Activation of neurotensin receptor type 1 attenuates locomotor activity

Abstract

Intracerebroventricular administration of neurotensin (NT) suppresses locomotor activity. However, the brain regions that mediate the locomotor depressant effect of NT and receptor subtype-specific mechanisms involved are unclear. Using a brain-penetrating, selective NT receptor type 1 (NTS1) agonist PD149163, we investigated the effect of systemic and brain region-specific NTS1 activation on locomotor activity. Systemic administration of PD149163 attenuated the locomotor activity of C57BL/6J mice both in a novel environment and in their homecage. However, mice developed tolerance to the hypolocomotor effect of PD149163 (0.1 mg/kg, i.p.). Since NTS1 is known to modulate dopaminergic signaling, we examined whether PD149163 blocks dopamine receptor-mediated hyperactivity. Pretreatment with PD149163 (0.1 or 0.05 mg/kg, i.p.) inhibited D2R agonist bromocriptine (8 mg/kg, i.p.)-mediated hyperactivity. D1R agonist SKF-81297 (8 mg/kg, i.p.)-induced hyperlocomotion was only inhibited by 0.1 mg/kg of PD149163. Since the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) have been implicated in the behavioral effects of NT, we examined whether microinjection of PD149163 into these regions reduces locomotion. Microinjection of PD149163 (2 pmol) into the NAc, but not the mPFC suppressed locomotor activity. In summary, our results indicate that systemic and intra-NAc activation of NTS1 is sufficient to reduce locomotion and NTS1 activation inhibits D2R-mediated hyperactivity. Our study will be helpful to identify pharmacological factors and a possible therapeutic window for NTS1-targeted therapies for movement disorders.

Keywords: D2R; Locomotor activity; NTS1; Neurotensin; Nucleus accumbens; PD149163.

Fig. 1

Effect of PD149163 (PD) on locomotor activity of mice in the open-field. (A) Total distance traveled in 1 h of mice placed in the open-field 30 min after PD149163 (i.p.). PD149163 dose-dependently reduced locomotor activity in the open-field. *p < 0.05 relative to saline-treated mice by Tukey tests. #p < 0.05 relative to 0.1 mg/kg-treated mice by Tukey test. n = 8 for 0.5 mg/kg-treated mice and n = 9 for other treatments. (B) Data were also analyzed by distance traveled per 10 min in the open-field which revealed the dose-dependent effects of PD149163 over time. Individual comparisons were made by Tukey tests and significance is indicated where p < 0.05. Symbols: * 0.1 and 0.5 mg/kg is different from saline, # all doses different from saline, & 0.1 mg/kg different from 0.5 mg/kg, $0.1 and 0.5 mg/kg different from 0.05 mg/kg. (C) Mice recover from the locomotor depressant effect of PD149163 (0.1 mg/kg, i.p.) after 3.5 h. Total distance traveled in 15 min was determined every hour. *p < 0.05 for PD149163-relative to saline-treated mice by Tukey tests. n = 6 for saline- and n = 7 for PD149163-treated mice. Data are presented as mean ± s.e.m.

Fig. 2

Effect of three or seven repeated PD149163 (0.1 or 0.5 mg/kg, i.p.) injections on locomotor activity in the open-field. Mice were placed in the open-field 30 min after the last injection on the test day (day 3 or day 7) and locomotor activity was recorded for 1 h. Acutely treated mice received PD149163 only on the test day and control mice received only saline. (A) Mice developed tolerance to the hypolocomotor effect of 0.1 mg/kg of PD149163 after seven repeated administrations. *p < 0.05 by Tukey tests. n = 6. (B) Mice also developed tolerance to the hypolocomotor effect of 0.1 mg/kg of PD149163 after three repeated administrations. *p < 0.05 by Tukey tests. n = 5 for saline-treated mice and n = 6 for other treatments. (C) Acute and three days of chronic administration of 0.5 mg/kg of PD149163 similarly reduced locomotor activity in the open-field relative to saline-treated mice. *p < 0.05 by Tukey tests. n = 8. (D) Acute (n = 6) and seven days of chronic (n = 8) treatment of 0.5 mg/kg of PD149163 similarly reduced locomotor activity in the open-field relative to saline-treated (n = 8) mice. *p < 0.05 by Tukey tests. Data are presented as mean ± s.e.m.

