Calcitonin inhibition of physiological and stimulated prolactin secretion in rats

A synthetic peptide derived from mouse pituitary calcitonin cDNA sequence exhibits potent inhibition of prolactin secretion and prolactin mRNA abundance in primary mouse pituitary cells

We have shown that gonadotrophs synthesize and secrete immunoreactive calcitonin (CT)-like peptide, and CT is a potent inhibitor of prolactin (PRL) secretion and gene transcription. CT cDNA cloned from LssT2 cells (pit-CT cDNA) exhibits 99% homology with mouse CT cDNA sequence, but exhibits four mismatches in the coding region of CT peptide (347-485 bp) with consequent changes in the amino acids at positions 5 and 17 of mouse CT. We have synthesized a putative 23 amino acid pit-CT peptide based on pit-CT cDNA sequence, and tested its effect on PRL secretion and mRNA abundance in primary mouse pituitary cells. The results suggest that synthetic pit-CT attenuates PRL mRNA abundance and inhibits PRL release from mouse anterior pituitary cells. Moreover, pit-CT is remarkably more potent than salmon (S)CT in attenuating PRL mRNA abundance. These results raise a possibility that this endogenous pituitary peptide may potentially serve as a therapeutic molecule for the treatment of prolactinomas.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Calcitonin inhibition of prolactin secretion in lactating rats: mechanism of action

The effects of intracerebroventricular (10 ng/rat) or intravenous (10 or 40 microg/15 min/rat) administration of salmon calcitonin (sCT) on the prolactin (PRL) response to suckling and the activity of tyrosine hydroxylase (TH) were examined in lactating rats. Plasma concentration of PRL increased dramatically in control rats after the onset of the suckling stimulus, while administration of sCT resulted in inhibition of PRL response to suckling. The action of sCT was much more effective with intracerebroventricular administration, which totally blocked PRL release, compared to intravenous administration. The intracerebroventricular administration of sCT increased TH activity of tuberoinfundibular dopamine neuron (TIDA) in the stalk-median eminence, as measured by DOPA accumulation, while completely suppressing the PRL response to suckling. Injection of alpha-methyl-p-tyrosine (alpha-MT; 50 mg/kg), an inhibitor of TH and thus dopamine synthesis, increased PRL levels, and suckling caused a further increase in plasma concentrations of PRL. Injection of sCT (intracerebroventricularly) did not inhibit the PRL response to suckling in the presence of a depletion of dopamine. These results suggest that sCT inhibition of PRL secretion in lactating rats is mediated mainly by TIDA neurons without involvement of other neuroendocrine mechanisms.

Localization of calcitonin receptor mRNA in the mouse brain: coexistence with serotonin transporter mRNA

To elucidate the sites of and mechanisms of analgesic effect of centrally injected calcitonin, we examined expression of calcitonin receptor mRNA in the mouse brain by in situ hybridization techniques. Calcitonin receptor mRNA was expressed in various brain regions, including the preoptic area, dorsomedial hypothalamic nucleus, lateral hypothalamic area, periaqueductal gray, dorsal raphe nucleus, locus coeruleus, lateral parabrachial nucleus, gigantocellular reticular nucleus alpha part, lateral paragigantocellular nucleus, raphe magnus nucleus and solitary tract nucleus, which are known to play important roles in pain modulation. In addition, a double in situ hybridization technique demonstrated the intense expression of calcitonin receptor mRNA on serotonergic neurons in some raphe nuclei and the lateral paragigantocellular nucleus, suggesting the involvement of central serotonergic pathways in analgesic effect of calcitonin.

Effects of amylin and salmon calcitonin on beta-endorphin-induced growth hormone and prolactin secretion in the rat

In this study we examined the possible interplay of amylin (AMY) and salmon calcitonin (sCT) in the central control of growth hormone (GH) and prolactin (PRL) secretion in male rats. For this purpose we first compared effects of central intracerebroventricular (i.c.v.) admininstration of various doses of AMY (2.5-2,500 ng/rat) and sCT (2.2-220 ng/rat) on beta-endorphin (beta-END, 0.5 microg/rat)-induced GH and PRL secretion. AMY and sCT dose-dependently inhibited beta-END-induced GH secretion, whereas only sCT was able to inhibit beta-END-induced PRL secretion. To examine whether the GH inhibitory effect of AMY was due to the possible cross-reactivity of AMY and sCT on the same receptors in the CNS, we pretreated some rats with the AMY antagonist (AMY8-37, 2. 5 microg/rat, i.c.v.). AMY8-37 significantly enhanced the GH-stimulatory action of beta-END. AMY8-37, administered prior to AMY and sCT, significantly removed the inhibitory effect of both AMY and sCT on beta-END-induced GH release, suggesting that both peptides mediate their response on GH through a common receptor. In vitro competition binding studies on rat hypothalamic membranes have shown that both AMY and sCT compete with [125I]rAMY binding with half inhibition (IC50) values of 3.6 x 10(-11) and 1.6 x 10(-10) M, respectively. Binding of [125I]sCT was inhibited by sCT with an IC50 of 1.09 x 10(-10) M and to a lesser extent by AMY with an IC50 of 1. 3 x 10(-6) M. Thus it is possible that the two peptides recognize a common hypothalamic receptor but with different affinities (sCT > AMY). Overall these data indicate that AMY behaves as a mimic of sCT in the central control of GH secretion. The failure of AMY, at variance with sCT, to modify the PRL-releasing activity of beta-END indicates that different receptor subtypes for sCT are involved in the endocrine effects of sCT and only those mediating the modulatory action of GH respond to AMY.

