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

Immunocytochemical analysis of cyclic nucleotides

Visualization of second messenger molecules is a powerful approach to study the role of second messengers in relation to the (sub)cellular localization and/or kinetics of the second messenger response. This unit describes the appropriate fixation procedures for the cyclic nucleotides cAMP and cGMP and the use of specific antibodies to localize these molecules.

Cellular targets of nitric oxide in the hippocampus

In the hippocampus, as in many other CNS areas, nitric oxide (NO) participates in synaptic plasticity, manifested as changes in pre- and/or postsynaptic function. While it is known that these changes are brought about by cGMP following activation of guanylyl cyclase-coupled NO receptors attempts to locate cGMP by immunocytochemistry in hippocampal slices in response to NO have failed to detect the cGMP elevation where expected, i.e. in the pyramidal neurones. Instead, astrocytes, unidentified varicose fibres and GABA-ergic nerve terminals are reported to be the prominent NO targets, raising the possibility that NO acts indirectly via other cells. We have re-investigated the distribution of cGMP generated in response to endogenous and exogenous NO in hippocampal slices using immunohistochemistry and new conditions designed to optimise cGMP accumulation and, hence, its detectability. The conditions included use of tissue from the developing rat hippocampus, a potent inhibitor of phosphodiesterase-2, and an allosteric enhancer of the NO-receptive guanylyl cyclase. Under these conditions, cGMP was formed in response to endogenous NO and was found in a population of pyramidal cell somata in area CA3 and subiculum as well as in structures described previously. The additional presence of exogenous NO resulted in hippocampal cGMP reaching the highest level recorded for brain tissue (1700 pmol/mg protein) and in cGMP immunolabelling throughout the pyramidal cell layer. Populations of axons and interneurones were also stained. According with these results, immunohistochemistry for the common NO receptor β1-subunit indicated widespread expression. A similar staining pattern for the α1-subunit with an antibody used previously in the hippocampus and elsewhere, however, proved to be artefactual. The results indicate that the targets of NO in the hippocampus are more varied and extensive than previous evidence had suggested and, in particular, that the pyramidal neurones participating in NO-dependent synaptic plasticity are direct NO targets.

Phosphodiesterases in the central nervous system: implications in mood and cognitive disorders

Cyclic nucleotide phosphodiesterases (PDEs) are a superfamily of enzymes that are involved in the regulation of the intracellular second messengers cyclic AMP (cAMP) and cyclic GMP (cGMP) by controlling their rates of hydrolysis. There are 11 different PDE families and each family typically has multiple isoforms and splice variants. The PDEs differ in their structures, distribution, modes of regulation, and sensitivity to inhibitors. Since PDEs have been shown to play distinct roles in processes of emotion and related learning and memory processes, selective PDE inhibitors, by preventing the breakdown of cAMP and/or cGMP, modulate mood and related cognitive activity. This review discusses the current state and future development in the burgeoning field of PDEs in the central nervous system. It is becoming increasingly clear that PDE inhibitors have therapeutic potential for the treatment of neuropsychiatric disorders involving disturbances of mood, emotion, and cognition.

Increased particulate phosphodiesterase 4 in the prefrontal cortex supports 5-HT4 receptor-induced improvement of object recognition memory in the rat

RATIONALE

Serotonin receptors (5-HT4Rs) are critical to both short-term and long-term memory processes. These receptors mainly trigger the cyclic adenosine monophosphate (cAMP)/protein kinase A signaling pathway, which is regulated by cAMP phosphodiesterases (PDEs).

OBJECTIVES

We investigated the mechanisms underlying the effect of the selective activation of 5-HT4R on information acquisition in an object recognition memory task and the putative regulation of PDE.

MATERIALS AND METHODS

The effect of RS 67333 (1 mg/kg, intraperitoneally [i.p.], injected 30 min before the sample phase) was examined at different delay intervals in an object recognition task in Sprague-Dawley rats. After the testing trial, PDE activity of brain regions implicated in this task was assayed.

