Julie A. Chowen

Gonadal hormones as promoters of structural synaptic plasticity: Cellular mechanisms

It is now obvious that the CNS is capable of undergoing a variety of plastic changes at all stages of development. Although the magnitude and distribution of these changes may be more dramatic in the immature animal, the adult brain retains a remarkable capacity for undergoing morphological and functional modifications. Throughout development, as well as in the postpubertal animal, gonadal steroids exert an important influence over the architecture of specific sex steroid-responsive areas, resulting in sexual dimorphisms at both morphological and physiological levels. We are only now beginning to gain insight into the mechanisms involved in gonadal steroid-induced synaptic changes. The number of synaptic inputs to specific neuronal populations is sexually dimorphic and this can be modulated by changes in the sex steroid environment. These modifications can be correlated with other morphological changes, such as glial cell activation, that are occurring simultaneously in the same anatomical area. Indeed, the close physical relationship between glial cells and neuronal synaptic contacts makes them an ideal candidate for participating in this process. Interestingly, not only can the morphology and immunoreactivity of glial cells be modulated by gonadal steroids, but a close negative correlation between the number of synapses and the amount of glial ensheathing of a neuron has been demonstrated, suggesting an active participation of these cells in this process. Glia have sex steroid receptors, are capable of producing and metabolizing steroids, and can produce other neuronal trophic factors in response to sex steroids. Hence, their role in gonadal steroid-induced synaptic plasticity is becoming more apparent. In addition, there is recent evidence that this process may involve certain cell surface molecules, such as the N-CAMs, since a specific isoform of this molecule, previously referred to as the embryonic form, is found in those areas of the brain which maintain the capacity to undergo synaptic remodelling. However, there is much work to be done in order to fully understand this phenomenon and before bringing it into a clinical setting in hopes of treating neurodegenerative diseases or injuries to the nervous system.

Role of astroglia in estrogen regulation of synaptic plasticity and brain repair

Astroglia are targets for estrogen and testosterone and are apparently involved in the action of sex steroids on the brain. Sex hormones induce changes in the expression of glial fibrillary acidic protein, the growth of astrocytic processes, and the degree of apposition of astroglial processes to neuronal membranes in the rat hypothalamus. These changes are linked to modifications in the number of synaptic inputs to hypothalamic neurons. These findings suggest that astrocytes may participate in the genesis of androgen-induced sex differences in synaptic connectivity and in estrogen-induced synaptic plasticity in the adult brain. Astrocytes and tanycytes may also participate in the cellular effects of sex steroids by releasing neuroactive substances and by regulating the local accumulation of specific growth factors, such as insulin-like growth factor-I, that are involved in estrogen-induced synaptic plasticity and estrogen-mediated neuroendocrine control. Astroglia may also be involved in regenerative and neuroprotective effects of sex steroids, since astroglia formation after brain injury or after peripheral nerve axotomy is regulated by sex hormones. Furthermore, the expression of aromatase, the enzyme that produces estrogen, is induced de novo in astrocytes in lesioned brain areas of adult male and female rodents. Since astroglia do not express aromatase under normal circumstances, the induction of this enzyme may be part of the program of glial activation to cope with the new conditions of the neural tissue after injury. Given the neuroprotective and growth-promoting effects of estrogen after injury, the local production of this steroid may be a relevant component of the reparative process.

Estradiol upregulates Bcl-2 expression in adult brain neurons

BCL-2, a protein which negatively modulates apoptosis, is up-regulated by estrogen in several tissues. To determine the effect of estradiol on Bcl-2 in the adult brain, its immunoreactive distribution was examined in the hypothalamic arcuate nucleus of female rats under different endocrine conditions. The number of Bcl-2–immunoreactive neurons was significantly increased (p< 0.001) on the day of estrus compared with proestrus, diestrus and metestrus, was decreased by ovariectomy and showed a dose–response increase after estradiol administration to ovariectomized rats. Progesterone, when injected simultaneously with estradiol, reduced the effect of estradiol. These findings indicate that ovarian hormones regulate Bcl-2 in hypothalamic neurons and suggest that this protein may be involved in the neuro-protective effects of estrogen.

Colocalization of glucagon-like peptide-1 (GLP-1) receptors, glucose transporter GLUT-2, and glucokinase mRNAs in rat hypothalamic cells : Evidence for a role of GLP-1 receptor agonists as an inhibitory signal for food and water intake

