Association of stimuli at long intervals in conditioned odor aversion

Odors associated with neonatal experiences with the dam have enhanced anxiolytic effects in rat

Odor stimuli are widely reported to promote relaxation and reduce anxiety in humans and rodents. However, it remains unclear if this anxiolytic efficacy can be further enhanced by association with positive experiences. Therefore, we compared the effects of a novel odor to a familiar odor previously paired with a positive experience on anxiety-like behaviors in rats. One group of Wistar-Imamichi female and male pups was exposed to an odor stimulus with their dams during postnatal days (PNDs) 8-12, whereas another control group was exposed to perfused air during the same period. Starting on PND 42, all animals were examined in the open field test (OFT) and elevated plus maze (EPM) test during exposure to scent-free air (vehicle), a novel odor, or the positive-familiar odor from postnatal exposure. In the EPM, female rats entered open arms with all 4 paws (complete entry) more frequently and spent more time on open arms during exposure to the positive-familiar odor than during exposure to air or a novel odor, whereas partial open arm entries with forepaws only were increased during exposure to both novel and positive-familiar odors compared to air. In contrast, male rats demonstrated no significant increase in open arm activity during positive-familiar odor exposure, but did show equally reduced grooming frequency during novel and familiar-positive odor exposure in the OFT. Exposure to positive-familiar odors may be an effective and safe method for anxiety reduction, especially in females.

Short-term memory reactivation of a weak CS-US association promotes long-term memory persistence in conditioned odor aversion

In conditioned odor aversion (COA), the association of a tasteless odorized solution (the conditioned stimulus [CS]) with an intraperitoneal injection of LiCl (the unconditioned stimulus [US[), which produces visceral malaise, results in its future avoidance. The strength of this associative memory is mainly dependent on two parameters, that is, the strength of the US and the interstimuli interval (ISI). In rats, COA has been observed only with ISIs of ≤15 min and LiCl (0.15 M) doses of 2.0% of bodyweight, when tested 48 h after acquisition (long-term memory [LTM]). However, we previously reported a robust aversion in rats trained with ISIs up to 60 min when tested 4 h after acquisition (short-term memory [STM]). Since memories get reactivated during retrieval, in the current study we hypothesized that testing for STM would reactivate this COA trace, strengthening its LTM. For this, we compared the LTM of rats trained with long ISIs or low doses of LiCl initially tested for STM with that of rats tested for LTM only. Interestingly, rats conditioned under parameters sufficient to produce STM, but not LTM, showed a reliable LTM when first tested for STM. These observations suggest that under suboptimal training conditions, such as long ISIs or low US intensities, a CS-US association is established but requires reactivation in the short-term in order to persist in the long-term.