Fig. 3

Effect of PD149163 (PD) on homecage activity of mice. Mice were individually housed with lights on a 6 AM and off at 6 PM. PD149163 (0.5 mg/kg, i.p.) or saline was administered at approximately 5 PM, just before the start of the nocturnal active phase, for three days. Baseline (day 0), post-injection (days 1-3) and recovery (day 4) activities were recorded. (A) A representative actogram of the pattern of activity of a mouse treated with saline for three days. Vertical bars represent bouts of activity and bar heights represent activity intensity. Black and white boxes above the actogram indicate dark and light phases, respectively. Diamonds indicate when the injections were administered. (B) A representative actogram of the pattern of activity of a mouse treated with PD149163 for three days. PD149163 given just before lights off, or start of a new nocturnal active phase, decreased homecage activity relative to saline-treated mice. (CE) PD149163 (0.5 mg/kg, i.p.) reduced activity duration on days 1-3 during the early dark phase (18-0 h or 6 PM-12 AM) but not during late dark phase (0-6 h or 12 AM-6 AM) or during the light phase (6-18 h or 6 AM-6 PM). *p < 0.05 by Tukey tests. (F-H) PD149163 (0.5 mg/kg, i.p.) reduced activity intensity during early dark phase (18-0 h) but not during late dark phase (0-6 h) or during the light phase (6-18 h). *p < 0.05 for effect of treatment by two-way repeated measures ANOVA. n = 5 for saline- and n = 7 for PD149163-treated mice. Data are presented as mean ± s.e.m.

Fig. 4

Effect of PD149163 (PD) on rotarod performance and grip strength. (A) PD149163 (0.1 and 0.5 mg/kg, i.p.) reduced the time mice were able to stay on the rotarod relative to baseline performance. Mice recovered from 0.1 mg/kg of PD149163 after 2.5 h. *p < 0.05 relative to time 0 performance (Tukey tests). n = 8 for 0.1 mg/kgand n = 9 for 0.5 mg/kg-treated mice. (B) PD149163 (0.5 mg/kg, i.p.) did not affect the grip strength of mice tested 30 min after injection relative to saline-treated mice. n = 8. (C) PD149163 (0.5 mg/kg, i.p.) did not affect the grip strength of mice tested 1 h after injection relative to saline-treated mice. n = 7. Data are presented as mean ± s.e.m.

Fig. 5

Effect of PD149163 (PD) on dopamine receptor-mediated hyperactivity. (A) PD149163 (0.05 or 0.1 mg/kg, i.p.) or saline (sal) was administered 15 min before D2R agonist bromocriptine (bromo, 8 mg/kg, i.p.) or vehicle (veh) treatment. Mice were placed in the open-field 3 h after the last injection for 1 h. Pretreatment with both doses of PD149163 inhibited the hyperactivity induced 3 h after the bromocriptine injection. *p < 0.05 (Tukey test). n = 8 for saline + vehicle-, n = 6 for saline + bromo-, n = 7 for PD 0.05 + bromo- and n = 8 for PD 0.1 + bromo-treated mice. (B) PD149163 (0.05 or 0.1 mg/kg, i.p.) or saline was administered 15 min before D1R agonist SKF81297 (SKF, 8 mg/kg, i.p.) or saline. Locomotor activity of mice was recorded in the open-field 15 min after the last injection. Only 0.1 mg/kg of PD149163 reduced SKF81297-induced hyperactivity. *p < 0.05 (Tukey test). n = 7 for saline + saline-, n = 6 for saline + SKF-, n = 6 PD 0.05 + SKF- and n = 5 PD 0.1 + SKF-treated mice. (C) PD149163 (0.05 mg/kg, i.p.) or saline was administered 2.5 h after bromocriptine (8 mg/kg, i.p.) or vehicle treatment. PD149163 still reduced D2R-mediated hyperactivity when administered after bromocriptine. *p < 0.05 (Tukey test). n = 6 for saline + vehicle-, n = 8 for saline + bromo-, and n = 7 for PD 0.05 + bromo-treated mice. Data are presented as mean ± s.e.m.

Fig. 6

Effect of microinjection of PD149163 (PD) into the NAc and mPFC on locomotion of mice. (A) Observed location of injectors after microinjection into the NAc. (B) Total distance traveled per 10 min of mice placed in the open-field 30 min after PD149163 (2 pmol) microinjection into the NAc. Intra-NAc administration of PD149163 reduced locomotor activity relative to saline-microinjected mice. *p < 0.05 by Tukey tests. n = 6. (C) Observed location of injectors after microinjection into the mPFC. (D) Total distance traveled per 10 min of mice placed in the open-field 30 min after PD149163 (2 pmol) microinjection into the mPFC. Intra-mPFC administration of PD149163 did not affect the locomotor activity as compared to mice microinjected with saline. n = 5 for saline- and n = 6 for PD149163-treated mice. Data are presented as mean ± s.e.m.