Ulrich Knigge

Serotonin receptors involved in vasopressin and oxytocin secretion

Serotonin (5‐HT), 5‐HT agonists, the 5‐HT precursor 5‐hydroxytryptophan, 5‐HT‐releasers and ‐reuptake inhibitors stimulate the release of vasopressin and oxytocin. We investigated the involvement of 5‐HT receptors in the serotonergic regulation of vasopressin and oxytocin secretion. Vasopressin and oxytocin secretion was stimulated by 5‐HT, the 5‐HT1A+1B+5A+7 agonist 5‐carboxamidotryptamine (5‐CT), the 5‐HT2A+2C agonist DOI, the 5‐HT2C+2A agonist mCPP, the 5‐HT2C agonist MK‐212, the 5‐HT3 agonist SR 57277 and the 5‐HT4 agonist RS 67506. The 5‐HT1A agonist 8‐OH‐DPAT, which had no effect on vasopressin secretion, stimulated oxytocin secretion. The 5‐HT‐induced release of vasopressin and oxytocin was inhibited by central infusion of the 5‐HT antagonists WAY 100635 (5‐HT1A), LY 53857 (5‐HT2A+2C), ICS 205‐930 (5‐HT3+4) and RS 23597 (5‐HT4). The 5‐HT2+6+7 antagonist metergoline in combination with the 5‐HT1A+2+7 antagonist methysergide inhibited the stimulatory effect of 5‐CT on both hormones, whereas the 5‐HT1A+1B antagonist cyanopindolol only inhibited the oxytocin response. The 5‐HT2A antagonist 4‐(4‐flourobenzoyl)‐1‐(4‐phenylbutyl)‐piperidine oxalate had no effect on DOI‐induced hormone response. The 5‐HT2C antagonist Y 25130 partly inhibited the stimulating effect of MK‐212. ICS 205‐930 and RS 23597 inhibited vasopressin and oxytocin secretion induced by RS 67506. WAY 100635 inhibited 8‐OH‐DPAT‐induced oxytocin secretion. We conclude that 5‐HT‐induced vasopressin secretion primarily is mediated via 5‐HT2C, 5‐HT4 and 5‐HT7 receptors, whereas 5‐HT2A, 5‐HT3 and 5‐HT5A receptors seem to be of minor importance. 5‐HT‐induced oxytocin secretion involves 5‐HT1A, 5‐HT2C and 5‐HT4 receptors; in addition an involvement of 5‐HT1B, 5‐HT5A and 5‐HT7 receptors seems likely, whereas 5‐HT2A and 5‐HT3 receptors seem to be less important.

Nordic guidelines 2014 for diagnosis and treatment of gastroenteropancreatic neuroendocrine neoplasms.

Abstract Background. The diagnostic work-up and treatment of patients with neuroendocrine neoplasms (NENs) has undergone major recent advances and new methods are currently introduced into the clinic. An update of the WHO classification has resulted in a new nomenclature dividing NENs into neuroendocrine tumours (NETs) including G1 (Ki67 index ≤ 2%) and G2 (Ki67 index 3–20%) tumours and neuroendocrine carcinomas (NECs) with Ki67 index > 20%, G3. Aim. These Nordic guidelines summarise the Nordic Neuroendocrine Tumour Groups current view on how to diagnose and treat NEN-patients and are meant to be useful in the daily practice for clinicians handling these patients.

Serotonin Stimulates Hypothalamic mRNA Expression and Local Release of Neurohypophysial Peptides

