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.

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.

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.

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.

Photoendocrine signal transduction in pineal photoreceptors of the trout. Role of cGMP and nitric oxide

This study describes the presence and distribution of cGMP-immunoreactivity and of the nitric oxide (NO) synthesizing enzyme, NO synthase (NOS), as demonstrated by use of the NADPH-diaphorase technique in directly light sensitive pineal organ of the trout. Cyclic GMP immunohistochemistry revealed immunoreactivity in pineal photoreceptor cells that were identified by double-labeling with S-antigen, whereas NADPH-positive structures were located adjacent to these photoreceptor cells. Since NO is known to stimulate synthesis of cGMP, these results indicate a role for NO in pineal function, e.g. in cGMP related events in the phototransduction process as well as in the light-dark control of melatonin synthesis.

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.

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.

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.

Vasopressin and sensory circumventricular organs

The subfornical organ, the area postrema and the organum vasculosum of the lamina terminalis are considered to be sensory circumventricular organs as they contain neuronal somata which are located outside the blood-brain barrier and are thus capable of serving as 'sensors' for blood-borne humoral messengers. The endocrine hormone, vasopressin (VP), not only causes strong antidiuresis by acting on the kidney, but also exerts centrally mediated effects as a neuromodulator. Several lines of evidence suggest that VP can influence regulatory functions mediated by the sensory circumventricular organs, since vasopressinergic somata and terminals as well as VP receptors have been reposted to be present in these structures. These biochemical prerequisites offer the possibility that blood-borne VP might on the one hand act as a feedback signal from the periphery and, on the other hand, synaptically released or locally produced VP could modulate the known functions of sensory circumventricular organs, such as thirst, fever or cardiovascular regulation. This review focuses on the possible physiological relevance of VP acting on sensory circumventricular organs in view of recent evidence obtained from biochemical and electrophysiological studies at the cellular level.

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.

Effect of glutamate and angiotensin II on whole cell currents and release of nitric oxide in the rat subfornical organ

Blood-borne angiotensin II (AngII) is known to mediate water-intake by its excitatory effect on neurons in the subfornical organ (SFO). Conversely, nitric oxide (NO) has exclusively inhibitory effects on rat SFO-neurons and on SFO-mediated water-intake. Extracellular and patch-clamp recordings from freshly dissociated rat SFO-neurons showed that glutamate activates AngII-sensitive SFO-neurons by opening ligand-gated cation channels. An immunocytochemical study showed that activation of glutamate receptors increased the concentration of the inhibitory second messenger cGMP in the SFO. A model is proposed suggesting that NO protects SFO-neurons from overexcitability by excitatory neurotransmitters.

Multiple receptor subtypes mediate the effects of serotonin on rat subfornical organ neurons

The subfornical organ (SFO) receives significant serotonergic innervation. However, few reports have examined the functional effects of serotonin on SFO neurons. This study characterized the effects of serotonin on spontaneously firing SFO neurons in the rat brain slice. Of 31 neurons tested, 80% responded to serotonin (1-100 microM) with either an increase (n = 15) or decrease (n = 10) in spontaneous activity. Responses to serotonin were dose dependent and persisted after synaptic blockade. Excitatory responses could also be mimicked by the 5-hydroxytryptamine (5-HT)2A/2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI; 1-10 microM) and could be blocked by the 5-HT2A/2C-receptor antagonist LY-53,857 (10 microM). LY-53,857 unmasked inhibitory responses to serotonin in 56% of serotonin-excited cells tested. Serotonin-inhibited cells were also inhibited by the 5-HT1A-receptor agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT; 1-10 microM; n = 7). The data indicate that SFO neurons are responsive to serotonin via postsynaptic activation of multiple receptor subtypes. The results suggest that excitatory responses to serotonin are mediated by 5-HT2A or 5-HT2C receptors and that inhibitory responses may be mediated by 5-HT1A receptors. In addition, similar percentages of serotonin-excited and -inhibited cells were also sensitive to ANG II. As such the functional relationship between serotonin and ANG II in the SFO remains unclear.

