Memorization of contextual and CS conditioned fear response (freezing) in a one-trial acquisition paradigm.

In fear-conditioned Wistar rats freezing was induced by the delivery of a series of footshocks paired to tones (CS) in a specific conditioning chamber (context). CS and contextual fear were acquired in the same single conditioning session without preexposition to the conditioning chamber (day 1). Different groups of animals were conditioned employing three increasing US (footshock) intensities (0.25, 0.5, 0.75 mA). During the retention sessions context and CS conditioned freezing (fear response) were measured using a paradigm that fulfilled the following conditions: i) CS freezing retention was measured in a context different from the conditioning one; ii) CS and context freezing were measured at increased delays after the training session (days 3 and 4, 14 and 15, 28 and 29). The results show that there are significant differences between CS and context freezing retention, which are clearly related to delay after the initial session and to US intensity. In particular: 1) conditioned freezing to a discrete tone is better retained than conditioned freezing to context (irrespective of US intensity); 2) context freezing is directly related to US intensity much more than to tone freezing; 3) context freezing is easier to extinguish than tone freezing. The results are discussed in relation to previous ones and to their relevance to freezing genesis neural correlates.