly. No significant change was found in mean fructosamine (baseline: 494 ± 127 µmol/L; month 6: 319 ± 113.3 µmol/L; P = .07) or MBG (baseline: 347.7 ± 111.0 mg/dL; month 6: 319.5 ± 113.3 mg/dL; P = .11), despite a significant decrease in median insulin dose (baseline: 1.5 [0.4-5.2] U/kg; 6 months: 0.3 [0.0-1.4] U/kg; P < .001). Adverse events included diarrhea (n = 11), hypoglycemia (n = 5), and worsening polyphagia (n = 2). CONCLUSIONS AND CLINICAL IMPORTANCE: Pasireotide LAR is the first drug to show potential as a long-term management option for cats with HS.

Somatostatin Agonist Pasireotide Inhibits Exercise-Stimulated Growth in the Male Siberian Hamster (Phodopus sungorus).

The Siberian hamster (Phodopus sungorus) is a seasonal mammal, exhibiting a suite of physiologically and behaviourally distinct traits dependent on the time of year and governed by changes in perceived day length (photoperiod). These attributes include significant weight loss, reduced food intake, gonadal atrophy and pelage change with short-day photoperiod as in winter. The central mechanisms driving seasonal phenotype change during winter are mediated by a reduced availability of hypothalamic triiodothyronine (T3), although the downstream mechanisms responsible for physiological and behavioural changes are yet to be fully clarified. With access to a running wheel (RW) in short photoperiod, Siberian hamsters that have undergone photoperiod-mediated weight loss over-ride photoperiod-drive for reduced body weight and regain weight similar to a hamster held in long days. These changes occur despite retaining the majority of hypothalamic gene expression profiles appropriate for short-day hamsters. Utilising the somatostatin agonist pasireotide, we recently provided evidence for an involvement of the growth hormone (GH) axis in the seasonal regulation of bodyweight. In the present study, we employed pasireotide to test for the possible involvement of the GH axis in RW-induced body weight regulation. Pasireotide successfully inhibited exercise-stimulated growth in short-day hamsters and this was accompanied by altered hypothalamic gene expression of key GH axis components. Our data provide support for an involvement of the GH axis in the RW response in Siberian hamsters.

Somatostatin Agonist Pasireotide Promotes a Physiological State Resembling Short-Day Acclimation in the Photoperiodic Male Siberian Hamster (Phodopus sungorus).

The timing of growth in seasonal mammals is inextricably linked to food availability. This is exemplified in the Siberian hamster (Phodopus sungorus), which uses the annual cycle of photoperiod to optimally programme energy expenditure in anticipation of seasonal fluctuations in food resources. During the autumn, energy expenditure is progressively minimised by physiological adaptations, including a 30% reduction in body mass, comprising a reduction in both fat and lean tissues. However, the mechanistic basis of this adaptation is still unexplained. We hypothesised that growth hormone (GH) was a likely candidate to underpin these reversible changes in body mass. Administration of pasireotide, a long-acting somatostatin receptor agonist developed for the treatment of acromegaly, to male hamsters under a long-day (LD) photoperiod produced a body weight loss. This comprised a reduction in lean and fat mass, including kidneys, testes and brown adipose tissue, typically found in short-day (SD) housed hamsters. Furthermore, when administered to hamsters switched from SD to LD, pasireotide retarded the body weight increase compared to vehicle-treated hamsters. Pasireotide did not alter photoperiod-mediated changes in hypothalamic energy balance gene expression but altered the expression of Srif mRNA expression in the periventricular nucleus and Ghrh mRNA expression in the arcuate nucleus consistent with a reduction in GH feedback and concurrent with reduced serum insulin-like growth factor-1. Conversely, GH treatment of SD hamsters increased body mass, which included increased mass of liver and kidneys. Together, these data indicate a role for the GH axis in the determination of seasonal body mass of the Siberian hamster.

Pasireotide for the Medical Management of Feline Hypersomatotropism.

