Reversal of quinpirole inhibition of ventral tegmental area neurons is linked to the phosphatidylinositol system and is induced by agonists linked to G(q)

Abstract

Putative dopaminergic (pDAergic) ventral tegmental area neurons play an important role in brain pathways related to addiction. Extended exposure of pDAergic neurons to moderate concentrations of dopamine (DA) results in a time-dependent decrease in sensitivity of pDAergic neurons to DA inhibition, a process called dopamine inhibition reversal (DIR). We have shown that DIR is mediated by phospholipase C and conventional protein kinase C through concurrent stimulation of D2 and D1-like receptors. In the present study, we further characterized this phenomenon by using extracellular recordings in brain slices to examine whether DIR is linked to phosphatidylinositol (PI) or adenylate cyclase (AC) second-messenger pathways. A D1-like dopaminergic agonist associated with PI turnover (SKF83959), but not one linked to AC (SKF83822), promoted reversal of inhibition produced by quinpirole, a dopamine D2-selective agonist. Other neurotransmitter receptors linked to PI turnover include serotonin 5-HT(2), α(1)-adrenergic, neurotensin, and group I metabotropic glutamate (mGlu) receptors. Both serotonin and neurotensin produced significant reversal of quinpirole inhibition, but agonists of α(1)-adrenergic and group I mGlu receptors failed to significantly reverse quinpirole inhibition. These results indicate that some agonists that stimulate PI turnover can facilitate desensitization of D2 receptors but that there may be other factors in addition to PI that control that interaction.

Fig. 1.

Reversal of quinpirole inhibition required coadministration of quinpirole and D1/D5 agonist linked to the phosphatidylinositol (PI) pathway, but not D1/D5 agonist linked to the adenylyl cyclase (AC) pathway. Relative change in firing rate (mean ± SE) in response to long-duration quinpirole application in the absence or presence of either SKF83822 or SKF83959 is plotted as a function of time. Effect of quinpirole at each time point was normalized by subtracting the change in firing rate (%) at the 5-min time point. A concentration of quinpirole that produced inhibition of 50% or greater was applied for 40 min. Quinpirole alone (■, [Q] = 45 ± 5.75 nM, n = 8) produced inhibition of the firing rate that did not significantly reverse over the 40-min duration of quinpirole application [1-way repeated-measures ANOVA, F_(7,49) = 1.00, P > 0.05]. In the presence of 100 μM SKF83822 in the pipette (●, [Q] = 44.64 ± 9.67 nM, n = 7), quinpirole produced an inhibition in firing rate, and this inhibition was not significantly changed for the duration of quinpirole application [1-way repeated-measures ANOVA, F_(7,42) = 1.55, P > 0.05]. In the presence of 100 μM SKF83959 in the pipette (▼, [Q] = 46.07 ± 9.48 nM, n = 14), there was a significant reduction in quinpirole inhibition over time, with the last time point significantly different from the first 3 time points [1-way repeated-measures ANOVA, F_(7,91) = 4.44, P < 0.05]. When 100 μM SKF83959 and 100 μM SCH39166 were included in the pipette (▽, [Q] = 50.83 ± 10.83 nM, n = 6), quinpirole significantly inhibited the firing rate, with no reversal over time [1-way repeated-measures ANOVA, F_(7,35) = 4.32, P < 0.05].

Fig. 2.

Reversal of quinpirole inhibition by D1/D5 agonist linked to the PI pathway was blocked by a conventional protein kinase C (cPKC) inhibitor. Relative change in firing rate (mean ± SE) in response to long-duration quinpirole application is plotted as a function of time. Effect of quinpirole at each time point was normalized by subtracting the change in firing rate (%) at the 5-min time point. The response to a concentration of quinpirole that produced inhibition (crossed open circles and dashed line) and the response to SKF83959 that reversed quinpirole inhibition (▽) (both from Fig. 1) are shown for reference. Saline containing 100 μM SKF83959 and 1 μM Gö6976 was used to fill the recording pipette. After 20-min administration of Gö6976 and SKF83959, a concentration of quinpirole that produced inhibition of 50% or greater was applied for 40 min. In the presence of Gö6976, there was no reversal of quinpirole inhibition produced by SKF83959, and the firing rate was significantly inhibited (●, [Q] = 65 ± 15.8 nM, n = 7) [1-way repeated-measures ANOVA, F_(7,42) = 4.12, P < 0.05].

Fig. 3.

