Chronic Morphine Withdrawal On The Discharge Characteristics Of Neurons In The Nucleus Accumbens Medium Spinous Process

The nucleus accumbens (NAc), a major component of brain mesocorticolimbic dopamine systerm. plays a central role in rewarding and drug addiction. It receives excitatory glutamatergic inputs from prefrontal cortex (PFC), hippocampus and amygdala. It is also innervated by dopaminergic terminals originated from ventral tegmental area. Carrying integrated information, the medium spiny neurons (MSNs) project to the nuclei of the basal ganglia to influence voluntary behavior. Medium spiny neurons are the principal cells of the NAc. which take up about 90%～95% of the total cell amount.NAc plays a critical role in drug addiction. Neuroadaptations in nucleus accumbens during withdrawal from repeated opioid exposure is considered to be one of the key mechnism of drug dependence. Neuroadaptations induced by drug withdrawal includes alterations in the efficiency of synaptic transmission, gene transcription and protein expression and neuronal morphology (the latter two decided the firing property). Synaptic strength and firing property are two major determinants of the functional output of neurons during drug addiction and withdrawal. Therefore, the change of neuronal plasticity (synaptic plasticity and plasticity of the firing property) is considered to be neural molecular basis of drug addition. Ample evidences have shown that the concentration of glutamate, which might modulate neuronal plasticity via synaptic and extrasynaptic receptors, was significantly increased in many brain areas including NAc during morphine chronic withdrawal. Our previous studies suggested that chronic morphine withdrawal downregulated groupⅡmetabotropic glutamate receptors (mGluR2/3) to induce PFC-MSN synaptic potentiation via increased glutamate release. Up to present, it is still not clarified about the alternation in intrinsic excitability of MSNs during chronic morphine withdrawal.In present study, we examined the change in intrinsic excitability of MSNs in the Shell of NAc (NAcShell MSNs) using whole-cell patch-clamp recordings in slice preparations from rats treated with repeated morphine exposure. The purposes of this study are to describe the alternation of intrinsic excitability of MSNs following chronic morphine withdrawal and possible underlying mechanism, therefore providing candidate therapeutic strategies for treating morphine addiction. We first observed that the intrinsic excitability was significantly enhanced during chronic morphine withdraw. We also found that the activation of extrasynaptic N-methyl-D-aspartate receptors (NMDARs) mediated potentiation in MSN intrinsic excitability via negatively regulating the sustained potassium currents (Iks) during chronic morphine withdrawal. The main results are as followings:1 NMDAR activation enhanced the intrinsic excitability and spike adaptation of NAcShell MSNsActivation of NMDAR significantly enhanced the intrinsic excitability and spike adaptation of NAcShell MSNs. The enhancement of intrinsic excitability is presented as decreased rheobase current, increased spike number and the first instantaneous frequency (IF) induced by a given depolarizing current. The enhancement of spike adaptation is presented as decreased spike train duration by a given depolarizing current. NR2A/B-containing NMDAR have different effects on the firing properties of NAcShell MSNs:selective activation of NR2A-containing NMDAR enhanced the spike adaptation, whereas selective activation of NR2B-containing NMDAR enhanced the intrinsic excitability.2 Chronic morphine withdrawal enhanced the intrinsic excitability and spike adaptation of NAcShell MSNsA. Chronic morphine withdrawal significantly enhanced both the intrinsic excitability and spike adaptation of NAcShell MSNs population as a whole.The enhancement of intrinsic excitability is presented as decreased rheobase current, decreased the first action potential latency and the first IF induced by a given depolarizing current. The enhancement of spike adaptation is presented as decreased spike train duration by a given depolarizing current. Pretreated with LY379268, a mGluR2/3 agonist, can significantly block these changes. Gaven that MSNs are not homogenous but heterogeneous in their biochemical, morphological and intrinsic physiological properties, we identified three distinct cell types from naive rats with their firing properties in response to depolarizing pulses in naive rats. They are repetitive spike discharge without tiring ceasing (typeⅠ), repetitive spike discharge with firing ceasing (typeⅡ). and non-repetitive spike firing (typeⅢ).B. Chronic morphine withdrawal selectively enhanced the intrinsic excitability of type I NAcShell MSNs.The enhancement of intrinsic excitability is presented as decreased rheobase current, decreased the first action potential latency, increased spike number and the first IF induced by a given depolarizing current. Pretreated with LY379268 can significantly block these changes.C. Chronic morphine withdrawal selectively enhanced spike adaptation of typeⅡNAcShell MSNs.The enhancement of spike adaptation is presented as a decreased spike train duration and an increased the first IF induced by depolarizing currents. Pretreated with LY379268 can significantly block these changes.Therefore, the downregulation of mGluR2/3 potentiates MSN intrinsic excitability and spike adaptation during chronic morphine withdrawalal.3 Extrasynaptic NMDAR activation mediates potentiation in MSN intrinsic excitability via negatively regulating the IksChronic morphine withdrawal significantly increased in the total area of spontaneous slow inward currents (SICs, an extrasynaptic NMDAR mediated current), and pretreated with LY379268 or PPF (that is able to decrease extracellular glutamate) can significantly block the change, indicating that the extrasynaptic NMDARs are activated during chronic morphine withdrawal. The steady-state current density of Iks from the morphine chronic withdrawal (MOR CW) group was decreased significantly versus that from the saline group. Selective activation of extrasynaptic NMDARs significantly inhibited the steady-state current density of Iks from the saline group, indicating that extrasynaptic NMDAR activation negatively regulates the sustained Ik. Furthermore, selective activation of extrasynaptic NMDARs induced a further decrease in the Iks density in the MOR CW group than in the saline group. Taken together, these results suggest that the enhancement in MSN intrinsic excitability and spike adaptation during chronic morphine withdrawal likely relies on inhibition of the Iks through extrasynaptic NMDAR activation.Conclusion:Chronic morphine withdrawal significantly enhanced the intrinsic excitability and spike adaptation of NAcShell MSN. The possible signaling process of this enhancement is that chronic morphine withdrawal downregulate mGluR2/3 leading increasing glutamate release to spill over. Activation of extrasynaptic NMDAR mediated potentiation in MSN intrinsic excitability and spike adaptation via negatively regulating the Iks during chronic morphine withdrawal.