RESULTS

RS 67333-treated rats spent more time exploring the novel object after a 15-min (P < 0.001) or 4-h delay (P < 0.01) but not after a 24-h delay, whereas control animals showed no preference for the novel object for delays greater than 15 min. We characterized the specific patterns and kinetic properties of PDE in the prefrontal and perirhinal cortices as well as in the hippocampus. We demonstrated that particulate PDE activities increase in both the prefrontal cortex and hippocampus following 5-HT4R stimulation. In the prefrontal cortex, PDE4 activities support the RS 67333-induced modification of PDE activities, whereas in the hippocampus, all cAMP-PDE activities varied. In contrast, particulate PDE variation in the hippocampus was not found to support improvement of recognition memory after a 4-h delay.

CONCLUSIONS

We provide evidence that the increase in particulate PDE4 activity in the prefrontal cortex supports the 5-HT4R-induced increase in information acquisition.

Phosphodiesterase 2 and 5 inhibition attenuates the object memory deficit induced by acute tryptophan depletion

The underlying mechanism of short-term memory improvement after inhibition of specific phosphodiesterases (PDEs) is still poorly understood. The present study aimed to reveal the ability of PDE5 and PDE2 inhibitors, that increase cyclic guanosine monophosphate (cGMP) and both cyclic adenosine monophosphate (cAMP) and cGMP, respectively, to reverse an object recognition deficit induced by acute tryptophan depletion. Acute tryptophan depletion is a pharmacological challenge tool to lower central serotonin (5-hydroxytryptamine; 5-HT) levels by depleting the availability of its dietary precursor tryptophan. Short-term object memory was tested in male Wistar rats by exposing them to the object recognition task. First, the effects of acute tryptophan depletion upon object recognition 2 h after administration of the nutritional mixture were established. Subsequently, acute tryptophan depletion was combined with the PDE5 inhibitor vardenafil (1, 3 and 10 mg/kg) or with the PDE2 inhibitor BAY 60-7550 (0.3, 1 and 3 mg/kg), 30 min prior to testing. Acute tryptophan depletion significantly lowered plasma tryptophan levels and impaired object recognition performance. Vardenafil (3 and 10 mg/kg) and BAY 60-7550 (3 mg/kg) were able to attenuate the acute tryptophan depletion induced object recognition impairment. Thus, both PDE5 and PDE2 inhibition improved short-term object recognition performance after an acute tryptophan depletion induced deficit. The underlying mechanisms, however, remain poorly understood and further studies are needed to determine whether the present findings can be explained by a direct effect of enhanced cAMP and cGMP levels upon 5-HT activity, or even other neurotransmitter systems, and possibly an interaction with synthesis of nitric oxide or effects upon cerebral blood flow function.

Regulation of long-term depression by increases in [guanosine 3',5'-cyclic monophosphate] in the hippocampal CA1 region of freely behaving rats

A role for guanosine 3',5'-cyclic monophosphate (cGMP) and the protein kinase G (PKG) pathway in synaptic long-term depression (LTD) in the hippocampal CA1 region has been proposed, based on observations in vitro, where, for example, increases of [cGMP] result in short-term depression (STD) coupled with a reduction in presynaptic glutamate release. To date, no evidence exists to support that LTD in the intact, freely behaving animal involves these mechanisms. We examined the effect of increases of [cGMP] on basal transmission and electrically-induced STD at hippocampal CA1 synapses in vivo. We found that elevating [cGMP] dose-dependently caused a chemically-induced STD which occluded electrically-induced STD. Repeated administration of Zaprinast, an inhibitor of cGMP-degrading phosphodiesterase, resulted in persistent LTD (>24 h). Paired-pulse analysis supported a presynaptic mechanism of action. Application of an inhibitor of soluble guanylate cyclase prevented LTD induced by low-frequency stimulation (LFS), and impaired LFS-STD elicited in the presence of Zaprinast. These data suggest the involvement of cGMP in LTD in the CA1 region of freely behaving adult rats.

Alterations of the expression and activity of midbrain nitric oxide synthase and soluble guanylyl cyclase in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonism in mice

