Assessment of glutamatergic synaptic transmission and plasticity in brain slices: relevance to bioelectronic approaches.

BACKGROUND: Glutamatergic neurons represent the largest neuronal class in the brain and are responsible for the bulk of excitatory synaptic transmission and plasticity. Abnormalities in glutamatergic neurons are linked to several brain disorders and their modulation represents a potential opportunity for emerging bioelectronic medicine (BEM) approaches. Here, we have used a set of electrophysiological assays to identify the effect of the pyrimidine nucleoside uridine on glutamatergic systems in ex vivo brain slices. An improved understanding of glutamatergic synaptic transmission and plasticity, through this type of examination, is critical to the development of potential neuromodulation strategies. METHODS: Ex vivo hippocampal slices (400 µm thick) were prepared from mouse brain. We recorded field excitatory postsynaptic potentials (fEPSP) in the CA1's stratum radiatum by stimulation of the CA3 Schaeffer collateral/commissural axons. Uridine was applied at concentrations (3, 30, 300 µM) representing the physiological range present in brain tissue. Synaptic function was studied with input-output (I-O) functions, as well as paired-pulse facilitation (PPF). Synaptic plasticity was studied by applying tetanic stimulation to induce post-tetanic potentiation (PTP), short-term potentiation (STP) and long-term potentiation (LTP). Additionally, we determined whether uridine affected synaptic responses carried solely by n-methyl-d-aspartate receptors (NMDARs), particularly during the oxygen-glucose deprivation (OGD) paradigm. RESULTS: The presence of uridine altered glutamatergic synaptic transmission and plasticity. We found that uridine affected STP and LTP in a concentration-dependent manner. Low-dose uridine (3 µM) had no effect, but higher doses (30 and 300 µM) impaired STP and LTP. Moreover, uridine (300 µM) decreased NMDAR-mediated synaptic responses. Conversely, uridine (at all concentrations tested) had a negligible effect on PPF and basal synaptic transmission, which is mediated primarily by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). In addition, uridine (100 µM) exerted a protective effect when the hippocampal slices were challenged with OGD, a widely used model of cerebral ischemia. CONCLUSIONS: Using a wide set of electrophysiological assays, we identify that uridine interacts with glutamatergic neurons to alter NMDAR-mediated responses, impair synaptic STP and LTP in a dose-dependent manner, and has a protective effect against OGD insult. This work outlines a strategy to identify deficits in glutamatergic mechanisms for signaling and plasticity that may be critical for targeting these same systems with BEM device-based approaches. To improve the efficacy of potential neuromodulation approaches for treating brain dysfunction, we need to improve our understanding of glutamatergic systems in the brain, including the effects of modulators such as uridine.

The Class I HDAC inhibitor RGFP963 enhances consolidation of cued fear extinction.

Evidence indicates that broad, nonspecific histone deacetylase (HDAC) inhibition enhances learning and memory, however, the contribution of the various HDACs to specific forms of learning is incompletely understood. Here, we show that the Class I HDAC inhibitor, RGFP963, enhances consolidation of cued fear extinction. However, RGFP966, a strong inhibitor of HDAC3, does not significantly enhance consolidation of cued fear extinction. These data extend previous evidence that demonstrate the Class I HDACs play a role in the consolidation of long-term memory, suggesting that HDAC1 and/or HDAC2, but less likely HDAC3, may function as negative regulators of extinction retention. The development of specific HDAC inhibitors, such as RGFP963, will further illuminate the role of specific HDACs in various types of learning and memory. Moreover, HDAC inhibitors that enhance cued fear extinction may show translational promise for the treatment of fear-related disorders, including post-traumatic stress disorder (PTSD).

HDAC3-selective inhibitor enhances extinction of cocaine-seeking behavior in a persistent manner.

