Presence and possible site of action of secretin in the rat pituitary and hypothalamus

Distribution and Functional Implication of Secretin in Multiple Brain Regions

Secretin is a polypeptide hormone initially identified for its gastrointestinal functions. However, emerging evidences show wide distribution of secretin and secretin receptor across various brain regions from cerebral cortex, hippocampus, hypothalamus to cerebellum. In this mini review, we will firstly describe the region-specific expression pattern of secretin and secretin receptor in the brain, followed by a summary of central physiological and neurological functions mediated by secretin. Using genetic manipulation and pharmaceutical approaches, one can elucidate the role of secretin in mediating various neurological functions from simple behaviors, such as water and food intake, to more complex functions including emotion, motor, and learning or memory. At last, current weakness and future perspectives of secretin in the central nervous system will be discussed, aiming to provide the potency of using secretin or its analog for treating various neurological disorders.

Secretin, at the hub of water-salt homeostasis

Water and salt metabolism are tightly regulated processes. Maintaining this milieu intérieur within narrow limits is critical for normal physiological processes to take place. Disturbances to this balance can result in disease and even death. Some of the better-characterized regulators of water and salt homeostasis include angiotensin II, aldosterone, arginine vasopressin, and oxytocin. Although secretin (SCT) was first described >100 years ago, little is known about the role of this classic gastrointestinal hormone in the maintenance of water-salt homeostasis. In recent years, increasing body of evidence suggested that SCT and its receptor play important roles in the central nervous system and kidney to ensure that the mammalian extracellular fluid osmolarity is kept within a healthy range. In this review, we focus on recent advances in our understanding of the molecular, cellular, and network mechanisms by which SCT and its receptor mediate the control of osmotic homeostasis. Implications of hormonal cross talk and receptor-receptor interaction are highlighted.

The physiological roles of secretin and its receptor

Secretin is secreted by S cells in the small intestine and affects the function of a number of organ systems. Secretin receptors (SR) are expressed in the basolateral domain of several cell types. In addition to regulating the secretion of a number of epithelia (e.g., in the pancreas and biliary epithelium in the liver), secretin exerts trophic effects in several cell types. In this article, we will provide a comprehensive review on the multiple roles of secretin and SR signaling in the regulation of epithelial functions in various organ systems with particular emphasis in the liver. We will discuss the role of secretin and its receptor in health and biliary disease pathogenesis. Finally, we propose future areas of research for the further evaluation of the secretin/secretin receptor axis in liver pathophysiology.

Secretin stimulates biliary cell proliferation by regulating expression of microRNA 125b and microRNA let7a in mice

BACKGROUND & AIMS

Proliferating cholangiocytes secrete and respond to neuroendocrine hormones, including secretin. We investigated whether secretin secreted by S cells and cholangiocytes stimulates biliary proliferation in mice.

METHODS

Cholestasis was induced in secretin knockout (Sct(-/-)) and wild-type (control) mice by bile duct ligation (BDL). At days 3 and 7 after BDL, control and Sct(-/-) mice received tail-vein injections of morpholinos against microRNA 125b or let7a. One week later, liver tissues and cholangiocytes were collected. Immunohistochemical, immunoblot, luciferase reporter, and real-time polymerase chain reaction assays were performed. Intrahepatic bile duct mass (IBDM) and proliferation were measured. Secretin secretion was measured in conditioned media from cholangiocytes and S cells and in serum and bile.

RESULTS

Secretin secretion was increased in supernatants from cholangiocytes and S cells and in serum and bile after BDL in control mice. BDL Sct(-/-) mice had lower IBDM, reduced proliferation, and reduced production of vascular endothelial growth factor (VEGF) A and nerve growth factor (NGF) compared with BDL control. BDL and control mice given morpholinos against microRNA 125b or let7a had increased IBDM. Livers of mice given morpholinos against microRNA 125b had increased expression of VEGFA, and those treated with morpholinos against microRNA let7a had increased expression of NGF. Secretin regulated VEGF and NGF expression that negatively correlated with microRNA 125b and let7a levels in liver tissue.