The study was aimed at investigating the expression and the activity of neuronal nitric oxide synthase, and of soluble guanylyl cyclase and phosphodiesterase activities that regulate guanosine 3',5'-cyclic monophosphate level in the midbrain, in a mouse model of PD using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine injections. Adult male mice of the C57/BL strain were given three i.p. injections of physiological saline or three i.p. injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine solution in physiological saline at 2 h intervals (summary 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine dose: 40 mg/kg), and were killed 3, 7, or 14 days later. mRNA, protein level, and/or activities of neuronal nitric oxide synthase, soluble guanylyl cyclase, phosphodiesterase and guanosine 3',5'-cyclic monophosphate were determined. Immunohistochemistry showed about 75% decrease in the number of tyrosine hydroxylase-positive neurons in the substantia nigra pars compacta. Mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine showed increased midbrain guanylyl cyclase and total nitric oxide synthase activities at 3, 7, and 14 days post-treatment. The specific neuronal nitric oxide synthase inhibitor 7-nitroindazole (10 microM) and the specific inducible nitric oxide synthase inhibitor 1400W (10 microM) inhibited the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced excess in nitric oxide synthase activity by 63-70 and 13-25%, respectively. The increases in total midbrain nitric oxide synthase activity were accompanied by elevated guanosine 3',5'-cyclic monophosphate, enhanced expression of neuronal nitric oxide synthase and of the beta1 subunit of guanylyl cyclase at both mRNA and protein levels that persisted up to the end of the observation period, and by enhanced neuronal nitric oxide synthase and guanylyl cyclase beta1 immunoreactivities in substantia nigra pars compacta 7 and 14 days after the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. The increases in guanylyl cyclase activity were found to occur exclusively due to increased maximal enzyme activity. No 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced change in phosphodiesterase activity has been detected in any brain region studied. 7-Nitroindazole prevented a significant increase in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced midbrain guanosine 3',5'-cyclic monophosphate level and neurodegeneration of dopaminergic neurons. These results raise the possibility that the nitric oxide/guanylyl cyclase/guanosine 3',5'-cyclic monophosphate signaling pathway may play a role in maintaining dopaminergic neurons function in substantia nigra pars compacta.

The selective PDE5 inhibitor, sildenafil, improves object memory in Swiss mice and increases cGMP levels in hippocampal slices

Previous studies have shown memory enhancing effects of phosphodiesterase type 5 (PDE5) inhibitors in rats. However, differences in nitric oxide (NO)-mediated cyclic GMP (cGMP) signaling in the hippocampus have been described between rats and mice. In the present study we investigated the memory enhancing effects of the PDE5 inhibitor, sildenafil on memory performance in Swiss mice using the object recognition task. Sildenafil (0.3, 1 and 3 mg/kg) was administered orally directly after the first trial. The memory for the objects was retested 24 h later when mice show no memory for the familiar object. Sildenafil improved the object discrimination performance of Swiss mice at a dose of 1 mg/kg. Hippocampal slices of Swiss mice incubated with sildenafil (10 microM) increased cGMP levels in varicosities in the CA3 region of the hippocampus and a number of short, thin fibers. Addition of DEA/NO, an NO donor (10 microM), in the presence of sildenafil (10 microM) strongly increased cGMP immunoreactivity of varicosities in the CA3 region. Double immunostaining of cGMP with the presynaptic marker synaptophysin did not reveal any co-localization of these markers under any circumstance. Taken together, inhibition of PDE5 improves object recognition memory in mice. Furthermore, a postsynaptic role of cGMP could be involved in this respect.

Dissociable effects of acetylcholinesterase inhibitors and phosphodiesterase type 5 inhibitors on object recognition memory: acquisition versus consolidation

RATIONALE

Phosphodiesterase enzyme type 5 (PDE5) inhibitors and acetylcholinesterase (AChE) inhibitors have cognition-enhancing properties. However, it is not known whether these drug classes affect the same memory processes.

OBJECTIVE

We investigated the memory-enhancing effects of the PDE5 inhibitor sildenafil and AChE inhibitors metrifonate and donepezil in the object recognition task to find out whether acquisition or consolidation processes were affected by these drugs.

METHODS

The object recognition task measures whether rats remembered an object they have explored in a previous learning trial. All drugs were given orally 30 min before or immediately after learning to study acquisition and consolidation, respectively.

RESULTS

Sildenafil given immediately after the first trial improved the memory performance after 24 h and resulted in an inverted U-shaped dose-effect curve with the peak dose at 3 mg/kg. When given before the first trial, sildenafil also improved the memory performance. However, the dose needed for the best performance under this condition was 10 mg/kg, suggesting that the dose-effect curve shifted to the right. This can be explained by the metabolic clearance of the high dose of sildenafil. Donepezil had no memory improving effect when given after the first trial. However, when given before the first trial, a gradually increasing dose-effect curve was found which had its maximum effect at the highest dose tested (1 mg/kg). Likewise, only when metrifonate (30 mg/kg) was given before the first trial did rats show an improved memory performance.

