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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 glutamate release from the ventral tegmental area (VTA) synaptosomes of rats and the frequency of sEPSCs in mechanically dissociated VTA-DA neurons from rats.(A) Effects of morphine on glutamate release from VTA synaptosomes. Average concentration of glutamate release before and after application of morphine (10 μM) from VTA synaptosomes (n = 6 samples from eight rats, p = 0.46). (B) Effects of morphine on the frequency of sEPSCs in mechanically dissociated VTA-DA neurons. Panel 1, left: images of an acutely dissociated single neuron from the VTA under phase contrast microscopy. Scale bar: 10 μm. Panel 1, right: representative current traces showing a large hyperpolarization-activated current (Ih) in whole-cell voltage-clamp recording. Holding potential: −70 mV. Panel 2: typical current traces of sEPSC 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 = 6 cells from six rats, p = 0.65). Data are shown as the mean ±s.e.m. *p < 0.05.DOI:http://dx.doi.org/10.7554/eLife.09275.006
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fig4: Effects of morphine on glutamate release from the ventral tegmental area (VTA) synaptosomes of rats and the frequency of sEPSCs in mechanically dissociated VTA-DA neurons from rats.(A) Effects of morphine on glutamate release from VTA synaptosomes. Average concentration of glutamate release before and after application of morphine (10 μM) from VTA synaptosomes (n = 6 samples from eight rats, p = 0.46). (B) Effects of morphine on the frequency of sEPSCs in mechanically dissociated VTA-DA neurons. Panel 1, left: images of an acutely dissociated single neuron from the VTA under phase contrast microscopy. Scale bar: 10 μm. Panel 1, right: representative current traces showing a large hyperpolarization-activated current (Ih) in whole-cell voltage-clamp recording. Holding potential: −70 mV. Panel 2: typical current traces of sEPSC 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 = 6 cells from six rats, p = 0.65). Data are shown as the mean ±s.e.m. *p < 0.05.DOI:http://dx.doi.org/10.7554/eLife.09275.006

Mentions: The mechanism underlying the promoting effect of morphine on presynaptic glutamate release in VTA-DA neurons may involve different processes. One suggestion is that morphine may directly act at glutamatergic terminals to promote glutamate release. To test this hypothesis, we studied the effect of morphine on glutamate release from the VTA synaptosomes of rats, which are sealed particles containing vesicles, viable mitochondria, and all components necessary to store, release, and retain neurotransmitters (Breukel et al., 1997), using on-line fluorometry. However, we did not find that morphine (10 μM) had a direct effect at glutamatergic terminals to promote glutamate release. The average concentration of glutamate was 8.1 ± 0.1 nmol/mg before and 8.5 ± 0.7 nmol/mg after morphine application (n = 6 samples from eight rats, paired t test, p > 0.05, Figure 4A). We also studied the effect of morphine on the frequency of sEPSCs from mechanically dissociated single VTA-DA neurons in rats, which retained functional synaptic terminals (Akaike and Moorhouse, 2003; Ye et al., 2004; Deng et al., 2009). The left top image of panel 1 of Figure 4B shows mechanically dissociated neurons from the VTA. These VTA-DA neurons were identified by only using Ih currents (the graph on the right of panel 1 of Figure 4B) without TH staining because in dissociated DA neurons, it was difficult to fix the cell after recording for TH staining. The results showed that morphine (10 µM) had no significant effect on the frequency of sEPSCs (panels 2 and 3 of Figure 4B). The average frequency of sEPSCs was 2.1 ± 0.3 Hz before and 2.0 ± 0.2 Hz for 10–15 min after morphine application (10 µM) (n = 6 cells from six rats, paired t test, p > 0.05, compared to control before morphine, panel 4 of Figure 4B). These results suggest that morphine may promote presynaptic glutamate release in VTA-DA neurons in an indirect way.10.7554/eLife.09275.006Figure 4.Effects of morphine on glutamate release from the ventral tegmental area (VTA) synaptosomes of rats and the frequency of sEPSCs in mechanically dissociated VTA-DA neurons from 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 glutamate release from the ventral tegmental area (VTA) synaptosomes of rats and the frequency of sEPSCs in mechanically dissociated VTA-DA neurons from rats.