CONCLUSIONS

After liver injury, secretin produced by cholangiocytes and S cells reduces microRNA 125b and let7a levels, resulting in up-regulation of VEGF and NGF. Modulation of cholangiocyte expression of secretin could be a therapeutic approach for biliary diseases.

Metabolic effects of secretin

Secretin (Sct), traditionally a gastrointestinal hormone backed by a century long research, is now beginning to be recognized also as a neuroactive peptide. Substantiation by recent evidence on the functional role of Sct in various regions of the brain, especially on its potential neurosecretion from the posterior pituitary, has revealed Sct's physiological actions in regulating water homeostasis. Recent advances in understanding the functional roles of central and peripheral Sct has been made possible by the development of Sct and Sct receptor (SctR) knockout animal models which have led to novel approaches in research on the physiology of this brain-gut peptide. While research on the role of Sct in appetite regulation and fatty acid metabolism has been initiated recently, its role in glucose homeostasis is unclear. This review focuses mainly on the metabolic role of Sct by discussing data from the last century and recent discoveries, with emphasis on the need for revisiting and elucidating the role of Sct in metabolism and energy homeostasis.

Modulation of paraventricular firing rate by secretin in vivo

The paraventricular nucleus (PVN) of hypothalamus is a major integrative center in homeostatic control. Morphological studies have revealed a high level of secretin and secretin receptor expression in the PVN. To investigate the direct electrophysiological effects of secretin in the PVN, in vivo extracellular recordings were performed in the present study. In 24 out of the 46 paraventricular neurons, micro-pressure ejection of secretin increased the firing rate from 3.07±0.43 Hz to 4.86±0.70 Hz. In another 8 out of the 46 paraventricular neurons, secretin decreased the firing rate from 2.61±0.46 Hz to 1.41±0.25 Hz. In the remaining 14 paraventricular neurons, secretin did not alter the firing rate significantly. The present findings provided direct electrophysiological evidence for the possible functions of secretin in the PVN.

Combined administration of secretin and oxytocin inhibits chronic colitis and associated activation of forebrain neurons

BACKGROUND

The pathogenesis of inflammatory bowel disease is unknown; however, the disorder is aggravated by psychological stress and is itself psychologically stressful. Chronic intestinal inflammation, moreover, has been reported to activate forebrain neurons. We tested the hypotheses that the chronically inflamed bowel signals to the brain through the vagi and that administration of a combination of secretin (S) and oxytocin (OT) inhibits this signaling.

METHODS

Three daily enemas containing 2,4,6-trinitrobenzene sulfonic acid (TNBS), which were given to rats produced chronic colitis and ongoing activation of Fos in brain neurons.

KEY RESULTS

Fos was induced in neurons in the paraventricular nucleus of the hypothalamus, basolateral amygdala, central amygdala, and piriform cortex. Subdiaphragmatic vagotomy failed to inhibit this activation of Fos, suggesting that colitis activates forebrain neurons independently of the vagi. When administered intravenously, but not when given intracerebroventricularly, in doses that were individually ineffective, combined S/OT prevented colitis-associated activation of central neurons. Strikingly, S/OT decreased inflammatory infiltrates into the colon and colonic expression of tumor necrosis factor-alpha and interferon-gamma.

CONCLUSIONS & INFERENCES

These observations suggest that chronic colonic inflammation is ameliorated by the systemic administration of S/OT, which probably explains the parallel ability of systemic S/OT to inhibit the colitis-associated activation of forebrain neurons. It is possible that S and OT, which are endogenous to the colon, might normally combine to restrict the severity of colonic inflammatory responses and that advantage might be taken of this system to develop novel means of treating inflammation-associated intestinal disorders.

Multiple actions of secretin in the human body

The discovery of secretin initiated the field of endocrinology. Over the past century, multiple gastrointestinal functions of secretin have been extensively studied, and it was discovered that the principal function of this peptide in the gastrointestinal system is to facilitate digestion and to provide protection. In view of the late identification of secretin and the secretin receptor in various tissues, including the central nervous system, the pleiotropic functions of secretin have more recently been an area of intense focus. Secretin is a classical hormone, and recent studies clearly showed secretin's involvement in neural and neuroendocrine pathways, although the neuroactivity and neural regulation of its release are yet to be elucidated. This chapter reviews our current understanding of the pleiotropic actions of secretin with a special focus on the hormonal and neural interdependent pathways that mediate these actions.

