Central roles of nociceptin/orphanin FQ and nocistatin: allodynia as a model of neural plasticity

Identification of NIPSNAP1 as a nocistatin-interacting protein involving pain transmission

4-Nitrophenylphosphatase domain and non-neuronal SNAP25-like protein homolog 1 (NIPSNAP1) is a molecule of physiologically unknown function, although it is predominantly expressed in the brain, spinal cord, liver, and kidney. We identified NIPSNAP1 as a protein that interacts with the neuropeptide nocistatin (NST) from synaptosomal membranes of mouse spinal cord using high-performance affinity latex beads. NST, which is produced from the same precursor protein as an opioid-like neuropeptide nociceptin/orphanin FQ (N/OFQ), has opposite effects on pain transmission evoked by N/OFQ. The calculated full-length pre-protein of NIPSNAP1 was 33 kDa, whereas the N-terminal truncated form of NIPSNAP1 (29 kDa) was ubiquitously expressed in the neuronal tissues, especially in synaptic membrane and mitochondria of brain. The 29-kDa NIPSNAP1 was distributed on the cell surface, and NST interacted with the 29-kDa but not the 33-kDa NIPSNAP1. Although intrathecal injection of N/OFQ induced tactile allodynia in both wild-type and NIPSNAP1-deficient mice, the inhibition of N/OFQ-evoked tactile allodynia by NST seen in wild-type mice was completely lacking in the deficient mice. These results suggest that NIPSNAP1 is an interacting molecule of NST and plays a crucial role in pain transmission.

Intrinsic and synaptic long-term depression of NTS relay of nociceptin- and capsaicin-sensitive cardiopulmonary afferents hyperactivity

The nucleus tractus solitarius (NTS) in the caudal medulla is a gateway for a variety of cardiopulmonary afferents important for homeostatic regulation and defense against airway and cardiovascular insults and is a key central target potentially mediating the response habituation to these inputs. Here, whole-cell and field population action potential recordings and infrared imaging in rat brainstem slices in vitro revealed a compartmental pain-pathway-like organization of capsaicin-facilitated vs. nocistatin-facilitated/nociceptin-suppressed neuronal clusters in an NTS region, which receives cardiopulmonary A- and C-fiber afferents with differing capsaicin sensitivities. All capsaicin-sensitive neurons and a fraction of nociceptin-sensitive neurons expressed N-methyl-D: -aspartate (NMDA) receptor-dependent synaptic long-term depression (LTD) following afferent stimulation. All neurons also expressed activity-dependent decrease of excitability (intrinsic LTD), which converted to NMDA receptor-dependent intrinsic long-term potentiation after GABA(A) receptor blockade. Thus, distinct intrinsic and synaptic LTD mechanisms in the NTS specific to the relay of A- or C-fiber afferents may underlie the response habituation to persistent afferents hyperactivity that are associated with varying physiologic challenges and cardiopulmonary derangements-including hypertension, chronic cough, asthmatic bronchoconstriction, sustained elevated lung volume in chronic obstructive pulmonary disease or in continuous positive-airway-pressure therapy for sleep apnea, metabolic acidosis, and prolonged exposure to hypoxia at high altitude.

Nonassociative learning as gated neural integrator and differentiator in stimulus-response pathways

Nonassociative learning is a basic neuroadaptive behavior exhibited across animal phyla and sensory modalities but its role in brain intelligence is unclear. Current literature on habituation and sensitization, the classic "dual process" of nonassociative learning, gives highly incongruous accounts between varying experimental paradigms. Here we propose a general theory of nonassociative learning featuring four base modes: habituation/primary sensitization in primary stimulus-response pathways, and desensitization/secondary sensitization in secondary stimulus-response pathways. Primary and secondary modes of nonassociative learning are distinguished by corresponding activity-dependent recall, or nonassociative gating, of neurotransmission memory. From the perspective of brain computation, nonassociative learning is a form of integral-differential calculus whereas nonassociative gating is a form of Boolean logic operator--both dynamically transforming the stimulus-response relationship. From the perspective of sensory integration, nonassociative gating provides temporal filtering whereas nonassociative learning affords low-pass, high-pass or band-pass/band-stop frequency filtering--effectively creating an intelligent sensory firewall that screens all stimuli for attention and resultant internal model adaptation and reaction. This unified framework ties together many salient characteristics of nonassociative learning and nonassociative gating and suggests a common kernel that correlates with a wide variety of sensorimotor integration behaviors such as central resetting and self-organization of sensory inputs, fail-safe sensorimotor compensation, integral-differential and gated modulation of sensorimotor feedbacks, alarm reaction, novelty detection and selective attention, as well as a variety of mental and neurological disorders such as sensorimotor instability, attention deficit hyperactivity, sensory defensiveness, autism, nonassociative fear and anxiety, schizophrenia, addiction and craving, pain sensitization and phantom sensations, etc.

