2.7. Acoustic Startle and Prepulse Inhibition (PPI)

In experiment 2, the same design of TBI and PTSD was examined for acoustic startle reflex and PPI. Following SPS and/or a CCI depth of 2.5 mm, mice were placed in isolation boxes inside a clear acrylic tube affixed over a piezoelectric accelerometer (San Diego Instruments) that captured animal movement in response to precisely calibrated white noises of varying intensity emitted from a loudspeaker. For each trial, a testing box was left empty, acting as an ambient sensor to capture any background noise that may offer interference to the collected readings. At no point did the ambient sensor register movement higher than that of a box containing an animal. Testing began 5 minutes after mice were placed in the chambers. The test consisted of 66 trials divided into 11 blocks. Each block consisted of null trials with background 65 dB noise only, startle only trials consisting of the pulse (120 dB) alone, and prepulse trials of 72, 80, or 84 dB preceding a 120 dB pulse (fixed interstimulus interval (ISI) = 100 ms). Trial order within each block was pseudorandomized so that no trial type occurred consecutively, and trials were separated by a variable 15 s ITI (maximum ITI = 20 s, minimum ITI = 10 s). All testing occurred between 0900 and 1100 h. A schematic of acoustic startle and PPI apparatus is shown in Figure 4(a).

Comorbid TBI + PTSD does not affect acoustic startle response or prepulse inhibition. (a) Schematic of experimental design, acoustic testing chamber, and startle protocol. (b) Acoustic startle response. There were no changes in acoustic startle response across groups when the 120 dB pulse was played without a prepulse. (c) Prepulse inhibition. There was a significant interaction between prepulse level and treatment group on inhibition of the startle response to the pulse; however, there were no group differences in overall PPI. Background noise was constantly generated at 65 dB. Error bars ± SEM. dB = decibel.