Dual effects of D-amphetamine on dopamine neurons mediated by dopamine and nondopamine receptors

Abstract

By increasing dopamine (DA) release and activating feedback mechanisms, amphetamine and related psychostimulants are known to inhibit DA cell firing. Here, we report that D-amphetamine also has an excitatory effect on DA cells, which under control conditions, is masked by the inhibitory effect of D-amphetamine and is revealed when D2-like receptors are blocked. Thus, using in vivo single-unit recording in rats, we found that the selective D2 antagonist raclopride not only blocked the inhibition induced by D-amphetamine but also enabled D-amphetamine to excite DA cells. The excitation, expressed as an increase in both firing rate and bursting, persisted when both D1- and D2-like receptors were blocked by SCH23390 and eticlopride, suggesting that it is not mediated by DA receptors. The norepinephrine uptake blocker nisoxetine mimicked the effect of D-amphetamine, especially the increase in bursting, whereas the 5-HT uptake blocker fluoxetine produced no significant effect. Adrenergic alpha1 antagonists prazosin and WB4101 and the nonselective alpha antagonist phenoxybenzamine completely blocked increase in bursting induced by D-amphetamine and partially blocked the increase in firing rate. The alpha2 antagonist idazoxan and the beta antagonist propranolole, however, failed to prevent D-amphetamine from producing the excitation. Thus, revising the traditional concept, this study suggests that D-amphetamine has two effects on DA cells, a DA-mediated inhibition and a non-DA-mediated excitation. The latter is mediated in part through adrenergic alpha1 receptors.

Fig. 1.

Excitation of DA neurons after raclopride reversal of d-amphetamine-induced inhibition.A, Representative recordings from a nigral DA neuron showing that raclopride (Rac) not only reversed the inhibition induced by d-amphetamine (Amph) but further increased the activity of the cell to above baseline. Four different parameters were measured: firing rate (spikes/10 sec), number of spikes in bursts (spikes/10 sec), number of bursts (bursts/10 sec), and ISI variation coefficient (percent of mean ISI/10 sec). Of the four measurements, the number of spikes in bursts was increased most significantly. B, Summary of data from eight cells tested with d-amphetamine followed by raclopride. After raclopride, both firing rate and the number of spikes in bursts were significantly increased compared with predrug baseline (F_(18,268) = 26.914,p < 0.0005; andF_(18,268) = 17.434,p < 0.0005, respectively).

Fig. 2.

d-Amphetamine excites DA neurons when D2-like receptors are blocked. A, Typical recordings showing that, in a nonanesthetized rat, the activity of a DA cell was also increased after raclopride (Rac) reversal of d-amphetamine (Amph)-induced inhibition.B, Recordings from a different DA neuron showing no significant increase in the activity of the cell after raclopride reversal of the direct DA agonist apomorphine (Apo)-induced inhibition. C, Recordings from another DA cell showing that raclopride alone produced only a small increase in firing. After raclopride, however,d-amphetamine significantly increased both firing rate and bursting. D, Recordings from still another DA neuron showing that the excitatory effect of d-amphetamine persisted after both D1- and D2-like receptors were blocked by SCH23390 (SCH) and eticlopride (Etic).

Fig. 3.

The α1 antagonist prazosin blocks d-amphetamine-induced bursting of DA cells.A, Typical recordings from a DA neuron showing that pretreatment with prazosin (Praz) completely blocked the ability of d-amphetamine [Amph; injected after raclopride (Rac)] to increase bursting. In the same cell, however, d-amphetamine was still able to induce a small increase in firing rate. B, Recordings from a different DA cell showing that prazosin completely reversedd-amphetamine-induced increase in bursting and transiently reversed the increase in firing rate. C, Summary of data showing the differences between the effects ofd-amphetamine in control (filled circles; n = 22) and prazosin-pretreated rats (open circles; n = 7). Prazosin pretreatment partially blocked the increase in firing rate (left) and completely blocked the increase in bursting (right) induced by d-amphetamine (1 mg/kg, injected after 50–200 μg/kg of raclopride; see Results for detailed statistics).

Fig. 4.

The selective NE uptake blocker nisoxetine increases DA cell bursting. A, Typical recordings from a DA cell showing that nisoxetine (Nis) alone produced a significant increase in bursting and a small and transient increase in firing rate. Raclopride (Rac) after nisoxetine further increased firing rate and bursting. The 5-HT uptake blocker fluoxetine (Flu) produced no further effect. Prazosin (Praz) reversed the increase in DA cell activity induced by both nisoxetine and raclopride. B, Recordings from a different DA cell showing that the 5-HT uptake blocker fluoxetine produced no significant effect on either firing rate or bursting. Subsequent injection of raclopride also failed to produce a significant effect. C, Summary of data from all 12 cells tested with nisoxetine (2–5 mg/kg) followed by raclopride (0.1 mg/kg). Nisoxetine had no significant effect on firing rate (left); it, however, significantly increased the number of spikes in bursts (right). Raclopride injection further increased both firing rate and bursting (see Results for detailed statistics).

Fig. 5.

Effects of peripheral activation of α1 receptors on DA neurons. Typical recordings showing that intravenous injection of phenylephrine (Phenyl) produced no effect on bursting and a small decrease in firing rate. In the same cell, subsequent injection of d-amphetamine (Amph) markedly increased both firing rate and bursting. Rac, Raclopride