In vitro autoradiographic localization of (125)i-secretin receptor binding sites in rat brain

Secretin-dependent signals in the ventromedial hypothalamus regulate energy metabolism and bone homeostasis in mice

Secretin, though originally discovered as a gut-derived hormone, is recently found to be abundantly expressed in the ventromedial hypothalamus, from which the central neural system controls satiety, energy metabolism, and bone homeostasis. However, the functional significance of secretin in the ventromedial hypothalamus remains unclear. Here we show that the loss of ventromedial hypothalamus-derived secretin leads to osteopenia in male and female mice, which is primarily induced by diminished cAMP response element-binding protein phosphorylation and upregulation in peripheral sympathetic activity. Moreover, the ventromedial hypothalamus-secretin inhibition also contributes to hyperphagia, dysregulated lipogenesis, and impaired thermogenesis, resulting in obesity in male and female mice. Conversely, overexpression of secretin in the ventromedial hypothalamus promotes bone mass accrual in mice of both sexes. Collectively, our findings identify an unappreciated secretin signaling in the central neural system for the regulation of energy and bone metabolism, which may serve as a new target for the clinical management of obesity and osteoporosis.

A gut-brain axis mediates sodium appetite via gastrointestinal peptide regulation on a medulla-hypothalamic circuit

Salt homeostasis is orchestrated by both neural circuits and peripheral endocrine factors. The colon is one of the primary sites for electrolyte absorption, while its potential role in modulating sodium intake remains unclear. Here, we revealed that a gastrointestinal hormone, secretin, is released from colon endocrine cells under body sodium deficiency and is indispensable for inducing salt appetite. As the neural substrate, circulating secretin activates specific receptors in the nucleus of the solitary tracts, which further activates the downstream paraventricular nucleus of the hypothalamus, resulting in enhanced sodium intake. These results demonstrated a previously unrecognized gut-brain pathway for the timely regulation of sodium homeostasis.

Secretin receptor deletion in the subfornical organ attenuates the activation of excitatory neurons under dehydration

In mammals, thirst is strongly influenced by the subfornical organ (SFO), a forebrain structure that integrates circulating signals including osmotic pressure and sodium contents. Secretin (SCT), a classical gastrointestinal hormone, has been implicated as a humoral factor regulating body-fluid homeostasis. However, the neural mechanism of secretin in the central nervous system in managing thirst remains unclear. In this study, we report that the local ablation of SCT receptor (SCTR) in the SFO reduces water but not salt intake in dehydrated mice and this effect could not be rescued by exogenous SCT administration. Electrophysiology with single-cell RT-PCR indicates that SCT elicits inward currents in the SFO neuronal nitric oxide synthase (SFO^nNOS) neurons via SCTR in the presence of glutamate receptor antagonists. We further show that the SCTR in the SFO permits the activation of SFO^nNOS neurons under distinct thirst types. Projection-specific gene deletion of SCTR in SFO to the median preoptic nucleus (MnPO) pathway also reduces water intake in dehydrated animals. SCT signaling thus plays an indispensable role in driving thirst. These data not only expand the functional boundaries of SCTR but also provide insights into the central mechanisms of homeostatic regulation.

Involvement of Secretin in the Control of Cell Survival and Synaptic Plasticity in the Central Nervous System

With emerging evidence showing a wide distribution of secretin (SCT) and its receptor (SCTR) in the central nervous system (CNS), the putative neuropeptide role of SCT has become more appreciated since the disruption of SCT/SCTR axis affects various neural functions. This mini review thus focuses on the effects of SCT on cell survival and synaptic plasticity, both of which play critical roles in constructing and maintaining neural circuits with optimal output of behavioral phenotypes. Specifically, SCT-dependent cellular and molecular mechanisms that may regulate these two aspects will be discussed. The potential complementary or synergistical mechanisms between SCT and other peptides of the SCT superfamily will also be discussed for bridging their actions in the brain. A full understanding of functional SCT/SCTR in the brain may lead to future perspectives regarding therapeutic implications of SCT in relieving neural symptoms.

