Intracerebroventricular met-enkephalin administration modulates adjuvant arthritis

Endogenous opioids upregulate brain-derived neurotrophic factor mRNA through delta- and micro-opioid receptors independent of antidepressant-like effects

Systemic administration of delta-opioid receptor (DOR) agonists decreases immobility in the forced swim test (FST) and increases brain-derived neurotrophic factor (BDNF) mRNA expression in rats, indicating that DOR agonists may have antidepressant-like effects. The aim of this study was to investigate the effects of central administration of endogenous opioid peptides on behavior in the FST and on brain BDNF mRNA expression in rats. Effects of endogenous opioids were compared with those produced by intracerebroventricular administration of a selective non-peptidic DOR agonist (+)BW373U86. Antidepressant-like effects were measured by decreased immobility in the FST. BDNF mRNA expression was determined by in situ hybridization. Centrally administered (+)BW373U86 decreased immobility and increased BDNF mRNA expression in the frontal cortex through a DOR-mediated mechanism, because these effects were blocked by the DOR antagonist naltrindole, but not by the micro-opioid receptor (MOR) antagonist naltrexone (NTX) or the kappa-opioid receptor antagonist nor-binaltorphimine. Of all the endogenous opioids tested, only leu- and met-enkephalin produced behavioral effects like those of (+)BW373U86 in the FST. Unlike (+)BW373U86, the enkephalins upregulated BDNF mRNA expression in the hippocampus through DOR- and MOR-mediated mechanisms. beta-Endorphin, endomorphin-1 and endomorphin-2 significantly increased BDNF mRNA levels in the frontal cortex, hippocampus and amygdala without reducing immobility; and most of these effects were reversed by NTX. This study is the first to provide evidence that endogenous opioids can upregulate BDNF mRNA expression through the DOR and MOR, and that leu- and met-enkephalin have similar pharmacological profiles to synthetic DOR agonists in producing antidepressant-like effects.

Tissue levels of leu-enkephalin in rats with adjuvant arthritis

To study the level of leu-enkephalin in bone and joint tissues and in the spinal cord of rats with adjuvant arthritis, arthritis was induced in Lewis rats by the injection of Mycobacterium butyricum in Freund's incomplete adjuvant (FIA). Immunoelectron microscopy (IEM) was used to monitor the cellular distribution of leu-enkephalin in control and arthritis groups, and radioimmunoassay (RIA) was used to measure the concentration in the tissues. The results of IEM showed increased levels of leu-enkephalin in the matrix of the sciatic nerve, in nerve fibres in the synovial membrane and periosteum, as well as in fibroblasts and endothelial cells of the periosteum in arthritic groups. In macrophage-like cells of the synovial membrane as well as monocyte and polymorphonuclear lineage cells in the bone marrow, the level of leu-enkephalin was decreased in the arthritic group. The results of RIA showed that the concentration of leu-enkephalin was lower in the ankle and increased in the spinal cord of arthritic animals compared with controls. In conclusion, leu-enkephalin levels were decreased in joints and in bone marrow, but increased in nerve tissues in the group with arthritis. Further studies are needed to show whether leu-enkephalin is involved in a process that serves to limit the effect of immunisation.

Systemic immune challenge induces preproenkephalin gene transcription in distinct autonomic structures of the rat brain

The involvement of enkephalins in the immune response was investigated in rats injected intravenously with interleukin-1beta (2 microg/kg). In situ hybridization with a riboprobe complementary to intron A of the preproenkephalin (ppENK) gene showed distinct transcriptional activation within several brain regions known to be activated by immune stimuli, including the nucleus of the solitary tract, the area postrema, the paraventricular hypothalamic nucleus, and the oval nucleus of the bed nucleus of the stria terminalis, and dual labeling confirmed that a large proportion of the intron expressing neurons co-expressed c-fos mRNA. Rats injected with saline (controls) showed little or no heteronuclear transcript in these structures. The induced signal was strongest after 1 hour but was present in some structures 30 minutes after interleukin-1beta injection. At 3 hours, transcriptional activity returned to basal levels. High basal expression of the heteronuclear transcript that appeared unchanged by the immune stimulus was seen in regions not primarily involved in the immune response, such as the striatum, the olfactory tubercle, and the islands of Calleja and in the immune activated central nucleus of the amygdala. The heteronuclear transcript colocalized with ppENK mRNA, demonstrating that it occurred in enkephalinergic neurons and was not the result of alternative transcription from the ppENK gene in other cells. These results demonstrated that enkephalin transcription is induced in central autonomic neurons during immune challenge, suggesting that enkephalins are involved in the centrally orchestrated response to such stimuli.

