L-740,093, a new antagonist of the CCK-B receptor, potentiates the antinociceptive effect of morphine: electrophysiological and behavioural studies

Single-chain Fragment variable antibody targeting cholecystokinin-B receptor for pain reduction

The cholecystokinin B receptor and its neuropeptide ligand are upregulated in chronic neuropathic pain models. Single-chain Fragment variable antibodies were generated as preferred non-opioid targeting therapy blocking the cholecystokinin B receptor to inhibit chronic neuropathic pain models in vivo and in vitro. Engineered antibodies of this type feature binding activity similar to monoclonal antibodies but with stronger affinity and increased tissue penetrability due to their smaller size. More importantly, single-chain Fragment variable antibodies have promising biotherapeutic applications for both nervous and immune systems, now recognized as interactive in chronic pain. A mouse single-chain Fragment variable antibody library recognizing a fifteen amino acid extracellular peptide fragment of the cholecystokinin B receptor was generated from immunized spleens. Ribosome display, a powerful cell-free technology, was applied for recombinant antibody selection. Antibodies with higher affinity, stability, solubility, and binding specificity for cholecystokinin B not A receptor were selected and optimized for in vivo and in vitro efficacy. A single dose of the lead candidate reduced mechanical and cold hypersensitivity in two rodent models of neuropathic pain for at least seven weeks. Continuing efficacy was evident with either intraperitoneal or intranasal dosing. Likewise, the lead single-chain Fragment variable antibody totally prevented development of anxiety- and depression-like behaviors and cognitive deficits typical in the models. Reduction of neuronal firing frequency was evident in trigeminal ganglia primary neuronal cultures treated in vitro with the cholecystokinin B receptor antibody. Immunofluorescent staining intensity in the trigeminal neuron primary cultures was significantly reduced incrementally after overnight binding with increasingly higher dilutions of the single-chain Fragment variable antibody. While it is reported that single-chain Fragment variable antibodies are removed systemically within 2-6 h, Western blot evidence indicates the His-tag marker remained after 7 weeks in the trigeminal ganglia and in the dorsolateral medulla, providing evidence of brain and ganglia penetrance known to be compromised in overactivated states. This project showcases the in vivo efficacy of our lead single-chain Fragment variable antibody indicating its potential for development as a non-opioid, non-addictive therapeutic intervention for chronic pain. Importantly, studies by others have indicated treatments with cholecystokinin B receptor antagonists suppress maintenance and reactivation of morphine dependence in place preference tests while lowering tolerance and dose requirements. Our future studies remain to address these potential benefits that may accompany the cholecystokinin B receptor biological therapy. Both chronic sciatic and orofacial pain can be unrelenting and excruciating, reducing quality of life as well as diminishing physical and mental function. An effective non-opiate, non-addictive therapy with potential to significantly reduce chronic neuropathic pain long term is greatly needed.

Potentiation of opioid analgesia in dopamine2 receptor knock-out mice: evidence for a tonically active anti-opioid system

Dopamine systems are intimately involved with opioid actions. Pharmacological studies suggest an important modulatory effect of dopamine and its receptors on opioid analgesia. We have now examined these interactions in a knock-out model in which the dopamine(2) (D(2)) receptor has been disrupted. Loss of D(2) receptors enhances, in a dose-dependent manner, the analgesic actions of the mu analgesic morphine, the kappa(1) agonist U50,488H and the kappa(3) analgesic naloxone benzoylhydrazone. The responses to the delta opioid analgesic [d-Pen(2),d-Pen(5)]enkephalin were unaffected in the knock-out animals. Loss of D(2) receptors also potentiated spinal orphanin FQ/nociceptin analgesia. Antisense studies using a probe targeting the D(2) receptor revealed results similar to those observed in the knock-out model. The modulatory actions of D(2) receptors were independent of final sigma receptor systems because the final sigma agonist (+)-pentazocine lowered opioid analgesia in all mice, including the D(2) knock-out group. Thus, dopamine D(2) receptors represent an additional, significant modulatory system that inhibits analgesic responses to mu and kappa opioids.

