Monoamine system disruption induces functional somatic syndromes associated symptomatology in mice.

Functional somatic syndromes (FSS), a clinical condition manifesting a variety of unexplained somatic symptoms, has been proposed as an inclusive nosology encompassing individual syndromes such as fibromyalgia syndrome and irritable bowel syndrome. Accumulating evidence suggests that disturbance of the endogenous monoamine system could be involved in the aetiology of FSS. Therefore, the purpose of present study was to investigate whether the disturbance of the monoamine system would cause FSS-associated symptomatology in mice. The optimal dose of reserpine, an inducer of endogenous monoamines reduction, was first explored in mice. General body condition (body weight, rectal temperature, and ptosis) and FSS-associated symptomatology (paw withdrawal threshold, small intestinal transit, and locomotor activity) were measured. The concentration of monoamines was measured in central and peripheral tissues. Mice dosed with reserpine (0.25 mg/kg s.c., once daily for 3 consecutive days) exhibited a decrease in paw withdrawal threshold, delay in small intestinal transit, and reduction of locomotor activity without deterioration of general body condition on day 5 after the first reserpine injection. The concentration of monoamines was decreased in the central nervous system and skeletal muscle, but not in the small intestine. A reserpine dose of 0.5 mg/kg or more caused deterioration of general body condition. In conclusion, the optimal protocol of reserpine treatment for inducing pain symptom without deterioration of general physical condition is 0.25 mg/kg s.c., once daily for 3 consecutive days in mice. This protocol causes not only pain but also FSS-associated symptomatology which are associated with disruption of the endogenous monoamine system. The reserpine-treated animal may be useful for the research of not only fibromyalgia syndrome but also FSS, especially for the research focusing on the hypothesis that FSS is associated with the disturbance of endogenous monoamine system.

Effects of betaine on lipopolysaccharide-induced memory impairment in mice and the involvement of GABA transporter 2.

BACKGROUND: Betaine (glycine betaine or trimethylglycine) plays important roles as an osmolyte and a methyl donor in animals. While betaine is reported to suppress expression of proinflammatory molecules and reduce oxidative stress in aged rat kidney, the effects of betaine on the central nervous system are not well known. In this study, we investigated the effects of betaine on lipopolysaccharide (LPS)-induced memory impairment and on mRNA expression levels of proinflammatory molecules, glial markers, and GABA transporter 2 (GAT2), a betaine/GABA transporter. METHODS: Mice were continuously treated with betaine for 13 days starting 1 day before they were injected with LPS, or received subacute or acute administration of betaine shortly before or after LPS injection. Then, their memory function was evaluated using Y-maze and novel object recognition tests 7 and 10-12 days after LPS injection (30 µg/mouse, i.c.v.), respectively. In addition, mRNA expression levels in hippocampus were measured by real-time RT-PCR at different time points. RESULTS: Repeated administration of betaine (0.163 mmol/kg, s.c.) prevented LPS-induced memory impairment. GAT2 mRNA levels were significantly increased in hippocampus 24 hr after LPS injection, and administration of betaine blocked this increase. However, betaine did not affect LPS-induced increases in levels of mRNA related to inflammatory responses. Both subacute administration (1 hr before, and 1 and 24 hr after LPS injection) and acute administration (1 hr after LPS injection) of betaine also prevented LPS-induced memory impairment in the Y-maze test. CONCLUSIONS: These data suggest that betaine has protective effects against LPS-induced memory impairment and that prevention of LPS-induced changes in GAT2 mRNA expression is crucial to this ameliorating effect.

Nociceptin and its metabolite attenuate U0126-induced memory impairment through a nociceptin opioid peptide (NOP) receptor-independent mechanism.

