Neutral stimuli that are repeatedly paired with reward act as predictors of impending reward, and can acquire incentive salient properties, measured using Pavlovian-conditioned approach. Individual differences in Pavlovian-conditioned approach have been observed in a subset of animals. In sign-trackers, the CS becomes ‘attractive’ and ‘wanted’, whereas in goal-trackers, the CS retains informational properties that signal the availability of reward. Most studies have investigated sign- and goal-tracking using food and drug reward, as these phenotypes may confer vulnerability or resistance to the development of addictive behaviours, respectively. To date, the expression of sign- and goal-tracking in response to a sexually-conditioned cue has been limited to male Japanese quail.

We are the first to assess individual differences in Pavlovian-conditioned approach in response to a cue paired with sexual reward leading to the ejaculatory state. We found evidence of sign-tracking, as subjects approached, engaged, and spent more time near the cue paired with the opportunity to ejaculate with a sexually-receptive female. Goal-tracking was also observed in subjects that approached and spent more time near the location where sexual reward was delivered. 

Next, we compared the stability of sign- and goal-tracking to a sucrose- and sex-paired cue, and whether phenotypic differences are consistent across different types of natural reward. Sucrose goal-trackers fluctuated between cue- and goal-directed responses, though a statistical trend revealed a tendency to spend more time near the location where sexual reward was delivered. Sucrose sign-trackers appeared to ‘shift’ their behavioural phenotype, as they demonstrated goal-directed behaviour in response to a sexually-conditioned cue.  

Lastly, we explored whether the chronic systemic administration of oxytocin influences the expression of Pavlovian-conditioned approach in response to a sexually-conditioned cue. Oxytocin did not enhance nor diminish sign-tracking behaviour, however it potentiated goal-tracking responses in intermediate subjects that typically fluctuate between cue- and goal-directed behaviours.

Collectively, we demonstrate that individual differences in Pavlovian-conditioned approach develop in response to a sexually-conditioned cue in male rats, likely as they acquire incentive salience due to their pairing with sexual reward. Furthermore, sign- and goal-tracking are expressed differentially based on the type of natural reward and neuropeptide influence.