Sensitization of midbrain dopamine neuron reactivity promotes the pursuit of amphetamine

Abstract

Stimulant drugs such as amphetamine are readily self-administered by humans and laboratory animals by virtue of their actions on dopamine (DA) neurons of the midbrain. Repeated exposure to this drug systemically or exclusively in the cell body region of these neurons in the ventral tegmental area (VTA) leads to long-lasting changes in dopaminergic function that can be assessed by increased locomotor activity and enhanced DA overflow in the nucleus accumbens (NAcc) after re-exposure to the drug. Three experiments were conducted to evaluate the possibility that this enduring sensitized reactivity underlies compulsive drug self-administration. In all experiments, rats were pre-exposed to amphetamine and, starting 10 d later, their intravenous self-administration of the drug was assessed. In the first experiment, rats previously exposed to amphetamine systemically or exclusively in the VTA subsequently worked harder than untreated animals to obtain the drug when the work required to obtain successive infusions was increased progressively. In the second experiment, this progressively increasing workload was found to decrease the magnitude of amphetamine-induced DA overflow observed with successive infusions until responding ceased. Rats previously exposed to amphetamine were more resistant to this decline and more apt to maintain responding. Finally, in experiment three, a noncontingent priming injection of the drug produced a greater NAcc DA response and a greater parallel increase in lever pressing in drug compared with saline pre-exposed rats. Together, these results demonstrate a direct relation between drug-induced sensitization of midbrain dopamine neuron reactivity and the excessive pursuit and self-administration of an abused substance.

Fig. 1.

Rats previously exposed to amphetamine intraperitoneally or into the VTA, but not into the NAcc, will subsequently work more to obtain the drug. In all cases, data are shown as mean (± SEM) number of infusions obtained. The number of presses required to obtain the successive infusions is also shown.A, Rats pre-exposed to amphetamine or saline intraperitoneally were tested for their self-administration of the 200 μg/kg per infusion dose of the drug under a PR schedule. Bar graphs were derived from means of the values obtained for each subject on each of the six PR test days. These are shown to the right as group means. *p < 0.05 as revealed byt test. n = 8–11 per group.B, Rats pre-exposed to amphetamine or saline intraperitoneally were tested for their self-administration of one of five doses of the drug. Bars were derived from means of the values obtained for each subject on each of five PR test days. *p values < 0.05–0.025 versus saline pre-exposed rats, as revealed by post hoc tests following ANOVA.n = 5–12 per group. C, Rats pre-exposed to VTA amphetamine, VTA saline, or amphetamine in sites adjacent to the VTA were tested as in A. Injection cannula tip placements in the mesencephalon of rats in the different groups are shown at the far right. Overlaid line drawings (all adapted from Paxinos and Watson, 1986) each depict the caudal surface of a coronal section extending −5.2 to −6.0 mm from bregma. *p < 0.01 versus saline pre-exposed rats as revealed by post hoc tests after ANOVA.n = 9–11 per group. D, Rats pre-exposed to NAcc amphetamine or saline were tested as inA. Injection cannula tip placements are illustrated as in C. Overlaid line drawings each depict the caudal surface of a coronal section extending +2.2 to +3.0 mm from bregma.n = 9–10 per group.

Fig. 2.

