Chronic mild stress impact: are females more vulnerable?

Despite the knowledge that women are more susceptible than men to stress-related mental illness, such as major depression, there is no comprehensive estimation of the role of gender in the detrimental effects of chronic stress that might cause depression. Sex differences regarding the association of behavioral parameters with serotonergic and hypothalamic-pituitary-adrenal axis activities were investigated in the chronic mild stress model of depression. Additionally, the impact of chronic mild stress exposure on an additional/novel short-term stressful procedure, such as the forced swim test was examined in male and female rats. Female rats were found to be more vulnerable to chronic mild stress and that was depicted with disruption of sucrose intake, decreases in open field activity, increased corticosterone levels, alteration in estrous cycle and decreased serotonergic activity in hippocampus and hypothalamus. On the contrary, in males the current chronic mild stress protocol elicited only behavioral changes, such as disruption in sucrose intake and decreased open field activity. Interestingly, in response to forced swim test, females previously subjected to chronic mild stress, were found to cope better by exhibiting increased active behavior in the second forced swim test session and higher hypothalamic serotonergic activity in comparison with corresponding males. On the other hand, males were more affected by previous chronic mild stress exposure and that was manifested by decreased active behavior in the first forced swim test session and increased corticosterone levels following second forced swim test session. These data indicate that although females are more vulnerable in the application of chronic mild stress than males, in response to an additional-novel stressor (forced swim test) they show better response. Therefore, both sex/gender and combination of stressful procedures should be carefully considered in the study of the pathophysiology of stress-related mental illnesses.

Tamoxifen exerts testosterone-dependent and independent effects on thymic involution.

Circulating testosterone concentrations and seminal vesicles weights, as well as thymus and spleen weights and histology were assessed in male Wistar rats from the infantile to post-pubertal period. The widely used anti-estrogenic agent tamoxifen was then administered in adult intact and castrated male rats and its long-term effects on thymic involution and splenic growth were examined. The results showed that: (1) age-related involution of the male thymus from the juvenile period through puberty to post-puberty depends on the rising testosterone levels and represents mainly a decrease of thymic lymphoid-cell elements; (2) tamoxifen administration reverses thymic involution in intact adult male rats and this effect is related to a dose-dependent, tamoxifen-induced castration and decrease of testosterone levels; (3) the changes of circulating testosterone levels, either resulting from maturity, or induced by tamoxifen or by castration, have a minimal effect on splenic growth and weight; and (4) in contrast to intact animals, administration of tamoxifen at pharmacological doses to adult castrated rats results in thymic regression. Underscoring the critical role of testosterone on thymic involution, these findings show that tamoxifen is able to reverse ageing changes in the thymus by suppressing testosterone production, while conversely, exerts thymolytic effects in the absence of androgens.

Monoaminergic dysregulation on diestrus-2 and estrus through high emotional reactivity.

Rats with great differences in emotional reactivity, during weighing and handling for vaginal smear screening were examined on diestrus-2 (DE-2), proestrus (PE), and estrus (E). Rats with high emotional reactivity (HR), interpreted as trait anxiety, had different serotonergic and dopaminergic profile in hypothalamus-preoptic area (HY-PA) and striatum (Str) and thymus weight lower than that found in rats with low emotional reactivity (LR). In HY-PA of rats with HR when compared to rats with LR, increased 5-hydroxyindoleacetic acid (5-HIAA), 5-HIAA/serotonin (5-HT) ratio, and 3,4-dihydroxyphenylacetic acid (DOPAC) and in Str increased DOPAC and DOPAC/dopamine (DA) ratio were found only on DE-2, paralleled by increased adrenal weight and decreased thymus weight. In Str, a significant effect of HR on 5-HIAA was found only on E, in parallel with increased 5-HT and decreased DOPAC and DOPAC/DA ratio when compared to rats with LR. The results suggest that activation of 5-HT and DA in HY-PA and DA in Str through HR is apparent only on DE-2 while, conversely, on E suppression of striatal DA it is apparent with 5-HT dysregulation. These findings might have some relevance to the predisposition of women with trait anxiety to premenstrual syndrome.

Relationship between the thymus and neurochemical changes in the hypothalamus-preoptic area and prefrontal cortex in female rats with delayed puberty.

