Sex-specific Difference of Hippocampal Synaptic Plasticity in Response to Sex Neurosteroids

Numerous studies provide increasing evidence, which supports the ideas that every cell in the brain of males may differ from those in females due to differences in sex chromosome complement as well as in response to hormonal effects. In this study, we address the question as to whether actions of neurosteroids, thus steroids, which are synthesized and function within the brain, contribute to sex-specific hippocampal synaptic plasticity. We have previously shown that predominantly in the female hippocampus, does inhibition of the conversion of testosterone to estradiol affect synaptic transmission. In this study, we show that testosterone and its metabolite dihydrotestosterone are essential for hippocampal synaptic transmission specifically in males. This also holds true for the density of mushroom spines and of spine synapses. We obtained similar sex-dependent results using primary hippocampal cultures of male and female animals. Since these cultures originated from perinatal animals, our findings argue for sex-dependent differentiation of hippocampal neurons regarding their responsiveness to sex neurosteroids up to birth, which persist during adulthood. Hence, our in vitro findings may point to a developmental effect either directly induced by sex chromosomes or indirectly by fetal testosterone secretion during the perinatal critical period, when developmental sexual priming takes place.

The serine protease inhibitor neuroserpin is required for normal synaptic plasticity and regulates learning and social behavior

The serine protease inhibitor neuroserpin regulates the activity of tissue-type plasminogen activator (tPA) in the nervous system. Neuroserpin expression is particularly prominent at late stages of neuronal development in most regions of the central nervous system (CNS), whereas it is restricted to regions related to learning and memory in the adult brain. The physiological expression pattern of neuroserpin, its high degree of colocalization with tPA within the CNS, together with its dysregulation in neuropsychiatric disorders, suggest a role in formation and refinement of synapses. In fact, studies in cell culture and mice point to a role for neuroserpin in dendritic branching, spine morphology, and modulation of behavior. In this study, we investigated the physiological role of neuroserpin in the regulation of synaptic density, synaptic plasticity, and behavior in neuroserpin-deficient mice. In the absence of neuroserpin, mice show a significant decrease in spine-synapse density in the CA1 region of the hippocampus, while expression of the key postsynaptic scaffold protein PSD-95 is increased in this region. Neuroserpin-deficient mice show decreased synaptic potentiation, as indicated by reduced long-term potentiation (LTP), whereas presynaptic paired-pulse facilitation (PPF) is unaffected. Consistent with altered synaptic plasticity, neuroserpin-deficient mice exhibit cognitive and sociability deficits in behavioral assays. However, although synaptic dysfunction is implicated in neuropsychiatric disorders, we do not detect alterations in expression of neuroserpin in fusiform gyrus of autism patients or in dorsolateral prefrontal cortex of schizophrenia patients. Our results identify neuroserpin as a modulator of synaptic plasticity, and point to a role for neuroserpin in learning and memory.

Structure-function-behavior relationship in estrogen-induced synaptic plasticity

This article is part of a Special Issue "Estradiol and Cognition". In estrogen-induced synaptic plasticity, a correlation of structure, function and behavior in the hippocampus has been widely established. 17ß-estradiol has been shown to increase dendritic spine density on hippocampal neurons and is accompanied by enhanced long-term potentiation and improved performance of animals in hippocampus-dependent memory tests. After inhibition of aromatase, the final enzyme of estradiol synthesis, with letrozole we consistently found a strong and significant impairment of long-term potentiation (LTP) in female mice as early as after six hours of treatment. LTP impairment was followed by loss of hippocampal spine synapses in the hippocampal CA1 area. Interestingly, these effects were not found in male animals. In the Morris water maze test, chronic administration of letrozole did not alter spatial learning and memory in either female or male mice. In humans, analogous effects of estradiol on hippocampal morphology and physiology were observed using neuroimaging techniques. However, similar to our findings in mice, an effect of estradiol on memory performance has not been consistently observed.

[Gender and the effects of steroid hormones in the central nervous system]

Degenerative diseases of the central nervous system, the incidence and prevalence of which vary between men and women, often manifest in the hippocampus. Neurosteroids are hormones that are synthesized in the CNS, and it is here that they exert their influence. Estrogen and testosterone are examples of neurosteroid hormones. In the hippocampus, an area of the brain closely associated with learning and memory, the local synthesis of estrogen in females, but not in males, is essential for the plasticity and stability of the synapses. The inhibition of estrogen synthesis in the female hippocampus causes a reduction in long-term potentiation (LTP), an electrophysiological parameter of learning and memory, thus resulting in a significant loss of synapses. In light of this, the fact that estrogen has been attributed with many neuroprotective functions in degenerative diseases of the CNS suggests that therapeutic concepts involving the use of estrogen are possibly only effective in women, but not in men. These findings similarly provide a basis for explaining the gender dimorphism that has been found in certain degenerative illnesses of the CNS.