Spontaneous activity of preoptic neurons in slice preparations of the hypothalamus of European hamsters (Cricetus cricetus) and Wistar rats under different states of hypothermia

Hypothermia was induced in European hamsters (hibernators) and Wistar rats (nonhibernators), and changes in the firing rate and spike duration of extracellularly recorded action potentials were investigated in hypothalamic slices in vitro. At slice temperatures close to normal body temperature (37 +/- 3 degreesC), 32 and 57% of spontaneously active neurons in the medial preoptic area were classified as warm-sensitive in rats and hamsters, respectively. With decreasing slice temperature, the number of active neurons decreased progressively without a significant difference between rats and hamsters. At a slice temperature of 10 degreesC, 57% of all hypothalamic neurons in rats and 42% in hamsters were still spontaneously active. The average temperature at which activity ceased completely when the temperature was decreased further (the cut-off temperature) was 7.9 +/- 0.3 degreesC (n = 14) in rats but was significantly lower at 4.9 +/- 0.4 degreesC (n = 8) in hamsters (P < 0.001). Firing rates and temperature coefficients did not differ in their temperature dependence between rats and hamsters. Action potential duration increased with decreasing slice temperature in both species, but the increase in duration was significantly greater in rats.

Spinal neuronal thermosensitivity in vivo and in vitro in relation to hypothalamic neuronal thermosensitivity

In the spinal cord, temperature signals are generated which serve as specific inputs in the central nervous control of body temperature. Because of the spatially distinct organization of afferent and efferent neuronal systems at the spinal level, the afferent pathway for temperature signal transmission could be identified in vivo in the ascending, anterior and lateral tracts with a relationship of about 75:25% between warm and cold sensitive neuraxons. Analysis of spinal neuronal thermosensitivity in vitro on spinal cord tissue slices has been concerned, so far, with the superficial laminae of the dorsal horn as the site of origin of ascending nerve fibers conveying mostly temperature and pain signals, and with lamina X as a site of origin of afferent as well as efferent neurons. A relationship of about 95:5% between warm and cold sensitive neurons was found at the segmental level, indicating that warm sensitivity is the prevailing, primary property of spinal neurons, whereas cold sensitivity seems to be mainly generated by synaptic interaction as a secondary modality. Dynamic responses to temperature changes were frequently displayed in vitro at the spinal segmental level in lamina I + II but not in lamina X, even by neurons whose static activity was little influenced by local temperature. Dynamic thermosensitivity was found less frequently in ascending tract neuraxons and was not observed in hypothalamic neurons receiving temperature signal inputs from the spinal cord, and thus, does not seem to be relevant for the thermosensory function of spinal cord neurons, unlike peripheral warm and cold receptors. A majority of spinal warm sensitive neurons displayed both static and dynamic warm sensitivity as an inherent property after synaptic blockade. In the further analysis of spinal cord thermosensitivity, the in vitro approach permits application of the same electrophysiological and neuropharmacological methods as were established for the analysis of hypothalamic thermosensitivity. In addition, the topography of the spinal cord will provide additional structural and possibly histochemical information to characterize the functions of neurons independently of their thermal properties.

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.

Heterogeneous actions of vasopressin on ANG II-sensitive neurons in the subfornical organ of rats