Partial reversal of phencyclidine-induced impairment of prepulse inhibition by secretin

BACKGROUND

Secretin is a "gut-brain" peptide whose neural function is as yet poorly understood. Several clinical studies have reported modestly increased social interaction in autistic children following intravenous secretin administration. Very recently secretin also was administered to schizophrenic patients and found to increase social interaction in some individuals.

METHODS

In light of this finding, we assessed the ability of secretin to reverse phencyclidine- (PCP) induced impairment in prepulse inhibition (PPI), a leading animal model of sensorimotor gating deficits in schizophrenia.

RESULTS

Similar to atypical antipsychotics, secretin (1, 3, 10, 30, and 100 microg/kg) partially and dose-dependently reversed the PCP-induced deficit in PPI without significantly affecting baseline startle when administered intraperitoneally (IP) 10 minutes following IP administration of PCP (3 mg/kg).

CONCLUSIONS

This finding may be relevant to observations of antipsychotic efficacy of secretin in schizophrenic patients as well as our previous report that systemically administered secretin is capable of modulating conditioned fear, even at quite low doses.

Secretin: hypothalamic distribution and hypothesized neuroregulatory role in autism

1. This study aims (1) to determine whether secretin is synthesized centrally, specifically by the HPA axis and (2) to discuss, on the basis of the findings in this and previous studies, secretin's possible neuroregulatory role in autism. 2. An immunocytochemical technique with single-cell resolution was performed in 12 age/weight-matched male rats pretreated with stereotaxic microinjection of colchicine (0.6 microg/kg) or vehicle into the lateral ventricle. Following 2-day survival, rats were anesthetized and perfused for immunocytochemistry. Brain segments were blocked and alternate frozen 30-microm sections incubated in rabbit antibodies against secretin, vasoactive intestinal peptide, glucagon, or pituitary-adenylate-cyclase-activating peptide. Adjacent sections were processed for Nissl stain. Preadsorption studies were performed with members of the secretin peptide family to demonstrate primary antibody specificity. 3. Specificity of secretin immunoreactivity (ir) was verified by clear-cut preadsorption control data and relatively high concentrations and distinct topographic localization of secretin ir to paraventricular/supraoptic and intercalated hypothalamic nuclei. Secretin levels were upregulated by colchicine, an exemplar of homeostatic stressors, as compared with low constitutive expression in untreated rats. 4. This study provides the first direct immunocytochemical demonstration of secretinergic immunoreactivity in the forebrain and offers evidence that the hypothalamus, like the gut, is capable of synthesizing secretin. Secretin's dual expression by gut and brain secretin cells, as well as its overlapping central distribution with other stress-adaptation neurohormones, especially oxytocin, indicates that it is stress-sensitive. A neuroregulatory relationship between the peripheral and central stress response systems is suggested, as is a dual role for secretin in conditioning both of those stress-adaptation systems. Colchicine-induced upregulation of secretin indicates that secretin may be synthesized on demand in response to stress, a possible mechanism of action that may underlie secretin's role in autism.

Secretin depolarizes nucleus tractus solitarius neurons through activation of a nonselective cationic conductance

The recent suggestion that secretin may be useful in treating autism and schizophrenia has begun to focus attention on the mechanisms underlying this gut-brain peptide's actions in the central nervous system (CNS). In vitro autoradiographic localization of125I-secretin binding sites in rat brain shows the highest binding density in the nucleus tractus solitarius (NTS). Recent evidence suggests that intravenous infusion of secretin causes fos activation in NTS, a relay station playing important roles in the central regulation of autonomic functions. In this study, whole cell patch-clamp recordings were obtained from 127 NTS neurons in rat medullary slices. The mean resting membrane potential of these neurons was -54.7 ± 0.3 mV, the mean input resistance was 3.7 ± 0.2 GΩ, and the action potential amplitude of these neurons was always >70 mV. Current-clamp studies showed that bath application of secretin depolarized the majority (80.8%; 42/52) of NTS neurons tested, whereas the remaining cells were either unaffected (17.3%; 9/52) or hyperpolarized (1.9%; 1/52). These depolarizing effects were maintained in the presence of 5 μM TTX and found to be concentration dependent from 10-12to 10-7M. Using voltage-clamp techniques, we also identified modulatory actions of secretin on specific ion channels. Our results demonstrate that while secretin is without effect on net whole cell potassium currents, it activates a nonselective cationic conductance (NSCC). These results show that NTS neurons are activated by secretin as a consequence of activation of a NSCC and support the emerging view that secretin can act as a neuropeptide within the CNS.

