Melanocortin receptors mediate alpha-MSH-induced stimulation of neurite outgrowth in neuro 2A cells

Melanocortins (MC), neuropeptides derived from pro-opiomelanocortin, have been implicated in enhancing neurite outgrowth via an as yet unknown mechanism. Recently, five MC receptors have been identified, three of which, the MC3-R, the MC4-R and the MC5-R, are expressed in the nervous system. In this study, alpha-MSH and the melanocortin analog [D-Phe7]ACTH (4-10) were able to stimulate neurite outgrowth in the neuroblastoma cell line Neuro 2A. ACTH (4-10), gamma2-MSH and ORG2766 were inactive. In addition, the MC4-R antagonist [D-Arg8]ACTH (4-10), inhibited the alpha-MSH effect, indicating that the MC4-R mediated stimulation of neurite outgrowth by alpha-MSH. Indeed, the presence of MC4-R mRNA in Neuro 2A cells was demonstrated by a RNase protection assay. Heterologous expression of the MC5-R in Neuro 2A cells lead to the recruitment of a responsiveness to gamma2-MSH, but did not increase the effect of alpha-MSH on neurite outgrowth. This finding indicated that the function of MC4-R can also be exerted by another MC receptor, suggesting that the coupling to Gs, which they have in common, plays an essential role in the neurite outgrowth promoting effect. This was further substantiated by the fact that forskolin treatment per se induced neurite outgrowth in a similar fashion. These data imply that the neurotrophic properties of alpha-MSH are likely to result from Gs-coupled MC receptor activity in neuronal cells.

Suppression of gene expression on the simian virus 40 major late promoter by human TR4 orphan receptor. A member of the steroid receptor superfamily

The key expression of the simian virus 40 (SV40) major late promoter could be repressed by the human TR4 orphan receptor via the +55 region of the SV40 major late promoter (nucleotide numbers 368-389, 5' -GTTA-AGGTTCGTAGGTCATGGA-3'). Using the coupled in vitro transcribed and translated TR4 orphan receptor with a molecular mass of 67.3 kilodaltons, electrophoretic mobility shift assay showed specific binding with a dissociation constant of 1.09 nM between the TR4 orphan receptor and the SV40 +55 oligonucleotides. In addition, chloramphenicol acetyltransferase assay demonstrated that this SV40 +55 region can function as a repressor via the TR4 orphan receptor, suppressing the transcriptional activities of both SV40 early and late promoters. Together, our data suggest that the TR4 orphan receptor may play an important role for the suppression of the SV40 gene expression.

The nuclear hormone-receptor family in the brain: classics and orphans

Nuclear hormone receptors comprise a superfamily of over 40 transcription factors. About half of them are classical receptors for lipophilic ligands such as steroids and vitamins. Almost all of these true receptors are present in the brain, where they transduce chemical signals from endocrine organs or signals of nutritional origin into cellular responses. The other members resemble the classical receptors in structure, but have no known ligands, and are hence called 'orphan receptors'. The issue of whether ligands for nuclear orphan receptors exist is controversial. Evidence is emerging that orphan receptors might be activated by signal transduction pathways or might be constitutive enhancers or repressors that interact with the classical receptors. Thus, nuclear orphan receptors are placed in strategic positions in the regulation of gene expression in the nervous system.

Somatic mutations in the brain: relationship to aging?

Genetic instability is generally thought to underlie the process of aging and is predominantly associated with meiosis and mitosis. This review will discuss DNA damage and repair, somatic mutations and somatic recombination events in non-dividing neurons in relation to aging. In general it can be concluded that mutagenesis operates at high frequency in the brain. Present data do not provide clear evidence for accumulating DNA damage or a change in DNA repair activity in the brain with age. However, a linear age-related increase in frameshift mutations has been shown to occur in vasopressin neurons of the rat, revealing a novel post-mitotic mechanism.

