Evolution of the Corticotropin-releasing Hormone Signaling System and Its Role in Stress-induced Phenotypic Plasticity

Developing animals respond in variation in their habitats by altering their rules of development and/or their morphologies (i.e., they exhibit phenotypic plasticity). In vertebrates, one mechanism by which plasticity is expressed is through activation of the neuroendocrine system, which transduces environmental information into a physiological response. Recent findings of ours with amphibians and of others with mammals show that the primary vertebrate stress neuropeptide, corticotropin-releasing hormone (CRH), is essential for adaptive developmental responses to environmental stress. For instance, CRH-dependent mechanisms cause accelerated metamorphosis in response to pond-drying in some amphibian species, and intrauterine fetal stress syndromes in humans precipitate preterm birth. CRH may be a phylogenetically ancient developmental signaling molecule that allows developing organisms to escape deleterious changes in their larval/fetal habitat. The response to CRH is mediated by at least two different receptor subtypes and may also be modulated by a secreted binding protein.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73287/1/j.1749-6632.1999.tb07877.x.pd

Evidence that urocortin I acts as a neurohormone to stimulate alpha MSH release in the toad Xenopus laevis

Contains fulltext : 32483.pdf (publisher's version ) (Closed access)We have raised the hypothesis that in the South African clawed toad Xenopus laevis, urocortin 1 (UCN1), a member of the corticotropin-releasing factor (CRF) peptide family, functions not only within the brain as a neurotransmitter/neuromodulator but also as a neurohormone, promoting the release of alpha-melanophore-stimulating hormone (alphaMSH) from the neuroendocrine melanotrope cells in the intermediate lobe of the pituitary gland. This hypothesis has been investigated by (1) assessing the distribution of UCN1 and CRF by light immunocytochemistry, (2) determining the subcellular presence of UCN1 in the neural lobe of the pituitary gland by immuno-electron microscopy applying high-pressure freezing and cryosubstitution, and (3) testing the effect of UCN1 on MSH release from toad melanotrope cells using in vitro superfusion. In the X. laevis brain, the main site of UCN1-positive somata was found to be the Edinger-Westphal nucleus. UCN1 immunoreactivity (ir) also occurs in the nucleus posteroventralis tegmenti, central gray, nucleus reticularis medius, nucleus motorius nervi facialis, and nucleus motorius nervi vagi. UCN1 occurs together with CRF in the nucleus motorius nervi trigemini, and in the magnocellular nucleus, which send a UCN1- and CRF-containing fiber tract to the median eminence. Strong UCN1-ir and CRF-ir were found in the external zone of the median eminence. From the internal zone of the median eminence, UCN1-ir fibers, but few CRF-ir fibers, were found to project to the pituitary neural lobe, where they form numerous neurohemal axon terminals. Ultrastructurally, two types of terminal containing UCN1-ir secretory granules were distinguished: type A contains large, moderately electron-dense, round secretory granules and type B is filled with smaller, strongly electron-dense, ellipsoid secretory granules. In vitro superfusion studies showed that UCN1 stimulated the release of alphaMSH from melanotrope cells in a dose-dependent manner. Our results support the hypothesis that in X. laevis, UCN1 released from neurohemal axon terminals in the pituitary neural lobe functions as a stimulatory neurohormone for alphaMSH release from melanotrope cells of the pituitary intermediate lobe

The influence of visual and tactile stimulation on growth and metamorphosis in anuran larvae

1.   Sensory modalities that allow tadpoles to assess their environment, and subsequently mediate their development, are not well understood. 2.   By putting clay model tadpoles into the tanks with live tadpoles we have enhanced tactile and visual stimuli for tadpoles of three species ( Rana sylvatica , Bufo americanus and Xenopus laevis ) in a controlled fashion. The goal was to determine whether visual and tactile cues in the absence of chemical signals influenced tadpole growth and development. 3.   The response to enhanced visual and tactile stimuli was strong in Rana , intermediate in Xenopus , but absent in Bufo tadpoles. Rana tadpoles that experienced both stimuli enhanced developed the fastest and metamorphosed at the smallest body size. Development was slower in the treatments with only one stimulus enhanced, and slowest in the controls. 4.   Our results suggest that tadpoles use both vision and mechanoreception for environment assessment, and that they are able to modify their growth and developmental rates in response to sensory enrichment. 5.   Tadpoles exposed to the combination of visual and tactile stimulation showed the highest whole-body content of the stress hormone corticosterone, suggesting that the enhanced stimuli were experienced as stressful. Corticosterone is known to synergize with thyroid hormone to promote metamorphosis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74112/1/j.1365-2435.2005.01051.x.pd

An ontogenetic perspective on the relationship between age and size at maturity

1. Understanding the relationship between age and size at maturity is essential because these traits are pivotal determinants of an organism’s fitness. 2. The relationship between age and size is commonly addressed using optimization and quantitative genetic approaches. Here we argue that the value of such studies is often limited by an insufficient consideration of organismal ontogeny. 3. On the basis of a simple conceptual framework of hierarchical resource allocation, we identify key aspects of ontogeny that prove critical to a fuller understanding of the relationship between age and size, and which, to date, have been insufficiently explored. In particular, these include intrinsic variation in growth rate within and among populations, and the physiological nature of the maturation process that co-ordinates growth and reproductive function in an organism. 4. We also provide some guidance to the empirical investigation of these aspects, anticipating that a wider theoretical, but especially empirical appreciation of ontogenetic detail will greatly increase the explanatory and predictive power of life-history studies