The neurotransmitter serotonin (5‐HT) stimulates the secretion of vasopressin and oxytocin, and 5‐HT is involved in the mediation of the vasopressin and oxytocin response to stress. In male Wistar rats, we investigated the 5‐HT receptors involved in the 5‐HT‐induced increase of mRNA expression of vasopressin and oxytocin in the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). The 5‐HT precursor, 5‐hydroxytryptophan, injected in combination with the 5‐HT reuptake inhibitor, fluoxetine, increased oxytocin mRNA expression in the PVN, and the concentration of vasopressin and oxytocin in plasma, whereas mRNA in the SON was not affected. Intracerebroventricular infusion of 5‐HT agonists selective for the 5‐HT1A, 5‐HT1B, 5‐HT2A and 5‐HT2C receptor increased oxytocin mRNA in the SON and PVN. Infusion of agonists selective for the 5‐HT2A + 2C receptor increased vasopressin mRNA in the PVN, whereas none of the 5‐HT agonists affected vasopressin mRNA in the SON. All the 5‐HT agonists infused increased peripheral oxytocin concentration and vasopressin was increased by stimulation of the 5‐HT2A, 5‐HT2C and 5‐HT3 receptor. Intracerebroventricular infusion of 100 nmol 5‐HT increased the extracellular hypothalamic concentration of vasopressin as measured by microdialysis in the PVN. To evaluate the involvement of hypothalamic‐pituitary system in the 5‐hydroxytryptophan and fluoxetine‐induced vasopressin secretion, rats were immunoneutralized with a specific anti‐corticotropin‐releasing hormone antiserum. This treatment reduced plasma vasopressin and oxytocin responses. We conclude that stimulation with 5‐hydroxytryptophan or 5‐HT agonists increases mRNA expression of oxytocin in the PVN and the SON via stimulation of at least 5‐HT1A, 5‐HT1B, 5‐HT2A and 5‐HT2C receptors. Vasopressin mRNA in the PVN was increased only via the 5‐HT2 receptor, whereas vasopressin mRNA in the SON does not seem to be affected by 5‐HT stimulation. Corticotropin‐releasing hormone appears to be partly involved in the mediation of 5‐HT induced vasopressin and oxytocin secretion.

Serotonergic stimulation of corticotropin-releasing hormone and pro-opiomelanocortin gene expression.

The neurotransmitter serotonin (5‐HT) stimulates adrenocorticotropic hormone (ACTH) secretion from the anterior pituitary gland via activation of central 5‐HT1 and 5‐HT2 receptors. The effect of 5‐HT is predominantly indirect and may be mediated via release of hypothalamic corticotropin‐releasing hormone (CRH). We therefore investigated the possible involvement of CRH in the serotonergic stimulation of ACTH secretion in male rats. Increased neuronal 5‐HT content induced by systemic administration of the precursor 5‐hydroxytryptophan (5‐HTP) in combination with the 5‐HT reuptake inhibitor fluoxetine raised CRH mRNA expression in the paraventricular nucleus (PVN) by 64%, increased pro‐opiomelanocortin (POMC) mRNA in the anterior pituitary lobe by 17% and stimulated ACTH secretion five‐fold. Central administration of 5‐HT agonists specific to 5‐HT1A, 5‐HT1B, 5‐HT2A or 5‐HT2C receptors increased CRH mRNA in the PVN by 15–50%, POMC mRNA in the anterior pituitary by 15–27% and ACTH secretion three‐ to five‐fold, whereas a specific 5‐HT3 agonist had no effect. Systemic administration of a specific anti‐CRH antiserum inhibited the ACTH response to 5‐HTP and fluoxetine and prevented the 5‐HTP and fluoxetine‐induced POMC mRNA response in the anterior pituitary lobe. Central or systemic infusion of 5‐HT increased ACTH secretion seven‐ and eight‐fold, respectively. Systemic pretreatment with the anti‐CRH antiserum reduced the ACTH responses to 5‐HT by 80% and 64%, respectively. It is concluded that 5‐HT via activation of 5‐HT1A, 5‐HT2A, 5‐HT2C and possibly also 5‐HT1B receptors increases the synthesis of CRH in the PVN and POMC in the anterior pituitary lobe, which results in increased ACTH secretion. Furthermore, the results indicate that CRH is an important mediator of the ACTH response to 5‐HT.

Multidetector computed tomography and neuroendocrine pancreaticoduodenal tumors

Purpose: To investigate the accuracy of dedicated pancreatic multidetector computed tomography (MDCT) in the diagnosis of neuroendocrine pancreaticoduodenal tumors (NPTs). Material and Methods: MDCT and other imaging studies in patients with suspected NPTs were identified. Thirty dedicated MDCT studies were done in 23 patients. Fourteen patients (16 operations) subsequently had surgery. Imaging reports were reviewed and findings compared with surgical findings and findings in other imaging studies. Results: Patients with surgery: 19 NPTs (16 extrapancreatic gastrinomas and 3 pancreatic NPTs) were identified at surgery. MDCT identified 16 and somatostatin receptor scintigraphy (SRS) 11 out of 19 tumors. Endoscopic ultrasound detected 11 out of 14 NPTs. Patients without surgery: In 4 out of 9 patients, no NPTs were identified at MDCT. Conclusion: Dedicated MDCT of the pancreas can identify many NPTs, including small duodenal and periduodenal tumors, and the detection rate is better than reported in the older literature on CT.