The aim of this study was to investigate the effects of the antidiuretic hormone arginine vasopressin (AVP), which is released in vivo during dehydration and hypovolemia to prevent further water loss, on the activity of neurons in the subfornical organ (SFO). The SFO is a brain structure with an open blood-brain barrier and is critically involved in angiotensin II (ANG II)-dependent water intake. SFO neurons were recorded extracellularly in tissue slices of the rat brain and were tested for responsiveness to AVP and ANG II. About one-half of 159 neurons tested with an AVP concentration of 10(-6) M in the superfusion medium were responsive, and approximately equal proportions were excited and inhibited. Neurons exhibiting the different response types did not differ from each other with respect to spontaneous discharge rate, latency, and duration of the response. Excitatory and inhibitory responses to AVP were dose dependent and reversible, and their threshold concentrations (10(-8) to 10(-9) M) were similar. Superfusion with a medium low in Ca2+ and high in Mg2+ showed that the excitatory effect is most likely direct, whereas the inhibitory effect largely depends on inhibitory synaptic interaction. About one-half of the SFO neurons excited by ANG II (10(-7) M) were responsive to AVP (10(-6) M), and equal proportions were inhibited and excited. Both excitatory and inhibitory AVP actions were blocked by the V1-receptor antagonist, Manning compound, and neurons responsive to AVP did not respond to the V2-receptor agonist [deamino-Cys1,D-Arg8]vasopressin. It is concluded that AVP, probably released from synaptic terminals, may increase or decrease the activity of neurons in the SFO, many of which are activated by ANG II. In contrast to previous experiments on ducks, in which the exclusively excitatory effect of the avian antidiuretic hormone arginine vasotocin on ANG II-sensitive SFO neurons correlates well with the dipsogenic effect of both peptides, a greater functional heterogeneity exists among AVP-responsive neurons in the rat SFO.

Electrophysiological responses of neurons in the rat spinal cord to nitric oxide

The effects of nitric oxide-containing solution and different nitric oxide donors were investigated on spontaneously active neurons using extracellular recording technique in areas of rat spinal cord slices where high levels of nitric oxide synthase are present. In lamina X, 93% of all neurons investigated (n = 84) increased their firing rate and 2% decreased it by superfusion with the nitric oxide donor sodium nitroprusside. In contrast, 49% of all neurons in laminae I and II (n = 90) were inhibited and only 28% were activated. Both effects were due to the postsynaptic action of sodium nitroprusside, because they could still be observed in medium containing 0.3 mM Ca2+ and 9 mM Mg2+, known to block synaptic transmission. Application of 8-bromo-cyclic-GMP caused an excitation of every neuron which was excited by sodium nitroprusside and an inhibition of every cell which was inhibited by sodium nitroprusside (n = 25). This effect was different from the effect of 8-bromo-cyclic-AMP, which mimicked only the excitatory, but not the inhibitory response of sodium nitroprusside. These results provide evidence that nitric oxide in the spinal cord can directly cause an excitation or an inhibition of the electrical activity of spinal neurons. Another, more general conclusion from our results is that the nitric oxide-induced production of cyclic-GMP alone does not allow any prediction about an excitatory or inhibitory effect on the neuronal activity, which has to be determined separately.

Temperature sensitivity of neurones in slices of the rat spinal cord

1. The inherent temperature sensitivity of 343 spontaneously active neurones recorded from rat spinal cord (SC) slices was investigated electrophysiologically. Recordings were made from 321 neurons from transverse and 22 neurons from longitudinal slices and their thermosensitivity was determined by relating changes in firing rate to changes in slice temperature. 2. Of the neurones from transverse slices, 53% were warm sensitive, 2% were cold sensitive and 45% were temperature insensitive. In longitudinal slices, 68% were warm sensitive and the remaining neurones were temperature insensitive. 3. When classified according to their recording sites in transverse slices, warm-sensitive neurones in laminae I and II had the same mean temperature coefficient compared with those recorded from lamina X, despite the fact that the latter had a significantly higher spontaneous activity. 4. The intrinsic temperature sensitivity of the majority of warm-sensitive neurones was confirmed by blocking their synaptic input. 5. A transient overshoot in activity, i.e. a dynamic response characteristic following rapid temperature stimuli (0.4 degree C s-1) was observed in 73% of the warm-sensitive and 59% of the temperature-insensitive neurones in laminae I and II in response to rapid warming, but only rarely (< 10%) in lamina X. 6. Temperature-sensitive SC neurones share response characteristics with temperature-sensitive neurones in the preoptic and anterior hypothalamic (PO/AH) area and with peripheral temperature receptors. Functionally, these neurones may represent the cellular basis for the temperature sensory function of the spinal cord that has been well characterized in vivo in homeothermic species.

