Assembly of neuronal circuits, neuronal replacement, and the cellular mechanisms of behavior.
Our laboratory is interested in the assembly of neuronal circuits and the mechanisms by which brain circuits give rise to behavior. We focus on the process of neuron addition into the brain of vertebrates, and seek to understand how new neurons integrate into the circuits of the adult brain, and their role in information processing and storage. To address these questions our laboratory develops new technologies to genetically manipulate the development and biophysical properties of neurons. To investigate how behavior arises from the activity of neurons in brain circuits, we are generating transgenic songbirds to manipulate key genes involved in the assembly of circuits that mediate vocal learning behavior.
Most neurons in the brain are born before birth and are never replaced. In contrast, certain populations of neurons are continuously replaced throughout the life of the animal. Do neurons acquired in adult life participate in a special form of memory storage that requires the replacement of old neurons? In mammals, neuronal replacement occurs at high levels in two brain areas be involved in olfactory perception and spatial memory. In songbirds, the capacity to learn their songs varies during adult life, and this variation is correlated to radical structural changes in the brain nuclei controlling song, which include massive neuronal replacement. Recently we have developed several new tools that allow us to genetically control the function of neurons. By using these techniques we are manipulating the birth, death, and electrical function of newly generated neurons in the brain of behaving animals, both in the olfactory system of mice, and in the song system of songbirds.
- Regulation of neuronal integration into brain circuits.
The brain of adult vertebrates harbors a population of neuronal stem cells that continues to proliferate throughout the life of the animal, and whose progeny migrate through the brain, differentiate into neurons, and establish synaptic contacts with other neurons in the circuit. We are interested in understanding the cellular and molecular mechanisms that control the integration of these neurons into neuronal circuits. We are currently testing the hypothesis that synaptic input into newly born adult neurons guides the integration of these cells into existing circuits. In addition, we are investigating the mechanisms that neurons use to adapt their intrinsic and synaptic properties as they integrate into circuits and communicate with other neurons. To study the role of electrical and synaptic activity on neuronal integration we have developed new tools to manipulate the biophysical properties of neurons by genetically modifying the activity of ion channels and neurotransmitter receptors.
- Genetic control of the assembly of circuits involved in vocal learning.
Vocal learning depends on the ability of brain circuits to perceive and imitate sound sequences and use these sequences for communication. Songbirds such as canaries and zebra finches have been a favorite experimental system for the study of vocal learning in animals for decades. These animals exhibit a robust and spontaneous vocal learning behavior, and they have dedicated brain circuits, known as the song system, that participate in the learning and production of song. Zebra finches listen to the songs that their fathers produce, and imitate these sounds until they acquire a stable adult-like song. In this respect, the time course and strategy of vocal learning in zebra finches is very similar to the manner in which human infants learn to speak. These observations suggest that the zebra finch could be an ideal system where to start investigating the genetic and biological basis of vocal learning. Recently, my laboratory has succeeded in the development of a series of techniques that allow us to genetically modify the brain of songbirds. These technical advances open new opportunities for the study of the relationship between genes and learning in an animal species with a robust behavioral repertoire. We are currently generating transgenic songbirds to manipulate key genes involved in the assembly of circuits involved in vocal learning behavior.
Figure 1: Genetic manipulation of the electrical properties of neurons.
Newly-generated neurons (green) in the hippocampus of adult mice are rendered hyperexcitable by delivering into them a voltage-gated channel via recombinant retroviruses. Enhanced excitability increases the number of inhibitory synapses (arrows) on the genetically modified neurons (green). By genetically controlling the electrical properties of neurons we investigate how neuronal activity regulates the integration of cells into brain circuits, and the connections between neurons.
Figure 2: Genetic labeling of neuronal circuits in transgenic animals
Transgenic mice were generated by random insertion of enhancer detector probes encoding a visible marker. In this transgenic line, clonally-related cells are organized in vertical columns of pyramidal neurons in the neocortex. These transgenic lines allow us to investigate the rules by which neurons assemble into circuits in the brain during development, and how they connect to each other.
Figure 3: Genetic labeling of synapsis.
The excitatory synaptic inputs (yellow) of newly-generated neurons in the olfactory bulb were genetically labeled by delivering a retrovirus encoding the postsynaptic marker psd95:GFP. The genetic labeling of synaptic proteins in vivo allows us to quantify the connections of neurons within a circuit.
Figure 4: Genetic manipulation in the brain of transgenic mice and songbirds
Our lab has developed several techniques to genetically manipulate the development and function of neurons during the assembly of neuronal circuits. We are using transgenic animals to investigate the rules by which neurons migrate, choose their final locations, and establish connections with each other. We are generating transgenic songbirds to investigate the genetic basis of the assembly of brain circuits involved in vocal communication.