In female rats, aged 55-58 days with delayed puberty due to deficient growth and environmental stress, 5-hydroxyindoleacetic acid levels and serotonin turnover rate in the hypothalamus-preoptic area as well as body weight, body weight gain and relative weight of ovaries, uterus, adrenals and preputial glands were lower while serotonin and 5-hydroxyindoleacetic acid levels in the prefrontal cortex were higher when compared to normal rats with the latest onset of puberty aged 42-52 days. In rats with delayed puberty, multiple regression analysis revealed a significant negative dependence on dopamine turnover in the hypothalamus-preoptic area for body weight gain and, of all organs, for the relative weight of the thymus. A similar negative significant dependence on serotonin turnover rate in the prefrontal cortex was also found for the relative weight of thymus and spleen. The same analysis in the opposite direction revealed a significant negative dependence of 3,4-dihydroxyphenylacetic acid levels and dopamine turnover rate in the hypothalamus-preoptic area as well as serotonin turnover rate in the prefrontal cortex only on thymus weight. After separation of delayed pubertal rats into two groups, based on absolute ovarian weight, the rats in the low ovarian weight range and no signs of puberty exhibited: lower body weight gain, lower body weight, and lower relative weight only of thymus, ovaries and preputial glands in parallel with an increased dopamine turnover rate in the hypothalamus-preoptic area and serotonin turnover rate in the prefrontal cortex compared to the delayed pubertal rats in the high ovarian weight range and early signs of puberty. The results suggest that in rats with delayed puberty: (1) serotonergic activation in the hypothalamus-preoptic area is lower compared to normal puberty rats; (2) dopaminergic activation in the hypothalamus-preoptic area negatively affects body weight gain, thymus weight and initiation of puberty and (3) thymus weight is negatively implicated in dopaminergic activation in the hypothalamus-preoptic area and serotonergic activation in the prefrontal cortex and positively related to ovarian weight and early signs of puberty.

Effect of cyproheptadine treatment on conditioned avoidance response in female rats.

1. The percentage of conditioned avoidance response was higher during proestrus compared to diestrus. 2. Cyproheptadine (CPH) significantly enhanced avoidance behavior during diestrus. 3. On the other hand, CPH treatment did not alter avoidance behavior during proestrus. 4. Serum progesterone and testosterone levels were determined at the end of 60 trials for acquisition of conditioned avoidance response after prolonged (12-15 days) CPH treatment (0.5 mg/kg for 24 h per os (p.o.). 5. Prolonged CPH treatment lowered adrenal testosterone levels, and rats with impaired avoidance had higher testosterone and progesterone levels. 6. The results of this study indicate a positive role for CPH in the acquisition of avoidance response during diestrus, and a negative effect of progesterone and adrenal testosterone on the avoidance response.

Stress through handling for vaginal screening, serotonin, and ACTH response to ether.

The effect of duration of handling for vaginal smear screening on the adrenal weight and acute ACTH response to ether were examined in 4-day-cycling female rats, sacrificed at 97-103 days of age on diestrus-2 after evaluation of resistance to handling, thymus weight, and hypothalamic serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA). Prolonged handling paralleled increased resistance (behavioral response) to handling and adrenal weight but was inversely related to thymus weight. The hypothalamic 5-HT, 5-HIAA, and 5-HIAA/5-HT ratio, compared to controls with similar conditions of handling, were not modified after 2.5 min of ether despite the ACTH rise. In ether-stressed rats, the ACTH response to ether was lower after prolonged handling compared to short handling paralleling decreased thymus weight. In contrast, 5-HT, 5-HIAA, and the 5-HIAA/5-HT ratio were higher, paralleling increased resistance and adrenal weight. The results suggest chronic activation of the hypothalamo-pituitary-adrenal axis with positive serotonergic involvement after prolonged handling and resistance during vaginal screening and a negative implication of this activation on the acute ACTH response to ether.

The effect of sumatriptan on brain monoamines in rats.

Clinical data suggests that sumatriptan is effective in the acute treatment of migraine. The vascular effects of the drug have been invoked to explain this antimigraine efficacy. However, the effect of sumatriptan on brain monoamines has not previously been investigated. In order to study these hypothetical effects, we administered the drug to 24 male rats, subcutaneously, at three doses (0.3, 0.6, and 0.9 mg/kg of body weight), and 30 minutes later, all animals were decapitated. Dopamine, serotonin, and their metabolites 3,4 dihydroxyphenylacetic acid, 5-hydroxyindoleacetic acid, and homovanillic acid concentrations were measured in the frontal cortex, hypothalamus, striatum, and hippocampus, by high performance liquid chromatography. Plasma concentrations of the drug were also determined. The control group was treated with NaCl 0.9%, given subcutaneously. Sumatriptan, at the dose of 0.3 mg/kg did not alter the brain monoamine concentrations; however, at the dose of 0.6 mg/kg, sumatriptan decreased serotonin concentration in the hypothalamus and increased the turnover of dopamine and serotonin in the hypothalamus and striatum, while at the dose of 0.9 mg/kg, it augmented only the turnover of serotonin in the hypothalamus. No dose-dependent effect of the drug was found. This subcortical antidopaminergic and antiserotoninergic effect of sumatriptan may be involved in its antimigraine action.

The effects of valproate in brain monoamines of juvenile rats after stress.

