Hydrolysis-resistant Analog Research Articles

Spatially restricted ecto-5'-nucleotidase expression promotes the growth of uterine leiomyomas by modulating Akt activity.

Found in as many as 80% of women, uterine leiomyomas are a frequent cause of abnormal uterine bleeding, pelvic pain, and infertility. Despite their significant clinical impact, the mechanisms responsible for driving leiomyoma growth remain poorly understood. After obtaining IRB permission, expression of ecto-5'-nucleotidase (NT5E, CD73) was assessed in matched specimens of myometrium and leiomyoma by real-time qPCR, Western blot, and immunohistochemistry (IHC). Adenosine concentrations were measured by enzyme-linked assay. Primary cultures were used to assess the impact of adenosine and/or adenosine receptor agonists on proliferation, apoptosis, and patterns of intracellular signaling invitro. When compared to matched specimens of healthy myometrium, uterine leiomyomas were characterized by reduced CD73 expression. Largely limited to thin-walled vascular structures and the pseudocapsule of leiomyomas despite diffuse myometrial distribution. Restricted intra-tumoral CD73 expression was accompanied by decreased levels of intra-tumoral adenosine. Invitro, incubation of primary leiomyoma cultures with adenosine or its hydrolysis-resistant analog 2-chloro-adenosine (2-CL-AD) inhibited proliferation, induced apoptosis, and reduced proportion of myocytes in S- and G2-M phases of the cell cycle. Decreased proliferation was accompanied by reduced expression of phospho-Akt, phospho-Cdk2-Tyr15, and phospho-Histone H3. Enforced expression of the A2B adenosine receptor (ADORA2B) and ADORA2B-selective agonists similarly suppressed proliferation and inhibited Akt phosphorylation. Collectively, these observations broadly implicate CD73 and reducedextracellular concentrations ofadenosine as key regulators of leiomyoma growth and potentiallyidentify novel strategies for clinically managing these common tumors.

Adenine Nucleotides Control Proliferation In Vivo of Rat Retinal Progenitors by P2Y1 Receptor.

Previous studies demonstrated that exogenous ATP is able to regulate proliferation of retinal progenitor cells (RPCs) in vitro possibly via P2Y1 receptor, a G protein-coupled receptor. Here, we evaluated the function of adenine nucleotides in vivo during retinal development of newborn rats. Intravitreal injection of apyrase, an enzyme that hydrolyzes nucleotides, reduced cell proliferation in retinas at postnatal day 2 (P2). This decrease was reversed when retinas were treated together with ATPγ-S or ADPβ-S, two hydrolysis-resistant analogs of ATP and ADP, respectively. During early postnatal days (P0 to P5), an increase in ectonucleotidase (E-NTPDase) activity was observed in the retina, suggesting a decrease in the availability of adenine nucleotides, coinciding with the end of proliferation. Interestingly, intravitreal injection of the E-NTPDase inhibitor ARL67156 increased proliferation by around 60% at P5 rats. Furthermore, immunolabeling against P2Y1 receptor was observed overall in retina layers from P2 rats, including proliferating Ki-67-positive cells in the neuroblastic layer (NBL), suggesting that this receptor could be responsible for the action of adenine nucleotides upon proliferation of RPCs. Accordingly, intravitreal injection of MRS2179, a selective antagonist of P2Y1 receptors, reduced cell proliferation by approximately 20% in P2 rats. Moreover, treatment with MRS 2179 caused an increase in p57KIP2 and cyclin D1 expression, a reduction in cyclin E and Rb phosphorylated expression and in BrdU-positive cell number. These data suggest that the adenine nucleotides modulate the proliferation of rat RPCs via activation of P2Y1 receptors regulating transition from G1 to S phase of the cell cycle.

Exploring Light and Life

Exploring Light and Life

Endogenous fatty acid ethanolamides suppress nicotine-induced activation of mesolimbic dopamine neurons through nuclear receptors.

