Somatic and neuroendocrine responses to standard and biologically salient acoustic startle stimuli in monkeys

Abstract

The startle response, a simple defensive response to a sudden stimulus signaling proximal threat, has been well studied in rodents and humans, but has been rarely examined in monkeys. The first goal of the present studies was to develop a minimally immobilizing startle measurement paradigm and validate its usefulness by testing two core features of the startle response (habituation and graded responsivity) in squirrel monkey subjects. Two different types of startle stimuli were used: standard broad-band noise bursts, and species-specific alarm vocalizations ("yaps") which are elicited in response to threat in both wild and captive animals. The second goal of the present studies was to test whether yaps produce enhanced startle responsivity due to their increased biological salience compared to simple, non-biologically relevant noise bursts. The third goal of the present studies was to evaluate the hypothalamic-pituitary-adrenal (HPA) axis response to startle stimuli, as little is known about the stress-activating role of startle stimuli in any species. These experiments determined that the whole-body startle response in relatively unrestrained squirrel monkeys habituates across repeated stimulus presentations and is proportional to stimulus intensity. In addition, differential habituation was observed across biologically salient vs. standard acoustic startle stimuli. Responses to "yaps" were larger initially but attenuated more rapidly over trials. Responses to "yaps" were also larger in the early subepochs of the response window but then achieved a lower level than responses to noise bursts in the later subepochs. Finally, adrenocorticotropic hormone and cortisol concentrations were significantly elevated above baseline after startle stimuli presentation, though monkeys did not exhibit differential HPA axis responses to the two types of startle stimuli. The development of monkey startle methodology may further enhance the utility of this paradigm in translational studies of human stress-related psychiatric disorders.

Figure 1

Comparative time-domain and spectrographic representations of the standard noise burst (panels A and C) and biologically salient yap (panels B and D) startle stimuli. It is evident that these are highly contrastive physical stimuli. The noise burst offsets precede the onsets of the energetic portion of the yaps. Yap onsets are relatively graded in comparison to noise bursts. Yaps are also harmonically complex (possess distinct formants), segment, and extend in time.

Figure 2

The custom-built whole-body acoustic startle device is depicted in this drawing. The upper chamber is removed from the base and the animal transferred from below, after which a false floor is inserted. The chamber is then slowly righted and set onto the base. As the false floor is removed the animal steps down onto the instrumented platform. The platform, itself, is cantilevered from a low-friction hinge. An adjustable flat steel spring provides preload; a pair of small shock-absorbers (not shown) damp post-movement oscillations; the accelerometer transduces platform movements. Wide-band headphones (not shown) supply acoustic stimuli through large holes cut into the sidewalls of the upper chamber and guarded by acoustically transparent metal screens. Safety features include tape strips indicating the otherwise transparent enclosure walls and a padded ceiling. Further details are supplied in the text.

Figure 3

Monkey subjects (N=12) differentially habituated to repeated presentation of biologically salient yap vs. standard noise burst startle stimuli [TYPE by TRIAL interaction: (F(9,72) = 4.15, p < 0.007 _H-F]. Figure 3 depicts mean ± SEM whole body startle responses to 10 repeated stimulus trials collapsed across bins for both the noise burst and yap test sessions.

Figure 4

In a pattern formally similar to habituation over trials, responses to yaps tended to be larger in the early subepochs of the response window but achieved a lower level than responses to noise bursts in the later subepochs [TYPE by BIN interaction approached significance: (F(5,40) = 2.96, p = 0.071 _H-F]. Figure 4 depicts mean ± SEM whole body startle responses for the 6 subepochs, collapsed across all 10 trials for both the noise burst and yap test sessions (N=12 monkey subjects).

Figure 5

Monkey subjects (N=12) exhibited whole body startle responses monotonically related to stimulus intensity for both biologically salient yap and standard noise burst startle stimuli [F(2,16) = 7.05, p < 0.014 _H-F]. Figure 5 depicts mean ± SEM whole body startle responses to three different stimulus intensities for both the noise burst and yap test sessions.