CONCLUSION

Our data strongly suggest that PDE5 inhibitors improve processes of consolidation of object information, whereas AChE inhibitors improve processes of acquisition of object information.

Phosphodiesterase type 5 inhibition improves early memory consolidation of object information

The nitric oxide (NO)-cyclic GMP (cGMP) signaling pathway is assumed to play an important role in processes underlying learning and memory. We used phosphodiesterase type 5 (PDE5) inhibitors to study the role of cGMP in object- and spatial memory. Our results and those reported in other studies indicate that elevated hippocampal cGMP levels are required to improve the memory performance of rodents in object recognition and passive avoidance learning, but not in spatial learning. The timing of treatment modulates the effects on memory and strongly supports a role for cGMP in early stages of memory formation. Alternative explanations for the improved memory performance of PDE5 inhibitors are also discussed. Immunocytochemical studies showed that in vitro slice incubations with PDE5 inhibitors increase NO-stimulated cGMP levels mainly in hippocampal varicose fibers. Reviewing the available data on the localization of the different components of the NO-cGMP signaling pathway, indicates a complex interaction between NO and cGMP, which may be independent of each other. It is discussed that further studies are needed, immunocytochemical and behavioral, to better understand the cGMP-mediated molecular mechanisms underlying memory formation.

Species differences in the localization of cGMP-producing and NO-responsive elements in the mouse and rat hippocampus using cGMP immunocytochemistry

The aim of the study was to compare the localization of the nitric oxide (NO)-cGMP pathway in hippocampus of mice and rats using cGMP- and soluble guanylyl cyclase (GC) immunocytochemistry and in situ hybridization of the cGMP-hydrolysing phosphodiesterase types 2, 5 and 9. In vitro incubation of hippocampus slices in the absence of a guanylyl cyclase stimulator or a phosphodiesterase inhibitor resulted in cGMP-positive astrocytes mainly in the CA1 area in mouse slices. In contrast, no cGMP immunoreactivity was observed under these conditions in the rat hippocampus. Treatment with an NO synthase inhibitor or inhibitors of soluble or particulate GC did not abolish cGMP immunoreactivity in astrocytes. Incubation with the NO donors sodium nitroprusside or diethylamino NONOate, or with the NO-independent activators of soluble GC, YC-1 and BAY 41-2272, in combination with phosphodiesterase inhibitors, resulted in an increase in cGMP immunoreactivity in numerous astrocytes throughout the mouse hippocampus. In contrast, under these conditions cGMP immunoreactivity was primarily observed in varicose fibers in rat hippocampus. Comparison of the cellular localization of the beta1 subunit of soluble GC and the mRNAs of PDE2, PDE5 and PDE9 revealed that in both species the beta1 subunit was observed in pyramidal and granule cells, which also expressed the mRNAs of the three phosphodiesterase families. Although the beta1 subunit was observed in astrocytes, none of the phosphodiesterases were detected in these cells. We conclude that, although the expression profiles of the soluble GC beta1 subunit and cGMP-hydrolysing phosphodiesterase mRNAs were identical, the cellular patterns of cGMP immunoreactivity differ between rat and mouse hippocampus.

Pairing elevation of [cyclic GMP] with inhibition of PKA produces long-term depression of glutamate release from isolated rat hippocampal presynaptic terminals