Calcitropic peptides: neural perspectives

In mammals and higher vertebrates, calcitropic peptides are produced by peripheral endocrine glands: the parathyroid gland (PTH), thyroid or ultimobranchial gland (calcitonin) and the anterior pituitary gland (growth hormone and prolactin). These hormones are, however, also found in the neural tissues of lower vertebrates and invertebrates that lack these endocrine organs, suggesting that neural tissue may be an ancestral site of calcitropic peptide synthesis. Indeed, the demonstration of CNS receptors for these calcitropic peptides and their induction of neurological actions suggest that these hormones arose as neuropeptides. Neural and neuroendocrine roles of some of these calcitropic hormones (calcitonin and parathyroid hormone) and related peptides (calcitonin gene related peptide, stanniocalcin and parathyroid hormone related peptide) are thus the focus of this review.

Antinociceptive effects of repeated systemic injections of calcitonin in formalin-induced hyperalgesic rats

Calcitonin (CT) produces long-lasting analgesia in patients suffering from painful diseases following repeated systemic injections, but there have been only a few contradictory reports on the antinociceptive action of systemic injections of CTs in animal experiments. This study was conducted to elucidate an antinociceptive action of systemic CT in rats. An injection of dilute formalin induced hyperalgesia for about 2 h. Single topical injections of 0.12 and 1.2 U, but not 0.012 U, of [Asu1.7] eel CT (eCT) into the same site of formalin injection inhibited the hyperalgesia. Repeated systemic injections of eCT (4 and 40, but not 0.4, U kg-1 day-1) for 7 days inhibited the hyperalgesia, while the single injection was without effects at doses tested. Although the highest dose of eCT (40 U kg-1 day-1) inhibited an increase in body weight following repeated injections, lower doses (0.4 and 4 U kg-1 day-1) were without effects. The suppression of hyperalgesia following repeated systemic injections of eCT (4 U kg-1 day-1) lasted for at least 24 h, and subsided by 3 days following the last eCT injection. These results indicate that the repeated systemic injections of eCT produce a long-lasting inhibition of formalin-induced hyperalgesia in rats. This inhibitory effect is similar to CT analgesia in human subjects in terms of a necessity for repeated administration, effective dose and long-lasting effects.

Calcitonin inhibits testosterone and luteinizing hormone secretion through a mechanism involving an increase in cAMP production in rats

Effects of calcitonin peptides, including human calcitonin (hCT), salmon calcitonin (sCT), and calcitonin gene-related peptide (CGRP), on the secretion of testosterone and luteinizing hormone (LH) in male rats were studied. Male rats were injected intravenously with human chorionic gonadotropin (hCG), calcitonin peptides, or hCG plus calcitonin peptides. Blood samples were collected at several intervals following hormone challenge. In an in vitro experiment, testis blocks were incubated with hCG (0, 0.05, 0.5, or 5 IU/ml) or hCG (0.5 IU/ml) plus calcitonin peptides (0-10(-9) or 10(-6) M) at 34 degrees C for 30 minutes. Both medium and plasma samples were extracted by ether and analyzed for testosterone by radioimmunoassay (RIA). The concentration of calcium in each plasma sample was measured by an automatic calcium analyzer. The anterior pituitary gland (AP) was incubated with or without calcitonin peptides (0-10 nM) at 37 degrees C for 30 minutes. They were then incubated with gonadotropin releasing hormone (GnRH, 10 nM) for a further 30 minutes. The concentration of LH in AP medium was measured by RIA. The accumulation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) in both testicular tissues and APs were measured by RIA. A single intravenous injection of calcitonin peptides decreased the basal and hCG-stimulated levels of plasma testosterone gradually from 60 to 180 or 360 minutes after challenge. The plasma calcium was not altered by the injection of calcitonin peptides and/or hCG. Administration of calcitonin peptides in vitro resulted in a dose-dependent inhibition of both basal and hCG-stimulated release of testosterone.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcitonin and its antinociceptive activity: animal and human investigations 1975-1992