Secretin as a neuropeptide

The role of secretin as a classical hormone in the gastrointestinal system is well-established. The recent debate on the use of secretin as a potential therapeutic treatment for autistic patients urges a better understanding of the neuroactive functions of secretin. Indeed, there is an increasing body of evidence pointing to the direction that, in addition to other peptides in the secretin/glucagon superfamily, secretin is also a neuropeptide. The purpose of this review is to discuss the recent data for supporting the neurocrine roles of secretin in rodents. By in situ hybridization and immunostaining, secretin was found to be expressed in distinct neuronal populations within the cerebellum and cerebral cortex, whereas the receptor transcript was found throughout the brain. In the rat cerebellum, secretin functions as a retrograde messenger to facilitate GABA transmission, indicating that it can modulate motor and other functions. In summary, the recent data support strongly the neuropeptide role of secretin, although the secretin-autism link remains to be clarified in the future.

Secretin, glucagon, gastric inhibitory polypeptide, parathyroid hormone, and related peptides in the regulation of the hypothalamus- pituitary-adrenal axis

Secretin, glucagon, gastric inhibitory polypeptide (GIP), and parathyroid hormone (PTH) belong, together with vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase (AC)-activating polypeptide, to a family of peptides (the VIP-secretin-glucagon family), which also includes growth hormone-releasing hormone and exendins. All the members of this peptide family possess a remarkable amino-acid sequence homology, and bind to G-protein-coupled receptors, whose signaling mechanism primarily involves AC/protein kinase A and phospholipase C/protein kinase C cascades. VIP and pituitary AC-activating polypeptide play a role in the regulation of the hypothalamus-pituitary-adrenal (HPA) axis, and in this review we survey findings that also other members of the VIP-secretin-glucagon family may have the same function. Secretin and secretin receptors are expressed in the hypothalamus and pituitary gland, and secretin inhibits adrenocorticotropic hormone (ACTH) release. No evidence is available for the presence of secretin receptors in adrenal glands, but secretin selectively depresses the glucocorticoid response to ACTH of dispersed zona fasciculata-reticularis (ZF/R) cells. Glucagon and glucagon-like peptide-1 are contained in the hypothalamus, and all the components of the HPA axis are provided with glucagon and glucagons-like-1 receptors. These peptides exert a short-term inhibitory effect on stress-induced pituitary ACTH release and depress the ZF/R cell response to ACTH by inhibiting the AC/protein kinase A cascade; they also stimulate hypothalamic arginine-vasopressin release. GIP receptors are present in the ZF/R of the normal adrenals, and are particularly abundant in some types of adrenocortical adenomas and hyperplasias. GIP, through the activation of the AC/protein kinase A cascade, evokes a sizeable glucocorticoid secretagogue effect, leading to the identification of a food/GIP-dependent Cushing's syndrome. PTH and PTH-related protein are expressed in the hypothalamus and pituitary gland, and PTH and PTH-related protein receptors in all the components of the HPA axis. Both peptides enhance ACTH and arginine-vasopressin release, as well as stimulate aldosterone and glucocorticoid secretion of dispersed zona glomerulosa and ZF/R cells, respectively. The involvement of growth hormone-releasing hormone and exendins in the functional regulation of the HPA axis has not yet been extensively investigated.