Data suggest both presynaptic and postsynaptic changes contribute to activity-dependent long-term synaptic plasticity. We have shown that pairing elevation of intracellular [cyclic GMP], using the type V phosphodiesterase inhibitor zaprinast, with inhibition of cyclic AMP-dependent protein kinase (PKA), is sufficient to elicit chemical long-term depression (CLTD) of synaptic transmission at Schaffer collateral-CA1 and mossy fibre-CA3 synapses in rat hippocampus. CLTD does not require synaptic activity, and selective postsynaptic drug injections do not affect it, suggesting it is presynaptically induced and expressed. To directly evaluate this hypothesis, we tested whether CLTD of transmitter release can be expressed in isolated presynaptic nerve terminals. Presynaptic nerve terminals (synaptosomes) were isolated from rat hippocampi by Percoll density gradient centrifugation. Synaptosomes were loaded with [3H]glutamate, and basal and depolarisation-induced release of [3H]glutamate measured in control medium versus medium containing zaprinast (20 microm) plus or minus the PKA inhibitor H-89 (10 microm). Zaprinast produced a significant decrease in basal [3H]glutamate release. However, only combining zaprinast with H-89 significantly depressed K+-evoked [3H]glutamate release. After a 20-min drug washout, basal release returned to normal in all conditions, but K+-evoked [3H]glutamate release was persistently reduced only by the combination of zaprinast plus H-89. Long-term reduction of [3H]glutamate release from synaptosomes was completely prevented by the PKG inhibitor KT5823 (5 microm). These data demonstrate the existence of a presynaptic, cyclic GMP-PKG dependent cascade capable of expressing LTD of glutamate release from isolated hippocampal nerve terminals.

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.

Regional distribution of protein and activity of the nitric oxide receptor, soluble guanylyl cyclase, in rat brain suggests multiple mechanisms of regulation

Nitric oxide (NO) is an unconventional neuromodulator that signals by intercellular diffusion. Its effects are often mediated by activation of its cytosolic receptor, the hemoprotein soluble guanylyl cyclase (sGC). Regional distribution of heterodimeric (alpha/beta) sGC at both the activity and protein level and its regulation are still unclear. Here, sGC was analyzed in rat brain by Western blot and NO donor-stimulated cyclic GMP accumulation. sGCalpha(1) and sGCbeta(1) immunoreactive protein signals strongly correlated with each other. However, V(max) values depended on the type of NO donor used. Sodium nitroprusside, the most widely used compound and formally an NO(+) donor, was up to 20-fold less effective in stimulating sGC activity than the NO donor diethylamine NONOate. In contrast to the rather even distribution of sGC proteins and SNP-stimulated cGMP accumulation in various regions of rat brain, diethylamine NONOate-stimulated sGC activity varied up to 8-fold between the different brain regions tested. In conclusion, we show that expression of both sGCalpha(1) and sGCbeta(1) subunits is tightly coregulated in rat brain, while yet unknown additional mechanisms affect the V(max) of sGC.

Skin pigmentation enhancers

The highest incidences of cancer are found in the skin, but endogenous pigmentation is associated with markedly reduced risk. Agents that enhance skin pigmentation have the potential to reduce both photodamage and skin cancer incidence. The purpose of this review is to evaluate agents that have the potential to increase skin pigmentation. These include topically applied substances that simulate natural pigmentation: dihydroxyacetone and melanins; and substances that stimulate the natural pigmentation process: psoralens with UVA (PUVA), dimethylsulfoxide (DMSO), L-tyrosine, L-Dopa, lysosomotropic agents, diacylglycerols, thymidine dinucleotides, DNA fragments, melanocyte stimulating hormone (MSH) analogs, 3-isobutyl-1-methylxanthine (IBMX), nitric oxide donors, and bicyclic monoterpene (BMT) diols. These agents are compared with regards to efficacy when administered to melanoma cells, normal human epidermal melanocytes, animal skin, and human skin. In addition, mechanisms of action are reviewed since these may reveal issues related to both efficacy and safety. Both dihydroxyacetone and topically applied melanins are presently available to the consumer, and both of these have been shown to provide some photoprotection. Of the pigmentation stimulators, only PUVA and MSH analogs have been tested extensively on humans, but there are concerns about the safety and side effects of both. At least some of the remaining pigmentation stimulators under development have the potential to safely induce a photoprotective tan.

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.

Chemically induced, activity-independent LTD elicited by simultaneous activation of PKG and inhibition of PKA