Calcitonin (CT) is a polypeptide hormone produced in the thyroid gland that regulates, blood calcium levels and bone calcium metabolism. The unexpected finding of binding sites for calcitonin in several areas of the brain oriented attention to activities of CT in the central nervous system and also to its antinociceptive action. The first report of this last effect was in 1975, and the many different experimental and clinical data on this topic reported since then are reviewed here. The heterogenous findings have been organized according to the logical classification of animal and human studies. For each of these headings, subheadings such as acute and chronic pain, different kinds of administration and different procedures used to record the results, are considered. The several proposed mechanisms of action, involving serotoninergic, catecholaminergic, Ca2+ fluxes, protein phosphorylation, beta-endorphin production, cyclooxygenase inhibition and histamine interference are also reviewed. Calcitonin, neurotensin, substance P, VIP and, recently, CGRP are some of the non-opioid peptides that have been reported to interfere with pain and that open up a new, alternative way of investigating antinociceptive drugs different than opioid or opioid-like agents. An examination of the state-of-investigation of calcitonin's antinociceptive activity in the last 17 years shows that many experimental studies indicate the existence of this effect, including studies in humans, and this opens up perspectives for therapy with a new class of antinociceptive agents.

Involvement of central serotonergic pathways in analgesia elicited by salmon calcitonin in the mouse

The contribution of central serotonergic pathways to the analgesic activity induced by salmon calcitonin in the writhing test was investigated. Salmon calcitonin was administered to mice after lesioning of the ascending and descending serotonergic pathways by means of i.p. administration of p-chloroamphetamine (40 mg/kg, for 2 days) or p-chlorophenylalanine (300 mg/kg, for 3 days). The analgesic effect induced by salmon calcitonin at the doses of 10 and 20 IU/kg was not evident in mice previously treated with p-chloroamphetamine or p-chlorophenylalanine. However, the analgesic effect of salmon calcitonin 40 IU/kg was not significantly modified by p-chloroamphetamine or p-chlorophenylalanine pretreatment. Salmon calcitonin did not alter the depletion of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid after p-chloroamphetamine or p-chlorophenylalanine administration. Similarly, this hormone did not change the NSD 1015-induced accumulation of 5-hydroxytryptophan or the tranylcypromine-induced accumulation of 5-HT. These results indicate that although salmon calcitonin does not influence the synthesis and metabolism of 5-HT, it does require the integrity of the serotonergic system in order to cause analgesia.

Biodistribution of calcitonin encapsulated in liposomes in mice with particular reference to the central nervous system

The biodistribution of [125I]porcine calcitonin (pCT) encapsulated in reverse-phase evaporation vesicles (REVs) in mice upon the intravenous administration was examined. It was found that sulfatide significantly improved the stability of REVs in vivo, and altered the relative distribution of [125I]pCT encapsulated in liposomes in mice. These sulfatide-containing REVs were able to target [125I]pCT into the liver and central nervous system (CNS) reasonably well, with the maximal effect of about 40% and 2% of the injected doses occurring at 30 min and 90 min, respectively, after injection. Neither free [125I]pCT, nor sulfatide-free liposome-encapsulated [125I]pCT, nor a mixture of free [125I]pCT and empty sulfatide liposomes was effective. [125I]pCT was widely distributed in the CNS, with predominance in hypothalamus, brainstem, striatum and spinal cord. The results indicate that pCT encapsulated in sulfatide-containing liposomes is able to pass through the blood-brain barrier (BBB), and calcitonin, thus encapsulated, may be applicable to studies on its functions in the CNS.

Molecular cloning of two receptors from rat brain with high affinity for salmon calcitonin

Two receptors with high affinity for salmon calcitonin were cloned from the nucleus accumbens region of rat brain. The deduced 479 amino acid sequence of cDNA clone L2175-D20 (designated C1a receptor) is 78% and 66% identical with those reported for human and porcine calcitonin receptors, respectively. Clone U3237-A2 codes for a receptor (designated C1b) that is identical to C1a except for a 37 amino acid insert in the second extracellular domain. COS-7 cells transfected with either transcript bound [125I]salmon calcitonin with high affinity (Kd = 8 pM for C1a; Kd = 48 pM for C1b) and responded to salmon calcitonin with increases in cAMP. Tissue distribution studies revealed C1a transcript in rat brain, skeletal muscle, kidney and lung, whereas C1b was predominantly found in brain.