Rat oxytocin receptor in brain, pituitary, mammary gland, and uterus: partial sequence and immunocytochemical localization

Partial complementary DNAs of an oxytocin (OT) receptor were cloned from rat brain and uterus. The complementary DNAs encoded for the same amino acid sequence, which showed a high degree of homology with the human and porcine uterine OT receptors, except for a region in the third intracellular loop. Antibodies were raised against nonoverlapping sequences of the third intracellular loop of this rat OT receptor. Using these antisera, OT receptor expression was demonstrated in the brain, pituitary, mammary gland, and uterus by immunocytochemistry. In the brain, several areas including the ventromedial hypothalamus, the bed nucleus of the stria terminalis, the ventral pallidum, the paraventricular nucleus, and the dorsal part of the supraoptic nucleus, demonstrated OT-receptor immunoreactivity. However, no immunoreactivity was detected in two areas of the brain known to contain dense OT-binding sites by receptor autoradiography studies: the ventral hippocampus and the central nucleus of the amygdala. In the pituitary, both the anterior and posterior lobes were positive for OT receptor immunoreactivity, whereas the intermediate lobe was negative. These results demonstrate that the same receptor type is expressed in both peripheral OT target tissues and the brain, and also suggest the possibility that a different OT receptor subtype may be present in some areas of the brain.

Expression of nuclear hormone receptors in the rat supraoptic nucleus

To investigate the role of nuclear hormone receptors on neuropeptide gene expression in the hypothalamo-neurohypophyseal system (HNS) of the rat, a survey was made of members of the nuclear hormone receptor superfamily that are expressed in the supraoptic nucleus (SON). A polymerase chain reaction cloning strategy based on homologies in the DNA-binding domain of AGGTCA-binding factors was devised for the identification of receptors in microdissected SON tissue. Cloning of the amplified products led to the identification of five true receptors, thyroid hormone receptor-alpha (THR alpha), retinoic acid receptor-alpha, retinoic acid receptor-gamma, retinoid X receptor-alpha, and retinoid X receptor-gamma, as well as four orphan receptors, apolipoprotein AI regulatory protein (ARP-1), chicken ovalbumin upstream promoter transcription factor I (COUP-TF I), estrogen-related receptor 2, and testis receptor 4 (TR4). Dot-blot screening of amplified gene fragment analysis showed that THR alpha, ARP-1, TR4, and COUP-TF I were the most abundant factors expressed in the SON region, in the order THR alpha > ARP-1 > TR4 approximately COUP-TF I. THR alpha has previously been localized to HNS neurons. In situ hybridization analysis showed that ARP-1, COUP-TF I, and TR4 were not expressed in magnocellular neurons at appreciable levels, but rather in surrounding structures. Furthermore, in lactating female rats there were no significant differences in the composition of the nine identified nuclear hormone receptors in the SON region compared with control animals. From these experiments, it is concluded that there is a multitude of hypothalamically expressed nuclear hormone receptors, but that only THR alpha is expressed at relatively high abundance in HNS neurons. This indicates that the peptide-producing magnocellular neurons of the SON express a specific set of transcription factors of the nuclear hormone receptor family.

Localization of transcripts of the related nuclear orphan receptors COUP-TF I and ARP-1 in the adult mouse brain

The chicken ovalbumin upstream promoter transcription factor, COUP-TF I, and the protein ARP-1 (COUP-TF II) are two highly homologous orphan receptors of the nuclear hormone receptor family. In this study we investigated their expression patterns in the adult nervous system of the mouse. In situ hybridizations were performed on brain sections with 35S-labeled cRNA probes derived from the 3'-non-coding regions of the mARP-1 and mCOUP-TF I mRNAs. Both COUP-TF I and ARP-1 were shown to be expressed in the adult brain and they displayed restricted and distinct expression patterns. COUP-TF I transcripts were predominantly found in the rostral and caudal parts of the adult mouse brain, whereas ARP-1 transcripts prevaled in the middle part of the brain. High expression of COUP-TF I was detected in the olfactory nucleus, in neocortex layers I/II and V/VL, in the dentate gyrus and in areas CA1/CA3/CA4 of the hippocampus, and in the granular layer of the cerebellum. Only low amounts of COUP-TF I mRNA were detected in the ventral, the laterodorsal and in the interanteromedial thalamic nuclei. Small amounts of COUP-TF I transcripts were also found in the epithelial layer of the ventricle and in arachnoid membranes. High expression of ARP-I was detected in the reticular, the ventral lateral and the gelatinosus thalamic nuclei. Other hot spots of ARP-1 mRNA expression were the amygdaloid nucleus and the arachnoid membranes. Lower amounts of ARP-1 transcripts were found in the anterior and lateral hypothalamic areas, in the suprachiasmatic nucleus, and in the choroid plexus.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of melanocortin peptides on neural melanocortin receptors

