Stimulus quality affects expression of the acoustic startle response and prepulse inhibition in mice.

The relationship between stimulus intensity and startle response magnitude (SIRM) can assess the startle reflex and prepulse inhibition (PPI) with advantages over more commonly used methods. The current study used the SIRM relationships in mice to determine differences between white noise and pure tone (5 kHz) stimuli. Similarly to rats, the SIRM relationship showed a sigmoid pattern. The SIRM-derived reflex capacity (RMAX) and response efficacy (slope) of the white noise and pure tone stimuli in the absence of prepulses were equivalent. However, the pure tone startle response threshold (DMIN) was increased whereas the stimulus potency (1/ES50) was decreased when compared to white noise. Prepulses of both stimulus types inhibited RMAX and increased DMIN, but the white noise prepulses were more effective. Both stimulus intensity gating and motor capacity gating processes are shown to occur, dependent on prepulse intensity and stimulus onset asynchrony. Prepulse intensities greater than 10 dB below the startle threshold appear to produce PPI via stimulus intensity gating, whereas a motor capacity gating component appears at prepulse intensities near to the startle threshold.

Entrainment of circadian rhythm to a photoperiod reversal shows retinal dystrophy in RPE65(-/-) mice.

Light entrainment of circadian rhythms is mediated by classical "visual" photoreceptors (rods and cones) as well as "nonvisual" photoreceptive elements (light-detecting cells that do not contribute to classical "vision"). This paper aimed to assess whether light entrainment of locomotor circadian rhythms in mice with impaired rods and cones differs from normal controls and whether this technique, alongside existing techniques, could be used to assess visual function. The study was primarily interested in differences between the entrainment of circadian rhythms of normal-sighted C57Bl/6J mouse and the C57Bl/RPE65 knockout mouse (RPE65(-/-)), although C3H/HeJ (rd/rd) mice were included as a preexisting model of retinal degeneration. Circadian rhythms of motor activity before and after a 12-h light reversal were assessed in custom-built cages that continuously monitored movement. The controls showed a significantly higher mesor and amplitude when compared to the RPE65(-/-) and rd/rd mice. Despite the loss of rods and cones, the RPE65(-/-) and rd/rd maintained a 24-h circadian rhythm entrained to light similar to controls and were capable of circadian reentrainment to a 12-h light reversal. Importantly, this light reentrainment of the circadian phase occurred at a significantly slower rate in the retinal degenerate models than in the controls. The RPE65(-/-) model demonstrates a retinal degenerate reentrainment phenotype when compared to the rd/rd model. It is suggested that these retinal degenerate mice retain the ability to detect light for the purposes of circadian rhythm entrainment. However, alterations of specific parameters of the circadian rhythm with loss of rods and cones may provide measures of loss of visual function (sight).