Antinociceptive activity of salmon calcitonin: electrophysiological correlates in a rat chronic pain model

Experimental and clinical evidence testifies to an antinociceptive action of salmon calcitonin (sCT), administered in different ways, on the central nervous system. These studies were performed almost exclusively in acute pain models. The purpose of the present study was to investigate the effects of sCT, injected directly into the lateral cerebral ventriculi, on the firing of single nociceptive thalamic neurons, detected by electrophysiological techniques in an experimental model of prolonged or chronic pain, such as rats rendered arthritic by injection of Freund's adjuvant into the left hindfoot. The noxious test stimuli used were either extension or flexion of the ankle or mild lateral pressure on the heel. With increasing doses of sCT (5, 10, 20, 40 micrograms, 5 microliters/i.c.v.) it was possible to observe correspondingly increasing inhibitory and long-lasting effects on the evoked firing, with a significant dose-effect relationship. In agreement with electrophysiological findings, preliminary data, obtained with a patch clamp technique, on depression of calcium fluxes through neuronal membrane, induced by sCT, oriented the attention to a direct action of sCT on CNS.

Cells showing immunoreactivity for calcitonin or calcitonin gene-related peptide (CGRP) in the central nervous system of some invertebrates

In the central nervous system of some species of several invertebrate phyla, including land planarians (Platyhelminthes), ribbon worms (Nemertina), slugs (Mollusca), polychaetes, earthworms and leeches (Annelida), pill bugs (Arthropoda), and beard worms (Pogonophora), salmon calcitonin-immunoreactive cells and rat calcitonin gene-related peptide (CGRP)-immunoreactive cells were found by immunohistochemistry. These immunoreactive cells were located in the region surrounding the neuropile, although the sizes of the cells varied according to species. Some of them were round or polygonal and regarded as apolar nerve cells because of their lack of cytoplasmic processes, whereas others were spindle-shaped or elongated, being comparable with unipolar nerve cells because of extension of their cytoplasmic processes in the direction of the neuropile. In some cases, it was noted that the cytoplasmic processes had complicated branches or formed loop-like structures at their ends. These observations suggest that a calcitonin-like or CGRP-like substance is extensively present in invertebrates as well as vertebrates.

Bimodal action of [Asu1,7]eel-calcitonin on phosphoinositide hydrolysis in cultured anterior pituitary cells

[Asu1,7]Eel-calcitonin, a semisynthetic analog of eel-calcitonin displaying high stability and full biological activity, was used to study the effect of calcitonin on phosphoinositide turnover in cultured anterior pituitary cells. Incubation of cells with [Asu1,7]eel-calcitonin produced a slight, concentration-dependent increase in [3H]inositol monophosphate accumulation, without modifying thyrotropin-releasing hormone (TRH)-stimulated phosphoinositide hydrolysis. This effect was correlated with a stimulatory action on prolactin secretion. Conversely, a long-term preincubation with [Asu1,7]eel-calcitonin reduced basal as well as TRH-induced [3H]inositol monophosphate formation. This effect was concentration-dependent, was not due to an increase of cyclic AMP intracellular levels, and was attenuated in the presence of maximally effective concentrations of TRH. Such a long incubation in the presence of [Asu1,7]eel-calcitonin resulted in a marked inhibition of prolactin secretion. The present data confirm and extend previous findings showing an interference of calcitonin with the intracellular cascade consequent to membrane phospholipase C activation and further support a role for calcitonin in the modulation of hormone secretion at the pituitary.

Central nervous system binding sites for calcitonin and calcitonin gene-related peptide

Alternative splicing of the primary RNA transcript of the calcitonin gene leads to the generation of two distinct peptides, calcitonin (CT) and calcitonin gene-related peptide (CGRP). These peptides share only limited sequence homology and generally subserve different biological functions through their own distinct binding sites, which differ in specificity and distribution. Additionally, a binding site with high-affinity binding for both peptides that has a restricted pattern of distribution has been identified. The present article reviews the biochemical and morphological characteristics of centra CT and CGRP binding sites.

Inhibitory effects of centrally administered /ASU1-7/eel calcitonin on basal and stimulated prolactin release in rats

We investigated the effects of /ASU1-7/eel calcitonin (ASU1-7eelCT) on basal and stimulated prolactin (PRL) release in male rats. /ASU1-7/eelCT was administered intracerebroventricularly (icv) into freely moving rats with indwelling catheters. The administration of /ASU1-7/eelCT (2.5 micrograms/rat, icv) significantly inhibited basal PRL secretion. When PRL secretion was stimulated by exposing rats to restraint stress, /ASU1-7/eelCT (250 ng; 800 ng; 2.5 micrograms/rat, icv) dose-relatedly inhibited the PRL surges at 10 min after stress. The same doses of icv /ASU1-7/eelCT were effective in inhibiting morphine (6 mg/kg, intracarotid, ia-induced PRL release. No effect on stress-induced PRL secretion was observed when the peptide was administered intracarotid at the dose of 10 micrograms/rat. These results demonstrate that /ASU1-7/eelCT, as we previously observed with salmon calcitonin (sCT), has central inhibitory activity on PRL secretion, probably through enhancement of hypothalamic inhibitory pathways involved in the control of PRL.

