Habituation, prepulse inhibition, fear conditioning, and drug modulation of the acoustically elicited pinna reflex in rats.

A reliable component of the acoustic startle response in animals is a flexion of the ears, the pinna reflex. One goal of the present investigation was to develop a preparation and apparatus suitable for analyzing the pinna reflex in the awake rat. A second goal was to examine the pinna reflex under behavioral and pharmacological conditions known to affect other response systems, such as whole-body startle, and determine whether the pinna response is a valid model for analyzing behavioral plasticity. The amplitude of the pinna reflex in spinally transected rats was directly related to stimulus intensity and exhibited short-term habituation, prepulse inhibition, and enhancement by prior fear conditioning. Also, pinna response amplitude increased following systemic administration of strychnine and was decreased following clonidine. It is concluded that this preparation and response system are valid for studying various forms of behavioral modification. Implications of these findings are discussed in terms of the potential utility of this response system in investigating the cellular correlates of behavioral plasticity in mammals.