The Hypothalamic-Pituitary-Adrenal Axis and Serotonin Metabolism in Individual Brain Nuclei of Mice with Genetic Disruption of the NK1 Receptor Exposed to Acute Stress

Abstract

Mice lacking the substance P (SP) neurokinin-1 (NK1) receptor (NK1R-/-mice) were used to investigate whether SP affects serotonin (5-HT) function in the brain and to assess the effects of acute immobilisation stress on the hypothalamic-pituitary-adrenocortical (HPA) axis and 5-HT turnover in individual brain nuclei. Basal HPA activity and the expression of hypothalamic corticotropin-releasing hormone (CRH) in wild-type (WT)- and NK1R-/- mice were identical. Stress-induced increases in plasma ACTH concentration were considerably higher in NK1R-/- mice than in WT mice while corticosterone concentrations were equally elevated in both mouse lines. Acute stress did not alter the expression of CRH. In the dorsal raphe nucleus (DRN), basal 5-HT turnover was increased in NK1R-/- mice and a 15 min stress further magnified 5-HT utilisation in this region. In the frontoparietal cortex, medial prefrontal cortex, central nucleus of amygdala, and the hippocampal CA1 region, stress increased 5-HT and/or 5-hydroxyindoleacetic acid (5-HIAA) concentrations to a similar extent in WT and NK1R-/- mice. 5-HT turnover in the hypothalamic paraventricular nucleus was not affected by stress, but stress induced similar increases in 5-HT and 5-HIAA in the ventromedial and dorsomedial hypothalamic nuclei in WT and NK1R-/- mice. Our findings indicate that NK1 receptor activation suppresses ACTH release during acute stress but does not exert sustained inhibition of the HPA axis. Genetic deletion of the NK1 receptor accelerates 5-HT turnover in DRN under basal and stress conditions. No differences between the responses of serotonergic system to acute stress in WT and NK1R-/- mice occur in forebrain nuclei linked to the regulation of anxiety and neuroendocrine stress responses.

Keywords: ACTH; Brain; CRH; Mouse; NK1 receptor; Serotonin; Stress; Substance P.

Fig. 1

ACTH (upper panel) and corticosterone (middle panel) concentrations in plasma of wild-type (WT) (NK1R+/+) mice (empty circles) and NK1R-knockout (NK1R−/−) mice (solid circles) exposed to acute immobilisation stress for 12 and 120 min (n = 7/group). One group of WT mice (n = 7) and NK1R−/− mice (n = 7) were immobilised for 120 min and then returned to their home cages for 120 min (120 min stress + 120 min rest). The increase in plasma ACTH concentrations in immobilised mice was markedly higher in NK1R−/− mice than in WT mice (time effects: P < 0.001; interaction: P = 0.002; group effect: P = 0.003) (upper panel). Identical increases in corticosterone concentrations were detected in both types of mice (time effect: P < 0.0001; interaction: P = 0.705; group effect: P = 0.182) (middle panel). Values are presented as the means ± SEM. * P < 0.05, ** P < 0.01 and ***P < 0.001, statistical comparison to the basal value within the same group. †P < 0.05 and ††P < 0.01, statistical comparison to the control, wild-type mice at the respective time point, calculated by two-way analysis of variances (ANOVA) for repeated measurements followed by post hoc multiple pairwise comparisons (Hochberg adjusted). Lower panel: CRH mRNA in the hypothalamus of NK1R+/+ (control) mice (empty columns) and NK1R−/− mice (solid columns). CRH mRNA levels in both, WT and NK1R−/− mice exposed to immobilisation did not significantly differ from basal CRH mRNA levels

Fig. 2

Effect of acute immobilisation stress for 15 min on serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in the dorsal raphe nucleus (DRN) (upper panel), median raphe nucleus (MRN) (middle panel) and nucleus raphe magnus (NRM) (lower panel) in wild-type (NK1R+/+) mice and NK1R knockout (NK1R−/−) mice (n = 7/group). Basal 5-HT utilisation in the DRN was elevated in NK1R−/− mice compared with WT mice. Stress further increased 5-HT concentration in the DRN (F_3,24 = 12.961; P < 0.001). Immobilisation resulted in significantly elevated 5-HIAA concentrations in both, WT and NK1R−/− mice. In NK1R−/− mice, stress was associated with an additional rise in 5-HIAA concentrations (F_3,24 = 10.222; P < 0.001) (upper panel). Stress did not alter 5-HT concentrations in the MRN (F_3,24 = 1.486, P > 0.05). Basal and stress-induced 5-HIAA concentrations in the MRN were higher in NK1R−/− mice than in WT mice (F_3,24 = 5.383; P < 0.01) (middle panel). 5-HT turnover in the NRM was not affected by stress (lower panel). Values are presented as the means ± SEM. *P < 0.05 and **P < 0.01, statistical comparison to the appropriate value detected in control, non-stressed NK1R+/+ and NK1R−/− mice, respectively. †P < 0.05 and ††P < 0.01, statistical comparison to the appropriate value detected in non-stressed or stressed NK1R+/+ mice, calculated with one-way ANOVA followed by a post hoc Bonferroni test