Abstract
Substance withdrawal presents a barrier to substance use disorder (SUD) recovery in part due to symptoms implicating neurotransmitters and their metabolites. Despite this relationship, research investigating alternative treatments such as psychotherapy incorporating equines (PIE) have primarily targeted vital signs and cortisol concentrations. Previous research has also proposed benefits of human-horse physiological coupling, but the role of neurotransmitters in the relationship between humans and horses during PIE has not yet been investigated. The purpose of this study was to characterize neurotransmitter concentrations of humans and horses throughout PIE in withdrawing SUD patients and determine neurotransmitter involvement in human-horse physiological coupling. Saliva samples were collected from humans (n = 6) and horses (n = 4) immediately prior to and following PIE sessions in the first and second week of SUD treatment. Concentrations of 3-methoxytyramine (3-MT) in the second week of treatment increased (P = 0.04) in human participants, and concentrations of serotonin (5-HT) and Homovanillic acid (HVA) decreased (P = 0.04; P = 0.03), which may be indicative of withdrawal symptoms improvement. Horses exhibited increased concentrations of 5-HT (P = 0.02) and decreased concentrations of 3-MT (P = 0.01), which could reflect positive interactions within PIE. Patients and horses also developed correlations in 5-HT (0.99, P = 0.05), DA (0.20, P < 0.01), 3,4-dihydroxy-phenyl acetic acid (DOPAC) (0.70, P = 0.01), and glutamate (0.66, P = 0.02) concentrations over the course of PIE sessions, indicating human-horse coupling associated and providing support of physiological synchronization between humans and horses participating in PIE.