(A) Effects of morphine on glutamate release from VTA synaptosomes. Average concentration of glutamate release before and after application of morphine (10 μM) from VTA synaptosomes (n = 6 samples from eight rats, p = 0.46). (B) Effects of morphine on the frequency of sEPSCs in mechanically dissociated VTA-DA neurons. Panel 1, left: images of an acutely dissociated single neuron from the VTA under phase contrast microscopy. Scale bar: 10 μm. Panel 1, right: representative current traces showing a large hyperpolarization-activated current (Ih) in whole-cell voltage-clamp recording. Holding potential: −70 mV. Panel 2: typical current traces of sEPSC 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 = 6 cells from six rats, p = 0.65). Data are shown as the mean ±s.e.m. *p < 0.05.DOI:http://dx.doi.org/10.7554/eLife.09275.006
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fig4: Effects of morphine on glutamate release from the ventral tegmental area (VTA) synaptosomes of rats and the frequency of sEPSCs in mechanically dissociated VTA-DA neurons from rats.(A) Effects of morphine on glutamate release from VTA synaptosomes. Average concentration of glutamate release before and after application of morphine (10 μM) from VTA synaptosomes (n = 6 samples from eight rats, p = 0.46). (B) Effects of morphine on the frequency of sEPSCs in mechanically dissociated VTA-DA neurons. Panel 1, left: images of an acutely dissociated single neuron from the VTA under phase contrast microscopy. Scale bar: 10 μm. Panel 1, right: representative current traces showing a large hyperpolarization-activated current (Ih) in whole-cell voltage-clamp recording. Holding potential: −70 mV. Panel 2: typical current traces of sEPSC 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 = 6 cells from six rats, p = 0.65). Data are shown as the mean ±s.e.m. *p < 0.05.DOI:http://dx.doi.org/10.7554/eLife.09275.006
Mentions: The mechanism underlying the promoting effect of morphine on presynaptic glutamate release in VTA-DA neurons may involve different processes. One suggestion is that morphine may directly act at glutamatergic terminals to promote glutamate release. To test this hypothesis, we studied the effect of morphine on glutamate release from the VTA synaptosomes of rats, which are sealed particles containing vesicles, viable mitochondria, and all components necessary to store, release, and retain neurotransmitters (Breukel et al., 1997), using on-line fluorometry. However, we did not find that morphine (10 μM) had a direct effect at glutamatergic terminals to promote glutamate release. The average concentration of glutamate was 8.1 ± 0.1 nmol/mg before and 8.5 ± 0.7 nmol/mg after morphine application (n = 6 samples from eight rats, paired t test, p > 0.05, Figure 4A). We also studied the effect of morphine on the frequency of sEPSCs from mechanically dissociated single VTA-DA neurons in rats, which retained functional synaptic terminals (Akaike and Moorhouse, 2003; Ye et al., 2004; Deng et al., 2009). The left top image of panel 1 of Figure 4B shows mechanically dissociated neurons from the VTA. These VTA-DA neurons were identified by only using Ih currents (the graph on the right of panel 1 of Figure 4B) without TH staining because in dissociated DA neurons, it was difficult to fix the cell after recording for TH staining. The results showed that morphine (10 µM) had no significant effect on the frequency of sEPSCs (panels 2 and 3 of Figure 4B). The average frequency of sEPSCs was 2.1 ± 0.3 Hz before and 2.0 ± 0.2 Hz for 10–15 min after morphine application (10 µM) (n = 6 cells from six rats, paired t test, p > 0.05, compared to control before morphine, panel 4 of Figure 4B). These results suggest that morphine may promote presynaptic glutamate release in VTA-DA neurons in an indirect way.10.7554/eLife.09275.006Figure 4.Effects of morphine on glutamate release from the ventral tegmental area (VTA) synaptosomes of rats and the frequency of sEPSCs in mechanically dissociated VTA-DA neurons from 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