The last stage of development: The restructuring and plasticity of the cortex during adolescence especially at puberty.

There is considerable evidence of reorganization in the prefrontal cortex during adolescence in humans, as well as in rodents, where the cellular basis can be explored. Studies from my laboratory in the rat medial prefrontal cortex are reviewed here. In general, growth predominates before puberty. Pruning mainly occurs at puberty and after with decreases in the number of synapses, dendrites, and neurons. Perineuronal nets, extracellular structures that control plasticity, are pruned peripubertally only in female rats, which may further open the adolescent prefrontal cortex to environmental influences. This is supported by our recent evidence that exposure to mild stress early, but not late, in adolescence decreases prepulse inhibition. Additionally, exposure to methamphetamine in females early in adolescence increases the number of a major class of inhibitory interneurons, parvalbumin neurons, while the opposite occurs late in adolescence. In females, even estrogen receptor beta mRNA decreases at puberty in the prefrontal cortex. Interestingly, rats of both sexes perform better after puberty on a test of cognitive flexibility in the water maze. Thus, evidence is accruing that adolescence is not a single entity but rather an ongoing set of processes, and environmental effects will differ depending on timing and sex.

Impact of pubertal onset on region-specific Esr2 expression.

In female rats, pubertal onset is associated with maturation of the medial prefrontal cortex (mPFC) and mPFC-mediated behaviours. These behavioural and anatomical changes are likely a result of the effects of oestrogens at the nuclear oestrogen receptor (ER)ß, which is expressed at higher levels than the ERα isoform in the adult mPFC. Researchers have previously quantified ERß protein and Esr2 RNA in rodents during early postnatal development and adulthood, although an adolescent-specific trajectory of this receptor in the mPFC has not been documented. Given that Esr2 expression can fluctuate in the presence or absence of oestrogens, puberty and the subsequent rise in gonadal hormones could influence levels of ERß in the adolescent brain. To further explore this, we used RNAscope® technology to quantify the amount of Esr2 mRNA in pre-pubertal adolescent, recently post-pubertal adolescent and adult female rats. We show that Esr2 expression decreases significantly in the mPFC, striatum and motor cortex between pre-pubertal adolescence and adulthood. In the mPFC, this decrease occurs rapidly at pubertal onset, with no significant decrease in Esr2 levels between the recently post-pubertal and adult cohort. By contrast, the striatum and motor cortex had no significant differences in the amount of Esr2 mRNA between pre- and post-pubertal females. Insofar as the amount of Esr2 expression is proportional to functional ERß, these results suggest ERß decreases in a region-specific pattern in response to pubertal onset and highlight a role for this receptor in the maturational events that occur in the female rat mPFC at puberty.

Factors influencing the cognitive and neural effects of hormone treatment during aging in a rodent model.

Whether hormone treatment alters brain structure or has beneficial effects on cognition during aging has recently become a topic of debate. Although previous research has indicated that hormone treatment benefits memory in menopausal women, several newer studies have shown no effect or detrimental effects. These inconsistencies emphasize the need to evaluate the role of hormones in protecting against age-related cognitive decline in an animal model. Importantly, many studies investigating the effects of estrogen and progesterone on cognition and related brain regions have used young adult animals, which respond differently than aged animals. However, when only the studies that have examined the effects of hormone treatment in an aging model are reviewed, there are still varied behavioral and neural outcomes. This article reviews some of the important factors that can influence the behavioral and neural outcomes of hormone treatment including the type of estrogen administered, whether or not estrogen is combined with progesterone and if so, the type of progesterone used, as well as the route, mode, and length of treatment. How these factors influence cognitive outcomes highlights the importance of study design and avoiding generalizations from a small number of studies. This article is part of a Special Issue entitled Hormone Therapy.

The effects of long-term treatment with estradiol and medroxyprogesterone acetate on tyrosine hydroxylase fibers and neuron number in the medial prefrontal cortex of aged female rats.

Menopausal women often initiate hormone treatment to alleviate the symptoms of menopause. Research suggests that these treatments also affect cognition, and studies in young animals indicate that hormone treatment can alter several neuroanatomical measures. However, very little is known about the effects of long-term hormone treatment on the aging female brain. This study investigated the effects of hormone treatment on neuron number and tyrosine hydroxylase (TH) in the rat medial prefrontal cortex (mPFC). Female Long Evans rats were ovariectomized at middle age (12-13 months) and placed in one of four groups: no replacement (NR) (n = 12), 17ß-estradiol (E(2)) (n = 12), E(2) and progesterone (n = 7), or E(2) and medroxyprogesterone acetate (MPA) (n = 10). Animals were euthanized at 20 months, and the brains were Nissl stained; a subset was immunostained for TH [NR (n = 5); E(2) (n = 6); E(2) + MPA (n = 4); E(2) + progesterone (n = 6)]. E(2) was administered through the drinking water, and progestagens were administered via pellets inserted at the nape of the neck. Neuron number and TH fiber density were quantified in the mPFC. Hormone treatment did not alter neuron number. Treatment with E(2) and MPA resulted in greater TH densities than NR in layer 1 (P < 0.05). In layers 2/3, animals receiving E(2) had greater TH densities than NR animals (P < 0.01). These results indicate that long-term hormone treatments alter dopaminergic fibers and potentially the functioning of the aging mPFC.

Effects of long-term treatment with estrogen and medroxyprogesterone acetate on synapse number in the medial prefrontal cortex of aged female rats.

