Detection of particulate guanylate cyclase in rat neurohypophysis after stimulation with ANF and BNP: an ultracytochemical study

Evidence of a guanylyl cyclase natriuretic peptide receptor in the gills of the new zealand hagfish Eptatretus cirrhatus (Class Agnatha)

Natriuretic peptide binding sites were examined in the gills of the hagfish Eptatretus cirrhatus (Class Agnatha, subfamily Eptatretinae) using radio-ligand binding techniques, molecular cloning and guanylyl cyclase assays. Iodinated rat atrial natriuretic peptide ((125)I-rANP) and iodinated porcine C-type natriuretic peptide ((125)I-pCNP) bound specifically to the lamellar folds and cavernous tissue of E. cirrhatus gills, and 0.3 nmol l(−1) rat ANP competed for 50 % of specific (125)I-rANP binding sites. Affinity cross-linking of (125)I-rANP to gill membranes followed by sodium dodecylsulphate-polyacrylamide gel electrophoresis revealed a single binding site of 150 kDa. In the presence of Mn(2+), 0.1 nmol l(−1) rANP inhibited cGMP production, whereas 1 micromol l(−1) rANP stimulated cGMP production rates. At 1 micromol l(−1), pCNP also stimulated cGMP production. The production of cGMP was also measured in the presence and absence of ATP with either Mn(2+) or Mg(2+). Reverse transcriptase polymerase chain reaction (RT-PCR) of hagfish gill RNA, followed by cloning and sequencing of PCR products, produced a partial cDNA sequence of a natriuretic peptide guanylyl cyclase receptor. The deduced amino acid sequence indicated 87–91 % homology with other natriuretic peptide guanylyl cyclase receptors. This study indicates the presence of a natriuretic peptide guanylyl cyclase receptor in the gills of E. cirrhatus that is similar to the natriuretic peptide guanylyl cyclase receptors in higher vertebrates. These observations demonstrate that the coupling of natriuretic peptide receptors with guanylyl cyclase has a long evolutionary history.

Guanylyl cyclase-B represents the predominant natriuretic peptide receptor expressed at exceptionally high levels in the pineal gland

The generation and function(s) of the signalling molecule cyclic GMP (cGMP) in brain are still poorly understood. One mechanism to raise intracellular cGMP levels is binding of C-type natriuretic peptide (CNP) to a membrane guanylyl cyclase (GC), termed GC-B. Here, we demonstrate an exceptionally strong expression of GC-B in the pineal gland. Crosslinking experiments performed with 125I-Tyr(0)-CNP and membranes from various rat tissues identified the receptor as a 130-kDa protein, expressed at highest levels in pineal membranes. Receptor autoradiography on brain sections confirmed a striking density of CNP binding sites in pineal tissue, whereas binding sites for the related atrial natriuretic peptide (ANP) predominate in other regions of the brain. Incubations of freshly dissected whole pineal glands in either the absence or presence of natriuretic peptides followed by immunohistochemical analyses of cGMP revealed strong accumulations of cGMP in response to CNP but not to ANP in the majority of pinealocytes. Stimulation of soluble GC (sGC) activity by use of sodium nitroprusside (SNP) resulted in a very similar pattern of cGMP immunostaining, indicating a co-expression at high levels of particulate and soluble forms of GC. These findings point to a major role of cGMP signalling in pinealocytes and suggest an important regulatory function for CNP.

Coronary vasodilator effects of BNP: mechanisms of action in coronary conductance and resistance arteries

Brain natriuretic peptide (BNP), a hormone secreted predominantly in ventricular myocytes, may influence coronary vascular tone. We studied the coronary vasodilatory response to BNP under physiological conditions and after preconstriction with endothelin-1 (ET-1) in anesthetized pigs. Average peak-flow velocity (APV) was measured using intracoronary Doppler, and cross-sectional area (CSA) was measured using intravascular ultrasound. Coronary blood flow (CBF) was calculated. Intracoronary BNP induced dose-dependent increases in CSA, APV, and CBF similar in magnitude to those induced by nitroglycerin (NTG). The magnitude of BNP-induced vasodilation was accentuated after preconstriction with ET-1. Pretreatment with either the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester or the cyclooxygenase inhibitor indomethacin attenuated the coronary vasodilator effect of BNP in resistance arteries without influencing epicardial vasodilation. Pretreatment with the ATP-sensitive potassium-channel blocker glibenclamide enhanced epicardial vasodilation in response to BNP. We conclude that BNP exerts coronary vasodilator effects, predominantly in epicardial conductance vessels. An accentuated vasodilatory response to BNP occurs in ET-1-preconstricted arteries. BNP-induced vasodilation in coronary resistance arteries may be partially mediated via nitric oxide and/or prostaglandin release.

