Embryonic GABA(B) receptor blockade alters cell migration, adult hypothalamic structure, and anxiety- and depression-like behaviors sex specifically in mice

Neurons of the paraventricular nucleus of the hypothalamus (PVN) regulate the hypothalamic- pituitary-adrenal (HPA) axis and the autonomic nervous system. Females lacking functional GABA_B receptors because of a genetic disruption of the R1 subunit have altered cellular characteristics in and around the PVN at birth. The genetic disruption precluded appropriate assessments of physiology or behavior in adulthood. The current study was conducted to test the long term impact of a temporally restricting pharmacological blockade of the GABA_B receptor to a 7-day critical period (E11–E17) during embryonic development. Experiments tested the role of GABA_B receptor signaling in fetal development of the PVN and later adult capacities for adult stress related behaviors and physiology. In organotypic slices containing fetal PVN, there was a female specific, 52% increase in cell movement speeds with GABA_B receptor antagonist treatment that was consistent with a sex-dependent lateral displacement of cells in vivo following 7 days of fetal exposure to GABA_B receptor antagonist. Anxiety-like and depression-like behaviors, open-field activity, and HPA mediated responses to restraint stress were measured in adult offspring of mothers treated with GABA_B receptor antagonist. Embryonic exposure to GABA_B receptor antagonist resulted in reduced HPA axis activation following restraint stress and reduced depression-like behaviors. There was also increased anxiety-like behavior selectively in females and hyperactivity in males. A sex dependent response to disruptions of GABA_B receptor signaling was identified for PVN formation and key aspects of physiology and behavior. These changes correspond to sex specific prevalence in similar human disorders, namely anxiety disorders and hyperactivity.

Thyroid hormone-dependent development in Xenopus laevis: a sensitive screen of thyroid hormone signaling disruption by municipal wastewater treatment plant effluent

Because thyroid hormones (THs) are conserved modulators of development and physiology, identification of compounds adversely affecting TH signaling is critical to human and wildlife health. Anurans are an established model for studying disruption of TH signaling because metamorphosis is dependent upon the thyroid system. In order to strengthen this model and identify new gene transcript biomarkers for TH disruption, we performed DNA microarray analysis of Xenopus laevis tadpole tail transcriptomes following treatment with triiodothyronine (T(3)). Comparison of these results with previous studies in frogs and mammals identified 36 gene transcripts that were TH-sensitive across clades. We then tested molecular biomarkers for sensitivity to disruption by exposure to wastewater effluent (WWE). X. laevis tadpoles, exposed to WWE from embryo through metamorphosis, exhibited an increased developmental rate compared to controls. Cultured tadpole tails showed dramatic increases in levels of four TH-sensitive gene transcripts (thyroid hormone receptor β (TRβ), deiodinase type II (DIO2), and corticotropin releasing hormone binding protein (CRHBP), fibroblast activation protein α (FAPα)) when exposed to T(3) and WWE extracts. TRβ, DIO2, and CRHBP were identified as TH sensitive in other studies, while FAPα mRNA transcripts were highly TH sensitive in our array. The results validate the array and demonstrate TH-disrupting activity by WWE. Our findings demonstrate the usefulness of cross-clade analysis for identification of gene transcripts that provide sensitivity to endocrine disruption. Further, the results suggest that development is disrupted by exposure to complex mixes of compounds found in WWE possibly through interference with TH signaling.

GABA regulates corticotropin releasing hormone levels in the paraventricular nucleus of the hypothalamus in newborn mice

The paraventricular nucleus of the hypothalamus (PVN) is a major regulator of stress responses via release of corticotropin releasing hormone (CRH) to the pituitary gland. Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis is characteristic of individuals with major depressive disorder (MDD). Postmortem data from individuals diagnosed with MDD show increased levels of CRH mRNA and CRH immunoreactive neurons in the PVN. In the current study, an immunohistochemical (IHC) analysis revealed increased levels of CRH in the PVN of newborn mice lacking functional GABA(B) receptors. There was no difference in the total number of CRH immunoreactive cells. By contrast, there was a significant increase in the amount of CRH immunoreactivity per cell. Interestingly, this increase in CRH levels in the GABA(B) receptor R1 subunit knockout was limited to the rostral PVN. While GABAergic regulation of the HPA axis has been previously reported in adult animals, this study provides evidence of region-specific GABA modulation of immunoreactive CRH in newborns.

