introduction: response to morphine and role of nitric oxide (no) on expression of morphine response has been studied in vertebrates. but, little evidence is provided in the matter in earlier invertebrates. this investigation for the first time evaluated the effect of no on expression of morphine potency in paramecium caudatum. methods: animal after isolation from natural media and specific iden...

High doses of insurmountable antagonists or frequent administration of high doses of agonists are required to alter the potency of opioid agonists to produce discriminative stimuli. In the present study, insurmountable antagonism and repeated agonist treatment were combined to remove or disable a large enough proportion of mu-opioid receptors to alter the potency or maximal effect for four agon...

Several variables have been reported to affect the expression of sex differences in the analgesic potency of morphine. Although the effect of genetic background on morphine analgesia has been well documented, the relevance of genotype to sex differences in morphine analgesia has rarely been considered. The present study investigated morphine dose-response relationships in male and female mice o...

Single morphine injections induce a state of acute opioid dependence measured by an increase in naloxone potency to precipitate withdrawal. Repeated morphine exposure (daily/weekly intervals) results in further potentiation of naloxone potency, perhaps due to conditioning mechanisms. The current study tested the hypothesis that previously neutral stimuli could elicit a conditioned potentiation ...

It is well known that the analgesic potency of morphine is reduced in neuropathic pain. In this study, we demonstrate that the decreased effectiveness of systemic morphine in neuropathic pain might be caused by the loss of morphine analgesia at the periphery. When given s.c. or i.t., the dose-response curves for morphine analgesia in Hargreaves thermal test were shifted rightward in partial sci...

Morphine loses analgesic potency after repeated administration. The underlying mechanism is not fully understood. Glia are thought to be involved in morphine tolerance, and P2X7 purinergic receptor (P2X7R) has been implicated in neuron– glia communication and chronic pain. The present study demonstrated that P2X7R immunoreactivity was colocalized with the microglial marker OX42, but not the ast...

Morphine loses analgesic potency after repeated administration. The underlying mechanism is not fully understood. Glia are thought to be involved in morphine tolerance, and P2X(7) purinergic receptor (P2X(7)R) has been implicated in neuron-glia communication and chronic pain. The present study demonstrated that P2X(7)R immunoreactivity was colocalized with the microglial marker OX42, but not th...

BACKGROUND The involvement of the mu-opioid receptor (muOR) system in the control of breathing, anesthetic potency, and morphine- and anesthesia-induced respiratory depression was investigated in mice lacking the muOR. METHODS Experiments were performed in mice lacking exon 2 of the muOR gene (muOR-/-) and their wild-type littermates (muOR+/+). The influence of saline, morphine, naloxone, and...

Regulation of calcium flux has been suggested to play a role in acute and chronic effects of opioids. Previous studies have shown calcium channel blockers can inhibit opioid agonist-induced downregulation of mu-opioid receptors and may reduce the magnitude of tolerance. In the present study, we determined if calcium channel blockade would affect increases in opioid receptor density and function...

POTENCY RATIO BETWEEN OPIOID AGONISTS AND PARTIAL AGONISTS Sir,—The paper by Klepper and colleagues (1986) again seems to indicate the belief of this group of workers in a fixed potency ratio between opioid agonists and partial agonists, although such a concept is theoretically unlikely (Kay, 1985). Support for a variable potency ratio is apparent in their paper, however, in that nalbuphine in ...