Although it is widely agreed that cyclic AMP is necessary for the full expression of long-term potentiation of synaptic strength, it is unclear whether cyclic AMP or cyclic AMP-dependent protein kinase (PKA) play roles in the induction of long-term depression (LTD). We show here that two PKA inhibitors, H-89 (10 microM) and KT5720 (1 microM), are unable to block induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices in vitro. Rather, H-89 enhanced the magnitude of LTD induced by submaximal low-frequency stimulation. Raising [cGMP] with zaprinast (20 microM), a selective type V phosphodiesterase inhibitor, reversibly depressed synaptic potentials. However, coapplication of H-89 plus zaprinast converted this to a robust LTD that depended critically on activation of cyclic GMP-dependent protein kinase (PKG). Chemically induced LTD is activity-independent because it could be induced without stimulation and in tetrodotoxin (0.5 microM). Additionally, chemical LTD did not require activation of N-methyl-D-aspartate or GABA receptors and could be reversed by LTP. Stimulus-induced LTD occluded chemical LTD, suggesting a common expression mechanism. In contrast to bath application, postsynaptic infusion of H-89 into CA1 pyramidal neurons did not enhance LTD, suggesting a presynaptic site of action. Further evidence for a presynaptic locus was supplied by experiments where H-89 applied postsynaptically along with bath application of zaprinast was unable to produce chemical LTD. Thus simultaneous presynaptic generation of cyclic GMP and inhibition of PKA is sufficient to induce LTD of synaptic transmission at Schaffer collateral-CA1 synapses.

Effect of arginine on basal and high potassium-induced efflux of [3H]D-aspartate from rat striatal slices

There are conflicting reports in the literature regarding the effects of nitric oxide as well as the involvement of the cyclic GMP pathway on the transmitter release. To study the influence of the availability of the nitric oxide precursor arginine on the glutamate transmission process, rat striatal slices preloaded with the tritiated glutamate analogue D-aspartate were used. L-Arginine stimulated in a concentration-dependent way (0.01-10.0 mM) the high potassium-induced efflux of [3H]D-aspartate. The basal release was increased only by 10 mM L-arginine. Neither the basal nor the depolarization-induced efflux of [3H]D-aspartate was affected by D-arginine. The L-arginine effect was abolished by the nitric oxide synthase inhibitor L-arginine methyl ester and was not modified by cyclic GMP. Only at high concentrations of L-arginine (10 mM) could an elevation of cyclic GMP level be demonstrated. The results are discussed in terms of direct presynaptic action of nitric oxide on [3H]D-aspartate efflux and a possible modulation of glutamate release by the availability of arginine.

Bicyclic monoterpene diols induce differentiation of S91 melanoma and PC12 pheochromocytoma cells by a cyclic guanosine-monophosphate-dependent pathway

Previously, we showed that 5-norbornene-2,2-dimethanol (5-NBene-2,2-DM) is an effective inducer of melanogenesis in cultured cells and guinea-pig skin [Brown et al. (1998) J. Invest. Dermatol., 110:428-437]. This study shows that 2,3-cis/exo-pinanediol (2,3-cs/ex-PinD) is a more effective inducer of melanogenesis than 5-NBene-2,2-DM in S91 mouse melanoma cells. Furthermore, 2,3-cs/ex-PinD appears to penetrate guinea-pig skin better than 5-NBene-2,2-DM and to induce higher levels of pigmentation. Both 5-NBene-2,2-DM and 2,3-cs/ex-PinD induce synthesis of nitric oxide (NO) in S91 cells, and the melanogenic activity of both compounds is reduced by inhibitors of the NO/cyclic guanosine monophosphate (cGMP)/protein kinase(PK) G signaling pathway, but not by inhibitors of the PKC or PKA pathways. Thus, these bicyclic monoterpene diols appear to induce melanogenesis by the same pathway in S91 cells as that shown previously for ultraviolet radiation in melanocytes (Romero-Graillet et al. (1996) J. Biol. Chem., 271:28052-28056). These compounds also induce NO synthesis, neurite outgrowth, and tyrosine hydroxylase activity in PC12 pheochromocytoma cells. Neurite outgrowth in PC12 cells is blocked by the guanylate cyclase inhibitor, LY83583 (6-anilino-2,8-quinolinequinone), indicating that, similar to S91 cells, the induction of morphological differentiation of PC12 cells by bicyclic monoterpene diols is regulated by a cGMP-dependent pathway.