Non-opioid actions of lamotrigine within the rat dorsal horn after inflammation and neuropathic nerve damage

Some opioid-resistant pain conditions can be alleviated by voltage-dependent Na(+) channel blockers such as lamotrigine. The mu-opioid-receptor agonist morphine can modulate cation entry into cells to affect overall cellular excitability, an effect which can in turn be endogenously antagonised by the neuropeptide cholecystokinin (CCK). However, lamotrigine may also modulate cellular excitability by non-specifically blocking voltage-dependent ion channels. We have looked for interactions of lamotrigine with the opioid/CCK pathway within the spinal dorsal horn, to rule out the possibility that lamotrigine may attenuate nociceptive responses via actions on this pathway. Both lamotrigine and the mu-opioid agonist DAMGO inhibited mustard oil-evoked cell firing by approximately 50% compared with control levels. Co-application of CCK8S reversed DAMGO-, but not lamotrigine-induced inhibition of cell firing and this reversal was prevented with the selective CCK(B) receptor antagonist PD 135158. Although lamotrigine inhibited both brush- and cold-evoked cell firing in neuropathic animals, lamotrigine inhibition of mustard oil-evoked cell firing in the same animals was not significantly greater than that observed in controls. These results suggest that the antinociceptive properties of lamotrigine within the spinal dorsal horn are unlikely to be mediated via interactions with the opioid/CCK pathway.

Acute pain mechanisms

The systems activated by tissue-injuring stimuli are complex. The nociceptive primary afferents have little spontaneous activity under normal conditions; however, after tissue injury, they display longlasting, ongoing activity. This results, in part, because the injury elicits the release of active factors that sensitize or excite the peripheral nerve terminal. A threshold that is lowered to the extent that body temperature and the pressure of edema are adequate stimuli results in spontaneous pain. This phenomenon is mediated by a variety of blood-borne active factors released during plasma extravasation, by agents released from local inflammatory cells, and by neurotransmitters released from the peripheral terminals of the primary afferent fibers themselves. Well-defined projections into the dorsal horn convey the "pain message" to at least two well-defined populations of neurons: those that are nociceptive specific and those that display an intensity-linked discharge over a range of stimuli from innocuous to noxious. Convergence from various fiber types, modalities, and end organs permits the encoding of afferent traffic with respect to intensity and location. The convergence of axons from somatic and visceral structures reflects the mechanism for the so-called "referred pain state." Most importantly, these dorsal horn systems have a dynamic component in addition to the hard-wiring; their output can be regulated both up and down. The up-regulation provides the basis for much of the facilitated processing that is believed to account for a significant percentage of the postinjury pain state. The facilitated state has a unique pharmacology, with the underlying mechanisms reflecting a cascade of actions that starts with the NMDA receptor and proceeds through the spinal release of intermediaries, such as prostaglandins and nitric oxide. Conversely, the ability to down-regulate the dorsal horn stimulus response function accounts for the powerful control exerted by a wide variety of diverse factors, including the spinal delivery of opioid and nonopioid analgesics and the "endogenous analgesia system." These linkages reflect the complexity of the encoding mechanisms that transduce the tissue injury into the behavioral sequela known as pain. This article also emphasizes that, although considerable progress has been made in the past decade, the current pace of research promises greater insights.

Prolongation of morphine analgesia by competitive NMDA receptor antagonist D-CPPene (SDZ EAA 494) in rats

A possible future clinical application of NMDA receptor antagonists is the control of the development of opiate analgesic tolerance. Therefore, the ability of NMDA receptor antagonists to modify the acute analgesic effects of opiates becomes increasingly important. The present study sought to evaluate the analgesic potency of combined administration of morphine (5-20 mg/kg) and a competitive NMDA receptor antagonist D-CPPene (SDZ EAA 494; 3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid; 0.3-5.6 mg/kg) in the tail-flick and tail-pinch tests with rats. It was found that D-CPPene significantly increased the duration of morphine analgesia, but there was hardly any evidence for potentiation of morphine analgesia shortly after morphine administration. This effect could only in part be attributed to the D-CPPene-induced disruption of the development of 'learned hyperresponsiveness' (i.e., acquisition of decreased latencies to escape from repeated exposures to noxious stimulation). In addition, the plasma concentration of morphine was not affected by concurrent treatment with D-CPPene.