BACKGROUND: Feline hypersomatotropism (HST) is a cause of diabetes mellitus in cats. Pasireotide is a novel multireceptor ligand somatostatin analog that improves biochemical control of humans with HST. HYPOTHESIS/OBJECTIVES: Pasireotide improves biochemical control of HST and diabetes mellitus in cats. ANIMALS: Hypersomatotropism was diagnosed in diabetic cats with serum insulin-like growth factor-1 (IGF-1) concentration >1,000 ng/mL by radioimmunoassay and pituitary enlargement. METHODS: Insulin-like growth factor 1 was measured and glycemic control assessed using a 12-hour blood glucose curve on days 1 and 5. On days 2, 3, and 4, cats received 0.03 mg/kg pasireotide SC q12h. IGF-1, insulin dose, and estimated insulin sensitivity (product of the area under the blood glucose curve [BGC] and insulin dose) were compared pre- and post treatment. Paired t-tests or Wilcoxon signed rank tests were employed for comparison where appropriate; a linear mixed model was created to compare BGC results. RESULTS: Insulin-like growth factor 1 decreased in all 12 cats that completed the study (median [range] day 1: 2,000 ng/mL [1,051-2,000] and day 5: 1,105 ng/mL [380-1,727], P = .002, Wilcoxon signed rank test). Insulin dose was lower on day 5 than on day 1 (mean reduction 1.3 [0-2.7] units/kg/injection, P = .003, paired t-test). The product of insulin dose and area under the BGC was lower on day 5 than day 1 (difference of means: 1,912; SD, 1523; u × mg/dL × hours, P = .001; paired t-test). No clinically relevant adverse effects were encountered. CONCLUSIONS: Short-acting pasireotide rapidly decreased IGF-1 in cats with HST and insulin-dependent diabetes. The decrease in IGF-1 was associated with increased insulin sensitivity.

Pasireotide (SOM230) prevents sulfonylurea-induced hypoglycemia in rats.

Persistent hypoglycemia is a serious condition that is frequently reported in patients undergoing sulfonylurea treatment, often necessitating hospitalization in the event of overdose. Somatostatin is a regulatory hormone with a broad range of physiological actions that include the inhibition of insulin and glucagon secretion, predominantly via activation of the somatostatin receptor subtypes sstr5 and sstr2, respectively. Previous studies have demonstrated that octreotide, a potent somatostatin analogue with high affinity for sstr2 and moderate affinity for sstr5, significantly increases serum glucose levels and prevents recurrence of hypoglycemic episodes in patients with sulfonylurea-induced hypoglycemia. Pasireotide (SOM230) is a multireceptor-targeted somatostatin analogue with a 39-, 30- and 5-fold higher binding affinity for sstr5, sstr1 and sstr3, respectively, and a slightly lower (0.4-fold) affinity for sstr2 compared with octreotide. This study evaluated the effects of pasireotide and octreotide in rats with glyburide-induced hypoglycemia. In fasted rats, pasireotide (10 and 30 µg/kg) prevented glyburide-induced hypoglycemia in a dose-dependent manner for up to 6 h. Qualitatively similar results were observed in non-fasted rats. However, the antihypoglycemic effect of pasireotide was stronger in non-fasted rats, resulting in transient hyperglycemia. In contrast to pasireotide, octreotide 10 µg/kg did not prevent glyburide-induced hypoglycemia in fasted and non-fasted rats, while octreotide 30 µg/kg resulted in small but significant increases in blood glucose at 3 h post-dose only. These findings suggest that pasireotide could have a more potent effect than octreotide in the management of patients with severe hypoglycemia caused by hyperinsulinemia.

Pasireotide (SOM230) protects the retina in animal models of ischemia induced retinopathies.

