Study On The Mechanism Of Dopamine Involved In The Nociceptive Modulation And Morphine Addiction In The Parafascicular Nucleus Of Rats

Objective: Medications used for the relief of pain have potential for addiction,which suggest analgesic and addiction are related. To investigate whether painmodulation and morphine addiction is related to dopamine (DA) system of theparafascicular nucleus (Pf), the methods of praxiology and electrophysiology wereused to investigate the mechanism of DA involved in the pain modulation andmorphine addiction in the Pf of rats. Hope for the mechanism of morphine addictionprovided new scientific evidences.Methods: Behavioral method was used to evaluate natural morphinewithdrawal model. Electrophysiological method of extracellular recording was usedto observe the discharged changes of the pain-related neurons in the Pf of normaland morphine addiction rats, i.e. Net-increased value (NIV) of pain excited neuron(PEN) or pain inhibited neuron (PIN), Latency of PEN and Inhibitory duration (ID)of PIN, and observed the effects of DA and droperidol on pain-related neurons.Experiment is divided into two parts. Part1, the effects of DA on the pain-evokedresponse of PEN and PIN in the Pf of normal rats: The rats were randomly andequally divided into three groups (n=12per group): control group, intra-Pfadministration of saline0.5μl; DA group, intra-Pf administration of DA5μg/0.5μl;droperidol group, intra-Pf administration of droperidol0.15μg/0.5μl. Trains ofelectric impulses were used for noxious stimulation, and were applied to the sciaticnerve. The electrical activities of pain-related neurons in the Pf were recorded usinga glass microelectrode. Part2, the effects of DA on the pain-evoked response of PENand PIN in the Pf of morphine addiction rats: after establishment of morphine addiction rat models, we evaluate the model of the morphine addiction rat by usingethological observation method. Other methods are same as part1.Results:1. In normal rats, the average NIV of PEN increased and the averagelatency shortened; the average NIV of PIN decreased and the average ID prolongedafter the intra-Pf administration of DA. The average NIV of PEN decreased and theaverage latency prolonged; the average NIV of PIN increased and the average IDshortened after the intra-Pf administration of droperidol.2. Morphine addiction rats, withdrawal symptoms and total scores comparedwith the control group had significant differences. The NIV increased and the latencyshortened of the PEN; the NIV decreased and the ID prolonged of the PIN in the Pfof morphine addiction rats compared with normal rats. In morphine addiction rats,the average NIV of PEN decreased and the average latency prolonged; the averageNIV of PIN increased and the average ID shortened after the intra-Pf administrationof DA. The average NIV of PEN increased and the average latency shortened; theaverage NIV of PIN decreased and the average ID prolonged after the intra-Pfadministration of droperidol.Conclusion: According to above experimental results, the followingconclusions we can get.1. In normal rats, intra-Pf administration of DA excited PEN,inhibited PIN; intra-Pf administration of droperidol inhibited PEN, excited PIN. Theresults proved DA has facilitated the transmission of pain in the Pf of normal rats.2.Rats with progressive doses of morphine injection during6days, the withdrawalscores of natural withdrawal rats have significantly difference with the rats in controlgroup, so the model of the morphine addiction rats can be regard as a successfulmodel. The NIV and the latency of the PEN; the NIV and the ID of the PIN in the Pfof morphine addiction rats changed compared with normal rats. The result provedmorphine addiction caused the changes of neuronal activity of the Pf. In morphineaddiction rats, DA inhibited PEN, excited PIN; droperidol excited PEN, inhibition PIN. The results proved DA inhibited pain in morphine addiction rats. Experimentsshow that the Pf is involved in pain modulation and morphine addiction.