Local inhibition of hippocampal nitric oxide synthase does not impair place learning in the Morris water escape task in rats

Recent studies have provided evidence that nitric oxide (NO) has a role in certain forms of memory formation. Spatial learning is one of the cognitive abilities that has been found to be impaired after systemic administration of an NO-synthase inhibitor. As the hippocampus has a pivotal role in spatial orientation, the present study examined the role of hippocampal NO in spatial learning and reversal learning in a Morris task in adult rats. It was found that N omega-nitro-L-arginine infusions into the dorsal hippocampus affected the manner in which the rats were searching the submerged platform during training, but did not affect the efficiency to find the spatial location of the escape platform. Hippocampal NO-synthase inhibition did not affect the learning of a new platform position in the same water tank (i.e. reversal learning). Moreover, no treatment effects were observed in the probe trials (i.e. after acquisition and after reversal learning), indicating that the rats treated with N omega-nitro-L-arginine had learned the spatial location of the platform. These findings were obtained under conditions where the NO synthesis in the dorsal hippocampus was completely inhibited. On the basis of the present data it was concluded that hippocampal NO is not critically involved in place learning in rats.

Distribution of nitric oxide synthase and nitric oxide-receptive, cyclic GMP-producing structures in the rat brain

The structures capable of synthesizing cyclic GMP in response to nitric oxide in the rat brain were compared relative to the anatomical localization of neuronal nitric oxide synthase. In order to do this, we used brain slices incubated in vitro, where cyclic GMP-synthesis was stimulated using sodium nitroprusside as a nitric oxide-donor compound, in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine. Nitric oxide-stimulated cyclic GMP synthesis was found in cells and fibers, but was especially prominent in varicose fibers throughout the rat brain. Fibers containing the nitric oxide-stimulated cyclic GMP production were present in virtually every area of the rat brain although there were large regional variations in the density of the fiber networks. When compared with the localization of nitric oxide synthase, it was observed that although nitric oxide-responsive and the nitric oxide-producing structures were found in similar locations in general this distribution was complementary. Only occasionally was nitric oxide-mediated cyclic GMP synthesis observed in structures which also contained nitric oxide synthase. We conclude that the nitric oxide-responsive soluble guanylyl cyclase and nitric oxide synthase are usually juxtaposed at very short distances in the rat brain. These findings very strongly support the proposed role of nitric oxide as an endogenous activator of the soluble guanylyl cyclase in the central nervous system and convincingly demonstrate the presence of the nitric oxide-cyclic GMP signal transduction pathway in virtually every area of the rat brain.

Topographical relationship between neuronal nitric oxide synthase immunoreactivity and cyclic 3',5'-guanosine monophosphate accumulation in the brain of the adult Xenopus laevis

Previous immunohistochemical staining procedures of the brain and pituitary in Xenopus laevis, using an antiserum against neuronal nitric oxide (NO) synthase (nNOS) and nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry, have revealed NOS activity in neurons and fibers in a number of brain areas, as well as in fibers in the pituitary. In the present study we have localized the target structures of the NOergic system in the Xenopus brain by visualizing the sites of NO-sensitive cyclic 3',5'-guanosine monophosphate (cGMP) accumulation, according to a method for cGMP visualization in rat brain slices. Brain slices of unfixed Xenopus are incubated in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine and the NO donor sodium nitroprusside, followed by fixation and cryosectioning. Sections were then processed for immunohistochemistry using rabbit and sheep antisera against cGMP and a sheep antiserum against nNOS. Visualization of single and double labeling of cGMP immunoreactive and/or nNOS immunoreactive structures was performed with combined CY3/fluorescein isothiocyanate fluorescence microscopy. Following this procedure, we provide immunohistochemical evidence for the distribution of cGMP-accumulating neurons in the brain of adult Xenopus. In most brain areas, the distribution of nNOS and cGMP immunoreactive structures (neuron somata and fibers) is distinct and separate, for instance in the dorsal pallium, the lateral thalamic nuclei, the optic tectum, the locus coeruleus and the reticular formation. However, nNOS and cGMP immunoreactive structures are often found in the vicinity of each other, and in the optic tectum even in adjacent neuron fibers and somata. The present observations are in line with the presence of an NO-dependent soluble guanylate cyclase in distinct brain areas of Xenopus laevis, corroborating similar data in the mammalian brain. Further, our observations may add to the understanding of the anatomical connectivity pattern and functional relevance of the NOergic system in the amphibian brain.