The neuropeptide somatostatin and selective analogs for the sst(2/5) receptor subtypes provided neuroprotection against retinal chemical ischemia ex vivo and AMPA [(RS)-α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid hydrobromide] induced retinal toxicity in vivo, when employed in micromolar concentrations (Mastrodimou et al., 2005; Kiagiadaki and Thermos, 2008). The aim of the present study was to investigate the neuroprotective properties of a new metabolically stable agent pasireotide (SOM230) in the above mentioned retinal models of ischemia. Adult Sprague Dawley (250-350 g) rats were employed. For the ex vivo experiments, retinal eye cups were incubated with PBS or the chemical ischemia mixture [iodoacetic acid (5 mM)/sodium cyanide (25 mM)] in the absence or presence of SOM230 (10(-7)-10(-5) M) alone or in the presence of the sst(2) antagonist CYN-154806 (10(-7) or 10(-5) M). In the in vivo model, the animals received intravitreally: PBS (50 mM), AMPA (42 nmol/eye) or AMPA (42 nmol) in combination with SOM230 (10(-7)-10(-5) M). Immunohistochemistry studies using antisera against bNOS, a marker for brain/neuronal NOS containing amacrine cells, protein kinase C (PKC) a marker for rod bipolar cells, and TUNEL studies in conjunction with FACS analysis were employed to examine retinal cell loss and protection. Chemical ischemia led to a loss of bNOS and PKC immunoreactivity which was reversed by SOM230. Partial and full protection of bNOS and PKC immunoreactive neurons, respectively, was observed even at the low concentration of 10(-7) M. The neuroprotective actions of SOM230 (10(-7) or 10(-5) M) were reversed by CYN-154806 (10(-7) or 10(-5) M, respectively). Similarly, SOM230 (10(-7), 10(-6), 10(-5) M) provided neuroprotection in the in vivo model. The dose of 10(-7) M prevented the loss of the bNOS cells and provided almost full protection. These data were substantiated by TUNEL staining and fluorescence-activated cell sorting (FACS) analysis. SOM230 appears very efficacious in its neuroprotective properties in both models of retinal ischemia affording neuroprotection at the concentration or dose of 100 nM. These data suggest that SOM230 might represent a useful pharmacological compound for the treatment of retinal disease.

Stimulatory effect of SOM230 on human and rat adrenal corticosteroid secretion in vitro.

SOM230 (pasireotide, Signifor), a recently developed somatostatin analog, has been tested in ACTH-secreting pituitary tumors with promising results. No study has yet evaluated whether this analog also directly affects adrenal steroid production. The aim of the current study was to evaluate whether SOM230 modulates corticosteroid secretion by normal adrenals in vitro. Primary cultures from normal human and rat adrenals were incubated with 10-100 nM SOM230 with and without 10nM ACTH. Dose-response studies with 1 nM-1 µM SOM230 were performed on rat adrenals. Cortisol/corticosterone levels in medium were measured after 4 and 24h. SOM230 (10nM) significantly increased corticosteroid levels after 24h incubation in both human (36.4 ± 0.43 ng/well vs 27.7 ± 3.17 ng/well, p<0.05) and rat (16.2 ± 1.16 ng/well vs 11.6 ± 0.92 ng/well p<0.05) adrenals; lesser effects were observed with 100 nM SOM (33.4 ± 2.59 ng/well vs 27.7 ± 3.17 ng/well p<0.05; 13.4 ± 0.82 ng/well vs 11.6 ± 0.92 ng/well, N.S. vs baseline secretion for human and rat adrenals, respectively). Dose-response curves confirmed maximal effect at 10nM SOM230. The corticosteroid secretory response to ACTH was unaffected by SOM230 co-incubation. In conclusion, SOM230 exerts a moderate stimulatory effect on adrenal corticosteroid secretion in vitro. This argues against a direct adrenal involvement in the clinical efficacy of SOM230 in patients with ACTH-secreting pituitary tumors and widens the known range of action of SOM230.

Octreotide and the novel multireceptor ligand somatostatin receptor agonist pasireotide (SOM230) block the adrenalectomy-induced increase in mitotic activity in male rat anterior pituitary.

