Atrial natriuretic factor-responding and cyclic guanosine monophosphate (cGMP)-producing cells in the rat hippocampus: a combined micropharmacological and immunocytochemical approach

Presynaptic and postsynaptic roles of NO, cGK, and RhoA in long-lasting potentiation and aggregation of synaptic proteins

Recent results suggest that long-lasting potentiation at hippocampal synapses involves the rapid formation of clusters or puncta of presynaptic as well as postsynaptic proteins, both of which are blocked by antagonists of NMDA receptors and an inhibitor of actin polymerization. We have investigated whether the increase in puncta involves retrograde signaling through the NO-cGMP-cGK pathway and also examined the possible roles of two classes of molecules that regulate the actin cytoskeleton: Ena/VASP proteins and Rho GTPases. Our results suggest that NO, cGMP, cGK, actin, and Rho GTPases including RhoA play important roles in the potentiation and act directly in both the presynaptic and postsynaptic neurons, where they contribute to the increase in puncta of synaptic proteins. cGK phosphorylates synaptic VASP during the potentiation, whereas Rho GTPases act both in parallel and upstream of cGMP, in part by maintaining the synaptic localization of soluble guanylyl cyclase.

Effects of two selective phosphodiesterase type 5 inhibitors, sildenafil and vardenafil, on object recognition memory and hippocampal cyclic GMP levels in the rat

The present study investigated the effects of two cyclic GMP-specific phosphodiesterase enzyme type 5 inhibitors, sildenafil and vardenafil, on the memory performance in the object recognition task. Both compounds were given per orally (1, 3 and 10 mg/kg sildenafil; 0.1, 0.3, 1 and 3 mg/kg vardenafil) immediately after the exposure to two identical objects. The memory for the objects was tested 24 h later. Vehicle-treated rats spent equal times exploring a new and the familiar object demonstrating that they did not remember the familiar one. However, sildenafil improved the object discrimination performance of the rats with a high discrimination performance at a dose of 3 mg/kg. Rats treated with vardenafil also showed an improved object discrimination performance. Compared with sildenafil, vardenafil appeared to be even more potent in this respect since it already produced a high discrimination performance at a dose of 0.3 mg/kg. The effects of both compounds on cyclic GMP and cyclic AMP accumulation were studied in rat hippocampal slices incubated in vitro. Cyclic GMP levels were increased after incubation with the highest concentration of 100 microM vardenafil (together with 0.1 mM sodium nitroprusside), although no changes in cyclic GMP levels were detected after incubation with different concentrations of sildenafil. Both compounds had no effect on cyclic AMP levels. Additional cyclic GMP immunocytochemistry showed that incubation with vardenafil (in the presence of sodium nitroprusside) resulted in a concentration-dependent staining of cyclic GMP. Staining was predominantly found in neuronal fibres in the hippocampal CA2/CA3 region. It was already detected at a concentration of 0.1 microM vardenafil. Also positive fibres were detected after incubation with sildenafil but at a higher concentration of 10 microM. Taken together, these results suggest that inhibition of phosphodiesterase enzyme type 5 improves object recognition memory. This effect might be explained by increased levels of central cyclic GMP.

Immunocytochemistry of cGMP in the Cerebellum of the Immature, Adult, and Aged Rat: the Involvement of Nitric Oxide. A Micropharmacological Study

