Testosterone is essential for cocaine sensitization in male rats

Abstract

Most studies agree that males and females respond differently to drugs of abuse. In females, estradiol enhances the behavioral response to cocaine. However, studies on the role of testosterone and the locomotor response to psychostimulants in the male rat are inconclusive. Our study was designed to determine the behavioral effects of testosterone on the development and persistence of cocaine sensitization in male rats. We tested different doses of cocaine (10, 15 and 30mg/kg) to determine which dose induced locomotor sensitization in intact (INT) and gonadectomized (GDX) animals. We also investigated if GDX males with testosterone replacement (GDX-T) showed a similar locomotor response to cocaine as INT males. Our data showed that gonadectomy enhanced the locomotor response to a single cocaine injection. This effect was not observed in gonadectomized rats that received testosterone replacement. However, GDX rats did not show a progressive increase in their locomotor response to repeated cocaine administration (15 and 30mg/kg) (sensitization) as did INT and GDX-T animals. It is possible that in GDX males, the initial high locomotor response to cocaine creates a ceiling effect that limits further increase in cocaine-induced hyperactivity. These findings indicate that testosterone not only modulates the behavioral response to a single and to repeated cocaine injections, but is also essential for male rats to become sensitized to cocaine.

Figure 1. Basal locomotor activity of intact and gonadectomized male rats

Total horizontal (A) and stereotyped (B) activity of intact (INT) (white bars) and gonadectomized (GDX) (black bars) adult male rats is presented as the mean sum ± SEM of the first 30 minutes after saline injection on days 1, 5 and 13; N: INT = 8, GDX = 8. Gonadectomy did not alter basal locomotor activity on any of the days. Repeated saline injections did not alter basal LMA throughout the testing period.

Figure 2. Locomotor response of intact and gonadectomized male rats to different doses of cocaine

Total horizontal (A) and stereotyped (B) activity of INT and GDX male rats is presented as the mean sum ± SEM of the first 30 minutes after a single injection of saline (0.9%) or cocaine (10, 15 or 30mg/kg); N: Saline: INT = 8, GDX = 8; 10mg/kg: INT = 11, GDX = 10; 15mg/kg: INT = 30 (STRCNT) 31(HACTV), GDX = 29; 30mg/kg: INT = 9, GDX = 9. Cocaine increased locomotor activity of all animals compared to their saline counterpart. Gonadectomy enhanced the locomotor response to 15 and 30mg/kg of cocaine when compared to intact males. * p<0.05 compared to LMA of INT animals with the same dose of cocaine.

Figure 3. Locomotor response of intact and gonadectomized male rats to repeated cocaine (10mg/kg) administration

Animals received daily injections of cocaine (10mg/kg) for 5 consecutive days and a challenge injection on day 13. Horizontal (A and C) and stereotyped (B and D) activity was recorded on days 1 (white bar and circle), 5 (grey bar and circle) and 13 (black bar and circle) for 30 min prior to injection (0 – 30 min) and 1 hour after injection (31 – 90 min). Data are presented on the left of each panel as the mean sum ± SEM of the first 30 minutes after cocaine injection and on the right of each panel as mean ± SEM in 5 min intervals; N: INT = 11, GDX = 10. Locomotor activity was not significantly different on days 5 and 13 from day 1, indicating that neither INT nor GDX male rats become sensitized with a 10mg/kg dose of cocaine.

Figure 4. Locomotor response of intact and gonadectomized male rats to repeated cocaine (15mg/kg) administration

Animals received daily injections of 15mg/kg cocaine for 5 consecutive days and a challenge injection on day 13. Horizontal (A and C) and stereotyped (B and D) activity was recorded on days 1 (white bar and circle), 5 (grey bar and circle) and 13 (black bar and circle) for 30 min prior to injection (0 – 30 min) and 1 hour after injection (31 – 90 min). Data are presented on the left of each panel as the mean sum ± SEM of the first 30 minutes after cocaine injection and on the right of each panel as mean ± SEM in 5 min intervals; N: INT = 30 (STRCNT) – 31 (HACTV), GDX = 29. INT male rats show an increased locomotor response after repeated injections, while GDX animals do not. These results indicate that 15mg/kg of cocaine induces sensitization in INT but not in GDX male rats. * p<0.05 compared to LMA on day 1.