Monitoring for dynamic biological processing by intramolecular bioluminescence resonance energy transfer system using secreted luciferase

Proteolytic processing plays crucial roles in physiological and pathophysiological cellular functions such as peptide generation, cell cycle, and apoptosis. We developed a novel biophysical bioluminescence resonance energy transfer (BRET) system between a secreted Vargula luciferase (Vluc) and an enhanced yellow fluorescent protein (EYFP) for visualization of cell biological processes. The bioluminescence spectrum of the fusion protein (Vluc-EYFP) is bimodal (lambdamax = 460 nm (Vluc) and 525nm (EYFP)), indicating that the excited-state energy of Vluc transfers to EYFP (in short, BRET). The BRET signal can be measured in the culture medium and pursue quantitative production of two neuropeptides, nocistatin (NST) and nociceptin/orphanin FQ (N/OFQ) in living cells. NST and N/OFQ are located in tandem on the same precursor, but NST exhibits antagonistic action against N/OFQ-induced central functions. Insertion of a portion of the NST-N/OFQ precursor (Glu-Gln-Lys-Gln-Leu-Gln-Lys-Arg-Phe-Gly-Gly-Phe-Tyr-Gly) in Vluc-EYFP makes the fusion protein cleavable at Lys-Arg in NG108-15 cells, and proprotein convertase 1 enhances this digestion. The change in BRET signals quantifies the processing of the fusion protein. Our novel intramolecular BRET system using a secreted luciferase is useful for investigating peptide processing in living cells.

Pain imaging: future applications to integrative clinical and basic neurobiology

We have entered a new era in understanding CNS circuitry involved in acute and chronic pain. The ability to objectively measure a pain or analgesic state of the brain using non-invasive methods that define neural activation provides the possibility for top-down approaches to drug discovery. These brain maps represent the specific brain state. In the future, correlations with such states and behavioral, genetic, epigenetic or other chemical markers may help define specific diagnostic tools and novel approaches to drug discovery.

Attenuation of neuropathic pain by the nociceptin/orphanin FQ antagonist JTC-801 is mediated by inhibition of nitric oxide production

At the spinal level, the involvement of nociceptin/orphanin FQ (N/OFQ) in pain transmission is controversial. JTC-801, a selective nonpeptidergic N/OFQ antagonist, is a good tool to examine the involvement of endogenous N/OFQ in pathophysiological conditions. In the present study, we studied the effect of JTC-801 on neuropathic pain induced by L5 spinal nerve transection in mice. Thermal hyperalgesia was evident on day 3 postsurgery and maintained during the 10-day experimental period. Oral administration of JTC-801 relieved the thermal hyperalgesia in neuropathic mice in a dose-dependent manner. Following L5 nerve transection, the increase in nitric oxide synthase (NOS) activity was observed in the superficial layer of dorsal horn and around the central canal in the spinal cord by NADPH diaphorase histochemistry. Using the novel fluorescent nitric oxide (NO) detection dye diaminofluorescein-FM, we confirmed that NO production increased in the spinal slice prepared from neuropathic mice and that the increase was more prominent in the ipsilateral side to the nerve transection than in the contralateral side. These increases in NOS activity and NO production in neuropathic mice were blocked by pretreatment of oral JTC-801. Although intraperitoneal injection of the nonselective NOS inhibitor NG.-nitro-L-arginine methyl ester transiently, but significantly, attenuated neuropathic hyperalgesia, inducible NOS-deficient mice showed neuropathic pain after L5 spinal nerve transection. These results suggest that N/OFQ is involved in the maintenance of neuropathic pain and that the analgesic effect of JTC-801 on neuropathic pain is mediated by inhibition of NO production by neuronal NOS.

