Region-specific regulation of voltage-gated intrinsic currents in the developing optic tectum of theXenopustadpole

Abstract

Hamodi AS, Pratt KG. Region-specific regulation of voltagegated intrinsic currents in the developing optic tectum of the Xenopus tadpole. J Neurophysiol 112: 1644 -1655, 2014. First published July 2, 2014; doi:10.1152/jn.00068.2014.-Across the rostrocaudal (RC) axis of the Xenopus tadpole optic tectum exists a developmental gradient. This gradient has served as a useful model to study many aspects of synapse and dendrite maturation. To compliment these studies, we characterized how the intrinsic excitability, the ease in which a neuron can fire action potentials, might also be changing across the same axis. Whole-cell recordings from tectal neurons at different points along the RC axis revealed a graded increase in intrinsic excitability: compared with neurons at the caudal end of the tectum, neurons at the rostral end fired more action potentials in response to current injection and expressed greater peak Na ϩ and K ϩ currents, the major intrinsic currents in these neurons that underlie the action potential. We also observed, along the same axis and in the same direction, a previously described increase in the amount of synaptic drive received by individual neurons (Wu GY, Malinow R, Cline HT. Science 274: [972][973][974][975][976] 1996). Thus as synaptic activity ramps up across the RC axis, so does intrinsic excitability. The reduction of overall circuit activity induced a compensatory scaling up of peak Na ϩ and K ϩ currents only in the caudal portion of the tectum, suggesting a region-specific, compensatory form of plasticity. development; instrinsic currents; optic tectum; region specific; Xenopus tadpole AT THE CAUDAL EDGE OF THE optic tectum of the Xenopus tadpole is a proliferative zone where progenitor cells give rise to tectal neurons. At a rate that is dependent on levels of visually driven activity, these newborn neurons migrate rostrally out of the proliferative zone and into the tectum proper, where they get incorporated into the functioning retinotectal circuit (Sharma and Cline 2010). As new members of the retinotectal circuit, they begin receiving direct synaptic input from retinal ganglion cells of the contralateral eye, the lateral line (via the brain stem), as well as other tectal neurons. This ongoing incorporation of newborn neurons from the proliferative zone creates a spatial developmental gradient, in which the most immature neurons are localized at the caudal end of the tectum, and the most mature are localized to the rostral end (Cline 2001;Lazar 1973;Wu et al. 1996). Many aspects of neural development have been described across this rostrocaudal (RC) axis: tectal neuron dendrites become increasingly complex and arborized (Wu et al. 1999), glutamatergic synapses gain ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (Wu et al. 1996), AMPA receptors gain glutamate receptor 2 (GluR2) subunits (Aizenman et al. 2002), spontaneous synaptic frequency increases (Wu et al. 1996), and GABAergic transmission becomes increasingly hyperpolarizing (Khakhalin and Aizenman 2012)....

Preparations and Protocols for Whole Cell Patch Clamp Recording of <em>Xenopus laevis</em> Tectal Neurons

“…Current-voltage relationships (I-V curves) can be generated from the same neuron by stepping the neuron to increasingly depolarized potentials and recording the resulting voltage-activated Na + and K + currents, referred to as “mixed current recordings” (Figure 4D, 4E). It has been well established that for these amphibian neurons, peak Na + and K + currents can be quantified via mixed current recordings because the peaks do not temporally overlap with one another 4,7 .…”

“…With an electrophysiology rig outfitted with an amplifier that supports voltage- and current-clamp recording modes, whole cell patch clamp recordings allow a neuron’s electrophysiology to be characterized at high resolution. As a result, whole cell patch clamp recordings from tectal neurons across the key stages of retinotectal circuit formation have provided a detailed and comprehensive understanding of the development and plasticity of intrinsic 4,5,6,7 and synaptic 8,9,10,11 properties. Combining whole cell patch clamp tectal neuron recordings, the ability to express genes or morpholinos of interest in these neurons 12 , and a method to assess visual guided behavior via an established visual avoidance test 13 promotes the identification of links between molecules, circuit function, and behavior.…”

A population of gap junction-coupled neurons drives recurrent network activity in a developing visual circuit

“…A positive correlation between the maximum Na ϩ currents expressed by an individual neuron and its ability to fire action potentials has been observed in DL neurons (Hamodi and Pratt 2014;Pratt and Aizenman 2007). Thus we would predict that SL1 neurons would display a higher level of intrinsic excitability compared with DL neurons.…”

“…4A). In prior studies using this preparation, currents were pharmacologically isolated, and the inward component was determined to be mediated by a voltage-gated Na ϩ current, and the outward component by a voltage-gated K ϩ current (Aizenman et al 2003;Ciarleglio et al 2015;Hamodi and Pratt 2014), and it is likely that Na ϩ and K ϩ also mediate these currents in SL1 neurons, especially since the shape of the current (I)-voltage (V) curves for SL1 and DL neurons are quite similar (Fig. 4B).…”

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

“…In combination with this experimental asset, whole cell recordings of postsynaptic tectal neurons can be readily carried out in vivo to characterize responses to light or, alternatively, using the whole brain preparation that allows for the RGC and HB inputs to be activated in a controlled manner. As a result, the tadpole model has been used to study many aspects of neural circuit development including the maturation of synapses (Wu et al 1996) and intrinsic currents (Hamodi and Pratt 2014;Pratt and Aizenman 2007), functional refinement (Tao and Poo 2005), direction-selective plasticity (Engert et al 2002), visual acuity (Aizenman et al 2003;Dong et al 2009;Schwartz et al 2011), and spike-timing-dependent plasticity (Richards et al 2010;Zhang et al 1998). This work, however, does not address the neurons that lie beyond the readily accessible periventricular layer.…”