Electrophysiological and immunocytochemical evidence for a cGMP-mediated inhibition of subfornical organ neurons by nitric oxide

The activation of neurons in the subfornical organ (SFO) by angiotensin II (AngII) is well established and is widely regarded as the basis for the AngII-induced increase in water intake. Application of the nitric oxide (NO) donor sodium nitroprusside (SNP) led to an inhibition of the spontaneous electrical activity in 96% of the neurons sensitive for SNP (n = 50). In addition, the firing rate in 60% of the neurons inhibited by SNP decreased in response to superfusion with the natural substrate of the NO synthase (NOS) L-arginine whereas 70% increased their frequency after application of the NOS blocker NG-monomethyl-L-arginine (L-NMMA; n = 10). The inhibitory effect of SNP could be mimicked by application of membrane-permeable 8-Br-cGMP. The presence of nNOS, the neuronal isoform of NOS, was demonstrated immunocytochemically and using the NADPH-diaphorase technique on SFO slices. Using a highly selective antibody against cGMP in formaldehyde-fixed tissue, the NO donors SNP, 3-morpholinosydnonimine (SIN-1), and S-nitroso-N-acetyl-DL-penicillamine (SNAP) caused a strong increase in cGMP formation when applied under the same conditions as used for the electrophysiological recordings. These electrophysiological results suggest an important role for NO in SFO-mediated responses and offer a plausible explanation for the in vivo-observed opposite effects of AngII and NO on water intake.

Effects of GABA agonists and antagonists on temperature-sensitive neurones in the rat hypothalamus

1. Extracellular recordings were obtained from 94 warm-sensitive, 6 cold-sensitive and 117 temperature-insensitive neurones in slices of the hypothalamic medial preoptic area of rats, to determine the effect of the GABAA agonist muscimol, the GABAA antagonist bicuculline, the GABAB agonist baclofen and the GABAB antagonist phaclofen on tonic activity and temperature sensitivity. 2. Muscimol and baclofen dose-dependently inhibited the tonic activity of 69% (36/52) and 97% (36/37) of the hypothalamic neurones, respectively, regardless of their type of thermosensitivity. In contrast, the GABAA antagonist bicuculline increased the tonic activity of the majority of neurones (58/83), while the GABAB antagonist phaclofen increased neuronal activity only in the high dose of 100 microM. 3. The temperature sensitivity of hypothalamic neurones was only changed by ligands of GABAB receptors, and this effect was restricted to warm-sensitive neurones. The temperature coefficient (TC) was significantly increased by the GABAB agonist baclofen (delta TC = 0.69 +/- 0.11 imp s-1 degree C-1, P < 0.01, n = 18). In contrast, the GABAB antagonist phaclofen (10 microM) decreased the temperature sensitivity (delta TC = -0.67 +/- 0.09 imp s-1 degree C-1, P < 0.01, n = 10) in doses which did not affect tonic activity. 4. The increase in temperature sensitivity due to the GABAB agonist baclofen was significantly enhanced by co-perfusion of the GABAA antagonist bicuculline, indicating an interaction of GABAA and GABAB receptor-mediated mechanisms with regard to neuronal thermosensitivity. 5. The results suggest that neurones in the medical preoptic area are subject to GABA-mediated tonic inhibition resulting in modulation of tonic activity and temperature sensitivity of warm-sensitive neurones possibly involved in the control of body temperature. The data support the hypothesis that the hypo- or hyperthermic action of an endogenous substance is related to its effect on the thermosensitivity rather than on tonic activity of hypothalamic neurones.