Nicotine stimulates the activity of mesolimbic dopamine neurons, which is believed to mediate the rewarding and addictive properties of tobacco use. Accumulating evidence suggests that the endocannabinoid system might play a major role in neuronal mechanisms underlying the rewarding properties of drugs of abuse, including nicotine. Here, we investigated the modulation of nicotine effects by the endocannabinoid system on dopamine neurons in the ventral tegmental area with electrophysiological techniques in vivo and in vitro. We discovered that pharmacological inhibition of fatty acid amide hydrolase (FAAH), the enzyme that catabolizes fatty acid ethanolamides, among which the endocannabinoid anandamide (AEA) is the best known, suppressed nicotine-induced excitation of dopamine cells. Importantly, this effect was mimicked by the administration of the FAAH substrates oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), but not methanandamide, the hydrolysis resistant analog of AEA. OEA and PEA are naturally occurring lipid signaling molecules structurally related to AEA, but devoid of affinity for cannabinoid receptors. They blocked the effects of nicotine by activation of the peroxisome proliferator-activated receptor-alpha (PPAR-alpha), a nuclear receptor transcription factor involved in several aspects of lipid metabolism and energy balance. Activation of PPAR-alpha triggered a nongenomic stimulation of tyrosine kinases, which might lead to phosphorylation and negative regulation of neuronal nicotinic acetylcholine receptors. These data indicate for the first time that the anorexic lipids OEA and PEA possess neuromodulatory properties as endogenous ligands of PPAR-alpha in the brain and provide a potential new target for the treatment of nicotine addiction.

Pharmacological Characterization of Hydrolysis-Resistant Analogs of Oleoylethanolamide with Potent Anorexiant Properties

Oleoylethanolamide (OEA) is an endogenous lipid mediator that reduces food intake, promotes lipolysis, and decreases body weight gain in rodents by activating peroxisome proliferator-activated receptor-alpha (PPAR-alpha). The biological effects of OEA are terminated by two intracellular lipid hydrolase enzymes, fatty-acid amide hydrolase and N-acylethanolamine-hydrolyzing acid amidase. In the present study, we describe OEA analogs that resist enzymatic hydrolysis, activate PPAR-alpha with high potency in vitro, and persistently reduce feeding when administered in vivo either parenterally or orally. The most potent of these compounds, (Z)-(R)-9-octadecenamide,N-(2-hydroxyethyl,1-methyl) (KDS-5104), stimulates transcriptional activity of PPAR-alpha with a half-maximal effective concentration (EC50) of 100 +/- 21 nM (n = 11). Parenteral administration of KDS-5104 in rats produces persistent dose-dependent prolongation of feeding latency and postmeal interval (half-maximal effective dose, ED50 = 2.4 +/- 1.8 mg kg(-1) i.p.; n = 18), as well as increased and protracted tissue exposure compared with OEA. Oral administration of the compound also results in a significant tissue exposure and reduction of food intake in free-feeding rats. These results suggest that the endogenous high-affinity PPAR-alpha agonist OEA may provide a scaffold for the discovery of novel orally active PPAR-alpha ligands.

Store Calcium Mediates Cholinergic Effects on mIPSCs in the Rat Main Olfactory Bulb

The significance of endoplasmic reticulum (ER) store calcium in modulating transmitter release is slowly gaining recognition. One transmitter system that might play an important role in store calcium modulation of transmitter release in the CNS is acetylcholine (ACh). The main olfactory bulb (OB) receives rich cholinergic innervation from the horizontal limb of the diagonal band of Broca and blocking cholinergic signaling in the bulb inhibits the ability of animals to discriminate between closely related odors. Here we show that exposing OB slices to carbamylcholine (CCh), a hydrolysis-resistant analog of Ach, increases gamma-aminobutyric acid (GABA) release at dendrodendritic synapses onto the mitral cells. This increase in transmitter release is mediated by the activation of the M1 class of muscarinic receptors and requires the mobilization of calcium from the ER. The site of action of CCh for this effect is developmentally regulated. In animals younger than postnatal day 10, the major action of CCh appears to be on mitral cells, enhancing GABA release by reciprocal signaling resulting from increased glutamate release from mitral cells. In animals older than postnatal day 10, CCh appears to modulate transmitter release from dendrites of the interneurons themselves. Our results point to modulation of inhibition as an important role for cholinergic signaling in the OB. Our data also strengthen the emerging idea of a role for store calcium in modulating transmitter release at CNS synapses.