In this study we describe the localization of formaldehyde-fixed cGMP-immunoreactivity (cGMP-IR) in rat cerebellar tissue slices incubated in vitro. In the absence of phosphodiesterase inhibition, cGMP-immunofluorescence was of low intensity in tissue slices prepared from immature cerebella. Addition of isobutylmethylxanthine (IBMX) to the incubation medium resulted in the appearance of cGMP-IR in clusters of astrocytes in the internal granular layer. Addition of N-methyl-d-aspartate (NMDA), kainic acid, atrial natriuretic factor (ANF), or sodium nitroprusside (SNP) gave an intense cGMP-IR in Bergmann fibres, Bergmann cell bodies, and astrocytes in the internal granular layer. Astrocytes in the white matter showed cGMP-IR after incubation of the slice in the presence of ANF or nitroprusside, but not after NMDA or kainic acid. In addition, after SNP stimulation of cGMP production, cGMP-IR was found in fibres which were not positive for glial fibrillary acidic protein (GFAP). In the adult cerebellar slice, intense basal cGMP-immunostaining was observed in Bergmann fibres, Bergmann cell bodies, and astrocytes in the granular layer. No cGMP-IR was observed in Purkinje cells. Stimulation of the cGMP-content in the glial structures by NMDA, ANF, or SNP, was suggested by the immunocytochemical results. However, when measured biochemically, only the effect of SNP was statistically significant, and immunocytochemistry showed that SNP clearly stimulated cGMP synthesis in neuronal cell structures. In the cerebellum of the aged rat a reduced cGMP-IR was found compared to the adult, in the same structures which showed cGMP-IR in the adult. Basal cGMP-immunostaining was reduced in the presence of haemoglobin, methylene blue, by inhibiting nitric oxide synthesis with NG-monomethyl-l-arginine (NGMAr), or by depletion of external Ca2+. Also the stimulatory effect of NMDA and of ANF (partly) on the cGMP-IR was inhibited by these compounds. cGMP-IR after stimulation of guanylate cyclase by SNP was reduced by the concomitant presence of haemoglobin or methylene blue, but not by NGMAr, or by omission of Ca2+. Our results point to an important role for cGMP in the functioning of glial tissue in the cerebellum and also suggest a role for nitric oxide as an intercellular mediator in the functioning of glutamate and ANF in the cerebellum.

In vivo NO/cGMP signalling in the hippocampus

In the hippocampus of freely-moving rats, basal extracellular levels of cGMP are inhibited by L-NARG or ODQ whereas they are increased by NO donors or phosphodiesterase inhibitors. Activation of NMDA receptors also augments cGMP dialysate levels in a MK-801 and L-NARG sensitive manner, an effect dramatically diminished during ageing. Experiments with AMPA, AMPA receptor antagonists and cyclothiazide revealed complex relationships with GABAergic circuits that potently control the NO/cGMP pathway. Furthermore, the activity of this neurochemical cascade is also modulated by hippocampal nicotinic receptors via enhancement of endogenous glutamate release and stimulation of NMDA receptors. From a behavioural point of view, increased hippocampal excitation leads to the appearance of epileptic-like manifestations that, however, seem unrelated to the increase of NO/cGMP formation.

Aquaporins in the central nervous system

In this review, we have tried to summarize most available data dealing with the aquaporin (AQP) family of water channels in the CNS. Two aquaporins have been identified so far in the CNS, AQP1 and AQP4. AQP1 is restricted to the choroid plexus of the lateral ventricles, which raises a role for this aquaporin in cerebrospinal fluid formation. AQP4 is the predominant water channel in the brain and it is more widely distributed than originally believed, with a marked prevalence over periventricular areas. In the first part of this review, we examine the complete distribution pattern of AQP4 in the CNS including its rostro-caudal localization to end with its subcellular location. After discussing scarce data dealing with regulation of aquaporins in the CNS, we focus in potential roles for aquaporins. Novel recent data highlights very important roles for this aquaporin in the normal and pathological brain including, among others, role in potassium buffering, body fluid homeostasis, central osmoreception and development and restoration of brain edema.

The effects of phosphodiesterase inhibition on cyclic GMP and cyclic AMP accumulation in the hippocampus of the rat

The effects of selective and non-selective 3',5'-cyclic nucleotide phosphodiesterase (PDE) inhibitors on cGMP and cAMP accumulation were studied in rat hippocampal slices incubated in vitro. The following PDE inhibitors were used: vinpocetine and calmidazolium (PDE1 selective), erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, PDE2 selective), SK&F 95654 (PDE3 selective), rolipram (PDE4 selective), SK&F 96231 (PDE5 selective), the mixed type inhibitors zaprinast and dipyridamole, and the non-selective inhibitors 3-isobutyl-1-metylxanthine (IBMX) and caffeine. cGMP levels were increased in the presence of different concentrations of IBMX, EHNA, dipyridamole, vinpocetine and rolipram. cGMP immunocytochemistry showed that incubation with different inhibitors in the presence and/or absence of sodium nitroprusside resulted in pronounced differences in the extent and regional localization of the cGMP response and indicate that PDE activity in the hippocampus is high and diverse in nature. The results suggest an interaction between cGMP and cAMP signalling pathways in astrocytes of the rat hippocampus.