Abstract: This study was designed to determine the possible role of brain glucagon‐like peptide‐1 (GLP‐1) receptors in feeding behavior. In situ hybridization showed colocalization of the mRNAs for GLP‐1 receptors, glucokinase, and GLUT‐2 in the third ventricle wall and adjacent arcuate nucleus, median eminence, and supraoptic nucleus. These brain areas are considered to contain glucose‐sensitive neurons mediating feeding behavior. Because GLP‐1 receptors, GLUT‐2, and glucokinase are proteins involved in the multistep process of glucose sensing in pancreatic β cells, the colocalization of specific GLP‐1 receptors and glucose sensing‐related proteins in hypothalamic neurons supports a role of this peptide in the hypothalamic regulation of macronutrient and water intake. This hypothesis was confirmed by analyzing the effects of both systemic and central administration of GLP‐1 receptor ligands. Acute or subchronic intraperitoneal administration of GLP‐1 (7–36) amide did not modify food and water intake, although a dose‐dependent loss of body weight gain was observed 24 h after acute administration of the higher dose of the peptide. By contrast, the intracerebroventricular (i.c.v.) administration of GLP‐1 (7–36) amide produced a biphasic effect on food intake characterized by an increase in the amount of food intake after acute i.c.v. delivery of 100 ng of the peptide. There was a marked reduction of food ingestion with the 1,000 and 2,000 ng doses of the peptide, which also produced a significant decrease of water intake. These effects seemed to be specific because i.c.v. administration of GLP‐1 (1–37), a peptide with lower biological activity than GLP‐1 (7–36) amide, did not change feeding behavior in food‐deprived animals. Exendin‐4, when given by i.c.v. administration in a broad range of doses (0.2, 1, 5, 25, 100, and 500 ng), proved to be a potent agonist of GLP‐1 (7–36) amide. It decreased, in a dose‐dependent manner, both food and water intake, starting at the dose of 25 ng per injection. Pretreatment with an i.c.v. dose of a GLP‐1 receptor antagonist [exendin (9–39); 2,500 ng] reversed the inhibitory effects of GLP‐1 (7–36) amide (1,000 ng dose) and exendin‐4 (25 ng dose) on food and water ingestion. These findings suggest that GLP‐1 (7–36) amide may modulate both food and drink intake in the rat through a central mechanism.

Synaptic input organization of the melanocortin system predicts diet-induced hypothalamic reactive gliosis and obesity.

The neuronal circuits involved in the regulation of feeding behavior and energy expenditure are soft-wired, reflecting the relative activity of the postsynaptic neuronal system, including the anorexigenic proopiomelanocortin (POMC)-expressing cells of the arcuate nucleus. We analyzed the synaptic input organization of the melanocortin system in lean rats that were vulnerable (DIO) or resistant (DR) to diet-induced obesity. We found a distinct difference in the quantitative and qualitative synaptology of POMC cells between DIO and DR animals, with a significantly greater number of inhibitory inputs in the POMC neurons in DIO rats compared with DR rats. When exposed to a high-fat diet (HFD), the POMC cells of DIO animals lost synapses, whereas those of DR rats recruited connections. In both DIO rats and mice, the HFD-triggered loss of synapses on POMC neurons was associated with increased glial ensheathment of the POMC perikarya. The altered synaptic organization of HFD-fed animals promoted increased POMC tone and a decrease in the stimulatory connections onto the neighboring neuropeptide Y (NPY) cells. Exposure to HFD was associated with reactive gliosis, and this affected the structure of the blood-brain barrier such that the POMC and NPY cell bodies and dendrites became less accessible to blood vessels. Taken together, these data suggest that consumption of an HFD has a major impact on the cytoarchitecture of the arcuate nucleus in vulnerable subjects, with changes that might be irreversible due to reactive gliosis.

Endocrine Glia: Roles of Glial Cells in the Brain Actions of Steroid and Thyroid Hormones and in the Regulation of Hormone Secretion

The development and functioning of the nervous system are known to be influenced in various ways by endocrine signals. In turn, neural tissue modulates internal homeostasis, not only by electrical signaling, but also by regulating the release of endocrine messengers. However, the mechanisms underlying these processes are not fully understood. Recent evidence indicates that glia may play a significant role in the link between the endocrine and nervous systems. Glial cells express nuclear receptors for both thyroid and steroid hormones and participate in the metabolism of these hormones, resulting in the production of neuroactive metabolites. Furthermore, glial cells synthesize endogenous neuroactive steroids, including pregnenolone and progesterone, from cholesterol. Thyroid hormones, glucocorticoids, gonadal steroids, and neurosteroids affect myelinization by acting on oligodendroglia and modulate astroglia morphology, differentiation, and gene expression in different brain areas. Under physiological conditions, hormonal effects on glia may have important consequences for neuronal development, metabolism, and activity and for the formation and plasticity of synaptic connections. In addition, glucocorticoids, gonadal steroids, and neurosteroids may affect regenerative processes in neurons by modulating glial responses after injury. These effects include the activation of microglia, which is regulated by glucocorticoids, and the proliferation of reactive astroglia, which is regulated by gonadal hormones and neurosteroids. Glial cells are also involved in the modulation of hormone release. Pituicytes and microglia in the neurohypophysis may influence hormonal secretion by regulating neurovascular contacts, while astroglia in the hypothalamus regulate the number of synaptic inputs to specific neuronal populations involved in pituitary hormone release, such as LHRH and oxytocinergic neurons. Furthermore, tanycytes and astrocytes in the arcuate nucleus and median eminence release trophic factors that regulate hormone secretion by hypothalamic neurons.