Misidentification of prostamides as prostaglandins

Prostaglandins and endogenous cannabinoid metabolites share the same lipid backbone with differing polar head groups at exactly the position through which a large molecule is attached to provide antigenicity and thus raise antisera. Hence, we hypothesized that antisera raised against prostaglandins linked to a large molecule such as BSA at the carboxyl functional group would also recognize endogenous cannabinoid metabolites and lead to highly misleading interpretations of data. We found major cross-reactivity of commercial antisera raised to prostaglandins with endocannabinoid metabolites. Furthermore, in a well-characterized cell line (WISH) or primary amnion tissue explants, endocannabinoid treatment led to increased production of endocannabinoid metabolites as opposed to primary prostaglandins. This was apparent only after separation of products by thin-layer chromatography, because they measured as prostaglandins by radioimmunoassay. These findings have major implications for our interpretation of data in situations in which these prostaglandin-like molecules are formed, and they stress the need for chromatographic or spectrometric confirmation of prostaglandin production detected by antibody-based methods.

Medicinal Chemistry and Pharmacology of Cyclic ADP‐Ribose

AbstractFor Abstract see ChemInform Abstract in Full Text.

Medicinal chemistry and pharmacology of cyclic ADP-ribose.

Cyclic ADP-ribose (cADPR) is a signaling molecule that has been shown to regulate calcium mobilization from intracellular stores in a wide variety of biological systems. Synthesis of structural analogs of cADPR has provided insights into structure-activity relationships as well as produced pharmacological research tools with useful properties such as, hydrolysis-resistance and cell permeability. The first generation of cADPR analogs was synthesized by a chemo-enzymatic approach that took advantage of the broad substrate specificity of Aplysia ADP-ribosyl cyclase. Analogs synthesized by this approach provided useful structure-activity information, including the importance of the 8-position of the adenine in determining agonistic or antagonistic activity and of the 3'-hydroxyl group of the southern ribose for activity. Hydrolysis resistant analogs were generated by replacing the southern ribose with a carbocyclic structure or by replacing the adenine ring with 7-deaza- or 3-deaza-adenine. Approaches to synthesize cADPR analogs by total chemical approaches have been recently reported. These approaches allow the synthesis of analogs with stable linkages between N1 of adenine and the northern ribose (or surrogate) that are not possible with the enzymatic strategy. This review will focus on the synthesis and properties of analogs that have been shown to have utility in dissecting the role of cADPR in calcium signaling.

GTP gamma S increases Nav1.8 current in small-diameter dorsal root ganglia neurons.

Tetrodotoxin-resistant (TTX-R) sodium current in small-size dorsal root ganglia (DRG) neurons is upregulated by prostaglandin E(2) and serotonin through a protein kinase A (PKA)/protein kinase (PKC) pathway, suggesting G protein modulation of one or more TTX-R channels in these neurons. Recently, GTP(gammaS), a hydrolysis-resistant analogue of GTP, was shown to increase the persistent current produced by the TTX-R Na(v)1.9. In this study, we investigated the modulation of another TTX-R channel, Na(v)1.8, by GTP(gammaS) in small-diameter DRG neurons from rats using whole-cell voltage clamp recordings. Because it has been suggested that fluoride, often used in intracellular recording solutions, may bind to trace amounts of aluminum and activate G proteins, we recorded Na(v)1.8 currents with and without intracellular fluoride, and with the addition of deferoxamine, an aluminum chelator, to prevent fluoride-aluminum binding. Our results show that GTP(gammaS) (100 micro M) caused a significant increase in Na(v)1.8 current (67%) with a chloride-based intracellular solution. Although the inclusion of fluoride instead of chloride in the pipette solution increased the Na(v)1.8 current by 177%, GTP(gammaS) further increased Na(v)1.8 current by 67% under these conditions. While the effect of GTP(gammaS) was prevented by pretreatment with H7 (100 micro M), a non-selective PKA/PKC inhibitor, the fluoride-induced increase in Na(v)1.8 current was not sensitive to H7 (100 micro M), or to inclusion of deferoxamine (1 mM) in the intracellular solution. We conclude that G protein activation by GTP(gammaS) increases Na(v)1.8 current through a PKA/PKC mechanism and that addition of fluoride to the pipette solution further enhances the current, but is not a confounding variable in the study of Na(v)1.8 channel modulation by G proteins independent of a PKA/PKC pathway or binding to aluminum.

Modulation of phrenic motoneuron excitability by ATP: consequences for respiratory-related output in vitro.