Involvement of Histamine in Suckling-Induced Release of Oxytocin, Prolactin and Adrenocorticotropin in Lactating Rats

We have previously shown that histaminergic neurons participate in mediation of the prolactin (PRL), adrenocorticotropin (ACTH) and oxytocin responses to physiological stimuli such as stress and dehydration. Since suckling is a potent stimulus for the secretion of these three hormones, we investigated the mediating role of neuronal histamine in suckling-induced release of oxytocin, PRL and ACTH in conscious lactating rats. The animals were pretreated with the histamine synthesis inhibitor alpha-fluoromethylhistidine, the H1 receptor antagonist mepyramine, the H2 receptor antagonist cimetidine or the H3 receptor agonist R (alpha) methylhistamine, which by binding to H3 autoreceptors inhibits histamine release and synthesis. After the lactating rats were separated from their pups for 240 min, the pups were returned for a suckling period of 20 min. Thereafter the mothers were sacrificed by decapitation and trunk blood was collected for determination of hormones. Lactating rats not exposed to suckling served as controls. Suckling increased oxytocin 2-fold, PRL 50-fold and ACTH 5-fold. Blockade of histamine synthesis by alpha-fluoromethylhistidine or histamine release by R(alpha)methylhistamine reduced the suckling-induced secretion of the three hormones significantly. Blockade of postsynaptic H1 receptors by mepyramine inhibited the hormone responses to suckling, while the blockade of postsynaptic H2 receptors by cimetidine decreased the suckling-induced oxytocin and PRL release but did not affect the ACTH release. None of the compounds affected oxytocin, PRL or ACTH secretion in lactating mothers not exposed to suckling. In addition, suckling significantly increased the mRNA of the histamine synthesizing enzyme histidine decarboxylase in the ventrolateral tuberomammillary nucleus by 1.5-fold, indicating that the stimulus of suckling enhances the neuronal histamine synthesis. We conclude that suckling increases neuronal histamine synthesis and that histaminergic neurons by activation of postsynaptic H1 and H2 receptors are involved in the hypothalamic mediation of oxytocin, PRL and ACTH responses to suckling. These findings further substantiate a role of neuronal histamine in the neuroendocrine regulation of pituitary hormones in response to physiological stimuli.

Histamine H1 and H2 Receptor Activation Stimulates ACTH and β-Endorphin Secretion by Increasing Corticotropin-Releasing Hormone in the Hypophyseal Portal Blood

Histamine (HA) stimulates the release of adrenocorticotropic hormone (ACTH) and beta-endorphin (beta-END) via activation of central postsynaptic H1 or H2 receptors. The effect of HA is indirect and may involve the hypothalamic regulating factors corticotropin-releasing hormone (CRH), arginine vasopressin, or oxytocin (OT). We studied the effect of specific HA H1 or H2 receptor agonists on the concentration of CRH and OT in hypophyseal portal blood in urethane-anesthetized male rats. In addition we investigated the effect of the agonists on ACTH and beta-END immunoreactivity in peripheral plasma in conscious male rats pretreated with antiserum to CRH. Intracerebroventricular administration of the H1 receptor agonist 2-thiazolylethylamine (2-TEA) or the H2 receptor agonist 4-methylhistamine (4-MeHA) increased the CRH concentration in pituitary portal blood by 80-90% when compared to preinfusion levels (p < 0.05). Central infusion of saline had no effect. The level of OT in the pituitary portal blood was not affected by 2-TEA or 4-MeHA when compared to saline-treated rats. Intracerebroventricular infusion of 2-TEA or 4-MeHA increased the ACTH concentration in peripheral plasma 3- or 4-fold, respectively (p < 0.01). Pretreatment with a specific CRH antiserum (abCRH) inhibited the responses by 50 and 70%, respectively (p < 0.01). Intracerebroventricular administration of 2-TEA or 4-MeHA increased the beta-END immunoreactivity in peripheral plasma 3- or 2-fold, respectively (p < 0.01). These effects were inhibited by 80-90%, when rats were pretreated with abCRH (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative Gene Expression of Somatostatin Receptors and Noradrenaline Transporter Underlying Scintigraphic Results in Patients with Neuroendocrine Tumors

