The mammalian brain is arguably the most complex organ of any living being, and how its more than 100 billion neurons and several trillion connections among them develop remains largely a mystery. The PIs and other researchers have shown that one of the essential features of brain development is the occurrence of spontaneous waves of electrical activity that propagate across large structures in the brain and which serve to allow developing neurons to communicate with each other. The PI's laboratory uses optical and electrophysiological methods applied to the mouse brain to study how these spontaneous waves are initiated in the brain at the appropriate stages of development. The proposed experiments aim to show that specific populations of pacemaker neurons initiate these waves, and that these pacemaker populations change during early development. This change in pacemaker identity is thought to allow waves of activity to occur over longer periods of development than would be possible with a single pacemaker type. The expected results of these studies are to understand: (1) which neurons serve as pacemakers for spontaneous activity at each stage of development; (2) How the transition between pacemakers occurs; and (3) whether this transition involves a form of learning by the embryonic brain. Disruptions of spontaneous waves of activity in the human brain is likely to be the cause of many clinical abnormalities of brain development, and understanding how activity is initiated should allow us to gain insights into the basic mechanism involved in such ailments. This project will provide outstanding training opportunities for graduate students studying developmental neuroscience.
The development of the brain is regulated in part by electrical activity generated by neurons of the brain in the absence of any sensory inputs. This spontaneous electrical activity takes the form of propagating waves that move across large regions of the brain during critical period of early development. The actual patterns of propagation of these waves instructs several critical aspects of brain development, such as the strengthening of immature weak synaptic connections amongh neurons and the establishment of spatially ordered patterns of neuronal connections. It is therefore of critical importance of understand how these spontaneous waves of activity are regulated in terms of their frequency, the spatial patterns of their propagation, and the developmental stages at which they occur. We have studied spontaneous waves in living slices prepared from the developing mouse cerebral cortex. In the mouse brain, spontaneous waves are genereated about once per minute, starting on the day before birth and ending at the end of the first postnatal week. Our experiments have been designed to determine what cells of the brain are responsible for initiating waves, and what properties of those cells allow them to act as wave initiators. We have discovered that all cortical waves arise from the piriform cortex, a region in the ventral part of the developing brain which in the adult becomes olfactory cortex and other structures. When this region is surgically separated from the rest of the brain in living slice preparations, it continues to generate waves at the normal frequencies, but waves in all other regions are eliminated. We next asked what types of neurons in the piriform cortex initiate the waves. We found that at earlier stages (1 day before birth to 3 days after birth), neurons that use the neurotransmitter GABA are wave initiators. GABA is normally an inhibitory transmitter in the adult brain, but is excitatory early in development. However, between 3-8 days after birth, this GABA initiator ceases to function and waves are initiated by the other major neuron type, the glutamatergic neurons. These results were obtained by using specific blockers of GABA and glutamate action, and by genetically eliminating GABA from the brain. This indicates a complex and dynamic wave initiation system, where one initiator circuit operates at early stages, but then later gives way to a different initiating circuit. These results resolve several controversies in the current literature, and pave the way for us to study exactly how these neurons acquire and then lose their ability to initiate spotnaneous waves. In the next set of experiments, we asked how GABAergic neurons are specialized to initiate waves. We found that these neurons had two properties that distinguish them from other neurons at those stages. First, they generate spontaneous electrical activity between waves and second, they have intrinsic computational properties that prevent them from lessening their output responses and their inputs gradually increase in amplitude (‘gain scaling’). Both properties were measured directly in the neurons using optical and electrical recording, and then the measured parameters were used in mathematical models of circuit function to show that both can give rise to circuits that spontaneously initiate propagating waves of activity. These results show that the intrinsic electrical properties of developing neurons can give them the ability to initiate waves of activity, and thus regulate this important aspect of brain development. As a scientist who also teaches undergraduate neurobiology classes and directs a degree-granting undergraduate program in neurobiology, I am committed to involving undergraduate students in my research. During the past three years, I have mentored 8 undergraduate student in my laboratory while they carry out semi-independent research projects. Each of the experiments described above had significant participation of undergraduates in the work and, of the 8 students, 6 have been authors on peer-reviewed scientific papers. I have also welcomed two high-school students into the laboratory, one a minority student and the other a student with disabilities. Both of these students have participated in related projects, and I expect that both will become authors on papers in the near future.
