Figure 3. Effects of morphine on the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and paired pulse facilitation (PPF) of VTA-DA neurons in rats.
(A) Effects of morphine on the frequency of sEPSCs in the presence of extracellularly applied picrotoxin (PTX) in VTA-DA neurons. Left panel: typical current traces of sEPSCs before and after morphine (10 μM) in the presence of extracellularly applied PTX. Middle panel: typical time course of the frequency of sEPSCs before and after morphine (10 μM) in the presence of extracellularly applied PTX. Right panel: average frequency of sEPSCs before and after morphine (10 μM) in the presence of extracellularly applied PTX (n = 6 cells from four rats, p < 0.05, compared to control before morphine). (B) Effects of morphine on sEPSCs in the presence of intracellularly applied PTX in VTA-DA neurons. Top of panel 1: inhibitory postsynaptic currents (IPSCs) in the normal intracellular recording solution and the presence of intracellularly applied PTX. Bottom of panel 1: typical current traces of sEPSCs before and after DNQX (10 μM) in the presence of intracellularly applied PTX. Panel 2: typical current traces of sEPSCs before and after morphine (10 μM) in the presence of intracellularly applied PTX. Panel 3: typical time course of the frequency of sEPSCs before and after morphine (10 μM) in the presence of intracellularly applied PTX. Panel 4: average frequency of sEPSCs before and after morphine (10 μM) in the presence of intracellularly applied PTX (n = 8 cells from five rats, p < 0.05, compared to control before morphine). Panel 5: average amplitude of sEPSCs before and after morphine (10 μM) in the presence of intracellularly applied PTX (n = 8 cells from five rats, p = 0.24, compared to control before morphine). Panel 6: typical current traces of sEPSCs before and after DNQX (10 μM) in the presence of intracellularly applied PTX and morphine. (C) Effects of morphine on the PPF in VTA-DA neurons. Left panel: representative traces of the PPF before and after morphine (10 μM), and superimposition of the two traces normalized to the first excitatory postsynaptic current (EPSC) before and after morphine (10 μM) in the presence of intracellularly applied PTX. Middle panel: average amplitude of the first EPSC in control and morphine (10 μM) in the presence of intracellularly applied PTX (n = 6 cells from four rats, p < 0.05, compared to control before morphine). Right panel: average PPF before and after morphine (10 μM) in the presence of intracellularly applied PTX (n = 6 cells from four rats, p < 0.05, compared to control before morphine). (D) Effects of morphine on the frequency of sEPSCs when VTA-DA neurons were clamped the membrane potential at the reversal potential of Cl− channels in VTA-DA neurons. Left panel: typical current traces of sEPSCs before and after morphine (10 μM) when DA neurons was clamped the membrane potential at the reversal potential of Cl− channels. Middle panel: typical time course of the frequency of sEPSCs before and after morphine (10 μM) when DA neurons was clamped the membrane potential at the reversal potential of Cl− channels. Right panel: average frequency of sEPSCs before and after morphine (10 μM) when DA neurons was clamped the membrane potential at the reversal potential of Cl− channels (n = 6 cells from four rats, p < 0.05, compared to control before morphine). Data are shown as the mean ±s.e.m. *p < 0.05.