Aim: To measure, by a quantitative approach, the gene expression underlying the results of somatostatin receptor (sst) scintigraphy (111In-DTPA-octreotide) and noradrenaline transporter (NAT) scintigraphy (123I-MIBG) in patients with neuroendocrine (NE) tumors. Methods: The gene expression of somatostatin receptors 1–5 (sst) and NAT was measured quantitatively by real-time PCR in a group of patients with NE tumors (n = 14) and compared to a group of patients with colorectal adenocarcinomas (n = 15). If available, scintigraphic results were compared with gene expression results (9 octreotide and 3 MIBG scintigraphies). Results: The sst2 was upregulated in 13 of 14 patients (93%) with NE tumors, and the absolute level of gene expression was highest for sst2. Gene expression alterations of NAT and the other sst subtypes were more variable. Gene expression of sst2 was in all cases in agreement with positive octreotide scintigraphies. In 2 of 3 cases where MIBG scintigraphy was positive, NAT was also upregulated. Sst2 was generally downregulated in the colorectal tumor group with the gene expression of the other receptors being more heterogeneous. Conclusions: In general, changes in gene expression of sst2 corresponded with scintigraphic results. Our data support that sst2 is the best target for visualization of NE tumors, whereas NAT is only a useful target in a subpopulation of NE tumors. Comparison of scintigraphic results with quantitative gene expression may be used to achieve a better understanding of the link between them, which in turn could aid in planning and development of noninvasive molecular imaging of key molecular processes.

Role of hypothalamic histaminergic neurons in mediation of ACTH and beta-endorphin responses to LPS endotoxin in vivo

The involvement of hypothalamic histaminergic neurons in the mediation of the ACTH and beta-endorphin (beta-END) response to lipopolysaccharide (LPS) endotoxin was investigated in conscious male rats. LPS stimulated the release of ACTH and beta-END dose-dependently and increased the hypothalamic concentration of the histamine (HA) metabolite tele-methylhistamine significantly and that of HA slightly, indicating an increased turnover of neuronal HA. Pretreatment with the HA synthesis inhibitor alpha-fluoromethyl-histidine administered intracerebroventricularly (i.c.v.) or intraperitoneally (i.p.) inhibited the ACTH and beta-END response to LPS about 60%, whereas i.p. administration of the H3 receptor agonist R(alpha)methylHA, which inhibits HA synthesis and release, decreased the response about 50%. Pretreatment with the H1 receptor antagonist mepyramine (67 micrograms x 2 i.c.v.) inhibited the hormone response to LPS about 50%, while pretreatment with equimolar doses of the H2 receptor antagonists cimetidine (67 micrograms x 2 i.c.v.) or ranitidine (83 micrograms x 2 i.c.v.) had no effect on the LPS-induced release of ACTH and beta-END. When the H1 receptor antagonists mepyramine and cetirizine were administered i.p. in doses (10 mg/kg) which penetrate the blood-brain barrier the hormone response to LPS was inhibited 50% and 30%, respectively. Administered i.p. the H2 receptor antagonists had no effect on the hormone response to LPS. We conclude that hypothalamic histaminergic neurons in rats are involved in the mediation of the ACTH and beta-END response to LPS stimulation via activation of central postsynaptic H1 receptors.

Histaminergic Neurons Are Involved in the Orexigenic Effect of Orexin-A

Orexin-A is an orexigenic peptide expressed mainly in the hypothalamus. Orexin-A increases and anti-orexin-A antibodies decrease food intake. However, the exact mechanism by which orexin-A exerts its orexigenic action is not fully elucidated. The histaminergic system is known to play a role in feeding behavior and we hypothesized that it could be involved in the orexigenic effect of orexin-A. To study this, we used histamine knockout animals and pharmacological blockade of the histaminergic system and studied the effect of orexin-A on feeding behavior and gene expression of neuropeptide Y (NPY). Orexin-A was administered intracerebroventricularly and food intake measured in wild-type, histamine H1-receptor knockout or histidine decarboxylase knockout mice. Additionally, we administered orexin-A to wild-type mice with pharmacologically blocked H1-receptors or pharmacologically stimulated autoinhibitory H3-receptors. By quantitative real-time PCR we measured the effect of orexin-A on NPY mRNA expression in wild-type and knockout mice. Orexin-A dose-dependently stimulated food intake when administered to wild-type mice in doses up to 0.03 µg. Orexin-A in a dose of 0.01 µg increased food intake 10-fold in wild-type mice, whereas no increase in food intake was seen in either knockout mice or pharmacologically manipulated mice. Orexin-A increased NPY mRNA 4-fold in wild-type mice, whereas no change was observed in knockout mice. We conclude that the orexigenic effect of orexin-A is dependent on an intact histaminergic neuronal system and seems to involve an H1-receptor mechanism.