Limitations of grouping subjects based on biological sex (males versus females) and a new approach: insights from intra-nucleus accumbens core dopamine-induced psychostimulant activity

Background In the field of substance use disorder research, sex-as-a-biological-variable (SABV) is employed to determine the mechanisms governing sex differences. Based on our recently developed MISSING (Mapping Intrinsic Sex Similarities as an Integral quality of Normalized Groups) model, we hypothesized that grouping subjects by biological sex does not represent the most effective way to group behavioral data objectively. To test our hypothesis, we conducted experiments to compare the psychostimulant effect of intra-nucleus accumbens (NAc) dopamine on groups based on 1) biological sex (current model) and 2) behavioral clustering (MISSING model) for effectiveness in identifying groups of subjects that a) are distinct with regards to behavioral variables, and b) confirm NAc dopamine neurochemical expression/activity topography (NEAT). Methods For the current model, we separated subjects (n = 37 Sprague Dawley rats, male n = 20, female n = 17) by biological sex prior to all assessments. For the MISSING model, we conducted normal mixtures clustering of baseline activity, dopamine activity (as distance traveled in cm over 60 min) and dopamine activity normalized-to-baseline activity (NBA) of all subjects to identify behavioral clusters. Results Separating groups by biological sex revealed groups (males and females) that were not clearly distinct with regards to behavioral variables and do not confirm NAc dopamine NEAT. Separating groups using the MISSING model revealed groups (behavioral clusters) that were clearly distinct with regards to behavior and confirm NAc dopamine NEAT. Conclusions Our results reveal the limitations of grouping subjects based on biological sex. We discuss a new approach.