Re-examining the ontogeny of the context preexposure facilitation effect in the rat through multiple dependent variables

The capability to acquire context conditioning does not emerge until weaning, at least when the defining features of the context lack explicit and salient olfactory cues. Contextual learning deficits in preweanling rats have been associated with functional immaturity of the dorsal hippocampus. According to recent studies, the so-called context preexposure facilitation effect (CPFE) – a hippocampus-dependent effect – is not observed until postnatal day 23 (PD23). In these studies the footshock intensity employed was higher (1.5 mA) than in adult studies, and context conditioning was inferred from a single behavioral measure (percentage of freezing). The present study examined the CPFE on PD17 and PD23 by analyzing multiple dependent variables, including fecal boli and an ethogram covering the complete behavioral repertoire of the rat. A non-shocked control group was included in the design and two footshock intensities were employed (0.5 and 1.5 mA). Results showed clear evidence of contextual fear conditioning in preweanling and weanling rats, as well as evidence of conditioned fear in non-preexposed rats from both age groups. In some cases, some dependent variables, such as grooming or vertical exploration, were more sensitive than freezing for detecting evidence of memory. Strong fear responses were detected in weanling (but not preweanling) rats, when rats were evaluated in a different context from the one employed at conditioning. These results indicate that preweanling rats are capable of acquiring contextual conditioning, even in a context lacking explicit odor cues, and highlight the importance of multiple dependent variables for analyzing the ontogeny of memory.