Insight into Evolution and Conservation Patterns of B1-Subfamily Members of GPCR

The diverse, evolutionary architectures of proteins can be regarded as molecular fossils, tracing a historical path that marks important milestones across life. The B1-subfamily of GPCRs (G-protein-coupled receptors) are medically significant proteins that comprise 15 transmembrane receptor proteins in Homo sapiens. These proteins control the intracellular concentration of cyclic AMP as well as various vital processes in the body. However, little is known about the evolutionary correlation and conservational blueprint of this GPCR subfamily. We performed a comprehensive analysis to understand the evolutionary architecture among 13 members of the B1-subfamily. Multiple sequence alignment analysis exhibited six multiple sequence aligned blocks and five highly aligned blocks. Molecular phylogenetics indicated that CRHR1 and CRHR2 share a typical ancestral relationship and are siblings in 100% bootstrap replications with a total of 24 nodes observed in the cladogram. CRHR2 has the maximum number of extremely conserved amino acids followed by ADCYAP1R1. The longest continuous number sequence logos (74) were found between sequence location 349 and 423, and consequently, the maximum and minimum logo height recorded was 3.6 bits and 0.18 bits, respectively. Finally, to understand the model and pattern of evolutionary relatedness, the conservation blueprint, and the diversification among the members of a protein family, GPCR distribution from several species throughout the animal kingdom was analysed. Together, the study provides an evolutionary insight and offers a rapid method to explore the potential of depicting the evolutionary relationship, conservation blueprint, and diversification among the B1-subfamily of GPCRs using bioinformatics, algorithm analysis, and mathematical models.

Secretin Regulates Excitatory GABAergic Neurotransmission to GnRH Neurons via Retrograde NO Signaling Pathway in Mice

In mammals, reproduction is regulated by a wide range of metabolic hormones that maintain the proper energy balance. In addition to regulating feeding and energy expenditure, these metabolic messengers also modulate the functional performance of the hypothalamic-pituitary-gonadal (HPG) axis. Secretin, a member of the secretin-glucagon-vasoactive intestinal peptide hormone family, has been shown to alter reproduction centrally, although the underlying mechanisms have not been explored yet. In order to elucidate its central action in the neuroendocrine regulation of reproduction, in vitro electrophysiological slice experiments were carried out on GnRH-GFP neurons in male mice. Bath application of secretin (100 nM) significantly increased the frequency of the spontaneous postsynaptic currents (sPSCs) to 118.0 ± 2.64% compared to the control, and that of the GABAergic miniature postsynaptic currents (mPSCs) to 147.6 ± 19.19%. Resting membrane potential became depolarized by 12.74 ± 4.539 mV after secretin treatment. Frequency of evoked action potentials (APs) also increased to 144.3 ± 10.8%. The secretin-triggered elevation of the frequency of mPSCs was prevented by using either a secretin receptor antagonist (3 μM) or intracellularly applied G-protein-coupled receptor blocker (GDP-β-S; 2 mM) supporting the involvement of secretin receptor in the process. Regarding the actions downstream to secretin receptor, intracellular blockade of protein kinase A (PKA) with KT-5720 (2 μM) or intracellular inhibition of the neuronal nitric oxide synthase (nNOS) by NPLA (1 μM) abolished the stimulatory effect of secretin on mPSCs. These data suggest that secretin acts on GnRH neurons via secretin receptors whose activation triggers the cAMP/PKA/nNOS signaling pathway resulting in nitric oxide release and in the presynaptic terminals this retrograde NO machinery regulates the GABAergic input to GnRH neurons.

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.

Activation of Supraoptic Oxytocin Neurons by Secretin Facilitates Social Recognition

BACKGROUND

Social recognition underlies social behavior in animals, and patients with psychiatric disorders associated with social deficits show abnormalities in social recognition. Oxytocin is implicated in social behavior and has received attention as an effective treatment for sociobehavioral deficits. Secretin receptor-deficient mice show deficits in social behavior. The relationship between oxytocin and secretin concerning social behavior remains to be determined.

METHODS

Expression of c-Fos in oxytocin neurons and release of oxytocin from their dendrites after secretin application were investigated. Social recognition was examined after intracerebroventricular or local injection of secretin, oxytocin, or an oxytocin receptor antagonist in rats, oxytocin receptor-deficient mice, and secretin receptor-deficient mice. Electron and light microscopic immunohistochemical analysis was also performed to determine whether oxytocin neurons extend their dendrites into the medial amygdala.

RESULTS

Supraoptic oxytocin neurons expressed the secretin receptor. Secretin activated supraoptic oxytocin neurons and facilitated oxytocin release from dendrites. Secretin increased acquisition of social recognition in an oxytocin receptor-dependent manner. Local application of secretin into the supraoptic nucleus facilitated social recognition, and this facilitation was blocked by an oxytocin receptor antagonist injected into, but not outside of, the medial amygdala. In the medial amygdala, dendrite-like thick oxytocin processes were found to extend from the supraoptic nucleus. Furthermore, oxytocin treatment restored deficits of social recognition in secretin receptor-deficient mice.

