Identification of critical amino acid residues for human iNOS functional activity.

Nitric oxide (NO) is a short-lived signaling molecule that mediates a variety of biological functions, including vascular homeostasis, neurotransmission, antimicrobial defense and antitumor activities. Three known NOS isoforms (eNOS, nNOS and iNOS) have been cloned and sequenced. Here, we show that upon expression in Escherichia coli using a novel expression vector, an iNOS sequence containing three mutations (A805D, F831S and L832P) within the iNOS reductase domain produced very little functionally active iNOS protein compared to the wild type (wt) iNOS. Each of these point mutations also was individually constructed into the wt iNOS sequence. The activity of the iNOS protein containing the A805D mutation was comparable to wt, while a drastic reduction in iNOS activity was observed for the F831S and L832P mutants. A comparison of the molecular models of the reductase domain of the wt and mutant iNOS revealed a reduced core packing density for the F831S and L832P mutations compared to wt. In addition, the modeling also suggests altered hydrogen bonding, van der Waals and hydrophobic interactions of these mutants.

Tramadol and several anticonvulsants synergize in attenuating nerve injury-induced allodynia.

Neuropathic pain results from injury or dysfunction of the central or peripheral nervous system. The treatment of neuropathic pain is challenging, in part because of its multiple etiologies. The present study explores combinations of the analgesic tramadol and each of four anticonvulsants in the treatment of surgically induced (ligation of the L5 spinal nerve) allodynia in rats. Each of the five drugs studied exhibited a dose-dependent antiallodynic effect. When studied in combination, tramadol and each of two of the anticonvulsants (topiramate and RWJ-333369) interacted synergistically at all three ratios studied, whereas tramadol and each of the other two anticonvulsants (gabapentin and lamotrigine) exhibited a synergistic antiallodynic effect at only one of three ratios investigated. In addition, tramadol and topiramate were found to interact synergistically in a nociceptive pain model, the mouse hot-plate test. These studies suggest the benefit of using combinations of analgesics and anticonvulsants in the relief of neuropathic pain.

Direct inhibition of Ih by analgesic loperamide in rat DRG neurons.

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are responsible for the functional hyperpolarization-activated current (I(h)) in dorsal root ganglion (DRG) neurons, playing an important role in pain processing. We found that the known analgesic loperamide inhibited I(h) channels in rat DRG neurons. Loperamide blocked I(h) in a concentration-dependent manner, with an IC(50) = 4.9 +/- 0.6 and 11.0 +/- 0.5 microM for large- and small-diameter neurons, respectively. Loperamide-induced I(h) inhibition was unrelated to the activation of opioid receptors and was reversible, voltage-dependent, use-independent, and was associated with a negative shift of V(1/2) for I(h) steady-state activation. Loperamide block of I(h) was voltage-dependent, gradually decreasing at more hyperpolarized membrane voltages from 89% at -60 mV to 4% at -120 mV in the presence of 3.7 microM loperamide. The voltage sensitivity of block can be explained by a loperamide-induced shift in the steady-state activation of I(h). Inclusion of 10 microM loperamide into the recording pipette did not affect I(h) voltage for half-maximal activation, activation kinetics, and the peak current amplitude, whereas concurrent application of equimolar external loperamide produced a rapid, reversible I(h) inhibition. The observed loperamide-induced I(h) inhibition was not caused by the activation of peripheral opioid receptors because the broad-spectrum opioid receptor antagonist naloxone did not reverse I(h) inhibition. Therefore we suggest that loperamide inhibits I(h) by direct binding to the extracellular region of the channel. Because I(h) channels are involved in pain processing, loperamide-induced inhibition of I(h) channels could provide an additional molecular mechanism for its analgesic action.

Use of a murine cell line for identification of human nitric oxide synthase inhibitors.

INTRODUCTION: Nitric oxide (NO) has been implicated in a wide range of physiological and pathological processes. Low concentrations of this mediator play homeostatic roles, whereas many acute and chronic responses are associated with excessive production of NO. This upregulation is due in part to the induction of inducible nitric oxide synthase (iNOS) by proinflammatory cytokines in several different cell types, including macrophages and their CNS derivative, microglia. METHODS: The crystal structures of the oxygenase domains of mouse and human iNOS were superimposed using the "align by homology" feature in Sybyl (SYBYL 7.0, Tripos Inc.). NOS isoform expression was assessed by TaqMan, Western blotting, and activity assays. RESULTS: We demonstrate that there is a high degree of three-dimensional overlap between the mouse and human iNOS active centers and propose that the murine isoform can serve as a suitable substitute for the human in assays. We also demonstrate that LPS stimulation of the mouse macrophage cell line RAW 264.7 induces the expression of iNOS, but not nNOS or eNOS, at the levels of mRNA transcription and protein expression. Furthermore, the pharmacology and calcium dependency of the NO formation support the finding that it is due to iNOS alone. Also reported is the demonstration of LPS-induced RAW 264.7 macrophages in simple cell-based and cell-free screening assays for iNOS inhibitors. Both assays were reproducible, as demonstrated by Z' factors of 0.69 and 0.71, and had high signal to noise ratios of 11- and 6-fold for the cell-based and cell-free assay, respectively. DISCUSSION: Our computational analyses indicate that there is a high degree of three-dimensional overlap between the oxygenase domains of human and murine iNOS. This observation together with the selective induction of murine iNOS in RAW 264.7 macrophages demonstrates the potential utility of the mouse iNOS assay to identify inhibitors of the human enzyme.