Hypoprolactinemic effect of calcitonin gene-related peptide in the rat

The effect of calcitonin gene-related peptide (CGRP) on prolactin (PRL) secretion was studied in female rats. Prepubertal female rats were submitted to the stressful stimuli of injection, blood puncture and thermal stress. Lactating rats were exposed to suckling stimulus. The effects of CGRP on PRL release were compared to those of calcitonin (CT). CGRP i.p. or s.c. prevents the increase in circulating PRL induced by stress. This effect was observed two hrs and even 24 hrs after CGRP administration. It was elicited at all doses tested, 2.5, 1.25 and 0.6 micrograms per animal. The time course of the actions of CGRP and CT are different. It is proposed that CGRP and CT act on different receptors.

The calcitonin gene peptides: biology and clinical relevance

The calcitonin/CGRP multigene complex encodes a family of peptides: calcitonin, its C-terminal flanking peptide, katacalcin, and a third novel peptide, calcitonin gene-related peptide (CGRP). The 32-amino acid peptide calcitonin inhibits the osteoclast, thereby conserving skeletal mass during periods of potential calcium lack, such as pregnancy, growth, and lactation. This hormonal role is emphasized by observations that lower circulating calcitonin levels are associated with bone loss and that calcitonin replacement prevents further bone loss. Structurally, CGRP resembles calcitonin and has been implicated in neuromodulation and in the physiological regulation of blood flow. Here we review the molecular genetics, structure, and function of the calcitonin-gene peptides as analyzed in the laboratory and focus on more recent clinical studies relating to disorders and therapeutics.

Calcitonin suppresses growth hormone (GH) response to growth hormone-releasing hormone (GHRH) in man

Calcitonin (CT) receptors have been found in the hypothalamus, suggesting a neuroendocrine role for this peptide. We have recently shown that, in the rat, central administration of salmon calcitonin (sCT) suppresses basal and GHRH-stimulated GH secretion. To further investigate how sCT alters GH secretion, we studied the effects of sCT (100U MRC, im) or placebo on basal and GHRH (50 micrograms, iv)-stimulated GH secretion in 6 normal men. GHRH was administered 1 h after sCT injection. Basal GH levels were not altered by sCT administration. However, GH response to GHRH was markedly suppressed by sCT (area under the curve - sCT: 574.6 +/- 69.7 vs placebo: 1057.2 +/- 284.8 micrograms. min/L; p less than 0.02). Cortisol levels were higher in sCT-treated subjects compared to controls, from 45 to 105 min after sCT injection (p less than 0.05). However, no correlation was found between GH response to GHRH and cortisol levels. No changes in glucose, calcium and PTH levels were seen. These results demonstrate that sCT inhibits GHRH-induced GH secretion in man by a mechanism apparently independent of changes in peripheral cortisol, glucose, calcium and PTH levels.

Pituitary cells secrete calcitonin in the reverse hemolytic plaque assay

Rat pituitary cells were evaluated in the reverse hemolytic plaque assay for calcitonin (CT) secretion. The secretion of CT could be demonstrated by the formation of hemolytic plaques around single pituitary cells when a specific CT antibody was used. Approximately 0.1 percent of the cells secreted CT in the basal state. Phorbol stimulated CT secretion by up to 25-fold. The diameter of the hemolytic plaques around pituitary cells from genetically obese (Zucker) rats was significantly greater than normal rats (24 versus 37 microns). This study demonstrates that pituitary cells secrete CT and that the secretion may be regulated by pharmacological agents (phorbol) and physiological signals (obesity).

Anatomical mapping of the rat hypothalamus for calcitonin-induced anorexia

The primary physiological function of calcitonin, a peptide hormone secreted by the thyroid gland, is to modulate plasma calcium concentrations. Calcitonin also has several effects on the central nervous system including an inhibition of feeding behavior. In the present study synthetic salmon calcitonin (15 ng in 0.3 microliter) was found to produce a marked suppression of eating when infused in several hypothalamic areas. The greatest inhibition was produced by infusions into the paraventricular nucleus of the hypothalamus, the perifornical area and several areas on the floor of the hypothalamus. A less marked inhibition of eating was produced by infusions in the nucleus accumbens. Infusions in the olfactory tubercule, the ventrolateral hypothalamus, the medial forebrain bundle and the posterior nucleus of the hypothalamus had no effect. It is concluded that the anorectic effects of calcitonin on the central nervous system are mediated by several hypothalamic and extrahypothalamic sites.

