Individual differences in the immobility behavior in juvenile and adult rats are associated with monoaminergic neurotransmission and with the expression of corticotropin-releasing factor receptor 1 in the nucleus accumbens

• Behavioral differences in the FST are associated with differential expression of CRFR1 in the nucleus accumbens.
• Behavioral differences in the FST are related to accumbal dopaminergic neurotransmission.
• Individual differences in depression-related behaviors are not echoed by anxiety-like responses.
• Age is a key modulator of anxiety- and depression-like behaviors.

The study of individual differences provides an important methodological approach to analyze the neurobehavioral spectrum of a given cohort in order to understand brain function and disease. Based on immobility time in the forced swimming test (FST) juvenile and adult rats were classified as subgroups with low and high immobility. Afterwards, we compared behavior, neurochemical parameters, and gene expression profiles in some brain areas of rats with low and high immobility only. No differences in the open field test (OFT) were observed between subgroups. Regarding neurochemistry, juvenile animals with low immobility showed higher accumbal dopamine turnover and lower hippocampal norepinephrine concentrations, whereas adult rats only differed for accumbal dopamine, although in an opposite direction from that observed in juveniles. Moreover, the expression of accumbal corticotrophin-releasing factor receptor 1 (CRFR1) was significantly different in animals with low and high immobility at both ages, with animals less immobile showing higher levels of CRFR1 mRNA levels. Taken together, our findings suggest that differences in monoaminergic neurotransmission and CRFR1 expression are associated with the coping strategy adopted by the animal and with the tendency to develop depression-related behaviors. Concerning monoaminergic neurotransmission such association is modulated by age, and such modulation could be related to the differential behavioral results observed between juvenile and adult rats.