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Morphine disinhibits glutamatergic input to VTA dopamine neurons and promotes dopamine neuron excitation.

Chen M, Zhao Y, Yang H, Luan W, Song J, Cui D, Dong Y, Lai B, Ma L, Zheng P - Elife (2015)

Bottom Line: However, it is not known whether morphine has an additional strengthening effect on excitatory input.We also studied the contribution of the morphine-induced disinhibitory effect on the presynaptic glutamate release to the overall excitatory effect of morphine on VTA-DA neurons and related behavior.Our results suggest that the disinhibitory action of morphine on presynaptic glutamate release might be the main mechanism for morphine-induced increase in VTA-DA neuron firing and related behaviors.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Medical Neurobiology, Collaborative Innovation Center for Brain Science, School of Basic Medical Sciences and Institutes of Brain Science, Fudan Univeristy, Shanghai, China.

ABSTRACT
One reported mechanism for morphine activation of dopamine (DA) neurons of the ventral tegmental area (VTA) is the disinhibition model of VTA-DA neurons. Morphine inhibits GABA inhibitory neurons, which shifts the balance between inhibitory and excitatory input to VTA-DA neurons in favor of excitation and then leads to VTA-DA neuron excitation. However, it is not known whether morphine has an additional strengthening effect on excitatory input. Our results suggest that glutamatergic input to VTA-DA neurons is inhibited by GABAergic interneurons via GABAB receptors and that morphine promotes presynaptic glutamate release by removing this inhibition. We also studied the contribution of the morphine-induced disinhibitory effect on the presynaptic glutamate release to the overall excitatory effect of morphine on VTA-DA neurons and related behavior. Our results suggest that the disinhibitory action of morphine on presynaptic glutamate release might be the main mechanism for morphine-induced increase in VTA-DA neuron firing and related behaviors.

No MeSH data available.


Related in: MedlinePlus

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.DOI:http://dx.doi.org/10.7554/eLife.09275.005
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fig3: 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.DOI:http://dx.doi.org/10.7554/eLife.09275.005

Mentions: In order to study the effect of morphine on glutamatergic input to VTA-DA neurons, we examined the effect of morphine on the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in VTA-DA neurons in rats. First, we observed the effect of morphine on the frequency of sEPSCs when the GABAA receptor antagonist picrotoxin (PTX) was added to a bath solution to remove spontaneous inhibitory postsynaptic currents (sIPSCs). Consistent with earlier reports (Manzoni and Williams, 1999; Margolis et al., 2005), in the presence of extracellularly applied PTX (100 μM), morphine (10 μM) decreased the frequency of sEPSCs (Figure 3A). The average frequency of sEPSCs decreased from 4.2 ± 0.7 Hz before to 3.5 ± 0.7 Hz for 10–15 min after morphine application (n = 6 cells from four rats, paired t test, p < 0.05, compared to control before morphine, right panel of Figure 3A). However, bath application of GABAA receptor antagonists can lead to a wide blocking effect on GABAA receptors present on different types of neurons, including DA-neurons and GABA neurons, in VTA slices. To circumvent this wide influence and target the inhibition of GABAA receptors to VTA-DA neurons, we added the GABAA receptor antagonist PTX (100 µM) to the internal solution of microelectrodes as described by Akaike et al. (1985) when recording sEPSCs. To demonstrate the effectiveness of the blockade of GABAA receptors by intracellularly applied PTX, we first examined the effect of intracellularly applied PTX (100 μM) on inhibitory postsynaptic currents (IPSC) in VTA-DA neurons. The results showed that under this experimental paradigm, IPSCs disappeared (top of panel 1 in Figure 