The novel somatostatin receptor agonist pasireotide binds with high affinity to somatostatin receptors SSTR1, 2, 3, and 5. Acting principally through the latter, it inhibits basal and CRH-stimulated ACTH secretion from the AtT20 corticotroph cell line and ACTH release from a proportion of human corticotroph adenomas both in vitro and in vivo. Data supporting an additional antiproliferative effect has led to pasireotide being explored as a potential therapy for patients with Cushing's disease. We have compared the effects of pasireotide and octreotide on adrenalectomy-induced mitotic and apoptotic activity in the male rat anterior pituitary. Adrenalectomized rats were treated with daily sc injections of vehicle, pasireotide, or octreotide. Changes in proliferation and apoptosis were determined 2-6 d postoperatively. Pasireotide and octreotide had no effect on baseline pituitary cell turnover and no measurable effects on apoptosis. However, the wave of increased mitotic activity normally seen in the pituitary after adrenalectomy was completely abolished. Nevertheless, pasireotide and octreotide did not diminish the increase in ACTH-immunopositive cell index after adrenalectomy, indicating that cell division and differentiation of hormonally null cells in the pituitary are under independent control. In conclusion, basal cell turnover in the pituitary is not inhibited by pasireotide or octreotide. Bilateral adrenalectomy stimulates differentiation of preexisting null cells into ACTH-positive cells. Cell division after bilateral adrenalectomy occurs in a specific subpopulation of hormonally null cells that are equally sensitive to the antiproliferative effects of pasireotide and octreotide, implicating SSTR2 receptors in this antimitotic response.

Inhibitory effect of pasireotide and octreotide on lymphocyte activation.

Somatostatin (SST) regulates the function of the central and peripheral nervous system, the endocrine and exocrine organs, as well as the vascular and immune system. These actions are mediated by five specific membrane somatostatin receptors. This study compares the effects on human lymphocytes of two long-acting somatostatin analogues that have different receptor affinity: octreotide and pasireotide. Both analogues have an antiproliferative effect on human lymphocyte proliferation, but they act at different concentration and, while octreotide enhances IL10 and inhibits gamma IFN pasireotide inhibits IL2 and gamma IFN. In both sets of experiment the different behaviour of the two analogues could be due to their different affinity to the SSTR subtypes. Finally this study suggest that the growth inhibitory action of somatostatin analogues is an apoptotic phenomenon and it can be mediated by SSTR2a, in the case of octreotide, and by SSTR3 when pasireotide is used or it can be mediated by the heterodimerization of the two receptor.

Regulation of CRH-induced secretion of ACTH and corticosterone by SOM230 in rats.

OBJECTIVE: Adrenocorticotropic hormone (ACTH)-dependent Cushing's syndrome is biochemically characterized by increased plasma concentrations of ACTH inducing hypersecretion of cortisol. Somatostatin is known to inhibit ACTH secretion, and in vitro data have shown the inhibition of ACTH secretion by agonists activating sst2 and sst5 receptors. The present study aimed to determine the inhibitory effect of the multireceptor ligand SOM230, compared with the sst2-preferring agonist octreotide, on corticotropin-releasing hormone (CRH)-stimulated secretion of ACTH and corticosterone in rats. METHODS: Secretion of ACTH and corticosterone was induced by i.v. application of CRH (0.5 microg/kg) in rats pretreated 1 h before by i.v. application of SOM230 (1, 3, or 10 microg/kg), octreotide (10 microg/kg) or NaCl 0.9%. RESULTS: SOM230 (3 and 10 microg/kg) inhibited CRH-induced ACTH release by 45+/-3% and 51+/-2%, respectively, and corticosterone release by 43+/-5% and 27+/-16%, respectively. 10 microg/kg of octreotide tended to be less potent at inhibiting ACTH release (34+/-6% inhibition) and did not alter the secretion of corticosterone. CONCLUSION: SOM230 has a stronger inhibitory effect on ACTH and corticosterone secretion than octreotide in rats. This difference can be explained by its higher affinity to sst1, sst3 and especially sst5 receptors compared with octreotide.

Short- and long-term effects of octreotide and SOM230 on GH, IGF-I, ACTH, corticosterone and ghrelin in rats.