On the basis of the high level of P2X receptor expression found in phrenic motoneurons (MN) in rats (Kanjhan et al., J Comp Neurol 407: 11-32, 1999) and potentiation of hypoglossal MN inspiratory activity by ATP (Funk et al., J Neurosci 17: 6325-6337, 1997), we tested the hypothesis that ATP receptor activation also modulates phrenic MN activity. This question was examined in rhythmically active brain stem-spinal cord preparations from neonatal rats by monitoring effects of ATP on the activity of spinal C4 nerve roots and phrenic MNs. ATP produced a rapid-onset, dose-dependent, suramin- and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid 4-sodium-sensitive increase in C4 root tonic discharge and a 22 +/- 7% potentiation of inspiratory burst amplitude. This was followed by a slower, 10 +/- 5% reduction in burst amplitude. ATPgammaS, the hydrolysis-resistant analog, evoked only the excitatory response. ATP induced inward currents (57 +/- 39 pA) and increased repetitive firing of phrenic MNs. These data, combined with persistence of ATP currents in TTX and immunolabeling for P2X2 receptors in Fluoro-Gold-labeled C4 MNs, implicate postsynaptic P2 receptors in the excitation. Inspiratory synaptic currents, however, were inhibited by ATP. This inhibition differed from that seen in root recordings; it did not follow an excitation, had a faster onset, and was induced by ATPgammaS. Thus ATP inhibited activity through at least two mechanisms: 1) a rapid P2 receptor-mediated inhibition and 2) a delayed P1 receptor-mediated inhibition associated with hydrolysis of ATP to adenosine. The complex effects of ATP on phrenic MNs highlight the importance of ATP as a modulator of central motor outflows.

Synthesis of enzymatically and chemically non-hydrolyzable analogues of dinucleoside triphosphates Ap(3)A and Gp(3)G.

Dinucleoside polyphosphates are ubiquitous compounds tightly involved in the regulation of a number of key biological processes. Hydrolysis-resistant analogues of Ap(3)A and Gp(3)G, two important members of that family of nucleotides, have been synthesized. P(1),P(2):P(2),P(3)-Bis-methylene diadenosine and diguanosine triphosphates were prepared from O,O-dialkyl methaneselenophosphonates using an original methodology. Whereas the 2-fold addition of the methanephosphonate anion to the activated phosphorus species cannot be performed, multiple condensation of lithiated methaneselenophosphonate with electrophilic trivalent phosphorus compounds is revealed to be very effective. A one-pot condensation/esterification/oxidation sequence involving O,O-dialkyl methaneselenophosphonates provides a highly efficient route to the PCH(2)PCH(2)P backbone. This new development in selenophosphonate chemistry offers a great potential for further regioselective functionalization of polyphosphate mimics.

Effects of ATP on TTX-sensitive and TTX-resistant sodium currents in rat sensory neurons.

Differential effects of ATP on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium currents in rat dorsal root ganglion neurons were studied using the whole-cell variation of path-clamp technique. Currents were evoked by step depolarizations to 0 mV from a holding potential of -80 mV. ATP suppressed TTX-S sodium currents while it increased TTX-R sodium currents. The effects were concentration-dependent and were reversible upon washout with ATP-free external solution. ATP-gamma-S, a hydrolysis-resistant ATP analog, also affected two types of sodium currents similarly to ATP, excluding the possibility that the effects were caused by the products of ATP hydrolysis, namely adenosine. ATP by modulating sodium currents may exert profound effects on the transmission of sensory information such as nociception.

The phosphoinositide signaling cascade is involved in photoreception in the leech Hirudo medicinalis.

The effects of BAPTA, heparin, and neomycin on electrical light responses were studied in the photoreceptors of Hirudo medicinalis. Light activation produces a fast increase in intracellular Ca2+ concentration (Cai) as detected with the fluorescent Ca2+ indicator calcium green-5N. Chelating intracellular calcium by injections of 10 mmol(-1) BAPTA suppresses spontaneous quantum bumps, reduces light sensitivity by more than 2 log(10) units, and substantially increases the latent period of light responses. BAPTA strongly inhibits the plateau phase of responses to long steps of light. Injections of 45-100 mg ml(-1) of heparin act in a similar manner to BAPTA, affecting the latency of the light responses even more. De-N-sulfated heparin, an inactive analog, is almost ineffective at the same concentration compared with heparin. Heparin diminishes the light-induced Cai elevation significantly, whereas de-N-sulfated heparin does not. Intracellular injections of 50-100 mmol l(-1) of the aminoglycoside neomycin, which inhibits phospholipase-C-mediated inositol 1,4,5-trisphosphate formation, acts similar to BAPTA and heparin. Pressure injections of the hydrolysis resistant analog of inositol 1,4,5-trisphosphate, inositol 2,4,5-trisphosphate, strongly depolarize leech photoreceptors and mimic an effect of light adaptation. These results suggest a close similarity between phototransduction mechanisms in leech photoreceptors and existing models for visual transduction in other invertebrate microvillar photoreceptors.