Nonspecific histone deacetylase (HDAC) inhibition has been shown to facilitate the extinction of drug-seeking behavior in a manner resistant to reinstatement. A key open question is which specific HDAC is involved in the extinction of drug-seeking behavior. Using the selective HDAC3 inhibitor RGFP966, we investigated the role of HDAC3 in extinction and found that systemic treatment with RGFP966 facilitates extinction in mice in a manner resistant to reinstatement. We also investigated whether the facilitated extinction is related to the enhancement of extinction consolidation during extinction learning or to negative effects on performance or reconsolidation. These are key distinctions with regard to any compound being used to modulate extinction, because a more rapid decrease in a defined behavior is interpreted as facilitated extinction. Using an innovative combination of behavioral paradigms, we found that a single treatment of RGFP966 enhances extinction of a previously established cocaine-conditioned place preference, while simultaneously enhancing long-term object-location memory within subjects. During extinction consolidation, HDAC3 inhibition promotes a distinct pattern of histone acetylation linked to gene expression within the infralimbic cortex, hippocampus, and nucleus accumbens. Thus, the facilitated extinction of drug-seeking cannot be explained by adverse effects on performance. These results demonstrate that HDAC3 inhibition enhances the memory processes involved in extinction of drug-seeking behavior.

Effect of PF-04217329 a prodrug of a selective prostaglandin EP(2) agonist on intraocular pressure in preclinical models of glaucoma.

Better control of intraocular pressure (IOP) is the most effective way to preserve visual field function in glaucomatous patients. While prostaglandin FP analogs are leading the therapeutic intervention for glaucoma, new target classes also are being identified with new lead compounds being developed for IOP reduction. One target class currently being investigated includes the prostaglandin EP receptor agonists. Recently PF-04217329 (Taprenepag isopropyl), a prodrug of CP-544326 (active acid metabolite), a potent and selective EP(2) receptor agonist, was successfully evaluated for its ocular hypotensive activity in a clinical study involving patients with primary open angle glaucoma. In the current manuscript, the preclinical attributes of CP-544326 and PF-0421329 have been described. CP-544326 was found to be a potent and selective EP(2) agonist (IC(50) = 10 nM; EC(50) = 2.8 nM) whose corneal permeability and ocular bioavailability were significantly increased when the compound was dosed as the isopropyl ester prodrug, PF-04217329. Topical ocular dosing of PF-04217329 was well tolerated in preclinical species and caused an elevation of cAMP in aqueous humor/iris-ciliary body indicative of in vivo EP(2) target receptor activation. Topical ocular dosing of PF-04217329 resulted in ocular exposure of CP-544326 at levels greater than the EC(50) for the EP(2) receptor. PF-04217329 when dosed once daily caused between 30 and 50% IOP reduction in single day studies in normotensive Dutch-belted rabbits, normotensive dogs, and laser-induced ocular hypertensive cynomolgus monkeys and 20-40% IOP reduction in multiple day studies compared to vehicle-dosed eyes. IOP reduction was sustained from 6 h through 24 h following a single topical dose. In conclusion, preclinical data generated thus far appear to support the clinical development of PF-04217329 as a novel compound for the treatment of glaucoma.

Ocular hypotensive activity of BOL-303259-X, a nitric oxide donating prostaglandin F2α agonist, in preclinical models.

The aim of the study was to investigate the ocular hypotensive activity of a nitric oxide (NO)-donating latanoprost, BOL-303259-X, following topical administration. The effect of BOL-303259-X (also known as NCX 116 and PF-3187207) on intraocular pressure (IOP) was investigated in monkeys with laser-induced ocular hypertension, dogs with naturally-occurring glaucoma and rabbits with saline-induced ocular hypertension. Latanoprost was used as reference drug. NO, downstream effector cGMP, and latanoprost acid were determined in ocular tissues following BOL-303259-X administration as an index of prostaglandin and NO-mediated activities. In primates, a maximum decrease in IOP of 31% and 35% relative to baseline was achieved with BOL-303259-X at doses of 0.036% (9 µg) and 0.12% (36 µg), respectively. In comparison, latanoprost elicited a greater response than vehicle only at 0.1% (30 µg) with a peak effect of 26%. In glaucomatous dogs, IOP decreased from baseline by 44% and 10% following BOL-303259-X (0.036%) and vehicle, respectively. Latanoprost (0.030%) lowered IOP by 27% and vehicle by 9%. Intravitreal injection of hypertonic saline in rabbits increased IOP transiently. Latanoprost did not modulate this response, whereas BOL-303259-X (0.036%) significantly blunted the hypertensive phase. Following BOL-303259-X treatment, latanoprost acid was significantly elevated in rabbit and primate cornea, iris/ciliary body and aqueous humor as was cGMP in aqueous humor. BOL-303259-X lowered IOP more effectively than latanoprost presumably as a consequence of a contribution by NO in addition to its prostaglandin activity. The compound is now in clinical development for the treatment of glaucoma and ocular hypertension.

