Revealing a latent variable: individual differences in affective response to repeated injections

Area of Science:

• Neuroscience
• Behavioral Neuroscience
• Psychopharmacology

Background:

• Latent variables in experimental designs can compromise reproducibility.
• Individual differences in affective responses to stress are a potential confounding factor.
• The novelty-seeking phenotype in rats (high responders/HR and low responders/LR) models differential emotional reactivity.

Purpose of the Study:

• To investigate the impact of chronic injection stress on depressive-like behavior in HR and LR rats.
• To determine if individual differences in stress reactivity influence responses to repeated intraperitoneal injections.
• To highlight the importance of accounting for emotional variability in animal studies.

Main Methods:

• Classified experimentally naïve outbred rats into HR and LR groups based on locomotor reactivity to a novel environment.
• Administered 14 daily intraperitoneal saline injections to HR and LR rats.
• Assessed social behavior and depressive-like behavior using the forced swim test.

Main Results:

• Repeated saline injections did not alter basal phenotypic differences in social behavior between HR and LR rats.
• Injections significantly increased immobile time in the forced swim test for HRs.
• Injections significantly decreased immobile time in the forced swim test for LRs, compared to handled controls.

Conclusions:

• Individual differences in stress reactivity significantly affect depressive-like responses to repeated intraperitoneal injections in rats.
• Emotional variability within standard rat populations can impact study reliability and replicability.
• Recommends including an uninjected control group in studies involving repeated drug administration to account for variability.