Ap 4A induces apoptosis in human cultured cells

Diadenosine oligophosphates (Ap nA) have been proposed as intracellular and extracellular signaling molecules in animal cells. The ratio of diadenosine 5′,5‴-P 1,P 3-triphosphate to diadenosine 5′,5‴-P 1,P 4-tetraphosphate (Ap 3A/Ap 4A) is sensitive to the cellular status and alters when cultured cells undergo differentiation or are treated with interferons. In cells undergoing apoptosis induced by DNA topoisomerase II inhibitor VP16, the concentration of Ap 3A decreases significantly while that of Ap 4A increases. Here, we have examined the effects of exogenously added Ap 3A and Ap 4A on apoptosis and morphological differentiation. Penetration of Ap nA into cells was achieved by cold shock. Ap 4A at 10 μM induced programmed cell death in human HL60, U937 and Jurkat cells and mouse VMRO cells and this effect appeared to require Ap 4A breakdown as hydrolysis-resistant analogues of Ap 4A were inactive. On its own, Ap 3A induced neither apoptosis nor cell differentiation but did display strong synergism with the protein kinase C activators 12-deoxyphorbol-13- O-phenylacetate and 12-deoxyphorbol-13- O-phenylacetate-20-acetate in inducing differentiation of HL60 cells. We propose that Ap 4A and Ap 3A are physiological antagonists in determination of the cellular status: Ap 4A induces apoptosis whereas Ap 3A is a co-inductor of differentiation. In both cases, the mechanism of signal transduction remains unknown.

Modulation of long-term potentiation in the CA1 area of rat hippocampus by platelet-activating factor.

We have previously shown that platelet-activating factor (PAF: l-O-alkyl-2-sn-3-glycerophosphocholine) selectively augments EPSCs in cultured hippocampal neurons by a presynaptic mechanism, suggesting that this agent could be involved in hippocampal long-term potentiation (LTP). We have also examined the possible involvement of PAF in LTP in the CA1 region of rat hippocampal slices using the hydrolysis resistant analog, methyl-carbamyl-PAF (C-PAF), and PAF receptor antagonists. When 2 μM BN--52021, a synaptosomal PAF receptor antagonist, was applied, LTP was inhibited and 20 min application of 1 μM MC-PAF in the absence of tetanic stimulation produced a slowly developing potentiation of extracellular recorded EPSP’s which persisted for more than 2h. Extra- or intracellular administration of C-PAF also appears to slow the washout of LTP-generating ability which occurs during whole-cell recording, making it possible to induce LTP for as long as 60 min after cell penetration. A decrease in extracellular magnesium (0.1 mM) for 15 min induced LTP which was not inhibited by a PAF antagonist, indicating the existence of PAF dependent and independent LTP. These data suggest that PAF and PAF activated process are important in LTP and modulate the threshold for LTP induction.

Measurement of agonist-stimulated [35S]GTP gamma S binding to cell membranes.

AbstractThis chapter describes a functional assay which measures the increase in guanine nucleotide exchange at G-proteins in cell membranes, resulting from agonist binding to G-protein-coupled receptors (GPCRs), by monitoring the binding of [35S]GTPγS, a radiolabeled, hydrolysis-resistant analog of GTP (guanosine triphosphate), in the presence of unlabeled GDP (guanosine triphosphate).KeywordsScintillation VialScintillation FluidFilter DiskAgonist BindingGuanosine TriphosphateThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Mitogenic signaling from P1 and P2 purinergic receptors to mitogen-activated protein kinase in human fetal astrocyte cultures