Characterization of nociceptin/orphanin FQ-induced pain responses by the novel receptor antagonist N-(4-amino-2-methylquinolin-6-yl)-2-(4-ethylphenoxymethyl) benzamide monohydrochloride

At the spinal level, nociceptin/orphanin FQ (Noc/OFQ) produces pronociceptive and allodynic effects at low doses (picogram range), while causing antinociceptive effects at high doses (microgram range). The discrepancy of pain modulation by Noc/OFQ at low and high doses raised a question whether Noc/OFQ exerted actions through the same Noc/OFQ receptor. In the present study, we examined the involvement of the Noc/OFQ receptor in pain responses with the novel nonpeptide antagonist N-(4-amino-2-methylquinolin-6-yl)-2-(4-ethylphenoxymethyl) benzamide monohydrochloride (JTC-801). Allodynia and hyperalgesia evoked by intrathecal administration of Noc/OFQ (50 pg/mouse) were dose dependently blocked by simultaneous administration of JTC-801 with IC(50) values of 32.2 and 363 pg, respectively. JTC-801 did not induce allodynia by itself. Subcutaneous injection of formalin into a hindpaw evoked biphasic pain behaviors such as flinching and biting in mice. Noc/OFQ at 10 pg increased the second-phase pain behaviors evoked by 1% formalin, whereas it strongly inhibited both the first-phase and second-phase pain evoked by 2% formalin at 1 microg. Although the pronociceptive effect by 10 pg of Noc/OFQ was dose dependently blocked by JTC-801 with an IC(50) value of 4.58 pg, the antinociceptive effects by 1 microg of Noc/OFQ were not antagonized by JTC-801. Furthermore, both phases of 2% formalin-induced pain behaviors were relieved by JTC-801. These results demonstrate that pronociceptive responses induced by a low dose of Noc/OFQ may be mediated through the Noc/OFQ receptor in the spinal cord and that JTC-801 can be a useful antagonist to examine the involvement of endogenous Noc/OFQ and mediation of the Noc/OFQ receptor under physiological and pathophysiological conditions including pain.

Central and peripheral roles of prostaglandins in pain and their interactions with novel neuropeptides nociceptin and nocistatin

While acute pain has a fundamental role to operate a protective system, chronic pain associated with inflammation and nerve injury often outlasts its biological usefulness. Therefore, there has recently been great interest in the neurochemical mechanisms of hyperalgesia to noxious stimuli and tactile pain (allodynia) to innocuous stimuli with a hope to relieve persistent, intractable pain. Over several decades non-steroidal anti-inflammatory drugs and opioids have been employed for clinical management of pain. The introduction of molecular biology to pain research has enabled us to describe the mechanism of pain at the molecular level and to develop analgesics with selectivity for targets and with less adverse effects. This review focuses on current knowledge concerning mechanisms and pathways for pain induced by prostaglandins and their interactions with novel neuropeptides nociceptin/orphanin FQ and nocistatin derived from the same opioid precursor protein.

Bidirectional actions of nociceptin/orphanin FQ on A delta-fibre-evoked responses in rat superficial spinal dorsal horn in vitro

The present study investigated the modulatory actions of nociceptin/orphanin FQ on excitatory glutamatergic transmission in spinal dorsal horn. In transverse spinal cord slices with an attached dorsal root, mono- and polysynaptic A delta-fibre-evoked extracellular field potentials were recorded from superficial dorsal horn. Nociceptin/orphanin FQ showed bidirectional effects on monosynaptic transmission with a potentiation at lower concentrations (100-300 nM) and a dose-dependent depression at higher concentrations (1-3 microM). The polysynaptic field potential was dose-dependently depressed by nociceptin/orphanin FQ (100 nM-3 microM). None of the actions of nociceptin/orphanin FQ was reversed by the non-specific opioid receptor antagonist naloxone, the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovaleric acid or the peptide nocistatin. The bidirectional actions of nociceptin/orphanin FQ on the monosynaptic field potential may provide an in vitro model for the bidirectional actions of nociceptin/orphanin FQ in behavioural studies showing hyperalgesia at low doses of intrathecal nociceptin/orphanin FQ and analgesia at higher doses.