Intracerebroventricular secretin enhances pancreatic volume and bicarbonate response in rats

BACKGROUND

Although secretin has been found within the brain, its central role in pancreatic exocrine function has not been previously addressed. The hypothesis that intracerebroventricular secretin enhances pancreatic volume and bicarbonate output at doses that have no effect when given intravenously was tested.

METHODS

Sprague-Dawley rats had a cannula stereotactically placed into the left lateral cerebral ventricle 24 hours before study. At laparotomy the bile and pancreatic ducts were separately cannulated and excluded for tared collections and bicarbonate assay.

RESULTS

Increasing doses of intracerebroventricular secretin (0.005, 0.05, and 0.5 microgram/1.0 microliter) induced a significant dose-related increase in bicarbonate output (2.95, 3.32, and 4.02 microEq/30 min, respectively) above basal (2.62 microEq/30 min) compared with control or intracerebroventricular saline treated animals. Pancreatic volume increased to 59.7 microliters at the lowest intracerebroventricular dose and increased (p < 0.025) to 65.8 microliters at the 0.05 intracerebroventricular secretin dose when compared with basal (59.4 microliters). To show that this was not a systemic effect of secretin, intravenous infusion of secretin at 0.005 and 0.05 microgram/kg/hr failed to stimulate either volume or bicarbonate output compared with that observed with intracerebroventricular secretin over the same dose range.

CONCLUSIONS

These observations indicate that intracerebroventricular secretin stimulates pancreatic volume and bicarbonate output and suggest that central secretin may play a role in the regulation of exocrine pancreatic secretion.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

Regulatory peptides produced in the anterior pituitary

Several peptide families have been detected in the anterior pituitary of several species, including man, and for many of them evidence for local synthesis has been found. Although a paracrine action seems evident for a few, the precise function of most of these peptides remains unknown.

Secretin receptors in the neuroglioma hybrid cell line NG108-15. Characterization and regulation of their expression

Secretin receptors in membranes from the neuroblastoma-glioma hybrid cell line NG108-15 were investigated by 125I-secretin binding and adenylate cyclase activation. On both parameters the corresponding relative potencies of parent peptides were, respectively: secretin greater than helodermin greater than peptide histidine isoleucinamide = vasoactive intestinal peptide. With secretin analogs and secretin fragments, the order of potency for binding was: secretin = [Val5]secretin greater than [Ala2]secretin = [Ala11]secretin greater than [Ala4, Val5] secretin greater than [Ala4]secretin greater than [D-Phe4] secretin greater than [D-Phe2]secretin = secretin (2-27) greater than secretin (3-27) greater than secretin (7-27). Also, on adenylate cyclase, [D-Phe4]secretin, [D-Phe2]secretin, secretin (2-27) and secretin (3-27) were partial agonists while secretin (7-27) was ineffective. The differentiating agent N6,2'-O-dibutyryladenosine 3',5'-monophosphate (1 mM) increased the density of secretin receptors and secretin-stimulated adenylate cyclase activity after a lag period of 4 h. After incubation for 24 h, receptor number and enzyme activity were increased 4- and 3-fold, respectively. These effects were inhibited totally by 1 microgram/ml cycloheximide and halved by 5 micrograms/ml actinomycin D. They were mimicked by 1 mM sodium butyrate but were not reproduced by either 8-bromoadenosine 3',5'-monophosphate or the phosphodiesterase inhibitor rac-4-(3-Butoxy-4-methoxybenzyl)-2-imidazolidinone.

Localization of vasoactive intestinal peptide- and peptide histidine isoleucine amide-like immunoreactivities in the rat superior cervical ganglion and its nerve trunks