Calcitonin as a feeding suppressant: localization of central action to the cerebral III ventricle

Calcitonin suppresses food and water intake. To further study this effect of calcitonin, rats were subjected to various intra-cerebroventricular (ICV) applications of calcitonin. The results show: (1) Intra-third ventricular (III-ICV) infusion of calcitonin dose-dependently decreased food intake with short- and long-term effects; (2) Potency was decreased by using non-siliconized materials; (3) Potency decreased with age of rats; (4) Infusion into the aqueduct and cisterna magna decreased short- and long-term food intake less than III-ICV administration; (5) Aqueduct obstruction did not affect feeding suppression by III-ICV calcitonin. Aqueduct obstruction did not affect dipsogenic response to III-ICV infusion of angiotensin II; (6) Results of water intake and food to water intake ratios suggest a greater calcitonin effect on food intake than on water intake. The evidence suggests that the hypothalamus is a main locus for suppression of food intake by ICV administered calcitonin.

Increased forearm bone mineral content after bromocriptine treatment in hyperprolactinemia

We studied 15 hyperprolactinemic women to evaluate possible modifications of bone mineral content after pharmacological treatment. Patients received a dopamine agonist (bromocriptine) for six months after which there was a significant decrease of prolactin plasma levels (P less than 0.01) and a significant increase of bone mineral content (P less than 0.05).

Immunoreactive calcitonin in brain regions and pituitary of sheep

In this study we have investigated the presence of immunoreactive calcitonin in the central nervous system and pituitary of sheep. The calcitonin concentrations were determined radioimmunologically by two different antibodies. We have demonstrated calcitonin in extracts of areas of the central nervous system, whole pituitary, thyroid gland and plasma of 21 sheep. The concentrations were (ng/g wet weight, mean values +/- SE): thyroid 16.0 +/- 4.4, pituitary 2.03 +/- 0.34, reticular formation 1.64 +/- 0.25, substantia nigra 1.53 +/- 0.46, dentate nucleus 1.11 +/- 0.27, putamen 1.05 +/- 0.35, hippocampus 0.97 +/- 0.17, fornix 0.96 +/- 0.15, anterior thalamus 0.92 +/- 0.28, mammillary body 0.88 +/- 0.12, cerebellum 0.86 +/- 0.09, caudate nucleus 0.84 +/- 0.11, posterior hypothalamus 0.83 +/- 0.19, epiphysis 0.75 +/- 0.25, thalamus centralis 0.71 +/- 0.10, almond nucleus 0.69 +/- 0.16, medulla oblongata 0.67 +/- 0.15, anterior hypothalamus 0.66 +/- 0.20, precentral gyrus 0.66 +/- 0.16, globus pallidus 0.63 +/- 0.31, postcentral gyrus 0.36 +/- 0.08 and plasma (ng/ml) 0.058 +/- 0.013. Our results demonstrate that immunoreactive calcitonin is present in the central nervous system (CNS) of sheep, compatible with a neurotransmitter function for this hormone.

Effect of (Asu1,7)E-CT, synthetic analogue of eel-calcitonin, on nociceptive transmission

The effect of (Asu1,7)E-CT a deaminodicarba-analogue of the synthetic eel-calcitonin on the nociceptive transmission has been studied in mice. The analogue was intracerebroventricularly (0.0.5-0.01-0.02 U.I./kg) or intravenously (0.02-0.04-0.05 -0.1 U.I./kg) injected. This synthetic derivative of eel-calcitonin increased the antinociceptive effect also after peripheral administration. Moreover, preliminary studies on the time-course of this analogue showed that the dose of 0.1 U.I./kg i.v. injected, was able to elicit antinociceptive effect, already 5 min after the administration.

Calcitonin receptors in the rat mesencephalon mediate its analgesic actions: autoradiographic and behavioral analyses

Autoradiographic analyses of salmon calcitonin (sCT) binding in the rat mesencephalon revealed an exceptionally high concentration of receptors in the ventral and ventrolateral segments of the periaqueductal gray matter (PAG) extending along the entire rostral-caudal axis. Relatively heavy labeling was also seen along a band extending ventrolaterally through the mesencephalic reticular formation. Other receptor-rich areas include the nucleus linearis, pars compacta and lateralis of the substantia nigra, locus coeruleus, parabrachial nuclei and nucleus raphe pontis of the pontine reticular formation. Injections of sCT into the PAG induced a dose-dependent increase in hot-plate latencies. All rostral-caudal levels of these brain regions appeared to be equally responsive. Injections into the midline pontine reticular formation were also effective in increasing response latencies. Unilateral injections into the hypothalamus, medial thalamus, ventral thalamus and mesencephalic reticular formation proved to be ineffective. Human calcitonin (hCT) was considerably less potent. These biological effects are consistent with the potencies of both peptides in displacing 125I-sCT from slide-mounted sections of rat PAG. Naloxone failed to antagonize sCT-induced analgesia, suggesting an opiate independent mechanism for this peptide in eliciting analgesia.

