Roles of estrogen on the sentonergic neurotransmission involed in the Anxiety-Like behaviors of ovariectomized rats

Abstract

There are evidences of an important role of estrogen on the regulation of anxiety but the mechanism is not yet clear. The current study aimed to investigate the roles of estrogen on the serotonergic transmission in the brain areas involving the anxiety-like behaviors in ovariectomized rats. Female rats were divided into 3 groups, the ovariectomized rats treated with 17β-estradiol (10 µg/kg, s.c.) (Ovx+E2) or with vehicle (Ovx) and proestrous rats (Pro). Four weeks after ovariectomy, all rats were tested with the elevated T-maze (ETM) and followed by the open field, in order to measure for anxiety level and locomotor activity, respectively. To study the effects of estrogen on serotonergic neural transmission, the brains were removed immediately after behavioral tests, for measurement of 5-HT and its metabolite (5-HIAA) levels by HPLC technique, and tryptophan hydroxylase (TPH) enzyme and serotonin reuptake transporter (SERT) protein levels by Western blot analysis. The 5-HT, its metabolite, and SERT protein levels were measured in frontal cortex, nucleus accumbens, septum, amygdala, hippocampus, whereas TPH protein levels measured in midbrain. Finally, to examine whether estrogen had an effect on the function of 5-HT[subscript 2A/2C] receptor, the 5-HT[subscript 2A/2C] receptor antagonist (0.3, 1.0, and 3.0 mg/kg, i.p.) was injected to a separate set of animals from each group, 30 min before tested with ETM. In the ETM test, Ovx+E₂ rats impaired inhibitory avoidance as compared with Ovx rats, indicating chronic estrogen administration has an effect on decreasing conditioned anxiety related to generalized anxiety disorder. In contrast, the Pro rats prolonged escape as compared with both Ovx and Ovx+E₂ rats, indicating natural high level of estrogen has an effect on decreasing unconditioned anxiety related to panic disorder. The measurements of brain 5-HT and its metabolite levels revealed the increased turnover rates (5-HIAA/5-HT ratio) in the hippocampus and the nucleus accumbens of Ovx+E₂ rats than those of Ovx and Pro rats. The TPH protein levels in the midbrain of Ovx+E₂ and Pro rats significantly lower than Ovx rats and no significant different in SERT protein levels in all measured brain areas. Additionally, the function of 5-HT[subscript 2A/2C] receptor may also affect by estrogen as the 5-HT[subscript 2A/2C] receptor antagonist at the dosage of 3.0 mg/kg significantly decreased both avoidance I and 2 latencies of ETM in Pro rats. Further, the open field showed that the same dosage of 5-HT[subscript 2A/2C] receptor antagonist tended to increase time in the inner zone and decrease time in outer Zone of Ovx rats. Finally, locomotor activity was not affected by any treatment. In conclusion, the chronic estrogen administration to Ovx rats and the natural high levels of estrogen in Pro rats demonstrated the anxiolytic response in different form of anxiety. Despite the difference, this study has clearly demonstrated that estrogen has an effect on anxiolytic-like behavior in related to the function of serotonergic neural transmission.