Study On The Relationship Between Thienorphine's Low Dependence And Central Opioid Receptor, Monoamine Neurotransmitter Of The Related Brain Region And Synaptic Plasticity

Objective and significance:With the number of drug users increasing yearly, drug abuse has become a global social and medical problem. However, detoxification treatment has so far failed to achieve satisfactory long-term results, so much so that the relapse rate in drug use tops 95 % among patients undergoing detoxification treatment. Post-detoxification relapse prevention has become a focus to addictive medicine worldwide. Medical researchers are faced with the important task to develop a new medicine for the prevention of opioid addiction, reduction of psychological curving and post-detoxification relapse prevention.However, the available anti-opioid relapse drugs are insufficient at home and abroad. As a commonly used anti-opioid relapse agent, naltrexone calls for large dosage, but its effective period is so short that its medication's failing rate is running high. Only 20-30% of all the detoxification patients treated with naltrexone can manage to remain drug free for six months. In recent years, a partial opioid agonist known as Buprenorphine (Bup) has been proved effective for its good and long detoxification effects in alleviating the protracted withdrawal syndrome, and obstructing drug craving. However, for its poor oral absorption and some dependence potentials, Bup can hardly be accepted as a desirable detoxificaion agent.Thienorphine (Thien) is a Bup derivant with independent intellectual property rights synthesized by our institute. It has been proved that Thien is a new type of non-selective opioid receptor partial agonists which has stronger analgesic effect than buprenorphine, good oral absorption, a long term of treatment and clinical safety. Moreover, at an effective dosage only 1/10-1/20 that of naltrexone, Thien can antagonize the effects of morphine for as long as 14 days. Its t1/2 was 108 h (26 times longer than that of naltrexone). This prolonged anti-morphine effect, which helps cut medication's failing rate and maximize the post-detoxification abstinence rate. Another prominent pharmacological characteristic of Thien is its low psychological and physical dependence. Those features determine that Thien may become a choice medicine in dealing with opioid relapse. Once introduced to clinic use, Thien will be widely used in the medical treatment of drug abuse and will be likely to replace naltrexone and Bup. China's drug rehabilitation rate will be increased by our breakthroughs. Thien will be also likely to enter the world market and contribute to worldwide anti-drug efforts.However, the mechanisms of Thien's low dependence remain largely unknown. Previous available results obtained from our institute indicated that Thien had no selective affinity toμ,κandδopioid receptors, and can easily bind to the three receptors. Nevertheless, its dissociation toμ,κopioid receptors was quite slow. We also found that Thien had stronger capability of activatingμandκopioid receptors than that ofδopioid receptors. Thien can down-regulate the number of MOR in the cell membrane in a dose-dependent manner after incubating with CHO-μcells. But these results are still insufficient to explaining the mechanism of Thien's low dependence. So we try to explain the mechanisms of Thien's low dependence based on current mechanism of opioid dependence including the function of Norepinephrine (NE) in the locus coeruleus (LC) related with physical dependence, the release of monoamine neurotransmitter in the reward system related with psychological dependence and the change of synaptic plasticity. To clarify this question is valuable not only for developing nerves theory of drug dependence but also guiding clinical practice and synthesizing more and better anti-opioid relapse agents using Thien as the leading compound.Content and Methodology:1. Study relationship between Thien's pharmacological properties and its activation of central opioid receptor in vivo. We utilize mu and kappa receptor antagonist to oppose Thienorphine antinociceptive effects and establish mice physical dependence model and pretreat with kappa receptor antagonist to observe dependence symptoms. The aim of this part is to investigate receptor mechanism of Thien's low dependence on the basis of the theory on activation ofκreceptor against dependence induced by activation ofμreceptor.2. Investigate the effect of Thien on the content of monoamine neurotransmitter and activity of enzymes in the brain region correlatd with dependence. We analysis the release course of monoamine neurotransmitter in the Locus coeruleus (LC), Nucleus accumbens (NAc) and Striatum of dialyzed rats by HPLC-ECD method and measure activity of Monoamine oxidase (MAO) after Thien acute and chronic administration. Our objective of this part is to approach mechanisms of Thien's low dependence from neuromechanism level compared with Mor.3. Explore the possible role of Thien-induced synaptic plasticity. We observe the ultra-microstructure of neurons and the synaptic interface structure in NAc and hippocampus CA1 (Hip CA1) using transmission electron microscope and measure the activity of Synaptophysin (SYP) in the brain region correlatd with dependence of rats treated with Thien administration by Western blot way. Our purpose of this part is to get the mechanisms of Thien's low dependence from synaptic plasticity aspect.Results and conclusions:1.Nor-BNI (a kappa receptor antagonist) can inhibite Thien's antinociceptive effects which inhibition rate decreased from 100% to 70.94% (writhing model) and from 69.79% to 41.19% (P<0.05) (tail flick model); Naloxonazine(a mu receptor antagonist) can also antagonize Thien analgesic effect which inhibition rate dropped from 79.26% to 41.02% (P <0.05) (writhing model) and from 86.08% to 41.97% (P < 0.05) (tail flick model). In mice model of physical dependence, Mice pretreated with Nor-BNI on Thien group did not show jumping and other physical dependence symptoms. Thien can activate kappa receptors to take its effect in vivo. It is not certain that relationship between Thien's kappa receptors activation and its low dependence. Maybe distribution ofκopioid receptor in the central nervous system (CNS) is different with that ofμopioid receptor, so Thien's kappa receptors activation can not interfere with the physical dependence induced byμopioid receptor in the locus coeruleus.2.Compared with saline group, Thien's acute and chronic administration did not affect the content of NE in the LC and the content of DA in the NAC and the Striatum of freely moving rats using microdialysis, while Mor increased the content of NE and DA. These results may display one of neuromechanism of Thien's low dependence. Compared with saline and Mor group, Thien's chronic administration increased the content of DOPAC and HVA (DA metabolites) and reinforce the activity of MAO in the Striatum of rats. The results indicated that Thien may accelerate DA metabolism course owing to strengthening the activity of MAO and block the change of neurotransmitter and did not show dependence.3. Thien's chronic administration increased the content of SYP in the NAc of rats, but did not affect the content of SYP in the hippocampus CA1 area (Hip CA1) of rats, while Mor decreased the content of SYP in the Hip CA1 of rats. These results showed Thien's effect on neural plasticity is lower than that of Mor. Length of synaptic active zone (NAc and Hip CA1) and the post-synaptic density (NAc) of Thien-treated rats were significantly lower than that of saline group. Length of synaptic active zone in NAc and Hip CA1 of Thien-treated rats were greater than that of Mor group, but the width of synaptic cleft and the post-synaptic density have an opposite result. These result indicated Thien and Mor can decrease synaptic transmission efficiency, but decrease level of Thien was lower than that of Mor. This might be one of mechanisms of Thien's low dependence from synaptic plasticity aspect.In a word, we investigate the possible mechanism of Thien's low dependence from three directions including receptor, neurotransmitter and synaptic plasticity. These results provide experimental foundation for explaining of Thien's pharmacological characteristic and clinical practice.