Postsynaptic neural activity regulates neuronal addition in the adult avian song control system

Abstract

A striking feature of the nervous system is that it shows extensive plasticity of structure and function that allows animals to adjust to changes in their environment. Neural activity plays a key role in mediating experience-dependent neural plasticity and, thus, creates a link between the external environment, the nervous system, and behavior. One dramatic example of neural plasticity is ongoing neurogenesis in the adult brain. The role of neural activity in modulating neuronal addition, however, has not been well studied at the level of neural circuits. The avian song control system allows us to investigate how activity influences neuronal addition to a neural circuit that regulates song, a learned sensorimotor social behavior. In adult white-crowned sparrows, new neurons are added continually to the song nucleus HVC (proper name) and project their axons to its target nucleus, the robust nucleus of the arcopallium (RA). We report here that electrical activity in RA regulates neuronal addition to HVC. Decreasing neural activity in RA by intracerebral infusion of the GABAA receptor agonist muscimol decreased the number of new HVC neurons by 56%. Our results suggest that postsynaptic electrical activity influences the addition of new neurons into a functional neural circuit in adult birds.

Keywords: birdsong; songbird; testosterone.

Fig. 1.

Inhibition of RA neural activity by muscimol infusion decreases HVC neuronal addition. Birds were injected with BrdU to label adult-born neurons, and muscimol, a GABA_A receptor agonist, was infused unilaterally near RA to inhibit its neural activity. (A) A coronal schematic of the song-control system showing major song system projections. Black arrows show projections in the motor output circuit; red arrows show bilateral projections; blue arrows show recursive projections; and green dashed arrow shows a weak projection from RA to HVC. (B) Experimental timeline. (C) A representative image of BrdU and Hu immunolabeling in HVC. BrdU is shown in red, and Hu, a neuronal marker, is shown in green. Arrows show BrdU-positive neurons. The arrowhead indicates a BrdU-positive cell that does not colabel with Hu. (D) The number of BrdU-positive neurons in HVC at time of death. The number of Hu- and BrdU-colabeled neurons in HVC is significantly lower in the hemisphere ipsilateral to muscimol infusion than in the contralateral, uninfused hemisphere and is lower than either hemisphere in vehicle-infused controls. *P ≤ 0.05. Contralateral hemispheres are shown in light gray, and ipsilateral are in black. All data are presented as mean ± SEM.

Fig. 2.

Reduction in HVC neuronal addition is not an effect of cannula placement or muscimol diffusion. (A) FCM diffusion in HVC. D, dorsal; M, medial. White arrows outline HVC. FCM is shown in red; Hoechst 33342, a nuclear marker, is shown in blue. Muscimol did not diffuse to HVC. (B) The diffusion of muscimol–TMR in RA. White arrows outline RA. The FCM area of diffusion is outlined with a dotted line. The cannula did not damage RA, and muscimol diffused to most of RA. (C) Representative raw electrophysiological trace recordings from ipsilateral and contralateral HVC and RA. Electrical activity was suppressed by muscimol infusion in ipsilateral RA (thus, no spike trace shown), but not ipsilateral HVC. (D) The spontaneous firing rates in HVC and RA both ipsilateral and contralateral to muscimol infusion. Spontaneous firing in HVC does not differ between HVC ipsilateral and contralateral to muscimol infusion (see Table S1 for values), further suggesting that muscimol did not diffuse to HVC. Muscimol completely suppressed activity in ipsilateral, but not contralateral, RA (Table S1).

Fig. 3.

Muscimol infusion transiently alters song behavior. (A) Representative sonograms of songs produced by the same birds before and during unilateral infusion of muscimol or vehicle into RA. (B) Whole song pitch of birds infused with vehicle and muscimol. (C) Whole song entropy of birds infused with vehicle and muscimol. Pitch and entropy of whole songs produced 14 d after onset of muscimol infusion were significantly different compared with songs produced before and 21 d after infusion of muscimol (all comparisons, n = 7, P < 0.05; see Table S3 for values). Pitch and entropy of songs recorded before experiment and 21 d after infusion onset were not significantly different between vehicle- and muscimol-infused birds (all comparisons, n = 10, P > 0.1; Tables S3 and S4). Preexperiment song is shown in light gray; song recorded 14 d after infusion onset is in dark gray; and 21 d after infusion onset is in black. *P < 0.05.