Mice with chronic norepinephrine deficiency resemble amphetamine-sensitized animals

Abstract

Acute pharmacological blockade of alpha1 adrenoreceptors (ARs) attenuates the locomotor response to amphetamine (LRA). We took a genetic approach to study how norepinephrine (NE) signaling modulates psychostimulant responses by testing LRA in dopamine beta-hydroxylase knockout (Dbh-/-) mice that lack NE. Surprisingly, Dbh-/- animals were hypersensitive to the behavioral effects of amphetamine. Amphetamine (2 mg/kg) elicited greater locomotor activity in Dbh-/- mice compared to controls, whereas 5 mg/kg caused stereotypy in Dbh-/- mice, which is only observed in control mice at higher doses. Prazosin, an alpha1AR antagonist, attenuated LRA in Dbh+/- mice but had no effect in Dbh-/- mice. Changes in the sensitivity of dopamine (DA)-signaling pathways may contribute to the altered amphetamine responses of Dbh-/- mice because they were relatively insensitive to a D1 agonist and hypersensitive to a D2 agonist. Daily amphetamine administration resulted in behavioral sensitization in both Dbh+/- and Dbh-/- mice, demonstrating that NE is not required for the development or expression of behavioral sensitization. Daily prazosin administration blunted but did not completely block locomotor sensitization in Dbh+/- mice, suggesting that alpha1AR signaling contributes to, but is not required for sensitization in Dbh+/- control animals. We conclude that in contrast to acute alpha1AR blockade, chronic NE deficiency induces changes similar to sensitization, perhaps by altering DA-signaling pathways.

Figure 1

Locomotor response of Dbh+/− and Dbh−/− mice to a novel environment. (a) Naive mice (Dbh+/−, n = 13; Dbh−/−, n = 13) were placed in activity chambers, injected with vehicle (1.5% DMSO/1.5% Cremophor EL in 0.9% NaCl) at 90 min, 0.9% NaCl at 120 min, and activity was recorded for an additional 4 h. *, P < 0.05; †, P < 0.001 compared to Dbh+/−. (b) In the course of the amphetamine sensitization experiments, mice (Dbh+/−, n = 16; Dbh−/−, n = 8) were placed in the activity chambers once per day for 5 days. Shown are total ambulations over the first 60 min (before any drug treatment). *, P < 0.05 compared to Dbh−/− day 1; †, P < 0.001 compared day 1 for that genotype; #, P < 0.01 compared to Dbh−/− day 2.

Figure 2

Locomotor response of Dbh+/− and Dbh−/− mice to amphetamine. Naive mice were placed in activity chambers and injected with vehicle at 90 min and amphetamine at 120 min, and activity was recorded for an additional 4 h. Amphetamine doses were: 1 mg/kg (Dbh+/−, n = 13; Dbh−/−, n = 13) (a); 2 mg/kg (Dbh+/−, n = 32; Dbh−/−, n = 24) (b); 3 mg/kg (Dbh+/−, n = 24; Dbh−/− n = 24) (c); and 5 mg/kg [Dbh+/−, n = 20; Dbh−/−, n = 17, Dbh−/− DOPS + carbidopa (D + C), n = 4] (d). *, P < 0.05 compared to Dbh+/−; †, P < 0.001 compared to Dbh+/−.

Figure 3

Stereotypy in Dbh+/− and Dbh−/− mice in response to 5 mg/kg amphetamine. Naive mice (Dbh+/−, n = 6; Dbh−/−, n = 6) were placed in activity chambers and injected with vehicle at 90 min and amphetamine at 120 min, and mice were videotaped for 2 additional h. Behavior was scored in 10-s bins. AM, ambulating; RE, rearing; SN, sniffing; HB, head bobbing; NB, nail biting; CR, circling; GR, grooming; QWS, quiet wake/sleeping. *, P < 0.05; **, P < 0.01; †, P < 0.001 compared to Dbh+/−.

Figure 4

Effects of prazosin on amphetamine-induced locomotion in Dbh+/− and Dbh−/− mice. Naive mice were placed in activity chambers and injected with vehicle (Dbh+/−, n = 32; Dbh−/−, n = 24; replicated from Fig. 1b) or prazosin (0.5 mg/kg; Dbh+/−, n = 18; Dbh−/−, n = 9) at 90 min and amphetamine (2 mg/kg) at 120 min, and activity was recorded for an additional 4 h. *, P < 0.05; †, P < 0.001 compared to vehicle control for that genotype.

Figure 5

Locomotor response of Dbh+/− and Dbh−/− mice to a D1 and D2 agonist. Naive or sensitized mice were placed in activity chambers and injected with vehicle at 90 min and the D1 agonist SKF81297 (5 mg/kg; Dbh+/−, n = 10; Dbh−/−, n = 10) or the D2 agonist quinpirole (2.5 mg/kg; Dbh+/−, n = 19; Dbh−/−, n = 15) at 120 min, and activity was recorded for an additional 2 h. Shown are the total ambulations for 2 h after drug administration. **, P < 0.01 compared to Dbh+/−.

Figure 6

Amphetamine sensitization in Dbh+/− and Dbh−/− mice. (a) Mice (Dbh+/−, n = 23; Dbh−/−, n = 8) were placed in activity chambers and injected with vehicle at 90 min and amphetamine (2 mg/kg) at 120 min, and activity was measured for an additional 2 h. This paradigm was repeated every day for 6 days, mice were rested on day 7, then retested on days 8, 15, and 43. Shown are the total ambulations for 2 h after amphetamine administration. *, P < 0.05 compared to Dbh−/− day 1; †1, P < 0.001 compared to Obh+/− day 1; †2, P < 0.001 compared to Dbh+/− day 2; *4, P < 0.05 compared to Obh+/− day 4. (b) For “Dbh+/− prazonsin (prz) 0.5” and “Dbh+/− prz 1”, mice were given prazosin (0.5 mg/kg or 1 mg/kg) at 90 min on days 2–6. For “Dbh+/− prz 1^#”, mice were given prazosin (1 mg/kg) at 90 min on days 1–6. All mice were given amphetamine (2 mg/kg) at 120 min. Dbh+/− vehicle (veh) (n = 23, replicated from a), Dbh+/− prz 0.5 (n = 8), Dbh+/− prz 1 (n = 4), and Dbh+/− prz 1^# (n = 3). Shown are the total ambulations for 2 h after amphetamine administration. *, P < 0.05; †, P < 0.001 compared to day 1 for that treatment.