Uridine ameliorates the pathological phenotype in transgenic G93A-ALS mice.

There is strong evidence from studies in humans and animal models to suggest the involvement of energy metabolism defects in neurodegenerative diseases. Uridine, a pyrimidine nucleoside, has been suggested to be neuroprotective in neurological disorders by improving bioenergetic effects, increasing ATP levels and enhancing glycolytic energy production. We assessed whether uridine treatment extended survival and improved the behavioral and neuropathological phenotype observed in G93A-ALS mice. In vitro and in vivo pharmacokinetic analyses in mutant SOD models provided optimal dose and assurance that uridine entered the brain. A dose-ranging efficacy trial in G93A mice was performed using survival, body weight, open-field analysis, and neuropathology as outcome measures. Urinary levels of 8-hydroxy-2'-deoxyguanosine, identifying DNA oxidative damage, were measured and used as a pharmacodynamic biomarker. Uridine administration significantly extended survival in a dose-dependent manner in G93A mice, while improving the behavioral and neuropathological phenotype. Uridine increased survival by 17.4%, ameliorated body weight loss, enhanced motor performance, reduced gross lumbar and ventral horn atrophy, attenuated lumbar ventral horn neuronal cell death, and decreased reactive astrogliosis. Consistent with a therapeutic effect, uridine significantly reduced urinary 8-hydroxy-2'-deoxyguanosine in G93A mice. These data suggest that uridine may be a therapeutic candidate in ALS patients.

A novel nitric oxide releasing prostaglandin analog, NCX 125, reduces intraocular pressure in rabbit, dog, and primate models of glaucoma.

PURPOSE: Nitric oxide (NO) is involved in a variety of physiological processes including ocular aqueous humor dynamics by targeting mechanisms that are complementary to those of prostaglandins. Here, we have characterized a newly synthesized compound, NCX 125, comprising latanoprost acid and NO-donating moieties. METHODS: NCX 125 was synthesized and tested in vitro for its ability to release functionally active NO and then compared with core latanoprost for its intraocular pressure (IOP)-lowering effects in rabbit, dog, and nonhuman primate models of glaucoma. RESULTS: NCX 125 elicited cGMP formation (EC(50) = 3.8 + or - 1.0 microM) in PC12 cells and exerted NO-dependent iNOS inhibition (IC(50) = 55 + or - 11 microM) in RAW 264.7 macrophages. NCX 125 lowered IOP to a greater extent compared with equimolar latanoprost in: (a) rabbit model of transient ocular hypertension (0.030% latanoprost, not effective; 0.039% NCX 125, Delta(max) = -10.6 + or - 2.3 mm Hg), (b) ocular hypertensive glaucomatous dogs (0.030% latanoprost, Delta(max)= -6.7 + or - 1.2 mm Hg; 0.039% NCX 125, Delta(max) = -9.1 + or - 3.1 mm Hg), and (c) laser-induced ocular hypertensive non-human primates (0.10% latanoprost, Delta(max) = -11.9 + or - 3.7 mm Hg, 0.13% NCX 125, Delta(max) = -16.7 + or - 2.2 mm Hg). In pharmacokinetic studies, NCX 125 and latanoprost resulted in similar latanoprost-free acid exposure in anterior segment ocular tissues. CONCLUSIONS: NCX 125, a compound targeting 2 different mechanisms, is endowed with potent ocular hypotensive effects. This may lead to potential new perspectives in the treatment of patients at risk of glaucoma.

L-Carnosine: multifunctional dipeptide buffer for sustained-duration topical ophthalmic formulations.