Regional specific effects of nitric oxide donors and cGMP on the electrical activity of neurons in the rat spinal cord

Numerous functional studies establish the role of nitric oxide (NO) as a neuromodulator in the central nervous system which affects synaptic transmission. However, there are only a few reports indicating a direct and postsynaptic effect of nitric oxide on the electrical activity of neurons in the central nervous system. The aim of this study was to characterize the effect of nitric oxide on spontaneously active neurons in spinal cord slices using an extracellular recording technique. Because in the lumbar rat spinal cord the NO producing enzyme NO-synthase is primarily located in the superficial dorsal horn (laminae I+II) and around the central canal (lamina X), we restricted our recordings to these areas. While the majority of neurons increased their electrical activity during superfusion with the NO-donor sodium nitroprusside (SNP) in lamina X, neurons in laminae I+II were mainly inhibited by SNP. The excitatory and the inhibitory effects were dose-dependent and reversible and were mimicked by other NO-donors and membrane permeable cyclic guanosine monophosphate (8Br-cGMP) on the same neurons. The spinal cord slice preparation contains functional NO-synthase (NOS), because selective blockade of NOS increased the spontaneous activity of those neurons from laminae I+II which were inhibited by SNP and this effect could be reversed by superfusion with the natural substrate for NOS, L-arginine. It is concluded that NO can activate and inhibit the activity of spinal cord neurons by raising cGMP levels and that these effects are lamina specific. A general consequence of our results is that the NO-induced production of cGMP alone does not allow any prediction about an excitatory or inhibitory effect of NO on the discharge rate of neurons. Thus the NO mediated increase and decrease in neuronal activity is probably the result of intracellular mechanisms downstream from the production of cGMP which results in the activation or inhibition of different ion channels on neurons in laminae I+II and X.

Angiotensin II receptor subtypes in the duck subfornical organ: an electrophysiological and receptor autoradiographic investigation

The pharmacology of angiotensin II (AngII) receptors was investigated in the brain of ducks using receptor autoradiographic and electrophysiological methods. Using 125I[Val5]AngII as a ligand, specific binding was observed in sections of the duck adrenal gland and in several brain areas involved in body fluid homeostasis. Displacement studies using the same antagonists as used for classifying mammalian AngII receptor subtypes revealed that the rank order of potencies in competition with AngII receptors in the adrenal gland and in the subfornical organ was: AngII > CGP-42112A > losartan > PD-123319. Electrophysiological recordings from spontaneously active neurons of duck SFO slices revealed that the majority of neurons could be excited by AngII (10(-7) M). The excitatory effect of AngII could be partially inhibited by CGP-42112A (10(-5) M), which proved to be more effective than equimolar losartan and far more effective than PD-123319. These data suggest that the neuronal AngII receptors in the SFO are pharmacologically distinct from the mammalian AT1- and AT2-receptors. Further, central AngII receptors of ducks share common pharmacological characteristics with AngII receptors in the duck adrenal gland and peripheral organs of other bird species.

Effects of angiotensin II and its blockers Sar1-Ile8-angiotensin II and DuP 753 on drinking in ducks in relation to properties of subfornical organ neurons

Properties of systemically applied angiotensin II in stimulating water intake of normally hydrated ducks were studied and the results compared with properties of angiotensin II-responsive neurons of the subfornical organ which are considered as targets for circulating angiotensin II acting as a dipsogen. Following intravenous infusion of hypertonic saline (2000 mosmol.kg-1 at 0.3 ml.min-1 for 1 h), intravenous infusion of 0.3 ml.min-1 isotonic saline with angiotensin II (200 ng.min-1), starting 1 h later, stimulated drinking in each case at an angiotensin II plasma level of about 1400 pg.ml-1. Without hypertonic priming, the same angiotensin II infusion did not stimulate drinking in each experiment; however, if effective, repeated infusions of ANGII induced stable dipsogenic responses. Angiotensin II infusions did not alter plasma levels of antidiuretic hormone. Sar1-Ile8-angiotensin II, a non-selective angiotensin II antagonist, acted weakly as a partial agonist when infused at a dose 200-fold higher than angiotensin II and effectively blocked the dipsogenic action of angiotensin II; this corresponds to the inhibition of angiotensin II-induced excitation by Sar1-Ile8-angiotensin II observed in duck subfornical organ neurons. DuP 753 (losartan), an angiotensin II antagonist specifically blocking AT1 receptors in mammals, had equivocal effects on angiotensin II-induced drinking in ducks at rates 50- and 200-fold higher than angiotensin II, which corresponds to the weak inhibitory action of this compound on angiotensin II-induced neuronal excitation in the duck SFO. Blood pressure was only marginally elevated by the applied angiotensin II dose and Sar1-Ile8-angiotensin II had no effect.