CONCLUSIONS

The results of our study demonstrate that secretin-induced dendritic oxytocin release from supraoptic neurons enhances social recognition. The newly defined secretin-oxytocin system may lead to a possible treatment for social deficits.

Effects of secretin on neuronal activity and feeding behavior in central amygdala of rats

Previous studies have shown that secretin and secretin receptors are expressed in central amygdala neurons. By using both in vivo extracellular recording as well as behavioral test, we investigated the direct electrophysiological effects of secretin in the central amygdala and its involvement in feeding behavior. Micro-pressure ejection of secretin increased the spontaneous firing rate by 104.22±26.18% in 13 out of the 27 central amygdala neurons. In other 6 out of the 27 neurons, secretin decreased the firing rate by 68.80±12.10%. Firing patter analysis showed that secretin did not change the firing pattern significantly. Further electrophysiological recordings revealed that secretin decreased the firing rate of glucose-sensitive neurons. In behavioral test, microinjection of secretin into the central amygdala significantly reduced cumulative food intake through cAMP-activated protein kinase activation. Based on the present electrophysiological and behavioral findings, we hypothesized that secretin may suppress food intake by its modulation of spontaneous firing of central amygdala neurons.

Proposed toxic and hypoxic impairment of a brainstem locus in autism

Electrophysiological findings implicate site-specific impairment of the nucleus tractus solitarius (NTS) in autism. This invites hypothetical consideration of a large role for this small brainstem structure as the basis for seemingly disjointed behavioral and somatic features of autism. The NTS is the brain's point of entry for visceral afference, its relay for vagal reflexes, and its integration center for autonomic control of circulatory, immunological, gastrointestinal, and laryngeal function. The NTS facilitates normal cerebrovascular perfusion, and is the seminal point for an ascending noradrenergic system that modulates many complex behaviors. Microvascular configuration predisposes the NTS to focal hypoxia. A subregion--the "pNTS"--permits exposure to all blood-borne neurotoxins, including those that do not readily transit the blood-brain barrier. Impairment of acetylcholinesterase (mercury and cadmium cations, nitrates/nitrites, organophosphates, monosodium glutamate), competition for hemoglobin (carbon monoxide, nitrates/nitrites), and higher blood viscosity (net systemic oxidative stress) are suggested to potentiate microcirculatory insufficiency of the NTS, and thus autism.

Distribution of secretin receptors in the rat central nervous system: an in situ hybridization study

Secretin shows a wide distribution in the brain. Functional significance of central secretin is stressed since it has been associated with autism and schizophrenia. The presence of the secretin receptor was previously demonstrated in the brain by different methods. Neurons in the cerebellum, hypothalamic paraventricular and supraoptic nuclei, and in the vascular organ of lamina terminalis were shown to express secretin receptor mRNA by using in situ hybridization with digoxigenin-labeled probe. In this work, we used a very sensitive radioactive in situ hybridization technique and systematically mapped the expression of secretin receptor mRNA in the brain. The densest labeling was observed in the nucleus of solitary tract and in the laterodorsal thalamic nucleus, where decreasing number of receptors was seen in the vascular organ of lamina terminalis, and the lateral habenular complex, and then in the supraoptic nucleus. Only a few scattered labeled cells were observed in the median frontal gyrus, entorhinal cortex, hypothalamic paraventricular nucleus, perifornical region, lateral hypothalamic area, head of the caudate nucleus, spinal trigeminal nucleus, and cerebellum. Secretin receptor mRNA showed a far wider distribution than was known before, suggesting a more significant functional relevance than thought earlier.

Intranasal application of secretin, similarly to intracerebroventricular administration, influences the motor behavior of mice probably through specific receptors

Secretin and its receptors show wide distribution in the central nervous system. It was demonstrated previously that intravenous (i.v.) and intracerebroventricular (i.c.v.) application of secretin influenced the behavior of rat, mouse, and human. In our previous experiment, we used a special animal model, Japanese waltzing mice (JWM). These animals run around without stopping (the ambulation distance is very limited) and they do not bother with their environment. The i.c.v. secretin attenuated this hyperactive repetitive movement. In the present work, the effect of i.c.v. and intranasal (i.n.) application of secretin was compared. We have also looked for the presence of secretin receptors in the brain structures related to motor functions. Two micrograms of i.c.v. secretin improved the horizontal movement of JWM, enhancing the ambulation distance. It was nearly threefold higher in treated than in control animals. The i.n. application of secretin to the left nostril once or twice a day or once for 3 days more effectively enhanced the ambulation distance than i.c.v. administration. When secretin was given twice a day for 3 days it had no effect. Secretin did not improve the explorative behavior (the rearing), of JWM. With the use of in situ hybridization, we have found very dense secretin receptor labeling in the cerebellum. In the primary motor cortex and in the striatum, only a few labeled cells were seen. It was supposed that secretin exerted its effect through specific receptors, mainly present in the cerebellum.

