Acoustic Startle

TECHNOLOGY UPDATE Acoustic Startle

KATHLEEN PRITCHETT, DVM, DIPLOMATE, ACLAM1 AND GUY B. MULDER, DVM, MS, DIPLOMATE, ACLAM2

Purpose. The startle reflex is noted in many species, including humans. Startle is a pattern of facial and skeletal muscle contractions, accompanied by an elevation of heart rate, which is mediated by neurons in the brainstem. This reflexive pattern is evoked by a sudden intense stimulus which may be tactile, auditory, visual, or olfactory. The muscle contractions associated with the startle reflex may provide protection against injury and may serve to prepare the animal to respond with fight or flight behavior (1). Startle responses may be made stronger or attenuated through the use of cues associated with the startling event. This provides a measure of an animal’s sensorimotor learning ability. The administration of anxiolytics may also attenuate the startle response (2). In rodents, the acoustic startle response is the most commonly evaluated. In animals with a normal hearing apparatus, acoustic stimuli of greater than approximately 80 decibels (dB) are necessary to elicit the reflex. Evaluation of the acoustic startle response has been proposed as part of a battery of behavioral tests for phenotyping genetically modified mice (3, 4). Methods. The basic equipment necessary to conduct acoustic startle reflex testing is a sound-attenuating chamber and a means to generate sound (Fig. 1 and 2). The chamber can be Plexiglas or metal but should be lined with sound attenuating material such as a foam liner. Usually, sound inside the chamber registers no more than 65-70 dB. The chamber may also be supplied with white noise at approximately 65 dB. The chamber is equipped with a speaker, through which the auditory startle stimulus is administered. In ad- dition, the chamber may contain an apparatus to expose the animal to a puff of air, a light, or a floor that can administer a mild foot Figure 1. Acoustic startle apparatus. Note the sound attenuating foam within shock. The startle response is measured by a piezoelectric strain gauge the chamber. When the lid is closed, the animals are also shielded from other in the floor of the chamber. When the animal startles, the strain environmental cues. (Photograph courtesy of Coulbourn Instruments, Allen- gauge measures the amplitude of the flinch. town, Pa.) To test the magnitude of the acoustic startle response, the animal is first placed within the chamber, either quiet or with a low level of white noise for an acclimation period of several minutes. After that acclimation period, the animal is exposed briefly (i.e.: 40 ms) to the startling noise, which may be either louder white noise or a particu- lar tone within the rodent’s normal hearing range. The strain gauge measuring the startle reflex is often attached to a computer or other device which records the animal’s response. A variable number of trials are conducted often utilizing variable sound intensities rang- ing from 70 to 120 dB, as animals may become habituated to the noise and the magnitude of their startle response will decrease. Typi- cally, animals respond reliably at sound intensities greater than 90 dB (5-7). The maximum sound intensity rodents are exposed to is typi- cally 120 dB. At approximately 120 dB and greater, animals may risk hearing loss and the vibrational effect of the noise becomes detectable and even deaf rodents may respond with a startle response (7). Variations. (i) Acoustic startle threshold. Acoustic startle threshold is the minimal sound intensity that produces a startle response. In this test, animals are exposed to variable sound intensities to de- termine the minimal sound intensity level that results in the animal Figure 2. Interior of acoustic startle apparatus. The animals are placed in a Charles River Laboratories, 251 Ballardvale St., Wilmington, Massachusetts 01887-10001; holder, which is placed on a sensor platform in the chamber. This platform is University Laboratory Animal Resources, University of California-Irvine, 147 BSA, Irvine, connected to a computer and measures the magnitude of the startle response. California 92612-13102 (Photograph courtesy of Coulbourn Instruments, Allentown, Pa.)

54 CONTEMPORARY TOPICS © 2004 by the American Association for Laboratory Animal Science Volume 43, No. 1 / January 2004 flinching. For example, an animal with impaired hearing will show shock apparatus is used, the device should be properly maintained an elevated acoustic startle threshold, and a deaf animal may not and tested regularly to ensure that the apparatus reliably delivers the show a startle response until 120 dB when vibrational effects be- desired foot shock. come detectable. (ii) Prepulse inhibition. One variation of the acoustic startle test References is prepulse inhibition. In this variation, the animal is warned of the 1. Koch, M. 1999. The neurobiology of startle. Progress in Neurobiology upcoming startling stimulus by the use of a softer noise first. When 59:107-128. an animal is exposed to the prepulse 30-500 milliseconds before the 2. Davis, M., W. A. Falls, S. Campeau, et al. 1993. Fear-potentiated startle stimulus, it decreases the magnitude of the startle response startle: a neural and pharmacological analysis. Behavioural Brain Re- considerably. Animals may be habituated to view many cues as warn- search 58:175-198. ing of the startle stimulus. Some trials use puffs of air; others use 3. Crawley, J. N. 1999. Behavioral phenotyping of transgenic and knock- flashes of light. out mice: experimental design and evaluation of general health, sensory (iii) Fear-potentiated startle. In this variation of acoustic startle, functions, motor abilities, and specific behavioral tests. Brain Research animals are conditioned to associate a cue, such as a light, a tone, or 835:18-26. 4. Nolan, P. M., D. Kapfhamer and M. Bucan. 1997. Random mutagen- a puff of air, with a painful stimulus such as a foot shock. This varia- esis screen for dominant behavioral mutations in mice. Methods tion greatly enhances the magnitude of the startle response when 13:379-395. the animals are presented with the cue and then the startle stimulus. 5. Glowa, J. R. and C. T. Hansen. 1994. Differences in response to an Animal welfare considerations. Since behavioral testing appara- acoustic startle stimulus among forty-six rat strains. Behavior Genetics tuses are often difficult or impossible to disassemble and thoroughly 24:79-84. disinfect due to their complexity, or the materials used, this must be 6. Bullock, A. E., B. S. Slobe, V. Vaszquez, et al. 1997. Inbred mouse taken into consideration when planning tests. The use of shared strains differ in the regulation of startle and prepulse inhibition of the equipment or serial trials involving animals of different health sta- startle response. Behavioral Neuroscience 111:1353-1360. 7. Crawley, J. N. 2000. What’s wrong with my mouse?: behavioral tuses may expose “clean” animals to unwanted pathogens. If a foot phenotyping of transgenic and knockout mice. Wiley-Liss, New York.