In vivo studies of the cerebral glutamate receptor/NO/cGMP pathway

Overwhelming evidence indicates that the glutamate/nitric oxide (NO) synthase/soluble guanylyl cyclase system is of primary importance in a variety of physiological and pathological processes of the brain. Most of our knowledge on this neurochemical pathway derives from in vitro and ex vivo studies but the recent improvement of microdialysis techniques combined with extremely sensitive measurements of the amplified end-product cyclic GMP (cGMP) has given new impulses to the investigation of this cascade of events, its modulation by neurotransmitters and its functional relevance, in a living brain. The first reports, appeared in the early 90's, have demonstrated that microdialysis monitoring of cGMP in the extracellular environment of the cerebellum and hippocampus exactly reflects what is expected to occur at the intracellular level; thus, in vivo extracellular cGMP is sensitive to NO-synthase and soluble guanylyl cyclase inhibitors, can be increased by NO-donors or phosphodiesterase blockers and is modulated by glutamate receptor stimulation in a NO-dependent fashion. Since then, other microdialysis studies have been reported showing that the brain NO synthase/guanylyl cyclase pathway is mainly controlled by NMDA, AMPA and metabotropic glutamate receptors but can be also influenced by other transmitters (GABA, acetylcholine, neuropeptides) through polysynaptic circuits interacting with the glutamatergic system. The available data indicate that this technique, applied to freely-moving animals and combined with behavioural tests, could be useful to get a better insight into the functional roles played by NO and cGMP in physiological and pathological situations such as learning, memory formation, epilepsy, cerebral ischemia and neurodegenerative diseases.

Nitric oxide as a retrograde messenger during long-term potentiation in hippocampus

Nitric oxide (NO) is widespread in the nervous system and is thought to play a role in a variety of different neuronal functions, including learning and memory (see other chapters, this volume). A number of behavioral studies have indicated that NO is involved in several types of learning such as motor learning (Yanagihara and Kondo, 1996), avoidance learning (Barati and Kopf, 1996; Myslivecek et al., 1996), olfactory learning (Okere et. al., 1996; Kendrick et al., 1997), and spatial learning (Holscher et al., 1995; Yamada et al., 1996) (for review of earlier papers see Hawkins, 1996). Moreover, NO is thought to be involved in neuronal plasticity contributing to these different types of learning in different brain areas including the cerebellum (chapter by R. Tsien, this volume) and hippocampus. In this chapter we review evidence on the role of NO in long-term potentiation (LTP), a type of synaptic plasticity in hippocampus that is believed to contribute to declarative forms of learning such as spatial learning.

Effects of the 3',5'-phosphodiesterase inhibitors isobutylmethylxanthine and zaprinast on NO-mediated cGMP accumulation in the hippocampus slice preparation: an immunocytochemical study

The effect of inhibition of 3',5'-phosphodiesterase (PDE) activity on the cGMP accumulation was studied in control and nitric oxide (NO) stimulated hippocampal slices incubated in vitro using immunohistochemical visualisation of cGMP. Isobutylmethylxanthine (IBMX) was used as a non-selective PDE inhibitor and zaprinast was used as a selective inhibitor of cGMP-specific PDE activity. In the absence of PDE inhibitors cGMP-immunoreactivity (cGMP-IR) was found in blood vessel walls only. After incubation with the NO-donor sodium nitroprusside (SNP) cGMP-IR was found in a few isolated varicose fibres which were distributed throughout the slice. Incubation in the presence of either 1 mM IBMX or 10 microM zaprinast resulted in cGMP-IR in small numbers of varicose fibres distributed throughout the hippocampal slice. SNP in combination with IBMX resulted in cGMP-IR in small numbers multitude of varicose fibres throughout the slice; occasionally cell somata were observed. After incubation with SNP and zaprinast cGMP-IR was found in varicose fibres, although with a more restricted distribution and less numerous than in the presence of IBMX. In the latter combination, varicose fibres were observed predominantly in the CA2/CA3 region and in the stratum lacunosum molecular of the hippocampus, and cell somata were occasionally observed throughout the hippocampus. The differential distribution of cGMP-IR in the presence of different PDE inhibitors is consistent with the notion that there are regional differences in the localization of cGMP hydrolyzing enzymes in the hippocampus.