Electrical stimulation of the preganglionic cervical sympathetic trunk causes an increase in dopa synthesis in the postganglionic neurons in the superior cervical ganglion (SCG). This transsynaptic biochemical effect can be blocked only partially by cholinergic antagonists, suggesting the involvement of a noncholinergic preganglionic sympathetic neurotransmitter(s). A survey of a large number of possible candidates for this neurotransmitter revealed that, in addition to cholinergic agonists, only a small group of peptides (all members of the secretin-glucagon family) stimulated dopa synthesis in the SCG. The effective peptides included vasoactive intestinal peptide (VIP), peptide histidine isoleucine amide (PHI), and secretin. Consequently we looked for the presence of immunoreactivities for these three peptides in the SCG. VIP- and PHI-like immunoreactive fibers were found in the SCG and in its major pre- and postganglionic nerve trunks. The distributions of the two immunoreactivities were very similar. Immunoreactive fibers were seen both singly and in bundles. In some instances, fibers were found apposed to neuronal cell bodies in the ganglion, and occasionally dense plexuses of fibers were found surrounding the neurons. In addition, punctate immunoreactive profiles were found apposed to the neurons in what appeared to be terminal fields. A small number of immunoreactive neuronal cell bodies were also seen in the ganglion. In a few instances, it was possible to establish, in serial sections, that the same cell body was immunostained with both VIP and PHI antisera. No secretin like-immunoreactive fibers or cells were observed. The presence of VIP-like and PHI-like-immunoreactive fibers in the cervical sympathetic trunk and in the SCG strengthens the possibility that these peptides, or a related molecule(s), serve as preganglionic neurotransmitters in this ganglion.

Brain neuropeptides: actions on central cardiovascular control mechanisms

The many peptides we have not considered (e.g. gastrin, motilin, FMRFamide, carnosine, litorin, dermorphin, casomorphin, eledoisin, prolactin, growth hormone, neuromedin U, proctolin, etc.) were omitted due to lack of information as far as any putative central cardiovascular effects are concerned. However, even for some of these peptide pariahs intriguing snippets of information are available now (e.g. ref. 85), although as we write, the list of possible candidates for investigation grows longer. On an optimistic note, it is becoming clear that many brain neuropeptides may have important effects on cardiovascular regulation. It seems feasible that 'chemically coded' pathways in the brain might be the neuroanatomical correlate of a 'viscerotopic' organization of cardiovascular control mechanisms, whereby the activity of the heart and flows through vascular beds are individually controlled, but in an integrated fashion, utilizing particular combinations of neurotransmitters and neuropeptides within the brain. Such possibilities can only be investigated, properly, by measurement of changes in cardiac output and regional haemodynamics in response to appropriate interventions, in conscious, unrestrained animals.

Preoptic injection of VIP, but not of secretin or PHI, inhibits LH and stimulates prolactin secretion in the ovariectomized rat

The microinjection of vasoactive intestinal peptide (VIP) into the medial preoptic area rapidly inhibited pulsatile luteinizing hormone (LH) secretion within 1 h in ovariectomized rats. A transient but significant increase in prolactin (PRL) secretion occurred after VIP injection coincidentally with the initiation of the inhibition of LH pulses. Preoptic microinjections of secretin or peptide histidine isoleucine amide (PHI) did not have such effects. These results suggest specific actions of VIP, despite its structural similarity to secretin and PHI, on the preoptic mechanisms governing LH and PRL secretion in ovariectomized rats.

The depressor effect of secretin administered either centrally or peripherally in conscious rats

The role of secretin in systemic blood pressure was examined in conscious rats. Either i.c.v. (0.5 or 1.0 micrograms/2 microliter 0.9% saline) or i.v. (1.0 or 2.0 micrograms/10 or 25 microliter saline) administration of secretin in conscious rats resulted in a significant fall in blood pressure. In consideration of previous findings that many other peptides such as substance P, enkephalin, bradykinin, neurotensin and vasoactive intestinal polypeptide cause pressor effects when administered centrally, but produce depressor responses after peripheral administration, the hypotension induced as both central and peripheral effects of secretin is unique and significant.

Effects of preoptic injections of gastrin, cholecystokinin, secretin, vasoactive intestinal peptide and PHI on the secretion of luteinizing hormone and prolactin in ovariectomized estrogen-primed rats

The effects of intracerebral injections of brain-gut peptides in the preoptic region on the secretion of luteinizing hormone (LH) and prolactin (PRL) were examined in ovariectomized, estrogen-primed rats. Gastrin or cholecystokinin-octapeptide injection in the preoptic region induced a moderate increase of circadian rise of LH secretion in some animals but no appreciable change in PRL secretion. Secretin injection was strikingly effective in increasing circadian rise of LH and PRL secretion. Vasoactive intestinal peptide injection completely abolished the occurrence of circadian rise of LH secretion whereas PHI injection facilitated LH secretion undergoing the circadian rhythm. The results suggest that those peptides act as neurotransmitters or neuromodulators in the preoptic region and are implicated in the regulation of LH and PRL secretion.

