Environmental enrichment increases response strength and paired-pulse depression of auditory cortex neurons

The anatomical and neurochemical consequences of enrichment on information processing have not been studied in auditory cortex. Understanding the direction of the physiologic plasticity associated with enrichment will facilitate the development of therapies tailored to specific clinical populations with auditory processing disorders. The experiments within this dissertation provide the first evidence of evoked potential plasticity in auditory cortex with an enrichment paradigm, and describe nine significant findings about the influence of housing conditions on awake rat auditory cortical evoked potentials. In chapters two and three, we recorded responses to repeated tones and noise bursts to document the time-course of changes in response strength and paired-pulse depression. In chapter four, rats were housed in conditions designed to isolate several environmental factors that may have contributed to the expression of enrichment-induced plasticity. Other rats received immunotoxic cholinergic lesions to determine if enrichment-induced plasticity is modulated by acetylcholine input from nucleus basalis. Collectively, the conclusions of this dissertation are: (1) Enrichment increases noise and tone-evoked response strength; however, (2) greater plasticity effects can be observed with narrowband stimuli that do not elicit saturated responses. (3) Housing condition increased or decreased cortical responses in less than 2 weeks in young and adult rats. (4) There was no difference in the scale of plasticity between adolescent and sexually mature rats. (5) Paired-pulse depression also increased or decreased depending on current housing condition (6) Enrichment also increased response strength and forward masking in rats with substantial cholinergic damage. (7) Rats that could hear the enriched environment, but not interact with it, exhibited increased tone-evoked responses and paired-pulse depression. (8) Social housing and exercise experienced in isolation had no effect on response strength. (9) Rats housed in a social environment had more paired-pulse depression than rats housed with a running wheel, but were not different than rats passively exposed to the sounds associated with the enriched environment. These results indicate that sensory experience modulates response strength and forward masking in the auditory cortex along a continuum of environmental experiences. The implications of these results are relevant to several clinical disorders, including autism, schizophrenia, and dyslexia.