OBJECTIVE: The present study investigated the effects of long-term hormone treatment, including the most commonly prescribed progestin, medroxyprogesterone acetate, during aging on synaptophysin-labeled boutons, a marker of synapses, in the medial prefrontal cortex (mPFC) of rats. METHODS: Female Long Evans hooded rats were ovariectomized at middle age (12-13 mo) and were placed in one of four groups: no replacement (n = 5), 17ß-estradiol alone (n = 6), estradiol and progesterone (n = 7), or estradiol and medroxyprogesterone acetate (n = 4). Estradiol was administered in the drinking water and progestogens were administered via subcutaneous pellets that were replaced every 90 days. After 7 months of hormone replacement, the animals were euthanized, and the brains were stained for synaptophysin, a membrane component of synaptic vesicles. The density of synaptophysin-labeled boutons was quantified in the mPFC using unbiased stereology and multiplied by the volume of the mPFC to obtain the total number. RESULTS: Animals receiving estradiol and medroxyprogesterone acetate had significantly more synaptophysin-labeled boutons in the mPFC than did animals not receiving replacement (P < 0.03) and those receiving estradiol and progesterone (P < 0.02). In addition, there was a nonsignificant trend for animals receiving estradiol alone to have more synapses than those receiving estradiol and progesterone. CONCLUSIONS: This study is the first to examine the effects of estradiol and medroxyprogesterone acetate during rat aging on cortical synaptic number. Estradiol with medroxyprogesterone acetate, but not progesterone, resulted in a greater number of synapses in the mPFC during aging than did no replacement.

Long-term replacement of estrogen in combination with medroxyprogesterone acetate improves acquisition of an alternation task in middle-aged female rats.

Studies have shown that ovarian hormones protect against some of the cognitive deficits associated with aging. Although much of the literature in rodents has focused on hippocampal dependent tasks, studies suggest that tasks dependent on the prefrontal cortex are also influenced by ovarian hormones. The present study investigated the effects of ovarian hormone treatment during aging on a delayed alternation t-maze. Female Long Evans hooded rats were ovariectomized at middle age (11-12 months) and placed in 1 of 5 treatment groups: no replacement, chronic estradiol (E(2)), cyclic E(2), chronic E(2) and progesterone, or chronic E(2) and medroxyprogesterone acetate (MPA). Following 6 months of hormone treatment, animals were trained to alternate in a t-maze. After reaching criterion, a series of delays from 5 to 90 s were introduced in random order. Rats receiving E(2) with MPA reached criterion significantly faster than animals not receiving treatment and those who received chronic or cyclic E(2) only. There was a nonsignificant trend for animals receiving E(2) and progesterone to reach criterion in fewer sessions than animals receiving E(2) only. Mode of administration, cyclic or chronic, did not affect performance. Hormones did not affect performance on the delayed alternation. This study, in combination with previous research, indicates that hormone effects cannot be generalized across tasks, age, or duration, and long-term estrogen in combination with MPA can be beneficial for some tasks.

Effects of long-term treatment with 17 beta-estradiol and medroxyprogesterone acetate on water maze performance in middle aged female rats.

Although previous research has indicated that hormone replacement therapy benefits memory in menopausal women, several recent studies have shown either detrimental or no effects of treatment. These inconsistencies emphasize the need to evaluate the role of ovarian hormones in protecting against age-related cognitive decline in an animal model. The present study investigated the effects of long-term hormone treatment during aging on the Morris water maze. Female Long Evans hooded rats were ovariectomized at middle age (12-13 months) and were immediately placed in one of five groups: no replacement, chronic 17 beta-estradiol only, chronic 17 beta-estradiol and progesterone, chronic 17 beta-estradiol and medroxyprogesterone acetate (MPA), or cyclic 17 beta-estradiol only. 17 beta-estradiol was administered in the drinking water in either a chronic or cyclic (3 out of 4 days) fashion. Progesterone and MPA were administered via subcutaneous pellets. Following 6 months of hormone treatment, animals were tested on the Morris water maze. Animals performed four trials a day for 4 days and after the final day of testing a subset of animals completed a probe trial. Across 4 days of testing, rats receiving 17 beta-estradiol in combination with MPA performed significantly worse than all other groups receiving hormone replacement. In addition on the last day of testing, chronic 17 beta-estradiol administration was more beneficial than cyclic administration and no replacement. Thus compared to other hormone-treated groups, long-term 17 beta-estradiol treatment in combination with MPA results in impaired performance on the spatial Morris water maze.

Aging and sex influence the anatomy of the rat anterior cingulate cortex.

Cognitive processes supported by the prefrontal cortex undergo an age-related decline. Until very recently, nonhuman animal models of aging have relied on the exclusive use of male subjects. This study was designed to investigate the influence of age, sex, and ovarian hormonal state on anatomy of the rat medial prefrontal cortex (anterior cingulate cortex). Dendritic tree extent and spine density were examined in young adult (3-5 mos.) and aged (20-24 mos.) male and female rats. Young adult females were examined either at proestrus or estrus, and aged females were examined in one of two reproductively senescent (estropausal) phases, persistent estrus or persistent diestrus. Neither the estrous cycle nor state of estropause influenced spine density or dendritic tree extent. However, the anatomy of the anterior cingulate cortex of young adult rats was sexually dimorphic, with males having greater dendritic spine density as well as arborization. While there was a reduction in density and tree extent with age for both sexes, this reduction was more pronounced for males, resulting in a disappearance of most sex differences with age. Thus the results of this study suggest that aging of the rodent cerebral cortex may follow a sexually dimorphic pattern.