Calcitonin inhibits the phosphorylation of various proteins in rat brain synaptic membranes

The present report examines the effect of different calcitonins and analogs on the in vitro phosphorylation of brain synaptic membrane proteins. The findings demonstrate that calcitonin is a potent inhibitor of several brain synaptic proteins and that salmon and eel calcitonins are considerably more potent than thyrocalcitonin in eliciting this effect. Deletion of leucine from position 16 in salmon calcitonin sequence resulted in a drastic loss of inhibitory activity, indicating the importance for a hydrophobic residue at position 16 in the intact calcitonin molecule. The mechanism of calcitonin inhibition of protein phosphorylation was likely due to the blockade of stimulation of protein kinases by calmodulin.

Calcitonin and calcitonin gene-related peptide alter the excitability of neurons in rat forebrain

Individual neurons in the hypothalamus, thalamus, cortex, and other forebrain areas of urethane-anesthetized, male rats were iontophoretically tested for their membrane sensitivity to salmon calcitonin (CT), human CT, and CT gene-related peptide (CGRP). Extracellular recording of unit activity revealed that depression of neuronal firing was the predominant effect of iontophoretically applied salmon CT (35 of 74 cells tested). Few neurons responded to salmon CT with an increase in firing rate (N = 3). When CGRP was iontophoretically applied a pattern of response resembling that of salmon CT was observed. CGRP was predominantly inhibitory and excited those neurons whose firing rate was increased by salmon CT. Inhibition was also the predominant effect of human CT. However, no neurons were excited by human CT. The results clearly demonstrate that a subpopulation of neurons with membrane sensitivity to salmon CT, human CT, and CGRP are present in the rat forebrain. This finding suggests that modulation of neuronal activity may underlie the behavioral and biochemical effects of these peptides when administered centrally. Endogenous CGRP and CT-like peptides in rat brain may be capable of regulating these events as neurotransmitters or neuromodulators.

The analgesic activity of calcitonin and the central serotonergic system

The effect of peripherally administered cyproheptadine or reserpine and the administration of 5,7-dihydroxytryptamine (5,7-DHT) in the nucleus raphe dorsalis on the analgesic activity of salmon calcitonin (sCT) injected into the lateral ventricle were investigated in male rats. Cyproheptadine or reserpine, given respectively 30 min or 24 h before the peptide, completely abolished the analgesic activity at all the times studied. However, when reserpine was given before the peptide it increased the effect of sCT at 30 (P less than 0.01), 60 (P less than 0.001), 120 (P less than 0.01) and 180 (P less than 0.01) min. 5,7-DHT injected in the nucleus raphe dorsalis 15 days before the peptide led to complete abolition of the analgesic activity. If neurotoxin was injected 4 days before sCT, the effect of the peptide was significant (P less than 0.05) only at 60 min. The results obtained confirm that the analgesic activity of sCT may involve central serotonergic pathway(s), and that the midbrain raphe nuclei 5-HT content is an important focus for this activity.

Pentagastrin promotes prolactin release in patients with medullary carcinoma of the thyroid

Both calcitonin and gastrin have been found in the mammalian central nervous system, including the pituitary. Following a pentagastrin infusion in several patients with medullary carcinoma of the thyroid, we noted a coincident increase in plasma calcitonin and prolactin (PRL) levels. In order to evaluate further the influence of pentagastrin on human PRL release, a pentagastrin infusion was administered to 13 patients with active medullary carcinoma of the thyroid (MTC), eight subjects with inactive MTC, eight family members without MTC, and ten normal subjects. Plasma mean +/- SE PRL levels were significantly (P less than 0.01) increased in the active MTC patients from 7.6 +/- 0.5 to 12 +/- 1.4 ng/mL by 15 minutes post pentagastrin. Plasma mean +/- SE calcitonin levels increased in parallel with the plasma PRL levels from 0.28 +/- 0.1 to a peak of 1.9 +/- 0.9 ng/mL at 5 minutes post pentagastrin. A significant (P less than 0.05) correlation was found between the percentage increase in plasma calcitonin concentrations and plasma PRL levels at five and ten minutes post pentagastrin stimulation in this group of active MTC patients. Significant increases in serum calcitonin levels in the other groups post pentagastrin were of lesser magnitude and were not associated with a significant increase in PRL release. This latter observation suggested that neither the stress of the infusion nor the multiple endocrine neoplasia type 2 nor the pentagastrin was responsible for the observed increase in plasma PRL levels in the active MTC patients. These findings suggest, but do not prove, that calcitonin is a PRL-releasing factor in humans.