Expression of the glucagon-like peptide-1 receptor gene in rat brain

Abstract: Evidence that glucagon‐like peptide‐1 (GLP‐1) (7–36) amide functions as a novel neuropeptide prompted us to study the gene expression of its receptor in rat brain. Northern blot analysis showed transcripts of similar size in RINm5F cells, hypothalamus, and brainstem. First‐strand cDNA was prepared by using RNA from hypothalamus, brainstem, and RINm5F cells and subsequently amplified by PCR. Southern blot analysis of the PCR products showed a major 1.4‐kb band in all these preparations. PCR products amplified from hypothalamus were cloned, and the nucleotide sequence of one strand was identical to that described in rat pancreatic islets. In situ hybridization studies showed specific labeling in both neurons and glia of the thalamus, hypothalamus, hippocampus, primary olfatory cortex, choroid plexus, and pituitary gland. In the hypothalamus, ventromedial nuclei cells were highly labeled. These findings indicate that GLP‐1 receptors are actually synthesized in rat brain. In addition, the colocalization of GLP‐1 receptors, glucokinase, and GLUT‐2 in the same areas supports the idea that these cells play an important role in glucose sensing in the brain.

The expression of GLP‐1 receptor mRNA and protein allows the effect of GLP‐1 on glucose metabolism in the human hypothalamus and brainstem

In the present work, several experimental approaches were used to determine the presence of the glucagon‐like peptide‐1 receptor (GLP‐1R) and the biological actions of its ligand in the human brain. In situ hybridization histochemistry revealed specific labelling for GLP‐1 receptor mRNA in several brain areas. In addition, GLP‐1R, glucose transporter isoform (GLUT‐2) and glucokinase (GK) mRNAs were identified in the same cells, especially in areas of the hypothalamus involved in feeding behaviour. GLP‐1R gene expression in the human brain gave rise to a protein of 56 kDa as determined by affinity cross‐linking assays. Specific binding of 125I‐GLP‐1(7–36) amide to the GLP‐1R was detected in several brain areas and was inhibited by unlabelled GLP‐1(7–36) amide, exendin‐4 and exendin (9–39). A further aim of this work was to evaluate cerebral‐glucose metabolism in control subjects by positron emission tomography (PET), using 2‐[F‐18] deoxy‐d‐glucose (FDG). Statistical analysis of the PET studies revealed that the administration of GLP‐1(7–36) amide significantly reduced (p < 0.001) cerebral glucose metabolism in hypothalamus and brainstem. Because FDG‐6‐phosphate is not a substrate for subsequent metabolic reactions, the lower activity observed in these areas after peptide administration may be due to reduction of the glucose transport and/or glucose phosphorylation, which should modulate the glucose sensing process in the GLUT‐2‐ and GK‐containing cells.

Leptin signaling in astrocytes regulates hypothalamic neuronal circuits and feeding

We found that leptin receptors were expressed in hypothalamic astrocytes and that their conditional deletion led to altered glial morphology and synaptic inputs onto hypothalamic neurons involved in feeding control. Leptin-regulated feeding was diminished, whereas feeding after fasting or ghrelin administration was elevated in mice with astrocyte-specific leptin receptor deficiency. These data reveal an active role of glial cells in hypothalamic synaptic remodeling and control of feeding by leptin.

Gonadal Hormone Regulation of Insulin-Like Growth Factor-I Like Immunoreactivity in Hypothalamic Astroglia of Developing and Adult Rats

The influence of gonadal steroids on insulin-like growth factor I (IGF-I)-like immunoreactivity was assessed in the rat arcuate nucleus, an area of the hypothalamus that regulates pituitary secretion. IGF-I-like immunoreactivity was observed in hypothalamic cells with the morphological aspects of tanycytes and astrocytes. The surface density of IGF-I-like immunoreactive glia increased with puberty in the arcuate nucleus of male and female rats, while decreasing with age in other brain areas. Gender differences in the surface density of IGF-I-like immunoreactive glia were detected in adult animals, with males and androgenized females having significantly higher values than normal females. In the latter, the surface density of IGF-I-like immunoreactive glia was increased in the afternoon of proestrus and in the morning of estrus compared to the morning of proestrus, diestrus and metestrus. In addition, IGF-I-like immunoreactivity showed a dose-dependent increase in ovariectomized rats injected with 17 beta-estradiol, but not in those receiving 17 alpha-estradiol. The effect of 17 beta-estradiol was blocked by simultaneous administration of progesterone, while this hormone alone had no effect. These results indicate that IGF-I-like immunoreactivity in arcuate glial cells is affected by the hormonal environment and suggest that IGF-I-like immunoreactive glia may be involved in neuroendocrine events within the hypothalamus.