Identification and characterization of mesotocin and V1a-like vasotocin receptors in a urodele amphibian, Taricha granulosa

The cDNA sequences encoding the mesotocin receptor (MTR) and vasotocin 1a receptor (VTR-1a) were identified in a urodele amphibian, the rough-skinned newt, Taricha granulosa. Saturation binding of [(3)H]oxytocin (OT) to the Taricha MTR (tMTR) was best fit by a two-state model; a high affinity-low abundance site and a lower affinity-high abundance site. Competition-binding studies found the following rank-order affinities for the tMTR: mesotocin (MT)>OT≈vasotocin (VT)>vasopressin (VP)>isotocin (IT). Inositol phosphate (IP) accumulation studies demonstrated functional activity of both the tMTR and Taricha VTR-1a (tVTR-1a) in a heterologous cell culture system. The rank-order potencies for the tMTR were MT>OT>VT≈VP>IT. The combined binding and IP results indicate that VT may act as a partial agonist of the tMTR. Rank-order potencies for the tVTR-1a were VT>VP>MT≈OT>IT. For both receptors, stimulation of IP accumulation was blocked by d(CH(2))(5)[Tyr(Me)(2)]AVP (Manning compound) and d(CH(2))(5)[Tyr(Me)(2),Thr(4),Tyr-NH(2)]OVT (OTA). OTA was a more potent antagonist for the transiently expressed tMTR while Manning compound was relatively more potent at inhibiting IP accumulation in tVTR-1a expressing cells. In contradiction to earlier assumptions, the absolute IC(50) of Manning compound was lower for the tMTR (27nM±13) than the tVTR-1a (586nM±166) indicating its potential higher affinity for the tMTR, a finding with special relevance to interpretation of comparative studies investigating the behavioral and physiological actions of neurohypophysial peptides in non-mammalian species.

Identification of mesotocin and vasotocin nucleotide sequences in two species of urodele amphibian

We amplified and identified, for the first time in urodele amphibians, cDNA sequences that encode preprovasotocin (preproVT) and prepromesotocin (preproMT) from two distinct urodelian species, Taricha granulosa (the rough-skinned newt) and Plethodon shermanii (the spotted salamander). Each of these cDNA sequences encoded proteins that contained the characteristics of known neurohypophysial peptide precursors; each sequence consisting of (1) a signal peptide, (2) VT- or MT-like peptides, (3) neurophysin, and for the preproVTs, (4) copeptin. In T. granulosa, cDNA sequences encoded for the nine amino acids that define VT or MT. In P. shermani, cDNA sequences encoded for the VT peptide and a previously unidentified isoform of MT, ([Val(4)]-MT).

Cloning, heterologous expression and pharmacological characterization of a kappa opioid receptor from the brain of the rough-skinned newt, Taricha granulosa

A full-length cDNA that encodes a kappa (kappa) opioid receptor has been isolated from the brain of a urodele amphibian, the rough-skinned newt Taricha granulosa. The deduced protein contains 385 amino acids and possesses features commonly attributed to G protein-coupled receptors, such as seven putative transmembrane domains. The newt kappa receptor has 75% sequence identity to kappa opioid receptors cloned from mammals, and 66% sequence identity to the kappa opioid receptor reported for the zebrafish, with the greatest divergence in the extracellular N-terminus, the second and third extracellular loops and the intracellular C-terminus. Membranes isolated from COS-7 cells expressing the newt kappa receptor possessed a single, high-affinity (Kd = 1.5 nM) binding site for the kappa-selective agonist U69593. In competition binding assays, the expressed newt kappa receptor displayed high affinity for the kappa-selective agonists GR89696, dynorphin A(1-13), U69593, U50488 and BRL52537, as well as the kappa-selective antagonist nor-binaltorphimine and the non-selective antagonist naloxone. Rank order potencies and affinity constants were similar in competition binding studies that used either whole brain homogenates or membranes isolated from COS-7 cells expressing the newt kappa receptor. The expressed receptor displayed essentially no affinity for the delta-selective agonist DPDPE ([d-penicillamine, d-penicillamine]enkephalin), but showed moderate affinity for the mu-selective agonist DAMGO ([d-Ala-MePhe, Gly-ol]enkephalin) and moderately high affinity for nociceptin (orphanin FQ), the endogenous ligand for the opioid receptor-like (ORL)1 receptor. These findings support the conclusions that a gene for the kappa opioid receptor is expressed in amphibians and that the pharmacology of the newt kappa receptor closely matches mammalian kappa opioid receptors. However, the functional dichotomy between the classic opioid receptors (kappa, delta, mu) and ORL1 appears less strict in amphibians than in mammals.