Secretin: Should we revisit its metabolic outcomes?

Metabolic pathologies such as Type 2 Diabetes have become a major health problem for worldwide populations. Unfortunately, efforts to cure and especially to prevent these significant global problems have so far been met with disappointment. Recently, the involvement of the gut-derived hormonal dysregulation in the development of obesity-related disturbances has been intensively studied. For instance, studies of gut-derived peptides such as peptide YY 3-36, glucagon-like peptide-1, oxyntomodulin and, more recently, ghrelin have significantly improved our understanding of mechanisms underlying weight and metabolic regulation. Even though early reports of the existence of secretin, the first peptide hormone to be described, date back as far as 1825, so much and yet so little is still known about its physiological role in mammals, including humans. However, recent years have provided a better understanding of how the release of secretin is regulated by enteral secretagogues. On the other hand, most basic questions about its role in the post-prandial regulation of metabolic functions in normal and pathophysiological conditions remain to be elucidated. The present work intends to review the physiology of secretin along with its central and peripheral outcomes on metabolic functions.

Secretin receptor-deficient mice exhibit impaired synaptic plasticity and social behavior

Secretin is a peptide hormone released from the duodenum to stimulate the secretion of digestive juice by the pancreas. Secretin also functions as a neuropeptide hormone in the brain, and exogenous administration has been reported to alleviate symptoms in some patients with autism. We have generated secretin receptor-deficient mice to explore the relationship between secretin signaling in the brain and behavioral phenotypes. Secretin receptor-deficient mice are overtly normal and fertile; however, synaptic plasticity in the hippocampus is impaired and there are slightly fewer dendritic spines in the CA1 hippocampal pyramidal cells. Furthermore, secretin receptor-deficient mice show abnormal social and cognitive behaviors. These findings suggest that the secretin receptor system has an important role in the central nervous system relating to social behavior.

Signaling mechanisms of secretin receptor

Secretin, a 27-amino acid gastrointestinal peptide, was initially discovered based on its activities in stimulating pancreatic juice. In the past 20 years, secretin was demonstrated to exhibit pleiotropic functions in many different tissues and more importantly, its role as a neuropeptide was substantiated. To carry out its activities in the central nervous system and in peripheral organs, secretin interacts specifically with one known receptor. Secretin receptor, a member of guanine nucleotide-binding protein (G protein)-coupled receptor (GPCR) in the secretin/VIP/glucagon subfamily, possesses the characteristics of GPCR with seven conserved transmembrane domains, a relatively large amino-terminal extracellular domain and an intracellular carboxyl terminus. The structural features and signal transduction pathways of the secretin receptor in various tissues are reviewed in this article.

The role of secretin in the cerebellum

There is increasing interest in the role played by secretin in the central nervous system. Recent evidence suggests that this peptide is widely expressed in the brain but some areas, notably the cerebellum, show a prominent expression of the peptide and its specific receptor. In this article we summarize our current understanding of the expression pattern and action of secretin in the cerebellum. We discuss the findings supporting the endogenous release of this peptide from Purkinje neurons and its role as a retrograde messenger modulating GABAergic synaptic transmission via multiple mechanisms. In addition, we would like to propose other possible, but still to be confirmed, functions of secretin in the cerebellum..

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.

The effectiveness of secretin in the management of autism

Autism is a complex neurological disorder that severely impacts a child's ability to communicate and interact socially. Many treatments have attempted to alleviate the symptoms of autism, but with limited success. After reports of improvements in autistic children who received secretin, this hormone became popular as a possible treatment for autism. Since then, the interest in secretin has greatly increased, as well as the demand for secretin by parents of autistic children. However, there is still limited experimental evidence that supports its effectiveness. Many biological studies and clinical trials were conducted to test the effectiveness of secretin in treating autism. This review discusses the autistic disorder, instruments used in the trials, and reports the findings of some of these studies.

