Paeoniflorin inhibits excitatory amino acid agonist-and high-dose morphine-induced nociceptive behavior in mice via modulation of N-methyl-D-aspartate receptors
Paeoniflorin, an active component from Paeonia lactifloria, is converted to paeonimetabolines by human intestinal bacteria [47]. Only very trace amount of paeoniflorin in rat hippocampus was detected by high-performance liquid chromatography after intravenous administration of Paeoniae Radix extract [48] Since the bioavailability for oral administration of paeoniflorin is low, the intracerebroventricular injection is applied in the present study to acquire high potency of paeoniflorin. Previously, it was reported that PF inhibited formalin-induced nociception in mice [32]. It has been suggested that formalin-induced nociception could be blocked by NMDA antagonists [13, 49]. In the present study, we determined if PF could inhibit formalin-induced licking and biting behavior by acting on the NMDA receptor.
PF produced significant inhibition of both neurogenic (early phase) and inflammatory (late phase) pain responses caused by formalin injection in mice. Co-administration of PF increased the antinociceptive effect of MK-801 on formalin-induced nociception. The enhancement of PF on the inhibitory effects of MK-801 on formalin-induced licking behavior is significant both in early phase and in late phase.
However, PF did not affect 100 nmol trans-(±) – ACPD-induced nociceptive behavior (data not shown). Since trans-(±)-ACPD is an mGluRs agonist, effects of PF effects may be mediated primarily by acting on iGluRs but not mGluRs.
To provide more direct evidence concerning the interaction of PF with iGluRs, we determined if PF could diminish EAA agonist-induced nociceptive behavior in mice. PF attenuated biting and scratching behavior induced by glutamate, NMDA, and AMPA in a dose-dependent manner. Co-administration of PF augmented effects of MK-801 on reducing nociceptive behavior induced by the 3 EAA agonists. MK-801 is a NMDA receptor antagonist in the glutamate category involved with the central nervous system [50]. Co-administration of PF and MK-801 revealed a synergistic inhibitory effect on formalin-induced licking behavior. The effect observed from the combination of PF and MK-801 might be due to their effect on NMDA receptor with different binding sites.
Our results indicate that the antinociception effects of PF are due in part to targeting the glutamatergic system, especially via interaction with iGluRs. Such conclusion is supported by evidence showing that PF produced significant attenuation of the biting and scratching behavior induced by NMDA, AMPA, and glutamate. Besides, the spinal NMDA receptors may be primarily involved in eliciting the licking and biting behavior that followed intrathecal injection of high-dose morphine.
PF not only delayed the onset but also decreased the total number of clonic seizure-like excitation induced by morphine, the antinociception caused by PF was due to an interaction with iGluRs, more specifically via an interaction with the NMDA receptor.
Impaired glutaminergic neurotransmission has been implicated in several neurological diseases such as acute stroke, trauma, epilepsy, schizophrenia, depression, chronic pain and opioid dependence [51]. The excitatory effect of glutamate is thought to be mediated in part through NMDA receptors [3, 4, 10]. Substances which block NMDA receptors could have potential clinical use in pain management. However, antagonists that completely block NMDA receptors cause numerous side effects, such as ataxia, motor incoordination, memory impairment and psychotomimetic effects. Therefore, developing new substances that can inhibit excitation, and maintain the normal physiological function of NMDA receptors would be ideal candidates for the treatment of NMDA-induced diseases. Such agents might be predicted to be devoid of CNS side effects at doses producing potent antinociception at peripheral NMDA receptors [52]. The antinociception effects of PF on amino acid agonists and high-dose morphine-induced nociceptive behavior are mediated by targeting iGluRs, especially modulation of the NMDA 2B receptor. The NMDA receptor family consists of NR1, NR2 (A, B, C, and D), and NR 3 (A and B) subunits, which determine the characteristics of native NMDA receptors. Among NMDA receptor subunits, the NR2B subunit for pain is particularly noteworthy. A drug that has NR2B selective antagonistic properties, may be effectively used for the treatment of chronic pain [24]. In this study, it revealed that NR2B was significantly inhibited by PF and antisense ODN for NR2B. PF significantly increased the suppressive effects of the antisense oligodeoxynucleotide (ODN) subunits, NR1, NR2A, NR2B on EAA and high dose morphine-induced nociceptive behavior. PF targeting NR2A and NR2B subunits was confirmed in in silico experiments. Docking energy data revealed that paeoniflorin had stronger binding activity in NR2A and NR2B than NR2C of NMDA receptors. Notably, the result of docking procedure was corresponding to the experiment of an animal model.
Remarkably, no side effects such as ataxia, motor incoordination, memory impairment and psychomimetic effects were found in mice during the period of antisense ODNs of NMDAR subunits co-administered with PF. This observation reveals the superiority of PF.