3B, n = 5 cells from two rats), demonstrating that GABAA receptors in the recorded VTA-DA neurons were blocked by intracellularly applied PTX. In addition, to further confirm that the spontaneous events we measured in the presence of intracellularly applied PTX were in fact sEPSCs, we observed the effect of the AMPA receptor antagonist DNQX on spontaneous events in the presence of intracellularly applied PTX. The results showed that the spontaneous events were completely blocked by DNQX (10 μM) (bottom of panel 1 in Figure 3B). We repeated the experiment in three cells from different slices and obtained similar results. On this basis, we observed the effect of morphine on the sEPSCs in the presence of intracellularly applied PTX. Raw current traces (panel 2 of Figure 3B) and the time course of sEPSCs (panel 3 of Figure 3B) before and after morphine application in the presence of intracellularly applied PTX showed that morphine (10 μM) increased the frequency of sEPSCs. The average frequency of sEPSCs increased from 4.8 ± 0.6 Hz before to 5.5 ± 0.6 Hz for 10–15 min after morphine application (n = 8 cells from five rats, paired t test, p < 0.05, compared to control before morphine, panel 4 of Figure 3B). However, morphine (10 μM) had no significant effect on the amplitude of sEPSCs. The average amplitude of sEPSCs was 16.6 ± 1.3 pA before and 15.3 ± 1.3 pA for 10–15 min after morphine application (n = 8 cells from five rats, paired t test, p > 0.05, compared to control before morphine, panel 5 of Figure 3B). To confirm that the increased spontaneous events we measured in the presence of intracellularly applied PTX after morphine were also in fact sEPSCs, we observed the effect of the AMPA receptor antagonist DNQX on spontaneous events after morphine application in the presence of intracellularly applied PTX. The results showed that the spontaneous events after morphine application were completely blocked by adding DNQX (10 μM) (panel 6 of Figure 3B). We repeated the experiment in three cells from different slices and obtained similar results. We also used the first excitatory postsynaptic current (EPSC) of paired pulse facilitation (PPF) as an index of EPSC (Maejima et al., 2001), and the PPF of paired EPSC as an indicator of presynaptic glutamate release (Zucker and Regehr, 2002) to confirm the effect of morphine on presynaptic glutamate release in VTA-DA neurons in the presence of intracellularly applied PTX. As shown in the left panel of Figure 3C, morphine (10 μM) increased the amplitude of the first EPSC, which was accompanied by a clear change in the presynaptic parameter PPF. The average amplitude of the first EPSCs was 124.1 ± 9.0 pA before and 161.9 ± 10.8 pA for 10–15 min after morphine application (n = 6 cells from four rats, paired t test, p < 0.05, compared to control before morphine, middle panel of Figure 3C). The average PPF was decreased from 1.5 ± 0.2 before to 1.2 ± 0.1 for 10–15 min after morphine application (n = 6 cells from four rats, paired t test, p < 0.05, compared to control before morphine, right panel of Figure 3C). These results supported the suggestion that morphine increased presynaptic glutamate release in VTA-DA neurons in the presence of intracellularly applied PTX. More importantly, when we clamped the membrane potential of VTA-DA neurons at the reversal potential of Cl− channels to remove sIPSCs, as an alternative to the application of a GABAA antagonist, either by the intracellular or bath approach, morphine still exerted a promoting effect on presynaptic glutamate release in VTA-DA neurons (Figure 3D). The average frequency of sEPSCs in this condition increased from 4.4 ± 0.3 Hz before to 5.5 ± 0.4 Hz for 10–15 min after morphine application (n = 6 cells from four rats, paired t test, p < 0.05, compared to control before morphine, right panel of Figure 3D).10.7554/eLife.09275.005Figure 3.Effects of morphine on the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and paired pulse facilitation (PPF) of VTA-DA neurons in rats.