Nociceptin binds to nociceptin opioid peptide (NOP) receptors. We reported that although high doses of nociceptin impaired memory function and that these effects were mediated via NOP receptors, low doses of nociceptin attenuated the memory impairment, and these attenuating effects were not mediated via NOP receptors. Even very low doses of nociceptin were biologically active and suggested a certain binding site for this peptide, but the mechanism underlying this attenuating effect has not yet been elucidated. In the present study, we investigated the effect of an intrahippocampal injection (i.h.) of nociceptin on memory impairment induced by U0126, a MEK inhibitor, and Rp-cAMPS, a PKA inhibitor in a step-down type passive avoidance test. U0126 (2.63 nmol/mouse, i.h.) impaired memory formation and training-dependent phosphorylation of ERK2 in the hippocampus. Co-administration of nociceptin (10 fmol/mouse) significantly attenuated memory impairment, while it did not attenuate the inhibition of training-dependent phosphorylation of ERK2 induced by U0126. On the other hand, nociceptin did not attenuate memory impairment induced by Rp-cAMPS (0.448 nmol/mouse, i.h.). Nociceptin (1 fmol/mouse) also attenuated U0126 (5.26 nmol/mouse)-induced memory impairment in NOP receptor knockout mice. Nociceptin was reported to metabolize into fragments (1-13) and (14-17) in vivo, which showed pharmacological activities without affecting NOP receptors. Our findings showed that nociceptin (14-17) (1 fmol/mouse) also attenuated U0126-induced memory impairment, while nociceptin (1-13) (0.1-10 fmol/mouse) did not attenuate memory impairment. These results suggest a novel action site or mechanism for the attenuating effects of nociceptin and its metabolite, and the sequence of nociceptin (14-17) is a critical structure.

Distinct effects of nociceptin analogs on scopolamine-induced memory impairment in mice.

Nociceptin, also known as orphanin FQ, binds to opioid receptor like-1 (NOP) receptors. Nociceptin and NOP receptor play important roles in several physiological functions in the central nervous system. We reported that although high doses of nociceptin impaired learning and memory and these effects were blocked by nocistatin, naloxone benzoylhydrazone and [NPhe(1)]nociceptin(1-13)NH(2), low doses of nociceptin improved scopolamine- or mecamylamine-induced impairment of learning and memory, and these ameliorating effects were not blocked by these antagonists. In the present study, to confirm our previous findings, the effects of [Arg(14), Lys(15)]nociceptin and [(pF)Phe(4)]nociceptin(1-13)NH(2), highly potent and long-lasting nociceptin analogs, on the memory impairment induced by scopolamine using the Y-maze and step-down type passive avoidance tests were investigated. [Arg(14), Lys(15)]Nociceptin (0.1 and/or 1 pmol/mouse, i.c.v.) significantly improved impairment of memory function. Although this analog was about 30 times more potent than nociceptin, the doses ameliorating these memory impairments were comparable to those of the natural ligand nociceptin. The ameliorating effects of [Arg(14), Lys(15)]nociceptin were not blocked by an NOP receptor antagonist, [NPhe(1)]nociceptin(1-13)NH(2). Interestingly, another potent nociceptin analog, [(pF)Phe(4)]nociceptin(1-13)NH(2) could not improve impairment of memory function. These results confirmed that there are novel mechanisms underlying these ameliorating effects and these seem not to be mediated via an NOP receptor.

Nociceptin/orphanin FQ reverses mecamylamine-induced learning and memory impairment as well as decrease in hippocampal acetylcholine release in the rat.

Nociceptin/orphanin FQ is an endogenous neuropeptide that plays important roles in several physiological functions including pain, anxiety, locomotion, learning, and memory. We previously reported that low doses of nociceptin improved the scopolamine-induced impairment of learning and memory in the passive avoidance test and the spontaneous Y-maze alternation task in mice. In the present study, the effects of nociceptin on learning and memory impairment as well as the decrease in acetylcholine release induced by mecamylamine were investigated in rats. Mecamylamine (49 micromol/kg, s.c.), a nicotinic acetylcholine receptor antagonist, impaired learning and memory in the step-through type passive avoidance test and decreased acetylcholine release in the hippocampus, as determined by in vivo microdialysis. The administration of nociceptin (10 fmol/rat, i.c.v.) reversed the impairment of learning and memory and blocked the decrease in acetylcholine release induced by mecamylamine. This ameliorating effect on the mecamylamine-induced impairment of learning and memory was not blocked by [NPhe(1)]nociceptin(1-13)NH(2) (1 nmol/rat, i.c.v.), an opioid receptor-like 1 (NOP) receptor antagonist. These results suggest that nociceptin improves the impairment of learning and memory as well as decrease in acetylcholine release induced by mecamylamine, and that these effects may not be mediated by NOP receptors.