Role of Early Acoustic Experience in Development of the Rat Primary Auditory Cortex

Abstract

Neocortical architecture is established by both intrinsic, genetic factors and experience- dependent factors. Postnatal sensory experience plays a significant role in the maturation and refinement of cortical sensory fields, such as the primary auditory cortex (A1). In this thesis, I investigated the effects of manipulating postnatal acoustic experience on the functional and morphological properties of neurons in the thalamocortical auditory pathway of adult rats. In Experiment 1, I used two converging electrophysiological techniques to determine the effects of patterned acoustic deprivation (through exposure to continuous, moderate-level white noise; cWN) on the functional properties of neurons in the central auditory system. In Experiment 2, I used Golgi-Cox staining to visualize morphological correlates of experience-dependent changes in neuron functioning. Long- and short-term plasticity mediate synaptic strengthening in sensory cortices in response to postnatal sensory experience. I assessed levels of long-term plasticity (using long- term potentiation; LTP) and short-term plasticity (using paired-pulse facilitation/depression; PPF/PPD) in vivo (under deep urethane anesthesia) in the A1 of normally reared rats and rats reared in the absence of patterned acoustic input through cWN exposure. Rats reared under cWN showed significantly greater LTP of field postsynaptic potentials (fPSPs) for thalamocortical, but not intracortical synapses in A1 compared to age-matched controls, indicative of immature, more plastic synaptic connectivity. Both groups showed similar, moderate levels of PPD (across interstimulus intervals ranging from 25 to 1000 ms) prior to LTP induction. Across groups, PPD was significantly enhanced after LTP induction, indicative of a presynaptic component of thalamocortical LTP in A1. I also assessed the morphology of layer II/III pyramidal neurons in A1 using Golgi-Cox staining and two-dimensional neuron reconstruction. Morphological features, including dendritic length, arbor complexity, and spine density, did not differ significantly between rats reared under cWN and age-matched controls. Rats reared under cWN showed a significantly greater proportion of filopodia to mature spines on apical dendrites compared to age-matched controls. Together, these data indicate that patterned acoustic experience results in a reduction of plasticity in A1, indicative of more mature, hard-wired synaptic connectivity. Furthermore, LTP in A1 in vivo is mediated in part by presynaptic mechanisms, such as increases in transmitter release probability at thalamocortical synapses.