The Effects Of Morphine Addiction On The Hippocampal Combinatorial Synaptic Plasticity

Drug addiction is characterized by persistent and compulsive drug craving, drug seeking and persistence tendency to relapse, which is the major problem hampering clinical treatment of drug addiction. Ample evidence adds to a growing consensus that drug-associated contextual cues play an important role in triggering drug craving and relapse and the retrieval of addiction memory by conditioned contextual cues may be the underlying mechanism.Hippocampus is known to be critical for the formation of certain type of memory including contextual cues-associated addiction memory. Hippocampal activity-dependent or experience-dependent synaptic plasticity, including Long-term potentiation (LTP) and Long-term depression (LTD), is believed to be the cellular and molecular mechanism which underlies hippocampus-dependent learning and memory. Hippocampal synaptic plasticity in the studies of morphine addiction is often examined by using one pathway recording technique, in which LTD is induced by low frequency stimulation (LFS) and LTP is induced by high frequency simulation (HFS). The results of those studies suggest that there should be a crosstalk between the potentiation or depression of hippocampal synaptic plasticity and the mechanism of addictive behavior. However behavioral learning, e.g. the classic conditioning, is based on the association between conditioned and unconditioned stimuli and in the experiment of locomotor sensitization to morphine, locomotor sensitization may be expressed sucessfully only in the conditioned context, which is based on unconditioned stimulus (morphine injection) in the conditioned environment (open field). Recent study indicates that hippocampal combinatorial synaptic plasticity can be induced by a similar conditioning procedure that conditioned stimuli between two Schaffer-CA1 pathways within a certain timing window induces LTP and LTD simultaneously, which illustrates the flexibility and stability of hippocampal synaptic plasticity. Since conditioned contextual cues may make critical contribution to relapse, hippocampal combinatorial synaptic plasticity based on conditioning of two pathways should be a candidate mechanism for long-lasting and unforgettable addiction memory.In the present study, using in vivo electrophysiological, behavioral and biochemical methods, we carried out three sets of experiments. In the first set of experiments, we examined the distribution of hippocampal combinatorial synaptic plasticity in two converging Schaffer-CA1 pathways in rat; in the second set of experiments, we examined the effects of morphine addiction on hippocampal combinatorial synaptic plasticity; in the third set of experiments, we examined the effects of repetitive withdrawal on hippocampal combinatorial synaptic plasticity. We also used immunoblots (Western Blotting) to asses the expression of GluR1 (Glutamate receptor1) and GluR2/3 (Glutamate receptor2/3) subunits of the AMPAR, which is the major receptor responsible for the expression and maintenance of LTP and LTD, in the total membrane fraction and the synaptosome fractions of the hippocampus after one time and third times of morphine withdrawal.In the first set of experiment, we demonstrated for first time that there are four types of combinatorial synaptic plasticity, i.e. LTD/LTD, LTP/LTD, LTD/LTP and LTP/LTP. The results implicate that LTD and LTP may coexist and concomitantly encode information, which provides the stability of the hippocampal synaptic plasticity and makes contribution to limit the tendency to drive synaptic strength towards potentially epileptogenic level of LTP or minimum value and shows the flexibility of encoding process which may be similar to encoding information by digital 0 and 1. Combinatorial plasticity may encode the information more efficiently and may enhance the capacity for the storage of memory traces compared with encoding process mediated by LTP or LTD independently, which provide the possibility to reset synaptic weight and update synaptic configuration delicately.In the second set of experiment, we first introduced the dual Schaffer-CA1 pathways experiments to examine hippocampal combinatorial synaptic plasticity after repeated morphine exposure. We observed that repeated morphine exposure facilitated LTD (LTD/LTD), which was gradually restored following morphine withdrawal. However on withdrawal day 4, LTP/LTD was induced. Thus, from drug taking to withdrawal, homeostatic adaptations and contextual conditioning may be the mechanism underlying contextual cues-associated addiction memory. Since LTD or LTP was fully blocked by antagonists to NMDAR and D1/D5 receptors, it seems that morphine may employ dopamine system to modulate glutamatergic LTP and LTD which may be the molecular mechanism of the conditioned contextual cues which can retrieve addiction memory.In the third set of experiments, we demonstrated for first time that LTP after three times of withdrawal was enhanced as compared with that after one time of withdrawal and combinatorial plasticity was remarkable LTP/LTP as compared with LTP/LTD after one time of withdrawal. Consistent with this result, locomotor activity after three times of withdrawal was largely increased as compared with that after one time of withdrawal. The data of the expression of GluR1 and GluR2/3 are in preparation. Our findings suggest that the more times of withdrawal, the more possibilities of combinatorial plasticity would be LTP/LTP that may be responsible for stronger contextual cues-associated addiction memory and drug-seeking behavior. These results reveal that acute withdrawal may be critical in the formation of contextual cues-associated addiction memory and effective block of hippocampal LTP induction may contribute to the extinction of contextual cues-associated addiction memory and thereby decrease the tendency to relapse evoked by conditioned contextual cues.