SOM230 is a novel somatostatin analog which shows affinity to 4 of the 5 known somatostatin receptors (SSTR1-5). In binding experiments, SOM230 has a higher affinity to SSTR1, SSTR3 and SSTR5 and a slightly lower affinity to SSTR2 compared to octreotide. In addition, SOM230 has a >7-fold longer plasma half-life than octreotide (11 vs 1.5 h). It was suggested that SOM230 with its broader binding and activity profile compared to octreotide should have a stronger (usually inhibitory) effect on the secretion of hormones. In several animal species, SOM230 was a more potent inhibitor of GH and IGF-I than octreotide. This is in line with a strong expression of both SSTR2 and SSTR5. In the pituitary of patients with primary Cushing's disease, the SSTR5 is more frequently expressed than SSTR2. Accordingly, in rats SOM230 caused a stronger inhibition of ACTH and corticosterone secretion than octreotide. In contrast, most recent experiments showed that octreotide was more potent than SOM230 to inhibit ghrelin secretion in rats. This effect could be explained by the strong expression of SSTR2 in the rat stomach, whereas expression of SSTR3, SSTR4 and SSTR5 was poor or absent. Based on these data it can be concluded that in tissues (or tumors), where several SSTRs are expressed, SOM230 will generally have a stronger effect than octreotide. In cases where SSTR2 is the most important receptor mediating a response (e.g. ghrelin release in rats), the stronger inhibitory effect of octreotide can be explained by its higher affinity for SSTR2. In contrast to the long-lasting inhibitory effect of SOM230 on GH and IGF-I secretion, the inhibitory effects of both compounds on ghrelin show strong tachyphylaxis. These data are in line with the hypothesis that activation of the SSTR2 alone results in a rapid desensitization of the response. If, however, additional SSTR subtypes (especially SSTR5) are expressed and activated by multiligand analogs like SOM230, this might not only form the basis for a stronger response, but also the basis for a reduced tachyphylaxis.

Ghrelin acts on leptin-responsive neurones in the rat arcuate nucleus.

Leptin decreases food intake and increases energy expenditure in rodents by inhibiting neurones in the hypothalamic arcuate nucleus. The growth hormone secretagogue (GHS) ghrelin is known to stimulate food intake and to be the endogenous ligand for the GHS-receptor, which is strongly expressed in the arcuate nucleus, like the leptin receptor (Ob-R). In this study, we analysed the effect of systemic ghrelin administration on Fos expression in the arcuate nucleus on neurones expressing Ob-R. Injection of ghrelin (0.2 mg/kg, i.p) significantly increased the number of neurones expressing Fos protein in the ventromedial arcuate nucleus. Fifty-seven percent of all Fos-positive cells in the ventromedial arcuate nucleus were also positive for Ob-R staining. Furthermore, we investigated electrophysiologically the effect of ghrelin and leptin on the activity of arcuate neurones in an in-vitro slice preparation. Ghrelin stimulated the electrical activity dose-dependently in 80% of all cells tested (n=49) with a threshold concentration of 10(-11) M; only 8% were inhibited and 12% did not respond. The effect of ghrelin (10(-7) M) was weakly antagonized by the peptidic GHS-receptor antagonist (D-Lys3)-GHRP-6 (10(-4) M), which also showed a much weaker affinity (IC(50), 0.9 x 10(-6) M) to the GHS-receptor than ghrelin (IC(50), 0.3 x 10(-9) M). Ghrelin increased the electrical activity in 76% of all cells which were inhibited by leptin (n=17). These data show that ghrelin interacts with the leptin hypothalamic network in the arcuate nucleus. The opposite effect of leptin and ghrelin on neurones in the arcuate nucleus may serve as a neurophysiological correlate of the orexigenic and anorectic effects of ghrelin and leptin.

Circulating relaxin acts on subfornical organ neurons to stimulate water drinking in the rat.

Relaxin, a peptide hormone secreted by the corpus luteum during pregnancy, exerts actions on reproductive tissues such as the pubic symphysis, uterus, and cervix. It may also influence body fluid balance by actions on the brain to stimulate thirst and vasopressin secretion. We mapped the sites in the brain that are activated by i.v. infusion of a dipsogenic dose of relaxin (25 microg/h) by immunohistochemically detecting Fos expression. Relaxin administration resulted in increased Fos expression in the subfornical organ (SFO), organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus, and magnocellular neurons in the supraoptic and paraventricular nuclei. Ablation of the SFO abolished relaxin-induced water drinking, but did not prevent increased Fos expression in the OVLT, supraoptic or paraventricular nuclei. Although ablation of the OVLT did not inhibit relaxin-induced drinking, it did cause a large reduction in Fos expression in the supraoptic nucleus and posterior magnocellular subdivision of the paraventricular nucleus. In vitro single-unit recording of electrical activity of neurons in isolated slices of the SFO showed that relaxin (10(-7) M) added to the perfusion medium caused marked and prolonged increase in neuronal activity. Most of these neurons also responded to 10(-7) M angiotensin II. The data indicate that blood-borne relaxin can directly stimulate neurons in the SFO to initiate water drinking. It is likely that circulating relaxin also stimulates neurons in the OVLT that influence vasopressin secretion. These two circumventricular organs that lack a blood-brain barrier may have regulatory influences on fluid balance during pregnancy in rats.