Novel treatments for autistic spectrum disorders

In no area of developmental pediatric practice is there more controversy regarding the choice of treatment than related to children with autistic spectrum disorders (ASD). Complementary and alternative medical therapies (CAM) are often elected because they are perceived as treating the cause of symptoms rather than the symptoms themselves. CAM used for autism can be divided by proposed mechanism: immune modulation, gastrointestinal, supplements that affect neurotransmitter function, and nonbiologic intervention. Secretin as a therapy for autism is discussed as an example of how a clinical observation rapidly grew to a widespread treatment before well-designed studies demonstrated absence of effect. The plausibility for behavioral effect was not substantiated by clinical studies. CAM used for treatment of autism is examined in terms of rationale, evidence of efficacy, side effects, and additional commentary. Families and clinicians need access to well-designed clinical evidence to assist them in choice of therapies.

Age-related and regional differences in secretin and secretin receptor mRNA levels in the rat brain

In the present study expression levels of secretin and secretin receptor mRNAs in several brain regions of rats ranging in age from postnatal days 7 to 60 were investigated by quantitative real-time PCR. Expression of secretin and secretin receptor was detected in the central amygdala, hippocampus, area postrema, nucleus of the tractus solitary and cerebellum. The cerebellum expressed secretin receptor at significantly higher levels than that found in other brain regions within all the ages examined. In contrast, secretin mRNA was significantly higher in the nucleus of the tractus solitary than in the other four brain regions examined in postnatal day-21, -30 and -60 rats. Within most brain regions, both secretin and secretin receptor mRNAs were more abundant in postnatal day-7 and -14 rats as compared to postnatal day-21, -30 and -60 rats. Thus, secretin and its receptor are widely expressed in rat brain and the expression of both genes is developmentally regulated during the first few weeks following birth.

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.

A dyslexia susceptibility locus (DYX7) linked to dopamine D4 receptor (DRD4) region on chromosome 11p15.5

Dyslexia is a disability in acquiring reading and spelling skills that is independent of general intelligence and educational opportunity, and is highly heritable. It is known that dyslexia often co-occurs with attention deficit hyperactivity disorder (ADHD), and the 7-repeat allele of the 48-bp tandem repeat in exon 3 of the dopamine D4 receptor (DRD4) has been implicated in ADHD. We, therefore, investigated DRD4 as a candidate gene for dyslexia by testing for linkage and association with 14 markers at and around the DRD4 locus on chromosome 11p15.5. Using 100 families having at least two siblings affected with dyslexia, model-free linkage analysis revealed evidence for linkage to the DRD4-exon 3 repeat (two-point MFLOD = 2.27, P = 0.001) and to HRAS located just proximal to DRD4 (two-point MFLOD = 2.68, P = 0.0004). Evidence for linkage was maximal between DRD4 and HRAS (three-point MFLOD = 3.57, P = 0.00005). However, linkage disequilibrium analysis showed no significant evidence for association between dyslexia and DRD4 or HRAS. In particular, dyslexic subjects showed no significant increase of the DRD4 7-repeat allele associated with ADHD. It is possible that other DRD4 variants, not in strong linkage disequilibrium with the exon 3 repeat polymorphism, or alternatively, another gene very closely linked to DRD4, may influence susceptibility to dyslexia.

Secretin in a patient with treatment-resistant schizophrenia and prominent autistic features

Secretin, a gastrointestinal (GI) peptide, may offer therapeutic benefit in autism. Autistic features can also be present in schizophrenia and a recent study suggested a role for adjunctive secretin in treatment-resistant schizophrenia. The current report describes one patient with undifferentiated schizophrenia and prominent autistic features who received a single dose of secretin and demonstrated substantial yet transient improvement. The case illustrates the potential role of secretin as a novel adjunctive treatment strategy in schizophrenic patients with autistic features.

A secretin i.v. infusion activates gene expression in the central amygdala of rats

For the last 100 years secretin has been extensively studied for its hormonal effects on digestion. Recent observations that the deficits in social reciprocity skills seen in young (3-4-year-old) autistic children are improved after secretin infusions suggest an additional influence on neuronal activity. We show here that i.v. administration of secretin in rats induces Fos protein expression in the neurons of the central amygdala as well as the area postrema, bed nucleus of the stria terminalis, external lateral parabrachial nucleus and supraoptic nucleus. However, secretin infusion did not induce Fos expression in the solitary tract nucleus or paraventricular nucleus, regions normally activated by related peptides such as cholecystokinin. The peak blood levels of secretin that induce Fos protein expression in rat brain are similar to the peak blood levels observed during i.v. treatment with secretin in humans. The amygdala is known to be critical for developing reciprocal social interaction skills and abnormalities in this brain region have been demonstrated in autistic children.

