Early Auditory Experience Modifies Neuronal Firing Properties in the Zebra Finch Auditory Cortex

Abstract

Songbirds learn to sing much as humans learn to speak. In zebra finches, one of the premier songbird models, males learn to sing for later courtship through a multistep learning process during the developmental period. They first listen to and memorize the song of a tutor (normally their father) during the sensory learning period. Then, in the subsequent sensory-motor learning phase (with large overlap), they match their vocalizations to the memorized tutor song via auditory feedback and develop their own unique songs, which they maintain throughout their lives. Previous studies have suggested that memories of tutor songs are shaped in the caudomedial nidopallium (NCM) of the brain, which is analogous to the mammalian higher auditory cortex. Isolation during development, which extends the sensory learning period in males, alters song preference in adult females, and NCM inactivation decreases song preference. However, the development of neurophysiological properties of neurons in this area and the effect of isolation on these neurons have not yet been explained. Here, we performed whole-cell patch-clamp recording on NCM neurons from juvenile zebra finches during the sensory learning period, 20, 40, or 60 days post-hatching (DPH) and examined their neurophysiological properties. In contrast to previous reports in adult NCM neurons, the majority of NCM neurons of juvenile zebra finches showed spontaneous firing with or without burst firing patterns, and the percentage of neurons that fired increased in the middle of the sensory learning period (40 DPH) and then decreased at the end (60 DPH) in both males and females. We further found that auditory isolation from tutor songs alters developmental changes in the proportions of firing neurons both in males and females, and also changes those of burst neurons differently between males that sing and females that do not. Taken together, these findings suggest that NCM neurons develop their neurophysiological properties depending on auditory experiences during the sensory song learning period, which underlies memory formation for song learning in males and song discrimination in females.

Keywords: auditory cortex; auditory experience; critical period; firing properties; song learning; songbird; zebra finch.

Figure 1

Caudomedial nidopallium (NCM) neurons with or without spontaneous firing. (A) A histogram of the number of NCM neurons plotted against their spontaneous firing rates (SFRs) (73 cells from 39 males and 74 cells from 39 females). Fifty-three neurons showed no spontaneous firing. Representative traces of neuronal activities from firing neurons with lower-SFR (B) and higher-SFR (C) and a silent neuron (D). Firing neurons were further divided into burst-type or non-burst-type neurons, based on interspike intervals (ISIs). Representative traces of burst-type firing in a with lower SFR (E) and higher-SFR (F) neuron. Inserts: expanded traces from the red square showing burst firing with firing at less than 30 ms ISI (horizontal scale bar, 200 ms; vertical scale bar, 10 mV).

Figure 2

The proportion of neuronal types changes during development. (A) Proportions of firing neurons recorded from normally raised male and female juveniles at 20, 40, and 60 days post-hatching (DPH). The number of neurons recorded is shown in parentheses (N = 5, 18, and 16 for males at 20, 40, and 60 DPH; 11, 14, and 14 for females at 20, 40, and 60 DPH birds, respectively). (B) Proportions of burst-type neurons recorded from normally raised male and female juveniles at 20, 40, or 60 DPH (N = 4, 17 and 12 for males at 20, 40, and 60 DPH; 10, 12, and 10 for females at 20, 40 and 60 DPH birds, respectively).

Figure 3

Types of neurons with an SFR and burst-type were not correlated with a location within the NCM (X² test). Anatomical locations of recorded neurons are color-coded with the neuronal type with SFR (A) or burst-type (B) from the male (left) or female (right) juveniles at 20 (top), 40 (middle), or 60 (bottom) DPH in a schematic including the NCM, 400–800 mm from the midline. HP, hippocampus; L, field L; LH, lamina hyperstriatica. B: The anatomical location of burst and non-burst-type NCM neurons.

Figure 4

mDlx-RFP-expressing neurons are higher firing neurons in males, but comprise a variety of types in females. (A) A DIC image of a representative slice of a patch-clamp recording from an mDlx-RFP-injected juvenile (left), the mDlx-RFP (middle), and merged (right). (B) Representative waveforms recorded from an mDlx-RFP-positive neuron. (C) Proportions of the three types of GABAergic neurons, separated based on SFR in male and female juveniles at 60 DPH.

Figure 5

Isolation alters developmental change in the proportions of the type of neurons. Proportions of firing neurons recorded from male and female isolated juveniles at 20, 40, and 60 DPH (A), and proportions of burst- and non-burst-type neurons (B) in isolated male and female juveniles at 20, 40, and 60 DPH.

Figure 6

Types of neurons with an SFR and burst type were not correlated with a location in the NCM in isolated juveniles (X² test). Anatomical locations of recorded neurons are color-coded with neuronal type with SFR (A) or burst type (B) from the male (left) or female (right) juveniles at 20 (top), 40 (middle), or 80 (bottom) DPH in a schematic including the NCM, 400–800 mm from the midline.