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.

Orexin A activates leptin-responsive neurons in the arcuate nucleus.

Orexins, also named hypocretins, are newly described neuropeptides, which are produced almost exclusively in neurons of the lateral hypothalamus and have been shown to increase food intake after intracerebroventricular injection. Leptin, the ob-gene product released from white adipocytes, is suspected to reduce food intake mainly by acting on neurons in the arcuate nucleus of the hypothalamus. Application of orexin A activated 85% (66 out of 78) of all neurons of the rat arcuate nucleus investigated electrophysiologically in an in vitro slice preparation, by a direct excitatory postsynaptic effect. Leptin inhibited electrical activity in 10 out of 22 orexin-sensitive neurons in this brain region and excited only 3 neurons. These data give the first indication as to where and how orexin might interact with the leptin-responsive hypothalamic network.

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.

Neuronal actions of oxytocin on the subfornical organ of male rats.

The aim of this study was to investigate effects of oxytocin (OT) on electrical neuronal activities in rat subfornical organ (SFO) and compare its action with the well-described excitatory effects of blood-borne angiotensin II (ANG II) on the same SFO neurons. With the use of extracellular recordings from spontaneously active neurons in slice preparations of the SFO of male rats, 11.7% of tested neurons (n = 206) were excited and 9.7% were inhibited by superfusion with 10(-6) M OT. Both excitatory and inhibitory effects of OT were dose dependent with similar threshold concentrations and were blocked by a specific OT-receptor antagonist but not by a vasopressin receptor antagonist. Blocking synaptic transmission with low calcium medium suppressed only inhibitory effects of OT. All but one of the OT-sensitive neurons were also excited by superfusion with ANG II at a concentration much lower than required for OT, suggesting that synaptically released OT rather than blood-borne OT alters the activity of SFO neurons in vivo. The results support the hypothesis that neurally released OT may modulate SFO-mediated functions by acting on OT-sensitive neurons.

Functional evidence for subfornical organ-intrinsic conversion of angiotensin I to angiotensin II.

Using extracellular electrophysiological recording in an in vitro slice preparation, we investigated whether ANG I can be locally converted to the functionally active ANG II within the rat subfornical organ (SFO). ANG I and ANG II (10(-8)-10(-7) M) excited approximately 75% of all neurons tested with both peptides (n = 25); the remainder were insensitive. The increase in firing rate and the duration and the latency of the responses of identical neurons, superfused with equimolar concentrations of ANG I and ANG II, were not different. The threshold concentrations of the ANG I- and ANG II-induced excitations were both 10(-9) M. Inhibition of the angiotensin-converting enzyme by captopril (10(-4) M; n = 8) completely blocked the ANG I-induced excitation, a 10-fold lower dose was only effective in two of four neurons. The AT1-receptor antagonist losartan (10(-5) M; n = 6) abolished the excitation caused by ANG I and ANG II. Subcutaneous injections of equimolar doses of ANG I and ANG II (200 microliters; 2 x 10(-4) M) in water-sated rats similarly increased water intake by 2.4 +/- 0.5 (n = 16) and 2. 7 +/- 0.4 ml (n = 20) after 1 h, respectively. Control rats receiving saline drank 0.07 +/- 0.06 ml under these conditions. Pretreatment with a low dose of captopril (2.3 x 10(-3) M) 10 min before the injection of ANG I caused a water intake of 2.8 +/- 0.5 ml (n = 10), whereas a high dose of captopril (4.6 x 10(-1) M) suppressed the dipsogenic response of ANG I entirely (n = 11). These data provide direct functional evidence for an SFO-intrinsic renin-angiotensin system (RAS) and underline the importance of the SFO as a central nervous interface connecting the peripheral with the central RAS.

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.