Vasotocin acts as a dipsogen in ducks at concentrations stimulating subfornical organ neurons in vitro

The effects of systemic infusions of the avian antidiuretic hormone arginine vasotocin on water intake of domestic ducks were investigated under steady conditions of water balance in which angiotensin II was effective as a dipsogen. The study proceeded from the consistent stimulatory effect of arginine vasotocin on angiotensin II-responsive neurons found in the subfornical organ of ducks, suggesting brain-intrinsic vasotocinergic control of these neurons which are also accessible to circulating agents because of the lacking blood-brain barrier. Levels of circulating arginine vasotocin of about 2700 pg.ml-1 which were close to the threshold for activation of subfornical organ neurons in vitro, induced weak but significant drinking responses. Even at this high arginine vasotocin level circulatory effects were absent, thereby excluding their interference with water intake. Arginine vasotocin plasma levels of about 60 pg.ml-1 significantly attenuated the dipsogenic action of angiotensin. While drinking in response to high pharmacological levels of arginine vasotocin is assumed to mimic a stimulatory innervation of angiotensin-responsive subfornical organ neurons by brain-intrinsic vasotocinergic axons, attenuation of angiotensin-induced drinking by high physiological arginine vasotocin levels cannot be explained by its action on central neurons, but may be secondary to body fluid retention caused by the antidiuretic action of arginine vasotocin.

Excitatory action of the bird antidiuretic hormone vasotocin on neurons in the subfornical organ

The responsiveness of spontaneously active neurons in the subfornical organ (SFO) of adult ducks to angiotensin II (ANGII) and the bird specific antidiuretic hormone, arginine vasotocin (AVT), the analog of the mammalian arginine vasopressin (AVP), were investigated in brain slices with extracellular recording technique. 65% (n = 66) of the neurons increased their activity after superfusion with ANGII, the rest were unresponsive. Application of AVT activated 52% (n = 68) of the investigated neurons and like ANGII never caused an inhibition of the spontaneously active SFO neurons. A close correlation exists between the ANGII and AVT sensitivity of duck SFO neurons, because 29 out of 33 neurons were excited by AVT as well as ANGII. The relatively weak antagonistic effect of the V1-type receptor antagonist Pmp-Tyr (Me)-Arg8-vasopressin on the AVT induced excitation suggests a different pharmacology of the bird AVT receptor as compared to the mammalian AVP receptor. The excitatory response of ANGII and AVT on the very same neurons suggest a similar function of both peptides on SFO mediated effects in vivo, such as an increase in water intake. However, peripheral AVT concentrations, unlike ANGII concentrations in the blood are not high enough to activate SFO neurons from the blood side of the blood brain barrier. Therefore AVT is presumably released from synapses of neurons originating within or projecting to the SFO. The identity of the ANGII and AVT reactive neurons suggests that synaptically released AVT should facilitate SFO mediated drinking.

Opposite effects of angiotensin II and nitric oxide on neurons in the duck subfornical organ

Angiotensin II (ANGII) is known to activate neurons in the subfornical organ (SFO) of mammals and birds and this activation is regarded as the basis for the ANGII induced increase in water intake. Application of the nitric oxide (NO) donor sodium nitroprusside inhibited the activity in 10 out of 12 duck SFO neurons, 8 of which were in addition excited by ANGII. These data, in combination with the histochemical detection of NO synthase in the duck SFO, demonstrate the involvement of NO in SFO mediated responses and might represent the cellular basis for the observed opposite effects of ANGII and NO on water intake.