Melanocortins (MCs) have various physiological actions on the brain. The recent cloning of neural MC receptors opened new avenues to study the effects of these neuropeptides on the nervous system. Here we investigated the structure-activity relationships (SARs) of peptides derived from adrenocorticotropic hormone (ACTH) with cloned MC3 and MC4 receptors in vitro and correlated these with central effects of MCs in vivo. Analysis of the effects of various MC peptides on cAMP accumulation in and binding to cells that expressed either the rat MC3 receptor or the human MC4 receptor demonstrated that ACTH-4-9-NH2 was the core sequence of ACTH able to activate these receptors. Furthermore, gamma-melanocyte-stimulating hormone (MSH) displayed selectivity for the MC3 receptor, whereas [D-Phe7]ACTH-4-10 more efficiently activated the MC4 receptor than the MC3 receptor. The activities of MC fragments that lacked the three carboxyl-terminal amino acids (residues 11-13) of ACTH-1-13 were much lower than that of alpha-MSH, for both receptors. Furthermore, the three amino-terminal amino acids (residues 1-3) of alpha-MSH were more important for full activation of the MC4 receptor, compared with the MC3 receptor. The SAR for the MC4 receptor resembled that for the induction of excessive grooming behavior by MC peptides. Therefore, we suggest that this behavioral response is mediated by MC4 receptors. The SAR for the MC3 receptor did not overlap with that for in vivo effects of MCs. ORG2766, an ACTH-4-9 analog that is very potent in an active avoidance task, did not activate, antagonize, or bind to the MC3 and MC4 receptors. This suggests the presence of still other MC receptors, in addition to the MC3 and MC4 receptors, in the brain. These data identify peptides with selectivity for either the MC3 receptor or the MC4 receptor, which may be used for development of novel MC receptor-specific ligands. Furthermore, this is the first report that discusses behavioral effects of MCs in light of data on cloned MC receptors.

Identification of antagonists for melanocortin MC3, MC4 and MC5 receptors

Antagonists for the melanocortin receptor family were identified by analysis of the effects of four melanocortin analogues on alpha-MSH(alpha-melanocyte-stimulating hormone)-induced cAMP accumulation in 293 human embryonal kidney (HEK) cells that expressed either the rat melanocortin MC3 receptor, the human melanocortin MC4 receptor or the ovine melanocortin MC5 receptor. Two peptides, [D-Arg8]ACTH(adrenocorticotrope hormone)-(4-10) and [Pro8,10,Gly9]ACTH-(4-10), antagonized the action of alpha-MSH on the melanocortin MC4 and MC5 receptors, but not the melanocortin MC3 receptor. [Ala6]ACTH-(4-10) inhibited the alpha-MSH activation of the melanocortin MC3 and MC5, but only weakly antagonized the activation of the melanocortin MC4 receptor. [Phe-I7]ACTH-(4-10) antagonized the melanocortin MC3, MC4 and MC5 receptors equally well. These antagonists were also tested to block a behavioral response induced by alpha-MSH. alpha-MSH-induced excessive grooming behavior in rats was inhibited by [Phe-I7]ACTH-(4-10), [D-Arg8]ACTH-(4-10) and [Pro8,10,Gly9]ACTH-(4-10), but not by [Ala6]ACTH-(4-10). This suggests that alpha-MSH-induced excessive grooming behavior is mediated by melanocortin MC4 receptors.