The effect of calcitonin on prolactin secretion during gestation

The relationship between calcitonin (CT) and prolactin (PRL) was studied by means of the injection of salmon calcitonin (SCT) i.p. on day 1 of gestation. An estrogen inhibitor - clomiphene - was also administered to certain groups of animals on day 4 and 5 of gestation. SCT did not affect PRL levels on day 1 of gestation nor on days 5 or 7, but it prevented the rise of PRL levels observed in animals submitted to injection stress on days 4 and 5. In animals treated with clomiphene, the inhibition by SCT on PRL increase after injection stress was partially abolished. SCT while not affecting basal PRL level prevented the rise observed after stress and this effect occurred some days later. Thus SCT could exercise a delayed neuroendocrine control. This action of SCT seemed to be partially dependent upon the presence of estrogens.

1,25-Dihydroxyvitamin D3 normalizes maternal food consumption and pup growth in rats

Maternal food consumption, maternal body weight loss, and pup growth were studied in the following six groups of rats: vitamin D-deficient, vitamin D3-replete, vitamin D3-replete but pair-fed with the vitamin D-deficient rats and rats given either 50, 150, or 450 pmol/day of 1,25-dihydroxyvitamin D3 as their sole source of vitamin D by continuous infusion from an Alzet osmotic minipump. As expected, vitamin D-deficient rats were hypocalcemic and lost body weight, and their pups stopped growing at 1 wk of age. Food consumption by the vitamin D-deficient rats was one-third that of the vitamin D3-replete rats. Although normalization of plasma calcium levels was not perfect, 1,25-dihydroxyvitamin D3 treatment led to normal maternal food consumption, prevented maternal body weight loss, and promoted normal pup growth. Pups from the vitamin D3-replete rats pair-fed with the vitamin D-deficient rats did not grow properly and their dams lost body weight. These data indicate that 1,25-dihydroxyvitamin D3 is fully capable of replacing vitamin D3 in promoting maternal food consumption in lactating rats and that maintaining adequate food consumption is a major factor in the stimulatory effect of vitamin D3 on pup growth and hence milk production. The anorexia and reduced milk production of vitamin D-deficient lactating rats did not result from changes in plasma glucose or triglyceride levels.

Localization of calcitonin binding sites in rat central nervous system: evidence of its neuroactivity

The distribution of calcitonin (CT) binding sites in serial sections of the rat brain and spinal cord has been examined by an 'in vitro' autoradiographic technique using a radioisotope-sensitive sheet film and [125]salmon CT. Autoradiograms of the diencephalic region had the highest grain density throughout the entire hypothalamus, with the exception of the nuclei ventromedialis, posterior and mammillaris, which were not labeled at all. In the brainstem, large amounts of grains were found in the ventrolateral division of the periaqueductal gray, in the locus coeruleus, in the nucleus tractus spinalis nervi trigemini and in the raphe obscurus, pallidus and magnus, while a widespread and lower grain density was observed in the reticular formation. In the spinal cord the labeling was discretely localized in laminae IV, V and VI of the dorsal horn. The observed distribution of CT binding sites is closely related to the neuroendocrine and analgesic effects of exogenous CT and reinforces the concept of a possible neuromodulatory role proposed for the peptide at brain level.

Central nervous system effect of calcitonin: stimulation of prolactin release in rats

Effect of [Asu 1,7]eel calcitonin (CT) on prolactin (PRL) release was examined in male rats under urethane anesthesia. Intravenous injection of 4-20 micrograms [Asu1,7]eel CT did not modify plasma PRL levels. Injections of 0.5-2.5 micrograms [Asu1,7]eel CT into the lateral ventricle produce a significant and dose-related increase of plasma PRL within 10 min of injection. When intraventricularly injected in an equimolar dose (0.74 nmol/10 microliters), eel CT11-32, eel CT15-32, [Asu1,7]eel CT1-16 and [Asu1,7]eel CT1-9 showed 44.8, 25.7, 19.9 and 10.1% the potencies of [Asu1,7]eel CT, respectively, in stimulating activity of PRL release. The rise of plasma PRL after [Asu1,7]eel CT injection were significantly less or abolished not only in hypothalamic-lesioned rats but also in rats with complete deafferentation. Pretreatment with alpha-methyl-p-tyrosine (250 mg/kg, 12 h before) but not with p-chlorophenylalanine (300 mg/kg, 72 and 24 h before) resulted in a suppression of [Asu1,7]eel CT-induced PRL release. These results suggest the following: first, PRL release is stimulated by centrally injected [Asu1,7]eel CT, the action site of which may exist in the extrahypothalamic area; second, brain catecholamines may be involved in the mechanism of [Asu1,7]eel CT-evoked PRL release; third, the C-terminal portion of the peptide may play an important role in stimulating PRL release.