A Novel Methodology to Probe the Structural and Functional Correlates of Synaptic Plasticity

Dendritic spines are mushroom-shaped appendages on the dendritic branches of neurons. They are invaluable to the function of the brain as they form the major site for excitatory signal transmission in the mammalian brain. These ubiquitous structures have several invaluable and unique characteristics – namely that their morphological and functional characteristics are activity-dependent and undergo remodeling as the spine experiences stimulation. This activity-dependent regulation then in turn modulates the excitatory postsynaptic potential that propagates into the adjacent parent dendrite, and which ultimately reaches the somatic compartment. The mediation of this modulatory effect on the postsynaptic signal by dendritic spines renders them invaluable to the brain’s ability to change neuronal circuits as it learns. The relationship between the structural and functional change in dendritic spines as plasticity is induced remains poorly understood; while efforts have been made to examine the morphology of dendritic spines during plasticity as well as the change to receptor insertion on the postsynaptic density, a comprehensive methodology to interrogate the concomitant changes to several aspects of dendritic spine structure and function as plasticity occurs has not been established. In this study, such a methodology was developed in order to facilitate future study of how a dendritic spine’s diffusional neck resistance, head volume, calcium-sensitive channels (on the postsynaptic density), and excitatory postsynaptic potential amplitude change concurrently as the spine undergoes activity-dependent regulation. This activity-dependent regulation also occurs in groups of spines called “clusters” in vivo, and the structural and functional dynamics of spines as these groups are formed also remains unknown. In order to to facilitate future in vivo studies on how clustered dendritic spines may change dynamically in both structure and function, a methodology for surgically accessing and recording calcium-based activity from the primary auditory cortex was developed, as the frequency-specific tuning of dendritic spines in this cortical area forms a compelling environment in which to study the relationship between spine form and function.