Hippocampal synaptic connectivity in phenylketonuria

In humans, lack of phenylalanine hydroxylase (Pah) activity results in phenylketonuria (PKU), which is associated with the development of severe mental retardation after birth. The underlying mechanisms, however, are poorly understood. Mutations of the Pah gene in Pah(enu2)/c57bl6 mice result in elevated levels of phenylalanine in serum similar to those in humans suffering from PKU. In our study, long-term potentiation (LTP) and paired-pulse facilitation, measured at CA3-CA1 Schaffer collateral synapses, were impaired in acute hippocampal slices of Pah(enu2)/c57bl6 mice. In addition, we found reduced expression of presynaptic proteins, such as synaptophysin and the synaptosomal-associated protein 25 (SNAP-25), and enhanced expression of postsynaptic marker proteins, such as synaptopodin and spinophilin. Stereological counting of spine synapses at the ultrastructural level revealed higher synaptic density in the hippocampus, commencing at 3 weeks and persisting up to 12 weeks after birth. Consistent effects were seen in response to phenylalanine treatment in cultures of dissociated hippocampal neurones. Most importantly, in the hippocampus of Pah(enu2)/c57bl6 mice, we found a significant reduction in microglia activity. Reorganization of hippocampal circuitry after birth, namely synaptic pruning, relies on elimination of weak synapses by activated microglia in response to neuronal activity. Hence, our data strongly suggest that reduced microglial activity in response to impaired synaptic transmission affects physiological postnatal remodelling of synapses in the hippocampus and may trigger the development of mental retardation in PKU patients after birth.

Sexual dimorphism in estrogen-induced synaptogenesis in the adult hippocampus

It has long been known that estradiol influences synaptic plasticity in the female hippocampus. The density of dendritic spines varies during the estrous cycle and correlates positively with varying levels of estradiol in serum. In accordance, ovariectomy results in a loss of spines that can be rescued by estradiol treatment in animals, suggesting that estradiol originating from the ovaries induces spine formation in the hippocampus. More recent studies point to a role of hippocampus-derived estradiol in synaptogenesis in the female hippocampus, rather than of estradiol of ovarian origin. In our studies, we have shown that inhibition of hippocampal estrogen synthesis results in spine synapse loss in female animals and, more importantly, also in ovariectomized animals. Surprisingly, inhibition of local estradiol synthesis had no effect on synapse formation in males, in spite of a similar capacity to synthesize estradiol in male and female hippocampal neurons. In females, neuro-sexual steroid production is promoted by hypothalamic, cyclic GnRH release and likely underlies the estrus cyclicity of spine synapse density in the hippocampus. As a result, peripheral serum concentrations of estradiol determine the amount of estradiol synthesis in the hippocampus. This paradigm may also be true in males. In support of this hypothesis, we found that the content of estradiol in hippocampal tissue is higher in female compared to that in male animals, with low levels of estradiol in serum and tonic and acyclical GnRH release. In summary, our data point to important sex-specific differences in sexual steroid-induced synaptogenesis.

Endocrine regulation of estrogen synthesis in the hippocampus?

Estradiol synthesis in the ovaries is regulated via feedback mechanisms mediated by gonadotrophin-releasing hormone (GnRH) and gonadotrophins, secreted by the hypothalamus and the pituitary, respectively. Estradiol synthesis also takes place in the hippocampus. In hippocampal slice cultures of female animals, GnRH regulates estradiol synthesis dose-dependently. Hence, both hippocampal and ovarian estradiol synthesis are synchronized by GnRH. Hippocampus-derived estradiol is essential to synapse stability and maintenance because it stabilizes the spine cytoskeleton of hippocampal neurons. Inhibition of hippocampal estradiol synthesis in mice, however, results in loss of spines and spine synapses in females, but not in males. Stereotaxic application of GnRH to the hippocampus of female rats confirms the regulatory role of GnRH on estradiol synthesis and synapse density in the female hippocampus in vivo. This regulatory role of GnRH necessarily results in estrus cyclicity of spine density in the hippocampus of females.