Distribution of leucine-enkephalin in bone and joint tissues

The distribution and concentration of leu-enkephalin in periosteum, cortical bone, bone marrow and synovial membrane of normal rats were analysed. Periosteum, cortical bone and bone marrow of the rat femurs were collected as well as the ankles. The distribution of leu-enkephalin was analysed by immunoelectron microscopy and the concentration of leu-enkephalin was measured with radioimmunoassay. Immunoelectron microscopy showed that leu-enkephalin is abundant in monocytes of bone marrow, nerve fibers and endothelial cells in the periosteum and also in macrophage-like-cells of the synovial membrane. The concentration of leu-enkephalin measured by RIA showed highest concentration in bone marrow followed by periosteum and cortical bone. The study supports that leu-enkephalin is present and can be quantified in different compartments of bone and joint tissues suggesting that leu-enkephalin may be involved in the physiological regulation of nociception and immunoregulation.

Neuropeptides in experimental and degenerative arthritis

Classical symptoms of both inflammatory and degenerative arthritides may contribute to neurogenic responses like wheal, flare, edema, and pain. Rheumatoid arthritis (RA) is an autoimmune disease with an immunogenetic background. Neurogenic inflammation has been considered to play an essential role in RA, in part because of the symmetrical involvement (cross-spinal reflexes) and the predominant involvement of the most heavily innervated small joints of the hands and the feet (highly represented in the hominiculus). In contrast, osteoarthritis (OA) is considered to arise as a result of degeneration of the hyaline articular cartilage, which secondarily results in local inflammation and pain. However, it is possible that the age-related and predominant (compared to nociceptive nerves) degeneration of the proprioceptive, kinesthetic and vasoregulatory nerves can represent the primary pathogenic events. This leads to progressive damage of tissue with extremely poor capacity for self-regeneration. Inflammation, be it primary/autoimmune or secondary/degenerative, leads to peripheral sensitization and stimulation, which may further lead to central sensitization, neurogenic amplification of the inflammatory responses and activation of the neuro-endocrine axis. Neuropeptides serve as messengers, which modulate and mediate the actions in these cascades. Accordingly, many neuropeptides have been used successfully as experimental treatments, most recently VIP, which effectively controlled collagen-induced arthritis in mice. Therefore, it can safely be concluded that better treatment/control of disease activity and pain can be achieved by blocking the cascade leading to initiation and/or amplification of inflammatory process combined with effects on central nociceptive and neuroendocrine responses.

Adjuvant-induced arthritis: IL-1 beta, IL-6 and TNF-alpha are up-regulated in the spinal cord

Adjuvant-induced arthritis (AIA) is a widely used animal model of human rheumatoid arthritis (RA). We have previously shown that increased neuropeptide expression is observed in the spinal cord of AIA rats. To study the potential role of cytokines in the spinal cord of AIA, we wanted to determine whether there are changes of glial and cytokine expression (IL-1 beta, IL-6, TNF-alpha and IFN-gamma) in the spinal cord of AIA rats. Our data indicated that macroglia and MHC class II immunostaining were enhanced, astrocytes expressing GFAP were increased in number and immunostaining intensity. Using in situ hybridization and immunohistochemical methods, both mRNA and protein levels of IL-1 beta, IL-6 and TNF-alpha were significantly increased in the spinal cord of arthritic rats. Increased cytokine expression was presented in the reactive astrocytes and microglia.

Intracerebroventricular administration of somatostatin prevents and attenuates adjuvant arthritis

The effects of somatostatin on the development of adjuvant arthritis induced by Mycobacterium butyricum were studied. Somatostatin was injected into the lateral cerebral ventricle every day for 14 days beginning on the first day of mycobacteria inoculation in the preventive group. In the treatment group, somatostatin was injected from day 17 until day 30 post-mycobacteria inoculation. Arthritis was evaluated by measuring ankle joint circumference and diameter as well as microscopic examination of ankle joint sections. Somatostatin profoundly inhibited the development of adjuvant arthritis and an anti-inflammatory action was observed in the treatment group. These results suggest that somatostatin has a central action that can prevent or attenuate symptoms associated with arthritis.