The rate at which an operant is produced has often functioned as a fundamental measure of the efficacy of a reinforcer. Previous research has shown that operant behavior is typically organized into bouts implying that rate of responding is a composite of bout-initiation rate, within-bout response rate, and mean bout length. However, it is still unclear whether this organization of behavioral responses into bouts is a product of the motivational processes or a property that arises from the location of an organism in space. To test this proximity hypothesis, two-response sequences were intermittently reinforced: either pressing one lever twice (manipulandum proximal to response termination) or pressing each of two levers, located on either side of an operant chamber, once (manipulandum distal to response termination). In Experiment 1, rats were first trained to lever press for food on a VI schedule before being exposed to the alternation paradigm. Experiment 1 consisted of three phases. In Phase 1, food-deprived rats learned the alternation paradigm under a tandem variable time (VT) 150-s fixed-ratio (FR) 1 schedule of reinforcement. Phase 2 and 3 increased the FR requirement from 1 to 3 or 5 and removed food deprivation, respectively, to examine their effect on response-rate components. In Experiment 2, rats switched between trials consisting of pressing a single lever repeatedly or alternating between two levers for reward. Following stable behavior, lever pressing was extinguished in both trial types to the effect of extinction on response-rate components. Overall, behavioral bouts persisted under the alternation paradigm suggesting that they reflect motivational states and not just location. Additionally, bout-initiation rate decreased with increased response effort and decreased deprivation. Taken together, these results provide support for the use of response-bout analysis to evaluate the value of a reinforcer and its sensitivity to pharmacological manipulations.

Brain volume increases throughout early development in predictable patterns and is an important indicator of brain health. Hippocampal development and volume are related to many complex phenotypes, such as depression and anxiety. While basic brain development is genetically driven, environmental influences also influence individualized brain growth and regression. Epigenetics is one mechanism by which development is impacted by environment. Both animal and post-mortem human studies suggest that early life environments shape epigenetic regulation of genes involved in depression and anxiety in the hippocampus. Further, much research suggests that these environmentally driven changes in epigenetics are also reflected in buccal cells. However, little is known about the relationship between peripheral and brain epigenetics, especially in young and healthy cohorts. In an effort to close the gap between the peripheral epigenome and brain structure in a pediatric population, it was investigated whether DNA methylation (DNAm) levels of stress-related genes (NR3C1, FKBP5, and SLC6A4) measured in buccal cells predict hippocampal volume in a healthy, pediatric population (N = 255; females = 113; age range < 2 months – 14 years, Mage = 5.17, SDage = 3.61). Using multiple linear regression, results indicate that DNAm values across the whole gene and individual CpG sites of NR3C1, FKBP5, and SLC6A4 predict bilateral hippocampal volume. Results also indicate an age interaction such that the relationship between hippocampal volume and HPA gene DNAm is stronger in younger participants (0-6 years old), whereas serotonin transporter gene DNAm is stronger in older participants (6-14 years old). These results indicate that buccal DNAm of NR3C1, FKBP5, and SLC6A4 may be useful predictors of hippocampal volume early in development. These results validate the utility of peripheral epigenetics in the study of early life stress and brain structure. Further, these results emphasize the importance of considering developmental stages between which the relationship between brain and peripheral epigenetics may differ and highlight the possibility that diverse biological systems may be more or less relevant at different ages.