Nitric oxide-mediated cGMP synthesis in Helix neural ganglia

The central nervous system of the mollusc Helix pomatia was stimulated with NO donors sodium nitroprusside (SNP), S-nitroso-N-acetylpenicillamine (SNAP) or hydroxylamine, in the presence of a phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (IBMX). Radioimmunoassay revealed that all of the three NO donors significantly increased cGMP levels by 22-27-fold above basal levels. Compared with controls, strong cGMP immunoreactivity was observed in axons and cytoplasm of the stimulated neurons. About 80% of cGMP-immunoreactive neurons colocalized with NADPH-diaphorase activity. Some glial cells and giant neurons were not stained by NADPH-diaphorase histochemistry but were cGMP-immunoreactive. The results suggest the existence of a NO/cGMP pathway and indicate NO as an intra- and intercellular signaling molecule in the Helix central nervous system.

Nitric oxide synthase and background adaptation in Xenopus laevis

Adaptation of the skin colour to the background light condition in the amphibian Xenopus laevis is achieved by migration of pigment granules in the skin melanophores, a process regulated by alpha-MSH secretion from melanotrope cells in the pituitary pars intermedia (PI). alpha-MSH secretion in turn, is regulated by various stimulatory and inhibitory messengers synthesized in brain nuclei, especially the hypothalamic suprachiasmatic and magnocellular nuclei and the locus coeruleus in the hindbrain. In the present study, the roles in background adaptation of nitric oxide (NO) and NO synthase (NOS) enzyme activity were evaluated. In situ, using both immunohistochemistry with anti-human brain NOS (bNOS) serum in paraffin-embedded material and using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry in cryo-sections, we showed NOS in neurons in the optic tectum and in the locus coeruleus. NADPH-d reactivity was also found in neurons in the lateral amygdala, the ventral hypothalamic nucleus and in fibers in the median eminence. Using a Western blot stained with an anti-human bNOS serum, we demonstrated a 150 kDa band in Xenopus hindbrain lysates, which is similar to the NOS protein present in the rat anterior pituitary, but which was not detectable in the lysates from both the neurointermediate and distal lobes in Xenopus. No differences in histochemical staining pattern or on Western blotting were observed between animals adapted to a black or a white background. Paraffin sections of the endocrine PI and pars distalis did not reveal bNOS-like immunoreactivity. NADPH-d reactivity was observed in the endothelia of this gland. However, using a new procedure of thin cryo-sections of pituitary neurointermediate lobes, we observed bNOS-immunoreactive fibers as well as cyclic 3',5' guanosine monophosphate (cGMP)-accumulating fibers in the PI. The PI may be regulated by NOergic neurons from higher brain centers. The possibility that NOergic neurons in the locus coeruleus are involved in the innervation of the PI needs further investigation. The latter neurons are probably not noradrenergic because double labeling studies show no co-localization of NADPH-d reactivity and tyrosine hydroxylase immunoreactivity in locus coeruleus neurons.

Possible role of nitric oxide-cyclic GMP pathway in object recognition memory: effects of 7-nitroindazole and zaprinast

The effects of 7-nitroindazole, a putative selective inhibitor of neuronal nitric oxide (NO) synthase and zaprinast, a cGMP-selective phosphodiesterase inhibitor, were evaluated on recognition memory of rats in the object recognition test. This test is based on the differential exploration of a new and a familiar object. Two doses of 7-nitroindazole (10 and 30 mg/kg) and zaprinast (3 and 10 mg/kg) were used. The substances were administered i.p. immediately after the exposure to two identical objects, i.e., at the start of the delay interval. After a delay interval of 1 h, control rats spent more time exploring the new object which demonstrates that they recognized the familiar one. Both doses of 7-nitroindazole impaired the discrimination between the two objects after the 1 h interval. After a 4 h interval, control rats did not discriminate between the objects. The highest dose of zaprinast facilitated object recognition after the 4 h interval. In addition, this dose of zaprinast (10 mg/kg) reversed the recognition memory deficit induced by 7-nitroindazole (10 mg/kg) at the 1 h interval. The highest dose of 7-nitroindazole slightly increased mean arterial blood pressure 1 h after its administration. 4 h after administration of zaprinast (10 mg/kg), mean arterial blood pressure was also slightly increased, but not after 1 h after zaprinast administration. However, these effects on blood pressure do not explain the differential effects on object recognition memory. These results therefore suggest that NO-cGMP signal transduction is involved in object recognition memory independently of its cardiovascular role. Finally, since 7-nitroindazole affected mean arterial blood pressure it can not be regarded as a selective inhibitor of neuronal NO synthase.