Human and rat TR4 orphan receptors specify a subclass of the steroid receptor superfamily

We have identified a member of the steroid receptor superfamily and cloned it from human and rat hypothalamus, prostate, and testis cDNA libraries. The open reading frame between first ATG and terminator TGA can encode 615 (human) and 596 (rat) amino acids with calculated molecular mass of 67.3 (human) and 65.4 (rat) kDa. The amino acid sequence of this protein, called TR4 orphan receptor, is closely related to the previously identified TR2 orphan receptor. The high homology between TR2 and TR4 orphan receptor suggests that these two orphan receptors constitute a unique subfamily within the steroid receptor superfamily. These two orphan receptors are differentially expressed in rat tissues. Unlike TR2 orphan receptors, the TR4 orphan receptor appears to be predominantly located in granule cells of the hippocampus and the cerebellum, suggesting that it may play some role(s) in transcriptional regulation in these neurons.

Frameshift mutations at two hotspots in vasopressin transcripts in post-mitotic neurons

Mutations in DNA underlie carcinogenesis, inherited pathology, and aging and are generally thought to be introduced during meiosis and mitosis. Here we report that in post-mitotic neurons specific frameshift mutations occur at high frequency. These mutations were identified in vasopressin transcripts in magnocellular neurons of the homozygous Brattleboro rat and predominantly consist of a GA deletion in GAGAG motifs. Immunocytochemistry provides evidence for similar events in wild-type rats. However, the diseased state of the Brattleboro rat, resulting in a permanent activation of vasopressin neurons, enhanced the mutational rate. These data reveal hitherto unrecognized somatic mutations in nondividing neurons.

Repression of estrogen-dependent stimulation of the oxytocin gene by chicken ovalbumin upstream promoter transcription factor I

The orphan receptor chicken ovalbumin upstream promoter transcription factor I (COUP-TF I) fully prevented not only the activation of the oxytocin gene by retinoic acid and thyroid hormone but also completely repressed the estrogen-dependent stimulation in transfected P19 EC cells. DNase I footprinting showed that the COUP-TF I protein bound to the 5'-flanking region of the oxytocin gene at the site of the distal composite hormone response element, which mediates the responses to estrogen, retinoic acid, and thyroid hormone. Electrophoretic mobility shift assay using this composite hormone response element as probe showed that COUP-TF I and the estrogen receptor competed for binding but did not form a heterodimer. The binding by COUP-TF I was stronger than the binding of the estrogen receptor. Thus, the mechanism of repression involves occupancy of integrated binding sites. By mutagenesis of the composite hormone response element, the COUP-TF I binding site and the estrogen response element could be separated, resulting in functional dissociation of the repressive action of COUP-TF I and the induction by estrogen. The results show that repression of gene expression by COUP-TF I is not limited to receptors that act through heterodimerization but also extends to the homodimer-forming estrogen receptor in a context-dependent manner. This interaction between COUP-TF I and the estrogen receptor may provide a physiological mechanism of selective antagonism of gene regulation by estrogens.

Functional analysis of an alternatively spliced estrogen receptor lacking exon 4 isolated from MCF-7 breast cancer cells and meningioma tissue

An alternatively spliced mRNA coding for a variant estrogen receptor (ER) missing exon 4 (ERdelta4) was detected in the breast tumor cell line MCF7 and meningioma tissue by using the reversed transcriptase PCR technique. The trans-activational properties of this mutant ER were assessed in embryo carcinoma P19EC and human choriocarcinoma JEG3 cells by co-transfection of the ERdelta4 expression vector with an oxytocin promoter construct containing an estrogen-responsive element. ERdelta4 did not trans-activate the oxytocin promoter in either a hormone-dependent or -independent manner. Co-transfection of ERdelta4 together with the wtER did not show any interference of ERdelta4 on the stimulation of the oxytocin promoter by the wtER. ERdelta4 was translated in vitro. Its capacity to bind estradiol, and the binding of the variant to a synthetic estrogen-responsive element were compared to those of the wild-type receptor. ERdelta4 did not bind to a synthetic estrogen-responsive element, nor did it bind estradiol. Hence, ERdelta4 appears to be a silent variant and we speculate that it is without any role in tumor progression.