Neuroanatomical distribution of vasotocin and mesotocin in two urodele amphibians (Plethodon shermani and Taricha granulosa) based on in situ hybridization histochemistry

Previous research suggests that considerable species-specific variation exists in the neuroanatomical distributions of arginine vasotocin (AVT) and mesotocin (MST), non-mammalian homologues of vasopressin and oxytocin. An earlier study in rough-skinned newts (Taricha granulosa) indicated that the neuroanatomical distribution of cells labeled for AVT-immunoreactivity (ir) was greater in this urodele amphibian than in any other species. It was unknown whether the widespread distribution of AVT-ir is unique to T. granulosa or a feature common among salamanders. Using in situ hybridization (ISH) histochemistry and gene-specific riboprobes, the current study labeled AVT and MST mRNA in T. granulosa and the red-legged salamander (Plethodon shermani). In T. granulosa, AVT ISH-labeled cells were found to be widespread and localized in brain areas including the dorsal and medial pallium, lateral and medial septum, bed nucleus of the stria terminalis, amygdala, preoptic area, ventral hypothalamus, nucleus isthmus, tectum mesencephali, inferior colliculus, and hindbrain. In P. shermani, the distribution of AVT ISH-labeled neurons matched that of T. granulosa, except in the lateral septum, ventral hypothalamus, and inferior colliculus, but did however include labeled cell bodies in the lateral pallium. The distribution of MST ISH-labeled cells was more restricted than AVT ISH labeling and was limited to regions of the preoptic area and ventral thalamus, which is consistent with the limited distribution of MST/OXY in other vertebrates. These findings support the conclusion that urodele amphibians possess a well-developed vasotocin system, perhaps more extensive than other vertebrate taxa.

A subset of kappa opioid ligands bind to the membrane glucocorticoid receptor in an amphibian brain

Previous studies demonstrated that a membrane receptor for glucocorticoids (mGR) exists in neuronal membranes from the roughskin newt (Taricha granulosa) and that this receptor appears to be a G protein-coupled receptor (GPCR). The present study investigated the question of whether this mGR recognizes nonsteroid ligands that bind to cognate receptors in the GPCR superfamily. To address this question, ligand-binding competition studies evaluated the potencies of various ligands to displace [3H]corticosterone (CORT) binding to neuronal membranes. Initial screening studies tested 21 different competitors and found that [3H]CORT binding was displaced only by dynorphin 1-13 amide (an endogenous kappa-selective opioid peptide), U50,488 (a synthetic kappa-specific agonist) and naloxone (a nonselective opioid antagonist). Follow-up studies revealed that the kappa agonists bremazocine (BRE) and ethylketocyclazocine (EKC) also displaced [3H]CORT binding to neuronal membranes, but that U69,593 (a kappa specific agonist) and nor-BNI (a kappa specific antagonist) were ineffective. The Ki values measured for the opioid competitors were in the subnanomolar to low micromolar range and had the following rank-order: dynorphin > U50,488 > naloxone > BRE > EKC. Because these ligands displaced, at most, only 70% of [3H]CORT specific binding, it appears that some [3H]CORT binding sites are opioid insensitive. Kinetic analysis of [3H]CORT off-rates in the presence of U50,488 and/or CORT revealed no differences in dissociation rate constants, suggesting that there is a direct, rather than allosteric, interaction with the [3H]CORT binding site. In summary, these results are consistent with the hypothesis that the high-affinity membrane binding site for [3H] CORT is located on a kappa opioid-like receptor.