Regional differences in the blood-brain-barrier of the subfornical organs of rats and ducks (Anas platyrhynchos)

Recently published electrophysiological data investigated the effect of blood borne and brain intrinsic substances on the activity of neurons in the duck subfornical organ (SFO). This study defines histologically the region in the duck SFO, where blood borne substances can possibly influence neuronal activity. Intravenous injection of Evans blue, a dye which labels brain structures devoid of a blood brain barrier (BBB), resulted in diffuse labelling of the duck SFO from the anterior commissure to the end of the organ in rostrocaudal extension. In addition, specifically labelled neurons could be observed just rostral to the diffuse Evans blue labelling and in an area dorsomedial to the large central blood vessel. The majority of the somata of these heavily stained neurons were located inside the BBB, whereas in the areas with diffuse Evans blue labelling, thus being outside the BBB, labelled cells were rarely observed. Intravenous injection of Evans blue in rats resulted similarly in diffuse labelling of the parenchyma of the medial and caudal part of the SFO, with only a few, but heavily stained cells with fusiform somata. The rostral region of the rat SFO, which is known to have a functional BBB, shows hardly any diffuse labelling, but there the majority of neurons show strong Evans blue fluorescence. It is concluded that the heavily labelled somata inside the BBB have axonal or dendritic projections to BBB-free areas, where they can take up the dye. This study gives a functional description of the extension of the SFO areas without a BBB of rats and ducks. It is concluded that blood borne agents can affect those SFO neurons which have their somata located outside the BBB as well as those located inside the BBB which have terminals projecting to BBB free regions.

Age-related differences in the structure of human pineal calcium deposits: results of transmission electron microscopy and mineralographic microanalysis

Pineal tissue calcifications (male, ages 14, 47, 62, 82), which were metallographically embedded and polished at controlled levels and studied by transmission electron microscopy and microanalytic spectroscopy, showed age-related differences. Results show that concentrically arranged crescent-shaped lamellae increase in number and decrease in width with age. Calcium (Ca) and phosphorus (P) per point measurements in dark and light lamellae at various distances from the core show Ca/P molar ratios between 1.49-1.62 in the 82-year-old specimen as compared to 1.26 to 1.41 in the younger specimens. The 62-year-old specimens show a decrease in P and an increase in Ca from periphery to center. These data and other descriptive details suggest that the sum of these changes represent remodelling of the mineralogical structure within the same calcification throughout the life span.

Primary culture of circumventricular organs from the rat brain lamina terminalis

A primary culture system of cells derived from two circumventricular organs (CVO) of the rat brain was established. The subfornical organ (SFO) and the organum vasculosum of the lamina terminalis (OVLT) were dissected from the rostral wall of the third ventricle and its cells taken into culture after mechanical dissociation. The cells were cultured in a modified microculture chamber system ensuring relatively high cell density despite their low absolute number. When animals were injected with Evans blue prior to cell preparation, the macroscopically visible penetration of the dye into the parenchyma of the CVOs could be used as guidance during tissue isolation and labelled cells could be identified in culture. Cultured CVO neurones and astrocytes were identified using antibodies against cell type specific marker proteins. The histochemical NADPH-diaphorase staining was used for the detection of nitric oxide synthase in tissue sections of both CVOs and in their cultured neurones. In addition, angiotensin II (ANG II)-evoked elevations of the intracellular Ca2+ concentration ([Ca2+]i) in single cultured OVLT neurones were measured. The described methods will be useful for further characterization of CVO neurones and astrocytes.

Calcium, calcification, and melatonin biosynthesis in the human pineal gland: a postmortem study into age-related factors

It is believed that pineal calcification may be age-associated and that the well-demonstrated age-related decline in melatonin biosynthesis may be an expression of an alteration in calcium homeostasis in the pinealocyte. Prior correlations of melatonin to calcium deposition and age were made on the basis of radiological or semiquantitative analysis. In this postmortem study of 33 subjects (age range 3 months to 65 years) calcium deposits measured by atomic absorption spectrometry correlated positively with age in day and night samples (day: r = 0.56, P < 0.05; night: r = 0.818, P < 0.001). Nighttime (2200 h to 0800 h) pineal melatonin content (HPLC fluorometry) was higher than daytime melatonin levels (nighttime 3.80 +/- 0.3 vs. daytime 0.85 +/- 0.4 ng/mg protein). Nighttime calcium levels in the supernatant correlated negatively with melatonin content (r = -0.59, P < 0.05).