To investigate potential trophic actions of extracellular ATP in human astrocytes, we have examined mitogenic signaling by purinergic receptors in cultures prepared from first trimester rostral central nervous system tissue. We found that ATP and ATP γS, a hydrolysis-resistant analog, stimulated DNA synthesis, thereby indicating that P2 purinergic receptors can stimulate mitogenic signaling in these cells. In addition, ATP activated a mitogen-activated protein kinase (MAPK) termed ERK (extracellular signal-regulated protein kinase), a key component of signal transduction pathways involved in cellular proliferation and differentiation. The activation of MAPK was mediated at least in part by P2 purinergic receptors, because a P2 purinoceptor antagonist, suramin, inhibited the ATP-evoked stimulation by 50%, whereas a P1 purinergic-receptor antagonist, 8-(para-sulfonphenyl)-theophylline, was without effect. In contrast to rat astrocytes, adenosine/P1 purinergic-receptor agonists, 2-chloroadenosine and 5′-N-ethylcarboxyamidoadenosine, stimulated MAPK activity and DNA synthesis in human astrocytes. A selective inhibitor of protein kinase C, Ro 31–8220, blocked the ability of ATP and adenosine analogs to stimulate MAPK, thereby indicating that protein kinase C is upstream of MAPK in both P2- and P1-receptor signaling pathways. An inhibitor of the MAPK activator MEK, PD 098059, effectively blocked ATP- and 2-chloroadenosine-induced DNA synthesis, thereby indicating that the ERK/MAPK cascade mediates mitogenic signaling by P2 and P1 purinergic receptors in human fetal astrocytes. These findings suggest a role for P1 and P2 purinergic receptors in the proliferation of human fetal astrocytes.

Opioid Inhibition of Adenylyl Cyclase in Membranes from Pertussis Toxin-Treated NG108-15 Cells

Gi/Go proteins are uncoupled from receptors by ADP-ribosylation with pertussis toxin (PTX). However, PTX treatment of δ opioid receptor-containing NG108-15 cells reduces, but does not eliminate, opioid inhibition of adenylyl cyclase. The present study explored potential mechanisms of this residual inhibition. Overnight treatment of NG108-15 cells with 100 ng/ml PTX eliminated both PTX-catalyzed [adenylyl-32P]NAD+-labeling of G proteins and agonist stimulation of low Km GTPase in membranes. Although PTX-treatment decreased the maximal opioid inhibition of adenylyl cyclase by 50-65%, the inhibition that remained was concentration-dependent and antagonist-reversible. This inhibition persisted in the absence of GTP (even though opioid inhibition of adenylyl cyclase in untreated membranes was GTP-dependent), but was eliminated by hydrolysis-resistant guanine nucleotide analogs, indicating that G-proteins were still involved in the coupling mechanism. However, assays of agonist-stimulated [35S]GTPγS binding in the presence of excess GDP indicated that PTX pretreatment eliminated stimulation of guanine nucleotide exchange by opioid agonists. These results suggest that in membranes from PTX-treated NG108-15 cells, a subpopulation of G proteins may transduce an inhibitory signal from agonist-bound opioid receptors without involvement of guanine nucleotide exchange.

23] Applications of caged compounds of hydrolysis-resistant analogs of cAMP and cGMP

Publisher Summary Cyclic nucleotides—that is, cAMP and cGMP—control a variety of important cellular processes. The study of cAMP- and cGMP-signaling pathways has been greatly facilitated by the design of chemical derivatives, dubbed “caged”compounds. Caged cAMP and caged cGMP have also been used to study the activation kinetics of the cGMP-gated channel from rod photoreceptor cells, the signaling pathways in olfactory sensory neurons (OSNs), 15'16 the enhancement of CI – currents in ventricular cells, and the motile response of fish melanophores. This chapter discusses the properties of these compounds and their usefulness for electrophysiological studies of cyclic nucleotide-gated (CNG) channels in situ using heterologously expressed α subunits of CNG channels from bovine OSNs and cone photoreceptor cells. For many physiological studies, it would be desirable to use caged compounds that liberate cAMP or cGMP derivatives that are hydrolysis resistant or have higher biological efficiency. Derivatives that meet both these criteria are 8-Br-cAMP and 8-Br-cGMP. They are poorly hydrolyzable by phosphodiesterases, and 8-Br-cGMP activates cGK and CNG channels more effectively than cGMP.