Activity-dependent long-term enhancement of transmitter release by presynaptic 3',5'-cyclic GMP in cultured hippocampal neurons

Long-term potentiation (LTP) in hippocampus is a type of synaptic plasticity that is thought to be involved in learning and memory. Several lines of evidence suggest that LTP involves 3',5'-cyclic GMP (cGMP), perhaps as an activity-dependent presynaptic effector of one or more retrograde messengers (refs 2-12, but see ref. 13). However, previous results are also consistent with postsynaptic effects of cGMP. This is difficult to test in hippocampal slices, but more rigorous tests are possible in dissociated cell culture. We have therefore developed a reliable method for producing N-methyl-D-aspartate (NMDA) receptor-dependent LTP at synapses between individual hippocampal pyramidal neurons in culture. We report that inhibitors of guanylyl cyclase or of cGMP-dependent protein kinase block potentiation by either tetanic stimulation or low-frequency stimulation paired with postsynaptic depolarization. Conversely, application of 8-Br-cGMP to the bath or injection of cGMP into the presynaptic neuron produces activity-dependent long-lasting potentiation. The potentiation by cGMP involves an increase in transmitter release that is in part independent of changes in the presynaptic action potential. These results support a presynaptic role for cGMP in LTP.

Nitric oxide and carbon monoxide as possible retrograde messengers in hippocampal long-term potentiation

We have been investigating the hypothesis that the membrane-permeant molecules nitric oxide (NO) and carbon monoxide (CO) may act as retrograde messengers during long-term potentiation (LTP). Inhibitors of either NO synthase or heme oxygenase, the enzyme that produces CO, blocked induction of LTP in the CA1 region of hippocampal slices. Brief application of either NO or CO to slices produced a rapid and long-lasting increase in the size of synaptic potentials if, and only if, the application occurred at the same time as weak tetanic stimulation of the presynaptic fibers. The long-term enhancement by NO or CO was spatially restricted to synapses from active presynaptic fibers and appeared to involve mechanisms utilized by LTP, occluding the subsequent induction of LTP by strong tetanic stimulation. The enhancement by NO or CO was not blocked by the NMDA receptor blocker APV, suggesting that NO and CO act downstream from the NMDA receptor. In other systems, both NO and CO produce many of their effects by activation of soluble guanylyl cyclase and cGMP-dependent protein kinase. An inhibitor of soluble guanylyl cyclase blocked the induction of normal LTP. Conversely, the membrane-permeable analog 8-Br-cGMP produced a rapid onset and long-lasting synaptic enhancement if, and only if, it was applied at the same time as weak presynaptic stimulation. Similarly, two inhibitors of cGMP-dependent protein kinase blocked the induction of normal LTP, and a selective activator of cGMP-dependent protein kinase produced activity-dependent long-lasting synaptic enhancement. 8-Br-cGMP also produced an activity-dependent, long-lasting increase in the amplitude of evoked synaptic currents between pairs of hippocampal neurons in dissociated cell culture. In addition, 8-Br-cGMP, like NO, produced a long-lasting increase in the frequency of spontaneous miniature synaptic currents. These results are consistent with the hypothesis that NO and CO, either alone or in combination, serve as retrograde messengers that produce activity-dependent presynaptic enhancement, perhaps by stimulating soluble guanylyl cyclase and cGMP-dependent protein kinase, during LTP in hippocampus.

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

We investigated the ultracytochemical localization of particulate guanylate cyclase (GC) in the rat neurohypophysis after activation with rat atrial natriuretic factor (rANF) or porcine brain natriuretic peptide (pBNP). Under our experimental conditions, the presence of GC reaction product indicated that rANF and pBNP were strong activators of particulate GC since samples incubated in basal conditions without rANF or pBNP did not reveal any GC reaction product. The rANF-stimulated GC was localized both to pituicytes and to nerve fibers and endings whereas the pBNP-stimulated GC was present exclusively in nerve fibers and endings. Recently, two subtypes of receptors for natriuretic peptides have been identified as two isoforms of particulate GC [24,50]. Our data indicate that the receptors of the two hormones have a partially distinct distribution in the rat neurohypophysis. In pituicytes, GC reaction product was found on plasma membrane of finger-like processes and on the membranes surrounding the lipid droplets. In nerve fibers and endings, GC reaction product was associated with intracellular membranes. This finding suggests that the enzyme could mediate an internal inhibitory action of these hormones on the release of vasopressin and oxytocin.