1. Clinical data suggest that valproate (VPA) may be useful in prophylaxis of affective disorders, which show disturbances of the serotoninergic system. On the other hand, chronic stress has an adverse effect on affective disorders, those with disturbances of the serotonergic system, especially. 2. In order to study the effects of VPA on brain monoamines and acute stress, 200 mgr VPA/Kgr was administered intraperitoneal (ip) to juvenile male rats; the control group was treated with NaCL 0.9% ip. After 30 min, all animals were evoked on predictable neurogenic or systemic stress (30 min foot shock, or 15 min ether stress, respectively), and 48 hours later, VPA or NaCL were administered ip again; 30 min afterwards, the rats were decapitated. Rats without stress were also sacrificed 30 min after VPA or NaCL administration. 3. Measurements of brain monoamines noradrenaline (NA), dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA), were done in Frontal Cortex (FC), Hypothalamus (HY) and Striatum (S), by High Performance Liquid Chromatography (HPLC). 4. Compared with the control stress group the level of 5-HIAA in the FC was significantly increased (P < 0.01) in VPA stress rats; in the HY and in S the increase of 5-HIAA was not significant. No remarkable differences were observed in NA, DA, 5-HT and DOPAC concentrations, in any of the brain regions. No changes in brain monoamine levels were found in non stress rats, either. 5. The augmentation of 5-HIAA level after VPA administration and after stress, in correlation with the decrease of 5-HIAA that is observed in depression, support the hypothesis that VPA may be effective in affective disorders by influencing the serotoninergic system.

Behavioural and biochemical effects of haloperidol during the oestrous cycle of the rat.

The effects of two doses (1 and 2 mg/kg, i.p.) of haloperidol (HAL) on catalepsy, on concentrations of DA and DOPAC in frontal cortex, nucleus accumbens and striatum and on serum levels of oestradiol were investigated in intact female rats during the 4-day oestrous cycle. Catalepsy induced by haloperidol did not vary much during phases of the cycle. The turnover of DA in the cortex induced by haloperidol was significantly greater on proestrus and smaller on oestrus. The effect of haloperidol on the turnover of DA in the nucleus accumbens and in striatum was marginally affected by the oestrous cycle being greatest on oestrus. The levels of serum oestradiol were higher on proestrus and lower on oestrus. No significant differences were detected between diestrus and metestrus. After haloperidol there was a dramatic increase in serum oestradiol on oestrus, a slight increase on metestrus and diestrus and a decrease on proestrus. However, serum levels of oestradiol were not significantly different between phases of the cycle in rats treated with haloperidol. The results indicate that the oestrous cycle has a detectable influence on DAergic mechanisms in the frontal cortex and possibly in the tuberoinfudibular system, brought about by treatment with haloperidol.

Apomorphine-induced behaviour during the oestrous cycle of the rat.

The effect of various doses of apomorphine (APO) (25, 250, 400 and 750 micrograms/kg, s.c.) on open field behaviour, stereotyped behaviour, body temperature and concentrations of serum oestradiol was studied in cycling females and in ovariectomized rats. With the exception of grooming, the hormonal variations during the cycle, or the ovariectomy, did not have an effect on behaviour related to stimulation of presynaptic dopamine (DA) receptors. The endocrine status on proestrus (PE), characterized by an increase in serum oestradiol, did influence hyperlocomotion and hypothermia induced by apomorphine; the former being attenuated and the latter increased, as compared to the other phases of the cycle. Ovariectomy resulted in an increase in the stimulatory effect of apomorphine on locomotion. Stereotypy induced by apomorphine was unaltered by hormonal variations during the cycle and it was slightly attenuated by removal of the ovaries. During phases of low levels of oestrogen (oestrus, metestrus) apomorphine significantly increased the levels of serum oestradiol, determined 30 min after the administration of drug. It is concluded that the various DAergic mechanisms in brain are differentially affected by hormonal variations during the cycle and by ovariectomy.

Estrogen treatment to ovariectomized rats modifies morphine-induced behavior.

Two weeks after surgery, ovariectomized (OVX) rats were treated for 3 days with either 17 beta-estradiol (10 or 100 micrograms/kg, SC, per day) or the oil vehicle. They were then tested for morphine-induced hyperactivity (4 mg/kg, IP), analgesia and catalepsy (15 and 20 mg/kg, IP) 24 or 72 hr after the last steroid or oil injection. Estradiol treatment did not affect the locomotion or the sensitivity to nociceptive stimuli of OVX rats and did not induce a cataleptic state in animals. Estradiol- (100 micrograms/kg) treated OVX rats exhibited attenuated morphine-induced hyperlocomotion regardless of the time that had elapsed after estradiol treatment cessation, attenuated morphine-induced catalepsy at 24 hr after estradiol treatment and unaltered morphine-induced analgesia. OVX rats treated with a lower estradiol dose (10 micrograms/kg) exhibited significantly increased morphine-induced analgesia and slightly increased catalepsy. The results show that the sensitivity of brain opiate systems controlling some of the behavioral effects of morphine is modified following estradiol treatment to OVX rats.