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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

Effect of morphine on spontaneous firing of VTA-DA neurons in rats and the influence of the NMDA receptor antagonist APV and the AMPA receptor antagonist DNQX on the effect of morphine on spontaneous firing of VTA-DA neurons in rats.(A) Effect of morphine on spontaneous firing of VTA-DA neurons. Left panel: representative spontaneous firing traces before and after morphine (10 μM). Middle panel: time course of spontaneous firing before and after morphine (10 μM) (n = 6 cells from five rats). Right panel: average frequency of spontaneous firing before and after morphine (n = 6 cells from five rats, p < 0.05, compared to control before morphine). (B) Influence of the NMDA receptor antagonist APV and the AMPA receptor antagonist DNQX on the effect of morphine on spontaneous firing in VTA-DA neurons. Left panel: representative spontaneous firing traces before and after morphine (10 μM) in the presence of APV (50 μM) and DNQX (10 μM). Middle panel: time course of spontaneous firing before and after morphine in the presence of APV (50 μM) and DNQX (10 μM) (n = 6 cells from five rats). Right panel: average frequency of spontaneous firing before and after morphine in the presence of APV (50 μM) and DNQX (10 μM) (n = 6 cells from five rats, p = 0.34). Data are shown as the mean ±s.e.m. *p < 0.05.DOI:http://dx.doi.org/10.7554/eLife.09275.004
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fig2: Effect of morphine on spontaneous firing of VTA-DA neurons in rats and the influence of the NMDA receptor antagonist APV and the AMPA receptor antagonist DNQX on the effect of morphine on spontaneous firing of VTA-DA neurons in rats.(A) Effect of morphine on spontaneous firing of VTA-DA neurons. Left panel: representative spontaneous firing traces before and after morphine (10 μM). Middle panel: time course of spontaneous firing before and after morphine (10 μM) (n = 6 cells from five rats). Right panel: average frequency of spontaneous firing before and after morphine (n = 6 cells from five rats, p < 0.05, compared to control before morphine). (B) Influence of the NMDA receptor antagonist APV and the AMPA receptor antagonist DNQX on the effect of morphine on spontaneous firing in VTA-DA neurons. Left panel: representative spontaneous firing traces before and after morphine (10 μM) in the presence of APV (50 μM) and DNQX (10 μM). Middle panel: time course of spontaneous firing before and after morphine in the presence of APV (50 μM) and DNQX (10 μM) (n = 6 cells from five rats). Right panel: average frequency of spontaneous firing before and after morphine in the presence of APV (50 μM) and DNQX (10 μM) (n = 6 cells from five rats, p = 0.34). Data are shown as the mean ±s.e.m. *p < 0.05.DOI:http://dx.doi.org/10.7554/eLife.09275.004

Mentions: We observed the effect of morphine (10 µM) on the spontaneous firing frequency of VTA-DA neurons in rats. We select 10 µM of morphine because this concentration is commonly used in in vitro experiments as it elicits a significant effect useful for further analyses (Akaishi et al., 2000), and the effect of morphine at this concentration can be essentially abolished by the opioid receptor antagonist naloxone (Valentino and Dingledine, 1982). VTA-DA neurons were identified based on Ih currents (Figure 1A) and tyrosine hydroxylase (TH) staining (Figure 1B). Details of the identification method are given in the ‘Materials and methods’ section. Using original recordings of spontaneous firing (left panel of Figure 2A) and the time course of spontaneous firing (middle panel of Figure 2A) in VTA-DA neurons for comparison, we could see that morphine (10 μM) increased the frequency of spontaneous firing in VTA-DA neurons. The average frequency of spontaneous firing increased from 1.0 ± 0.3 Hz before to 1.4 ± 0.4 Hz for 10–15 min after morphine application (n = 6 cells from five rats, paired t test, p < 0.05, compared to control before morphine, right panel of Figure 2A). In order to determine the role of glutamatergic input in the morphine-induced increase in the spontaneous firing frequency of VTA-DA neurons, we observed the influence of the N-methyl-D-aspartic acid (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV) (50 μM) and the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6, 7-Dinitroquinoxalie-2, 3-dione (DNQX) (10 μM) on the effect of morphine. In the presence of APV and DNQX, morphine no longer increased spontaneous firing frequency (Figure 2B). The average frequency of spontaneous firing was 0.7 ± 0.1 Hz before and 0.8 ± 0.1 Hz for 10–15 min after morphine application in the presence of APV and DNQX (n = 6 cells from five rats, paired t test, p > 0.05, compared to control with APV and DNQX before morphine, right panel of Figure 2B). These results suggest that the morphine-induced increase in the spontaneous firing frequency of VTA-DA neurons requires AMPA and NMDA receptor-mediated glutamatergic input, consistent with a recent report using the NMDA antagonist APV and the AMPA receptor antagonist 6-Cyano-7-nitroquinoxaline-2, 3-dione (CNQX) in in vivo experiments (Jalabert et al., 2011).10.7554/eLife.09275.003Figure 1.Identification 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)

