Thyroid hormones regulate the frequency of miniature end‐plate currents in pre‐ and prometamorphic stages of the tadpole tail

Abstract

Thyroid hormones (THs), primarily 3,3',5-triiode-(L)-thyronine (T(3)), have been clearly established as natural inducers of apoptosis during metamorphosis of anuran embryos. We decided to use this phenomenon to test the hypothesis that, prior to genomic activation, T(3) has acute actions in the neuromuscular junction (NMJ) of the tail of amphibian embryos. We detected a dramatic increase in the production of miniature end-plate currents (MEPCs) 2-5 min after continuous application of T(3) (250 nM) using focal recordings under voltage clamp. Furthermore, this increase in the spontaneous release of neurotransmitter, evaluated by the MEPC frequency, was maintained for several hours. Reverse-T(3), the "inhibitory" form of THs, prevented this increase in MEPC frequency, suggesting that this is probably a highly specific action of T(3). In addition, the elevation in MEPC frequency induced by T(3) was unchanged in the presence or absence of extracellular calcium. The T(3)-mediated increase in MEPC frequency was blocked by niflumic acid, a nonsteroidal antinflammatory fenamate used to prevent the apoptotic volume decrease observed in many systems. The present study demonstrated that T(3) induces a remarkable nongenomic action in the NMJ of the tadpole tail at pre- and promatamorphic stages.

Triiodothyronine (T3) action on aquatic locomotor behavior during metamorphosis of the bullfrog Rana catesbeiana

“…This structural remodeling results from regulated apoptosis mediated through T 3 (Yaoita and Brown, 1990), and has been proposed is preceded by a loss of cell volume regulation. In a previous study, we demonstrated that T 3 acutely affects neuromuscular junction physiology in anuran larvae (Rojas, Bonilla, Báez, and Lasalde, 2003). We now consider how changes in cell volume during development might affect MEPC generation at the neuromuscular junction.…”

“…3B). Although there is general agreement on the fact that most of T 3 effects are via thyroid receptor regulation of nuclear target genes, a number of reports suggest non-genomic effects of thyroid hormones (Rojas, Bonilla, Báez, and Lasalde, 2003; Saelim, John, Wu, Park, Bai, et al , 2004). These non-genomic effects are of rapid onset (seconds to minutes), do not depend on intracellular binding of T 3 and nuclear thyroid receptors, and are independent of gene transcription and protein synthesis (Davis and Davis, 2002).…”

“…These rapid actions of T 3 are likely related to non-genomic actions, possibly involving an effect on spontaneous neurotransmitter release at the neuromuscular junctions of larval bullfrogs. Exposure of the neuromuscular junction of the tail to T 3 for as little as 3–5 min sharply increases the spontaneous release of neurotransmitter (Rojas, Bonilla, Báez, and Lasalde, 2003). In addition, exposure of larvae to T 3 , during 2 h or 24 h significantly reduced locomotor activity more in larvae with DI < 2.8, as compared to larvae with DI≥2.8, supporting the idea that T 3 has a differential physiological effect during anuran stages of development.…”

“…Indeed, most studies on the changing locomotion have focused on the gross morphology of the muscular structures rather than on the physiological characteristics of the involved muscles. Surging T 3 levels during metamorphosis have been shown to produce rapidly acting non-genomic effects (independent of gene transcription and protein synthesis) on the neuromuscular junctions of larval tail muscle (Rojas, Bonilla, Báez, and Lasalde, 2003). It is conceivable that T 3 might have a direct, rapidly acting effect on locomotor activity in larval Rana catesbeiana , and that this effect would be developmentally stage-specific.…”

“…THs are heavily implicated in this apoptosis during amphibian metamorphosis (Huang, Marsh-Armstrong and Brown, 1999; Yoshida, Okada, Kinoshita, Hara, Sasaki, et al , 2002; Brown et al , 2005). THs influence presynaptic activity at neuromuscular junctions during larval development (Rojas, Bonilla, Báez, and Lasalde, 2003), but the mechanism is not yet well understood, and any linkage back to TH-mediated apoptosis has not been studied. Since cell volume regulation is a primary target for apoptosis, we hypothesized that developing larvae would exhibit different sensitivities to disruptions in cell volume regulation according to the developmental stage.…”