Differential and synergistic effects of selective norepinephrine and serotonin reuptake inhibitors in rodent models of pain.

There is increasing recognition that norepinephrine (NE) and serotonin (5-HT) reuptake inhibitors (NRIs and SRIs) are efficacious in treating some types of pain. To date, studies have not systematically evaluated the relative activity at the NE and/or 5-HT transporter required for maximal efficacy in rodent pain models. Known selective NE and 5-HT reuptake inhibitors reboxetine, desipramine, fluoxetine, and paroxetine were evaluated in both in vitro and in vivo assays. Using the spinal nerve ligation model of neuropathic pain, the compounds differentially reversed tactile allodynia. Evaluation of a broader spectrum of reuptake inhibitors in the para-phenylquinone (PPQ)-induced abdominal constriction model, a model of acute visceral pain, demonstrated that both the SRIs and the NRIs significantly blocked abdominal constrictions. However, the magnitude of this effect was greater following treatment with compounds having greater affinity for NRI compared with SRI affinity. In addition, isobolographic analyses indicated significant synergistic effects for all combinations of desipramine and fluoxetine in the PPQ model of visceral pain. Collectively, the present results support the hypothesis that compounds with greater NRI activity should be more effective for the treatment of pain than compounds having only SRI activity, and this hypothesis is also supported by clinical data. These studies also suggest that the potency and effectiveness of NRIs might be enhanced by the presence of 5-HT activity.

An estrogen receptor-beta agonist is active in models of inflammatory and chemical-induced pain.

ERB-041 (2-(3-Fluoro-4-hydroxyphenyl)-7-vinyl-1,3 benzoxazol-5-ol) is a selective estrogen receptor-beta agonist with activity in rodent models of rheumatoid arthritis and endometriosis. Clinical trials for these diseases are underway: however, the role of estrogen receptor-beta in modulating pain associated with inflammation remains unknown. These studies demonstrate that acutely administered ERB-041 is anti-hyperalgesic in preclinical models of chemical-induced and acute inflammatory pain, thus suggesting that ERB-041 may be useful for modulating pain associated with some types of inflammation.

High-throughput screening for the discovery of inhibitors of fatty acid amide hydrolase using a microsome-based fluorescent assay.

Fatty acid amide hydrolase (FAAH) is a membrane-associated enzyme that catalyzes the hydrolysis of several endogenous bioactive lipids, including anandamide (AEA), N-palmitoylethanolamine (PEA), oleamide, and N-oleoylethanolamine (OEA). These fatty acid amides participate in many physiological activities such as analgesia, anxiety, sleep modulation, anti inflammatory responses, and appetite suppression. Because FAAH plays an essential role in controlling the tone and activity of these endogenous bioactive lipids, this enzyme has been implicated to be a drug target for the therapeutic management of pain, anxiety, and other disorders. In an effort to discover FAAH inhibitors, the authors have previously reported the development of a novel fluorescent assay using purified FAAH microsomes as an enzyme source and a fluorogenic substrate, arachidonyl 7-amino, 4-methyl coumarin amide (AAMCA). Herein, the authors have adapted this assay to a high-throughput format and have screened a large library of small organic compounds, identifying a number of novel FAAH inhibitors. These data further verify that this fluorescent assay is sufficiently robust, efficient, and low-cost for the identification of FAAH inhibitory molecules and open this class of enzymes for therapeutic exploration.

A novel scintillation proximity assay for fatty acid amide hydrolase compatible with inhibitor screening.

A binding assay for human fatty acid amide hydrolase (FAAH) using the scintillation proximity assay (SPA) technology is described. This SPA uses the specific interactions of [3H]R(+)-methanandamide (MAEA) and FAAH expressing microsomes to evaluate the displacement activity of FAAH inhibitors. We observed that a competitive nonhydrolyzed FAAH inhibitor, [3H]MAEA, bound specifically to the FAAH microsomes. Coincubation with an FAAH inhibitor, URB-597, competitively displaced the [3H]MAEA on the FAAH microsomes. The released radiolabel was then detected through an interaction with the SPA beads. The assay is specific for FAAH given that microsomes prepared from cells expressing the inactive FAAH-S241A mutant or vector alone had no significant ability to bind [3H]MAEA. Furthermore, the binding of [3H]MAEA to FAAH microsomes was abolished by selective FAAH inhibitors in a dose-dependent manner, with IC50 values comparable to those seen in a functional assay. This novel SPA has been validated and demonstrated to be simple, sensitive, and amenable to high-throughput screening.