Coadministration of quinpirole and serotonin produced the reversal of quinpirole inhibition, which was suppressed by the 5-HT₂ receptor antagonist ketanserin. Relative change in firing rate (mean ± SE) in response to long-duration quinpirole application is plotted as a function of time. Effect of quinpirole at each time point was normalized by subtracting the change in firing rate (%) at the 5-min time point. The effect of quinpirole alone (from Fig. 1) is shown for comparison (crossed open circles and dashed line). A concentration of quinpirole that produced 50% inhibition or greater was applied for 40 min. In the presence of 50 μM serotonin in the superfusate (▼, [Q] = 112.5 ± 22.16 nM, n = 8), there was a significant reduction in quinpirole inhibition over time, with the last 3 time points significantly different from the 5-min time point [1-way repeated-measures ANOVA, F_(7,1) = 7.22, P < 0.05]. In the presence of 50 μM serotonin in the superfusate and 50 μM ketanserin in the recording pipettes (●, [Q] = 78.57 ± 13.83 nM, n = 6), quinpirole produced a significant inhibition in firing rate, with no reversal over time [1-way repeated-measures ANOVA, F_(7,1) = 3.17, P < 0.05]. No reversal of quinpirole inhibition was observed when 50 μM ketanserin was present in the recording pipettes (▽, [Q] = 56.25 ± 15.7 nM, n = 4) [1-way repeated-measures ANOVA, F_(7,1) = 4.51, P < 0.05].

Fig. 4.

Coadministration of quinpirole and neurotensin produced the reversal of quinpirole inhibition, which was suppressed by neurotensin receptor antagonist SR142948. Relative change in firing rate (mean ± SE) in response to long-duration quinpirole application is plotted as a function of time. Effect of quinpirole at each time point was normalized by subtracting the change in firing rate (%) at the 5-min time point. The effect of quinpirole alone (from Fig. 1) is shown for comparison (crossed open circles and dashed line). A concentration of quinpirole that produced inhibition of 50% or greater was applied for 40 min. In the presence of 10 nM neurotensin in the superfusate (■, [Q] = 51.88 ± 8.96 nM, n = 8), there was a small but significant reduction in quinpirole inhibition over time, with the last 3 time points significantly different from the 5-min time point [1-way repeated-measures ANOVA, F_(7,1) = 4.39, P < 0.05]. In the presence of 10 nM neurotensin in the superfusate and 1 μM SR142948 in the recording pipettes (●, [Q] = 49.17 ± 10.9 nM, n = 6), quinpirole produced a significant inhibition in firing rate, with no reversal over time [1-way repeated-measures ANOVA, F_(7,1) = 18.98, P < 0.05]. No reversal of quinpirole inhibition was observed when 1 μM SR142948 was present in the recording pipettes (▼, [Q] = 78 ± 14.28 nM, n = 5) [1-way repeated-measures ANOVA, F_(7,1) = 3.56, P < 0.05].

Fig. 5.

Coadministration of quinpirole and agonist of either α₁-adrenergic or group I metabotropic glutamate (mGlu) receptors did not produce the reversal of quinpirole inhibition. Relative change in firing rate (mean ± SE) in response to long-duration quinpirole application is plotted as a function of time. Effect of quinpirole at each time point was normalized by subtracting the change in firing rate (%) at the 5-min time point. The effect of quinpirole alone (from Fig. 1) is shown for comparison (crossed open circles and dashed line). A and B: a concentration of quinpirole that produced inhibition of 50% or greater was applied for 40 min. In the presence of the α₁-adrenergic receptor antagonist phenylephrine (10 μM; A), quinpirole produced a significant reduction in firing rate, with no reversal over time (■, [Q] = 76.67 ± 8.33 nM, n = 6) [1-way repeated-measures ANOVA, F_(7,1) = 6.43, P < 0.05]. In the presence of the group I mGlu receptor antagonist DHPG (10 μM; B), no significant reversal of quinpirole-induced inhibition was observed (■, [Q] = 91.67 ± 15.37 nM, n = 6) [1-way repeated-measures ANOVA, F_(7,1) = 0.81, P > 0.05]. C: a single concentration of quinpirole was applied for 40 min alone or in the presence of either DHPG or serotonin. Quinpirole alone (■, 45 nM, n = 7) produced a significant inhibition in firing rate, with no reversal over time [1-way repeated-measures ANOVA, F_(7,42) = 8.85, P < 0.05]. In the presence of 10 μM DHPG, quinpirole (■, 90 nM, n = 8) significantly inhibited the firing rate, and this inhibition persisted for the duration of drug application [1-way repeated-measures ANOVA, F_(7,49) = 4.32, P < 0.05]. In the presence of 50 μM serotonin, a single dose of quinpirole (▼, 112.5 nM, n = 5) produced an initial inhibition in firing rate, and this inhibition partially reversed with time [1-way repeated measures ANOVA, F_(7,28) = 7.22, P < 0.05].