Using pentobarbital to assess the sensitivity and independence of response-bout parameters in two mouse strains.

A recently developed model posits that a bout of operant responding comprises three different components: bout initiation rate, within-bout response rate and bout length. Each parameter is thought to be affected by different classes of variables. Pentobarbital was used to assess the independence and sensitivity of these parameters in two mouse strains, BALB/c and C57BL/6, selected because of their different behavioral characteristics. With or without a running wheel present, BALB/c and C57BL/6 mice nose-poked under a Percentile 10:0.5 schedule designed to select high response rates while holding reinforcement rate constant. Baseline rates of nose-poking were higher for BALB/c mice than for C57BL/6 mice, but no strain difference occurred in baseline distance run. Nose-poking occurred at a higher rate when the wheel was absent from the chamber for both strains, and this was due to longer bout lengths and higher bout initiation rates. Nose-poke rates were increased by the 5.6-17 mg/kg doses of pentobarbital, especially in C57BL/6 mice. Pentobarbital only decreased running. No strain differences in pentobarbital sensitivity were observed for running. Whether reinforcement was extrinsic or intrinsic to the response was hypothesized to influence pentobarbital's effects. The different bout parameters helped dissect pentobarbital's effects and were selectively affected by pentobarbital.