Antinociceptive effects of morphine and naloxone in mu-opioid receptor knockout mice transfected with the MORS196A gene

Abstract

BackgroundOpioid analgesics such as morphine and meperidine have been used to control moderate to severe pain for many years. However, these opioids have many side effects, including the development of tolerance and dependence after long-term use, which has limited their clinical use. We previously reported that mutations in the mu-opioid receptors (MOR) S196L and S196A rendered them responsive to the opioid antagonist naloxone without altering the agonist phenotype. In MORS196A knock-in mice, naloxone and naltrexone were antinociceptive but did not cause tolerance or physical dependence. In this study we delivery this mutated MOR gene into pain related pathway to confirm the possibility of in vivo transfecting MORS196A gene and using naloxone as a new analgesic agent.MethodsThe MOR-knockout (MOR-KO) mice were used to investigate whether morphine and naloxone could show antinociceptive effects when MORS196A gene was transfected into the spinal cords of MOR-KO mice. Double-stranded adeno-associated virus type 2 (dsAAV2) was used to deliver the MORS196A-enhanced green fluorescence protein (EGFP) gene by microinjected the virus into the spinal cord (S2/S3) dorsal horn region. Tail-flick test was used to measure the antinociceptive effect of drugs.ResultsMorphine (10 mg/kg, s.c.) and naloxone (10 mg/kg, s.c.) had no antinociceptive effects in MOR-KO mice before gene transfection. However, two or three weeks after the MOR-S196A gene had been injected locally into the spinal cord of MOR-KO mice, significant antinociceptive effects could be induced by naloxone or morphine. On the other hand, only morphine but not naloxone induced significant tolerance after sub-chronic treatment.ConclusionTransfecting the MORS196A gene into the spinal cord and systemically administering naloxone in MOR-KO mice activated the exogenously delivered mutant MOR and provided antinociceptive effect without causing tolerance. Since naloxone will not activate natural MOR in normal animals or humans, it is expected to produce fewer side effects and less tolerance and dependence than traditional opioid agonists do.

Prolonged morphine administration alters protein expression in the rat myocardium

“…To exclude the possibility that the observed protein changes could have been mediated by signaling pathways independent of opioid receptor activation, the opioid antagonist naloxone was employed in subsequent experiments [ 37 ]. Naloxone alone or a combination of naloxone and morphine was administered to rats and the levels of selected cytoprotective proteins in myocardial preparations were assessed by immunoblotting (Figure 5 ).…”

Convallatoxin enhance the ligand-induced mu-opioid receptor endocytosis and attenuate morphine antinociceptive tolerance in mice

“…Male wild-type C57BL/6 (B6) mice (25–30 g) and MOR-KO mice 16 (provided by Dr. Pao-Luh Tao, National Health Research Institutes, Taiwan) were kept in a temperature-controlled animal room with a 12:12 h light-dark cycle. The protocol was approved by the Institutional Animal Care and Use Committee of the National Health Research Institutes, Taiwan.…”

Lentiviral Vectors and Adeno‐Associated Virus Vectors: Useful Tools for Gene Transfer in Pain Research

“…Intraparenchymal injection into the spinal cord DH can result in significant transduction of neurons (Eaton et al, ; Garraway et al, ; Chen et al, ). The expression of GFP was confined to the ipsilateral DH, with minimal expression detected in the ipsilateral ventral horn or on the contralateral side.…”