Regulation of the hypothalamic-pituitary-adrenal axis circadian rhythm by endocannabinoids is sexually diergic

Abstract

We have examined the effects of acute administration of the cannabinoid receptor type 1 (CB(1)) antagonist AM251 on the rat hypothalamic-pituitary-adrenal (HPA) axis with respect to both gender and time of day. Blood samples were collected from conscious male and female rats every 5 min using an automated blood sampling system, and corticosterone concentrations were determined. In male rats, there was a distinct diurnal effect of AM251 with a greater activation of the HPA axis in the morning (diurnal trough) compared with the evening (diurnal peak). At both times of the day, circulating corticosterone concentrations were elevated for approximately 4 h after AM251 administration. In female rats, there was also diurnal variation in the activation of the HPA axis; however, these effects were not as profound as those in males. Corticosterone concentrations were only slightly elevated at the diurnal trough and for a shorter time period than in males (2 compared with 4 h). Moreover, there was no effect of AM251 on corticosterone concentrations when administered at the diurnal peak. Subsequent studies, only in males, in which both ACTH and corticosterone were measured, confirmed that the effects of AM251 on corticosterone were mediated by ACTH. Moreover, the elevation of both ACTH and corticosterone could be replicated using another CB(1) antagonist, AM281. These data demonstrate that the extent and duration of HPA axis activation after CB(1) blockade are clearly dependent on both gender and time of day.

Figure 1

Blood corticosterone profiles from male rats treated with a cannabinoid receptor antagonist during the diurnal peak or trough. Top, Group means ± se from male rats treated with a sc bolus of AM251 (1 mg/kg) or vehicle (5% ethanol, 5% Cremophor, 90% saline) during the diurnal trough; middle, group means ± se from male rats treated with a sc bolus of AM251 (1 mg/kg) or vehicle (5% ethanol, 5% Cremophor, 90% saline) during the diurnal peak; bottom, profile from a male rat treated with AM251 during the diurnal trough. Note the pulsatile nature of corticosterone release. Vertical dotted lines indicate time of sc injection. Horizontal gray bars indicate the period of lights off. Horizontal black bars indicate where the hourly means of the AM251-treated rats were significantly greater than the vehicle-treated rats by repeated-measures ANOVA (see text for details).

Figure 2

Blood corticosterone profiles from female rats treated with a cannabinoid receptor antagonist during the diurnal peak or trough. Top, Group means ± se from female rats treated with a sc bolus of AM251 (1 mg/kg) or vehicle (5% ethanol, 5% Cremophor, 90% saline) during the diurnal trough; bottom, group means ± se from female rats treated with a sc bolus of AM251 (1 mg/kg) or vehicle (5% ethanol, 5% Cremophor, 90% saline) during the diurnal peak. Vertical dotted lines indicate time of sc injection. Horizontal gray bars indicate the period of lights off. Horizontal black bars indicate where the hourly means of the AM251-treated rats were significantly greater than the vehicle-treated rats by repeated-measures ANOVA (see text for details).

Figure 3

Effect of the AM251 on plasma ACTH and corticosterone in male rats treated during the diurnal peak of trough. Male rats were given a single sc bolus of AM251 (1 mg/kg) or vehicle (VEH) in either the morning (left panels) or the evening (right panels). Plasma concentrations of both ACTH (top panels) and corticosterone (lower panels) are presented. Values are group means ± se. Horizontal gray bars indicate the period of lights off. Arrows indicate time of sc administration. *, Time points in AM251-treated animals that are significantly greater than for controls by repeated-measures ANOVA (see text for details).

Figure 4

Effect of the AM281 on plasma ACTH and corticosterone in male rats treated during the diurnal peak of trough. Male rats were given a single sc bolus of AM281 (1 mg/kg) or vehicle (VEH) in either the morning (left panels) or the evening (right panels). Plasma concentrations of both ACTH (top panels) and corticosterone (lower panels) are presented. Values are group means ± se. Horizontal gray bars indicate the period of lights off. Arrows indicate time of sc administration. *, Time points in AM281-treated animals that are significantly greater than for controls by repeated-measures ANOVA (see text for details).