A cyclic GMP-stimulated cyclic nucleotide phosphodiesterase gene is highly expressed in the limbic system of the rat brain

Cyclic AMP and cyclic GMP serve as second messengers in a variety of neural cells, modulating their metabolic and electrical activity. The cyclic GMP-stimulated cyclic nucleotide phosphodiesterase, an enzyme whose hydrolytic activity is allosterically regulated by cyclic GMP in peripheral tissues, could play an important role in the regulation of cyclic nucleotide levels in the brain. To study the presence and distribution of cyclic GMP-stimulated phosphodiesterase in the rat brain, we cloned a portion of rat liver cyclic GMP-stimulated phosphodiesterase complementary DNA by polymerase chain reaction, using degenerate phosphodiesterase-specific oligonucleotide primers. Northern blot analysis of rat tissues reveals abundant expression of cyclic GMP-stimulated phosphodiesterase messenger RNA in the brain. Northern blot analysis of brain subregions shows especially strong expression in hippocampus and cortex, modest expression in the remainder of the forebrain and in the midbrain, and little expression in cerebellum and hindbrain. In situ hybridization studies with cyclic GMP-stimulated phosphodiesterase riboprobes confirm these northern blot results, and delineate cell groups with high levels of expression. Medial habenular nucleus is intensely labeled, as is hippocampus in the vicinity of pyramidal and granule cell bodies in areas CA1, CA2, CA3, and dentate gyrus. Other elements of the limbic system also contain cyclic GMP-stimulated phosphodiesterase messenger RNA, including olfactory and entorhinal cortices, subiculum, and amygdala. Additional cortical regions show more diffuse expression of cyclic GMP-stimulated phosphodiesterase messenger RNA, as do the basal ganglia. Cerebellum, thalamus, and hypothalamus do not show appreciable specific labeling. These studies demonstrate the presence of cyclic GMP-stimulated phosphodiesterase messenger RNA in specific regions of the rat brain, and suggest that the cyclic GMP-stimulated phosphodiesterase might modulate neuronal activity by regulating intracellular cyclic AMP levels in response to changes in intracellular cyclic GMP levels.

A new approach to the immunocytochemistry of cAMP. Initial characterization of antibodies against acrolein-fixed cAMP

A method is described to couple cyclic adenosine 3',5' monophosphate (cAMP) to a carrier protein by means of acrolein. Antibodies against this conjugate were raised in mice. These antibodies proved to be highly specific for acrolein-fixed cAMP in a gelatin model system. Slices (300 microns in thickness) from rat cerebral cortex were incubated in vitro and the dopaminergic control of adenylate cyclase activity was drug-manipulated. This manipulation was visualized by application of the cAMP-antisera on cryostat sections of the acrolein fixed slices.

Atrial natriuretic peptide binds to ANP-R1 receptors in neuroblastoma cells or is degraded extracellularly at the Ser-Phe bond

ANP-R1 receptors for atrial natriuretic peptide (ANP) showed the following rank order of affinity in intact human neuroblastoma cells NB-OK-1: human ANP-(99-126) approximately human ANP-(102-126) approximately rat ANP-(99-126) (K1 17-32 pM) > human ANP-(103-126) > porcine brain natriuretic peptide (BNP). Analogues truncated at the C-terminal extremity or devoid of a disulphide bridge, such as rat ANP-(103-123), rat C-ANP-(102-121), rat ANP-(111-126), rat ANP-(99-109) and rat [desCys105,Cys121]ANP-(104-126) and chicken C-type natriuretic peptide, were not recognized. The occupancy of these high affinity ANP-R1 receptors led to marked cyclic GMP accumulation in the presence of 3-isobutyl 1-methylxanthine. An ectoenzymic activity, partly shed in the incubation medium, provoked the stepwise release of Phe-Arg-[125I]Tyr, Arg-[125I]Tyr and [125I]Tyr from rat [125I]ANP-(99-126), at an optimal pH of 7.0. Its inhibition by 1,10-phenanthroline, EDTA and bacitracin but not by thiorphan suggests the contribution of at least one neutral metalloendopeptidase, distinct from EC 3.4.24.11, for which ANP showed high affinity.

