Acoustic startle, prepulse inhibition, and fear-potentiated startle measured in rhesus monkeys

Modulation of the acoustic startle response is a simple and objective indicator of emotionality and attention in rodents and humans. This finding has proven extremely valuable for analysis of neural systems associated with fear and anxiety. Until recently, there have been few efforts to develop acoustic startle measurement in nonhuman primates. We report here development of a whole-body acoustic startle protocol in rhesus monkeys.

Eight juvenile rhesus monkeys were tested in a new whole-body (somatic) acoustic startle protocol. Startle responses were assessed in three paradigms: 1) stimulus intensity-response amplitude, 2) prepulse inhibition (PPI), and 3) fear-potentiated startle.

Initial studies revealed that the amplitude of whole-body startle in monkeys, as in rodents and humans, is directly proportional to acoustic stimulus intensity and gradually habituates with repeated exposures. Presentation of a weak acoustic stimulus 45-2020 msec before a startle stimulus reduces startle amplitude by 40%-50%, depending on interstimulus interval length (PPI). We have also measured significant potentiated startle amplitude in the presence of a visual stimulus after pairing it with an inescapable pulse of pressurized air (fear-potentiated startle).

Our data demonstrate that acoustic startle in nonhuman primates successfully bridges rodent and human research in two broad areas: stimulus-response relationships and behavioral plasticity represented by habituation, PPI, and fear potentiation. The opportunity now emerges to link concepts developed in rodents to the more complex neuroanatomical and cognitive processes common to monkeys and humans.