Morphine disinhibits glutamatergic input to VTA dopamine neurons and promotes dopamine neuron excitation.

Chen M, Zhao Y, Yang H, Luan W, Song J, Cui D, Dong Y, Lai B, Ma L, Zheng P - Elife (2015)

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.DOI:http://dx.doi.org/10.7554/eLife.09275.005
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fig3: 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.DOI:http://dx.doi.org/10.7554/eLife.09275.005
Mentions: In order to study the effect of morphine on glutamatergic input to VTA-DA neurons, we examined the effect of morphine on the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in VTA-DA neurons in rats. First, we observed the effect of morphine on the frequency of sEPSCs when the GABAA receptor antagonist picrotoxin (PTX) was added to a bath solution to remove spontaneous inhibitory postsynaptic currents (sIPSCs). Consistent with earlier reports (Manzoni and Williams, 1999; Margolis et al., 2005), in the presence of extracellularly applied PTX (100 μM), morphine (10 μM) decreased the frequency of sEPSCs (Figure 3A). The average frequency of sEPSCs decreased from 4.2 ± 0.7 Hz before to 3.5 ± 0.7 Hz for 10–15 min after morphine application (n = 6 cells from four rats, paired t test, p < 0.05, compared to control before morphine, right panel of Figure 3A). However, bath application of GABAA receptor antagonists can lead to a wide blocking effect on GABAA receptors present on different types of neurons, including DA-neurons and GABA neurons, in VTA slices. To circumvent this wide influence and target the inhibition of GABAA receptors to VTA-DA neurons, we added the GABAA receptor antagonist PTX (100 µM) to the internal solution of microelectrodes as described by Akaike et al. (1985) when recording sEPSCs. To demonstrate the effectiveness of the blockade of GABAA receptors by intracellularly applied PTX, we first examined the effect of intracellularly applied PTX (100 μM) on inhibitory postsynaptic currents (IPSC) in VTA-DA neurons. The results showed that under this experimental paradigm, IPSCs disappeared (top of panel 1 in Figure 3B, n = 5 cells from two rats), demonstrating that GABAA receptors in the recorded VTA-DA neurons were blocked by intracellularly applied PTX. In addition, to further confirm that the spontaneous events we measured in the presence of intracellularly applied PTX were in fact sEPSCs, we observed the effect of the AMPA receptor antagonist DNQX on spontaneous events in the presence of intracellularly applied PTX. The results showed that the spontaneous events were completely blocked by DNQX (10 μM) (bottom of panel 1 in Figure 3B). We repeated the experiment in three cells from different slices and obtained similar results. On this basis, we observed the effect of morphine on the sEPSCs in the presence of intracellularly applied PTX. Raw current traces (panel 2 of Figure 3B) and the time course of sEPSCs (panel 3 of Figure 3B) before and after morphine application in the presence of intracellularly applied PTX showed that morphine (10 μM) increased the frequency of sEPSCs. The average frequency of sEPSCs increased from 4.8 ± 0.6 Hz before to 5.5 ± 0.6 Hz for 10–15 min after morphine application (n = 8 cells from five rats, paired t test, p < 0.05, compared to control before morphine, panel 4 of Figure 3B). However, morphine (10 μM) had no significant effect on the amplitude of sEPSCs. The average amplitude of sEPSCs was 16.6 ± 1.3 pA before and 15.3 ± 1.3 pA for 10–15 min after morphine application (n = 8 cells from five rats, paired t test, p > 0.05, compared to control before morphine, panel 5 of Figure 3B). To confirm that the increased spontaneous events we measured in the presence of intracellularly applied PTX after morphine were also in fact sEPSCs, we observed the effect of the AMPA receptor antagonist DNQX on spontaneous events after morphine application in the presence of intracellularly applied PTX. The results showed that the spontaneous events after morphine application were completely blocked by adding DNQX (10 μM) (panel 6 of Figure 3B). We repeated the experiment in three cells from different slices and obtained similar results. We also used the first excitatory postsynaptic current (EPSC) of paired pulse facilitation (PPF) as an index of EPSC (Maejima et al., 2001), and the PPF of paired EPSC as an indicator of presynaptic glutamate release (Zucker and Regehr, 2002) to confirm the effect of morphine on presynaptic glutamate release in VTA-DA neurons in the presence of intracellularly applied PTX. As shown in the left panel of Figure 3C, morphine (10 μM) increased the amplitude of the first EPSC, which was accompanied by a clear change in the presynaptic parameter PPF. The average amplitude of the first EPSCs was 124.1 ± 9.0 pA before and 161.9 ± 10.8 pA for 10–15 min after morphine application (n = 6 cells from four rats, paired t test, p < 0.05, compared to control before morphine, middle panel of Figure 3C). The average PPF was decreased from 1.5 ± 0.2 before to 1.2 ± 0.1 for 10–15 min after morphine application (n = 6 cells from four rats, paired t test, p < 0.05, compared to control before morphine, right panel of Figure 3C). These results supported the suggestion that morphine increased presynaptic glutamate release in VTA-DA neurons in the presence of intracellularly applied PTX. More importantly, when we clamped the membrane potential of VTA-DA neurons at the reversal potential of Cl− channels to remove sIPSCs, as an alternative to the application of a GABAA antagonist, either by the intracellular or bath approach, morphine still exerted a promoting effect on presynaptic glutamate release in VTA-DA neurons (Figure 3D). The average frequency of sEPSCs in this condition increased from 4.4 ± 0.3 Hz before to 5.5 ± 0.4 Hz for 10–15 min after morphine application (n = 6 cells from four rats, paired t test, p < 0.05, compared to control before morphine, right panel of Figure 3D).10.7554/eLife.09275.005Figure 3.Effects of morphine on the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and paired pulse facilitation (PPF) of VTA-DA neurons in rats.

Bottom Line: However, it is not known whether morphine has an additional strengthening effect on excitatory input.We also studied the contribution of the morphine-induced disinhibitory effect on the presynaptic glutamate release to the overall excitatory effect of morphine on VTA-DA neurons and related behavior.Our results suggest that the disinhibitory action of morphine on presynaptic glutamate release might be the main mechanism for morphine-induced increase in VTA-DA neuron firing and related behaviors.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Medical Neurobiology, Collaborative Innovation Center for Brain Science, School of Basic Medical Sciences and Institutes of Brain Science, Fudan Univeristy, Shanghai, China.

ABSTRACT
One reported mechanism for morphine activation of dopamine (DA) neurons of the ventral tegmental area (VTA) is the disinhibition model of VTA-DA neurons. Morphine inhibits GABA inhibitory neurons, which shifts the balance between inhibitory and excitatory input to VTA-DA neurons in favor of excitation and then leads to VTA-DA neuron excitation. However, it is not known whether morphine has an additional strengthening effect on excitatory input. Our results suggest that glutamatergic input to VTA-DA neurons is inhibited by GABAergic interneurons via GABAB receptors and that morphine promotes presynaptic glutamate release by removing this inhibition. We also studied the contribution of the morphine-induced disinhibitory effect on the presynaptic glutamate release to the overall excitatory effect of morphine on VTA-DA neurons and related behavior. Our results suggest that the disinhibitory action of morphine on presynaptic glutamate release might be the main mechanism for morphine-induced increase in VTA-DA neuron firing and related behaviors.

No MeSH data available.


Related in: MedlinePlus