OBJECTIVES: The use of l-carnosine as an excipient in topical ophthalmic formulations containing gellan gum, a carbohydrate polymer with in-situ gelling properties upon mixing with mammalian tear fluid, was developed as a novel platform to extend precorneal duration. Specific utilisation of l-carnosine as a buffer in gellan gum carrying vehicles was characterised. METHODS: Buffer capacity was evaluated using 7.5, 13.3, and 44.2 mm l-carnosine in a pH range of 5.5-7.5. Accelerated chemical stability was determined by HPLC at l-carnosine concentrations of 5-100 mm. Combinations of 7.5 mm l-carnosine with 0.06-0.6% (w/v) gellan gum were characterised rheologically. l-Carnosine-buffered solutions of gellan gum were tested for acute topical ocular tolerance in vivo in pigmented rabbits. A unique formulation combining timolol (which lowers intraocular pressure) in l-carnosine-buffered gellan gum was compared with Timoptic-XE in normotensive dogs. KEY FINDINGS: l-Carnosine exhibited optimal pharmaceutical characteristics for use as a buffer in chronically administered topical ocular formulations. Enhancement trends were observed in solution-to-gel transition of l-carnosine-buffered vehicles containing gellan gum vs comparators. Topical tolerability of l-carnosine-buffered gellan gum formulations and lowering of intraocular pressure were equivalent with timolol and Timoptic-XE. CONCLUSIONS: Functional synergy between excipients in gellan gum formulations buffered with l-carnosine has potential for topical ocular dosage forms with sustained precorneal residence.

In vivo evaluation of 11beta-hydroxysteroid dehydrogenase activity in the rabbit eye.

PURPOSE: Steroids are used in a diverse range of conditions in clinical ophthalmology and one of the most significant complications is corticosteroid-induced glaucoma, which is characterized by an increase in intraocular pressure (IOP). 11beta-Hydroxysteroid dehydrogenase-1 (11beta-HSD1) is known to catalyze the interconversion of hormonally inactive cortisone to hormonally active cortisol and is widely expressed in the eye, particularly ciliary epithelium. Carbenoxolone (CBX), an 11beta-HSD1 inhibitor, has been shown to reduce IOP in healthy volunteers and patients with ocular hypertension (OHT). The purpose of this study was to: (1) develop an in vivo model for the assessment of cortisone to cortisol conversion in the eye, that is, 11beta-HSD1 activity and (2) assess the pharmacokinetic/pharmacodynamic relationship following topical treatment with 11beta-HSD1 inhibitors using an in vivo rabbit model. METHODS: Potent and selective 11beta-HSD1 inhibitors were topically administered to the rabbit eye and exogenous cortisone to endogenous cortisol conversion in the eye was assessed in rabbits. Tissues were then evaluated for cortisone, cortisol, and 11beta-HSD1 inhibitor levels by LC/MS/MS. Concomitantly cortisol activity in ocular tissue samples was determined using a secondary mechanistic pLuc-GRE assay. RESULTS: Topical treatment with potent and selective 11beta-HSD1 inhibitors resulted in complete inhibition in the conversion of cortisone to cortisol in the rabbit eye as well as decreased pLuc-GRE luciferase activity. The reduction of cortisone conversion was time- and dose-dependent as well as dependent on dosing volume (suggestive of increased spillover and washout with greater dosing volume). CONCLUSIONS: In conclusion, topical delivery of 11beta-HSD1 inhibitors can reduce or inhibit the conversion of cortisone to cortisol in the eye, indicating that the rabbit eye possesses an active enzyme for glucocorticoid synthesis. Dosing concentration and volume play an important role in the pharmacokinetic and pharmacodynamic effects of topically delivering an 11beta-HSD1 inhibitor. The rabbit model is useful for mechanistically assessing the conversion of cortisone to cortisol in the eye.

Ocular pharmacokinetics and hypotensive activity of PF-04475270, an EP4 prostaglandin agonist in preclinical models.