Figure 5. Locomotor response of intact and gonadectomized male rats to repeated cocaine (30mg/kg) administration

Animals received daily injections of 30mg/kg cocaine for 5 consecutive days and a challenge injection on day 13. Horizontal (A and C) and stereotyped (B and D) activity was recorded on days 1 (white bar and circle), 5 (grey bar and circle) and 13 (black bar and circle) for 30 min prior to injection (0 – 30 min) and 1 hour after injection (31 – 90 min). Data are presented on the left of each panel as the mean sum ± SEM of the first 30 minutes after cocaine injection and on the right of each panel as mean ± SEM in 5 min intervals; N: INT = 9, GDX = 9. INT male rats show an increase locomotor response after a withdrawal period. On the other hand, GDX animals show decreased horizontal activity after repeated injections when compared to day 1. * p<0.05 compared to LMA on day 1.

Figure 6. Locomotor response of intact, gonadectomized and gonadectomized male rats with testosterone replacement to a single saline or cocaine (15mg/kg) injection

Total horizontal (A) and stereotyped (B) activity of INT (white bars), GDX (black bars) and GDX-T (grey bars) male rats, presented as the mean sum ± SEM of the first 30 minutes after a single injection of saline (0.9%) or cocaine (15mg/kg); N: Saline: INT = 8, GDX = 8, GDX-T = 7; Cocaine: INT = 30 (STRCNT) 31(HACTV), GDX = 29, GDX-T = 28. Testosterone treatment of GDX animals did not alter basal locomotor activity. Cocaine administration increased LMA of all animal groups. Hormone treatment altered the locomotor response to a single cocaine injection. The increased locomotor response of GDX male rats to a single 15mg/kg cocaine injection was attenuated by testosterone replacement. * p<0.05 compared to LMA of INT animals with the same dose of cocaine.

Figure 7. Basal and cocaine-induced locomotor activity of gonadectomized male rats with testosterone replacement

Animals received daily injections of saline or 15mg/kg cocaine for 5 consecutive days and a challenge injection on day 13. Horizontal (A and B) and stereotyped (C and D) activity was recorded on days 1 (white bar and circle), 5 (grey bar and circle) and 13 (black bar and circle) for 30 min prior to injection (0 – 30 min) and 1 hour after injection (31 – 90 min). Data are presented on the left of each panel as the mean sum ± SEM of the first 30 minutes after cocaine injection and on the right of each panel as mean ± SEM in 5 min intervals; N : Saline = 7; Cocaine = 28. Basal LMA was increased on day 13 compared to days 1 and 5. GDX-T male rats show an increased locomotor response after repeated cocaine administrations. * p<0.05 compared to LMA on day 1; ** p<0.05 compared to LMA on day 1 and day 5.

Figure 8. Plasma levels of total and free testosterone on the first and last day of injection of intact and gonadectomized male rats with testosterone replacement

Total (A and B) and free (C and D) testosterone levels were measured by radioimmunoassay in INT (white bars) and GDX-T (grey bars) animals. Data are presented as mean ± SEM; N: Total Testosterone: Day 1: Saline: INT = 9, GDX = 8; Cocaine: INT = 8, GDX-T = 9; Day 13: Saline: INT = 22, GDX-T = 21; Cocaine; INT = 32, GDX-T = 33; Free Testosterone: Day 1: Saline: INT = 9, GDX = 8; Cocaine: INT = 8, GDX-T = 9; Day 13: Saline: INT = 10, GDX-T = 9; Cocaine; INT = 10, GDX-T = 10. Values for GDX animals were negligible and not included in the graph. GDX-T animals had higher total and free testosterone levels than intact male rats when treated with saline. A single cocaine injection (Day 1) did not alter total (A) or free (C) testosterone plasma levels when compared to their saline counterpart. Repeated cocaine administration (Day 13; B and D) decreased plasma testosterone of GDX-T compared to their saline counterparts, although only total testosterone reached statistical significance. * p<0.05 compared to INT animals with the same drug treatment; # p < 0.01 compared to the saline counterpart.