Serotonin and Tryptophan Serum Concentrations in Shelter Dogs Showing Different Behavioural Responses to a Potentially Stressful Procedure

Abstract

In mammals, serotonin (5-HT) levels depend on the availability of tryptophan (TRP). Low 5-HT concentrations have been linked to behavioural disorders in dogs. This study aimed at investigating possible differences in dogs’ serum TRP and 5-HT concentrations according to their behavioural response to a potentially stressful procedure. Thirty-nine physically healthy shelter dogs, 15 females and 24 males, mean age = 5.6 years, were categorized by a certified veterinary behaviourist according to their behavioural response to medical examination and blood collection, in: relaxation, stress signals, tension without growling, tension with growling, escape attempts, and aggression attempts. Extraction and quantification of 5-HT and TRP were performed using a HLPC method. Data were statistically analysed, applying Chi-square and Spearman tests. Results showed no significant difference in TRP (χ2 = 2.084, p = 0.555) nor 5-HT (χ2 = 0.972, p = 0.808) serum concentrations among different categories of dogs; however, some categories were underrepresented (relaxation = 20.5%, stress signals = 30.8%, tension without growling = 43.6%, tension with growling = 5.1%, escape attempts = 0%, aggression attempts = 0%). No correlation between serum TRP and 5-HT concentrations was found (ρ = 0.086, p = 0.602). Serum 5-HT levels do not seem to be associated with dogs’ behavioural response to a stressful situation nor with serum TRP concentrations. The relationship between serum TRP and 5-HT concentrations and behaviour needs further research.

The Variation of Serotonin Values in Dogs in Different Environmental Conditions

“…The average serotonin value in medium breed shelter dogs was 397.62 ng/mL, similar to the results obtained by Alberghina et al [ 24 ], who determined average serotonin values of 400 ng/mL. Riggio et al [ 34 ] observed no association between serum TRP and 5-HT levels in their sample of shelter dogs. Because it is widely accepted that TRP and 5-HT concentrations are correlated in the brain, negative findings may be at least partially explained by the role of the blood–brain barrier, regulating the passage of TRP from peripheral to central circulation and that of 5-HT in the opposite direction.…”

Urinary neurotransmitter analysis and canine behavior assessment

“…Although, urinary neurotransmitter deviations have been previously associated with different neuropsychological conditions in humans, those results were not transferable to the investigated canine population, and its value for canine behavioral medicine could not be confirmed from these results. This lack of conformity is following some earlier serum serotonin level studies and may be explained by the fact that a direct correlation between the central nervous system (CNS) and the urinary neurotransmitter levels only have been shown to a limited extent in previous studies (18,19,54,55). During neurotransmitter transfer through the body, transporters of the blood-brain barrier (BBB) and the kidney modulate their concentration, whereby the BBB is not permeable for every neurotransmitter (18).…”

The effects of melatonin on the concentrations of inflammatory cytokines and proteins, serotonin, cortisol and melatonin in ovariohysterectomised female dogs

“…Oral melatonin administration did not affect cortisol concentration in intact female dogs. Two possible explanations have been suggested to be responsible for the effect of melatonin on reducing cortisol concentration: (a) melatonin increases serotonin concentration, and increased serotonin, in turn, decreases cortisol concentration in animals with aggressive behaviour and during stress conditions (Riggio et al., 2021 ) and (b) it is possible that melatonin reduces cortisol secretion and concentration through its receptors in the adrenal gland (Frank et al., 2004 ).…”

“…Serotonin is a neurotransmitter with various regulatory functions in physiologic, emotional and behavioural processes. There is an association between aggressive behaviour and low levels of serum serotonin and 5‐hydroxyindoleacetic acid, the primary metabolite of serotonin in dogs, in the cerebrospinal fluids of dogs (Riggio et al., 2021 ). Serotonin is the key intermediate in melatonin synthesis from tryptophan (Zhao et al., 2019 ).…”