Astrocytic neurotransmitter receptors in situ and in vivo

In the brain, astrocytes are associated intimately with neurons and surround synapses. Due to their close proximity to synaptic clefts, astrocytes are in a prime location for receiving synaptic information from released neurotransmitters. Cultured astrocytes express a wide range of neurotransmitter receptors, but do astrocytes in vivo also express neurotransmitter receptors and, if so, are the receptors activated by synaptically released neurotransmitters? In recent years, considerable efforts has gone into addressing these issues. The experimental results of this effort have been compiled and are presented in this review. Although there are many different receptors which have not been identified on astrocytes in situ, it is clear that astrocytes in situ express a number of different receptors. There is evidence of glutamatergic, GABAergic, adrenergic, purinergic, serotonergic, muscarinic, and peptidergic receptors on protoplasmic, fibrous, or specialized (Bergmann glia, pituicytes, Müller glia) astrocytes in situ and in vivo. These receptors are functionally coupled to changes in membrane potential or to intracellular signaling pathways such as activation of phospholipase C or adenylate cyclase. The expression of neurotransmitter receptors by astrocytes in situ exhibits regional and intraregional heterogeneity and changes during development and in response to injury. There is also evidence that receptors on astrocytes in situ can be activated by neurotransmitter(s) released from synaptic terminals. Given the evidence of extra-synaptic signaling and the expression of neurotransmitter receptors by astrocytes in situ, direct communication between neurons and astrocytes via neurotransmitters could be a widespread form of communication in the brain which may affect many different aspects of brain function, such as glutamate uptake and the modulation of extracellular space.

Localization of natriuretic peptide-activated guanylate cyclase mRNAs in the rat brain

Physiological actions of atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are elaborated by membrane-bound natriuretic peptide receptors (NPRs). These receptors possess intracellular guanylate cyclase domains that mobilize cyclic guanosine monophosphate upon binding of peptide. Two distinct NPR subtypes have been described in brain: the NPR-A selectively binds ANP, whereas NPR-B exhibits high affinity for CNP. To define further the potential domains of ANP and CNP action in brain, the present study used in situ hybridization histochemistry to map NPR-A and NPR-B mRNA-expressing cell populations. Significant levels of neuronal NPR-A mRNA expression were observed only in the mitral cell layer of the olfactory bulb, medial habenula, subfornical organ, and area postrema. Expression of NPR-A mRNA was observed in forebrain white matter tracts, suggesting synthesis in glial cells. In contrast, NPR-B mRNA was widely expressed throughout the neuraxis. In the telencephalon, signal was abundant throughout limbic cortex and neocortex, olfactory bulb, hippocampus, and amygdala. Intense NPR-B mRNA hybridization was observed in preoptic-hypothalamic neuroendocrine circuits and in motor nuclei of cranial nerves. Intermediate expression of NPR-B mRNA was observed in brainstem nuclei controlling autonomic function. Labeling for NPR-B but not NPR-A mRNA was observed in pituicytes in the neural lobe of the pituitary and in scattered cells of the anterior pituitary. These results suggest that CNP is the primary biologically active natriuretic peptide in brain. In contrast with NPR-B, NPR-A appears to be expressed largely in restricted cell populations containing high levels of ANP and in circumventricular organs. These data implicate the NPR-A in autoregulation of ANP neurons and central registration of cardiac ANP release.

Detection of membrane-bound guanylate cyclase activity in rat C6 glioma cells at different growth states following activation by natriuretic peptides

We studied the activity and the ultracytochemical localization of membrane-bound guanylate cyclase (GC) after stimulation with rat atrial natriuretic peptide (rANP), porcine brain natriuretic peptide (pBNP), rat brain natriuretic peptide (rBNP), or porcine C-type natriuretic peptide (CNP) in rat C6 glioma cells during proliferation or following exposure of confluent cells to dibutyryl cyclic AMP (db-cAMP) or retinoic acid (RA). Under our experimental conditions all peptides were activators of GC as demonstrated by the accumulation of cGMP within cells. During proliferation of C6 cells, the amounts of cGMP remained approximately constant. However, at subconfluency, confluency and postconfluency, the GC reaction product was located at different sites in C6 cells. At subconfluency, GC reaction product was on membranes of protoplasmic extensions, at postconfluency, GC reaction product was in association with membranes of cell bodies, and at confluency, both localizations of GC reaction product were detected. Incubation of confluent cells in culture medium containing db-cAMP or RA induced the appearance of long and slender protoplasmic extensions. Under these conditions, the GC reaction product was localized exclusively to these processes. These data suggest that GC is differentially located depending on the state of growth of glial cells, and that in differentiating glial cells GC is preferentially located in cell processes.