Maternal exposure to organophosphate flame retardants alters locomotor and anxiety-like behavior in male and female adult offspring.

Endocrine disrupting chemicals (EDCs) are chemicals found in our environment that interrupt typical endocrine function. Some flame retardants (FRs) are EDCs as shown in their interaction with steroid and nuclear receptors. Humans are consistently exposed to flame retardants as they are used in everyday items such as plastics, clothing, toys, and electronics. Polybrominated diphenyl ethers were used as the major FR until 2004, when they were replaced by organophosphate flame retardants (OPFRs). Previous research in rodent models utilizing a commercial flame retardant mixture containing OPFRs reported alterations in anxiety-like behavior in the elevated plus maze (EPM) for rodents perinatally exposed to OPFRs. In the present study we utilize wild-type mice maternally exposed (gestational day 7 to postnatal day 14) to either an OPFR mixture of tris(1,3-dichloro-2-propyl), triphenyl phosphate, and tricresyl phosphate or a sesame seed oil vehicle. These mice were evaluated for anxiety-like behavior in adulthood on the open field test (OFT) and the light/dark box (LDB) as well as the EPM. Outcomes from the OFT and LDB indicate that males and females maternally exposed to OPFRs exhibit altered locomotor activity. Results of the EPM were sex-specific as we did not observe an effect in females; however, effects in males differed depending on exposure condition. Males maternally exposed to OPFRs exhibited an anxiolytic-like phenotype in contrast to their vehicle counterparts. This effect in perinatally OPFR-exposed males was not due to alterations in locomotor activity. Our research illustrates that there are sex- and exposure-dependent effects of perinatal OPFR exposure on adult locomotor and anxiety-like behaviors in a mouse model.

Serotonin 5-HT2C receptor-mediated inhibition of the M-current in hypothalamic POMC neurons.

Hypothalamic proopiomelanocortin (POMC) neurons are controlled by many central signals, including serotonin. Serotonin increases POMC activity and reduces feeding behavior via serotonion [5-hydroxytryptamine (5-HT)] receptors by modulating K(+) currents. A potential K(+) current is the M-current, a noninactivating, subthreshold outward K(+) current. Previously, we found that M-current activity was highly reduced in fasted vs. fed states in neuropeptide Y neurons. Because POMC neurons also respond to energy states, we hypothesized that fasting may alter the M-current and/or its modulation by serotonergic input to POMC neurons. Using visualized-patch recording in neurons from fed male enhanced green fluorescent protein-POMC transgenic mice, we established that POMC neurons expressed a robust M-current (102.1 ± 6.7 pA) that was antagonized by the selective KCNQ channel blocker XE-991 (40 µM). However, the XE-991-sensitive current in POMC neurons did not differ between fed and fasted states. To determine if serotonin suppresses the M-current via the 5-HT(2C) receptor, we examined the effects of the 5-HT(2A)/5-HT(2C) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) on the M-current. Indeed, DOI attenuated the M-current by 34.5 ± 6.9% and 42.0 ± 5.3% in POMC neurons from fed and fasted male mice, respectively. In addition, the 5-HT(1B)/5-HT(2C) receptor agonist m-chlorophenylpiperazine attenuated the M-current by 42.4 ± 5.4% in POMC neurons from fed male mice. Moreover, the selective 5-HT(2C) receptor antagonist RS-102221 abrogated the actions of DOI in suppressing the M-current. Collectively, these data suggest that although M-current expression does not differ between fed and fasted states in POMC neurons, serotonin inhibits the M-current via activation of 5-HT(2C) receptors to increase POMC neuronal excitability and, subsequently, reduce food intake.

Genes associated with membrane-initiated signaling of estrogen and energy homeostasis.

During the reproductive cycle, fluctuations in circulating estrogens affect multiple homeostatic systems controlled by hypothalamic neurons. Two of these neuronal populations are arcuate proopiomelanocortin and neuropeptide Y neurons, which control energy homeostasis and feeding. Estradiol modulates these neurons either through the classical estrogen receptors (ERs) to control gene transcription or through a G protein-coupled receptor (mER) activating multiple signaling pathways. To differentiate between these two divergent ER-mediated mechanisms and their effects on homeostasis, female guinea pigs were ovariectomized and treated systemically with vehicle, estradiol benzoate (EB) or STX, a selective mER agonist, for 4 wk, starting 7 d after ovariectomy. Individual body weights were measured after each injection day for 28 d, at which time the animals were euthanized, and the arcuate nucleus was microdissected. As predicted, the body weight gain was significantly lower for EB-treated females after d 5 and for STX-treated females after d 12 compared with vehicle-treated females. Total arcuate RNA was extracted from all groups, but only the vehicle and STX-treated samples were prepared for gene microarray analysis using a custom guinea pig gene microarray. In the arcuate nucleus, 241 identified genes were significantly regulated by STX, several of which were confirmed by quantitative real-time PCR and compared with EB-treated groups. The lower weight gain of EB-treated and STX-treated females suggests that estradiol controls energy homeostasis through both ERalpha and mER-mediated mechanisms. Genes regulated by STX indicate that not only does it control neuronal excitability but also alters gene transcription via signal transduction cascades initiated from mER activation.