Atrial natriuretic peptide increases cyclic guanosine monophosphate immunoreactivity in the carotid body

The mammalian carotid body is a peripheral arterial chemoreceptor organ involved in the regulation of respiration, and in the modulation of blood pressure through reflex control of peripheral vascular resistance and cardiac output. In addition to its responsiveness to blood gases, the organ is also sensitive to hyperosmotic solutions, and we have recently shown that a systemic hormonal regulator of natriuresis and diuresis, atrial natriuretic peptide, is a potent inhibitor of chemoreceptor activity evoked by hypoxia in the cat carotid body. The present study demonstrates atrial natriuretic peptide immunoreactivity in type I cells of the carotid body, and shows further that a biologically active atrial natriuretic peptide fragment, atriopeptin III, increases cyclic guanosine monophosphate immunoreactivity in type I cells in a dose-dependent manner. Moreover, double-labeling techniques demonstrate co-existence of atrial natriuretic peptide immunoreactivity with the atriopeptin III-enhanced cyclic guanosine monophosphate reaction product. These findings indicate the probable existence of atrial natriuretic peptide receptors coupled to membrane-bound guanylate cyclase on the parenchymal type I cells. Our findings support the view that cyclic guanosine monophosphate functions as a second messenger in this organ, and may serve as a functional activity marker in identifying type I cells which respond to atrial natriuretic peptide.

On the stimulation of soluble and particulate guanylate cyclase in the rat brain and the involvement of nitric oxide as studied by cGMP immunocytochemistry

The localization of the particulate and soluble guanylate cyclase in the rat brain was studied using cGMP-immunocytochemistry. The cGMP was fixed to tissue protein using a formaldehyde fixative, and an antibody against cGMP was used which was raised against a cGMP-formaldehyde-thyroglobulin conjugate. We used the atrial natriuretic factor (ANF) as a model compound to stimulate the particulate enzyme and sodium nitroprusside (SNP) to stimulate the soluble enzyme. Sequential immunostaining for cGMP and glial fibrillary acidic protein (GFAP) showed that the great majority of the ANF-responsive, cGMP-producing cells were astrocytes. These ANF-responsive cells were found in discrete parts of the CNS; not all astrocytes in these regions were responsive to ANF. SNP stimulated cGMP in abundantly present neuronal fibres throughout the CNS; few neuronal cell bodies showed increased cGMP production after SNP. Moreover, SNP also raised cGMP in astrocytes, however, not all astrocytes showed the response to SNP. These results suggest that cells might be present in the CNS which contain both the soluble and the particulate guanylate cyclase. It was demonstrated that in the immature cerebellum, the cGMP was raised in glial structures in response to N-methyl-D-aspartate (NMDA), ANF, SNP, and kainic acid. The response to NMDA and kainic acid was sensitive to inhibition of the nitric oxide synthesis from L-arginine by NG-methyl-L-arginine. Surprisingly the response to ANF localized in the molecular layer and the granular layer was also sensitive to inhibition by NG-methyl-L-arginine, whereas the response to ANF in the deep nuclei was not. A small depolarization induced by 10 to 20 mmol/l K+ induced an increase in cGMP in chopped hippocampus tissue which showed a biphasic temporal characteristic. The initial, fast (30 sec), peak was shown to be localized in varicose fibres throughout the hippocampus, whereas the slower response (10 min) was localized in astrocytes. These studies demonstrate that the different enzymes which synthesize cGMP are differently localized. However, there is also a time dependency in the activation of the guanylate cyclases, which becomes apparent in different structures at different times. The possible role of cGMP as a regulator of ion homeostase is discussed.

Immunocytochemical localization of cAMP and cGMP in cells of the rat carotid body following natural and pharmacological stimulation

Although the chemoreceptive function of the carotid body has been known for many decades, the cellular mechanisms of sensory transduction in this organ remain obscure. Common elements in the transductive processes of many cells are the cyclic nucleotide second messengers, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Studies from our laboratory have revealed stimulus-induced changes in cyclic nucleotide levels in the carotid body as measured by RIA, but such changes in second messenger levels have not been localized to specific cellular elements in the organ. The present immunocytochemical study utilized the avidin-biotin-peroxidase method to investigate the distribution of cAMP and cGMP in the rat carotid body and to assess changes in the intensity of immunostaining following in vitro stimulation by hypoxia, forskolin, sodium nitroprusside, high potassium, and atrial natriuretic peptide. Both cAMP and cGMP immunoreactivity were localized to type I cells of organs maintained in vivo and fixed by perfusion. Organs exposed to 100% O2-equilibrated media in vitro produced low but visible levels of cAMP immunoreactivity in a majority of type I cells; hypoxia (5% O2-equilibrated media) for 10 min moderately increased the level of immunoreactivity; forskolin (10(-5) M), or forskolin combined with hypoxia, dramatically increased cAMP levels in virtually all cells. Moderate levels of cGMP immunoreactivity in control carotid bodies in vitro were strikingly reduced by hypoxia; a significant increase in cGMP levels occurred following incubation in high potassium (100 mM), and under these conditions, the decrease in cGMP immunoreactivity with hypoxia was much more pronounced.(ABSTRACT TRUNCATED AT 250 WORDS)

