Neuronal Intrinsic Physiology Changes During Development of a Learned Behavior

Figure 1. The developmental timeline and song circuit of zebra finches. A, Standard timeline for song development by juvenile male zebra finches. The sensorimotor stage begins around 35 dph. By ∼90 dph, a male’s song is largely crystalized, remaining stable for the duration of adult life. B, Example song spectrograms from experimental subjects. All birds were tutored to the same song pattern. C, An abridged sagittal schematic of the song network in the male zebra finch. Note that HVC contains two major populations of projection neurons (HVC_X neurons and HVC_RA neurons) and interneurons (HVC_INT). LMAN, lateral portion of the magnocellular nucleus of the anterior nidopallium; RA, robust nucleus of the arcopallium; DLM, medial dorsolateral nucleus of the thalamus; NCM, caudal medial nidopallium; NIF, forebrain nucleus interface of the nidopallium; CMM, caudal medial mesopallium; Av, nucleus avalanche; field L; Ov, ovoidalis; Uva, nucleus uvaeformis; MLd, dorsal lateral nucleus of the mesencephalon; nXIIIts, nucleus XII, tracheosyringeal part.

Electrophysiolgical trace analysis. A, Example from an adult HVC_X neuron. B, Example from an adult HVC_RA neuron. All traces were analyzed using an automated script that measured a variety of features for each voltage trace. See Materials and Methods for a full list of the features analyzed, along with a detailed explanation of measurment procedures. Many of the depolarizing features (e.g., spike ampltide, adaptation rate) were analyzed at a current injection of 200 pA (blue traces). Many hyperpolarizing features (e.g., SR, rebound depolarization) were analyzed at −200 pA (black traces).

HVC_X recordings and coresponding model traces. A, Voltage traces of HVC_X neurons recorded at subsong, plastic song, or adult stages of development. Features that showed developmental changes included an increase in the Sag ratio (solid arrow) and an increase in rebound depolarization that resulted in rebound spiking (dotted arrow). B, Model traces of the three neurons depicted in A. The models attribute the sag to the h-current and rebound firing to the T-type Ca²⁺ current.

Developmental changes in HVCx neurons. Each scatterplot depicts a physiologic feature of interest where each point represents measurement from a single neuron. Outlined circles represent features measured from the experimentaly recorded traces presented in Figure 3. Outlined squares indicate the measurments from the modeled traces presented in Figure 3. The means and SEMs are ploted over each data set. Legends for developmental changes that were statistically significant are indicated in green font while features that were developmentally stable are indicated in red.

HVC_RA recordings and coresponding model traces. A, Voltage traces of HVC_RA neurons recorded at subsong, plastic song, or adult stages of development. One of the more prominent changes observed across development was a systematic decrease in the resting potential of HVC_RA neurons. B, Modeled traces of the three neurons depicted in A. The models attributed the shift in resting potential to the leak current.

Developmental changes in HVC_RA neurons. Each scatterplot depicts a physiologic feature of interest where each point represents the measurement from a single neuron. Outlined circles represent features measured from the experimentaly recorded traces presented in Figure 5. Outlined squares indicate the measurments from the modeled traces presented in Figure 5. The means and SEMs are plotted over each data set. Legends for developmental changes that were statistically significant are indicated in green font while features that were developmentally stable are indicated in red.

Developmental variation in ion channel parameters relative to adult. Model parameters that differed from the adult cell model are listed along the x-axis. Parameters that did not change are not displayed but can be found in Table 2. All parameter values were manually fit to representative data from Figures 3, 5. In this plot, conductance (g) values were first normalized to the capacitance of the model neuron. All values are displayed as a ratio comparing the juvenile model parameters to the adult (on a logarithmic scale). A value of zero indicates no difference relative to the adult. A value >0 indicates an increase in the magnitude of the parameter, and a value <0 indicates a decrease in the magnitude of the parameter relative to the adult. The models predict that HVC_X and HVC_RA neurons undergo different changes in ionic conductances as a function of development.

HVC_INT neurons are comparatively stable across development. Voltage traces of HVC_INT neurons recorded at subsong, plastic song, or adult stages of development. As a population, HVC_INT neurons show variability in many of the measured features; however, the distributions of these measured features were relatively stable across developmental timepoints (Table 1). Cells from all three age groups show high firing frequencies, a large sag and rebound firing. The cells shown here provide examples of the variability observed in HCV_INT neurons, however, there was no consistent classifiable difference across groups for any of the features analyzed.