GABA Expression and Regulation by Sensory Experience in the Developing Visual System

Abstract

The developing retinotectal system of the Xenopus laevis tadpole is a model of choice for studying visual experience-dependent circuit maturation in the intact animal. The neurotransmitter gamma-aminobutyric acid (GABA) has been shown to play a critical role in the formation of sensory circuits in this preparation, however a comprehensive neuroanatomical study of GABAergic cell distribution in the developing tadpole has not been conducted. We report a detailed description of the spatial expression of GABA immunoreactivity in the Xenopus laevis tadpole brain at two key developmental stages: stage 40/42 around the onset of retinotectal innervation and stage 47 when the retinotectal circuit supports visually-guided behavior. During this period, GABAergic neurons within specific brain structures appeared to redistribute from clusters of neuronal somata to a sparser, more uniform distribution. Furthermore, we found that GABA levels were regulated by recent sensory experience. Both ELISA measurements of GABA concentration and quantitative analysis of GABA immunoreactivity in tissue sections from the optic tectum show that GABA increased in response to a 4 hr period of enhanced visual stimulation in stage 47 tadpoles. These observations reveal a remarkable degree of adaptability of GABAergic neurons in the developing brain, consistent with their key contributions to circuit development and function.

Experience-Dependent Bimodal Plasticity of Inhibitory Neurons in Early Development

“…These studies were conducted by randomly sampling tectal neurons without knowledge of neurotransmitter phenotype, and therefore likely reflect changes in the majority excitatory neuron population. Little is known about inhibitory neurons in the developing tectum, except that GABA is hyperpolarizing in the tadpole stages studied here (Akerman and Cline, 2006; Tao and Poo, 2005) and visual experience increases GABA levels (Miraucourt et al, 2012), consistent with other reports (Hendry and Jones, 1988; Kreczko et al, 2009; Morales et al, 2002; Sarro et al, 2008). In addition, electrophysiological recordings indicate that tectal neurons do not yet display characteristic features of mature inhibitory neurons, such as fast spiking, at these developmental stages (Ciarleglio et al, 2015).…”

“…Inhibitory and excitatory neurons in the optic tectum are generated in a common proliferative zone and integrate into the optic tectal circuit at the same time (Akerman and Cline, 2006; Muldal et al, 2014). About 30% of optic tectal neurons are GABA-immunoreactive (Antal, 1991; Miraucourt et al, 2012), comparable to other brain regions and species (Caputi et al, 2013). Prior work has shown that visual experience promotes the structural and functional development of tectal neurons and the connectivity required for visual information processing (Ruthazer and Aizenman, 2010).…”

“…To determine whether inhibitory and excitatory tectal neurons can be distinguished based on distinct dendritic arbor structure or their distribution in the tectum, we collected in vivo time-lapse images of GFP-expressing neurons at daily intervals over 3 days, followed by posthoc identification of inhibitory and excitatory neurons based on GABA immunolabeling (Miraucourt et al, 2012; Wu and Cline, 1998). We imaged tectal neurons 8-10 days after electroporating GFP-expressing constructs, when they were incorporated into the functional retinotectal circuit, and synaptic GABAergic currents are hyperpolarizing (Akerman and Cline, 2006).…”

Reversible developmental stasis in response to nutrient availability in theXenopus laevisCNS

“…We defined stage 46 based on the NF criteria of having two to 2.5 turns of the intestine, slight rounding of the operculum edges, and the presence of the cement gland as a dominant anterior feature in the head (Nieuwkoop and Faber, 1956). Stage 47 encompasses considerable morphological and behavioral changes (Gilbert and Frieden, 1981;Karpinka et al, 2015;McKeown et al, 2013;Miraucourt et al, 2012;Nieuwkoop and Faber, 1956;Sharma and Cline, 2010;Shen et al, 2011), so we further divided stage 47 into early and late substages (Fig. 1B).…”

The horizontal brain slice preparation: a novel approach for visualizing and recording from all layers of the tadpole tectum

“…By developmental stage 49 (ϳ16 dpf), the tectal neuron somata have organized into six compact somatic layers and have begun to display a range of different morphologies (Lazar 1973). Furthermore, a detailed immunohistochemical study indicates that even before developmental stage 49, the neurons across the tectum can be categorized as expressing either GABA or CaMKII and that the spatial pattern of GABA-expressing neurons has redistributed from clustered to being more evenly dispersed throughout the somatic layers (Miraucourt et al 2012). Although these morphological and immunohistochemical studies indicate that, even at relatively early developmental stages, tectal neurons are differentiating and reorganizing, the electrophysiology of the neurons beyond the deepest, most periventricular somatic layer, henceforth referred to as the "deep-layer" neurons, has not been described.…”