Heterologous biosynthesis and processing of preprovasopressin in Neuro2A neuroblastoma cells

To obtain a model for the sorting and processing of preprovasopressin (preproVP), rat VP cDNA was transfected in murine Neuro2A neuroblastoma cells, which do not express VP. The precursor of VP was expressed and processed into the authentic VP gene products VP, neurophysin (NP) and glycopeptide (GP) as determined with reversed phase HPLC and radioimmunoassay. In addition, Neuro2A-specific forms of NP and GP were observed, which may be produced in the constitutive secretory pathway in these cells.

The human vasopressin-oxytocin gene family: no evidence for additional neurophysin-related genes

Over the last 20 years several observations at the peptide level have indicated the possible existence of additional members of the vasopressin (VP)-oxytocin (OT) gene family in mammals. In this study, the human genome was analyzed for the existence of genes structurally related to the VP and OT genes. Human genomic blots probed under low stringency conditions with exon B of the human OT gene, that codes for the conserved constant region of neurophysin, revealed the presence of two distinct bands in addition to the known VP and OT gene fragments. Five clones were obtained from a library of genomic EcoRI fragments ranging from 4-8 kb, that comprised both low stringency signals, by low stringency hybridization with the OT exon B probe. One clone of 7 kb hybridized at high stringency conditions to bands of the same size as previously detected with OT exon B on a human genomic blot. However, no similarity was observed between the open reading frames of this clone and the neurophysin portion of the OT gene. Another clone of 4.8 kb was identical to a fragment of the gene for the human bone morphogenetic factor hBMP-6, a member of the TGF-beta family. The hBMP-6 gene was not detected by low stringency hybridization of the human genomic blot with the OT exon B probe. No significant similarity was found between the amino acid sequences of human OT neurophysin and hBMP-6. Therefore, no evidence can be provided that the human genome contains additional neurophysin-related genes.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of proopiomelanocortin (POMC) messenger ribonucleic acid and POMC-derived peptides in human peripheral blood mononuclear cells: no evidence for a lymphocyte-derived POMC system

A number of recent studies suggest that cells of the immune system, e.g. peripheral blood mononuclear cells (PBMC), can synthesize and process POMC and secrete POMC-derived peptides, such as ACTH and endorphins, upon immune and hormonal challenges. From this, it has been proposed that POMC-derived peptides originating from lymphoid cells can function as hormones, for instance in a lymphoid-adrenal axis. In view of the important physiological implications of this proposal, the present study was designed to investigate the expression of the POMC gene in human PBMC and the production by these cells of alpha-, beta-, and gamma-endorphins (alpha E, beta E, and gamma E) peptides that are established end products of the posttranslational processing of POMC. PBMC of individual donors were used uncultured (fresh cells) or cultured for 24 and 48 h in the presence and absence of Concanavalin-A (Con-A), bacterial lipopolysaccharide, phytohemagglutinin, or CRH, and vasopressin, conditions that reportedly stimulate POMC activity in those cells, to investigate the presence of POMC transcripts by analysis of total RNA with Northern blotting and the reverse transcriptase polymerase chain reaction (RT-PCR). Large scale preparations containing over 10(9) cells (fresh, cultured with and without Con-A) originating from several donors were examined for the presence of POMC transcripts by analysis of poly(A)+ RNA on Northern blots and for the presence of alpha E, beta E, and gamma E by gel filtration over Sephadex G-75 and reverse phase HPLC, followed by assay of the fractions in four endorphin RIA systems with different specificities. On the Northern blots of total RNA, no POMC transcripts were detectable. In poly(A)+ RNA preparations, no full-length POMC mRNA was found, and it was estimated that the concentration of POMC mRNA, if present, was below approximately 0.005 transcript/cell in Con-A-stimulated cells and still lower in unstimulated cells. In accord with literature data, an 800- to 900-nucleotide POMC transcript was detected in cultured PBMC, and the levels of this transcript were stimulated by Con-A. In all samples analyzed with RT-PCR, a transcript spanning most of exons 2 and 3 was detectable only on Southern blots of the RT-PCR product, but not on agarose gels stained with ethidium bromide. Chromatographic analysis of endorphin immunoreactivities in cell extracts revealed no qualitative differences between the immunoreactive profiles of fresh PBMC or PBMC cultured with or without Con-A.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of aminopeptidase activity involved in the conversion of vasopressin by rat brain membranes

