Fig. 1

Schematic summary of potential biomarkers of the experiential state of an animal. These biomarkers are potentially involved in the shift to a more positive experiential state. The blue and red arrows represent the full spectrum of experiential state from negative (red end) to positive (blue end). The biomarkers were selected because they have been associated with neurobiological disorders (mood instability, anxiety, or depression) or psychological processes involved in emotion, temperament, or personality. Several of the candidate biomarkers of experience act in the central nervous system and are also present in the peripheral circulation, such as orexins, BDNF, oxytocin, IGF-1, and endocannabinoids. However, only peripheral levels of IGF-1 and endocannabinoids have been associated with changes in central processes that are linked to emotion or neurobiological disorders (illustrated by the orange arrows). MicroRNAs, glycoRNAs, and NEAT1 could be efferent signals that are associated with experiential state and therefore could be measured in biological fluids such as serum, plasma, or saliva (green arrows). The experiential state of an animal could also affect its level of thiol oxidation, SIH, and CRT (green arrows) which could then be used as biomarkers (green arrows). The HPA axis and the stress response are not good biomarkers of experiential state because they are modulated by a large array of internal and external factors. Telomere attrition could be a long-time marker of experiential state, with a more positive experiential state resulting in less attrition. In addition of being produced in the brain tissue, BDNF is also produced in the peripheral tissue such as plasma and saliva. Abbreviations: BDNF: brain-derived neurotrophic factor, CRT: circadian rhythm of core body temperature, ECBs: endocannabinoids, HPA axis: hypothalamic–pituitary–adrenal axis, IGF-1: insulin-like growth factor 1, SIH: stress-induced hyperthermia