Emerging insights into hypothalamic-pituitary-gonadal axis regulation and interaction with stress signalling.

Reproduction and fertility are regulated via hormones of the hypothalamic-pituitary-gonadal (HPG) axis. Control of this reproductive axis occurs at all levels, including the brain and pituitary, and allows for the promotion or inhibition of gonadal sex steroid secretion and function. In addition to guiding proper gonadal development and function, gonadal sex steroids also act in negative- and positive-feedback loops to regulate reproductive circuitry in the brain, including kisspeptin neurones, thereby modulating overall HPG axis status. Additional regulation is also provided by sex steroids made within the brain, including neuroprogestins. Furthermore, because reproduction and survival need to be coordinated and balanced, the HPG axis is able to modulate (and be modulated by) stress hormone signalling, including cortiscosterone, from the hypothalamic-pituitary-adrenal (HPA) axis. This review covers recent data related to the neural, hormonal and stress regulation of the HPG axis and emerging interactions between the HPG and HPA axes, focusing on actions at the level of the brain and pituitary.

Oestrogen modulates hypothalamic control of energy homeostasis through multiple mechanisms.

The control of energy homeostasis in women is correlated with the anorectic effects of oestrogen, which can attenuate body weight gain and reduce food intake in rodent models. This review investigates the multiple signalling pathways and cellular targets that oestrogen utilises to control energy homeostasis in the hypothalamus. Oestrogen affects all of the hypothalamic nuclei that control energy homeostasis. Oestrogen controls the activity of hypothalamic neurones through gene regulation and neuronal excitability. Oestrogen's primary cellular pathway is the control of gene transcription through the classical oestrogen receptors (ERs) (ERalpha and ERbeta) with ERalpha having the primary role in energy homeostasis. Oestrogen also controls energy homeostasis through membrane-mediated events via membrane-associated ERs or a novel, putative membrane ER that is coupled to G-proteins. Therefore, oestrogen is coupled to at least two receptors with multiple signalling and transcriptional pathways to mediate immediate and long-term anorectic effects. Ultimately, it is the interactions of all the receptor-mediated processes in hypothalamus and other areas of the central nervous system that will determine the anorectic effects of oestrogen and its control of energy homeostasis.

Cross-talk between membrane-initiated and nuclear-initiated oestrogen signalling in the hypothalamus.

It is increasingly evident that 17beta-oestradiol (E(2)), via a distinct membrane oestrogen receptor (Gq-mER), can rapidly activate kinase pathways to have multiple downstream actions in central nervous system (CNS) neurones. We have found that E(2) can rapidly reduce the potency of the GABA(B) receptor agonist baclofen and mu-opioid receptor agonist DAMGO to activate G-protein-coupled, inwardly rectifying K(+) (GIRK) channels in hypothalamic neurones, thereby increasing the excitability (firing activity) of pro-opiomelanocortin (POMC) and dopamine neurones. These effects are mimicked by the membrane impermeant E(2)-BSA and a new ligand (STX) that is selective for the Gq-mER that does not bind to ERalpha or ERbeta. Both E(2) and STX are fully efficacious in attenuating the GABA(B) response in ERalpha, ERbeta and GPR 30 knockout mice in an ICI 182 780 reversible manner. These findings are further proof that E(2) signals through a unique plasma membrane ER. We have characterised the coupling of this Gq-mER to a Gq-mediated activation of phospholipase C leading to the up-regulation of protein kinase Cdelta and protein kinase A activity in these neurones, which ultimately alters gene transcription. Finally, as proof of principle, we have found that STX, similar to E(2), reduces food intake and body weight gain in ovariectomised females. STX, presumably via the Gq-mER, also regulates gene expression of a number of relevant targets including cation channels and signalling molecules that are critical for regulating (as a prime example) POMC neuronal excitability. Therefore, E(2) can activate multiple receptor-mediated pathways to modulate excitability and gene transcription in CNS neurones that are critical for controlling homeostasis and motivated behaviors.