Previously it has been shown that vasopressin (VP) and oxytocin are converted by aminopeptidase activity in brain membranes into fragments with potent CNS activities. This report concerns the properties of this enzyme activity, addressed as VP-converting aminopeptidase (VP-AP) activity, in membranes of the rat brain. The VP-AP activity had a pH optimum at pH 7.0 and had a Km of 17 microM for its action on VP. Amastatin was the most potent aminopeptidase inhibitor. Enzyme activity was inhibited by relatively low concentrations of metal chelators. Treatment of brain membranes by EDTA resulted in loss of enzyme activity that was completely reversed by 10 microM Zn2+, indicating that VP-AP activity is a metallopeptidase. Several VP analogues and fragments, in particular VP(1-8), inhibited the action of enzyme activity on VP. Among peptides unrelated to VP, angiotension I, somatostatin, and porcine ACTH(1-39) markedly inhibited enzyme activity. Solubilization of VP-AP activity from brain membranes and gel filtration on Sephadex G200 showed two peaks of activity, one eluting with an apparent mass of about 140 kDa, the other in the void volume. Gel filtration fractions were able to convert [3H][Phe3]VP in a step-wise fashion. The VP-AP-like activity was found in many tissues outside the brain. Highest activity was present in lung, kidney, parts of the gastrointestinal tract, ovary, and uterus. The results indicate that VP-AP activity is a widely distributed enzyme with probably multiple functions, one of which involves the metabolism of vasopressin in the brain.

A composite hormone response element mediates the transactivation of the rat oxytocin gene by different classes of nuclear hormone receptors

Transcription factors of the steroid/thyroid hormone receptor superfamily are mediators of development and regulation of the brain. Previous studies have shown that the hypothalamic oxytocin (OT) gene is a potential target of these receptors, since its promoter is stimulated by estrogens and thyroid hormone. Here it is shown that the rat OT promoter is stimulated (at least 20-fold) by retinoic acid through two distinct regions in the 5'-flanking region. The major retinoic acid response element was located between nucleotides -172 and -148 and a minor one between nucleotides -112 and -77, as concluded from the transactivation of 5'-deletion mutants and binding to promoter elements by the retinoic acid receptor. Since the -172/-148 element also conferred estrogen and thyroid hormone responsiveness, it can be considered a composite hormone response element. This element contains a natural variant of the direct repeat of the half-site AGGTCA with spacing zero (DR-0) as well as a palindrome. Analysis of the core sequences of this element by site-directed mutagenesis showed that each of the three TGACC motifs integral to this element contributes to the multihormone sensitivity, but the contribution of each motif is different for the individual receptors. In neonatal rats, vitamin A deficiency and retinoic acid supplementation did not cause changes in hypothalamic OT mRNA levels and OT peptide levels in the pituitary gland and plasma. Gel-retarded protein-DNA complexes were formed between the composite hormone response element and extracts of the hypothalamic supraoptic and paraventricular nuclei. The composite hormone response element has a unique configuration and integrates responses of multiple members of the steroid/thyroid hormone receptor superfamily.