NMDA receptor activation in rat hippocampus induces cyclic GMP formation through the L-arginine-nitric oxide pathway

When slices of young rat hippocampus were exposed briefly (2 min) to N-methyl-D-aspartate (NMDA), a rise in the levels of cyclic GMP took place. This response was dependent on NMDA concentration (EC50 approximately 30 microM) and the maximal elevations exceeded the unstimulated levels by 25-fold. The response to 100 microM NMDA was inhibited by two competitive antagonists of the conversion of arginine to nitric oxide, L-NG-methylarginine and L-NG-nitroarginine (IC50 approximately 6 microM and 100 nM respectively). The inhibitions produced by both antagonists were reduced or abolished when the incubation medium was supplemented with L-arginine (100-300 microM). Slices of adult hippocampus produced smaller increases (5-fold) in cyclic GMP levels in response to 100 microM NMDA than those found in the immature tissue, but the response could similarly be inhibited by NG-methylarginine. The results indicate that NMDA receptor activation in the hippocampus induces the generation of nitric oxide from arginine and that this novel intercellular messenger mediates the increases in cyclic GMP levels.

Second messenger enzymes in glial cells: a cytochemical point of view

Knowledge about second messenger metabolizing enzymes in neuroglia is still rather fragmentary. Therefore, the aim of the present investigation was to localize adenylate cyclase, guanylate cyclase, cyclic nucleotide phosphodiesterase and protein kinase A in glial cells of the rat hippocampus and cerebellum. Enzyme histochemical and immunohistochemical methods were used to detect the enzymes at the light and electron microscopic level. Astroglial cells were found to contain all 4 enzymes. Especially the microvascular glial cell processes were reactive. Oligodendroglial cells were only stained for adenylate cyclase acticity. Intracellularly, microtubules and intracellular membranes were frequently stained. The results point to the regulation of glial cell metabolism and of transport processes by cyclic nucleotides.

A functional parameter to study heterogeneity of glial cells in rat brain slices: cyclic guanosine monophosphate production in atrial natriuretic factor (ANF)-responsive cells

Stimulation of guanylate cyclase in vitro by atrial natriuretic factor (ANF) or sodium nitroprusside was studied in rat brain tissue slices biochemically as well as by means of cyclic guanosine monophosphate (cGMP) immunocytochemistry. The ANF-responsive, cGMP-producing cells were studied in the olfactory bulb, the septal area, the hippocampus, the medial amygdala, and the medial preoptic area. These cells, having the ANF-stimulated particulate guanylate cyclase, were characterized as astroglial cells on the basis of their glial fibrillary acidic protein (GFAP) immunostaining, although not all astroglial cells in these areas could be identified as cGMP-immunoreactive cells. Sodium nitroprusside-stimulated soluble guanylate cyclase activity was demonstrated in neuronal cell bodies and varicose fibers and was associated with blood vessel walls. Upon maturation, a significant decrease in cGMP production was found after stimulation by 100 nM ANF-(103-126) in the olfactory bulb, the medial amygdala, and the hippocampus, but not in the septal area; no change was found in these areas in cGMP content after stimulation of cGMP production by 10 microM sodium nitroprusside. Via cGMP immunocytochemistry, no qualitative differences were seen in the ANF-responsive, cGMP-producing cells upon maturation.

Localization of cGMP in the cerebellum of the adult rat: an immunohistochemical study

The localization of cGMP in the cerebellum of the adult rat after fixation with formaldehyde was studied with an antibody raised against a cGMP-formaldehyde-thyroglobulin conjugate. Three different protocols were used: (1) in vitro incubation of 300 microns cerebellar slices followed by fixation and cryostat sectioning; (2) in vitro incubation of 100 microns cerebellar slices followed by fixation with no further sectioning; (3) perfusion fixation of the anesthetized rat followed by vibratome sectioning. All 3 protocols gave essentially the same results: cGMP-immunoreactivity was found predominantly in Bergmann fibers in the molecular layer, in Bergmann cell bodies in the Purkinje cell layer (but not in Purkinje cells), and in astroglial cells in the granular layer.