Rats previously exposed to amphetamine maintain responding and a significant drug-induced increase in NAcc DA for a longer period of time when self-administering the drug under a PR schedule requiring progressively increasing work to obtain successive infusions. A, Cumulative record of infusions obtained on the fourth to the fifth day of PR testing. Amphetamine pre-exposed rats obtained significantly more infusions than saline pre-exposed rats (dashed lines: 9.5 ± 1.6 vs 4.4 ± 0.9;p < 0.005). B, Baseline.C, Rats transferred to self-administration chamber.L, Lever inserted into chamber. B,Extracellular concentrations of DA in the NAcc before and during amphetamine self-administration during PR testing for rats described above. Data are shown as mean picograms per microliter (± SEM) plotted over time. Dialysate samples were collected every 5 min. Numbered arrows indicate the group mean time of 1, the last infusion (4.4 ± 0.9) obtained by saline pre-exposed rats, and the infusion marking when amphetamine pre-exposed rats had obtained 50% (4.5 ± 0.9) of the infusions they would obtain in the session, 2, the infusion marking when amphetamine pre-exposed rats had obtained 75% (7.5 ± 1.2) of their infusions, and 3, the last infusion (9.5 ± 1.6) obtained by amphetamine pre-exposed rats. Amphetamine pre-exposed rats continued to respond significantly longer than saline pre-exposed rats (p < 0.005).C, DA concentrations at (0 min) and after (5 and 10 min) the infusion obtained at times 1, 2, and3 in B. Data are plotted as a percentage of levels observed at the time of the individual infusion. Amphetamine, compared with saline, pre-exposed rats resisted the decline in the ability of the drug infusions to maintain a NAcc DA response.Numbers in parentheses indicate the increasing group mean number of lever presses required to obtain the respective infusions. *p values < 0.01–0.001 versus levels at the time of infusion as revealed by post hoc tests after ANOVA. D, Line drawings of coronal sections showing the location of the active portion of the microdialysis probes in the NAcc (solid lines, amphetamine pre-exposed; dashed lines, saline pre-exposed). Numbers indicate millimeters from bregma.n = 8–11 per group.

Fig. 3.

Rats for which the work required to obtain infusions is kept constant continue to press and show significant drug-induced increases in NAcc DA through to the last infusion of the session. Data are illustrated as in Figure 2 but for the FR and no drug control groups. FR control rats were trained to emit a constant amount of work (5 presses; FR5) to receive successive amphetamine infusions, were tested for 3 hr, and obtained 8.5 ± 0.5 infusions. Numbered arrows in Bindicate the group mean time of the infusion marking when these animals obtained 1, 50% (4.2 ± 0.2), 2, 75% (6.3 ± 0.3), and 3, 100% (last infusion, 8.5 ± 0.5) of their infusions. Numbers in parenthesesin C indicate the constant number of lever presses required to obtain the infusions self-administered at times1, 2, and 3 inB. These animals maintained a significant increase in NAcc DA through to the last infusion of the session. *pvalues < 0.05–0.01 versus levels at the time of infusion as revealed by post hoc tests after ANOVA. No drug was delivered to no drug controls. These animals were tested for 100 min and showed little pressing and no effect on NAcc DA. Active portions of microdialysis probes are shown in D (solid lines, FR control; dashed lines, no drug control). n = 5–6 per group.

Fig. 4.

Rats previously exposed to amphetamine show enhanced NAcc DA overflow and lever pressing in response to a single priming injection of the drug. Top, Extracellular concentrations of DA in the NAcc before and after a single noncontingent priming injection of amphetamine (1.0 mg/kg, i.p.) on the fourth to the fifth day of PR testing. Data are shown as mean picograms per microliter (± SEM). The priming injection was made as the levers were introduced into the chambers (arrow). Each scheduled completion had no intravenous consequences.Inset shows data as mean (+SEM) maximal NAcc DA overflow induced by amphetamine. *p < 0.005 as revealed byt test. Bottom, Nonreinforced lever presses (mean ± SEM) after the priming injection (arrow) in the rats described above.Inset shows the mean (+SEM) number of infusions earned (not obtained) with the corresponding number of presses required. *p < 0.05 as revealed by t test.B, Baseline. C, Rats transferred to self-administration chamber. L, Lever inserted into chamber. Line drawings of coronal sections to the side show the location of the active portion of the microdialysis probes in the NAcc.Numbers indicate millimeters from bregma. Solid lines, Amphetamine pre-exposed; dashed lines, saline pre-exposed. n = 8 per group.