Neuroplastic alteration of TTX-resistant sodium channel with visceral pain and morphine-induced hyperalgesia.

The discovery of the tetrodotoxin-resistant (TTX-R) Na(+) channel in nociceptive neurons has provided a special target for analgesic intervention. In a previous study we found that both morphine tolerance and persistent visceral inflammation resulted in visceral hyperalgesia. It has also been suggested that hyperexcitability of sensory neurons due to altered TTX-R Na(+) channel properties and expression contributes to hyperalgesia; however, we do not know if some TTX-R Na(+) channel property changes can be triggered by visceral hyperalgesia and morphine tolerance, or whether there are similar molecular or channel mechanisms in both situations. To evaluate the effects of morphine tolerance and visceral inflammation on the channel, we investigated the dorsal root ganglia (DRG) neuronal change following these chronic treatments. Using whole-cell patch clamp recording, we recorded TTX-R Na(+) currents in isolated adult rat lumbar and sacral (L6-S2) DRG neurons from normal and pathologic rats with colon inflammatory pain or chronic morphine treatment. We found that the amplitudes of TTX-R Na(+) currents were significantly increased in small-diameter DRG neurons with either morphine tolerance or visceral inflammatory pain. Meanwhile, the result also showed that those treatments altered the kinetics properties of the electrical current (ie, the activating and inactivating speed of the channel was accelerated). Our current results suggested that in both models, visceral chronic inflammatory pain and morphine tolerance causes electrophysiological changes in voltage-gated Na channels due to the chronic administration of these medications. For the first time, the present investigation explored the adaptations of this channel, which may contribute to the hyperexcitability of primary afferent nerves and hyperalgesia during these pathologic conditions. The results also suggest that neurophysiologic mechanisms of morphine tolerance and visceral hyperalgesia are related at the TTX-R Na(+) channel.

IRAS, a candidate for I1-imidazoline receptor, mediates inhibitory effect of agmatine on cellular morphine dependence.

Agmatine, an endogenous ligand for the I1-imidazoline receptor, has previously been shown to prevent morphine dependence in rats and mice. To investigate the role of imidazoline receptor antisera-selected protein (IRAS), a strong candidate for I1R, in morphine dependence, two CHO cell lines were created, in which mu opioid receptor (MOR) was stably expressed alone (CHO-mu) or MOR and IRAS were stably co-expressed (CHO-mu/IRAS). After 48 h administration of morphine (10 microM), naloxone induced a cAMP overshoot in both cell lines, suggesting cellular morphine dependence had been produced. Agmatine (0.1-2.5 microM) concentration-dependently inhibited the naloxone-precipitated cAMP overshoot when co-pretreated with morphine in CHO-mu/IRAS, but not in CHO-mu. Agmatine at 5-100 microM also inhibited the cAMP overshoot in CHO/mu and CHO-mu/IRAS. Efaroxan, an I1R-preferential antagonist, completely blocked the effect of agmatine on the cAMP overshoot at 0.1-2.5 microM in CHO-mu/IRAS, while partially reversing the effects of agmatine at 5-100 microM. L-type calcium channel blocker nifedipine entirely mimicked the effects of agmatine at high concentrations on forskolin-stimulated cAMP formation in CHO-mu and naloxone-precipitated cAMP overshoot in morphine-pretreated CHO-mu. Therefore, IRAS, in the co-transfected CHO-mu/IRAS cell line, appears necessary for low concentrations of agmatine to cause attenuation of cellular morphine dependence. An additional effect of agmatine at higher concentrations seems to relate to both transfected IRAS and some naive elements in CHO cells, and L-type voltage-gated calcium channels are not ruled out. This study suggests that IRAS mediates agmatine's high affinity effects on cellular morphine dependence and may play a role in opioid dependence.

[Comparison of the antagonistic effects of 6 beta-naltrexol and naltrexone against morphine analgesia].

AIM: To compare the antagonistic effects of 6 beta-naltrexol and naltrexone against morphine analgesia. METHODS: The effects of 6 beta-naltrexol and naltrexone against morphine analgesia were observed in mouse heat radiant tail-flick assay and in mouse (55 +/- 1) degrees C hot plate test. The displacement of 6 beta-naltrexol and naltrexone on binding to CHO-mu receptor was observed by radioligand binding study. RESULTS: 6 beta-naltrexol antagonized morphine analgesia but the potency was (6.1 +/- 1.7)% that of naltrexone. The effective duration of 6 beta-naltrexol was 3-4 times that of naltrexone and the peak time of the response was about 0.5-1 h after s.c. equivalent efficacy dose (ED95) in two models. Like naltrexone, 6 beta-naltrexol was effective by oral administration and the potency ratio of p.o./s.c. was 1/3. As an antagonist to opioid receptor, the displacement of 6 beta-naltrexol was about 12.5% that of naltrexone, which was almost in agreement with the efficacies against morphine analgesia in mouse. CONCLUSION: Compared with naltrexone, 6 beta-naltrexol was less potent but duration was longer.

Modulation by alpha-difluoromethyl-ornithine and aminoguanidine of pain threshold, morphine analgesia and tolerance.

The effects of alpha-difluoromethyl-ornithine (DFMO) and aminoguanidine, which might influence the metabolism of endogenous agmatine, on pain threshold, morphine analgesia and tolerance were investigated in mice. In the mouse acetic acid writhing test, intracerebroventricular (i.c.v.) injection of DFMO or aminoguanidine significantly elevated the pain threshold as indicated by a decrease in the number of writhings. DFMO or aminoguanidine obviously increased the analgesic effect of morphine in the mouse acetic acid writhing test and the mouse heat radiation tail-flick assay. These effects of DFMO and aminoguanidine were antagonized by idazoxan (3 mg/kg, i.p.), which is a selective antagonist of the imidazoline receptor. In the mouse heat radiation tail-flick assay, aminoguanidine significantly prolonged the tail-flick latency of animals, suggesting that the pain threshold was elevated. Furthermore, both DFMO and aminoguanidine enhanced morphine analgesia and inhibited acute morphine tolerance in the mouse heat radiation tail-flick assay. Neither DFMO nor aminoguanidine inhibited the activity of nitric oxide synthase in different brain areas in mice in vivo. These results indicate that the substances involved in the metabolism of endogenous agmatine could modulate the pain threshold, morphine analgesia and tolerance, indicating the possible role of endogenous agmatine in the pharmacological effects of morphine.

A biphasic opioid function modulator: agmatine.

Recently it has been revealed that some agents that are not able to interact with opioid receptors play an important role in regulating the pharmacological actions of opioids. Especially, some of them show biphasic modulation on opioid functions, which enhance opioid analgesia, but inhibit tolerance to and substance dependence on opioids. We would like to call these agents which do not interact with opioid receptors, but do have biphasic modulation on opioid functions as biphasic opioid function modulator (BOFM). Mainly based on our results, agmatine is a typical BOFM. Agmatine itself was a weak analgesic which enhanced analgesic action of morphine and inhibited tolerance to and dependence on opioid. The main mechanisms of agmatine were related to inhibition of the adaptation of opioid receptor signal transduction induced by chronic treatment of opioid.