cGMP immunocytochemistry in aorta, kidney, retina and brain tissues of the rat after perfusion with nitroprusside

The distribution of cyclic guanosine 3',5' monophosphate (cGMP) producing cells in various organs of the rat were studied immunocytochemically using antibodies raised against formaldehyde-fixed cGMP. Sodium nitroprusside (SNP), a direct activator of guanylate cyclase and vasodilator, was used to enhance cGMP levels. In order to reach all organs optimally, whole body perfusion was performed using a modified Krebs-Ringer buffer at 37 degrees C, aerated with 5% CO2/95% O2, also containing isobutyl methyl xanthine (IBMX); a phosphodiesterase inhibitor. After 15-min pre-perfusion, SNP was added to the perfusate, followed by fast fixation with ice-cold 4% paraformaldehyde-phosphate buffer. After vehicle perfusion, only the retina showed cGMP immunoreactivity in the photoreceptor and ganglion layer, while other organs lacked cGMP immunoreactivity. After 15-min perfusion with SNP (10 microM), enhanced cGMP immunostaining was seen in smooth muscles of the aorta, amacrine-like cells in the retina, glomeruli of the kidney cortex, blood vessels in the dura mater, as well as cells in the pineal and in the median eminence. The results indicate that the distribution and the reactivity of cGMP producing cells, situated outside the blood brain barrier, can be studied by immunocytochemistry after pharmacological manipulations of the intact tissue with a nitrovasodilator using whole body perfusion.

cGMP-Producing, Atrial Natriuretic Factor-Responding Cells in the Rat Brain

Using an in vitro incubation method, we stimulated cGMP production in rat brain slices by rat ANF-(103 - 126). The localization of the cells responding to this ANF stimulation with an increase in cGMP production was studied by cGMP immunocytochemistry. ANF-responding cells were found in specific loci throughout the central nervous system of the rat. Regions containing the highest number of these cells were: the olfactory bulb, the lateral septum, the bed nucleus of the accessory olfactory tract, the mediobasal amygdala, the central grey area, the medial vestibular nucleus, and the nucleus of the solitary tract. Scattered ANF-responding, cGMP-immunoreactive cells were found in the hippocampus, the cingulate cortex, the ventral pallidum, the medial preoptic area, and the endopeduncular nucleus. ANF-responding cells in these areas had the same morphology, that is, multipolar with numerous processes. The nature of these ANF-responding cells was studied by sequential staining with an antiserum against glial fibrillary acidic protein (GFAP). In the hippocampus it was demonstrated that all ANF-responding cells are astroglial cells. However, not all astroglial cells in this area showed a cGMP response, demonstrating a regional heterogeneity. ANF-responding cells, having the appearance of neuronal cell bodies, could be found in the subfornical organ, and the hypothalamic paraventricular nucleus. Fibres producing cGMP immunoreactivity in response to ANF were found in the median preoptic nucleus, the medial preoptic area, and the dorsal hypothalamus.

Formaldehyde fixation of cGMP in distinct cellular pools and their recognition by different cGMP-antisera. An immunocytochemical study into the problem of serum specificity

Three different antisera raised against the same formaldehyde fixed cGMP conjugate were tested for their specificity in two non-biological and two biological model systems. The first non-biological model system was based on nucleotides fixed to gelatin by formaldehyde and the other non-biological model was nitrocellulose paper as a carrier for nucleotides coupled to proteins by formaldehyde. All antisera proved specific for cGMP in both models. As biological models we used the in vitro incubated hippocampus slice and the in vitro incubated aortic ring. In hippocampus slices all three antisera showed cGMP-producing cells after atrial natriuretic factor stimulation. However, there were significant differences in the visualization of cGMP-immunoreactivity between the three antisera when sodium nitroprusside or potassium were used to stimulate cGMP production. Nevertheless, these differential staining patterns all showed cGMP-immunoreactivity using the conventional immunocytochemical control tests. In the aorta ring all three antisera showed the same strong increase in cGMP-immunoreactivity after in vitro stimulation with sodium nitroprusside. These results were corroborated by biochemical assay of cGMP. We conclude that these three antisera all demonstrate cGMP-immunoreactivity in the biological models used. The different staining patterns that occur are caused by differences in the microchemical milieu of the formaldehyde-fixed cGMP. The use of different antibodies to cGMP may give information about this microchemical milieu which may eventually contribute to a better understanding of different intracellular cGMP pools.