Serotonin modulates nicotinic responses of adrenal chromaffin cells

5-Hydroxytryptamine (5-HT) specifically and reversibly inhibits nicotine-induced currents and catecholamine release in bovine adrenal chromaffin cells in culture. Pharmacological analysis indicates that the inhibition is not mediated by known 5-HT receptor subtypes. The inhibition is noncompetitive over a range of nicotine concentrations between 1 and 100 microM. Preincubation with either 5-HT or substance P significantly protects the response from nicotine-induced desensitization. It is concluded that 5-HT inhibits nicotinic acetylcholine receptors on bovine adrenal chromaffin cells, probably by binding to a noncompetitive site on the receptor itself. Because both blood and the chromaffin cells contain 5-HT, the inhibition provides an opportunity for negative control of catecholamine secretion from the adrenals.

Temperature sensitivity of neurons in slices of the rat PO/AH hypothalamic area: effect of calcium

The temperature sensitivity of spontaneously active neurons in slices of the preoptic and anterior hypothalamic areas (PO/AH) of rats was assessed by extracellular recording during sinoidal temperature displacements. Classifying cells according to their temperature coefficients (TC, impulses.s-1.degree C-1) revealed that 48% of the 177 cells tested were warm sensitive, 5% were cold sensitive, and 47% were temperature insensitive. Increasing the calcium concentration ([Ca2+]) of the artificial cerebrospinal fluid (aCSF) from 0.9 to 1.5 mM Ca2+ decreased the firing rate (FR) and TC of PO/AH neurons. Decreasing extracellular [Ca2+], however, elicited concentration-dependent increases in both parameters. This means in physiological terms that the number and temperature sensitivity of PO/AH neurons is strongly dependent on the extracellular [Ca2+]. Furthermore, these results suggest that caution is warranted when comparing numbers and temperature sensitivities of PO/AH neurons. After blocking synaptic transmission by superfusing the slice with aCSF containing reduced [Ca2+] and elevated [Mg2+], 8 out of 35 neurons ceased their spontaneous activity within 6 min, and 27 remained spontaneously active in low [Ca2+]-high [Mg2+] solution, but displayed unstable spontaneous activity and temperature sensitivity. It is concluded that aCSF with reduced [Ca2+] and elevated [Mg2+] is not a suitable tool to discriminate between inherent and synaptically mediated thermosensitivity.

Temperature sensitivity of neurons in slices of the rat PO/AH area: effect of bombesin and substance P

The effects of bombesin (Bom) and substance P (SP) were investigated in 156 temperature-sensitive and -insensitive neurons in slices of the preoptic and anterior hypothalamic area (PO/AH) of rats. Application of Bom increased the firing rate (FR) in 68% (n = 38) of the warm-sensitive and in 62% (n = 39) of the temperature-insensitive neurons. One cold-sensitive neuron was excited; a second was not affected by the peptide. No neuron decreased its activity after Bom application. SP excited 80% (n = 15) of the warm-sensitive neurons and 48% (n = 29) of the temperature-insensitive neurons. Two cold-sensitive neurons were inhibited by SP, a third one was not affected. The opposite effect on thermoregulation in vivo caused by the two peptides cannot be explained simply by their relatively similar excitatory effects on the FR of PO/AH neurons. After Bom application the temperature coefficient (TC) was significantly elevated in 7 out of 11 warm-sensitive neurons and in 19 out of 21 temperature-insensitive neurons. After SP application the TC was significantly reduced in 6 out of 7 warm-sensitive and 1 out of 12 temperature-insensitive neurons. Bom caused grouped discharges (bursts) in 7 out of 42 PO/AH neurons; SP never produced bursts in the discharge pattern. The increase of the TC of warm-sensitive and the transformation of temperature-insensitive into warm-sensitive neurons by Bom might be regarded as the neurophysiological basis for the decreased body temperature after Bom application. It is concluded that the temperature sensitivity of PO/AH neurons is not an unchangeable inherent property of certain cells but may be altered or even evoked by physiological processes like the release of neuromodulators.