Functional neural networks underlying latent inhibition and the effects of the metabolic enhancer methylene blue
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The present research reports the first comprehensive map of brain networks underlying latent inhibition learning, the first application of structural equation modeling to cytochrome oxidase data, and the first effects of methylene blue, a known metabolic enhancer, on latent inhibition. In latent inhibition, repeated exposure to a stimulus results in a latent form of learning that inhibits subsequent associations with that stimulus. As neuronal energy demand to form learned associations changes, so does the induction of the respiratory enzyme cytochrome oxidase. Therefore, cytochrome oxidase can be used as an endpoint metabolic marker of the effects of experience on regional brain metabolic capacity. Quantitative cytochrome oxidase histochemistry was used to map brain regions in mice trained on a tone-footshock fear conditioning paradigm with either tone preexposure (latent inhibition), conditioning only (acquisition), conditioning followed by tone alone (extinction), or no handling or conditioning (naïve). In normal latent inhibition, the ventral cochlear nucleus, medial geniculate, CA1 hippocampus, and perirhinal cortex showed modified metabolic capacity due to latent inhibition. Structural equation modeling was used to determine the causal influences in an anatomical network of these regions and others thought to mediate latent inhibition, including the accumbens and entorhinal cortex. An uncoupling of ascending influences between auditory regions was observed in latent inhibition. There was also a reduced influence on the accumbens from the perirhinal cortex in both latent inhibition and extinction. These results suggest a specific network with a neural mechanism of latent inhibition that involves sensory gating, as evidenced by modifications in metabolic capacity, effective connectivity between auditory regions, and reduced hippocampal influence on the accumbens. The effects of methylene blue on disrupted latent inhibition were also investigated. Reduced tone-alone presentations disrupted the latent inhibition effect and led to an increase in freezing behavior. Repeated low-dose administration of methylene blue decreased freezing levels and facilitated the disrupted latent inhibition effect. Methylene blue administration also resulted in changes in metabolic capacity in limbic and cortical regions. A unique functional neural network was found in methylene blue-restored latent inhibition that emphasized sensory gating of auditory information, attention processing, and cortical inhibition of behavior.