Prostaglandins are widely used to lower intraocular pressure (IOP) as part of the treatment regimen for glaucoma. While FP and EP2 agonists are known to lower IOP, we investigated the ocular hypotensive activity and ocular drug distribution of PF-04475270, a novel EP4 agonist following topical administration in normotensive Beagle dogs. PF-04475270 is a prodrug of CP-734432, which stimulated cAMP formation in HEK293 cells expressing EP4 receptor and beta-lactamase activity in human EP4 expressing CHO cells transfected with a cAMP response element (CRE) with an EC(50) of 1 nM. Prodrug conversion and transcorneal permeability were assessed in rabbit corneal homogenates and a human corneal epithelial cell (cHCE) model. The compound underwent rapid hydrolysis to CP-734432 in corneal homogenates, and exhibited good permeability in the cHCE model. The descending order of ocular exposure to CP-734432 after topical dosing of PF-04475270 in dogs was as follows: cornea > aqueous humor >or= iris/ciliary body. When administered q.d., PF-04475270 lowered IOP effectively in the dog IOP model both after single and multiple days of dosing. A maximum decrease in IOP with PF-04475270 was between 30 and 45% at 24h post-dose relative to that observed with vehicle. In conclusion, PF-04475270 is a novel ocular hypotensive compound which is bioavailable following topical dosing, effectively lowering IOP in dogs. EP4 agonists could be considered as potential targets for lowering IOP for the treatment of glaucoma and ocular hypertension.

Correlation of in vitro and in vivo kinetics of nitric oxide donors in ocular tissues.

In the eye, nitric oxide (NO) is involved in the regulation of intraocular pressure (IOP) and ocular blood flow. The main purpose of this study was to measure the kinetics of NO release from NO donors in ocular cells and tissues using in vivo and in vitro models and demonstrate the link between the kinetics of NO release with the functional effect, IOP. Nitric oxide release was measured in human ocular cells using a fluorescent dye, diaminofluorescein (DAF), following treatment with short-acting sodium nitroprusside (SNP) and longer-acting S-nitroso-N-acetylpenicillamine (SNAP) NO donors. Both SNP and SNAP were also administered topically to rabbits; IOP was measured and levels of NO and cGMP were assessed as biomarkers over a time course in the aqueous humor (AH) and iris/ciliary body (ICB). Time- and concentration-dependent increases in NO level were produced by SNP and SNAP in human ocular cells. Both NO and cGMP levels appeared to be elevated following treatment with the aforementioned NO donors in rabbit ocular tissues. Transient IOP lowering was accompanied with these biochemical estimations in rabbits, with time of maximal effect being shifted to the right for longer-acting SNAP as compared with short-acting SNP. In vitro and in vivo NO/cGMP assay results displayed a correlation between short- and longer-acting NO donors, discriminating their respective temporal actions in the eye. Due to their translatability, the in vitro DAF assay and in vivo NO fluorometric assay can therefore be potentially useful in screening novel NO donors with different temporal/kinetic profiles.

Drug transporter and cytochrome P450 mRNA expression in human ocular barriers: implications for ocular drug disposition.

Studies were designed to quantitatively assess the mRNA expression of 1) 10 cytochrome P450 (P450) enzymes in human cornea, iris-ciliary body (ICB), and retina/choroid relative to their levels in the liver, and of 2) 21 drug transporters in these tissues relative to their levels in human small intestine, liver, or kidney. Potential species differences in mRNA expression of PEPT1, PEPT2, and MDR1 were also assessed in these ocular tissues from rabbit, dog, monkey, and human. P450 expression was either absent or marginal in human cornea, ICB, and retina/choroid, suggesting a limited role for P450-mediated metabolism in ocular drug disposition. In contrast, among 21 key drug efflux and uptake transporters, many exhibited relative expression levels in ocular tissues comparable with those observed in small intestine, liver, or kidney. This robust ocular transporter presence strongly suggests a significant role that transporters may play in ocular barrier function and ocular pharmacokinetics. The highly expressed efflux transporter MRP1 and uptake transporters PEPT2, OCT1, OCTN1, and OCTN2 may be particularly important in absorption, distribution, and clearance of their drug substrates in the eye. Evidence of cross-species ocular transporter expression differences noted in these studies supports the conclusion that transporter expression variability, along with anatomic and physiological differences, should be taken into consideration to better understand animal ocular pharmacokinetic and pharmacodynamic data and the scalability to human for ocular drugs.