Septal-preoptic unit responses to microelectrophoresis of cholecystokinin, gastrin, vasoactive intestinal peptide and secretin in the rat

Effects of microelectrophoretic application of cholecystokinin-8, gastrin-17, vasoactive intestinal peptide and secretin on activities of septal-preoptic neurons were examined in ovariectomized rats. All of the 4 peptides produced either excitatory or inhibitory responses in some neurons tested. No consistent relationship was observed between effects of different peptides, even between the peptides of the same family. These results provide electrophysiologic evidence for the action in the septal-preoptic region of these peptides, and suggest that there may be specific interneurons sensitive to a corresponding peptide with some overlapping.

Secretin stimulates cyclic AMP formation in the rat brain

The effects of secretin on cyclic AMP levels in the rat brain were determined. Incubation of rat brain frontal cortex slices with secretin or the structurally related peptides peptide histidine leucine (PHI) or vasoactive intestinal polypeptide (VIP) in the presence of 10 mM theophylline resulted in a dose-dependent increase in the cyclic AMP levels. The half-maximal increase in cyclic AMP occurred using a 1 microM dose of secretin or a 2 microM dose of PHI or VIP. Preincubation of slices with secretin-(5-27) produced a dose-dependent inhibition of the secretin but not VIP- or PHI-stimulated increase in the cyclic AMP content. Also, in receptor binding studies, secretin-(5-27) produced a dose-dependent inhibition (Ki = 400 nM) of 125I-secretin but not of 125I-VIP binding to rat brain membranes. Guanyl-5'-yl imidodiphosphate decreased the affinity of radiolabelled secretin binding as a result of an increased rate of dissociation of bound 125I-secretin. These data suggest that secretin receptors in the rat brain may be coupled to adenylate cyclase in a stimulatory manner and that secretin-(5-27) may function as a central secretin receptor antagonist.

Paracrine interactions in the anterior pituitary

The topographical affinity between certain cell types in rat anterior pituitary as well as the presence of biogenic amines, neuropeptides, growth and tissue factors in specific cell types suggest participation of paracrine control mechanisms in the regulation of anterior pituitary hormone secretion. Due to the recent advances in the separation of pituitary cell types and the development of three-dimensional cell cultures, direct experimental evidence for control by intercellular messengers has become available. The stimulation of PRL release from superfused pituitary cell aggregates by LHRH has been shown to be mediated by gonadotrophs. Gonadotrophs appear to secrete a factor with PRL-releasing activity. Gonadotrophs also modulate the stimulation of PRL release by angiotensin II. Interaction of somatotrophs with an unknown small-sized cell type strongly amplifies the GH response to adrenaline, GRF and VIP. The latter phenomenon requires the permissive action of glucocorticoids. Some of these in vitro observations can be correlated with recently reported in vivo actions of LHRH, PRL and angiotensin II and with pathophysiological changes in the pituitary.

Galanin: evidence for a hypothalamic site of action to release growth hormone

Galanin is a 29 amino acid peptide that was isolated and characterized from porcine intestinal extracts. The presence of galanin-like immunoreactivity in neuronal elements in the hypothalamus and median eminence suggested a role for it in the hypothalamic control of anterior pituitary function. A hypothalamic site of action of galanin to stimulate growth hormone (GH) release is suggested by our observation that doses as low as 50 picomoles when infused into the third cerebroventricle of conscious, unrestrained ovariectomized rats resulted in significantly elevated plasma levels of GH. This effect was specific for GH and was dose-related. The failure of galanin to alter GH release from dispersed, cultured anterior pituitary cells in vitro further suggests that endogenous galanin plays a neuromodulatory role at the level of the median eminence, possibly affecting the release of known GH-releasing and/or inhibiting factors.