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

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 GABA_A 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.The ongoing birth and incorporation of neurons into functional neural circuits in the central nervous system of higher vertebrates was first demonstrated conclusively in songbirds and subsequently in rodents, nonhuman primates, and humans (reviewed in ref. 1). Because new neuron generation continues throughout adulthood, the fundamental importance and the clinical implications of neurogenesis are clear. The mechanisms by which new neurons integrate into functional neural circuits, however, are not well understood.Songbirds provide a tractable model for understanding the mechanisms that regulate new neuron addition into functional circuits. Song is a learned sensorimotor behavior that is important for songbird reproduction. Song learning and production are regulated by a discrete, well-characterized neural circuit that includes HVC (proper name) and its target nucleus, the robust nucleus of the arcopallium (RA), both located in the avian forebrain (Fig. 1A) (2). In the adult Gambel’s white-crowned sparrow (WCS), the song control system shows extreme seasonal neuroplasticity (reviewed in ref. 3). Early in the breeding season, HVC and RA of WCS nearly double in volume. The increase in HVC volume results largely from an increase in new neuron incorporation, whereas the increase in RA volume results from increases in neuron size and spacing, but not number. RA neurons also show increased spontaneous electrical activity in the breeding season (4, 5). WCS typically produce only one song type that is longer and more stereotyped in structure during the breeding season (6, 7).Open in a separate windowFig. 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.HVC contains three types of neurons: HVC→RA and HVC→area X projection neurons and interneurons. During seasonal growth, most, if not all, neurons incorporated into HVC project to RA (ref. 8, but see ref. 9). Neural progenitor cells are born at the dorsal and ventral portion of the lateral ventricle and migrate from the ventricular zone (VZ) to HVC within 1 wk after birth (10). Over the next 2–3 wk, new neurons send axonal projections to RA (11). These new HVC→RA projection neurons are functional; they can fire action potentials in response to sound stimuli (12).Environment and experience play important roles in both brain development and adult neurogenesis. For example, during embryonic development, neural activity in the visual cortex is required for target selection by axons from the lateral geniculate nucleus (13). In adult rodents, voluntary exercise and hippocampal-dependent learning enhance neurogenesis in the dentate gyrus (reviewed in ref. 1). In adult songbirds, auditory experience and song production influence neuronal turnover in HVC (14–16). This literature suggests that adult neurogenesis in the vertebrate brain is activity-dependent. In vitro studies show that excitatory stimuli act directly on hippocampal neural progenitor cells and promote survival of new neurons (reviewed in ref. 17). Thus, activity-dependent mechanisms likely influence neuronal recruitment in vivo in both developing and adult brains.One factor that may influence the recruitment to and survival of new neurons in HVC is the electrical activity of their postsynaptic targets in RA (5). We hypothesized that inhibiting the electrical activity in RA neurons in vivo would reduce neuronal addition to adult HVC. We show that decreasing RA electrical activity does indeed reduce neuronal addition to HVC, indicating that target activity is essential for appropriate neuronal addition to HVC.