Expression of the pro-opiomelanocortin gene in dorsal root ganglia, spinal cord and sciatic nerve after sciatic nerve crush in the rat

Neuropeptides related to alpha-melanocyte-stimulating hormone (alpha-MSH) stimulate nerve outgrowth following peripheral nerve injury and may play an important physiological role in peripheral nerve regeneration. The mechanism of action underlying the neurotrophic effect of pharmacologically administered alpha-MSH is unknown. Here we investigate the hypothesis that reexpression of the proopiomelanocortin (POMC) gene, the prohormone of alpha-MSH/adrenocorticotropic hormone (ACTH)-like peptides, is part of the endogenous repertoire of peripheral nerve responses following injury. The effect of sciatic nerve crush on the expression of POMC mRNA between 0.5 h and 14 days after crush was investigated using polymerase chain reaction (PCR) and Northern blot analysis. The presence of a POMC transcript in dorsal root ganglia (DRG), spinal cord and in the sciatic nerve at the crush site could be demonstrated in both control and lesioned animals by PCR using primers located in exon 1 and 3 of the POMC gene. Minute quantities of two POMC transcripts (1200 nt and 800 nt) could be detected by Northern blot analysis of total RNA prepared from DRG, spinal cord and the sciatic nerve of control animals and of animals subjected to nerve crush. POMC mRNA expression was, however, not increased following nerve crush. Probes specific for exons 1 and 2 or specific for exon 3 of the POMC gene were employed to demonstrate that the 800 nt transcript represents the truncated POMC mRNA previously shown to be present in extra-pituitary tissue. The larger 1200 nt transcript comigrates with the full length POMC mRNA expressed in the pituitary gland. The present results demonstrate the expression of small amounts of POMC mRNA in all compartments of the sciatic nerve. The absence of an induction of POMC expression in response to nerve crush suggests that the stimulating effect of exogenously applied alpha-MSH does not mimic a POMC derived neurotrophic peptide induced in the nerve following nerve injury.

The identification of a cis-acting element involved in cyclic 3',5'-adenosine monophosphate regulation of bovine vasopressin gene expression

Cyclic adenosine 3',5'-monophosphate (cAMP) has been implicated as an intracellular messenger mediating osmotic regulation of expression of the gene encoding the neuropeptide vasopressin (VP) in the hypothalamus. We have used a heterologous transient transfection system to demonstrate that cAMP regulates the bovine VP gene promoter following transfection into CV1 cells. Mutational analysis identified a bovine VP cAMP-responsive element (BVP-CRE) 120-112 base-pairs upstream of the start of transcription. DNase I footprint analysis using nuclear protein extract from CV1 cells showed protection at the site of the BVP-CRE. Protection of the BVP-CRE was also observed using purified AP1 protein, while there was a weak interaction with the BVP-CRE using purified rat CREB protein. Nuclear proteins purified from the rat supraoptic nucleus bind to the BVP-CRE. As transgenic mouse studies have shown that the bovine VP gene is subject to appropriate physiological regulation in the mouse hypothalamus (Ang, H. L., Funkhouser, J., Carter, D. A., Ho, M. Y., and Murphy, D. (1991) Soc. Neurosci. Abstr. 513, 12), these data indicate a role for the BVP-CRE element in mediating VP gene expression in vivo. These data demonstrate that cAMP regulates bovine VP gene expression in vitro via a cis-acting element within the VP promoter, and this activation may be mediated by members of the AP1/ATF/CREB family of transcription factors.

The structure of neuropeptide receptors

Recently, the primary structures of 17 different receptors for neuropeptides and small peptide hormones have been elucidated by molecular cloning. All but one belong to the superfamily of G-protein coupled receptors which share a topography consisting of seven transmembrane domains. Comparison of primary structures shows that two classes of peptide receptors exist. One referred to as the 'neurokinin-type receptors', possesses many of the typical, conserved amino acid sequence motifs of the aminergic transmitter receptors (e.g. beta-adrenoceptor). The other, referred to the 'secretin-type receptors', displays unrelated and distinctly different sequence motifs which are conserved between the three presently known members of this class. These are the secretin, calcitonin and parathyroid hormone/parathyroid hormone-like polypeptide receptors. One may speculate that many other peptides with a core of biological activity in the N-terminal or middle region may have receptors of the secretin-type.