Effect of morphine on spontaneous firing of VTA-DA neurons in rats and the influence of the NMDA receptor antagonist APV and the AMPA receptor antagonist DNQX on the effect of morphine on spontaneous firing of VTA-DA neurons in rats.(A) Effect of morphine on spontaneous firing of VTA-DA neurons. Left panel: representative spontaneous firing traces before and after morphine (10 μM). Middle panel: time course of spontaneous firing before and after morphine (10 μM) (n = 6 cells from five rats). Right panel: average frequency of spontaneous firing before and after morphine (n = 6 cells from five rats, p < 0.05, compared to control before morphine). (B) Influence of the NMDA receptor antagonist APV and the AMPA receptor antagonist DNQX on the effect of morphine on spontaneous firing in VTA-DA neurons. Left panel: representative spontaneous firing traces before and after morphine (10 μM) in the presence of APV (50 μM) and DNQX (10 μM). Middle panel: time course of spontaneous firing before and after morphine in the presence of APV (50 μM) and DNQX (10 μM) (n = 6 cells from five rats). Right panel: average frequency of spontaneous firing before and after morphine in the presence of APV (50 μM) and DNQX (10 μM) (n = 6 cells from five rats, p = 0.34). Data are shown as the mean ±s.e.m. *p < 0.05.DOI:http://dx.doi.org/10.7554/eLife.09275.004
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fig2: Effect of morphine on spontaneous firing of VTA-DA neurons in rats and the influence of the NMDA receptor antagonist APV and the AMPA receptor antagonist DNQX on the effect of morphine on spontaneous firing of VTA-DA neurons in rats.(A) Effect of morphine on spontaneous firing of VTA-DA neurons. Left panel: representative spontaneous firing traces before and after morphine (10 μM). Middle panel: time course of spontaneous firing before and after morphine (10 μM) (n = 6 cells from five rats). Right panel: average frequency of spontaneous firing before and after morphine (n = 6 cells from five rats, p < 0.05, compared to control before morphine). (B) Influence of the NMDA receptor antagonist APV and the AMPA receptor antagonist DNQX on the effect of morphine on spontaneous firing in VTA-DA neurons. Left panel: representative spontaneous firing traces before and after morphine (10 μM) in the presence of APV (50 μM) and DNQX (10 μM). Middle panel: time course of spontaneous firing before and after morphine in the presence of APV (50 μM) and DNQX (10 μM) (n = 6 cells from five rats). Right panel: average frequency of spontaneous firing before and after morphine in the presence of APV (50 μM) and DNQX (10 μM) (n = 6 cells from five rats, p = 0.34). Data are shown as the mean ±s.e.m. *p < 0.05.DOI:http://dx.doi.org/10.7554/eLife.09275.004
Mentions: We observed the effect of morphine (10 µM) on the spontaneous firing frequency of VTA-DA neurons in rats. We select 10 µM of morphine because this concentration is commonly used in in vitro experiments as it elicits a significant effect useful for further analyses (Akaishi et al., 2000), and the effect of morphine at this concentration can be essentially abolished by the opioid receptor antagonist naloxone (Valentino and Dingledine, 1982). VTA-DA neurons were identified based on Ih currents (Figure 1A) and tyrosine hydroxylase (TH) staining (Figure 1B). Details of the identification method are given in the ‘Materials and methods’ section. Using original recordings of spontaneous firing (left panel of Figure 2A) and the time course of spontaneous firing (middle panel of Figure 2A) in VTA-DA neurons for comparison, we could see that morphine (10 μM) increased the frequency of spontaneous firing in VTA-DA neurons. The average frequency of spontaneous firing increased from 1.0 ± 0.3 Hz before to 1.4 ± 0.4 Hz for 10–15 min after morphine application (n = 6 cells from five rats, paired t test, p < 0.05, compared to control before morphine, right panel of Figure 2A). In order to determine the role of glutamatergic input in the morphine-induced increase in the spontaneous firing frequency of VTA-DA neurons, we observed the influence of the N-methyl-D-aspartic acid (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV) (50 μM) and the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6, 7-Dinitroquinoxalie-2, 3-dione (DNQX) (10 μM) on the effect of morphine. In the presence of APV and DNQX, morphine no longer increased spontaneous firing frequency (Figure 2B). The average frequency of spontaneous firing was 0.7 ± 0.1 Hz before and 0.8 ± 0.1 Hz for 10–15 min after morphine application in the presence of APV and DNQX (n = 6 cells from five rats, paired t test, p > 0.05, compared to control with APV and DNQX before morphine, right panel of Figure 2B). These results suggest that the morphine-induced increase in the spontaneous firing frequency of VTA-DA neurons requires AMPA and NMDA receptor-mediated glutamatergic input, consistent with a recent report using the NMDA antagonist APV and the AMPA receptor antagonist 6-Cyano-7-nitroquinoxaline-2, 3-dione (CNQX) in in vivo experiments (Jalabert et al., 2011).10.7554/eLife.09275.003Figure 1.Identification 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