Secretin activation of chromogranin A gene transcription. Identification of the signaling pathways in cis and in trans

Secretin evokes catecholamine secretion from PC12 pheochromocytoma cells. We tested whether secretin activates transcription of the major vesicular core protein chromogranin A (CgA). Secretin stimulated both endogenous CgA gene transcription (approximately 4-6-fold) as well as transfected CgA promoter activity (approximately 8-10-fold; EC50, approximately 7 nm) in PC12 cells. Studies on CgA promoter 5'-deletion mutant/luciferase reporter constructs, point mutations of the CgA cAMP response element (CRE), and their transfer to a heterologous promoter implicated CRE in cis as both necessary and sufficient for secretin-stimulated CgA gene transcription. Secretin-induced CgA gene transcription was inhibited/abolished by cytosolic Ca2+ chelation, chemical blockade of phospholipase C, protein kinase A (PKA), or mitogen-activated protein (MAP) kinase extracellular signal regulated kinase (ERK) 1/2 and the expression of dominant negative mutants of ERK1/2, CRE binding protein (CREB) kinase RSK2, or CREB. Secretin also augmented (approximately 4-fold) phosphorylation of ERK1/2. Trans-activation (approximately 21-fold) of GAL4-CREB fusion protein by secretin indicates involvement of CREB in secretin signaling to gene transcription. Electrophoretic mobility shift assays also identified CREB as the mediator of secretin-induced CgA gene transcription, and pCREB supershifts indicated Ser-133 as the active CREB moiety in vitro. This conclusion was reinforced in vivo by results of chromatin pCREB immunoprecipitation assays. We conclude that secretin signals to CgA gene transcription through the CRE domain in cis and through cAMP, Ca2+, PKA, MAP kinase, and the transcription factor CREB in trans. Thus, multiple signal transduction pathways seem to subserve the function of stimulus-transcription coupling after this peptidergic stimulus to chromaffin cells.

G protein-coupled receptor kinase 6 (GRK6) selectively regulates endogenous secretin receptor responsiveness in NG108-15 cells

1. To determine the role of G protein-coupled receptor kinases (GRKs) in the regulation of endogenous secretin receptor responsiveness, we have transiently overexpressed both wild-type (WT) and dominant negative mutant (DNM) GRKs in NG108-15 mouse neuroblastoma x rat glioma hybrid cells and investigated the effects of this on agonist-stimulated adenylyl cyclase activity. 2. Overexpression of WT GRK6 selectively inhibited secretin-stimulated cyclic AMP accumulation (fold stimulation of cyclic AMP above basal following 15 min incubation with 100 nM secretin was 12.1+/-2.0 and 6.2+/- 0.8 in control and WT GRK overexpressing cells, respectively) without affecting cyclic AMP responses mediated by the adenosine A(2) receptor agonist 5'-(N-ethylcarboxamido) adenosine (NECA) or the prostanoid-IP receptor agonist iloprost, or the direct activator of adenylyl cyclase, forskolin. On the other hand DNM GRK6 (Lys(215)Arg) overexpression produced the opposite effect--a selective increase in the secretin-stimulated cyclic AMP response was observed in cells overexpressing DNM GRK6 compared to plasmid-transfected cells (fold stimulation of cyclic AMP above basal following 15 min incubation with 100 nM secretin was 12.6+/-2.7 and 29.6+/-5.6 for control and DNM GRK6-overexpressing cells, respectively). 3. Overexpression of WT GRK5 likewise inhibited the secretin-stimulated cyclic AMP response, however, this effect was not as selective as with GRK6, since adenosine A(2) receptor responsiveness was also suppressed by GRK5 overexpression. Unlike DNM GRK6, overexpression of DNM GRK5 failed to modulate secretin or A(2) adenosine receptor signalling suggesting that endogenous GRK5 is unlikely to regulate desensitization of these receptors in NG108-15 cells. 4. Overexpression of WT GRK2 did not affect secretin-stimulated cyclic AMP accumulation. Instead, GRK2 overexpression selectively inhibited A(2) adenosine receptor responsiveness, confirming our previous findings. 5. Together these results suggest a selective role of endogenous GRK6 in regulating secretin receptor responsiveness in NG108-15 cells. In addition, these data indicate that GRKs exert a surprising degree of selectivity in the regulation of natively expressed GPCR responses.