Distinguished Lecture in Marine Neuroscience

The research goals of my laboratory are to understand cellular and molecular mechanisms by which neuroendocrine cells regulate changes in neuropeptide synthesis and secretion. The model system that we use to explore these issues is the neuroendocrine bag-cells of the marine mollusk Aplysia. The bag cell neurons (BCNs) secrete a 36-amino acid peptide hormone called egg-laying hormone (ELH) in response to a repetitive pattern of action potential firing called an electrical afterdischarge. ELH diffuses to the ovotestis and target sites in the central nervous system to trigger ovulation and egg-laying behavior. Because of the accessibility of the BCNs for electrophysiology and our laboratory's development of the only radioimmunoassay for measurement of ELH, we are in a unique position to monitor electrical and secretory activity simultaneously. This is not yet possible with most neuroendocrine systems in vertebrates because of the disperse location of those neurons. We have shown that although the afterdischarge triggers ELH secretion, significant release of hormone is maintained for tens of minutes following termination of action-potential firing. This finding suggests that, unlike in most other neurosecretory cells, action potentials do not drive neuropeptide release from bag cell neurons. Rather, they trigger some prolonged intracellular event(s) that maintains secretion 1-2 hours. What cellular events mediate the effects of the afterdischarge on prolonged ELH secretion? We are currently investigating the roles of calcium, protein kinase (PK) A and PKC. Our recent results show that action-potential firing leads to prolonged membrane depolarization and release of Ca2+ from intracellular stores. These events could play important roles in controlling neurohormone secretion - but do not fully account for the prolonged nature of ELH release. Furthermore, inhibiting PKA and PKC attenuates ELH secretion, suggesting roles for these kinases in regulating ELH secretion. And stimulation of an afterdischarge leads to rapid and prolonged activation of PKC, consistent with the hypothesis that PKC partly controls prolonged ELH release. We are also exploring the molecular mechanisms that mediate effects of membrane excitation on neuropeptide synthesis. Our recent findings show that activation of the afterdischarge stimulates a 2-fold increase in ELH synthesis; and this effect is through translation of already existing message, not through upregulated transcription of the ELH gene. Notably, transcription of a non-ELH gene plays an important role in mediating the effects of afterdischarge on ELH synthesis. Our most recent findings show that the 5' UTR of the ELH mRNA contains an internal ribosomal entry site (IRES) that gets activated in response to membrane excitability by a cap-independent mechanism, most likely leading to afterdischarge-induced ELH synthesis.

To learn more about Dr. Waynes' research click here.

ABSTRACT

Gonadotropin releasing hormone (GnRH) neurons are the command cells in the central nervous system (CNS) that control reproductive physiology and sexual behavior in all vertebrates studied to date. These neurons are born outside of the CNS and migrate into the forebrain during embryogenesis. Inappropriate migration of these neurons during development results in infertility in adulthood. Mechanisms that regulate GnRH neuronal migration and cell physiology are poorly understood because these neurons are relatively few in number and scattered in the hypothalamus and adjacent brain areas, making it unfeasible to study their biology in living tissue of normal animals. To overcome this obstacle, we generated a stable line of transgenic zebrafish in which the GnRH promoter drives expression of green fluorescent protein (GFP). This new animal model provides us with the unique opportunity to identify GnRH neurons for analyses of neuronal migration and electrical activity in the intact, living embryo. The aim of this work is to reveal the functional relationship between neuronal migration and spontaneous action potential firing in vivo. My collaborators and I are using a combination of experimental tools to understand the biology of GnRH neuronal migration: molecular biology to manipulate gene expression; optical imaging to monitor neuronal migration and axonal pathfinding; and whole-cell patch clamp electrophysiology to analyze electrical activity. We are at the beginning of this scientific adventure, and the data I will present are from new and unpublished studies.

Location :
The workshop will be held on the Rosenstiel School of Marine and Atmospheric Sciences campus in the Campus Auditorium of the Marine Science Center. Parking is available just outside of the main gate and is free to all visitors. If you have any questions, please contact the Neuroscience Program Coordinator at 305-243-3368 or send email to neurosci@med.miami.edu.

Sponsored by the Neuroscience Program and the Marine and Freshwater Biomedical Sciences Center of the University of Miami.