Olfactory learning in the rat immediately after birth: Unique salience of first odors

Abstract

An infant rat’s chance of survival is increased when it remains close to the nest. Early olfactory learning supports such adaptive behavior. Previous experiments indicated that non-associative odor exposure immediately after birth promoted later attachment to a similarly scented artificial nipple. The goal of the current experiments was to extend these findings on olfactory learning in the hours after birth by: exposing pups to more than one odor exposure (Exp. 1), dissecting the role of timing versus order of odor exposure (Exp. 2), testing the odor specificity of these effects (Exp. 3 and 4), and evaluating associative odor conditioning soon after birth (Exp. 5). Without explicit prior odor experience, pups only hours old do not respond much to a novel odor. Prior non-associative odor experience increases later motor activity to that same odor and to novel odors. Furthermore, these findings may be specific to certain amodal dimensions of the (in our case) lemon odor exposure. Single odor non-associative and associative conditioning was equally effective immediately after birth and during the third postnatal hour. Nevertheless, pups given two mere odor exposures responded to the first one more than the second at test, regardless of whether the exposures began immediately or two hours after birth. Possible mechanisms for these findings concerning early olfactory learning are discussed.

Distinct neurobehavioral dysfunction based on the timing of developmental binge-like alcohol exposure

“…For example, the primary olfactory system (including the olfactory bulb and piriform cortex) is functional at birth in rodents (Brunjes, 1994, Miller and Spear, 2009, Sarma et al, 2011), and in fact is necessary for infant survival (Moriceau and Sullivan, 2004). In contrast, the hippocampal formation which receives robust olfactory input via the entorhinal cortex, is a relatively late developing system with hippocampal-dependent behaviors not emerging until near weaning in rodents (Freeman et al, 1994, Rudy, 1994, Raineki et al, 2010).…”

Mere odor exposure learning in the rat neonate immediately after birth and 1 day later

“…Previous experiments did not find consistent sex effects in olfactory learning immediately after birth (Miller & Spear, 2008, 2009). Therefore, although sex was evenly distributed across groups, this was not a factor in the present design.…”

“…Sensitization, or increased responsiveness after successive exposures, is unlikely however since previous experiments have found that animals given two odor exposures prior to activity testing did not exhibit levels of activity higher than pups with only one odor exposure (Miller & Spear, 2009). Given the apparent lack of odor specificity in the effect of such neonatal odor exposure (Miller & Spear, 2009), it is also possible that such responding could indicate odor-induced neurophysiological changes in the olfactory bulb. Yet, odor recognition and neural changes in the olfactory bulb are not mutually exclusive explanations.…”

“…Motor activity may signify either an appetitive response (an attempt to move toward the odor) or an aversive response (an attempt to move away from the odor). It is interesting to note, however, that when odor exposure on P0 was paired with pulses of appetitively reinforcing milk, motor activity was increased beyond that of pups given exposure to the odor alone (Miller & Spear, 2009). Furthermore, several studies have documented odor preferences resulting from mere odor exposure in rat pups during their second postnatal week (e.g., Alberts & May, 1984; Caza & Spear, 1984).…”

“…Given the absence of prior experience with the dam, it was not surprising that P0 pups responded differently than P1 pups in the presence of an odor not previously experienced. It may be advantageous for a pup to respond uniquely to the first odor it is exposed to because the first odors are likely to be the dam or nest odors (Miller & Spear, 2009; Romantshik, Porter, Tillmann, & Varendi, 2007). If prior dam exposure were the driving force behind the current age-related differences we would expect that giving P0 rats exposure to a dam prior to odor exposure should result in a pattern of results more similar to what was seen in P1 rats.…”