**Background:** The stress response is mediated by the hypothalamic–pituitary–adrenal (HPA) and sympathetic–adreno–medullary (SAM) axes, with catecholamines (CAs) playing a key role in neuroimmunoendocrine communication. Tyrosine hydroxylase (TH) is the rate-limiting enzyme in CA synthesis. Mice hemizygous for the TH gene (TH-HZ) have reduced TH activity and lower CA production, leading to impaired sensorimotor skills, increased anxiety, premature immunosenescence, and oxidative-inflammatory stress. The authors aimed to investigate how a punctual (acute) restraint stress affects behavior, immunity, and oxidative state in TH-HZ mice compared to WT mice, and to examine sex differences in these responses.
**Methods:** Adult (9 ± 1 month) female and male TH-HZ and WT ICR-CD1 mice (n=6 per group) were used. In the basal condition, mice underwent a battery of behavioral tests (sensorimotor: visual placing, hindlimb extensor reflex, wood rod test, tightrope test; anxiety: elevated plus maze, marble burying test; exploration: holeboard test, T-maze, corner test). Peritoneal leukocytes were then collected to assess immune function (macrophage and lymphocyte chemotaxis via Boyden chamber, macrophage phagocytosis of latex beads, natural killer activity against YAC-1 cells, lymphoproliferation to ConA and LPS via ³H-thymidine incorporation), oxidative stress parameters (catalase, glutathione reductase, glutathione peroxidase activities; reduced [GSH] and oxidized [GSSG] glutathione levels; xanthine oxidase activity), and catecholamine concentrations (adrenaline, noradrenaline, dopamine by ELISA). Mice were then subjected to 10 minutes of restraint stress in a cylindrical acrylic tank, after which the same behavioral tests and immune/oxidative assessments were repeated (post-stress condition). Data were analyzed using independent- and dependent-samples t-tests.
**Key Results:** In basal conditions, TH-HZ mice of both sexes showed impaired sensorimotor abilities (e.g., TH-HZ males: greater latency to leave first segment in wood rod test, p<0.01; higher time of permanence, p<0.001; more fell off tightrope, p<0.05), higher anxiety (TH-HZ females: less time in open arms of EPM, p<0.001; TH-HZ males: smaller % time in open arms, p<0.01), and reduced exploration (TH-HZ females and males: lower total locomotion in HBT, p<0.001; fewer head-dippings, p<0.001) compared to WT. Immune function was impaired in TH-HZ mice (macrophage chemotaxis, phagocytic efficacy and index, lymphocyte chemotaxis, basal and LPS proliferative responses, NK activity: all p<0.001 vs. WT). TH-HZ mice also exhibited a pro-oxidant state (e.g., lower GSH, higher GSSG/GSH ratio, higher XO activity). After stress, WT females improved innate immunity (macrophage chemotaxis p<0.05, phagocytic efficacy p<0.01, phagocytic index p<0.001, NK activity p<0.001) but showed impaired lymphocyte chemotaxis (p<0.001) and basal lymphoproliferation (p<0.001). WT males showed impaired innate immunity after stress (macrophage chemotaxis p<0.001, phagocytic efficacy p<0.05, phagocytic index p<0.01). TH-HZ females showed no immune improvement after stress; instead, macrophage and lymphocyte chemotaxis (p<0.001) and lymphoproliferation to ConA and LPS (p<0.001) were impaired. TH-HZ males showed further deterioration (phagocytic efficacy p<0.01, phagocytic index p<0.01, lymphocyte chemotaxis p<0.05, lower ConA response p<0.001, lower NK activity p<0.01). Oxidatively, WT females increased both antioxidant defenses (GR p<0.001, GPx p<0.01, GSH p<0.001) and oxidant compounds (XO p<0.001, GSSG p<0.001) after stress, maintaining balance. TH-HZ females failed to regulate oxidants, establishing clear oxidative stress (higher GSSG/GSH ratio p<0.001). Catecholamine analysis showed TH-HZ mice had lower adrenaline, noradrenaline, and dopamine in peritoneal leukocytes at baseline (p<0.05 to p<0.001). After stress, WT females increased adrenaline (p<0.01) and noradrenaline (p<0.05); WT males increased adrenaline (p<0.001) but decreased dopamine (p<0.001). TH-HZ mice showed no increase in any catecholamine after stress; adrenaline decreased in TH-HZ males (p<0.001). Sex differences were prominent: males generally had worse behavioral performance, higher anxiety, impaired immunity, and a more pro-oxidant profile than females across genotypes.
**Clinical Implications:** This study demonstrates that adequate catecholamine synthesis is essential for mounting an appropriate acute stress response. TH haploinsufficiency severely compromises the ability to respond to stress, leading to deteriorated behavior, immunity, and redox balance. The findings also highlight significant sex differences in stress responses, with males showing poorer adaptive capacity. These results may inform understanding of conditions involving catecholamine dysregulation (e.g., Parkinson's disease, chronic stress disorders) and suggest that sex-specific approaches may be important in managing stress-related health outcomes. The TH-HZ mouse model may be useful for studying accelerated aging and stress vulnerability.