Abstract
The acoustic startle response (ASR) and its modulation by non-startling prepulses, presented shortly before the startle-eliciting stimulus, is a broadly applied test paradigm to determine changes in neural processing related to auditory or psychiatric disorders. Modulation by a gap in background noise as a prepulse is especially used for tinnitus assessment. However, the timing and frequency-related aspects of prepulses are not fully understood. The present study aims to investigate temporal and spectral characteristics of acoustic stimuli that modulate the ASR in rats and gerbils. For noise-burst prepulses, inhibition was frequency-independent in gerbils in the test range between 4 and 18 kHz. Prepulse inhibition (PPI) by noise-bursts in rats was constant in a comparable range (8–22 kHz), but lower outside this range. Purely temporal aspects of prepulse–startle-interactions were investigated for gap-prepulses focusing mainly on gap duration. While very short gaps had no (rats) or slightly facilitatory (gerbils) influence on the ASR, longer gaps always had a strong inhibitory effect. Inhibition increased with durations up to 75 ms and remained at a high level of inhibition for durations up to 1000 ms for both, rats and gerbils. Determining spectral influences on gap-prepulse inhibition (gap-PPI) revealed that gerbils were unaffected in the limited frequency range tested (4–18 kHz). The more detailed analysis in rats revealed a variety of frequency-dependent effects. Gaps in pure-tone background elicited constant and high inhibition (around 75%) over a broad frequency range (4–32 kHz). For gaps in noise-bands, on the other hand, a clear frequency-dependency was found: inhibition was around 50% at lower frequencies (6–14 kHz) and around 70% at high frequencies (16–20 kHz). This pattern of frequency-dependency in rats was specifically resulting from the inhibitory effect by the gaps, as revealed by detailed analysis of the underlying startle amplitudes. An interaction of temporal and spectral influences, finally, resulted in higher inhibition for 500 ms gaps than for 75 ms gaps at all frequencies tested. Improved prepulse paradigms based on these results are well suited to quantify the consequences of central processing disorders.
Highlights
The acoustic startle response (ASR) of mammals is a fast contraction of facial and skeletal muscles evoked by a sudden and intense sound (Swerdlow et al, 1999)
Stimulus and response are processed in the primary pathway of the ASR, a simple reflex-eliciting neuronal circuit containing only a few synapses located in the lower brainstem (Davis et al, 1982; Koch, 1999)
The present study examined in detail the dependence of the startle response on temporal and spectral characteristics of modulatory acoustic stimuli in rats and gerbils as these characteristics obviously influence methodologies based on ASR, which become more and more relevant especially for tinnitus assessment in rodents
Summary
The acoustic startle response (ASR) of mammals is a fast contraction of facial and skeletal muscles evoked by a sudden and intense sound (Swerdlow et al, 1999). Processing in the primary startle pathway is under the control of various modulatory influences from different areas of the brain, e.g., the pedunculopontine tegmental nucleus and laterodorsal tegmental nucleus reaching the primary pathway at the level of the PnC (Fendt et al, 2001). This is the neuronal basis for a variety of modulations of the magnitude of ASR behavior, most prominently the modulation by a non-startling prepulse presented shortly before the startleeliciting noise. Comparable results were found in mice (Willott and Carlson, 1995)
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